Nozomu Otsuka

Formation of Al2O3 whiskers by thermal decomposition of 9Al2O3·2B2O3 whiskers

Since the mechanical properties of A1203 whiskers are superior to those of other materials [1], it is considered to be a candidate material for whiskers reinforced to composites. Three major preparation methods for A1203 whiskers have been reported so far: first, vapour reaction methods such as the oxidation of A1 metal [2] or AI-Fe alloy [3] with hydrogen or steam; secondly, hydrolysis of A1C13 [4] or A1F3 [5] with hydrogen; and thirdly, the crystallization method from a boehmite gel-seeded oL A1203 [6]. Recently, the preparation of 9A1203.2B203 whiskers by the flux method has been reported [7]. This compound is known to decompose into o : A1203 and liquid phase when it is fired above 1440 °C in air [8]. In this letter the morphological and crystallographic change of 9AI203.2B203 to A1203 by thermal decomposition is reported, mainly by means of transmission electron microscopy (TEM). The starting material used was 9AlzO3.2BzO3 whiskers produced by Shikoku Chemicals Co. It was fired at various temperatures from 1400 to 1600 °C for 2 h. The TEM instruments used were JEM 2000EX (Jeol Co.) and H-9000 (Hitachi Co.). The observation was performed at an accelerating voltage of 200 kV in the former instrument and 300 kV in the latter. Fig. 1 shows a micrograph, high-resolution micrograph and selected-area electron diffraction (SAED) pattern of the as-received specimen. From the SAED, the particles were found to be a single crystal elongated to [001] with a side plane of {120}. The crystal structure of 9A1203.2B203 [9] resembles closely that of mullite, A14+2xSi2_2xOlo_ x [10], and