Matthew Mackey

Layer confinement effect on charge migration in polycarbonate/poly(vinylidene fluorid-co-hexafluoropropylene) multilayered films

Forced assembly microlayer coextrusion was used to produce polycarbonate/poly(vinylidene fluoride-co-hexafluoropropylene) [PC/P(VDF-HFP)] layered films for dielectric capacitor applications. Low field dielectric spectroscopy was systematically carried out on the layered films and controls. A low frequency relaxation was found that shifted to higher frequency and decreased in intensity as the P(VDF-HFP) layer thickness was reduced. The interfacial Maxwell-Wagner polarization, being layer thickness independent, could not account for this reduced low frequency relaxation behavior as the layer thickness decreased. Charge diffusion models by Sawada and Coelho, however, satisfactorily predicted the observed layer thickness effect, indicating that the migration of impurity ions in the P(VDF-HFP) layer caused the low frequency relaxation. A new, convenient fitting procedure was developed for the Sawada model yielding an ion concentration and diffusion coefficient of 2 × 1021 ions/m3 and 2 × 10−13 m2/s, respective...

Polymeric nanolayered gradient refractive index lenses: technology review and introduction of spherical gradient refractive index ball lenses

Abstract. A nanolayered polymer films approach to designing and fabricating gradient refractive index (GRIN) lenses with designer refractive index distribution profiles and an independently prescribed lens surface geometry have been demonstrated to produce a new class of optics. This approach utilized nanolayered polymer materials, constructed with polymethylmethacrylate and a styrene-co-acrylonitrile copolymer with a tailorable refractive index intermediate to bulk materials, to fabricate discrete GRIN profile materials. A process to fabricate nanolayered polymer GRIN optics from these materials through thermoforming and finishing steps is reviewed. A collection of technology-demonstrating previously reported nanolayered GRIN case studies is presented that include: (1) the optical performance of a f/# 2.25 spherical GRIN plano-convex singlet with one quarter (2) the weight of a similar BK7 lens and a bio-inspired aspheric human eye GRIN lens. Original research on the fabrication and characterization of a Luneburg inspired GRIN ball lens is presented as a developing application of the nanolayered polymer technology.

Internal field distributions in multilayer polycarbonate/poly(vinylidene fluoride)-hexafluoropropylene films at onset of breakdown

Multilayer polymer films comprising alternating layers of polycarbonate (PC) and polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) show enhanced dielectric strength relative to single component films of either source polymer. Previous failure analysis on films subjected to breakdown under divergent field conditions revealed that multilayer films produced distinct surface treeing patterns whereas monolithic films did not. The choice of surface layer (PC or PVDF-HFP) contacted by a needle electrode influenced the nature of these treeing patterns. Additionally, damage within the film was largely localized to the interfaces between layers. To help explain these empirical results, we model the divergent field based on the geometry of our experimental setup and calculate the internal electric field distribution using the boundary integral equation method (BIEM). All fundamental charges, including: free, bound, trapped, and space charges are accounted for in the calculations, based on current and voltage data recorded during prior breakdown measurements. The calculations show that when PC is used as the surface layer in contact with the needle anode, there is significant field intensification in the top PC layer, in excess of 2000 V/μm. This is many times higher than the measured dielectric strength of monolithic PC and is at least partially due to charge injection from the needle anode. In contrast, the PVDF-HFP sub-layer in this configuration has very low field. These observations are consistent with breakdown occurring near the surface of the film, resulting in large-range surface treeing. When PVDF-HFP is the top layer, field intensification occurs deeper in the film, which is again consistent with the observed optical and FIB/SEM imaging results where less surface treeing and more internal damage is observed. The calculations suggest that the large contrast in field between adjacent layers generates a nexus for localized breakdown at the layer interfaces, again consistent with large internal voids formed by layer delamination in films subjected to divergent field breakdown.

Enhanced dielectric properties due to space charge-induced interfacial polarization in multilayer polymer films

With the recent advancement of power electronics, polymer film capacitors have become increasingly important. However, the low temperature rating (up to 85 °C) and low energy density (5 J cm−3 at breakdown) of state-of-the-art biaxially oriented polypropylene (BOPP) films have been limiting factors for advanced power electronics. Based on our recent work, multilayer films (MLFs), which consist of a high energy density polymer [e.g., poly(vinylidene fluoride) (PVDF)] and a high breakdown/low loss polymer [e.g., polycarbonate (PC)], have shown potential to achieve high energy density (13–17 J cm−3), enhanced breakdown strength, high temperature tolerance, and low loss simultaneously. In this study, the dielectric properties of PC/PVDF 50/50 32- and 256-layer (32L and 256L) films were investigated. The breakdown strength of the 32L film was as high as 800 MV m−1 at room temperature, as compared to 600 MV m−1 of PVDF and 750 MV m−1 of PC. The temperature rating of the 32L film reached 120 °C, higher than that of BOPP. In addition, it was observed that the 32L film with thicker PC layers exhibited a higher breakdown strength and a lower DC conductivity than the 256L film with thinner PC layers at elevated temperatures. These differences were attributed to the difference in the interfacial polarization of space charges, which was further verified by thermally stimulated depolarization current spectroscopy. From this study, we conclude that interfacial polarization endows MLFs with the desirable dielectric properties for next generation film capacitors.