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Reduction of Ionic Conduction Loss in Multilayer Dielectric Films by Immobilizing Impurity Ions in High Glass Transition Temperature Polymer Layers Huadong Huang, Xinyue Chen, Kezhen Yin, Imre Treufeld, Donald Schuele, Michael Ponting, Deepak Langhe, Eric Baer, and Lei Zhu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00211 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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ACS Applied Energy Materials

Reduction of Ionic Conduction Loss in Multilayer Dielectric Films by Immobilizing Impurity Ions in High Glass Transition Temperature Polymer Layers

Huadong Huang,†,# Xinyue Chen,†,# Kezhen Yin,† Imre Treufeld,† Donald E. Schuele,† Michael Ponting,‡ Deepak Langhe,‡,* Eric Baer,†,* and Lei Zhu†,*

†

Center for Layered Polymeric Systems (CLiPS) and Department of Macromolecular Science

and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States ‡

#

PolymerPlus, LLC, 7700 Hub Pkwy, Valley View, Ohio 44125, United States

These authors contributed to this work equally.

* Corresponding authors. E-mail addresses: [email protected] (L. Zhu), [email protected] (E. Baer), and [email protected] (D. Langhe)

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Abstract Current development of advanced power electronics for electric vehicles demands high temperature, high energy density, and low loss polymer dielectrics.

Multilayer films (MLFs),

which are comprised of alternating high temperature/low loss linear dielectric polymer such as polysulfone (PSF) and high energy density polymer such as poly(vinylidene fluoride) (PVDF), are promising for this application, because high temperature tolerance, high energy density, and low loss can be achieved simultaneously.

This study explored the reduction of impurity ion

conduction loss in PSF/PVDF MLFs (e.g., the dissipation factor is as low as 0.003 at 1 Hz and 100 °C) without sacrificing high dielectric constant and high energy density.

Various electric

poling processes were explored at a temperature slightly below the glass transition temperature (Tg ~ 185 °C) of PSF.

Compared with pure alternating current (AC) and pure direct current

(DC) poling methods, unipolar (DC+AC) poling was found to be the most effective in polarizing impurity ions from the PVDF layers into the PSF layers.

Because of the low segmental

mobility below Tg, impurity ions were largely “locked” in PSF.

The immobilization of impurity

ions was thermally stable up to 120 °C.

Because DC-link capacitors work with unipolar charge

and discharge processes, these PSF/PVDF MLFs with low dielectric losses are promising for the application of advanced power electronics for the automobile industry.

Keywords: Multilayer polymer films, dielectric loss, polysulfone, poly(vinylidene fluoride), electric poling


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Introduction Rapid advances in electrical and power applications have called for breakthroughs for polymer film capacitors, whose light weight, high voltage/high current tolerance, low loss, and ease of large-scale manufacturing properties are attractive over ceramic and electrolytic capacitors.1,

2

The state-of-the-art polymer dielectric is biaxially oriented polypropylene

(BOPP), primarily owing to its ultralow dielectric loss (e.g., dissipation factor, tanδ ~ 0.0002 at 1 kHz), high dielectric breakdown strength (730 MV/m), and long service life.3,

4

However,

modern power electronics require a long-term operation at high temperatures (e.g., ≥120 °C) for polymer film capacitors with low dielectric losses.5

BOPP film cannot meet this requirement

because of its low temperature rating around 85 °C, above which a significant reduction of lifetime is observed.

In addition, the energy density of BOPP is low because of its low

dielectric constant (εr = 2.25).

Therefore, recent research thrusts have focused on high

temperature, high energy density, and low loss polymer dielectric films.6 Presently, most efforts are dedicated to enhancing energy densities of polymer films via increasing either dielectric constant or breakdown strength. Numerous new materials have been explored, including polymer/inorganic nanocomposites7-10 and relaxor-like ferroelectric polymers.6, 11-14

Nonetheless, significantly higher dielectric losses (e.g., tanδ > 0.01) than that

of BOPP are observed as results of internal conduction of space charges15, 16 and/or nonlinear ferroelectric switching of dipoles and domains.11 From a practical point of view, low loss is more important than high energy density because high dielectric loss will cause significant heat generation during capacitor operation and lead to reduction in breakdown strength and lifetime, especially at high temperatures.1, 17

This is why current film capacitor industry mostly focuses

on BOPP films with ultralow dielectric losses.

As such, the development of new polymer



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dielectric materials must pay close attention to the dielectric loss, namely, tanδ < 0.005 at low fields (< 1 MV/m) and discharge efficiency > 90% at high fields (> 200 MV/m). Instead of synthesizing new dielectric polymers, the multilayer film (MLF) technology combines

different

polymers

in

a

unique

layered

thicknesses/compositions and dielectric properties.18,

19

structure

with

tunable

layer

For example, high glass transition

temperature (Tg) polycarbonate (PC) or polysulfone (PSF) are linear dielectric polymers with low dielectric loss and high breakdown strength.

High dielectric constant poly(vinylidene

fluoride) (PVDF) and its random copolymers have high dielectric constant and thus high energy density.20,

21

MLFs of the low loss PC (or PSF) and the high energy density PVDF have

demonstrated

enhanced

dielectric

characteristics,

where

advantageous

synergistically enhanced and disadvantageous properties can be mitigated.18

properties

are

First, dielectric

constant of MLFs (εr,MLF) obey the series capacitor rule:22 ଵ ఌ౨,౉ైూ

=

ఝభ ఌభ

+

ఝమ

(1)

ఌమ

where φ1/φ2 and εr,1/εr,2 are volume fraction and dielectric constant of PC (or PSF) and PVDF, respectively.

Because of the high dielectric constant of PVDF (~10-12), the εr,MLF can be

readily tuned by varying the volume fraction of each component (φ1/φ2). Second, conductions from space charges (i.e., electrons and impurity ions) in PVDF layers cause significant dielectric loss at high temperatures.23, 24

One strategy to mitigate these losses is to reduce the PVDF layer

thickness to the nanometer scale (i.e., < 100 nm) and confine the transport of impurity ions.23 Nonetheless, it is not desirable to decrease the PC or PSF layer thickness, because electric insulation will be undermined and the breakdown strength of the MLFs decreases.

This is

demonstrated by comparing dielectric breakdown strengths between PSF/PVDF 30/70 32-layer (32L) and 256-layer (256L) films.25

Namely, the 32L film with thicker PSF layers exhibit 4

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higher breakdown strengths than the 256L film with thinner PSF layers.

The underlying

mechanism is later attributed to the interfacial polarization of space charges in the PVDF layers,26 which modifies the local electric field and prevents electrode-injected hot electrons from penetrating through the film.

More importantly, the interfaces in these MLFs orient

perpendicular to the electric field direction, rather than orient randomly in their immiscible blends.

It is observed that PC/P(VDF-HFP) MLFs exhibit much higher dielectric breakdown

strengths than their blend films, even higher than the linear additive values.27

Scheme 1. Schematic representation of electric poling near the glass transition temperature of PSF (Tg,PSF) to drive impurity ions in the PVDF layer into neighboring PSF layers.

Obviously, there is a dilemma for MLFs.

On one hand, thin PVDF layers are needed to

mitigate the conduction loss from space charges, and thick PC (or PSF) layers are desired to keep high breakdown strength/high insulation.

However, a combination of thin PVDF and thick PC

(or PSF) layers will decrease the dielectric constant and thus energy storage, according to Eqn. (1).

In this work, a viable strategy is proposed to resolve this dilemma. By using unipolar

electric poling at a temperature (165 °C) slightly below the Tg (~185 °C) of PSF, impurity ions can be driven from relatively thick (~400 nm) PVDF layers into the PSF layers (Scheme 1). 5 ACS Paragon Plus Environment


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After cooling, impurity ions can be effectively locked inside the glassy PSF layers.

Therefore,

ion conduction loss can be effectively reduced without compromising the εr,MLF. Because of the frozen mobility of impurity ions in the high Tg PSF, reduced dielectric loss can persist up to 120 °C for these electrically poled MLFs.

Because the direct current (DC)-link capacitors work

with unipolar charge-discharge cycles, this study provides a useful guidance for developing low loss MLF DC-link capacitors for power electronic applications.

Experimental Section Materials.

PSF (Udel® P-3703) and PVDF (Solef® 6010) resins were purchased from

Solvay Specialty Polymers (Alpharetta, GA).

Linear low density polyethylene (LLDPE) resins

were purchased from Dow Chemical Company.

PSF and PVDF resins were dried under

vacuum at 80 °C for 24 h prior to multilayer coextrusion.

LLDPE resins were used as received.

Preparation of PSF/PVDF 50/50 (vol./vol.) 33-Layer MLFs.

Multilayer coextrusion

with three single-screw extruders was utilized to fabricate alternating MLFs of PSF/PVDF at a coextrusion temperature of 285 °C, which was determined by the rheological compatibility of the two polymers.

The polymer rheology was characterized using a melt flow indexer (MFI), on

Kayeness Galaxy 1, at a shear rate similar to the extrusion condition (10 s-1; data not shown). Each single-screw extruder was equipped with a melt pump to control the volume ratio of PSF and PVDF (50/50 vol./vol.).

An ABA (with a thickness ratio of 1:2:1) three-layer feedblock

was used to converge the PSF/PVDF/PSF melts, followed by four times of multiplication via a splitting, spreading, and restacking processes.19

Before the exit die, two sacrificial LLDPE skin

layers were laminated onto both surfaces of the MLF before extrusion from an exit die (20 inch wide with a 500 µm slit opening) to avoid surface defects.

These sacrificial LLDPE layers were


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removed prior to subsequent dielectric characterization.

The final MLF thickness was

controlled to be ~12 µm by tuning the uptake speed onto a chiller roll (ca. 20 feet per minute). The integrity of these PSF/PVDF MLFs was demonstrated in a previous report.25 Instruments and Characterization Methods.

Normal broadband dielectric

spectroscopy (BDS) measurements were performed on a Novocontrol Concept 80 broadband dielectric spectrometer to study temperature-dependent dielectric behavior of impurity ions in PVDF layers. Approximately, 20 nm thick gold (Au) electrodes were evaporated using an Angstrom Engineering Evovac deposition system (model 01406, Kitchener, Ontario, Canada) onto both sides of the films, which were dried at 60 °C for 24 h prior to coating. diameter was 1.0 cm (area 0.785 cm2).

The electrode

Frequency-scan real and imaginary relative

permittivities (εr′ and εr″) in the range of 10-2~106 Hz were measured as a function of temperature from 20 to 170 °C. Novocontrol high-voltage BDS (HV-BDS) equipped with a high voltage interface, HVB4000, was used to polarize the impurity ions in PVDF layers.

The interface could provide

±2000 V (peak-to-peak) high voltage with frequency up to 104 Hz.

Similar to normal BDS

measurements, the film samples were coated with ~50 nm aluminum (Al) electrodes on both sides. The polarization process of impurity ions in PVDF was carried out as the following. The film sample was heated at a heating rate of 5 °C/min from 100 °C to the poling temperature (160-170 °C), where a pure alternating current (AC), a pure direct current (DC), or a unipolar (DC+AC) electric field was applied to drive the impurity ions in the PVDF layers into the PSF layers.

While holding the poling electric field, the sample was cooled to 100 °C to avoid the

relaxation of impurity ions.

To quantitatively evaluate the poling efficiency, high-voltage (2.10

MV/m DC + 2.10 MV/m AC) frequency-scans in the range of 10-2~104 Hz at 100 °C were



measured before and immediately after electric poling.

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Meanwhile, high-voltage

frequency-scans were also measured after annealing at 100 °C for 1 and 6 h to detect if impurity ions could relax back into the PVDF layers or not. Finally, high-voltage frequency-scans at different temperatures from 20 to 150 °C were carried out to investigate the thermal stability of PSF-immobilized impurity ions. The melting behavior of as-extruded and annealed MLFs was studied using a TA Instrument Q2500 differential scanning calorimetry (DSC). The instrument was calibrated with indium and tin standards.

The experiments were carried out in a nitrogen atmosphere using about 2 mg

film samples sealed in aluminum pans.

The samples were heated from -50 to 200 °C at a

heating rate of 10 °C/min.


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Figure 1. (A) Real (εr′) and (B) imaginary (εr″) relative permittivities as a function of frequency for PSF/PVDF 50/50 33L film at various temperatures. The inset shows the Arrhenius plots of lnτ and lnf vs. 1/T for fast impurity ions. (C) Ionic resistance (R1)/ capacitance (C1) and PVDF capacitance (C2) as a function of temperature. (D) Experimental and calculated D-E loops for the PSF/PVDF 50/50 33L film at 100 °C. The frequency is 10 Hz with a sinusoidal waveform.

Results and Discussion Simulation of Temperature-Dependent Conduction of Impurity Ions in PVDF Layers. On the basis of our previous work,23, 24 it was found that impurity ion conduction in the PVDF layers, rather than the amorphous PVDF dipole switching,28 accounted for high dielectric loss at elevated temperatures for MLFs.

To determine optimal electric poling

conditions, temperature-dependent conduction loss from impurity ions in PVDF layers should be 9 ACS Paragon Plus Environment


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understood. Figure S1 in the Supporting Information and Figure 1 show frequency-scan real (εr′) and imaginary (εr″) relative permittivities for the PSF/PVDF 50/50 33L film below and above 100 °C, respectively.

Around room temperature (Figure S1B), the αa relaxation for

amorphous PVDF dipoles was seen at a frequency above 106 Hz.29,

30

The low-frequency

relaxation peak centered at around 3.7 Hz was assigned as the αc relaxation from the α-form PVDF crystals, which is associated with the dipole-wagging motion along the chain axes.29, 30 Upon increasing temperature above 40 °C, both αa and αc relaxation peaks shifted to higher frequencies, and another peak emerged at low frequencies.

This low-frequency εr″ peak was

assigned as the conduction loss from fast impurity ions in PVDF (e.g., small cations such as Na+).26

Associated with these εr″ peaks, step-wise changes in εrʹ were seen in Figure S1A.

As

the temperature increased to 100 °C and above, the complete peak from the fast impurity ion conduction shifted from ~1 Hz at 100 °C to ~1000 Hz at 170 °C (Figure 1B).

From the

Arrhenius plot (i.e., lnf vs. 1/T; f is the peak frequency) in the inset of Figure 1B, the activation energy (Ea) was determined to be 1.27 eV, which is consistent with ion transport in a polymer matrix.23,

31

The upturn at low frequencies could be attributed to the conduction of slow

impurity ions in PVDF and PSF (e.g., anions or large cations);26 note that the peaks should appear at much lower frequencies and thus could not be seen in this plot (Figure 1B).

Although

the εr′ was seen to increase with decreasing frequency in Figure 1A due to ion conduction, significant loss was observed in εr″ (Figure 1B).

For example, at 100 °C, the εr′ only increased

from 4.6 at 200 Hz to 5.2 at 1 Hz; however, the εr″ increased from 0.06 to 0.3, and the corresponding tanδ increased nearly five times, from 0.013 to 0.058.

This could be attributed to

the nonlinear dielectric behavior of impurity ion transport in polymer matrices.28

Therefore,

ionic conduction should be avoided for low loss dielectrics.


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Figure 2. The top panel shows an equivalent resistor-capacitor (RC) circuit for the 32-layer (PSF/PVDF)16 with contributions from pure capacitors, C0 for PSF and C2 for PVDF, and a lossy capacitor (i.e., R1 and C1 in series) for ionic conduction. Note that the electronic conductions for PSF and PVDF are ignored because they are much lower than the impurity ion conduction. The left panel shows a simplified 2-layer circuit for the 32-layer circuit with parameters C0′, C1′, R1′, and C2′, where C0 = 16 C0′, C1 = 16 C1′, C2 = 16 C2′, R1 = R1′/16. Frequency-scan original and ZView-simulated BDS curves, (A) εr′ and (B) εr″, for the PSF/PVDF 50/50 33L film at 100 °C.

To understand the ionic conduction behavior in PVDF layers, a phenomenological simulation using the resistor-capacitor (RC) circuit was carried out by the ZView software. Since both surface PSF layers had a half thickness of that for the inside PSF (and PVDF) layers, the 33-layer PSF/PVDF system was the same as the 32-layer system with equal PSF and PVDF thicknesses. Electronic conductions for PSF and PVDF layers could be neglected, because the electronic conductivities (σe) of PSF (σe = 10-16 S/m) and PVDF (σe = 10-14 S/m) were much lower than the ionic conductivity (σion = 5.1×10-12 S/m) in PVDF at 100 °C.

The pure

capacitances for PSF (C0 = 5.38 nF) could be calculated by C = εrε0A/d, where ε0 and εr are vacuum and PSF permittivities (assuming constant at εr,PSF = 3.1), A is the area (0.785 cm-2), and 11 ACS Paragon Plus Environment


d is the PSF (or PVDF) layer thickness (394 nm).

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Impurity ion conduction in PVDF could be

simulated with a resistor (R1) and a capacitor (C1) in series.32

Because the temperature was

below the Tg,PSF, ionic conduction in PSF layers could be neglected. The 32-layer circuit was further equivalent to a 2-layer circuit as shown in the left panel of Figure 2, where C0 = 16C0′, C1 = 16C1′, C2 = 16C2′, R1 = R1′/16.

Experimental εr′ and εr″ curves for the fast ions at 100 °C

could be simulated with parameters C2 (from εr′) and C1′/R1′ (from εr″); C2 = 2.152 ×10-8 F, R1 = 9.7×106 Ω, and C1 = 2.67×10-8 F (see Figure 2). temperatures are plotted in Figure 1C. temperature.

Results of C2, C1, and R1 at different

The C2 had a slight decrease upon increasing

The ionic resistance R1 decreased exponentially, and the ionic capacitance C1 had

an initial decrease from 100 to 120 °C, beyond which it leveled off.

Based on the series R1C1

circuit, the ionic conductivity could be obtained as: σion = d/(AR1); see the inset of Figure 1C. Obviously, the σion increased exponentially as a function of temperature, which could be attributed to higher ionic mobility at elevated temperatures.

From R1 and C1 values, the

relaxation time (τ) could be obtained: τ = R1C1. The inset of Figure 1B shows the Arrhenius plot of lnτ vs. 1/T, from which the Ea of fast impurity ions in PVDF layers could be determined as 1.33 eV. Again, this value is comparable to that obtained from the lnf ~ 1/T plot (inset of Figure 1B). To further confirm the simulation results in Figure 1C, we calculated the D-E loop for the PSF/PVDF 50/50 33L film using parameters obtained from BDS simulation.

As described in

Figure S2A, the relationship between the charges passing through the entire circuit (Q0) and the applied AC voltage [V(t)] was obtained.

The electric displacement D can be defined as D =

Q0/A and the applied electric field (E) can be expressed as E = V(t)/h = V0 sin(ωt)/h (h is the total film thickness). The calculated D-E loop at 100 °C and 10 Hz for the PSF/PVDF 50/50 33L


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film is plotted in Figure 1D.

Compared with the experimental loop, the calculated loop showed

a reasonable fit. The slightly higher slope for the experimental loop was attributed to the increased dielectric constant for PSF under a high electric field (e.g., the dielectric constant at 200 MV/m is around 3.7 as calculated from the D-E loop slope; data not shown).

This result

suggests that the simulated ionic resistance and capacitance in Figure 1C are reasonable.

Figure 3. (A) Simulated time-dependent VDC on a PVDF layer at different temperatures when a 1000 VDC voltage is applied to the 32-layer equivalent circuit. (B) Simulated frequency-dependent peak VAC on a PVDF layer at different temperatures when a 1000 VAC voltage is applied to the 32-layer equivalent circuit. (C) Calculated energy densities in a single PSF or PVDF layer (394 nm thick and 0.785 cm2 area) and (D) total energy density of a PSF/PVDF bilayer and loss% as a function of frequency at 100 °C.

Voltage Drops in the PVDF Layer Due to Impurity Ion Conduction. Once the ionic conduction behavior is understood, the voltage drop on the PVDF layer during DC and AC 13 ACS Paragon Plus Environment


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poling conditions can be simulated using the equivalent circuits in Figure 2.

This is important

for the determination of optimal electric poling conditions to drive impurity ions into the PSF layers.

During simulation using the LTspice software, a 1000 VDC or a 1000 VAC was applied to

the 32-layer equivalent circuit (Figure 2), and the VDC or VAC on a single PVDF layer was monitored as a function of time or frequency at different temperatures. listed in Table 1.

Detailed results are

When a 1000 VDC voltage was applied to the 32-layer equivalent circuit

(Figure 3A), the VDC on a single PVDF layer quickly reached the maximum value (VDC,max), and then gradually decreased with time to the minimum value (VDC,min). The stepwise voltage drop over time on a single PVDF layer was a result of internal discharging of the pure PVDF capacitor C2 via the lossy capacitor (R1C1), as shown in Figure 2. attributed to impurity ion conduction.

The discharging process was mainly

Judging from the values summarized in Table 1, the

voltage drop on a PVDF layer was about 40-50%.

Meanwhile, the discharge time (tdis)

dramatically decreased with increasing temperature. This could be attributed to increased ionic conduction at elevated temperatures (see inset of Figure 1C).

In this sense, a pure DC poling

may not be efficient to polarize impurity ions in PVDF layers.

Simulation Results for the DC and AC Voltages on a Single PVDF Layer DC output voltage AC peak voltage Temperature a VDC,max tdis VAC,min Frequency range VDC,min VAC,max (°C) (V) (V) (V) fl - fh (Hz) b (V) (s) 100 12.7 6.4 0.9 12.7 6.4 0.07~35.5 110 13.3 7.6 0.4 13.3 7.6 0.16~50.1 120 13.7 8.0 0.1 13.7 8.0 0.63~159 130 14.1 8.2 0.04 14.1 8.2 1.8~355 140 14.5 8.3 0.02 14.5 8.3 8.5~1000 150 15.0 8.5 0.008 15.0 8.5 6.3~2820 160 15.4 8.7 0.005 15.5 8.7 17.8~6310 165 15.9 9.0 0.002 16.0 9.0 28.2~7080 170 16.4 9.2 0.001 16.7 9.2 44.7~17800 a tdis is the discharge time, where the VDC reaches the low value VDC,min. Table 1.


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b

fl is the lower critical frequency and fh is the higher critical frequency. Figure 3B shows the peak VAC on a PVDF layer when a 1000 VAC voltage with different

frequencies was applied to the 32-layer equivalent circuit. For a given temperature, the peak VAC depended on the poling frequency, i.e., low below a lower critical frequency (fl) and high above a higher critical frequency (fh). temperature (Table 1).

These critical frequencies depended on the poling

Again, this frequency-dependent behavior could be attributed to the

ionic conduction in PVDF.

Below the fl, ionic conduction in PVDF induced discharge of the

pure capacitor C2 via the lossy capacitor R1C1. Above the fh, impurity ions could not respond to the fast switching field, and thus no internal discharge could happen.

To drive the impurity ions

from PVDF layers into PSF layers, a frequency slightly below the fl should be used; otherwise, impurity ions would not migrate in the PVDF layer. The effect of impurity ion conduction on electric energy storage was also studied using the equivalent circuits in Figure S2B under an applied 1000 VAC (electric field ~ 78.1 MV/m) at 100 °C.

Detailed calculation should refer to the discussion in Section 3 of the Supporting

Information. As shown in Figure 3C, the PSF layer exhibited a high energy density (Ue,PSF) at low frequencies (< fl) and a low Ue,PSF at high frequencies (> fh), whereas the PVDF layer showed an opposite trend. This was ascribed to the frequency-dependent voltage profiles in PSF and PVDF layers.

As shown in Figure 3D, the total stored energy density (Ue,tot) for the

PSF/PVDF bilayer was higher at low frequencies (< fl) than that at high frequencies (> fh), indicating ionic conduction could enhance energy storage. However, a significant energy loss% (with a maximum of ~15%) was observed around 20 Hz due to impurity ion conduction.

From

Figure 3D, it seemed that high energy density and low loss could be achieved when the frequency was low enough (e.g.,

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