Superheating of crystals

Superheating is a basic phenomenon that needs to be described using irreversible thermodynamics and can be examined by standard and temperature-modulated calorimetry. Superheating of crystals occurs if the heat conduction into or out of a sample is faster than the phase transition. 

The phase changes of semicrystalline macromolecules that are usually not in equilibrium must make use of irreversible thermodynamics (11,97). On the other hand, even a system that is initially in equilibrium may go through a nonequilibrium transition because of kinetic restrictions, such as the superheating of crystals (98) or the more widely recognized supercooling of polymer melts on crystallization (10). The latter cases are often treated by kinetic arguments by introducing nucleation barriers (10,88,89,99). 

Thus, melting of a crystalline substance without superheating is a superficial effect. Pre-melting phenomena are apparently also related to the formation of liquid films on the surfaces of crystals, if not to other incidental causes (for example, impurities), and are not pertinent to Frenkel s theory. Heterophase fluctuations are quite large where the difference between two phases and the surface tension between them tend to zero—near the critical point and near the Curie point. The first case is commonly known, the second was earlier investigated quantitatively in Landau s fine work [13, 14]. 

Fig. 8. Superheating of polyethylene extended chain crystals. Curves 1) at 421.7 K, 2) at 419.2, 3) at 417.7 K, 4) at416.7K, 5) at 414.7 K. The equilibrium melting temperature is 414.6 K. Drawn after Ref.40). "w is the weight fraction molten, obtained by isothermal calorimetry...

Hellmuth, E. and Wunderlich, B. Superheating of linear high-polymer polyethylene crystals. J. Appl. Phys. 36, 3039 (1965)... 

First-order solid-state amorphization occurs due to an entropy catastrophe [39] causing melting of superheated graphite and decompressed diamond below Pg when the entropy of the ordered crystal would exceed the entropy of the disordered liquid. This condition is resolved with the occurrence of a kinetic transition to a (supercooled) glass whereby the exact kinetic conditions during carbon transformation will be critically Pg-depen-dent [39]. It is important to consider the crystal to liquid transition and the effect of a superheated crystal whereof the ultimate stability is determined by the equality of crystal and liquid entropies [40]. When this condition is met, a solid below its Pg will melt to an amorphous solid, particularly... 

Nanocalorimetry has been used by Olson and coworkers to investigate size-dependent melting of Bi nanoparticles. It has been discovered that for particles smaller than 7 nm the measured melting temperatures are about 50 K above the value predicted by the homogeneous melting model. The discrepancy is analyzed in terms of a possible size-dependent crystal structure change and the superheating of the solid phase [18],... 

Hellmuth E, Wunderlich B (1965) Superheating of Linear High-Polymer Polyethylene Crystals. J Appl Phys 36 3039-3044. 

Melting Kinetics at Different Superheating of Extended-chain Crystals of Polyethylene Using Isothermal DSC Experimemts... 

Superheatability of poly(ethylene terephthalate) was produced, or in some cases restrained in the sample annealed at about 250 °C, which was not observable for the as-polymerized sample This was not an effect of crystal size, but must be caused by tie molecules between different crystals or different locations on the same lamellae. Superheating effects due to reduction in entropy of melting are calculated theoretically in a situation where each chain molecule can simultaneously belong to many different crystalline and noncrystalline regions. In the case of natural rubber, the chain scission of molecules with stress, i.e., a large deformation with an extention ratio of more than 2, occurs easily by irradiation, since the molecules are entropy restricted... 

Toda, A., Hikosaka, M., and Yamada, K. (2002) Superheating of the melting kinetics in polymer crystals a possible nucleation mechanism. Polymer, 43, 1667—1679. 

Crystals at T > T, are known as superheated crystals. To measure C, it is important to estimate the stability limit of crystals and melting must be avoided. Owing to n, surfaces and interfaces of crystals have a higher energetic state and melt first at T. Atoms in the crystals remain in metastable states. Hence if surfaces are avoided, alloys can be superheated. In order to avoid the contribution of interfaces and surfaces, single crystals, such as plating metals, are selected. With these methods, a superheated crystal is obtained and its Cjj can be measured. 

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