|dc.description.abstract||Dielectrics, which are materials responding to an external electric field with a polarization, have been widely used in industries. Dielectrics with high permittivity and high breakdown strength are required for the applications including high charge capacitors and energy storage devices, where the dielectric composites could found their position as the potential candidates. As the commonly used matrix for dielectric composite, glasses and polymers exhibit high breakdown strength, but small permittivity. To increase the permittivity and energy storage density, a great deal of effort has gone into developing the high breakdown strength matrix filled with high permittivity ceramics or conductive materials to create new types of dielectrics that is easier to process while maintaining useful dielectric properties.
For the purpose of getting the optimized composites for dielectric and energy storage applications, both polymer based and glass based composites were fabricated and studied in the research. By the using of different matrix and fillers and optimization of fabrication process, the dielectric composites with excellent performances were obtained. According to the analysis of the data from testing, these composites were proved to be the potential candidates for the applications including high charge capacitors, energy storage device and even wearable electronics.
As the first part of research, for the purpose of effectively increase dielectric constant, conductor-polymer was considered to be the next part of research for making potential dielectric composites. The 2-D conductor Ti3C2TX was selected as the filler due to its high conductivity and P(VDF-TrFE) was used as the polymer matrix because of the relatively good permittivity and high breakdown strength to create the polymer based composite by solution casting. The hot pressing and silicon coupling agent was used to modify the morphology of the polymer composites. It was found that the percolation threshold is dependent on the testing frequency and is about 11.9 wt% at 100Hz, which is smaller than the composites using spherical conductive particle fillers. At room temperature, the dielectric constant of 10% composite is about 1570 at low frequency range, which is more than 100 times of the matrix material.
Although the increase of dielectric constant by combining 2-D conductive fillers and polymer matrix was proved by previous studies, the application of the composites is still limited by the high loss and low breakdown strength. Therefore, ceramic-polymer dielectric composites with different concentrations (0-30 vol%) were studied to create a composite with high energy and low loss. CaCu3Ti4O12 ceramic was selected to be the fillers due to its extremely high permittivity and P(VDF-CTFE) was used as the polymer matrix because of the relatively good permittivity and high breakdown strength. The silicon coupling agent was used to improve the uniformity of the polymer composites. It was observed that as the amount of silicon coupling agent increases, there is a obviously increase in the breakdown strength and simultaneously a slightly decrease in permittivity, which lead to a overall increasing energy storage density. Overall, the best energy density was found to be 4.61 J/cm3 from the composite film with 15 vol% CaCu3Ti4O12 concentration.
Finally, the research focus was turned to the detailed studies glass based dielectric composites due to the fact that the type of materials have the ability to keep the balance between high permittivity and energy storage density. The ceramic-glass with different glass concentrations (0-20 wt%) was prepared using nano-sized BaTiO3 as the ferroelectric ceramic fillers and SiO2 as the glass matrix by both randomly mixing and chemical coating methods. The variety of nanopowders were used to make composite pellets by conventional sintering under different conditions, such as molding pressure, sintering temperature, and ceramic powder size. In addition, the vacancies were introduced to BaTiO3 nanopowders by the methods of vacuum pretreatment and hydrogen pretreatment. By summarize the results, several conclusions about processing effects were obtained. Firstly, compare with the powders obtained other mixing method, the samples produced from core-shell particles by Stöber process exhibit the better performance including higher permittivity, lower loss and higher breakdown strength; Secondly, the permittivity of composites decreases while the breakdown strength increases with the increasing concentration of glass; Thirdly, compare with other sizes of particles, the composites made by 200 nm ceramic powders are showing highest breakdown simultaneously keep the high permittivity; finally, the dielectric properties are strongly depended on the the vacancies introduced by pretreatment, the same pretreat temperatures will lead to different results for different size of particles. From the analysis of all summarized data, the highest permittivity was found to be higher than 3100 while the highest energy density was reached to be 1.6 J/cm3 for the composite with the 2.5 wt% glass, 200 nm particle size by a 950°C pretreat temperature, which makes them attractive for both high charge capacitors and energy storage devices. More than that, by changing the method of sintering to SPS, another type of dielectric with extremely high dielectric constant, relatively high loss and low breakdown strength have also be created. At 1k Hz, a maximum dielectric constant of 811,895 and a loss of 0.457 were detected from the composites made by SiO2 atomic layer deposited 140 nm BaTiO3 nanopowders. Due to the layered structure of these SPS composites, some samples with ordinary dielectric constants and low loss which is close to the properties of insulators were also produced simultaneously with the same condition. Most of SPS composites have shown a very low breakdown strength below 0.1 MV/m; by the adding of ZrO2, the breakdown strength of composites can be increased to 1.622 MV/m with a polarization of 19 μC/cm2. Furthermore, Ba0.5Sr0.5TiO3-SiO2 composites were fabricated by the same process with SiO2 coating layers and solid state sintering. The BaTiSiO5 phase were also found due to the reaction of SiO2 and BST which lead to the change in XRD patterns, decrease of Tc and lower dielectric constant of the composites. With 1 wt% SiO2 additive, the maximum polarization, breakdown strength are about 20.28 µC/cm2 and 260 kV/cm, which suggests an energy storage density about 2.56 J/cm3 with a efficiency as high as 79%. Tunability of the BSTS composites was reduced by introduction of SiO2, but the breakdown strength of the composites was improved remarkably, which is much advantageous for high power tunable applications.||en_US