J. Davis, D. Brown, W. Henderson, P. Anbalagan
Georgia Institute of Technology,
Keywords: nanoparticles, 3D simulation, permittivity, energy density
Summary:Using experimentally calibrated 3-D quasi-electrostatic simulations, various microstructural architectures of cold-pressed nanoparticle (NP) compacts for low-frequency energy storage applications are investigated. This work is built upon our investigations into metal-insulator composites  and dielectric composites with high permittivity (high-k) grain boundaries . There is extensive research focused on enhancing energy density of composites using cold-pressed NP compacts composed of particulates ranging from single insulators NPs to insulator-coated high-k ferroelectric NPs to polymer-coated metallic NPs. For example, material researchers in  have discovered a 10-fold increase in the permittivity over bulk values at low frequencies for NPs constructed from SiO2, Si3N4, Al2O3, or TiO2. The authors in  claim the increase is due to highly polarizable rotational dipoles that develop on the outer surface of each NP. Motivated to enhance material properties further, researchers also synthesize coated nanoparticles in an attempt to attain electrical attributes that may not be achieved with single phase NPs. For example, researchers have coated high-k materials, such as BaTiO3, with high-breakdown field strength materials, such as Al2O3, in an attempt to have compacts with both high capacitance and high breakdown voltages . Similarly, researchers in  have also coated Au NPs with polymer insulators and have shown large increases in the dielectric constant of the resulting compact . Our 3D simulation platform uses a control-volume approach with a finite difference grid [1,2]. This platform allows us to efficiently calculate the complex electric field patterns of 3-phase (i.e. host matrix, coating, and core) NP compacts as shown in Fig. 1. These simulations are used to demonstrate (a) that highly polarizable surface dipoles on certain NPs in  can be modelled by a thin high-k surface coating; (b) that high porosity can limit both relative dielectric constants and breakdown voltages; and (c) that certain core-coating NP architectures are superior for increasing the effective permittivity of cold-pressed NP compacts. In this paper, silicon dioxide NPs are modeled as spherical dielectrics with low-k cores (d = 15nm) with thin (~1nm) high-k outer coatings. This equivalent 2-phase (coating-core) model of these NPs captures the impact of surface dipoles on silicon dioxide NP by predicting effective permittivities that are generally consistent with measured data from . In addition, cold-pressed compacts can also have varying degrees of compaction and, therefore, porosity. 3-D simulations calculate the large impact that porosity can have on the effective dielectric constant of a NP compact. Moreover, simulations reveal that a microstructural architecture with low-k insulator cores with highly polarizable surfaces is more effective than either coated metallic NPs or coated high-k NPs in achieving higher effective permittivities. A study of the electric fields reveal that the lower effective permittivity of metallic and high-k cores is a consequence of the high electric fields being concentrated in the thin low-k surface regions and material voids. Furthermore, setting the maximum allowable electric field in each dielectric region can be used to estimate the energy density limits in each of these NP architectures.