Electronic structure based design of thermal shock resistant nanolaminates

当材料承受快速的温度变化时，就会发生热冲击，导致工程部件内部的热应力积聚，最终导致其损坏和失效。尽管这在许多工程应用中出现，但关于这些材料的基于知识的设计的基础研究仍处于起步阶段。热冲击性能可以用热冲击参数来描述，包括抗弯强度、导热系数、泊松比、热膨胀线性系数和弹性（杨氏）模量。为了提高抗热冲击性能，必须最大化热冲击参数，但这是非常具有挑战性的，因为这些性能通常是相互关联的，并且有各种外部因素影响它们。采用相关的实验和理论研究策略，我们试图建立Ti2AlC、Zr2AlC、Zr3AlC2和Ta4AlC3的热冲击参数、化学成分和外在因素之间的因果关系。因此，我们系统地研究了价电子浓度（IVB和VB基团的过渡金属）、尺寸效应（Ti2AlC vs. Zr2AlC）和堆叠顺序（Zr2AlC和Zr3AlC2中不同数量的Zr-C和Al层）对热冲击参数的影响。在这个项目中要研究的外部因素是晶粒尺寸、应力和大气暴露。我们将利用薄膜合成来探讨化学变化对这些热机械性能的影响。施加衬底偏置电位和不同的衬底温度，成分的影响将与微观结构解耦。残余应力数据将被获取，因为它们对抗热震材料很重要，因为残余应力与热应力成分竞争。我们将研究排气温度（以及相关的大气暴露引起的成分变化）对热机械性能的影响。这在抗热震材料中从未被尝试过，但在一般情况下，环境-涂层相互作用对表面化学性质的改变对理解性能至关重要。所有实验都将伴随着密度泛函理论的研究，旨在更深入地理解电子结构水平上已建立的实验因果关系。所有理论数据将进行实验验证。几个开创性的步骤，如计算这些相的线性热膨胀系数在debye - grisen理论中，在这些低对称性系统的电子和声子水平上的热导率，并明确考虑外部因素以及温度。本课题所获得的基本因果关系，可望为今后抗热震材料的量子力学设计提供坚实的基础。

Thermal shock occurs when materials are subjected to rapid changes in temperature, leading to thermal stress buildup within an engineering component and eventually to its damage and failure. Even though this arises in many engineering applications, fundamental studies regarding the knowledge based design of these materials is at infancy. The thermal shock resistance can be described by the thermal shock parameter, containing flexural strength, thermal conductivity, Poisson’s ratio, linear coefficient of thermal expansion, and elastic (Young’s) modulus. To increase the thermal shock resistance, the thermal shock parameter must be maximized, but this is very challenging as these properties are often interconnected and there are various extrinsic factors affecting them. Using a correlative experimental and theoretical research strategy, we seek to establish the causalities between thermal shock parameter, chemical composition, and extrinsic factors for Ti2AlC, Zr2AlC, Zr3AlC2, and Ta4AlC3. Hence, we systematically investigate the influence of valence electron concentration (transition metals from IVB and VB groups), size effects (Ti2AlC vs. Zr2AlC), and stacking sequence (different amount of Zr-C and Al layers in Zr2AlC and Zr3AlC2) on thermal shock parameter. The extrinsic factors to be investigated in this project are grain size, stress, and atmosphere exposure. We will use thin film synthesis to explore the chemical variation influence on these thermomechanical properties. Applying a substrate bias potential and different substrate temperatures, the compositional influence will be decoupled from microstructure. The residual stress data will be acquired as they are of importance for thermal shock resistant materials since the residual stress competes with the thermal stress components. We will investigate the effect of venting temperature (and the associated atmosphere exposure induced changes in composition) on the thermomechanical properties. This has never been attempted for thermal shock resistant materials, but in general ambient-coating interactions modifying the surface chemistry are essential for understanding the performance. All experiments will be accompanied by density functional theory investigations, aiming at deeper understanding of the established experimental causalities on the electronic structure level. All theoretical data will be validated experimentally. Several pioneering steps will be made, such as calculation of the linear coefficient of thermal expansion for these phases within the Debye-Grüneisen theory, the thermal conductivity on the electronic and phonon level for these low symmetry systems, and explicit consideration of the extrinsic factors as well as temperature. With the fundamental causalities obtained in this project, it is expected that a solid basis will be obtained for future quantum mechanical design of thermal shock resistant materials.