Temperature Induced Phase Changes

Another property pecuHar to SMAs is the abiUty under certain conditions to exhibit superelastic behavior, also given the name linear superelasticity. This is distinguished from the pseudoelastic behavior, SIM. Many of the martensitic alloys, when deformed well beyond the point where the initial single coalesced martensite has formed, exhibit a stress-induced martensite-to-martensite transformation. In this mode of deformation, strain recovery occurs through the release of stress, not by a temperature-induced phase change, and recoverable strains in excess of 15% have been observed. This behavior has been exploited for medical devices. 

Poly(A alkyl acrylamide)s and poly(7V-isopropylacrylamide) in particular are the other type of LCST polymers our group has studied. Poly(iV-isopropylacrylamide) is soluble below 31 C in water but insoluble above that temperature. Our group has used this temperature induced phase change has been used as a way to isolate, recover and reuse water-soluble polymer-bound catalysts. It is also a way to make a smart catalysts, catalysts that can turn off an exothermic reaction without external temperature control. Such on/off behavior is seen for both catalysts and substrates. 

Topographical changes caused by temperature-induced phase changes (Barbosa et al., 2009 Hyun et al., 2004) or by the adsorption of compounds on the surface in response to a stimulus (Koga et al., 2006) can be monitored by atomic force microscopy (AFM). Altered adhesion properties of a responsive peptide surface can also be probed by AFM force measurements (Hyun et al., 2004). 

Raison, J.K. (1973) The influence of temperature-induced phase changes on kinetics of respiratory and other membrane-associated enzyme. J. Bioenerg. 4, 258-309... 

Raison, J. K., Lyons, J. M., Meibom, R. J., and Keith, A. D., 1971a, Temperature-induced phase changes in mitochondrial membranes detected by spin labeling, /. Biol. Chem. 246 4036. 

Other independent variables (changes in temperature or medium composition) should produce effects analogous to pressure and therefore induce phase changes and shifts in association equilibria. Combination of these different variables could be used to investigate protein-subunit interactions and conformational changes, to determine the fundamental physical-chemical parameters of these changes. 

A common technique for separating the water and the oil in a microemulsion is a temperature-induced phase separation, yielding an excess water phase (increasing the temperature) or an excess oil phase (decreasing the temperature). It is the simplest way to separate the oil and the water. Nevertheless this method is quite time consuming and often not complete. In case of enzymatic reaction, a change in temperature can lead to a loss of enzyme stabiUty. 

Temperature-induced phase transitions are typically associated with small volume changes so that the change of the internal energy is dominated by the entropy term TAS. For pressure-induced phase transitions with volume changes of the order of 10%, the PAF term dominates and the entropy term can be neglected at room temperature. The corresponding work AW is defined as ... 

Chemical methods can also be applied for the measurement of the interior temperature of microsystems. A fluorescent molecular thermometer using temperature-induced phase transitions seems to be quite attractive,but it needs a transparent window on the wall of the microchannel for the optical measurement. However, this may change the heat transfer ability of the wall. 

The effect of s—d hybridization on the relative stability of the structures of metallic Ca and Sr has been studied as a function of temperature and pressure within the context of the pseudopotential theory for metals. The inclusion of hybridization is found to favour the f.c.c. structure at all pressures and, in particular, is necessary to explain the observed f.c.c. structure in these metals at zero temperature and pressure. Phase boundaries are calculated by equating the free energy of the f.c.c. structure to that of the b.c.c. structure. Temperature-induced phase transitions are predicted to occur at 555 K in Ca and 625 K in Sr as compared with the actual temperatures of 721 and 830 K. From the changes in free energy of the reactions of the alkaline-earth metals with gases as a function of temperature, it is confirmed that at 298— 1400 K almost all of the residual gases in electrovacuum devices combine with the metals to form stable compounds. The dehydration of calcium... 

As in X-ray spectrometry, there is a need to reduce the particle size to below the level at which it will adversely affect the diffracted intensities. In practice, this means to about the 5 pm level. Since the action of grinding the sample can at times induce phase changes, it is important that the grinding action is not too vigorous. This can be achieved by slow grinding in an agate ball mill with an inert liquid medium such as cyclohexane. This helps to keep the temperature down and also acts as a grinding aid. 

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