|Scanning electron micrographs of barium titanate (BaTiO3) nanocomposites with polycarbonate (left, top and bottom) and Viton (right, top and bottom) polymer matrices. The images show the dramatic improvement in film uniformity. Click to enlarge.|
Researchers at Georgia Tech have developed a new technique for creating films of barium titanate (BaTiO3) nanoparticles in a polymer matrix that could allow fabrication of improved capacitors that are able to store twice as much energy as conventional devices.
Because of its high dielectric properties, barium titanate has long been used 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. For capacitors and related applications, the amount of energy you can store in a material is related to those two factors.—Joseph W. Perry, a professor in the Georgia Tech School of Chemistry and Biochemistry and the Center for Organic Photonics and Electronics
The new nanocomposite materials have been tested at frequencies of up to one megahertz, 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.
The key to developing thin-film capacitor materials with higher energy storage capacity is the ability to uniformly disperse nanoparticles in as high a density as possible throughout the polymer matrix. However, nanoparticles such as barium titanate tend to form aggregates that reduce the ability of the nanocomposite to resist electrical breakdown. Other research groups have tried to address the dispersal issue with a variety of surface coatings, but those coatings tended to come off during processing, or to create materials compatibility issues.
The Georgia Tech research team decided to address the issue by using organic phosphonic acids to encapsulate the particles. The tailored organic phosphonic acid ligands, designed and synthesized by a research group headed by Seth Marder—a professor in the Georgia Tech School of Chemistry and Biochemistry—provide a robust coating for the particles, which range in size from 30 to 120 nanometers in diameter.
Phosphonic acids bind very well to barium titanate and to other related metal oxides. 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.—Joseph Perry
Though large crystals of barium titanate could also provide a high dielectric constant, they generally do not provide adequate resistance to breakdown, and their formation and growth can be complex and require high temperatures. Composites provide the necessary electrical properties, along with the advantages of solution-based processing techniques.
Though the new materials may already offer enough of an advantage to justify commercializing, Perry believes there are additional opportunities for boosting their performance. The research team also wants to scale up production to make larger samples—now produced in two-inch by three-inch films—available to other researchers who may wish to develop additional applications.
Perry and Marder are working with Bernard Kippelen, a professor in the Georgia Tech School of Electrical and Computer Engineering, on the use of these new nanocomposites in organic thin-film transistors in which solution-based techniques are used to fabricate inexpensive electronic components.
The results were reported in the April 2007 edition of the journal Advanced Materials. The research was supported by the Office of Naval Research and the National Science Foundation. Georgia Tech has filed a patent application on the nanoparticle encapsulation technique.
EEStor, the developer of a new high-power-density ceramic ultracapacitor (the Energy Storage Unit—EESU) for use in vehicles, is also working with barium titanate powders. The first commercial application of the EESU is intended to be used in electric vehicles under a technology agreement with ZENN Motors Company. (Earlier post.)
“Phosphonic Acid-Modified Barium Titanate Polymer Nanocomposites with High Permittivity and Dielectric Strength”; P. Kim, S. C. Jones, P. J. Hotchkiss, J. N. Haddock, B. Kippelen, S. R. Marder, J. W. Perry; Advanced Materials Volume 19, Issue 7 , Pages 1001 - 1005