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Studies of Mechanical and Chemical Thermodynamics and Kinetics...

Before joining the faculty in Geological Sciences at Brown in 2003, I spent 17+ years as a professor in materials science and engineering. In that context, while pursuing research applying materials science theory and experiment to geophysical and petrological problems, I also pursued the obverse, that is, the applications of petrology, geochemistry and structural geology to the development of engineering materials and materials processes. Some of this work continues at Brown.

Refractory Float Glass: Reactions of multicomponent silicate melts with multicomponent metal alloys: All flat glass used in architecture and in automobiles is made by the "float" process, which was invented by Pilkington, Ltd., in the 1950's. In this process, a Na2O-CaO-SiO2 "window" glassmelt is poured out onto a large tank filled with high-purity liquid tin (the tank is enclosed in a controlled, highly reducing environment so as to avoid oxidizing the tin); surface energy and real-time deformation of the silicate melt produce atomically smooth glass of controlled thickness. But the glassmelt reacts continuously with the tin: components of the silicate melt are reduced to metallic state, alloying the tin bath; ionic tin is added to the silicate glassmelt. The technology just barely "works," and, because of the reactions, is limited to a small number of glassmelt compositions that are not particularly refractory (the high-temperature end of a commercial float tank is held at ~1100oC).

The reduction experiments described on the "Chemical Diffusion … Melts" page are directly applicable to understanding, at least in part, the float glass reaction. We have pursued research in this vein, coming up with the first holistic model for the float reaction (Fig. 8) and defining a family of exothermic liquid-metal alloys for float media that allow far more refractory glassmelts to be processed via the float method. One sees that the float reaction involves both reduction of the silicate melt as well as solution formation, i.e., homogenization of the silicate melt that now includes cations added from reaction with the metal. These processes act in kinetic series. As importantly, getting back to geophysical issues, understanding dynamics such as these provokes questions concerning the thermodynamic and kinetic aspects of, e.g., core-formation processes.

Figure 8: Reaction schematic for the float-glass process where a sodium aluminoborosilicate melt is in contact with a liquid gold-tin alloy. The dynamic involves (at x =0) oxidation of tin to Sn2+ in a redox couple with component ions of the glassmelt and its subsequent incorporation into the glassmelt (ion interdiffusion); (at x = x') oxidation of Sn2+ to Sn4+ by the acceptance of electron holes; (at x = x'') "release" of electron holes to diffuse outward in a local reduction reaction that changes the molecular structure of the glassmelt. The reactions at the various interfaces are written following the polymerization model for silicate melts by Prof. Paul Hess; one can so account for the species in the melt that are mobilized in the reaction. The extent of reaction overall can be controlled through the combination of the useful alloying of the metal float medium (e.g., to lower the chemical activity of Sn) and the doping of the glassmelt to increase the concentration of h·.

Representative Publications and Patent:
Cook, G.B. and R.F. Cooper (1999). Redox dynamics in the float-processing of glasses I: Reaction between undoped and iron-doped borosilicate glassmelts and a gold-tin alloy. J. Non-Crys. Solids 249 , 210-227.

Cooper, R.F. and G.B. Cook (2000). Float processing of high-temperature complex silicate glasses and float baths used for same. U.S. Patent #6,065,309.

Bledsoe, M.D., A.J. Wild and R.F. Cooper (2005). Float reaction between sodium aluminoborosilicate glassmelts and copper-base liquid metal alloys. J. Non-Cryst. Solids 351, 838-848.

Petromimetics: Structural Ceramic Composites and "Geopolymers": The desire to make structural materials ever lighter and for use at higher and higher temperatures has prompted the development in the past 20 years of polyphase ceramic composites. The problem with ceramics, of course, is that they are brittle, that is, they have an exceedingly low fracture toughness because their lack of intrinsic ductility makes them very flaw sensitive. The composite approach addresses the flaw-sensitivity issue: elastic energy is converted to surface energy, not by the propagation of a single crack (which leads to catastrophic failure of the component), but rather by the delamination of the fiber reinforcement phase from the matrix phase and the retention of integrity in the part by the fibers. Further, frictional dissipation as fibers are pulled from the matrix absorbs substantial energy, too. One of the best examples of the approach was the use of polymer-derived, continuous silicon carbide fibers embedded in an aluminosilicate (anorthite) glass-ceramic: an oxidation reaction between the matrix and the fiber placed a ~100-nm cylindrical-sheath interphase of nanocrystalline carbon between fiber and matrix: cracks would shred this interphase, the fibers would remain intact and the part could be as tough at 1300oC as aluminum alloys are at room temperature, provided that there was little oxygen in the environment. With oxygen reaching the interphase through matrix microcracks, the carbon interphase would rapidly convert to a silica interphase (strongly bonded to fiber and matrix) and the material would be as brittle as a monolithic ceramic. Our approach was to mimic the SiC/silicate composites with aluminum oxide (corundum) fibers embedded in a silicate matrix, but create a "functional" interphase from a high-temperature, synthetic mica (e.g., fluorophlogopite: KMg3[Si3Al]O10F2). The idea(s) evolved from concepts of oxide/silicate equilibria as developed in the fields of metamorphic and igneous petrology; as such, we refer to the approach as petromimetics; i.e., mimicking rocks. The approach creates 1300oC+ materials that remain both mechanically tough and thermodynamically stable even in air; the toughness persists even in a moist-air environment despite the fact that water seeks to attack the F- in the mica (Figs. 9 and 10).

Figure 9: Engineered laminate of Al2O3 (alumina), MgAl2O4 (spinel) and KMg3[Si3Al]O10F2 (fluorophlogopite mica). The composite is a fracture tough, high-temperature ceramic. In this specimen, the width of the mica layer (phase with the bladed morphology) is approximately 80mm.

Figure 10: Ambient-temperature mechanical response of alumina/spinel/fluorophlogopite/spinel/alumina laminates as a function of processing temperature (which affects mica morphology). In these flexure experiments, the tough laminates experience stable crack growth confined to the mica interphase. As a consequence, unloading and reloading the specimen allows monitoring the evolving compliance with delamination.

Representative Publications and Patent:
King, T.T. and R.F. Cooper (1994). Ambient-temperature mechanical response of alumina-fluoromica laminates. J. Am. Ceram. Soc. 77, 1699-1705.

King, T.T., W. Grayeski and R.F. Cooper (2000). Thermochemical reactions and equilibria between fluoromicas and silicate matrices: A petromimetic perspective on structural ceramic composites. J. Am. Ceram. Soc. 83, 2287-2296.

Chyung, K., R.F. Cooper, K.P. Gadkaree, R.L. Stewart and M.P. Taylor (1986). Reinforcement of alkaline earth aluminosilicate glass ceramics. U.S. Patent #4,615,987.

"Geopolymers" are tektosilicate compounds that are depolymerized by low-temperature reaction with strong bases. The reactions are distinctly exothermic, and the product created is invariably amorphous. Geopolymers are being considered as a primary candidate material to replace Portland cement in concrete. Our interests in geopolymers relate back to ceramic composites: one can imagine using the gel state of the reacting tektosilicate-base slurry to easily and effectively densify a polyphase composite. Post-densification, high-temperature reaction can take the assembly through numerous metamorphic reactions, creating—again following petromimetics—a thermodynamically stable material with engineered mechanical and thermochemical response. Our geopolymer work is pursued in collaboration with Dr. Balakrishnan Nair at Ceramatec, Inc., Salt Lake City, UT.

Brown faculty collaborators:


Other project collaborators:

Cook, G.B., Bledsoe, M.D., A.J. Wild, King, T.T., W. Grayeski, Chyung, K., K.P. Gadkaree, R.L. Stewart, M.P. Taylor

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