Silicon phase changes

The ability to control pressure in the laboratory environment is a powerful tool for investigating phase changes in materials. At high pressure, many solids will transfonn to denser crystal stmctures. The study of nanocrystals under high pressure, then, allows one to investigate the size dependence of the solid-solid phase transition pressures. Results from studies of both CdSe [219, 220, 221 and 222] and silicon nanocrystals [223] indicate that solid-solid phase transition pressures are elevated in smaller nanocrystals. 

Solvates may be readily detected and desolvation may be readily distinguished from a phase change using thermomicroscopy. The appearance of turbidity within the crystal upon heating is a sign of solvent being driven off, but a much more conclusive test involves covering the crystal with silicone gel or paraffin oil, which trap the bubbles of released solvent, as shown in Fig. 4.6. 

The MD calculations using the Tersoff model are carried out under constant-volimie and -temperature conditions. The melting temperatures predicted by tiie Tersoff model are much hi er tiim tiie real phase-change temperatures. The model is, however, widely applied in tiie recent simulations, because it can reproduce well the structural and dynamical properties of elemental semiconductors such as silicon md cM bon ranging from crystalline to amorphous structures In tiie Tersoff model, tiie potential energy between two neighboring atoms i and j can be expressed as... 

We reported that the free silicon phase is inter-connected to form a network structure when B=80-90mol%, while it dispersed isolatedly when B>90mol% [14]. And the Seebeck coefficients is almost independent of phase composition and changes from 100 to 300 iVK as the measuring temperature up to 1 lOOK [14]. 

As pointed out earlier, silicon microvalves produced with conventional microfabrication technology have been used to control flows as well as generate them. Because of the mechanisms used to drive the valves and pumps, there are limits on how small a device can be made. At present, there does not seem to be a completely satisfactory solution for mechanically operated microvalves and pumps. Interesting designs have been proposed for thermofluidic drives involving phase changes [47] and there have been recent advances with a LIGA-based micropump that shows promise in the valve area as well [46]. 

Fang, G. Li, H. Liu, X. Preparation and properties of lauric acid/silicon dioxide composites as form-stable phase change materials for thermal energy storage. Mater Chem Phys 122 (2010) 533-536. 

Li, H. Fang, G. Liu, X. Synthesis of shape-stabilized paraffin/silicon dioxide composites as phase change material for thermal energy storage. J Mater Sci 45 (2010) 1672-1676. 

Figure 7.8 High-resolution transmission electron micrograph showing the presence of a grain boundary glassy phase in silicon nitride. These observations have shown the glassy phase changes its thickness during creep. (Courtesy of Q. Jin and D. S. Wilkinson. McMaster University.)...

The most diverse data exist about the best-known compound silicon carbide (SiC), perhaps because oxidation (in air), dissociation, sublimation, and phase change obscure the picture. No aluminum silicide has been described. 

Zhang L, Wang EN, Goodson KE, Kenney TW (2005) Phase change phenomena in silicon microchannels. Int J Heat Mass Transf 48 1572-1582... 

The scheme also assumes that the redistribution of the secondary phase changes the size distribution of cavities within the material, which is supported by experimental observations [23,46], Final proof of the cavitation creep model is the fact that Eq. (2) fits the experimental data over a wide range of stresses for both SN 88 and NT 154. Thus, the model shovm in Figure 13.16 provides an understanding of the overall creep behavior of silicon nitride. Cavitation is the main creep mechanism in silicon nitride, and in other similarly bonded ceramics. 

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