Stiffness Threshold in Network Glasses

这是一个更新的项目，利用拉曼散射，M -斯堡尔光谱和t调制差示扫描量热测量（MDSC）研究硫族化物和硫化物玻璃的刚度阈值的物理性质。具体来说，拉曼模频移和Lamb-M - ssbauer因子将作为玻璃成分的函数进行系统测量，以建立局部弹性常数C与状态振动密度w-1的一阶逆矩的平均配数依赖关系。目前已经可以通过数值实验对玻璃中C和w-1的随机网络模型进行模拟，为与实验室实验进行比较提供了方便的指导。将在MDSC测量中研究玻璃化转变时结构弛豫的性质，以确定热流DHnr随成分的非逆转变化。在硫系玻璃上的初步实验表明，DHnr在刚度阈值处也显示出最小值，c=2.4，这表明当玻璃状液体达到机械临界时，只有极小的焓变化伴随着结构松弛。平均场约束计数程序还揭示，对于具有末端原子的网络，如单次配位原子（卤素），刚度跃迁通常会转移到较低的平均配位c 2.4。这些想法将在合适的拉曼、斯堡尔和MDSC实验中进行定量实验测试。这是一个更新的项目，以研究在硫化物和硫化物玻璃的刚度阈值的物理性质。许多含有S、Se或Te元素的玻璃被看作是由原子链碎片组成的网络（平均原子配位为2）。这些链片段可以通过使其他原子合金化而逐渐交联。用网络的平均配位数来测量，这种合金网络的机械刚度随着交联程度的增加而逐渐增加。当协调达到2.4时，理论计算预测网络将从软盘过渡到刚性，超过2.40后网络将变硬。我们建议对特定玻璃系统进行实验研究，以解码潜在刚度转变的物理性质。这种转变，对应于玻璃系统中的神奇成分，是玻璃科学中普遍感兴趣的，因为一些最好的玻璃形成物发生在这个神奇成分附近。此外，对这种转变的理解也将有助于解开在玻璃化转变中发生的结构停滞的神秘和难以捉摸的本质。这些信息还将有助于调整光学、热学、电子和机械应用的玻璃成分。＊＊＊

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. ***