Frequency-dependent dielectric properties of nanodielectric materials for energy storage applications.

摘要

Due to the miniaturization of electronic devices, seeking a high-k nanodielectric material becomes more urgent and important. High-k nanodielectrics have potential to improve the performance of traditional dielectrics in wide applications ranging from capacitors, dielectric resonators to cable insulators and provide key dielectric components in MEMS and NEMS systems and devices which cannot be achieved by the traditional dielectric materials. In this thesis, the main focus is on the theoretical modeling of the frequency-dependent dielectric properties of nanodielectrics such as BaTiO3 (BT) nanoparticles based nanocomposites (ex: BT/P(VDF-HFP) and BT/Parylene composites).;To be able to study the frequency-dependent dielectric properties of the nanodielectric composites, we first need to obtain the dielectric spectrum of the constituent phases involved in the composite system. For single domain and single crystals of BT, a Debye type of dissipation and soft mode theory has been adopted to obtain more precise frequency dependent dielectric spectrum of BT. For two-phase composites, Wiener Rule, Lichtenecker model, Maxwell-Wagner model, Yamada, and modified Kerner model were applied to evaluate frequency dependent dielectric spectrum of nanocomposites. For dielectric constants of BT/PVDF nanocomposites which belong to 0-3 type of nanoparticle composites, Lichtenecker model, Maxwell-Wagner model and Yamada model show reasonable agreements with the experimental data up to 50% volume fraction of the nanoparticles. At the higher volume fraction of the nanoparticles, the experimental data shows decreasing trend dielectric constant of the composites due to increase of porosity of the system. In this case a three-phase model (nanoparticles/pores/matrix) was developed to predict dielectric properties of the system at higher volume fraction of nanoparticles (up to 80%). The results showed reasonable agreements for a wide range of frequency.;For layered structure, the dielectric constants of a composite can be predicted using Wiener's lower bound. First, Lichtenecker logarithmic rule and modified Kerner model were applied to take into account of the packing density to evaluate frequency dependent dielectric spectrum of self-assembled pure BT thin film. Then, for the 2-2 type layered structure; Wiener lower bound was modified and applied to obtain the dielectric properties of BT/Parylene layered nanocomposites, and a simple rule of mixture has been developed using Lichtenecker logarithmic rule to predict the dielectric loss of the multi-layered composites. The predicted dielectric properties of the BT/Parylene nanocomposites agreed reasonably with the experimental results, whereas a little discrepancy in dielectric loss tangent shown at frequency 100 kHz.;The developed models may lead to better understanding of structure/property relation of the system and provide the guidance for optimization of the material system in design of new dielectrics for energy storage applications.;The last part of this thesis focuses on the new complex ceramics developed in our research team. With discovery of a new class of complex ceramics aMn 3Ti4O14.25 (BMT) for better dielectric properties in our research team, we also found multi-functionality of this new material such as ferromagnetic, ferroelectric and multiferroic behaviors. Here we only consider the ferroelectric behavior of BMT. It is found that this new ceramic has very high permittivity. The high permittivity is attributed to electron related polarization in conjunction with electron correlation polarization playing key role in the ferroelectric behavior of BaMn3Ti 4O14.25.;To verify the ferroelectric behaviors of the new material system, the electric displacement (Polarization) versus electric field hysteresis and strain versus electric field loops of pure aMn3Ti4O 14.25 films were measured for frequencies ranging from 5 to 5000 Hz in our Advanced Materials Lab. At a low frequency of 5Hz, the material exhibits paraelectric behavior, when the frequency increases, the hysteresis loop occurs and the material shows ferroelectric behavior. At high frequency of 5000Hz, the hysteresis loop reduces to linear dielectric behavior due to difficulty of domain switch at high frequencies. It is observed that the coercive field (Ec) and remnant polarization (P r) increases with frequency. At 5000 Hz there is no hysteresis, and this new material acts as a linear dielectric material.;A micromechanics based model was adopted to evaluate the ferroelectric hysteresis behavior of the new aMn3Ti4O14.25 material. In this case, the variation of the frequency dependent remnant polarization Pr and coercive field Ec was simulated using a simple equation, and then, subsequently it was implemented in the micromechanics based model to study the hysteresis loops at different frequencies. The predicted results showed reasonable agreements with experiments for a wide range of frequency.