Ferroelastic Behavior Research Articles

Microstructure and the influence of spontaneous strain in LaCoO3, La0.8Sr0.2CoO3 and La0.8Ca0.2CoO3

Transmission Electron Microscopy has been conducted on rhombohedral perovskite oxides with composition LaCoO3, La0.8Sr0.2CoO3, and La0.8Ca0.2CoO3. Thin foils prepared within the first weeks of sintering showed macroscopic strain and a very high defect density, which included fault-related superlattice structures. Samples prepared several months after sintering, showed no significant macroscopic strain and the main defect was found to be domains due to reflection twinning in the pseudo-cube plane. The behaviour is likely to be related to accomodation of spontaneous strain arising upon cooling of the material from the sintering temperature. The presence of twin domains illustrates the lowering of lattice symmetry as compared to the ideal cubic perovskite structure, and is relevant for explaining recent observations of ferroelastic behaviour of these materials.

Domain switching in monoclinic YNbO 4-modified ZrO 2(3Y)

Behaviors of domain switching in zirconia-based ceramics are observed only on the tetragonal phase. Despite the presence of domain switching in monoclinic hafnia ceramics, the similar behavior of monoclinic zirconia have frequently been considered impractical since zirconia ceramics are prone to fracture when they contain a large amount of the monoclinic phase. YNbO{sub 4} is a compound having a monoclinic structure at room temperature and a tetragonal structure at temperatures exceeding 900 C. A related investigation proposed that introducing YNbO{sub 4} into ZrO{sub 2} ceramics, either as a minor second phase or as a solid solution, would more easily promote the nucleation of monoclinic ZrO{sub 2}. Extra high fracture toughness has been observed in Zirconia doped with YNbO{sub 4}. Their studies observed ferroelastic domain switching in tetragonal YNbO{sub 4}-modified ZrO{sub 2}(3Y), similar to those of previous studies. However, their results also indicated an extraordinary elastic behavior of monoclinic YNbO{sub 4}-modified ZrO{sub 2}(3Y) under uniaxial compression. Therefore, the authors investigate the effect of incorporating YNbO{sub 4} into ZrO{sub 2}(3Y) and the ferroelastic domain switching behavior in the monoclinic phase of this material system.

Mechanical twinning in calcite considered with the concept of ferroelasticity

The mechanical twinning of calcite is compared with ferroelastic behavior. For calcite a paraelastic prototype phase does not exist, therefore a virtual prototype is defined with cubic m3m symmetry. Using this condition the strain tensors of all domain states of mechanical twinning are calculated. With the use of the strain tensors, application of Sapriel's strain compatibility law gives the crystallographic orientations of all possible twin planes between different domain states. The findings indicate that the twin walls are {1 0 0} and {1 1 0} planes with respect to the morphological rhombohedral lattice. These twin plane orientations from the equated strain tensors are in accordance with the r-and e-twin systems commonly observed in calcite.

Mechanical properties of PZT ceramics with tailored microstructure

PZT ceramics with tailored microstructure were fabricated using 3 and 5 valence ions as dopants. It was shown that the type of additive does not influence the microstructural development. Vacancies in the Pb-lattice are being formed for all dopants to assure charge equilibrium. Thus, Pb-vacancies are identified to dominate the microstructural development of PZT ceramics. Samples with 2 mol% additive which were sintered to near theoretical density exhibit a homogeneous microstructure with an average grain size of 2 μm. The morphotropic phase boundary (MPB) of doped PZT is shifted towards higher PbZrO 3 content as compared to undoped PZT. This effect is explained in terms of crystallographic considerations. The investigations on the mechanical properties are focused on the area of the MPB that is relevant for industrial applications due to the extraordinary high electrical properties. For the first time an entire evaluation of the mechanical properties have been conducted considering the fracture toughness of poled and unpoled samples, R-curve behavior, temperature dependent bending strength and ferroelastic behavior. From these measurements the influence of domain switching processes on the mechanical properties are deduced and a correlation between mechanical, ferroelastic and ferroelectric behavior and the microstructure is supposed.

Microscopic and macroscopic ferroelectric–ferroelastic and piezoelectric behavior of PZT ceramics

Synchrotron X-ray experiments on rhombohedral PZT ceramics with electric fields in between −2·4 and 2·4 kVmm −1 were performed in order to seperate between field-induced ferroelectric-ferroelastic and piezoelectric effects. The results are compared with results of macroscopic measurements. Additionally the influence of different dopands (La, Ag) and grain sizes on the intrinsic piezoelectric effect are investigated and coercive field strength are given.

Structure, twinning, and ferroelasticity of tetramethylammonium-hexafluoroplatinate(IV), (TMA)2[PtF6

Abstract (TMA)2[PtF6] is found to be tetragonal at room temperature (space group I4/m, a = 8.073(9) Å, c = 11.401(2) Å). The structure is related to the K2[PtCl6] structure type (space group Fm[unk]m) and is characterized by a ferrorotative tilt of the [PtF6] octahedra by an angle of 12.6(1)° and of the TMA tetrahedra of 12.1(2)° with respect to their position in the cubic aristotype structure. The double refraction (amounting to −0.0019(1) at room temperature) decreases parabolically, when the crystal is heated, and vanishes at 436 K indicating a phase transition to the cubic K2[PtCl6] structure type. Twinning with domains having straight twin boundaries and differently directed tetragonal c-axes (pseudo-cubic axes) sometimes occurs in as-grown crystals. Ferroelastic behaviour can be demonstrated under the polarizing microscope by application of external stress: (i) domain boundaries can be moved through the crystal; (ii) if a cube-shaped crystal is pressed perpendicular to the c-axis, the c-axis switches into the direction of pressure.

Micromechanical modeling of ferroelasticity and ferroelectricity and finite-element results for nonlinear piezoelectric applications

Piezoelectricity in polycrystalline materials like PZT ceramics is a complex phenomenon of both linear and nonlinear electromechanical coupling based on the single crystal piezoeffect and on domain switching, respectively. The latter can be caused by mechanical and/or electrical loading and yields to changes of the polarization texture of the polycrystalline material. The ferroelastic and ferroelectric hystereses can be understood by means of a micromechanical model. In order to include the grain-to-grain interaction Finite-Element (FE) method is its own recommendation. The micromechanical model has been implemented into the FE code ANSYS©. Experimental and simulation results are compared for purely ferroelastic and ferroelectric behavior.

Study on ferroelastic characteristics of gamma irradiated lithium potassium sulfate crystal

A detailed study of the optical absorption vs. γ-irradiation dose for lithium potassium sulfate crystal, LiKSO 4 cut out perpendicular to x, y and z directions is presented. The optical density was measured within the wavelength range from 200 to 600 nm at different γ-doses up to 300 kGy. The investigated optical parameters, e.g., absorption coefficient, relative absorbance and optical energy gap, were found to be gamma dose dependent as well as crystal orientation. The results were discussed in the view of interaction of point imperfections, introduced by irradiation, with the ferroelastic behavior of LiKSO 4 crystal.

Correlation between Surface Texture and Chemical Composition in Undoped, Hard, and Soft Piezoelectric PZT Ceramics

The electrical and mechanical properties of piezoelectric lead zirconate titanate (PZT) ceramics are strongly influenced by domain‐wall motion, which can be tailored via the substitution of ions within the perovskite structure. Different domain mobilities are achieved by lead and/or oxygen vacancies, according to the valency and ionic radius of the dopants. To quantify the surface near domain mobility, hard (silver‐doped), soft (lanthanum‐doped), and undoped PZT ceramics have been prepared. An applied mechanical stress after sintering causes texturing near the surface, because of the ferroelastic behavior of the 90° domains. The texture is quantified via X‐ray diffractometry (XRD) analysis of the tetragonal FT(002)‐ and FT(200)‐peak intensities, using least‐squares refinement with Gaussian profile functions. The samples are subsequently annealed to remove the surface texture and again characterized via XRD measurements. However, annealed samples still reveal a preferred domain orientation that can be removed only by a second annealing of the pulverized samples. A comparison of the tetragonal additive systems clearly reveals the greatest domain mobility for lanthanum‐doped PZT ceramics, whereas the silver‐doped and undoped samples have similar ferroelastic behavior. Furthermore, the surface texture of all the compositions is dependent on the applied mechanical stress and cannot be removed completely by heat treatment.

Ferroelastic domain switching behaviour of single crystals

We investigated ferroelastic domain switching due to stress in single crystals grown by a slow evaporation method. The ferroelastic domain switching stress due to the saturation effect is about 0.9 MPa. The temperature dependence of the stress-strain hysteresis was also studied. From this result, it was determined that the single crystal undergoes a phase transition near K. In other words, the phase transition due to an external mechanical stress applied to crystals causes a transformation from the ferroelastic to the paraelastic phase. From these results, it was confirmed that crystals have ferroelastic characteristics below .

Orientation and Anisotropy in Piezoelectric Ceramics

Preferred orientation and anisotropy are an inherent part of the utility of materials that demonstrate ferroic (ferroelectric, ferromagnetic, or ferroelastic) behavior. Many of these materials also demonstrate the complex interaction between polarization and deformation called piezoelectricity. In this paper, the orientation and anisotropic properties of a poled piezoelectric ceramic are assessed and compared to the properties of the ceramic in the unpoled state. The degree of orientation and the resulting mechanical (elastic), piezoelectric and fracture toughness anisotropy will be discussed with their correspondence to piezoelectricity.

Plastic deformation of zirconia single crystals: a review

The high-temperature deformation behaviour of zirconia single crystals stabilised with yttria is reviewed. Cubic or fully stabilised zirconia (FSZ), which is considered the matrix of high-strength partially stabilised zirconia (PSZ), deforms plastically down to 400°C without confining hydrostatic pressure. The relevant deformation behaviour above about 1200°C is characterised by athermal dislocation motion mainly on cube slip planes and diffusion-controlled recovery. Tetragonal polydomain zirconia or t′ zirconia consists of plate-like tetragonal domains alternately stacked to form large colonies. These colonies are arranged in a characteristic way to fill the whole crystal volume, t′ zirconia shows ferroelastic behaviour preceding dislocation plasticity. In tension, a tetragonal single crystal forms containing residual defects. The data available indicate that the coercive stress strongly depends on temperature. In situ straining experiments in a high-voltage electron microscope show an instantaneous switching of the individual tetragonal domains with the domain boundaries moving sidewise through the domains. Dislocations moving after the ferroelastic deformation are strongly bowed. The nature of the pinning agents is not clear yet. PSZ crystals are thought to consist of a cubic matrix and precipitates of the tetragonal phase of a structure similar to that of the colonies in t′ zirconia. Complete dislocations in the cubic matrix moving on cube planes are partial dislocations in two of the three tetragonal variants of the precipitates. They have then to produce a stacking fault or antiphase boundary like defect. The strong age-hardening and overageing experimentally observed can be explained by a decreasing width of the matrix channels between the precipitates and an increasing width of the domains within the colonies. Recent in situ studies in a high-voltage electron microscope have shown that the precipitates in PSZ may undergo ferroelastic deformation, too. Besides, in a number of cases the matrix of PSZ crystals turned out to be tetragonal rather than cubic. Thus, the formation of a tetragonal single crystal containing residual defects as during the ferroelastic deformation in t′ zirconia should affect the subsequent dislocation plasticity. The stacking fault or antiphase boundary like defects have experimentally been observed on 110 planes, however, not on the usual 100 slip planes. Thus, in spite of numerous experimental results a number of questions are still open particularly concerning the plastic deformation of PSZ crystals.

Ferroelasticity of t‘‐Zirconia: I, High‐Voltage Electron Microscopy Studies of the Microstructure in Polydomain Tetragonal Zirconia

The microstructure of polydomain tetragonal zirconia (t‐ZrO2), i.e., a ZrO2 modification exhibiting ferroelastic behavior, is studied by high‐voltage electron microscopy. This material consists of three domain variants of the tetragonal phase with their c‐axes nearly orthogonal to each other. Always two variants of these platelike domains are alternately arranged, forming elongated regular colonies. Hence, in both variants the common habit plane of the domains is a {110} twin plane. The colonies are of columnar shape with a longitudinal axis. They are bound by {110} planes, too, which are twin planes for the domains in the contiguous colonies. Owing to their particular structure and the helical arrangement of the adjoining colonies, the material remains coherent and pseudocubic over large macroscopic regions, although it is formed by different tetragonal domains.

Hinged and chiral polydiacetylene carbon crystals

Structures and properties are calculated for hinged and chiral polydiacetylene carbon phases, which consist entirely of carbon chains found in organic polydiacetylenes. Diffraction and spectroscopic results are consistent with those for the controversial reported carbon phases of unknown structure, which might have accidentally formed by the reaction of acetylenic chains. Predicted properties include (1) a semiconductor-to-metal transition upon doping, (2) a soft deformation mode that results in negative linear and area compressibilities, negative Poisson ratios and expected shape-memory and ferroelastic behavior for the hinged phases, and (3) optical activity, second harmonic generation and torsional and transverse compressional piezoelectricity for the chiral phase.

Ferroelasticity of the displacive tetragonal phase in Y2O3 partially stabilized ZrO2 (Y-PSZ) single crystals

Ferroelasticity of the tetragonal displacive (t′) phase was studied on 4.7 mol% Y2O3 partially stabilized zirconia single crystals. Samples were deformed at 1400 °C at constant strain rate to induce the ferroelastic behavior. Domain reorientation due to the applied stress has been studied as a function of the compression axis and aging time at 1600 °C. Domain switching was found in the as-received and 2-h-aged crystals deformed along the 〈100〉 direction, in which an exceptional high flow stress was reached (>700 MPa). Transmission electron microscopy observations were performed on deformed and undeformed crystals to study the microstructural changes associated with the domain switching. Incremental strain steps on the stress-strain curves and surface texture on the lateral faces of the deformed samples were correlated with the microstructural evidence.

Ferroic Phase Transitions in (Na1/2Bi1/2)TiO3 Crystals

The ferroic phase‐transition behavior of two (Na1/2Bi1/2)TiO3(NBT) crystals grown by flux and by the Czochralski method has been investigated in the present study. Although both the tetragonal and the rhombohedral phases of NBT are expected to be ferroelastic, these crystals exhibit different ferroelastic behavior. The two NBT crystals also show differences in the amount of temperature hysteresis and the thermal expansion coefficients. These differences can be attributed to nonstoichiometry and structural variations dependent on the growing method. The present investigation has revealed a second maximum at −450°C in dielectric constant ((T)) curves, which could indicate that the intermediate tetragonal phase is either polar or antipolar. This maximum, however, originates from space‐charge polarization and interaction between the charge carrier and the electrode, such that the tetragonal phase, in fact, is para‐electric. The diffuse phase transition (DPT) of the NBT crystal, therefore, is from a paraelectric and ferroelastic tetragonal phase to a ferroelectric and ferroelastic rhombohedral phase. The crystallographic supergroup‐subgroup relationships in the ferroic phase transitions of NBT crystals are discussed.

Infinitesimal loops and tweed structure

Some results are collected for phase transitions in which the elastic strain plays a decisive role. Both ferroelastic and antiferroelastic behaviour results in patterning which has a tweed-like appearance. The ‘infinitesimal loop approximation’ first used by Kroupa in 1962 is used to try to sort out the possible effects. Transformations resulting in pure shear produce ferroelastic tweed, whereas transformations based on the nucleation and growth of platelike precipitates seem likely to produce antiferroelastic tweed.

An original oxide of antimony and tungsten related to aurivillius phases

Aurivillius 's phases exhibit original [Bi 2O 2] n layers; up to now such phases exist only with bismuth. The aim of this paper is to report new related phase, containing only antimony, which is promissing because its ferroelastic properties. Synthesis, crystal growth and structure determination of the original double oxide Sb 2WO 6 has been performed. It crystallizes in the triclinic system, space group P1, with a=5.554(1), b=4.941(2), c=9.209(3) A ̊ , α=90.05(3), β=96.98(2), γ=90.20(2)°, V=250.8(7) A ̊ 3 . TEM and optical microscopy investigations allow to evidence a ferroelastic behaviour at room temperature with a phase transition in the range 300–400°C. The crystal structure of Sb 2WO 6 is built up by [WO 4] n layers of WO 6 octahedra sharing corners sandwiched by two [Sb 2O 2] n layers; it can be described as [WO 2Sb 2O 4] n thick layers held together by weak Sb...O interactions. The antimony atoms exhibit typical 3+1 one sided coordination. Via structural relationships with Sb 2O 3, VOSb 2O 4 and Bi 2WO 6, Sb 2WO 6 is related with the Aurivillius phases announcing how [Bi 2O 2] n layers with the shape encountered in red PbO can be formed or distorted.

Ferroelasticity of t’-ZrO 2

Different materials show ferroelastic behaviour when two conditions are satisfied. Firstly, a diffusionless phase transition from a high-symmetry modification must occur, which generates the ferroelastic material. The phase transformation must lower the point-group symmetry and must generate a lattice distortion. Usually, the ferroelastic material is composed of at least two different variants (domains) with different orientations. The domains are separated by domain walls. Secondly, it must be possible to reorientate the domains by external stresses. The latter poperty is equivalent to «switching» the domains by the migration of domain walls. One modification of zirconia, t'-ZrO 2 , exhibits the ferroelastic behaviour

Ferroelastic detwinning of the high-temperature superconductor YBa2Cu3O7-δ

Abstract Crystals of YBa2Cu3O7-δ (YBC) were prepared using a flux technique. The as-grown crystals were typically on the order of 200 μm × 200 μm × 50 μm, where the 50 μm dimension is in the c-direction. Ferroelastic behavior was observed by applying a uniaxial stress along the [100]tet-direction using a specially-designed fixture which was mounted to a programmable hot/cold stage of a polarizing reflected-light microscope. Domain wall motion was observed for temperatures greater than 200°C, and complete poling was accomplished at stresses on the order of 50–100 MPa at temperatures of 350–400°C. An ionic diffusion-activated mechanism is suggested.