Principal Investigator	Darin Zimmerman
Co-principal investigators	Nicholas Miskovsky
	JunKun Ma
Senior Associates	Gary Weisel
	Paul Cutler
Undergraduate researchers	Kelly Feather, Charles Smith,
	John Diehl, Chris Lynch, Kyle Cravener, Jeremy Cardellino

Funding

NSF (Grant RUI-DMR 0406584)	$300K
Penn State University	$74K
Period	July, 2004 - June, 2008

The PSA Microwave Sintering Group has used the above setup to study the microwave heating of various powdered metal samples (pressed into small cylindrical pellets).   The magnetron is on the right side and the TE102 (or TE103) resonant cavity is on the left.  A quartz sample tube is inserted in the cavity and has a pyrometer aimed along it.  After heating, we view the surfaces and interiors of our (now partially sintered) samples using scanning electron microscopy.

In addition to these heating and SEM investigations, we are using a second resonant cavity (pictured above) and a network analyzer to measure the complex permittivities and permeabilities of various powdered samples. We are also modeling the electromagnetics of our cavities and the heating of our samples using the multiphysics package created by Comsol.

Project Summary This project has proceeded along two main lines of inquiry.  During the first two years of work, we investigated the heating and sintering of powdered metal compacts by microwave radiation.  A few years earlier, experiments by Roy and co-workers at Penn State UP had demonstrated that metal-powder compacts are heated in both electric and magnetic microwave fields.  We confirmed these results in early work at Penn State Altoona.  Our research then turned to the question of how metal-powder compacts are heated so efficiently by microwave radiation.  We addressed the question in three ways: doing an empirical study of the complex permittivity and permeability of powder metal compacts as a function of temperature, particle size, particle oxidation, and packing density using resonant-cavity techniques; identifying the separate contributions of the E and H fields to the heating and subsequent sintering of powder metal compacts using a single-mode microwave system; evaluating the first stages of sintering by direct examination of heated samples using scanning and transmission electron microscopy. Our first published paper (Ma, 2007) demonstrated that the absorption of 2.45 GHz microwaves by pure copper powder compacts depends strongly on particle size and relies on the particle separation (which in green samples is provided by the native oxide layer).  We also used relatively straightforward analytic calculations to show that the initial heating rate of a copper powder metal compact can be explained if the sample is viewed as a collection of single-particle absorbers.  Of course any ensuing heating leads the individual absorbers to coalesce and the interior of the sample is screened from the microwave field, effectively quenching the heating.  When using pure-metal powder samples, this transition happens so quickly that it is difficult to study in any detail. During our second period of work, begun about a year ago, we decided to investigate the transition from single-particle behavior to bulk behavior by fabricating powder metal-insulator composites of varying metal volume fractions.  Our second article (Zimmerman, 2008) presents our first survey of the percolation behavior of metal-insulator composites, in terms of the DC conductivity (measured with standard techniques) and the permittivity and permeability (measured with a single-mode microwave cavity operating at 2.45 GHz).  In addition to varying the average particle size of the metal powders, we also varied the metal volume fraction, which effectively varied the average particle and cluster separation.  Our observations of the electromagnetic properties as a function of volume fraction were consistent with the predictions of generalized effective medium theory.  They also characterized the small differences in percolation behavior between the conductivity, the permittivity and the permeability. Our future work will seek to extend our study of percolation in metal-insulator composites by strengthening our characterization capabilities and by deepening our understanding of electromagnetic transport.  We will build on earlier work by ourselves and others to address a central question: How do the various transport processes (electron tunneling, hopping, magnetic dipole interaction, and so on) contribute to the nature of the percolation transition?  We adopt a two-pronged approach.  First, we will do an empirical study of the DC conductivity and microwave permittivity and permeability as a function of metal volume fraction.  We also will investigate how these results vary with particle size and shape and sample temperature.  Second, we will compare our empirical results to the results of generalized effective medium theory and to those of recent transport theory.  One of our main objectives is to characterize the universality or non-universality of the percolation behavior in these systems.  Our consideration of elastomer composites will provide a handle on electromagnetic transport by showing how it is modified by mechanical stresses.  Although our work focuses on the physics of electromagnetic transport, we also will seek to identify and explore applications of these composite materials for use within various electrical and mechanical devices, including chemical sensors, temperature sensors, pressure sensors, magnetic switches, and vibration/shock dampeners.

Papers J. Ma, J.F. Diehl, E.J. Johnson, K.R. Martin, N.M. Miskovsky, C.T. Smith, G.J. Weisel, B.L. Weiss, and D.T. Zimmerman, “Systematic Study of Microwave Absorption, Heating, and Microstructure Evolution of Porous Copper Powder Metal Compacts,” Journal of Applied Physics, 101 (2007) 074906. D.T. Zimmerman, J. D. Cardellino, K. T. Cravener, K. R. Feather, N. M. Miskovsky, and G. J. Weisel, "Microwave Absorption in Percolating Metal-insulator Composites," Applied Physics Letters 93 (2008) 214103. Talks J. Ma, C.T. Smith, G.J. Weisel, B.L. Weiss, N.M. Miskovsky, D.T. Zimmerman, “Single Mode Microwave Heating of Copper Powder Metal Compacts, COMSOL Users Conference 2006, October 22–24, Cambridge, MA. D.T. Zimmerman, E.J. Johnson, J. Ma, K.R. Martin, N.M. Miskovsky, C.T. Smith, G.J. Weisel, and B.L. Weiss, “Microwave Heating and Pre-sintering of Copper Powder Metal Compacts in Separated Electric and Magnetic Fields,” PM2006: World Congress on Powder Metallurgy, September 24–28, 2006 Busan, South Korea.  D.T. Zimmerman, J. Diehl, E.J. Johnson, K.R. Martin, N.M. Miskovsky, C.T. Smith, G.J. Weisel, B.L. Weiss, "Systematic Study of Microwave Absorption, Heating, and Microstructure Evolution of Porous Copper Powder Metal Compacts," 2008 March Meeting of the American Physical Society, March 10–14, New Orleans, Louisiana. Posters C.M. Lynch, E.J. Johnson, J. Ma, N.M. Miskovsky, G.J. Weisel, B.L. Weiss, and D.T. Zimmerman, “Complex Permittivity of Powder Metal Compacts by Cavity Perturbation Technique,” presented at the 2006 March Meeting of the American Physical Society, March 13–17, Baltimore, MD. K.R. Martin, E.J. Johnson, J. Ma, N.M. Miskovsky, C.T. Smith, G.J. Weisel, B.L. Weiss, and D.T. Zimmerman, “Microwave Heating and Pre-sintering of Copper Powder Metal Compacts in Separated Electric and Magnetic Fields,” presented at the 2006 March Meeting of the American Physical Society, March 13–17, Baltimore, MD. K.R. Martin, J. Cardellino, E.J. Johnson, N.M. Miskovsky, G.J. Weisel, D.T. Zimmerman, J. Ma, "Percolation Studies of Metal-insulator Composites at Microwave Frequencies," presented at the 2008 March Meeting of the American Physical Society, March 10–14, New Orleans, Louisiana.

Darin Zimmermans homepage
Gary Weisel's homepage	Materials Research Institute

Copyright © 2009 Penn State Altoona; Physics Gary J. Weisel, Associate Professor of Physics 128A Smith Building, 3000 Ivyside Park, Altoona, PA 16601 Phone: (814) 949-949-5175; E-mail: GXW20@psu.edu

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