Microstructure and multiferroic properties of BaTiO3/CoFe2O4 films on Al2O3/Pt substrates fabricated by electrophoretic deposition

BaTiO₃/CoFe₂O₄ bilayer films on high temperature resistant Al₂O₃/Pt substrates were fabricated using the electrophoretic deposition (EPD) method. Powder synthesis, suspension preparation and kinetics of deposition were investigated in detail. Two different sintering temperatures were tested and the resultant microstructure and properties of ceramic films were investigated. The composite films sintered at a temperature of 1200 °C/2 h yielded optimal microstructures and properties. The obtained bilayer films had a dense structure with no phase diffusion or passive layer. Besides to ferroelectric and magnetic properties, magnetoelectric (ME) coupling properties were also confirmed by the magnetic anisotropy and magnetic field induced polarization investigation. The composite films showed a increased P_s from 16.2 to 16.85 μC/cm² with variations ΔP_s of 4% when the magnetic field was applied in parallel direction and a decreased P_s from 16.2 to 14.9 μC/cm² with variations ΔP_s of ?8% when a normal direction field was applied.     

Enhancement of piezoelectric and ferroelectric performances in (Na0.85K0.15)0.5Bi0.5TiO3 films with BaTiO3 interlayers

In this work, in order to optimize the electrical performance of (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) thin films, 20 nm-thick BaTiO₃ (BTO) layer was utilized by deliberately coating in the NKBT film-substrate interface or in the NKBT film, i.e., BTO layers coated in sequence with NBKT layers. The BTO layer, especial coated in the NKBT film, was beneficial for crystallization process and more preferable to form a denser film morphology. The BTO-coated NKBT composite films exhibited much enhancement in electrical properties compared to the films without BTO layer. Accordingly, a high effective piezoelectric coefficient d₃₃^* of 75 pm/V and remnant polarization P_r of 22.1 μC/cm², as well as a low leakage current density of 1.2 × 10^?5 A/cm² were obtained in the 460 nm-thick composite film with BTO layers coated in the NBKT film. It meant that this kind of BTO-coated NKBT composite film could perform as a potential candidate for the lead-free piezoelectric applications. The observed enhancement in the electrical properties with the introduction of BTO layer could be mainly explained by the weakened influence of domain pinning in the film-electrode interface and grain boundaries due to the decreased strain in the film-electrode interface and better crystallinity in the highly (110)-oriented NKBT films, thereby enhancing motion of domain-walls.     

Effect of repeated composite sol infiltrations on the dielectric and piezoelectric properties of a Bi0.5(Na0.82K0.18)0.5TiO3 lead free thick film

Bi_0.5(Na_0.82K_0.18)_0.5TiO₃ lead free thick films have been produced using a combination of screen printing and subsequent infiltration of corresponding composite sol. Their structure, dielectric, ferroelectric and piezoelectric properties were investigated with variation in the number of composite sol infiltrations and the nanopowder loading in composite sol. Dielectric constant, remanent polarization, and piezoelectric coefficient have been shown to increase with increasing numbers of composite sol infiltration. Dielectric and ferroelectric properties of the thick films are found to be strongly dependent on the powder concentration of composite sols. The resulting 40 μm thick films infiltrated with 1.5 g/ml composite sols have maximum relative permittivity of 569 (at 10 kHz), remanent polarization of 21.3 μC/cm², coercive field of 80 kV/cm, and longitudinal effective piezoelectric coefficient d_33eff of 109 pm/V. The performance of these lead free piezoelectric thick films is comparable to the corresponding bulk ceramics.     

Effects of thickness on structures and electrical properties of K0.5Na0.5NbO3 thick films derived from polyvinylpyrrolidone-modified chemical solution

Lead-free ferroelectric K_0.5Na_0.5NbO₃ (KNN) films with different thicknesses were prepared by polyvinylpyrrolidone (PVP)-modified chemical solution deposition (CSD) method. The KNN films with thickness up to 4.9 μm were obtained by repeating deposition-heating process. All KNN thick films exhibit single perovskite phase and stronger (1 1 0) peak when annealed at 650 °C. The variation of dielectric constant with thickness indicates that there exists a critical thickness for the dielectric constant in the KNN films which should lie in 1.3–2.5 μm. The similar trend is observed for the ferroelectric and piezoelectric properties of KNN films. Both the remnant polarization P_r and the piezoelectric coefficient d₃₃ of KNN thick films increase with the film thickness and become saturated after the critical thickness.     

0-3 type magnetoelectric 0.94Na0.5Bi0.5TiO3-0.06BaTiO3:CoFe2O4 composite ceramics with a deferred thermal depolarization

Developing Na_0.5Bi_0.5TiO₃-based magnetoelectric (ME) coupling composites with higher depolarization temperature is highly valuable for the environment-friendly smart electronic devices. We have developed a new kind of 0-3 type 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃:xCoFe₂O₄ (NBTBT:xCFO, x = 0, 0.1, 0.2, 0.3) composite ceramics with a deferred depolarization temperature, together with an additional strong ME coupling of 9.2 mV/cm·Oe for the NBTBT:0.2CFO. The basic structure, ferroelectric/ferromagnetic properties, and the depolarization temperature of the NBTBT:xCFO composite ceramics were investigated. It was found that an enhancement of depolarization temperature (>25 °C) was obtained in these 0-3 type composites relative to the pure NBTBT ones (115 °C vs 90 °C). The mechanism of the enhanced depolarization temperature of the composites is discussed. The present results demonstrate that NBTBT:xCFO composites have great potential for ME devices.     

Low-temperature-sintered Bi0.5Na0.5TiO3-based lead-free ferroelectric ceramics with good piezoelectric properties

Li₂CO₃ has been used as a sintering aid for fabricating lead-free ferroelectric ceramic 0.93(Bi_0.5Na_0.5TiO₃)-0.07BaTiO₃. A small amount (0.5 wt%) of it can effectively lower the sintering temperature of the ceramic from 1200 °C to 980 °C. Unlike other low temperature-sintered ferroelectric ceramics, the ceramic retains its good dielectric and piezoelectric properties, giving a high dielectric constant (1570), low dielectric loss (4.8%) and large piezoelectric coefficient (180 pC/N). The “depolarization” temperature is also increased to 100 °C and the thermal stability of piezoelectricity is improved. Our results reveal that oxygen vacancies generated from the diffusion of the sintering aid into the lattices are crucial for realizing the low temperature sintering. Owing to the low sintering temperature and good dielectric and piezoelectric properties, the ceramics, especially of multilayered structure, should have great potential for practical applications.     

Magnetoelectric effect of Ni0.8Zn0.2Fe2O4/Sr0.5Ba0.5Nb2O6 composites

Novel magnetoelectric composites were prepared by incorporating the dispersed Ni_0.8Zn_0.2Fe₂O₄ ferromagnetic particles into Sr_0.5Ba_0.5Nb₂O₆ relaxor ferroelectric matrix. Dense composite ceramics were obtained with the co-presence of Sr_0.5Ba_0.5Nb₂O₆ and Ni_0.8Zn_0.2Fe₂O₄, and they could be electrically and magnetically poled to exhibit a significant magnetoelectric effect. A maximum magnetoelectric voltage coefficient of 26.6 mV/cm/Oe was observed from the composites with 70 mol% Sr_0.5Ba_0.5Nb₂O₆, which was much greater than that of magnetoelectric compounds and solid solutions. It is convenient to use the Pb-free (Sr,Ba)Nb₂O₆ relaxor as the matrix for ferromagnetic/ferroelectric composites in the future development of magnetoelectric materials.     

Synthesis and electrical properties of lead-free piezoelectric Bi0.5Na0.5TiO3 thin films prepared by Sol-Gel method

Lead-free Bi_0.5Na_0.5TiO₃ (BNT) piezoelectric thin films were deposited on Pt/TiO_x/SiO₂/Si substrates by Sol-Gel method. A dense and well crystallized thin film with a perovskite phase was obtained by annealing the film at 700 °C in a rapid thermal processing system. The relative dielectric constant and loss tangent at 12 kHz, of BNT thin film with 350 nm thickness, were 425 and 0.07, respectively. Ferroelectric hysteresis measurements indicated a remnant polarization value of 9 μC/cm² and a coercive field of 90 kV/cm. Piezoelectric measurements at the macroscopic level were also performed: a piezoelectric coefficient (d_33effmax) of 47 pm/V at E = 190 kV/cm was obtained. The piezoresponse force microscopy data confirmed that BNT thin films present ferroelectric and piezoelectric behavior at the nanoscale level.     

Textured (K0.5Na0.5)NbO3 ceramics prepared by screen-printing multilayer grain growth technique

The screen-printing multilayer grain growth (MLGG) technique is successfully applied to alkaline niobate lead-free piezoelectric ceramics. Highly textured (K_0.5Na_0.5)NbO₃ (KNN) ceramics with 〈0 0 1〉 orientation (f = 93%) were fabricated by MLGG technique with plate-like NaNbO₃ templates. The influence of sintering temperature on grain orientation and microstructure was studied. The textured KNN ceramics showed very high piezoelectric constant d₃₃ = 133 pC/N, and high electromechanical coupling factor k_p = 0.54. These properties were superior to those of conventional randomly oriented ceramics, and reach the level of those of textured KNN ceramic prepared by tape-casting technique. Compared with other grain orientation techniques, screen-printing is a simple, inexpensive and effective method to fabricate grain oriented lead-free piezoelectric ceramics.     

Magnetoelectric composite thick films of PZT–PMnN + NiZnFe2O4 by aerosol-deposition

Highly dense magnetoelectric composite films with 10 μm-thick of high piezoelectric voltage coefficient material, 0.9Pb(Zr₅₇Ti₄₃)O₃–0.1Pb(Mn_1/3Nb_2/3)O₃ (PZT–PMnN) and magnetostrictive material, Ni_0.8Zn_0.2Fe₂O₄ (NZF), were fabricated on a platinized Si substrate using aerosol deposition (AD). With increasing magnetic NZF content, dielectric and ferroelectric properties were gradually decreased while magnetizations were improved. The 20% NZF added composite thick film were found to exhibit the maximum ME coefficient. This optimal NZF content is the same as that of bulk ME composite materials. It is noticeable that AD can control the content ratio of ME composite films by controlling the powder composition. The fabricated ME composite films have high ME voltage coefficient coupling because of high density without severe inter-reactions of two phases.     

Dielectric,ferroelectric and field-induced strain behavior of K0.5Na0.5NbO3-modified Bi0.5(Na0.78K0.22)0.5TiO3 lead-free ceramics

Lead-free piezoelectric (1 ? x)Bi_0.5(Na_0.78K_0.22)_0.5TiO₃–xK_0.5Na_0.5NbO₃ (BNKT–xKNN, x = 0–0.10) ceramics were synthesized using a conventional, solid-state reaction method. The effect of KNN addition on BNKT ceramics was investigated through X-ray diffraction (XRD), dielectric, ferroelectric and electric field-induced strain characterizations. XRD revealed a pure perovskite phase with tetragonal symmetry in the studied composition range. As the KNN content increased, the depolarization temperature (T_d) as well as maximum dielectric constant (?_m) decreased. The addition of KNN destabilized the ferroelectric order of BNKT ceramics exhibiting a pinched-type hysteresis loop with low remnant polarization (11 μC/cm²) and small piezoelectric constant (27 pC/N) at 3 mol% KNN. As a result, at x = 0.03 a significant enhancement of 0.22% was observed in the electric field-induced strain, which corresponds to a normalized strain (S_max/E_max) of ～434 pm/V. This enhancement is attributed to the coexistence of ferroelectric and non-polar phases at room temperature.     

Effect of CuO addition on magnetic and electrical properties of Sr2Bi4Ti5O18 lead-free ferroelectric ceramics

The effect of CuO addition on magnetic and electrical properties of Sr₂Bi₄Ti₅O₁₈ (SBT) lead-free bismuth layered structure ferroelectric ceramics have been studied and reported. Interestingly, the prepared samples exhibit multiferroic behavior with the coexistence of magnetic and ferroelectric phase transition temperature. Magnetic phase transition with Neel׳s temperature (T_N) of 657 K is observed at 0.75 mol% of CuO added SBT ceramics, which is higher than the well known multiferroic BiFeO₃ (643 K) and the ferroelectric phase transition with Curie temperature (T_C) of 587 K is observed at 1 mol% of CuO added SBT ceramics, which is relatively higher than the reported pure SBT ceramics (558 K). Further, the electrical properties such as dielectric, ferroelectric, piezoelectric, leakage current density characteristics and optical properties were investigated as a function of x (x=0, 0.25, 0.5, 0.75 and 1 mol%). Presence of strong magnetic super-exchange interactions in CuO and the creation of oxygen vacancies play a vital role in the enhancement of magnetic and electrical properties of CuO doped SBT ceramics. Moreover, the present results indicate that, small amount (0.25 mol%) of CuO addition in SBT ceramics enhances the electrical properties significantly and vice versa, large amount (0.75 mol%) of CuO addition enhances the magnetic properties. Thus, the presence of magneto-electric coupling effect was observed in CuO doped Sr₂Bi₄Ti₅O₁₈ ferroelectric ceramics.     

Coexistence of three ferroelectric phases and enhanced piezoelectric properties in BaTiO3–CaHfO3 lead-free ceramics

Perovskite ferroelectrics possess the fascinating piezoelectric properties near a morphotropic phase boundary, attributing to a low energy barrier that the results in structural instability and easy polarization rotation. In this work, a new lead-free system of (1-x)BaTiO₃-xCaHfO₃ was designed, and characterized by a coexistence of ferroelectric rhombohedral-orthorhombic-tetragonal (R-O-T) phases. With the increase amount of CaHfO₃ (x), a stable coexistence region of three ferroelectric phases (R-O-T) exists at 0.06 ≤ x ≤ 0.08. Both large piezoelectric coefficient (d₃₃～400 pC/N), inverse piezoelectric coefficient (d₃₃^*～547 pm/V) and planar electromechanical coupling factor (k_p～58.2%) can be achieved for the composition with x = 0.08 near the coexistence of three ferroelectric phases. Our results show that the materials with the composition located at a region where the three ferroelectric R-O-T phases coexist would have the lowest energy barrier and thus greatly promote the polarization rotation, resulting in a strong piezoelectric response.     

Polarization-induced electro resistance and magneto resistance in La0.67Ca0.33MnO3/BaTiO3 composite film

La_0.67Ca_0.33MnO₃/BaTiO₃ composite films have been grown on Nb-doped SrTiO₃ substrates by the sol–gel method. The magnetic and ferroelectric properties in the composite films are investigated. A three-state memory is formed by applying a vertical electric field across the La_0.67Ca_0.33MnO₃/BaTiO₃ heterostructure, this behavior is attributed to the polarization-mediated resistive switching effect. In addition, the transport properties of La_0.67Ca_0.33MnO₃ thin film can be modulated by an external magnetic field, a 10.3 K shift of the metal insulator transition temperature is obtained with the change of applied magnetic field from 0 T to 6 T. Consequently, in La_0.67Ca_0.33MnO₃/BaTiO₃ heterostructure, the resistance behavior can be modulated by piezoelectric effect, ferroelectric polarization and magnetic field simultaneously.     

Raman and dielectric spectroscopic analysis of magnetic phase transition in Y(Fe0.5Cr0.5)O3 multiferroic ceramics

Here, we report the Raman and dielectric spectroscopic studies as a function of temperature of orthorhombically distorted Y(Fe_0.5Cr_0.5)O₃ (YFC) ceramics, measured from 80 to 300 K. The dc-magnetization measurements under field cooled (FC)-zero field cooled (ZFC) protocol indicate a small onset of magnetic ordering at T_N∼270 K. The field dependent magnetization plot recorded at 50 K, 150 K and 200 K show a clear opening in hysteresis loops. The linear dependence of magnetization plot at high field without any saturation of magnetization indicates the coexistence of weak ferromagnetic (WFM) component within the canting antiferromagnetic (CAFM) matrix. Temperature evolution of Raman line-shape parameter of B_2g(4) phonon mode clearly exhibits an anomalous behavior of phonon shift near T_N∼270 K, indicating the spin-phonon coupling in the ceramics. From the temperature dependent dielectric permittivity (ε(T)) study, two dielectric relaxation peaks are detected below 200 K and above 250 K. The appearance of former relaxation peak is responsible for polaronic conduction mechanism, while the later one is associated with magnetic phase transition which might be relevant to the presence of magnetoelectric coupling in YFC ceramics. The observed P-E hysteresis loops at room temperature indicate weak ferroelectric nature of the ceramics.     

Preparation of multiferroic composites of BaTiO3–Ni0.5Zn0.5Fe2O4 ceramics

Magneto-electric coupling in ceramic composites formed by ferroelectric and ferromagnetic phases can be obtained via an adequate mechanical coupling between the individual piezoelectric and magnetostrictive phases (product property). In the present work, the possibility of forming diphase ferroelectric–ferromagnetic ceramics has been investigated. Composites of xBaTiO₃–(1 − x)Ni_0.5Zn_0.5Fe₂O₄ with x = 0.5, 0.6 and 0.7 were prepared according two different procedures: (i) by direct mixing powders of perovskite BaTiO₃ and Ni_0.5Zn_0.5Fe₂O₄ spinel prepared by solid state and (ii) by coprecipitating Fe^III–Ni^II–Zn^II nitric salts in a NaOH solution in which the BaTiO₃ powders were previously dispersed. Optimum processing parameters for good homogeneity, densification and for a reduction of the chemical reactions at the interfaces ferroelectric-ferrite were found. A temperature and composition-dependent magnetic order is present in all the composites, with a dilution effect of the magnetisation due to the presence of the non-ferromagnetic phase. A diffuse ferroelectric–paraelectric transition due to the BaTiO₃ phase was identified by the temperature-dependence of the permittivity and losses, showing that at room temperature the material preserves a ferroelectric order. The interfaces play important roles in the dielectric properties, causing space charge effects and Maxwell–Wagner relaxation, particularly at low frequencies and high temperatures. The combined ferroelectric and magnetic ordering will result in magneto-electric coupling in this material; further investigations are necessary.     

Polarization and strain behaviors of 0.74BiNaTiO3–0.26SrTiO3/Bi0.5(Na0.8K0.2)0.5TiO3 ceramic composite

We investigated the temperature- and frequency-dependent polarization and strain of two bismuth-based perovskite materials, a matrix material and a seed material, with which we formed a composite whose properties we likewise investigated. The chosen matrix material is 0.74Bi_0.5Na_0.5TiO₃–0.26SrTiO₃ (BNT-ST) which has a transition point of ~65 °C, from the relaxor to the ferroelectric phase (T_R-F). The seed material was Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ (BNKT), which possesses a T_R-F of 120 °C. Different polarization and strain behaviors were observed in the BNT-ST/BNKT composite at different test temperatures. At T=25 °C (<T_R-F of the relaxor BNT-ST), the composite exhibited a hysteretic polarization loop and parabolic strain curves which involve an ergodic relaxor-to-normal ferroelectric phase transition with application of an external electric field and the reverse ferroelectric-to relaxor phase transition with removal of the field. When T=80 and 100 °C (>T_R-F °f the relaxor BNT-ST and <T_R-F of the ferroelectric BNKT), the BNT-ST/BNKT has a slim polarization loop and strain magnitudes that are slightly increased from those of pure BNT-ST. When T=120 °C (~T_R-F of the ferroelectric BNKT), the composite has a very slim polarization loop and strain behavior with values that are almost same as those of pure BNT-ST. In addition, the P-S relation for the BNT-ST/BNKT is identical to that of BNT-ST as the operating frequency increases up to 100 Hz. This may be because the polarization of BNT-ST is lower than that of BNKT. The electric field-induced polarization and strain of the BNT-ST/BNKT composite with respect to the temperature and frequency are related to the thermal stability of the ferroelectric seed and the degree of the phase transition in the relaxor matrix.     

Enhancement of multiferroic properties of Aurivillius Bi5Ti3FeO15 ceramics by Co doping

Multiferroic Bi₅Ti₃Fe_1−xCo_xO₁₅ (BFCT-x, where x = 0, 0.1, 0.3, 0.5, 0.7) ceramics were synthesized via a conventional solid-state reaction process and their microstructural, ferroelectric, magnetic and magnetoelectric coupling properties were investigated in detail. All samples show layered perovskite Aurivillius phase with an orthorhombic structure. The highest remanent polarization (2P_r) (35 μC/cm²) has been observed in BFCT-0 ceramic while the BFCT-0.3 ceramic shows the highest remanent magnetization (M_r) (0.13 emu/g) and magnetoelectric coefficient (11.47 mV cm⁻¹ Oe⁻¹). The enhancement of magnetic properties and the magnetoelectric coupling of these ceramics are attributed to the structural distortion caused by Co substitution which subsequently led to ferromagnetic interactions via the Dzyaloshinskii-Moriya interaction.     

Dielectric properties of some low-lead or lead-free perovskite-derived materials: Na0.5Bi0.5TiO3–PbZrO3, Na0.5Bi0.5TiO3–BiScO3 and Na0.5Bi0.5TiO3–BiFeO3 ceramics

Na_0.5Bi_0.5TiO₃ (NBT) and its modifications are known to be new lead-free ferroelectric materials and are promising for environment friendly devices. The systems under investigation were (i) NBT (trigonal/ferroelectric)–PbZrO₃ (orthorhombic/antiferroelectric); (ii) NBT (trigonal/ferroelectric)–BiScO₃ (trigonal/paraelectric); and (iii) NBT (trigonal/ferroelectric)–BiFeO₃ (trigonal/antiferromagnetic).The lattice parameters change as expected from the respective ionic radii values. For NBT–PZ, the dielectric permittivity shows a large frequency and temperature dispersion suggesting a relaxor-like behaviour dependent on the thermal annealing of the samples. For NBT–BS, the Curie temperature increases with BS content as well as the diffuseness of the phase transition, connected with the introduced disorder. For NBT–BF, the overall behaviour of the permittivity of NBT is maintained up to 50% BF but anomalies of the permittivity appeared close to 600 °C, which might be connected with the onset of magnetic influence for high BF content.