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Ferroelectric and magnetic properties of the PMN-PT-nickel zinc ferrite multiferroic ceramic composite materials
Materials Chemistry and Physics 157 (2015) 116e123
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Ferroelectric and magnetic properties of the PMN-PT-nickel zinc ferrite multiferroic ceramic composite materials Dariusz Bochenek a, *, Przemysław Niemiec a, Ryszard Skulski a, Artur Chrobak b, Paweł Wawrzała c University of Silesia, Institute of Technology and Mechatronics, 2, Sniez_ na St., Sosnowiec 41-200, Poland University of Silesia, Institute of Physics, 4, Uniwersytecka St., Katowice 40-007, Poland c Silesian University of Technology, Teachers College, 9-9A, Hutnicza St., Gliwice 44-100, Poland a
b
h i g h l i g h t s Multiferroic composites PMN-PT-ferrite have been obtained. Two compositions of PMN-PT were used to obtain composites. Ferroelectriceferromagnetic properties at room temperature were achieved. The shape of the M(T) curves is typical for this type composites.
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Article history: Received 4 January 2015 Received in revised form 17 February 2015 Accepted 12 March 2015 Available online 18 March 2015
Multiferroic ceramic composites based on PMN-PT and NieZn ferrite have been obtained and described in presented work. PMN-PT powders were synthesized by solegel method while nickelezinc ferrite was obtained by classical ceramic method. Two compositions of PMN-PT were used i.e., 0.72PbMg1/3Nb2/3O30.28PbTiO3 with rhombohedral symmetry and 0.63PbMg1/3Nb2/3O3-0.37PbTiO3 with tetragonal one. In both cases the classical methods of calcination and final pressureless densification were used for obtaining of final ceramic composites. XRD, microstructure, EDS, dielectric, electrical and magnetic studies were performed for the obtained ceramic composite materials which confirmed ferroelectric and ferrimagnetic properties at room temperature. © 2015 Elsevier B.V. All rights reserved.
Keywords: Ceramics Sintering Microstructure Dielectric properties Magnetic properties
1. Introduction Solid solutions (1x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-PT) with perovskite structure exhibit very good ferroelectric/relaxor and piezoelectric/electrostrictive properties which depend on the chemical composition (x) and technological methods [1]. A continuous transition from relaxor to normal ferroelectric properties takes place with increasing x. Morphotropic region is observed for 0.25 < x < 0.35 [2]. With increasing x the maximum of the dielectric permittivity shifts from about Tm ¼ 3oC for x ¼ 0 to about Tm ¼ 227 C for x ¼ 0.5 [3]. Morphotropic region of PMN-PT was investigated also in works [4,5]. The properties of solid solution
* Corresponding author. E-mail address: [email protected] (D. Bochenek). http://dx.doi.org/10.1016/j.matchemphys.2015.03.025 0254-0584/© 2015 Elsevier B.V. All rights reserved.
PMN-PT make them useful for many applications for example for high frequency actuators and high power piezoelectric transducers etc. [6,7]. However, there are the problems during obtaining PMNPT ceramics related with arising of unwanted pyrochlore phase [8,9]. It is the result of high volatility of PbO at high temperatures and the differential reactivity between MgO and Nb2O5. The pyrochlore phase degrades the electrophysical properties. Using the solegel method we can decrease the volume of pyrochlore phase. Nickelezinc ferrite with composition Ni0.64Zn0.36Fe2O4 has the spinel-type structure and belongs to soft ferrites with a high value of magnetic permeability and high resistance r (105 Um) [10]. Devices obtained from this material are used for example for signal processing, filters and broad-band transformers. Mixing the upper described materials we have tried to obtain ceramic compositions which are multiferroic at room temperature
D. Bochenek et al. / Materials Chemistry and Physics 157 (2015) 116e123
and can be characterized by a magnetic response to a variable electric field, or inversely, a polarization change in the external magnetic field. Similar effect was reached for example in [11e13]. In general obtained are also multiphase composites with additional, for example, polymer phase (called ceramic-polymer composites) [14]. In the case of the ferroelectromagnetic materials, a lot of other techniques of powder synthesizing such as molten salt synthesis, reaction sintering, columbite, solegel and co-precipitation methods are used, besides the classical method [15]. The ferroelectric and mechanical properties, altogether with the degree of ferroelectric and ferromagnetic subsystems' coupling in this type materials are mainly related to the properties of each components of the solid solution and their percentage [16e18]. Recently for example there were described ceramic composites based on best known piezoelectric PZT and zinc ferrite [14,19] and PMN-PT-ferrite composites [20,21]. Below described materials which have been obtained as a combination of ferroelectric/relaxor component i.e., (1x)PbMg1/3Nb2/3O3-xPbTiO3 and ferrimagnetic component i.e., (Ni0.64Zn0.36Fe2O4) are an example of two component materials. 2. Experiment PMN-PT solid solution was obtained by solegel technology described in [8]. PMN was obtained as a result of the reaction between Mg(OC2H5)2 and Nb(OC2H5)5 in alcohol. In a second step lead acetate (II) and ethylene glycol were added to the mixture according to the formula: Pb(CH3COO)2 þ 1/3 Mg(Nb(OC2H5) 6)2 / Pb(Mg1/3Nb2/3)O2(OC2H5)2 þ 2CH3COOC2H5. PbTiO3 (PT) was obtained according to the formula: Pb(CH3COO)2 þ Ti(CH3CH2CH2O)4 / PbTiO2(CH3CH2CH2O)2 þ 2CH3COOC3H7. Then the two liquid compositions PMN and PT were mixed together in ethylene glycol (with proper proportions for x ¼ 0.28 and 0.37) to give the PMN-PT compound. Finally distilled water was added in order to begin hydrolysis of the obtained sol solution. After passing the sol to gel, it was dried and then ground and sintered at T ¼ 550 C/4 h in order to remove the organic parts. Dielectric and electromechanical properties of such obtained PMNPT samples were described in [8,22]. Nickelezinc ferrite Ni0.64Zn0.36Fe2O4 was obtained from the simple oxides Fe2O3, ZnO, NiO. The powders were mixed using Fritsch planetary ball mill for 15 h. The synthesis was conducted at conditions 1100 C/4 h using calcination technique. These two components were mixed in proportion 90% PMN-PT and 10% ferrite using Fritsch planetary ball mill for 15 h (wet in ethyl alcohol). After this the mixture was synthesized by calcination method with conditions: Tsynth ¼ 950 C, tsynth ¼ 8 h. Final pressureless densification (sintering) of composite ceramic samples was conducted at Ts ¼ 1100 C by ts ¼ 8 h. Two compositions i.e., 0.73PMN-0.28PT-ferrite (abbreviated to PP28-F) and 0.63PMN0.37PT-ferrite (abbreviated to PP37-F) were obtained. For electrical tests silver electrodes were applied on the both surfaces of the diskshaped samples using paste burning method. The X-ray tests at room temperature were performed using a Philips diffractometer (CuKa radiation). Microstructure, EDS (Energy Dispersive Spectrometry) and EPMA (Electron Probe Microbeam Analysis) tests were carried out using a scanning microscope HITACHI S-4700. Magnetic properties were investigated using SQUID (MPMS XL-7 Quantum Design) magnetometer within the temperatures range from 271 C to þ27 C and magnetic field up to 7 T. Dielectric and impedance measurements were performed using QuadTech 1920 LCR meter for a cycle of heating (at frequencies of the measurement field from 0.1 kHz to 1.0 MHz). Dielectric hysteresis loops PeE were investigated using SawyerTower circuit and a Matsusada Inc. Heops-5B6 precision high voltage amplifier. Electromechanical measurements were carried
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out using an optical displacement meter (Philtec Inc., D63) and high voltage amplifier Heops. Data were stored on a computer disc using an A/D, D/A transducer card and LabView computer program. 3. Results and discussion 3.1. Crystal structure and microstructure The density of 0.72PMN-0.28PT is 7.45 g/cm3, while the density of 0.63PMN-0.37PT is equal to 7.21 g/cm3. The densities of PP-F composite materials are similar (see Table 1). Fig. 1 shows X-ray diffraction patterns of powder PMN-PT after synthesis, the ferrite powder and PP-F powders at room temperature. For rhombohedral 0.72PMN-0.28PT the (200) line should be a single maximum (R3m space group), while for tetragonal 0.63PMN-0.37PT the (200) line should consist of two components (Pm and P4mm space groups). The X-ray analysis of obtained by us PMN-PT powders after synthesis exhibit rhombohedral symmetry for x ¼ 0.28 and tetragonal one for x ¼ 0.37. The XRD patterns of the synthesized powders of PMN-PT show lines belonging to perovskite phase with relatively small amount of pyrochlore phase. It is known that preparation of PMN using solegel method decreases the amount of unwanted pyrochlore phase [23]. Comparing the intensities of the reflections 29.08 (pyrochlore line 222) and of the reflections 31.2 (perovskite line 110) it has been stated than the amount of pyrochlore phase is higher for composite with x ¼ 0.37. X-ray diffraction pattern of ferrite powder Ni0.64Zn0.36Fe2O4 shows a single phase cubic spinel lines what stays in agreement with results of [24]. In case of the mixtures PP-F the X-ray the diffraction patterns we can see strong maxima originating from PMN-PT, as well as weak reflexes from the Ni0.64Zn0.36Fe2O4 ferrite. Since the synthesis of mixtures PP-F was performed by calcination method, the increase of the amount of the pyrochlore phase was observed and as a result for PP-F the amount of pyrochlore phase is higher than in PMN-PT without ferrite. In final compositions smaller amount of pyrochlore phase was observed for PP28-F (about 11%). Energy-dispersive X-ray spectroscopy EDS (surface and local analyses) confirmed the assumed chemical composition of PP-F and the presence of maxima from elements originating in PMN-PT and elements originating in ferrite (Fig. 2). The obtained EDS examinations are comparable to the assumed proportions of the initial components, calculated by stoichiometry, while obtaining the PMN-PT-ferrite material. In Table 2 (for PP28-F) and in Table 3 (for PP37-F) are summarized assumed and measured individual components of the composite samples. Fig. 3 presents microstructural SEM images of the ferrite ceramics (Fig. 3a), PMN-PT ceramic samples (Fig. 3b PP28 and Fig. 3c PP37), and fractured ceramic composite PP-F samples i.e., PP28-F
Table 1 Parameters of the PMN-PT ceramic samples and PP-F composite materials (dielectric properties for f ¼ 1 kHz, d33 for E ¼ 0.5 kV/mm and f ¼ 0.1 Hz).
r [g/cm ] 3
Tm [ C] εr εm tand at Tr tand at Tm PS [mC/cm2] PR [mC/cm2] EC [kV/mm] d33 [m/V] 1012
PMN-PT28
PMN-PT37
PP28-F
PP37-F
7.45 132 1860 6.290 0.025 0.024 25.10 21.00 0.67 400
7.21 187 1760 6090 0.021 0.032 25.50 22.16 1.04 457
7.46 123 1860 7.130 0.039 0.095 12.60 8.42 0.57 368
7.47 182 1150 5840 0.011 0.063 8.00 4.51 0.81 20
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D. Bochenek et al. / Materials Chemistry and Physics 157 (2015) 116e123 Table 2 Assumed and measured weight % and atom % quantity of the constituent components of the PP28-F composite sample. PP28-F
O Pb Ti Mg Nb Ni Zn Fe
Fig. 1. X-ray spectra of the PMN-PT, the ferrite and the PP-F ceramic composite materials.
and PP37-F (Fig. 3d and e respectively). PMN-PT ceramics characterized by grained microstructure when the microstructure of the ferrite sample has large grains. The microstructures of the PP28-F and PP37-F are characterized by rather small, well crystallized grains. Large grains of ferrite are surrounded by small
Assumption % at
Experimental %
Assumption % weight
Experimental % weight
59.7 18.0 5.0 4.3 8.6 0.9 0.5 2.9
60.8 16.1 4.9 3.5 9.2 1.3 1.1 3.1
16.0 60.1 3.9 1.7 12.9 1.2 0.7 3.6
18.3 55.5 4.5 1.6 13.5 1.5 1.3 3.8
grains of ferroelectric ceramic component what is clearly visible in Fig. 3e. These results were confirmed by additional research EDS and EPMA (dispersive X-ray energy spectroscopy) in the relevant areas of the surface of the PP-F samples. Examples of EPMA images for PP28-F and PP37-F composite samples are shown in Fig. 4. Fig. 4b and d represent EPMA images of the composite samples for the selected element e iron (Fe K) which is a component only in ferrite. The average size of grains is between 2 and 4 mm and the average grain size slightly decreases when the PMN/PT ratio changes from 72/28 to 63/37.
Fig. 2. EDS spectra for the PMN-PT ceramics, the ferrite and the PP-F ceramic composite materials.
D. Bochenek et al. / Materials Chemistry and Physics 157 (2015) 116e123 Table 3 Assumed and measured weight % and atom % quantity of the constituent components of the PP37-F composite sample. PP37-F
O Pb Ti Mg Nb Ni Zn Fe
Assumption % at
Experimental %
Assumption % weight
Experimental % weight
59.7 18.0 6.7 3.8 7.6 0.9 0.5 2.9
61.1 16.1 6.2 3.3 7.9 1.3 1.0 3.1
16.1 60.4 5.2 1.5 11.4 1.2 0.7 3.6
18.1 55.2 5.8 1.6 12.5 1.6 1.3 3.9
3.2. Magnetic properties Generally, at low temperatures the magnetic properties of the obtained PP-F composite ceramic samples are similar for various x. Fig. 5 shows the temperature dependences of mass magnetization M(T) for investigated compositions of PP-F. The M(T) curves were measured in FC mode (field-cooled) at magnetic field 0.1 T (1000 Oe). The small difference in the absolute values of the bulk magnetization is attributed to the mass content of the ferrite phase in the ceramic composite samples. The magnetic hysteresis loops for the two compositions of PP-F are very similar both at temperature 271 C as well as at temperature of 27 C (Fig. 6). Remnant magnetization MR is relatively low and little decreases with increasing temperature as well as saturation magnetization too. At the room temperature, the shapes of the hysteresis curves are typical for ferrimagnetic materials (ferrite) and the result of introduction of paramagnetic (ferroelectric/relaxor) component is similar like in [25,26] for bulk composites.
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3.3. Ferroelectric and dielectric properties The temperature dependences of dielectric permittivity (ε) measured at 1 kHz for the PMN-PT ceramics as well as PP-F ceramic composites are presented in Fig. 7. The maxima of the dielectric permittivity indicate phase transitions in the samples. PMN-PT ceramic samples with. x ¼ 0.28 and 0.37, show maxima at temperatures 132 C and 187 C, respectively, what correspond to the phase transition from the ferroelectric to the paraelectric (TC temperature). Dependences for PMN-PT ceramic samples and PP-F composite ceramic samples are presented in Fig. 7a. It is seen that the temperatures at which maxima of dielectric permittivity are shifted towards lower temperatures in comparison with PMN-PT without ferrite (see DT in Fig. 7a). In case of dielectric losses (Fig. 7b) these shifts are larger and the increase of dielectric losses in PP-F is observed. It can be related with many factors for example with stresses and defects on boundaries between grains of PMN-PT and grains of ferrite. The shift of the temperature of maximum towards lower temperatures as a result of hydrostatic pressure is described for example in work [27]. The shift of the maximum of dielectric permittivity connected with decrease of diffusion of the phase transition can also mean increased ordering of the structure of the composite. In the case of PPF ceramic composites maximum of dielectric permittivity occurs in a more narrow range of temperatures and does not depend on the frequency of the measuring field. The values of electric permittivity are higher for PP28-F as compared with PP37-F sample (at room temperature as well as the temperature of maximum). Above the phase transition temperature the ε(T) curves show anomaly (another maximum) with very broad peak of dielectric permittivity. Additionally, a strong frequency dependency is observed (in the same range of temperatures for two compositions), probably as a result of relaxation processes as well as a large increase in electric conductivity at high temperatures. These peaks shift towards higher temperatures with the increase of frequency (Fig. 8).
Fig. 3. SEM microstructure of the PMN-PT samples: (a) PP28, (b) PP37, nickelezinc ferrite sample (c) and composite samples: (d) PP28-F, (e) PP37-F.
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Fig. 4. EPMA images of the composite samples: (a, b) PP28-F, (c, d) PP37-F.
Fig. 5. Mass magnetization v.s., temperature (field-cooled mode FC) at magnetic field 0.1 T for PP-F composite samples.
The broad and high maxima of dielectric permittivity are also associated with overlapping of the dielectric PMN-PT and ferrite phases. Similar phenomena were observed for other ferroelectroeferrimagnetic materials for example PbFe1/2Nb1/2O3-ferrite [28]. Samples show lower values of maximum permittivity compared with their counterparts' pure PMN-PT ceramic samples. In order to investigate the relaxation phenomena in this material, the dielectric constant and loss tangent are presented as a function of frequency at different temperatures (Fig. 9). Generally, for ceramic composite samples of PP-F, the dielectric permittivity ε decreases with increasing frequency and it attains a constant value at high frequencies what is typical for a MaxwelleWagner type of interfacial polarization. The high values of ε observed at low frequencies can be attributed to the dipoles that result from changes in the valence states of the cations, and the space charge polarization due to the inhomogeneous microstructure (disorder regularities of the microstructure by the introduction of the ferrite grains). In polycrystalline samples, the role of inhomogeneities play impurities, pores, grains and grain boundaries. When the frequency increases, only the electronic polarization contributes to the total polarization what is caused by the fact, that dipoles of other types have low mobility and cannot follow the fast variation of the
Fig. 6. Magnetic hysteresis loops for composite materials: (a) PP28-F, (b) PP37-F.
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Fig. 7. Comparison of temperature dependencies of dielectric permittivity ε (a) and dielectric losses tand (b) for the PMN-PT28 and PMN-PT37 ceramic samples as well as for PP28-F and PP37-F ceramic composite samples (on heating, frequency 1 kHz).
Fig. 8. Temperature dependencies of electric permittivity ε for the ceramic composite samples: (a) PP28-F, (b) PP37-F (on heating).
Fig. 9. Frequency dependencies of dielectric permittivity of the composite samples: (a) PP28-F and (b) PP37-F.
electric field [29]. The phase transitions can also be seen clearly on the dielectric loss tangent curves for the PP28-F and PP37-F samples presented in Fig. 10. Plots show a sharper maximum at the TC in good agreement with the dielectric behavior of the materials with perovskite structures commonly known. For all samples, the dielectric losses are low at low temperatures (below phase transitions) and increase with increasing temperature. Comparing the two obtained composite materials we can say that the PP37-F sample at TC temperature exhibits lower values of dielectric losses than PP28-F. It can be a result of significant amount of pyrochlore phases in the PP37-F. In the ferroelectrics the broad hysteresis loops (PeE) and large
values of the remnant polarization PR are observed. In contrast relaxors exhibit so called slim loops and very low PR. Fig. 11a shows electric hysteresis loops for the PMN-PT ceramics for x ¼ 0.28 and 0.37 at room temperature. PMN-PT ceramics with tetragonal symmetry (x ¼ 0.37) has broader electric hysteresis loop (greater EC coercive field) compared with PMN-PT ceramic with rhombohedral symmetry (x ¼ 0.28). For the PMN-PT ceramic with x ¼ 0.37 the spontaneous polarization is PS ¼ 26.6 mC/cm2, remnant polarization is PR ¼ 22.30 mC/cm2 and coercive field EC ¼ 1.05 kV/mm. In case of the PMN-PT ceramic with x ¼ 0.28 this parameters are PS ¼ 26.56 mC/cm2, PR ¼ 21.15 mC/cm2 and EC ¼ 0.67 kV/mm respectively. Hysteresis loops for composite materials PP-F
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Fig. 10. Temperature dependencies of the dielectric losses tand for the composite materials: (a) PP28-F, (b) PP37-F (heating).
Fig. 11. PeE electric hysteresis loops (a) for the PMN-PT ceramic samples and (b) for the composite materials PP-F (at room temperature, frequency 1 Hz).
(Fig. 11b) exhibit similar trends, but with lower values of spontaneous polarization and remnant polarization, compared with the PMN-PT ceramic samples. It is a result that the polarization is a dipole moment divided by volume. In composites part of volume is non ferroelectric. The hysteresis loops of the ceramic composites PP-F are also narrower with lower values of the characteristic PS, PR and EC parameters (Table 1). PeE electric hysteresis loops at various temperatures for the composite materials PP-F are shown in Fig. 12. We think that the
changes of the shape of hysteresis loops are related with the change of remnant polarization and coercion field but also with the increase of electric conductivity [30]. 3.4. Electromechanical properties Fig. 13 shows the bipolar electric field-induced strain loops for the PMN-PT ceramics, and for the composite PP-F ceramics. The normalized strain coefficients (d33 ¼ S/E) measured using 0.5 kV/
Fig. 12. PeE electric hysteresis loops at various temperatures for composite materials: (a) PP28-F, (b) PP37-F (frequency 1 Hz).
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Fig. 13. Bipolar strain-electric field loops (SeE) for the PMN-PT ceramics (a) and for the composite materials PP-F (b) (at room temperature, frequency of 0.1 Hz, þ/ 3 kV/mm).
mm electric field and frequency 0.1 Hz are listed in Table 1. From Fig. 13 it is seen, that the loops have characteristic butterfly wings shape, typical for piezoelectric materials. The addition of ferrite explicitly degrades the piezoelectric properties, but the characteristic butterfly shape remains in all samples. The relatively high value of d33 coefficient for the composition of PP28-F is worth mentioning, comparable to the commercial PZT-5A ceramics [31].
4. Conclusion A new ferroelectriceferrimagnetic ceramic composite material 0.9PP-0.1F on the basis of the (1-x)PMN-xPT powder and nickelezinc ferrite powder has been obtained. Magnetic studies shown that the magnetic properties are very similar for various compositions of ferroelectric PP component. The shape of the M(T) curves is typical for a mixture of soft ferrimagnet (ferrite) and paramagnet (ferroelectric/relaxor). By designing this type of materials we can obtain ceramic composite materials simultaneously with magnetic properties and ferroelectric component (with suitable selection dielectric, ferroelectric, piezoelectric properties). Dielectric and ferroelectric parameters of the obtained PP-F ceramic composites are slightly inferior in comparison with their pure PMN-PT. The magnetic component (ferrite) influences on the structure of ceramic composites, which manifests itself in the reduction breadth region of the phase transition. In comparison with PMN-PT ceramic samples ferroelectric properties (polarization and the coercive field) are reduced. Relaxor ferroelectric-based materials with high dielectric and electromechanical properties can be used as materials for electromechanical actuators, high-power electromechanical transducers and sensors. New ferroelectromagnetic properties in materials of this type can be used to develop a new class of transducers/sensors integrating electroemagnetic interaction in a single device. The test of magnetoelectric coupling of the magnetic and electric subsystems for the composite samples will be the subject of the next article in future.
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Ferroelectric and magnetic properties of the PMN-PT-nickel zinc ferrite multiferroic ceramic composite materials Dariusz Bochenek a, *, Przemysław Niemiec a, Ryszard Skulski a, Artur Chrobak b, Paweł Wawrzała c University of Silesia, Institute of Technology and Mechatronics, 2, Sniez_ na St., Sosnowiec 41-200, Poland University of Silesia, Institute of Physics, 4, Uniwersytecka St., Katowice 40-007, Poland c Silesian University of Technology, Teachers College, 9-9A, Hutnicza St., Gliwice 44-100, Poland a
b
h i g h l i g h t s Multiferroic composites PMN-PT-ferrite have been obtained. Two compositions of PMN-PT were used to obtain composites. Ferroelectriceferromagnetic properties at room temperature were achieved. The shape of the M(T) curves is typical for this type composites.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 January 2015 Received in revised form 17 February 2015 Accepted 12 March 2015 Available online 18 March 2015
Multiferroic ceramic composites based on PMN-PT and NieZn ferrite have been obtained and described in presented work. PMN-PT powders were synthesized by solegel method while nickelezinc ferrite was obtained by classical ceramic method. Two compositions of PMN-PT were used i.e., 0.72PbMg1/3Nb2/3O30.28PbTiO3 with rhombohedral symmetry and 0.63PbMg1/3Nb2/3O3-0.37PbTiO3 with tetragonal one. In both cases the classical methods of calcination and final pressureless densification were used for obtaining of final ceramic composites. XRD, microstructure, EDS, dielectric, electrical and magnetic studies were performed for the obtained ceramic composite materials which confirmed ferroelectric and ferrimagnetic properties at room temperature. © 2015 Elsevier B.V. All rights reserved.
Keywords: Ceramics Sintering Microstructure Dielectric properties Magnetic properties
1. Introduction Solid solutions (1x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-PT) with perovskite structure exhibit very good ferroelectric/relaxor and piezoelectric/electrostrictive properties which depend on the chemical composition (x) and technological methods [1]. A continuous transition from relaxor to normal ferroelectric properties takes place with increasing x. Morphotropic region is observed for 0.25 < x < 0.35 [2]. With increasing x the maximum of the dielectric permittivity shifts from about Tm ¼ 3oC for x ¼ 0 to about Tm ¼ 227 C for x ¼ 0.5 [3]. Morphotropic region of PMN-PT was investigated also in works [4,5]. The properties of solid solution
* Corresponding author. E-mail address: [email protected] (D. Bochenek). http://dx.doi.org/10.1016/j.matchemphys.2015.03.025 0254-0584/© 2015 Elsevier B.V. All rights reserved.
PMN-PT make them useful for many applications for example for high frequency actuators and high power piezoelectric transducers etc. [6,7]. However, there are the problems during obtaining PMNPT ceramics related with arising of unwanted pyrochlore phase [8,9]. It is the result of high volatility of PbO at high temperatures and the differential reactivity between MgO and Nb2O5. The pyrochlore phase degrades the electrophysical properties. Using the solegel method we can decrease the volume of pyrochlore phase. Nickelezinc ferrite with composition Ni0.64Zn0.36Fe2O4 has the spinel-type structure and belongs to soft ferrites with a high value of magnetic permeability and high resistance r (105 Um) [10]. Devices obtained from this material are used for example for signal processing, filters and broad-band transformers. Mixing the upper described materials we have tried to obtain ceramic compositions which are multiferroic at room temperature
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and can be characterized by a magnetic response to a variable electric field, or inversely, a polarization change in the external magnetic field. Similar effect was reached for example in [11e13]. In general obtained are also multiphase composites with additional, for example, polymer phase (called ceramic-polymer composites) [14]. In the case of the ferroelectromagnetic materials, a lot of other techniques of powder synthesizing such as molten salt synthesis, reaction sintering, columbite, solegel and co-precipitation methods are used, besides the classical method [15]. The ferroelectric and mechanical properties, altogether with the degree of ferroelectric and ferromagnetic subsystems' coupling in this type materials are mainly related to the properties of each components of the solid solution and their percentage [16e18]. Recently for example there were described ceramic composites based on best known piezoelectric PZT and zinc ferrite [14,19] and PMN-PT-ferrite composites [20,21]. Below described materials which have been obtained as a combination of ferroelectric/relaxor component i.e., (1x)PbMg1/3Nb2/3O3-xPbTiO3 and ferrimagnetic component i.e., (Ni0.64Zn0.36Fe2O4) are an example of two component materials. 2. Experiment PMN-PT solid solution was obtained by solegel technology described in [8]. PMN was obtained as a result of the reaction between Mg(OC2H5)2 and Nb(OC2H5)5 in alcohol. In a second step lead acetate (II) and ethylene glycol were added to the mixture according to the formula: Pb(CH3COO)2 þ 1/3 Mg(Nb(OC2H5) 6)2 / Pb(Mg1/3Nb2/3)O2(OC2H5)2 þ 2CH3COOC2H5. PbTiO3 (PT) was obtained according to the formula: Pb(CH3COO)2 þ Ti(CH3CH2CH2O)4 / PbTiO2(CH3CH2CH2O)2 þ 2CH3COOC3H7. Then the two liquid compositions PMN and PT were mixed together in ethylene glycol (with proper proportions for x ¼ 0.28 and 0.37) to give the PMN-PT compound. Finally distilled water was added in order to begin hydrolysis of the obtained sol solution. After passing the sol to gel, it was dried and then ground and sintered at T ¼ 550 C/4 h in order to remove the organic parts. Dielectric and electromechanical properties of such obtained PMNPT samples were described in [8,22]. Nickelezinc ferrite Ni0.64Zn0.36Fe2O4 was obtained from the simple oxides Fe2O3, ZnO, NiO. The powders were mixed using Fritsch planetary ball mill for 15 h. The synthesis was conducted at conditions 1100 C/4 h using calcination technique. These two components were mixed in proportion 90% PMN-PT and 10% ferrite using Fritsch planetary ball mill for 15 h (wet in ethyl alcohol). After this the mixture was synthesized by calcination method with conditions: Tsynth ¼ 950 C, tsynth ¼ 8 h. Final pressureless densification (sintering) of composite ceramic samples was conducted at Ts ¼ 1100 C by ts ¼ 8 h. Two compositions i.e., 0.73PMN-0.28PT-ferrite (abbreviated to PP28-F) and 0.63PMN0.37PT-ferrite (abbreviated to PP37-F) were obtained. For electrical tests silver electrodes were applied on the both surfaces of the diskshaped samples using paste burning method. The X-ray tests at room temperature were performed using a Philips diffractometer (CuKa radiation). Microstructure, EDS (Energy Dispersive Spectrometry) and EPMA (Electron Probe Microbeam Analysis) tests were carried out using a scanning microscope HITACHI S-4700. Magnetic properties were investigated using SQUID (MPMS XL-7 Quantum Design) magnetometer within the temperatures range from 271 C to þ27 C and magnetic field up to 7 T. Dielectric and impedance measurements were performed using QuadTech 1920 LCR meter for a cycle of heating (at frequencies of the measurement field from 0.1 kHz to 1.0 MHz). Dielectric hysteresis loops PeE were investigated using SawyerTower circuit and a Matsusada Inc. Heops-5B6 precision high voltage amplifier. Electromechanical measurements were carried
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out using an optical displacement meter (Philtec Inc., D63) and high voltage amplifier Heops. Data were stored on a computer disc using an A/D, D/A transducer card and LabView computer program. 3. Results and discussion 3.1. Crystal structure and microstructure The density of 0.72PMN-0.28PT is 7.45 g/cm3, while the density of 0.63PMN-0.37PT is equal to 7.21 g/cm3. The densities of PP-F composite materials are similar (see Table 1). Fig. 1 shows X-ray diffraction patterns of powder PMN-PT after synthesis, the ferrite powder and PP-F powders at room temperature. For rhombohedral 0.72PMN-0.28PT the (200) line should be a single maximum (R3m space group), while for tetragonal 0.63PMN-0.37PT the (200) line should consist of two components (Pm and P4mm space groups). The X-ray analysis of obtained by us PMN-PT powders after synthesis exhibit rhombohedral symmetry for x ¼ 0.28 and tetragonal one for x ¼ 0.37. The XRD patterns of the synthesized powders of PMN-PT show lines belonging to perovskite phase with relatively small amount of pyrochlore phase. It is known that preparation of PMN using solegel method decreases the amount of unwanted pyrochlore phase [23]. Comparing the intensities of the reflections 29.08 (pyrochlore line 222) and of the reflections 31.2 (perovskite line 110) it has been stated than the amount of pyrochlore phase is higher for composite with x ¼ 0.37. X-ray diffraction pattern of ferrite powder Ni0.64Zn0.36Fe2O4 shows a single phase cubic spinel lines what stays in agreement with results of [24]. In case of the mixtures PP-F the X-ray the diffraction patterns we can see strong maxima originating from PMN-PT, as well as weak reflexes from the Ni0.64Zn0.36Fe2O4 ferrite. Since the synthesis of mixtures PP-F was performed by calcination method, the increase of the amount of the pyrochlore phase was observed and as a result for PP-F the amount of pyrochlore phase is higher than in PMN-PT without ferrite. In final compositions smaller amount of pyrochlore phase was observed for PP28-F (about 11%). Energy-dispersive X-ray spectroscopy EDS (surface and local analyses) confirmed the assumed chemical composition of PP-F and the presence of maxima from elements originating in PMN-PT and elements originating in ferrite (Fig. 2). The obtained EDS examinations are comparable to the assumed proportions of the initial components, calculated by stoichiometry, while obtaining the PMN-PT-ferrite material. In Table 2 (for PP28-F) and in Table 3 (for PP37-F) are summarized assumed and measured individual components of the composite samples. Fig. 3 presents microstructural SEM images of the ferrite ceramics (Fig. 3a), PMN-PT ceramic samples (Fig. 3b PP28 and Fig. 3c PP37), and fractured ceramic composite PP-F samples i.e., PP28-F
Table 1 Parameters of the PMN-PT ceramic samples and PP-F composite materials (dielectric properties for f ¼ 1 kHz, d33 for E ¼ 0.5 kV/mm and f ¼ 0.1 Hz).
r [g/cm ] 3
Tm [ C] εr εm tand at Tr tand at Tm PS [mC/cm2] PR [mC/cm2] EC [kV/mm] d33 [m/V] 1012
PMN-PT28
PMN-PT37
PP28-F
PP37-F
7.45 132 1860 6.290 0.025 0.024 25.10 21.00 0.67 400
7.21 187 1760 6090 0.021 0.032 25.50 22.16 1.04 457
7.46 123 1860 7.130 0.039 0.095 12.60 8.42 0.57 368
7.47 182 1150 5840 0.011 0.063 8.00 4.51 0.81 20
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D. Bochenek et al. / Materials Chemistry and Physics 157 (2015) 116e123 Table 2 Assumed and measured weight % and atom % quantity of the constituent components of the PP28-F composite sample. PP28-F
O Pb Ti Mg Nb Ni Zn Fe
Fig. 1. X-ray spectra of the PMN-PT, the ferrite and the PP-F ceramic composite materials.
and PP37-F (Fig. 3d and e respectively). PMN-PT ceramics characterized by grained microstructure when the microstructure of the ferrite sample has large grains. The microstructures of the PP28-F and PP37-F are characterized by rather small, well crystallized grains. Large grains of ferrite are surrounded by small
Assumption % at
Experimental %
Assumption % weight
Experimental % weight
59.7 18.0 5.0 4.3 8.6 0.9 0.5 2.9
60.8 16.1 4.9 3.5 9.2 1.3 1.1 3.1
16.0 60.1 3.9 1.7 12.9 1.2 0.7 3.6
18.3 55.5 4.5 1.6 13.5 1.5 1.3 3.8
grains of ferroelectric ceramic component what is clearly visible in Fig. 3e. These results were confirmed by additional research EDS and EPMA (dispersive X-ray energy spectroscopy) in the relevant areas of the surface of the PP-F samples. Examples of EPMA images for PP28-F and PP37-F composite samples are shown in Fig. 4. Fig. 4b and d represent EPMA images of the composite samples for the selected element e iron (Fe K) which is a component only in ferrite. The average size of grains is between 2 and 4 mm and the average grain size slightly decreases when the PMN/PT ratio changes from 72/28 to 63/37.
Fig. 2. EDS spectra for the PMN-PT ceramics, the ferrite and the PP-F ceramic composite materials.
D. Bochenek et al. / Materials Chemistry and Physics 157 (2015) 116e123 Table 3 Assumed and measured weight % and atom % quantity of the constituent components of the PP37-F composite sample. PP37-F
O Pb Ti Mg Nb Ni Zn Fe
Assumption % at
Experimental %
Assumption % weight
Experimental % weight
59.7 18.0 6.7 3.8 7.6 0.9 0.5 2.9
61.1 16.1 6.2 3.3 7.9 1.3 1.0 3.1
16.1 60.4 5.2 1.5 11.4 1.2 0.7 3.6
18.1 55.2 5.8 1.6 12.5 1.6 1.3 3.9
3.2. Magnetic properties Generally, at low temperatures the magnetic properties of the obtained PP-F composite ceramic samples are similar for various x. Fig. 5 shows the temperature dependences of mass magnetization M(T) for investigated compositions of PP-F. The M(T) curves were measured in FC mode (field-cooled) at magnetic field 0.1 T (1000 Oe). The small difference in the absolute values of the bulk magnetization is attributed to the mass content of the ferrite phase in the ceramic composite samples. The magnetic hysteresis loops for the two compositions of PP-F are very similar both at temperature 271 C as well as at temperature of 27 C (Fig. 6). Remnant magnetization MR is relatively low and little decreases with increasing temperature as well as saturation magnetization too. At the room temperature, the shapes of the hysteresis curves are typical for ferrimagnetic materials (ferrite) and the result of introduction of paramagnetic (ferroelectric/relaxor) component is similar like in [25,26] for bulk composites.
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3.3. Ferroelectric and dielectric properties The temperature dependences of dielectric permittivity (ε) measured at 1 kHz for the PMN-PT ceramics as well as PP-F ceramic composites are presented in Fig. 7. The maxima of the dielectric permittivity indicate phase transitions in the samples. PMN-PT ceramic samples with. x ¼ 0.28 and 0.37, show maxima at temperatures 132 C and 187 C, respectively, what correspond to the phase transition from the ferroelectric to the paraelectric (TC temperature). Dependences for PMN-PT ceramic samples and PP-F composite ceramic samples are presented in Fig. 7a. It is seen that the temperatures at which maxima of dielectric permittivity are shifted towards lower temperatures in comparison with PMN-PT without ferrite (see DT in Fig. 7a). In case of dielectric losses (Fig. 7b) these shifts are larger and the increase of dielectric losses in PP-F is observed. It can be related with many factors for example with stresses and defects on boundaries between grains of PMN-PT and grains of ferrite. The shift of the temperature of maximum towards lower temperatures as a result of hydrostatic pressure is described for example in work [27]. The shift of the maximum of dielectric permittivity connected with decrease of diffusion of the phase transition can also mean increased ordering of the structure of the composite. In the case of PPF ceramic composites maximum of dielectric permittivity occurs in a more narrow range of temperatures and does not depend on the frequency of the measuring field. The values of electric permittivity are higher for PP28-F as compared with PP37-F sample (at room temperature as well as the temperature of maximum). Above the phase transition temperature the ε(T) curves show anomaly (another maximum) with very broad peak of dielectric permittivity. Additionally, a strong frequency dependency is observed (in the same range of temperatures for two compositions), probably as a result of relaxation processes as well as a large increase in electric conductivity at high temperatures. These peaks shift towards higher temperatures with the increase of frequency (Fig. 8).
Fig. 3. SEM microstructure of the PMN-PT samples: (a) PP28, (b) PP37, nickelezinc ferrite sample (c) and composite samples: (d) PP28-F, (e) PP37-F.
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Fig. 4. EPMA images of the composite samples: (a, b) PP28-F, (c, d) PP37-F.
Fig. 5. Mass magnetization v.s., temperature (field-cooled mode FC) at magnetic field 0.1 T for PP-F composite samples.
The broad and high maxima of dielectric permittivity are also associated with overlapping of the dielectric PMN-PT and ferrite phases. Similar phenomena were observed for other ferroelectroeferrimagnetic materials for example PbFe1/2Nb1/2O3-ferrite [28]. Samples show lower values of maximum permittivity compared with their counterparts' pure PMN-PT ceramic samples. In order to investigate the relaxation phenomena in this material, the dielectric constant and loss tangent are presented as a function of frequency at different temperatures (Fig. 9). Generally, for ceramic composite samples of PP-F, the dielectric permittivity ε decreases with increasing frequency and it attains a constant value at high frequencies what is typical for a MaxwelleWagner type of interfacial polarization. The high values of ε observed at low frequencies can be attributed to the dipoles that result from changes in the valence states of the cations, and the space charge polarization due to the inhomogeneous microstructure (disorder regularities of the microstructure by the introduction of the ferrite grains). In polycrystalline samples, the role of inhomogeneities play impurities, pores, grains and grain boundaries. When the frequency increases, only the electronic polarization contributes to the total polarization what is caused by the fact, that dipoles of other types have low mobility and cannot follow the fast variation of the
Fig. 6. Magnetic hysteresis loops for composite materials: (a) PP28-F, (b) PP37-F.
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Fig. 7. Comparison of temperature dependencies of dielectric permittivity ε (a) and dielectric losses tand (b) for the PMN-PT28 and PMN-PT37 ceramic samples as well as for PP28-F and PP37-F ceramic composite samples (on heating, frequency 1 kHz).
Fig. 8. Temperature dependencies of electric permittivity ε for the ceramic composite samples: (a) PP28-F, (b) PP37-F (on heating).
Fig. 9. Frequency dependencies of dielectric permittivity of the composite samples: (a) PP28-F and (b) PP37-F.
electric field [29]. The phase transitions can also be seen clearly on the dielectric loss tangent curves for the PP28-F and PP37-F samples presented in Fig. 10. Plots show a sharper maximum at the TC in good agreement with the dielectric behavior of the materials with perovskite structures commonly known. For all samples, the dielectric losses are low at low temperatures (below phase transitions) and increase with increasing temperature. Comparing the two obtained composite materials we can say that the PP37-F sample at TC temperature exhibits lower values of dielectric losses than PP28-F. It can be a result of significant amount of pyrochlore phases in the PP37-F. In the ferroelectrics the broad hysteresis loops (PeE) and large
values of the remnant polarization PR are observed. In contrast relaxors exhibit so called slim loops and very low PR. Fig. 11a shows electric hysteresis loops for the PMN-PT ceramics for x ¼ 0.28 and 0.37 at room temperature. PMN-PT ceramics with tetragonal symmetry (x ¼ 0.37) has broader electric hysteresis loop (greater EC coercive field) compared with PMN-PT ceramic with rhombohedral symmetry (x ¼ 0.28). For the PMN-PT ceramic with x ¼ 0.37 the spontaneous polarization is PS ¼ 26.6 mC/cm2, remnant polarization is PR ¼ 22.30 mC/cm2 and coercive field EC ¼ 1.05 kV/mm. In case of the PMN-PT ceramic with x ¼ 0.28 this parameters are PS ¼ 26.56 mC/cm2, PR ¼ 21.15 mC/cm2 and EC ¼ 0.67 kV/mm respectively. Hysteresis loops for composite materials PP-F
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Fig. 10. Temperature dependencies of the dielectric losses tand for the composite materials: (a) PP28-F, (b) PP37-F (heating).
Fig. 11. PeE electric hysteresis loops (a) for the PMN-PT ceramic samples and (b) for the composite materials PP-F (at room temperature, frequency 1 Hz).
(Fig. 11b) exhibit similar trends, but with lower values of spontaneous polarization and remnant polarization, compared with the PMN-PT ceramic samples. It is a result that the polarization is a dipole moment divided by volume. In composites part of volume is non ferroelectric. The hysteresis loops of the ceramic composites PP-F are also narrower with lower values of the characteristic PS, PR and EC parameters (Table 1). PeE electric hysteresis loops at various temperatures for the composite materials PP-F are shown in Fig. 12. We think that the
changes of the shape of hysteresis loops are related with the change of remnant polarization and coercion field but also with the increase of electric conductivity [30]. 3.4. Electromechanical properties Fig. 13 shows the bipolar electric field-induced strain loops for the PMN-PT ceramics, and for the composite PP-F ceramics. The normalized strain coefficients (d33 ¼ S/E) measured using 0.5 kV/
Fig. 12. PeE electric hysteresis loops at various temperatures for composite materials: (a) PP28-F, (b) PP37-F (frequency 1 Hz).
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Fig. 13. Bipolar strain-electric field loops (SeE) for the PMN-PT ceramics (a) and for the composite materials PP-F (b) (at room temperature, frequency of 0.1 Hz, þ/ 3 kV/mm).
mm electric field and frequency 0.1 Hz are listed in Table 1. From Fig. 13 it is seen, that the loops have characteristic butterfly wings shape, typical for piezoelectric materials. The addition of ferrite explicitly degrades the piezoelectric properties, but the characteristic butterfly shape remains in all samples. The relatively high value of d33 coefficient for the composition of PP28-F is worth mentioning, comparable to the commercial PZT-5A ceramics [31].
4. Conclusion A new ferroelectriceferrimagnetic ceramic composite material 0.9PP-0.1F on the basis of the (1-x)PMN-xPT powder and nickelezinc ferrite powder has been obtained. Magnetic studies shown that the magnetic properties are very similar for various compositions of ferroelectric PP component. The shape of the M(T) curves is typical for a mixture of soft ferrimagnet (ferrite) and paramagnet (ferroelectric/relaxor). By designing this type of materials we can obtain ceramic composite materials simultaneously with magnetic properties and ferroelectric component (with suitable selection dielectric, ferroelectric, piezoelectric properties). Dielectric and ferroelectric parameters of the obtained PP-F ceramic composites are slightly inferior in comparison with their pure PMN-PT. The magnetic component (ferrite) influences on the structure of ceramic composites, which manifests itself in the reduction breadth region of the phase transition. In comparison with PMN-PT ceramic samples ferroelectric properties (polarization and the coercive field) are reduced. Relaxor ferroelectric-based materials with high dielectric and electromechanical properties can be used as materials for electromechanical actuators, high-power electromechanical transducers and sensors. New ferroelectromagnetic properties in materials of this type can be used to develop a new class of transducers/sensors integrating electroemagnetic interaction in a single device. The test of magnetoelectric coupling of the magnetic and electric subsystems for the composite samples will be the subject of the next article in future.
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