的sis Defense- Advisor Chelsey Hargather

高熵合金弹性和热力学性质的从头计算 及其合金成分

琼斯大厅227

摘要

High-entropy alloys (HEAs) are a class of engineering metals of recent scientific 感兴趣. HEAs usually consist of five or more alloying components in roughly equiatomic 比例可以形成单相或双相固溶体，如面心 cubic (FCC), body-centered cubic (BCC), or hexagonal close packed (HCP). 他们把 on the name high entropy because they exhibit unusually high entropy of mixing. 的 科学界对HEAs感兴趣的原因是这些合金可以 display promising combinations of material properties. 这些属性可以包括 更好的延展性和强度关系，更轻更强，氧化性更高 resistance, superior resistance to corrosion, and better fracture resistance when 与传统合金相比. Because of these qualities, HEAs are candidate materials to replace traditional alloys as structural engineering materials.

然而，在HEAs取代传统合金之前，它们的机械性能包括 thermodynamic and elastic properties must be understood. 在工业应用中， 对难熔合金的热力学和弹性性能的认识 crucial because their utility depends on these properties. 耐火材料的应用 合金取决于它们对不同过程(如高温)的耐受性 高氧化. This means that in their application, they must have low thermal expansion and high strength at elevated temperatures. 热膨胀与强度 在高温下是其热力学和弹性特性的一部分 一般. So to understand how these alloys will react under these types of conditions, these properties must be well understood. This thesis explores thermodynamic and elastic 利用密度泛函理论研究BCC耐火高熵合金的性能 计算与建模. By using Vienna Ab initio Simulation Package (VASP), thermodynamic and elastic properties such as Debye temperature, Helmholtz free energy, entropy, 焓，热容，热膨胀，弹性常数，体积模量，剪切 模量、杨氏模量和泊松比作为温度的函数来计算 并在本文中进行了讨论. First, the computational method is compared to well 已知的纯体系Ag、Ni、Al、Ta、Ti和V的实验值 验证，然后用于BCC耐火HEAs，包括三个季度和三个 五元系统，NbTaTiZr, NbTaTiV, AlMoNbV, HfNbTaTiZr, MoNbTaVW和AlNbTaTiV with 40和60原子单元用于四元体系，50和75原子单元用于四元体系 第五位的系统.

本工作计算了纯金属Al和Nb的熵和焓 be in close agreement with NIST JANAF reported experimental values. 这已经完成了 to validate the methodology of the thermodynamic modeling. 弹性性质 作为弹性常数，纯金属的体积、剪切和杨氏模量也接近 符合文献价值. After method validation on pure metals this work 然后将建模过程应用于更复杂的系统，如rhea和is 在AlMoNbV上进一步验证. For the first time, the finite temperature thermodynamic 计算了四元RHEA所有24种原子构型排列的性质. 最多1个.7% difference is found between the resulting properties as a function of 原子配置，表明SQS的原子配置几乎没有 effect on the calculated thermodynamic properties. 的 behavior of thermodynamic properties 从价电子浓度和原子密度的角度讨论了所研究的RHEAs 大小. Among the quaternary RHEAs studied, namely AlMoNbV, NbTaTiZr, and NbTaTiV, it is found that the presence of Zr contributes to higher entropy. 此外，在较低的 温度较高时，Zr的热容量和热膨胀率较高 to the alloys without Zr, possibly due to its valence electron concentration. 在更高的 温度，铝有助于更高的热容量和热膨胀，可能 由于其延展性. Among the 第五位的系统, the presence of Mo, W, and/or V causes the RHEA to have a lower thermal expansion than the other systems studied. 当比较 the systems with the NbTaTi core, the addition of Al increases thermal expansion, while the removal of Zr lowers the thermal expansion. 焓，熵和热 expansion of all six RHEA systems were calculated and results discussed.

的 elastic properties of all 24 atomic configurations of AlMoNbV were calculated and it was found that elastic constants had a standard deviation of up to 6.4%, elastic constant C44 being the highest, while C11 and C12 varied by 1.85%和2.28% 分别. 当比较 1-, 40, 50, and 64-atom cell 大小s, elastic constants 最多变化1.2% different, showing that cell 大小, shape, and orthogonality does not vary the results of the elastic properties significantly. 月，出 所研究的6种RHEAs具有最高的体积、剪切和杨氏模量，以及最低的热 expansion, giving promise to its use and utility in refractory settings. 这可能 be due to its high valence electron concentration. 讨论了其他结果和趋势 在这篇论文中. This thesis offers novel insights for cutting down computational 时间和资源，同时计算系统的属性，以前没有 以前发表过.

关键词:高熵合金，耐火材料，BCC体心立方，密度泛函 的ory, Elastic Properties, 的rmodynamic Properties

PhD Proposal Defense- Advisor: 大卫·伯利

第一性原理计算

Intrinsic and Extrinsic Point Defects in AlGaN

琼斯A 101

摘要

Aluminum Gallium Nitrogen (AlGaN) is a wide band gap semiconductor alloy which is useful in producing AlGaN/GaN high electron mobility transistors (HEMTs). 在半导体中, 点缺陷产生的能级既可以发生在带隙中，也可以发生在带附近 levels and affect the performance of HEMTs. Understanding these defects is therefore necessary for understanding the properties of AlGaN/GaN HEMTs under application. 在这里, 综合密度泛函理论(DFT)对大范围内禀的分析 并对闪锌矿(立方)AlN和AlGaN进行了外源性缺陷处理 局部矩反电荷法. An attempt is made at creating an effective defect 利用AlN和AlGaN的缺陷带隙，就像之前对GaN所做的那样.

关键词:AlGaN，立方，缺陷水平，带电缺陷，LMCC, Jahn-Teller畸变 过渡金属

的sis Defense- Advisor: Nikolai Kalugin

传递圆偏振中红外光到液氦温度和 High Magnetic Fields for Generation of Floquet-Bloch States in 2D Materials

 琼斯大厅156

摘要

在此之前，将中红外光发射到液氦中是非常具有挑战性的 temperatures and magnetic fields higher than 8T, and we have designed a system to 提供更大的功率和更高的精度比以前的解决方案，并使用它来检测 Floquet-Bloch features in a continuous wave system. 检测Floquet-Bloch特征 在石墨烯样品中，需要设计一个能够传递的系统 高功率中红外光在线性和圆偏振我们的样品 in a 18T, 1K environment at the bottom of a cryostat. 采用一系列专有技术 光纤，我们就能把中红外光传送下来 放入低温恒温器，并使用了一个特别设计的校准系统，它将保持 其位置从工作台向下到低温恒温器，以保证激光 光将被传送到样品上. We were able to demonstrate 带修改 在石墨烯的能带结构中，为进一步的弗洛克-布洛赫实验打开了大门 带修改. Additionally, this system would be able to be used for numerous 以前由于无法交付而无法实现的其他实验 mid infrared light down to a sample in a cryostat environment.

关键字: Floquet-Bloch; Magnetic Field; Graphene; Circular and Linear Polarization; Mid Infrared   

 

论文提案-顾问T. 大卫·伯利 

CORROSION RESISTANCE OF 316L STAINLESS STEEL IN HCl

Jones附件104

摘要

容器由退火316L奥氏体不锈钢材料制成 暴露于危险的放射性实验室废物时易受腐蚀. 几种副产物，如盐酸，通常是降解的结果 by radiolysis of the PVC wastes stored in the containers. 问题是 随后释放的这些副产物凝结在内壁周围 会显著导致316L不锈钢容器的腐蚀 by reducing the wall thickness and with time reduces the expected service life.

这项工作的主要目标是确定容器的设计寿命 当暴露在蒸汽中时，由316低碳奥氏体不锈钢材料制成 of HCl or immersed in varying concentrations of hydrochloric acid and FeCl₃ 溶液采用不同的测试方法如失重、动电位极化等 and electrochemical impedance spectroscopy techniques.

关键字: Corrosion, Stainless steel, 316L, Radiolysis, HCl …