Improved Breakdown Strength and Energy Density in Thin-Film

Improved dielectric constant, breakdown strength, and energy density are reported for polyimide/sub-10 nm Ba0.7Sr0.3TiO3 (BST) nanocomposites up to th...
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Improved Breakdown Strength and Energy Density in Thin-Film Polyimide Nanocomposites with Small Barium Strontium Titanate Nanocrystal Fillers Christopher W. Beier,† Jason M. Sanders,‡ and Richard L. Brutchey*,† †

Department of Chemistry and ‡Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *

ABSTRACT: Improved dielectric constant, breakdown strength, and energy density are reported for polyimide/sub-10 nm Ba0.7Sr0.3TiO3 (BST) nanocomposites up to the percolation threshold. Homogeneous nanocomposites were prepared via an in situ polymerization technique, whereby a suspension of BST is blended with 1,3-bis(4-aminophenoxy)benzene (BAPB) and pyromellitic dianhydride (PMDA) monomers prior to spincasting and thermal imidization. The dielectric properties the PMDA-BAPB/ BST nanocomposites were measured as a function of BST loading and displayed a 24% increase in breakdown strength and calculated energy densities more than twice that of the pure polymer at 10 vol % BST. The PMDA-BAPB/BST nanocomposites also display improved permittivities (εr = 6.2 at 18 vol % and 1 MHz) and low dielectric loss (tan δ 500 °C), low loss (tan δ = 0.015), and a modest dielectric constant (εr = 2.8).33−36 Variable concentrations of BST suspensions were added such that the resulting PMDA-BAPB/BST nanocomposites possessed nanocrystal loadings of 0, 5, 10, 13, 15, and 18 vol %. The as-prepared nanocomposite thin films were yellow in color (like the parent polyimide) and were optically transparent, suggesting a fairly homogeneous dispersion of the nanocrystals within the polyimide matrix (see Supporting Information, Figure S2). SEM images of the nanocomposite surface showed relatively uniform film morphology, and EDX spectroscopic mapping of the Ti K emission lines on the film surface corroborated a homogeneous distribution of titanium cations within the nanocomposite (Figure 3 and Supporting Information, Figure S3). XRD analyses of the nanocomposite thin films revealed the BST nanocrystals remained phase pure after processing and fabrication, with no indication of BaCO3 or SrCO3 formation (Figure 1). The PMDA-BAPB/BST composite system was thermally robust, as confirmed by TGA. The addition of the BST nanoparticulate filler resulted in a modest decrease in thermal stability relative to the neat PMDA-BAPB polyimide, which possessed a decomposition onset (determined by the first derivative of the wt% as a function of temperature) at 560 °C in air and 540 °C in nitrogen by TGA. The resulting PMDABAPB/BST nanocomposites were all stable up to 450 °C in air and 500 °C in nitrogen (see Supporting Information, Figure S4). The larger variation in thermal stability in air is a result of catalytic decomposition of the polyimide in the presence of oxides.9,37 FT-IR spectroscopy was used to verify complete imidization of the neat PMDA-BAPB polyimide under the processing conditions (see Supporting Information, Figure S5). The appearance of strong ν(CO) in-phase and out-of-phase imide stretching modes at 1780 and 1731 cm−1, respectively, and ν(C−N−C) axial and transverse stretching and out-ofplane deformation modes at 1375, 1169, and 725 cm−1, respectively, are indicative of imidization.38,39 Furthermore, the IR bands associated with the PAA intermediate (e.g., ν(CO) acid or amide modes at 1720 and 1670 cm−1, respectively, or v(C−N−H) amide mode at 1545 cm−1) were not observed for the neat polymer. As confirmation of the FT-IR data, the neat polymer displayed prominent Raman bands at 1512 and 1604 cm−1 corresponding to the ν(C6H4) modes and a band at 1788

Figure 4. Raman spectra of PMDA-BAPB/BST nanocomposites, offset for clarity. The neat PMDA-BAPB film was processed and prepared in an analogous way to the nanocomposites but without addition of BST.

increases within the polymer matrix, a characteristic ν(C−N− H) amide band appears with increasing intensity at 1560 cm−1 as a result of incomplete imidization. This phenomenon has been previously reported for polyimide/BaTiO3 nanocomposites and attributed to the nanocrystals interacting with PAA, reducing chain mobility, and consequently hindering imidization.9 Despite incomplete imidization and the presence of amide moieties within the nanocomposite, the small BST nanocrystals are able to provide improved device performance (vide infra). Dielectric Characterization. The dielectric constant and loss tangent of the nanocomposites were measured at a range of frequencies from 1 kHz to 1 MHz. The dielectric constant of the PMDA-BAPB/BST nanocomposite increased with increasing BST loading over all frequencies tested. For example, the dielectric constant increased from εr = 2.8 for the neat PMDABAPB polyimide to εr = 6.2 at 18 vol % BST loading (measured at 1 MHz). The measured dielectric constant of the PMDABAPB/BST nanocomposite as a function of BST loading was compared against those values predicted by Bruggeman’s effective medium model, 0 = Σ ν[(εi − εeff)/(εi + 2εeff)],30 where v is the volume fraction of the inclusion, εeff is the dielectric constant of the nanocomposite, and εi is the dielectric constant of BST and PMDA-BAPB, which were measured to be 315 and 2.8, respectively. As seen in Figure 5a, the experimentally measured data match well with those values 6961

dx.doi.org/10.1021/jp312519r | J. Phys. Chem. C 2013, 117, 6958−6965

The Journal of Physical Chemistry C

Article

Figure 5. (a) Weibull breakdown at 63.2% probability of failure (red, left axis) and relative permittivity compared with Bruggeman’s effective medium model (blue, right axis) as a function of BST loading. (b) Two-parameter Weibull plots of PMDA-BAPB/BST nanocomposites where dashed line represents Weibull breakdown at 63.2% probability of failure. Measurements were taken at 25 °C.

the thin films were subjected to a negative DC bias at a ramp rate of 250 V/s until breakdown was indicated by an instantaneous increase in current. Dielectric breakdown strength was calculated using a two-parameter Weibull distribution function:42 P = 1 −[−(E/EBD)β], where P is the cumulative probability of breakdown, E is the experimentally measured breakdown strength, EBD is the cumulative breakdown probability at 63.2%, and β is a shape parameter for 15− 20 independent measurements per volume fraction. The Weibull distributions of breakdown strengths are given in Figure 5a,b. The breakdown strength of the PMDA-BAPB/BST nanocomposites increases with BST loading up to 10 vol %, where it reaches a maximum value of 296 V/μm. This represents a 24% increase over the breakdown strength of the neat PMDA-BAPB polyimide (EBD = 238 V/μm). At BST nanocrystal volume fractions above 10%, the breakdown strength falls slightly to become comparable to that of the neat PMDA-BAPB before falling below that of the neat polymer between 15 and 18 vol % BST loading. The precipitous decrease in breakdown strength at these BST volume fractions is likely the result of a continuous network of BST nanocrystals percolating across the electrodes.14,43 In the percolation regime, EBD is dominated by the nanoparticulate filler and negatively impacts the high breakdown strength of the matrix material.43,44 Whereas an enhancement in breakdown strength from nanoparticulate fillers below the percolation threshold has been reported in several nanocomposite systems,45−51 such an effect has not been previously reported for any perovskite-based nanocomposites. Nanocomposites are known to possess a high interfacial area because of the high surface area of nanoparticulate fillers relative to larger fillers;4 in turn, the interface between the nanocrystals and polymer will be dominant even at low volume fractions.52,53 This short-range layer surrounding the nanoparticulate fillers is thought to allow charge dissipation and improve the internal electric field distribution, which help to suppress significant interfacial polarization that exists in the case of larger fillers.54,55 The dispersed nanoparticulate fillers also help decrease charge transport by acting as scattering centers.47,54,55 The mitigating effects of nanoparticulate fillers can help improve the breakdown strength of the resulting nanocomposites. Here the excellent dispersion of sub-10 nm BST nanocrystals within the PMDA-BAPB matrix has allowed for an increase in breakdown strength relative to the neat polymer up to 10 vol % loading. This may be rationalized by

predicted by the effective medium model within this range of BST loadings. Moreover, the PMDA-BAPB/BST nanocomposites exhibited excellent dielectric stability over these frequencies (Figure 6a,b), which is an improvement over

Figure 6. (a) Relative permittivity of PMDA-BAPB/BST devices as a function of frequency. (b) Dielectric loss of PMDA-BAPB/BST devices as a function of frequency. Measurements were taken at 25 °C.

P(VDF-HFP)/BaTiO3 nanocomposite systems.5,18 In each of the BST nanocrystal loadings tested, the dielectric loss of the nanocomposites remained below 0.04 over the frequency range of 1 kHz to 1 MHz. Such values are consistent with other perovskite-loaded polyimide composites4,13 and are markedly improved over PVDF systems, which have shown dielectric losses as high as 0.2 at 1 MHz.5 To study the effects of the BST nanocrystals on the dielectric breakdown strength of the PMDA-BAPB/BST nanocomposite, 6962

dx.doi.org/10.1021/jp312519r | J. Phys. Chem. C 2013, 117, 6958−6965

The Journal of Physical Chemistry C the strong interaction between the BST nanocrystals and the polymer matrix (vide supra), which likely reduces the localized space charge accumulation.3,54,56 As a result of favorable effects inherent to the nanoparticulate nature of the BST filler, the calculated energy density of the PMDA-BAPB/BST increased from 1.4 J/cm3 for the pure polyimide to 2.9 J/cm3 at 10 vol % BST, which represents an increase of 107% (see Supporting Information, Figure S7). Such improvements have never been realized for perovskite nanocrystals, where precipitous drops in breakdown strength with marginal increase in permittivity result in lowered energy density at low filler volume fraction. These results suggest that perovskite nanocrystals in the size regime of ∼10 nm may be needed to reap these benefits because previously reported polymer/perovskite nanocomposites with fillers as small as 30 nm showed diminished breakdown strength at low volume fractions;2,4,5,13 however, it is likely that the interplay between filler size and dispersion within the polymer matrix dominates these interfacial effects.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic of high-voltage testing, digital photographs of PMDA-BAPB/BST nanocomposites, SEM micrographs with EDX maps of all PMDA-BAPB/BST films, TGA curves in air and nitrogen, FT-IR spectra of neat PMDA-BAPB polymer, and nitrogen adsorption−desorption isotherm for BST pellets. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ACKNOWLEDGMENTS

This work is supported by the Department of Energy Office of Basic Energy Sciences under grant no. DE-FG02-11ER46826. R.L.B acknowledges the Research Corporation for Science Advancement for a Cottrell Scholar Award. We thank Maya Jig Grinding and Gage Co. for their engineering expertise and the Resource Center for Medical Ultrasonic Transducer Technology at USC for use of their impedance equipment. We are also thankful to Profs. M. Gunderson, A. Hodge, and M. Thompson for use of their facilities.

4. SUMMARY AND CONCLUSIONS Nanocrystals of BST have been successfully blended into a PMDA-BAPB polyimide system to understand the effects of loading small perovskite fillers on the dielectric properties of the nanocomposite. The relative permittivity of the resulting PMDA-BAPB/BST nanocomposites continually increased from εr = 2.8 for the neat PMDA-BAPB polyimide to εr = 6.2 at 18 vol % BST, whereas tan δ