CAREER: Revealing the Crystallization Kinetics of Marginal Glass Formers Through In Situ Microscopy and Nanocalorimetry Experiments

非技术描述：相变材料(PCM)是用于存储设备的材料，通常由锑和碲与其他元素合金化组成。数据存储在PCM的非晶态(玻璃状)和结晶态的碎片中，这可能被认为是“坏”玻璃，因为它们可以极快地结晶。基于PCM的存储器需要快速结晶，但其结晶物理机制和过程中的热流的实验数据很难获得。需要这些数据来测试模型，这些模型构成了理解相变材料行为的基础，从而能够开发出新的技术有用的材料。该项目利用显微镜和纳米量热学的最新进展，解决了实验数据中的差距。量热法被用来测量反应过程中吸收或释放的热量，现在先进的显微方法可以直接观察晶体的生长，而同时纳米热量计则可以获得结晶过程中的热流数据。研究结果对科学界研究低功耗、非易失性存储器材料具有一定的参考价值。从事这项研究的研究生和本科生正在为在研究实验室或高科技行业中表征和开发先进材料的职业生涯做准备。该项目支持来自传统上代表性不足的群体的高中生参加暑期教育计划。它还支持在俄勒冈州为中学女生建立一个扩展你的视野的会议。技术细节：结晶动力学是了解所有类别材料的玻璃稳定性的关键。对于边缘玻璃成形器的重要材料，如相变材料(PCM)，提取粘度、晶体生长速度和温度之间的重要关系所需的数据是不完整的。相变材料是用于光学和电阻率存储的(半导体)合金，因为它们可以在非晶态和晶态之间快速切换。晶体生长和粘度与温度变化之间的关系尚不完全清楚，因为结晶速度太快，无法测量结晶过程中的物理和热力学性质。最近许多关于晶体生长和粘度行为的报告从量热测量中提取出来，但并不能令人满意地符合现有的晶体生长模型。这种差异可能是由于晶化机制的改变和关于不同温度下形核和生长的相对贡献的有缺陷的假设所致。在这个项目中，正在用原位显微方法研究结晶，从而能够直接观察晶体生长并揭示相变机制的变化。透射电子显微镜技术的最新进展使这一努力成为可能，即使在晶体生长极其迅速的温度下也是如此。原位透射电子显微镜与同步纳米量热法提供了热力学和动力学数据之间的无缝连接。这项研究的知识促进了对相变材料中玻璃稳定性的了解，并对科学界研究用于低功耗、非易失性存储器和其他边缘玻璃成形材料(如大块金属玻璃)具有价值。参与该项目的研究生获得了相变热力学和动力学的基础知识，并使用了尖端的原位透射电子显微镜和量热表征技术。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL DESCRIPTION: Phase change materials (PCMs) are materials, generally consisting of antimony and tellurium alloyed with other elements, that are used in memory devices. Data is stored in amorphous (glassy) and crystalline bits of PCMs, which may be considered “bad” glasses because they can crystallize extremely rapidly. Rapid crystallization is required for PCM-based memory, but it makes it difficult to acquire experimental data on the physical mechanisms of crystallization and on the heat flow during the process. These data are needed to test the models that form the basis of understanding of PCM behavior that enables the development of new technologically-useful materials. This project addresses the gap in experimental data by utilizing recent advances in microscopy and nanocalorimetry. Calorimetry is used to measure the heat absorbed or released during a reaction, and now advanced microscopic methods can be used to directly observe crystal growth while simultaneous nanocalorimetry yields data on heat flow during crystallization. The results are of value to scientific communities researching materials for low-power, non-volatile memory. Graduate and undergraduate students engaged in this research are being prepared for careers in characterization and development of advanced materials in research laboratories or high-tech industries. The project supports participation of high school students from traditionally-underrepresented groups in science and engineering in a summer educational program. It also supports the establishment of An Expanding Your Horizons conference for middle school girls, the first of its kind in Oregon.TECHNICAL DETAILS: Crystallization kinetics are key to understanding glass stability across all classes of materials. The data needed to extract important relationships between viscosity, crystal growth rate, and temperature are incomplete for important materials that are marginal glass formers, such as phase change materials (PCMs). PCMs are (semiconducting) alloys used in optical- and resistivity-based memory owing to their fast switching between amorphous and crystalline states. The relationship between crystal growth and viscosity with changes in temperature is not fully understood because crystallization can be so rapid the measurement of physical and thermodynamic properties during crystallization is frustrated. Multiple recent reports of crystal growth and viscosity behavior extracted from calorimetric measurements do not satisfactorily fit existing models of crystal growth. This discrepancy may be due to changes in crystallization mechanism and flawed assumptions about the relative contribution of nucleation and growth in different temperature regimes. In this project, crystallization is being studied with in situ microscopic methods, enabling direct observation of crystal growth and revealing changes in phase transformation mechanisms. Recent advances in transmission electron microscopy (TEM) techniques have made this endeavor possible even at temperatures where crystal growth is extremely rapid. In situ TEM with simultaneous nanocalorimetry provides a seamless connection between thermodynamic and kinetic data. Knowledge resulting from this research advances the understanding of glass stability in PCMs and is of value to scientific communities researching materials for low-power, non-volatile memory and other marginal glass formers, such as bulk metallic glasses. Graduate students participating in this project gain foundational knowledge of the thermodynamics and kinetics of phase transformations and employ cutting-edge in situ TEM and calorimetric characterization techniques.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.