Production of alumina borate filaments

Abstract

Reinforcing materials by incorporation of inorganic fibers and whiskers [1] provides the possibilty to obtain composite materials of high mechanical strength. Besides structural strength chemical inertness at high temperatures in different environments is the main demand towards reinforcing additives. Metal oxide ceramics fulfill this requirement, making them one of the most prominent materials for fiber and whisker production. Endless fibers of various ceramics with diameters in the μm range can be spun while whiskers with smaller diameters and lenghts of several μm can be synthesized by a variety of methods. Al2O3 whiskers were produced by vapor-phase reactions [2] and vapor-liquidsolid deposition [3]. Mixtures of Al2O3 and B2O3 form alumina borate whiskers at high temperatures [4, 5]. Alumina borate needles can also be obtained in a potassium sulphide flux from aluminum salts and boric acid at 1100 ◦C [6]. Here we report a simple and efficient method to produce alumina borate filaments by a vapor solid reaction between fumed alumina and boron trifluoride, BF3. The reaction was carried out in a resistively heated flow reactor. In the center of the quartz flow tube (inner diameter 22 mm) a quartz boat with typically 100 mg fumed alumina (Degussa) was placed. The system was evacuated to 10−5 Torr to remove air and checked for leaks. The temperature of the furnace was ramped to 1200 ◦C before a flow of 5 sccm BF3 at a pressure of 50 Torr was established. After 1 h the gas flow was stopped and the reactor evacuated before the system was allowed to cool down to room temperature. The reaction product was collected from the quartz boat and weighed. Optical characterization of the material was performed by scanning electron and transmission electron microscopy (SEM/TEM). Energy dispersive X-ray fluorescence (EDX) spectra were measured in order to determine the elemental composition of the product. The fumed alumina powder which was used as the solid reactant is shown in the inset in the upper part of Fig. 1. It consists of ultrafine particles with diameters around 17 nm. The appearance of the fumed alumina is not altered by the heating process in vacuum or when it is exposed to N2 or H2 at 1200 ◦C. The reaction with BF3 at 1200 ◦C and 50 Torr, however, results in a complete transformation of the alumina powder into filamenteous material. The filaments which are displayed in Fig. 1 have diameters around 200–300 nm and are