Development of novel composite materials with defects induced gigantic positive and negative permittivity

Abstract

Dielectrics respond to an electrical field with a polarization. They are high demands for electronic circuits and electrical system. There is a particular need to develop dielectrics with a high breakdown field for several applications such as energy storage, capacitors, transistors. Moreover, in the past decades developing new materials with negative permittivity are in broad interest. The reason for such attention lies on the usage of those materials on applications like cloaking, tunable electronic, microstrip antenna and transistors. \\ For developing the dielectric materials for different applications, composites are used to manipulate materials properties. In the composites, polymers and glasses are extensively used due to their high breakdown field. These composites usually exhibit low permittivity due to their matrix properties. In this study, the defects are used to improved ceramic-glass composites permittivities and the conductivity nonlinearity change just as the conclusion. \\ The first part of this research, is the dielectric characterization of $CaCu_3Ti_4O_{12}$ ($CCTO$)/$SiO_2$ ceramics with high permittivity. Many efforts aimed at reducing high dielectric loss and increasing low breakdown field which is essential for energy storages. One of the most common approaches for improving the performance of $CCTO$ is using insulative additive aiming grainboundaries microstructure and properties. In this regards, $SiO_2$ as an approved insulator is the right candidate which we used to control dielectric loss of $CCTO$ and improve its breakdown field. The ceramic composite was made with core-shell structure by $SiO_2$ layers around $CCTO$ particles obtained by the sol-gel method. The composites with different percentage of $SiO_2$ and the various sintered temperature was made to evaluate the glass-ceramic behavior carefully. There is a significant change on microstructure when composites with an adequate amount of $SiO_2$ was sintered above 1060 $^0C$. Dielectric properties of ceramics were influenced by the formation of the second phase and defects in grain structure. Moreover, the composites pose improvement in nonlinearity which can be considered to be used in varistor applications. \\ The second part of research is focus on BTO ($BaTiO_{3}$)/$SiO_2$ Composites. The BTO ceramics which has ferroelectric properties exhibit high permittivity with strong temperature dependence. The BTO composites with $SiO_2$ are one of the most used materials in energy storage application which has lower the permittivity and the dielectric loss. In this work, the Spark Plasma Sintering (SPS) was used to induced defects and oxygen vacancies in BTO-$SiO_2$ ceramics. Ceramics was fabricated with two different size powders and pretreatment to help the understanding of each parameter contributions. Ceramics posses gigantic permittivity up to $10^5-10^6$ due to induced defects. It is interesting to find the permittivity can be negative by applying DC bias. Furthermore, the negative permittivity can be found in the low-frequency test. The negative permittivity result can be fitted with the Drude model very well that indicates the electrons are almost free. It is believed that the gigantic positive and negative permittivity is due to polaron electrons with pretty low energy barriers which can easily overcome by an electrical field. Based on our knowledge, these ceramics posses smallest plasma frequency among all epsilon negative materials. \\ To confirm the influence of oxygen vacancies on SPS ceramics properties, they were annealed in the air. The ceramic color and features show that the annealing changes the oxygen vacancies concentration which can provide ceramics with permittivity that surpasses the conventional sintering and acceptable dielectric loss. The fully annealed sample can result in high energy density with thin PE loop. In the other hand, the partially annealed samples still can possess similar properties to the SPS sample. The ability to tune the properties can lead to many possibilities in using such materials.