Improving dielectric strength to achieve a giant energy storage density in ferroelectric based ultra-capacitors

This is a project built on the recent successes made through the team’s collaborative efforts in developing ultracapacitors for energy storage applications by using spark plasma sintering (SPS) process. The successes were made through the collaboration among three teams: Dr. Tucker and Mr. Hill at MSFC, Dr. Thompson at the University of Alabama (UA), and Dr. Cheng at Auburn University (AU). A record high dielectric constant was observed in the SPS ceramic composites with a uniquely designed microstructure. The goal of this project is to significantly improve the dielectric strength (i.e. breakdown electric field or electric breakdown strength) of the SPS ceramic composites by using optimized microstructure and composition of the SPS ceramic composites.

In this project, Dr. Cheng will oversee the overall research progresses and work closely with Dr. Tucker, Mr. Hill and Dr. Thompson to coordinate the proposed research activities among three teams through emails, phone calls and meetings. Dr. Cheng will travel to Huntsville to meet Dr. Tucker and Mr. Hill twice during the project. All four will work together closely to publish the scientific findings and the new knowledge gained during the project. The publications will be co-authored by all four with students. Dr. Cheng will lead and work closely with all others to prepare the progress reports and the final report.

The ultracapacitors and the ceramics used to make the ultracapacitors will be prepared by Mr. Hill and Dr. Tucker using the SPS process at MSFC. The starting materials for making the ceramic composites will be BaTiO₃ (BTO) nanoparticles coated with a thin layer of an insulator such as SiO₂. The functionality of the insulator layer is to significantly improve the dielectric strength. That is, the insulator used should have a very high dielectric strength. Two processes will be used in this project to coat the SiO₂ insulator layer onto the BaTiO₃ nanoparticles: 1) Atomic Layer Deposition (ALD) that will be performed by VaporPulse Technologies as commercial service; 2) wet chemical process that will be carried in Dr. Cheng’s lab at AU. The SiO₂ coated BTO nanoparticles will be sent to Mr. Hill and Dr. Tucker at MSFC. Besides the insulator layer coated onto BTO nanoparticles, the pores in the ceramic composites are the key for the dielectric strength. Two approaches will be used to reduce the porosity in the ceramic composites: 1) using the BTO nanoparticles with a smaller size so that the size of pores will be smaller, 2) using the mixture of BTO nanoparticles with different sizes, which would reduce the porosity in the ceramic composites. For the mixture of the BTO nanoparticles, a numerical simulation will be performed in Dr. Cheng’s group. Based on the simulated results, experiments will be carried out. Based on the experimental results, the mixing features will be further optimized. Mr. Hill and Dr. Tucker will be responsible for the fabrication of the ceramic composites and ultracapacitor using SPS process at MSFC. The samples will be sent to Dr. Cheng and Dr. Thompson at AU and UA, respectively, for the properties and structure characterization. The electric and dielectric properties of the samples under different conditions will be determined and the energy-storage behavior of the ceramic composites and ultracapacitors will be evaluated in Dr. Cheng’s lab at AU. The microstructure characterization of the samples will be carried out in Dr. Thompson’s lab at UA. All results will be shared among three teams.

The process and composition of the ceramic composites and ultracapacitors will be optimized to achieve a high energy storage performance through discussions and evaluation of the experimental results by all team members. The optimized composition and process will be further validated through experiments.

We plan to test, study and characterize the ferroelectric based ultra-capacitors prepared using

spark plasma sintering (SPS) process. We would like to steer this ultra-capacitor study towards effective energy-storage devices with a significantly higher energy-storage density. In this project, we propose to improve the electrical breakdown strength of the ferroelectric based ultra- capacitors through optimizing microstructure and coming up with a deeper understanding of SPS process that will predict critical design parameters targeted towards energy storage.

The continuous increase in energy requirement has made the need for alternate and novel means of improving the efficiency of energy storage more imminent. Capacitors are devices that allow electrical energy to be stored and then released as required for shorter time periods under controlled conditions. Releasing stored electrical energy over a short time period is critical for various applications ranging from civilian to military including devices/components/systems in NASA’s Space Launch System (SLS) and Space Vehicles (SV). Current energy-storage capacitors exhibit excellent capability for releasing stored electrical energy over a short time period. The key drawback for the current energy-storage capacitors is its low energy-storage density. The energy-storage density measures the size/volume and mass required to store a certain amount of energy in the capacitors. Capacitors with a higher energy-storage density could further promote compact electrical and electronic systems toward miniaturization and integration. The energy-storage density of a capacitor is directly determined by the energy- storage density of the dielectric materials used to build the capacitor.

the energy-storage density in a dielectric material is associated with the relative permittivity and dielectric strength of the dielectric material. Regarding the relative permittivity, the ferroelectric based dielectric materials usually exhibit a much higher relative permittivity (~ 10² to 10⁴) than others (10⁰ to 10²). Therefore, it is interested to fabricate the ultra-capacitors using ferroelectric based dielectric materials. Among the ferroelectrics, BaTiO₃ (BTO) is a lead-free material exhibiting a very high permittivity.

In the study of dielectric materials for the energy-storage applications, it is found that using dielectric composites is a promising approach.

A dielectric composite is a mixture of different dielectric materials. The properties of a composite can be the sum, product, and complicated combinations of the properties of its constituents [3]. Therefore, the excellent dielectric performance/properties can be achieved in the dielectric composites.

In recent years, Dr. Cheng at AU, the PI of this project, has been working very closely with Mr. Curtis Hill, Dr. Dennis Tucker and Dr. Terry Rolin at MSFC on the dielectric composites for the development of high-performance ultra-capacitors. During the study, we identified a promising approach to prepare the dielectric composites for the development of high- performance ultra-capacitors. In the approach, nanosized BTO particles are used as the core dielectric material, while the surface of the BTO nanoparticles is coated with a thin layer of an insulator. The insulator used should be the dielectric materials exhibiting an ultra-high dielectric strength (E_b). The coated BTO nanoparticles are used as the start material to prepare the BTO- SiO₂ ceramics composites using the SPS process rather than the conventional ceramic sintering process. Regarding the properties of these ceramic composites prepared using SPS process, it has been experimentally demonstrated that the ceramic composites exhibit a giant permittivity. For example, the relative permittivity as high as 10⁶, which is the record high for the dielectric ceramics by the best of our knowledge, has been obtained in these ceramic composites. To achieve a high energy-storage density in these ceramic composites, the key now is to achieve a high dielectric strength. It is well known that the permittivity of a ceramic is mainly determined by the intrinsic factors, while the dielectric strength of a dielectric ceramic is mainly determined by the extrinsic factors. The current BTO-SiO₂ ceramic composites were fabricated using the BTO nanoparticles in diameter of 100 and 200 nm, respectively. The dielectric strength of these ceramic composites is limited by the pores in the ceramic composites. The presence of the pores in the ceramic composites is determined by the process and the size of the BTO particles used.

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