Laser irradiation of α-SiC ceramics

The process of laser irradiation of a surface of SiC ceramics in air was investigated. As a result of SiC oxidation and evolution of CO₂, porous SiO₂ forms on the target surface. During deposition of ablation products on the substrate, a loose SiO₂ film forms.     

BaO–TiO2 microwave ceramics

In this paper, the structure and dielectric properties of BaO–TiO₂ system ceramics were studied. By adding ZnO and Nb₂O₅ as sintering agents to the raw materials, the BaO–TiO₂ system ceramics were sintered at a temperature of 1260 °C for 2 h and have superior dielectric properties at 1 GHz: quality factor Q=12,500, relative dielectric constant ε_r≈37, temperature coefficient of dielectric constant α_ε=0±30 ppm/°C. XRD pattern shows that the main crystal phase of the ceramics is Ba₂Ti₉O₂₀, accompanied by a small number of additional phases: BaTi₄O₉, Ba₄Ti₁₃Zn₇O₃₄, Ba₄Ti₁₃O₃₀ and Ti₂Nb₁₀O₂₉, etc. The initial Ba/Ti ratio has a great effect on the dielectric properties of the ceramics, which can be explained by the variance in the formation of phases due to different Ba/Ti ratios.     

Cordierite ceramics—Spalling-resistant high-melting refractories

Conclusions The production technology of cordierite refractories from fireclay and magnesite differs from other processes in that the components have to be finely ground.The components must be accurately apportioned and the mass thoroughly mixed preferably in batch mixers. Otherwise, the production process of cordierite refractories does not differ from that of chamotte brick.The use of fireclays with a high Al₂O₃ content (38%) and magnesite eliminates the need for commercial alumina.     

Properties of AlN–TiN composite ceramics

Abstract

Results are presented of a study dealing with an AlN–TiN composite material for high temperature applications. Hardness, microhardness, compressive strength, electrical resistance, and thermal conductivity of the material were measured at room temperature and flexural strength was measured at temperatures from 20 to 1800°C. The composite material containing 40 wt-% AlN and 60 wt-% TiN and hot pressed at 1850–1950°C exhibits an increase in strength of 20% with a rise in temperature, which permits it to be used at elevated temperatures.     

Thermal shock properties of β-sialon ceramics

An indentation–quench method based on Vickers cracks for measuring thermal-shock properties has been applied to β-sialon materials. The thermal-shock properties have been correlated with the morphology of the β-sialon grains, the z value in the β-sialon solid solution Si_6−zAl_zO_zN_8−z and the amount of residual intergranular glass phase. z Values in the range 0.6–3.0 were tested, and the amount of residual yttrium-containing glass phase was varied between 0 and 20 vol.%. The best thermal-shock resistance was found at low z values, and was further improved by adding an intergranular glass phase. The poorest resistance to thermal shock was found for the highest z value where the presence of glass had no measurable influence. One composition (z=1.5, 10 vol.% glass) was selected for studying the influence of the microstructure on the thermal-shock properties. The microstructure was varied by applying different sintering conditions. An improvement of the thermal-shock properties and the fracture toughness was found in samples containing elongated β-sialon grains formed in situ. In general, in-situ reinforced β-sialon materials with low z values and containing an intergranular glass phase exhibited the best thermal-shock resistance and improved fracture toughness (K_1c>4 MPa m^{$\text{1}\text{2}$}).     

Pressureless sintering of PMN–PT ceramics

PMN–PT ceramics with PMN to PT ratios of 60:40, 65:35, and 70:30 were prepared from PMN–PT powders synthesized by the columbite precursor method, and their sintering and grain growth characteristics at temperatures less than 1000°C were investigated. Results indicate that the PMN–PT ceramics can be pressureless-sintered to a relative density of approximately 96% at 950°C. However, full densification was prevented by the onset of abnormal grain growth. The addition of 0·5 wt% PbO to 65:35 PMN–PT ceramics lowered their sintering temperature to 900°C, but caused abnormal grain growth at lower temperatures. Preliminary TEM analyses indicate the presence of submicron-sized MgO particles at some ceramic microstructure triple points. Further studies will be required to understand abnormal grain growth behavior and to devise means for full densification.     

Two-step sintering of fine alumina–zirconia ceramics

This work investigates the feasibility to the fabrication of high density of fine alumina–5 wt.% zirconia ceramics by two-step sintering process. First step is carried out by constant-heating-rate (CHR) sintering in order to obtain an initial high density and a second step is held at a lower temperature by isothermal sintering aiming to increase the density without obvious grain growth. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range between 1400 and 1450 °C is effective for the first step sintering (T₁) due to its highest densification rate. The isothermal sintering is then carried out at 1350–1400 °C (T₂) for various hours in order to avoid the surface diffusion and improve the density at the same time. The content of zirconia provides a pinning effect to the grain growth of alumina. A high ceramic density over 99% with small alumina size controlled in submicron level (0.62–0.88 μm) is achieved.     

Glass–ceramics and composites containing aluminum borate whiskers

Glass–ceramics and composites containing aluminum borate whisker crystals were developed using two different approaches: crystallization of an aluminum borosilicate glass or addition of an aluminum borate precursor powder to a glass frit. Two different glass frits were used, a commercially available borosilicate glass or the same aluminum borosilicate glass used in the crystallization experiments. X-ray diffraction analysis showed that Al₄B₂O₉ or Al₁₈B₄O₃₃ whiskers formed in all samples, indicating that the glass crystallized significantly with increasing heating temperature, and that the precursor can be effectively be used to generate in situ aluminum borate crystals within glass matrices. However, the samples produced by mixing the aluminum precursor with glass frits contained porosity after processing, indicating that pressureless viscous sintering was not efficient. The hardness of the glass–ceramic did not vary significantly with processing temperature, but the (indentation) fracture toughness measured showed a >100% increase (after heating at 1200 °C), demonstrating that whisker-shaped crystals are effective in increasing the mechanical toughness of the glass matrix. The hardness of the composites showed a dependence on the amount of aluminum borate crystals present.     

Synthesis of BCN ceramics from pyrrolidine–Borane complex

BCN ceramics were prepared from pyrrolidine–borane complex. Its complex was a new product and has not appeared on the market. The chemical compositions of the BCN ceramics were BC_1·5N_0·4 obtained at 1000°C in Ar, BC_0·9N_0·4 obtained at 1000°C in NH₃. Boron was a mixture of trigonal and tetragonal BN bonds. Electrical conductivity, of the BCN ceramics showed semiconductive properties. ©     

Transformation and thermal stability of Li-α-sialon ceramics

Two phase α/β and single phase α lithium sialons with different m and n values were produced by hot pressing at 1730–1750°C at 30 MPa for 30–40 min in a graphite resistance furnace. When the two-phase samples were heat-treated at lower (1200–1450°C) temperatures in different packing powders, an increase in the amount of α was observed, due to β-sialon in the as-sintered material reacting with grain boundary liquid to form more α. β→α transformation at low temperatures has not been reported previously in any sialon system and in the present case is believed to occur because the α-sialon phase field in the lithium sialon system shifts slightly towards the β-sialon line at lower temperatures. The thermal stability of lithium α-sialon is good in the centre of the single-phase α region when surrounded by a Li-containing powder bed. However, towards the edges of the single-phase region, compositional changes occur on heat-treatment. Thus, samples with high m, n values decompose into β-sialon plus other Li-containing phases. During heat-treatment of other compositions when surrounded by a BN powder bed, the composition of the α-sialon phase continually readjusts towards the α/β sialon phase region as a function of time and this is followed by decomposition of the α phase. Evaporation of the Li⁺ stabilising cation is believed to be the main reason for this behaviour. The effects of m and n value, heat treatment parameters and packing powder on the thermal stability of Li α-sialons are discussed.     

An aqueous gel-casting process for γ-LiAlO2 ceramics

Methyl cellulose (MC) was used as the gelation agent in the gel-casting of γ-LiAlO₂ because it could form strong gels during heating. The thermal gelation behavior of the MC solution was studied based on the measurement of its apparent viscosity as a function of temperature. The effects of MC solution concentration and solid loading on the rheological properties of the γ-LiAlO₂ slurries were studied systematically. It was found that all the slurries exhibited a shear-thinning behavior, which was considered to be favorable for the casting process. The compressive strength and relative density of the dried γ-LiAlO₂ green bodies were measured, and the microstructure of the green and sintered bodies was investigated.     

Joining transparent spinel ceramics using refractive index–matched glass

Ceramic joining by glass is a promising method of the preparation of large transparent ceramics from small blocks. The chemical composition of glass was optimised to match the coefficient of thermal expansion and refractive index of transparent magnesium aluminate (MgAl₂O₄) ceramics. A two-step joining method was developed to join MgAl₂O₄ ceramics with a reduced number of bubbles in the joint, and the thermal properties of the optimised glass were evaluated to determine the joining temperature. Two transparent MgAl₂O₄ blocks were joined by a glass layer that was approximately 20 µm thick. The joint area could not be distinguished with a naked eye. The transmittance of the joined body vertical to or parallel through the glass layer was approximately the same as that of the ceramics. The average three-point bending strength of the joint reached 202 ± 33 MPa, which was 64% that of the ceramic body.     

Investigation on the microstructure of self-reinforced Nd-α-sialon ceramics

α-Sialon ceramics doped with Nd₂O₃ were prepared by hot-pressing sintering at 1900 °C holding for 1 h with a heat preservation for 1 h at 1500 °C. Microscopic observations indicate that elongated α-sialon grains appear and are embedded in the fine equiaxed-grain matrix. A small amount of β-sialon phases and secondary crystallized phases M′ (Nd₂Si_3−xAl_xO_3+xN_4−x) also exist in the Nd-α-sialon ceramic. A core/shell structure can be found in the elongated α-sialon and β-sialon grains with a high aspect ratio, in which some misfit dislocations surround the core. The 21R AlN-polytype was observed by transmission electron microscopy (TEM), which could not be detected by X-ray diffraction (XRD) due to its trace amount. Different nucleation and growth modes of sialon grains were also discussed.     

Wear properties of Y–α/β composite sialon ceramics

The tribological properties of yttrium containing α/β composite sialon ceramics have been studied under non-lubricated conditions by means of block-on-ring and ball-on-disk type experiments against a commercial silicon nitride material. The sialon ceramics were produced by hot pressing powder mixtures of Si₃N₄, AlN, Al₂O₃ and Y₂O₃, resulting in composite ceramics containing different amounts of the α/β phases. The effects of microstructural differences on the mechanical properties of the ceramics, and their wear characteristics under a range of testing conditions have been assessed. It was found that Vickers hardness decreased whilst both fracture toughness and bending strength increased with increasing amount of β phase in the composite. Under mild testing conditions, material removal was considered to occur by polishing of the surface, and in this case the high α-sialon composites exhibited the highest wear resistance, reflecting their higher hardness. Under severe testing conditions, the wear behaviour was characterised as microcracking caused by the higher Hertzian stress levels, and resulted in grain removal or “dropping” from the surface of the materials. Under these conditions, the elongated microstructure and higher fracture toughness of the low α-sialon composites hinder the crack propagation and result in better wear characteristics when compared to the fine equiaxed α-sialon materials.     

Tin–Zinc oxide composite ceramics for selective CO sensing

Composite metal oxide gas sensors were intensely studied over the past years having superior performance over their individual oxide components in detecting hazardous gases. A series of pellets with variable amounts of SnO₂ (0–50 mol%) was prepared using wet homogenization of the component oxides leading to the composite tin-zinc ceramic system formation. The annealing temperature was set to 1100 °C. The samples containing 2.5 mol% SnO₂ and 50 mol% SnO₂ were annealed also at 1300 °C, in order to observe/to investigate the influence of the sintering behaviour on CO detection. The sensor materials were morphologically characterized by scanning electron microscopy (SEM). The increase in the SnO₂ amount in the composite ceramic system leads to higher sample porosity and an improved sensitivity to CO. It was found that SnO₂ (50 mol%) - ZnO (50 mol%) sample exhibits excellent sensing response, at a working temperature of 500 °C, for 5 ppm of CO, with a fast response time of approximately 60 s and an average recovery time of 15 min. Sensor selectivity was tested using cross-response to CO, methane and propane. The results indicated that the SnO₂ (50 mol%)-ZnO (50 mol%) ceramic compound may be used for selective CO sensing applications.