Capacitors are devices which store electrical charge, and as such are essential components of almost all electrical machinery and modern electronic devices. The continuing need for increased storage capacity of such devices, as well as for functional stability and miniaturization, make capacitive materials a competitive and promising area for expanded research and development, with exceptionally high potential for practical innovations and discoveries of a fundamental nature. The electronic components industry depends heavily on superior capacitor performance, and such components immediately impact the global market, paving the way for higher efficiency devices and often to new product innovations. The aim of the proposed investigation, therefore, is to explore the structural limitations of optimized capacitors based on barium titanate ceramics, and to use modern physical concepts, including nanotechnology, to develop superior capacitor systems and new devices based on these concepts. The high cost of fossil fuels of necessity focuses attention on the development of alternate energy conversion and storage strategies. An ambitious goal of the proposed research is to develop a capacitor system (a supercapacitor) that will convert solar energy and store it as electrical energy. Such solar rechargeable supercapacitors can revolutionize energy and transportation technologies and be widely applied to various sensor and storage devices. The proposed research is multidisciplinary and will involve participation by students with outreach to high school students and their teachers. The experience gained in these exercises will significantly advance their science background education and awareness of engineering materials.
Barium Titanate (BaTiO3) continues to be the preferred dielectric material for capacitor use, because of its inherently high dielectric constant, and almost limitless potential for chemical modification to enhance dielectric and storage properties. Superior BaTiO3 capacitative systems have originated from core-shell structuring of the grains, with chemical gradients induced by doping, leading to stress-strain micro-domains and high polarization. The proposed research will explore these issues in depth, experimentally and theoretically, with the aim of minimization of percolative recombination loss of the stored charge, yielding high breakdown voltages and high dielectric constants. Effects of grain dimensions down to the nanoscale will be analyzed in detail to identify and quantify gradient features in the prepared microstructures, and to develop analytical relationships linking these structures to exhibited properties. Based on these associations, the proposed study will also focus on fabrication methodologies for nanostructured supercapicitors. A futuristic goal will be to develop devices that can harvest sunlight and store it as electrical potential energy in the supercapacitor. A possible strategy might involve integration of a dye-sensitized solid-state solar cell with the nanostructured supercapacitor, requiring exploration of loss issues resulting from recombination of the separated charges. The proposed work is multidisciplinary in scope, involving condensed-matter physics, electronics and materials science, instrumentation analysis and chemistry. This gives wide scope for the training of graduate students, as well as outreach to and involvement of undergraduate and high school students in hands-on experiments.
Barium Titanate (BT) based ceramics, as high dielectric constant materials, are widely used as capacitive device components in microelectronics circuitry. Increasing trends toward smaller sized components impose a need for multilayer structural designs, micron-thick layers, and superior dielectric properties. The dielectric properties of the polycrystalline ceramic capacitors are greatly influenced by composition, internal stress, grain size, domain size, and electrical boundary conditions. To understand these influences, various models, mostly based on microstructure analysis, have been proposed, but a good understanding the integrated effects of these parameters on dielectric and other properties is still not fully realized. The aim of the proposed project, therefore, was to elucidate the physical mechanisms involved, through experimentation, data analysis, and theoretical modeling, to determine the conditions for high device performance and applications. A physical model that would demonstrate understanding of the core-shell modified BT structure, and be predictive of its properties, was a major project focus. With miniature ceramic capacitors, performance can be further enhanced by use of core-shell grain and Barrier Layer Capacitor (BLCs) structures. The BLCs are based on the reduction and partial re-oxidation of donor-doped BT compositions. On this project, the selective doping of BT with Nd and Zr oxides yielded well-defined core-shell and barrier layer structures, as normal sintered ceramics, not previously reported in the literature. The actual barrier layer formation results from interaction of unique features in the ceramic structure, notably an oxygen gradient from the interior to the outer surface, which also produces a gradient in the Nd and Ti3+ concentration. These gradient features are formed during the processing phase, and include not only gradients in elemental compositions, but in defects, domain structure, strain, resistivity and polarization. The identification, mapping and control of these gradient fields, in relationship to critical processing variables such as dopant level and sintering regime, was mainly accomplished. An important clue towards understanding the nature of these systems is the discovery of high permittivity, core/shell/ barrier layer features in the ferroelectric ceramics. The origin of the high dielectric permittivity was correlated to interfacial polarization, with Maxwell-Wager type relaxation, and to Interaction of ferroelectric micro-domains and core-shell/barrier induced polarization fields. A mechanistic understanding of these gradient features was explored, with the goal of potentially exploiting the unique properties of these materials. The scientific merit relates to the overall goal of predictive property coupling for a defined set of processing variables, using basic physical concepts applied to gradient/tensor relationships in the doped BaTiO3 systems. The broader Impact will allow modeling, predictive design and fabrication of useful devices that take advantage of the unique features of these gradient structure materials, for capacitive energy storage. The project also promoted the technical educational and prospects for graduate, REU, and high school students.
