Carbon-carbon heat pipe fabrication and CVD coating development

A lightweight, high-performance, heat pipe radiator meeting SP-100 thermoelectric requirements has been identified. To facilitate the development and fabrication of this design, the resolution of two fundamental feasibility issues was required. First, it was necessary to produce a carbon-carbon heat pipe tube with integral fins meeting both thermal and mechanical requirements while satisfying structural weight goals. Second, it was necessary to develop a coating that protects the carbon-carbon substrate from the 875-K potassium working fluid. These feasibility issues have been resolved. Carbon-carbon tubes with integral fins were produced using a T-300 fiber, interlocking weave, and pitch densification. A barrier coating was then applied to the inside diameter of these tubes utilizing the chemical vapor deposition (CVD) of niobium over a thin rhenium interlayer. The rhenium interlayer was critical to the success of this coating by providing gradation in coefficient of thermal expansion (CTE) mismatch, carrying a portion of the induced stress load, improving coating adhesion, and providing a carbon diffusion barrier. Coatings with good adhesion to the interlayer and to the carbon-carbon body have been produced.