Stiffness Threshold in Network Glasses

9701289 Boolchand这是一个新的项目，利用拉曼散射、穆斯堡尔谱和T调制差示扫描量热法研究硫化物和硫卤化物玻璃中硬化度阈值的物理性质。具体地说，系统地测量了拉曼模频移和Lamb-M穆斯堡尔因子作为玻璃组成的函数，以建立局域弹性常数C的平均配位数依赖关系和态振动密度的一逆矩w-1。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，这为与实验室实验进行比较提供了方便的指南。将在MDSC测量中研究玻璃化转变处的结构松弛的性质，以建立不可逆的热流DHnr随组成的变化。对硫化物玻璃的初步实验表明，DHnr在刚性阈值处也表现出最小值，c=2.4，这表明当玻璃液体是机械临界的时，只有微小的热焓变化伴随着结构松弛。平均场约束计数过程还表明，对于具有末端原子的网络，如单配位原子(卤素)，刚性跃迁一般转移到较低的平均配位c2.4。这些想法将在硫卤化物玻璃的适当的拉曼、穆斯堡尔和MDSC实验中得到定量的实验检验。%这是一个更新项目，目的是研究硫化物和硫卤化物玻璃中硬度阈值的物理性质。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)。这些链段可以通过合金化其他原子来逐步交联化。发现这种合金化网络的力学强度随网络的平均配位数所衡量的交联度而递增。当配位达到2.4时，理论计算预测网络将从软到硬的转变和超过2.40的刚性。我们建议对特定的玻璃系统进行实验研究，以破译潜在的刚度转变的物理性质。这一转变相当于玻璃系统中的一种神奇成分，在玻璃科学中引起了普遍的兴趣，因为一些最好的玻璃形成物出现在这种神奇成分附近。此外，对这一转变的理解也将有助于揭开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. ***