Tie Cheng Lu

A Study on Toughening and Strengthening of Mg-Al Spinel Transparent Ceramics

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.

A New Method for the Preparation of Transparent AlON Ceramic

Transparent AlON ceramics are prepared with single-phase AlON powder synthesized from micron-sized aluminum and nano-sized alumina. The ceramics are prepared through sintering at the temperature of 1880°C for 10, 20 and 40 hours in flowing-nitrogen atmosphere, respectively. The effects of powder and temperature holding-time on transparent ceramic preparation and microstructure and transparency are investigated. The results show that all samples are transparent and of a single-phase AlON and, with increasing holding-time, the average pore size and porosity decreases and consequently, the ceramics are dense and their transparency is improved.

MgAl2O4 Transparent Nano-Ceramics Prepared by Sintering under Ultrahigh Pressure

The first experimental work to produce transparent MgAl2O4 nano-ceramics was reported in this paper. The sintering characteristics of transparent nano-ceramics were investigated at relatively low temperature (800 ~ 1100°C) under ultrahigh pressure (2 ~ 5 GPa) using hydrostatic equipment. The morphologies and phases of ceramics were observed by means of SEM and XRD, respectively. The grain sizes of the ceramics are shown to be less than one hundred nanometers, far smaller than the sizes of common transparent ceramics. Furthermore, the higher the sintering temperature and pressure are, the greater the extent of densification is. At the same temperature, the higher the pressure is, the smaller the average grain size is. Under the same pressure, the higher the temperature is, the larger the average grain size is. The optimal sintering condition for preparing transparent nano-ceramics was also determined.

Preparation and Characterization of Transparent Nanocrystalline Ceramics

The transparent nanocrystalline MgAl2O4 spinel ceramics were synthesized at lowtemperature and high-pressure conditions with low-cost nano-sized MgAl2O4 powder. The sintering characteristics of transparent nano-ceramics were investigated at 500~700oC under 2 ~ 5 GPa. The optimal sintering condition for preparing transparent nano-ceramics was determined. The microstructure and phase composition of powder, as well as the microstructures, morphologies, optical properties, densities and mechanical properties of synthetic ceramics were investigated. The grain sizes of the synthetic ceramics are less than 100 nm, far smaller than those of usual transparent micron-ceramics, and the average grain size depends on the pressure and temperature. The transmittance at the saturation plateau can near 80%. The relative densities of all samples are less than 99%, however, they are highly transparent. The toughness, derived from energy dissipation, of the transparent nano-ceramics was investigated and was compared with that of transparent micron-ceramic.

Effects of Vacuum Heat Treatment of Nano Powder on Sintering of Transparent Nano-Crystalline MgAl2O4 Ceramics

Nano-sized MgAl2O4 powders were treated at 500~1000 oC in a vacuum of 10-3Pa for 2 h, and then sintered at 500~900 oC under 1~6 GPa. The powders with and without vacuum-heat-treatment were investigated by means of Transmission Electron Microscope (TEM) and Sedimentation Test. It was found that the vacuum-heat-treatment of nano powder could effectively get rid of the absorbed gas on the surface of nano-particles. Bulk nanostructured transparent ceramics without cracks were sintered easily with the treated nano-sized powder. The success rate of nano-sinterning of bulk transparent ceramics was greatly increased when a suitable vacuum-heat-treatment process was used for the starting nano-sized powder.

Sintering of Transparent Polycrystal Nd:YAG with MgO as Additive

In this paper, pure phase Nd-doped YAG powders were prepared by co-precipitation method with different amount of MgO (0~ 0.6 wt %) as additive. Transparent Nd:YAG ceramics were fabricated by vacuum sintering at 1750°C for 10 hours using the powders. Microstructure of the surfaces of thermally etched Nd:YAG ceramics was observed by SEM. Transmittance of the transparent Nd:YAG ceramics was measured over the wavelength region from 600nm to1200nm. The results showed that MgO as sintering aid can restrain abnormal grain growth and reduce pores in grains and at the same time, optical transmittance of the ceramics was increased. The optimal weight percentage of MgO added can be determined as 0.3wt % for the fabrication of transparent Nd:YAG ceramics.

Effect of Phase of YAG Powder Synthesized by Co-Precipitation on Transparent Ceramic Sintering

Effect factors of phase of nano-sized YAG powder prepared by co-precipitation method, and effect of powder phase on transparent ceramic sintering, have been investigated. In experiment, mother solutions, with different concentrations, of aluminum nitrate and yttrium nitrate were dripped into ammonium hydrogen carbonate solutions with different concentrations at different temperatures adopting different dripping speeds, respectively. Obtained precursors were calcined to powers at above 900

Preparation of Aluminum Oxynitride Powder by Solid State Reaction Method

for 2h, an appropriate amount of these powders were sintered to transparent ceramics via vacuum sintering. Phase and morphology of the powders, as well as micro-morphology of grains in ceramics, were measured by means of XRD, TEM and SEM, respectively. The results show that the concentrations of mother solution and precipitant solution are dominant effect factors in preparation of powder with YAG as main phase, and environment temperature and dripping speed only affect the phase purity of powder. In addition, the YAG powder with a little impure Y2O3 or YAlO3 phase cannot be highly densified via vacuum sintering.

Low-Temperature Synthesis and Micro-Morphology Observation of Transparent MgAl2O4 Nano-Ceramics

Single-phase aluminum oxynitride powders were obtained by varying the holding temperature (T) and the weight percentage of α-Al2O3 in Al2O3 (X) with nano-sized Al2O3 and AlN powders as raw materials. Influences of T and X on the phase composition of obtained powders were studied. The results showed that the content of aluminum oxynitride increased with increasing T or X. When X was 50% and T was 1750°C, obtained powders had better properties than those of others.

Preparation of MgAl2O4 Transparent Nano-Ceramics and their Light Transmission Properties

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.