Abstract: Chemical routes to synthesize inorganics start with solutions containing different metal-organic molecules that remain well mixed during evaporation to a solid precursor. The solid precursor decomposes and crystallizes during heating. Because decomposition (pyrolysis) occurs at very low temperatures relative to the melting temperature of the inorganic, a large free energy change exists for crystallization. This large free energy change is responsible for two very interesting phenomena. First, the size of the critical nucleus for spontaneous crystallization and growth is very small. Thus the grain size of the initial inorganic material produced during crystallization is <= 2 nanometers. Second, crystallization occurs at very low temperatures, i.e., either during or subsequent to pyrolysis. Thus diffusion is very limited, resulting in the crystallization of metastable phases, i.e., phases with unexpected (non-equilibrium) structures and/or phases with a solid-solution that is much greater than found for equilibrium conditions (high temperature heat treatments). With this understanding, nano-crystalline materials can be made that are very stable at high temperatures using the following procedure. The first step is to formulate a precursor composition that would produce at least two phases under equilibrium conditions. During and/or subsequent to pyrolysis, only one, metastable crystalline phase will form; it will have a nanometer grain size. Heating to higher temperatures where long range diffusion can occur will cause the single, metastable phase to partition to its stable phases. Since the second phase(s) partition with a smaller size than the initial metastable phase, a multi-phase composite is produced when the size of each phase can be <= 100 nano-meters.
| Limitations: |
 APPROVED FOR PUBLIC RELEASE |
| Description: |
Technical rept. no. 6, Dec 95-Nov 96 |
| Pages: |
12 |
| Report Date: |
28 FEB 97 |
| Contract Number: |
$F49620-96-1-0003 |
| Report Number: |
A971223 |
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