Isothermal nanocrystallization kinetics during polymorphic transformation

The Kolmogorov-Johnson-Mehl-Avrami (KJMA) isothermal phase transformation kinetics theory assumes negligible volume of the nucleus. For systems with nano-sized grain formation, the initial introduction of finite radius of the product phase nuclei has special relevance. The value of time exponent deviates from the ideal KJMA prediction for such cases. Discrete models may be advanced for explaining such deviation. The main limitation of such an approach is the discrete time step used. We have recently studied these aspects of isothermal transformation kinetics in a systematic way with the help of discrete adaptation of KJMA model. The transformation is assumed to be polymorphic and interface controlled. In this paper we present studies on 2-dimensional (2D) systems.

Heating-Induced Phase Transition of Bupropion Hydrobromide Polymorphs

Computer simulation of recrystallization in non-uniformly deformed metals

We report a quasi-three-dimensional observation of electron-beam-induced nanobubbles inside a 1000 nm thick layer of water using the liquid cell electron microscopy. In the early stage of observation, heterogeneous bubble nucleation occurred, and small bubbles coalesced with the adjacent bubbles when they come in contact with each other. However, for the first time, we found that after prolonged electron beam irradiation heterogeneous nucleation did not occur more, and then homogeneous nucleation started even though a solid surface was available for heterogeneous nucleation. We conclude that the Ostwald ripening effect prevents heterogeneous nucleation from occurring and that the lower surface tension due to the generation of ions and radicals boosts the homogeneous nucleation. It was also discovered that the generation sites of homogeneous nucleation are beneath the three-phase contact lines of existing interfacial bubbles.

Isothermal crystallization kinetics of Ni 24Zr 76 and Ni 24(ZrX) 76 amorphous alloys

Eliminating overgrowth effects in Kolmogorov–Johnson–Mehl–Avrami model through the correlation among actual nuclei

Abstract. Aerosols affect cirrus formation and evolution, yet quantification of these effects remain difficult based on in situ observations due to the complexity of nucleation mechanisms and large variabilities in ice microphysical properties. This work employed a method to distinguish five evolution phases of cirrus clouds based on in situ aircraft-based observations from seven U.S. National Science Foundation (NSF) and five NASA flight campaigns. Both homogeneous and heterogeneous nucleation were captured in the 1 Hz aircraft observations, inferred from the distributions of relative humidity in the nucleation phase. Using linear regressions to quantify the correlations between cirrus microphysical properties and aerosol number concentrations, we found that ice water content (IWC) and ice crystal number concentration (Ni) show strong positive correlations with larger aerosols (>500 nm) in the nucleation phase, indicating strong contributions of heterogeneous nucleation when ice crystals first start to nucleate. For the later growth phase, IWC and Ni show similar positive correlations with larger and smaller (i.e., >100 nm) aerosols, possibly due to fewer remaining ice-nucleating particles in the later growth phase that allows more homogeneous nucleation to occur. Both 200 m and 100 km observations were compared with the nudged simulations from the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 6 (CAM6). Simulated aerosol indirect effects are weaker than the observations for both larger and smaller aerosols for in situ cirrus, while the simulated aerosol indirect effects are closer to observations in convective cirrus. The results also indicate that simulations overestimate homogeneous freezing, underestimate heterogeneous nucleation and underestimate the continuous formation and growth of ice crystals as cirrus clouds evolve. Observations show positive correlations of IWC, Ni and ice crystal mean diameter (Di) with respect to Na in both the Northern and Southern Hemisphere (NH and SH), while the simulations show negative correlations in the SH. The observations also show higher increases of IWC and Ni in the SH under the same increase of Na than those shown in the NH, indicating higher sensitivity of cirrus microphysical properties to increases of Na in the SH than the NH. The simulations underestimate IWC by a factor of 3–30 in the early/later growth phase, indicating that the low bias of simulated IWC was due to insufficient continuous ice particle formation and growth. Such a hypothesis is consistent with the model biases of lower frequencies of ice supersaturation and lower vertical velocity standard deviation in the early/later growth phases. Overall, these findings show that aircraft observations can capture both heterogeneous and homogeneous nucleation, and their contributions vary as cirrus clouds evolve. Future model development is also recommended to evaluate and improve the representation of water vapor and vertical velocity on the sub-grid scale to resolve the insufficient ice particle formation and growth after the initial nucleation event.

We studied the competition between heterogeneous and homogeneous nucleation ofan aqueous suspension of charged colloidal spheres close to the container walls.Samples of equilibrium crystalline structure were shear-melted and the metastablemelt left to solidify after the cessation of shear. The crystallization kinetics wasmonitored using time-resolved scattering techniques: at low particle number densitiesn we applied an improved static light scattering method while at large particle concentrationsultra-small-angle x-ray scattering was applied for the first time. Our results show someunexpected behavior: the heterogeneous nucleation at the container walls is delayed incomparison to the homogeneous bulk nucleation and its rate density appears surprisinglyslightly smaller, demonstrating the complexity of the observed crystallization process.

This work addresses homogeneous and heterogeneous nucleation phenomena in atmospheric aerosol particles. First, the heterogeneous freezing of supercooled droplets containing feldspar particles has been investigated in a novel droplet freezing assay instrument. Second, the efflorescence of NaCl-dihydrate in supercooled NaCl and sea salt solute droplets has been studied with a technique combining electrodynamic balance (EDB) and Raman microscopy. The study was aimed to clarify the fundamental stochastic nature of homogeneous and heterogeneous nucleation in metastable aerosol systems. The ice nucleation efficiency of feldspar particles has been measured by depositing several hundred suspension droplets onto continuously cooled silicon substrate. Three types of experiments have been conducted: cooling ramp, isothermal freezing at constant temperature and freeze-thaw cycles. The observed freezing behavior have been interpreted in a framework of classical nucleation theory (Soccer Ball Model, SBM). The SBM was shown to adequately describe the shift of the freezing curves towards lower temperature with dilution, the cooling rate dependence and the ice nucleating active sites (INAS) surface density n_s (T) in a wide temperature range. Moreover, the SBM was capable of reproducing the variation of INAS surface density n_s (T) with concentration of IN in the suspension droplets and correctly predicting the leveling-off of the n_s (T) at low temperature. The freeze-thaw experiments have clearly shown that the heterogeneous freezing induced even by very active ice nucleating species still possesses a stochastic nature, with the degree of randomness increasing towards homogeneous nucleation. The reliability of the experimental approach has been also demonstrated during the Fifth Ice Nucleating intercomparison campaign (FIN02). Single solution droplets consisting of pure NaCl or simulated sea salt were suspended into the EDB maintained at constant temperature and relative humidity. A statistical analysis of several hundreds of individual efflorescence events allowed to describe the probability of NaCl dihydrate crystallization as a function of temperature, droplet volume, and solute concentration, and to derive the homogeneous and heterogeneous nucleation rate coefficients of NaCl dihydrate. Based on these measurements, the discrepancies between temperature dependent efflorescence of NaCl dihydrate observed in the previous experiments could be quantitatively explained for the first time. Furthermore, the experiments with the sea salt solution droplets showed an enhanced formation of NaCl dihydrate due to the faster crystallization of secondary sea salt components triggering the heterogeneous nucleation of NaCl dihydrate.

Ice formation induced by atmospheric particles through heterogeneous nucleation is not well understood. Onset conditions for heterogeneous ice nucleation and water uptake by particles collected in Los Angeles and Mexico City were determined as a function of temperature (200–273 K) and relative humidity with respect to ice (RHice). Four dominant particle types were identified including soot associated with organics, soot with organic and inorganics, inorganic particles of marine origin coated with organic material, and Pb/Zn‐containing particles apportioned to emissions relevant to waste incineration. Single particle characterization was provided by micro‐spectroscopic analyses using computer controlled scanning electron microscopy with energy dispersive analysis of X‐rays (CCSEM/EDX) and scanning transmission X‐ray microscopy with near edge X‐ray absorption fine structure spectroscopy (STXM/NEXAFS). Above 230 K, significant differences in onsets of water uptake and immersion freezing of different particle types were observed. Below 230 K, particles exhibited high deposition ice nucleation efficiencies and formed ice atRHicewell below homogeneous ice nucleation limits. The data suggest that water uptake and immersion freezing are more sensitive to changes in particle chemical composition compared to deposition ice nucleation. The data demonstrate that anthropogenic and marine influenced particles, exhibiting various chemical and physical properties, possess distinctly different ice nucleation efficiencies and can serve as efficient IN at atmospheric conditions typical for cirrus and mixed‐phase clouds.

Abstract. Binary heterogeneous nucleation of water-succinic/glutaric/malonic/adipic acid on nanometer-sized particles is investigated within the frame of classical heterogeneous nucleation theory. Homogeneous nucleation is also included for comparison. It is found that the nucleation probabilities depend on the contact angle and on the size of the seed particles. New thermodynamical properties, such as saturation vapor pressure, density and surface tension for all the dicarboxylic acid aqueous solutions are included in the calculations. While the new surface tension and density formulations do not bring any significant difference in the computed nucleation rate for homogeneous nucleation for succinic and glutaric acids, the use of the newly derived equations for the vapor pressure decrease the acid concentrations in gas phase by 3 orders of magnitude. According to our calculations, the binary heterogeneous nucleation of succinic acid-water and glutaric acid-water – although it requires a 3–4 orders of magnitude lower vapor concentrations than the homogeneous nucleation – cannot take place under atmospheric conditions. On the other hand binary homogeneous nucleation of adipic acid-water systems might be possible under conditions occuring in upper boundary layer. However, a more detailed characterization of the interaction between the surface and the molecules of the nucleating vapor should be considered in the future.

Nanocrystallization kinetics understood as multiple microprocesses following the classical theory of crystallization

Amorphization of silicon by ion implantation: Homogeneous or heterogeneous nucleation?

The impingement effect of an inert, immobile second phase on the recrystallization of a matrix

We present a study of homogeneous nucleation in polymer crystallization. Crystal nucleation is observed within discrete droplets of poly(ethylene oxide) that are formed by the dewetting of a thin film. The samples provide an ensemble of impurity-free droplets, with length scales that can easily be measured. Droplet crystallization is found in two distinct temperature regimes corresponding to homogeneous and heterogeneous nucleation. Here we focus on homogeneous nucleation and show that the nucleation rate scales with the volume of the crystallizable domain, indicating that we are observing homogeneous nucleation in the bulk of the droplets. In addition, the dependence of the nucleation rate on temperature is consistent with classical nucleation theory.