Key Engineering Materials Vols. 336-338

Abstract: Transparent Mg-Al spinel ceramics of four different nonstoichiometries were obtained utilizing two-stage sintering technique of vacuum sintering and post hot isostatic pressing using MgO·nAl2O3 powders with n of 1, 1.3, 1.5 and 1.8, respectively, as raw materials. The mechanical properties of the ceramics were measured and indentation shapes and radial crack propagations were observed. The results showed that change of molar ratio of Al2O3 to MgO leads to variation of indentation shapes and crack propagations of the ceramics, resulting in the change of mechanical properties of the ceramics. The flexural strengths of the ceramics increase first then decrease with increasing n. However, the hardness and fracture toughness of the ceramics increase with increasing n. As a result, an optimal n for high performance ceramic is suggested to be 1.5. In addition, mechanisms of toughening and strengthening of the transparent ceramics have also been discussed.

Abstract: HIP (hot isostatic pressed) high-purity alumina was modified by Ti-ion implantation in a MEVVA (Metal Vapor Vacuum Arc) implanter. The samples were implanted by Ti ions with nominal doses 5×1016 to 1×1018 ions/cm2 under ambient temperature. The effects of titanium implantation and the ion dose implanted on the microstructures and mechanical properties of the ceramics were studied. After implanted by Ti ions, the maximum nanohardness of the as-implanted ceramics were increased about 20%; the bending strength were increased about 22%; the life of the alumina cutting tools were about 2 times longer than before. The result showed that all the factors of the improvement of the ceramic surface states played an important role on the mechanical properties after implantation.

Abstract: Several nano-composite Ti(C, N)-based cermets were produced by vacuum sintering and subsequently heat-treated with hot isostatic pressing (HIP) in Ar. The influence of nano powder additions on the microstructures and properties of cermets has been studied It was found that, with increasing nano powder additions, the microstructures became finer and even, the amount of small grains with inner-rim, outer-rim and those with “white core-gray rim” increased. When nano TiC and TiN additions accounted for 20wt% of the amount of TiC and TiN, the samples exhibited the best properties. Some nano particles were found to distribute at (along) the grain boundaries of cermets, which strengthened the grain boundaries and improved the mechanical properties of Ti(C, N)-based cermets.

Abstract: Amorphous nano-sized silicoboron carbonitride (Si-B-C-N) powders with average grain size <50 nm were fabricated by high energy shaker mill using hexagonal boron nitride, graphite and amorphous silicon powders as starting materials. The powders were consolidated by spark plasma sintering at 1900° and 1950°C. Amorphous phase were partially retained in ceramic sintered at 1900°C. For ceramic sintered at 1950°C, amorphous Si-B-C-N ceramic transferred to hexagonal BN and cubic SiC.

Abstract: Polycrystalline bulk samples of ternary carbide Ti3AlC2 ceramics were fabricated by reactively hot-pressing a mixture of Ti, Al, and graphite powders with and without Sn additive. The effects of sintering temperature, time and addition of Sn on the purity, mechanical properties and microstructure of Ti3AlC2 were investigated. The result showed that the TiC content was strongly influenced by sintering temperature for the Ti3AlC2 samples without Sn additive, and the most suitable sintering temperature to create the lowest TiC content was 1400°C. The addition of Sn additive led to a distinct decrease in TiC content. The flexural strength of the testing materials had close relation with the TiC content and sintering time. A certain content of TiC second phase and longer sintering time were helpful to improving the flexural strength. The sample sintered at 1400°C for 2 h possessed the highest flexural strength.

Abstract: In this paper, particular CVD processes of so-called Se method with solid substances as starting materials was investigated to fabricate ZnSe. On the basis of detailed experimental researches, the growth model and dominant regime of deposition rate were discussed. ZnSe of high optical properties was produced under optimized process conditions. The grain size of CVD ZnSe by Se method is less than 10μm, and its average transmittance in the wavelength range of 0.55~20μm is up to 70%.

Abstract: In this paper, transparent ceramics Mn:MgAl2O4, which are potentially applicable in visibleband laser field, were prepared and their light transmission and photoluminescence properties were investigated. In experiment, Mn:MgAl2O4 powders with two kinds of doping concentrations of 0.5 and 1.0 at% were synthesized through a high-temperature calcination method with NH4Al(SO4)2·12H2O and MgSO4·7H2O as raw materials, and with MnSO4·H2O as dopant. Transparent ceramics Mn:MgAl2O4 were prepared through vacuum hot-pressing sintering at 1750oC for 1 h, and subsequent vacuumannealing at 1700oC for 2 h. The samples were measured by means of light transmission and photoluminescence. The results show that transparent ceramics Mn:MgAl2O4 can be prepared by a low-cost powder technique and vacuum hot-pressing sintering. Two channels of visible-band luminescence, one peak around 520nm and one wideband centered at 650nm, respectively, can be shown. The intensity of their luminescence increases with increasing doping concentration. Obviously, it possesses a potential application in the field of visible-band solid tunable laser material with high doping concentration.

Abstract: Perhydropolysilazane was synthesized by the ammonolysis of dichlorosilane-pyridine adduct, and used as precursor to prepare three-dimensional silica fiber reinforced silicon nitride matrix composites via preceramic polymer infiltration and pyrolysis at 1073K
in anhydrous ammonia atmosphere. The polymer-derived ceramic matrix was amorphous near-stoichiometric silicon nitride with an empirical formula of SiN1.34O0.02C0.01H1.21. The as-received composites were amorphous with a high density of 1.96g/cm3 after five infiltration-pyrolysis cycles. The process was successful to fabricate dense silicon nitride matrix composites reinforced by three-dimensional silica fiber due to the low viscosity, good wettability and high ceramic yield of perhydropolysilazane.

Abstract: The influence of pressure on the crystallization behavior in SiO2f/SiO2 composites hotpressed at 1350°C was studied. The crystalline phase composition analysis on SiO2f/SiO2 composites revealed that the formation of cristobalite was promoted when the hot-pressing pressure ≤ 12 MPa, however suppressed with higher pressure applied. It can be ascribed to the nucleation mechanism change from surface nucleation to bulk nucleation. Analysis on relative density as well as fracture microstructure of SiO2f/SiO2 composites confirmed the conclusion.

Abstract: SiO2/ SiO2 nanocomposites dipped with silicon resin was ablated and the physical state and phase transformation were characterized. Trace impurity in raw material and compound obtained by chemical reaction were analyzed. Moreover, the high-temperature dielectric properties were investigated. On the basis of above, it is found that the impurity carbon and silicon carbide are the key factors influencing dielectric properties.