The thermal plasma processing for ceramics was surveyed with a stress on plasma spraying, plasma CVD and plasma sintering. The development of the low pressure plasma process and RF plasma process in plasma spraying is worth of special mention. These processes will be applied to ceramics coating in near future. Researches on thermal CVD are being carried out very actively for synthesis of high purity ultra fine ceramic powders such as silicon nitride and silicon carbide. This process has a very high potential as a means for rapid growth of ceramic films on various substrates. Low pressure RF inductive plasma and high power microwave plasma have characteristics quite different from RF glow discharge plasma. It is very interesting that these plasmas are applied to synthesize diamond and cubic boron nitride films. Rapid sintering of ceramic powders in plasma atmosphere is one of the most important applications of plasma to materials processing, but many problems remain to be solved in the sintering process.

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This review examines the stages of the sol-gel process, including hydrolysis, condensation, gelation, aging, drying, and sintering. The species produced in the sol are polymeric, rather than being dense glass-like colloidal particles. Considerable control over the structure of the polymer is possible through an understanding of the chemistry of hydrolysis and condensation. The properties of the gel and its response to heat treatment are sensitive to the structure created in the sol stage. Solvent must be removed slowly to prevent the high capillary stresses from causing cracking. A model of drying is presented that explains the relationship between cracking, drying rate, gel size, and permeability. Heat treatment causes densif ication of the solid phase, as well as collapse of the pores. The sintering behavior of gels is complex, because the viscosity of the gel is affected by concurrent structural relaxation and changes in hydroxyl content.

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The formation of SiC-Si₃N₄ composite particles by the vapor phase reaction of Si(CH₃)₄-NH₃-H₂ system at 1200°C was investigated with emphasis on the effect of mixing temperature of Si(CH₃)₄ and NH₃ streams. Powders produced were amorphous. When Si(CH₃)₄ was mixed with NH₃ in a low temperature zone below 900°C, the particle size was 0.05 to 0.07μm and decreased with an increase in NH₃ concentration. When the mixing temperature was 1100°C, the particle size was nearly 0.02μm regardless of NH₃ concentration. The amorphous powders crystallized into β-SiC and α-Si₃N₄ (partially including β-Si₃N₄) by the heat treatment at 1550°C in Ar-N₂ atmosphere. The crystallized phase from amorphous powder produced in low temperature mixing was SiC phase, SiC-Si₃N₄ composite and Si₃N₄ phase depending on the NH₃ concentration used, among which the composite particles had hybrid structure consisting of Si₃N₄ core and SiC shell. On the other hand, SiC-Si₃N₄ composite particles were obtained even at a high NH₃ concentration in high temperature mixing, and they consisted of SiC core and Si₃N₄ shell. The following formation processes of composite particles in the vapor phase reaction were proposed: in low temperature mixing (T_m=700°-900°C), nitride particles were produced at first by the reaction between Si(CH₃)₄ and NH₃ immediately after the mixing and then carbide layer deposited on their surfaces from the unreacted Si(CH₃)₄, while in high temperature mixing (T_m=1100°C), Si(CH₃)₄ polymerized and decomposed into SiC particles in a low temperature zone and then the SiC particles were nitrided with NH₃ in a high temperature zone.

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Polycrystalline and epitaxial films of MnFe₂O₄ were prepared by organometallic chemical vapor deposition using metal acetylacetonates as starting materials. Mn(acac)² and Fe(acac)³ were evaporated at 165°-175° and 140°-143°C, respectively under a reduced pressure (5-10 Torr) and were transported to the deposition zone with N², gas, where MnFe₂O₄ deposited by thermal decomposition of acetylacetonates in the vapor phase including O² reaction gas. Polycrystalline MnFe₂O₄ films were grown on a silica glass substrate under the following deposition conditions; pressure: 5-10 Torr, deposition temperature: 600°-700°C, N² flow rate: 100ml/min, O² flow rate: 5ml/min. The growth rate of the film at 600°C was 130Å/min. The saturation magnetization and coercive force were 62-80emu/g and about 80 Oe, respectively. Epitaxial MnFe₂O₄ films were grown on {100} MgO single crystal at 10 Torr and 650°-700°C using lower concentrations of metal acetylacetonates. The saturation magnetization of the epitaxial film increased with increasing film thickness, though lower than that of the polycrystalline film.

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Niobium nitride films were deposited on fused silica substrates by the CVD process using reactant gas mixtures of NbCl₅, N₂, H₂ and Ar at a deposition temperature from 700° to 1100°C. The effects of deposition parameters on the film properties were investigated, and the deposition mechanism was discussed. Two regions with different rate-determining steps were found under the experimental conditions studied. At 1000°C and at a total gas flow rate from 1000 to 4000ml/min, the deposition rate was controlled by the mass transport in the gas phase, and the nitrogen content in the deposits increased with increasing deposition temperature and increasing mole ratio of N₂ to NbCl₅.

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An ultrafine Si₃N₄ powder was synthesized by the vapor phase reaction of SiCl₄ and NH₃ in a thermal plasma arc jet. The as-prepared powder was white and amorphous, and had an average particle size of 30 to 40nm. The production rate was 200g/h. The heat treatment above 1380°C decreased the specific surface area and increased the crystallinity, implying that the grain growth and the crystallization occurred simultaneously. The oxygen content was reduced to 1wt% at 1450°C. The powder thus obtained was the crystallized hexagonal grain with the average size 0.2μm. X-ray diffraction indicated the presence of α-Si₃N₄ phase more than 95% and β phase as remainder. The sintering at 1700°C with the aid of 5wt% Al₂O₃ and 5wt% Y₂O₃ resulted in the relative density 98%, Vicker's hardness 16 GN/m² and the fracture toughness 7 MN/m^3/2.

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SiC powder has been synthesized using CO₂ laser-heated vapor phase reactions in the silane-methane gas system. SiC particles form via a two-step reaction: silicon particle formation and subsequent carburization of the silicon particles. It is shown that the silicon particles form at the lower edge of the reaction zone, and carburization occurs in the hottest region, through which the CO₂ laser beam passes. The formation of silicon particles can be explained by silane pyrolysis and subsequent collisions and coagulation of the silicon particles. The carburization mechanism of the silicon particles is discussed in terms of diffusion through a SiC product layer and methane pyrolysis. The rate-controlling step is believed to depend on the reaction conditions such as temperature and gas stoichiometry.

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A new principle of chemical vapor deposition for high-speed film growth is presented by incorporating the thermophoretic precipitation of ultrafine particles produced in the gas phase. This new CVD method, as named PPCVD (Particle-Precipitation Aided Chemical Vapor Deposition), has been successfully applied to the preparation of TiO₂ films on a glass substrate by thermal decomposition of titanium-tetraisopropoxide (Ti(OC₃H₇)₄, TTIP). The TiO₂ films are amorphous and oriented anatase crystallites for the substrate temperature lower than 623 and higher than 723K, respectively. The growth rate is 40nm/s, which is 1 to 2 orders of magnitude greater than the conventional CVD process.

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Ultrafine powders of AlN and (AlN+Al) with particle sizes of less than 0.5μm were produced by arc melting of Al in atmospheres of N₂+Ar, N₂+H₂, N₂+NH₃ at 0.1MPa pressure. The size, surface area, chemical composition, and crystal structure of obtained powders were determined by electron microscope, BET method, chemical analysis and X-ray diffraction analysis respectively. The following results were obtained.
(1) AlN ratio in (AlN+Al) powders obtained in (N₂+Ar) atmospheres decreases from 0.3 to zero with decreasing nitrogen partial pressure.
(2) AlN ratio in (AlN+Al) powders obtained in (N₂+NH₃) atmospheres increases from 0.3 to 0.95 with increasing NH₃ partial pressure.
(3) Projections were formed on molten Al surface by evolved nitrogen gas from the melt and by reaction between nitrogen and molten Al.
The reaction between nitrogen plasma and molten Al plays an important role in the formation of ultrafine particles. The driving force for the formation of ultrafine particles can be explained as follows; In arc melting of Al in a nitrogen atmosphere, molten Al simultaneously contacts with two gas phases, one is atomic nitrogen in the arc gas phase and another is the molecular nitrogen in the non-arc gas phase. The extent of dissolution of atomic nitrogen in the molten Al is much larger than that of molecular one. Then, the dissolved nitrogen through the arc gives higher nitrogen content for the non-arc gas phase. That is, super-saturated nitrogen will evolve in non-arc gas phase carrying with Al vapor. A kind of enhanced evaporation of Al is induced. This Al vapor reacts with surrounding gas species near the arc such as N₂, N, NH₃, NH₂, NH in high temperature ranges (above 1600K) to form AlN or (AlN+Al) vapor which condense to ultrafine particles.

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Fine amorphous silicon nitride powders were prepared by vapor phase reaction between nitrogen-containing, chloride-free organosilicon compound and NH₃ at the production rate of-200g/h. Crystallization was carried out by heat treating the resulting amorphous powders in Ar atmosphere at 1500°C. High purity crystalline silicon nitride powders could be synthesized with metallic impurities Fe, Al, Ca<50ppm, carbon content <1wt% and oxygen content of -1wt%. The specific surface area, mean particle size and a phase content were-10m²/g, 0.6μm and >95wt%, respectively. Mechanical properties of hot-pressed silicon nitride from the resulting crystalline powders were comparable to hot-pressed bodies fabricated from high purity silicon nitride powders by imide decomposition and carbothermic reduction of silica.

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In order to obtain ZrF₄-based fluoride fibers expected as ultra low-loss optical waveguide at 2-4μm, the reduction in 3d transition metal impurities, e.g. Fe, Ni, Cu, etc. and oxygen impurities is indispensable. In this study, ZrF₄ powder was prepared by chemical vapour deposition in the system ZrBr₄-HF which was particularly effective for removing 3 d transition metals. The phase synthesized in the system ZrBr₄-HF was α-ZrF₄. The equilibrium constant and the reaction rate of vapour phase reaction in the system ZrBr₄-HF were high enough for powder preparation. Consequently, it is concluded that the system ZrBr₄-HF is suitable for the synthesis of ZrF₄ by chemical vapour deposition.

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By the RF-plasma method, preparation of silicon carbide and silicon nitride powders from alkoxysilanes (Si(OC₂H₅)₄, (CH₃)₂Si(OC₂H₅)₂ and (CH₃)₂Si(OCH₃)₂) and from SiCl₄-NH₃ system, respectively, was studied. In the preparation of silicon carbide, the reaction product consisted of β-SiC, free carbon and silicon oxide. The amount of SiC in the powder product increased with rising reaction temperature or with increasing [H₂]. The amount of SiC increased with decreasing C/Si ratio in alkoxysilanes and was estimated to reach 100% when the C/Si ratio is around 3. In the preparation of silicon nitride, the reaction product was white and non-crystalline powders with a particle size of 0.01-0.02μm. The composition of powders was virtually independent of [NH₃] and close to that of Si₃N₄. Heat treatment at 1550°C was necessary to obtain the crystalline α-Si₃N₄.

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Since δ-TaN, which has a cubic crystal structure, is a high temperature phase which transforms into ε-TaN and β-Ta₂N, both hexagonal, below 1700°C, it has been prepared by heating the hexagonal ε-TaN phase above 1700°C in high-pressure nitrogen. In this note, the formation of cubic δ-TaN by nitriding tantalum with argon-nitrogen plasma jet under reduced pressure is described. The tantalum sheet was held on the water cooled holder in the nitriding reactor. After the evacuation of the reactor, an argon-nitrogen (10%) gas mixture was introduced into the reactor and the pressure in the reactor was maintained at 240 Torr. The plasma jet impinged on the tantalum sheet resulting in nitridation. When the plasma jet impinged on the tantalum surface, tantalum was heated. The surface temperature was about 1850°C. When tantalum was nitrided for 10min, β-Ta₂N and δ-TaN were identified as products by X-ray diffraction. The lattice parameter of the cubic phase was 4.33-4.34Å and the composition was estimated to be TaN_0.85-TaN_0.95. The superconducting transition temperature of the specimen was about 9K and it was higher than that of the mixture of β-Ta₂N, ε-TaN and δ-TaN obtained by the heating and quenching of ε-TaN by the plasma jet. The quenching rate of the tantalum sheet after the impingement of the plasma jet was calculated using a quenching model of the plate, and it was estimated to be the order of magnitude of 10³Ks^-1, It is two orders of magnitude lower than that in the quenching of the particle of ε-TaN in the plasma jet.

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Highly pure boron nitride of low crystallinity could be synthesized from triammoniadecaborane and ammonia below 800°C under atmospheric pressure. Triammoniadecaborane reacted with ammonia at temperatures between 300° and 450°C, and formed an amorphous solid containing BN, NH and BH bonds. According to the data of IR spectrum, the BH absorption band at 2500cm^-1 decreased in intensity with increasing temperature, and disappeared at 800°C. The 800cm^-1 band assigned to the BNB bonding increased in intensity from 700° to 800°C. The degree of alignment of the B₃N₃ hexagonal layer increased as the reaction time increased from 2h to 10h at 800°C. The X-ray diffraction profiles of the product formed at 800°C for 10h showed the broad diffractions centered at 2θ=25.5° and 42.0° (Cu Kα). The crystallization of boron nitride synthesized at 800°C for 10h proceeded above 1180°C. The 100 reflection of hexagonal boron nitride was separated from the broad 10 diffraction of the specimen heated at 1550°C for 2h. The lattice constant (c⁰) and size of crystallite (L^c) of the specimen were 6.735 and 220Å, respectively, after heating at 1550°C under atmospheric pressure for 2h. The size of crystallite increased from 320 to 830Å as the temperature increased from 1100°C to 1500°C under 2.0GPa for 10min heating. Boron nitride synthesized by heating at 800°C for 10h crystallized to hexagonal boron nitride when heated at 1500°C and 2.0GPa for 10min.

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Precursors of carbon-containing SiC fibers (SiC-C fiber) were synthesized with high yield by the copyrolysis of poly (dimethylsilane) and 3.9-80wt% pitch. During copyrolysis the decomposition product of polysilane reacts with pitch molecule with no significant condensation among pitch molecules. SiC-C fibers were obtained by melt spinning of the precursors and heat-treatment up to 1400°C in vacuum. The tensile strength and Young's modulus of the SiC-C fiber decreased with increasing carbon content. The electric conductivity is two orders of magnitude larger than that of SiC fibers obtained from pure polycarbosilane.

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Ultrafine silicon carbide powders of high purity have been prepared by pyrolysis of tetramethyldisilane at temperatures 700° to 1400°C. The diameter of the particles changed from 5 to 200nm depending on the pyrolysis conditions. The ultrafine particles consist of small crystallites of β-type SiC, arranged randomly in the particles. The molar ratio (C/Si) of the powders can be also controlled within the range 0.9 to 1.2 by pyrolysis conditions such as pyrolysic temperature and reactant concentration. No admixtures of carbon such as carbon soot or film on the SiC particles were observed with a high-resolution electronmicroscope even in the powders having (C/Si) ratio more than 1.0. The formation mechanism of the ultraf ine SiC powders is discussed in relation to the structure.

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Fine yttria-partially-stabilized zirconia powders were prepared by the spray pyrolysis method from a water: alcohol solution of ZrO(NO₃)₂⋅2H₂O and YCl₃⋅6H₂O corresponding to 2mol% Y₂O₃. Effects of flow rates of atomizing air and solution, concentration of solution and reaction furnace temperature on the powder properties such as particle size, specific surface area, crystallite size and crystalline phase of as-synthesized powders were examined. It was observed that the as-synthesized powders are tetragonal phase containing 2±0.3 mol% Y₂O₃. The particle size was 0.2-5μm and a particle was agglomerates of fine crystallite of 165-210Å in size. The powder consisted of hollow spherical or lath like particles and the specific surface area was 3-11m²/g. The size of agglomerate decreased with increasing atomizing air flow rate, of which the pyrolysis was independent. With increasing rate of solution, the size of agglomerate increased, but this size was not sensitive to the increase in the concentration of solution. In both cases, the pyrolysis of the salts was incomplete. A similar phenomenon was observed in the powder preparation at low temperatures.

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A magadiite (layered polysilicate)-polyacrylonitrile (PAN) intercalation compound was first prepared to investigate its potential as a precursor for the synthesis of non-oxide ceramics by carbothermal reduction. Acrylonitrile monomer was intercalated and polymerized in the interlayer space of a C₁₂H₂₅N(CH₃)₃-magadiite complex. After the cyclization of PAN by the heat treatment in air, the compound was heated in Ar flow for 2h. Above 1300°C, β-SiC was formed without the formation of any crystalline SiO₂ phases. Since the reactions of a magadiite-carbon mixture involved the formation of cristobalite, it was shown that SiO₂ from magadiite was reduced to form β-SiC without crystallization, which was caused by the presence of PAN in the interlayer space.

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Silicon nitride was formed from a magadiite (layered polysilicate)-polyacrylonitrile (PAN) intercalation compound through a novel process of carbothermal reduction. SiO₂ from magadiite was reduced above 1400°C without the formation of any crystalline oxides. On heating above 1400°C, both α-and β-Si₃N₄ were detected, and the additional formation of α-and β-SiC was observed by heating at 1600°C. On the other hand, the reactions for the synthesis from a magadiite-carbon mixture involved the formation of cristobalite as well as α-and β-Si₃N₄, and β-SiC. In addition, it was also indicated that the mixture was less reactive for the formation of Si₃N₄. These observations suggest that the reaction process of the magadiite-PAN complex was different from that of the magadiite-carbon mixture, and that the use of the intercalation compound was favorable for Si₃N₄ production.

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Boron nitride was formed from boric acid-diethanolamine or -triethanolamine condensation products by their pyrolysis in a nitrogen flow. The condensation products obtained from boric acid-diethanolamine system (molar ratio 1:1 and 2:3) were glassy solids and had a polymeric structure containing the B-O-C bond. Monomeric triethanolamine borate, which contained both B-O-C and coordinate B-N bonds, was formed from boric acid-triethanolamine system. Pyrolysis of these condensation products in N₂ flow at 1400°C yielded boron nitride. A small amount of B₄C was also formed, depending on the molar ratio or on the carbon content of the condensation products. In order to clarify the effect of the nitrogen in the molecules, the pyrolysis was conducted in an argon flow. Both BN and B₄C were formed and the influence of nitrogen in the precursors on the pyrolyzed products was observed.

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Amorphous thin films in the systems Li₂O-SiO₂ and Li₂O-B₂O₃-SiO₂ are prepared by rf-magnetron sputtering, and their electrical properties are investigated in order to develop solid electrolytes for thin film solid state ionic devices. These films exhibit high electrical conductivities ranging from 10^-4 to 10^-7Sm^-1 at room temperature.

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Growth of ZrO₂ whiskers by the reaction of ZrCl₄ and O₂ or H₂O was examined at 1100°-1300°C. Only powder products were obtained at temperatures below 1200°C in both reaction systems. At 1250°-1300°C, however, monoclinic whiskers or needle crystals were produced. In ZrCl₄-O₂ system, the whiskers were grown on a mullite substrate together with flaky materials, bricky crystals and powder. Optimum conditions for the whisker growth were ZrCl₄ 1-3%, O₂ 0.25-0.35%, and total flow rate of the reactant gas 40-60cm³/min. The size of the whiskers was 0.1-2μm in width and 10-100μm in length. The growth axis was the ‹010› direction or perpendicular to the (104) plane. In ZrCl₄-H₂O system, no whiskers were formed on the substrate, but needle crystals (whisker-like) and powder were obtained at the outlet of the reaction tube. The monoclinic needles were very minute, their dimension being 0.05-0.5μm in width and 0.5-3μm in length. The growth axis was the ‹010› direction or perpendicular to the (104), (113) or (211) plane. Optimum conditions were ZrCl₄ 2-5%, H₂O 3-4%, and total flow rate of the reactant 30-60cm³/min. Electron microscopic observation revealed that the whiskers and the needles had no particles on the tops, suggesting that both of them grew by a VS mechanism.

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