Polymer based dielectric capacitors are used in pulse power systems due to their ultrafast charge-discharge speed, mechanical flexibility, good reliability, light weight, and low cost. Although the breakdown strength of the commercial biaxially oriented polypropylene film (BOPP) capacitor is greater than 700 MV/m and the dielectric loss is less than 0.02%, its energy density is only 2-3 J/cm³, and its long-term service temperature does not exceed 105 ℃. Poly(ether imide) (PEI) as a linear polymer with excellent thermal and chemical stability has a high breakdown strength (E_b) and an extremely low loss, thus is widely used at high temperatures. Nevertheless, the discharged energy density of pristine PEI as low as 2.90 J/cm³ inhibits the enhanced energy storage performance. Composites composed of dielectric ceramic particle and polymer matrix have the combined advantages of a large dielectric constant of ceramic and a high breakdown strength of polymer. In this paper, titanium dioxide (TiO₂) particles were introduced into the PEI matrix to prepare a TiO₂/PEI composite, and the dielectric properties were analyzed.

TiO₂ particles were prepared by a sol-gel method, and TiO₂/PEI composites were prepared by a tape casting method. TiO₂ particles with a target content were firstly dispersed in NMP solvent under ultrasound for 60 min to form a stable suspension, and then the PEI particles were added into the suspension in a small amount repeatedly and stirred for several hours until a uniform and stable solution was formed. Subsequently, the suspension was cast on a smooth glass plate, and the thickness of the composite was controlled by a scraper. The composites were further dried in a vacuum oven at 120 ℃ for several hours to thoroughly remove the solvent. Finally, the electrode with the diameter of 2 mm was coated onto the film via ion sputtering before testing.

The relative dielectric constant of the composite is improved as TiO₂ content is increased. When the volume fraction of TiO₂ content is 9%, the relative dielectric constant of composite increases to 4.9, which is 1.75 times greater than that of pristine PEI. Moreover, at 25-150 ℃, the dielectric constant of 5% TiO₂/PEI composite increases from 4.3 to 4.5, showing that its dielectric properties are stable and independent of temperature. The conductivity polarization caused by diffusion current in the Cole-Cole curve shows a straight line with a certain slope, indicating that there are a few conductive paths in the composite. Thus, the characteristic breakdown strength E_b of composite is significantly enhanced in comparison with pristine PEI. The E_b of 5% TiO₂/PEI composite reaches 615 MV/m. The improvement of E_b is since TiO₂ particles hinder the expansion path of the electrical branches inside the composite, reducing the possibility of electrical breakdown, and the uniform distribution of TiO₂ particles in the PEI matrix weakens the distortion of the local electric field of the composite, and the uniform distribution of electric field can effectively improve the E_b of the composite. Also, the addition of TiO₂ improves the elastic modulus of the composite, which is conducive to the improvement of E_b. Its energy storage density U_dis of composite is 7.10 J/cm³, which is 2.3 times higher than that of pristine PEI and the discharged efficiency η is 94.9% due to the synchronous improvement of dielectric constant and breakdown strength. However, excessive TiO₂ ceramic particles form some inevitable defects such as holes and agglomerations that increase the electrical breakdown paths, thus weakening the breakdown strength and energy storage performance. While the breakdown strength and energy storage performance of the composite film at 150 ℃ are tested. Although E_b is lower than that at 25 ℃, its value is still as high as 487 MV/m. Compared with the case at 25 ℃, U_dis and η at 150 ℃ also decrease. At 150 ℃, the maximum U_dis of 5% TiO₂/PEI composite is 5.05 J/cm³, and η is 88.5%. The stability comparison of U_dis and η of 5% TiO₂/PEI composite film under 300 MV/m is further conducted at different temperatures. As the temperature increases to 150 ℃, the U_dis stability of 5% TiO₂/PEI composite remains 98.5% with a η stability of 97.4% in comparison with an U_dis stability of 99.5% and a η stability of 99.2% for pristine PEI. The faint decrease of U_dis and η at high temperature is due to the unavoidable massive dielectric loss and joule heat during the frequent charge-discharge processes. All these results prove that benefiting from an inherent low dielectric loss, TiO₂/PEI composite basically possesses a satisfactory fatigue resistance at room temperature and even at high temperatures.

TiO₂ particles were prepared by a sol-gel method, and TiO₂/PEI composites were prepared by a tape casting method. The addition of TiO₂ particles increased the dielectric constant and breakdown strength of the composite, improving its energy storage performance. At room temperature, the breakdown strength of TiO₂/PEI composite reached 615 MV/m, and the energy storage density was 7.10 J/cm³, which was 2.3 times greater than that of pristine PEI. And the charge-discharge efficiency could reach 94.9%. Moreover, when the temperature increased to 150 ℃, the maximum energy storage density of 5% TiO₂/PEI composite film was 5.05 J/cm³, while the charge-discharge density stability remained 98.5% with an efficiency stability of 97.4%. TiO₂/PEI composite prepared could have a great potential in high temperature capacitors due to the excellent energy storage performance and stability.