Preparation Technology And Mechanism Of Tungsten Nano-powders By Carbon Assisting Hydrogen Reduction

There are many new processes of preparing tungsten nano-powders, however, hydrogen reduction is still the main process of preparing tungsten nano-powders in industry. The volatility of tungsten oxide is accelerated by water vapor which is produced during the process of preparing tungsten nano-powders by hydrogen reduction, which is the fundamental reason of rapid and abnormal growth of tungsten powders. In order to prepare the tungsten nano-powders, the reaction of carbon and water vapor was used to reduce the effect of water vapor, the tungsten oxide volatilization inhibitor was used to inhibit the volatility of tungsten oxide.Tungsten nano-powders were prepared by carbon assisting hydrogen reduction with AMT and glucose as raw materials. The reduction products, the mean particle size and residual carbon content of tungsten nano-powder depend on reduction temperature, reduction time and the tungsten carbon molar ratio was investigated. It is shown that the reduction products depend on reduction temperature. When the reduction temperature is 700 oC, 750 oC and 800 oC, the reduction products are W+WO2, W+W2C+WC and W, respectively. The mean particle size of tungsten powders decreases as the tungsten carbon molar ration increases, the residual carbon content of tungsten powders increases as the tungsten carbon molar ration increases, the optimum molar ration is 1:2.6. Tungsten nano-powders without obvious agglomeration were prepared by the carbon assisting hydrogen reduction of the precursor with tungsten carbon molar ratio of 1:2.6 at 800 oC for 1 h. The mean size and residual carbon content of tungsten nano-powders are 46 nm and 0.05 wt%, respectively. The relationship between reduction time and mean particle size is 2/10 ktdd +=, in which, d0 is 41.263, k is 0.529, the grain growth of tungsten powder was inhibited by carbon addition, the dominant growth mechanism of tungsten powder transforms from volatility-sedimentation mechanism combines with atomic diffusion mechanism to atomic diffusion mechanism.The effect of adding tungsten oxide powder grain growth inhibitors on the mean particle size of tungsten nano-powders was investigated. It is shown that adding V2O5 and Cr2O3 are helpful to inhibit the growth of tungsten nano-powder during reduction. When the content of inhibitors increases from 0 to 0.9%, the mean particle size of tungsten powders which is prepared by the mixure with tungsten carbon molar rate of 1:2.6 by hydrogen reduction decreases from 46 nm to 41 nm, there is no apparent effect of the content of addition on mean particle size of tungsten nano-powders while adding addition continuously. The growth mechanism of tungsten powder also is atomic diffusion mechanism, when the content of addition increases from 0 to 1.2%, the nucleation size of tungsten powders decreases from 43.73 to 39.02, there is no apparent effect of the content of addition on particle growth factor.