The Structure and Mechanism of Melt Crystallization of CZX-1, a Templated ZnCl2 Network Material

Halozeotypes;DSC;Diffuse Scattering;Crystallization;2-D X-ray Diffraction;Quaternions;Neutron Diffraction;Polarizing Microscopy

Josey, Amanda Ann ; Paul A. Maggard, Committee Member,Mike H. Whangbo, Committee Member,Jerry L. Whitten, Committee Member,James D. Martin, Committee Chair,Josey, Amanda Ann ; Paul A. Maggard ; Committee Member ; Mike H. Whangbo ; Committee Member ; Jerry L. Whitten ; Committee Member ; James D. Martin ; Committee Chair

University:North Carolina State University

关键词: Halozeotypes;    DSC;    Diffuse Scattering;    Crystallization;    2-D X-ray Diffraction;    Quaternions;    Neutron Diffraction;    Polarizing Microscopy;   

Others  :  https://repository.lib.ncsu.edu/bitstream/handle/1840.16/4935/etd.pdf?sequence=1&isAllowed=y

美国|英语

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【 摘 要 】

A glassforming templated derivative of ZnCl2, called CZX-1, is melt synthesized by combining stoichiometric amounts of ZnCl2, CuCl and (CH3)3NHCl in a 5:1:1 ratio.CZX-1 consists of templated sodalite cage structures with the empirical formula of CuZn5Cl121-.A cage-template charge interaction occurs where templates ((CH3)3NH1+) are positioned at one of four tetrahedrally distributed sites within a sodalite cage.Molten CZX-1 shows significant extended-range order including structural features up to 50 Å long.This network organization in molten CZX-1 does not allow for nanoparticle formation during melt crystallization. CZX-1 exhibits several temperature-induced phase transitions.Upon heating above 164 oC the templates disorder, forcing a phase transition of α-CZX-1 to β-CZX-1 at 170 oC.A peritectic point is observed at 175 oC where CZX-1 disproportionates into 20 % β-ZnCl2 and 80 % template- and copper(I)-rich liquid.β-ZnCl2 dissolves forming an isotropic liquid at 205 oC.Rapid quenching of the isotropic melt to below 164 oC produces α-CZX-1, while slow cooling produces β-ZnCl2 and the template- and copper(I)-rich liquid below 205 oC.The disproportionation of CZX-1 into β-ZnC2 and liquid is reversed if the material is rapidly cycled from below 164 oC to the isotropic melt.Time-resolved X-ray diffraction experiments are exploited to independently measure the rates of nucleation and crystal growth.These experiments clearly indicate that condensed-phase crystallization is not explained by Classical Nucleation Theory.Rather, melt crystallization kinetics fit a new model treating CZX-1 nucleation and growth separately as a serial process.Three types of nucleation are observed, (1) intrinsic: the inherent nucleation of CZX-1, (2) extrinsic: nucleation is influenced by outside parameters like defects or internal domains, and (3) growth-initiated: continuous nucleation that follows the rate of growth.The rate of growth increases with increasing temperature to a critical temperature of 164 oC where template disordering forces the rate of growth to drop dramatically - demonstrating that the respective structure of the melt and crystalline phases exhibit a strong controlling influence on the crystallization mechanism.