Preparation and mgel properties of NF-PZT composites obtained in-situ by sol gel combustion method_JECS 2012

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2. Author's personal copy Available online at www.sciencedirect.com Journal of the European Ceramic Society 32 (2012) 3325–3337 Preparation and magnetoelectric properties of NiFe2O4–PZT composites obtained in-situ by gel-combustion method Cristina Elena Ciomagaa,∗, Mirela Airimioaeib, Valentin Nicaa, Luminita M. Hriba, Ovidiu F. Caltuna, Alexandra R. Iordanb, Carmen Galassic, Liliana Mitoseriua,∗, Mircea N. Palamarub aFaculty of Physics, Al. I. Cuza University, Iasi 700506, Romania bFaculty of Chemistry, Al. I. Cuza University, Iasi 700506, Romania cCNR-ISTEC, Via Granarolo no. 64, I-48018, Faenza, Italy Received 20 September 2011; received in revised form 30 March 2012; accepted 30 March 2012 Available online 8 May 2012 Abstract Magnetoelectric composites of xNiFe2O4–(1−x)Pb(Zr,Ti)O3with x=2, 5, 10, 20, 30% were prepared by citrate–nitrate combustion using PZT- based template powders. In order to ensure a better connectivity of dissimilar phases, we have used chemical methods for preparation in situ composites, followed by adequate sintering procedure. The structural, microstructural and functional properties of di-phase magnetoelectric composites of NiFe2O4–PZT are reported. The XRD analysis is demonstrating the synthesis of pure ferrite phase directly on the ferroelectric templates. An excellent mixing was obtained in the composite powders, as proved by a detailed SEM analysis. The magnetic and dielectric behaviors of the ceramic composites vary with the ratio of the two phases. The dielectric behavior is greatly influenced by the magnetic phase. The magnetoelectric (ME) coefficient was measured as a function of applied DC magnetic field. The maximum ME coefficient (dE/dH) varies from 0.0011mV/(cmOe) to 0.5mV/(cmOe) with increasing of NF addition. © 2012 Elsevier Ltd. All rights reserved. Keywords: Sol–gel processing; Ferrites; PZT; Dielectric properties; Magnetic properties 1. Introduction properties, not existing in individual components are envisaged, make them to be used in the microelectronic industry.4The design and preparation of composite materials are realized also in the view of increasing the degree of multifunctionality, the final aim being a higher degree of miniaturization in devices for microelectronics and micromechanics. The functional properties of composites are derived both from the constituents’ properties as from their reciprocal inter- actions that might be: (i) sum property, in which a function is the weighted sum of the individual contributions, (ii) combina- torial property, when for specific circumstances, the amplitude of the property is higher in the composite than for individual components; (iii) product property, indicating effects absent in the individual components and present in the composite, as result of a kind of coupling between the components.1–3 The last ones are the most interesting because by acting on the microstructures and interfaces, radically new properties can be induced. Combination of two phases, such as a combina- tion of magnetostrictive and ferroelectric (thus, piezoelectric) The composites are artificial systems containing at least two phaseswithdifferentphysicalandchemicalpropertiesseparated at mesoscopic level in the final product, which are attracting an increasinginterestinviewofvariousapplications.Theirproper- ties are determined by the number, type and volume ratio of the phases, as well as by the individual properties, degree of inter- connectivity and interface properties. The composite is formed bythefillerembeddedintoamatrix(ceramic,polymer,metallic, etc.).1–3Whilethestructuralcompositeswerelargelyusedsince long time ago in applications using their superior performances (mainlymechanicones)3,the“electroniccomposites”aresmart materialsinwhichbetterelectromagneticperformanceandnew ∗Corresponding authors. Tel.: +40 232201102x2406; fax: +40 232201205. E-mail addresses: cristina.ciomaga@uaic.ro, CRISFEDOR@yahoo.com (C.E. Ciomaga), lmtsr@uaic.ro (L. Mitoseriu). 0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.03.041

3. Author's personal copy 3326 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 phases, can yield a desirable magnetoelectric (ME) property, in spite the individual components do not show it. An electric field will induce in the ferro-piezoelectric phase a deformation that will be transmitted to the magnetostrictive component giv- ing rise to a change of its magnetic properties, so that finally an electric field causes a change in the magnetic state, i.e. ME coupling. The ME effect is realized in composites based on the concept of product property.5In order to realize compos- ites with radically new properties (for example the transition dielectric-conductor)andexceptionalcharacteristicsincompar- ison to those of the individual components, the investigation of functional properties and the understanding of the intrin- sic/extrinsic contribution to the properties are presently of high interest. In spite of a large number of recent publications referring to compositesystems,therearemanyproblemsofchemical,struc- tural and thermodynamic compatibility of the components. In order to get diphase composites with high spatial homogeneity, with a specific type of interconnectivity, clean interfaces, high density in the view of a good interphase mechanical coupling and superior electrical properties the preparation routes must be carefully controlled; for example, in the case of MEs, the couplingbetweenthemagneticandferroelectricphasesarereal- ized by piezoelectric-magnetostriction effects, which imposes structural compatibility of the components to reduce internal stresses and the absence of porosity at the interfaces between the phases. Consequently, the complex problems of the multi- functional diphase composites with ME coupling is far to be solved and their investigation is still of high interest. Lead zirconate titanate Pb(Zr1−xTix)O3 solid solution belongs to the ferroelectric perovskites family with formula ABO3 with Ti4+and Zr4+ions randomly occupying the B positions,withexcellentelectromechanical,dielectricandpyro- electric characteristics that are strongly dependent on the stoichiometry. The NiFe2O4(NF) has an invers spinel struc- ture and was chosen in order to introduce large resistivity, high piezomagnetic coefficient, low anisotropy and high initial per- meability, which are very important and one of the prerequisites to increase the efficiency of ME effect. Different methods have been used to synthesize the ME di-phase ceramic composites formed of ferrites such as MnFe1.8Cr0.2O4, CuFe2O4, Ni0.5Co0.5Fe2O4 and BaTiO3, or Ba0.8Pb0.2TiO3relaxor. In the large majority of publications reporting data on com- posite ceramics the conventional sintering technique was using for producing the composite because of cost effectiveness, ease of fabrication and better control of the process parameters. The powders of magnetic oxides (e.g. ferrites) and piezoelectric ceramics are mixed firstly together, and then the mixed pow- ders are pressed into green bulks followed by sintering at high temperature.6–8The maximum values of the ME output for the compositions prepared by mixing oxides route was found to be around 200–300?Vcm−1Oe−1. The reported dielectric con- stant and losses present a frequency-dispersion characterized by a strong decrease of permittivity from thousands to ∼300 when frequency is ranging from Hz to MHz, with tangent losses >1 for f<10kHz and tangent losses >10 at f<1kHz at room temperature.6,7 Recently, in order to obtaining the ME ceramic composites with high density and purity, hot pressing and spark plasma sintering SPS techniques have been employed to replace the conventional sintering. Hot-pressed and SPS samples exhibited a large improvement in the ME voltage coefficient, as compared to the conventionally sintered samples.9 Several wet chemical methods, such as the hydrothermal method, co-precipitation process, and sol–gel technique, have been developed for the synthesis of oxide ceramic powders to improve their properties. Auto-combustion synthesis method attracted considerable attention in fabricating homogeneous and de-agglomerated fine ceramic powder. Availability of inex- pensive precursors, simple calculations, eases optimization of process parameters proved to be advantageous in auto- combustion synthesis. Di-phase ceramic 0.50BaTiO3–0.50Ni0.5Zn0.5Fe2O4 with limited reaction at interfaces, higher density and better dielectric properties (tanδ<5% and εr∼300–800 for f>104Hz at room tempera- ture) were obtained by co-precipitation of Fe, Ni, Zn salts in solutions containing BaTiO3 powders, than that obtained by mixing BaTiO3and Ni0.5Zn0.5Fe2O4powders.10 Theinsitusol–gelmethodfollowedbyaconventionalsinter- ing,usedforobtainingtheMEceramiccomposites,havegained attentionbecauseensuresabettermixingandmuchmoredense, well-developed and homogeneous microstructures, leading to serious reduction of the dielectric losses. The composition and purityofthetwoconstituentphasesismaintainedaftersintering at low temperature. In the present work, we tural, microstructural and functional xNiFe2O4–(1−x)PZTwithx=2,5,10,20,30wt%andZ/Tratio 47/53 magnetoelectric composites prepared in situ by sol–gel method. In the following the original results are presented and discussed in detail. composites as have studied properties the struc- of the 2. Sample preparation and experimental details The powder precursors of xNiFe2O4 and CoFe2O4, Pb(Zr,Ti)O3 NiFe2O4 ferroelectric or (1−x)Pb(Zr0.47Ti0.53)O3 with various x concentration were synthesized by in situ processing based on sol–gel method. We have started with Pb(Zr0.47Ti0.53)O3(PZT) (ρth=7.952g/cm3) template powders which were firstly prepared by a conventional solid state reaction method by using reagent grade PbO (Aldrich 211907, purity 99.9%), ZrO2(MEL SC101), and TiO2 (Degussa P25). The precursors were ball milled with zirconia milling media in water for 48h, freeze dried, sieved to 250?m and calcined at 800◦C/4h. The calcined powder was wet milled in ethanol (100h), then dried and sieved. The mean diameter (d50) of the PZT calcined powder was 0.71?m and the specific surface area was 2.67m2/g. NiFe2O4nanoparticles (NF) and xNF–(1−x)PZT (x=2, 5, 10, 20, 30%, w/w) nanocomposite were synthesized by auto- combustion method.11,12All chemicals are analytical grade and used without further purification. Ni(NO3)2·6H2O and

4. Author's personal copy 3327 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 Fe(NO3)3·9H2Osolutionsweremixedandafterthenitratesolu- tion obtained was mixed with aqueous citric acid (C6H8O7) under magnetic stirring. Citric acid was used with two important roles: as fuel for the combustion reaction and as a chelating agent to form complexes with metal ions, preventing the precipitation of hydroxylated compounds.13 The Ni:Fe mole ratio was 1:2 and the molar ratio NiFe2O4:citric acid was 1:3. The as-prepared PZT powder was dispersed in the nitrate/citric acid solution by magnetic stirring. The suspension of PZT–(nitrate/citric acid solution) was heated at 80◦C under constant stirring until the water evap- oration was completed and a viscous gel was obtained. After formation of gel, the temperature was increased gradually to ∼350◦C.Alargeamountofgases(CO2,H2O,N2)wasreleased andtheauto-combustionoccurred.Thecombustionreactionwas completedwithinafewsecondsanditwasformedafinepowder. This was completely crushed and powdered. The as-combusted brown powders were calcined at 500◦C for 8h resulting in fine composite powders of xNF–(1−x)PZT (x=2, 5, 10, 20, 30%, w/w). The composite powders were ground, and then cold iso- statically pressed at 300MPa into discs of 30mm diameter (green bodies). The samples were sintered at 1200◦C/1h, in Pb atmosphere maintained by a PbZrO3source in a closed Al2O3 crucible. After sintering the ceramic pellets were polished to remove the surface layers. In order to measure the dielectric properties selected sintered and ground samples were screen printed with silver electrodes, fired at 750◦C and finally poled into silicon oil at 120◦C, under a DC field of 3kV/mm for 40min. XRD patterns of the powders and ceramic composites were recorded using Shimadzu LabX 6000 diffractometer (CuK? radiation, λ=0.15406nm). The samples mounted in reflection mode were analyzed in ambient atmosphere with scanning rate of0.02◦andcountingtimeof1s/stepoverthe2θ =20–80◦range. The average crystallite size of the particles for each sample was calculated from the full width at half maximum (FWHM) of the XRD peak profile using a Lorentzian convolution. The error for the determination of crystallite size affects approx- imately by 2% the calculated values. The lattice parameters were determined using the linear multiple regressions based on the Least Squares procedure with a Si standard powder correction. The phase formation of the NiFe2O4ferrite onto the PZT ferroelectrictemplateswascheckedbyusingaFouriertransform infrared spectra (FT-IR), recorded in the range 4000–400cm−1 with 2cm−1resolution using a Bruker FTIR spectrophotometer TENSORTM27-type with an anvil ATR cell. Scanning electron microscopy (SEM) (Hitachi Instrument) was used to obtain the microstructure and phase compositions of the ceramic composites. The electrical characterization was performed on the disk shaped samples following the standard IEEE 176-1987. The elasto-piezo-dielectric constants were calculated from the val- ues of resonance and antiresonance frequencies of the planar mode. The dielectric properties vs. frequency at the 20Hz–2×106Hz frequency domain in the range temper- ature of 25–250◦C were determined by performing complex impedance measurements with an impedance bridge type Agilent E4980A Precision LCR Meter. In order to determine the M(H) hysteresis loops in the range0–1.4T(14,000Oe),aMicroMagTMmagnetometerVSM (Vibrating Sample Magnetometer) model 3900 system from Princeton Measurements Co. was used. The magnetoelectric propertieswerestudiedbymeasuringtheinducedelectricpoten- tial (Vin) at a small AC magnetic field superimposed (Hac) onto a DC magnetic bias (Hdc), both of which were parallel to the thickness direction of the pellets. The AC field was produced by a Helmholtz coil (470 turns with a diameter of 80mm), which was driven by the reference signal from lock-in. The amplitude of the AC and DC field was measured by means of a gaussmeter (Model 455 DSP Gaussmeter). The ME signals was measured withalock-inamplifier(ModelSR830DSPLock-InAmplifier). Data acquisition was performed by computer using a LabView interference program. 3. Results and discussions 3.1. Phase identification The X-ray diffraction measurements for xNF–(1−x)PZT ceramics composites with x=2, 5, 10, 20, 30%, at room temper- ature are shown in Fig. 1a and b. The Bragg reflections match the reported values of tetragonal PZT and face-centered cubic nickel ferrite systems with the space group P4mm and Fd3m, respectively (Powder Diffraction Files, cards no 50-0346 and 74-2081, respectively). Pure diphase powder composites with a good crystallinity were obtained after the calcination steps (Fig. 1a). Fig. 1b indicates the presence of NiFe2O4spinel phase and maintenance of Pb(Zr,Ti)O3 ferroelectric phase. It confirms the formation of diphase ceramic composites after sintering at 1200◦C/1h and the lack of secondary phases at interfaces. The tiny peak around 2θ ∼28◦(Fig. 1b) existing in the samples with low concentration of Ni ferrite, (for x=2 and 5%), could corre- spondtoZrO2andmaycomefromthestartingmaterialZrO2−x. The (311) peak intensity of the nickel ferrite phase increases with increasing the NF content in composites. From Fig. 1a we can observe that the peak (110) belonging to the PZT does not change even in composites, while the posi- tion of the peaks (111), (200), (211) is slightly splitted. This indicates tetragonal structure of the PZT ferroelectric phase. Thelatticeparametersoftheceramiccompositesaredetailed in Table 1. The XRD patterns reveal that there is no chemical reactionbetweenPZTandNF.Thus,theinsituprocessinggives a feasible method in order to obtain highly crystalline ceramic composites which leads to multifunctional properties. 3.2. FTIR TheIRabsorptionspectroscopywasusedtomonitorthesyn- thesis of these samples. For this purpose, spectra were recorded

5. Author's personal copy 3328 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 Fig.1. XRDpatternsofxNF–(1−x)PZTwithx=2,5,10,20,30%:(a)calcined powders at 500◦C for 8h; (b) sintered ceramics at 1200◦C for 1h with various compositions. for samples thermally treated at 200◦C and 1200◦C (Fig. 2a and b). Theabsorptionbandat∼3284cm−1inthespectraofsamples thermally treated at 200◦C (Fig. 2a) was associated to O H vibration. Peaks localized at 1567cm−1and 1390cm−1can be assigned to the stretching vibration of carboxylate and nitrate groups, indicating that the self-propagating high temperature synthesis process is incomplete.14 We can also see that the spectra of these compounds present two absorption bands at ∼756cm−1and ∼528cm−1that can be attributed to the stretching vibration of the M O bonds, characteristic for the parent PZT and ferrite phases. Fig.2. IRspectraofthexNF–(1−x)PZTcompositesafterthermaltreatmentat: (a) 200◦C and (b) 1200◦C/1h. For the samples sintered at 1200◦C/1h (Fig. 2b) only the absorption bands characteristic for the stretching vibrations of the M O bonds from two parent phases – NF and PZT are present in the spectra. Thus the shoulders from ∼583cm−1to ∼670cm−1may be associated with the stretching vibrations of Ti O bonds belonging to PZT phase.15,16The broader absorp- tion band from 487cm−1and the shoulder at ∼751cm−1are Table 1 Average crystallite size and lattice parameters of the ferroelectric phase from composite systems. xNF–(1−x)PZT ceramic samples x=2% x=5% x=10% x=20% x=30% Crystallite size (nm) Lattice parameters a (˚A) ferroelectric phase Lattice parameters c (˚A) ferroelectric phase 22 4.0173±0.0020 23 4.0221±0.0024 21 4.0313±0.0063 20 4.0303±0.0031 17 4.0577±0.0050 4.1279±0.0024 4.1244±0.0029 4.1284±0.0021 4.1287±0.0023 4.1054±0.0049

6. Author's personal copy 3329 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 Fig. 3. SEM micrograph of xNF–(1−x)PZT ceramic composites for x=30% NiFe2O4. Fig. 4. Variation of the piezoelectric properties of composites with NF content. composition with low concentration of Ni ferrite of 2% is 52×10−12m/V, but with increasing the NF addition, the d33 of the composites drops rapidly to 18×10−12m/V for the com- position with 30% NF content. The increasing ferrite content leads to the reduction of the piezoelectric coefficient as a result of the much smaller resistance of the ferrite in comparison to the PZT. The piezoelectric voltage constant, g33, presents also a decrease at increasing content of Ni ferrites in the composite. associated with stretching modes (Zr O stretch) in the ZrO6 octahedron of the PZT structure.17 The nickel ferrite has an inverse spinel structure, the Fe3+ ions being positioned on the tetrahedral sites and also on the octahedral ones, while the Ni2+ions are placed only in octahe- dral sites. In the absorption spectra of the NF ferrite with cubic spinelstructureandinthespectraofxNF–(1−x)PZTcomposite materials thermally treated at 1200◦C/1h, an absorption band at ∼529cm−1, characteristic of intrinsic stretching vibrations of the (Mtetra↔O) bonds from tetrahedral units, is found. Usu- ally the IR spectra of the ferrites present a smaller adsorption band attributed to the stretching vibrations of the (Mocta↔O) fromtheoctahedralunits,situatedatwavenumberssmallerthan 400cm−1. 3.5. Magnetic properties The magnetic properties of the xNF–(1−x)PZT ceramic compositeswithx=2,5,10,20,30%arepresentedinFig.5and proved the ferromagnetic nature of the materials. These prop- erties are a consequence of the “sum property” and interface effects. IfincaseofNiFe2O4thesaturationmagnetizationwasfound tobe∼45emu/g,forthexNF–(1−x)PZTmagnetoelectriccom- posites,thesaturationmagnetizationdecreasefrom13.63emu/g forx=30%to0.33emu/gforx=2%Niferritephase.Thecoerci- tive field (Hc) for all the composite samples is larger than that for the pure NiFe2O4phase, Hc∼200Oe for x=2% and Hc∼11.5Oe for NF, as the results of mixing of ferrimagnetic 3.3. SEM analysis The SEM micrograph of the xNF–(1−x)PZT ceramic com- posites with x=30% magnetic phase, sintered at 1200◦C/1h are shown in Fig. 3. The SEM image of the polished surface demonstratethatdenseandhomogeneousmagnetoelectriccom- posites were obtained with a good dispersion of the Ni ferrite spinel phase (black color for NF) in PZT ferroelectric phase matrix (grey color for PZT). The average grain size of the NF grains becomes larger at increasing content of NF. Thus, the sol–gel method influences the degree of connectivity of the two phases and this can yield to some differences in the functional properties. 3.4. Piezoelectric characterization The piezoelectric constants of PZT and the magnetostriction of NF are essential parameters that determine the mag- netoelectric effect. The piezoelectric constant (d33) of the xNF–(1−x)PZT composites decreases with the decrease of concentration of pure PZT ferroelectric phase, similar to the dielectric constant. Fig. 4 shows the piezoelectric constant d33of the compos- ites with various NF contents. The d33value of the ceramic Fig. 5. Magnetic hysteresis loops of the xNF–(1−x)PZT ceramic composites.

7. Author's personal copy 3330 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 Fig. 6. Dielectric dispersion at various temperatures in the xNF–(1−x)PZT ceramic composite with x=2, 5, 10, 20, 30% of NF: (a–e) real and (f–j) imaginary part of the dielectric constant.

8. Author's personal copy 3331 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 Table 2 Piezoelectric characteristics of xNF–(1−x)PZT ceramic composites. xNF–(1−x)PZT x=2% 7.58 x=5% 7.61 x=10% 7.40 x=20% 7.13 x=30% 6.81 ρ (g/cm3) d31(10−12m/V) d33(10−12m/V) g31(10−3m/N) g33(10−3m/N) ρ (%) 95.9 97.3 96.7 95.7 95.0 19.8 18.6 22.8 8.6 7.4 52 37 49 23 18 2.89 2.79 3.87 2.13 1.89 7.63 5.61 8.36 5.61 4.65 phase with PZT phase. As in the pure Ni ferrite, the sponta- neous magnetization in xNF–(1−x)PZT ceramic composites originates from the unbalanced antiparallel spins (ferrimagnetic character), which can give rise to the mentioned small coerciv- ities and saturation fields. frequency dispersion in the frequency range 103–106Hz, for all the compositions, which are manifests in a broad Debye- type maximum with a shift to higher frequency (Fig. 6f–j).26 The Maxwell–Wagner relaxation mechanism is associated with uncompensated surface charges at interfaces inside the com- posite ceramics. Such phenomenon is determined by the local inhomogeneity of sample caused by a high number of charged defectslocatedinsidetheceramicgrainsandatthegrainbound- aries, which produce local charge unbalance in the sample volume.Thesechargesarecreatedbyexternal(contacts)orinter- nal(grainboundaries,domainwalls,inhomogeneities)boundary layers and induce local conductivity variation. Due to the local compositional inhomogeneity and enhanced fluctuations of the local electrical properties in the ferroelectric-magnetic compos- ite systems, the barriers for the thermally activated dielectric relaxations and conductivity are much lowered, so that they give unavoidable contributions even at room temperature. As discussed in detail in Refs. 21–23, such extrinsic phenomena cause the peculiar dielectric properties observed in all the mag- netoelectric composites reported in the literature. Analyzing the frequency dependence of permittivity at vari- ous temperatures in the range 25–250◦C (Figs. 6a–j), it can be clearly seen that a typical thermally activated dispersion domi- nates the dielectric response of the xNF–(1−x)PZT system. Similar dielectric properties were reported for PZT-Ni fer- ritepreparedbysparkplasmasintering,24inPZT-(Ni,Zn)ferrite ceramicspreparedbypowder-in-solprecursorhybridprocessing route25and other composite systems.26,27 Dielectric losses are proportional to the dielectric imaginary part of the permittivity (ε??) in the studied system. Thus, the dielectric losses (tanδ) are below 0.16 for all the ceramic com- positions and increase with NF amount. The impedance spectroscopy plots at room temperature are shown in Fig. 7. It can be seen that impedance spectra for the compositions x=2, 5, 10% exhibit a single semicircular arcs, demonstrating a good homogeneity of the dielectric and con- ductive properties. For the compositions with low concentration of magnetic phase, x=2, 5, 10%, the influence of ferrite phase is weak, and the NF phase in small quantity is only discontinu- ouslyimbeddedinthePZTferroelectricmatrix.Thepresenceof thesinglesemicirculararcs,whosepatternchangeswithcompo- sition, indicate a modification of the resistance/reactance ratio when increasing the ferrite addition x. The composition with x=20,30%additionofNiferriteshowmorethanonecomponent in the complex impedance plot demonstrating that the ceramic samples present some degree of local electrical heterogeneity eveniftheyarehomogeneousfromstructuralandcompositional 3.6. Dielectric properties 3.6.1. Dielectric constant characterization The dielectric constant of xNF–(1−x)PZT ceramic com- posites with x=2, 5, 10, 20, 30% were checked at different temperatures 25–250◦C as a function of frequency, in the range of 100Hz to 2MHz, and are presented in Fig. 6. As expected, the dielectric constant of the xNF–(1−x)PZT system compos- ites decreases with increasing the ferrite phase addition as a consequence of the sum property, similar to the piezoelectric properties (Table 2). From Fig. 6a–e can be observed that the realpartsofdielectricpermittivity(ε?(f))arestronglyfrequency- dependent and their values dramatically increase with reducing frequency, arriving to ε?≈4×104, at f=100Hz for x=2% and ε?≈1×106,atf=100Hzforx=30%ofNiferriteatT=250◦C (Fig. 6a–e). The increasing of dielectric constant with increas- ingoftemperatureisathermallyactivatedprocess,whichresults in an increase of dielectric polarization. The dielectric constant is a combined effect of dipolar, electronic, ionic and interfacial polarizations. At lower frequency, the dipolar and interfacial polarizations contribute significantly to the dielectric constant and at higher frequency only the electronic polarization became significant. Hence, it increases at a higher value for 100Hz as compared to 10kHz or 2MHz. The dielectric behavior in composites can also be explained on the basis of the polariza- tion mechanism in ferrites because conduction beyond phase percolation limits is due to ferrite.18In ferrites, the rotational dispalcementofFe3+↔Fe2+dipolesresultsinorientationpolar- ization that may be visualized as an exchange of electrons between the ions, the dipoles align themselves with the alternat- ingfield.Thetransportpropertiessuchaselectricalconductivity anddielectricdispersionofferritearemainlyduetotheexchange mechanismofchargesamongtheionssituatedatcrystallograph- ically equivalent sites.19 The frequency dependence of the imaginary part of per- mittivity (ε??(f)) presents a few characteristic of dielectric and conductivity relaxations in the investigated frequency range 100Hz–2MHz: (i) an increasing of the imaginary part with increasing temperature at low frequency, mainly due to the interfacial polarization (Maxwell–Wagner relaxation phenom- ena) and also to the heterogeneity in the samples20and (ii) a

9. Author's personal copy 3332 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 Two types of dispersion phenomena are determined for the composite samples: (i) one at frequencies below 10kHz, a pronounced maximum of M??which shifts toward higher fre- quencies with increasing temperatures, and (ii) a rather constant values of M??up to 1kHz with a decreasing with increasing of frequency range. The fact that only at low frequency both the imaginary part of permittivity and dielectric modulus show maxima indicates a Maxwell–Wagner dispersion in this fre- quency range, while at higher frequencies both in the range of 102–104Hz and above 105Hz, combined dielectric relaxation and AC-conductivity relaxations are most probably responsi- ble with the observed dielectric response. In case of the sample compositewithx=30%thefrequencydependenceofimaginary part of dielectric modulus, M??, presents two well defined con- voluted components. Both of them are thermally activated: the maximaareshiftedtowardhigherfrequenciesasthetemperature is increasing. Both low and high-frequency observed relaxations in xNF–(1−x)PZT ceramic composites are thermally activated phenomena, for which the characteristic activation energy can be deduced from the maximum of the imaginary part of the dielectric modulus by the Arrhenius law: ?Ea where fM is maximum peak position for the high frequency relaxation, kBis the Boltzmann constant, τ0is a pre-exponential factor, and Eais the activation energy. The values of the pre- exponential factor and activation energies corresponding to ferroelectricregionaregiveninTable3.Thesevaluesoftheacti- vation energies obtained by us are in the range of ones reported in other composite materials.30–32 Fig.7. CompleximpedancespectroscopyforxNF–(1−x)PZTcompositeswith x=2, 5, 10, 20, 30% at room temperature. pointofview.Similarphenomenonwasreportedonferroelectric BaTiO3and ferrite (Ni0.3Zn0.7)Fe2.1O4, synthesized via solid state reaction route.20 ? 1 = τ0 exp (3) 3.6.2. Dielectric modulus characterization A better representation for understanding if more than one relaxation process is involved in the dielectric response of the composite ceramics is given by the formalism of the dielectric modulus (M*), defined as28,29: τM= , 2πfM kBT M∗(f) = M?(f) + iM??(f), where: (1) ε?(f) ε??(f) M?(f) = andM??(f) = (2) ε?2(f) + ε??2(f) Thefrequencydependenceofthecomplexmodulushasbeen investigated,takingintoaccountthecomplexdielectricconstant (ε*(f)=1/M*(f)) (Fig. 8a–j). Figs. 8a–e and f–j show the varia- tion of the real and imaginary part of the complex modulus with frequency of xNF–(1−x)PZT ceramic composites. From Fig. 8a–e, it is observed that M?(f) increases with fre- quencyupto10kHzandreachesaratherconstantvalueathigher frequency, while ε?, as expected, decreases quickly to an almost constant value. The variation of M??(f) as a function of frequency pro- vides, also, useful information concerning the charge transport mechanism such as electrical transport and conductivity relax- ation and has been successfully used to distinguish localized dielectric relaxation processes from long-range conductivity and short/long range polaron hoping in composite materials (Fig. 8f–j). A conductivity relaxation is indicated by the pres- ence of a peak in M??(f) spectra and no peak would appear in corresponding plot ε??(f), while the dielectric relaxation gives maxima both in the ε??(f) and M??(f). In the studied frequency range M??present maximum as in the frequency dependence of ε??(f), for all compositions, indicating the dielectric relaxation processes. ε?2(f) + ε??2(f) 3.6.3. AC-conductivity characterization The variation of the AC conductivity as a function of fre- quency for a few temperatures is shown in Fig. 9a–e. For all the compositions the conductivity (σ) increases with the increas- ing of frequency, indicating also a thermally activated process. We can identified two distinct frequency regions in the AC- conductivity dependence (e.g. Fig. 9a–e): (i) a low-frequency region for which the conductivity corresponding to low tem- peratures (T<150◦C) tends to an almost zero conduction (ii) a region at high frequencies above 103Hz with a large increase of conductivity for all temperatures. The increasing in tempera- ture at low frequency confirms the Maxwell–Wagner relaxation present in all studied ceramic composites. The linearity of AC conductivity plots (Fig. 9a–e), at lower temperature, indicates that the conduction occurs by hopping of small polaron type of charge carriers among the localized states. As we have mention before an important contribution to the conduction mechanism in the ceramic samples is explained on the basis of hopping of charge carriers between the Fe2+and Fe3+ions on the octahedral site from ferrite phase, and with addition of this one. The increase in frequency enhances the hopping frequency of charge carriers, resulting in an increase in the conduction process, thereby decreasing the resistivity. The

10. Author's personal copy 3333 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 Fig. 8. Frequency dependence of the complex dielectric modulus at a few temperatures in the range of 25–250◦C: (a–e) real part (M?) and (f–j) imaginary part (M??) for xNF–(1−x)PZT ceramic composites with x=2, 5, 10, 20, 30%.

11. Author's personal copy 3334 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 Table 3 Activation energies in ferroelectric region for xNF–(1−x)PZT ceramic composites with x=2, 5, 10, 20 and 30%. xNF–(1−x)PZT ceramic samples T (◦C) Ea(eV) from M??(f) τ0(s) 3.7×10−8 8.7×10−8 1.4×10−6 2.9×10−8 3.7×10−8 3.3×10−8 3.3×10−8 2.6×10−8 25–120 160––250 0.024 0.168 x=2% 25–140 160–250 0.026 0.15 x=5% x=10% x=20% 160–250 160–250 0.147 0.13 120–240 160–250 0.047 0.12 x=30% Fig. 9. Variation of AC conductivity with frequency for xNF–(1−x)PZT composites at a few temperatures. results are similar to that observed by other authors for different composites.28,31,33,34 TheMEoutputofthepresentxNF–(1−x)PZTceramiccom- posites is studied as a function of the addition of Ni ferrite and of DC magnetic field (Hdc) (Fig. 10). The ME coefficient (α) is calculated with the following formula35: 3.7. Magnetoelectric effect In the ME composites, the static ME voltage coefficient depends on the mechanical coupling, resistivity and content of the constituent phases. V (4) α = Hac× d

12. Author's personal copy 3335 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 Table 4 Comparative study of dielectric properties and magnetoelectric coefficient of composite systems. Reference ME composite Preparation route At room temperature ε?at 1kHz tanδ Ea(eV) dE/dH (mV/(cmOe)) 20 BaTiO3–(Ni0.3Zn0.7)Fe2.1O4 x=0.3 (x)Ni0.9Zn0.1Fe2O4+(1−x)PZT x=0.30 Solid state 10,000 at 100Hz 0.8 26 Solid state, double-sintering method Solid state 512 at 1kHz 0.3 0.082 0.68 25 Ni0.5Zn0.5Fe2O4–PbZr0.53Ti0.47O3 x=25% NiFe2O4+Pb0.93La0.07(Zr0.60Ti0.40)O3 x=0.30 (y)Ni0.5Zn0.3Co0.2Fe2O4+(1−y)BaTiO3 x=0.30 (x)Ni0.5Zn0.5Fe2O4+(1−x)BPZT x=0.30 CoFe2O4–Pb(Zr,Ti)O3 x=0.30 Ni0.2Co0.2Fe2O4–PbZr0.8Ti0.2O3 x=0.30 xNi0.8Zn0.2Fe2O4+(1−x)Pb0.93La0.07(Zr0.60Ti0.40)O3 x=0.30 Pb(Zr0.53Ti0.47)O3–(Ni0.5Zn0.5)Fe2O4 740 0.025 31 Solid state 720 0.5 0.035 40 Solid state 0.54 0.43 38 Solid state 700 0.5 0.69 12 Sol–gel 187 1.25 27 Solid state 680 0.05 0.39 0.76 30 Solid state 650 0.55 0.05 4.64 190 at 100◦C 32 Modified hybrid process Sol–gel 0.7 0.28 Present study xNF–(1−x)PZT x=0.30 600 0.16 0.0470.12 0.5 saturation, the magnetostriction and the mechanical strain thus produced would also generate a constant electric field in the piezoelectric phase which leads to the decrease of the ME coef- ficient values. The increase of the magnetoelectric signal with ferrite amount could be attributed to the increase of the mechanical deformation in the magnetoestrictive phase. The maximum α of ∼0.5mV/(cmOe) observed in the composition with x=30% of ferrite, may be originated, also, from more uniform distribution of ferrite and PZT phase. The same values of ME coeffi- cient were reported in literature on Ni0.5Zn0.5Fe2O4–BPZT composites,38CoFe2O4–BaTiO3andCoMn0.2Fe1.8O4–BaTiO3 ceramic composites.26,39,40 Thesynthesismethodforobtainingthemagnetoelectriccom- posites, the connectivity pattern of the component phases in the systems are of important for the electrical properties such as dielectric constant (ε?), tangent losses (tanδ), activation energy (Ea(eV)),andhencetheMEconstant(dE/dH).Table4presentsa comparativestudyinasyntheticwayofthevaluesobtainedover time on various ME composites, together with their preparation routes and sintering conditions. Fig. 10. Variation of ME coefficient as a function of static magnetic field for xNF–(1−x)PZT ceramic composites. where V is the voltage generated due to the magnetoelectric effect, Hacis the amplitude of the sinusoidal magnetic field and d is the thickness of the sample. TheαvaluesshowninFig.10areobtainedaftersubtractionof αvalueatHdc=0.Fromthisfigure,itisobservedthatαincreases up to 500Oe, attains a maximum value and then decreases with further increase of Hdc. This behavior was observed also by other researchers who studied magnetoelectric composites with nickelferrite.36,37TheexplanationgivenforthevariationofME coefficient with the applied DC magnetic field was as follow: at a certain value of Hdcmagnetic field, the magnetostriction coefficient of the ferrite phase reaches to saturation. Beyond 4. Conclusions In summary, we have studied the xNF–(1−x)PZT magnetic- ferroelectric ceramic composites with different compositions (x=2, 5, 10, 20, 30%) prepared in situ by sol–gel method using Pb(Zr,Ti)O3 template powders obtained by mixed oxide method. The XRD patterns confirmed the successful

13. Author's personal copy 3336 C.E. Ciomaga et al. / Journal of the European Ceramic Society 32 (2012) 3325–3337 formationofadi-phasecompositeforeachcomposition,formed by the spinel NiFe2O4and Pb(Zr,Ti)O3perovskite phases in the nominal proportion. The existence of the two distinct phases in the xNF–(1−x)PZT composite systems, the ferrite and the ferroelectric phase, was confirmed also by SEM analysis. The magnetic properties measured at room temperature showed that the saturation magnetization and the remnant mag- netizationofthecompositesincreasewithincreasingtheamount of Ni ferrite. Dielectric measurements with frequency showed a decrease of the dielectric constant at increasing ferrite addition as a consequence of the sum property. The dielectric constant vs. frequency at room temperature and at a few temperatures in the range 25–250◦C indicate the dielectric relaxation caus- ing by space charge effects and Maxwell–Wagner phenomena, particularly at low frequency and high temperature, and Debye relaxation in the range of frequency 103–2×106Hz. Using the sol–gel method, we have obtained homogeneous microstructure and a reducing of the dielectric losses. The complex impedance representation reveals that the com- positions with x=2, 5, 10% NF present electrical homogeneity, while compositions with higher Ni ferrite content show local electrical inhomogeneity due to the presence of more than one components.ThexNF–(1−x)PZTceramiccompositesaregood candidates for the ME applications, due to the fact that both the ferroelectricandmagneticphasespreservetheirbasicproperties in the bulk composite form. The dependence of the ME response of the xNF–(1−x)PZT ceramic composites on the applied magnetic fields is dom- inated by NF addition. The maximum ME coefficient ∼0.5mV/(cmOe) is observed for 30% Ni ferrite phase. These composites have a better sensitivity in the low field range. 8. Kadam SL, Kanamadi CM, Patankaer KK, Chougule BK. Dielectric behaviour and magnetoelectric effect in Ni0.5Co0.5Fe2O4+Ba0.8Pb0.2TiO3 ME composite. Mater Lett 2005;59:215–9. 9. NanC-W,BichirinMI,DongS,ViehlandD,SrinivasanG.Multiferroicmag- netoelectriccomposites:historicalperspective,status,andfuturedirections. J Appl Phys 2008;103:031101–31135. 10. Mitoseriu L, Buscaglia V, Viviani M, Buscaglia MT, Pallecchi I, Harnagea C, et al. J Eur Ceram Soc 2007;27:4379–82. 11. Airimioaei M, Ciomaga CE, Apostolescu N, Leontie L, Iordan AR, Mitoseriu L, et al. Synthesis and functional properties of the Ni1−xMnxFe2O4ferrites. J Alloys Compd 2011;509:8065–72. 12. 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