Characterization of AlN-based ceramic composites for use as millimeter-wave susceptor materials at high temperature: Die
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Characterization of AlN-based ceramic composites for use as millimeter-wave susceptor materials at high temperature: Dielectric properties of AlN:Mo with 0.25 vol% to 4.0 vol% Mo from 25 to 550 °C Brad W. Hoff1,a)b) , Steven C. Hayden2,b), Martin S. Hilario1, Rachael O. Grudt2, Frederick W. Dynys3, Anthony E. Baros1, Ian M. Rittersdorf4, Michele L. Ostraat2 1
Air Force Research Laboratory, Directed Energy Directorate, Kirtland AFB, New Mexico 87117, USA Aramco Research Center – Boston, Aramco Services Company, Cambridge, Massachusetts 02139, USA 3 NASA Glenn Research Center, Materials and Structures Division, Cleveland, Ohio 44135, USA 4 Naval Research Laboratory, Plasma Physics Division, Washington, District of Columbia 20375, USA a) Address all correspondence to this author. e-mail: [email protected] b) These authors contributed equally to this work. This paper has been selected as an Invited Feature Paper. 2
Received: 18 February 2019; accepted: 2 May 2019
Microstructural analysis and bulk dielectric property analysis (real and imaginary permittivity at 95 GHz) were performed at temperatures ranging from 25 to 550 °C for ceramic composites comprising a hot-pressed aluminum nitride matrix (containing yttria and trace carbon as sintering additives) with molybdenum powder as a millimeter-wave radiation-absorbing additive. Loading percentages in the range of 0.25 vol% to 4.0 vol% Mo were characterized. For the temperature regime evaluated, the temperature-related changes in real and imaginary components of permittivity were found to be relatively modest compared with those driven by Mo loading. Energy-dispersive X-ray spectroscopic analysis of Mo grains and surrounding regions showed the presence of a mixed-phase layer, containing Mo2C, at the AlN–Mo interface. The Mo2C-containing mixed-phase layer, typically a few micrometers thick, surrounded the Mo grains. Further characterization of this mixed-phase layer is required to determine its contribution to the dielectric properties of the composite.
Introduction In wireless power transfer systems using thermomechanical conversion schemes, such as those described in Ref. 1, hightemperature bulk susceptor materials are required to convert electromagnetic radiation to thermal power, which is, in turn, converted to electrical power. These susceptor materials take the form of composites formed from a high thermal conductivity ceramic matrix in combination with a radiationabsorbing additive, such as metal particles or lossy ceramic [2]. Here, we examine one such system comprising molybdenum particles supported in an aluminum nitride matrix. In systems such as those investigated in the present study, in which (i) the loading fraction of the radiation-absorbing additive is below the electrical percolation threshold and (ii) the bulk electrical conductivity of the matrix is negligible, dipole
relaxation is the predominant loss mechanism [3, 4]. When these systems are illuminated with electromagnetic radiation in the 1-GHz to 100-GHz regime, the primary polarization mecha
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