Single and polycrystalline mullite fibres grown by laser floating zone technique

The laser floating zone technique was used to grow large 2Al₂O₃–SiO₂ mullite fibres (up to 1.6 mm in diameter and 40 mm in length). The fibres grown at 10 mm/h are single crystalline in nature, while those pulled at higher rates (40 and 100 mm/h) are polycrystalline with a cellular microstructure. The crystals are highly [0 0 1] textured with respect to the fibre axis, as determined by X-ray diffraction analysis. The Raman spectra taken at different orientations corroborate the strong anisotropy observed by X-ray and SEM on both single crystalline and textured polycrystalline samples.Four point bending tests and ultramicroindentation Vickers experiments were performed at room temperature in order to characterize the mechanical properties. The presence of lamellar inclusions in the single crystalline fibres decreases the flexural strength (431 MPa) and the fracture toughness (1.2 MPa.m^1/2) compared to the polycrystalline ones (631 MPa and 1.6 MPa.m^1/2). However, the absence of grain boundaries in the single crystals leads to higher ultramicrohardness (H_V = 15.6 GPa) and Young's modulus (E = 170 GPa) than those of the polycrystalline fibres (14.2 and 145 GPa), where a glassy intergranular phase exists.     

Comparative study of conventional and microwave sintered mullite fibres: structural study

Mullite fibres were synthesised from a mixture of aluminium, aluminium chloride and acidic silica solutions as monophasic salts using a sol–gel technique. The viscosity and rheological behaviour of the mullite precursor sol were examined. Mullite fibres were synthesised using both conventional and microwave sintering techniques. The samples were characterised by X-ray diffraction and scanning electron microscopy. Pure mullite fibres were synthesised by microwave sintering at a relatively low temperature of 1200°C. The grain size of the fibre samples sintered using the microwave technique was finer than that produced by conventional sintering. These results show that microwave sintering is a promising technique for processing mullite fibres.     

Effect of sodium on the creep resistance of yttrium aluminium garnet (YAG) fibres

Aligned YAG fine fibres had been made previously from an aqueous sol–gel process, but it was suspected that sodium contamination in one of the starting materials lessened the creep resistance of the final product. Therefore, sodium-free gel precursor fibres were made using the same process, and upon firing in air at 900 °C these formed pure phase YAG fibres, of 4 μm diameter. When steamed over 200–500 °C for 3 h, the amorphous gel fibre formed single phase nanocrystalline YAG between 400 and 500 °C, an extraordinarily low temperature for this material to crystallise. Upon postfiring up to 1550 °C, grains averaging 0.5 μm and pores of 0.17 μm had formed, but despite this porosity and smaller grain size, the sodium-free fibre exhibited superior creep resistance than the sodium contaminated fibres reported previously by the authors, typically creeping at temperatures 50 °C higher. This demonstrates that even small levels of sodium contamination are harmful to creep resistance in YAG.     

Mechanical and fracture properties of zircon–mullite composites obtained by direct sintering

Refractory materials based on zircon (ZrSiO₄) are applied in high temperature applications (1400–1500 °C). They are demonstrated to have an excellent chemical attack resistance, such as corrosion or degradation due to molten glass or metals. On the other hand mullite (3Al₂O₃2SiO₂) is important both in traditional and advanced ceramics. Although multi-phase ceramic materials were always used, nowadays composite materials have an important industrial and technological development, to enlarge the designing capability of the manufacturer in properties and behaviors. The objective of the present work is to study the influence of the starting composition on the mechanical and fracture properties of zircon–mullite composites obtained by direct sintering of consolidated samples by slip cast of concentrated aqueous suspensions in plaster molds. Zircon–mullite composites using 15–45 wt% mullite were prepared and compared with pure zircon material obtained in the same conditions. Flexural strength (σ_f), dynamic elastic modulus (E), toughness (K_IC) and initiation fracture surface energy (γ_NBT) were evaluated. The results were explained by microstructure and the XRD analysis. The presence of mullite increased the zircon thermal dissociation. The ZrO₂ was a product of this reaction and also influence the mechanical and fracture properties of these materials through several combined mechanisms.Zircon composites prepared with 45 wt% of mullite in the starting powder showed a higher fracture toughness and initiation energy than ceramics derived from pure zircon. Microstructure consisting in mullite as a continuous predominant phase in which zircon and zirconia grains were distributed improved almost all the mechanical and fracture properties.     

Microstructure and mechanical properties of hydroxyapatite obtained by gel-casting process

In the present work, well-shaped HAp green bodies were obtained by the gel-casting process with 50 vol.% slurry. After drying, the microstructure and pore distribution of the green body were investigated. The density, compressive strength and flexural strength of the green body were 1.621 g/cm³, 32.6 ± 3.2 MPa and 13.8 ± 1.0 MPa, respectively. After pressureless sintering at the range of 1100–1300 °C for 2 h, the relative density of the final product ranges from 71.8 to 97.1% th. The maximum value of flexural strength, elastic modulus, hardness and fracture toughness were 84.6 ± 12.6 MPa, 138 ± 7 GPa, 4.45 ± 0.18 GPa and 0.95 ± 0.13 MPa m^1/2, respectively. SEM images show a compact and uniform microstructure; the average grain size was found by using the linear intercept method. XRD and FTIR determined the phase and the radical preserved after sintering.     

用凝胶法制备片状莫来石粉及其应用

用拟薄水铝石和非晶态ＳｉＯ_２微粉为原料，用溶胶－凝胶工艺制取了莫来石凝胶。通过干燥、煅烧，制得片状结晶莫来石粉．初步研究了凝胶法莫来石粉在钛酸铝陶瓷和烧结刚玉砖中的应用。实验结果表明，片状莫来石起了促进烧结、降低烧成温度的作用，并使钛酸铝－莫来石复相陶瓷和刚玉砖的强度得到显著提高．     

Microstructure and properties of lightweight fibrous porous mullite ceramics prepared by vacuum squeeze moulding technique

A novel fibrous porous mullite network with a quasi-layered microstructure was produced by a simple vacuum squeeze moulding technique. The effects of organic binder content, inorganic binder and adsorbent on the microstructure and the room-temperature thermal and mechanical properties of fibrous porous mullite ceramics were systematically investigated. An anisotropy microstructure without agglomeration and layering was achieved. The fibrous porous mullite ceramics reported in this study exhibited low density (0.40 g/cm³), low thermal conductivity (~0.095 W/(m K)), and high compressive strength (~2.1 MPa in the x/y direction). This study reports an optimal processing method for the production of fibrous porous ceramics, which have the potential for use as high-temperature thermal insulation material.     

Dense mullite zirconia composites obtained from the reaction sintering of milled stoichiometric alumina zircon mixtures by SPS

The main objective of this article is to obtain dense (porosity under 0.5%) polyphasic ceramics belonging to the Al₂O₃–SiO₂–ZrO₂ system by SPS sintering of high energy powders milled drily; the stoichiometric (54.45:45.54 zircon–alumina, weight basis) mixture was explored in this work. Particularly the principal sintering variables: sintering temperature and dwell time were investigated. The textural, structural and microstructural changes were evaluated together with the hardness and toughness of the obtained ceramics and their microstructure. The effect of the mechanical pre-treatment was carried out by X-ray diffraction and particle distribution evaluation. Due to the rapid heating process an incomplete reaction was achieved in several cases, as a consequence multiphasic ceramics with different alumina, mullite, zircon and zirconia contents were obtained.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

Microstructure and properties of HfC and TaC-based ceramics obtained by ultrafine powder

HfC and TaC-based ceramics were hot pressed at 1900 °C for 5-20 min starting from synthesized ultrafine powders. The addition of 5 vol.% of MoSi₂ improved the densification, which increased from around 85-90% for the pure matrices to 95% for the composites. The flexural strength was measured at room temperature and at 1500 °C under protective atmosphere. The added value of this work consists in the utilization of cheap synthesized powders for the realization of the composites with properties comparable to those obtained using more expensive commercial powders. The effect of the nanometric size of the starting powder showed to have the potential to improve the densification behavior and the mechanical properties, however it is necessary a further optimization of the synthesis condition in order to avoid the formation of agglomerates of unreacted powder.