Reaction Mechanism Of Al₈B₄C₇ Produced By Oxide Raw Materials

The ternary compound Al₈B₄C₇ not only is a successful antioxidant in carbon-containing refractories, but also can improve the hydration resistance of carbon-containing refractories. So far, single phase Al₈B₄C₇ can be produced, but the starting materials B and B4C used as the source of B in the product Al₈B₄C₇ are expensive in those synthesis methods. Therefore, those methods cannot be industrialized on a large scale because of the high cost. Using boron trioxide or borax as the source of B can reduce the cost of the synthesis. In this paper, we used boron trioxide or borax mixed equably with Al and C as the raw materials to produce Al₈B₄C₇, and the process and mechanism of the reactions were investigated to provide fundamental basis for the application of Al₈B₄C₇ in refractories industry.Using either aluminum, boron oxide and carbon powders or aluminum, borax and carbon powders as the raw materials can produce Al₈B₄C₇, and the finally offsprings were related to the temperature and the ratio of the raw materials. Al₈B₄C₇ appeared at 1300℃, and with the temperature increased, more Al₈B₄C₇ appeared in the sintered samples. The phase composition of the samples sintered at 1700℃was Al₈B₄C₇, Al₂O₃, Al4O4C and Al2OC.When using Al, B₂O₃ and C as the raw materials to produce Al₈B₄C₇ by two steps, the phase composition of the last sintered sample was Al₈B₄C₇, Al₄C₃, Al₄O₄C and Al₂OC.Three groups of samples, whose mole ratios of the raw materials Al, Na₂B₄O₇ and C were respectively (38/3):1:10, (38/3):1:13 and (38/3):1:16, were heated by different sintering time. With prolonging the sintering time, the Al₈B₄C₇ content in the sintered samples was increased. In other words, protracting the sintering time was propitious to imrove the yield of Al₈B₄C₇.In the preparation of the experiment, the raw materials ratios were designed by using C as the reducing agent or using Al as the reducing agent. But, the experiment led up to the fact that the reaction process was a complex process, in which both C and Al were acting as the reducing agent, namely, neither C nor Al worked as the reducing agent alone.The shape of most grains in sintered samples was ruleless, and there were also a few of grains having the shape of prism or sheet.The size of the grains was about several microns.