Materials Science Forum Vols. 675-677

Abstract: The product formation of Mo-66.7at.%Si powders mixture during mechanical alloying using a high-energy vibratory type ball-milling machine was investigated by X-ray diffractometer (XRD). The temperature of the vial was measured at intervals during ball milling. The results showed that some MoSi2 was formed by the mechanically induced self-propagating reaction during a milling time of 105~110min; at the same time the surface temperature of the vial is increased. In early stages of milling, as a result of extensive fracturing and cold welding of powder particles, the interfacial energy between Mo and Si powders and cold deformation energy led to the formation of supersaturated solid solution of Mo(Si). And then mechanically induced self-propagating reaction of MoSi2 occurred immediately. After this, MoSi2 was gradually synthesized by diffusion. MoSi2 andMo(Si) were the end products, and the end products underwent the amorphous trend.

Abstract: CaB2O4 was added into hexagonal boron nitride (hBN) to improve the sintering behaviors of hBN. CaB2O4 and hBN were mixed and then pressed into plates. The plates were sintered at 2000°C for 5h under a N2 ambience. The phase compositions with different CaB2O4 contents were examined with X-ray diffraction analysis. The fracture cross-sections of the hBN plates were investigated by SEM. The apparent density and Rockwell hardness were also measured. The results show that the hBN particles had a plate-like shape and the grain sizes of hBN increased with increasing CaB2O4 contents. The apparent density and Rockwell hardness decreased with increasing CaB2O4 contents. When the CaB2O4 content was 15(wt) %, the hBN has the average grain sizes of 3μm in diameter and 200nm in thickness, the apparent density of 1.06 g/cm3 and the Rockwell hardness of 3, respectively.

Abstract: The major industrial by-product largely generated in southern China is ceramic polishing powder (PP) produced from ceramic industries. In this paper, gross of silicon and aluminum dissolved from polishing powder and fly ash were studied by alkali dissolve experiments, pozzolanic activity of PP and fly ash were studied as well by determining chemical combined water and fast strength experiments. Results show that the cement mortar strength active index of PP is up to 82%. The average size of PP is 6.31 um; the fine granule of PP fills in the pore of mortars and improves the compressive strength of mortars. Microstructure of mortars added PP demonstrates that hydration gel is close-grained, dense and reticular. It reveals that “the granule domino effect” enhance the density of mortars added PP and conduce to the high compressive strength. Polishing powder can be used as cementitious material, especially the quality of early stage is higher than that of fly ash. Gross of vitreous phase is not the single factor to determine pozzolanic quality of polishing powder, ability of alive silicon and aluminum responding to Ca(OH)2 is one of the most important impact.

Abstract: A melt-spinnable precursor for SiC based fibers was prepared from blend polymers of polycarbosilane (PCS) and modified polymethylsilane (M-PMS). The blend polymers cured at 320°C are different from M-PMS and PCS. The ceramic yield of these blend polymers is about 83%. The C/Si ratio of M-PMS/PCS derived ceramics (pyrolyzed at 1250°C) is linear to the content of MPMS in M-PMS/PCS. After melt spinning, thermal oxidation curing, and pyrolysis, Si-C-O fibers were obtained. The diameter and the tensile strength of the resulted fibers are 16.5μm and 1.62GPa, respectively.

Abstract: Effects of rare-earth oxides addition (0.38~1.52 mol% of Sm2O3, Eu2O3 and Er2O3) on the property and microstructure of the hot-pressed (1300°C, 2h, 35 MPa) Al2O3 (12 wt %)/TiAl insitu composites have been investigated. The results show that the doping of rare-earth oxides has a positive effect on both mechanical properties and densities of Al2O3/TiAl composites. Densities enhanced with increasing of rare-earth oxides. The flexural strength and fracture toughness were higher than other samples when the rare-earth oxide content was 0.38 mol %. The matrix grains and Al2O3 particles were significantly refined, and Al2O3 particles evenly distributed in the matrix.

Abstract: The structural, elastic and thermodynamic properties of the ternary layered nitrides Ti4AlN3 under the pressure effect have been calculated by using the full-potential linearized augmented plane-wave method within the generalized gradient approximation (GGA). The calculated properties such as cell parameters and bulk modulus, can found in good agreement with the experimental and other theoretical data. Using the quasi-harmonic Debye model in which the phonon vibration effects are considered, the thermodynamic properties such as thermal expansion coefficient, Debye temperature and specific heats in the whole pressure range from 0 to 50GPa and temperature range from 0 to 1600 K has been obtained.

Abstract: Continuous silicon carbide fiber reinforced silicon carbide matrix (SiCf/SiC) composites were fabricated by a combining polymer impregnation and pyrolysis (PIP) and vapor silicon infiltration (VSI) process. The mechanical properties and microstructure of the SiCf/SiC composites were studied. The results show that the open porosity of the SiCf/SiC composites is near zero. The density and mechanical properties of SiCf/SiC increase with decreasing the porosity of SiCf/C. However, the SiC fibers are damaged during the VSI process due to the reaction between SiC fibers and vapor Si.

Abstract: The sintering behaviors and microwave dielectric properties of the (Ca0.3La0.4/3)(Li0.25Nd0.25)TiO3 (abbreviated CLLNT) ceramics doped by small amounts of ZnOB2O3- SiO2 glass and LiF were investigated in this paper. The combination adding of ZnO-B2O3- SiO2 glass and LiF improved the densifications of the CLLNT ceramics and the sintering temperature of the CLLNT ceramics were efficiently lowered from 1400°C to 1000°C. Especially, the 4.0wt% ZnO-B2O3-SiO2 glass and 1.0wt% LiF doped CLLNT ceramic sintered at 1000°C for 3h has optimum microwave dielectric properties of Kr=97, Q×f=1286GHz, and TCF=43 ppm/°C, which enable it a suitable candidate for LTCC applications.

Abstract: AlN ceramics were prepared by hot-pressing process with Y(NO3)3·6H2O as a sintering additive. Microstructure and mechanical properties of sintered AlN ceramics were measured and compared with that of monolithic AlN ceramic prepared by the same process. The results showed that the density and mechanical properties of monolithic AlN ceramics were drastically enhanced by adding Y(NO3)3·6H2O. Especially, the AlN ceramic with 3.24 wt% Y(NO3)3·6H2O (2 wt% Y2O3) displayed striking enhancement in mechanical propertie. However, such enhancement did not follow a consistent behavior with the increase of Y(NO3)3·6H2O content. A significant drop present at contents over inflection point (3.24 wt% Y(NO3)3·6H2O).

Abstract: Preparation of high wear-resistant ceramic parts was studied in this paper. The mechanical property was measured by the three-point bending tester, and the microstructures were analyzed with the scanning electronic microscope (SEM). The experimental results showed that vitrified bond CM2 was suitable for the preparation of high wear-resistant ceramic parts because of its lower refractoriness, and higher bonding strength sintering at 850oC. Moreover, containing 8- 12μm CBN abrasive grains, the ceramic parts reached higher strength and higher wear resistance than other samples.