Formation, microstructure and properties of aluminum borate ceramics obtained from alumina and boric acid

Abstract

The formation of aluminum borates (Al 18 B 4 O 33 and Al 4 B 2 O 9) from alumina and boron oxide occurs between 600 and 800°C. These materials have refractory properties and corrosion resistance. The objective of this work is to develop materials from the Al 2 O 3-B 2 O 3 system, employing alumina and boric acid as starting powders, to study the critical processing variables and describe the developed microstructure and properties. Three formulations (13, 19.5 and 26 wt% B 2 O 3) were studied. In order to confirm the formation of borates, the differential thermal analysis and thermogravimetric analysis were carried out. Afterwards, uniaxially pressed disc-shaped specimens were fired at four temperatures above the formation temperature. The textural properties of the ceramics were evaluated by the immersion method, this permit to evaluate the sintering processes. Then the degree of borate formation was confirmed by X-ray diffraction. Finally, the developed microstructures were characterized by scanning electron microscopy, and the diametral compression behavior was evaluated. A series of porous (≈50%) refractory materials from the Al 2 O 3-B 2 O 3 system were developed. The processing strategy resulted in materials with Al 18 B 4 O 33 as the main crystalline phase. Needle grains with diameters between 0.2 and 1 µm and an aspect ratio over 20:1 were obtained. Thus, based on the information gathered from our research, aluminum borate ceramic materials can be designed for structural, insulating or filtering applications employing only alumina and boric acid as boron oxide source.

Microstructural evolution and kinetics analysis of aluminum borate ceramics via solid‐state reaction synthesis

“…Aluminum borate ceramics (ABCs) have been applied in numerous regions such as ceramic/metal brazing joints, whisker‐reinforced composite materials, molten metal corrosion resistance among others, owing to their fancy rod‐like morphology, high Young modulus, tensile strength, low thermal conductivity, perfect thermal stability, and good corrosion resistance . Wang et al achieved the joining of aluminum by the epitaxial growth of aluminum borate whiskers (ABOws) between two alumina bulks; the maximum flexural strength of the joints reached 27 MPa tested at 800°C .…”

Synthesis of Novel Electrospun Composite Nano Powders of the Quaternary Ti-Al-O-B System

“…The decomposition of boric acid to B2O3 was completed at 250 °C (Eq. 3) [18]. The solid B2O3 particles can distribute homogeneously within a fiber due to the homogenous nature of the electrospinning solution.…”

“…Aluminum borates (Al18B4O33 and Al4B2O9) are the main stable phases present in the Al2O3-B2O3 binary system. Al18B4O33 is stable until up to 1900 °C and is generated with lower B2O3 amounts, whereas Al4B2O9 is produced when more B2O3 is present and is stable only below 1100 °C [18]. Aluminum borates have a strong tendency toward taking a needle-like morphology [19].…”

Oxidation‐led decomposition of hexagonal boron nitride coatings on alloy substrates at 900 °C: Ti‐6Al‐4V

“…Al 4 B 2 O 9 can form at temperatures as low as 780 °C, whilst temperatures greater than 1,000 °C are required for Al 18 B 4 O 33 . This material has been synthesised via various means that include a sol‐gel route followed by calcination, chemical vapour deposition, precipitation followed by annealing, ball milling followed by calcination, and direct reactions of mixtures at elevated temperatures in air or an argon atmosphere . Across this range of reaction systems, it is the consensus that the key reaction takes place between alumina and boron trioxide.…”

“…One popular proposed mechanism by which this occurs is via the solution–liquid–solid (SLS) growth mechanism . According to this theory, alumina or aluminum dissolves in molten boron trioxide and then when the solution becomes supersaturated, aluminum borate crystallites form and are precipitated out . In the case of aluminum, this may follow the thermite‐type reduction of boron trioxide by aluminum …”