Tailoring the microwave permittivity and permeability of composite materials |
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Authors: | Kenneth Bober Robert H Giles and Jerry Waldman |
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Institution: | (1) Submillimeter-Wave Technology Laboratory, University of Massachusetts, 01854 Lowell, Massachusetts |
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Abstract: | The microwave permittivity (ɛr) and permeability (μr) of composite materials are tailored by adding various loading agents to a host plastic and are subsequently modeled using
the Maxwell Garnett theory and second order polynomials. With the addition of manganese zinc ferrite, strontium ferrite, nickel
zinc ferrite, barium tetratitanate and graphite powders, materials with values of ɛ′, e″, μ′, μ″ as high as 22, 5, 2.5 and
1.7 have been obtained. Permittivity and permeability data are calculated at 2.0245 GHz from reflection and transmission measurements
performed in a 7 mm coaxial test line. The Maxwell Garnett (MG) theory successfully models ɛr if the filling factor is less than 0.30 and ratio |ɛ1| (host)/ |ɛ2| (powder) is greater than 0.04. As this ratio decreases, the MG theory is shown to be independent of ɛ2 and second order polynomials are used to effectively model the dielectric constant. Polynomials are also used for the ferrite
composites because it was determined that the MG theory was unable to model μr. This deficiency is attributed to the difference of domain structures that exist in powdered and sintered ferrites. |
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