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Determination of the Raman Spectra of Molten ZnCl2 and Thermophysical Properties of Polymerized C60 Solids Using Atomistic Computational Techniques

机译：原子计算技术测定熔融ZnCl2的拉曼光谱和聚合的C60固体的热物理性质

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摘要

Recent advances in high performance computing technology have enabled the implementation of computational materials science algorithms to examine the structure-property relations of a wide variety of materials. Fundamental insights gained from these studies can thus provide guidelines for the appropriate selection of materials (or combination of materials) for targeted engineering and technological applications. In this regard, this dissertation primarily focuses on using atomistic computational techniques for characterizing the vibrational properties of two distinct material-systems, namely molten ZnCl_2 and polymerized C_60 solids, and thus gain new knowledge on their structure and thermophysical properties respectively.;In the first study, ab initio methods were used to interpret the experimentally determined Raman spectrum of molten ZnCl_2, thereby providing never before available insights on the short range structural ordering in ZnCl_2, melts. In particular, using ab initio molecular dynamics (AIMD) the different structural motifs present in molten ZnCl_2 were determined at 600K. The accuracy of the result was confirmed by the good agreement between the structural factors and radial distribution functions as determined from AIMD and neutron diffraction (ND) experiments. To study the Raman signatures of the ZnCl_2 melt structures, appropriate solid-state ZnCl_2 prototypes were chosen to represent the different observed structural motifs. The respective Raman signatures of the different prototypes were calculated from density functional perturbation theory (DFPT) within the density functional theory (DFT) framework. The identification of the respective Raman signatures provided the ability to accurately deconvolute the experimentally determined Raman spectrum at 600K as well as identify the relative population of the different short range ordering structures. The findings of this study has implications for optimizing the composition and operating temperatures of ZnCl_2 based salts (e.g. KCl-NaCl-ZnCl_2-AlCl_3) for utilization as thermal storage fluids in concentrating solar power plants.;In the second study, classical molecular dynamics (MD) was used to compare and contrast phonon-driven thermophysical properties of polymerized and unpolymerized solid state polymorphs of C_60. To achieve this, a newly parameterized interatomic potential was developed for accurately modeling the short range as well as long range interactions in the C_60 solids. Using this potential, it was unambiguously shown that polymerized C_60 polymorphs exhibit a two order of magnitude enhancement in the thermal conductivity and an order of magnitude change in the elastic stiffness. The significant increase in the thermal conductivity was correlated to the presence of new THz thermal phonon modes, characterized by larger mean free paths. In addition, it was also seen that the Debye temperature of the C_60 structures was strongly dependent on the extent of polymerization. The new understanding obtained in this work provides valuable guidelines for the design and development of novel C_60 based phononic metamaterials for applications as vibrational and thermal management systems.
机译：高性能计算技术的最新进展已使计算材料科学算法的实现能够检查各种材料的结构-特性关系。因此，从这些研究中获得的基本见识可以为针对目标工程和技术应用适当选择材料（或材料组合）提供指导。在这方面，本论文主要集中在使用原子计算技术来表征两种不同的材料系统，即熔融的ZnCl_2和聚合的C_60固体的振动特性，从而分别获得有关其结构和热物理性质的新知识。在研究中，从头算方法被用来解释实验确定的熔融ZnCl_2的拉曼光谱，从而从未提供有关ZnCl_2熔体的短程结构有序的见解。特别是，使用从头算分子动力学（AIMD），在600K下确定了熔融ZnCl_2中存在的不同结构基序。由AIMD和中子衍射（ND）实验确定，结构因素与径向分布函数之间的良好一致性证实了结果的准确性。为了研究ZnCl_2熔体结构的拉曼特征，选择了合适的固态ZnCl_2原型来代表观察到的不同结构图案。根据密度泛函理论（DFT）框架内的密度泛函扰动理论（DFPT）计算了不同原型的各自拉曼签名。各个拉曼特征的识别提供了准确地对600K实验确定的拉曼光谱进行反卷积的能力，以及识别不同短程有序结构的相对种群的能力。这项研究的发现对于优化ZnCl_2基盐（例如KCl-NaCl-ZnCl_2-AlCl_3）的组成和操作温度具有重要意义，这些盐可用作聚光太阳能发电厂的储热流体。在第二项研究中，经典分子动力学（ MD）被用来比较和对比C_60的聚合和未聚合固态多晶型的声子驱动的热物理性质。为实现此目的，开发了一种新参数化的原子间电势，用于精确建模C_60固体中的短程和长程相互作用。利用这一潜力，可以明确地表明，聚合的C_60多晶型物的导热系数提高了两个数量级，弹性刚度变化了两个数量级。热导率的显着增加与新的THz热声子模式的存在有关，其特征在于较大的平均自由程。另外，还可以看出，C 60结构的德拜温度强烈地取决于聚合程度。在这项工作中获得的新理解为设计和开发基于C_60的新型声子超材料提供了有价值的指导，这些材料可用于振动和热管理系统。

著录项

  • 作者

    Al Sayoud, Abduljabar Q.;

  • 作者单位

    The University of Arizona.;

  • 授予单位 The University of Arizona.;
  • 学科 Engineering.
  • 学位 Ph.D.
  • 年度 2017
  • 页码 132 p.
  • 总页数 132
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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