It was found in the literature that the aluminum borate and other mullite-like structures have been widely studied due to their interesting properties. Mullite (3Al2O3–2SiO2) belongs to the special refractory materials group as well as the alumina and are notable for their low porosity and high purity crystal structures. As early as 1955, Dietzel et al. suggested that a substitution of Si by B in the mullite is possible, and forms a compound characterized as Al18B4O33. Aluminum borates, especially Al18B4O33and Al4B2O9are of interest for their noteworthy mechanical properties (high Young modulus, yield strength and low thermal expansion coefficient), their refractory behavior and their high chemical stability. When used as whiskers or fibers in advanced ceramics, the result is a material with comparable properties to SiC but with lower costs. Fan et al. suggested that Al4B2O9 is formed when liquid aluminum is dissolved in melted boron oxide, when they formed aluminum borate whiskers from Al + B2O3powders at 850 ℃ in an argon atmosphere. Boron oxide melts at 450 ℃, and at the working temperatures of the experiments in the present work it formed a liquid around the aluminum particles. These aluminum particles were covered by an aluminum oxide layer even though locally molten aluminum could flow out of the particle (Fig. 1a).
Fig. 8 SEM micrographs with different magnification of the coating surface obtained using aluminum particles after 5 h at 650℃ (a) Without boron; (b) With 20 % boron
However, this theory could not explain why the aluminum borate whiskers are formed around the particles . When the borate is formed by the reaction of molten aluminum and B2O3, the borate should not appear in the inside walls. Another proposed mechanism is the one offered by Qi et al, who proposed that the Al4B2O9 is formed by a reaction between the B2O3 and the alumina. This could explain the formation of the whiskers over the complete surface of the particles: as soon as the liquid aluminum comes into contact with the air it oxidizes, and the whole surface of the particles is therefore covered by an alumina layer able to react with the B2O3 in the reaction:
2xAl2O3+B2O3→Al4B2O9
In general to make these structures grow with a fiber shape uniaxial pressure must be applied, or a catalyst that favors nucleation and growth must be added. In these experiments, no impurities were found in the surface that could act as a catalyst, and no pressure was applied. The spontaneous growth of these 1D structures has already been observed under different conditions and several authors have tried to explain the formation mechanism of these structures by different theories. The spontaneous growth of 1D structures can be explained by the following reaction mechanisms: vapor–solid, vapor–liquid–solid or stress-induced recrystallization
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