Particle size characterization of ultrafine tungsten powder

Abstract

The aim of this research was to investigate the particle size measurement of as-supplied and lab-milled ultrafine tungsten powders. The ultrafine tungsten powder derived from X-ray diffraction pure WO 3 by hydrogen reduction process under “dry” reduction conditions was milled for 1, 2 and 4 h, respectively. Then the as-supplied and lab-milled tungsten powders were used to perform particle size measurement by laser diffraction method and Fisher sub-sieve sizer (FSSS) method and BET method through adsorption isotherms. Results show that the mean particle size values of as-supplied and lab-milled powders measured by laser diffraction method and FSSS method were misleading because of the defects of the measurement system or unsuitability of measurement theory. And the calculated particle size of d BET values from the formula 6/(S BET density) were smaller than that of by SEM method, which was resulted from the neglect of the effect of surface roughness and pore area on d BET calculation. The fractal dimension D of surface roughness and external surface area S t of pores were obtained from adsorption isotherms, which were used to modify d BET calculation formula. The modified formula was d BET( M) =k (D/2)×6/(S BET density ), where coefficient k was a constant, it was a function of pores area, D was fractal dimension value of surface roughness. As to the ultrafine tungsten powders in this investigation, k was defined as S BET/ S t, so modified d BET(M) may be expressed as 3 D/( S t density ). By the modified formula, the calculated d BET(M) values of four powders in this study were in good agreement with the SEM sizes.