Liquid phase sintering based on the dissolution-precipitation mechanism was applied to densify a 0.8 micrometer SiC powder with alumina (1.2 vol%)-yttria (0.9-3.3 vol%) additives. To uniformly distribute the sintering additives around the SiC particles, a heterocoagulated particle network was formed among negatively charged SiC particles, positively charged 0.2 micrometer alumina and yttrium ions in an aqueous suspension at pH 5. Yttrium ions were electrostatically adsorbed on the negatively charged SiC surfaces. The consolidated green compacts were highly sintered to 97-99% of theoretical density by hot-pressing at 1950°C. The mechanical properties (four-point strength, fracture toughness and Weibull modulus) were highly enhanced when a bimodal particle size system of SiC (0.8 micrometer-30 nanometer SiC) was sintered. The maximum strength of 75 vol% 0.8 micrometer SiC-25 vol% 30 nanometer SiC reached 1.1 GPa at room temperature. The fracture toughness was about 6 MPa·m^1/2 and the Weibull modulus was 5.9. When a small amount of SiC precursor polymer was infiltrated in the green compact, the strength and Weibull modulus were further improved.

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The present article reviews recent advances in synthesis of novel phases in the ternary Si-C-N system. A dense carbon nitride phase, C₂N₂(NH), was synthesized for the first time at high pressures and high temperatures in a laser heated diamond anvil cell (LH-DAC). Based on results of electron diffraction, EELS- and SIMS-measurements combined with theoretical calculations the structure of this new C-N-H compound was analysed to be a defect wurtzite structure of the sinoite (Si₂N₂O)-type. Farther, a variety of amorphous SiCN phases and the first ternary crystalline phases, namely Si(NCN)₂ and Si₂N₂(NCN), were synthesized at ambient pressure. In general, the high-pressure polymorphs of Si-C-N materials are predicted to exhibit a unique combination of high hardness, thermal stability and oxidation resistance with interesting optoelectronic properties.

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The present study was performed to determine suitable processing conditions to obtain optimized boron carbide slurries for gelcasting. We examined how dispersant, pH and other factors function in boron carbide slurries. The effect of the dispersant on the slurry stability was characterized in terms of zeta potential, sedimentation, and rheology measurements. Subsequently, the optimum conditions for the preparation of stable slurries with high solid content were determined. Gelcasting was used to consolidate the slurries. The effect of monomers on the slurry properties was studied. The properties of green and sintered B₄C samples were also investigated.

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This paper is focussed on the development of microstructure during liquid phase sintering and post-densification crystallisation heat treatment of ceramic materials based on the α- and β-Si₃N₄ structures. Grain shape and size distributions, assessed by quantitative microscopy in combination with stereological methods, and fine scale microstructures, investigated by electron diffraction and high resolution imaging and microanalysis in the transmission electron microscope, are discussed in relation to the fabrication process and the overall composition of the ceramic material.

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Reaction bonded silicon nitride ceramics have been produced from Si powders of different particle size and both with and without zirconia as a nitride catalyzing element. The effect of the ZrO₂ on the nitridation rate of the specimens was studied as a function of temperature, and the mechanism of enhanced nitridation was discussed. At all temperatures, the degree of nitridation of specimens sintered with ZrO₂ was higher than those without additions for all particle sizes. In the case of nitridation of specimens produced without ZrO₂, the degree of nitridation was dependent on particle size, with larger sizes resulting in less nitridation. For specimens with ZrO₂ the finest powder also showed highest transformation, but there was no difference in the degree of nitridation of medium and coarse powders. It was found that during heating of the specimens the ZrO₂ was transformed to ZrN at low temperature and enhanced nitridation is considered to be due to this phase re-converting to ZrO₂ at higher temperature and thereby supplying nitrogen to the interior of the specimens. At lower temperatures, the strength of the specimens with ZrO₂ were higher than those without, attributed to the enhanced nitridation. However, comparing strengths of fully nitrided specimens, those with ZrO₂ were lower than those without, thought to be due to the presence of ZrO₂ particles disrupting the connectivity of adjacent silicon nitride grains.

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In this article, the aqueous gelcasting of carbide ceramics (SiC, TiC, B₄C) was studied systematically. For SiC with the addition of Al₂O₃ and Y₂O₃ as sintering additives, tetramethylammonium hydroxide (TMAH) is used as the dispersant for making slurry with high solids lading and low viscosity. N,N-dimethyl acrylamide (DMAA) and N,N'-methylenebisacrylamide (MBAM) are applied as organic monomers and cross-linker respectively. Ammonium persulfate (APS) acts as initiator for the polymerization of acrylamide monomers. Gelcasted SiC samples are pressureless sintered at 2000°C with the relative density higher than 97%. The gelcasting of TiC/Ni system are studied with PEI as the dispersant. An azocompound, 2,2'-azobis[2-(imidazolin-2-yl) propane]dihydrochloride(AZIP · 2HCl) is selected as the initiator. The green samples with the relative density as high as 57.5% can be densified at 1530°C in vacuum. The linear shrinkage, relative density, apparent porosity, flexural strength, Young's modulus and Rockwell hardness(HRA) are (15.96 ± 0.65)%, (93.3 ± 0.7)%, (1.03 ± 0.16)%, (1062.77 ± 49.81) MPa, (323.93 ± 4.00)GPa and (84.74 ± 0.63)% respectively. PEI is also an effective dispersant for B₄C powder in aqueous media. With the addition of PEI, the solids loading of B₄C slurry higher than 50 vol% can be obtained. The pressureless sintering of B₄C with 10 mass% TiC as sintering additives is performed at 2250°C for 30 min in Ar atmosphere. The relative density of B₄C sintered samples is around 96.2%. Results showed that gelcasing is an effective processing technique for preparing carbides with complex shape and satisfied properties.

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An investigation has been made of the use of 2.45 GHz microwave energy to sinter Si₃N₄-ZrO₂(4 mol% Y₂O₃ stabilized) composites with 5 mass% MgO as a densifying aid at l600°C to avoid the formation of ZrN. XRD phase analysis showed the presence of ZrN in all the sintered samples. Higher temperature developed within the specimen favored the reaction between Si₃N₄and ZrO₂ to form ZrN. SEM microstructures revealed elongated β-Si₃N₄ grains with high aspect ratio. Indentation fracture toughness of the composites increased with increasing weight fraction of ZrO₂. Cracks were propagated intergranularly through Si₃N₄/ZrO₂ interfaces and transgranularly through ZrO₂ grains with many microcracks in the process zone of the main crack. The results indicated that the dominant toughening mechanisms operating were crack deflection and bridging in the case of 0 mass% ZrO₂ and 10 mass% ZrO₂ whereas crack deflection, bridging and microcrack toughening were observed when the amount of ZrO₂ in the composites increased to 25 and 40 mass%.

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This paper reports the gas pressure sintering and thermal conductivity of silicon nitride with 2 mol% Y₂O₃ and varied contents of MgSiN₂ in the range of 0 to 5 mol% at 1900°C under a nitrogen pressure of 1 MPa N₂. It was found that the density curves show a typical "V" shape against the MgSiN₂ content. The 1 mol% MgSiN₂ hinders significantly the densification, but the 5 mol% MgSiN₂ leads to complete densification. The inhibited densification is associated with the pronounced normal grain growth. However, the abnormal grain growth is inhibited with the addition of MgSiN₂. After sintering for 12 h, the sample without MgSiN₂ exhibits the coarsest and highest bimodal microstructure, whereas the one with 5 mol% MgSiN₂ exhibits the finest microstructure. Although the MgSiN₂ addition leads to a slightly enhanced thermal conductivity, it tends to reduce the thermal diffusivity. The thermal conductivity without porosity shows a decrease with the increase of MgSiN₂ content because of the inhibited grain growth.

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Si₃N₄-SiC composites, which are widely used as sidewall refractories in aluminum smelting cells, have been produced by reaction bonding and their corrosion performance assessed in simulated aluminum electrochemical cell conditions. Additional samples were produced with the silicon nitride bonding phase replaced by β SiAlON with compositions ranging from Z = 1-4 in Si_6-zAl_zO_zN_8-z. The formation of the Si₃N₄ and SiAlON bonding phases were studied by reaction bonding of silicon powders in a nitrogen atmosphere at low temperatures to promote the formation of silicon nitride, followed by a higher heating step to produce β SiAlON composites of different composition. The corrosion performance was studied in a laboratory scale aluminum electrolysis cell where samples were exposed to both liquid attack from molten salt bath and corrosive gas attack. For the Si₃N₄ bonded samples, the corrosion resistance was shown to be strongly dependent on the environment during corrosion testing, with samples in the gas phase showing higher corrosion than those immersed in the bath. Samples that had been pre-soaked in the bath and then tested in the gas phase showed the highest corrosion due to the combined effects of bath penetration and gas attack. For the SiAlON bonded samples, the corrosion results showed similar trends but were complicated by the presence of a strongly adherent layer on the samples which influenced volume measurements. This layer is believed to be alumina and appears to form through an interaction with the SiAlON and the bath components, since no such layer was observed for the silicon nitride samples.

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Mono-phase and hexagonal plate-like Al₄SiC₄ powders were successfully synthesized using a mixture of Al(OH)₃, SiO₂ and phenolic resin via the carbothermal reduction process. Two-step reaction was compartmentally observed during calcination; the carbothermal reduction of SiO₂ between 1300 and 1600°C and that of Al₂O₃ above 1500°C. X-ray diffraction data indicated the orientation of the compacted powders along c-axis due to the stacking of plate-like Al₄SiC₄ powders. Microscopic observation showed that the synthesized powders were mainly hexagonal platelets. The main synthetic mechanism of the ternary carbide powder during the carbothermal reduction process was gas-solid reaction. Possible reaction mechanisms which are responsible for the synthesis of Al₄SiC₄ by the carbothermal reduction process are discussed.

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This paper provides an overview of the crystallisation of an oxynitride glass likely to remain in a silicon nitride ceramic following firing. The crystallisation process was studied using both differential thermal analysis (DTA) and separate isothermal heat treatments in a tube furnace under nitrogen. The activation energy for the crystallisation process was determined by DTA. The nucleation temperature, T_g + 40°C, which corresponded to the maximum volume fraction of crystalline phases, agreed closely with the optimum nucleation temperature of T_g + 35°C, found from DTA. The optimum crystal growth temperature was observed to be 1210°C and yielded the α- and β-polymorphs of yttrium disilicate. Heat treatments at other temperatures indicated the development of phase assemblages which contained different polymorphs of yttrium disilicate as well as silicon oxynitride. Not all of the polymorphic transformations of yttrium disilicate were observed by DTA unless some crystallisation exotherms were deconvoluted, indicating that DTA analysis of the crystallisation of complex systems requires careful interpretation. It is, however, possible to simplify the system by substituting some yttrium by lanthanum. This stabilises the α-polymorph of yttrium disilicate. The activation energy for crystallisation was observed to be similar to that for viscous flow of Y-Si-Al-O-N glasses.

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In this study the production of Si₃N₄-B₄C composites using spark plasma sintering technique was studied. This technique was preferred in order to minimize the reactions between starting constituents. Fully dense Si₃N₄ ceramic was obtained by SPS. The bulk densities of all the produced composites was found to be significantly less than that of Si₃N₄, and this was attributed to the formation of the relatively low-density reaction products. In the B₄C-containing composites there was a trend for the bulk density to increase slightly with decreasing particle size of B₄C powder. Composites containing both B₄C and TiO₂ had somewhat higher bulk densities when compared with those just containing B₄C. This was related to the formation of relatively high-density reaction products in significant amounts. The fine B₄C added to the Si₃N₄ base composition containing Al₂O₃ and Y₂O₃ as sintering additives, even when incorporated in significant amounts, was consumed readily during the fast sintering process. Using coarse B₄C particles reduced the reaction kinetics to some extent. As a result of reactions between Si₃N₄ and B₄C particles SiC, h-BN and metallic Si were formed. When both B₄C and TiO₂ were added together additional phases of Ti (C, N) and TiB₂ were formed. Possible reactions that explain the formation of the in-situ phases were proposed through thermodynamic considerations.

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Immobilization of long-lived fission products (LLFP) such as radioactive Tc, Cs and Sr into α-SiAlON ceramics was evaluated using stable isotopes instead of radioactive isotopes, and the applicability of α-SiAlON ceramics as the inert matrix for transmutation of LLFP was investigated. In the case of single addition of SrO, SrCO₃, Cs₂CO₃ or ReO₂ to the starting materials, α-SiAlON single phase was not formed after hot-pressing. When Y₂O₃ was added with SrO, SrCO₃ or Cs₂CO₃ to the starting materials (α-Si₃N₄, AlN and α-Al₂O₃) in optimum compositions, α-SiAlON single phase was obtained after hot-pressing at 1700°C or 1800°C. From the EDS analysis, Sr and Y were detected from grains. It is suggested that Y would assist the expansion of interstices of α-SiAlON lattice, resulting in the incorporation of Sr²⁺ into α-SiAlON lattice. In the case of Cs addition with Y, Cs was not incorporated into interstices due to much larger ionic radius of Cs⁺. Bulk density of hot-pressed α-SiAlON ceramics obtained in this study was higher values and densification proceeded well. From these results, it was found that the immobilization of Sr in α-SiAlON was successfully achieved with co-doping of Y, and α-SiAlON ceramics would be an inert matrix for transmutation of long-lived radioactive Sr.

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Dy-doped α-SiAlON ceramics with a combined property of optical translucence and wear resistance is reported in the present presentation. A considerable test was made on the tribological behavior of the Dy-α-SiAlON to find out the effects of temperature and sliding conditions on the wear properties. It was shown that the friction coefficient was the lowest (0.26) at 100°C under an applied load of 5N. Accordingly, the wear rate was the lowest at 100°C for the same Dy-α-SiAlON sample. Obviously Dy-SiAlON shows an excellent wear resistance under a suitable sliding condition under a load of 5N and 100°C condition. This appears, therefore, attractive and important for Dy-SiAlON ceramics to be used as a type of special wear resistant materials, with an optical translucence.

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The preparation of Eu²⁺ doped Ca-α-SiAlON phosphor films were conducted on ITO glasses by electrophoretic deposition (EPD). The thickness of the SiAlON phosphor films was controlled by altering the deposition time. The photoluminescent (PL) properties of the SiAlON phosphor films were characterized by blue-light (λ = 450 nm) irradiation through the ITO glass substrates. The light emission from the deposit surfaces was characterized by comparing the intensities of the excited yellow light and the transmitted blue light. The chromaticity coordinates were calculated from the PL spectra and compared with actual colors perceived by human eyes.

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Al₂O₃-cubic boron nitride (cBN) composites were prepared from Al₂O₃ and cBN powders by spark plasma sintering (SPS) at temperatures of 1200 to 1600°C for 600 s under a pressure of 100 MPa. Densification, phase transformation, microstructure and mechanical properties of the Al₂O₃-cBN composites were investigated. Fully dense Al₂O₃-cBN composites containing 10 to 20 vol% cBN with a relative density of more than 98% were obtained at 1300°C for 600 s without the phase transformation from cBN to hexagonal BN (hBN). The transformation of cBN was more greatly accelerated in the Al₂O₃-cBN composite than in cBN. The cBN phase to Al₂O₃ inhibited the grain growth of Al₂O₃ in the Al₂O₃-cBN composite. Vickers hardness of dense Al₂O₃-cBN composites containing 10 to 20 vol% cBN sintered at 1300°C showed the maximum value of 26 GPa. The Al₂O₃-cBN composite containing 20 vol% cBN sintered at 1300°C had the highest fracture toughness of 4.1 MPa·m^1/2.

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TiN particle-dispersed Si₃N₄ ceramics are applied to bearing materials because of their excellent mechanical and thermal properties, chemical stability, etc. In particular, hybrid bearings composed of Si₃N₄ ceramics and metals are more popular because of low production costs. In such hybrid bearings, control of abrasive wear of metals by hard TiN particles is required. The objective of this study is development of TiN nanoparticle-dispersed Si₃N₄ ceramics that cause low damage to the mating metals. In order to disperse TiN nanoparticles in Si₃N₄ ceramics, TiO₂ nanoparticles and Si₃N₄ particles were mixed by mechanical treatment. TEM observation showed that TiO₂ nanoparticles were directly bonded to submicron Si₃N₄ particles. Si ₃N₄ ceramics in which TiN nanoparticles were uniformly dispersed were fabricated using the composite particles. The amount of damage caused by the Si₃N₄ ceramics with TiN nanoparticles, developed in this study, to the mating metals in ball-on-disk test is comparable to damage caused by the Si₃N₄ ceramics without TiN particles.

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Sialon glasses are effectively alumino-silicates containing modifying cations (Mg, Ca, Y and Lanthanides) in which nitrogen substitutes for oxygen in the glass network. They are found at triple point junctions and as intergranular films in silicon nitride based ceramics. This paper provides an overview of M-Si-Al-O-N glasses and outlines the effects of composition on structure and properties. As nitrogen substitutes for oxygen, increases are observed in glass transition (T_g) and dilatometric softening (T_ds) temperatures, viscosities, elastic moduli and microhardness and decreases can be seen in coefficient of thermal expansion as a result of extra crosslinking of the glass network. The gradients of the linear relationships between properties and nitrogen content are similar for a wide range of different sialon glass compositions and modifier types since each eq.% nitrogen substitution introduces the same number of additional crosslinks into the glass network.

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AlN/BN nanocomposites indicated such a unique property as machinable with high fracture strength. In order to investigate the deformation mechanism of AlN/BN nanocomposites, Hertzian contact tests were performed on monolithic AlN, and nanocomposite and microcomposite AlN/BN. The monolith and microcomposite comprised micro-sized grains of AlN and BN. In the microcomposite, plate-like BN particles preferentially oriented perpendicular to the hot-press direction. On the other hand, the nanocomposite showed a finer microstructure with fine BN particles and small matrix grains. These microstructural differences led to differences in the mechanism of contact damage. The damage of the monolithic AlN and the AlN/BN microcomposite exhibited collapsed Hertzian cone fracture and preferentially oriented large cracks perpendicular to the hot-press direction, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.

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Ag⁺-Na⁺ exchange equilibrium for carrier (sodium zirconium phosphate) was investigated at 60°C, and the chemical compositions and structure of carrier and silver-carried zirconium phosphate (AgZrP) were investigated by EDX and XRD. The silver valence state and antibacterial activity of AgZrP were also studied. The Ag⁺ exchange was favorable over the whole Ag⁺ concentration range. The molecular formulae of carrier [NaZr₂(PO₄)₃.H₂O] and AgZrP [Ag_0.5Na_0.5Zr₂(PO₄)₃.H₂O] have been determined, and the silver of AgZrP was in an ionic state. Both carrier and AgZrP displayed hexagonal crystal structures, but Ag⁺ exchange resulted in the increase of interplanar spacing and lattice parameter c, and the crystal orientation of AgZrP was affected by the existence of Ag⁺. Moreover, 100 mg.l^-1 of AgZrP possessed high antibacterial activity and was capable of killing all the Escherichia coli and more than 99.9% of the Staphylococcus aureus within 8.0 h of contact. Furthermore, AgZrP showed a broad-spectrum and long-acting antibacterial activity.

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Magnesium-niobium (Mg-Nb) thin films with a palladium (Pd) top layer were prepared on glass substrates by means of DC magnetron sputtering deposition, at room temperature. A Mg-Nb thin film with a Pd top layer can be switched to a transparent state from a mirror-like state by loading with 4% hydrogen gas. The optical properties of Mg_(1-x)Nb_x thin film switchable mirrors, with x ranging from 0.13 to 0.37, coated with a Pd layer were investigated using a UV-Vis-NIR optical photometer. The optical transmittance of the hydride states decreased with an increasing Nb fraction. Mg-Nb thin film switchable mirrors show color-neutral characteristics in the hydride state. The degradation mechanism of Mg-Nb thin films when loading hydrogen (H) gas was studied using X-ray photoelectroscopy (XPS) and scanning electron microscope (SEM) methods. We showed that Mg-Nb-H thin films are transparent, color-neutral semiconductors with a band cap of 2.2 ± 0.2 eV.

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