Experimental Determination of the Crystallization Phase-Boundary Velocity in the Halozeotype CZX-1

Abstract

Isothermal crystallization experiments were performed on the halozeotype CZX-1 with 2D temperatureand time-resolved synchrotron X-ray diffraction (TtXRD) and differential scanning calorimetry (DSC). These crystallization experiments demonstrate that the fundamental materials property, the velocity of the phase boundary of the crystallization front, v pb , can be recovered from the Kolmogorov Johnson and Mehl and Avrami (KJMA) model of phase-boundary controlled reactions by introducing the sample volume into the KJMA rate expression. An additional corrective term is required if the sample volume of the crystallization measurement is anisotropic. The concurrent disappearance of the melt and appearance of the crystalline phase demonstrate that no intermediates exist in the crystallization pathway. The velocity of the phase boundary approaches 0 as the glass transition (T g ≈ 30°C) is approached and at about 10°below melting point (T m = 173°C). The velocity of the phase boundary reaches a maximum of 30 μm s −1 at 135°C. Single or near-single crystals are grown under conditions where the v pb is much greater than the rate of nucleation.

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“…The KJMA expression provides excellent fits for diverse phase transition kinetic data, from which mechanistic detail is frequently inferred. Nevertheless, as we recently summarized [2], and here provide another case study, the KJMA parameters are significantly affected by experimental factors that do not necessarily reflect the true reaction mechanism.…”

“…This is likely why unconstrained fitting to the KJMA model yields values of the exponent n that are less than 3, which was expected for three-dimensional growth. It must be recognized, however, that the KJMA parameters n, kA and t0 are highly correlated [2]. It is not practical to conduct a set of aspect ratio and nucleation rate simulations for each type of material to obtain anisotropy corrections.…”

“…This is precisely the trend observed experimentally in Figure 7 with the large TtND and DSC pan experiments exhibiting the slowest rate constants and TtXRD single crystallite rate constants being the fastest. Clearly, the sample size must be reintroduced into the rate expression to obtain intrinsic, material specific rate constants [2,9].…”

“…Unlike microscopy or calorimetry, diffraction techniques uniquely afford phase identification concurrent with the temporal evolution of the transformation (e.g., the commensurate formation of calcite and vaterite from amorphous calcium carbonate [1]). Time-resolved diffraction using an area detector further affords the ability to discriminate between nucleation and growth components of a phase transition [2].…”

“…These comparative data are evaluated using the modified KJMA expression (M-KJMA), Equation (2) [2,9], demonstrating the corrections necessary to ensure rate constants are intrinsic to the sample and reaction conditions, not the specific technique of measurement.…”