This project will use isothermal time and temperature resolved two-dimensional X-ray diffraction (2-D t/TRXRD) techniques to independently measure the rates of intrinsic, and extrinsic nucleation, and crystal growth, as well as to perform simultaneous pair distribution function (PDF) analysis of the crystalline and amorphous fractions. This will be accomplished using high energy X-rays, area detectors with up to 30 Hz time resolution, development of melt-quenching techniques with rates greater than 500/min, and data processing methods to accommodate collection and analysis of massive amounts of data. The studies will be conducted with a series of halide materials that exhibit melting points between room temperature and 350 C, which are temperatures that are readily amenable to the proposed experimental methods. Specifically, the crystallization of clathrated networks (our halozeotypes), probing the crystallization of parent binary condensed phases, and considering step-wise structural ordering through the formation of molecular plastic crystalline phases will be studied. This project will provide invaluable training of students in fundamental scientific discoveries with implications for the atmospheric and geologic sciences as well as for the design and function of advanced materials.
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Fast time-resolved diffraction techniques are being developed, and facilities at Argonne and Brookhaven National Laboratories are being used to independently measure the rates of crystal nucleation and growth. This work is transforming classical understandings of the mechanisms by which crystals are formed from the molten state. Contrary to the assumptions of classical theory, this project will demonstrate that it is the organization of atomic and molecular level structure in the molten state, rather than surface energy that controls the rate of nucleation and crystal growth. New instrumentation and data processing tools will be developed, and series mechanistic studies performed to develop and refine a new structural-order-driven-crystallization (SODC) theory. Answers from these fundamental mechanistic studies of crystal growth will help decipher mysteries such as ice-crystal growth in clouds which dramatically impact global climates, and will be applicable to the design and function of advanced technologies such as phase change materials utilized for data storage for which understanding of the rapid amorphous-to-crystal transitions will lead to superior performance.
Understanding the mechanism by which crystals form is of critical importance to diverse areas of science and technology. For example understanding when and how ice crystals grow to form cirrus clouds is important to atmospheric science. What controls the formation of fine grain granite vs. large crystalline pegmatites remains an unsolved question in geology. And the speed at which crystals grow from an amorphous state in phase change materials determines read-write capabilities for optical data storage technologies. In spite of the broad importance to understanding the mechanism by which crystals grow, mechanistic understanding has only moderately advanced since the development of classical nucleation theory (CNT) in the first half of the 20th century. The research funded by this NSF grant has taken a two-pronged approach to advancing the understanding of the mechanism(s) of crystal growth. First it is important to understand the structural relationship between the liquid and crystalline state. Continuing our previous research in this area, we initiated a new study into the plastic crystalline phase of CBr4. Plastic crystals are a phase of mater intermediate between that of a liquid and a crystal, and can provide insight into the liquid to crystal transition process. Using high energy synchrotron diffraction studies, and various modeling techniques, we demonstrated extensive order in the diffuse scattering of this plastic crystalline phase, which disproves the concept of a free-rotor structure of plastic crystals. Instead plastic crystals (and liquids) must be describe with recognition of the importance of intermolecular correlations in both the plastic and liquid phases. In related work, we also are exploring the structure of concentrated salt solutions, specifically aqueous ZnCl2, and have demonstrated that in contrast to classic descriptions of solutions, concentrated aqueous solutions are better described as ionic liquids. This is most clearly demonstrated in the structure and characterization of the three equivalent hydrate solution which adopts a CsCl-type salt structure as [Zn(OH2)6][ZnCl4], a material that is liquid down to 8 °C. The second major effort of this research project was to use temperature and time resolved synchrotron X-ray diffraction (TtXRD) and differential scanning calorimetry (DSC) to measure the temperature dependent rate of crystal growth, independently from the influences of crystal nucleation. Using an area detector we clearly identified conditions where the rate of crystal growth is fast with respect to the rate of nucleation. Under these conditions single or near single crystals grow. Interestingly, under these conditions, it was observed that the rate of crystal growth of the halozeotype CZX-1 increases to a maximum at about 135 °C, then decreases to zero at about 10° below the melting point (Tm = 174 °C). This work demonstrates that the rate of crystal growth does not follow Arrhenius kinetics. Furthermore, the slowed rate of crystal growth as the melting temperature is approached is shown not to be caused by decreased nucleation (as supposed by CNT) since the rate of single crystal growth is slowed. We are currently working on a model that explains this process based on the entropy of activation. The kinetic measurement of the rate of crystallization was initially complicated because fitting data from crystal growth measurements using multiple techniques (multiple synchrotron beam lines and DSC) yielded different rate constants. It is not reasonable for a chemical reaction, such as crystallization to exhibit rate constants that are dependent on the technique used for measurement, instead of being intrinsic to the material being studied. To address this issue, we constructed an extensive set of crystal growth simulations whereby we could evaluate the parameters of the Kolmogorov, Johnson and Mehl, Avrami (KJMA) rate expression with respect to definable parameters of our simulation. These simulations clearly demonstrate that the classic KJMA model must be corrected with respect to the sample volume, as well as with respect to the shape of the container in which the crystal is growing. Application of this correction provides the material specific rate parameter, the velocity of the phase boundary. The sample volume corrected KJMA model affords an unprecedented measurement of the velocity of the phase boundary from bulk crystallization kinetic measurements. For the CZX-1 syster, the velocity of the phase boundary reaches a maximum of 3.2 × 10-5 m×s-1 at 135 °C. Given the unit cell length for CZX-1 is 10.85 Å, the maximal phase boundary velocity indicates the crystalline axes increase by approximately 29 unit cell lengths per millisecond. This work, which for the first time provides rate parameters for a condensed matter phase transformation that are intrinsic to the material being studied, opens great opportunity for further mechanistic study of diverse materials. And the clear separation of the rates of crystal growth from the rate of nucleation will now allow the careful investigation of the non-Arrhenius behavior of the rate of crystal growth.
