SiC-Based Black-Body Absorbers for Submillimeter Wavelengths

NASA's Jet Propulsion Laboratory, Pasadena, California

Open-cell silicon carbide foam coated with a ferrite-filled absorbing resin has been found to be useful for black-body absorbers at millimeter and submillimeter wavelengths. The panels can be used as calibration targets for radiometers that operate at these wavelengths.

Heretofore, black-body calibration targets have been made from lightweight carbon-coated polyurethane foams or from heavy ferrite materials with machined or molded, finely pointed, periodic surface features (e.g., arrays of cones or pyramids) (see Figure 1). These black-body loads have either excessive weight, deteriorate (crumble) over time in vacuum, or have poor thermal conductivity.

Figure 1. A Panel of SiC Foam Coated With Ferrite Resin is shown at the same magnification with three panels of older competing designs. Tests at frequencies from about 120 to 640 GHz showed that the SiC-foam/ferrite-resin panel performed comparably to the panel with the array of pyramids, which cost much more.

The new loads are SiC-based, can be fabricated easily, are lighter in weight than solid ferrite absorbers, do not outgas excessively, and exhibit sufficient thermal conductivity for the intended purposes. Moreover, the silicon carbide base material withstands high operating temperatures.

The absorber works by effecting random scattering and surface impedance matching with absorption of power to produce a low return loss to incident radiation over a broad range of wavelengths. The open-cell foam structure is responsible for the scattering and impedance-matching properties. The ferrite coating increases the absorption coefficient without significantly changing the scattering and impedance-matching properties.

Figure 2. Two Coated SiC Panels are mounted in a wedge configuration to make incident radio-frequency energy undergo multiple reflections to enhance absorption. The panels measure 9 x 9 and 9 x 12 in. (229 x 229 and 229 x 305 mm), respectively. The wedge can accommodate a fairly wide beam but the thickness of the panels [0.5 in. (13 mm)] places the lower limit of operating frequency at about 100 GHz. The slots on the sides enable the adjustment of the dihedral angle between about 20° and 45° to obtain the best performance for a given beam diameter.

The combination of roughness needed for random scattering and the impedance match to the radio-frequency field is optimized by choice of the foam cell size, which should be of the order of half the wavelength at the frequency of interest. SiC foam can be fabricated with foam cell sizes ranging from a few millimeters down to less than a tenth of a millimeter, corresponding to a frequency range from below 100 GHz to 1 THz or more.

The absorber is fabricated by pouring the castable ferrite resin over the open-cell SiC foam sheet and baking the sheet to cure the epoxy resin. Slabs can then be assembled in wedge or pyramidal geometries to enhance absorption (see Figure 2).

This work was done by Peter Siegel of Caltech and Robert Tuffias of Ultranet Corp. for NASA's Jet Propulsion Laboratory. NPO-20401