Thermochemistry of New, Technologically Important Inorganic Materials
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MRS BULLETIN/MAY 1997
Liberation of ceramic science from the tyranny of high-temperature equilibrium is thus leading to new materials synthesized more quickly, at lower cost, and
under environmentally more friendly conditions. There is of course a price to pay. First the synthetic procedures are more complex than traditional "mix, grind, fire, and repeat" ceramic processing. Second and more importantly, very little is known about the long-term stability of the materials formed, about their degradation during use, and about materials compatibility. Two examples of such problems are the potential corrosion of high Tc YBCO superconductors by ambient H 2 O and CO2, and the collapse to inactive phases of complex zeolitic and mesoporous catalysts under operating conditions. Chemical reactions in metastable materials are governed by an intertwined combination of thermodynamic driving forces and kinetic rates. For this rich landscape of new materials, neither the depths of the valleys nor the heights of the mountains are known. Often one cannot even tell which way is energetically downhill. What then is the role of thermochemical measurements for these new ceram-
Protection Tube
Main Heater Top Heater Exit Lead Inconel Block Thermopile Sample Chamber Pt Crucibles With Solvent Insulation Bottom Heater
Block Support
Figure 1. Schematic diagram of high-temperature oxide-melt solution calorimeter.
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Thermochemistry of New, Technologically Important Inorganic Materials
Figure 2. A comparison of the relatively dense tetrahedral framework of a-quartz (a) and the open framework of zeolite ZSM-5 (b). Although the structure of ZSM-5 has a density only 50% that of quartz for SiO2 composition, its energy is only 8-kJ higher.
ics? Enthalpies and free energies of formation, and the energetics of metastability, are useful in two major contexts. The first is thermochemical data for the calculation of phase relations, materials compatibility, and optimal synthesis conditions. Here the abundance of new materials has simply outrun the foundation of thermochemical data. Such equilibrium calculations are viewed by some as mundane. However they are very pertinent to the real world because they are essential for rational design of processing, fabrication, and encapsulation of components for lowest cost and longest life. A related application is to provide parameters for the thermodynamic driving forces in kinetic equations governing the rates of decomposition, crystallization, phase separation, and other processes. The second area of relevance to energetics is for providing insight into the factors relating structure, bonding, stability, and reaction mechanisms. Are many different structures accessible, what is their energetic cost, and what microscopic features (for example, cation size, bond angles, bond lengths, covalency) favor or limit the formation of a given structure? Such systematic under36
standing puts synthesis into a more rational and predictable context. This article illustrates the application of modern thermochemistr
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