Materials Science Forum Vol. 922

Abstract: To reinforce the mullite/Al₂O₃ composite ceramics through formation of mullite whiskers, the composite ceramics were prepared by pressureless sintering using different AlF₃ content. The microstructure, porosity, fracture toughness and thermal shock resistance of the composite ceramics were investigated. The results show that the addition of AlF₃ can promote the mullite whisker formations, and the whiskers with the size of 3～10μm in diameter and a length-diameter ratio of 10～15 are obtained by sintering at 1600°C with the AlF₃ content of 5wt%. Fracture toughness and thermal shock resistance of the composite ceramics are improved by the formation of mullite whisker. The fracture toughness of 4.79MPa•m^1/2 can be obtained, and the 95.18% flexural strength remained after thermal shock.

Abstract: Alumina has been widely used as a structural ceramic because of high hardness and chemical stability. However, due to the unique characteristics of low fracture toughness of ceramic materials, it has seen limited use as a dynamic structural material. Recently, zirconia toughened alumina (ZTA) has been receiving spotlight, which has various toughness mechanisms caused by the volume change associated in the phase transformation process of ZrO₂ particles dispersed in Al₂O₃ to increase the toughness of Al₂O₃. In this study, 8 mol% Y-ZrO₂ and 12 mol% Y-ZrO₂ with different crystal structures was dispersed in Al₂O₃ individually as a stabilizer and the mechanical properties of the ZTA were observed by differing the composition of the stabilizer. Experimental results show that the ZTA specimens with 12 mol% Y-ZrO₂ which contains a large amount of stable cubic crystal phases had relatively higher micro hardness values. Whereas, fracture toughness of ZTA specimens with 8 mol% Y-ZrO₂ which contains many unstable tetragonal crystal phases, was measured to have higher values than ZTA specimens with 12 mol% Y-ZrO₂, which was opposite to that of micro hardness.

Abstract: High emissive Ca²⁺/Fe³⁺-doped LaAlO₃ based ceramic materials were prepared by flame spraying and controlled crystallization method. The phase composition, microstructure, infrared optical properties of Ca²⁺/Fe³⁺-doped LaAlO₃ based ceramic powders were investigated. The physical mechanism for the significantly enhanced infrared emissivity of LaAlO₃ by doping with Ca²⁺ and Fe³⁺ was analysized. This high emissive Ca²⁺/Fe³⁺-doped LaAlO₃ based ceramic materials shows promising applications in high temperature thermal process field to enhance the radiative heat transfer and improve its thermal efficiency.

Abstract: The sapphire substrate have a significant impact on reducing the light scattering loss and in accordance with the crystalline surface and can improve the light emission efficiency is the surface roughness of the sapphire LED is an important factor. When the CMP process after measuring the transmittance, haze, and surface roughness had a transmittance of 84% or more transmittance at all wavelengths, regardless of the thickness it was confirmed that the haze also has a low value, regardless of the thickness. Was confirmed that the surface roughness is greater the thicker the thickness of the sapphire it affects the pressure of the CMP polishing and then take a lot of importing a low surface roughness value of the thickness of the sapphire CMP process the surface roughness value.

Abstract: We synthesized TiO₂ nanoparticles dispersed hydroxyapatite (TiO₂-modifed HAp) by solvothermal treatment of α-tricalcium phosphate (α-TCP) using water-isopropanol solution including tetraisopropyl orthotitanate (TTIP). Rod-shaped HAp with TiO₂ nanoparticles were formed by solvothermal treatment. HAp formation decreased with increasing the isopropanol fraction in the solution with TTIP. With increasing the treatment period, the rate of HAp formation in sample increased. The aspect ratio of HAp decreased with increasing the isopropanol fraction in the solution. Synthesized TiO₂-modifed HAp samples shows the photocatalytic decoloration of methylene blue under UV irradiation.

Abstract: The effect of nitrogen gas flow rate on the morphology of silicon nitride fibers obtained via carbothermal nitridation heat treatment method was investigated. A precursor containing silicon, oxygen and carbon was obtained by a sol-gel method from a mixture of tetraethyl orthosilicate, polyvinyl alcohol, H₂O and ethanol. A white wool-like product was obtained by heat treating the precursor placed in an alumina crucible under a 0.5 MPa nitrogen gas pressure at 1500oC with different nitrogen gas flow rates. The mass-based production rates of the samples obtained from the precursor powder were 20-30% for the different nitrogen gas flow rates. X-ray diffraction analysis revealed that the samples contained α-Si₃N₄ as the major phase along with β-Si₃N₄, Si₂N₂O and a small amount of amorphous product as minor phases. Unique twisted fibers with diameters of several hundreds of nanometers were found among the straight fibers by SEM observation. Elemental analysis using energy dispersive X-ray spectroscopy indicated that silicon and nitrogen were contained in the twisted fibers along with approximately 68 at.% of oxygen and several at.% of aluminum, which might have come from the crucible material. The SiAlON-like structures might have been formed by the partial dissolution of Al and O in the Si₃N₄ fibers. It was considered that the twisted morphology of some fibers might be formed by co-existing of β-SiAlON and/or amorphous phase regions in the Si₃N₄ fiber and resultant distortion of the fibers.

Abstract: In this paper, reaction bonded silicon carbide (RBSC) was prepared by silicon infiltration with silicon carbide and carbon black as raw materials. The effects of the mixing methods with different binders on density of green body were compared. The influences of phenolic resin content, forming pressure, sintering temperature of samples on the performance of green body, mechanical microstructure and properties of RBSC were studied. The result shows that the density of green body by wet-mixing with alcohol-soluble binder (phenolic resin) was much better than semi-dry-mixing with usual water-soluble binders (polyvinyl alcohol and carboxy methyl cellulose sodium). The bending strength of green body prepared with phenolic resin at the content of 12 wt.% reached to the maximum value. The density of RBSC increased generally with phenolic resin content increasing at temperature range from 1550 to 1650 °C. The bending strength of RBSC increased firstly and then decreased with phenolic resin content increasing at a sintering temperature of 1600 °C. The optimum condition for fabricating RBSC was sintering at 1600 °C with 12 wt.% phenolic resin, the density, porosity and bending strength of the obtained RBSC was 3.06 g·cm^-3, 0.05% and 370±54 MPa, respectively.

Abstract: Oxidation resistance and bending strength at high temperatures of 5 vol% Ni/(10 vol% ZrO₂+Al₂O₃) were investigated in this paper. Oxidation tests were conducted at temperature ranging from 1100 to 1300oC for 1 to 24 h in air. The oxidation resistance of the composites was studied via observation of oxidized-zone development from a cross-section view after oxidation. Three-point bending tests were conducted at temperatures ranging from room temperature to 1200oC in order to estimate its performance at high temperatures. Bending strength of the composites achieved 1200 MPa at room temperature and 460 MPa at 1200oC. Dispersion of ZrO₂ in Ni/Al₂O₃ composites enhanced both their room and high temperature bending strength. Nevertheless, ZrO₂ slightly degraded the oxidation resistance of the composites. The oxidation rate of Ni/(ZrO₂+Al₂O₃) was one order of magnitude higher than that of Ni/Al₂O₃.

Abstract: This paper discusses the relationship between torsion strength and bending strength of Al₂O₃ and Si₃N₄ ceramics. Measurement of the torsion strength was carried out by a simplified torsion test method proposed by Yasuda and Tsutsumi. In this experiment, the average torsion strength was approximately 2/3 of its average bending strength for both ceramics. By estimating the characteristic strength σ₀ in Weibull distribution from the strength data, σ₀ in torsion is almost the same as that in bending, which agrees to the basic discussion in mechanics of materials. The characteristic strength σ₀ is one of the normalized strength by eliminating the effects of specimen shape, dimensions and also its stress distribution, and therefore the apparent difference between torsion and bending strengths can be explained by the difference in effective surface area (Conversely, the measured torsion strength data by the simplified method must be reasonable). It reveals that biaxial fracture in tension/compression during torsion test is equivalent to uniaxial tensile fracture by this experiment.

Abstract: Transparent polycrystalline alumina with high bending strength has been fabricated using two-step pressureless sintering. The microstructure and mechanical property has been investigated by varying the MgO doping concentration and sintering parameters. The results showed that the grain size markedly decreased with the increasing content of MgO addition. Although the density of alumina could be increased by the higher temperature for the first sintering step, the larger grain would depress the bending strength. Hence, the optimum bending strength was achieved with 0.5 wt% MgO with the two-step sintering of 1450°C for 20 min and 1400°C for 20 h. The Vickers hardness of the transparent PCA increased doping content, and the variation tendency was well accelerated with the relative density, indicated that the porosity plays a more important on hardness than the grain size.