Nanocomposites doubles capacitor efficiency

Nanocomposite technology has led to the development of capacitors capable of storing twice the energy of existing devices, according to a report in the April 2007 edition of Advanced Materials.

A technique for creating films of barium titanate (BaTiO3) nanoparticles in a polymer matrix could allow fabrication of improved capacitors. The capacitors could be used in consumer devices such as mobile telephones and in defence applications requiring both high energy storage and rapid current discharge.

Because of its high dielectric properties, barium titanate has long been of interest for use in capacitors, but until recently materials scientists had been unable to produce good dispersion of the material within a polymer matrix.

By using tailored organic phosphonic acids to encapsulate and modify the surface of the nanoparticles, researchers at the Georgia Institute of Technology's Center for Organic Photonics and Electronics were able to overcome the particle dispersion problem to create uniform nanocomposites.

"Our team has developed nanocomposites that have a remarkable combination of high dielectric constant and high dielectric breakdown strength," said the Center's Prof Joseph Perry. "For capacitors and related applications, the amount of energy you can store in a material is related to those two factors."

The nanocomposite materials have been tested at frequencies of up to 1 MHz, and Perry says operation at even higher frequencies may be possible.

Though the new materials could have commercial application without further improvement, their most important contribution may be in demonstrating the new encapsulation technique — which could have broad applications in other nanocomposite materials.

"This work opens a door to effectively exploit this type of particle in nanocomposites using the coating technology we have demonstrated," explained Perry. "There are many ways we can envision making advances beyond what we've done already."

The tailored organic phosphonic acid ligands, designed and synthesised by a research group in the Georgia Tech School of Chemistry and Biochemistry, provide a robust coating for the particles, which range in size from 30 to 120 nm in diameter.

"Phosphonic acids bind very well to barium titanate and to other related metal oxides," Perry said. "The choice of that material and ligands were very effective in allowing us to take the tailored phosphonic acids, put them onto the barium titanate, and then with the correct solution processing, to incorporate them into polymer systems.

"This allowed us to provide good compatibility with the polymer hosts — and thus very good dispersion as evidenced by a three- to four-fold decrease in the average aggregate size."

Though the new materials may already offer enough of an advantage to justify commercialising, Perry believes there are additional opportunities for boosting their performance. The research team wants to make larger samples available to other researchers who may wish to develop additional applications.

"Beyond capacitors, there are many areas where high dielectric materials are important, such as field-effect transistors, displays and other electronic devices," Perry added.

"With our material, we can provide a high dielectric layer that can be incorporated into those types of applications."