Metamaterials, or artificial negative index materials (NIMs), have generated great attention due to their unique and exotic electromagnetic properties. A negative dielectric constant material, which is an essential key for creating the NIMs, was developed by doping ions into a polymer, a protonated poly(benzimidazole) (PBI).

The doped PBI showed a negative dielectric constant at frequencies of MHz due to its reduced plasma frequency and an induction effect. The magnitude of the negative dielectric constant and the resonance frequency were tunable by doping concentration. The highly doped PBI showed a larger absolute magnitude of the negative dielectric constant at just above its resonance frequency than the less doped PBI. The PBI doped with 60 wt% phosphoric acid solution showed a very large absolute magnitude of negative dielectric constant of –7.35 × 104 at 300 °C and 8.28 × 104 Hz.The PBI doped with 50 wt% phosphoric acid solution showed a smaller absolute magnitude of negative dielectric constant of –1.39 × 104 at 300 °C and higher frequency 1 × 105 Hz.

As temperature increased, the dielectric behavior changed from a relaxation spectrum to a resonance spectrum showing larger magnitude of negative dielectric constant at a lower frequency. The conductivity of the doped PBI measured as a function of both temperature and frequency followed the same trend as the dielectric constant. With respect to the dielectric constant and the conductivity data, it can be assumed that the origin of the negative dielectric constant is attributed to the resonance behavior of the high mobility of ions at elevated temperatures and high frequencies. The use of the developed negative dielectric material can be a novel approach for making unique optical and microwave devices, such as filters and switches, by creating homogenous negative dielectric constant materials with tunable resonance frequency and without complex geometric architectures consisting of capacitors and inductors or aggregated fillers. This innovation will also enable the material to be a structural element and a medium for power generation and energy storage, as well as a sensor and/or an actuator.

This work was done by Keith L. Gordon, Peter T. Lillehei, and Joycelyn S. Harrison of Langley Research Center; and Cheol Park and Jin Ho Kang of National Institute of Aerospace. LAR-17689-1

NASA Tech Briefs Magazine

This article first appeared in the February, 2015 issue of NASA Tech Briefs Magazine.

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