Evaluation of the Dielectric Properties of Aluminum Conductive Polymer Solid Capacitors at High Temperatures
Authors: Quintero Giraldo David , Matsuya Hisato , Iwai Mana , Kitano Sho , Fushimi Koji , Habazaki Hiroki 
Published: December 2024

Aluminum capacitors have a unique combination of properties that highlight them as potential devices for power electronic applications, such as high capacitance, low cost, and high energy density. When combined with a polymeric conductive media, such as PEDOT:PSS, it outperforms traditional electrolytic capacitors in terms of thermal stability, showing a high ripple current with a low equivalent series resistance (ESR). The low breakdown voltage of commercially available devices (~100 V) has limited their applications in many fields of electronics. Nonetheless, current scientific research has found that crystalline coatings, with subsequent immersion in hot water (95 °C), are a good alternative to enhance the breakdown potential (≥ 600 V) of conductive polymer solid capacitors, especially when operated at room temperature; in addition, the leakage current can be effectively controlled through a re-anodizing procedure [1,2]. The current market needs capacitors that can reach high withstand potentials (≥ 600 V) in an operation range of up to 200 °C, and reports on solid polymer capacitor performance at high potentials and temperatures are scarce. Here, the performance at high temperatures (≤ 200 °C) of crystalline anodic coatings with subsequent hot water immersion was assessed, and the results were compared with a post-hydrated sample with a re-anodizing process at 400 V. Finally, a possible degradation mechanism at high temperatures was explored.
The dielectric material was formed as follows. First, an electropolished high-purity aluminum sheet was immersed in hot water (95 °C) for 10 min to form a pseudo-boehmite layer. The anodization was conducted at 700 V in H3BO3 solution at 85 °C for 10 min, followed by immersion in hot water at 95 °C for 10 min. To control the dielectric defects, a re-anodizing process at 400 V was performed at 85 °C for 10 min in H3BO3 solution. The PEDOT:PSS conductive polymer was coated on the dielectric material using an aqueous dispersion solution of ~1% concentration. The dielectric properties were assessed by AC impedance spectroscopy and voltage sweeping in a model flat aluminum capacitor with a PEDOT:PSS conductive polymer as the cathode at high temperatures (≤ 200 °C).
The thermal stability analysis evidenced that the aluminum capacitor with a subsequent immersion in hot water assessed with a PEDOT:PSS conductive polymer remained dielectrically invariable up to 100 ˚C, with a breakdown potential of ~790 V. Nonetheless, the breakdown potential was reduced when the temperature was increased to 200 °C with an average value of ~473 V. Analogous behavior was observed in the aluminum capacitor with re-anodization at 400 V, where the breakdown potential remained constant with a value of ~ 670 V up to 100 °C; however, an increase in the temperature to 200 °C reduced its value until ~550 V. Hence, thermal analysis revealed that the post-anodizing treatments aimed at enhancing the breakdown potential (˃600 V) of crystalline coatings at ambient temperature [2] fail to sustain the withstand voltage above 600 V as the temperature increases to a maximum value of 200°C. Furthermore, an in-parallel examination in which the dielectric layer without the PEDOT:PSS coating was heated to 200 °C showed that thermal degradation mostly affects the dielectric layer.
Surface and cross-sectional analyses by SEM showed that a crucial aspect of the breakdown potential enhancement is on the PEDOT:PPS-dielectric layer interface. The formation of a nanovoid layer by immersion in hot water avoids the local current concentration at high voltages and modifies the electrical resistance in the PEDOT:PSS-dielectric material interface. SEM cross-sectional observations of the dielectric layer at high temperatures (≤ 200 °C) show that the nanovoid layer remains visible; nevertheless, a porosity analysis by pore-filling reveals that the porosity in the nanovoid layer increases with temperature, modifying the PEDOT:PSS-dielectric layer interaction. Although the reason for the low breakdown potential in solid polymer capacitors is still unclear, from the light of the present results, the PEDOT: PSS-dielectric layer interaction is shown to be an important factor.
This work was supported by the MEXT-Program for Creation of Innovative Core Technology for Power Electronics Grant Number JPJ009777.

References

[1] H. Matsuya, D. Quintero, S. Kitano, H. Habazaki, Structure and Electric Properties of Anodized Aluminum with PEDOT:PSS Conductive Polymer Cathode, ECS J Solid State Sci Technol 12 (2023) 073002.
[2] D. Quintero, H. Matsuya, M. Iwai, S. Kitano, K. Fushimi, H. Habazaki, Controlling Dielectric Film Defects to Increase the Breakdown Voltage of Conductive Polymer Solid Capacitors, ACS Appl Mater Interfaces 16 (2024) 1737–1748.