Critical nucleation temperature

Instantaneous boiling takes place only if the temperature of a liquid is higher than its supeiheat-limit temperature (also called the homogeneous-nucleation temperature), in which case, boiling occurs throughout the bulk of the liquid. This temperature is only weakly dependent on the initial pressure of the liquid and the pressure to which it depressurizes. As stated in Section 6.1., T has a value of about 0.89T,., where is the (absolute) critical temperature of the fluid. 

Using the properties of water Li and Cheng (2004) computed from the classical kinetics of nucleation the homogeneous nucleation temperature and the critical nu-cleation radius ra. The values are 7s,b = 303.7 °C and r nt = 3.5 nm. However, the nucleation temperatures of water in heat transfer experiments in micro-channels carried out by Qu and Mudawar (2002), and Hetsroni et al. (2002b, 2003, 2005) were considerably less that the homogeneous nucleation temperature of 7s,b = 303.7 °C. The nucleation temperature of a liquid may be considerably decreased because of the following effects dissolved gas in liquid, existence of corners in a micro-channel, surface roughness. 

There are aspects of cell membranes other than their permeability to water and solutes that also play a critical role in the responses of cells to freezing. The structure of the plasma membrane allows cells to supercool and probably determines their ice-nucleation temperature. The nucleation temperature along with the permeability of membranes to water are the chief determinants of whether cells cooled at... 

For an external pressure of one atmosphere, is about 0.89-0.90 times the critical temperature of a pure hydrocarbon. For hydrocarbon mixtures, the superheat-limit temperature may be closely approximated by a mole fraction average of the homogeneous nucleation temperature of the pure components. 

From nucleation theory (see Section IX), one can estimate the expected rate of formation of critical-sized vapor embryos in a liquid as a function of temperature. This rate is a very strong function of temperature emd changes from a vanishingly low value a few degrees below the homogeneous nucleation temperature to a very large value at this temperature. 

Miscible blends of poly(vinyl methyl ether) and polystyrene exhibit phase separation at temperatures above 100 C as a result of a lower critical solution temperature and have a well defined phase diagram ( ). This system has become a model blend for studying thermodynamics of mixing, and phase separation kinetics and resultant morphologies obtained by nucleation and growth and spinodal decomposition mechanisms. As a result of its accessible lower critical solution temperature, the PVME/PS system was selected to examine the effects of phase separation and morphology on the damping behavior of the blends and IPNs. 

In addition to requiring nucleating agents to promote crystallization, glass ceramics usually must be heat-treated at the critical nucleating, crystallization, and annealing temperatures to restructure the glass (Fig. 1). These temperatures can be conveniently determined, to a first approximation, by differential thermal analysis (DTA)i (Fig. 2). 

Prefreezing phenomena are less easily summarized than premelting ones partly because they are more diversified and partly because they have been less well characterized up to now. In quasi-crystalline melts, minute concentrations of crystalline regions can be inferred on the basis of observations on spontaneous nucleation at the critical supercooling temperatures P, which for many quasi-crystalline melts show a constant ratio to the... 

For a wide range of substances, including organic melts, the critical homogeneous nucleation temperature expressed in kelvins is approximately 0.8-0.85 r, although for hydrocarbons >Ci5 it may approach 0.957 . ... 

Owing to the presence of specific interactions, most blends have a phase separation diagram with a lower critical solution temperature, LOST, i.e. phase separation occurs upon heating. Two separation mechanisms are known spinodal decomposition (SD), and nucleation-and-growth (NG). The morphology generated in NG is dispersed, whereas that in SD is co-continuous. Cahn and HiUiard s theory describes well the SD kinetics [1, 2]. 

Figure 8. Left panel phase diagram of ice T> T (P)) and transition lines corresponding to the ice Ih-to-HDA, LDA-to-HDA, and HDA-to-LDA transformations T

Detailed calculations are presented in the articles (V.I. Talanin I.E. Talanin, 2010b). The temperature of the formation of A-microdefects corresponds to 1153 K. An increase in the crystal growth rate weakly decreases the critical radius of A-microdefects and shghtly affects the nucleation temperature. An increase in the crystal growth rate leads to an almost twofold decrease in the concentration of introduced defects. 

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