Perhaps one of the most exciting possibilities for geopolymers is their potential use as oxide matrices for structural ceramic composites. Geopolymers offer significant advantages associated with the processing of ceramic matrix composites (CMC), including low-cost, near-net-shape processing, a tailorable coefficient of thermal expansion, and corrosion resistance in acids. The third team working on an AFRL STTR Phase II project operates under the leadership of Dr. Balakrishnan Nair, of Ceramatec, Inc. Dr. Nair and his collaborators at Brown University (Providence, Rhode Island) and Rutgers University (New Brunswick, New Jersey) are developing an innovative, technologically simple, net-shape processing method for the fabrication of CMCs. They demonstrated the formation of a hydrothermally stable matrix phase using the geopolymer processing approach. To obtain improved mechanical properties and facilitate graceful failure in the geopolymer CMCs, the scientists then incorporated a distinct functional layer (interphase) at the fiber/matrix interfaces. The team reported the development of a novel geopolymer steel mesh composite with enhanced strength and ductility at the 2004 Annual Meeting of the American Ceramic Society (see Figure 2).

Current basic research efforts focus on refining and confirming the structural model, studying reaction mechanisms and kinetics in greater detail, exploring new composite systems, and examining additional high-temperature properties. The research team on which Prof Kriven participates has observed that geopolymers form pollucite, a highly creep-resistant oxide, when they decompose at elevated temperatures. This and other insights have prompted the group to investigate various hightech applications, such as refractory adhesives, versatile ceramic binders, and ceramics with tunable thermal expansion coefficients. Potential Air Force geopolymer materials applications range from lightweight fireproof aircraft insulation to strong, rapidly setting runway repair materials.

Geopolymer research has greatly benefited from the funding supplied by AFRL and other governmental agencies. Increasingly rapid advancement in this area will, however, require wider commercialization of geopolymers and investment of the resulting profits to expand research and development. According to Prof van Deventer, “With the recent development of durable products [constructed] from a range of low-cost waste materials, geopolymers are on the verge of commercialization.” As with all structural materials, cost remains a key selection factor, and geopolymers possess many inherent cost advantages, from their availability as plentiful and inexpensive raw materials to their reliance on simple, near-net-shape processing methods. Consequently, they may soon become another option for designers of future aerospace structures.

Dr. Ken Goretta, Dr. Joan Fuller, and Ms. Erin Crawley, of the Air Force Research Laboratory’s Air Force Office of Scientific Research, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp. Reference document OSR-H-05-05.