Structure and Dynamics in Boron-Containing Oxide Glasses andLiquids: High-Resolution and High-Temperature Nuclear Magnetic Resonance Studies

9802072 Stebbins这个项目的目标是使用核磁共振波谱来获得关于室温和高温下的硼酸盐、硼硅酸盐和硼铝酸盐玻璃和液体的定量结构和动力学信息，并根据这些新的发现来解释和模拟现有的宏观热力学和输运性质数据。玻璃和相应液体之间的根本区别在于，后者的结构可以随温度变化得足够快，从而保持平衡。这一过程通常会对性能产生很大影响，必须对这些性能进行评估，以准确描述玻璃熔化和加工过程中的熔体行为。结构随温度的变化也控制着玻璃退火过程中的机械松弛，而这反过来又是制造技术上有用的玻璃材料的关键步骤。关于这些宏观性质的大量数据集已经存在，并且已经制定了复杂的唯象模型来描述和量化松弛。然而，这些模型通常缺乏温度对分子结构或动力学的影响的经验数据，因此它们的预测价值有限：当将温度外推到玻璃熔窑的温度时，它们甚至可能大错特错。本文提出的研究将在常温下对各种核素进行核磁共振，以确定不同虚构温度对玻璃结构的影响，这将限制T效应对玻璃化转变附近液体结构的影响。这些核素的原位、高分辨率、高T核磁共振也将被用来确定温度高达1500℃的平均配位环境中的T效应。观察到的结构变化对熵、焓、活度、摩尔体积和其他性质的影响将使用简单的热力学模型进行评估。在温度较低但仍在液体范围内，静态核磁共振和MAS核磁共振将被用来观察结构物种之间的交换并测量它们的交换率。将测量自旋-晶格弛豫时间，以在更宽的温度范围内获得动力学信息。分子尺度动力学的这些结果将与根据现有粘度数据得出的剪切松弛时间进行比较，并与电导率和扩散系数数据进行比较。将制定和测试连接微观和宏观的模型。含硼氧化物玻璃及其相应的高温液体在工业和社会中具有广泛的重要性，可用于耐腐蚀和耐高温的容器、储罐和管道、结构复合材料的纤维、光学元件、计算机显示屏等。这项研究项目将对理解玻璃的形成和松弛产生重大影响，这是提高商业产品性能所需的。***

9802072 Stebbins The objective of this project is to use nuclear magnetic resonance (NMR) spectroscopy to acquire quantitative structural and dynamical information on borate, borosilicate, and boraluminate glasses and liquids at ambient and high temperature, and to interpret and model existing macroscopic thermodynamic and transport property data in light of these new findings. The fundamental difference between a glass and the corresponding liquid is that the structure of the latter can change with temperature rapidly enough to remain in equilibrium. This process results in often large effects on properties that must be assessed to accurately describe melt behavior during glass melting and processing. Structural change with temperature also controls the mechanical relaxation during annealing of glasses, which in turn is an essential step in making technologically useful glassy materials. Extensive data sets exist on these macroscopic properties, and sophisticated phenomenological models have been formulated to describe and quantify relaxation. However, these models in general lack empirical data on temperature effects on either molecular-scale structure or dynamics and thus are limited in their predictive value: when extrapolated hundreds (or even 1000) degrees up to temperatures of glass melting furnaces, they may even be grossly in error. The studies proposed here will use NMR on a variety of nuclides at ambient temperature to determine the effects of varying fictive temperature on glass structure, which will constrain T effects on liquid structure in the vicinity of the glass transition. In situ, high resolution, high T NMR on these nuclides will also be used to determine T effects on average coordination environments to temperatures as high as 1500 C. The effects of observed structural changes on entropy, enthalpy, activities, molar volumes, and other properties will be assessed using simple thermodynamic models. At lower temperatures but sti ll in the liquid range, static and MAS NMR will be used to observe exchange among structural species and measure their exchange rates. Spin-lattice relaxation times will be measured to gain dynamical information over wider ranges in temperature. These results for molecular-scale dynamics will be compared to shear relaxation times derived from existing data on viscosity, and to data on electrical conductivity and diffusivity. Models linking the microscopic and macroscopic will be formulated and tested. %%% Boron-containing oxide glasses, and their corresponding high temperature liquids, are of wide-ranging importance in industry and society, finding use in corrosion- and temperature resistant containers, tanks and pipes, fibers in structural composites, optical components, computer display screens, etc. Borosilicate glasses are also likely to be of major importance in the solidification and sequestering of radioactive wastes. This research project will have a major impact on the understanding of glass formation and relaxation needed to improve the properties of commercial products. ***