Thermal Analysis of Graphite and Silicon Carbide with Millimeter-Wave Radiometry

Millimeter-wave thermal analysis instrumentation is being developed for characterization of high temperature materials required for diverse fuel and structural needs in extreme high temperature reactor environments. A two-receiver 137 GHz system with orthogonal polarizations for anisotropic properties resolution has been implemented at MIT and is being tested with graphite and silicon carbide specimens at temperatures up to 1300 oC. Real time measurement sensitivity to submillimeter surface displacement and simulated anisotropic surface emissivity is demonstrated. INTRODUCTION The development of Generation IV (Gen IV) very high temperature reactor (VHTR) technology depends on the development and characterization of high temperature materials that can reliably meet the diverse fuel and structural requirements in extreme VHTR environments. New thermal analysis tools are needed as well as the compilation of data bases on the characteristics of old and new materials in various previously untested high temperature situations. Thermal analytical tools that could be used in the laboratory and eventually applied in situ for reactor monitoring would be of particular value. Millimeter-wave (MMW) thermal analysis methods and technologies can meet these needs and address the short comings of present material diagnostic technology due to anisotropic and small sample size issues. MMW radiation refers to electromagnetic wavelengths in the 0.1 – 10 mm range that are shorter than microwaves but longer than infrared radiation. At these wavelengths propagation is possible through infrared/optically opaque pathways and materials, but with spatial resolution much finer than with microwaves. Past work has applied MMW techniques to nuclear waste glass melter measurements [1], water-ice freezing dynamics [2], and precision high-temperature superconductor resistivity measurements [3]. In this paper, we extend the technology to thermal analysis of graphite and silicon carbide materials in high temperature environments relevant to the Next Generation Nuclear Plant (NGNP). EXPERIMENTAL The amplitude, phase, and polarization of MMW thermal emission and probe beam reflection are used to provide information on the thermal characteristics of the materials under observation. The use of two orthogonally polarized receivers makes possible the study of anisotropic features. A schematic illustration of the laboratory implementation of the MMW thermal analysis hardware is shown in Figure 1. The MMW receivers are heterodyne receivers that operate at a frequency of 137 ± 2 GHz (λ = 2.19 mm). Receiver 2 is polarized perpendicular to Receiver 1. Receiver 1 views the electric field direction at the test specimen that is in the plane of Figure 1 and Receiver 2 views the electric field direction that is perpendicular to the plane of Figure 1. In addition to detecting the test specimen thermal emission heterodyne receivers also leak a local oscillator (LO) signal that is used as a coherent probe beam. MMW Receiver Waveguide