Phase-field Model of Electromechanical and Optical Properties of Ferroelectric Domain Structures

该奖项支持理论和计算研究，以及开发用于研究铁电晶体的压电性和光透明度的计算模型和工具的教育。材料的压电性表征了材料受到机械应力时产生电压差的能力，或材料受到电压差或电场时产生机械运动的能力。固体的光透明度是测量入射可见光通过材料透射的比例，它受限于外表面的光反射和散射量以及内部界面和固体内部的光吸附量。铁电体是在没有外加电场的情况下含有高密度电偶极子或极化的材料，它们是具有高压电性的压电材料的主要类别。然而，具有最高压电性的铁电晶体往往是那些包含许多均匀电极化的空间区域，这些区域具有不同的极化方向，被所谓的铁电畴壁隔开。大多数畴壁散射和反射光，因此即使是单晶铁电材料也不是完全透明的，或者充其量是不透明的。PI将开发计算模型和工具来研究铁电晶体的压电性和光透明度。这些模型和工具将通过理解铁电畴壁取向和畴壁密度的作用来找到光学透明度和压电性的最佳组合。具有高压电性的透明铁电晶体在高通量光声生物医学成像、透明致动器、自能量收集触摸屏和隐形机器人设备等方面具有潜在的应用前景。该项目将培养研究生成为计算材料科学、压电物理和非均匀固体中的光传播领域的专家。研究生还将通过共同指导PI小组中材料科学与工程或物理专业本科生的研究，接受指导方面的培训。该奖项支持理论和计算研究以及教育，主要目标是从根本上理解铁电晶体中压电性和光透明度领域结构的作用。该奖项将支持在存在电子和离子缺陷的多域单晶和多晶陶瓷中开发铁电畴和压电的相场模型，以及在空间中使用频率域描述的光谱方法，用于求解光传播的麦克斯韦方程并获得任意铁电畴结构的光透射光谱。PI和他的研究生将使用计算工具研究不同铁电极化协议下不同频率下畴壁、压电性、电子载流子和光透明度的演变。开发的计算框架和基本理解的进步将被用来指导铁电畴结构的设计，以实现所需的机电和光学特性，并寻找具有高压电性和光透明度的铁电晶体。PI的小组过去曾接待过许多本科生进行计算材料研究方面的研究培训，其中包括两名最近获得NSF研究生奖学金的NSF- reu学生。在拟议的项目期间，PI小组将继续通过宾夕法尼亚州立大学NSF-REU计划从其所在机构和其他机构积极招募本科生，以进行研究培训，并为参与该项目的研究生提供指导培训。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NONTECHNICAL SUMMARYThis award supports theoretical and computational research, and education to develop computational models and tools for studying piezoelectricity and light transparency of ferroelectric crystals. The piezoelectricity of a material characterizes the ability of the material to generate an electric voltage difference when it is subject to a mechanical stress or to generate a mechanical motion when the materials is subjected to an electric voltage difference or electric field. Light transparency of a solid measures the fraction of the incident visible light transmitted through the material, and it is limited by the amount of light reflection and scattering on the outside surfaces as well as the internal interfaces and the light adsorption inside the solid. Ferroelectrics are materials that contain high density of electric dipoles or polarization in the absence of an applied electric field, and they are the major class of piezoelectric materials exhibiting high piezoelectricity. However, the ferroelectric crystals that possess the highest piezoelectricity tend to be those containing many spatial regions of uniform electric polarization with different polarization directions separated by so-called ferroelectric domain walls. Most of these domain walls scatter and reflect light, and thus even single crystal ferroelectric materials are not completely transparent or tend to be opaque at best. The PI will develop computational models and tools to study both piezoelectricity and light transparency of ferroelectric crystals. The models and tools will be employed to find the optimal combination of optical transparency and piezoelectricity through understanding the roles of the ferroelectric domain wall orientations and domain wall density. Transparent ferroelectric crystals with high piezoelectricity have potential applications in high-throughput photoacoustic biomedical imaging, transparent actuators, self-energy-harvesting touch screens, and invisible robotic devices. The project will train graduate students to become experts in the areas of computational materials science, physics of piezoelectricity, and light propagation in inhomogeneous solids. Graduate students will also be trained in mentoring by co-supervising the research of undergraduate students in materials science and engineering or physics in the PI’s group.TECHNICAL SUMMARYThis award supports theoretical and computational research, and education with the main goal to fundamentally understand the science underlying the roles of domain structures in both piezoelectricity and light transparency of ferroelectric crystals. The award will support the development of a phase-field model of ferroelectric domains and piezoelectricity in both multidomain single crystals and polycrystalline ceramics in the presence of electronic and ionic defects and a spectral method in space with frequency-domain description in time for solving the Maxwell equations of light propagation and obtaining the light transmission spectrum for arbitrary ferroelectric domain structures. The PI and his graduate students will use the computational tools to study the evolution of domain walls, piezoelectricity, electronic charge carriers, and the light transparency at different frequencies under different ferroelectric polarization poling protocols. The developed computational framework and advance in fundamental understanding will then be harnessed to guide the design of ferroelectric domain structures to achieve desired electromechanical and optical properties, and to search for ferroelectric crystals possessing both high piezoelectricity and light transparency. The PI’s group has hosted numerous undergraduate students in the past for research training in computational materials research, including two recent NSF-REU students subsequently awarded NSF graduate fellowships. During the proposed project period, the PI’s group will continue to actively recruit both undergraduate students from its home institution and those from other institutions through the Penn State NSF-REU program(s) for research training as well as for mentoring training for the graduate students involved in the project.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.