Interdiffusion of rhenium and poco graphite

Graphite is being considered as the prime candidate as the energy storage material for space propulsion. One of the anticipated problems is the loss of graphite at high temperature. Rhenium has been proposed as a diffusion barrier for graphite to prevent excessive loss of the material. However, the mutual diffusion of graphite and rhenium has not been measured, and much of the data is based on extrapolation with high degree of error. A technique to study the interdiffusion of rhenium and carbon without a marker was developed based on Rutherford Backscattering Spectroscopy (RBS) technique. The graphite samples used in this experiment were Poco graphite purchased from Unocal. The specimens used for rhenium deposition were in the form of round discs cut from a 25.4 mm diameter bar. The round discs were mechanically polished down to a thickness of 2 mm. A thin layer of rhenium was then sputter-deposited onto the polished Poco graphite surface using a RF sputtering system. The deposition rate was determined to be 10 nm/min at a RF power of 20 Watts and an Ar gas pressure of 2.67 Pa. The samples coated with rhenium were given different diffusion anneals at temperatures between 1373 K and 2273 K in a vacuum electron beam furnace. The diffusion constants were obtained by fitting the RBS profiles with appropriate diffusion equations. The diffusion constants for rhenium in Poco graphite range from 9x10-18 m2/s at 1373 K to 7x10-17 m2/s at 2273 K, whereas the diffusion constants for carbon in rhenium films range from 6x10-17 m2/s at 1373 K to 9x10-16 m2/s at 2273 K. Both Arrhenius plots of rhenium in graphite and carbon in rhenium clearly show a change in the slope with temperature. This feature indicates that there are two diffusion mechanisms involved. At lower temperatures, rhenium atoms diffuse along the open pores in Poco graphite, whereas at temperatures above 2073 K diffusion of rhenium through graphite lattice becomes dominant. Similarly, the carbon atoms diffuse along grain boundaries of rhenium at lower temperature. Diffusion of carbon atoms through rhenium lattice becomes important at temperatures above 1773 K