Ferroelectric-paraelectric Transition Temperature Research Articles

Direct measurement of electrocaloric effect based on multi-harmonic lock-in thermography

We report on a direct measurement method for electrocaloric effects, the heating/cooling upon application/removal of an electric field in dielectric materials, based on a lock-in thermography technique. By use of sinusoidal excitation and multi-harmonic detection, the actual temperature change can be measured by a single measurement in the frequency domain even when the electrocaloric effect shows a nonlinear response to the excitation field. We demonstrated the method by measuring the temperature dependence of the electric-field-induced temperature change in two Sr-doped BaTiO3 systems with different ferroelectric-paraelectric phase transition temperatures, where we introduce the procedure for extracting the pure electrocaloric contribution free from heat losses and Joule heating due to leakage currents. This method can be used irrespective of the type of dielectric material and enables simultaneous estimation of the polarization change and power dissipation during the application of an electric field, making it a convenient imaging measurement method for the electrocaloric effect.

Effect of Ni Substitution on Structural, Dielectric, and Ferroelectric Properties and Variation in Magnetocapacitance of Single-Phase Ba0.7Pb0.3TiO3 Ceramic

The present study demonstrates the influence of Ni substitution on the structural, dielectric, and ferroelectric properties of single-phase Pb-doped BaTiO3, that is, Ba0.7Pb0.3TiO3 (BPT) ceramic, and variations in the magnetocapacitance. The hydroxide co-precipitation method was employed to fabricate single-phase Pb-doped BaTiO3 and Ni-substituted Ba0.7Pb0.3TiO3, as Ba0.7Pb0.3TiO3-(x)Ni for x = 0.05, 0.1, and 0.15. Rietveld refinement reflects the single-phase tetragonal crystal structure of pure Ba0.7Pb0.3TiO3 and at higher Ni concentrations, the detection of rhombohedral crystal symmetry of the NiO phase confirms the biphasic structure of the composite. The ferroelectric-paraelectric transition temperature decreased from 274 °C for pure Ba0.7Pb0.3TiO3 to 162 °C for the composite with a higher Ni concentration of x = 0.15. A drastic change in the typical ferroelectric (P-E) loop of Ba0.7Pb0.3TiO3 was observed with the substitution of Ni. The composite with a higher concentration had the highest value of the magnetodielectric coupling coefficient, MD (%) = −2.86. This study reflects an easy and useful approach to achieve two-order parameter coupling with doping of acceptor ions in the ferroelectric phase. This kind of material substantially plays a vital role in the advanced electromagnetic devices for futuristic applications.

Role of B-site disorder in the properties of lead-free Na0.5Bi0.5(Mg1/3Nb2/3)O3 ceramic: A possible electrocaloric material with low leakage current

The electrocaloric (EC) effect in ferroelectric materials has been studied extensively to develop materials for solid-state cooling applications. The Na0.5Bi0.5TiO3-based ferroelectric material is selected for this particular application. The Ti4+ of the lead-free Na0.5Bi0.5TiO3 is completely replaced by (Mg1/3Nb2/3)4+ to enhance the dielectric breakdown strength and to observe the EC effect. The lead-free Na0.5Bi0.5(Mg1/3Nb2/3)O3 is synthesized via the columbite route with optimized processing conditions. The material shows a frequency-independent high ferroelectric–paraelectric transition temperature (460 °C) with a slim polarization vs electric field loop. The adiabatic temperature change (ΔT) of the compound is evaluated from the temperature-dependent polarization behavior, and calculated ΔT is ≈ 0.135 K at room temperature. The room-temperature EC strength and leakage current of the material are examined to check its suitability for solid-state refrigeration.

Enhanced Spontaneous Polarization by V4+ Substitution in a Lead-Free Perovskite CaMnTi2O6.

Spontaneous polarization (Ps) of novel order-disorder type lead-free ferroelectric CaMnTi2O6 was successfully enhanced by partial V4+ substitution for Ti4+. A synchrotron X-ray diffraction study revealed that the polar displacement of octahedrally coordinated (Ti, V) in CaMn(Ti1-xVx)2O6 (0 ≤ x ≤ 0.4) increases with V4+ substitution having Jahn-Teller activity owing to the d1 electronic configuration. Our magnetic study suggested the presence of antisite disorder between Ca2+ and square planar coordinated Mn2+ associated with Mn-V intermetallic charge transfer for x ≥ 0.4, resulting in decreases in spontaneous polarization and the ferroelectric-paraelectric transition temperature. This is the first report on the enhanced polarization owing to the Jahn-Teller distortion of V4+ without stereochemical Pb2+ or Bi3+.

Cascaded pyroelectric conversion: Optimizing the ferroelectric phase transition and electrical losses

The dramatic increase in the pyroelectric coefficient around the ferroelectric–paraelectric phase transition suggests high potential energy conversion efficiencies, but the reality is more complicated when thermal and electrical losses are considered. The performance of prototype mono-domain lead titanate thin films is simulated around phase transition using a phenomenological modeling approach. Thermodynamic properties are calculated using a modified Landau–Devonshire potential that is a function of temperature, applied electric field, and uniaxial tensile stress for bulk films. Significant performance enhancement near the ferroelectric–paraelectric phase transition is observed. However, increases in both the specific heat and the dielectric constant reduced the anticipated improvement. Critically, electrical losses during charging and discharging processes within the energy conversion cycle are included and calculated using the dielectric dissipation factor. Cascaded pyroelectric conversion cycles are considered where heat for each subsequent stage is provided by the previous stage, allowing for the segmentation of large temperature changes into multiple cycles. The implementation of a multi-stage or cascade approach could greatly increase the cycle efficiency over a large temperature range while utilizing lower, more realistic, electric field strengths. We show how each stage could be optimized through a secondary multi-caloric effect where the ferroelectric–paraelectric transition temperature is shifted using an applied biaxial stress. Loss tangents as low as 5% are shown to negate the benefits of cascading for high number of stages (n > 10). Using a stress-tuned optimum phase transition temperature for cascade stages provides roughly a 100% increase in thermal efficiency vs the unoptimized material for low electric field cycles.

Structural Tailing and Pyroelectric Energy Harvesting of P(VDF-TrFE) and P(VDF-TrFE-CTFE) Ferroelectric Polymer Blends.

The copolymer P(VDF-TrFE) is a normal ferroelectric because the bulky TrFE monomer improves its crystalline chain structure, while the terpolymer P(VDF-TrFE-CTFE) is a relaxor ferroelectric because the third monomer CTFE makes it amorphous. Herein, in order to induce a crystalline beta phase in the terpolymer, we blended a small amount of crystalline P(VDF-TrFE) into P(VDF-TrFE-CTFE) and investigated the effect of blending on the pyroelectric energy harvesting (PyEH) properties. The polarization-electric field hysteresis loops at different temperatures and energy densities were investigated. The PyEH energy density (ND) is compared with the electrical energy density (UE). The UE and ND at the ferroelectric–paraelectric transition temperature for the χ = 0.1 blend are reported as 3.18 and 5.04 J/cm3, respectively, which are higher than the other polymer blends. Interestingly, the ND of the χ = 0.9 blend is found to be 3.44 J/cm3 when operated at lower and higher temperatures, that is, at TL = 25 °C and TH = 40 °C, respectively, which is the highest possible energy density at the lowest possible transition temperature for the polymer blends.

Enhanced Electrostrictive Properties and Thermal Stability in Zn-Modified 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Ceramics

A 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 (PMNT) relaxor ferroelectric ceramic is a good dielectric and electrostrictive actuator material. Nevertheless, important engineering problems of further enhancing its electrostrictive effect and increasing its thermal stability must be solved to promote its applications. In this work, the Zn-doped PMNT (PMNT/xZn2+) ceramics were prepared by the niobite precursor synthesis method. The transition of the PMNT/xZn2+ ceramics from relaxor ferroelectric to normal ferroelectric was realized, increasing the residual polarization (Pr), saturation polarization (Pm), the ferroelectric-paraelectric transition temperature at emax (Tm) and electric field induced strain (S) from x = 0.0 (Pr = 5.3294 μC/cm2, Pm = 26.6690 μC/cm2, Tm = 45°C, S = 1.08‰) to x = 8.0 (Pr = 19.9005 μC/cm2, Pm = 30.3718 μC/cm2, Tm = 70°C, S = 1.42‰). In addition, the upper temperature limit of the materials with a large electrostrictive effect (greater than 1‰) was extended from 40°C to 80°C. The main reason for these improved properties is that the magnesium ions were replaced by the highly reactive zinc ions at the B-site of the ferroelectric perovskite structure during the sintering and formed a certain amount of component segregation that increased the size of the polar nanoregions/domains and improved the polarizability of the materials.

Analytical description of domain morphology and phase diagrams of ferroelectric nanoparticles

Analytical description of domain structure morphology and phase diagrams of ferroelectric nanoparticles is developed in the framework of Landau-Ginzburg-Devonshire approach. To model realistic conditions of incomplete screening of spontaneous polarization at the particle surface, it was considered covered by an ultra-thin layer of screening charge with effective screening length. The phase diagrams, calculated for spherical Sn2P2S6 nanoparticles in coordinates "temperature" and "effective screening length" by finite element modelling, demonstrate the emergence of poly-domain region at the tricritical point and its broadening with increasing the screening length for the particle radius over a critical value. Metastable and stable labyrinthine domain structures exist in Sn2P2S6 nanoparticles with radius (8–10) nm and more. We derived simple analytical expressions for the boundaries between paraelectric, single-domain and poly-domain ferroelectric phases, tricritical point and the necessary condition for the appearance of labyrinthine domains, and demonstrated their high accuracy in comparison with finite element modelling results.Analytical expressions for the dependence of the ferroelectric-paraelectric transition temperature on the particle radius in the single-domain and poly-domain states of the particle were compared with analogous dependences experimentally measured for SrBi2Ta2O9 nanoparticles and simulated for Sn2P2S6 nanocrystals by Monte Carlo method within the framework of axial next-nearest-neighbours Ising model. The analytical expression for the ferroelectric-paraelectric transition temperature in the poly-domain state quantitatively agrees with experimental and simulated results, and it accurately reproduces empirical Ishikawa equation at all temperatures, justifying it theoretically. Analytical description shows that phase diagrams and domain morphologies, which are qualitatively similar to the ones calculated in this work, can be expected in other ferroelectric nanoparticles covered by the screening charges, being rather different for the ferroelectrics with the first and second order ferroelectric-paraelectric transitions respectively.

Synthesis, structural and dielectric properties of SrBi2−xLaxNb2O9 ceramics prepared by hydrothermal treatment

SrBi2−xLa x Nb2O9 (x = 0.2, 0.4 and 0.6) Aurivillius materials were prepared by hydrothermal treatment. The powder prepared by this method is highly pure and not agglomerated. The morphology of the samples was characterized by SEM. The dielectric properties of all the compositions were investigated in the temperature range from 25 °C to 500 °C and in the frequency range between 100 Hz and 1 MHz. The dielectric properties at room temperature of the proposed materials can match up with La-doped SrBi2Nb2O9 ceramics prepared via the solid-state reaction method. Partial substitution of bismuth by lanthanum greatly affects the ferroelectric–paraelectric transition temperature, as the ferroelectric–paraelectric phase transition becomes diffuse and the Curie temperature shifts toward lower temperatures typically from 375 to 290 °C. The conductivity results obtained for the samples are explained taking into account the metal-binding energies.

Ferroelectric transitions in small particles

ABSTRACTIn this article, the experiments on growth of small ferroelectric-salt particles from their aqueous solutions are analyzed with the help of the Landau-Ginzburg-Devonshire theory of phase transitions. Contrary to the most common trend, the paraelectric-ferroelectric transition temperature is found to increase with the decreasing size of the particle. This is attributed to the electrocapillarity effect that is, decrease of the surface energy between the particle and its solution with the emergence of the charge on it. The theoretical analysis found that the electrocapillarity effect here is due to the depolarization field of the spontaneously polarized ferroelectric phase. Application of the theory to the experiments on quasi-static cooling and drying of aqueous solutions of ferroelectric salts resulted in quantitative explanation of the experiments. Suggestions are made regarding novel experiments, which may clarify properties of small ferroelectric-salt particles and help design new methods of their fabrication.

Strain‐Induced Structural Phase Transition and its Effect on Piezoelectric Properties of (BZT‐BCT)‐(CeO2) Ceramics

A strain‐induced structural phase transition and its subsequent effect on the piezoelectric properties of BZT‐BCT–x CeO2 (x = 0–0.1 wt%) ceramics have been reported in this manuscript. BZT‐BCT–x wt% CeO2 lead‐free piezoceramics were successfully been prepared using sol–gel method. X‐ray diffraction spectra of BZT‐BCT–x wt% CeO2 (x = 0–0.10 wt%) ceramics showed a purely single‐phase perovskite structure and a change in crystal structure between tetragonal and rhombohedral phases with increasing CeO2 content. The strain‐induced structural phase transition has resulted in excellent piezoelectric properties, d33 = 673 pC/N, kp ~ 56%, and Tc = 110°C for BZT‐BCT–0.08 wt% CeO2 ceramics. Greatly improved temperature stability of the piezoelectric properties has been observed over a temperature range between 20°C and 90°C. The ferroelectric–paraelectric transition temperature, Tc obtained in this study is 17°C higher than those reported earlier.

Effect of Na on the structural and electrical properties of lead-free sodium potassium niobate ceramics

K1-xNaxNbO3 (where x = 0.48, 0.53, 0.56 and 0.60) (KNN) perovskite structured ferroelectric ceramics were prepared by the solid-state reaction method. X-ray diffraction patterns indicate that single orthorhombic phase was formed for KNN. Dielectric anomalies around 200°C and 370°C were identified as ferroelectric-ferroelectric and ferroelectric-paraelectric transition temperatures for KNN compound. The electrical behavior of KNN ceramics was studied by the help of impedance spectroscopy technique in the different temperature range. The impedance response in KNN ceramics could be resolved into a single contribution, associated with the bulk/grain. The conductivity studies confirm that the activation energies are strongly frequency dependent. A decreasing trend of Z′ with rise in temperature suggests the presence of negative temperature coefficient of resistance (NTCR) in the material in the low frequency region but tends to merge in the high frequency region at almost all temperatures.

Metallic-like to nonmetallic transitions in a variety of heavily oxygen deficient ferroelectrics

The coupling between ferroelectric distortions and electron transport is an important factor in understanding ferroelectric/noncentrosymmetric materials with metallic conductivities and ferroelectric-based thermoelectrics. Here, multiple d0 ferroelectrics with a variety of crystal structures are doped via oxygen deficiency, resulting in metallic-like conduction in the paraelectric state. It is found that most of the studied systems show a metallic-like to nonmetallic transition near the paraelectric-ferroelectric transition. The metallic-like to nonmetallic transition temperature can be shifted using mechanisms that shift the paraelectric-ferroelectric transition temperature. It was found that the metallic-like to nonmetallic transition temperature could be shifted from 373 K to 273 K by varying (Ba1−xSrx)TiO3−δ from x = 0 to x = 0.3 and x = 1. The most probable mechanism for ferroelectric-electron transport coupling was determined to be Anderson localization associated with polarization with short-range order.

Investigation on impedance response, magnetic and ferroelectric properties of 0.20(Co1−xZnxFe2−yMnyO4)–0.80(Pb0.70Ca0.30TiO3) magnetoelectric composites

In the present work, we report on structural, dielectric, multiferroic, magnetoelectric properties and impedance spectroscopic studies of 0.20(Co1−xZnxFe2−yMnyO4)–0.80(Pb0.70Ca0.30TiO3) (x = 0.4, y = 0; x = 0, y = 0.2 and x = 0.4, y = 0.2) composites. The composites were prepared by metallo-organic decomposition (MOD) route. The coexistence of a cubic spinel phase and a tetragonal phase in the composites is confirmed by the X-ray diffraction measurement. Scanning electron micrographs show well distributed ferrite and ferroelectric phases in the composites. The dielectric properties of the composites have been studied in a wide range of frequency at different temperatures. The temperature dependent dielectric properties of the composites reveal that the ferroelectric–paraelectric transition temperature lies in the range of 499–546 °C. The impedance spectroscopic measurements of the composites indicate non-Debye type dielectric relaxation. The grain and grain boundary contributions in the composites have been separated using impedance spectroscopy analysis. The magnetization hysteresis (M−H) loop of the composites shows ferrimagnetic behavior with saturation magnetization values Ms ∼12–13.4 emu/g and coercive field Hc ∼71–273.5 Oe. The Curie temperature of the composites as determined from the inverse susceptibility vs. temperature curves is in the range of 297–480 °C. All the composites exhibit typical ferroelectric hysteresis loops indicating the presence of spontaneous polarization. A maximum value of magnetoelectric coupling co-efficient α ∼17.4 mV/cm Oe has been observed for the composite with x = 0.4 and y = 0.

Physical properties of bifunctional BST/LSMO nanocomposites

We report the fabrication of bifunctional nanocomposites consisting of ferroelectric Ba0.7Sr0.3TiO3 (BST) and ferromagnetic La0.67Sr0.33MnO3 (LSMO) at different concentrations via a high-temperature solid state route. The structural, dielectric, electrical, magnetodielectric (MD), magnetoelectric (ME) and magnetic properties of BST/LSMO nanocomposites were systematically investigated over a wide range of temperatures and frequencies. The X-Ray Diffraction analyses reveal the nanocrystalline nature of the heterostructures, wherein both perovskite phases co-exist. No parasitic phases were observed. The study of the dielectric properties shows that the nanocomposites exhibit relaxor ferroelectric character, with ferroelectric-paraelectric phase transition temperatures around 287–292 K that do not follow the Curie-Weiss law. The electrical measurements indicate that ac conductivities of the nanocomposites follow the Jonscher's universal power law, with activation energies of 0.42–0.63 eV based on Arrhenius-type behavior at high temperatures. The nanocomposites exhibit well-defined ferromagnetic hysteresis loops at room temperature (RT). The MD and ME measurements at RT indicate that BST/LSMO exhibits a nonlinear ME effect at low frequencies, with a threshold near 0.5 T. The magnetocapacitance (MCp) measurements evidence a quadratic dependence on magnetic field, further confirming the multiferroic nature of BST/LSMO. The order of MCp was found to be ∼7% per Tesla. The analysis of the MCp measurements indicates that one of the BST/LSMO compositions studied can be considered as a new multiferroic compound.

Tuning the functional properties of PMN-PT single crystals via doping and thermoelectrical treatments

Single crystals based on solid solutions of lead-magnesium-niobate (PMN) and lead titanate (PT) have emerged as highly promising multifunctional systems combining piezoelectric, pyroelectric, and electro-optic properties that surpass by far those of the best known lead-zirkonium-titanate ceramics. In this paper we present new findings on how the phase transition temperature and the dielectric and ferroelectric properties can be tuned depending on crystal composition, orientation, and thermoelectrical treatment. Mn-doped and pure 0.72PbMg1/3Nb2/3O3-0.28PbTiO3 (0.72PMN-0.28PT) single crystals with ⟨111⟩ and ⟨001⟩ orientations were investigated. A special attention was devoted to field cooling (FC), i.e., cooling under electric field from different temperatures. The results illustrate different findings that were not reported before: the Curie temperature, i.e., ferroelectric-paraelectric transition temperature, is enhanced after field cooling of the Mn-doped, ⟨001⟩-oriented crystal while such a shift is not observed in the ⟨111⟩-oriented and the non-doped crystals. In addition, substantial polarization suppression occurs in the Mn-doped crystals upon FC from high temperature regardless of orientation. Based on piezoforce microscopy of the domain structure that shows suppression of domain growth following field cooling from 200 °C, we propose a mechanism for polarization suppression based on domain pinning by charged defects. The practical importance of our results lies in showing the opportunity offered by a proper choice of crystal composition and poling conditions for tuning the functional properties of PMN-PT single crystals for a specific application. This should contribute to the understanding of their properties towards advanced sensor and transducers devices.

Magnetoelectric coupling in the paramagnetic state of a metal-organic framework

Although the magnetoelectric effects - the mutual control of electric polarization by magnetic fields and magnetism by electric fields, have been intensively studied in a large number of inorganic compounds and heterostructures, they have been rarely observed in organic materials. Here we demonstrate magnetoelectric coupling in a metal-organic framework [(CH3)2NH2]Mn(HCOO)3 which exhibits an order-disorder type of ferroelectricity below 185 K. The magnetic susceptibility starts to deviate from the Curie-Weiss law at the paraelectric-ferroelectric transition temperature, suggesting an enhancement of short-range magnetic correlation in the ferroelectric state. Electron spin resonance study further confirms that the magnetic state indeed changes following the ferroelectric phase transition. Inversely, the ferroelectric polarization can be improved by applying high magnetic fields. We interpret the magnetoelectric coupling in the paramagnetic state in the metal-organic framework as a consequence of the magnetoelastic effect that modifies both the superexchange interaction and the hydrogen bonding.

PFM characterization of (K0.5Na0.5)0.95La0.05(Nb0.9Ti0.05)O2.9 ceramics lead free

The results of ferroelectric properties studies of KNN doped with La and Ti and sintered at temperatures in the interval of 1100 °C–1190 °C are presented in this work. The doping was achieved by the substitution of La for ions in A sites and Ti for ions in the B sites. Values of 94 % of the theoretical density were accomplished. The effect of the sintering temperature and the inclusion of the La and Ti cations in the KNN structure is evident through the shift in the ferroelectric-paraelectric transition temperature of ∼110 °C with respect to that of pure KNN (420 °C). Microstructure and ferroelectric analyses were carried out using Piezoresponse Force Microscopy (PFM) and hysteresis loops with interesting results, ΔPr=9 (μC/cm2) and Pr/Pmax=0.41, even when the saturation of the materials is not reached.

Phase transition and temperature stability of piezoelectric properties in Mn-modified Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 ceramics

This study investigates the effect of two different Mn modifiers [MnO2 and Pb(Mn1/3Nb2/3)O3(PMnN)] on the of phase transitions in Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 ceramics. The temperature dependence of polarization derived from measured pyroelectric current indicated change in nature of phase transition with MnO2 doping. This phenomenon was supported by the temperature evolution of the linear softening of low lying hard lattice mode as revealed by Raman analysis. The grain size was found to increase with MnO2 doping (5X) while decrease with PMnN modification (0.5X). Interestingly, the piezoelectric constant of MnO2 modified composition showed negligible degradation (<1%) even after heat treatment very close to the ferroelectric-paraelectric transition temperature.

Characterization of nanostructured ceramics prepared by both high energy ball milling and fast firing sintering processes

High-energy ball milling technique was successfully applied to calcinated lead zirconate titanate (PZT 60/40) powders. After 20 h of ball milling, large PZT particles were completely broken down, reducing its initial size in three orders of magnitude. Experimental results show a huge sinterability enhancement of the PZT powders by using this technique, achieving its maximum sintering rate at ∼800°C. Relatively low densities (∼91%) were achieved in stoichiometric samples, while in 3% lead excess samples sintered at 950°C for 30, 45, 60, 90 and 120 min using a fast firing process and a post-annealing treatment at 800°C for 4 h, densities of ∼97% of the theoretical were achieved. PZT nanostructured ceramics prepared under optimized processing conditions (60 h of powder milling, 950°C of sintering temperature, 60 min of sintering time and a post-annealing process at 800°C during 4 h) show high dielectric constant (ε′) values (900) and low dielectric loss (tan δ) at room temperature and a ferroelectric-paraelectric transition temperature at 375°C.