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 \xb0C

Brad W Hoff,Martin S Hilario,Anthony E Baros,Rachael O Grudt,Steven C Hayden,Michele L Ostraat,Ian M Rittersdorf,Frederick W Dynys

doi:10.1557/jmr.2019.177

Abstract

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.

Highlights

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
To enable the multi-physics modeling [7] necessary for design and optimization of the ceramic susceptor tiles to be used in a millimeter-wave powered heat exchanger, key electromagnetic, thermal, and mechanical properties of the susceptor materials must be known at elevated temperatures
Physisorption measurements were performed at À196 °C on a Micromeritics ASAP 2460 (Micromeritics Instruments, Norcross, Georgia), and the Brunauer–Emmett– Teller (BET) method was used to determine the apparent surface areas

Summary

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. It is noted that carbon, a common sintering additive [15, 19] present in the AlN matrix at a loading fraction of 0.5 vol%, was found to react with Mo to form Mo2C in concentration thresholds of approximately 0.5 vol% and independent of increasing Mo concentration [10]. An elemental gradient was observed at the edges of the grain at the interface between Mo and AlN, resolved as discrete regions of progressively decreased molybdenum content and progressively increased AlN content from the aggregate boundary (Fig. 6, yellow to purple to cyan) These finite interfacial zones suggest intermixing of the two phases and will constitute the subject of a more intensive investigation by the authors via transmission electron microscopy. Complex permittivity measurements of the AlN:Mo ceramic composites were performed at 95 GHz and at temperatures ranging from 25 to 550 °C using the free-space technique. For the 0.25 vol%, 0.5 vol%, 1.0 vol%, 2.0 vol%, 3.0 vol%, and 4.0 vol% Mo samples, respectively

Summary and discussion

Methodology