Preparation of Mullite Whiskers and Their Enhancement Effect on Ceramic Matrix Composites

Mullite whiskers were facilely prepared by sintering kyanite at high temperature, with the addition of AlF3·3H20. The as-prepared whiskers have been characterized systematically in terms of phase composition, morphology, and structure. Results showed that the morphology and size of mullite whiskers were strongly depended on the content of AlF3·3H20 and sintering temperature. At temperatures in the range of 1 100 to 1 500 ℃ with 4 wt% addition of AlF3·3H20, the well-shaped mullite whiskers were obtained. For an instance, the mullite whiskers with 5-10 μm in length and 0.1-0.2 μm in cross-section could be formed at 1 400 ℃, with 4 wt% addition of A1F3·3H20. Moreover, results showed that the addition of mullite whiskers into ceramic matrix enhanced its fracture toughness significantly.

Difference of Mullite Whiskers between Ytterbia-Doped Aluminum Hydroxide-Silica Gel and Mechanochemical Process

A number of processing routes were developed for the synthesis of mullite whiskers. Mullite whiskers that were derived from ytterbia-doped aluminum hydroxide-silica gel were studied; another process for preparing mullite whiskers from Al2O3-SiO2 oxide mixtures by mechanochemical method was encountered by chance. The method of ytterbia-doped aluminum hydroxide-silica was used to enhance mullite formation from gel. For adding ytterbia, mullite formation can take place at 1000℃and almost single-phase mullite can be achieved at 1150℃, which is a significantly lower temperature than mechanochemical method. This enhancement of mullite formation is also attributed to refinement of aluminum hydroxide-silica by gel. Furthermore, well-developed single-crystal mullite whiskers were produced from the gel powder instead of normal-shaped grains. The presence of YbO might have promoted the formation of nanosized Al2O3 particles, which then acted as a template leading to mullite whiskers at higher sintering temperature.

Strengthening Mullite Ceramic Materials by Gas Infiltration

In order to improve the high-temperature performance of mullite ceramic materials,mullite ceramic bodies were placed in closed containers with AlF_(3)·3H_(2)O powder and kept at 1 600 ℃ for 6 h.AlF_(3)·3H_(2)O reacts with O_(2) to produce gaseous compounds AlOF and F,which penetrate into the bodies,promote Al2O3 and SiO_(2) to form mullite whiskers,and strengthen the mullite ceramic materials.The results show that the mullite ceramics have enhanced hot strength,increased bulk density and declined apparent porosity by adding a certain amount of AlF_(3)·3H_(2)O in a closed container.When the addition of AlF_(3)·3H_(2)O is 6%,the bulk density of the ceramic material reaches the maximum and the apparent porosity is the lowest;and when the addition of AlF_(3)·3H_(2)O is 8%,the hot strength of the material is the highest.

Relationships among Preparative Technique,Phase Composition and Bending Strength of Bauxite Porcelain

Bauxite porcelain can be sintered to make its bending strength reach 179Mpa by using sintered bauxite, clay and potash feldspar and albite as the main raw materials in oxidation atmosphere under the normal pressure. XRD, SEM and so on are utilized to study the relationships between factors like the composition of ingot, reduction particle size and heat insulating time at sintering temperature and the phase composition, microstructure and bending strength of ceramic body. The results show that the main phases in bauxite porcelain are corundum, mullite and glass, and the mullite is composed of two parts： primary mullite formed by clay conversion and secondary mullite whiskers precipitating from high temperature melt. The bending strength of ceramic body can be improved by enhancing the content of sintered bauxite in the formula： the smaller the particle size of the ceramic body is, the higher the bending strength will be. The high-temperature heat insulating technique is beneficial to the precipitation of certain quantitative secondary mullite whiskers so as to remarkably improve the strength of glass phase and ceramic body.

Composite thermal protection coating on woven silica fiber fabrics

A composite coating with inner and outer layers was prepared for the thermal protection of woven silica fiber fabrics.Using a sol mixture of a silica sol and AlF3/SiO2 particles mixed in the stoichiometric molar ratio for mullite,hollow silica spheres and short mullite fibers were added to the inner layer and outer layer,respectively.The phase composition and thermal evolution of the coating,along with the interfacial microstructure between the coating and the matrix,were characterized by means of X-ray diffraction,differential scanning calorimetry/thermogravimetry,scanning electron microscopy,and tensile strength testing.Mullite whiskers grew between 950°C and 1200°C and helped prevent thermal cracking during the drying and densification processes.The hollow silica spheres might play dual roles,weakening the adhesion between the coating and the fibers by reducing their direct contact,but strengthening the joining between the coating and substrate by embedding themselves among the fabrics.