Journal of Materials Science

, Volume 26, Issue 5, pp 1401–1408 | Cite as

Oxidation sizing of iron and iron-neodymium-boron powders

  • M. Stewart
  • B. Roebuck
  • M. G. Gee


A powder sizing test developed for use on WC powders has been extended for use on iron and iron-neodymium-boron powders. In this test the particle size is derived from the rate of oxidation, because finer powders oxidize quicker. The rate of oxidation is monitored in a thermogravimetric analyser, where the powders are subjected to a controlled heating rate from room temperature to 1100 °C. If the constants from the Arrhenius law are known the powder size can be determined by comparing experimental oxidation plots with theoretical curves. For the sizing of a commercial spherical iron powder, the oxidation technique compared favourably with direct sizing using scanning electron microscopy and image analysis. The values for the activation energy of 125 kJ mol−1 determined in this study agree with previous studies. Validation of the sizing technique on a hydrogen-decrepitated stoichiometric Nd2Fe14B powder proved difficult because it was not possible to determine a definitive size distribution independently. Metallography of partially oxidized samples showed that the process is two-stage, at low temperatures the neodymium oxidizes, and above 400 °C the powder behaves as pure iron. Theoretical curves based on one oxidation process with an activation energy of 100 kJ mol−1 gave the best fit to the experimental curves.


Iron Activation Energy Neodymium Pure Iron Iron Powder 
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Copyright information

© Chapman and Hall Ltd. 1991

Authors and Affiliations

  • M. Stewart
    • 1
  • B. Roebuck
    • 1
  • M. G. Gee
    • 1
  1. 1.Division of Materials MetrologyNational Physical LaboratoryTeddingtonUK

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