Nanoscale Phase Transition in Free-Standing Dielectric Thin Foils

非技术描述：电容器是电子学中必不可少的元件。一组氧化物材料通过改变其原子堆积排列来响应电场，可以在电容器应用中显示出优异的性能。该项目利用先进的电子显微镜技术，实时揭示氧化物在纳米尺度上的电响应的潜在微观结构机制。研究了机械应力对这种响应的影响。开发了一种新的成像技术，并将其应用于对比材料的原始原子结构和变化的原子结构。预计这项工作的结果将有助于设计高效电容器的新材料，这是可再生能源和电动汽车迫切需要的。在这个项目中，包括教育活动，以使本科生，特别是那些来自代表不足的群体的学生，参与到研究中。此外，研究小组还向参观爱荷华州立大学参加一年一度的科学碗地区比赛的高中生展示了智能手机充电器中的电容器。技术说明：使用反铁电材料作为介质层的超高能电容器是可能的。人们认为，在下一代电力电子中实现这种反铁电体电容器可以提高其结构稳定性和能源效率。反铁电电容器的超高能量密度是通过反铁电-铁电反复相变来实现的，到目前为止，这种相变只能通过极化和应变的宏观表征来研究。这项研究的目的是利用PI小组开发的独特的原位电子显微镜技术，在纳米空间分辨率下直接显示反铁电氧化物自支撑薄片中新相在这一关键相变过程中的形核和生长演化。研究了晶体取向和位错、晶界等结构缺陷对形核过程的影响。此外，还利用电子全息技术确定了极性铁电相的极化方向，并与非极性反铁电相进行了比较。这种对相变的基本理解对于反铁电体电容器在未来电力电子中的成功应用是至关重要的。

Non-technical description: Capacitors are essential elements in electronics. A group of oxide materials responding to electric fields by changing their atomic packing arrangements can display excellent properties for capacitor applications. This project utilizes advanced electron microscopy techniques to reveal the underlying microstructural mechanisms responsible for the electrical responses of the oxides at the nanometer scale in real time. The impact of mechanical stresses to such responses are examined. A new imaging technique is developed and applied to contrast the material's original and changed atomic structures. The outcome of the work is expected to be helpful in designing new materials for efficient capacitors, which are urgently needed in renewable energy sources and electric cars. In this project, educational activities are included to integrate undergraduate students, especially those from underrepresented groups, into the research. In addition, the research team presents demonstrations on capacitors in smart phone chargers to high school students visiting Iowa State University for the annual Science Bowl regional competition. Technical description: Ultrahigh energy capacitors are possible with antiferroelectric materials as dielectric layers. It is believed that implementation of such antiferroelectric capacitors in the next generation power electronics could improve their structural stability and energy efficiency. The ultrahigh energy density in antiferroelectric capacitors is achieved through repeated antiferroelectric - ferroelectric phase transition, which has only been investigated through macroscopic characterization of polarizations and strains so far. This research effort aims to directly visualize the nucleation and growth evolution of the new phase during this critical phase transition at nanometer spatial resolution in free-standing thin foils of antiferroelectric oxides using a unique in situ transmission electron microscopy technique developed by the PI's group. The influences of crystallographic orientation and structural defects, such as dislocations and grain boundaries, on the nucleation process are examined. In addition, electron holography technique is applied to determine the polarization direction of the polar ferroelectric phase, and to compare it with the nonpolar antiferroelectric phase. Such fundamental understanding of the phase transition is of critical importance for the successful implementation of antiferroelectric capacitors in future power electronics.