| Morphological changes of intermediate tungstic oxide particles, prepared by reducing WO₃with hydrogen and a mixture of hydrogen and nitrogen over the temperature range from 800℃ to 1000℃, were studied using scanning electron microscope. Two different morphologies of W_(18)O_(49), bundles of needle crystals and short pole-type particles, were obtained at 800℃, whereas W_(18)O_(49), obtained at 900℃ and above, consisted of pale-type particles and fine whiskers. The growth of W_(18)O_(49) particles was faster in the length than the width with increasing reduction temperature. The size and morphologies of W_(18)O_(49) particles were greatly influenced by those of WO₃particles. Transformation of W_(18)O_(49) to WO₂and WO₂to α-W was accompanied by the rounding of W_(18)O_(49) particles and the recrystallization of α-W in spherical shape respectively. Extremely fine α-W powder was obtained by reduction of WO₃powder at 500℃ for 4 hrs in hydrogen followed by immediate reduction at 650℃ for 1 hr. |
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