A COMPARATIVE STUDY OF METHODS FOR PARTICLE-SIZE ANALYSIS IN THE SUB-SIEVE RANGE *

Abstract

Eleven powders of widely different properties (various grades of WC, WO₃, tungsten, iron, cobalt, SiO₂, Al₂O₃), considered typical of the raw materials encountered in powder metallurgy, have been studied by “single value methods” (apparent and tap density, air permeametry, gas adsorption (BET)) and by “size-distribution methods” (electron- and light microscope sizing, turbidimetry, sedimentation balance (with gaseous and with liquid medium), centrifugal air classification, and sieve analysis). In addition, a series of WC powders having varying degrees of fineness has been studied by air permeametry, gas adsorption, optical reflectivity, turbidimetry, and sedimentation in air, to assess the reliable working ranges of these methods.

The following topics are treated in detail: air-permeability measurements and their relation to the results of BET and other tests; electron and light-microscope sizing techniques; turbidimetry; and optical reflectivity. A microscopic study of the breakdown of agglomerates in a tungsten carbide powder is reported. The importance of large agglomerates in all work concerned with particle-size distributions is stressed. For highly agglomerated powders the size distribution of the ultimate units (individual particles within agglomerates) is difficult or impossible to determine, but it is pointed out that this “true” distribution has much less technical significance than that of the agglomerated powder, provided that the degree of agglomeration in which the powder is analysed corresponds to, or can be correlated with, that prevailing in the process for which it is intended.

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