Spontaneous relaxor to ferroelectric transition in lead-free relaxor piezoceramics and the role of point defects

The occupationally disordered structures and associated local polar fluctuations in lead-free relaxors determine their electrical properties that are also sensitive to external stimuli. These stimuli can lead to phase transitions, and the associated enhancement in the electro-mechanical responses necessitate a better understanding of these transitions. Here we report a non-canonical spontaneous phase transition from a relaxor to a ferroelectric phase in (Na_1/2Bi_1/2)TiO₃-BaTiO₃ with temperature. With the help of experiments (dielectric permittivity, diffraction, differential scanning calorimetry, polarization and Raman spectroscopy), a complete picture of the temperature evolution of relaxor behavior leading to this spontaneous phase transition has been reported. Furthermore, it has been shown that internal chemical pressure from oxygen vacancies can be utilized to tailor these phase transitions. Finally, an electric field-temperature phase diagram has been proposed with an emphasis on the influence of the defect chemistry. This work provides new insights into the origin of these spontaneous phase transitions.     

Preparation and investigation of structure,magnetic and dielectric properties of (BaFe11.9Al0.1O19)1-x - (BaTiO3)x bicomponent ceramics

(BaFe_11.9Al_0.1O₁₉)_1-x - (BaTiO₃)_x with x?=?0, 0.25, 0.5, 0.75 and 1 bicomponent ceramics has been prepared from single-phase compounds of BaFe_11.9Al_0.1O₁₉ (x?=?0) (BFO) and BaTiO₃ (x?=?1) (BTO) by a standard ceramic technique. The constituent materials have been chosen considering their perspective ferrimagnetic and ferroelectric properties, respectively for BFO and BTO. Moreover, Ba-hexaferrites are reported to exhibit ferroelectricity at room temperature as well, and the combination of two ferroelectric phases is of interest. Systematic investigations of the structural, magnetic and dielectrical properties versus chemical composition (x) have been performed. The ferrimagnetic phase transition temperature is almost independent of the BTO content, which is determined by intensity of the Fe³⁺-O^2--Fe³⁺ indirect superexchange interactions in the BFO hexaferrite phase. However, the coercivity of composite samples is lower due to the contribution of the microstructure-dependent shape anisotropy to the total magnetic anisotropy energy. The permittivity vs. temperature behavior confirmed the existence of two ferroelectric phase transitions corresponding to structural phase transitions in BTO at ~ 400?K and BFO at ~ 700?K. It has been observed that the dielectrical properties of composite samples, including the temperatures of the phase transitions, critically depended on concentration x which affects the composite microstructure. This behavior has been discussed in terms of microstructure analysis and such parameters as the grain size, porosity and density.     

Effect of mechanical stress on phase stability and polarization states in ferroelectric barium titanate and lead titanate

The effect of normal and shear stress on phase transitions in BaTiO₃ and PbTiO₃ has been investigated using a modified Landau–Ginzburg–Devonshire phenomenological model based on assumption of constant stress boundary conditions. Stress–temperature phase diagrams have been developed, and the influence of stress on polarization stability has been analyzed. The results show monoclinic phases with various polarization states absent in stress-free BaTiO₃ may exist under uniaxial, biaxial, anisotropic three-dimensional, and shear stress conditions. For PbTiO₃, our calculations show that, under normal stress new phases cannot be generated and the only stable ferroelectric phase has tetragonal symmetry, but under shear stress orthorhombic, rhombohedral, and monoclinic phases can be stabilized.     

Mechanical relaxation processes in polyacrylonitrile polymers and copolymers

The dynamic mechanical properties of molded and cast polyacrylonitrile and copolymers have been measured by an automatic Rheovibron. The transitions are described for the amorphous, “paracrystalline,” and crystalline phases. Based upon a number of polymer theories of multiple phases in polymeric systems, the transitions have been assigned to specific polymer motions. Three transitions are defined for the alpha transition region. The mechanical response is compared to x-ray diffraction and infrared to ascertain the transition assignments in decreasing order beginning with the highest temperature transition as (1) the main transition for the amorphous phase (T₁), (2) dipole–dipole interaction which is related to the “paracrystalline phase” (T₂), and (3) secondary transition for the amorphous phase (T₃).     

Impact of water film thickness on kinetic rate of mixed hydrate formation during injection of CO2 into CH4 hydrate

In this work, nonequilibrium thermodynamics and phase field theory (PFT) has been applied to study the kinetics of phase transitions associated with CO₂ injection into systems containing CH₄ hydrate, free CH₄ gas, and varying amounts of liquid water. The CH₄ hydrate was converted into either pure CO₂ or mixed CO₂?CH₄ hydrate to investigate the impact of two primary mechanisms governing the relevant phase transitions: solid‐state mass transport through hydrate and heat transfer away from the newly formed CO₂ hydrate. Experimentally proven dependence of kinetic conversion rate on the amount of available free pore water was investigated and successfully reproduced in our model systems. It was found that rate of conversion was directly proportional to the amount of liquid water initially surrounding the hydrate. When all of the liquid has been converted into either CO₂ or mixed CO₂?CH₄ hydrate, a much slower solid‐state mass transport becomes the dominant mechanism. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3944–3957, 2015     

Modeling the phase behavior of ternary systems ionic liquid + organic + CO2 with a Group Contribution Equation of State

This work presents the results of the use of a Group Contribution Equation of State (GC‐EOS) to model experimental data obtained for ternary systems of the type bmim[BF₄] + organic solute + CO₂ with four different organic compounds, namely acetophenone, 1‐phenylethanol, 4‐isobutylacetophenone, and 1‐(4‐isobutylphenyl)‐ethanol. Our results show that the GC‐EOS is able to qualitatively predict not only L+V→L but also L₁+L₂→L phase transitions. As the two two‐phase boundaries L+V and L₁+L₂ of the experimentally found three‐phase region L₁+L₂+V almost coincide with the saturated vapor pressure curve of pure CO₂, the phase transitions L+V→L₁+L₂+V and L₁+L₂+V→L₁+L₂ have been represented as this vapor‐pressure curve by the model. The average absolute deviations between experimental and predicted values for all phase transitions have been found to be very satisfactory. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Phase transitions and electrical characterizations of (K0.5Na0.5)2x(Sr0.6Ba0.4)5−xNb10O30 (KNSBN) ceramics with ‘unfilled’ and ‘filled’ tetragonal tungsten–bronze (TTB) crystal structure

Alkali-doped strontium barium niobate (K_0.5Na_0.5)_2x(Sr_0.6Ba_0.4)_5?xNb₁₀O₃₀ (KNSBN) ceramics has been prepared by a conventional solid-state reaction method. The alkali-dopant concentration x has been varied from 0.24 to 1.15 so that the crystal structure was transformed from ‘unfilled’ to ‘filled’ tetragonal tungsten–bronze (TTB) structure. Apart from the change in the structural properties, the effects of the alkali-dopants on the phase transition as well as ferroelectric, piezoelectric and pyroelectric properties have also been investigated. Phase transitions have been studied in the temperature range of ?200 °C to 350 °C. The origins of these phase transitions are discussed. The addition of the alkali-dopants enhances the ferroelectric, piezoelectric and pyroelectric properties of the KNSBN ceramics. Alkali-doping also favors abnormal grain growth and thus results in a porous microstructure, which might contribute to the enhancement of the pyroelectric performance.     

PVT-studies on polymeric and oligomeric compounds

Summary The pressure dependence of transitions in perfluorododecyloctane-F12H8-and in perfluorododecylhexadecane-F12H16)-has been measured from 10 to 200 MPa, from room temperature up to 220° C. The two transitions within this range-the transition of the alkane block from crystalline to mesomorphic, T_ss, and the transition to melt, T_m-exhibit different pressure coefficients. As a pecularity, for F12H16 T_ss and T_m superimpose at 10 MPa, in accordance to atmospheric pressure DSC data. Pressure dependences of transitions are given as well as their melting enthalpies.It has been shown, that PVT-measurements open up the possibility to gain new insight into the nature of certain phase transitions. A transition formerly to be thought of as a melting transition, turned out to be actually composed of two contributions, one due to disordering and the other due to melting.     

Phase transition and piezoelectric properties of lead-free (Bi1/2Na1/2)TiO3–BaTiO3 ceramics

(1−x)(Bi_0.5Na_0.5)TiO₃–xBaTiO₃ ceramics (x=0.03, 0.06, 0.08, 0.12, 0.15, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, and 0.90) were fabricated by a conventional solid-state reaction and their phase transitions and piezoelectric properties were investigated using XRD analyses, Curie temperature, the frequency dependance of dielectric constant, and P–E curves. A complete solid solution with a perovskite structure was formed in the whole composition range of BNT–BT and more than two phase transitions arising from a compositional change were found. With increasing BaTiO₃ content, the sequence of phase transitions from a rhombohedral structure to unknown tetragonal structures and finally a tetragonal structure with the space group P4mm has been established. While the rhombohedral–tetragonal phase transition led to superior piezoelectric properties in BNT–BT ceramics, other phase transitions between tetragonal structures had a little effect on piezoelectric properties. The d₃₃, ε_r, k_p, k_t, and Q_m and values lied in ranges of 40–130 pC/N, 400–900, 10–30%, 15–30%, and 50–250, respectively.     

Ultrasonic and piezoelectric properties of the BT–LMT ceramic system

Ultrasonic and piezoelectric studies of lead free ceramic system structures of (1 − x)BaTiO₃–xLa(Mg_1/2Ti_1/2)O₃ (BT–LMT) are presented in this contribution. It was shown, that such materials with x = 0.025, 0.05, 0.075 and 0.1 LMT content undergo phase transitions and exhibit piezoelectric effect in the low temperature phases. Anomalies of ultrasonic velocity and attenuation at phase transitions have been observed in these materials. Measurements of temperature dependencies of ultrasonic velocity and attenuation revealed anomalies related to phase transitions in these materials. It was shown that, at room temperature and under dc bias electric field, these ceramics behave as a piezoelectrics because of electrostriction.     

Anisotropic field induced phase transitions and negative electrocaloric effect in rhombohedral Mn doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

The electrocaloric effect (ECE) of Mn doped Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT:Mn) single crystals with particular emphasis on the impact of crystallographic orientations and phase transitions were investigated systematically. Orientation-dependent phase transitions have been demonstrated by the dielectric and strain behaviors. Intriguingly, the negative ECE of –0.02?°C and –0.002?°C were obtained firstly in [001]-oriented PIN-PMN-PT:Mn crystals near the rhombohedral→tetragonal phase transformation and in [011]-oriented crystals near the rhombohedral→orthorhombic phase transformation, respectively. However, only the positive ECE was found in [111]-oriented crystals near the tetragonal→rhombohedral phase transition. Additionally, the maximum ECE temperature changes calculated in [001]-, [011]- and [111]-oriented crystals were 0.33?°C, 0.46?°C and 0.38?°C, respectively. Our results suggest that the negative ECE is attributed to electric field-induced phase transitions, whose critical field decreases with the increase of temperature. The phase transition-mediated coexistence of positive and negative effects in the relaxor-ferroelectric single crystals is beneficial to enhance the efficiency of the solid-state cooling devices.     

Field-induced antiferroelectric to ferroelectric transitions in (Pb1–xLax)(Zr0.90Ti0.10)1–x/4O3 investigated by in situ X-ray diffraction

Phase transitions and field-induced preferred orientation in (Pb_1-xLa_x)(Zr_0.90Ti_0.10)_1–x/4O₃ (PLZT x/90/10) ceramics upon electric field cycling using in situ X-ray diffraction were studied. The evolution of the {200}_pc and {111}_pc diffraction line profiles indicate that PLZT 4/90/10 and PLZT 3/90/10 compositions undergo an antiferroelectric (AFE)–ferroelectric (FE) phase switching. Both PLZT 4/90/10 and PLZT 3/90/10 exhibit irreversible preferred orientation after experiencing the field-induced AFE-to-FE phase switching. An electric field-induced structure develops in both compositions which has a reversible character during the field decreasing in PLZT 4/90/10 and an irreversible character in PLZT 3/90/10. In addition, structural analysis of pre-poled PLZT 3/90/10 ceramics show that it is possible to induce consecutive FE-to-AFE and AFE-to-FE transitions when fields of reversed polarity are applied in sequence. The field range required to induce the AFE phase is broad, and the phase transition is kinetically slow. This kind of transition has rarely been reported before.     

Impedance spectroscopy analysis of Pb5Al3F19

The ac conductivity of Pb₅Al₃F₁₉ between 293 and 743 K has been analyzed within a combined complex impedance, modulus and permittivity formalism. The antiferroelectric phase IV to ferroelastic phase III, and the phase III to paraelastic phase II, transitions have been characterized; the latter is shown to be accompanied by higher F⁻ ion mobility above T=368±5 K. A new dielectric anomaly observed iso-chronously in ε′ (f,T) at 670 K corresponds to an expected, but hitherto unobserved, transition from phase II to the paraelectric prototype phase I of Pb₅Al₃F₁₉.     

Phase behaviour of poly(benzophenoneimide)s having n-alkyloxymethyl side chains

A series of poly(benzophenoneimide)s (C_m-BP-PIs) having n-alkyloxymethyl side chains (−CH₂O-n-CmH_2m+1, m = 4, 6, 8) have been examined by X-ray diffraction, differential scanning calorimetry (DSC) and polarizing optical microscopy. The samples showed basically two phase transitions and both transition temperatures decreased with increasing side chain length. The first transitions were ascribed to glass transitions and the second ones were assigned to liquid crystal-to-isotropic transitions. Eicosane (n-C₂₀H₄₂) which is chemically very similar to the side chain was miscible with the side chains of C₈-BP-PI, which induced depression of the glass transition temperature. Received: 17 February 1997/Revised: 1 April 1997/Accepted: 4 April 1997     

Polymorphic Phase Transitions in Nanocrystalline Binary Metal Oxides

Binary metal oxides occur in different polymorphic states under applied pressure and temperature. Structural changes occur due to polymorphic transitions in binary metal oxides. It is essential to theoretically predict the conditions of polymorphic transitions so that materials can be effectively used in engineering applications. Temperature and pressure are the two main factors affecting the bulk state phase transformation of materials. For nanomaterials, it has been observed that particle size and temperature are the main factors affecting the phase transformation, e.g., γ‐Fe₂O₃ to α‐Fe₂O₃, monoclinic to orthorhombic transformation in MoO₃, anatase to rutile transformation in Titania, γ to α Alumina transformation. We compile from literature the main factors which affect the phase stability of a nanocrystalline binary metal oxide. A heuristic approach to formulate particle size is put forth. Factors like surface energy, surface tension, and particle shape are considered, and a value for critical particle size is formulated. The model fits well with the experimental results for nanocrystalline alumina, titania, zirconia, and Fe₂O₃. Such an approach can be applied to predict the particle size‐dependent stability of a phase at known temperature range.     

Catalytic behavior of BiPO4 in the multicomponent bismuth phosphate system on the propylene ammoxidation

We found that the multicomponent bismuth phosphate (MBP) with multiphase activates catalytically the ammoxidation reaction of propylene and consists of mainly BiPO₄ and metal molybdates, etc. The MBP has been characterized by several techniques such as DTA, XRD, and BET surface area and total pore volume measurements. The DTA curve clearly shows that there are phase transitions with sharp breaks of the curve. The specific surface area and total pore volume decrease with increasing of the BiPO₄ phase content, showing that the BiPO₄ phase remains essentially on the catalyst surface and blocks the active sites and catalyst pores. It would mean that the catalytic behavior in the MBP depends on the catalyst surface covered with the BiPO₄ phase adequately. The pathway of the selective oxidation due to the controlled amounts of the BiPO₄ phase favors the formation of acrylonitrile. The improvement of catalytic performances of the mixture of phases is explained by the synergy effect.     

Dielectric and impedance spectroscopic study of lithium doped potassium tantalum niobium oxide

The (K_0.90Li_0.10) (Nb_0.80Ta_0.20)_0.99Mn_0.01O₃ (KLTN) ceramic has been synthesized by the conventional solid-state reaction route. The Rietveld refinement X-Ray diffraction (XRD) patterns confirmed the single-phase orthorhombic crystal structure with space group Amm2. The frequency dependent electrical properties were examined by the complex dielectric and impedance spectroscopy in the temperature range 30 °C–500 °C where multiple structural phase transition was observed with high dielectric constant, low tangent loss and well saturated electric polarization. The shifting of the ferroelectric phase transition temperature by 30 °C in heating and cooling mode suggests the irreversible motion of the domains and domain walls and significant effects on grain boundaries on structural phase transition temperature. A series of phase transitions from orthorhombic to tetragonal (∼190 °C) and tetragonal to Cubic (∼390 °C in cooling and 420 °C in heating) have been obtained in the wide range of temperature. The different type of analogy such as Modulus formalism, complex impedance spectra, frequency dependent conductivity, the activation energy of charge carriers has been used to understand the microstructure-electrical properties relation.     

Partial miscibility in the system poly(para-chlorostyrene-co-ortho-chlorostyrene)/poly(2,6-dimethyl-1,4-phenylene oxide)

The compatibility of random copolymers of para-chlorostyrene and ortho-chlorostyrene (PO copolymers) with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) has been studied by differential scanning calorimetry (d.s.c.). Blends prepared by compression moulding of coprecipitated powders display either one or two glass transitions, dependent on the composition of the copolymer component of the blend. PO copolymers of para-chlorostyrene content from 23 to 64% are miscible with PPO in all proportions, using the customary criteria of a single calorimetric glass relaxation and optical clarity. Both homopolymers poly(para-chlorostyrene) (P_pClS) and poly(ortho-chlorostyrene) (P_oClS) are found to be incompatible with PPO; such blends exhibit two glass transitions at temperatures characteristic of the pure component phases. All compatible PO-PPO blends undergo phase separation upon annealing at elevated temperatures, indicating that a lower critical solution temperature (LCST) must exist. The phase separation is found to be reversible by annealing below the LCST, at temperatures which are still above the glass transitions of both blend components.     

Temperature and Composition‐Induced Structural Transitions in Bi1−xLa(Pr)xFeO3 Ceramics

Bi_1?xLa_xFeO₃ and Bi_1?xPr_xFeO₃ ceramics of the compositions near the morphotropic phase boundary have been studied by X‐ray and neutron diffraction techniques and differential thermal analysis. The structural phases characterized by the long‐range polar, antipolar, and nonpolar ordering as well as the phase coexistence regions have been identified using the diffraction data depending on the dopant concentration and temperature. As a result of these studies the three phase region has been observed for the Pr‐doped compounds and the phase diagrams have been constructed. The detailed evolution of the structural parameters permitted to itemize the factors affecting the structural phase transitions and clarify the origin of the enhanced electromechanical properties in these materials. The performed structural analysis disclosed different character of the chemical bonds in the La‐ and Pr‐doped BiFeO₃ compounds. Further, the role of the rare‐earth ions in the covalency of the chemical bonds is discussed.     

Shock-induced phase transitions of C70 fullerite

Shock-induced phase transitions of C₇₀ fullerite were studied at the pressure range up to 52 GPa with use of recovery assemblies of planar geometry. The starting material consists of two crystalline phases: phase with hexagonal close-packed (HCP) and phase with rhombohedral structure. We have found that C₇₀ fullerite undergoes a series of polymorphic phase transitions in conditions of step-like shock-wave compression. In the specimens, recovered from 9, 14 and 19 GPa, a dominant phase was fullerite C₇₀ with cubic structure. Also, some amount of C₇₀ with HCP structure was observed. The quantity of HCP phase was decreasing with increasing of intensity of shock loading. With further growth of shock pressure, destruction of C₇₀ molecules occurs. In the samples, recovered after shock loading, the main phase was graphite with a low degree of three-dimensional regularity.