Key Engineering Materials Vols. 512-515

Abstract: The Ta₂O₅-based ceramics were prepared by dry pressing/sintering technique using Ta₂O₅ and Al₂O₃ as the starting materials. The present work investigated the effect of alumina (Al₂O₃) additions on the composition, microstructure, thermal expansion coefficient (TEC) and bending strength of Ta₂O₅ ceramics. The thermal expansion of the samples was measured by the dilatometry method. It was found that Al₂O₃ additions can effectively inhibit the β to α phase transformation in Ta₂O₅ ceramics. Orthorhombic AlTaO₄ as the minor phase formed when 2.5 and 7.0 wt% Al₂O₃ was added. The addition of Al₂O₃ results in obvious change of TEC and an increase of bending strength. This work demonstrated that the addition of Al₂O₃ is an effective way to modify the TEC and mechanical strength of Ta₂O₅ ceramics.

Abstract: _{Subscript text}High-temperature chemical stabilities of Hf₆Ta₂O₁₇/Al₂O₃, Hf₆Ta₂O₁₇/YSZ and Hf₆Ta₂O₁₇/Sm₂Zr₂O₇ were studied for thermal barrier coatings (TBCs). The mixed powders of Hf₆Ta₂O₁₇/Al₂O₃ and Hf₆Ta₂O₁₇/YSZ were annealed at 1600°C for 8 hours in air, and the mixed powders of Hf₆Ta₂O₁₇/Sm₂Zr₂O₇ were annealed at 1200°C for 8 hours in air. The phases of the annealed mixed powders were observed by X-ray diffractometry (XRD). The sample of Hf₆Ta₂O₁₇/Al₂O₃ mixed powder was molded and sintered at 1600°C for 8 hours in air, and the microstructures of the sintered sample were investigated by scanning electron microscopy (SEM). The XRD results revealed that the crystal structure of Hf₆Ta₂O₁₇ had no remarkable changes in the mixed powders of Hf₆Ta₂O₁₇/YSZ and it had changes in the mixed powders of Hf₆Ta₂O₁₇/Al₂O₃. The monocline HfO₂/ZrO₂ phases were found in the mixed powders of Hf₆Ta₂O₁₇/Sm₂Zr₂O₇. The SEM results showed that the boundaries of Hf₆Ta₂O₁₇ and Al₂O₃ crystals were clear. The element composes in crystals were analyzed by energy dispersive spectrometer (EDS), and the EDS results showed that a little amount of Al diffused into Hf₆Ta₂O₁₇ crystals.

Abstract: Cu-TiB2 composite coatings were in-situ synthesized on the copper substrate by using a Nd: YAG laser. The microstructure of the coating and the bonding interface between the laser cladding layer and the substrate were studied by X-ray and SEM. The microhardness and the wear resisting property were tested. The results show that the TiB2 particles were well-proportioned and spherical existing in the coating layer, the bonding interface between the layer and substrate was metallurgical bonding. The microhardness reaches HV450 and the wear resistance is about 10 times as much as that of Cu substate.

Abstract: We recently synthesized different composition polycrystalline Ho2+xTi2-xO7-x/2 (x=0, 0.4 and 0.67), which is derivative fluorite compounds known as and pyrochlore phases in Ho3O2-TiO2 phase diagram by using conventional solid state synthesis methods. The samples were irradiated with 400 keV Ne2+ ions at cryogenic temperature (~77 K), using the Danfysik ion accelerator at the Ion Beam Materials Laboratory (IBML) of Los Alamos National Laboratory (LANL). The irradiation fluences in the experiments ranged from 5×10¹⁴-5×10¹⁵ ions/cm². An order-to-disorder (O-D) transformation was observed for α, β and pyrochlore phases, as determined using grazing incidence X-ray diffraction (GIXRD) at an incident angle of 0.25°. The O-D transformation threshold fluence for α phase was found to be noticeably lower than those for β phase and pyrochlore, and the O-D transformation threshold fluence for β phase was the highest. The O-D transformation threshold fluence was found to be coherent with the phase transformation temperature in the Ho₃O₂-TiO₂ temperature-composition (T-C) phase diagram.

Abstract: In this paper, the mineral composition and main mineral morphology of bauxite were studied from ZhongXiang deposit, HuBei province. Through Electron Microscope Identification, there are two main rock types, oolitic, bean-like and aphanitic or fine crystalline structure of bauxite. The main mineral morphology is micro-crystalline or flake, crumby forms. The mineral and chemical compositions of samples were investigated by XRD, EPMA analysis tests. The EPMA results showed that Al₂O₃ content is 47.95% ~ 75.04%, in association with other isomorphous impurities, such as Si, Fe, Ti and Mg in varying proportions. The mineral compositions of bauxite ore are mainly aluminum, iron, silicon minerals, which totally account for from 83.01% to 86.40%. The XRD revealed that diaspore, hematite and kaolinite are the major mineral components in the bauxite ores. The bauxite from ZhongXiang deposits is a type of high diaspore bauxite, partly with clay minerals. Furthermore, the function of bauxite as a good fire-resistant material was discussed.

Abstract: This paper studies the mineral compositions and textures of seven rock specimens from jadeite orebody and its country rock in Nammaw, Myanmar through XRD and observation of hand specimens and thin sections. The jadeite orebody is mainly composed of jadeite and minor zeolite minerals. Phlogopite schist and chromite-bearing amphibolite occur between the orebody and its country rock. The country rock is antigorite serpentinite. Outside of serpentinite is schist consisting of chlorite, hastingsite and polylithionite. The specimens of jadeite orebody show mainly following texture types: radiation texture, inequigranular crystalloblastic texture, granular-prismatic crystalloblastic texture, metasomatic texture and mylonitic texture. These textures indicate that the formation of the orebody is related to the intrusion of some fused mass or hydrothermal solution and then the orebody underwent dynamical metamorphism and hydrothermal metamorphism.

Abstract: In this paper, the colorimetry characteristics of ten red tourmaline samples were studied through the visible light spectrometer Color I5 of X-rite company of the United States. In order to give a quantitative description of the color of red tourmalines, the chromaticity index of the samples was measured, and the red tourmaline color parameters with possible principle and technology for evaluation were obtained, including chroma coordinates, main wavelength, chroma, brightness, curves of reflection rate and chroma figures. Combining with the electron probe microanalysis and the chemical analysis, the relationship between the content change of color-caused ions and the color parameters was discussed by analyzing the existing states and content of color-caused ions Mn²⁺(ion), Cr³⁺(ion) and Fe²⁺(ion).

Abstract: Although sericite belongs to 2:1 clay minerals, it hardly has intercalation properties because it has high layer charge density, which produces pretty strong electrostatic force. Therefore, the purpose of activation is to permanently reduce the layer charge of sericite and obtain numbers of exchangeable cations. The original sericite (S₀) was sintered at 800 °C for 1 h. After that, thermal-treated sericite (S₁) was activated with sodium fluosilicate and nitric acid, whose effects were characterized by X-ray diffraction (XRD) analysis, chemical analysis and zeta electric potential analysis. The dissolving-out amount of Al³⁺ of acid-treated product (S₂) reached circa 65 mg/g at the optimal processing conditions. After sodium modification with NaCl, CEC of final resulting product (S₃) was increased to 14.34 meq/100g compared with that of original sericite (4.94 meq/100g). The results of chemical analysis and zeta electric potential analysis indicated that acid activation with sodium fluosilicate and nitric acid decreased the layer charge of sericite.

Abstract: Thermal expansion behavior method in reheating process was used to evaluate firing temperature of white porcelains excavated in Yanggu area, Jangpyung-Ri No.1 (J1, 17~20 centuries) and Songhyun-Ri No.5 (S5, 14~15 centuries). For standard specimen used for evaluation of the firing temperature, we used starting materials (YC) that have similar chemical composition of J1 and S5 and then they were fired at various temperatures from 800 to 1300°C. Starting temperature of shrinkage and shrinkage rate were measured in thermal expansion behavior in reheating process of the of 1st fired specimens and the base line for original firing temperature was set based on the results from the measurement of the standard specimens. Effects of particle size of starting materials for standard specimens on the thermal expansion behavior in reheating process were also investigated and the base lines were also set. As a result, we found that difference in particle size for starting materials could cause different shrinkage rate and starting temperature of shrinkage in reheating process even with the same original firing temperature.

Abstract: Ti₃SiC₂, a ternary carbide, was proposed at this paper to use as the binder of polycrystalline diamonds to overcome the weaknesses of traditional metal binders and ceramic binders. Ti₃SiC₂ was first reported to be in-situ synthesized under high pressure (4GPa) and at high temperature (1400°C) (HPHT) from the mixtures of Ti, Si and graphite powders or the mixture of Ti, SiC and graphite powders. Ti₃SiC₂-damond composites were also made at HPHT from the previous mixtures and diamond particles. TiC_x, Ti₅Si₃C_x and TiSi₂ were main impurities and/or intermediate products of Ti₃SiC₂ samples synthesized at HPHT. Ti₃SiC₂ content increased as synthesized time increased from 10 min to 60 min. For as-synthesized composites, diamond particles were evenly distributed in matrix. The diamond particles are bonded well with the matrix by three types of interface.