(Bi_(0.5)Na_(0.5))TiO_3-BaTiO_3系陶瓷的介电弛豫性能

采用传统压电陶瓷固相合成法制得了纯钙钛矿相的( 1 -x) (Bi0 5Na0 5 )TiO3 -xBaTiO3 (x=0 02, 0 04,0 06, 0 08, 0 10) (简写作BNBT)系无铅压电陶瓷。研究了1kHz条件下室温到400℃的温度范围内BNBT试样的介电温谱以及3种不同频率下(1、10、100kHz)BNBT-6试样的介电温谱,发现材料在研究组成范围内均为弛豫型铁电体。采用HRTEM研究了该系统的畴结构,表明BNBT钙钛矿结构铁电体的介电性能与复合离子的有序无序排列密切相关,纳米尺度有序微畴对介电弛豫起着重要作用。     

Synthesis,structural evolution,and dielectric properties of a new perovskite solid solution (Pb0.5Sr0.5)(Zr0.5Ti0.5)O3–PbTiO3

To explore lead-reduced dielectric materials in the SrTiO₃–PbTiO₃–PbZrO₃ ternary system, a novel solid solution between relaxor ferroelectric (Pb_0.5Sr_0.5)(Zr_0.5Ti_0.5)O₃ and ferroelectric PbTiO₃, namely (1 − x)(Pb_0.5Sr_0.5) (Zr_0.5Ti_0.5)O₃–xPbTiO₃ (lead–strontium–zirconate–titanate [PSZT]–PT), has been synthesized in the perovskite structure by high-temperature solid-state reaction method in the form of ceramics. The crystal structure and phase symmetry of the materials synthesized were analyzed and resolved based on X-ray powder diffraction (XRD) data through both the Pawley and Rietveld refinements. The results of the structural refinements indicate that at low PT-concentration end of the solid solution system, for example, x = 0.05, the PSZT–PT solid solution exhibits a cubic structural symmetry (with the space group Pm-3m). As the PT concentration (x) increases, the structure of (1 − x)PSZT–xPT gradually transforms from the cubic to a tetragonal (P4mm) phase. In the composition range of x = 0.10–0.25, a mixture of the cubic and tetragonal phases was identified. As the concentration of PT increases, the proportion of the tetragonal phase increases at the expense of the cubic phase. For a composition of x > 0.25, a pure tetragonal phase is observed. The dielectric properties of the materials were studied by measuring the permittivity as a function of temperature at various frequencies. For the composition of x = 0.05, the temperature dependence of dielectric constant shows typical relaxor behavior. For x = 0.35, the dielectric peaks indicate a normal ferroelectric phase transition. Overall, a structural transformation from a central-symmetric, nonpolar cubic phase to a non-centrosymmetric, polar tetragonal phase is induced by the substitution of PT for PSZT in the pseudo-binary solid solution of (1 − x)PSZT–xPT, which also reveals an interesting relaxor to ferroelectric crossover phenomenon.     

Electromechanical Determination of the High-Field Phase Transition of Pb(Mg1/3Nb2/3)O3–PbTiO3–(Ba,Sr)TiO3 Relaxor Ferroelectrics

Ceramics in the (1 – x )[(1 – y )Pb(Mg_1/3Nb_2/3)O₃· y PbTiO₃] · x MeTiO₃ system, where Me is Sr or Ba, exhibit very large electrostrictive strains at reasonable drive fields. However, the optimum use temperature and frequency vary with the particular composition used. As relaxor ceramics, each composition has a broad transition from electrostrictive to partially piezoelectric behavior. The transition temperature ( T _t) can be roughly determined from strain or polarization properties; however, it can be more quantitatively determined from the effective electro-mechanical Q . A plot of induced transverse strain/induced polarization squared (effective Q ₁₂) as a function of temperature shows a sharp and unmistakable change in slope—this defines T _t. The slope of induced transverse strain/polarization (effective g₃₁) also shows a change in slope at T _t, although this is more gradual than that of effective Q . The indicated T _t correlates with those found from measurement of strain and polarization.     

Surface Breakaway Decomposition of Perovskite 0.91PZN–0.09PT during High-Temperature Annealing

The thermal stability of crystallite powder and bulk single crystals of relaxor 0.91Pb(Zn_1/3Nb_2/3)O₃–0.09PbTiO₃ (0.91PZN–0.09PT) solid solution in air and an PbO-rich environment has been investigated. At 700°C, perovskite PZN–PT decomposes only slightly to the pyrochlore phase. At 800°C, the rate of decomposition is accelerated, promoted by a surface breakaway decomposition process. This process occurs via the inward growth of faceted pyrochlore grains from the particle surface. At a certain point of the growth process, they will break away and detach themselves from the perovskite phase. The transformation stress also causes the adjacent perovskite phase to fracture concurrently, contributing to the breakaway event. At higher annealing temperatures (i.e., 900°–1000°C) and/or in the PbO-rich environment, a layer of PbO-rich liquid phase is formed on the surface of the particle. Because of limited wetting between the PbO-rich liquid phase and the earlier-formed pyrochlore grains, the latter detach themselves from the perovskite substrate. This action frees the substrate from the pyrochlore nuclei to initiate the breakaway decomposition event, which causes the rate of decomposition to slow substantially. At 1100°C both in air and the PbO-rich environment, the PbO-rich liquid formed vaporizes readily, which causes the decomposition rate to accelerate again. The present work shows that (i) the decomposition of perovskite PZN–PT to the pyrochlore phase during high-temperature annealing is a surface phenomenon and (ii) its rate is controlled by the relative rate of formation and vaporization of the protective liquid layer that is present under the annealing conditions.     

Structure and Dielectric Properties of Pb(Sc2/3W1/3)O3–Pb(Zr/Ti)O3 Relaxors

The structure and dielectric properties of (1− x )Pb(Sc_2/3W_1/3)O₃–( x )Pb(Zr/Ti)O₃ ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x ≤ 0.25 a doubled cell results from a 1:1 ordered distribution of the B-site cations. The structural order in Pb(Sc_2/3W_1/3)O₃ (PSW) can be described by a random-site model with one cation site occupied by Sc³⁺ and the other by a random distribution of (Sc_1/3³⁺W_2/3⁶⁺). The ordering is destabilized in solid solutions of PSW with PbZrO₃ (PSW–PZ), but stabilized by PbTiO₃ in the (1− x )PSW–( x )PT system. The changes in order are accompanied by alterations in the dielectric response of the two systems. For PSW–PZ the temperature of the permittivity maximum ( T _ɛ,max) increases linearly with x ; however, for PSW–PT T _ɛ,max decreases in the ordered region (up to x = 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of (0.75)PSW–(0.25)PT; when the order parameter was reduced from ∼1.0 to ∼0.65, T _ɛ,max increased by more than 60°C.     

Fracture and electric-field-induced crack growth behavior in NBT-6BT relaxor ferroelectrics

The fracture properties of 0.94(Na_0.5Bi_0.5)TiO₃-0.06BaTiO₃ (NBT-6BT) relaxor ferroelectrics were investigated using the Vickers indentation method and computation of crack tip opening displacement. It was found that an unpoled sample had a fracture toughness of around 1.35 MPa m^1/2. In contrast, an electrically poled sample exhibited anisotropy with a lower fracture toughness perpendicular to the poling direction and a higher value in the parallel direction, as compared to the unpoled sample. Upon cyclic electrical loading (with applied electric field amplitudes between 0.73E_C and 1.4E_C), the indented surface crack was found to propagate. In general, the crack grew rapidly during the initial cycles followed by crack arrest, and the principal driving force for crack growth was proposed to be residual stress around the indentation, as evidenced by the limited field dependence of crack growth. There was also a contribution from the electromechanical strain, which played a role at high cycles (>100 cycles) and high fields (>1.3 E_C). Evidence of a saturation threshold of crack propagation is an advantage for the electromechanical reliability of relaxor ferroelectrics in devices.     

High temperature‐insensitive ferro‐/piezoelectric properties and nanodomain structures of Pb(In1/2Nb1/2)O3–PbZrO3–Pb(Mg1/3Nb2/3)O3–PbTiO3 relaxor single crystals

For rhombohedral (R) Pb(In_1/2Nb_1/2)O₃–PbZrO₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PIN–PZ–PMN–PT) relaxor single crystal, high temperature‐insensitive behaviors under different external stimuli were observed (remnant polarization P_r from 30°C to 180°C and piezoelectric strain d₃₃* from 30°C to 116°C). When electric field E ≥ 50 kV/cm in the case of an activation field E_a = 40‐50 kV/cm was applied, it was found that the domain switching was accompanied by a phase transition. The high relaxor nature of the R phase PIN–PZ–PMN–PT was speculated to account for the large E_a and high piezoelectric response. The short‐range correlation lengths extracted from the out‐of‐plane (OP) and in‐plane (IP) nanodomain images, were 64 nm and 89 nm, respectively, which proved the high relaxor nature due to In³⁺ and Zr⁴⁺ ions entering the B‐site in the ABO₃‐lattice and enhancing the disorder of B‐site cations in the R phase PIN–PZ–PMN–PT. The switching process of R nanodomain variants under the step‐increased tip DC voltage was visually revealed. Moreover, the time‐dependent domain evolution confirmed the high relaxor nature of the R phase PIN–PZ–PMN–PT single crystal.     

Relaxor nature in Ba5RZr3Nb7O30 (R = La,Nd, Sm) tetragonal tungsten bronze new system

Ba₅RZr₃Nb₇O₃₀(R = La, Nd, Sm) lead‐free relaxor ferroelectrics were prepared by a standard solid‐state reaction process, and the influence of A and B site ion occupation on the dielectric characteristics especially the relaxor nature were investigated systematically. Tetragonal tungsten bronze structure with space group P4/mbm was determined for all compositions, ion cross distribution by Ba²⁺ and R³⁺ in A1 site was observed, while A2 site was only occupied by Ba²⁺. Selected area electron diffraction patterns confirmed the existence of incommensurate superlattice modulation. Furthermore, temperature and frequency dependences of the dielectric properties showed a broad permittivity peak with strong frequency dispersion, following well the Vogel‐Fulcher relationship. The maximum dielectric constant temperature increased gradually with decreasing A1 site ion size. Slim P‐E hysteresis loops were obtained at room temperature for all compositions. Meanwhile, micro ferroelectric domains were observed in Ba₅SmZr₃Nb₇O₃₀. For Ba₄R₂Zr₄Nb₆O₃₀ and Ba₅RZr₃Nb₇O₃₀ (R = Nd, Sm), the transition from normal ferroelectric to relaxor behavior originates from the increased t_A1, which is a result of cross distribution at A1 site. Compared with Ba₅RTi₃Nb₇O₃₀, Zr substitution at B site enhances the relaxor nature.     

Thermal expansion studies on ferroelectric materials

Thermal expansion data is reported over a wide temperature range (80–800 K) for BaTiO₃ (BT) and Pb(Fe_1/2 Nb_1/2)O₃ (PFN), belonging to different classes of the ferroelectric materials. The sharp structural phase transitions of BT result in lowering of the thermal expansion coefficient (α) at the transitions which is proportional to the change in polarization at the transitions. In PFN, a relaxor ferroelectric, lowering of α is spread over a wider temperature range across the dielectric maxima.     

A generalized arrhenius law in dilute KTaO3Li versus the Vogel-Fulcher approach

Abstract

The temperature-frequency dependencies (5 K-300 K, 100 Hz-1 MHz) of the dielectric permittivity ε′ and ε″ in dilute (x < 0.01) K_1-xLi_xTaO₃ were studied and analyzed in detail. From the positions of the permittivity temperature maxima at various frequencies it was obtained that the temperature dependence of the mean relaxation time of Li⁺ off-centre related 90° dipole reorientations doesn't obey an Arrhenius law, but follows the empirical Vogel-Fulcher law. Such behaviour is usually believed to be an evidence of freezing phenomenon and polar glass formation at low temperatures. Here we suggest an alternative explanation and show that taking into consideration the coupling of the Li⁺ dipole impurities to the soft mode allows to obtain an analytical expression for a “Generalized Arrhenius law”, which describes quantitatively the deviation from the Arrhenius law and agrees well with the experiment. The approach developed describes the dependence of the ′'(T) maxima too due to the hardening of the soft mode caused by random fields. This hardening suppresses the divergent dielectric permittivity at the ferroelectric-type phase transition and can possibly lead to the appearance of polar regions.