Microwave Absorption and Heating in Copper Powder-metal Compacts

Open Access
Feather, Kelly Renee
Graduate Program:
Materials Science and Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 01, 2010
Committee Members:
  • Darin Zimmerman, Thesis Advisor
  • composite materials
  • complex permeability
  • Microwave heating
  • metals
  • copper
  • sintering
  • complex permittivity
A decade ago, Roy and co-workers demonstrated that powder-metal compacts may be heated in both electric and magnetic microwave fields. However, what remained unclear was an explanation of the origin of the heating trends, especially with regard to the direct interaction of microwaves with powder-metal compacts. To address this question, we have systematically studied the absorption and dissipation of microwave energy in porous, copper-powder-metal compacts. This study was accomplished through two main experimental thrusts: first, by characterizing heating behavior of the compacts as a function of microwave field (electric vs. magnetic), sample density, particle size, particle separation (oxide layer), and by observing the resulting microstructure evolution due to heating. For the heating experiments, we employed a single-mode, 2.45 GHz microwave system in which a sample could be placed in a nearly pure electric- (E) or magnetic- (H) field with the sample surface temperature measured by infrared pyrometry. Second, we observed the trends in the effective conductivity, permittivity, and permeability of the compacts as a function of exposure-time to the separate E- and H-fields. By measuring the change in the quality factor and resonant frequency of a TM010 resonant microwave cavity with and without the samples, we determined the real and imaginary components of the permittivity and permeability by cavity perturbation theory. Our studies confirm or establish three key results: at 2.45 GHz, magnetic dissipation is favored over electric in powder-metal compacts made with particle-sizes greater than the skin depth; magnetic dissipation is strongly dependent on particle-size, peaking near 6 microns; and an intrinsic oxide layer or other inter-particle separation is necessary for microwave dissipation to occur.