Temperature effects nucleation

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. 

A study was made of nucleation in PE foams produced in a counter-rotating extruder using talc powders and masterbatches as nucleating agents and CFG and HCFC blowing agents. The results indicated the importance of shear force in nucleation, as proposed in the lump cavity nucleation model and demonstrated by melt temperature effects observed in these experiments. It was shown that shear enhancement via lump break-up was not a vital mechanism in nucleation. 6 refs. 

Crystal stmcture prediction by computer has made great steps forward in the last 10 years, with progress toward consistent success in blindfold tests. Fundamental uncertainties still remain, due to the unknown role of nucleation kinetics and to the neglect of temperature effects in the calculations. Success or failure still depends to some extent on hardly predictable factors and on the extent to which the experimental polymorph screening has been carried out. Presently, some of the best computational tools are not yet available to the general community of solid state scientists, being implemented in commercial, strictly copyrighted software. 

Experimentally nucleation rates can be determined in expansion chambers [35], The vapor is expanded in a fast and practically adiabatic process. Then it cools down. Since at low temperatures, the equilibrium vapor pressure is much lower, supersaturation is reached. Partially, this is compensated for by the pressure reduction during the expansion, but the temperature effect dominates. The density of nuclei can be measured by light scattering. 

Figure 7 DSC eooling traces (5 KmirT ) for aqueous -trehalose solutions (a) 8% wjw and (b) 21% wjw, showing freezing and eutectic crystallisation exotherms. Note the effect of concentration on the nucleation temperatures. Reproduced with permission from Roberts and Franks ...

Fig. 1 Effect of nucleation temperature on fractional crystallisation / microhardness...

Figure 32. Effect of the cooling rate on the nucleation temperature of a 319 aluminium alloy [44]. (With permission from Elsevier.)...

The theory of bubble nucleation in a superheated liquid was first applied to the concept of thermal inkjet by Allen et al. [7]. They were able to determine the minimum cmiditions for the first bubble nucleation by applying Hsu s theory [10]. Time dependent temperature profiles above a heater surface were obtained. By superimposing the activation curve with the thermal boimdary layer, the initial bubble size and the minimum temperature for nucleation were determined. Based on a one-dimensional model and by assuming the nucleation temperature to be the superheat limit of the liquid at 330°C transient temperature profiles for the heater structure and the bubble surface after nucleation were obtained. It was noticed that the decay time to ambient temperature from its initial state was only several microseconds after 6 ps heating pulse. The thermal effects of the passivation (protective coating) layer on the heater surface were also analyzed. The results showed that the effective pulse energy required for bubble nucleation increases with the thickness of the passivation layer. 

---