Abstract
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.
Highlights
Since the first demonstration of sintering a metal powder body by microwave radiation [1], efforts to understand the energy absorption of conducting metal powders has been ongoing
For any practical size of particles which are considered to be metallic on the basis of a high value of electrical conductivity, magnetic absorption via eddy current loss is much greater than loss in an equivalent electric field and this has been demonstrated experimentally [5,6]
It is conceivable to construct a cheap and simple sensor based on microwave perturbation in order to assess evolution of particle size of a powder within an industrial process or to assess a powder with the intention of microwave heating
Summary
Since the first demonstration of sintering a metal powder body by microwave radiation [1], efforts to understand the energy absorption of conducting metal powders has been ongoing. The theoretical basis for electromagnetic absorption in conducting metal powders is well established. A variety of studies have taken a first principles approach to the absorption of an individual particle within both electric and magnetic fields [2,3,4]. For any practical size of particles which are considered to be metallic on the basis of a high value of electrical conductivity, magnetic absorption via eddy current loss is much greater than loss in an equivalent electric field and this has been demonstrated experimentally [5,6]
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