Stiffness Threshold in Network Glasses

小行星9701289 这是一个更新的项目，研究的物理性质的刚度阈值的硫属化物和硫卤玻璃使用拉曼散射，穆斯堡尔光谱和T-调制差示扫描量热测量（MDSC）。 具体地，拉曼模式频率 位移和Lamb-M ssbauer因子 被 作为玻璃组成的函数系统地测量，以建立局部弹性常数C和振动态密度的一阶倒数矩w-1的平均配位数依赖性。 玻璃中C和w-1的随机网络模型模拟现在可以从数值实验中获得，提供了一个方便的比较指南 通过实验室实验。 结构松弛的本质 在玻璃化转变将进行研究 在 MDSC测量，以确定非反向热流DHnr随组成的变化。 对硫系玻璃的初步实验表明，DHnr也在刚度阈值c=2.4处显示出最小值 这表明当玻璃状液体处于机械临界状态时，只有极小的焓变化伴随着玻璃状液体的结构松弛。 平均场约束计数程序还表明，网络拥有终端原子，如单倍配位原子（卤素）的刚度过渡一般转移到一个较低的平均协调C 2.4。 这些想法将在合适的拉曼，穆斯堡尔谱和MDSC实验在硫卤玻璃的定量实验测试。 这是一个研究硫系和硫卤系玻璃中刚度阈值的物理性质的更新项目。 许多含有元素S或Se或Te的玻璃被视为由原子链碎片组成的网络（平均原子配位为2）。 这些链片段可以逐渐交联 使其他原子合金化。 发现这种合金化网络的机械刚度随着网络的平均配位数测量的交联度而逐渐增加。 当协调度达到2.4时，理论计算预测，超过2.40时，网络将发生从松弛到刚性的转变和硬化。 我们建议对特定的玻璃系统进行实验研究，以解码潜在的刚度转变的物理性质。 这种转变对应于玻璃系统中的神奇组合物，在玻璃科学中具有普遍的兴趣，因为一些最好的玻璃形成物发生在这种神奇组合物附近。 此外，对这种转变的理解也将有助于解开玻璃化转变中发生的结构停滞的神秘和难以捉摸的性质。 这些信息也将有助于调整玻璃组成 为 光学的， 热的，电子的 和 机械应用。 ***

9701289 Boolchand This is a renewal project to study the physical nature of the stiffness threshold in chalcogenide and chalcohalide glasses using Raman scattering, M ssbauer spectroscopy and T-Modulated Differential Scanning Calorimetry measurements (MDSC). Specifically, Raman mode frequency shifts and Lamb-M ssbauer factors will be measured systematically as a function of glass composition to establish the mean coordination number dependence of local elastic constant, C, and the first-inverse moment of the vibrational density of states, w-1. Random network model simulations of both C and w-1 in glasses are now available from numerical experiments, provide a convenient guide to compare with laboratory experiments. The nature of structural relaxation at the glass transition will be studied in MDSC measurements to establish the non-reversing heat-flow DHnr variation with composition. Preliminary experiments on chalcogenide glasses reveal that DHnr also displays a minimum at the stiffness threshold, c=2.4 suggesting that only minuscule enthalpy changes accompany structural relaxation of a glassy liquid when it is mechanically critical. Mean-field constraint counting procedures also reveal that for networks possessing terminal atoms such as one-fold coordinated atoms (halogens) the stiffness transition in general shifts to a lower mean coordination c 2.4. These ideas will be put to a quantitative experimental test in suitable Raman, M ssbauer and MDSC experiments in chalcohalide glasses. %%% This is a renewal project to study the physical nature of the stiffness threshold in chalcogenide and chalcohalide glasses. Many glasses containing the elements S or Se or Te are viewed as networks composed of atomic-chain fragments (with a mean atomic coordination of 2). These chain-fragments can be progressively cross-linked by alloying other atoms. Mechanical ri gidity of such alloyed networks is found to increase progressively with degree of cross-linking as measured by the mean coordination number of a network. When the coordination reaches 2.4, theoretical calculations predict a floppy to rigid transition and stiffening of the network beyond 2.40. We propose to carry out an experimental study on specific glass systems to decode the physical nature of the underlying stiffness transition. This transition, which corresponds to a magic composition in a glass system, is of general interest in glass science because some of the best glass formers occur near this magic composition. Furthermore, an understanding of this transition will also help unravel the mysterious and elusive nature of structural arrest that occurs at the glass transition. Such information will also assist in tuning the glass composition for optical, thermal, electronic and mechanical applications. ***