Study of copper on graphite with titanium or chromium bond layer
- PDF / 1,471,859 Bytes
- 8 Pages / 612 x 828 pts Page_size
- 32 Downloads / 148 Views
Improvement of copper to graphite adhesion by thin interfacial films of titanium and chromium was investigated. Graphite fibers and highly oriented pyrolytic graphite flats were sputter-coated first with 10 nm of titanium or chromium and then with 50 nm of copper. After annealing to 970 °C in argon/5%-hydrogen at atmospheric pressure for 5 min, copper without an interfacial bond layer agglomerated into nearly spherical particles, copper with the chromium bond layer agglomerated into particles with a contact angle less than 90°, indicating improvement in adhesion, and copper with the titanium bond layer exhibited a continuous metal film. In the latter case, most of the interfacial titanium was observed to have migrated into the copper and to the free surface, where the titanium reacted with contaminants in the annealing ambient.
I. INTRODUCTION Significant power usage aboard spacecraft requires a method of radiating excess heat into space. Spacebased radiators fabricated on Earth need to be both very thermally conductive and as lightweight as possible for launch. A unit of merit for heat rejection materials is specific thermal conductivity, defined to be the thermal conductivity normalized by mass. Candidate space radiator materials include those based on beryllium, titanium, niobium, and copper, either pure, in alloys, or in various composite material configurations.1 Copper matrix composites reinforced with various types of graphite fibers offer the intriguing property of allowing the physical properties of the finished material to be tailored. Graphite fibers are available with a range of thermal conductivities, tensile strengths, and strain to failure values.2 By incorporating the proper volume fraction of graphite fiber, for instance, the coefficient of thermal expansion and specific thermal conductivity within a given range could be chosen, while still providing a stiffer material than pure copper. Using high thermal conductivity fibers in a unidirectional fiber composite would allow highly anisotropic thermal properties to be engineered into the finished product.1 One difficulty in using graphite fibers with copper is the lack of adhesion between copper and the graphite surface. This is manifested at high temperatures by copper "beading up" on graphite surfaces, or at lower temperatures by void formation at copper-graphite interfaces. In short, pure copper tends not to "wet" the graphite. A contact angle of less than 90° between a bead of material and substrate is considered to be "wetting" behavior. In the early 1970s Mortimer and Nicholas investigated improving copper bonding to graphite and vitreous carbon by alloying the copper with small amounts of
active metals.3'4 The active metal additions were intended to segregate to the interface and produce carbide, with a probable improvement in copper adhesion to the reaction layer. Working with various carbides as well as the carbon substrates, Mortimer and Nicholas developed a surface and interfacial energy model to describe their results. Significantly, und
Data Loading...
