The Prometheus gas Brayton system was designed to operate for 15 to 20 years. In order to accomplish this goal some potentially mission-threatening issues were considered, including gas leakage of the working fluid and mass transport of contaminants to other plant or core components. The NRPCT Design team has introduced several areas of development concerns for carbon carbon composite as recuperator materials.
- Impact on mechanical properties during operation.
- Design requirements (permeability and heat transfer properties)
- Fabrication challenges
- Material availability assessment
- Joining issues
- Compatibility issues in the primary working fluid
Mechanical properties of carbon-carbon composites:
The structural properties of carbon-carbon composites are generally controlled by the orientation, volume fraction, and properties of the fibers. Additionally, process-induced stresses, heat treatments, and fiber-matrix interactions also affect the primary properties. In general, mechanical properties can be tailored by selecting the correct modulus fibers and arranging the fibers in the direction of the applied load.
The characteristics of the interfacial bonding between the matrix and fiber typically govern the mechanical properties of the C/C composite. When the bonding between the fibers and matrix is strong, cracks that form in the matrix will propagate across the fiber-matrix interface resulting in brittle fracture. Conversely, weak interfaces between the fibers and matrix allow matrix cracking to occur without crack propagation through the fibers. Intact fibers bridge the matrix cracks and maintain a load-bearing capability until the load initiates fiber fracture. This type of failure exhibits pseudo-plastic behavior due to matrix cracking and fiber movement.
Fiber treatments and processing methods are often varied to optimize mechanical properties and to tailor the interfacial characteristics of the C/C composite. Two processes have been developed to produce high-performance carbon-carbon composites; liquid impregnation and chemical vapor infiltration. The liquid impregnation method commonly uses thermosetting or thermoplastic polymers as the matrix precursor to form a prepreg. The prepreg is a ready-to-mold component, usually in laminate form, consisting of a matrix and fiber reinforcement. In order to achieve the high-temperature properties of a carbon material, the prepreg matrix must be transformed into a carbon residue. Pyrolysis strongly influences the strength of the fiber-matrix interface due to densification and shrinkage of the matrix. To achieve a higher density and stronger matrix, the C/C composite is cycled through several impregnation/pyrolysis processes. Graphitization treatment of the carbon matrix may be included to encourage densification and opening of porosity to aid reimpregnation cycles.
The CVI of carbon uses gaseous hydrocarbons such as methane to deposit a carbon matrix on the external and internal surfaces of a porous carbon fiber preform. Three types of carbon microstructure are commonly seen in CVI fabrication: smooth laminar, rough laminar, and isotropic. The microstructure is controlled by processing parameters: temperature, pressure, gas composition, and flow rate. An isotropic CVI matrix exhibits lower mechanical properties due to lower density and closed porosity. The smooth laminar matrix has a strong fiber-matrix interface and produces composites with high strength and stiffness but brittle fracture behavior. A rough laminar matrix exhibits pseudo-plastic fracture behavior due to a loosely bonded fiber-matrix interface. The major drawback of CVI is the very slow rate of deposition leading to high final cost. Therefore, using impregnation to produce relatively uniform open pores, followed by CVI for densification is an attractive option.
C/C composites are unique in that their mechanical properties do not typically degrade with increasing temperature until 2000C. The properties of the matrix and fiber-matrix interface often dictate the effect of temperature on the shear, cross-fiber tensile, and compressive strengths of the composites. Generally, these properties improve with increasing temperature, which is attributed to the annealing of matrix microcracks. Creep behavior has received little attention to date, but is predicted to be at least four orders of magnitude lower than that of most ceramics.
Polyacrylonitrile (PAN)-based fibers woven in three-directions have been shown to posses very high-strength. Increasing density through a number of impregnation/CVI cycles will also help to improve the mechanical properties of the composite. Current development of the C/C recuperator under Allcomp should give detailed information on the C/C mechanical properties. Information on the type of carbon-fiber, fiber lay-up, matrix precursor, and processing conditions should also be available.