PVDF: A Three

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Enhanced Dielectric Properties of LaNiO/BaTiO/PVDF: A Three-Phase Percolative Polymer Nanocrystal Composite Prem Wicram Jaschin, Rajasekhar Bhimireddi, and Kalidindi Bapiraju Varma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07786 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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Enhanced Dielectric Properties of LaNiO3/BaTiO3/PVDF: A Three-Phase Percolative Polymer Nanocrystal Composite P. W. Jaschin, R. Bhimireddi and K. B. R. Varma* Materials Research Centre, Indian Institute of Science, Bangalore - 560012

Abstract Polymer (polyvinylidene fluoride) nanocrystal composites based on lanthanum nickelate (percolative oxide) and barium titanate were fabricated to obtain material systems with high dielectric constant and low loss to be used for high charge storage applications. Lanthanum nickelate (LaNiO3) nanocrystallites were synthesized from a simple citrate-assisted sol-gel route that yielded agglomerated crystallites of an average size of 120 nm. The defective nature of the lanthanum nickelate nanocrystals was revealed by the transmission electron microscopy studies. Hot-pressing method was executed to fabricate the LaNiO3/PVDF nanocrystal composites and their dielectric characteristics showed a low percolation threshold in the region of fLN (volume fraction of lanthanum nickelate) = 0.10. The percolative conductive filler-polymer nanocrystal composite at the percolation threshold exhibited a dielectric constant (εr) and loss (D) of 55 and 0.263, respectively, at 10 kHz; the dielectric constant obtained was more than five times that of host matrix PVDF. To further improve upon the obtained dielectric properties from the two-phase composites, a high dielectric constant material, barium titanate (BaTiO3) nanocrystals, with an average size of 100 nm, were embedded in the polymer matrix as the third phase. The dielectric properties of the three-phase nanocrystal composites were measured as a function of volume fraction of lanthanum nickelate (which was limited within the percolation threshold) and the dielectric constant as high as 90 and the associated loss of 0.13 at 10 kHz were achieved from fLN = 0.09 and fBT = 0.20. The obtained dielectric constant from this system is nine times more than that of PVDF and three times that of a two-phase barium titanate/PVDF composites, which proves to be a promising material for charge-storage applications.

Keywords: Lanthanum nickelate, barium titanate, percolation theory, dielectric constant, three-phase polymer composite, polyvinylidene difluoride, nanocrystals

*Author for communication E-mail: [email protected]; Fax: +91 80 2360 0683; Tel: +91 080 2293 2914.

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Introduction Although mankind has devised a plethora of ways to generate and consume electrical energy, efficient methods to store remain a weak link in overcoming its universal applicability. On a large scale, this can be achieved by compressed air energy storage, pumped hydro-power, etc., but fuel cells, supercapacitors, batteries and dielectric capacitors are widely used as small-scale energy storage devices.1 Owing to high charge-discharge rate of dielectric capacitors and an innate high power density associated with these systems, dielectric capacitors are of significant importance for applications such as, weapon systems, medical devices and hybrid vehicles. But to acquire large power density, there are two basic prerequisites – a high dielectric constant and a large breakdown voltage (or low dielectric loss). In this context, there have been tremendous efforts in obtaining the optimum combination of dielectric constant and loss in various materials systems to obtain maximum power density. To procure a low loss electroactive material system, various forms of ceramics, polymers and composites have been immensely investigated. 2–9 Ceramic-polymer composites were proved to be promising candidates owing to their ease of fabrication and ability to retain mechanical flexibility, in addition to their improved dielectric properties and energy density. Among different types of fillers, a percolative conducting filler embedded polymer composite would yield high dielectric constant when the volume fraction of the filler lies close to the percolation threshold. This is more advantageous as compared to the conventional ceramic filler-polymer composites that require much higher volume fractions of fillers to obtain similar values of dielectric constant. A variety of materials that include metals (Ag, Al, Ni), carbon-based fillers (carbon black, graphene and multiwalled carbon nanotubes) and semiconducting materials (ZnO, SiC, CaCu3Ti4O12) have been incorporated in various polymer matrices yielding different percolation thresholds and dielectric constants.10–19 However, percolative composites with significantly improved dielectric constants are accompanied by very large dielectric loss, rendering the system unfit for charge storage applications. To circumvent this issue, an additional high dielectric conventional ceramic phase was introduced to the percolative system that is confined within the percolation threshold, forming a three-phase composite, and these systems exhibited tremendous improvement in their respective dielectric properties. For example, a three-phase composite of surface functionalised graphene (1.25 vol%) and barium titanate (BaTiO3, 30 vol%) embedded in a PVDF matrix has shown a dielectric constant of 65 and loss of 0.35 at a high frequency of 1 MHz, that could be used as a flexible dielectric material for high frequency applications.20 Even so, choosing the right conducting filler in a three-phase composite remains a key challenge as the dielectric loss of these systems can still be significant. Lanthanum nickelate (LaNiO3) is a well-known metallic perovskite oxide with no signs of metal-insulator transition down to 1.5 K and shows large Pauli-type magnetic susceptibility.21 LaNiO3 crystallizes in a centrosymmetric rhombohedral space group, R3c, associated with lattice constants, a = 5.4535 Å and c = 13.1010 Å, and exhibits a low resistivity, around 2.25 × 10–4 Ω cm, which is also


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isotropic.22,23 The metallic nature of the material is attributed to the hybridization of Ni 3d and O 2p orbitals.24 Such interesting characteristics of LaNiO3 resulted in its use as bottom electrodes for ferroelectric materials, electrocatalysts (for fuel cells and metal-air batteries), supercapacitors, etc.25–33 Owing to an ease of synthesis, low calcination temperature, microstructural and dimensional tunability, facile dispersion in polymers and its metallic nature all the way down to 1.5 K makes LaNiO3 an interesting compound to be employed as a conducting filler for designing and fabricating percolative polymer composites. In this article, three-phase polymer nanocrystal composites were fabricated that include lanthanum nickelate and barium titanate (BaTiO3) with polyvinylidene fluoride (PVDF) acting as the base polymer matrix, and their dielectric characteristics as a function of volume fractions of LaNiO3 and BaTiO3 are discussed in detail. PVDF was chosen as the base matrix due to its superior electrical characteristics, attributed to the ferroelectric nature in its crystalline form, and better thermal stability. Lanthanum nickelate nanocrystallites, obtained using a facile sol-gel synthesis, were used as the conductive filler to yield a percolative polymer composite. BaTiO3 nanocrystallites were also embedded in the polymer because of its intrinsic high dielectric constant.

Experimental Section Materials Titanium isopropoxide (Ti[OCH(CH3)2]4, Spectrochem, 98%), barium nitrate (Ba(NO3)2, SDFCL, 99%), lanthanum nitrate (La(NO3)·6H2O, Merck, 99%), nickel nitrate (Ni(NO3)2·6H2O, SDFCL, 99%), citric acid (SDFCL, 99.5%), acetylacetone (Spectrochem, 99%), and 34% ammonia solution (SDFCL) were used as precursors for the synthesis of BaTiO3 and LaNiO3. Poly (vinylidene fluoride) (Aldrich, 99%), was used for the fabrication of polymer composites. Synthesis of LaNiO3 A citrate assisted sol-gel technique was employed for the synthesis of LaNiO3 polycrystalline powder. To begin with, an equimolar ratio of 1:1 of La(NO3)3·6H2O and Ni(NO3)2·6H2O were dissolved in 10 ml of deionized water. 1M of citric acid (acting as a chelating agent) was dissolved in 10 ml of H2O and was added to the above solution. The pH of the solution was maintained at ~ 7 by adding a few drops of aqueous ammonia. The solution was heated above 90 oC with constant stirring to evaporate the water from the solution, resulting in a viscous gel. The gel was placed in a pre-heated box furnace at 450 oC for 30 min. The obtained porous black sample mixture was ground into fine powder and heated at 850 oC for 4h, which resulted in polycrystalline powder of LaNiO3. Synthesis of BaTiO3 A similar sol-gel technique was adopted to synthesise BaTiO3 nanocrystallites. An equimolar ratio (1:1) of titanium isopropoxide (TIP) and acetylacetone (to avoid decomposition of TIP to TiO2) were



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mixed and stirred for 10 min using a magnetic stirrer. 1M citric acid solution was later added to the above solution to form a titanium-citrate complex. 1M of barium nitrate was then dissolved in deionized water and added to the previously prepared titanium-citrate solution with constant stirring. The pH (~7) of the solution was maintained by adding the aqueous ammonium solution and slowly heated above 90 oC, with constant stirring, to form a viscous gel. The gel, hence, obtained was placed in a pre-heated box furnace at 450 oC for 30 min to burn out the residue. The resultant powder was further heat-treated at 850 oC /5h, resulting in the formation of pure BaTiO3 nano-crystalline powder. Fabrication of Polymer Nanocrystal Composites Polymer nanocrystal composites were fabricated using hot-pressing technique. Corresponding to the volume fraction of filler and polymer, appropriate weights of LaNiO3/BaTiO3 were added to PVDF and mixed thoroughly in acetone medium using a mortar and pestle for an hour. The mixture was dried and hot-pressed at 180 oC and under a pressure of 200 MPa for 20 min into pellets of 12 mm diameter and 1 mm thickness. In case of LN/BT/PVDF nanocrystal composite, appropriate amounts of LaNiO3 and BaTiO3 were mixed using mortar and pestle for a half hour period and subsequently followed by another half hour of ultrasonication in acetone medium before adding PVDF. Characterization X-ray powder diffraction (XRD) patterns were acquired at room temperature for the synthesized powder and polymer nanocrystal composites with PanAlytical X-pert Pro X-ray diffractometer using Cu-Kα radiation operating at 40 kV/30 mA. The secondary electron images were recorded using Inspect F50 FEI scanning electron microscopy and JEOL 2100F was used to acquire high resolution electron images and electron diffraction patterns. For electrical measurements, the pellets were painted with silver epoxy on both faces and allowed to dry. The dielectric properties of the nanocrystal composites were monitored using impedance gain phase analyser (Agilent 4294A) at a signal strength of 0.5 Vrms in the frequency range of 40 Hz to 110 MHz. The breakdown voltage of the polymer composites were measured using the procedure illustrated in ASTM D149.

Results and Discussions The X-ray diffraction (XRD) pattern obtained for the 850 oC/4h heat-treated product derived from the sol-gel technique confirms the crystallization of LaNiO3 phase, as shown in Fig.1. The peaks obtained are characteristic of the rhombohedral structure of LaNiO3 phase associated with the space group of R3c (ICSD No. 79-2451) and the corresponding lattice parameters were determined to be, a = 5.4566 Å and c = 13.1703 Å. The peaks were accordingly indexed in Fig. 1. However, there were minor traces of NiO observed in the recorded XRD pattern (depicted as * in Fig. 1). Rietveld refinement performed on the powder pattern (not shown here) yielded a maximum of 3 % weight percentage of NiO in the resultant material. Hence, its effect on the electrical properties may not be


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that significant. The scanning electron image of the as-prepared LaNiO3 polycrystalline powder is shown in the inset of Fig. 1. As a consequence of the adopted synthesis route, the particles appear agglomerated and are in the range of 100–150 nm in size. The as-prepared powder was subjected to transmission electron microscopy (TEM) studies to acquire further insight into their microstructure. The bright field TEM image of the polycrystalline LaNiO3 is shown in Fig. 2 (a), which depicts the agglomeration of the nanocrystals that appears to be attached to one-another across a grain boundary. The selected area electron diffraction (SAED) pattern obtained from the nanocrystals were indexed to the rhombohedral phase of LaNiO3, as depicted in Fig. 2 (b). A highly defective crystallinity was observed in the LaNiO3 nanocrystals under higher magnification (Fig. 2 (c)). The nanocrystals comprised a high density of stacking faults that are representative of Ruddlesden-Popper (RP) faults, prominently observed in other perovskite systems, such as, LaNiO3/LaAlO3 superlattices, SrTiO3, BaTiO3, BaSnO3.34–37 Several parallel and perpendicular arrays of stacking faults are observed in the nanocrystal, where the parallel stacking faults are predominantly terminated by the perpendicular ones or vice versa. The lattice fringes, as shown in Fig 2 (d), are associated with d-spacing of (012) planes of the rhombohedral LaNiO3. The stacking faults, hence, correspond to the stacking of LaO–NiO2– LaO–NiO2 structural units.38,39 Across the planes, a double LaO layer results in the half-displacement of the La-columns that leads to these planar faults.38 The LaNiO3/PVDF (LN/PD) nanocrystal composite was fabricated using hot-pressing technique with different volume fractions of LaNiO3 (0 < fLN < 0.20). Fig. S1 depicts the XRD pattern of a hot-pressed pellet with an intermediate composition, fLN = 0.10 (for representation) which confirms the presence of LaNiO3 phase (marked as ‘#’ in the figure) embedded in the polymer matrix. The broad peaks represented as ‘@’ are associated with α- and β-phase of PVDF. The peaks at 2θ = 20.5o and 36.3o corroborate the crystallization of β-phase, whereas the presence of α-phase is indicated by the diffraction peak at 2θ = 17.3o.40 The presence of β-phase PVDF is imperative due to the existence of various properties such as, piezoelectric, pyroelectric, ferroelectric and possess better dielectric characteristics. The scanning electron microscope image of the cross-section of the above nanocrystal composite is depicted in the inset of Fig. S1. It shows the distribution of the conducting filler (brighter in contrast) in the continuous polymer network where crystallites of LaNiO3 were observed to be present in the form of agglomerates or clusters. The agglomeration was already present among the nanocrystals post-synthesis and in addition, strong Van der Waals forces between the nanocrystals can also lead to such cluster formation. The dependence of effective dielectric constant (εr), loss (D) and conductivity (σ) as a function of frequency (ν) for LN/PD nanocrystal composites with various concentrations of crystalline LaNiO3 filler are shown in Fig 3 (a), (b) and (c), respectively. With the addition of LaNiO3, a systematic increase in the dielectric constant, loss and conductivity were observed till fLN = 0.10, and above which the composites exhibit a drastic increase that suggests their percolative nature. In the low



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frequency region (below 105 Hz), the dielectric constant shows a frequency independent behaviour for all the compositions; the dielectric constant drops only at higher frequency region, characteristic of the occurrence of a relaxation process. A high magnitude of dielectric constant of around 2170 at 10 kHz was obtained for the LN/PD nanocrystal composites with fLN = 0.20, which is around 217 times the dielectric constant of host PVDF at the same frequency. Such dramatic increase in the dielectric constant could be rationalized by the microcapacitor model,41 which considers the composite to contain a network of several local capacitors formed by a thin layer of the polymer between two adjacent conducting particles where each of these microcapacitors contributes to an abnormally large capacitance. As the filler content in the composite increases, the thickness between the conducting filler decreases resulting in an augmented effective dielectric constant of the composite. However, such high dielectric constant of the polymer composites is accompanied by the increase in the dielectric loss as well, as shown in the Fig. 3(b). At low content of LaNiO3 (fLN < 0.10), there is only a very gradual and slight increase in the dielectric loss of the nanocrystal composite (less than 0.13) and reaches up to 0.27 at 10 kHz for fLN = 0.10, which is still acceptable for high charge storage applications. But further increase in the volume fraction of LaNiO3 resulted in a steep rise in the loss exhibiting as high as 176 at 10 kHz for fLN = 0.20. Although these samples cannot be used for dielectric applications, the combination of high dielectric constant and high dielectric loss of polymer nanocrystal composites are promising materials for electromagnetic wave absorption applications. Furthermore, the ac conductivity of the nanocrystal composites follows a similar trend with the increase in volume fraction of LaNiO3, as depicted in Fig. 3(c). There is a sharp transition in the conductivity as the filler content is increased beyond fLN = 0.10, which suggests two regions with different mechanisms of conduction – an ohmic conduction and a non-ohmic conduction.42 At low filler content region, the conduction occurs through the barrier-tunnelling effect between the conducting particles across a thin layer of the polymer matrix obeying a non-ohmic conduction mechanism. As fLN increases, the probability of direct contact between the conducting particles increase and as a result exhibits ohmic conduction. Near the percolation threshold, the effective conductivity (σeff) of the composite is related to the frequency (υ) as shown below, ∝

(1)

where ω = 2πυ and u is the critical exponent. The log-log plot of the conductivity as a function of frequency for fLN = 0.10 is shown in Fig. 3 (d), and from the linear fit of the experimental data, the critical exponent was determined to be 0.75 (adjusted R2 of the fit = 0.98), which is slightly higher than the universal value predicted by percolation theory (uuni = 0.7).43 Fig. 4 (a) and 4 (b) depict the dependence of dielectric constant and electrical conductivity, respectively, of LN/PD composite as a function of volume fraction of LaNiO3 recorded at room temperature and at a frequency of 10 kHz. The dielectric constant gradually increased up to 55 at a


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lower volume fraction of LaNiO3 (till fLN = 0.10), but an abrupt increase was observed for fLN = 0.11 and above, reaching a maximum value as high as 2170 for fLN = 0.20. As mentioned before, the significant rise in the dielectric constant is also accompanied by a sharp increase in the effective electrical conductivity of the system that is illustrated in Fig. 4 (b) measured at a frequency of 10 kHz. Such drastic changes in conductivity (insulator to metallic transition) in composites are characteristic of percolation occurring in the system, where the concentration of the conducting fillers when higher than a percolation threshold, a continuous conducting pathway occurs in the composite. The percolation theory can be invoked in these circumstances to define the effective conductivity (σeff) and dielectric constant (εeff) of the composite that are expressed by a power law; σeff ∝ σPVDF(fc – fLN)–s, and

(2)

εeff ∝ εPVDF(fc – fLN) –t, for fLN ≤ fc

(3)

where σPVDF, εPVDF are the conductivity and dielectric constant associated with host PVDF, respectively; fc is the percolation threshold; fLN is the volume fraction of LaNiO3 in the polymer matrix; s and t are the critical exponents in the insulating (f < fc) and the conducting (f > fc) region, respectively. From the best fit of log-log plot of the power law on dielectric constant (as shown in the inset of Fig. 4 (a)), we obtain the percolation threshold as fc = 0.10 and the critical exponent, s = 0.69 (adjusted R2 of the fit = 0.95) which is slightly less than the universal ones.41,44 As for the log-log plots of power laws for conductivity in the nanocrystal composites, the best fit (shown in the inset of Fig. 4 (b)) gives fc = 0.10 and t = 1.64 (adjusted R2 of the fit = 0.98). The critical exponent exhibited by the conductivity in the composites is slightly higher than the universal values for a percolation system, which lies in the range of 0.8–1 in the insulating region.44 The inverse “Swiss-cheese model” can be applied to such systems to rationalize the deviation of critical exponents from their universal values where conduction mechanism is controlled by inter-particle tunnelling between conducting particles that are embedded in an insulating matrix. It should also be emphasized that the percolation threshold obtained from the fit lies less than the theoretically calculated value of ∼ 0.16 for a twophase random composite with conducting fillers. Such reduced percolation threshold values could be attributed to the cluster formation of the LaNiO3 nanocrystals within the polymer matrices. Clusters and agglomerates have been proved to decrease the percolative threshold in conductive filler-based polymer composites. Agglomerates in a matrix, tend to reduce the interparticle distance at a much lower loading levels, hence, resulting in a longer conduction path across the composite. Consequently, the percolation threshold also decreases as compared to a well mono-dispersed filler. Although there is a decrease in the threshold, the total interfacial polarization will be compromised resulting in reduction of the dielectric constant. Therefore, it is necessary to exercise a strict control over the formation of agglomerates during the synthesis stage. But, nevertheless, this process of controlling agglomerate formation (employing surfactants) may significantly affect the physical properties,



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including the percolation threshold, of the composites. Furthermore, the magnitude of percolation threshold is relatively less as compared to similar conductive and semi-conductive percolative polymer composites based on Al-polyethylene,10 Ag-polyimide,11 Ni-PVDF,12 SiC-BaTiO3-PVDF.18 The dielectric composites of percolative nature are limited by fc for charge storage applications and in this case, the enhancement of five times the dielectric constant of host PVDF could only be achieved. To circumvent this limitation, an additional filler (third phase) of BaTiO3 is embedded in the polymer matrix to further enhance the dielectric properties. For this, four different volume fractions of LaNiO3 that lie within the percolation threshold (fLN = 0.6, 0.7, 0.8 and 0.9) are chosen as the base two-phase composites upon which the third phase of BaTiO3 of volume fractions fBT = 0.10, 0.15 and 0.20 are added. To begin with, BaTiO3 nanocrystals were synthesised by citrateassisted sol-gel technique. X-ray diffraction pattern, shown in Fig. S2(a), of the synthesized crystallites confirms the formation of phase pure BaTiO3, crystallized in a cubic Pm3m with the associated lattice parameter a = 4.012 Å. An average crystallite size of 110 nm was observed from the TEM analyses of the synthesized particles and the phase purity was further corroborated by the electron diffraction pattern that is accordingly indexed (shown as an inset in Fig. S2). The three-phase polymer composites were then fabricated by hot-pressing technique and subjected to X-ray diffraction studies to confirm the presence of all the phases. For representation, the X-ray diffraction patterns of a two-phase BaTiO3/PVDF of the composition fBT = 0.20 and three phase BaTiO3/LaNiO3/PVDF of volume fractions fBT = 0.20, fLN = 0.09, are shown in Fig. S2 (b) and (c), respectively. It is interesting to note the presence of peaks attributed to partially crystallized PVDF in both the systems. The peak intensities of BaTiO3 remain strong due to its larger volume fraction and peaks corresponding to LaNiO3 are also visible in the XRD pattern. The uniform distribution of the fillers in the polymer matrix, devoid of any porosity, is observed from the SEM image of the cross section of fBT = 0.20, fLN = 0.09 nanocrystal composite, depicted in Fig. S3. The elemental mapping of the cross-sectional area of the composite shows a good dispersion of all the phases involved (shown in supplementary material as Fig. S4). The dielectric properties of these hot-pressed pellets were studied as a function of filler compositions. By considering a fixed fBT, the dielectric properties were analysed as a function of increasing conducting LaNiO3 filler embedded in the polymer matrices. Figs. 5 (a–c) and (d–f) depict the variation of dielectric constant and loss as a function of frequency for different fBT and fLN. In the Fig. 5 (a–c), one could observe a slight dispersion of dielectric constant with frequency at lower frequency region (< 1 kHz) and is attributed to the space charge polarization across the sample and the electrode. This dispersion is followed by a frequency independent region till 1 MHz and decreases beyond this frequency due to the relaxation process associated with the base polymer matrix. This frequency dispersion is minimal with lesser content of LaNiO3 and becomes more prominent with the increase in the volume fraction of LaNiO3. A systematic increase in the dielectric constant is also


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observed on addition of LaNiO3 nanocrystals to the BaTiO3/PVDF composites. Although, the changes in the dielectric constant of the nanocrystal composites on the addition of LaNiO3 are more apparent, the dielectric loss does not appear to show any significant difference, as observed in Fig. 5 (d–f). This feature could prove to be imperative if one could control the dissipation factor of the ceramic-polymer composite system, the addition of LaNiO3 can exhibit excellent charge storage capacities without compromising much on the dielectric loss of the composite. The observed loss did not cross 0.20 at 10 kHz for all the samples, which is still acceptable for various dielectric applications. Moreover, the increasing trend in the dielectric constant as a function of increase in the volume fraction of LaNiO3 for different frequencies is shown in Fig 6 (a–c) for fBT of 0.10, 0.15 and 0.20, respectively. A dielectric constant of 90 at 10 kHz was achieved for fBT = 0.20 and fLN = 0.09, and the dielectric loss associated with the nanocrystal composite was 0.13. The obtained value of dielectric constant is around nine times that of host PVDF and around three times that of BaTiO3/PVDF composite of fBT = 0.20. Even at high frequency (1 MHz), the dielectric constant is around 56 for the same composition which could prove to be a promising dielectric material for high-frequency applications. The change in the dielectric constant appears almost linear with the addition of LaNiO3, suggesting the ability to compositionally tune the property within the percolation threshold. On a related note, the obtained values for this LaNiO3/BaTiO3/PVDF ternary composite are better than that of other BaTiO3 (BT)based three phase composites, such as BT/Ag/PVDF,45,46 BT/SnO2/PVDF,47 Ba0.6Sr0.4TiO3/Ag/PVDF,

48

BaTiO3/ZnO/PVDF,49 and comparable to that of a few carbon-based three-phased

composites.20,50 Such changes in the dielectric constant is brought about by two different mechanisms – one from the high dielectric constant BaTiO3 nanocrystal inclusion and the other as a result of the percolative nature of LaNiO3. As observed from the frequency dependency of dielectric constant of these nanocrystal composites, the effect of interfacial polarization across the nanoparticles appears to be more prominent with the addition of BaTiO3 particles. This interfacial polarization, which is also known as the Maxwell-Wagner-Sillars polarization, is associated with the free charges trapped across the interface of the filler and polymer matrix under the influence of an external electric field. The migration and accumulation of these trapped charges lead to large polarization and therefore, an enhanced dielectric constant.3 The dielectric breakdown strength of the polymer composites, for a constant volume fraction of BaTiO3 at fBT = 0.20, is plotted as a function of conducting LaNiO3 filler concentration in Fig. 7. As expected, the breakdown strength systematically decreases with the increase in volume fraction of LaNiO3. Moreover, the absence of any anomalies in the breakdown strength versus volume fraction of LaNiO3 (covered in the present studies) implies that the compositions are within the percolation limit of the fillers. The corresponding energy density (Ue) of the polymer composites at a frequency of 100 Hz was determined using the following equation,

=

(4)



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where εr is the dielectric constant of the material (measured at 100 Hz), E refers to the breakdown strength and εo is the permittivity of free space (8.854 × 10–12 F/m). Unfortunately, the energy density also decreased with the incorporation of LaNiO3, and the composite associated with the composition fLN = 0.09 and fBT = 0.20 exhibited an energy density of 0.053 J/cm3. The obtained energy densities are comparable with Ag/C-based percolative composites.51

Conclusions To summarize, the citrate-assisted sol-gel technique yielded agglomerated nanocrystals of LaNiO3 with an average size of 120 nm that exhibited a highly defective microstructure in the form of parallel and perpendicular stacking faults across the nanocrystal associated with the double LaO layer during the crystal growth. A two-phase percolative nanocrystal composites of LaNiO3/PVDF were successfully fabricated using hot-press technique that showed a percolation threshold of fLN = 0.10 and an improved dielectric permittivity of 55 was achieved at 10 kHz along with an associated loss of 0.27. As the dielectric properties for the practical applications were limited by the percolative nature of LaNiO3, a ceramic filler in the form of BaTiO3 nanocrystals was added to obtain a three-phase nanocrystal composite. With LaNiO3 composition restricted to within the percolation threshold, a high dielectric constant of 90 at 10 kHz (and a low loss of 0.13) was obtained for the composition of fLN = 0.09 and fBT = 0.20. The achieved dielectric constant is around nine times that of pure PVDF and three times that of BaTiO3/PVDF nanocomposite. Hence, LaNiO3 proves to be an important percolative filler and these nanocrystal composites are potential candidates for high charge storage applications that can further be extended as flexible polymer dielectric composites for energy storage.

Associated Content Supplementary Information: XRD patterns of LaNiO3/PVDF two-phase composite (fLN = 0.10), assynthesized BaTiO3 powder, BaTiO3/PVDF polymer nanocrystal composites (fBT = 0.20) and LaNiO3/BaTiO3/PVDF three-phase polymer nanocrystal composites (fLN = 0.09 and fBT = 0.20); SEM images

of

fractured

surface

of

LaNiO3/PVDF

binary

composite

(fLN

=

0.10)

and

LaNiO3/BaTiO3/PVDF ternary composite (fLN = 0.09 and fBT = 0.20); elemental area mapping of fractured surface of LaNiO3/BaTiO3/PVDF ternary composite; composition, fLN = 0.09 and fBT = 0.20.

ORCID K. B. R. Varma: 0000-0001-5095-1734 P. W Jaschin: 0000-0002-1486-5035 Notes The authors declare no competing financial interest.


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Acknowledgement We thank Dr. P. Thomas, Joint Director, Central Power Research Institute, Bangalore, India, for measuring the breakdown voltage for the polymer nanocrystal composites. References 1. Chena, H.; Cong, T. N.; Yang, W.; Tan, C.; Li, Y.; Ding, Y. Progress in Electrical Energy Storage System: A Critical Review. Prog. Nat. Sci., 2009, 19, 291–312. 2. Hao, X. A Review on the Dielectric Materials for High Energy-Storage Application. J. Adv. Dielect, 2013, 3, 1330001. 3. Dang, ZÅM.; Yuan, JÅK.; Yao, SÅH.; Liao, RÅJ. Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage. Adv. Mater., 2013, 25, 6334–6365. 4. Chen, Q.; Shen, Y.; Zhang, S.; Zhang, Q.M. Polymer-Based Dielectrics with High Energy Storage Density. Annu. Rev. Mater. Res., 2015, 45, 433–458. 5. Thomas, P.; Varughese, K.T.; Dwarakanath, K.; Varma, K.B.R. Dielectric Properties of Poly(vinylidene fluoride)/CaCu3Ti4O12 Composites. Compos. Sci. Technol., 2010, 70, 539–545. 6. Wang, Z.; Fang, M.; Li, H.; Wen, Y.; Wang, C.; Pu, Y. Enhanced Dielectric Properties in Poly(vinylidene fluoride) Composites by Nanosized Ba(Fe0.5Nb0.5)O3 Powders. Compos. Sci. Technol., 2015, 117, 410–416. 7. Wang, Z.; Wang, T.; Wang, C.; Xiao, Y.; Jing, P.; Cui, Y.; Pu, Y. Poly(vinylidene fluoride) Flexible Nanocomposite Films with Dopamine-Coated Giant Dielectric Ceramic Nanopowders, Ba(Fe0.5Ta0.5)O3, for High Energy-Storage Density at Low Electric Field. ACS Appl. Mater. Interfaces, 2017, 9, 29130–29139. 8. Wang,

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Figures

Figure 1. XRD pattern for LaNiO3 powder after heat-treated at 850 oC/4h; SEM image of the synthesized powder is shown in the inset.


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Figure 2. (a) Bright field TEM image of LaNiO3 nanocrystals and their corresponding (b) selected area electron diffraction pattern. (c) and (d) High resolution TEM image of the nanocrystal which shows its defective nature in the form of stacking faults (indicated by the blue lines in (d)).



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Figure 3. Variation of (a) dielectric constant, (b) dielectric loss and (c) conductivity as a function of frequency for LaNiO3/PVDF nanocrystal composites for various LaNiO3 volume fractions. (d) log-log plot of conductivity as a function of frequency for LaNiO3 volume fraction of f = 0.10 (red line represents the linear fit).


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Figure 4. Variation of (a) dielectric constant and (b) conductivity as a function of volume fraction of LaNiO3 in LaNiO3/PVDF nanocrystal composite measured at 10 kHz (the insets represent the linear fit of log-log plot of power law dielectric constant and conductivity).



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Figure 5. Plot of (a), (b), (c) dielectric constant and (d), (e), (f) loss as a function of frequency for various volume fractions of LaNiO3 and BaTiO3.


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Figure 6. Variation of dielectric constant with volume fraction of LaNiO3 for different BaTiO3 concentrations, (a) fBT = 0.10, (b) 0.15 and (c) = 0.20.



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Figure 7. Plot of breakdown voltage and energy density of the LaNiO3/BaTiO3/PVDF composites, with fBT = 0.20, as a function of volume fraction of LaNiO3.


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XRD pattern for LaNiO3 powder after heat-treated at 850 oC/4h; SEM image of the synthesized powder is shown in the inset. 72x52mm (300 x 300 DPI)


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a) Bright field TEM image of LaNiO3 nanocrystals and their corresponding (b) selected area electron diffraction pattern. (c) and (d) High resolution TEM image of the nanocrystal which shows its defective nature in the form of stacking faults (indicated by the blue lines in (d)). 74x55mm (300 x 300 DPI)


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Variation of (a) dielectric constant, (b) dielectric loss and (c) conductivity as a function of frequency for LaNiO3/PVDF nanocrystal composites for various LaNiO3 volume fractions. (d) log-log plot of conductivity as a function of frequency for LaNiO3 volume fraction of f = 0.10 (red line represents the linear fit). 74x55mm (300 x 300 DPI)



Variation of (a) dielectric constant and (b) conductivity as a function of volume fraction of LaNiO3 in LaNiO3/PVDF nanocrystal composite measured at 10 kHz (the insets represent the linear fit of log-log plot of power law dielectric constant and conductivity). 99x153mm (300 x 300 DPI)


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Plot of (a), (b), (c) dielectric constant and (d), (e), (f) loss as a function of frequency for various volume fractions of LaNiO3 and BaTiO3. 50x25mm (300 x 300 DPI)



Variation of dielectric constant with volume fraction of LaNiO3 for different BaTiO3 concentrations, (a) fBT = 0.10, (b) 0.15 and (c) = 0.20. 25x6mm (300 x 300 DPI)


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Plot of breakdown voltage and energy density of the LaNiO3/BaTiO3/PVDF composites, with fBT = 0.20, as a function of volume fraction of LaNiO3. 76x58mm (300 x 300 DPI)



Three-phase polymer nanocrystal composite based on LaNiO3 and BaTiO3 nanocrystals embedded in a PVDF polymer matrix for high charge storage applications. 62x38mm (300 x 300 DPI)


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PVDF: A Three

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