Deposition of carbon nitride thin films on Si(100) and polycrystalline Zr substrates was performed by dc magnetron sputtering of a graphite target under a pure nitrogen ambient. The resulting carbon nitride films (CNx) are primarily amorphous with a small volume fraction of nanocrystallites. Both energy-dispersive x-ray analysis and electron energy loss spectroscopy measurements showed that the amorphous matrix has uniform nitrogen-to-carbon ratios ∼0.4–0.8 depending on deposition conditions. Carbon and nitrogen K edge structures obtained from electron energy loss spectroscopy studies suggest that the amorphous carbon nitride matrix is primarily sp2 bonded. Apart from the carbon–nitrogen stretching vibration, Raman spectra of CNx films closely resemble those of diamondlike carbon films. Intensity and peak width changes of Raman features in the 1300–1600 cm−1 range due to inorganic carbon (D and G peak) under different deposition conditions are explained in terms of the extent of structural disorder in these films.
Light weight materials such as foams can be used in thermal protection and/or thermal insulation systems. At high temperature (>1000 K), thermal radiation may be important or dominate heat transfer in the foam; however, studies based on more detailed thermal radiation analysis are limited. In this study, foams are considered to be semitransparent, because radiation can penetrate through the pore (or void) space and/or foam skeleton (solid matrix), depending on the materials from which the foams are made. Of particular interest in this study is to understand how the properties such as foam material, porosity, pore size, etc. affect thermal and radiant energy transfer. Physical and mathematical models are developed to account for conduction and radiation (absorption, emission and scattering) in the porous material. The spectral extinction coefficients of SiC foams are measured experimentally in the laboratory at room temperature, and the radiative transfer equation is solved using the spherical harmonics P1‐approximation. The predicted radiative heat fluxes in foams are compared with those based on the Rosseland diffusion approximation. Parametric calculations have been carried out, and the results are reported in the article for a range of parameters characterizing heat transfer in SiC foams of different porosities to identify desirable conditions for effectively reducing heat transfer in potential thermal protection concepts.
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