Parametric acoustic transmitting arrays : Muir, T.G., Willette, J.G. 52 (November 1972) 1481

Abstract

The correlation of structural anisotropy with normal spectral emissivity has been investigated experimentally and theoretically. Measurements include data on the two major crystallographic faces of pyrolytic graphite, pyrolytic boron nitride and single crystal beryllium. The results were obtained at temperatures near 800°C over a wavelength range from visible to near i.r. (0.60 to 11 μ) in both purified hydrogen and argon atmospheres. Pyrolytic graphite C-faces deposited at higher temperatures exhibit generally lower normal spectral emissivity (0.60 to 11 μ) corresponding to an increase in basal plane absorption as the deposition temperature increases. A further increase in basal plane absorption upon extreme annealing results in the lowest normal spectral emissivity of C-face pyrolytic graphite. This highly annealed C-face pyrolytic graphite exhibits a normal spectral emissivity dispersion which is close to the theoretically predicted curve of C-face single crystal graphite. Pyrolytic boron nitride in its lattice absorption region (λ ≳ 2.60 μ) exhibits a normal spectral emissivity of the A-face which is greater than that of the C-face. This corresponds to the greater absorption in the basal plane than in the prismatic plane. Single crystal beryllium in its free-carrier intraband absorption region (λ ≳ 2.50 μ) exhibits a normal spectral emissivity of the A-face, (1010), which is greater than that of the C-face, (0001). Thus, absorption is greater in the basal plane (E→ ⊥ c-axis) than in the prismatic plane (E→ ⊥ c-axis) corresponding to an inverse relationship of normal spectral emissivity to absorption in the intraband region.