Key Engineering Materials Vols. 336-338

Abstract: Silica aerogels were synthesized from hybrid silicon sources of TEOS and acid silica sol by two-step sol-gel method and supercritical drying. Different chemical compositions of the starting solution were altered in order to make an insight into the relationship between process parameters and the microstructural characteristics of resulting silica aerogels. The results indicate that a certain amount of acid silica sol may promote the gelation process while excessive nano-scaled silica particles easily result in dense structure and low specific surface area. The gelation time increases with the increasing water amount. Excessive water will lead to weak network structure of silica gels and thus larger shrinkage and lower porosity of the resulting silica aerogels.

Abstract: TiB2-Cu-Ni cermet composite was fabricated by self-propagating high-temperature synthesis combined with Pseudo Hot Isostatic Pressing. The microstructure of the composite is fine and uniform. The thermal shock resistance and ablation behavior of the TiB2-Cu-Ni composite was investigated by heating it for twenty seconds using a plasma torch arc heater. Fatal breakup took place in the monolithic TiB2 ceramic once the plasma arc flow faced the surface of the ceramic. Only a small crack was found on the ablation surface of the TiB2-Cu-Ni composite. The thermal stress fracture resistance parameter, R, and the critical energy release rate GIC of TiB2-Cu-Ni composite are at the same order with that of the W/Cu alloy. It showed that the properties of thermal shock resistance and the ablation of the composite are good. The fraction of mass loss of the homogeneous composite was 2.32 %, which was similar to that of traditional W/Cu alloy. The volatilization of the metal binder and mechanical erosion was the main mechanisms of the ablation.

Abstract: A novel approach was developed to prepare Ni-coated TiB2 cermet. Fine Ni particles with mean particle size of about 80 nm were impacted onto coarse TiB2 particles having a mean size of about 5 μm to form Ni-coated TiB2 powder by Hybridization. The conventional blended TiB2-Ni powder, as well as Ni-coated TiB2 powder, was sintered by hot pressing (HP) method and Spark Plasma Sintering (SPS) method. Compared with the conventional blended TiB2-Ni cermet, particle features and mechanical properties of the Ni-coated TiB2 cermet were investigated. The microstructure analysis reveals that the thickness of Ni film is around 4 nm. It is concluded that the mechanical properties of Ni-coated TiB2 cermet are superior to the blended TiB2-Ni cermet.

Abstract: In order to decrease the heat-treatment time and avoid particle growth during the post-sintering heat-treatment, the interaction of nitrogen with (Ti,W)(C,N)/(Ni,Mo) cermets was investigated at high pressure. Sintered (Ti,W)(C,N)/(Ni,Mo) cermets with different starting formulations were heated at 1000 °C using nitrogen pressures up to 110 MPa. At this temperature and nitrogen pressure the nitriding time for the formation of interaction zones more than 20 μm thick is decreased to 3 hours and no particle growth occurs. The heat-treatment leads to a dramatic improvement in surface hardness and a very small decline in transverse rupture strength of the (Ti,W)(C,N)/(Ni,Mo) cermets. The microhardness in the near-surface area shows a transition of the highest microhardness to a lower as the interior is approached.

Abstract: In this paper, the Ti(CN)-based cermets were fabricated with ultrafine Ti(CN) powders, additives WC, Cr2C3 and Mo, metal binder Ni by vacuum sintering and HIP treatment. The composition of the mixture was Ti(CN)-10Mo-20WC-xCr2C3-20Ni. Effect of Cr2C3 addition on microstructure and mechanical properties was investigated. Results indicated that with increasing Cr2C3 content, the relative densities after vacuum sintering decreased and the grain sizes of cermets increased. Cr2C3 addition to the cermets improved the hardness but had some negative effect on the performance of bending strength.

Abstract: Ni base alloy ceramic composite coating fabricated through vacuum fusion sintering process can acquire high strength and good thermal fatigue resistance. Composite coating was mainly composed of Ni base alloy and ellipsoidal ceramic particles with random orientation. The three-phases model is used to study the thermal expansion strains in composite coating. First, Eshebly-Mori-Tanaka method was used to determine effective eigen strain of two-phases model formed by the ellipsoidal matrix shell being around a ellipsoidal ceramic particle, then disturbance strain in two-phases model aroused thermal inconsistency is obtained. Finally thinking average stress in effective matrix vanish, the residual stress field in Ni base alloy and particles can be gotten by considering random orientation of two-phases models. It will exert influence on strength and fatigue life of the composite coating.

Abstract: By means of a finite element method model, the effects of property parameters of refractory on thermal stress of injection lance have been studied. The results show that the maximum thermal stress increases with the improvement of thermal conductivity and elastic modulus of refractory, while it decreases at first and then increases with the improvement of the coefficient of expansion of refractory.

Abstract: The spherical nano-sized bioactive particles in the system of CaO-P2O5-SiO2 were bio-mimetically synthesized using micro-emulsion method. The microstructures and properties of the bio- mimetic nano-materials were characterized using XRD, FTIR, SEM/EDAX and TEM techniques. It was indicated that the nano-particles possessed glassy structural characteristics. The porous composite for bone tissue reconstruction was prepared by compounding poly (hydroxybutyrate-2-co-2-hydroxyvalerate) (PHBV) and the nano-particles of bio-mimetic bioactive glasses (BMBG). Bone-like hydroxyl- carbonate-apatite (HCA) could formed on the surface of porous composite by immersing the composite in simulated body fluid (SBF) at 37°C for 8 hours. With increase of immersion time, the morphology of HCA changed from spherical into flake-like crystals. The study on cells attachment of the porous PHBV/BMBG composite proved that the material possessed satisfactory bioactivity, bio-mineralization function and cells biocompatibility.

Abstract: The objective of this study was to investigate the inhibitory effect of the synthetic nanohydroxyapatite (HA) on dental caries. The nano-HA was synthesized by using the depositing reaction of Ca (H2PO4)2. The artificial dental caries was made by using sour solution and inoculating Streptococcus mutans (S.mutans) to the Sprague-Dawley (SD) rat’s mouth respectively. After mineralization of solution of nano-HA for 10 days, the hardness of enamel was measured. The inhibitory effect of the synthetic nano-HA on dental caries was detected by gargling way to rinse the rat tooth with the solution of nano-HA in the animal test. Transmission electron microscopy (TEM) revealed that the dried HA particles were needle-like with ф5-20nm×60nm and the sintered HA particles were spherical with less than 100nm. The remineralized test indicated that the solution of nano-HA with different shapes enhanced the hardness of artificial caries and improved the remineralization of artificial caries. The animal test showed that the dried nano-HA had the inhibitory effect on dental caries. The good absorptive effect of the nano-HA on both the saliva protein and the glucans leads to the development of interventions that could reduce or modify bacterial colonization of tooth surfaces.

Abstract: Dense HA/TCP bioceramics were immersed in pure bovine serum, rabbit serum and dog serum to observe apatite formation. Deposited crystals were examined using SEM. Results showed that some needle-like crystals formed on surface of sterilized HA/TCP, and needle crystals developed into sheet crystals and stick crystals after ceramics were immersed in bovine serum and rabbit serum respectively. The growth of crystals was maybe affected by the content of calcium, various kinds of albumen and alkaline phosphatase in different serums and the different pH of serum.