BaTiO3 0â3 Nanocomposite

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Monomer derived Poly(furfuryl)/BaTiO3 0-3 nanocomposite capacitors: maximization of the effective permittivity through control at the interface Frederick A. Pearsall, Julien Lombardi, and Stephen P O'Brien ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13879 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017

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ACS Applied Materials & Interfaces

Monomer Derived Poly(Furfuryl)/BaTiO3 0-3 Nanocomposite Capacitors: Maximization of the Effective Permittivity Through Control at the Interface Frederick A. Pearsall1,2,3, Julien Lombardi1,2,3, Stephen O’Brien1,2,3* 1

The CUNY Energy Institute, City University of New York, Steinman Hall, 160 Convent Avenue, The City College of New York, New York, NY 10031, USA. 2

Department of Chemistry, The City College of New York, 1024 Marshak, 160 Convent Avenue, NY 10031, USA

3

Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, New York, NY 10016, USA.

*Corresponding author: [email protected] ABSTRACT: Frequency stable, high permittivity nanocomposite capacitors produced under mild processing conditions offer an attractive replacement to MLCCs derived from conventional ceramic firing. Here, 0-3 nanocomposites were prepared using gel-collection derived barium titanate nanocrystals, suspended in a poly (furfuryl alcohol) matrix, resulting in a stable, high effective permittivity, low loss dielectric. The nanocrystals are produced at 600C, emerging as fully crystallized cubic BTO, 8 nm, paraelectric with a highly functional surface that enables both suspension and chemical reaction in organic solvents. The nanocrystals were suspended in furfuryl alcohol inside a uniquely prepared mold, in which volume fraction of nanocrystal filler (𝜈f) could be varied. Polymerization of the matrix in situ at 70-900C resulted in a nanocomposite with a higher than anticipated effective permittivity (up to 50, with 𝜈f only 0.41, 0.5-2000 KHz), exceptional stability as a function of frequency, and very favorable dissipation factors (tan δ < 0.01, 𝜈f < 0.41; tan δ < 0.05, 𝜈f < 0.5). The increased permittivity is attributed to the covalent attachment of the poly(furfuryl alcohol) matrix to the surface of the nanocrystals, homogenizing the particle-matrix interface, limiting undercoordinated surface sites and reducing void space. XPS and FTIR confirmed strong interfacial interaction between matrix and nanocrystal surface. Effective medium approximations were used to compare this with similar nanocomposite systems. It was found that the high effective permittivity could not be attributed to the combination of two components alone, rather the creation of a hybrid nanocomposite possessing its own dielectric behavior. A non-dispersive medium was selected to focus on the frequency dependent permittivity of the 8 nm barium titanate nanocrystals. Experimental corroboration with known theory is evident until a specific volume fraction (𝜈f ~ 0.3) where, due to a sharp increase in the effective permittivity, approximations fail to adequately describe the nanocomposite medium. KEYWORDS: Nanocomposite, capacitor, dielectric, energy storage, barium titanate, poly(furfuryl alcohol), nanoparticle, effective medium approximation. 1. INTRODUCTION Nanocomposite metal-insulator-metal (MIM) capacitors are of great interest due to the possibility of

reducing the number of discrete components in modern printed circuit boards (PCBs): embedded capacitors may be able to replace some of the ubiquitous discrete capacitive components for use in

1 ACS Paragon Plus Environment


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conjunction with integrated circuits.1 However, for these embedded capacitors to be useful, the dielectric must have a high effective permittivity over a wide frequency range, high breakdown strength, low dissipation, low leakage current, high energy density, and must be stable over working temperature ranges. Ideally, embedded capacitors will have a lower processing temperature compatible with IC design, or present additional manufacturing avenues such as r2r processing.2 To a large extent, polymer and high-k nanocrystal composites (nanocomposites) are able to fulfill these material requirements, whereby the polymer reduces dissipation and increases breakdown strength, while the nanocrystal allows for a higher energy density and overall higher effective permittivity. Nanocrystalline barium titanate (BaTiO3 commonly referred to as BTO) has been shown to be an excellent candidate for a nanocomposite filler material.3,4,5,6,7 BTO nanocrystals have been used concomitantly with assorted polymer matrices in various studies, ranging from device applications to dielectric material investigation.2,8,9 Although measuring the effective permittivity of nanocomposites is relatively facile, determining the contribution of each component is nontrivial.10 While there have been many studies on single crystal bulk dielectric ceramics4, there are fewer experimentally verified reports regarding their nanoparticle counterparts. Effective medium approximations (EMAs) have been developed over the last century to help predict physical properties (conductivity, permittivity, elasticity) of two-phase composites.11 While useful, EMAs for dielectric analysis and prediction require further refinement. Part of the issue is the inability to independently verify single nanoparticle dielectric constants, especially for a range of sizes. In addition, effective medium approximations often have severe limitations, imposed by particle shape, void space, and volume fraction, often failing to correlate with experiment.12,13,14 Furfuryl alcohol (FA) has been shown to be a compatible solvent for 8 nm BTO dispersions, allowing for the development of BTO/FA sols of varying concentrations.15 Polymerization is initiated through heat16, creating a 3D cross linking polymer that forms a covalent network17,18,19 between nanoparticles. The precursor BTO/FA sol allows the monomer to polymerize in situ and in contact with the nanoparticles: the hydroxyl groups on the nanoparticle surface provide plenty of scope for attachment. Direct covalent bonding between polymer and the BT dangling hydroxyl groups further homogenizes the regions surrounding the nanoparticles, reducing void space as well as prohibiting particle-particle contact. The proposed mechanism for attachment and polymer

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Scheme 1. Proposed BTO and PFA surface bonding, and PFA polymerization reaction. BTO is represented by a grey sphere (not to scale). (1) Furfuryl alchohol (FA) reacts with surface ethoxy or hydroxyl groups to form an oxygen bridging bond; (2) additional FA molecules or poly(furfuryl) alchohol oligomers (PFA) bind to the surface through condensation reactions; (3) polymerization to form PFA without a bridging oxygen also commonly occurs through formaldehyde elimination; (4) cross-linking of polymer chains to form 3D network nanoparticle-polymer nanocomposite.

chain growth is shown in Scheme 1. This network of bonding between the two components greatly improves loss characteristics when compared to mixing.12 Although the dielectric value of poly(furfuryl alcohol) (PFA) is lower relative to PVDF, its derivatives, and other host polymers previously used for nanocomposite capacitors, PFA characteristically has a low loss across lower to mid frequencies, especially at frequencies > 100 kHz.2,15 A stable capacitance over a broad frequency range is the best indication that the contribution to the permittivity is intrinsic to the properties of the nanocomposite components, such that reliable information can be obtained about the material, as well as its potential for energy storage applications. At the frequency range here recorded (500Hz – 2MHz), directional and interfacial polarization dominate, as well as space charge accumulation. Space charge effects may significantly alter the dielectric characteristics (and may shrink and tilt the P-E hysteresis loop of the ferroelectric component).20 The attachment of the PFA to the nanocrystal surface as well as its dense cross-linking polymer network reduces the frictional energy loss adding to tan17 increasing the effective permittivity of the nanocomposite via the Maxwell-Wagner effect. Thus, PFA is a good alternative to PVDF and its


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derivatives, as well as other resins which may require curing agents. When 0-3 nanocomposites are studied as capacitors great care must be taken in the interpretation of the effective permittivity for several important reasons: (i) the contribution of the filler dielectric (such as BaTiO3, PbTiO3, (Pb,Zr)TiO3) will vary according to particle size, extent of crystallinity, the existence of domain walls in individual particles, crystal structure (e.g. tetragonal vs cubic), the contribution of the surface chemistry and absorptivity at the nanoscale (attached species, vacancies); (ii) the contribution of the host polymer (such as PVDF-hfp, PVP, resins) whose permittivity may vary as a function of frequency according to the polarization behavior of the polymer chains, molecular weight, and degree of cross-linking; (iii) the contribution of void space that naturally occurs between the two components; (iv) the relative volume fractions which will have potentially non-linear effects, such as for higher filler fractions in which particles are contiguous. These effects greatly increase the complexity of the system when comparing measurements to a single phase dielectric material, and give rise to ambiguity, as well as opportunity, in determining optimal strategies for nanocomposite design. In this study, 0-3 nanocomposites were fabricated, with well-defined components, in order to produce a high performance dielectric with a stable capacitance up to 2 MHz, with concurrent low loss, low ESR, and high voltage tolerance. This dielectric layer could in turn be analyzed both experimentally and using two effective medium approximations to determine further the contribution of the components to the effective permittivity of the device. We selected ultrafine nanocrystals of BaTiO3 (BTO) as the filler, for which we have previously developed modified sol-gel methods that produce highly crystalline nanoparticles with relatively narrow size distributions.15 BTO nanocrystals with an average diameter of 8 nm were synthesized. Structural characterization (XRD, TEM) confirm phase, and that they are individual single crystal nanoparticles. XPS confirms only barium, titanium, and oxygen are present ruling out the possibility of elemental impurities. Nanocrystals of BTO at these dimensions are considered to be paraelectric, due to the theoretically predicted and experimentally verified21 size dependent suppression of ferroelectricity, also supported by XRD and Raman which indicated the phase of BTO is essentially cubic. The choice to be below the ferroelectric limit, and to utilize single crystal nanoparticles, allows for a filler that is high in dielectric constant and low in hysteretic loss.

This enables impedance measurements to be performed up to high frequencies on a nanocomposite free from hysteretic loss, while still attempting to optimize the contribution of higher permittivity from the high-k nanocrystal filler. The nanocomposites were fabricated by dispersing the BTO nanocrystals in a PFA host matrix. This served as a robust low loss dielectric medium for generating stable capacitance over a wide frequency range. A 0-3 nanocomposite layer produces an effective permittivity based on the combined effect of its components, and an attempt is made through EMA to ascertain the contribution of each component. It is commonplace to use the approximations to determine the permittivity of the filler, in cases where the host matrix dielectric constant is known or easily verified. In this case, PFA served as an ideal candidate matrix due to the ability to obtain a constant capacitance value with very low loss, as a function of frequency up to 2 MHz. XPS measurements showed a change in barium and titanium chemical environments in the nanocomposites when compared to those in pure BTO. These changes manifest in a relative reduction of surface carbonate impurities and in a reduction of surface oxygen vacancies, corroborated by reduction in titanium oxidation state diversity. Measurements of the O1s region in BTO/PFA nanocomposites show a reduction in surface oxygen vacancies, providing further proof of covalent bonding between PFA and surface BTO titanium atoms. Direct covalent bonding to the BTO surface ensures particle contact does not occur, preventing field concentration in the matrix near the nanoparticle surface.12 FTIR shows an increase in microenvironments for certain PFA and BTO peaks, as well as an increase in Ti-O vibrations after nanocomposite fabrication when compared to its constituents. The intrinsic permittivity of BTO was calculated using two distinct effective medium approximations. These are the Lichtenecker equation22 and a recently developed interphase model.23 A brief introduction to these effective medium approximations is given in the following section. The relative attributes of each model are discussed and, in the results and discussion, the calculated dielectric constant values are compared to those reported in literature. An effective medium approximation (EMA) is a tool for treating and predicting the properties of a macroscopically inhomogeneous medium in which different physical properties vary in space. EMAs are designed to define averages (more precisely, to



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homogenize an inhomogeneous mixture) which one hopes will be representative of the system and relate to experimental measurements. The property of interest here is the dielectric constant of BTO nanoparticles within a nanocomposite; an appropriate system for EMA modeling. The Lichtenecker Equation. An earlier medium approximation for a two-phase composite is the Lichtenecker equation, developed originally when the parallel and series rules of mixtures could not adequately describe a dielectric medium:22,24 log 𝜀𝑒𝑓𝑓 = 𝜈ℎ log 𝜀ℎ + 𝜈𝑓 log 𝜀𝑓

(1)

where εeff, εh, εf are the permittivity of the composite, host, and filler respectively and 𝜈h, and 𝜈f are the volume fractions of the host and filler respectively. The Lichtenecker equation does not allow for modification according to the shape of particles, being developed for spherical inclusions, which suffices for the 8nm BTO system. The Interphase Model. A recent model developed by Ezzat et al, herein referred to as the interphase model, considers particle shape/orientation as well as interphase spacing.23 Instead of approximating a twophase composite medium as consisting of only filler and host, the interphase region between particles is treated as three-phase: host, filler, and air (void). While this region cannot be practically probed, the model considers the experimentally and computationally verified conclusion that beyond a critical filler volume fraction, void space increases:25,8 𝛽

𝛽

𝛽

𝛽

𝜀𝑒𝑓𝑓 = 𝜈𝑓 𝜀𝑓 + 𝜈ℎ 𝜀ℎ + 𝜈 ′ (1 − 𝜀ℎ )

(2)

where β is a dimensionless parameter which depends on the shape and orientation of the filler. For spherical nanoparticles β = 1/3 and 𝜈’ is 𝜈 ′ = 𝜈𝑓

1 − 𝜈𝑓 1 + 𝜈𝑓

(3)

Above, 𝜈’ compensates for the void space within a nanocomposite. As can be seen from eq 3, there is a rapid increase in void space with addition of filler. While any simple model cannot include all parameters required to adequately describe a nanocomposite, it is generally accepted that EMAs, when applied to 0-3 nanocomposites, can assume relatively good fits at low volume fractions of filler (𝜈f < 0.30) but at higher volume fractions of filler, it appears that almost all models do not corroborate with experiment. 26,12,9 This is usually attributed to an increase in void space within

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the framework of the of the composite. 5 The more recent models, such as the interphase model, attempt to account for this observed deviation and attempt to better compensate for it. The contribution of void space renders modeling more complicated, not only because of the introduction of a third phase to the composite (that of air, εr ~ 1), but that the voids serve as a location for amplification of mobile charge interference effects such as charge distribution, charge trapping, or ionic/molecular relaxation processes at the nanoparticle interface. Such processes may ostensibly increase permittivity, especially at low frequencies, but are in fact a major impediment to nanocomposite capacitor development and ultimate commercialization.12 2. EXPERIMENTAL METHODS Barium isopropoxide (Ba(OCH(CH3)2)) was purchased from Alfa Aesar, titanium isopropoxide (Ti(OCH(CH3)4)) and furfuryl alcohol (C5H6O2) were purchased from Sigma-Aldrich. 200 proof ethanol was purchased from Decon Labs, Inc. All chemicals were used as received without further purification. 8 nm BaTiO3 nanoparticles were synthesized using a low temperature solvothermal technique developed in our group referred to as gel-collection, under N2 in a glovebox.15 The nanoparticles were synthesized by adding stoichiometric amounts of barium isopropoxide (3.37 mmol or 0.861g) in 40mL of 200 proof ethanol and magnetically stirred until the powder dissolved. A molar equivalent of titanium isopropoxide was added to the solution under stirring. A mixture of deionized H2O and 200 proof ethanol was prepared and added under stirring. The solution was then transferred to a sealed container and left to age overnight. A solid gel rod began to form after 8 hours and the container was heated to 600C for 12 hours in order for condensation of the gel rod to occur. The gel rod is then washed 3 times with 200 proof ethanol and dried in a vacuum oven to obtain a nanopowder. This was used to fabricate pressed pellets, of volume fractions 0.41 ≤ 𝜈f ≤ 0.50, whereupon furfuryl alcohol was dropcasted. To obtain nanocomposites of filler volume fraction 0.15 ≤ 𝜈f ≤ 0.37 the barium titanate gel-rod was instead sonicated in furfuryl alcohol after washing with 200 proof ethanol, with concentrations ranging from 20 – 40 mg/mL BTO/FA. Varying volumes of the suspensions were deposited into a teflon mold which was designed and machined specifically for nanocomposite fabrication. The resulting 8 nm BTO powders synthesized by gel collection were characterized using X-ray powder diffraction (XRD); Morphology was obtained using


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(a)

(c)

(b)

(d)

Figure 1 SEM micrograph of BTO/PFA nanocomposite with volume fraction 𝜈f = 0.31 (a). TEM micrograph of the same BTO/PFA nanocomposite with 𝜈f = 0.31 (b). SEM micrograph of BTO/PFA nanocomposite with volume fraction 𝜈f = 0.41 (c). TEM micrograph of the same BTO/PFA nanocomposite with 𝜈f = 0.41 (d).

TEM - transmission electron microscopy and SEM scanning electron microscopy (Figure 1). FTIR spectroscopy (Figure 2) was carried out using a Perkin Elmer system. X-ray photoelectron spectroscopy (XPS) was performed on a Physical Electronics VersaProbe II (Figure 3 and supporting information). The XRD measurements were performed on a PANalytical X’Pert Pro using Cu Kα radiation. Samples for the TEM were prepared by dispersing the nanocrystalline gel rod in 200 proof ethanol and dropcasting the suspension on a carbon coated copper TEM grid. TEM micrographs were recorded on a JEOL 2100 microscope. SEM surface and cross sectional analysis was performed on a Supra 55 SEM. Pressed pellet capacitors were prepared using a Cyky 12T Laboratory manual powder metallurgy press machine and then dropcasting furfuryl alcohol to the surface of the pellet. Both pressed pellet and suspension prepared capacitors used MG chemicals silver conductive epoxy as contacts. LCR measurements were performed on all samples to obtain the frequency dependent permittivity of the

nanocomposites. Capacitance and loss tangent were measured over 500 Hz to 1 MHz using an Agilent E4980 Precision LCR Meter. 3. RESULTS & DISCUSSION The synthesized barium titanate is highly crystalline, single phase, and roughly spherical in shape, 8 nm in diameter with a narrow size distribution, as previously reported (See supporting information, Figure S1).15 The BTO nanoparticles aggregate within the composites as can be seen in Figure 1a and 1b. At filler volume fractions above 0.37 the aggregates become much more closely packed and the surface of the composites becomes rougher. At first glance it may seem the aggregated particles are touching, however, this is not the case. Figure 1b and 1d shows a grinded portion of a BTO/PFA nanocomposite under TEM. Although BTO nanocrystals are within very close proximity of each other, they are in fact not in contact. Figure 1d



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(a)

(c)

(b)

(d)

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Figure 2 XPS spectra of BaTiO3 barium in in a f = 0.41 nanocomposite (a), as synthesized BTO (b) and of titanium in a 𝜈f = 0.41 nanocomposite (c) and as synthesized BTO (d). Dashed lines show the fitted curves.

shows the homogeneity of the composite throughout a larger volume. XPS and FTIR were performed to investigate particle-polymer interactions at the particle surface. XPS readily gives information on metal oxidation states as well as the binding energies of the elements in question. Peak deconvolution with reference to expected literature values allows separation of surface and inner chemical species for elements such as titanium and barium. Here, XPS gives evidence for change in relative amounts of surface barium and a change in surface oxidation state for titanium. XPS measurements of pure BTO and a BTO/PFA nanocomposite were obtained for comparison. The fitted Ba 3d5/2 spectrum for the pure nanoparticles shows 2 peaks (Figure 2b). Barium in barium titanate is known to exist in lattice and surface/vacancy states,  and , respectively.27,28,29 The lowest binding energy peak at 778.72 eV corresponds to  (lattice) barium with the peak at 780.16 eV arising due to  (surface) barium. The  peak is also sometimes attributed to Ba vacancy point defects on the surface.30,31 Fig. 2a and 2b show dashed lines corresponding to lattice and surface barium. Fig. 2b shows the pure BTO Ba3d5/2 region. The Ba3d5/2 region for a 𝜈f = 0.41 nanocomposite is shown in Figure 2a. This region contains the same two peaks corresponding to  (bulk) and β (surface) barium; 778.87 eV and 780.25 eV respectively. The relative amounts of the surface barium have changed. The deconvolution of the barium region for the nanocomposite shows a decrease in surface/vacancy barium when compared to pure BTO. This is an indication that the barium surface vacancy defects have

been passivated and that surface barium is now in a more favorable environment, most likely due to oxygen from PFA, which is now surrounding the particles. Titanium 2p deconvolution of the pure BTO shows two separate peaks, corresponding to lattice and surface titanium, shown in Figure 2d. The lower binding energy peak, indicated by the rightmost dashed line, corresponds to surface Ti3+ which occurs to balance the charge due to oxygen vacancies in barium titanate.32 Oxygen vacancies in barium titanate are a native defect, known to exist in ferroelectric ceramics.33,34 The dashed line at 457.78 eV corresponds to Ti 4+.7 The most conclusive evidence of direct bonding to the PFA matrix is the Ti 2p region shown in Figure 2c. In Fig. 2c, asymmetry of the experimental curve disappears, due to the removal of Ti3+ while only Ti4+ remains. The disappearance of the Ti3+ due to oxygen vacancies illustrates these vacancies were localized to the nanoparticle surface. The posited attachment of PFA onto the BTO nanoparticle surface is shown by both the barium and especially the titanium XPS measurements shown in Fig. 2. XPS analysis of the C1s and O1 s regions of the nanoparticles and composite provided additional data pertaining to the nature of the surface, and presence of chemical groups following grafting of the polymer to the BTO. The analysis is presented in the supporting information (See Supporting Information, discussion and Figures S2-S7). FTIR was performed to find evidence for change in the local chemical environment of the nanocomposites when compared to the individual components. The FTIR spectrum of PFA, BTO, and PFA/BTO a nanocomposite is shown in Figure 3. The pure BTO spectrum, Figure 3c shows a strong and


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broad peak in the 3500 - 3400 cm-1 region, which arises from characteristic surface hydroxyl and water stretching.35,36 Peaks at 2965 cm-1 and 2930 cm-1 correspond to antisymmetric CH3 and antisymmetric CH2 stretches respectively, which have been previously characterized as attached surface hydrocarbons, in the form of -OR groups (R = CH3, CH2CH3, CH(CH3)2).15,37 The surface hydroxyl and alkoxyl peaks remain even after heating in a vacuum oven for 2 days at 1000C, indicating that these groups are strongly adsorbed to the surface of the BTO nanoparticles. At 1558 cm-1 a peak arises from the complex of alkoxy groups and Ti on the surface of BTO nanoparticles. 38,39 Several peaks point to the presence of CO32- which is commonly found on the BTO surface29; the peaks at 1758 cm-1 (C=O), and 1425 cm-1 (symmetric COO stretch) arise, due to the strong affinity of CO2 and BTO at this size regime.35,36,37,40 The peaks shown at 1055 cm-1 and 1018 cm-1 are attributed to alkoxy groups attached to titanium ions in BTO40,41. The broad peak at 570 cm-1 is due to O-Ti-O lattice vibrations in the BTO crystal.38,29 In the PFA spectrum (Fig. 3a), the peak at 3440 cm-1 belongs to -OH of the hydroxymethyl group42 The pure PFA spectrum contains bands at wavenumbers characteristic of alkenes and furan rings. Specifically, the band at 3124 cm-1 is indicative of furan rings, and the band at 793 cm-1 is attributed to 2,5-disubstituted furan rings, an indication that -CH2- bridging has occurred on the main chain of PFA, shown in Scheme 1, step 3).42,43 The peak at 2923 cm-1 corresponds to bridging methylene groups between furan rings, further corroborating polymerization.44 The BTO/PFA spectrum corresponds to a 0.41 filler volume fraction nanocomposite. In the nanocomposite spectrum (Fig. 3b) the 2921 cm-1 peak broadens. New and unique bonding, postulated to be at the interface of particle and polymer explains the broadening of the bridging methylene group absorption peaks. PFA =C-O-C= groups can be identified using the peaks at 1064 cm-1 and 1010 cm-1. The broad –OH stretching at ~3500 cm-1 is still visible, corresponding mostly to surface hydroxyl groups. The broad shape of this peak, emphasized by the leftmost dashed line in Fig. 3, corresponds to hydrogen bonded hydroxyl groups, an increase when compared to pure PFA, due to increased possibilities of such bonding at interfaces, i.e. polymerpolymer and polymer-particle. Peaks from PFA are visible in the 3100 – 2800 cm-1 region, corresponding to furan rings, bridging methylene groups, and other alkane groups. More striking is the broadening of the peaks near 1558 cm-1, emphasized by a dashed line, which indicate there are new titanium complexes being formed post-processing, namely those of particle surface and polymer. Possible interactions include

Figure 3 FTIR of pure PFA (a), BTO/PFA nanocomposite of f = 0.41 (b), and pure BTO (c). Dashed vertical lines are used to emphasize differences between pure components and nanocomposite.

direct covalent bonding of PFA hydroxyl groups to surface titanium as well as weak interaction between Ti and the sp2 carbon atoms of the furan ring. Bonding is supported from the XPS measurements of the Ti 2p region discussed in the previous subsection. FTIR also shows how the in situ polymerization of PFA not only anchors the polymer onto the surface, but it immobilizes the polymer matrix. Sharpening of the band at 1380 cm-1 in the nanocomposite, indicated by another dashed line, corresponds to a –CH ring deformation45 which is due to immobilized furan rings, kept rigid due to main chain and cross linking polymerization a key contribution to the stability of the dielectric constant in BTO/PFA nanocomposites as is discussed in the next section. Capacitance measurements were made over a frequency range of 500 Hz – 2 MHz (Figure 4). The effective permittivity was compared at four frequencies as can be seen in Figure 4c. At lower frequencies interstitial void space, trapped moisture, and electrodedielectric interface can contribute to charge effects.46 Indeed, barium titanate derived capacitors often display strong frequency dispersion behaviors. 47 However, the near constant values for all frequencies show these contributions are minimized for this system. All experimental values of the permittivity are averages of multiple (3-5) Teflon mold created capacitors. The final 3 values in Figure 4a were obtained using pellet capacitors. These values show a higher frequency dependence, attributed to incomplete furfuryl alcohol monomer infiltration of the pellet, anticipated due to the different processing technique, and further testament to the benefit of mold based processing. The pellet capacitors were necessary to achieve 𝜈f > 0.37. At



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(a)

(b)

(c)

(d)

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Figure 4 Permittivity vs frequency of nanocomposites; permittivity shown on left-axis, 𝜈f shown on right (a). Loss tangent of nanocomposites, 𝜈f on left shown only for pellet capacitors (b). Permittivity vs 𝜈f at varying frequencies, showing extremely low frequency dependence (c). Permittivity vs 𝜈f of measured PFA system compared with EMAs and previously reported permittivity, at 1 kHz (d).

this volume fraction (𝜈f > 0.37) increasing the concentration of BTO in the barium titanate/furfuryl alcohol suspensions did not increase the overall permittivity and showed a drastic increase in the dissipation. This is most likely related to particleparticle interaction, favoring agglomeration at a certain point within the suspension. This agglomeration does not occur in the pellet formed capacitors. The reason behind this is that the pressure used to press the capacitors together was not enough to prevent infiltration of the furfuryl alcohol monomer, while the electrostatic interaction of close or neighboring BTO nanoparticles are no longer competing with the forces common to suspensions including hydrodynamic forces, capillary forces, and Brownian motion. This comes with a compromise of the frequency-insensitive behavior as can be seen in Fig. 4a. Figure 4a shows the incredible stability of the nanocomposites over the

measured frequency range. Evidence of covalent bonding explains the stability of the permittivity. It is postulated that cross-linked, bound PFA prevents rotational contributions to the dissipation factor. PFA attachment and replacement of surface groups (hydroxyl, ethanol, isopropyl) further reduces significant rotational contributions to the dissipation factor.38 Anchored PFA on the nanoparticle surface allows interfacial polarization to increase the effective permittivity as the space between filler and host becomes homogenized via a shared electron linkage. Processing temperatures, both for contact and active material have been shown to have an effect on the dispersive properties of permittivity and dissipation. Recent literature has reported a processing temperature dependence on sputtered barium titanate films.48,49 Larger temperatures resulted in more dispersive capacitors. Here, the devices are processed at


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temperatures 70-90oC, a temperature well below the lowest used in the just cited literature technique (~350oC). Lower temperatures may suppress interdiffusion at the film-electrode interface which is a major contribution to capacitor device dissipation. By functionalizing the interface, MaxwellWagner interfacial contributions to the permittivity can be designed and taken advantage of. As can be seen in Figure 4c, increasing the volume fraction of nanoparticle filler increases the effective permittivity of the nanocomposites. However, this is only true until a critical filler volume fraction, whereupon any increase in filler will have an adverse influence on the effective permittivity, due to increasing residual porosity and void space.50,51 This occurs at a volume fraction approximately 0.1 below that of previously reported critical volume fractions.8,9,11 This is most likely due to the increased sensitivity inherent in the homogenized nanocomposite matrix when compared to conventional BTO nanocomposites. Since the increased permittivity is due to an interfacial polymer polarization and functionalized interface, any porosity or void space has severe consequences on the effective permittivity of the nanocomposite capacitor. The points shown in Figure 4d were taken at 1 kHz. Although experiment and approximations match at lower volume fractions, there is a sudden and rapid increase in the effective permittivity at 𝜈f > 0.33. This jump may be explained by the increase in particle agglomeration as the packing density increases. As illustrated in Figure 1, although nanoparticles form aggregates, these particles are not in contact with each other due to the in situ polymerization and therefore do not lose any particle-polymer interfacial surface area. This large surface area as well as the densification of agglomerates causes the effective permittivity to increase. The Lichtenecker model has been used previously to model the effective dielectric constant for BTO nanocomposites.8,52 In the references, an intrinsic dielectric constant of 84 was obtained for BTO and is therefore used here in both in eq 1 and the eq 2. Shown in Figure 4d, the nanocomposite effective dielectric constant is much higher than that of the commercial reference8, and is much higher than the permittivity calculated by the eq 1 and eq 2. The nanocomposites reach a critical volume fraction at lower values which is due to the homogenization of the nanocomposite medium, supported by XPS and FTIR data. Due to this homogenization, the medium is highly sensitive to the effect of porosity, which is known to occur as filler volume fraction increases, discussed in the previous subsection. The interphase model which takes into account inherent void space, theoretically predicted to

exist to some extent at all volume fractions, yields values that are too low when compared to the experimentally measured effective permittivities. Here the comparison with the reference is not perfect; the matrix used was PVDF-HFP, with r = 11.9, which should inherently increase the effective permittivity. However, as can be seen in Figure 4c and 4d, there is a point where the BTO/PFA nanocomposites have an effective permittivity above that of the values shown in the references (f > 0.33). This is also where a sharp gradient change is apparent. This “gradual knee”, is consistent with the theoretical predictions of Calame et al., and in the similarly observed range of 𝜈f = 0.35 – 0.50.25 The reason for the jump in permittivity is the onset of microscopic percolation, occurring for random sphere packing in clusters. The clusters of particles can extend polarization over a longer than individually dispersed nanoparticles lengthscale within the dielectric, giving rise to an increase in the effective permittivity. This is to be contrasted with macroscopic percolation occurring at higher 𝜈f and to a detrimental effect – macroscopic percolation reaches the electrodes causing leakage and reducing voltage tolerance. It would also occur more easily in an ordered packing array. In other words, localized groups of spheres within the polymer matrix increase the overall effective permittivity of the composite.25 We attribute our experimental observations of a marked jump in effective permittivity around 𝜈f = 0.35 – 0.50 to the same effect. 4. CONCLUSION Ultra-stable, high permittivity 0-3 nanocomposites were successfully fabricated using poly(furfuryl alcohol) as the 3-dimensional host and 8 nm synthetic barium titanate as the filler. Key to the improved performance is the optimization of the interface through covalent bond formation between nanocrystal and polymer (polymer grafting). XPS shows a change in the surface chemistry after in situ polymerization of furfuryl alcohol, providing evidence of homogenizing the surface titanium chemical environment due to reduction in oxygen vacancies after direct bonding between surface titanium ions with PFA. The hydrophobicity of the polymer further reduces the potential for H2O inclusion, further stabilizing permittivity at lower frequencies. Also shown is the reduction of surface relaxed barium after polymerization. FTIR shows evidence of a rigid polymer matrix, as well as an indication of Ti and PFA interaction. The effective permittivities of the reported nanocomposites are competitively high when compared to previous nanocomposite systems using BTO as the filler, where BTO size distributions and synthetic



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routes, surface chemistries, and polymer hosts differ. Not only are the permittivities high in this system, but also the dissipation factors are very low, low enough in some cases to be compared with commercial ceramic capacitors. The observed improvement in comprehensive dielectric performance is proposed to be due to the homogenized interface of polymer and particle, as well as due to a microscopic percolation threshold that increases the overall effective permittivity without compromising the dissipation loss of the nanocomposite capacitors.

Nanoparticles for Hybrid Dielectric Capacitors. ACS Appl. Mater. Interfaces 2014, 6 3477-3482. 6.

Li, Y.; Huang, X.; Hu, Z.; Jiang, P.; Li, S.; Tanaka, T. Large Dielectric Constant and High Thermal Conductivity in Poly(vinylidene fluoride)/Barium Titanate/Silicon Carbide Three-Phase Nanocomposites. ACS Appl. Mater. Interfaces 2011, 3, 4396-4403.

7.

Toomey, M. D.; Gao, K.; Mendis, G. P.; Slamovich, E. B.; Howarter, J. A. Hydrothermal Synthesis and Processing of Barium Titanate Nanoparticles Embedded in Polymer Films. ACS Appl. Mater. Interfaces 2015, 7, 28640-28646.

8.

Kim, P.; Doss, N. M.; Tillotson, J. P.; Hotchkiss, P. J.; Pan, M.-J.; Marder, S. R.; Li, J.; Calame, J. P.; Perry, J. W. High Energy Density Nanocomposites Based on SurfaceModified BaTiO3 and a Ferroelectric Polymer. ACS Nano 2009, 3, 2581-2592.

9.

Cho, S.-D.; Lee, S.-Y.; Hyun, J.-G.; Paik, K.W. Comparison of Theoretical Predictions and Experimental Values of the Dielectric Constant of Epoxy/BaTiO3 Composite Embedded Capacitor Films. J. Mater. Sci. Mater. Electron. 2005, 16, 77-84.

ACKNOWLEDGEMENTS This work was supported by the National Science Foundation, under NSF DMR award #1461499, and NSF CREST #1547830. SO and FP are grateful to Jackie Li for useful discussions. FP is grateful to Tai-De Li for help with the XPS. FP is grateful to Eugene Onoichenco for assistance with the Teflon mold. JL and FP are grateful to Jorge Morales for help with the EM. Supporting Information Available: X-ray powder diffraction and TEM/HRTEM of the BaTiO3 nanoparticles used in the preparation of the BTO-PFA nanocomposites (Figure S1 and description); further Xray photoelectron spectroscopy data (XPS) and indepth analysis of the system (Figure S2-S7). REFERENCES 1.

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