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Heat Capacities in Aerospace Materials

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The airline industry is espcially concerned with the transfer of heat resulting from temperature changes, due to the extreme conditions that aircraft operate in. While the air temperature at such lofty altitudes is below freezing, the jet engines keeping the plane airborne must operate at almost 2000 oC. Passengers in the plane must be kept comfortable at room temperature and shielded from both of these deadly environments. In addition, temperatures must not become too hot or too cold, or parts of the craft will melt or freeze, respectively. The aerospace industry, then, is constantly attempting to find new materials that can withstand the severe temperatures associated with flight

From the equation

q = C × m x (T2T1)

It can be seen that if a certain amount of heat is released (in one case, by the combustion of jet fuel), the enthalpy change of the surroundings (in this case, the engine) is positive. If q remains the same, the specific heat, mass, and temperature variables are inversely related to one another; that is, if the value of one variable increases, the others must decrease so q remains constant.

There are three possibilities in which this can occur:

• The temperature can increase. This is the least favorable result, as engine conditions as hot enough as it is, and an overheat would be disasterous to the engine system and the crew and passengers aboard. While the metal parts that comprise the engine can absorb some of this heat, jet engine conditions are hot enough that many metals will melt.
• The mass of the substance absorbing the heat can be increased; usually in airplanes this is done by increasing the surface area of the metal or other solid that makes up the frame of the engine. This, too, is disadvantageous, as every extra kilogram of material on a plane requires more fuel to propel it into the sky, and with current fuel prices, uneeded weight can be expensive.
• If the material is replaced with a new one that has a higher specific heat capacity (C), a larger quantity of heat can be absorbed by the substance before its temperature rises. This is the the best option that aerospace engineers and chemists have; a change in substance does not neccessarily mean an increase in density or increase in temperature.

Diagram of a standard jet engine.

Many new materials with high specific heat capacities are being looked at for implementation into jet engines. Ceramics have relatively high specific heat capacities compared to the metals that normally constitute an engine. However, the engine framework can't simply be replaced by pure ceramic material, or it will not function properly. The problem is solved by coating metal parts of the engine, usually the turbine, in ceramic material, which serves as an insulator. Because of this insulatoion, engines can then actually work at higher temperatures, increasing thermodynamic efficiency[1].

Ceramics and advanced materials such as CMCs, or ceramic matrix composites, which have improved mechanical properties, are also used in the space industry. Jet engine temperatures may seem high, but they pale compared to those during reentry to the Earth's atmosphere at ludicrously high velocities. Such is the case of the soon to be retired space shuttles, which make of ceramic tiles to resist the astronomical heat involved in reentry[2]. Future materials with even higher specific heat capacities and lighter densities will help fuel the aerospace industry's desire to build craft that fly farther and faster, to the sky and into space.

Reentry of an Apollo capsule, guarded by high specific heat capacity material.