Polyamorphism and Structural Transitions during Glass Formation

[00:72258]基弗越来越多的证据表明，玻璃态和液态虽然都是无定形的，但在结构和热力学上是不同的。这意味着玻璃化转变是一种“多晶”转变。此外，物质在玻璃态下发生结构转变的趋势可能会影响材料的非线性光学响应和机械行为。该项目包括研究玻璃成形材料在压力和温度下的结构演变。将确定在玻璃形成过程中结构弛豫涉及多晶结构转变的程度，而不是不变结构中的粘性减速。过冷熔体的结构表征将使用布里渊和拉曼光散射以及分子动力学模拟的组合来完成，这将允许人们识别结构的机械响应和结构构建块的性质。布里渊光散射将用于在分子尺度上确定玻璃成型熔体的高频复杂机械模量。实分量或存储模量提供了有关结构完整性和网络连通性的信息，而虚分量对应于小结构成分（例如原子跳变）在非周期运动中耗散的能量。同时拉曼散射将允许人们建立粘弹性行为与分子构建块的对称性和丰度之间的直接关联。分子动力学模拟将用于调和实验确定的拉曼光谱和粘弹性性质与分子结构的详细几何描述。多晶态的表现形式将通过检查一系列玻璃形成系统来揭示，选择系统地取样广泛的玻璃形成属性，如价，网络元素的键合类型和熔体脆性。系统将包括锗酸盐、磷酸盐、碲化物和硫属化合物。通过研究同时压力和温度对加热金刚石砧细胞转变过程的影响，将建立网络前体的性质与不同非晶多晶之间转变机制之间的关系。这个项目的目的是澄清多面体的概念和表现形式。有了这一澄清，我们对玻璃科学中重要问题的理解将得到推进，因为材料在玻璃态下经历结构转变的趋势可能会影响其性质，例如非线性光学响应和机械行为，这对高科技应用很重要。扩大玻璃的适用范围将使新技术成为可能，包括光子学、光通信和计算、药物和辐射输送、生物医学植入物、传感器、能源储存和发电、核废料遏制和轻质金属合金。

0072258KiefferThere is increasing evidence that the glassy and liquid states are structurally and thermodynamically distinct, although both amorphous. This implies that the glass transition is a "polyamorphic" transformation. Furthermore, a tendency of the substance to undergo structural transitions in the glassy state could affect materials properties such as non-linear optical responses and mechanical behaviors. This project comprises the investigation of the structural evolution in glass-forming materials as a function of pressure and temperature. The extent to which structural relaxation during glass formation involves polyamorphic structural transitions, as opposed to a viscous slowing within an invariant structure, will be determined. Structural characterization in supercooled melts will be accomplished using a combination of Brillouin and Raman light scattering and molecular dynamics simulations, which will allow one to identify the mechanical response of the structure and the nature of structural building blocks. Brillouin light scattering will be used to determine the high-frequency complex mechanical modulus of glass-forming melts at the molecular scale. The real component, or storage modulus, provides information on the structural integrity and network connectivity, while the imaginary component corresponds to the energy dissipated in aperiodic motions of small structural constituents (e.g., atomic hopping). Simultaneous Raman scattering will allow one to establish a direct correlation between this visco-elastic behavior and the symmetries and abundance of molecular building blocks. Molecular dynamic simulations will be used to reconcile the experimentally determined Raman spectra and visco-elastic properties with a detailed geometric description of the molecular structure. The manifestation of polyamorphism will be revealed by examining a series of glass-forming systems, chosen to systematically sample a wide range of glass-forming attributes, such as the valences, bonding types of the network elements, and melt fragility. Systems will include germanates, phosphates, tellurites, and chalcogenides. The relationship between the nature of the network former and the mechanisms of transitions between different non-crystalline polymorphs will be established by studying the effects of simultaneous pressure and temperature on the transition processes using heated diamond anvil cells.The objective of this project is to clarify the concept and manifestations of polyamorphism. With this clarification, our understanding of important issues in glass science will be advanced, since a tendency of a material to undergo structural transitions in the glassy state could affect its properties, such as non-linear optical responses and mechanical behaviors, which are important for high tech applications. Expanding the range of applicability of glasses will enable new technologies, including photonics, optical telecommunication and computing, drug and radiation delivery, bio-medical implants, sensors, energy storage and generation, nuclear waste containment, and light-weight metallic alloys.