Key Engineering Materials Vols. 336-338

Abstract: By introducing ZrO2 (3Y) micron powder into the thermit, Al2O3/18%, 21%, 24%, 27% and 30%ZrO2 composite ceramics were obtained from in-situ growth in the melts produced by combustion synthesis. As the volume fraction of ZrO2 is below 27%, the ceramics is comprised of lath-shaped and rod-shaped α-Al2O3 matrix, within the rod-shaped grain the nano-micron fibers are oriented in essentially the same direction. As volume fraction of ZrO2 is over 27%, the ceramics is mainly composed of lamina-shaped grains with alternating α-Al2O3 and t-ZrO2 layers. The flexural strength and the fracture toughness measured simultaneously the maximum values with increasing volume fraction of ZrO2 to 24%, indicating that flexural strength and fracture toughness are simultaneously under the control of rod-shaped grains in which ZrO2 nano-micron monocrystal fibers are embedded.

Abstract: Natural and raw high-silica kaolins are firstly processed into fine kaolin powders with oxidation-reduction bleaching method before mixed with Al2O3 powders and then baked to produce mullite composite oxides at temperature between 1050-1150°C. The mullite composite oxides are further made into composite mullite nanocrystals by using open-system hydrothermal crystallization method under normal pressure. Analysis of the final products show that the mullite nanocrystals are texturally homogenous at an average size around 80-100 nm and have high thermal stability.

Abstract: Novel mesoporous ZnO-SiO2 (MZS) composites with rod-like morphology were synthesized from coprecipitation method, which was the first time to apply this method in the successful synthesis of ordered mesoporous composites to our knowledge. This direct-synthesis method was more simple, convenient and efficient, compared with the post-synthesis ones. Structural characterization and spectroscopic study were discussed.

Abstract: Transparent bulk CdSe/SiO2 nanocomposites, containing semiconductor CdSe nanocrystals well dispersed in silica glass matrix, were obtained by using sol-gel method and in-situ growth technique. The nanocomposites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-VIS absorption spectra and X-ray Photoelectron Spectroscopy (XPS). X-ray diffraction and transmission electron microscope measurements confirmed a cubic crystallographic structure of the nanoparticles, with mean diameter smaller than 10 nm. UV-VIS absorption spectra displayed a marked blue shift of the absorption band edge that was attributed to quantum confinement effects. XPS together with XRD proved that silica matrix plays an important role in enhancing the NCs’ stability.

Abstract: Three kinds of colloidal silica (CS)/silane sol solutions were synthesized in variation with parameters such as different acidity and reaction time. Sol solutions were prepared from HSA CS/ methyltrimethoxysilane (MTMS), LS CS/MTMS and LS CS/MTMS/γ -Glycidoxypropyltri methoxysilane (ES) solutions. In order to understand their physical and chemical properties, sol-gel coating films were fabricated on glass. Coating films on glass, obtained from LS/MTMS sol, had high contact angle, also, much enhanced flat surface in the case of LS/MTMS sol was observed in comparison with HSA/ MTMS sol. From all sol-gel solutions, seasoning effect of for enhancing properties of sol-gel coating layer on glass was observed while such sol-gel solutions were left for 7days. In initial stage of sol-gel reaction, all most of sol solutions used in this work seem to be unstable, formation of coating films was a little hazy and rough. However, improved coating films as observed in 4days later. LS/MTMS/ES sol solutions were synthesized with ES, adding to LS/MTMS sol. Contact angle of LS/MTMS/ES sol-gel coating films decreased, since ES played a role in forming hydrophilic hydroxyl sol. The elastic portion of coating films prepared from LS/MTMS/ES sol increased with addition of ES, but thermal stability decreased a little.

Abstract: Low density SiO2-xZrO2 aerogels with x=35wt%, 65wt%, 75wt%, 90wt%, 95wt% were prepared by CO2 supercritical drying technique with tetraethylorthosilicate (TEOS) and zirconyl nitrate dihydrate (ZrO(NO3)2 .2H2O) by hydrolytic polycondensation under different chemical conditions. The prepared aerogels are performed by X-ray Diffraction (XRD), Transmission electron microscopy (TEM), Fourier transformed infrared spectroscopy (FT-IR) and BET surface areas to characterize and analyze the morphology and pore structure of SiO2-ZrO2 aerogels. The results showed that the SiO2-ZrO2 areogels are the typical of nano mesopores and the average pore size is about 50 nm. The specific surface areas varied from 345.5 to 615.5 m2/g with (SBET)MAX = 615.5 m2/g with 20wt% Zirconia; Moreover a mass of Si-O-Zr bands formed in the aerogels and the formation mechanism of Si-O-Zr bands are also discussed.

Abstract: Low-density xSiO2-(1-x)Al2O3 xerogels with x=0.9, 0.8, 0.7, 0.6 (mole fractions) were prepared by sol-gel and non-supercritical drying. Silica alkogels, which were the framework of binary composite materials, formed from tetraethyl orthosilicate (TEOS) by hydrolytic condensation with a molar ratio of TEOS: H2O: alcohol: hydrochloric acid: ammonia =1: 4: 10: 7.5×10-4: 0.0375. Aluminum hydroxide derived from Al(NO3)3·9H2O and NH4OH acting in the alcohol solution under the condition of catalyst. After filtrating and washing, the precipitation was mixed into silica sols to form SiO2-Al2O3 mixed oxide gels with different silicon and aluminum molar ratio. The structural change and crystallization of the binary xerogels were investigated after heat treatment at 600
for 2 h by the means of X-ray diffraction. Nitrogen adsorption experiment was performed to estimate specific surface area, porous volume and pore size distribution. The structural change of xerogels was observed by FT-IR spectroscopy. The resulting mixed xerogels possess of mesoporous structure which is characteristic of cylindrical pores, high specific surface area of 596-863 m2/g and a relatively narrow pore distribution of 2.8-30 nm. Al2O3 is introduced into the SiO2 phase and some of Al-O-Si bonds form.

Abstract: Nanosized yttria stabilized tetragonal zirconia powders were prepared by chemical co-precipitation. By adding certain amount of wax emulsion, we obtained green compacts with the relative density ranged from 47% to 60%. Then the green compacts were sintered in air at the temperature from 1350°C to 1550°C. It turned out that the 3Y-TZP compacts were nearly fully densified after sintering at 1350°C and the grain sizes were between 200 to 300 nm. This result suggested that the addition of wax emulsion could improve the wettability between powders, elevated the density of the green compacts so that the coordination number and the contact points between particles were increased. With more transfer passages, the sintering behavior is remarkably improved.

Abstract: Sintering behavior of pure and Nb-doped barium titanate powders prepared by high-gravity reactive precipitation (HGRP) process was investigated. A thermal dilatometer was employed to monitor the sintering shrinkage continuously during sintering. TEM and SEM were used to characterize the microstructures of the samples. The experimental results confirmed that Nb-doped BaTiO3 ceramics sintered at 1220°C for 2 h reached a larger densification of 97.0% relative density compared with 94.6% of pure BaTiO3 ceramics sintered at 1265°C for 2 h, and Nb-doped BaTiO3 ceramics had a smaller grain size (1.2μm) than that of pure BaTiO3 ceramics (2μm).