Inorganic Materials最新文献

We examine possible interaction of molten aluminum oxide with a controlled (nitrogen) atmosphere in the pressure range 10⁴ to 10⁵ Pa at a temperature of 2400 K. Stochastic simulation is used to identify gas phase components and determine their concentration. We analyze the main chemical reactions that determine processes in the system. The results suggest that, even though neither molecular, nor atomic nitrogen directly interacts with molten aluminum oxide in the conditions under consideration, it is capable of entering into many chemical reactions in the form of its oxides or together with Al-containing dissociative melt vaporization products. We have calculated pressure ranges where the reactions under consideration and reverse reactions—aluminum oxide formation and precipitation in the melt—are possible on account of gas phase processes. Analysis of these processes makes it possible to optimize conditions for the use of nitrogen as a controlled atmosphere, the most important of which is to reduce the concentration of (atomic and molecular) oxygen and that of AlO₂ oxidizing oxide, which destroy the material of crystal growth system.

We have studied the effect of Al₂O₃ nanofiber on the properties of fused silica ceramics such as are used to produce containers for molten semiconductor materials. The addition of 0.05 to 0.15 wt % alumina nanofiber has been shown to cause an increase in the mechanical strength of ceramics prepared via slip casting. The addition of 0.15 wt % nanofiber increased the compressive strength of the ceramic material obtained at a sintering temperature of 1200°C from 90 ± 4 to 143 ± 9 MPa and its bending strength from 28 ± 2 to 42 ± 3 MPa.

We report a new, convenient and simple low-temperature process for synthesis of γ-lanthanum(III) sulfide (γ-La₂S₃). In molten sodium thiocyanate with small KCN additions at a temperature of 350°C, lanthanum oxide converts into phase-pure γ-La₂S₃ with a cubic structure, as evidenced by X-ray powder diffraction data. The method is rather easy to implement and requires neither high temperatures nor vacuum apparatus.

The formation of compounds that include the impurities typical of the raw materials used in the Acheson process is studied using thermodynamic modeling. It is shown that the yield of silicon carbide in the Acheson process is significantly influenced by moisture and sulfur content in the coke. The sequence of gasification of elements from the reaction mixture during heating was determined. It is shown that sodium chloride significantly affects the state of metallic impurities facilitating gasification of aluminum, magnesium and calcium. The amount of volatile silicon chloride formed in the presence of sodium chloride was estimated.

We have studied the LiF–KCl–KBr–LiKCrO₄ stable tetrahedron of the quinary reciprocal system Li⁺,K⁺||F^–,Cl^–,Br^–,({text{CrO}}_{4}^{{2 - }}). According to experimental data obtained by differential thermal analysis, three solid phases crystallize in the stable tetrahedron: LiF, LiKCrO₄, and a continuous series of KCl_xBr_1–x solid solutions. The solid solutions are stable, and there are no invariant equilibrium points.

This paper reports on the use of titanium/hafnium diboride composite powders for producing metal–ceramic coatings. The composite powders were prepared by high-energy mechanochemical synthesis. The starting components used were PTOM-1 titanium powder (as a matrix powder) and hafnium diboride powder (as a reinforcing agent). The composite powders were characterized by microstructural analysis and their particle size composition was determined. Examination of their morphology showed that the number of hafnium diboride particles embedded in the surface layer of the titanium particles increased with increasing reinforcing agent content in the mixture for mechanochemical synthesis. Assessment of the particle size composition of the powders showed that the size range 10–60 μm accounted for the largest volume fraction of the particles: from 7 to 10%. The microhardness of the coatings produced by microplasma spraying was determined using their transverse polished microsections. Scanning electron microscopy images showed that increasing the percentage of hafnium diboride in the starting mixtures increased the amount of HfB₂ in the coatings. In all of the polished microsections examined, the coating was firmly adherent to the substrate material, without through pores. As the hafnium diboride content was raised from 10 to 60 wt %, the microhardness of the coatings increased in proportion to it. The composite powder containing 60 wt % hafnium diboride had the highest microhardness: 1076 HV.

We have studied the effect of α-Al₂O₃ powders added as sintering aids (15 wt %) on the hardness and porosity of Al₂O₃–3YSZ ceramics. The alumina powders were synthesized via combustion of aluminum nitrates or by solution combustion synthesis (SCS) with the use of glycine and urea as organic reducing agents. The use of aggregates of submicron alumina particles that remained intact after SCS was shown to prevent preparation of high-density ceramics. After sintering at 1550°C, the relative density of the ceramics was 75 to 85% on account of considerable closed porosity. The microhardness of the ceramics ranged from 1 to 11.1 MPa and correlated with the variations in their density and porosity. A reference sample, containing pure-grade alumina additions, had the highest relative density, 98%, without closed porosity.

This paper reports the preparation of cast iron aluminides from thermite-type highly exothermic mixtures at atmosphere pressure in air. We have studied general aspects of combustion during the synthesis process and evaluated the ingot yield of the synthesized iron aluminides and the relative weight loss of the starting mixture components and final products in the combustion process. The chemical and phase compositions of three synthesized cast phase-pure iron aluminides—Fe₃Al, FeAl, and Fe₂Al₅—have been determined, their microstructures have been studied, and their microhardness has been measured.

This paper presents results on the synthesis, phase composition, and optical properties of Al₅O₆N aluminum oxynitride doped with Fe ions in a broad concentration range: from 0.01 to 5.0 at % (relative to aluminum). All of the samples, prepared by firing mixtures of Al₂O₃, AlN, and Fe₂O₃ at a temperature of 1750°C in flowing nitrogen, consisted of almost phase-pure γ-AlON, with small amounts of aluminum nitride and unidentified phases. The band gap E_g of AlON:Fe depends on Fe concentration and ranges from 5.76 to 5.88 eV. We have identified AlON:Fe luminescence due to native defects and extrinsic emission centers and measured the luminescence intensity of emission bands as a function of Fe concentration in AlON. The presence of Fe in AlON leads to an increase in optical absorption and a decrease in intrinsic luminescence intensity in the material.

We have developed a technique for the synthesis of CeO₂ nanopowder with the use of a powdered cellulose (PC)/Ce(NO₃)₃/NaCl composite. It comprises the following steps: preparation of PC/Ce(NO₃)₃/NaCl, burnout of the cellulose template (600°C), and removal of the sodium chloride via washing with water. Using IR and UV spectroscopies, X-ray diffraction, and electron microscopy, we have assessed the effect of sodium chloride concentration in the parent composite on the physicochemical properties of the resulting CeO₂. The material prepared from PC/Ce(NO₃)₃/NaCl consisted of two types of CeO₂ particles: particles 15–40 nm in diameter (forming irregularly shaped structures) and particles 1.5–2.2 nm in diameter. These latter formed the surface of spherical aggregates ranging in size from 30 to 200 nm. Increasing the amount of NaCl in PC/Ce(NO₃)₃/NaCl has been shown to cause an increase in the fraction of spherical aggregates in the nanopowders. The size of the spherical aggregates and that of the particles aggregated on their surface are essentially independent of the amount of sodium chloride in the parent composite. The nanopowder prepared without sodium chloride (PC/Ce(NO₃)₃) consisted of only particles of the former type. In the material prepared from PC/Ce(NO₃)₃/NaCl, cerium dioxide was present in the form of cerianite and amorphous phase. A tendency has been found for the content of the amorphous phase in CeO₂ to increase as the amount of sodium chloride in PC/Ce(NO₃)₃/NaCl increases. If the parent composite was free of NaCl, no amorphous phase was formed. No Ce(III) has been detected in the material prepared from PC/Ce(NO₃)₃/NaCl, as distinct from that prepared from PC/Ce(NO₃)₃. Increasing the amount of sodium chloride in PC/Ce(NO₃)₃/NaCl increases the thickness of the hydroxyl–hydrate shell in the nanopowder. The carbon-containing impurities identified in the nanopowders have been shown to be the result of sorption from the ambient atmosphere. The catalytic activity of CeO₂ nanopowder for hydrogen peroxide decomposition as a model reaction rises in proportion to the increase in the amount of sodium chloride in the parent composite.