1, SURVICE Engineering, Belcamp, Maryland, United States
2, U.S. Army Research Laboratory, APG, Aberdeen, Maryland, United States
Microwave sintering has been successfully employed to rapidly sinter ceramics, metals, and ceramic-metal composites (cermets) at reduced temperatures, yielding dense materials with fine grain structures. However, one of the greatest challenges with microwave sintering is that the material needs to have suitable dielectric and magnetic characteristics such that it couples with the microwaves and heats the material effectively.
Most commercial microwave sintering techniques employ external susceptors, or thermal sources, to heat the sample to the targeted temperature. Alternatively, internal microwave susceptors can aid in sintering ceramic materials. This can be accomplished by combining microwave-transparent materials with microwave-susceptible materials. For this reason, SiC-B4C composites are being investigated (as SiC is microwave-susceptible and B4C is microwave-transparent). The light weight, high strength, high corrosion resistance, and high temperature stability of SiC-B4C make it suitable for potential armor applications. In addition, desirable thermoelectric properties (i.e. high electrical conductivity, low thermal conductivity, high Seebeck coefficient, etc.) make them strong candidates for thermoelectric applications.
The incorporation of carbon precursors to SiC-B4C composites has the potential to further enhance thermoelectric properties while also serving as an additive to improve sinterability. Therefore, microwave sintering has been performed using SiC, B4C, and carbon black as starting materials. Powders of 85% SiC, 10% B4C and 5% carbon black were ball milled to obtain homogenous mixtures, which were then pressed into pellets and cold isostatically pressed. In order to determine the optimum sintering conditions, samples were processed in a 2.45 GHz single mode microwave cavity using various magnetic/electric field ratios. In this study, microwave sintered composites will be compared to conventionally sintered composites using scanning electron microscopy, x-ray diffraction, and micro-hardness measurements. In the future, this technique could be applied to a variety of applications, such as thermoelectric energy storage ceramics, nanocomposites, and multi-functional material systems.