Crystal nudeation kinetics

Fundamental studies on the kinetics of crystal nudeation and growth in glassy melts under the influence of surface tension-driven (Marangoni) convection that, under Ig conditions, would be superseded by gravity-driven (B nard or Rayleigh) convection. 

From these curves, the overall crystallization kinetics and the nudeation kinetics were deduced and are summarized in the activation diagram in Figure 37. ° In most measured curves shown in Figure 36, two contributions - one for the mdting of isothermally formed crystals and the other for the cold crystallization or reorganization - are separated suffi-dently for analysis. The presence of a nudeation -induced increase in cold crystallization is visible up to 290 K (black line with stars). For temperatures up to 230K, the nudeation effect of cold crystallization saturates at times before major crystallization at the annealing temperature is seen in the total latent heat. 

For different values of n ing(i ) = i2", other kinetic expressions can be developed. Figure 8.10 [18] shows the type of powder produced on spray diydng a solution that consists of metal salts of barium and iron in the ratio 1 12 (i.e., barium ferrite). Here we see the remains of the spherical droplets with a surface that consists of the metal salt precipitates, which form a narrow size distribution of platelet crystals (see Figure 8.10(a) and (b)). This narrow crystal size distribution is predicted by the population balance model if nudeation takes place over a short period of time. When these particles are spray roasted (in a plasma gun), the particles are highly sintered into spherical particles (see Figure 8.10(c)). 

Under eqiiiUbrium vapor pressure of water, the crystalline tdhydroxides, Al(OH)3 convert to oxide—hydroxides at above 100°C (9,10). Below 280°—300°C, boehmite is the prevailing phase, unless diaspore seed is present. Although spontaneous nudeation of diaspore requires temperatures in excess of 300 °C and 20 MPa (200 bar) pressure, growth on seed crystals occurs at temperatures as low as 180 °C. For this reason it has been suggested that boehmite is the metastable phase although its formation is kinetically favored at lower temperatures and pressures. The ultimate conversion of the hydroxides to corundum [1302-74-5], Al O, the final oxide form, occurs above 360°C and 20 MPa. 

A kinetic treatment of glass formation VI1 transient nudeation in non-isothermal crystallization during cooling. J. Non-Cryst. Solids, 50, 189-202. 

Differential scanning calorimetry (DSC) can be used to study the onset of crystallization on cooling from the melt where nudeated polymers have higher onset temperatures (Figures 2 and 3). Isothermal studies provide kinetic data where Avrami analysis (equation 2, where x = reduced... 

To study the kinetics of nudeation under isothermal conditions, experiments were performed for which the sample was cooled so fast (10 000 Ks" ) that no homogeneous nudeation occurs. If a sample were amorphous after annealing, it would show zero overall latent heat, but the cold crystallization enthalpy as well as its melting enthalpy would depend on the number of active nuclei in the sample. 

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