Formation mechanism of Sialon in alumina-ferro-silicon-nitride composite under nitrogen atmosphere at high temperatures

Sialon bonded Al₂O₃ composites were successfully synthesized using ferro-silicon nitride and different alumina sources at 1500 °C and 1600 °C under N₂ atmosphere. Fused corundum, sintered alumina and the mixture of both were used as different alumina sources to evaluate their effects on the formation of Sialon phases. The samples were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM) and Energy-dispersive X-ray spectroscopy (EDAS). The results show that the Sialons (β-Sialon (Si₂Al₄O₄N₄) and 15R-Sialon (SiAl₄O₂N₄)) contents are dependent on the sintering temperature and alumina sources. Formation mechanism of Sialon in samples prepared with different alumina sources is different. Sintered alumina can react with Si₃N₄ directly to form Sialon. In sample prepared with fused corundum, AlON is formed first and then it reacts with Si to form Sialon. Sintered alumina exhibits better reactivity than fused corundum. Sialon is more stable than AlON under N₂ atmosphere at high temperature with the existence of carbon.     

Ruby under pressure

The ruby luminescence method is widely used for pressure measurement in the diamond anvil cell and other optically transparent pressure cells. With this application in mind, we briefly review the ground-state physical properties of corundum (α-Al₂O₃) with some emphasis on its behavior under high pressure, survey the effects of temperature and stress on the R-line luminescence of ruby (Cr-doped corundum), and address the recent efforts towards an improved calibration of the R-line shift under hydrostatic pressures beyond the 50 GPa mark.     

Preparation, characterization and gas-sensitive properties of nano-crystalline Cr2O3Fe2O3 mixed oxides

A series of Fe₂-_xCr_xO₃(x = 0, 0.4, 0.8, 1.2, 1.6, 2.0) mixed oxides have been prepared with the chemical coprecipitation method and characterized by specific surface area, transmission electron microscopy (TEM), powder X-ray diffraction (XRD), IR and Mössbauer spectroscopy. Single-phase Fe-Cr mixed oxide nano-crystalline powders with corundum structure are obtained, and the results of the five characterization methods are well accordant with each other. Furthermore, gas-sensitive properties of the sensors made of the oxide powders have been studied.     

Preparation and Characterization of High-quality α-Al2O3/Al4.59Si1.41O9.7/Al2TiO5 Multiphase Material

The α-Al2O3/Al4.59Si1.41O9.7/Al2TiO5 multiphase material with high performance was prepared by reaction sintering. The influence of different formulas on the structures and properties of multiphase material was investigated to determine the optimal formula. Crystalline structure and cell parameters of sintered samples were characterized by XRD. Microstructure of all samples was observed by SEM. The properties of sintered samples were investigated. The results show that corundum phase (α-Al2O3 ), mullite solid solution phase (Al4.59Si1.41O9.7 ) and aluminium titanate phase (Al2TiO5 ) were formed in the sintered samples. Different formulas would not change the symmetry of each crystalline phase, but caused slight change in their cell parameters. The optimal formula was determined to be coarse corundum of 36%, fine corundum of 21%, total amount of three kinds of clay of 33%, and TiO2 of 12%, which possessed a comparatively ideal microstructure and optimal properties.     

煤催化气化工艺中碱金属腐蚀刚玉质耐火材料的实验研究

针对煤催化气化反应器设计中遇到的材料问题,采用碳酸钾及催化气化工况富含碱金属气化灰渣为实验原料,进行了碱金属腐蚀刚玉质耐火材料的实验研究。着重考察了有氧空气气氛和无氧还原性气氛、反应温度、反应时间、碱金属存在形态对腐蚀行为的影响,并结合SEM-EDX、XRD、核磁Al谱等表征手段,研究了反应前后碱金属和刚玉质耐火材料的成分、物相及结构变化。结果表明,碱金属对耐火材料腐蚀行为受反应的温度、气氛、时间及碱金属存在形态等因素影响较大。分析表明,有氧空气气氛下腐蚀尤为严重的原因是,原料中存在的钾与刚玉耐火材料发生化学反应生成新的钾的铝酸盐物相,且其含量随温度升高、反应时间延长而增加。     

Accumulation features and TL of TLD-500 detectors in a wide temperature range at pulsed and continuous high-dose irradiation

The results of a comparative research of thermoluminescence (TL) of TLD-500 detectors based on anion-defective corundum irradiated with continuous and pulsed X-ray and pulsed electron beams in a range of doses of 0.3 ÷ 10⁷ Gy, dose rates of 0.02–2.6·10¹¹ Gy/s, and in a temperature range of 300–950 K are presented. It is found that, in contrast to continuous irradiation, upon pulsed irradiation with a duration of 10 ns and dose rate of P_P ≥ 5·10⁶ Gy/s, the first linear region of dose dependences for TL peaks at 450, 580 and 830 K is, instead of saturation, followed by a second one with a smaller slope at doses near 2, 200 and 10³ Gy. Moreover, the slope of the second region increases with growing P_P. It was also found that dose dependence for the peak at 830 K in the area of the first linear region at 10–10³ Gy remains invariable at P_P ≤ 10¹⁰ Gy/s. It is shown that the upper limit of doses registered by TLD-500 detectors can be increased to 2·10³ and 6·10⁶ Gy for continuous and pulsed irradiation, respectively. New broadband UV luminescence with a maximum hν = 4.1 eV and half width H = 0.85 eV was registered within the TL peak spectrum at 830 K. Besides, the optical depletion spectrum in which a single band with hν = 5.2 eV and H = 1.6 eV is observed was investigated for a trap causing a peak at 830 K.     

Phonon deformation potentials for the corundum structure of sapphire

A theoretical Raman polarization analysis is proposed for the corundum structure of sapphire (α‐Al₂O₃) and validation experiments conducted with the purpose of retrieving the full set of phonon deformation potentials (PDPs). From the theoretical side, the change in force constants under stress/strain has been expressed in matrix form, and close‐form solutions were obtained for the eigenvalues that take into account the local dependence of oblique phonons on crystallographic orientation (i.e. uncoupling the effects of local crystal orientation and stress tensor from the shifts of Raman bands). From the experimental side, controlled (uniaxial) stress fields were applied to sapphire parallelepiped bars (along known crystallographic axes) while Raman spectra were systematically recorded along the bar thickness. An untextured alumina polycrystal with fine grain size was also investigated according to the same procedure. As a result of this set of experiments, PDPs for both A_1g and E_g vibrational bands could be retrieved. Validation of PDP constants was obtained by measuring the steeply graded stress fields developed ahead of a surface crack propagated along an arbitrary crystallographic direction in the R‐plane of the sapphire crystal. Copyright © 2011 John Wiley & Sons, Ltd.     

Thermische Untersuchungen der Kristallisation von Korund

With thermal analysis (DTA/TG) the crystallisation behaviour of corundum with influence of the fluor compounds were investigated. The formation of aluminium fluoridhydroxide in the temperature range 400-600°C are observed. Through the influence of AIF₃ or HF the strong exothermic effect increased and a rapid loss of mass were noticed. The particle of corundum grow more, when the exothermic effects are large. We supposed, that also these exothermic effect will be due to the decomposition of may be adsorbed AIOF gas molecules. This revised version was published online in July 2006 with corrections to the Cover Date.