Barium titanate ceramic powders have been incorporated in polyamide 11 to form homogeneous dispersion of particles in the matrix. Barium titanate/polyamide 11 nanocomposites have been synthesized using a solvent casting method with ultrasonic stirring to homogeneously disperse inclusions in the matrix. Composites with volume fraction of barium titanate / ranging from 0.01 to 0.4 were elaborated. Films were fabricated using a hot press method. Only the inclusions were poled in the matrix to form a ferroelectric particles/ unpoled matrix composite. Interactions between the particles and the matrix, pyroelectric and piezoelectric response were studied as a function of / by dynamic dielectric spectroscopy. Composites show interesting pyro-piezoelectric activity. Pyroelectric merit factor increases linearly and it reaches a limit value of 0.3 for a volume fraction / = 0.1

Capsal, Jean-Fabien

Dandurand, Jany

Dantras, Eric

Lacabanne, Colette

English

International audienceBarium titanate ceramic powders have been incorporated in polyamide 11 to form homogeneous dispersion of particles in the matrix. Barium titanate/polyamide 11 nanocomposites have been synthesized using a solvent casting method with ultrasonic stirring to homogeneously disperse inclusions in the matrix. Composites with volume fraction of barium titanate / ranging from 0.01 to 0.4 were elaborated. Films were fabricated using a hot press method. Only the inclusions were poled in the matrix to form a ferroelectric particles/ unpoled matrix composite. Interactions between the particles and the matrix, pyroelectric and piezoelectric response were studied as a function of / by dynamic dielectric spectroscopy. Composites show interesting pyro-piezoelectric activity. Pyroelectric merit factor increases linearly and it reaches a limit value of 0.3 for a volume fraction / = 0.1

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Electroactive influence of ferroelectric nanofillers on polyamide 11 matrix properties

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   Open Archive Toulouse Archive Ouverte (OATAO)  OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.  This is an author-deposited version published in: http://oatao.univ-toulouse.fr/  Eprints ID :  2487  To link to this article :  URL : http://dx.doi.org/10.1016/j.jnoncrysol.2007.01.097 To cite this version : Capsal, Jean-Fabien and Dantras, Eric and Dandurand, Jany and Lacabanne, Colette ( 2007) Electroactive influence of ferroelectric nanofillers on polyamide 11 matrix properties. Journal of Non-Crystalline Solids, vol. 353 (n° 47 - 51). pp. 4437-4442. ISSN 0022-3093 Any correspondence concerning this service should be sent to the repository administrator: staff-oatao@inp-toulouse.fr   Electroactive influence of ferroelectric nanofillers on polyamide11 matrix propertiesJean-Fabien Capsal, Eric Dantras *, Jany Dandurand, Colette LacabanneLaboratoire de Physique des Polyme`res, Institut Carnot CIRIMAT, Universite´ Paul Sabatier, Toulouse cedex 09, FranceAbstractBarium titanate ceramic powders have been incorporated in polyamide 11 to form homogeneous dispersion of particles in the matrix.Barium titanate/polyamide 11 nanocomposites have been synthesized using a solvent casting method with ultrasonic stirring to homo-geneously disperse inclusions in the matrix. Composites with volume fraction of barium titanate / ranging from 0.01 to 0.4 were elab-orated. Films were fabricated using a hot press method. Only the inclusions were poled in the matrix to form a ferroelectric particles/unpoled matrix composite. Interactions between the particles and the matrix, pyroelectric and piezoelectric response were studied as afunction of / by dynamic dielectric spectroscopy. Composites show interesting pyro-piezoelectric activity. Pyroelectric merit factorincreases linearly and it reaches a limit value of 0.3 for a volume fraction / = 0.1.PACS: 61.41.+e; 62.23.Pq; 77.84.LfKeywords: Nanocomposites; Pyroelectric; Piezoelectric; Relaxation1. IntroductionDuring last years, some ferroelectric polymers werestudied as P(VDF) [1], P(DVF-TrFE)[2], PA 11 [3,4]. Toincrease the electroactive properties of polymers, ferroelec-tric fillers were introduced in these polymer matrix to formferroelectric polymer/ferroelectric fillers composites [5,6].Most of these works concern P(VDF-TrFE) based compos-ites. We propose to associate ferroelectric properties ofinorganic materials with the attractive mechanical proper-ties of polymers. Barium titanate (BT) is one of the mostused ferroelectric ceramic, known for its high permittivity,high piezoelectric and pyroelectric properties [7]. As thetetragonality of BT decreases with the particles mean diam-eter [8,9], we choose particles of about 700 nm in diameter.PA 11 has been used as matrix in its non poled state andceramic powder was incorporated to form homogeneousdispersion of particles in the matrix (0–3 composites). Fer-doi:10.1016/j.jnoncrysol.2007.01.097* Corresponding author. Tel.: +33 561556456; fax: +33 561556221.E-mail address: dantras@cict.fr (E. Dantras).roelectric properties of the composites poled under 20 kV/mm (only ceramic phase poled) have been studied bychanging the volume fraction of BT. The challenge is toinsure a synergy between the electroactive properties ofthe ceramic and the attractive engineering behavior of PA11.2. Experimental section2.1. Materials and sample preparationPolyamide 111 (PA 11) will be studied in its unpoledstate. Barium titanate nanopowders (BaTiO3) are wellknown for their high ferroelectric properties. The meandiameter of these nanoparticules is 700 nm. PA 11 powderwas dissolved in a solution of dimethyl acethyl amide(DMAc) and the required barium titanate powder was1 PA 11 was supplied by Arkema (France) and BaTiO3 by Inframatmaterials (USA).dispersed to form a mixture by ultrasonic stirring. The sam-ples were dried over night at 110 C to remove the solvent.The nanocomposites were hot pressed to form thin films ofthickness from 70 to 100 lm. Volumic ratios (/) of nanoce-ramic in nanocomposite films were 0.01, 0.024, 0.1, 0.2, and0.4. For dynamic dielectric measurements, gold electrodeswere evaporated on both sides of thin films by cathode pul-verization. For pyroelectric and piezoelectric measurementsnanocomposites were poled to orient the dipoles; a macro-scopic polarization P was obtained. The films were polar-ised under vacuum (at room temperature under lowpressure p = 2 · 104 hPa) to prevent any voltage break-down. Electric signal was generated by a ±10 V generatorand then amplified using a ·2000 voltage amplifier. Theelectric field E is increased progressively until 20 kV/mmand kept constant 30 min. It was removed before short-cir-cuited. During poling, the current versus the applied fieldwas measured [10]. The samples were short-circuited torelax the internal stress induced during the poling and thenannealed at 100 C to eliminate thermally stimulated cur-rent during the pyroelectric measurements.2.2. Scanning electron microscopyJEOL JSM 6700F – Scanning electron microscope withfield emission gun (SEM-FEG) was used to study BT disper-sion in the polyamide matrix. As BT density is higher thanthe polyamide density, backscattered electron detectionwas used. Energy dispersive X-ray spectroscopy was madeon the samples in order to verify the BT trace in the matrix.2.3. Standard differential scanning calorimetryStandard differential scanning calorimetry (DSC) mea-surements were performed using a DSC/TMDSC 2920set up. The sample temperature was calibrated using theonset of melting of tin (Tm = 231.88 C), indium(Tm = 156.6 C) and cyclohexane (Tm = 6 C) with a heat-ing rate of qh = +5 C min1. The heat-flow was calibratedwith the heat fusion of indium (DH = 28.45 J g1), its base-line was corrected with sapphire. DSC experiments weresystematically carried out over a temperature range fromthe equilibrium state (in order to remove the effect of pre-vious thermal history) Teq = Tm + 20 C down to theglassy state T0 = Tg  70 C with a constant cooling rateqc = +20 C min1, and followed by a linear heating rateqh = 10 C min1. For each sample, the glass transitiontemperature (inflection point method) range but also thespecific heat jump was measured by a standard DSC duringa heating scan (qh = +10 C min1).2.4. Isothermal dielectric spectroscopyDynamic dielectric spectroscopy (DDS) were performedusing a BDS400 covering a frequency range of 102 Hz–3 · 106 Hz with 10 points per decade. Experiments werecarried out in a temperature range from 150 C to150 C. Dielectric isothermal spectra were measured every5 C. Before each frequency scan, temperature was keptconstant to ±0.2 C. The real e0T and imaginary e00T partsof the relative complex permittivity eT were measured asa function of frequency f at a given temperature T. Exper-imental data are fitted by the Havriliak–Negami (HN)function with an additional conductivity term [11]eT ¼ e1 þest  e1½1þ ðixsHNÞaHN bHN ir0ðestxÞ ; ð1Þwhere e1 is the relative real permittivity at infinitefrequency, est is the relative real permittivity at nullfrequency, sHN is the relaxation time, x is the angular fre-quency, aHN and bHN are the Havriliak–Negamiparameters.2.5. Pyroelectric and piezoelectric measurementsPyroelectric measurements were performed by a femto-amperemeter. The samples were short-circuited during5 min at room temperature and cooled to 170 C. Theywere heated up to 100 C with a constant heating rate(qh = +7 C min1) to eliminate thermally stimulated cur-rents. Then, they were cooled again to 170 C and thepyrocurrent was recorded as a function of temperature.The pyroelectric coefficient p is proportional to the heatingrate and to the sample surface as [12]p ¼ iðT Þb  S ; ð2Þwhere i(T) is the current (A) as a function of temperature, Sis the surface (m2), b (or qh) is the heating rate (C min1).Piezoelectric measurements were carried out using a PM200 piezometer supplied by Piezotest,UK, with a strengthof 0.25 N at 110 Hz in frequency. These measurementswere carried out after the pyroelectric tests to preventany poling effect (internal stresses).3. Results and discussion3.1. DispersionThe dispersion of barium titanate nanopowder in thepolymeric matrix has been studied by scanning electronmicroscopy (SEM) in order to check the experimental pro-tocol. Fig. 1(a) shows a good dispersion of BT in the matrixfor a low volume fraction (/ = 0.024) at a microscopicscale. The magnification of one part of Fig. 1(a) is repre-sented in Fig. 1(b). Spherical particles of BT (nanometricscale) are homogeneous dispersed in PA11. The chemicalnature of inclusion has been studied by energy dispersiveX-ray spectroscopy. The EDX spectrum is reported inFig. 1. As expected, the characteristic peaks of BaTiO3are pointed out. A platinum trace caused by pollution ismarked. An equivalent study has been performed on anamorphous area: peak characteristic of C and N atoms(major chemical constituents of PA 11) are present. Nano-Fig. 1. FEG-SEM pictures of PA11 + BaTiO3 nanocomposites for (a) /= 0.024 (·1000), (b) / = 0.024 (·5000), (c) EDXS of inclusions and (d) /= 0.1100 120 140 160 180 2001234560.0 0.1 0.2 0.3 0.4170175180185190195200Tf (°C)φHeat flow (arb. unit) φ=0.4φ=0.2φ=0φ=0.1φ=0.02421Temperature (°C)Endo12φFig. 2. PA 11 melting peak in 0–3 nanocomposites obtained by DSC.100 110 120 130 140 150BaTiO3 700 nmφ= 0.4φ= 0.2φ= 0.1φ= 0.024Heat flow (arb. unit) PA 11φ= 0.01Temperature (°C)Fig. 3. DSC thermograms of PA11, BaTiO3 700 nm nanoparticles andnanocomposites with volume fraction / ranging from 0.01 to 0.4.composites with higher volume ratio (/ = 0.1) of BaTiO3were also elaborated. SEM experiments were carried outand the micrographs confirm that even at high volume frac-tion of nanoparticles in the matrix the dispersion is stillcorrect at a nanometric scale. These results were confirmedby thermogravimetry analysis. The values of inorganic res-idue remain constant for several samples. These measure-ments have shown an accuracy of the dispersion of lessthan 3% in weight even for high volume ratio (/ = 0.4).Samples are homogeneous at a macroscopic scale.3.2. Thermal transitions3.2.1. Polyamide 11 meltingDifferential scanning calorimetry measurements wereperformed on the nanocomposites in order to estimatethe influence of BaTiO3 volume fraction on the PA 11 crys-tallinity ratio. The thermograms showing the PA 11 melt-ing peaks are reported in Fig. 2. The crystallinity ratio,vc, deduced from the melting enthalpy (DHm) is estimatedat 27%. Nanoparticles, even at high volume ratio, do notmodify the crystallinity of the matrix. But, we note thatthe melting peak of nanocomposites is shifted to lower tem-peratures as the volume ratio increases. It has been attrib-uted to structural defects within crystallites.3.2.2. BaTiO3 Curie pointDSC thermograms of PA 11, BaTiO3 and nanocompos-ites, in the temperature range (100–160 C) are shown inFig. 3. We observe that the nanoparticles of barium tita-nate exhibit an endothermic peak at 130 C characteristicof a first order transition. Polyamide does not have anytransition in this temperature range. This event has beenattributed to the ferro/paraelectric transition, also calledCurie point.The amplitude of the ferroelectric to paraelectric transi-tion increases with the volume ratio of barium titanate.Thermal manifestation associated with the Curie pointappears with / = 0.024 volume fraction of barium titanate.This fact shows that ferroelectrics properties can beobserved even at low volume fraction.3.3. Dielectric propertiesBroadband dielectric spectroscopy was performed toestimate the influence of nanofillers on the molecularmobility of the polyamide amorphous phase. The Havril-iak–Negami equation permits us to extract the relaxationtimes associated with the various modes. The evolutionof the relaxation time of the c and a modes has beenreported on an Arrhenius diagram (Fig. 4). The c modeis associated with the local mobility of the aliphaticsequences and the a mode is attributed to the dielectricmanifestation of the glass transition of the polyamide 11[13]. b mode is not reported due to its high sensitivity withthermal history and water content. Fig. 4 shows the evolu-tion of relaxation times as a function of BaTiO3 ratio. Itconfirms that molecular mobility is independent of thenanoceramics ratio. As there are very few interactions0.0 0.1 0.2 0.3 0.4 0.5024681012141618 real part of the permittivity ε'volume fraction of BT (φ)Fig. 5. Increase of relative permittivity of nanocomposites with thevolume fraction of BaTiO3. Dot line is the fit of Bruggeman model.0.00 0.05 0.10 0.15 0.200.00.20.40.60.81.0piezoelectric coefficient d (pC/N)volume fraction of BT (φ)0.00 0.05 0.10 0.15 0.200.00.40.81.21.6pyroelectric coefficient p (µC/K/m2)volume fraction of BT (φ)20°C25°C10°CabFig. 6. Ferroelectric properties versus volume fraction of PA11/BaTiO3nanocomposites (a) piezoelectric coefficient and (b) pyroelectric coefficient(lines are drawn as guides for the eyes).6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.51E-91E-81E-71E-61E-51E-41E-30.010.1110100  PA11+40% vol BaTiO3 700nm PA11+10% vol BaTiO3 700nm PA11+2.4% vol BaTiO3 700nm PA11relaxation times τ (s)1000/Temperature (K-1)mode γ mode αFig. 4. Relaxation times versus temperature of polyamide 11 andnanocomposites with volume fraction / = 0.024, / = 0.1 and / = 0.4.between nanoparticules and matrix, the molecular mobilityof PA 11 is not disturbed.One of the routes to greatly increase the dielectric per-mittivity is to incorporate particles with high dielectric per-mittivity. The evolution of dielectric permittivity withvolume ratio of BaTiO3 is predicted by the Bruggemanmodel [14].Fig. 5 shows that the evolution of e 0 at room tempera-ture as a function of volume fraction is well fitted withthe Bruggeman model. From this model we can estimatethe relative permittivity of the barium titanate nanopow-der. It has been found that e 0 is equal to 1500 at room tem-perature. Recent experiments of SPS (Spark plasmasintering) on nanopowders [15] allows us to measure thevalue of e 0 in the bulk state. The dielectric permittivity e 0has been estimated at 2000 (room temperature). These dataare coherent with the Bruggeman model.3.4. Electroactive propertiesIn order to determine the ferroelectric properties, thesamples were polarised as described in experimental sec-tion. As the polarization field of stretched PA 11 is100 kV/mm, only BaTiO3 nanofillers in the nanocompos-ites are poled. Fig. 6 reports the values of piezoelectriccoefficient at room temperature. The piezoelectric coeffi-cient d increases linearly with the volume fraction untilthe value of 1 pC/N (/ = 0.2). We recall that d is equalto 5 pC/N for ferroelectric polyamide 11 [16]. This valueis of the same order of magnitude than the one obtainedfor nanocomposites with non poled PA 11 with polednanoparticles. Pyroelectric activity has been reported inFig. 6. We observe a quasi linear increase of p3 with the vol-ume fraction of BaTiO3. This behavior is described usingthe linear mixture rule of the pyroelectric (p) and piezoelec-tric coefficient (d) [17]p ¼ acLEpC  /þ appP  ð1 /Þ; ð3Þd ¼ acLEdC  /þ apdP  ð1 /Þ; ð4Þwhere the subscripts C and P denote the ceramic and poly-mer respectively, LE ¼ 3eð2þ/Þeþð1/Þec is the local field coeffi-cient and a the poling ratio.Since polyamide is in its non poled state, pP = 0 lC/K/m2 and dP = 0 pC/N. In a first approximation, we find with0.200.150.100.050.00020406080100120volume fraction of BT (φ)p/L E (µC/K/m2 )predictedobservedFig. 8. Pyroelectric coefficient normalized by the local field coefficient ofeach sample versus / at 25 C. Solid line is the fit of Eq. (2).0.00 0.05 0.10 0.15 0.200.000.050.100.150.200.250.300.350.40Pyroelectric figure of merit (µC/m2 K)volume fraction of BT (φ)Fig. 9. Merit factor p/e of the 0–3 nanocomposites versus volume fractiona good agreement a linear increase of the pyroelectric coef-ficient and piezoelectric coefficient with / as previouslyshown in our experimental data. According to the model,it means that ac = 1, particles are supposed to be fullypoled. In this work, particles are poled at saturation inregards of their crystallographic orientations with respectto the applied field.We note that the pyroelectric coefficient is notconstant with the temperature. A maximum value ofp = 1.5 lC/K/m2 is obtained at 20 C and decreases top = 0.9 lC/K/m2 at 25 C. It has been attributed to theorthorhombic to tetragonal phase transition of thebarium titanate nanoparticles [5]. It has been confirmedby DDS measurements of BaTiO3 powders as shownin Fig. 7. DDS measurements pointed out three dielec-tric manifestations of solid–solid transitions: respectivelythe rhombohedral to orthorhombic phase transition,the orthorhombic to tetragonal phase transition andthe tetragonal to cubic (Curie point) around 130 C.Eqs. (3) and (4) permit us to fit p/LE and d/LE versus/. These equations confirm the linear increase of piezo-electric and pyroelectric coefficient with BT volumefraction.By fixing the pyroelectric coefficient of BT at200 lC/K/m2 we observe in Fig. 8 that the increaseof p of the composites is higher than the predictedone.Fig. 9 shows the evolution of the merit factor (Fp = p/e 0)of the nanocomposite as a function of volume ratio ofBaTiO3. Fp increases to a value 0.3 for / = 0.1. As the volt-age response of a pyroelectric sensor is proportional to themerit factor, we conclude that nanocomposites with / =0.1 exhibit the best properties as regards to its mechanicalproperties.-150 -100 -50 0 50 100 15016186543 real part of the permittivity ε'log fTemperature (°C)Fig. 7. Real part of the permittivity of BaTiO3 700 nm nanopowders as afunction of temperature and frequency.(line is drawn as guide for the eyes).4. ConclusionBaTiO3/PA11 nanocomposites with BaTiO3 volumefraction ranging from 0.01 to 0.4 have been elaboratedand characterized. As the nanocomposites were poled at20 kV/mm, only ceramic part was poled. Piezoelectricand pyroelectric activities of the composite were studiedby varying nanofillers volume fraction. Suitable ferroelec-tric properties of the composites were obtained close tothe ferroelectric properties of poled PA11. Pyroelectricmerit factor increases linearly and it reaches a limit valueof 0.3 for a volume fraction / equal to 0.1.References[1] L. Ibos, A. Bernes, C. Lacabanne, Ferroelectrics 320 (2005) 483.[2] G. Teyssedre, C. Lacabanne, Thermochim. Acta 226 (1993) 65.[3] S. Ikeda, T. Saito, M. Nonomura, T. Koda, Ferroelectrics 171 (1995)329.[4] L. Ibos, C. Maraval, A. Berne`s, G. Teysse`dre, C. Lacabanne, S.-L.Wu, J.I. Scheinbeim, J. Polym. Sci. B 37 (1999) 715.[5] T. Furukawa, K. Ishida, E. Fukada, J. Appl. Phys. 50 (1979) 4904.[6] K.H. Lam, H.L.W. Chan, Comp. Sci. Technol. 65 (2005) 1107.[7] T.A. Perls, T.J. Diesel, W.I. Dobrov, J. Appl. Phys. 29 (1958) 1297.[8] S. Wada et al., Ceram. Eng. Sci. Proceed. 26 (2005) 89.[9] T. Yan, Z.-G. Shen, W.-W. Zhang, J.-F. Chen, J. Mater. Chem. Phys.09 (2005) 450.[10] F. Bauer Nuclear Instruments and Methods in Physics ResearchSection B: Beam Interactions with Materials and Atoms, 105 (1995),212.[11] S. Havriliak, S. Negami, J. Polym. Sci. Polym. Symp. 14 (1966) 99.[12] L. Ibos et al., J. Polym. Sci. Part B 37 (1999) 715.[13] L. Ibos, A. Bernes, G. Teyssedre, C. Lacabanne, S.-L. Wu, J.-I.Scheinbeim, Electrets (1999) 623.[14] D.A.G. Bruggeman, Ann Phys Leipzig 24 (1935) 635.[15] S. Guillemet-Fritsch, M. Boulos, B. Durand, V. Bley, T. Lebey, J.Eur. Ceram. Soc. 25 (2005) 2749.[16] S. C Mathur, J.I. Scheinbeim, B.A. Newman, J. Appl. Phys. 56 (1984)2419.[17] T. Furukawa, K. Fujino, E. Fukada, Jpn. J. Appl. Phys. 15 (1976)2119.

r e a l p a r t o f t h e p e r m i t t i v i t y ε ' log f Temperature (°C)

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Electroactive influence of ferroelectric nanofillers on polyamide 11 matrix properties

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