Thermodynamics of Ferroelectric Solid Solutions with Morphotropic Phase Boundaries

A thermodynamic analysis is presented for the diffusionless phase diagrams of ferroelectric solid solutions that display a morphotropic phase boundary (MPB) separating adjacent tetragonal and rhombohedral phases. Equations are developed for the shape of the MPB, the locations of triple and tricritical points, and for the line along which the anisotropy of polarization vanishes. The appearance of lower symmetry orthorhombic and monoclinic phases is considered and the topologies of energy surfaces in the region of the phase diagram where these phases may stabilize are illustrated. The theory is applied to the solid solution of lead zirconate with lead titanate (PZT) and relationships between polar anisotropy and the transformation strain, dielectric susceptibility and piezoelectric properties, are discussed. The analysis is used to reproduce phase boundary lines for solid solutions of lead titanate with lead magnesium niobate (PMN‐PT) and lead zinc niobate (PZN‐PT) and composition–temperature diagrams along isopleths in the ternary system PMN‐PZT are estimated. The anisotropies of polarization in solid solutions based on lead titanate and barium titanate are contrasted. The results provide a thermodynamic framework useful for guiding experimental investigations of ferroelectric solid solutions and for generating energy functions used in constitutive modeling and phase field simulations of microstructure and properties.     

Detection of Indentation Induced FE-to-AFE Phase Transformation in Lead Zirconate Titanate

Instrumented indentation was combined with microscopy and spectroscopy analysis to investigate the local mechanically induced ferroelectric to anti-ferroelectric phase transformation of niobium-modified lead zirconate titanate 95/5. Indentation experiments to a depth of 2 μm were performed using a Berkovich pyramidal three-sided diamond tip. Subsequent Raman spectroscopy and piezoelectric force microscopy revealed that indentation locally induced the ferroelectric to antiferroelectric phase transformation. Piezoelectric force microscopy demonstrated the ability to map the individual phases within and near indented regions on the niobium-modified lead zirconate titanate ceramics.     

Hydrogen-Induced Semiconductor Transformation of Lead Zirconate Titanate Ferroelectric Ceramics

Semiconductor transformation of lead zirconate titanate (PZT)-5H ferroelectric ceramics induced by hydrogen has been investigated. The results showed that hydrogen caused the PZT-5H ferroelectric ceramics to transform from a yellow insulator into a black n -type semiconductor, and the leakage current and carrier concentration increased but the resistivity decreased with increasing hydrogen concentration. During charging in H₂ at a temperature higher than the Curie point, hydrogen would restrain the PZT-5H ferroelectric ceramics to transform from cubic into tetragonal phase, resulting in the disappearance of ferroelectricity at room temperature. Charging at a temperature below the Curie point, however, did not change the crystal structure of the tetragonal ferroelectric ceramics. The properties and color of PZT-5H ferroelectric ceramics were restored after outgassing at a high temperature.     

Relaxor Ferroelectric Behavior in Barium Strontium Titanate

The development of barium strontium titanate‐based tunable dielectrics is currently hindered by high losses in the paraelectric phase. Barium strontium titanate (BST) thin films and ceramics show a range of ferroelectric transition behavior, from normal, diffuse, and relaxor‐like ferroelectric responses, depending on the sample preparation route. Rayleigh analysis, the temperature‐dependent dielectric response, and the optical second harmonic generation were used to characterize the ferroelectric response of bulk and thin film BST. Ferroelectricity is observed to persist in BST for 30°C above the global phase transition temperature in ceramics and over 50°C in thin films. Piezoresponse force microscopy on BST ceramics with extensive residual ferroelectricity reveals the coexistence of nanoscale polar regions, typical of relaxor ferroelectrics, as well as micrometer scale domain structures. The nature of the phase transition was probed using electron energy loss spectroscopy and found to correlated with the nanoscale A‐site chemical inhomogeneity in the samples.     

Aging in Tetragonal Ferroelectric Barium Titanate

The aging phenomenon in tetragonal ferroelectric bairum titanate was investigated by optical microscopy and by electrical measurements. Transmission electron microscopy was used to observe in detail domain reactions which occurred during aging. Aging may be characterized as an exhaustion-type process because of the increase in the apparent activation energy with time. It is concluded that aging is the relief of the residual stresses of the ferroelectric transition by the thermally activated nucleation of 90° domains.     

Ferroelectric-Domain Structure in Piezoelectric Ceramics

Twin-related (tetragonal (T), rhombohedral (R)) adjacent ferroelectric domains, which require a smaller energy for the rotation of the polarization vector, have been found to occur in lead zirconate titanate type piezoceramics at the morphotropicphase boundary, explaining the triplet splitting observed in electron diffraction patterns. The values of elastically stored energy density in the walls of the twinning domains are estimated, and a packing model of the ferroelectric domain structure as a T₁RT₂RT₁…succession is proposed to explain the coexistence of the two ferroelectric phases in the morphotropicphase boundary region. A crystallographic analysis of the transformation from the paraelectric to the ferroelectric phase is performed on the basis of martensite transformation theory. The calculated results are in good agreement with the observed transmission electron microscopy data. In particular, the crystallography and morphology of the "herringbone" domain structure, often observed in piezoelectrio ceramics, are well explained by this analysis.     

Dielectric Aging in Tetragonal Solid Solutions of Calcium Titanate in Barium Titanate

The dielectric aging phenomenon was observed in tetragonal polycrystalline solid solutions of calcium titanate in barium titanate between 30° and 90°C, and the 90° ferroelectric domain microstructures were analyzed using replica electron microscopy and statistical procedures. The results show a correlation between aging rate and 90° domains that may be satisfactorily interpreted in terms of the internal residual stress relaxation theory of aging.     

A short solid-state synthesis leading to titanate compounds with porous structure and nanosheet morphology

A novel method was developed for the synthesis of titanate nanosheets with high surface area. A solid-state mixture of NaOH and TiO₂ was reacted at 600 °C for several minutes. The aqueous dispersion of the resulting melt was aged at room temperature for periods up to 14 days. After hydrochloric acid treatment and washing procedures, the reaction product was characterized by X-ray diffraction, Raman spectroscopy, N₂-sorption and small-angle X-ray scattering measurements. The titanate compound not subjected to the aging process was amorphous and possessed a microporous framework, while the aged samples displayed nanosheet morphology and high specific surface area (396-509 m² g⁻¹). It was revealed that the very short heat treatment is of crucial importance for the titania-titanate phase transformation, while the aging process is needed for the morphological evolution of the titanate samples. The effects of the aging time on the structure and the morphology are discussed.     

Calculations of finite size effects in barium titanate

The polarisation and permittivity of barium titanate are calculated as a function of temperature from Landau-Ginzburg-Devonshire theory, taking into account effects due to the finite volume of coherently polarising regions. For regions of less than 10 nm in diameter, in which spatially uniform thermal fluctuations of the polarisation become significant, there are marked departures from the single crystal behaviour, particularly in the vicinity of the ferroelectric phase transitions. The behaviour is similar to aspects of that reported for fine-grained ceramics and thin films, suggesting that limited coherence of the magnitude of polarisation is responsible for a reduction in the permittivity and the appearance of diffuseness of the ferroelectric phase transitions.     

Phase change and ferroelectric nature of microwave-heated lead cobalt titanate nanoparticles prepared by sol-gel method

The lead cobalt titanate (PCT) nanoparticles with varying cobalt concentration from 0.2 to 0.8 were synthesized via sol-gel technique. Furthermore, these samples were undergone microwave heat treatment at 1023 K for ¾ hours. The powder diffraction study established the structural transformation of PCT nanoparticles from tetragonal (T) to rhombohedral (R) phases at higher cobalt contents. The surface morphology was examined by transmission electron microscope. Moreover, the ferroelectric property was elucidated by means of polarization versus electric field loop with varying temperature. The results revealed that 0.2, 0.6, and 0.8 compositions established the real ferroelectric behavior, whereas 0.4 composition attributed the banana shape hysteresis loop revealing high lossy dielectric material.     

Phase coexistence and broad depolarization response in (Pb,La)(Zr,Sn,Ti)O3 single crystals

Lead lanthanum zirconate stannate titanate (PLZST) antiferroelectric materials have been intensively studied in the last few decades due to their adjustable phase transition and outstanding performance. Despite the strong interest it has drawn, there is a lack of study of domain structure evolution in antiferroelectric materials with morphotropic phase boundary (MPB) composition, which is of great importance for understanding the phase transition between ferroelectric and antiferroelectric phase. PLZST single crystals with the composition in the MPB region were grown by the flux method in this work. We presented the direct evidence that rhombohedral ferroelectric and tetragonal antiferroelectric phase coexist in the as-grown single crystals. The evolution of phase structure and domain morphology during the temperature-induced phase transition was observed by in situ optical visualization, accompanied by a broadening depolarization response. An enhanced broadening pyroelectric response with a wide temperature window was detected. A modified Ginzburg-Landau-Devonshire theory was employed to elucidate the structural and physical origin of the broadening thermal depolarization. It is anticipated that a further understanding of the phase transition behavior in PLZST single crystals may be helpful in developing novel ferroelectrics and antiferroelectrics with MPB compositions.     

Coexistence of Relaxor and Normal Ferroelectric Phases in Morphotropic Phase Boundary Compositions of Lanthanum-Modified Lead Zirconate Titanate

Hot-stage transmission electron microscopy studies of lanthanum-modified lead zirconate titanate (Pb_{1- x} La _x -(Zr_0.53Ti_0.47)_{1- x /4}O₃ (PLZT 100 x /53/47) have been performed for compositions close to the morphotropic phase boundary (MPB). These studies have revealed the coexistence of relaxor and normal ferroelectric phases. Lanthanum substitution disrupts long-range ferroelectric order, resulting in short-range polar ordering. Investigations also have been performed using dielectric spectroscopy, electrically induced polarization, and strain methods. Three phases have been found for 10/53/47: a low-temperature tetragonal phase, a high-temperature cubic phase, and an intermediate-temperature pseudocubic phase that exhibits relaxor ferroelectric behavior. Induced strain studies of 10/53/47 show a gradual change from butterfly-shaped to slim-quadratic loops with increasing temperature. In addition, the remanent strain decreases with increasing temperature, and reversible strain shows a peak at 80°C that corresponds to the appearance of intermediate phase.     

New Phase Transitions in Perovskite Oxides: BiFeO3, SrSnO3, and Pb(Fe2/3W1/3)1/2Ti1/2O3

This paper reports on two new aspects of oxide perovskites: the first part is devoted to new phase transitions, especially at high temperatures; multiferroics such as BiFeO₃ and Pb–Fe–W–titanate are emphasized but nonmagnetic materials such as SrSnO₃ are included. The work summarized on bismuth ferrite emphasizes its metal–insulator transition near 1200 K (atmospheric pressure) and 47 GPa (room temperature); that on SrSnO₃ emphasizes order–disorder phase transitions; and that on lead–iron tungstate–titanate exhibits a classic second-order ferroelectric phase transition, of which rather few are known in the literature. The second part of the paper presents a discussion of constant phase elements for oxide perovskite ceramics; this is a modern way of characterizing their dielectric relaxation, particularly near phase transition temperatures.     

Dielectric and ferroelectric properties of Ba(ZrxTi1−x)O3 ceramics

It is known that Curie temperature of barium titanate system can be altered by the substitution of dopants into either A- or B-site. Dopants could pinch transition temperature, lower Curie temperature, and raise the rhombohedral–orthorhombic and orthorhombic–tetragonal phase transition close to room temperature. This isovalent substitution could improve the ferroelectric properties of the BaTiO₃-based system. In this study, barium zirconate titanate Ba(Zr_xTi_1−x)O₃ (BZT; x = 0, 0.02, 0.05 and 0.08) ceramics were prepared by conventionally mixed-oxide method. The ferroelectric properties of BZT ceramics were investigated. Increasing Zr content in the BaTiO₃-based compositions caused a decrease in Curie temperature (T_c). At T_c, the highest relative permittivity of BZT with an addition of 0.08 mol% of Zr was 12,780. The BZT specimens with the additions of 0.05 mol% and 0.08 mol% of Zr presented the remanent polarization at 25 μC/cm² and 30 μC/cm², respectively.     

PTCR effect in donor doped barium titanate: review of compositions,microstructures, processing and properties

Abstract

Abstract

Barium titanate is widely used in the fabrication of thermistors with a positive temperature coefficient of resistivity (PTCR). The resistivity can increase by several orders of magnitude near the phase transition temperature T_c for the ferroelectric tetragonal to the paraelectric cubic phase transformation. There is general agreement that the anomaly in the change of electrical resistivity of donor doped BaTiO₃ around T_c is due to the grain boundary effect. The Heywang–Jonker model and other mechanisms involving the nature of the electrical barrier formed across the grain boundaries of polycrystalline BaTiO₃ are reviewed. The compositional effect on BaTiO₃ based PTCR properties is listed and discussed. The influences of manufacturing methods under different stages including the initial doping methods, sample forming methods and final heat treatments on PTCR properties are compared. The complex interrelationships between compositions, microstructures, processing and PTCR characteristics are well discussed.     

Isothermal Phase Transitions in Ceramic Lead Zirconate

Constant temperature permittivity measurements of ceramic lead zirconate near 230°C revealed that the paraelectric to ferroelectric, ferroelectric to antiferroelectric, and antiferroelectric to paraelectric phase transitions were isothermal transformations. On heating, the antiferroelectric phase transformed into a paraelectric phase. Cooling produced a transformation from the paraelectric into an intermediate ferroelectric phase which existed over a temperature range of about 5°C. On further cooling, the ferroelectric transformed into the antiferroelectric phase. Thermal expansion, relative permittivity, and dissipation factor data are reported.     

Synthesis of High Magnetoelectric Coefficient Composites Using Annealing and Aging Route

In this paper, aging temperature and time dependence of ferroelectric, ferromagnetic, and magnetoelectric properties of lead zirconate titanate and nickel ferrite composite (1– x ) Pb(Zr_0.52Ti_0.48)O₃– x NiFe_1.9Mn_0.1O₄ (PZT–NFM, x =0.03, 0.05, 0.1) are reported. The magnetoelectric composites of different compositions were fabricated by using a process based on the controlled precipitation route. The processing is a combination of conventional mixed oxide sintering and thermal treatment. The thermal treatment consists of annealing at a high temperature (800°C for 10 h), followed by aging in the range of 300–400°C for 3–15 h. X-ray diffractometry patterns show variation in the amount of spinel phase with different aging temperature and time. Scanning electron microscopy investigation showed that the annealing and aging treatment increases the homogeneity of NFM in the PZT matrix. The magnitude of piezoelectric constant ( d ₃₃), piezoelectric voltage constant ( g ₃₃), and the dielectric constant exhibited significant variation in magnitude with aging time and temperature. Aging at 400°C for 5 h exhibited a maximum magnitude of d ₃₃ and g ₃₃ and minimum magnitude of dielectric constant. The magnitude of the magnetoelectric (ME) voltage coefficient (d E /d H ) for 0.9PZT–0.1NFM samples sintered at 1125 °C was found to be of the order of 78 mV/cm Oe. This magnitude increased to 140 mV/cm Oe after annealing and aging at 300°C for 5 h. This significant enhancement in the magnetoelectric coefficient is probably due to the homogenization and precipitation of the spinel (NFM) phase in the perovskite matrix.     

锆钛酸铅（PZT）粉体合成的研究进展

锆钛酸铅（PbZrxTi1-xO3,PZT）陶瓷是一类重要的铁电、压电、介电材料,其粉体的相组成、化学组成、热稳定性和烧结活性影响着陶瓷制品的铁电、压电和介电性能。本文详细综述了合成PZT粉体的固相反应法和湿化学方法的发展现状,并对几种合成方法的特点进行了评介,为低温合成纯相PZT粉体和PZT一维纳米结构指出了可能的方法。     

Nanocrystalline Barium Strontium Titanate Ceramics Synthesized via the “Organosol” Route and Spark Plasma Sintering

Dense nanocrystalline barium strontium titanate Ba_0.6Sr_0.4TiO₃ (BST) ceramics with an average grain size around 40 nm and very small dispersion were obtained by spark plasma sintering at 950°C and 1050°C starting from nonagglomerated nanopowders (~20 nm). The powders were synthesized by a modified “Organosol” process. X‐ray diffraction (XRD) and dielectric measurements in the temperature range 173–313 K were used to investigate the evolution of crystal structure and the ferroelectric to paraelectric phase transformation behavior for the sintered BST ceramics with different grain sizes. The Curie temperature T_C decreases, whereas the phase transition becomes diffuse for the particle size decreasing from about 190 to 40 nm with matching XRD and permittivity data. Even the ceramics with an average grain size as small as 40 nm show the transition into the ferroelectric state. The dielectric permittivity ε shows relatively good thermal stability over a wide temperature range. The dielectric losses are smaller than 2%–4% in the frequency range of 100 Hz–1 MHz and temperature interval 160–320 K. A decrease in the dielectric permittivity in nanocrystalline ceramics was observed compared to submicrometer‐sized ceramics.     

CeO2掺杂钛酸铋钠钾陶瓷的制备工艺与介电性能

为了进一步探索合成工艺对钛酸铋钠系无铅材料的结构及介电特性的影响,本文以甘氨酸为燃料,利用固相-燃烧法制备了CeO2掺杂的Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 (BNKT)陶瓷.XRD表明,在固相合成工艺中引入燃烧法制备BNKT陶瓷,比传统固相法降低预烧温度150 ℃,掺杂的CeO2扩散进入了BNKT钙钛矿的晶格,且当掺杂量为0%~0.3%时,形成纯的钙钛矿相结构;SEM表明,CeO2掺杂使晶粒尺寸趋于平均,对晶粒生长有抑制作用;介电温谱表明,随着CeO2掺杂量增加,介电常数εr和退极化温度Td、相转变温度Tm降低,介电反常峰逐渐弱化,且室温至300 ℃,介电损耗tanδ始终在0.3%以下,并从微结构缺陷空位形成机制角度,结合铁电畴壁运动状态,分析讨论了对材料介电特性的作用规律.