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.
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.
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
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SiC-based black-body absorbers for submillimeter wavelengths
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Overview
The document discusses advancements in the development of SiC-based black-body absorbers specifically designed for millimeter and submillimeter wavelengths. Authored by Peter H. Siegel and Robert Tuffias, the report outlines a novel approach to creating efficient calibration targets for radiometers used in these frequency ranges.
The core innovation involves the use of open-cell silicon carbide (SiC) foam, which is coated with a ferrite-filled absorbing resin. This combination results in a lightweight, durable structure that exhibits high thermal conductivity and an enhanced absorption coefficient. The design aims to achieve low return loss to incident radiation, making it suitable for a broad spectrum of wavelengths.
The document highlights two primary techniques for achieving the desired dielectric match to incident RF fields: machining or casting the surface into finely pointed periodic structures, and molding or coating the material on large aperture structures with gradual geometric tapering. These methods ensure that the absorbers can effectively handle RF energy, allowing for multiple bounces before the energy can escape or scatter.
The proposed black-body load is easy to fabricate, requiring only the specially prepared SiC foam and the castable ferrite resin. The flexibility in the cell size of the foam allows for tailoring the frequency range of operation, covering from below 100 GHz to at least 1 THz and potentially higher. This adaptability makes the absorbers applicable not only in high-frequency scenarios but also in various other applications, provided the base material is shaped appropriately.
The report emphasizes that these new absorbers perform comparably to more complex and expensive periodic absorbing surfaces, making them a cost-effective solution for calibration targets. The simplicity of the design and fabrication process is a significant advantage, potentially leading to broader adoption in both research and practical applications.
In summary, the document presents a significant technological advancement in the field of RF absorption, showcasing a simple yet effective solution for creating black-body calibration targets that meet the demanding requirements of millimeter and submillimeter wave applications. The work was conducted at the Jet Propulsion Laboratory under NASA's contract, reflecting its relevance to aeronautics and space exploration.
