Placing a ferroelectric material between two electrodes and increasing the electric field causes charge to build up. During discharge, the amount of energy available depends on how strongly the material's electrons orient, or become polarized, in response to the electric field. However, most such materials typically cannot withstand a large electric field before the material fails. The fundamental challenge, therefore, is to find a way to increase the maximum possible electric field without sacrificing the polarization.
The researchers turned to an approach that they had previously developed to "turn off" conductivity in a material. By bombarding a thin film with high-energy charged particles known as ions, they were able to introduce isolated defects. The defects trap the material's electrons, preventing their motion and decreasing the film's conductivity by orders of magnitude.
"In ferroelectrics, which are supposed to be insulators, having charge that leaks through them is a major issue. By bombarding ferroelectrics with beams of high-energy ions, we knew we could make them better insulators," said Jieun Kim, a doctoral researcher in Martin's group. "We then asked, could we use this same approach to make a relaxor ferroelectric withstand bigger voltages and electric fields before it catastrophically fails?"
Kim first fabricated thin films of lead magnesium niobite-lead titanate. He then targeted the films with high-energy helium ions at the Ion-Beam Analysis Facility operated by the Accelerator Technology and Applied Physics (ATAP) Division at Berkeley Lab. The helium ions knocked target ions from their sites to create point defects. Measurements showed that the ion-bombarded film had more than twice the energy storage density of previously reported values and 50 percent higher efficiencies.
"We were originally expecting the effects to be mostly from reducing the leakage with isolated point defects. However, we realized that the shift in the polarization-electric field relationship due to some of those