MadSci Network: Chemistry |
Jonathan; Carbon fibers are commonly used in military and large commercial aircraft brakes. They have been used in some racing car brakes as well. For these usages, both the rotor and stator (or brake pad) are made from carbon fibers that are bonded by amorphous carbon. Such brakes can operate to 2500C, far hotter than brakes that use metallic rotors. Aircraft and racing cars need to keep weight to a minimum. For this reason they use brake friction materials that can be used at high operating temperatures, that have high specific heat capacities, and which have high thermal conductivities. Carbon fibers are particularly useful for such brakes. However, they are too expensive and their friction is too sensitive to contamination (water and organic vapors) for most automotive applications. You indicated that you want to calculate temperature increases due to braking. You should be aware that, during hard braking, about 85% of the braking work goes into heating the brake assembly (the remaining 15% mostly goes into heating the tires and pavement). On a panic brake stop, where the wheels lock and the tires skid, about 5% goes into heating the brakes and 95% into the tires and pavement. How hot a brake gets is determined by several factors, one of which is the specific heat capacity of the brake materials. Most materials have specific heats that vary with temperature. Typically, they rise with increasing temperatures. Carbon has a large variation in its specific heat capacity. It is very low at cryogenic temperatures, but rises rapidly in the brake temperature ranges. Variation above 600C is relatively small. The specific of carbon seems to vary somewhat with its structure, be it diamond, graphite, or amorphous carbon. A carbon-carbon (carbon fiber based) friction material should have a specific heat that varies with temperature much like graphite. You can estimate the specific heat using the following values from the Handbook of Chemistry and Physics: Temperature, ºC Specific Heat 20 0.17 85 0.177 138 0.254 642 0.445 896 0.454 You can plot these values to generate a curve for the specific heat versus temperature. Then you might wish to use an approximation technique to estimate the temperature increase due to braking. I, too, did not find any data on the specific heat of carbon--using the Internet. You might try a technical library, and look for a monograph on carbon.
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