Key Engineering Materials Vols. 368-372

Abstract: Hexagonal MgAl2O4 spinel fibers were synthesized at 1500oC for 6h in nitrogen atmosphere. The structure and morphology of the fibers were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The fibers grew via vapor- liquid- solid mechanism. The twin mechanism was suggested to play a key role in the formation process of the hexagonal fibers.

Abstract: Dielectric constant and dielectric dissipation of MgAl2O4 transparent nano-ceramics were measured at different frequencies. The results indicated that the transparent nano-ceramic has a very low dielectric constant and low dielectric dissipation in frequency range of 1K~9MHz, but the dissipation of is independent of frequency. The dielectric constant of the transparent nano-ceramic does not vary signifycantly with temperature.

Abstract: To determine the optimal doping concentration, a series of Mn:MgAl2O4 were successfully prepared through vacuum-sintering with the highest doping concentration up to 35%. Samples with a low doping concentration exhibit a light green color. When the doping concentration increases, its color becomes deeper and deeper and when the doping concentration reaches 14%, their colors become brown and deeper brown when their concentrations further increase. The crystal phases of the samples were manifested by XRD measurements and their photoluminescence properties were studied. Combined with phase analysis, optimal doping concentration of transparent ceramics has been determined. In addition, the mechanism for fluorescence of Mn:MgAl2O4 transparent ceramics has also been discussed.

Abstract: 5.0at% and 10.0at% Mn-doped MgAl2O4 transparent ceramics have been prepared through vacuum sintering (1750°C /2h) and subsequent hot isostatic pressing (HIP) (1650°C /2h). The effect of microstructure on the light transmittance of the sintered MgAl2O4 is discussed. Results showed that the grains became dense and grain boundary became clear and most of pores inside and between grains were removed, so the transmittance of two samples increased. The optical transmittance of the 5.0at% specimens was increased by one times in the visible light wavelength region. HIP treatment has a great effect on improving the transparency of samples with low doping concentrations.

Abstract: Yttrium aluminum garnet (YAG) nanopowders were synthesized by n-amyl co-precipitation method. Transparent YAG ceramics were achieved by sintering the green body at 1800°C and then 1600°C for ten hours with TEOS as sintering additive. The transmittance of the sample was 75% at visible wavelength of 800nm.

Abstract: Pure-phase 2at% Nd:YAG powder was synthesized by alcohol-water co-precipitation method. 2at% Nd:YAG ceramics were sintered at 1780 °C for 10 h under vacuum using the powder. Sintering ability of the powder was studied by comparing the results of alcohol-water solvent with those of distilled water solvent. The effects of different volume ratios of alcohol to water (R) on transparent ceramics have been also studied. Transparent ceramics having a transmittance of 40% in the visible wavelength have been sintered without sintering additive using the powder (R=0.8), which is much better than ceramics (16%) prepared using the powder derived from distilled water solvent under the same sintering conditions. In addition, the different R does not have much influence on the transmittance of ceramics in the present work. Compared with the results of distilled water solvent, the powders, using alcohol-water mixtures with different R as solvent, are more beneficial to the sintering of high transparent ceramics.

Abstract: 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.

Abstract: By means of co-precipitation, pure YAG phase was synthesized from the reaction of aluminum yttrium and cerium nitrates with ammonia as precipitator. In order to obtain the special coating oxide precursor, the metal ions solution and ammonia were respectively added with fixed ratio by two peristaltic pumps to maintain an expired pH value and reaction rate. The as-received precursor was annealed at 700°C and 900°C respectively. The structure, phase evolution and morphology of the as-prepared YAG nanopowders were tested by XRD, BET and SEM. At lower temperature (900°C), coating oxide precursor was directly transformed to nanopowders of pure YAG phase. The high salt solution concentration was critical for the control of chemical homogeneity within the precursors. The YAG nanopowders were less-aggregated and better-dispersed. The luminescent properties were examined and discussed.

Abstract: Nd:Gd2O3-HfO2 powder has been prepared with a co-precipitation method. The size of Nd:Gd2O3-HfO2 powder is near 50nm. Transparent Nd:Gd2O3-HfO2 ceramics were successfully fabricated by sintering the powder compact at 1800°C under H2 atmosphere. The synthesis process and optical properties were investigated in detail. By measuring the optical properties of transparent Nd:Gd2O3-HfO2 ceramics, we found that the absorption bond of Nd ions near 808nm is enlarged, and the FWHM of the emission bond near 1060nm is near 25nm. It was believed that Nd:Gd2O3-HfO2 transparent ceramic is potential laser material for the usage in solid-state heat-capacity laser(SSHCL) application.

Abstract: 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.