Engineering and mapping entropy changes at the nanoscale in oxide materials

从有序到无序的转变是固体中许多结构相变的决定性特征。当在绝热条件下驱动时，与有序过程相关的熵的变化可以引起温度变化[1]。在电热材料家族中，冷却是通过电场引起偶极有序性的变化来实现的，通常是在铁电和反铁电转变附近。近年来，在铁电薄膜中报道的引人注目的电热性能[2]重新唤起了人们对这些用于固态制冷应用的材料的兴趣，并证明了热泵的概念。尽管已经在许多材料系统中探索了电热效应，但由于在进行纳米级分辨率的直接测量方面存在挑战，这种效应的微观结构来源还没有被很好地理解。因此，可能会伴随微结构成核和生长过程的热变化并没有在纳米尺度上被直接绘制和关联。此外，偶极子水平上的异常动力学已被提出用来解释反铁电材料中的大电热效应，但这些仍有待实验验证[3]。尽管人们对电热材料的兴趣重新抬头，但对微观水平上的这种效应知之甚少。这个项目旨在通过纳米尺度的空间分辨温度映射来绘制和理解偶极无序，并有两个关键目标：第一，将熵温度变化与渐进的微结构发展相关联；第二，探索梯度场作为产生局部无序及其相关温度变化的手段。计划中的实验研究将包括：(I)使用扫描热显微镜(SthM)对经历转变的热能材料进行纳米尺度的分辨温度测绘。SthM设备是一种先进的原子力显微镜(AFM)成像模式，可在纳米结构媒体中心(CNM)使用，候选人将建立现有的知识库，以绘制与微结构发展相关的空间纳米级温度分布。(Ii)研究梯度场(例如应变梯度)对局部无序的影响及其对热量效应的影响。众所周知，AFM探针可以在纳米尺度上施加高达Gpa的应力；由此产生的应变场是高度不均匀的，并可以通过压电效应和挠性电效应影响极性有序[4]。CNM的内部AFM系统将被用来施加局部压力并监测由于诱导应变梯度而产生的热量效应。该项目主要属于物理科学主题中的功能陶瓷和无机物研究领域。参考：[1]Moya等人。《自然材料》13,439-450(2014)。[2]Mischenko等人。《科学》311,5765(2006)。先进材料20,3165-3169(2015)。[4]H.Lu等人。科学杂志336,6077(2012)。

"The transition from order to disorder is a defining characteristic for many structural phase transformations in solids. Changes in entropy associated with ordering processes can elicit changes in temperature, when driven under adiabatic conditions [1]. In the family of electrocaloric materials, cooling is achieved through electric field induced changes in dipole ordering, typically in the vicinity of ferroelectric and antiferroelectric transitions. In recent years, noteworthy electrocaloric performance reported in ferroelectric thin-films [2] has revived interest in these materials for solid state cooling applications and proof of concept heat pumps have been demonstrated. Even though electrocaloric effects have been explored across numerous material systems, the microstructural origins of the effect are not well understood due to challenges in carrying out direct measurements with nanoscale resolution. As a result, the thermal changes that may be expected to accompany the microstructural nucleation and growth processes have not been directly mapped and correlated at the nanoscale. Furthermore, unusual dynamics at the dipole level have been suggested to rationalise large electrocaloric effects in antiferroelectric materials, but these remain to be verified experimentally [3].Despite the resurgence of interest in electrocaloric materials, little is known about this effect at the microscopic level. This project aims to map and understand dipolar disorder through nanoscale spatially resolved temperature mapping and has two key objectives: firstly, to correlate entropic temperature changes with progressive microstructural development, and secondly, to explore gradient fields as a means to generate local disorder and associated temperature change. Planned experimental investigations will include:(i) Carrying out nanoscale resolved temperature mapping of caloric materials undergoing transition using Scanning Thermal Microscopy (SThM). The SThM facility is a cutting edge atomic force microscopy (AFM) imaging mode available at the Centre for Nanostructured Media (CNM) and the candidate will build on the existing knowledge base to map spatial nanoscale temperature distributions associated with microstructural development.(ii) Investigating the effect of gradient fields (e.g. strain gradients) on local disorder and their implications for caloric effects. It is well known that AFM probes can be used to apply up to GPa of stress at the nanoscale; the resulting strain fields are highly inhomogeneous and can affect polar ordering through piezo- and flexo-electric effects [4]. The in-house AFM system at CNM will be used to apply local pressure and to monitor caloric effects that develop because of induced strain gradients.This project falls primarily within the remit of the 'Functional Ceramics and Inorganics' research area in the Physical Sciences Theme.References: [1] Moya et al. Nature Materials 13, 439-450 (2014). [2] Mischenko et al. Science 311, 5765 (2006).[3] W. Geng et al. Advanced Materials 20, 3165-3169 (2015). [4] H. Lu et al. Science 336, 6077 (2012)."