Lead-free Relaxor Ferroelectrics Research Articles

Stable Ferroelectric Behavior of Nb-Modified Bi0.5K0.5TiO3-Bi(Mg0.5Ti0.5)O3 Lead-Free Relaxor Ferroelectric Ceramics

Crystal structure, dielectric, ferroelectric, piezoelectric, and electric field-induced strain properties of lead-free Nb-modified 0.96Bi0.5K0.5TiO3-0.04Bi(Mg0.5Ti0.5)O3 (BKT-BMT) piezoelectric ceramics were investigated. Crystal structure analysis showed a gradual phase transition from tetragonal to pseudocubic phase with increasing Nb content. The optimal piezoelectric property of small-signal d 33 was enhanced up to ∼ 68 pC/N with a lower coercive field (E c) of ∼ 22 kV/cm and an improved remnant polarization (P r) of ∼ 13 μC/cm2 for x = 0.020. A relaxor-like behavior with a frequency-dependent Curie temperature T m was observed, and a high T m around 320°C was obtained in the investigated system. This study suggests that the ferroelectric properties of BKT-BMT was significantly improved by means of Nb substitution. The possible shift of depolarization temperature T d toward high temperature T m may have triggered the spontaneous relaxor to ferroelectric phase transition with long-range ferroelectric order without any traces of a nonergodic relaxor state in contradiction with Bi0.5Na0.5TiO3-based systems. The possible enhancement in ferroelectric and piezoelectric properties near the critical composition x = 0.020 may be attributed to the increased anharmonicity of lattice vibrations which may facilitate the observed phase transition from a low-symmetry tetragonal to a high-symmetry cubic phase with a decrease in the lattice anisotropy of an undoped sample. This highly flexible (at a unit cell level) narrow compositional range triggers the enhancement of d 33 and P r values.

Near room temperature bismuth and lithium co-substituted BaTiO3 relaxor ferroelectrics family

Eco-friendly lead-free relaxor ferroelectrics, Ba1-x (Bi0.5Li0.5)xTiO3 (BBLT) with x = 0.3, 0.4 and 0.5, have been synthesized by using the conventional solid-state reaction route. The crystal structure and phase purity were verified by x-ray diffraction (XRD) patterns using Rietveld refinement which revealed the presence of tetragonal (95–80%) and orthorhombic phases (5–20%) in the matrix. The temperature dependent dielectric constant and tangent loss dispersion spectra, slim ferroelectric-hysteresis, and nano-scale microstructures suggest the presence of short-range ordering (SRO)/polar nano-regions (PNRs) in the matrix. The dielectric spectra suggest that the dielectric dispersion increases with increase in Bi/Li concentrations which may be due to compositional fluctuations, ordering of cations at nano-scale and presence of metastable states of SRO and long-range ordering (LRO) regions. The nonlinear Vogel-Fulcher relationship is fitted for maximum dielectric constant temperatures (Tm) which provides activation energy and freezing temperature of all compositions. The current density (J) versus electric field (E) (J-E) data obey bulk-limited current conduction mechanisms that may be due to a diffusive displacement of Ti4+ at octahedral center, the creation of random fields due to in-homogeneity at A-site, and trap-assisted hoping of charge carriers under high E-fields.

Effects of (Bi0.5Na0.5)ZrO3 addition on relaxation behavior and electrical properties of the KNN–LS ceramics

[0.94K0.48Na0.52Nb–(0.06−x)LiSbO3–xBi0.5Na0.5ZrO3, x = 0, 0.01, 0.02, 0.03, 0.04, 0.05] lead-free piezoelectric ceramics were prepared by a traditional two-step solid-state reaction method. The effects of BNZ addition on phase structure, electrical properties and dielectric relaxation of KNN–LS ceramics were investigated in this paper. X-ray diffraction patterns exhibit that the ceramics have a homogeneous perovskite structure with no other phases. (Bi0.5Na0.5)ZrO3 doping can promote grain growth, increase density and enhance the electrical properties of the KNN–LS ceramics. When the addition amount (x) of BNZ is 0.04 mol, the ceramic exhibits optimum electrical properties: d33 = 286pC/N, Kp = 42%, er = 2400, tanδ = 0.044. The dielectric constant and diffusive phase coefficient of the ceramics increase with the increase of BNZ. This indicates that the ceramics are with excellent relaxation behavior. When x is 0.01 mol, the dispersion coefficient γ reaches the maximum (1.55605) and the curie temperature of the ceramics is up to 350 °C. These results confirm that the ceramics are with the potentialities to be applied in the high temperature lead-free relaxor ferroelectrics.

Peculiar properties of phase transitions in Na0.5Bi0.5TiO3−0.06BaTiO3 lead-free relaxor ferroelectrics seen via acoustic emission

Na[Formula: see text]Bi[Formula: see text]TiO3-0.06BaTiO3 relaxor ferroelectrics (RFEs) ceramic samples were investigated by means of dielectric and acoustic emission methods in the temperature range of 70–350[Formula: see text]C and in dependence on frequency. Dielectric curve exhibits two anomalies: strongly dispersive the local maximum, within 130–180[Formula: see text]C and slightly dispersive the smeared high-temperature maximum within 300–310[Formula: see text]C. Acoustic emission exhibits the five groups of bursts: near 123[Formula: see text]C, 150–180[Formula: see text]C, near 225[Formula: see text]C, 300–310[Formula: see text]C and 327[Formula: see text]C. The first and third groups of acoustic emission bursts correspond to phase transitions of R[Formula: see text]c-P[Formula: see text]bm phases coexisting in this temperature, range, while the fourth and fifth groups of acoustic emission bursts correspond to P[Formula: see text]bm-P[Formula: see text] /mbm-Pm[Formula: see text] m phase transitions. The second group of acoustic emission bursts corresponds to strongly dispersive local maximum at which no phase transition has been proved in Na[Formula: see text]Bi[Formula: see text]TiO3-0.06BaTiO3. The existence and the location of the intermediate temperature, [Formula: see text], in Na[Formula: see text]Bi[Formula: see text]TiO3-0.06BaTiO3 compound are discussed.

Giant electrostrictive effects of NaNbO3-BaTiO3 lead-free relaxor ferroelectrics

A giant electrostrictive effect was observed in (1 − x)NaNbO3-xBaTiO3 relaxor ferroelectric ceramics, which exhibit a high electrostrictive coefficient Q33 of ∼0.046 m4/C2 twice as large as those of Pb- and Bi-based perovskite relaxor ferroelectric ceramics. The theoretical analysis suggests that Q33 should be strongly correlated with chemical species of cations in a perovskite structure in which a strong ionic bond is of great benefit compared with a covalent bond. A hysteresis-free large electrostrictive strain of ∼0.148% up to at least 70 Hz was obtained in the x = 0.25 sample, demonstrating significant advantages over piezoelectric effects in high-precision ceramic actuators.

Visualization of polar nanoregions in lead-free relaxors via piezoresponse force microscopy in torsional dual AC resonance tracking mode.

Polar nanoregions (PNRs) play a key role in the functionality of relaxor ferroelectrics; however, visualizing them in lead-free relaxor ferroelectrics with high lateral resolution is still challenging. Thus, we studied herein the local ferroelectric domain distribution of the lead-free bismuth-based (1 -x)(Bi1/2Na1/2TiO3-Bi1/2K1/2TiO3) -x(Bi1/2Mg1/2TiO3) piezoceramics which show a relaxor behavior using dual AC resonance tracking (DART) piezoresponse force microscopy (PFM). By using excitation frequencies at either side of the contact resonance peak of the torsional cantilever vibration, an enhanced contrast in the amplitude and phase images of the piezoresponse can be achieved. Additionally, this tracking technique reduces the topographical crosstalk while mapping the local electromechanical properties. The true drive amplitude, drive phase, contact resonant frequency and quality factor can be estimated from DART-PFM data obtained with vertically or torsionally vibrating cantilevers. This procedure yields a three-dimensional quantitative map of the local piezoelectric properties of the relaxor ferroelectric samples. With this approach, torsional DART allowed for the visualization of fine substructures within the monodomains, suggesting the existence of PNRs in relaxor ferroelectrics. The domain structures of the PNRs were visualized with high precision, and the local electromechanical characteristics of the lead-free relaxor ferroelectrics were quantitatively mapped.

Effect of Ce doping on the electrocaloric effect of SrxBa1−xNb2O6 single crystals

The electrocaloric effect (ECE) of SrxBa(1−x)Nb2O6 (SBN100x) single crystals, with a tetragonal tungsten bronze structure and a high ECE near room temperature, is studied by direct measurements. It is shown that although the onset of the ECE peak is closer to room temperature in SBN80 than in SBN75, the effect of the increase of the strontium content is very detrimental to the ECE performances with a decrease to ΔTEC = 0.23 K for SBN80 from the reported value of ΔTEC = 0.42 K under 10 kV/cm for SBN75 [F. Le Goupil et al., “Anisotropy of the electrocaloric effect in lead-free relaxor ferroelectrics,” Adv. Energy Mater. (published online)]. However, when 1.40% of cerium is introduced in SBN61, the temperature of depolarisation is shifted below 30 °C, while an ECE above 0.6 K is maintained over more than 70 K for a low electric field of 28 kV/cm. The maximum ECE ΔTEC = 0.85 K is measured at 61 °C. In addition to having an ECE peak close to room temperature, the ECE measured in Ce-doped SBN61 is comparable with the best reported values for lead-free materials [Y. Bai et al., J. Appl. Phys. 110, 094103 (2011); X.-S. Qian et al., Adv. Funct. Mater. 24, 1300 (2014)], when linearly extrapolated to higher electric fields.

Soft modes and local structural transitions in Pb-free Ba(Ti0.8Zr0.2)O3-x (Ba0.7Ca0.3)TiO3 (x = 0.5): Pressure- and temperature-dependent Raman studies

We report our Raman studies of a new lead-free relaxor ferroelectrics, Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCTZO). The Raman modes of BCTZO are compared with those of BaTi0.8Zr0.2O3 (BTZO), and BaTiO3 (BTO). Also, they are compared with the eigenmodes of BTO calculated by using an ab-initio quantum-mechanical frozen-phonon method. The sharp mode at 321 cm-1 of BTO, reported as a coupled mode showing an interference effect, becomes progressively broader in BTZO and BCTZO. This behavior, together with a broadening of the 527-cm-1 mode, suggests that the mode-coupling is weakened in BTZO and BCTZO. The structural transitions of BCTZO were investigated as functions of pressure at pressures below 20 GPa and of temperature at temperatures below 600 K. Three characteristic pressure-induced transitions, on each at 2.5, 5.0, and 13.0 GPa, were found. The transitions are suggested by the drastic changes in phonon modes (two softening modes, one each at ~ 300 and ~ 530 cm-1) and by the transformation of the intensity profile. A temperature-induced transition was found at a Curie temperature of ~ 380 K, where the average structure changes from tetragonal to cubic. It is accompanied by a softening mode at ~ 530 cm-1. The phonon spectrum of BCTZO suggests that its local environment is close to that of BTZO. However, the characteristic pressures of BCTZO are close to those of BTO. The sequence of pressure-induced transitions in both BCTZO and BTZO illustrate rich interplay between the long-range averaged structure and the short-range local order such that four distinguishable phases are suggested: tetragonal, locally ordered but compensated cubic, disordered cubic, and ideal cubic. We found that the critical pressures are plausibly related to the average crystal lattice.

Research on dielectric tunability of relaxor ferroelectrics

Based on the high dielectric tunability, high dielectric constant and small dielectric loss at paraelectric phase, lead-free relaxor ferroelectrics have been used in microwave devises widely. According to the present theory of dielectric tunability, the expression of dielectric tunability is derived by adjusting parameters properly. The derived expression can be used to deal with experimental results. It is found that there exists a great difference between experimental result and the theoretical result obtained under the assumption of unchanged paraelectric phase under an electric field, while the experimental result and the theoretical result of Johnson in 1962 is consistent. A modified relation of dielectric constant with higher electric field is derived by considering the Gibbs free energy modified with an external electric field and a spontaneous polarization. The result from the modified equation is in agreement with the experimental result. A concept of degree of tunability is proposed to express quantitatively the relationship between dielectric tunability and concentration of dopants.

New Lead-Free Relaxor Ferroelectrics Derived from (K0.5Na0.5)NbO3 for High Temperature Applications

In order to develop high-temperature lead-free relaxor ferroelectrics, 0.95(K0.5Na0.5) NbO3–0.05Ba(Mg1/3Nb2/3)O3 (0.95KNN–0.05BMN) solid solution was synthesized by conventional solid-state sintering. The diffuse phase transition and the temperature of the maximum dielectric permittivity shifting towards higher temperature with increasing frequency, which are two typical characteristics for relaxor ferroelectrics, were observed in 0.95KNN–0.05BMN solid solution. The dielectric relaxor behavior obeys a modified Curie-Weiss law. The relaxor nature is attributed to the appearance of polar nano-regions. The results indicate that this material is indeed a lead-free relaxor ferroelectric with high Tm (∼200°C), useful for high temperature applications.

Synthesis and dielectric properties of FeNbTiO6 ceramics

Abstract A new lead-free relaxor ceramic FeNbTiO6 was first synthesized following a multicalcination procedure. The ferroelectric relaxor behavior and dielectric properties of FeNbTiO6 sample were systemically investigated. XRD patterns show that the material exhibits a single-phase with tetragonal rutile structure. Diffuse phase transition and frequency dispersion of the dielectric constant, which are two typical characteristics for relaxor ferroelectrics, are observed in FeNbTiO6 ceramic. Dielectric relaxor behavior obeys a modified the Curie–Weiss law. A remarkably good fit to the Vogel–Fulcher relation further supports such a relaxor nature. The results show that the FeNbTiO6 ceramic is one of the promising lead-free candidate relaxor ferroelectrics for high-temperature applications.

Phase structure, dielectric properties, and relaxor behavior of (K0.5Na0.5)NbO3–(Ba0.5Sr0.5)TiO3 lead-free solid solution for high temperature applications

The (1−x)(K0.5Na0.5)NbO3–x(Ba0.5Sr0.5)TiO3 (KNN-BST) solid solution has been synthesized by conventional solid-state sintering in order to search for the new lead-free relaxor ferroelectrics for high temperature applications. The phase structure, dielectric properties, and relaxor behavior of the (1−x)KNN-xBST solid solution are systematically investigated. The phase structure of the (1−x)KNN-xBST solid solution gradually changes from pure perovskite phase with an orthorhombic symmetry to the tetragonal symmetry, then to the pseudocubic phase, and to the cubic phase with increasing addition of BST. The 0.90KNN-0.10BST solid solution shows a broad dielectric peak with permittivity maximum near 2500 and low dielectric loss (<4%) in the temperature range of 100–250 °C. The result indicates that this material may have great potential for a variety of high temperature applications. The diffuse phase transition and the temperature of the maximum dielectric permittivity shifting toward higher temperature with increasing frequency, which are two typical characteristics for relaxor ferroelectrics, are observed in the (1−x)KNN-xBST solid solution. The dielectric relaxor behavior obeys a modified Curie–Weiss law and a Vogel–Fulcher relationship. The relaxor nature is attributed to the appearance of polar nanoregions owing to the formation of randon fields including local electric fields and elastic fields. These results confirm that the KNN-based relaxor ferroelectrics can be regarded as an alternative direction for the development of high temperature lead-free relaxor ferroelectrics.

High Tm lead-free relaxor ferroelectrics with broad temperature usage range: 0.04BiScO3−0.96(K0.5Na0.5)NbO3

In order to develop high temperature lead-free relaxors, xBiScO3−(1−x)(K0.5Na0.5)NbO3[xBS−(1−x)KNN] ceramics were proposed using a tolerance factor approach. To verify this proposal, xBS−(1−x)KNN ceramics were synthesized by conventional solid-state sintering. A stable perovskite phase was obtained when KNN content was greater than 96 mol %. The diffuse phase transition and frequency dispersion of the dielectric constant, which are two typical characteristics of relaxor ferroelectrics, were observed in xBS−(1−x)KNN ceramics. The dielectric relaxor behavior follows a modified Curie–Weiss law relationship. In addition, 0.04BS-0.96KNN ceramics show a broad and stable permittivity maximum near 2500 from 100 to 300 °C and lower dielectric loss (<5%) at broad temperature usage range (100–300 °C). The results indicate that this material may have great potential for high temperature capacitors in automobile applications.

Lead-Free Relaxor Ferroelectric Ceramics in NaNbO3-Sr0.5NbO3-LiNbO3 Solid Solution System

Ceramics of (1-x)NaNbO3-(x)Sr0.5NbO3 solid solutions exhibit strongly diffused permittivity maxima near the room temperature, but show weak frequency dispersion of permittivity and small frequency shift of the permittivity maximum temperature, T m . Addition of LiNbO3 greatly enhances the frequency dependence of both T m and permittivity values.

Lead-free relaxor ferroelectrics with ‘TTB’ structure

Preparation and dielectric characterisations of a great number of TTB-type ceramics have allowed us to show that some of them present relaxor behaviour, due to the occupation of the same crystallographic site by two different cations or anions. One of the cation in the octahedral site has to be ferroelectrically active. The relaxor effect is correlated to either cationic distribution disorder in the same site or dilution of the ferroelectric character.

Some Solid State Chemistry Aspects of Lead-Free Relaxor Ferroelectrics

Some solid state chemistry aspects are investigated in relaxor ferroelectrics of both perovskite and tetragonal tungsten bronze (TTB) structure. Various issues regarding composition, cationic charge, electronic configuration, and structure are discussed. The relaxor behavior appears when two different cations occupy the same crystallographic site. One of the cations in the octahedral site has to be ferroelectrically active. The relaxor effect is correlated to cationic distribution disorder in the same site, depending on composition and structure (perovskite or TTB).