Structure and Thermodynamics of Quasicrystals

[111198]含过渡金属的富铝合金形成复杂的晶体结构和准晶体。这些合金表现出不同寻常的有用的机械性能，如摩擦系数低，低温下硬度大，高温下延展性好。准晶体已经用于强化外科合金和煎锅表面。进一步的应用，如涂层球轴承提出。对准晶体的最大兴趣在于它们所提出的基本科学问题。尖锐的衍射峰与晶体学上禁止的十面体和二十面体对称共存构成了最初吸引研究人员进入该领域的悖论。在认识到准周期性允许这样的对称性之后，人们的兴趣转向解释准周期序的起源和性质。这一至今仍未解决的问题是本理论研究的重点。前人的研究主要采用原子间势计算总能量和全从头算方法计算金属间合金。总能量计算被纳入复杂的蒙特卡罗计算机技术称为准晶格气体模拟。这些方法被用于预测AlCuCo和AlNiCo十方准晶相的结构，与实验结构数据有很好的定性一致性。这些方法揭示了除了优化、能量最小化结构之外的基本激励。原则上，可以得到随温度和成分变化的整个热力学演化过程。目前的拨款建立在这一成功的基础上，通过研究三元十角相。精确的原子模型和可靠的能量计算将用于检查匹配规则、能量学和相邻十角层的耦合。将对模型与实验数据进行详细的定量比较。用于十二面体相的基本技术将适用于稳定的二十面体相。抽象随机铺层模型继续发展，以高协维铺层作为非晶结构模型，以协维1铺层作为堆叠膜模型。含过渡金属的富铝合金形成复杂的晶体结构和准晶体。这些合金表现出不同寻常的有用的机械性能，如摩擦系数低，低温下硬度大，高温下延展性好。准晶体已经用于强化外科合金和煎锅表面。进一步的应用，如涂层球轴承提出。对准晶体的最大兴趣在于它们所提出的基本科学问题。尖锐的衍射峰与晶体学上禁止的十面体和二十面体对称共存构成了最初吸引研究人员进入该领域的悖论。在认识到准周期性允许这样的对称性之后，人们的兴趣转向解释准周期序的起源和性质。这一至今仍未解决的问题是本理论研究的重点。目前的赠款继续进行三元十角相的研究。精确的原子模型和可靠的能量计算将用于检查匹配规则、能量学和相邻十角层的耦合。将对模型与实验数据进行详细的定量比较。用于十二面体相的基本技术将适用于稳定的二十面体相

0111198WidomAluminum-rich alloys containing transition metals form intricate crystal structures and quasicrystals. These alloys display unusual and useful mechanical properties such as low coefficients of friction, great hardness at low temperatures, and ductility at high temperatures. Quasicrystals already strengthen surgical alloys and coat surfaces of frying pans. Further applications such as coatings for ball bearings have been proposed.The greatest interest in quasicrystals lies in the fundamental scientific issues they raise. Coexistence of sharp diffraction peaks with crystallographically forbidden decagonal and icosahedral symmetry posed the paradox that initially drew researchers to the field. After recognizing that quasiperiodicity allows such symmetries, interest turned to explaining the origin and nature of quasiperiodic order. This problem, which remains unsolved today, is the focus of this theoretical research.Prior research applied methods of total energy calculations using interatomic potentials and full ab-initio calculations to intermetallic alloys. Total energy calculations were incorporated into sophisticated Monte Carlo computer techniques known as quasilattice gas simulations. These methods were developed to predict structures of AlCuCo and AlNiCo decagonal quasicrystal phases, achieving excellent qualitative agreement with experimental structural data. These methods reveal fundamental excitations in addition to optimal, energy minimizing, structures. In principle, the entire thermodynamic evolution with temperature and composition can be obtained.The current grant builds on this success by studying ternary decagonal phases. Accurate atomistic models and reliable energy calculations will be used to examine matching rule energetics and the coupling of adjacent decagonal layers. Detailed quantitative comparisons will be made of models with experimental data. The basic techniques used for decagonal phases will be adapted to stable icosahedral phases.Development of abstract random tiling models continues with study of high codimension tilings as model amorphous structures and study of codimension-1 tilings as models of stacked membranes.%%%Aluminum-rich alloys containing transition metals form intricate crystal structures and quasicrystals. These alloys display unusual and useful mechanical properties such as low coefficients of friction, great hardness at low temperatures, and ductility at high temperatures. Quasicrystals already strengthen surgical alloys and coat surfaces of frying pans. Further applications such as coatings for ball bearings have been proposed.The greatest interest in quasicrystals lies in the fundamental scientific issues they raise. Coexistence of sharp diffraction peaks with crystallographically forbidden decagonal and icosahedral symmetry posed the paradox that initially drew researchers to the field. After recognizing that quasiperiodicity allows such symmetries, interest turned to explaining the origin and nature of quasiperiodic order. This problem, which remains unsolved today, is the focus of this theoretical research.The current grant continues the study of ternary decagonal phases. Accurate atomistic models and reliable energy calculations will be used to examine matching rule energetics and the coupling of adjacent decagonal layers. Detailed quantitative comparisons will be made of models with experimental data. The basic techniques used for decagonal phases will be adapted to stable icosahedral phases.***