Key Engineering Materials Vols. 336-338

Abstract: MgAl2O4 transparent nano-ceramic samples have been successfully prepared by ultrahighpressure- sintering method. The ultrafine MgAl2O4 nano-powder was obtained by high-temperaturecalcination method. The samples were sintered at relatively low temperatures (540, 620 and 700°C) under ultrahigh pressures (3, 3.7 and 5GPa) with a six-pressure-source and hexahedral anvil machine that is customarily used for diamond synthesis. The average grain sizes of the prepared samples range from 50 to 75nm. All of the samples sintered under different conditions are transparent. The sample sintered at 620°C under 3.7GPa has the highest density as well as the highest light transmittance (about 50%) at 1300 ~ 1700 nm. The results showed that the ultrahigh-pressure-sintered samples exhibit unique light transmission properties though their densities are not much higher (<97%), compared to the samples, of which densities are higher (>99.5%), obtained by conventional hot-pressing or hot isotopic pressing sintering methods. The light transmittance, its affecting factors and transparent mechanism of the MgAl2O4 samples have been discussed.

Abstract: Zirconia nanopowders (stabilized by 1.5 and 3 mol% Y2O3) with particle size below 10 nm were compacted by cold isostatic pressing. Pressureless sintering and hot isostatic pressing were applied to obtain dense nanocrystalline ceramics. The influence of the pore size in powder compacts on sintering behaviour was investigated. Green bodies pressed at 1000 MPa had a maximum pore size of 5 nm. These bodies were densified to a relative density of over 99.6% with an average grain size about 85 nm by pressureless sintering at 1100 °C. Indentation techniques were used to evaluate the hardness and fracture toughness of zirconia nanoceramics. The decrease in the yttria content from 3 to 1.5 mol% resulted in the toughness increasing from 5.3 to 11.1 MPa m1/2. The differences in fracture toughness of zirconia ceramics prepared with different yttria contents and by different sintering methods were discussed and their possible causes were proposed.

Abstract: Low frequency ultrasonic treatment was applied to desorb surface organics from the surface of nanosized TiO2 photocatalyst, which was prepared by hydrothermal method using a mixture of different amounts of Ti(OBu)4, ethanol, acetone and pyridine raw materials. The UV-Vis, TG-DTA and GC-MS analysis were used to investigate the surface organics. It was found that the absorption intensity around 250nm in UV-Vis spectra of the upper transparent solution, which represents the amount of organics desorbed from the TiO2 surface, increased with increasing treatment time. Along with the TG-DTA and GC-MS analysis, the kinds and amounts of partial surface organics were roughly clarified, and these organics may correspond with the photocatalytic degradation rate for methyl orange (MO) solution under visible light irradiation (≥450nm).

Abstract: Transparent MgAl2O4 nano-ceramics were prepared by ultrahigh-pressure-sintering method, using improved nano-sized powder synthesized by high-temperature calcination. The synthesis temperatures were exactly measured by nichrome-NiSi thermocouple passing through the samples. The micromorphology of the ceramics was observed by means of dynamic force microscopy (DFM), and the relation between transparency and micro-morphology was investigated. The results show that, the sintering temperatures, now calibrated, of transparent nano-ceramics are just 540~700°C, which are much lower than those of conventional nano-ceramics and transparent ceramics, and the uniformity of grain sizes and orderliness of the grain array are beneficial to the transparency and integrity of the ceramics.

Abstract: This study developed and evaluated a shoe cushioning system to reduce impact force exerted during running. Load-displacement curves for the shoe cushioning system were obtained from an instrumented testing machine and the results were compared with various pockets with air, water or nano-particles. The ground reaction force, force and contact pressure between the shoe and foot were collected from five young male participants, who are trained and untrained person. This study used the three cushioning systems such as ordinary shoe, with pocket containing 5 g nano-particles, with pocket containing 7 g nano-particles. For the trained participant, there was no significant difference in the cushioning performance among Shoes at a low running speed. But at the normal speed, shoes with cushion system were better in the cushioning performance. These results showed that shoes constructed with a shock cushioning system using nano-particles have a better performance for shock absorption. The significant difference was observed at a high running speed according to the composition ratio of water and nano-particles inside the pocket of shoe.

Abstract: ZrO2-TiO2 thin films, which were heat-treated with an applied electric field, were prepared by sol-gel method. The phase transformation behavior of ZrO2-TiO2 composite thin films was studied by DTA, XRD, SEM and Raman spectroscopy. In an electric field, the phase transformation temperature of amorphous phase to anatase and that of anatase to rutile are all decreased.

Abstract: TiO2/(O′+β′)-Sialon multiphase ceramics with different phase composition of TiO2 were prepared by pressureless sintering under high-purity N2 atmosphere with (O′+β′)-Sialon powder and nano TiO2 (anatase) powder as raw materials, Yb2O3 or Tb2O3 as additive. For each sample, the weight percentage of anatase in TiO2 was calculated from XRD data and the kinetics of anatase-rutile transformation was investigated, wherein the emphasis was placed on the influence of Yb2O3 and Tb2O3. The results indicate that the added Tb2O3 and Yb2O3 serve the significant function of inhibition and promotion on the phase transformation, and the effects are enhanced and attenuated with increasing additive content, respectively. For the sample without additive, the transformation process follows apparent first-order kinetic model. The addition of Yb2O3 or Tb2O3 results in completely different transformation kinetic law. For the samples with Yb2O3 added, the transformation is an apparent second-order reaction, whereas a unique kinetic model, CA=kt1/2+C, is valid for the samples containing Tb2O3. In the two cases, the effect of the additive content on the transformation can be perfectly reflected by the apparent rate constant.

Abstract: Transparent glass ceramics with composition of xErF3-10PbF2-90SiO2 (mol%, x=0, 1, 2, 3, 4) were prepared by sol-gel method. The thermal behavior of the xerogels was investigated by DSC. Through non-isothermal experiments, the apparent activation energy for the crystallization of β-PbF2 phase was evaluated by the Kissinger’s method and Chen’s method. XRD and TEM were carried out to study the growth of β-PbF2 nanocrystals and the effect of Er3+ doping on it.

Abstract: The high-energy ball milling of Ni-66.7 at% Si elemental powder mixtures was carried out using a FRITSCH Puluerisette-5 mill. XRD was used to identify the phase evolutions during high-energy ball milling. The XRD patterns showed that the intensities of nickel peaks and silicon peaks gradually decreased with milling time increased from 1 h to 10 h, simultaneously, the peaks of NiSi2 and NiSi appeared. With milling time further increased to 45 h, single phase of NiSi2 formed. New peaks were observed after 75 h milling, which were indexed as FeSi. This reveals that long time high-energy ball milling will induce Fe contamination and that Fe exhibits greater activity and reacts with NiSi2 forming FeSi and NiSi. Moreover, the peaks intensity of FeSi and NiSi increased with milling time increased from 75 h to 180 h. The phase evolutions of Ni-66.7at% Si during high energy ball milling were discussed.

Abstract: Several rare earth solid solutions of Y1.9R0.1Mo4O15 (R=Sm, Nd, Dy) with interesting thermal expansion behavior were prepared and studied by X-ray and neutron diffraction. These solid solutions crystallize in a monoclinic space group P21/c. The lattice parameters a, b and c of Y1.9R0.1Mo4O15 increase with increasing of the ionic radius of substituted elementary. The high-temperature XRD data for Y1.9Sm0.1Mo4O15 show that there is an unusual low thermal expansion behavior in the 25~100°C range followed by a normal thermal expansion at higher temperature up to 600°C. Since no phase transition was observed, the temperature dependence of Mo2-O14 bond distance seems to be responsible for the anomalous thermal behavior.