Sintering behavior of Cr2O3–Al2O3 ceramics

We investigated the sintering behavior of Cr₂O₃–Al₂O₃ ceramic materials. In our observation of the isothermal shrinkage behavior of Cr₂O₃–Al₂O₃ ceramic, the activation energy of sintering reaction was measured to be 102 kJ/mol, that is, the near value of the activation energy of diffusion of Al ions in Al₂O₃ single crystal. Therefore the diffusion of cations is believed to control the sintering behavior of this material. With the addition of TiO₂, (the compound chosen to accelerate the diffusion of cations) to Cr₂O₃–Al₂O₃, the sintering behavior was accelerated.     

The corrosion resistance of microsilica-containing Al2O3–MgO and Al2O3–spinel castables

Microsilica addition in Al₂O₃–MgO and Al₂O₃–spinel castables helps to improve their flowability and partially accommodate their residual expansion after firing. Nevertheless, there is a lack of conclusive statements in the literature regarding the effects of microsilica on one of the main requisites for steel ladle refractories: corrosion resistance. In the present work, the performance of alumina–magnesia and alumina–spinel with or without microsilica when in contact with a steel ladle slag was evaluated based on three aspects: the material's physical properties, its chemical composition and the microstructural features before the slag attack. According to the attained results, microsilica induced liquid formation and pore growth during sintering, favoring the physical slag infiltration. Moreover, due to this liquid, CA₆ was formed in the matrix, mainly for the Al₂O₃–spinel composition, which also favored the castable dissolution into the molten slag.     

Stability in the system CaO–Al2O3–H2O

The solubility of AH₃, CAH₁₀, C₂AH_7.5, and C₃AH₆ was determined experimentally at 7 to 40 °C and up to 570 days. During the reaction of CA, at 20 °C and above initially C₂AH_7.5 formed which was unstable in the long-term. The solubility products calculated indicate that the solubilities of CAH₁₀, C₂AH_7.5 and C₄AH₁₉ increase with temperature while the solubility of C₃AH₆ decreases. Thus at temperatures above 20 °C, C₃AH₆ is stable, while at lower temperature also CAH₁₀ and C₄AH₁₉ are stable, depending on the C/A ratio.At early hydration times, CAH₁₀ can be stable initially at 30 °C and above, as the formation of amorphous AH₃ stabilises CAH₁₀ with respect to C₃AH₆ + 2AH₃. With time, as the solubility AH₃ decreases due to the formation of microcrystalline AH₃, CAH₁₀ becomes unstable at 20 °C and above.     

Electrical resistivity of Cr–Al2O3 and ZrxAly–Al2O3 composites with interpenetrating microstructure

AC and DC resistivity of Cr–Al₂O₃ and Zr_xAl_y–Al₂O₃ composites with varying metal content were measured. A strong percolation behavior was observed in the Cr–Al₂O₃ system, where the AC resistivity varied nine orders of magnitude close to the percolation threshold of 28 vol.%. AC measurements were less dependant on the contact resistance than DC measurements. The best reproducibility was obtained at a frequency of 100 kHz. AC resistivity values of insulating composites differed from DC values and may also be frequency-dependant. DC measurements up to 600 °C indicate that the intermetallic phases ZrAl₃ and ZrAl are PTC conductors. The electrical properties of Zr_xAl_y–Al₂O₃ samples with a metal content of 29 vol.% were anisotropic, with a much higher resistivity in the pressing direction.     

Effect of TiO2 addition amount on BaO–CaO–Al2O3 -SiO2 glass-bonded Al2O3 ceramics

A new type of glass-ceramic BaO–CaO–Al₂O₃–SiO₂ (BCAS) was developed to join Al₂O₃ ceramics, adding TiO₂ to the glass-ceramics can promote the crystallization behavior of the glass-ceramics. Through the observation of the joints, rutile TiO₂ whiskers can grow on the surface of Al₂O₃ ceramics, and the grown TiO₂ whiskers are one-dimensional needle-like whiskers growing in different directions in the joints, providing mechanical support for the joints. The aspect ratio of TiO₂ whiskers was changed by controlling the addition of TiO₂, and the crystallization behavior and microstructure of the joints were studied. The experimental results show that when the amount of TiO₂ added is 10% (wt%), the density of TiO₂ whiskers in the joint is the largest, the strengthening effect on the joint is the best, and the shear strength can reach 94.33 MPa.     

Surface and bulk crystallization in Nd2O3–Al2O3–SiO2–TiO2 glasses

Neodymium aluminosilicate (Nd₂O₃–Al₂O₃–SiO₂; NdAS) glasses have been investigated for the effect of concentration of TiO₂ on the crystallization mechanism and for the effect of surface condition on crystal growth. NdAS glasses with 0–10 wt.% TiO₂ were heat-treated for nucleation and crystal growth and were examined for phase separation and morphology of surface crystals as well as for crystal growth rate. All the glasses exhibit surface crystallization, however, the glass having 8 wt.% TiO₂ also exhibits internal crystallization after a two-stage heat treatment. Surface crystallization was dependent on the condition of the glass surface and the amount of TiO₂. The crystal growth on the cut surface was faster than on the fractured surface and the growth rate in surface increased with increasing TiO₂. The phase separation found in NdAS glasses containing above 8 wt.% TiO₂, was confirmed to be an important factor controlling the internal crystallization process in Nd₂O₃–Al₂O₃–SiO₂–TiO₂ glasses.     

Thermal Stability and Crystallization Kinetics of MgO–Al2O3–B2O3–SiO2 Glasses

To support commercialization of the MgO–Al₂O₃–B₂O–SiO₂-based low-dielectric glass fibers, crystallization characteristics of the relevant glasses was investigated under various heat-treatment conditions. The study focused on the effects of iron on the related thermal properties and crystallization kinetics. Both air-cooled and nucleation-treated samples were characterized by using the differential thermal analysis/differential scanning calorimeter method between room temperature and 1200°C. A collected set of properties covers glass transition temperature (T_g), maximum crystallization temperature (T_p), specific heat (ΔC_p), enthalpy of crystallization (ΔH_cryst), and thermal stability (ΔT=T_p—T_g). Using the Kinssiger method, the activation energy of crystallization was determined. Crystalline phases in the samples having various thermal histories were determined using powder X-ray diffraction (XRD) and/or in situ high-temperature XRD method. Selective scanning electron microscope/energy-dispersive spectroscopy analysis provided evidence that crystal density in the glass is affected by the iron concentration. Glass network structures, for air-cooled and heat-treated samples, were examined using a midinfrared spectroscopic method. Combining all of the results from our study, iron in glass is believed to function as a nucleation agent enhancing crystal population density in the melt without altering a primary phase field. By comparing the XRD data of the glasses in two forms (bulk versus powder), the following conclusions can be reached. The low-dielectric glass melt in commercial operation should be resistant to crystallization above 1100°C. Microscopic amorphous phase separation, possibly a borate-enriched phase separating from the silicate-enriched continuous phase can occur only if the melt is held at temperatures below 1100°C, that is, below the glass immiscibility temperature. The study concludes that neither crystallization nor amorphous phase separation will be expected for drawing fibers between 1200°C and 1300°C in a commercial operation.     

Melt extraction processing of structural Y2O3–Al2O3 fibers

Compounds in the system Y₂O₃-Al₂O₃ are promising materials for optical, electronic and structural applications. In this study, a melt extraction process with a new approach to making ceramic fibers was used to produce amorphous fibers in the Y₂O₃–Al₂O₃ system within the 20–30-micron size range. Smooth and uniform cross section fibers with relatively high tensile strength were obtained depending on the wheel velocity. X-ray diffraction of as-extracted fibers revealed the non-crystalline nature of the yttria-alumina compositions. The crystallization and glass transition temperatures of non-crystalline fibers were determined using differential thermal analysis (DTA). Crystalline phases were identified by X-ray diffraction in the fibers after heat treatment.     

Fabrication of WSi2–Al2O3 and W5Si3–Al2O3 composites by combustion synthesis involving thermite reduction

Formation of WSi₂–Al₂O₃ and W₅Si₃–Al₂O₃ composites was studied by thermite-based combustion synthesis. The addition of two thermite combinations composed of WO₃+2Al and 0.6WO₃+0.6SiO₂+2Al into the W-Si reaction systems facilitated the combustion wave propagating in a self-sustaining manner and contributed to the in situ formation of tungsten silicides along with Al₂O₃. Experimental results showed that the former thermite mixture is more exothermic than the latter, and a decrease in the combustion temperature and flame-front velocity with increasing silicide phase formed in the composite. Depending on the reaction stoichiometry, the combustion wave velocity varied from 9.5 to 3.7 mm/s and temperature from 1650 to 1280 °C. A complete phase conversion and a broad range of the molar ratio of WSi₂/Al₂O₃ from 0.8 to 4.0 were achieved for the production of the WSi₂–Al₂O₃ composites. Due to the lower formation exothermicity, the W₅Si₃–Al₂O₃ composites were produced with a narrower range of W₅Si₃/Al₂O₃ from 0.4 to 2.0, beyond which combustion failed to proceed. Moreover, there exist WSi₂ and unreacted W in the as-synthesized W₅Si₃–Al₂O₃ composites.     

Creep behaviour of Al2O3–SiC nanocomposites

Compared with monolithic fine grained Al₂O₃, Al₂O₃ nanocomposites reinforced with SiC nanoparticles display especially high modulus of rupture as well as reduced creep strain. Taking into account the fracture mode change, the morphology of ground surfaces showing plastic grooving, the low sensitivity to wear and the low dependence of erosion rate with grain size, it can be reasonably assumed that the strength improvement is associated with an increase of the interface cohesion (due to bridging by SiC particles) rather than with a grain size refinement involving substructure formation (as initially suggested by Niihara). In the present work, creep tests have been performed and the results agree with such a reinforcement of the mechanical properties by SiC particle bridging Al₂O₃–Al₂O₃ grain boundaries. Indeed, particles pinning the grain boundaries hinder grain boundary sliding resulting in a large improvement in creep resistance. In addition, SiC particles, while counteracting sliding, give rise to a recoverable viscoelastic contribution to creep. Because of the increased interface strength, the samples undergoing creep support stress levels, greater than the threshold value required to activate dislocation motion. The high stress exponent value as well as the presence of a high dislocation density in the strained materials suggests that a lattice mechanism controls the deformation process. Finally, a model is proposed which fits well with the experimental creep results.     

Processing and crystallisation of rapidly solidified Al2O3–Y2O3 fibres

Abstract

Amorphous fibres of the Al₂O₃–Y₂O₃ system were prepared by a melt extraction technique, and subjected to crystallisation. The quality of the melt extracted fibres is controlled by the wheel edge and rotational speed, with both having a significant effect on fibre diameter and avoidance of irregularities and instabilities along the fibre length. Tensile strength in the glassy state varied from 0·6 to 1·0 GPa. Crystallisation activation energies calculated from scan-rate dependence of DTA peaks are 741 and 1374 kJ mol^-1 for E1 (Al₂O₃–yttrium aluminium garnet (YAG) eutectic), 390 kJ mol^-1 for YAG, and 438 kJ mol^-1 for E2 (YAG–yttrium aluminium perovskite (YAP) eutectic) by the Kissinger method; and 698 and 1346 kJ mol^-1 for E1, 352 kJ mol^-1 for YAG, and 399 kJ mol^-1 for E2 by the Augis–Bennett method.     

Characterization of chemosynthetic H3PO4–Al2O3–2SiO2 geopolymers

Pure chemosynthetic Al₂O₃–2SiO₂ powders fabricated by a sol–gel method exhibit high phosphoric acid-activated properties and high compressive strengths. The phosphoric acid-activated properties could be characterized by compressive strength. The phase structure evolution of synthetic powders and the resulting geopolymers were investigated by DTA-TG, XRD, FTIR and MAS NMR analysis. These results show that the phosphoric acid-activation region of the synthetic powders was in the range of 200–800 °C, which was much lower than the temperature at which kaolinite was converted into metakaolinite. ³¹P MAS NMR analysis revealed that [PO₄] tetrahedra were part of the geopolymer structure.     

SiAlON–Al2O3 ceramics as potential biomaterials

When used as implants, Al₂O₃ is unable of directly achieving good chemical bonding with soft and hard tissues. To overcome this problem, SiAlON–Al₂O₃ ceramics were prepared in this study by direct nitridation. Phase composition, porosity, bulk density, and compression strengths were examined, and biological properties were evaluated by cell culture on ceramic surface. Major phase of SiAlON–Al₂O₃ ceramics was identified as Si₄Al₂O₂N₆, formed by reaction of Si, Al and Al₂O₃ under nitrogen atmosphere at high temperature. As Al₂O₃ content increased, porosity and compressive strength decreased. Therefore, Si₄Al₂O₂N₆ phase could improve sintering, leading to formation of composites with better properties. The porosity and compression strength were found suitable for requirement of biomaterials. Cell culture experiments showed that cells could proliferate and survival well on ceramic surface, indicating good biocompatibility of Si₄Al₂O₂N₆ phase in SiAlON–Al₂O₃ ceramics. Overall, these data look promising and might provide novel strategies for development of future SiAlON–Al₂O₃ bioceramics.     

Microstructure evolution during air bonding of Al2O3 to Al2O3 joints using bismuth–borate–zinc glass

Alumina ceramics with 95 wt.% purity were sealed together using a bismuth based glass, 40Bi₂O₃–40B₂O₃–20ZnO (mol.%). The wettability of the glass on the Al₂O₃ substrate was investigated. The results showed a contact angle of ≤36.5° was achieved when the temperature was ≥630 °C. Subsequently, sealing cycles were performed at temperatures of 520–700 °C for 30 min. The dependence of microstructure evolution of the joints on temperature was investigated. Bi₂₄B₂O₃₉ was detected to be the product in the joints sealed at 530–580 °C, while ZnAl₂O₄ was identified to be the main product when sealing at temperature of ≥650 °C due to the reaction between the Al₂O₃ substrate and ZnO from the glass. The influence of dwelling time at 700 °C on microstructure evolution of the joints was also studied. The results showed that the size of ZnAl₂O₄ increased with increasing holding time.     

High temperature sintering of SiC with oxide additives: I. Analysis in the SiC–Al2O3 and SiC–Al2O3–Y2O3 systems

The vaporization behaviour of pure Al₂O₃, Y₂O₃ and SiC as well as SiC–Al₂O₃ and SiC–Al₂O₃/Y₂O₃ mixtures has been analysed by thermodynamic calculations in an open system. Pure Al₂O₃ and Y₂O₃ evaporate congruently in the 1200–2300 K temperature range. Pure SiC vaporizes in a non-congruent manner leading to graphite formation as by-product. A SiC–Al₂O₃ mixture evaporates congruently according to the main vaporization reaction, 2 SiC(s) + Al₂O₃(s) +Al₂O(g) ⇆ 2 SiO(g) + 2 CO(g) +4 Al(g), but the overall composition changes: for SiC rich samples, the mixture tends towards pure SiC in time, and for Al₂O₃ rich samples towards pure Al₂O₃. A SiC–Al₂O₃/Y₂O₃ mixture shows similar behaviour.     

A Significant Role of Tb2O3 on the Optical Properties and Radiation Shielding Performance of Ga2O3–B2O3–Al2O3–GeO2 Glasses

Journal of Inorganic and Organometallic Polymers and Materials - This research article focuses on the significant role of Tb2O3 content on the optical properties and radiation shielding performance...     

Al2O3/Al2O3扩散连接机理的研究

采用热压方式进行氧化铝陶瓷材料的扩散连接，并深入探讨了其连接机理，结果表明：采用适当的连接工艺，可使此种材料产生塑性形变，接触面积增加，空洞收缩和闭合，扩散进行充分，1420℃，15MPa热压60min条件下，获得的最高强度为357MPa，超过母材强度平均连接强度为250MPa，是母材平均强度的85％。     

Al2O3-3Al2O3·2SiO-ZrO2耐火材料的相组成

Al<,2>O<,3>-SiO<,2>-ZrO<,2>系耐火材料应用范围广,特别被用作陶瓷辊,具有力学强度高、抗热震性能优良、耐碱类化合物侵蚀和高温蠕变率低的特性.Al<,2>O<,3>-SiO<,2>-ZrO<,2>系耐火材料性能很大程度上取决于其结晶相和玻璃相的总量和化学成分,采用定量XRPD和XRF研究了原料中Al<,2>O<,3>/SiO<,2>比和氧化铝颗粒尺寸分布对结晶相和玻璃相的总量及其化学成分的影响.耐火材料由莫来石、刚玉、ZrO<,2>的多晶体和总量各异的玻璃相组成.莫来石含量及其晶胞参数和成分随烧成温度改变,但主要受原料中Al<,2>O<,3>/SiO<,2>比的影响.