EAPSI: In situ Structural Characterization of Lead-free, Bismuth-based Piezoelectric Thin Films

迫切需要发现和了解在所有应用中对环境无害、无毒和可持续的铅基材料替代品的行为（就像铅基涂料和焊料所做的那样）。压电材料（那些随着外加电场改变形状的材料，反之亦然）对于许多设备和应用都很重要，例如致动器、加速度计、过滤器、振动控制、燃油喷射、喷墨打印、超声波产生和传感以及谐振器，目前还没有合适的替代品。钛酸铋钠（BNT）是一类很有前景的替代品。在用于微电子应用的薄膜中，这些新材料的基本压电效应还没有被很好地理解，因此无法可靠地实现器件。因此，该项目的目标是利用东京工业大学Hiroshi Funakubo教授实验室提供的新型表征方法，探索与bnt基压电薄膜中压电性相关的机制。Funakubo教授的表征工具允许在施加电场的同时测量原子结构，从而直接了解这些特殊材料的压电应变机制。在块状形式下，这些基于bnt的材料属于一类被称为弛豫铁电体的材料，它们在电场诱导位移机制方面显示出诱人的特性，而这些特性才刚刚开始被理解。许多基于bnt的组合物表现出应变和极化行为，这是可逆场诱导弛豫到铁电相变的特征；这些被称为遍历松弛剂。然而，薄膜实施例中的bnt基材料不表现出在本体中发现的增强特性。特别是，在薄膜中研究的遍历弛豫组合物没有表现出可逆的场致弛豫到铁电相变的迹象，而是表现得与普通铁电体相似，并且具有相对较低的位移。为了使这种有前途的材料系统在实际应用中可行，需要了解体和薄膜之间的这种不同响应。因此，新的原位微结构表征方法（2D x射线衍射和拉曼光谱）将被用于阐明基于bnt的压电薄膜的电场诱导应变机制。该奖项隶属于东亚和太平洋暑期研究所项目，由美国国家科学基金会和日本科学促进会共同资助，支持一名美国研究生进行暑期研究。

There is a pressing need to discover and understand the behavior of environmentally benign, non-toxic, and sustainable replacements for lead-based materials in all applications (as already has been done for lead-based paints and solders). Suitable replacements have yet to be realized for piezoelectric materials (those that change shape with applied electric field and vice versa) that are important for a large range of devices and applications, such as actuators, accelerometers, filters, vibration control, fuel injection, ink jet printing, ultrasound generation and sensing, and resonators. One class of materials that shows great promise as replacements is based on bismuth sodium titanate (BNT). In thin films for microelectronic applications, the fundamental piezoelectric effect in these new materials is not yet understood well enough to allow devices to be reliably implemented. Accordingly, the objective of this project is to explore the mechanisms associated with piezoelectricity in BNT-based piezoelectric thin films using novel characterization methods available at Professor Hiroshi Funakubo's laboratory at the Tokyo Institute of Technology. Professor Funakubo's characterization tools allow for the measurement of atomic structure while applying electric fields, giving direct insight into the piezoelectric strain mechanisms of these exceptional materials.In bulk form, these BNT-based materials belong to a class of materials called relaxor ferroelectrics, which show enticing properties with regards to electric field induced displacement mechanisms that are just beginning to be understood. Many BNT-based compositions show the strain and polarization behavior that is characteristic of a reversible field-induced relaxor-to-ferroelectric phase transition; these are known as ergodic relaxors. However, BNT-based materials in thin film embodiments do not exhibit the enhanced properties that are found in the bulk. In particular, the ergodic relaxor compositions that have been studied in thin films do not show the signs of a reversible field-induced relaxor-to-ferroelectric phase transition, but rather behave similar to normal ferroelectrics and have comparatively low displacements. This differing response between bulk and thin films needs to be understood for this promising material system to be viable for real-world applications. Therefore, novel in situ microstructural characterization methods (2D X-ray diffraction and Raman spectroscopy), available at the host institution, will be applied to elucidate the electric field induced strain mechanisms in BNT-based piezoelectric thin films.This award under the East Asia and Pacific Summer Institutes program supports summer research by a U.S. graduate student and is jointly funded by NSF and the Japan Society for the Promotion of Science.