UC Berkeley

Polymer Nanocomposite Materials with High Dielectric Permittivity and Low Dielectric Loss Properties

by

Anju Toor

Doctor of Philosophy in Mechanical Engineering

University of California, Berkeley

Professor Tarek Zohdi, Co-chair

Professor Albert P. Pisano, Co-chair

Materials with high dielectric permittivity have drawn increasing interests in recent years

for their important applications in capacitors, actuators, and high energy density pulsed power.

Particularly, polymer-based dielectrics are excellent candidates, owing to their properties

such as high breakdown strength, low dielectric loss, flexibility and easy processing. To

enhance the dielectric permittivity of polymer materials, typically, high dielectric constant

filler materials are added to the polymer. Previously, ferroelectric and conductive fillers have

been mainly used. However, such systems suffered from various limitations. For example,

composites based on ferroelectric materials like barium titanate, exhibited high dielectric

loss, and poor saturation voltages. Conductive fillers are used in the form of powder

aggregates, and they may show 10-100 times enhancement in dielectric constant, however

these nanoparticle aggregates cause the dielectric loss to be significant. Also, agglomerates

limit the volume fraction of fillers in polymer and hence, the ability to achieve superior

dielectric constants. Thus, the aggregation of nanoparticles is a significant challenge to their

use to improve the dielectric permittivity.

We propose the use of ligand-coated metal nanoparticle fillers to enhance the dielectric

properties of the host polymer while minimizing dielectric loss by preventing nanoparticle

agglomeration. The focus is on obtaining uniform dispersion of nanoparticles with no

agglomeration by utilizing appropriate ligands/surface functionalizations on the gold

nanoparticle surface. Use of ligand coated metal nanoparticles will enhance the dielectric

constant while minimizing dielectric loss, even with the particles closely packed in the polymer

matrix. Novel combinations of materials, which use 5 nm diameter metal nanoparticles

embedded inside high breakdown strength polymer materials are evaluated. High breakdown

strength polymer materials are chosen to allow further exploration of these materials for

energy storage applications.

In summary, two novel nanocomposite materials are designed and synthesized, one involving

polyvinylidene fluoride (PVDF) as the host polymer for potential applications in

energy storage and the other with SU-8 for microelectronic applications. Scanning elec-

tron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray

spectroscopy and ultramicrotoming techniques were used for the material characterization

of the nanocomposite materials. A homogeneous dispersion of gold nanoparticles with low

particle agglomeration has been achieved. Fabricated nanoparticle polymer composite films

showed the absence of voids and cracks. Also, no evidence of macro-phase separation of

nanoparticles from the polymer phase was observed. This is important because nanoparticle

agglomeration and phase separation from the polymer usually results in poor processability

of films and a high defect density. Dielectric characterization of the nanocomposite materials

showed enhancement in the dielectric constant over the base polymer values and low

dielectric loss values were observed.