铋掺杂对铌酸钾钠无铅压电陶瓷结构和性能的影响

用常压烧结法制备铋掺杂铌酸钾钠无铅压电陶瓷(K0.5Na0.5)1-3xBixNbO3(KNBN)。研究不同铋掺杂量对KNN陶瓷结构、形貌、致密度及电学性能的影响。结果表明:在1 120℃烧结的含铋量为1%(摩尔分数)的陶瓷表现出最好的铁电和压电性能及较好的介电性能,即压电常数最大121pC/N,P-E回线形状达到饱和,且剩余极化为12.67μC/cm2,矫顽场Ec为13.58kV/cm,介电常数为575,损耗为5.82%(频率为1kHz)。陶瓷样品在131℃从正交结构转变到四方结构,Curie温度为400℃。     

Piezoelectric and Dielectric Properties of Ceramics in the System Potassium—Sodium Niobate

High radial coupling coefficients are observed for compositions having up to 50 mole % sodium niobate additions to potassium niobate. The activity diminishes with additional sodium niobate content and disappears beyond about 98 mole % additions. Dielectric constants are relatively low, varying from 450 to 125, depending on composition. A.-c. losses are high and d.-c. resistivities are ∼10¹²ohm-cm. The low dielectric constants and high coupling obtained in certain regions of the system make these materials promising for solid ultrasonic delay line use, especially where thin sectioned plates are required, as in high-frequency thickness extensional or thickness shear mode transducers.     

Domain Switching During Electromechanical Poling in Lead Zirconate Titanate Ceramics

In many polycrystalline piezoelectric ceramics, domain switching during the poling process leads to the development of a macroscopic polarization and piezoelectric behavior. Traditionally, poling involves the application of electric fields across two parallel electrodes. In the present work, a radial mechanical compressive stress is applied transverse to the electric field, increasing the potential for domain alignment during poling by taking advantage of ferroelasticity. Experiments demonstrate that poling of lead zirconate titanate using a combination of an electric field and a transverse mechanical compressive stress increases the d ₃₃ coefficient from 435 to 489 pC/N. Using neutron diffraction and pole figure inversion methods, the degree of non-180° domain switching is described using pole density distributions of the tetragonal c -axis (002). The degree of 002 domain alignment parallel to the electric field after the electromechanical poling process increases from 1.30 multiples of a random distribution (mrd) to >1.40 mrd at stresses exceeding 40 MPa.     

Temperature Gradient Introduced Ferroelectric Self‐Poling in BiFeO3 Ceramics

The as‐prepared BiFeO₃ ceramic shows a piezoelectric d₃₃ coefficient of ?14 pC/N, that is, an obvious ferroelectric self‐poling phenomenon. The temperature gradient between the two surfaces of BiFeO₃ ceramic was intentionally enlarged when BiFeO₃ was prepared with a rapid liquid sintering method. This temperature gradient and the corresponding thermal strain can introduce defect dipoles through separating bismuth vacancies from oxygen vacancies. A mass of these dipoles introduce a macroscopic internal electric field (E_in) which downward poles BiFeO₃ ceramic during its cooling down process. As expected, an E_in of >10 kV/cm is confirmed by the asymmetrical polarization/strain versus electric field curves.     

Mechano-Synthesis of Lead–Magnesium–Niobate Ceramics

The synthesis of Pb(Mg_1/3Nb_2/3)O₃ (PMN) with high-energy milling was studied by X-ray powder diffraction (XRD) using the Rietveld-refinement method. The results are discussed in terms of the qualitative and quantitative composition of the crystalline and amorphous phases as a function of milling time. The mechano-synthesis of PbO, Nb₂O₅, and MgO leads to the formation of perovskite PMN. In the initial stage of milling, particle size reduction and a high degree of amorphization were observed, together with the simultaneous formation of perovskite and pyrochlore-type structures. A mechanism for the formation of PMN by the mechano-synthesis route is proposed.     

Preparation of Plate‐Like Sodium Niobate Particles by Hydrothermal Method

Sodium niobate NaNbO₃ hydrate (NN‐hydrate) particles with a plate‐like morphology were prepared at 140°C for 2 h in 12 mol/L of NaOH by the hydrothermal method. Bar‐like Na₈Nb₆O₁₉·13H₂O particles were synthesized at as low as 100°C for 2 h. This work demonstrates that by carefully optimizing the reaction condition, we can selectively fabricate niobate structures, including the bar‐like, plate‐like, fibers and cube particles through a direct reaction between NaOH solution and Nb₂O₅. It was found that Nb₆O₁₉^8? formed was an important premise for formation of the NN‐hydrate, and lower [OH^–] was not favorable in preparing the NN‐hydrate as there was an optimum [OH^?]. Through researching effects of the reaction temperature, time, concentration of NaOH, and content of Nb₂O₅ on the NN‐hydrate structure and evolution, the formation mechanism from solid reactants to the intermediate were investigated. After calcining at 800°C, the synthesized NN‐hydrate particles made a phase almost transform to the perovskite NaNbO₃, and the morphology of these calcined particles was still plate‐like.     

Preparation of Morphology‐Controlled Plate‐Like Sodium Niobate Particles by Hydrothermal Synthesis

Sodium niobate (NaNbO₃) particles with plate‐like morphology and hexagonal unit cells were prepared by the hydrothermal method. The result of SEM showed that the hexagonal NaNbO₃ were characterized by plate‐like morphology with a diameter of 5–15 μm and a thickness of 1–2 μm. The crucial influences on the morphology and crystal phase of the NaNbO₃, such as concentration of [OH^?], surfactant, and K⁺:Na⁺ ratio, were established. By further calcination treatment, the plate‐like hexagonal NaNbO₃ particles could be completely transformed into perovskite structure without morphology change. The XRD and EBSD results indicate that the major face of the calcined particles is parallel to the crystallographic (001)_pc (pseudo cubic index) plane. Compared with the traditional high‐temperature molten salt method, this work provides a simpler way to prepare the template for fabricating textured ceramics.     

One‐Step Surfactant‐Free Hydrothermal Synthesis of Platelike Sodium Niobate Template Powders

A one‐step surfactant‐free hydrothermal route is developed to prepare platelike NaNbO₃ template powders. At optimal KOH concentration, pure platelike NaNbO₃ with rhombohedral structure (width and thickness of 20 and 2 μm, respectively) is obtained at 200°C for 16 h. After calcination at 600°C for 4 h, the structure of the hydrothermally synthesized NaNbO₃ changes from rhombohedral to orthorhombic, whereas the initial platelike morphology is maintained. Such characteristics in terms of phase structure, elemental composition, and morphology render our hydrothermally synthesized NaNbO₃ suitable for textured ceramic fabrications.     

Potassium–Sodium Niobate‐Based Lead‐Free Piezoelectric Ceramic Coatings by Thermal Spray Process

Potassium–sodium niobate (KNN)‐based piezoelectric ceramic coatings with single perovskite phase and dense morphology were obtained by thermal spray processing. The structure, morphology, and properties of the coatings deposited at different conditions were investigated, and excellent piezoelectric performance properties were demonstrated. The piezoelectric coefficient observed in the KNN‐based coatings in this study is about one order of magnitude higher than other thermal sprayed lead‐free piezoelectric coatings as reported in literature. With analyses on the differences in the characteristics between KNN and lead zirconate titanate (PZT) compositions and the reaction mechanisms of thermal spray and ceramic synthesis, the reasons for the successful formation of single‐phase perovskite structure with high crystallinity in the thermal sprayed KNN‐based coatings while not in PZT are explained.     

Microstructure of Lead-Magnesium Niobate Ceramics

Lead-magnesium niobate ceramics were processed with excess MgO and by precalcining MgO and Nb₂O₅. Transmission electron microscopy revealed sub-micrometer MgO inclusions in the perovskite phase. The pyrochlore phase exists as submicrometer rectangular crystals in an amorphous PbO matrix. The composition of the pyrochlore phase was calculated to be Pb_2.25Mg_0.27Nb_1.79O₇.     

Domain Structure and Enhanced Electrical Properties in Sodium Bismuth Titanate Ceramics Sintered from Crystals with Different Morphologies

Sodium bismuth titanate (Na_0.5Bi_0.5TiO₃, NBT) crystals with different morphology, wires, plates and cubes, were synthesized by hydrothermal method. The domain structure in as‐synthesized poled NBT ceramics was observed. The results demonstrate that the domain width of the poled NBT ceramics sintered from wire crystals is slightly larger than that of the NBT ceramics sintered from cube crystals, while the NBT ceramics sintered from plate crystals possess the largest domain width. In particular, the poled NBT ceramics sintered from plate crystals exhibit the optimum piezoelectric coefficient and remnant polarization of 87 pC/N and 36.7 μC/cm², respectively, which are 55% and 37% higher than those of the NBT ceramics sintered from cube crystals. The expanded domain width and large grain size are responsible for the improvement of ferroelectric, piezoelectric, and dielectric properties in NBT ceramics.     

Interfacial Structure of Disordered-Ordered Domain Boundaries in Lanthanum-Doped Barium Magnesium Niobate

Microstructural studies on the interfacial boundaries of disordered and 1:2 ordered domains in lanthanum-doped (Ba_0.975La_0.025)(Mg_0.34Nb_0.66)O₃ were performed using high-resolution transmission electron microscopy and synchrotron powder X-ray diffractometry. Disordered and 1:2 ordered domains both coexisted in a single grain. Each domain occupied its own region, and the interfaces were atomically sharp and coherent. The wavelength of the superlattice modulation in the 1:2 ordered domain was ∼0.71 nm. The transition from the 1:2 ordered region to the disordered one could be differentiated at the interface by using the superlattice modulations in the 1:2 ordered domain. From these observations, a deducible interfacial model also was presented.     

Ferroelectric Domain Structure of Lanthanum-Modified Lead Titanate Ceramics

The ferroelectric domain configurations in lanthanummodified lead titanate ceramics have been studied by transmission electron microscopy in conventional, analytical, and high-resolution modes. Results indicate a preponderance of (101), twin-related 90° domains of equilibrium width 50 to 100 nm. These values are consistent with those derived from consideration of elastic strain energy. Below a critical grain size of approximately 0.3 μm, single-domain grains are found. A domain wall energy of 1 mJ/m² was calculated based on this observation. Within such twins are occasionally found ordered domains, displaying little or no misorientation with respect to one another. In addition, (001), 180° domains were observed.     

Dielectric Properties of Lead-Magnesium Niobate Ceramics

Dielectric properties are reported for lead magnesium niobate (PbMg_1/3Nb_2/3/O₃) ceramics which were prepared as single phase (i.e., without pyrochlore) with an improved technique. Dielectric constants of 18000 for pure PMN and 31000 for PMN with 10% PbTiO₃ were achieved; these values are 50% larger than those reported in the literature. The dielectric constant of PMN ceramics was found to increase with both sintering temperature and excess MgO, and subsequent analysis of the microstructures confirmed that this was due to an increase in grain size. This grain-size dependence is explained as a consequence of low-permittivity grain boundaries.     

Domain Structure Evolutions During the Poling Process for [011]‐Oriented PMN–xPT Crystals Across the MPB Region

The poling effect on the [011]‐oriented (1?x)Pb(Mg_1/3Nb_2/3)O₃‐xPbTiO₃ (PMN–xPT) single crystals across the morphotropic phase boundary (MPB) was studied. The dielectric and piezoelectric properties were investigated as a function of the poling field. Domain structure evolutions during the poling process were recorded. In the unpoled PMN–xPT phase diagram, an apparent rhombohedral (R)‐tetragonal (T) phase boundary exists. With room‐temperature poling, the structure transformation sequence strongly depends on the composition. The crystal experiences a direct transition to the 2R/2T domain state in the rhombohedral or tetragonal phase field beyond the MPB region, whereas within the MPB zone it is hard to achieve the 2R/2T engineered configuration although the initial state is either rhombohedral or tetragonal as well. The piezoelectric responses of the MPB·PMN–xPTs are extraordinary weak (d₃₃ ~ 250 pC/N), in contrast to the [011]‐oriented multidomain PMN–xPTs with ultrahigh‐piezoelectric coefficient (d₃₃ > 1000 pC/N). We demonstrate that a slight composition variation near the MPB will significantly influence the domain evolution route and piezoelectricity for the [011]‐oriented PMN–xPT crystals. We also confirm the feasibility to realize the 2R/2T engineered domain configuration for the [011]‐oriented MPB crystals, which will extend the desired portion of the Bridgeman‐grown boules with optimal piezoelectric properties.     

Dielectric and Pyroelectric Properties of Cadmium Niobate Ceramics

The low-temperature dielectric and pyroelectric properties of pyrochlore Cd₂Nb₂O₇ ceramics have been investigated over the temperature range from 10 to 300 K. Dielectric data confirmed that two ferroelectric transitions occurred in the Cd₂Nb₂O₇ ceramics at temperatures near 80 and 192 K. The higher-temperature ferroelectric transition is complex, with evidence for three separate transitions occurring within a narrow temperature range. The temperature and frequency dependencies of the dielectric constant were consistent with both second-order (diffuse) and improper ferroelectric (ferroelastic) effects. Pyroelectric data also confirmed the multiple-transition behavior, with anomalies in the pyroelectric coefficient at temperatures corresponding to the onset of the diffuse ferroelectric transition and the ferroelastic effect. Dielectric loss data (analyzed by both Arrhenius and Cole-Cole formalisms) indicated the presence of three separate relaxation-type dielectric loss mechanisms in this temperature range. Unambiguous explanations for the observed dielectric and pyroelectric phenomena could not be made, but domain effects are believed to be responsible for at least part of the complex nature of the ferroelectric transitions in this material.     

Acoustic Characterization of Poling Effects in PZT Ceramics

Acoustic velocity and attenuation were measured in poled and unpoled lead zirconate titanate (PZT) ceramics prepared by sintering and hot-pressing under-different conditions. Hot-pressed PZT was found to have attenuation values approximately 1 order of magnitude smaller than simered PZT. For both materials, poling caused a decrease in attenuation. Depolarization and phase transition phenomena were also observed at elevated temperatures using a novel laser-ultrasound technique in combination with conventional pulse-echo measurements.     

Two‐step Synthesis of Platelike Potassium Sodium Niobate Template Particles by Hydrothermal Method

Two‐step hydrothermal synthesis of platelike potassium sodium niobate (K, Na)NbO₃ (KNN) template particles was investigated. Platelike K₄Na₄Nb₆O₁₉·9H₂O (KNN‐hydrate) particles were synthesized in 4 mol/L aqueous alkali at 150°C by the sodium dodecyl benzene sulfonate (SDBS) surfactant‐assisted hydrothermal method, which were used as crystal nucleus in the second step of hydrothermal synthesis. The two‐step synthesized KNN‐hydrate particles with 0.6 μm thickness and 7 μm width were prepared at 80°C after 10 h of the second step. After calcination of the KNN‐hydrate particle at 600°C, platelike KNN particles were obtained, which were used as templates for textured ceramics. Particles obtained by the two‐step synthesis showed regular morphology and uniform distribution, with a marked improvement in grain size.     

Grain Size Effects on Piezoelectric Properties and Domain Structure of BaTiO3 Ceramics Prepared by Two‐Step Sintering

A series of highly dense barium titanate (BaTiO₃) ceramics with the average grain size (GS) from 0.29 to 8.61 μm are successfully prepared by two‐step sintering, and the GS effect on piezoelectric coefficient (d₃₃) is systematically discussed in this work. It is found that when GS above 1 μm, d₃₃ can be enhanced with decreasing GS, reaching a maximum value of 519 pC/N around 1 μm due to the high activity of domain wall mobility. Subsequently, d₃₃ rapidly drops with a further decrease in GS owing to the reduced domain density. The results suggest that it is possible to prepare high‐performance BaTiO₃ ceramics by controlling the GS and domain configuration properly, which brings great revitalization to the BaTiO₃‐based piezoceramics.