Functional Geopolymer Composites for Structural Ceramic Applications

"Continuous fiber-reinforced ceramic matrix composites (CMCs) are attractive candidate materials for structural components in military/commercial airframe or engine/turbine components due to their high-temperature mechanical properties. However, currentCMCs have two major limitations that have prevented replacement of current materials, namely (1) very high processing/materials costs and (2) insufficient corrosion resistance under hydrothermal oxidizing conditions. Geopolymers, in whichamorphous/semi-crystalline aluminosilicates are dissolved into an inviscid, highly concentrated alkaline solution, offer an approach for the development of easily and cost-effectively processed matrix materials for alumina fiber composites. This Phase ISTTR proposal is targeted at demonstrating the feasibility of developing a geopolymer-based CMC with appropriate high-temperature performance. Our approach will include chemical design (i.e., aluminosilicate phase selection and solid-solution composition)and thermal processing of geopolymers so as to create, after firing, CMCs at chemical equilibrium ("petromimetics") that, too, have more refractory behavior than current geopolymer systems. What is envisioned is a hybridization of present glass-ceramicand geopolymer processing. The work will establish a chemical processing and design/microstructure/property database for this relatively new class of materials, which will enable functional CMC design. Specifically, the role of a highly doped andreactive intermediate gel phase on properties of the final geopolymer will be studied. Cost-effective processing routes for CMCs with adequate high-temperature mechanical properties are attractive to a variety of applications where high-temperaturemechanical performance is required. The use of CMCs in aircraft or stationary engines and turbines have the potential to raise operating temperatures which will result in a significant step up in efficiency than is possible through marginal improvementsusing currently used materials such as nickel-based superalloys. Fur