High temperature polymer film dielectrics for aerospace power conditioning capacitor applications

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Abstract

Polymer dielectrics are the preferred materials of choice for capacitive energy-storage applications because of their potential for high dielectric breakdown strengths, low dissipation factors and good dielectric stability over a wide range of frequencies and temperatures, despite having inherently lower dielectric constants relative to ceramic dielectrics. They are also amenable to large area processing into films at a relatively lower cost. Air Force currently has a strong need for the development of compact capacitors which are thermally robust for operation in a variety of aerospace power conditioning applications. While such applications typically use polycarbonate (PC) dielectric films in wound capacitors for operation from −55 °C to 125 °C, future power electronic systems would require the use of polymer dielectrics that can reliably operate up to elevated temperatures in the range of 250–350 °C. The focus of this research is the generation and dielectric evaluation of metallized, thin free-standing films derived from high temperature polymer structures such as fluorinated polybenzoxazoles, post-functionalized fluorinated polyimides and fluorenyl polyesters incorporating diamond-like hydrocarbon units. The discussion is centered mainly on variable temperature dielectric measurements of film capacitance and dissipation factor and the effects of thermal cycling, up to a maximum temperature of 350 °C, on film dielectric performance. Initial studies clearly point to the dielectric stability of these films for high temperature power conditioning applications, as indicated by their relatively low temperature coefficient of capacitance (TCC) (∼2%) over the entire range of temperatures. Some of the films were also found to exhibit good dielectric breakdown strengths (up to 470 V/μm) and a film dissipation factor of the order of <0.003 (0.3%) at the frequency of interest (10 kHz) for the intended applications. The measured relative dielectric permittivities of these high temperature polymer films were in the range of 2.9–3.5.

Introduction

The inadequacy of Commercial-Off-The-Shelf (COTS) capacitors in meeting the challenges of high temperature aerospace power electronics applications renders it absolutely necessary to design a new generation of dielectric materials for the development of capacitive energy-storage devices. These applications require better reliability and flexibility in the power system design as well as high temperature stability to lower the demand for the cooling system. The increasing demand for stable dielectric performance over a large temperature range arises from the closer proximity of aircraft power electronics to heat sources involving turbine engines, generators and motors. The primary focus of high temperature dielectric film research is geared towards the development of compact capacitors which are mechanically robust and thermally stable for wide-temperature aerospace and avionic power conditioning capacitor applications [1]. Polymer dielectrics are the preferred materials of choice for such capacitor applications because of their potential for high breakdown strengths, low dissipation factors (dielectric loss factors) and good dielectric stability over a wide range of frequencies and temperatures, despite having inherently lower dielectric constants relative to ceramic capacitors. Besides, they are also amenable to large area processing into films at a relatively lower cost. An added advantage of a metallized polymer film capacitor design is its ‘self-healing’ or ‘clearing’ capability [2], [3] that facilitates a graceful failure mechanism which is generally not the case with pure ceramic capacitors.

Aerospace power conditioning applications typically use polycarbonate (PC) dielectric films in wound capacitors for operation from −55 °C to 125 °C. For higher operating temperatures up to 200 °C, capacitors incorporating poly(p-phenylenesulfide) (PPS) films have been evaluated because of their low dissipation factor and dielectric strengths approximating 500 V/μm [4]. Recently, an amorphous high temperature fluorenyl copolyester (FPE) has been evaluated for space power conditioning [5] and is also being touted as a potential replacement for PC in aerospace power conditioning applications; besides an operating temperature capability up to or exceeding 250 °C, it is reported to have a relatively high dielectric strength (400 V/μm) as a thin film, a dielectric constant of 3.3 and a low dissipation factor of 0.0003 (0.03%) at 1 kHz. However, its cost and current, even more stringent (∼350 °C) thermal management requirements for future aerospace power conditioning capacitor applications necessitate further research in the area of high temperature polymer dielectrics. High energy dissipation occurring at higher temperatures can result in considerable heat rise within the dielectric, which, in turn, might cause dielectric degradation due to thermal runaway. Thus, it is reasonable to assume that polymer dielectrics with high glass transition temperatures and high thermal stability will play a significant role in providing electro-mechanical stability for wide-temperature power electronics applications.

As part of our capacitor research program toward meeting the requirements of dielectric stability and reliability at high temperatures, we report herein the film fabrication and variable temperature dielectric evaluation of some high performance polymers synthesized in our laboratories. The candidate high temperature polymers evaluated in this program are based on fluorinated polybenzoxazoles and polyimides as well as cardo-type (fluorenyl) polyesters incorporating diamond-like hydrocarbon structural units in the polymer backbone. The structures of the polymers are depicted in Fig. 1.

The fluorinated polybenzoxazole is a 1:1 random copolymer consisting of a hydroxyphenyl-6F-PBO unit as well as a 12F-PBO unit (OH-6F-PBO/12F-PBO copolymer). PI-ADE refers to a fluorinated polyimide post-functionalized with an adamantyl ester pendant and FDAPE refers to a fluorenyl polyester incorporating another diamond-like hydrocarbon group, i.e., 4,9-diamantyl unit in the polymer backbone. These polymers have high glass transition temperatures as well as good thermal stabilities and fulfill the requirements for potential evaluation in wide-temperature power electronics applications of interest to the Air Force.

Section snippets

Synthesis and general characterization of the polymers

The detailed synthesis of the fluorinated polybenzoxazoles has been described elsewhere [6]. PI-ADE was synthesized from the post-polymer reaction of a hydroxy polyimide with adamantane-1-carboxylic acid chloride [7]. The synthesis and characterization of the fluorenyl polyester FDAPE have also been reported in our earlier studies [8], [9]. The glass transition temperatures of the solvent-cast polymer films were determined by DMA (Dynamic Mechanical Analysis). The thermal and thermo-oxidative

Results and discussion

Besides the potential for electro-mechanical stability at high temperatures due to their high glass transition temperatures and thermal stability, some established criteria for micro-electronic packaging applications of fluorinated polybenzoxazoles [6], [10] and fluorinated polyimides [10], [11] are equally important for their dielectric applications in wide-temperature power electronics. The versatility of the hydroxyl pendant in fluorinated polyimides can be utilized to develop novel,

Conclusions

A few classes of high temperature polymer dielectric films have been investigated with regard to their potential utilization in wide-temperature aerospace power conditioning applications. Preliminary studies have demonstrated their dielectric stability for operation at high temperatures in the 250–350 °C range. Variable temperature dielectric studies have shown that their temperature coefficient of capacitance over a wide-temperature range has been relatively small (1–3%). A relatively low

Acknowledgement

The financial support to NV (University of Dayton Research Institute) from AFRL/RZPE, Wright-Patterson Air Force Base, Dayton, OH is gratefully acknowledged.

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