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Na2Ti307 and K2Ti409 have layered structures with alkali metals in their interlayer regions. H2Ti307 and 2 409• 20 were obtained by exchanges of the alkali metals for protons in HC1 solution. Water molecules were additionally incorporated in case of K2Ti409. Reversible ion exchange of the protonated forms to the original Na2Ti307 and K2Ti409 were examined by using NaOH, NaCl, KOH, and KC1 solutions with various concentrations. Dehydrations were also studied on the protonated titanates. A new compound, H2Ti8017, having a channel-type framework was formed on dehydration of 2 409• 20. The ion exchange and dehydration behaviors are related to the structure of layers in the titanates.The hydrolyses of Na2Ti307 (ca. 750 mg) and K2Ti409 (ca. 1000 mg) were performed in 200 cm3 of 0.5 M HC1 at 60 °C for 3 day. The acid solution was changed everyday in order to remove completely alkali from the compounds. The products were

Space planes require high-performance heat-resistant materials which can withstand ultrahigh temperatures and extremely large temperature gradients. To meet these needs, functionally gradient materials (FGMs) were proposed about 10 years ago in Japan.Figure 1 shows a conceptual diagram of functionally gradient materials, taking into account the relaxation of thermal stress. For the surface that contacts high-temperature gases at thousands of degrees, ceramics are used to provide adequate heat resistance. For the surface that provides cooling, metallic materials are used to furnish the necessary thermal conductivity and mechanical strength. In addition, the composition of these materials is formulated to provide optimum distribution of composition, structure, and porosity to effectively relax thermal stress.Since fiscal 1987, an R&D project entitled “Research on Fundamental Techniques to Develop Functionally Gradient Materials for Relaxation of Thermal Stress,” which aimed to develop ultra heat-resistant materials, had been carried out with special coordination funds from the Science and Technology Agency. The five-year project had two phases; Phase I was carried out from 1987 to 1989, and Phase II from 1990 to 1991.

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