Measurement of average particle size in metal powders by microwave cavity perturbation in the magnetic field

The magnetic absorption of metallic powders, particularly at microwave frequencies, is of great theoretical and practical interest and has been the subject of previous research examining the dependence of absorption on the ratio of the particle skin depth to radius. Here, the validity of the theoretical approach concerning the peak in the absorption spectrum is verified using a 3D simulation of a hexagonal, close-packed particle matrix. Clear experimental data is given for the real and imaginary parts of the magnetic permeability of metal alloy powders (Ti6Al4V), of varying size, obtained by using the cavity perturbation technique across three separate frequencies in the GHz range. The results are shown to be congruent with existing theory. Further verification of the absorption peak is given by the testing of the powder at lowered conductivity by elevating the temperature. The results demonstrate the applicability of the relatively simple microwave cavity perturbation approach to the determination of the average particle size in a metal powder when compared with other, more complex and time-consuming methods.