Boron dusts produced by arc vaporization using boron rods

Elemental boron can be obtained by reducing boron oxide with Mg or by chemical vapor deposition (CVD) of the gas mixture of boron halide and hydrogen [1±3]. The solid boron appears in at least three crystal forms, a-rhombohedral, â-rhombohedral and tetragonal, which are based on icosahedron B12 unit. In addition to these modi®cations, there exists amorphous boron, which is also composed of a random arrangement of B12 icosahedrons [4±7]. The arc vaporization method is frequently used for producing the particles and ®lms of high-melting materials, such as carbon, W and Mo [8, 9], but it has not been applied to boron because it is an insulator at room temperature. This letter reports the experimental results on arc vaporization of boron using a pair of boron electrodes and is concerned with the conditions for generating an arc discharge and with the nature of the resultant materials. The boron electrodes used in this study were prepared by machining sintered boron produced by hot-pressing. The raw material for the boron was amorphous with a small amount of â-rhombohedral phase (Herman Stark y300 mesh 99% B), containing oxygen and Mg impurities, which was hotpressed at a temperature of 1850 8C under a pressure of 15 MPa in an atmosphere of argon, as reported elsewhere [10]. The sintered boron bodies had a strength suf®cient for subsequent handling, despite their low densities of around 1.0 g=cm. It is dif®cult to generate the arc discharge at room temperature because the electrical conductivity of boron is very low. The conductivity of the rectangular-shaped boron samples from the sintered body were measured, and Fig. 1 shows the electrical conductivity as a function of 1=T. The conductivity increased rapidly with the temperature reaching 0.1 ohmy1 cmy1 at 700 8C. This required that the boron electrodes be pre-heated to a temperature at least above 600 8C to make it possible to generate the discharge. Fig. 2 shows the apparatus used in this study. This consists of a quartz reaction tube for generating the arc discharge and an electric furnace for heating the tube. A pair of the boron electrodes, 7 mm in diameter, between two carbon joints that were connected to an a.c. electric source, were held along the axis of the quartz tube, which had an inner diameter of 25 mm. The sealed quartz tube was evacuated to remove air before admitting the desired gas ow and heated to a temperature of 600 8C. The electrodes were contacted manually with each other, which caused them to become red-hot by resistive self-heating. When the electrodes were separated to a distance of 3±5 mm, the arc discharge occurred with a violent emission of light, accompanied with the vaporization of boron. The experiments were performed at a voltage of 100 V and a current of about 10 A. It appeared that the occurrence and stability of the discharge depended on the magnitudes of the electric voltage and the current. Voltages below 70 V made it dif®cult to generate the discharge, while higher currents induced an excess heating of the electrodes, resulting in the softening of the boron. Crystallographic measurements were carried out using an MXP3VA-BR X-ray diffractometer (XRD, Mac Science) with a graphite monochromator.