Opinion essay format

Professor D. Professor W. Professor R. North Dr. Emin, Division NMDissertation directory. Grosvenor Professor J. Gregson, VG Elemental Ltd. Organization basic essay format, Kobe Steel Ltd. Professor Dr. Werkstoffwissenscbaften, Pulvermetallurgisches Laboratorium, llcisenbergstr. Professor T. Page, Department of Metallurgy and.

Universitat Slavery essay titles, A, Professor F. Institut fur fur Metallforschung, Max-Planck-lnstitut G. Schneider, Dr. Werkstoffwissenschaften, Pulvermetallurgiaches Laboratorium, Heisenbergstr. Thompson, Dept. Professor M. Professor H. Chemical routes for the preparation of selected non-oxide ceramic powders, namely silicon carbide, silicon nitride, aluminium nitride, titanium diboride, boron carbide and boron nitride have been reviewed.

Although these techniques include conventional ones such as powder mixing, this article is primarily concerned with nonconventional ceramic syntheses, that is, polymer pyrolysis, non-aqueous liquid phase reactions, gas phase reactions and sol-gel processing. Powder properties such as particle size, corresponding to different synthetic techniques are described. This review concludes that chemical synthesis can make a significant contribution to the development of ceramic precursors for non-oxide powders.

These techniques are referred to as non-conventional when they exclude powder mixing, fusion and coprecipitation methods, that is, the three formal essay kal% c4% b1plar% c4% b1 synthetic routes to ceramic powders. Non-conventional techniques have the potential for preparation of powders with greater chemical purity and homogeneity than is attainable from conventional syntheses.

They also allow fabrication of ceramic coatings and fibres, geometries which can be difficult to make by conventional syntheses. The nonconventional syntheses can often be assigned to one of six categories [1]. Thus, sol-gel processing involves either use of aqueous oxide colloids or hydrolysis of metal-organic compounds, usually alkoxide solutions.

Publications on sol-gel have dominated the scientific literature on ceramic synthesis but these two techniques are applicable, almost exclusively, to oxide systems. Hydrothermal synthesis of ceramic powders has not attracted the attention, as measured by the output of publications, which has been afforded to solgel and it is also restricted to oxide systems.

The remaining three 3 R. Freer ed. Prinmedin the Netherlands. This short review describes chemical preparative routes for selected carbide, nitride and boride powders with particular emphasis on recent developments in non-conventional syntheses.

Use of Powder Reactants Two of the three principal methods for production of titanium diboride involve powder reactants. The first method is carbothermic reduction of titanium dioxide and boron oxide at around K while in the second conventional synthesis titanium dioxide is reduced by carbon at about K in the presence of boron carbide. Both reactions have the same disadvantage, namely, considerable grinding of the titanium diboride product is required for particle size reduction.

Gas Phase Reactions The third principal method for synthesis of scholarship essay letter format diboride involves hydrogen reduction of titanium tetrachloride and a boron halide at high-temperature, typically K.

The low temperature decomposition is an advantage in this synthesis as is the availability of a soluble ceramic precursor which had coating applications [4]. Use of Powder Reactants Many methods have been used for production of hexagonal boron nitride powders on the laboratory scale and for manufacture [5]. Although reactants are cheap it rhetorical essay format necessary to leach out formal essays from magazines filler from the boron nitride product, an operation which enhances the process costs.

Boron nitride has also been prepared by reacting calcium cyanamide, Ca CN2with boron oxide between K but the product contains calcium oxide which is dissolved in an acid-wash. Silica is more readily available than silicon metal and another manufacturing route involves carbothermic reduction of silica in a mixture containing fine silica and carbon powders followed by nitridation at K.

The nitridation of ultra pure silicon is quite impossible to complete due to the formation of protective niare required during layer. Hard agglomerates have to be ground to obtain the powder.

How to Write an Opinion Essay: Step-by-Step Guide

One of the non-stoichiometry of the reactants by comparison with reaction 2 presented in Table 2, an excess of carbon always or above close to being used. Table 3 shows the routes used for the production of the 40 different powders tested for this paper.

Table 3 : Silicon nitride yowders classified by their synthesis rout. Chemical impurities. Table 4 shows powders. Direct nitridation of silicon involves the retention of other metals such as aluminium or calcium, alloyed in the reacting silicon. Carbon level is generally in the range 0. Iron, used as a catalyst in the nitridation reaction, is found at a relatively high level in some powders; unreacted silicon is detected in all the powders.

Such decrease of the metallic impurity content is obtained by using purer starting silicon as well as by washing the powder in acids.

Very pure silicon nitride powder is therefore available but XPS analysis reveals the presence of surface fluorine species in LC12SX for instance. Oxygen content is also 19 difficult to control in this synthesis route because of the writing a dissertation chapter 1 of the hard agglomerates formed during the reaction at high temperature.

The A powder synthesis reaction 2 is mainly characterised by the high levels of oxygen 2. The powders produced from silicon halides reactions 3 and 4 are the purer ones when considering the metals content excepted for the reaction 1 - LC12SXbut traces of chlorine are, however, found. For this series of powders, grades recently introduced for instance TS1O for TS7give also improved purity : lower carbon and chlorine levels. The physical characteristics of selected powders are given in Table 5, which shows powders with high surface area, small primary crystal size as well as low beta silicon nitride content between 5 and 10 percent are today largely available.

The compaction behaviour of the powders can be evaluated on the basis of the tap densities Rho t or of the green densities reached after uniaxial compression Rho 0.

Taking into account these two parameters, silicon nitride powders provided by the nitridation of silicon generally show a better behaviour than other series due to their wider size distribution the tap density of LC12SX is an exception. SEM pictures Figure 1 show that grains are usually irregular, as a result of the post-milling process. Microscopic examination of the grains of SH Figure 2 reveals large-scale occurrence of whiskers, the presence of which can be probably attributed to growth through vapour transport.

The presence of whiskers in this powder leads, as a consequence, to a very low tap density and to very bad compaction behaviour, which impeded the shaping of samples by uniaxial pressing under the standard conditions used in this study. Therefore, the sintering evaluation of this powder was carried out by hot-pressing only. The powders formed by diimide decomposition as well as the A powder silica carbothermal reduction are more regularly shaped Figures 3 and 4 but do not have high compacted densities, due to their narrow size distribution.

Such a figure seems to be the minimum acceptable value for raw materials to be used for conventional sintering. Figure 6 shows the strengths of materials prepared by powder compaction followed by conventional sintering under rhetorical essay format nitrogen over-pressure 0.

The data presented in Figure 5 show that the heat treatment conditions used in these experiments do not allow high density to be reached, and therefore inhibit the attainment of good mechanical performance see circles on Figure 6. However, such a procedure provides a good comparitive tool for assessing the sinterability of the powders.

The strengths can be satisfactorily correlated with the bulk densities Figure ewhich means 22 that the most fully densified materials around 2. Fisure 5 : Bulk densities of conventionally sintered nmpats for the 3 systems of additives. The results of the mechanical characterisation bend strength are also presented in Figure 6, together with corresponding data for hot-pressing experiments.

Sintering under 0. When processed under 1. It is, however, found that unexpectedly low values are obtained for some high bulk density compacts. One example is the hot pressed A powder, which strength reaches only for a bulk density of MPa, and Kic 3.

An additional cause of decreasing strength is grain growth of beta-Si3N4 occurring in wide particle size distribution powders such as AE or AE Figure 6 : Strength resistance of sintered compacts as a function of hulk denaitv. As in pressureless sintering, the starting density is dependent on the green density of the compact, and is increased by a wider particle size disin the starting powder, and the amount of sintetribution ring during heating and on application of pressure.

Thereafter, densification will depend mainly on the inverse of the particle size, but if the particle size range is wide, coarsening of the particles may occur, leading to a loss of sinterability as densification proceeds. Although the value of m is dependent on the mechanism of densification, change of mechanisms for powders processed under identical conditions are less likely to occur than changes in rate for a given mechanism.

Indeed value of the figure of merit is clearly smaller and does not increase with density. The ideal silicon nitride powder for the fabrication of high density components by the SSN process is the powder which will provide a high green density to reduce the firing shrinkage and improve the precision of shape of the productreach close to theoretical density to eliminate porosity and improve the mechanical propertiesin a reasonable time, with a controlled microstructure complicated in the case of silicon nitride by the phase change from alpha to beta-Si3N4 and corresponding morphological changesbut avoiding abnormal grain growth and formal essays from magazines porosity.

This must be achieved without the presence of a large amount of low viscosity liquid, in addition to that intended as a result of the deliberate dopants. Oxide or cation impurities in the silicon nitride may increase the volume and decrease the viscosity of the liquid phase, promoting densification but degrading high temperature properties, further complicated by the presence of halogens or carbon from the synthesis or processing procedures used during the manufacture.

From consideration of the data presented in this paper though without the evaluation of high temperature behaviour some recently introduced grades of silicon nitride powder show promise. Cambier, F. Bunk and H. Hausner, Verlag DKG, Gazzara, C. Wotting, G. Brook, R. Cannon, W. The TG curves generally show a sigmoidal shape. It is a characteristic feature of the reaction that it comes to an almost complete stop when only a fraction of the original Si has reacted. The reaction is markedly dependent on the N2 pressure; higher pressure gives higher initial rate but a lower final extent of reaction.

SEM investigations show that the silicon surface is covered with nitride crystals already after a short time of reaction, and subsequent nitridation takes place on the outside of this initial nitride layer. The observations have been interpreted in terms of a model with nucleation and branching growth, coupled with a self-blocking channel model. The four parameters of the model show characteristic dependences on the nitrogen formal essays from magazines.

The temperature dependences are less clear because of the short temperature span used. Introduction Silicon nitride powders for ceramic applications may be produced by a number of methods, including both gas phase and solution reactions, see, e. The direct reaction between silicon and nitrogen, however, appears as one of the simplest and most economical routes. This reaction is also of interest when silicon nitride articles are produced by first forming the article from silicon powder by pressing or slip casting, followed by treatment in nitrogen at a suitable temperature.

This communication does not, however, deal directly with the production of silicon nitride powder or ceramic articles. It is intended as a more fundamental study of the kinetics of the direct reaction, but silicon was used in the form of a fine powder in order to retain some similarity to processes of ceramic interest.

Primed in the Netherlands. Materials The powder was prepared from electronic-grade silicon stock Ventron-Alfa Products, Karlsruhe, Germany by crushing in an iron mortar, followed by ball-milling in a polyethylene container using iron balls.

Subsequently the silicon was wet-sieved using stainless steel sieves to a size fraction of gm. Spectrometric analysis of the dried powder showed 15 ppm Fe as the only detectable contaminant. Assuming spherical particles, the specific -face area was calculated to 6. In most of the thermogravimeuic runs, this powder was used with addition of 1.

Starck, Berlin, Germany with particle size. The method minimizes the free energy and the theoretical diagrams are compared to experimental observations [ Heat and mass transfers, and diffusion kinetics in the gas phase are related to the morphology and composition of the deposits A recent review is available [ Depending on conditions, hot cracking of a mixture of iodides B13 and CI4 gives the tetragonal boron phase B48B2C2 or the orthorhombic carbide B8C [23,43,44].

Using the dangerous gaseous mixture B2H6 diboraneCH4 and H2 a plasma induced CVD [48] or a toroidal glow discharge [49], produce amorphous boron-carbon alloy films, with low hydrogen content, similar to those obtained by using BBr3 [42].

This deposit is very promising as coating materials for fusion reactor walls 49]. To hinder interactions with metals they should be protected by boron carbide deposits obtained by CVD, using BC13 or BBr3 as boron source cf. Boron carbide fibers can be prepared directly by reaction of boron obtained by the H2 reduction of BC13 on carbon fibers formal essay kal% c4% b1plar% c4% b1 Complex fiber e.

C structures should by consolidated by in-depth low pressure CVD [57] or by cracking of organic precursors, like carboranes [58]. Crystal growth 4. B4C single crystals, mm long, are obtained by CVD reaction b [59], or by reduction of B by graphite in an electrical arc [1,60].

A 6 mm diameter sintered boron carbide rod can be zone melted under Ar [61]. Single crystals needles up to 1 mm in length could have 90 also a high carbon content: B2. Mixtures of coarse platelets and whiskers are now industrially available [64]. Powders Powders may be prepared i using reactants treated by physical technics laser, rhetorical essay format, electron beam ii or by heating, cracking or reacting diverse inorganic or organic chemicals.

Powders of B4C stoechiometry, opinion essay format of excess B or C, were prepared through optimization dissertation deutsch gas flow rates and laser intensity. The powders are loosely agglomerated, of high purity, consisting of equiaxed particles and exhibiting a MPa, narrow size distribution. Microstructure revealed examples of uniform and exaggerated grain growth. Mechanical properties were good [].

Finally, B4C powders, with a particle size of 6. These powders have a X-ray pattern characteristic of B4C [79] ; using the same technic we obtained non-organized boron carbide deposits cf. Carbothermalreduction of boric anhydride. A mixture of acetylene every clouds has a silver lining essay, or sugar with ethylene glycol, is heated with boric acid at Organization basic essay format, in a stream of Ar ; the resulting B4C powder is fine grained tn [80].

Similarly mixtures of boric acid with ethylene glycol [81] or glycerin [ may be used. Fine carbide powder 0. The resulting solid contains very fine specific surface Ceramic-ceramic multiphase compounds may be obtained by the same method [ This method of powder synthesis should be promising, but it is not well known. Boron carbide is ill-crystallized and may be mixed with SiC, Formal essays from magazines Mixed phases may be produced: - B4C-BN by cracking of borate esters [, an amine adduct of decaborane in a stream of Ar [90] B4C-SiC-SiO2 by cracking carborane-siloxane polyborodiphenylsiloxane [ Conclusions In industry, and even in the laboratory, the boron carbide powder obtained by arc melting, with a boron rich stoechiometry B4.

Direct synthesis has no pratical use. Concerning powders for the future, laser or plasma, give very fine, narrow sized, pure powders, with different stoechiometries, and reactive powders, which should be of great interest for sintering. The method is still in the R. Cracking of organic precursors does not seem to organization basic essay format appropriate for an economic preparation of powders : yields are low, sometimes the final product is polyphased, the precursors ae difficult to synthesise and very expensive.

But should students have homework yes or no development of twophase ceramic materials from single polymers blends as a method of tailoring desired I think that boron organic precursors should be convenient scholarship essay letter format sintering additives for industrial, good quality, boron carbide powders ; we showed that boron carbide can be pressureless sintered with the addition of both carbon and silicon carbide organic polycarbosilane precursors.

It was necessary to introduce a minimal amount of carbon which could act in deoxidizing the boron carbide starting grains and to create a second phase which inhibits grain coarsening, thus improving mechanical properties [12]. An organic boron precursor should eliminate the residual SiC. Ceramists need classified whiskers mean diameter pm and narrow sized distributed platelets of boron carbide, which could be used to reinforce ceramics borides, carbides Lipp, A.

Production, Properties, application', Techn. Beauvy, M. Schwetz, K. SchweMz ILA. Th6venot F. Reinmuth, K. Bouchacourt, M. Conard, J. Thivenot, F. Cueilleron, J. Matkovich ed. Benton, S. Lifshits, E. Ostapenko, I. SSSR, Neorg. Brodhag, C. Vincenzimi ed. Ceramic powders, Elsevier, Amsterdam, pp. Bougoin, M. Treutler, C. Cochran, A.

Metals, 22, Ploog, K. La"iue S. Rey, J. Amberger, E, Druminsi, M. Bernard, C. Ducarroir, K. Chemical Vapor Deposition Fft Intern. Ducarroir, M. Vandenbulcke, L. Basutcu, M. Vandenbulcke, L, Vuillard, G. Vandenbulcke, L "Theoretical influence of the deposition conditions on the uniformity of boron carbide coatings', Proc. Bloem, G. Verspui and L. Wolff eds. Vandenbulcke, L 'Theoretical and experimental studies on the chemical vapor deposition of boron carbide', Ind.

Chem Prod. Jansson, U. Thin solid Films, Rebenne, H. Am Ceram. Phase and chemical composition', Thin solid films, in press. Olsson, M. Sdderberg, S. Morphology and unicrostructure', Thin solid films, in press. Cholet, V. Ploog, K. I Tetragonales und orthorhombisches Borcarbid', J. Campbell, A. Kevill, D.

Veprek, S. Plasma Chem. Toyoda, H. I Domaine d'existence de la phase carbure de bore', J. Eschnauer, H. Thbault, J. Naslain, R. Pailier, R. Park, Y. Smith, W. Newkirk, L. Walker, B. Jr, Rice, R. Mierzejewska, S. Sugaya, T. Borchert, W. Brief essay format, A. Aselage, C. Beckel, I. Howard and C. Physics, New York, pp. Mackinnon, I. Weaver, S. Morse ed. TMS Annu. Gatti, A. No 1, Ahmad, I. Mitchell ed.

Motoc, C. Bosch, F. Wickens, A. London7, Becherescu, D. Bucharest15 2 Vogt, G. Cruiziat, B. Lyon F. Mac Kinnon, I. Lon, on Fauchais ed. Plasmas, 3rd, Paper S. Limoges F8 pp. Knudsen, A. WO 86 04, US Appl. Ramsay, J. Dufek, G. Harris, G. Wada, H. Koshida, R.

Kokai JP 62, Tanemoto, H. Kouvetakis, J. Ritter, J. Wada, K- Ito, S. Mirabelli, M. Ceram Soc. Rees, W. Jr and Seyferth, D. Wynne, K. Van Deusen, and R. The electronic and thermal transport properties of refractory icosahedral boron-rich solids suggest the utilization of these unique solids as high-temperature semiconductors. Such applications will require high-quality, well-characterized materials.

A variety of techniques have been used to prepare powders, ceramics, and single crystals of boron carbides and the icosahedral boron pnictides. X-ray diffraction, Raman spectroscopy, and ultrasound measurements have been used to probe boron carbide sample quality.

Each of these measurements supports the structural model of boron carbides developed from analysis of transport data. Initial electrical conductivity measurements on these samples show no thermal hysteresis. The boron-rich pnictides and a-boron are wide bandgap insulators, while boron carbides are hopping-type semiconductors with exceptional transport properties. These mechanical and electrical properties derive from the unusual structures and bonding arrangements in the solids.

These structures, depicted in Figure 1, are based on boron-rich icosahedra [1,2]: twelve-atom, twenty-sided polyhedra, with each side an equilateral triangle.

The simplest structure, that of a-boron, is shown in Organization basic essay format la. Here, a Bl2 icosahedron is located at each vertex of the rhombohedral unit cell. The icosahedra are bonded to one another by very strong twocenter bonds along the unit cell edges and by inter-icosahedral threecenter bonds that lie along the long diagonal of the unit cell.

These three-center bonds are represented by dark triangles in Figure la. The icosahedral boron-rich pnictides and the boron carbides are derived from this simple structure. In the pnictides, depicted in Figure lb, the three-center bonds of a-boron are replaced by a two-atom "chain" containing two phosphorus or arsenic atoms.

In the boron carbides, Figure 1c, a three-atom chain lies along the long cell diagonal. At 97 R. Proi In the Netherlands. Structural changes that accompany changes in carbon concentration will be discussed in detail below. Structures of icosahedral borides. Despite the similarity of structure, the electronic properties of the boron carbides and the boron pnictides are quite different. The boron pnictides are wide-bandgap, transparent insulators [4] at room temperature. The electronic structure of the boron-rich pnictides is discussed elsewhere in this volume [5].

If doping can be controlled appropriately, these materials have the potential for utilization as high-temperature semiconductors [6]. Several applications currently utilize the high-temperature electronic properties of boron carbides. Most of our work has focussed on the potential of boron carbides in very-hightemperature thermoelectric energy conversion [8], where boron carbides are the most efficient p-type material at high temperatures.

Additionally, the thermally-activated electrical conductivity provides the basis for thermistors for use as automotive engine temperature sensors [9]. Details of the theory of the bipolaronic conduction mechanism [11], the electronic and thermal transport measurements [12], and the L 99 electronic structure of boron carbides [5] formal essay kal% c4% b1plar% c4% b1 presented elsewhere in this volume. Here, it is sufficient to note that one result of the transport analysis is a model organization basic essay format changes in the structure of boron carbides as carbon content is varied.

According to the transport-based model and a recent analysis of the change in free energy of boron carbides as boron substitutes for carbon [13], B-B-C chains first replace C-B-C chains as carbon content is reduced from B4 C. Further reduction in carbon content is accomplished by replacing some of the icosahedral carbon atoms with boron. This model accounts for the p-type behavior of boron carbides over the entire single phase region. The replacement of C-B-C chains by C-B-B chains is also believed to be related to the transition in the thermal conductivity from crystal-like at high carbon concentration to glass-like at low carbon content [11,12,14].

Other structural models, based on x-ray diffraction data [] or 1 3 C NMR results [18], differ with that proposed above. Our model, in particular, runs counter to the usual proposition that boron carbide with the composition B1 3 C2 is comprised of B1 2 icosahedra and C-B-C chains [15,17].

Clearly, the use of boron-rich solids as electronic materials will place new demands on the quality of materials. In this paper, we shall describe the preparation of powders, ceramics, and single crystals of the icosahedral boron-rich solids.

In addition, we will describe characterization techniques that we have employed to determine the sample quality. We will review the results of measurements [] that provide evidence to support our structural model for the boron carbide solid solution. Finally, we shall comment on several areas for future work to enhance or expand the useful properties of these interesting materials. In order to prepare small amounts of material with variable stoichiometry, we have used the solid-state reaction of a high file a police report, sub-micron amorphous boron powder 2allery Chemical Co.

These powders were primarily for use in precision lattice parameter determinations. Following this treatment, some amount of unreacted starting material was occasionally observed by x-ray diffraction. The resulting powder patterns have very narrow diffraction peaks and, for sample compositions in the single phase region, show no evidence of second phases.

Powders of boron carbide are very difficult to sinter into dense ceramics. We have used uniaxial hot pressing to prepare dense ceramic samples. Boron powders, with either natural abundanre or isotopicallyenriched in 11 B or 1 0 B Eagle-Picher Industries were mixed by shaking for several hours with the high purity graphite powder. The use of ball mills in the mixing process was avoide as it leads to contamination of the sample.

Graphite rams were used to apply pressure, and either graphite felt graphoil or tungsten foil was inserted between the boron carbide and the rams to prevent bonding high purity BN crumbled when used between the rams and the sample. Here, the atmosphere was switched to gettered argon and full pressure psi applied. After a 30 minute formal essay kal% c4% b1plar% c4% b1 for the interior of the die to reach the set temperaturethe sample was held at temperature for 15 minutes. Typically, the sample must be forced from the die liner, and the graphoil or tungsten foil bonds to the sample and must be removed by grinding.

Small, shaped pieces of boron carbide were prepared by cutting with a low-speed diamond saw. Boron carbide crystals are also of interest for structural and transport studies. Due to the refractory nature of boron carbide and problems associated with the incongruent melting [23] of the solid solution, growth of crystals from the melt is prohibitively difficult.

Solution growth from metallic fluxes has, however, been used successfully to produce small several mm crystals of boron carbide 24]. Transition metals with filled or nearly-filled d shells particularly Cu and Pd have been used most successfully; these metals either do not form metal borides or they form less stable borides with no direct boron-boron bonding. Details of the growth conditions are given in Ref.

PBN crucibles and tubes were used to minimize contamination and to provide flat temperature profiles. Crystals were recovered by dissolution of the metal matrix in aqua regia. I0I Considerably less work has been performed on the preparation of samples of the icosahedral boron-rich pnictides.

Powders of these solids may be prepared by the thermal decomposition in vacuum of BP or BAs [24]. These crystals exhibited a continuous range of opacity, from clear to black. These differences may be due to differences in Pd-rich inclusion density in the crystals. Because of the high vapor pressure of P or As at temperatures required for sintering or hot pressing, attempts to prepare dense ceramics of the boron-rich pnictides formal essay checker not been successful.

Single crystal x-ray diffraction studies have been described elsewhere [1, Precision lattice parameter determinations employed a Si internal standard. Assignment of observed boron carbide reflections was based on the JCPDS B4 C standard [25], and lattice parameter refinement was performed using Siemens least squares software. Raman spectra were obtained using a computer-controlled, scanning, double monochromator equipped with holographic gratings and a photoncounting photomultiplier tube [26].

Small single crystal samples were illuminated using a microscope rhetorical essay format, while larger samples were illuminated with the laser beam focused to a 0. The Background features Rayleigh scattering, fluorescence were removed by an empirical fitting procedure, and count axes have been scaled so as to normalize one of the more intense Raman bands in each spectrum. Ultrasonic travel time data were obtained on thin boron carbide bus service essay with flat and parallel sides ranging from approximately 1.

Both longitudinal and shear data were taken by the pulse-echo overlap method [27]. Longitudinal wave velocities were determined on all three faces of the specimen using 50 and 20 MHz transducers and the average was taken for moduli calculations.

The shear wave velocity could only be determined on the large face of the specimens using a 20 MHz transducer. This technique was particularly useful in identifying samples containing small cracks. Four point electrical conductivity measurements were also made on the thin slabs of boron carbide described above. Gold wires were wrapped around the samples and were held in place at notches cut into the boron carbide with mm spacings. Good electrical contact was assured by holding the sample between two flat, spring loaded PBN disks.

The sample assembly and lead and thermocouple wires were contained within a quartz tube through which gettered Ar was passed. This tube was placed into the heated zone of a tube furnace equipped with a Na heat pipe.

The programmable current source, digital voltmeter, and furnace controller were digitally-controlled using a minicomputer. Current levels were auto-selected 5-lOOmA to provide voltage drops between 1 and 10 mV. The current during each measurement was precisely determined by measuring the voltage drop across a standard resistor in series with the sample. Forward and backward voltage drops were averaged to eliminate the effect of thermoelectric voltages.

This criterion was usually met on heating. At low temperatures results are seen to be in reasonably good agreement with the work of Bouchacourt and Thevenot [34 on hot-pressed samples. Several factors may contribute to the large scatter in the rest of the data. First, the composition of boron carbides is frequently not well known.

Thus, for compositions reported as B4 C, a values larger than 5. Second, small impurity concentrations can have a very large effect on the lattice parameter 2]. Also, internal stresses may affect lattice parameter values. Thus, boron carbide lattice parameters are indicative of carbon concentration for pure samples, but are sensitive to small impurity concentrations and internal stresses.

We shall discuss another technique for verifying carbon content below. Hexagonal lattice parameters of boron carbides vs. The variation of the lattice parameters with carbon concentration also supports our structural model of the boron carbide solid solution.

The general increase in the size of the unit cell as carbon content decreases is consistent with the replacement of carbon atoms covalent radius 0. The a axis changes uniformly over the entire single phase region, with about 0. The c axis, on the other hand, increases by 0. We can understand the q axis behavior in terms of the structural model. Figure 3 shows the hexagonal unit cell of boron carbides viewed along one of the basal plane unit.

The icosahedra in these solids are more compressible than the chains, so that the size of the unit cell is controlled by the size of the chains [1]. By replacing C-B-C chains with C-B-B chains, with a larger boron atom at one end and a B-B bond that is much weaker and softer than a C-B bond [14], we expect a longer chain unit and thus an increase in the q axis length. Further substitution of B for C occurs within icosahedra, with a much smaller effect on the size rhetorical essay format the unit cell.

Hexagonal unit cell of boron carbide, viewed along a basal plane unit vector. The c axis is vertical on the page, and the three-atom chains are parallel to c. Raman Spectroscopy has proven to be a very powerful tool in the analysis of boron-rich solids.

Brief essay format

In addition to local structural information, Raman spectra provide a semi-quantitative probe for carbon content in boron carbides and a check for the presence of amorphous carbon. In an earlier study of the Raman spectra of hot-pressed boron carbides, Shelnutt et al.

These Raman bands scholarship essay letter format clearly shown to be related to the presence of amorphous carbon in the samples. Indeed, they were absent from the spectra of chemical-vapor deposited material [36]. These carbon bands are also absent or vanishingly small in the Raman spectra of our hot-pressed boron carbide samples, indicating a substantial improvement in sample quality compared to the material used in the earlier studies.

If a small amount of excess carbon is intentionally added during sample preparation, the amorphous carbon bands are readily observed. We do note, however, that infraredactive vibrational modes with frequencies near cm"1 are present in our samples.

Thus, the IR mode comes from the boron carbide structure, and is most likely a chain-related vibration [37,38]. The broad bands with frequencies greater than cm "1 are relatively insensitive to the carbon content. Below B13C2, a band near cm"1 grows. These changes in the Raman spectra provide a semi-quantitative, nondestructive probe for carbon content in boron carbides.

We have used this formal essay kal% c4% b1plar% c4% b1 to identify the carbon content in boron carbide single crystals discussed below and to study carbon gradients in hot-pressed samples due to contact with graphite or a tungsten foil during the pressing. Carbon from the graphoil readily diffuses into boron-rich boron carbides. Typically, pm must be removed from each end of a b sample before the surface spectrum matches one taken from the interior side of the sample.

Tungsten foil was C used in an attempt to eliminate such diffusion. Ta form Raman Shift, cm-1 eutectic liquids with boron at Fig.

Raman spectra of boron temperatures lower than the carbides with indicated pressing temperatures. Thus, compositions.

Based on an empirical comparison of formal essay checker spectra of single crystals of a-boron, Bl2As2and B4 C and of B1 2 P2 powders [19], we have assigned Raman bands with frequency greater than cm-I to icosahedral vibrations, and bands with frequencies between and cm-1 to chain vibrations.

These assignments are corroborated by normal coordinate calculations 39] and by the variation in intensity of the chain bands as the single crystals are rotated with respect to the polarization of the laser beam.

The Raman spectra shown in Figure 4 thus provide evidence of structural changes with composition in boron carbides.

Consistent with the assignments described above, the narrow bands centered at cm-1 are due to vibrations of the C-B-C chains [19]. The narrow Raman bands have virtually disappeared at B13 C2, the composition at which all of the C-B-C chains have been replaced.

The icosahedral bands of boron carbides are broad compared to the other icosahedral borides. Broad bands are more characteristic of a disordered solid or amorphous material than a well-ordered, crystalline material. We have interpreted this result 19] as being indicative of carbon atoms distributed among inequivalent sites in B 1 1 C icosahedra. Thus, in a-boron or the boron-rich pnictides, B1 2 icosahedra exist in a well-ordered environment and the Raman bands are narrow.

In BxC, B1 1 C icosahedra are inherently disordered due to the distribution of carbon in inequivalent sites. This disorder is manifested veterans essay ideas the broad icosahedral bands.

Because the bands are broad over the entire single phase region, carbon must be present in the icosahedra at all compositions. Thus, the Brief essay format spectra of boron carbides support our structural model: 1 near B4 C, B1 1 C icosahedra and C-B-C chains are the predominant structural units, 2 between B4 C and B13C2, boron replaces carbon in the chains, forming C-B-B chains, and 3 only for carbon concentrations below B 1 3 C2 does boron begin to replace carbon in icosahedral sites.

We have attempted to prepare single crystals with lower carbon content in order to perform additional structural studies. B:C ratios from to and a variety of metal solvents Cu, Pd, Pt, Ni have been employed in the solution growth procedure. We find that, regardless of B:C ratio and solvent choice, the resulting crystals have Raman spectra characteristic of B4 C. In addition, the lattice parameters of a number of individual crystals and of a best professional resume writing services jobs generated from crushed crystals are very close to the values for B 4 C.

For the more boron-rich starting compositions, 6-boron crystals are formed with the B4 C crystals. We conclude that, formal essays from magazines the wide single phase region of boron carbides, only B4 C crystals may be grown by the metal flux technique. This suggests that one reason for the width of the opinion essay format phase region of boron carbides formal essay kal% c4% b1plar% c4% b1 the relative instability of a-boron.

The free energy of boron is lowered by dissolution in the molten metal, and appears to be lowered even further by the presence of carbon in solution [40]. This lowering of the free energy of boron in solution drives the composition of boron carbide in equilibrium with the solution to its most energetically favorable value, near B4 C [13].

In order to better understand the thermal transport behavior of boron carbides, we have measured the velocity of longitudinal and shear acoustic waves as a function of composition. The results of these measurements are shown in Figure 5. The longitudinal sound velocity in boron carbides, 1. The cracking in samples 1. The elastic constants Fig. Longitudinal and shear derived from the sound velocity acoustic velocities VI and VS of measurements using Equations 1 boron carbides vs.

The values for B4 C are in reasonable agreement with the literature [41]. We understand the large decrease in modulus below BI3 C2 to reflect a change in the stiffness of the most compressible structural unit, the icosahedron, as B1 1 C icosahedra are replaced by B12 icosahedra [11].

Once again, the composition dependence of the sound velocities can be explained using the structural model. Thus, the simple kinetic theory result Equation 7 for the thermal conductivity appears to be inadequate for boron carbides. An alternate formulation of the thermal transport is described elsewhere in this volume [11]. The room temperature electrical conductivity was measured with a table-top version of the four scholarship essay letter format apparatus. The conductivity does not vary monotonically with carbon concentration; rather, the opinion essay format conductivity is observed at B1 3 C2 - 23 0-cm Once again, these observations are 1E6 in agreement with the predicted composition dependence [6,7,11].

Previous measurements of the temperature dependence of the electrical transport properties of boron carbides always lE5 displayed some amount of thermal hysteresis [42]. This result ". Experimental difficulties large background b U noise were encountered during the initial heating of the B4 C IE3 sample.

When this was corrected, the heating and cooling curves nlo a Heating were quite reproducible and returned to the room temperature A A Cooling values determined in the tabletop apparatus. The B13C 2 conductivities were also 1E2 reproducible with thermal cycling. Conduction in the boron Fig. Electrical conductivity vs. B1 3C2. Heating and cooling sequence The temperature dependence of as indicated. Interestingly, the conductivity of the B1 3 C 2 sample has more curvature than B4 C at both the low- and hightemperature ends.

These differences may reflect the increasing disorder present in B 1 3 C2 compared to B4 C, as 1 the CBB chains have organization basic essay format orientation up or downwhereas CBC chains are symmetrical, and 2 the chain-bonding sites within an icosahedron may be more accessible to icosahedral carbon when bonded to the boron end of a CBB chain. We shall continue these measurements on other low-carbon boron carbides to further explore such effects.

We have shown that high quality boron carbide powders, ceramics, and crystals can be made, and that x-ray diffraction, Raman spectroscopy, and ultrasonic measurements are useful probes of phase purity, composition, and sample quality. The structural model for the boron carbide solid solution is supported by a vast array of measurements, including the lattice parameters, Raman spectra, acoustic velocity, and electrical conductivity data described in this paper.

The essential features of this model are 1 the presence of B1 1 C icosahedra throughout the single phase region, and 2 the replacement of C-B-C chains with C-B-B chains formal essay kal% c4% b1plar% c4% b1 carbon content is reduced from B4 C.

Thus, many questions regarding the properties of boron carbides have been addressed. Our work in the future will address several remaining structural questions. In particular, we will try to understand the IR spectra of boron carbides in terms of our structural model, and we shall use 1 3C MASS NMR to further study the location of carbon in the solid solution as a function of composition. In addition, we shall continue to measure the high-temperature electronic transport properties conductivity and Seebeck coefficient of our samples to study the effect of composition and disorder.

Finally, we shall study the possibility of preparing n-type boron carbides by doping. These ntype boron carbides would then form the basis for an all boron carbide thermoelectric couple.

We foresee a growing effort in the preparation of large, pure icosahedral boron pnictide materials. We shall attempt to prepare ceramics by hot isostatic pressing and crystals by melt growth steps in writing essay pressure. Given suitable samples, we shall attempt to identify suitable n- and p-type dopants and try to control doping to appropriate levels for high temperature semiconductor applications.

Morosin, B. Emin, T. Howard, and C. Wood, eds. Aselage, and C. Less-Common Metals, 82, Slack, G. Switendick, A. Emin, D. Wood, C. See, for example, Ceramic Bulletin, A8, Hunold, K. B 29, ; and Wood, C. B 31, Will, G. B35, ; Kirfel, A.

Opinion: It's been 75 years since US flag flew over Iwo Jima

B35, Yakel, H. Larson, A. Duncan, T. Tallant, D. Solids,; Tallant, D. B, accepted. Aselage, T. Gieske, J. Bose, D. Materials and Processes, 2, Less-Common Metals, 67, III Papadakis, E. Galbraith Laboratories, Inc. Chambers, W. Growth 74, Mierzejewski, S. Less-Common Metals, 8, Less-Common Metals, 59, Matkovich, V.

Less-Common Metals, 47, 39 Less-Common Metals 26, 25 Shelnutt, J. Becher, H. Werheit, H. Assignment reference, C.

Storms, E. Less-Common Metals Thompson, Dept. Professor M. Professor H. Chemical routes for the preparation of selected non-oxide ceramic powders, namely silicon carbide, silicon nitride, aluminium nitride, titanium diboride, boron money happiness essay and boron nitride have been reviewed.

Although these techniques include conventional ones such as powder mixing, this article is primarily concerned with nonconventional ceramic syntheses, that is, polymer pyrolysis, non-aqueous liquid phase reactions, gas phase reactions and sol-gel processing.

Powder properties such as particle size, corresponding to different synthetic techniques are described. This review concludes that chemical synthesis can make a significant contribution to the development of ceramic precursors scholarship essay letter format non-oxide powders.

These techniques are referred to as non-conventional when they exclude powder mixing, fusion and coprecipitation methods, that is, the three conventional synthetic routes to ceramic powders. Non-conventional techniques have the potential for preparation of powders with greater chemical purity and homogeneity than is attainable from conventional syntheses. They also allow fabrication of ceramic coatings and fibres, geometries which can be difficult to make by conventional syntheses.

The nonconventional syntheses can often be assigned to one of six categories [1]. Thus, sol-gel processing involves either use of aqueous oxide colloids or hydrolysis prentice hall algebra 1 answers metal-organic compounds, usually alkoxide solutions.

Publications on sol-gel have dominated the scientific literature on ceramic synthesis but these two techniques are applicable, almost exclusively, to oxide systems.

Hydrothermal synthesis of ceramic powders has not attracted the attention, as measured by the output of publications, which has been afforded to solgel and it is also restricted to oxide systems.

The remaining three 3 R. Freer ed. Prinmedin the Netherlands. This short review describes chemical preparative routes for selected carbide, nitride and boride powders with particular emphasis on recent developments in non-conventional syntheses.

Use of Powder Reactants Two of the three principal methods for production of titanium diboride involve powder reactants. The first method is carbothermic reduction of titanium dioxide and boron oxide at around K while in the second conventional synthesis titanium dioxide is reduced by carbon at about K in the presence of boron carbide.

Both reactions have the same disadvantage, namely, considerable grinding of the titanium diboride product is required for particle size reduction. Gas Phase Reactions The third principal method for synthesis of titanium diboride involves hydrogen reduction of titanium tetrachloride and a boron halide at high-temperature, typically K. The low temperature decomposition is an advantage in this synthesis as is the availability of a soluble ceramic precursor which had coating applications [4].

Use of Powder Reactants Many methods have been used for production of hexagonal organization basic essay format nitride powders on the laboratory scale and for manufacture [5].

Although reactants are cheap it is necessary to leach out the filler from the boron nitride product, an operation which enhances the process costs. Boron nitride has also been prepared by reacting calcium cyanamide, Ca CN2with boron oxide between K but the product contains calcium oxide which is dissolved in an acid-wash. Silica is more readily available than silicon metal and another manufacturing route involves carbothermic reduction of silica in a mixture containing fine silica and carbon powders followed by nitridation at K.

The nitridation of ultra pure silicon is quite impossible to complete due to the formation of protective niare required during layer. Hard agglomerates have to be ground to obtain the powder. One of the non-stoichiometry of the reactants by comparison with reaction 2 presented in Table 2, an excess of carbon always or above close to being used. Table 3 shows the routes used for the production of the 40 different powders tested for this paper.

Table 3 : Silicon nitride yowders classified by their synthesis rout. Chemical impurities. Table 4 shows powders. Direct nitridation of silicon involves the retention of other metals such as aluminium or calcium, alloyed in the reacting silicon. Carbon level is generally in the range 0. Iron, used as a catalyst in the nitridation reaction, is found at a relatively high level in some powders; unreacted silicon is detected in all the powders.

Such decrease of the metallic impurity content is obtained by using purer starting silicon as well as by washing the powder in acids. Very pure silicon nitride powder is therefore available but XPS analysis reveals the presence of surface fluorine species in LC12SX for instance. Oxygen content is also 19 difficult to control in this synthesis route because of the brief essay format of the hard agglomerates formed during the reaction at high formal essay checker.

The A powder synthesis reaction 2 is mainly characterised by the high levels of oxygen 2. The powders produced from silicon halides reactions 3 and 4 are the purer ones when considering the metals content excepted for the reaction 1 - LC12SXbut traces of chlorine are, however, found. For this series of powders, grades recently introduced for instance TS1O for TS7give also improved purity : lower carbon and chlorine levels.

The rhetorical essay format characteristics of selected powders are given in Table 5, scholarship essay letter format shows powders with high surface area, small primary crystal size as well as low beta silicon nitride content between 5 and 10 percent are today largely available. The compaction behaviour of the powders can be evaluated on the basis of the tap densities Rho t or of the green densities reached after uniaxial compression Rho 0.

Taking into account these two parameters, silicon nitride powders provided by the nitridation of silicon generally show a better behaviour than other series due to their wider size distribution the tap density of LC12SX is an exception. SEM pictures Figure 1 show that grains are usually irregular, as a result of the post-milling process. Microscopic examination of the grains of SH Figure 2 reveals large-scale occurrence of whiskers, the presence of which can be probably attributed to growth through vapour transport.

The presence of whiskers in this powder leads, as a consequence, to a very low tap density and to very bad compaction behaviour, which impeded the shaping of samples by uniaxial pressing under the standard conditions used in this study.

Therefore, the sintering evaluation of this powder was carried out by hot-pressing only. The formal essays from magazines formed by diimide decomposition as well as the A powder silica carbothermal reduction are more regularly shaped Figures 3 and 4 but do not have high compacted densities, due to their narrow size distribution. Such a figure seems to be the minimum acceptable value for raw materials to be used for conventional sintering. Figure 6 shows the strengths of materials prepared by powder compaction followed by conventional sintering under different nitrogen over-pressure 0.

The data presented in Figure 5 show that the heat treatment conditions used in these experiments do not allow high density to be reached, and therefore inhibit the attainment of good mechanical performance see circles on Figure 6. However, such a procedure provides a good comparitive tool for assessing the sinterability of the powders.

The strengths can be satisfactorily correlated with the bulk densities Figure ewhich means 22 that the most fully densified materials around 2. Fisure 5 : Bulk densities of conventionally sintered nmpats for the 3 systems of additives. The results of the mechanical characterisation bend strength are also presented in Figure 6, together with corresponding data for hot-pressing experiments.

Formal essays from magazines

Sintering under 0. When processed under 1. It is, however, found that unexpectedly low values are obtained for some high bulk density compacts. One example is the hot pressed A powder, which strength reaches only for a bulk density of MPa, and Kic 3. An additional cause of decreasing strength is grain growth of beta-Si3N4 occurring in wide particle size distribution powders such as AE or AE Figure 6 : Strength resistance of sintered compacts as a function of hulk denaitv.

As in pressureless sintering, the starting density is dependent on the green density of the compact, and is increased by a wider particle size disin the starting powder, and the amount of sintetribution rhetorical essay format during heating and on application of pressure.

Thereafter, densification will depend mainly on the inverse of the particle size, but if the particle size range is brief essay format, coarsening of the particles may occur, leading to a loss of sinterability as densification proceeds. Although the value of m is dependent on the mechanism of densification, change of mechanisms for powders processed under identical conditions are less likely to occur than changes in rate for a given mechanism.

Indeed value of the figure of merit is clearly smaller and does not increase with density. The ideal silicon nitride powder for the fabrication of high density components by the SSN process is the powder which will provide a formal essay checker green density to reduce the firing shrinkage and improve the precision of shape of the productreach close to theoretical density to eliminate porosity and improve the mechanical propertiesin a reasonable time, with a controlled microstructure complicated in the case of silicon nitride by the phase change from alpha to beta-Si3N4 and corresponding morphological changesbut avoiding formal essays from magazines grain growth and trapped porosity.

This must be achieved without the presence of a large amount of low viscosity liquid, in addition to that intended as a result of the deliberate dopants. Oxide or cation impurities in the silicon nitride may increase the volume and decrease the viscosity of the liquid phase, promoting densification but degrading high temperature properties, further complicated by the presence of halogens or carbon from the synthesis or processing procedures used during the manufacture.

From consideration of the data presented in this paper though without the evaluation of high temperature behaviour some recently introduced grades of silicon nitride powder show formal essay kal% c4% b1plar% c4% b1. Cambier, F. Bunk and H. Hausner, Verlag DKG, Gazzara, C. Wotting, G. Brook, R. Cannon, W. The TG curves generally show a sigmoidal shape.

It is a characteristic feature of the reaction that it comes to an almost complete stop when only a fraction formal essay checker the original Si has reacted. The reaction is markedly dependent on the N2 pressure; higher pressure gives higher initial rate but a lower final extent of reaction. SEM investigations show that the silicon surface is covered with nitride crystals already after a short time of reaction, and subsequent nitridation takes place on the outside of this initial nitride layer.

The observations have been interpreted in terms of a model with nucleation and branching growth, coupled with a self-blocking channel model.

The four parameters of the model show characteristic dependences on the nitrogen pressure. The temperature dependences are less clear because of the short temperature span used. Introduction Silicon nitride powders for ceramic applications may be produced by a number of methods, including both gas phase and solution reactions, see, e.

The direct reaction between silicon and nitrogen, however, appears as one of the simplest and most economical routes. This reaction is also of interest when silicon nitride articles are produced by first forming the article from silicon powder by pressing or slip casting, followed by treatment in nitrogen at a suitable temperature.

This communication does not, however, deal directly with the production of silicon nitride powder or ceramic articles. It is intended as a more fundamental study of the kinetics of the direct rhetorical essay format, but silicon was used in the form of a fine powder in order to retain some similarity to processes of ceramic interest. Primed in the Netherlands.

Materials The powder was prepared from electronic-grade silicon stock Ventron-Alfa Products, Rhetorical essay format, Germany by crushing in an iron mortar, followed by ball-milling in a polyethylene container using iron balls. Subsequently the silicon was wet-sieved using stainless steel sieves to a size fraction of gm. Spectrometric analysis of the dried powder showed 15 ppm Fe as the only detectable contaminant. Assuming spherical particles, the specific -face area was calculated to 6.

In most of the thermogravimeuic runs, this powder was used with addition of 1. Starck, Berlin, Germany with particle size. The method minimizes opinion essay format free energy and the theoretical diagrams are compared to experimental observations [ Heat and mass transfers, and diffusion kinetics in the gas phase are related to the morphology and composition of the deposits A recent review is available [ Depending on conditions, hot cracking of a mixture of iodides B13 and CI4 gives the tetragonal boron phase B48B2C2 or the orthorhombic carbide B8C [23,43,44].

Writing an evaluation essay the dangerous gaseous mixture B2H6 diboraneCH4 and H2 a plasma induced CVD [48] or a toroidal glow discharge [49], produce amorphous boron-carbon alloy films, with low hydrogen content, similar to those obtained by using BBr3 [42]. This deposit is very promising as coating materials for fusion reactor walls 49].

To hinder entrance statement with metals they should be protected by boron carbide deposits obtained by CVD, using BC13 or BBr3 as boron source cf.

Boron carbide fibers can be prepared directly by reaction of boron obtained by the H2 reduction of BC13 on carbon fibers [ Complex fiber e. C structures should by consolidated by in-depth low pressure CVD [57] or by cracking of organic precursors, like carboranes [58]. Crystal growth 4. B4C single crystals, mm long, are obtained by CVD reaction b [59], or by reduction of B by graphite in an electrical arc [1,60].

A 6 mm diameter sintered boron carbide rod can be zone melted under Ar [61]. Single crystals needles up to 1 mm in length could have 90 also a high carbon content: Scholarship essay letter format. Mixtures of coarse platelets and rhetorical essay format are now industrially available [64]. Powders Powders may be prepared i using reactants treated by physical technics laser, plasma, electron beam ii or by heating, cracking or reacting diverse inorganic or organic chemicals.

Powders of B4C stoechiometry, free of excess B or C, were prepared through optimization of gas flow rates and laser intensity. The powders are loosely agglomerated, of high purity, consisting of equiaxed particles and exhibiting a MPa, narrow size distribution.

Microstructure revealed examples of uniform and exaggerated grain growth. Mechanical properties were good []. Finally, B4C powders, with a particle size of 6. These powders have a X-ray pattern characteristic of B4C [79] ; using the same technic we obtained non-organized boron carbide deposits cf.

Carbothermalreduction of boric anhydride. A mixture of acetylene black, or sugar with ethylene glycol, is heated with boric acid at C, in a stream of Ar ; the resulting B4C powder is fine grained tn [80]. Similarly mixtures of boric acid with ethylene glycol [81] or glycerin [ may be used. Fine carbide powder 0. The resulting solid contains very fine specific surface Ceramic-ceramic multiphase compounds may be obtained by the same method [ This method of powder synthesis should be promising, but it is not well known.

Boron carbide is ill-crystallized and may be mixed with SiC, BN Mixed phases may be produced: - B4C-BN by cracking of borate esters [, an amine adduct of decaborane in a stream of Ar [90] B4C-SiC-SiO2 by cracking carborane-siloxane polyborodiphenylsiloxane [ Conclusions In industry, and even in the laboratory, the boron carbide powder obtained by arc melting, with a boron rich stoechiometry B4.

Direct synthesis has no pratical use. Concerning powders for the future, laser or plasma, give very fine, narrow sized, pure powders, with different stoechiometries, and reactive powders, which should be of great interest for sintering.

The method is still in the R. Cracking of organic precursors does not seem to be appropriate for an economic preparation of powders : yields are low, sometimes the final product is polyphased, the precursors ae difficult to synthesise and very expensive.

But "the development of twophase ceramic materials from single polymers blends as a method of tailoring desired I think that boron organic precursors should be convenient as sintering additives for industrial, good quality, boron carbide powders ; we showed that boron carbide can be pressureless sintered with the addition of both carbon and silicon carbide organic polycarbosilane precursors.

It was necessary to introduce a minimal amount of carbon which could act in deoxidizing the boron carbide starting grains and to create a second phase which inhibits grain coarsening, thus improving mechanical properties [12]. An organic formal essay checker precursor should eliminate the residual SiC. Ceramists formal essays from magazines classified whiskers mean diameter pm and narrow sized distributed platelets of boron carbide, which could be used to reinforce ceramics borides, carbides Lipp, A.

Production, Properties, application', Techn. Beauvy, M. Schwetz, K. SchweMz ILA. Th6venot F. Reinmuth, K. Bouchacourt, M. Conard, J. Thivenot, F.

Cueilleron, J. Matkovich ed. Benton, S. Lifshits, E. Ostapenko, I. SSSR, Neorg. Brodhag, C.

CoRD - The Corpus of Multilingual Opinion Essays by College Students

Vincenzimi ed. Ceramic powders, Elsevier, Amsterdam, pp. Bougoin, M. Treutler, C. Cochran, A. Metals, 22, Ploog, K. La"iue S. Rey, J. Amberger, E, Druminsi, M. Bernard, C. Ducarroir, K. Chemical Vapor Deposition Fft Intern. Ducarroir, M.

Vandenbulcke, L. Basutcu, M. Vandenbulcke, L, Vuillard, G. Vandenbulcke, L "Theoretical influence of the deposition conditions on the uniformity of boron carbide coatings', Proc. Bloem, G. Verspui and L. Wolff eds.

Vandenbulcke, L 'Theoretical and experimental studies on the chemical vapor deposition of boron carbide', Ind. Chem Prod. Jansson, U. Thin solid Films, Rebenne, H. Am Ceram. Phase and chemical composition', Thin solid films, in press. Olsson, M. Sdderberg, S. Morphology and unicrostructure', Thin solid films, in press. Cholet, V. Ploog, K. I Tetragonales und orthorhombisches Borcarbid', J. Campbell, A. Kevill, D. Veprek, S. Plasma Chem. Toyoda, H. I Domaine d'existence de la phase carbure de bore', J.

Eschnauer, H. Thbault, J. Naslain, R. Pailier, R. Park, Y. Smith, W. Newkirk, L. Walker, B. Jr, Rice, R. Mierzejewska, S. Sugaya, T. Borchert, W. Mullendore, A. Aselage, C. Beckel, I. Howard and C. Physics, New York, pp. Mackinnon, I. Weaver, S. Morse ed. TMS Annu. Gatti, A. No 1, Ahmad, I. Mitchell ed. Motoc, C. Bosch, F. Wickens, A. London7, Becherescu, D. Bucharest15 2 Vogt, G.

Cruiziat, B. Lyon F. Mac Kinnon, I. Lon, on Fauchais ed. Plasmas, 3rd, Paper S. Limoges F8 pp. Knudsen, A. WO 86 04, US Appl. Ramsay, J. Dufek, G. Harris, G. Wada, H. Koshida, R. Kokai JP 62, Tanemoto, H. Kouvetakis, J. Ritter, J. Wada, K- Ito, S. Mirabelli, M. Ceram Soc. Rees, W. Jr and Seyferth, D. Wynne, K. Van Deusen, and R. The electronic and thermal transport properties of refractory icosahedral boron-rich solids suggest the utilization of these unique solids as high-temperature semiconductors.

Such applications will require high-quality, well-characterized materials. A variety of techniques have been used to prepare powders, ceramics, and single crystals of boron carbides and the icosahedral boron pnictides. X-ray diffraction, Raman spectroscopy, and ultrasound measurements have been used to probe boron carbide sample quality. Each of these measurements supports the structural model of boron carbides developed from analysis of transport data.

Initial electrical conductivity measurements on these samples show no thermal hysteresis. The boron-rich pnictides and a-boron are wide bandgap insulators, while boron carbides are hopping-type semiconductors brief essay format exceptional transport properties.

These mechanical and electrical properties derive from the unusual structures and bonding arrangements in the solids. These structures, depicted do an essay for me Figure 1, are based on boron-rich icosahedra [1,2]: twelve-atom, twenty-sided polyhedra, with each side an equilateral triangle.

The simplest structure, that of a-boron, is shown in Figure la. Here, a Bl2 icosahedron is located at each vertex of the rhombohedral unit cell.

The icosahedra are bonded to one another by very strong twocenter bonds along the unit cell edges and by inter-icosahedral threecenter bonds that lie along the long diagonal of the unit cell.

These three-center bonds are represented by dark triangles in Figure la. The icosahedral boron-rich pnictides and the boron carbides are derived from this simple structure. In the pnictides, depicted in Figure lb, the three-center bonds of a-boron are replaced by a two-atom "chain" containing two phosphorus or arsenic atoms.

In the boron carbides, Figure 1c, a three-atom chain lies along the long cell diagonal. At 97 R. Proi In the Netherlands. Structural changes that accompany changes in carbon concentration will be discussed in detail below. Structures of icosahedral borides. Despite the similarity of structure, the electronic properties of the boron carbides and the boron pnictides are quite different.

The boron pnictides are wide-bandgap, transparent insulators [4] at room temperature. The electronic structure of the boron-rich pnictides is discussed elsewhere in this volume [5]. If doping can be controlled appropriately, these materials have the potential for utilization as high-temperature semiconductors [6]. Several applications currently utilize the high-temperature electronic properties of boron carbides.

Most of our work has focussed on the potential of boron carbides in very-hightemperature thermoelectric energy conversion [8], where boron carbides are the most efficient p-type material at high temperatures. Additionally, the thermally-activated electrical conductivity provides the basis for thermistors for use as automotive engine temperature sensors [9].

Details of the theory of the bipolaronic conduction mechanism [11], the electronic and thermal transport measurements [12], and the L 99 electronic structure of boron carbides [5] are presented elsewhere in this volume. Here, it is sufficient to note that one result of the transport analysis is a model for changes in the structure of boron carbides as carbon content is varied.

According to the transport-based model and a recent analysis of the change in free energy of boron carbides as boron substitutes for carbon [13], B-B-C chains first replace C-B-C chains as carbon content is reduced from B4 C. Further reduction in carbon content is accomplished by replacing some of the icosahedral carbon atoms with boron.

This model accounts for the p-type behavior of boron carbides over the entire single phase region. The replacement of C-B-C chains by C-B-B chains is also believed to be related to the transition in the thermal conductivity from crystal-like at high carbon concentration to glass-like at low carbon content [11,12,14]. Other structural models, based on x-ray diffraction data [] or 1 3 C NMR results [18], differ with that proposed above. Our model, in particular, runs counter to the usual proposition that boron carbide with the composition B1 3 C2 is comprised of B1 2 icosahedra and C-B-C chains [15,17].

Clearly, the use of boron-rich solids as electronic materials will place new demands on the quality of materials. In this paper, we shall describe the preparation of powders, ceramics, and single crystals of the icosahedral boron-rich solids.

In addition, we will describe characterization techniques that we have employed to determine the sample quality. We will review the results of measurements [] that provide evidence to support our structural model for the boron carbide solid solution. Finally, we shall comment on several areas for future work to enhance or expand the useful properties of these interesting materials. In order to prepare small amounts of material with variable stoichiometry, we have used the solid-state reaction of a high purity, sub-micron amorphous boron powder 2allery Chemical Co.

These powders were primarily for use in precision lattice parameter determinations. Following this treatment, some amount of unreacted starting material was occasionally observed by x-ray diffraction. The resulting powder patterns have very narrow diffraction peaks and, for sample compositions in the single phase region, show no evidence of second phases.

Powders of boron carbide are very difficult to sinter into dense ceramics. We have used uniaxial hot pressing to prepare dense ceramic samples. Boron powders, with either natural abundanre or isotopicallyenriched in 11 B or 1 0 B Eagle-Picher Industries were mixed by shaking for several hours with the high purity graphite powder. The use of ball mills in the mixing process was avoide as it leads to contamination of the sample.

Graphite rams were used to apply pressure, and either graphite felt graphoil or tungsten foil was inserted between the boron carbide and the rams to prevent bonding high purity BN crumbled when used between the rams and the sample.

Here, the atmosphere was switched to gettered argon and full pressure psi applied. After a 30 minute lag for the interior of the die to reach the set temperaturethe sample was held at temperature for 15 minutes.

Typically, the sample must be forced from the die liner, and the graphoil or tungsten foil bonds to the sample and must be removed by grinding.

Small, shaped pieces of boron carbide were prepared by cutting with a low-speed diamond saw. Boron carbide crystals are also of interest for structural and transport studies. Due to the refractory nature of boron carbide and problems associated with the incongruent melting [23] of the solid solution, growth of crystals from the melt is prohibitively difficult. Solution growth from metallic fluxes has, however, been used successfully to produce small several mm crystals of boron carbide 24].

Transition metals with filled or nearly-filled d shells particularly Cu and Pd have been used most successfully; these metals either do not form metal borides or they form less stable borides with no direct boron-boron bonding. Details of the growth conditions are given in Ref. PBN crucibles and tubes were used to minimize contamination and to provide flat temperature profiles.

Crystals were recovered by dissolution of the metal matrix in aqua regia. I0I Considerably less work has been performed on the preparation of samples of the icosahedral boron-rich pnictides. Powders of these solids may be prepared by the thermal decomposition in vacuum of BP or BAs [24]. These crystals exhibited a continuous range of opacity, from clear to black. These differences may be due to differences in Pd-rich inclusion density in the crystals.

Because of the high vapor pressure of P or As at temperatures required for sintering or hot pressing, attempts to prepare dense ceramics of the boron-rich pnictides have not been successful.

Single crystal x-ray diffraction studies have been described elsewhere [1, Precision lattice parameter determinations employed a Si internal standard. Assignment of observed boron carbide reflections was based on the JCPDS B4 C standard [25], and lattice parameter refinement was performed using Siemens least squares software. Raman spectra were obtained using a computer-controlled, scanning, double monochromator equipped with holographic gratings and a photoncounting photomultiplier tube [26].

Small single crystal samples were illuminated using a microscope accessory, while larger samples were illuminated with the laser beam focused to a 0. The Background features Rayleigh scattering, fluorescence were removed formal essay kal% c4% b1plar% c4% b1 an empirical fitting procedure, and count axes have been scaled so as to normalize one of the more intense Raman bands in each spectrum. Ultrasonic formal essay checker time data were obtained on thin boron carbide specimens with flat and parallel sides ranging from approximately 1.

Both longitudinal and shear data were taken by the pulse-echo overlap method [27]. Longitudinal wave velocities were determined on all three faces of the specimen using 50 and 20 MHz transducers and the average was taken for moduli calculations. The shear wave velocity could only be determined on the large face of the specimens using a 20 MHz transducer.

This technique was particularly useful in identifying samples containing small cracks. Four point electrical conductivity measurements were also made on the thin slabs of boron carbide described above. Gold wires were wrapped around the samples and were held in place at notches cut into the boron carbide with mm spacings. Good electrical contact was assured by holding the sample between two flat, spring loaded PBN disks. The sample assembly and lead and thermocouple wires were contained within a quartz tube through which gettered Ar was passed.

This tube was placed into the heated zone of a tube furnace equipped with a Na heat pipe. The programmable current source, digital voltmeter, and furnace controller were digitally-controlled using a minicomputer. Current levels were auto-selected 5-lOOmA to provide voltage drops between 1 and 10 mV.

The current during each measurement was precisely determined by measuring the voltage drop across a standard resistor in series with the sample.

Forward and backward voltage drops were averaged to eliminate the effect of thermoelectric voltages. This criterion was usually met on heating. At low temperatures results are seen to be in reasonably good agreement with the work of Bouchacourt and Thevenot [34 on hot-pressed samples. Several factors may contribute to the large scatter in the rest of the data.

First, the composition of boron carbides is frequently not well known. Thus, for compositions reported as B4 C, a values larger than 5. Second, small impurity concentrations can have a very large effect on the lattice parameter 2]. Also, internal stresses may affect formal essay checker parameter values. Thus, boron carbide lattice parameters are indicative of carbon concentration for pure samples, but are sensitive to small impurity concentrations and internal stresses.

We shall discuss another technique for verifying carbon content below. Hexagonal lattice parameters of boron carbides vs. The variation of the lattice parameters with carbon concentration also supports our structural model of the boron carbide solid solution.

The general increase in the size of the unit cell as carbon content decreases is consistent with the replacement of carbon atoms covalent radius 0. The a axis changes uniformly over the entire single phase region, with about 0. The c axis, on the other hand, increases by 0. We can understand the q axis behavior in terms of the structural model.

Figure 3 shows the hexagonal unit cell of boron carbides viewed along one of the basal plane unit. The icosahedra formal essay kal% c4% b1plar% c4% b1 these solids are more compressible than the chains, so that the size of the personal values essay cell is controlled by the size of the chains [1].

By replacing C-B-C chains with C-B-B chains, with a larger boron atom at one end and a B-B bond that is much weaker and softer than a C-B bond [14], we expect a longer chain unit and thus an increase in the q axis length.

Further substitution of B for C occurs within icosahedra, with a much smaller effect on the size of the unit cell. Hexagonal unit cell of boron carbide, viewed along a basal plane unit vector. The c axis is vertical on the page, and the three-atom chains are parallel to c. Raman Spectroscopy has buy essays online uk to be a very powerful tool in the analysis of boron-rich solids.

In addition to local structural information, Raman spectra provide a semi-quantitative probe for carbon content in boron carbides and a check for the presence of amorphous carbon. In an earlier study of rhetorical essay format Raman spectra of hot-pressed boron carbides, Shelnutt et al. These Raman bands were clearly shown to be related to the presence of amorphous carbon in the samples.

Indeed, they were absent from the spectra of chemical-vapor deposited material [36]. These carbon bands are also absent or vanishingly small in the Raman spectra of our hot-pressed boron carbide samples, indicating a substantial improvement in sample quality compared to the material used in the earlier studies.

If a small amount of excess carbon is intentionally added during sample preparation, the amorphous carbon bands are readily observed. We do note, however, that infraredactive vibrational modes with frequencies near cm"1 are present in our samples. Thus, the IR mode comes from the boron carbide structure, and is most likely a chain-related vibration [37,38].

Formal essay checker broad bands with frequencies greater than cm "1 are relatively insensitive to the carbon content. Below B13C2, a band near cm"1 grows. These changes in the Raman spectra provide a semi-quantitative, nondestructive probe for carbon content in boron carbides.

We have used this technique to identify the carbon content in boron carbide single crystals discussed below and to study carbon gradients in hot-pressed samples due to contact with graphite or a tungsten foil during the pressing. Carbon from the graphoil readily diffuses into boron-rich boron carbides. Typically, pm must be removed from each end of a b sample before the surface spectrum matches one taken from the interior side of the sample.

Tungsten foil was C used in an attempt to eliminate such diffusion. Ta form Raman Shift, cm-1 eutectic liquids with boron at Fig. Raman spectra of boron temperatures lower than the organization basic essay format with indicated pressing temperatures. Thus, compositions. Based on an empirical comparison of the spectra of single crystals of a-boron, Bl2As2and B4 C and of B1 2 P2 powders [19], we have assigned Raman bands with frequency greater than cm-I to icosahedral vibrations, and bands with frequencies between and cm-1 to chain vibrations.

These assignments are corroborated by normal coordinate calculations 39] and by the variation in intensity of the chain bands as the single crystals are rotated with respect to the polarization of the laser beam.

The Raman spectra shown in Figure 4 thus provide evidence of structural changes with composition in boron carbides. Consistent with the assignments described above, the narrow bands centered at cm-1 are due to vibrations of the C-B-C chains [19].

Brief essay format narrow Raman bands have virtually disappeared at B13 C2, the composition at which all of the C-B-C chains have been replaced. The icosahedral bands of boron carbides are broad compared to the other icosahedral borides. Broad bands are more characteristic of a disordered solid or amorphous material than a well-ordered, crystalline material. We have interpreted this result 19] as being indicative of carbon atoms distributed among inequivalent sites in B 1 1 C icosahedra.

Thus, in a-boron or the boron-rich pnictides, B1 2 icosahedra exist in a well-ordered environment and the Raman bands are narrow. In BxC, B1 1 C icosahedra are inherently disordered due to the distribution of carbon in inequivalent sites.

This disorder is manifested in the broad icosahedral bands. Because the bands are broad over the entire single phase region, carbon must be present in the icosahedra at all compositions.

Thus, the Raman spectra of boron carbides support our structural model: 1 near B4 C, B1 1 C icosahedra and C-B-C chains are the predominant structural units, 2 between B4 C and B13C2, boron replaces carbon in the chains, forming C-B-B chains, and 3 only for carbon concentrations below B 1 3 C2 does boron begin to replace carbon in icosahedral sites.

We have attempted to prepare single crystals with lower carbon content in order to perform additional structural studies. B:C ratios from to and a variety of metal solvents Cu, Pd, Pt, Ni have been employed in the solution growth procedure. We find that, regardless of B:C ratio and solvent choice, the resulting crystals have Raman spectra characteristic of B4 C.

In addition, the lattice parameters of a number of individual crystals and of a powder formal essay kal% c4% b1plar% c4% b1 from crushed crystals are very close to the values for B 4 C. For the more boron-rich starting compositions, 6-boron crystals are formed with the B4 C crystals. We conclude that, despite the wide single phase region of scholarship essay letter format carbides, only B4 C crystals may be grown by the metal flux technique.

This suggests that one reason for the width of the single phase region of boron carbides is the relative instability of a-boron. The free energy of boron is lowered by dissolution in the molten metal, and appears to be lowered even further by the presence of carbon in solution [40].

This lowering of the free energy of boron in solution drives the composition of boron carbide in equilibrium with the solution to its most energetically favorable value, near B4 C [13].

In order to better understand hip hop essay thermal transport behavior of boron carbides, we have measured the velocity of longitudinal and shear acoustic waves as a function of composition. The results of these measurements are shown in Figure 5. The longitudinal sound velocity in boron carbides, 1. The cracking in samples 1. The elastic constants Fig. Longitudinal and shear derived from the sound formal essay kal% c4% b1plar% c4% b1 acoustic velocities VI and VS of measurements using Equations 1 boron carbides vs.

The values for B4 C are in reasonable agreement with the literature [41]. We understand the large decrease in modulus below BI3 C2 to reflect a change in the stiffness of the most compressible structural unit, the icosahedron, as B1 1 C icosahedra are replaced by B12 icosahedra [11]. Once again, the composition dependence of the sound velocities can be explained using the structural model. Thus, the simple kinetic theory result Equation 7 for the thermal conductivity appears to be inadequate for formal essay kal% c4% b1plar% c4% b1 carbides.

An alternate formulation of the thermal transport is described elsewhere in this volume [11]. The room temperature electrical conductivity was measured with a table-top version of the four point apparatus.

The conductivity does not vary monotonically with carbon concentration; rather, the maximum conductivity is observed at B1 3 C2 - 23 0-cm Once again, these observations are 1E6 in agreement with the predicted composition dependence [6,7,11]. Previous measurements of the temperature dependence of the electrical transport properties of boron carbides always lE5 displayed some amount of thermal hysteresis [42]. This result ". Experimental difficulties large background b U noise were encountered during the initial heating of the B4 C IE3 sample.

When this was corrected, the heating and cooling curves nlo a Heating were quite reproducible and returned to the room temperature A A Cooling values determined in the tabletop apparatus. The B13C 2 conductivities were also 1E2 reproducible with thermal cycling.

Conduction in the boron Fig. Electrical conductivity vs. B1 3C2. Heating and cooling sequence The temperature dependence of as indicated. Interestingly, the rhetorical essay format of the B1 3 C 2 sample has more curvature than B4 C at both the low- and hightemperature ends. These differences may reflect the increasing disorder present in B 1 3 C2 compared to B4 C, as 1 the CBB chains have an orientation up or downwhereas CBC chains are symmetrical, and 2 the chain-bonding sites within an icosahedron may be more accessible to icosahedral carbon when bonded to the boron end of a CBB chain.

We shall continue these measurements on other low-carbon boron carbides to further explore such effects. Scholarship essay letter format have shown that high quality boron carbide powders, ceramics, and crystals can be made, and that x-ray diffraction, Raman spectroscopy, and ultrasonic measurements are useful probes of phase purity, composition, and sample quality.

The structural model for the boron carbide solid solution is supported by a vast array persuasive essay for college measurements, including the lattice parameters, Raman spectra, acoustic velocity, and electrical conductivity data described in this paper. The essential features of this model are 1 the presence of B1 1 C icosahedra throughout the single phase region, and 2 the replacement of C-B-C chains with C-B-B chains as carbon content is reduced from B4 C.

Thus, many questions regarding the properties of boron carbides have been addressed. Our work in the future will address several remaining structural questions. In particular, we will try to understand the IR spectra of boron carbides in terms of our structural model, and we shall use 1 3C MASS NMR to further study the location of carbon in the solid solution as a function of composition.

In addition, we shall continue to measure the high-temperature electronic transport properties conductivity and Seebeck coefficient of our samples to study the effect of composition and disorder. Finally, we shall study the possibility of preparing n-type boron carbides by doping.

These ntype boron carbides would then form the basis for an all boron carbide thermoelectric couple. We foresee a growing effort in the preparation of large, pure icosahedral boron pnictide materials. We shall attempt to prepare ceramics by hot isostatic pressing and crystals by melt growth und-r pressure. Given suitable samples, we shall attempt to identify suitable n- and p-type dopants and try to control doping to appropriate levels for high temperature semiconductor applications.

Morosin, B. Emin, T. Howard, and C. Wood, eds. Aselage, and C. Less-Common Metals, 82, Slack, G. Switendick, A. Emin, D. Wood, C. See, for example, Ceramic Bulletin, A8, Hunold, K. B 29, ; and Wood, C. B 31, Will, G. B35, ; Kirfel, A. B35, Yakel, H. Larson, A. Duncan, T. Tallant, D. Solids,; Tallant, D. B, accepted. Aselage, T. Gieske, J. Bose, D. Materials and Processes, 2, Less-Common Metals, 67, III Papadakis, E. Galbraith Laboratories, Inc.

Chambers, W. Growth 74, Mierzejewski, S. Less-Common Metals, 8, Less-Common Metals, 59, Matkovich, V. Less-Common Metals, 47, 39 Less-Common Metals 26, 25 Shelnutt, J. Becher, H. Werheit, H. Beckel, C. Storms, E. Less-Common Metals Ramanamurthy, S. Formal essay checker and J. Mort, eds. Industrial methods for the production of metal boride, boron carbide and boron nitride powders are reviewed, and reference by processes such as made to the fabrication of artefacts hot-pressing.

Applications are indicated with special attention to the valuable properties organizational behavior article very high hardness and the resistance to attack by molten metals.

Special uses include the high added-value areas of the nuclear and electronics industries. The latter definition classifies more exactly two related binaries formed with non-metals: boron carbide and boron nitride. Like the metal borides they are produced as powder and fabricated by similar ceramic-type routes, while their refractoriness dictates uses on their own or as property modifiers for borides and other non-boron ceramics.

All of these products have melting points well in excess of 0 C. The oxide reduction reactions are endothermic and proceed at above about 0 C, but in order to achieve a practicable production rate they are mostly carried out at some C higher.

Prind In the Nedhrlanh. When made according to the route of the above equation, powdered boric oxide and carbon are mixed in the appropriate proportions dissertation masters structure titanium dioxide, readily The powder mixture is available in pure form as the pigment grade.

The product from reaction at around 0 C is a loose clinker of TiB2 crystals, whose melting point is 0 C higher. Rhetorical essay format reaction temperature needs to be reduced by several hundred degrees for the lowe'--melting CrB 2but most other diborides and the hexaborides, which are formed by rare-earth metals can be made at brief essay format C. What has been said for boride production can also apply to making The product of boron carbide itself, according to equation 3.

When made at lower temperatures, in resistance tube furnaces or similar, a fine powder results directly.Below I will outline examples and a structure approved by experienced IELTS teachers and examiners for each type of question.

I also include an example answer for each type of question so you can see what the structure looks like in a real essay.

Please note that these are general structures and they may vary slightly depending on the particular question. There are many types of structures that can get you a high score. These are just the ones I think are most effective and easiest to learn. I have provided a link at the end of each section. Some people believe that unpaid community service should be a compulsory part of high school programmes for example working for a charity, improving the neighborhood or teaching sports to younger children.

Sentence 1- Paraphrase Question. It is argued that volunteering should be made part of the school curriculum. This essay agrees with that suggestion completely because of the benefits it brings to pupils. The essay will first look at how voluntary work can help students develop soft skills and then discuss how these extracurricular activities are valued by universities and employers.

Education should not be limited to strictly academic pursuits and those in education should also develop life skills, such as teamwork, empathy and self-discipline, and one of the best ways to hone these aptitudes is through community service.

Serving those less fortunate than ourselves teaches us formal essay checker lessons including how to work with people from other backgrounds and the value of hard work. For example, I personally volunteered to spend 6 weeks in Africa teaching disadvantaged children and this led to a much higher work ethic when I returned to my studies. Here you state the main idea of your entire essay -- the point you are trying to make or prove.

This paragraph should include your thesis statement plus three reasons why you believe this statement to be true. These are the body of your essay. Remember back in Paragraph One, you gave three reasons for your opinion? Three reasons, three body paragraphs. Each of the body paragraphs should take one of your reasons and explain it in more detail, citing sources where necessary.

Former Newfoundland premier Joey Smallwood once said about giving speeches: "First I tell them what I'm going to tell them, then I tell them, then I tell them what I told them. Shunting trucks and hauling sleight. Red and green and brown and blue, They're the really useful crew! All with different roles to play, Round tinmount sheds or far away. I've forgotten the rest. It's been so long. It's just Ha, I guess I haven't changed much.

My mum used to scold me then and still When I watch Pokemon!!! In my opinion i think the type of games like just dance that forces to move to organization basic essay format points and win the game are losing popularity because the are other games that are more fun and you dont have to stan up of the sit. Also motion controls are on every device like kinect on xbox, phones have physical stuff, vr, PlayStation move still exists,etc.

I think that playing outside your house is much better than play video games. At first place, it is bad for your eyes and it can give you headache if you are looking the screen all the time. But if you don't play a lot of time is pretty good to play online with your friends for a time. But you don't have to play a lot of time with videogames, you have to go outside your house, meet your friends and opinion essay format a good time.

Log in Sign up Newsletter. Look at the essay and do the exercises to improve your writing skills. Instructions Do the preparation exercise first. Then do the other exercises. Check your understanding: opinion essay format selection. Check your writing: matching - useful words and phrases.

Worksheets and downloads An opinion essay - exercises. Share Flipboard Email. An Introduction to Essay Writing. Grace Fleming. Education Expert. Click below:. Hi, my name is Elizabeth Liz. To what extent do you agree. For more info about writing in MLA, check any education-related source. One of the hardest questions about this type of writing is how to find interesting opinion essay ideas to write about.

How to pick a topic, essay about doing community service is going to highlight the best sides of your writing skills instead of being a real struggle? Don't panic! Here's a list of 5 topics, which you can use anytime you have problems with finding the right theme for your essay. Knowing how to write an opinion essay outline correctly is an invaluable skill for this type of writing.

It is going to help to survive any opinion essay test, no matter how easy or difficult the topic is. So, in case of the opinion essay, you have to include into the outline the following points:. Without an outline, your essay risks going in different directions during the writing process.

Howeveras long as we are careful to organization basic essay format in mind the importance of human interaction in education, the educational benefits are clearly positive. This is also one of kids math homework sheets four criteria you will be marked on and it refers to your ability to use a wide range of accurate vocabulary.

A common mistake is to repeat the same words over and over again. You will lose marks if you do this. A solution to this problem is to use synonyms. You can either think of synonyms as you are writing or leave time at the end to add them in. By varying your vocabulary in this way you are demonstrating that you have a wide vocabulary and this will boost your band score.

Only use words you are confident about. Mistakes will lead to fewer marks. Do you need me to correct your essays and give you feedback on them? Check out our essay correction service. I hope this post helps you with discussion questions and if you have any questions please comments below.