Conclusions of Carbon-carbon composites as recuperator materials

Carbon-carbon materials provide high-strength at low weight and can withstand temperatures forecasted for recuperator operation in a non-oxidizing environment. Also, C/C composites present the opportunity to tailor their thermal or mechanical properties, which can be controlled by the different fiber, matrix, or processing options. Although this option exists, the method of production often becomes the cost-limiting factor in determining recuperator design.

C/C composites present risks that can prohibit their adoption into the direct gas Brayton system. Foremost is the concern over compatibility issues. Incorporating a C/C composite into the system would add yet another potential source of contaminants. The transport of carbon could lead to interstitial contamination of refractory metals and under some conditions carburization of superalloys. Also, the corrosion of C/C composites reduces its strength and performance by the gasification of solid carbon. The corrosion reaction produces impurities that can drive the coolant chemistry into a regime where superalloys carburize, decarburize, or oxidize.

Integrating a carbon-carbon composite into an otherwise all-metallic energy conversion system raises concerns over joining. The CTE mismatch at C/C to metal interfaces may create excessive thermal stresses causing failure at the joint and catastrophic gas leakage. The joining technology between C/C and metal is underdeveloped and unproven for high-temperature joint reliability over a long mission lifetime.

Finally, the use of carbon-carbon as a recuperator material presents several fabrication issues. Due to reduced cost and ease of fabrication, a plain plate-fin design has been considered instead of a more efficient strip-fin design. Because of the lower heat transfer characteristics of plate-fin designs, C/C recuperators are expected to carry an increase in volume compared to their metallic counterparts. Development of C/C recuperators must also overcome obstacles such as fabricating uniform fin heights and properly joining plates to fins.

In order to resolve the issues facing a C/C recuperator core, experimental testing must be performed. Provided the C/C material passes preliminary tests, the compatibility with fission products would need to be assessed. Capsule testing under isothermal conditions is a fast and inexpensive way to determine this data. Representative C/C material would be enclosed in a refractory or superalloy capsule with fission product elements or mixtures of elements and heated to recuperator operating temperatures. The effect of the fission product on mass transport, corrosion, or mechanical properties would then be determined.

A more detailed approach to fission product compatibility is to incorporate the materials in the long-term helium loop. Potential fission products would be free to circulate in the primary system and deposit on surfaces of the components. The fission products are suspected of playing an important role on accelerated corrosion and deterioration of C/C composite. This experiment would help understand fission product volatility, compound free energy, and the critical temperature at which the dominant species would affect the C/C materials.

Due to the likelihood of a C/C to metallic joint, joint reliability at high-temperatures must be studied. The first tier studies would function as a screening phase and would include tensile testing, bend-ductility testing, and non-destructive testing of the joint. Second tier studies would reflect long-term thermal effects under prototypical conditions. Testing would include: fracture toughness, creep, creep-fatigue, creep/stress rupture, and hermeticity.

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