Beam-Solid Interactions of Pyrolytic Graphite as a Function of Angle of Cut Surface
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BEAM-SOLID INTERACTIONS OF PYROLYTIC GRAPHITE AS A FUNCTION OF ANGLE OF CUT SURFACE PEARL W. YIP, DONALD J. JAKLITSCH, and JANET S. PERKINS Army Materials Technology Laboratory, Watertown, MA 02172-0001 ABSTRACT A study to examine beam-solid interaction as a function of the cut surfaces of pyrolytic graphite samples (0, 30, 60, and 90 degrees relative to the basal plane) was made using a high intensity, carbon dioxide laser beam operating at a wavelength of 10.6 pm. Crystal growth of redeposited carbon and impurities in the burn area as well as a melt on the burn edge were observed and examined using Scanning Electron (SEM) and Scanning Auger Microscopy (SAM). Similarities and differences relating to the structure of the material were further investigated using X-ray diffraction. Thermal transport properties as a function of angle were also examined. INTRODUCTION The development of high power laser beams has provided a clean definitive method for studying the effect of high radiant energies on non-isotropic and refractory materials. Studies of such behavior may reveal unusual material responses and suggest methods for modifying properties. Such a study has been initiated by the authors. Pyrolytic graphite (PG) was selected for this study because of its high temperature resistance, the large anisotropy in the electrical and thermal properties through the a- and c- crystallographic faces [1] (Fig. 1). Also such information may be applicable to other forms of carbon, e.g., low and high-modulus carbon fibers and highly oriented pyrolytic graphite (HOPG) which is often used for intercalation studies [2]. EXPERIMENTAL
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A 2-second pulse from a high intensity (8.2 kW/cm ) CO 2 laser beam was used to irradiate one surface of 1/8-inch thick PG samples. Samples were cut to obtain orientations of 0 , 300, 600, and 90 relative to the basal plane. A reason for use of a "wind tunnel" (0.1 Mach nitrogen gas flow) was to sweep volatile, smoke- and mist-forming secondary products out of the path of the laser beam to prevent attenuation or scattering of the beam. The irradiated ("burn") areas were compared with virgin samples and with each other using several diagnostic techniques. These comparisions reveal the characteristics of high energy laser beam interactions with anisotropic graphite with various cut-surfaces exposed. k pulsed ruby laser, operating at 694 nm with an intensity of 17.1 kW/cm and a pulse duration of 0.9 ns, was used to measured the thermal diffusivity of the samples [3]. From these data, the thermal conductivities of the PG samples at the various orientation were obtained. BEAM
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Fig. 1 a-face
A schematic drawing of a pyrolytic
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Mat. Res. Soc. Symp. Proc. Vol. 74. 11987 Materials Research Society
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RESULTS AND DISCUSSION a) X-ray Diffraction Analyses X-ray diffraction analyses, Fig. 2a, of the virgin material show that these pyrolytic graphite samples are highly oriented. For the surface along the basal plane (0 ), only the (00t) peaks are observed. Indeed, the (002) p
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