Annealing, reversible

We noted in Section VII-2B that, given the set of surface tension values for various crystal planes, the Wulff theorem allowed the construction of fhe equilibrium or minimum firee energy shape. This concept may be applied in reverse small crystals will gradually take on their equilibrium shape upon annealing near their melting point and likewise, small air pockets in a crystal will form equilibrium-shaped voids. The latter phenomenon offers the possible advantage that adventitious contamination of the solid-air interface is less likely. 

Compaction has also been observed as a result of neutron irradiation and extended exposure to iatense uv (excimer) laser light (131,132). The compaction tends to relax over months at room temperature and can be reversed quickly by annealing at sufficiently high temperatures (133). 

An increased yield stress is required [62] in order to reverse the unfavourable conformations of the molecular chains that develop during annealing. This explanation is supported by the energy changes observed in annealed polymers. The enthalpy difference, as determined by DSC was AH = 1.8 J/g (Sect. 4.2), whereas the additional work required for yielding in an annealed sample was... 

Differentiation between the two forms of Ag2C03 is not easy and, from the many methods used, electron spin resonance spectroscopy and thermal analysis have been most successfully applied [757]. The imperfections mentioned above occur in the low temperature decomposition product and are identified as being responsible for enhanced activity in readsorbing C02. Annealing of the residue removes these defects and reduces the reversibility of reaction. 

Fig. 3. Forward (top) and reverse (bottom) voltages of InGaN/GaN MQW LED with and without annealing after etching as a function of ICP source power.

The overall compositions of films 2 and 4 was such that homogeneous alloys were not expected, and the X-ray diffraction peaks showed evidence of phase-separation with two maxima corresponding to the compositions recorded in column 8, Table II. When palladium was deposited on top (and the film was rather light) then the apparent surface composition after annealing was 10% Rh and X-ray diffraction indicated a phase also containing 10% Rh (film 2). This convenient result was not observed when the order of deposition was reversed (film 4). These differences between apparent surface composition and the overall composition of the homogeneous alloy (or one of the phases in the miscibility gap) are discussed in Moss and Gibbens (34), with further examples. The main point to be made here is the rather variable nature of the surface composition compared with that expected, due to the operation of a number of factors. 

Figure 10. Irreversible capacity losses (A) and reversible capacity at 10-th cycle (B) for two specific current values of modified Graphite-type materials annealed at different temperatures.

The double helix can be denatured by heating (melting). Denatured DNA, like denatured protein, loses its structure, and the two strands separate. Melting of DNA is accompanied by an increase in the absorbance of UV light with a wavelength of 260 nm. This is termed hyperchromicity and can by used to observe DNA denaturation. DNA denaturation is reversible. When cooled under appropriate conditions, the two strands find each other, pair correctly, and reform the double helix. This is termed annealing. 

---