Electric Fatigue in Ferroelectric Lead Zirconate Stannate Titanate Ceramics Prepared by Spark Plasma Sintering

Ferroelectric lead zirconate stannate titanate ceramics were prepared by spark plasma sintering (SPS). Compared with its counterpart densified by conventional sintering (CS), the SPS material shows a smaller remanent polarization and maximum strain as well as a higher coercive field. Electric fatigue in both materials was investigated. In contrast to CS samples, the SPS specimens show a lower resistance to bipolar electric cycling, characterized by a faster decrease in remanent polarization and maximum strain at cycle number below 10^6.5 and a subsequent slower reduction of the properties at high cycle numbers up to 10⁸.     

Preparation and Properties of Lead Zirconate Stannate Titanate Sintered by Spark Plasma Sintering

Lanthanum-doped lead zirconate stannate titanate ceramics were successfully compacted to full density by spark plasma sintering (SPS). SPS samples densified at 900° or 950°C exhibit nearly full density and fine grain size (about 300 nm). Compared with samples from conventional sintering (CS), SPS samples show larger permittivity accompanied by a deterioration in dielectric loss and special strain hysteresis loops similar to those of ferroelectric relaxors, with a diffuse AFE–FE phase transition and less field-induced longitudinal strain. The differences in the properties of SPS and CS materials are attributed to the variations of the resultant microstructures, especially the grain size, of the ceramics.     

Electric Fatigue in Lead Zirconate Titanate Ceramics

Electric fatigue is a major obstacle for some potential applications of ferroelectric materials based on reversals of spontaneous polarization, such as memory devices and high strain actuators. Our studies of finegrained hotpressed lead zirconate titanate with lanthanum dopant (PLZT 7/68/32) show that fast fatigue is actually caused by contaminated surfaces instead of intrinsic structure deterioration or the change of domain states. All of the specimens with conventionally cleaned surfaces showed significant fatigue after 10₅ switching cycles, but specimens cleaned with a new cleaning procedure did not fatigue even after more than 10⁸ switching cycles. This type of fatigue is found to be due to generated microcracking at the ceramic–electrode interface.     

Synthesis of Lead Zirconate Titanate Stannate Antiferroelectric Thick Films by Sol-Gel Processing

The effects of different sintering procedures on the preparation of antiferroelectric thick films and the structure–property relations in these films were studied. An acetic acid-based sol–gel processing with multistep annealing and suitable lead oxide overcoat layers was developed to fabricate both niobium-doped and lanthanum-doped lead zirconate titanate stannate antiferroelectric thick films. The 5-μm-thick Pb_0.99Nb_0.02(Zr_0.85Sn_0.13Ti_0.02)_0.98O₃ films demonstrate typical square hysteresis loops with a maximum polarization of 40 μC/cm², zero remanent polarization, an antiferroelectric-to-ferroelectric phase transition field of 153 kV/cm, and a ferroelectric-to-antiferroelectric phase transition field of 97 kV/cm. The dielectric constant and dielectric loss are 283 and 1.7%, respectively. The 5-μm-thick Pb_0.97La_0.02(Zr_0.65Sn_0.31Ti_0.04)O₃ films display typical slanted hysteresis loops with very small hysteresis, a maximum polarization of 35.0 μC/cm², and zero remanent polarization. The dielectric constant and dielectric loss are 434 and 2.0%, respectively.     

Field-Forced Antiferroelectric-to-Ferroelectric Switching in Modified Lead Zirconate Titanate Stannate Ceramics

Electric-field-forced antiferroelectric-to-ferroelectric phase transitions in several compositions of modified lead zirconate titanate stannate antiferroelectric ceramics are studied for ultra-high-field-induced strain actuator applications. A maximum field-induced longitudinal strain of 0.85% and volume expansion of 0.95% are observed in the ceramic composition Pb_0.97La_0.02(Zr_0.66Ti_0.09Sn_0.25)O₃ at room temperature. Switching from the antiferroelectric form to the ferroelectric form is controlled by the nucleation of the ferroelectric phase from the antiferroelectric phase. A switching time of <1 μs is observed under the applied field above 30 kV/cm. The polarization and strains associated with the field-forced phase transition decrease with increasing switching cycle, a so-called fatigue effect. Two types of fatigue effects are observed in these ceramic compositions. In one, the fatigue effects only proceed to a limited extent and the properties may be restored by annealing above the Curie temperature, while in the other, the fatigue effects proceed to a large extent and the properties cannot be restored completely by heat treatment. Hydrostatic pressure increases the transition field and the switching time. But when the applied electric field is larger than the transition field, the induced polarization and strain are not sensitive to increasing hydrostatic pressure until the transition field approaches the applied field.     

Phase Transitions in Nanocrystalline Barium Titanate Ceramics Prepared by Spark Plasma Sintering

The bulk dense nanocrystalline BaTiO₃ (BT) ceramics ranging from 20 to 100 nm have been successfully prepared by the spark plasma sintering (SPS) method. Raman spectra and X-ray diffraction were used in combination with electron microscopy to study the evolution of lattice structure and phase transformation behavior with grain growth from nanoscale to micrometer scale for BT ceramics. The results reveal that the SPS technique provides exceptional opportunity to compact ceramics to full density with nanograin size. It is also demonstrated that all structural modifications in nanocrystalline BT and low-symmetry structures still exist in 20 nm nanograin BT ceramics. The ferroelectric properties of crystalline structures were investigated by scanning force microscopy in piezoresponse mode. Piezoelectric hysteresis loop was recorded, demonstrating that 20 nm BT ceramics has a remanent polarization and is switchable by an electric field. Thus, if a critical grain size exists for ferroelectricity, it is less than 20 nm for polycrystalline BT ceramics.     

Sintering Model for Mixed-Oxide-Derived Lead Zirconate Titanate Ceramics

The properties of lead zirconate titanate (PZT) ceramics are determined by the microstructure and chemical homogeneity of Zr, Ti, and dopants within the grains as well as the presence of secondary grain boundary phases. Stoichiometric 53/47 PZT and compositions with 3 mol% PbO excess were prepared by the mixed-oxide process, and were densified by pressureless sintering in oxygen. The influence of PbO content and different La concentrations on the densification behavior was analyzed by dilatometric measurements. Quantitative image analysis showed a different relative density and grain size dependence for samples containing >0.5 mol% additives compared to samples with <0.5 mol% La. On the basis of a model experiment and by using different analytical methods (microprobe analysis, HRTEM, STEM, and Auger spectroscopy) three types of inhomogeneities could be detected in conventionally prepared PZT ceramics: the existence of Ti and La enrichment in the core of PZT grains, and PbO-rich secondary phases in triple junctions as well as in grain boundary films. The results of the microstructural characterization and the analysis of the densification behavior were finally combined to deduce a sintering model based on a Pb-vacancy concentration gradient within the PZT grains.     

Synthesis of Dense Lead Titanate Ceramics with Submicrometer Grains by Spark Plasma Sintering

Dense PbTiO₃ ceramics consisting of submicrometer-sized grains were prepared using the spark-plasma-sintering (SPS) method. Hydrothermally prepared PbTiO₃ (0.1 μm) was used as a starting powder. The powder was densified to ≳98% of the theoretical X-ray density by the SPS process. The average grain size of the spark-plasma-sintered ceramics (SPS ceramics) was ≲1 μm, even after sintering at 900°–1100°C, because of the short sintering period (1–3 min). The measured permittivity of the SPS ceramics showed almost no frequency dependence over the range 10¹–10⁶ Hz, mainly because pores were absent from the ceramics. The coercive field of the SPS ceramics was somewhat higher than that of conventionally sintered ceramics, which could be attributed to the small-grained microstructures of the SPS ceramics.     

Spark Plasma Sintering of Bismuth Titanate Ceramics

Spark plasma sintering (SPS) was used to fabricate bismuth titanate (Bi₄Ti₃O₁₂) ceramics. The densification, microstructure development and dielectric properties were investigated. It was found that the densification process was greatly enhanced during SPS. The sintering temperature was 200°C lower and the microstructure was much finer than that of the pressureless sintered ceramics, and dense compacts with a high density of over 99% were obtained at a wide temperature range of 800°–1100°C. Dielectric property measurement indicated that the volatilization of Bi³⁺ was greatly restrained during SPS, resulting in an unprecedented low dielectric loss for pure Bi₄Ti₃O₁₂ ceramics.     

Fatigue of Lead Zirconate Titanate Ceramics II: Sesquipolar Loading

Piezoelectric actuators generally are driven with unipolar electric load cycles. Although the obtainable strain is increased by small excursions into the negative field regime, this type of load cycle is rarely considered, as its long-time reliability has been questioned. Here, we investigate the degradation of lead zirconate titanate during cycling between high positive and low negative electric fields. Measurements of the large and small signal parameters are used to quantify changes of the material. The fatigue behavior shifts from one best described with existing models of unipolar fatigue to bipolar-like fatigue with increasing field amplitude.     

电疲劳对掺镧锆钛酸铅陶瓷电致畴变行为的影响

在不同电疲劳周次下,对未极化掺镧锆钛酸铅铁电陶瓷在不同直流电场加载下得到的原位X射线衍射(XRD)谱和撤掉不同直流电场后得到的非原位XRD谱的(002)与(200)晶面衍射峰强与电场强度(EA)的关系进行研究。通过计算90°畴变体积分数发现,在电疲劳过程中,电场撤掉前后的90°畴变体积分数均随EA呈蝶形曲线变化。在低电场作用下,铁电畴更趋向于沿平行试样表面方向,而在EA=2 000 V/mm作用下,当电疲劳至106时,材料中有约5%的铁电畴被钉扎,有约6%的铁电畴在电场撤掉瞬间转回平行试样表面方向。电场撤掉后的90°畴变体积分数能直接反映铁电材料剩余极化程度。     

Fatigue of Ferroelectric Polarization and the Electric Field Induced Strain in Lead Lanthanum Zirconate Titanate Ceramics

Fatigue of ferroelectric polarization and the electric field induced strain is investigated in lead lanthanum zirconate titanate ceramics. Fatigue rate increases as the coercive field or the hysteretic nature of the electric field induced polarization of the ferroelectric ceramics increases. The ferroelectric polarization lost during fatigue can be partly restored by decreasing the frequency and increasing the amplitude of the applied ac electric field. The ferroelectric fatigue is modeled phenomenologically by an increase of the energy barrier for the ferroelectric switching and atomistically by a coupling of the spontaneous polarization with defects and/or space charge in the orientation of ferroelectric fatigue.     

Low-Temperature Sintering of Lead Zinc Niobate–Lead Zirconate Titanate Ceramics Using Nano-Sized Powder Prepared by the Stirred Media Milling Process

Lead zinc niobate–lead zirconate titanate (PZN–PZT) nano-sized powders with a diameter of ∼35 nm were fabricated by a high-energy stirred media mill using 50 μm diameter zirconia beads as the milling media at a rotation speed of 4000 rpm for 1 h. The sintering temperature of PZN–PZT was greatly reduced, and a fully densified bulk body was obtained at only 750°C when stirred media milled nanopowder was used. The control of evaporation of lead oxide was very important to obtain high electrical properties due to the increased surface area of nano-sized powders. The ferroelectric hysteresis, piezoelectric d ₃₃ coefficient, and dielectric properties of sintered ceramics using nanopowder were measured and compared with the values obtained from a sintered specimen using conventional milled powders. Remanent polarization, d ₃₃ coefficient, and relative dielectric constant of 750°C sintered stirred media milled powders containing 2% of excess PbO and 1% of 4PbO–B₂O₃ liquid phase were 10.3 μC/cm², 277 pC/N, and 1310, respectively.     

Preparation of Textured Bismuth Titanate Ceramics Using Spark Plasma Sintering

Textured bismuth titanate ceramics were successfully produced using spark plasma sintering and platelike bismuth titanate particles prepared using a molten-salt method. The microstructure and dielectric properties of the samples were investigated. The results showed that the dielectric property of the textured bismuth titanate ceramics was anisotropic in various directions and that spark plasma sintering was an effective sintering technology to obtain textured, dense bismuth titanate ceramics at a low temperature.     

Fabrication of Transparent Lead Lanthanum Zirconate Titanate Ceramics from Fine Powders by Two-Stage Sintering

Transparent lead lanthanum zirconate titanate (PLZT) ceramics were fabricated from fine powders using an inexpensive two-stage sintering technique. The powders were prepared by hydrolysis from low-cost inorganic precursors. In the two-stage sintering method, uniaxially pressed green pellets were densified to nearly theoretical values in an oxygen gas atmosphere during the first-stage sintering, at 1000°C for 1 h, and then residual, free lead oxide in the pellets was removed by second-stage sintering at 1100°C for 12 h. Transparent ceramic with an average grain size of 1.6 μm and a porosity of 1.3% was obtained. The transparency and dielectric characteristics of the present samples were compared with those of hot-pressed samples: The study of the polarization–field hysteresis loops of the present samples yielded a remanent polarization of 6.8 μC/m² and a coercive field of 1.6 kV/cm. The low coercive field of PLZT ceramics could potentially reduce the driving voltage of electrooptic devices in many applications.     

PMN–PT Ceramics Prepared By Spark Plasma Sintering

(1− x )Pb(Mg_1/2Nb_2/3)O₃− x PbTiO₃ (PMN–PT) ceramics of stoichiometric composition were fabricated by conventional pressureless sintering (CS) and spark plasma sintering (SPS). The CS ceramics exhibited a change from relaxor to normal ferroelectric behavior (FE) with increasing PT content. However, low dielectric constants, frequency dispersion, and diffuse phase transition behavior typical for relaxors were obtained for all SPS ceramics. FE and piezoelectric measurements further demonstrated low remanent polarization and strain, high coercive field, and low electromechanical response from SPS materials. Normal dielectric and enhanced FE performance appeared following high-temperature heat treatment after SPS. The effects of grain size, microstructure, and chemical heterogeneity formed during fast sintering are considered.     

Translucent Mg-α-Sialon Ceramics Prepared by Spark Plasma Sintering

The translucent Mg-α-sialon ceramics have been prepared by spark plasma sintering (SPS) α-Si₃N₄ powder with AlN and MgO as the additives at 1850°C for 5 min. The sample possesses a uniform, dense microstructure under the rapid densification of SPS process. The translucent Mg-α-sialon ceramics achieve the maximum transmittance of 66.4% for the sample of 0.5 mm in thickness in the medium infrared region, which could be attributed to the equiaxed microstructure and few glassy phase confirmed by the observation of transmission electron microscopy. The material also exhibits good mechanical properties of high hardness (21.4±0.3 GPa) and fracture toughness (6.1±0.1 MPa·m^1/2).     

Ferroelastic Properties of Lead Zirconate Titanate Ceramics

To increase the reliability of multilayer actuators, calculation of the mechanical stress inside the device during operation is important. This paper shows that the small-signal value of the elastic constant s is not sufficient to describe the complicated behavior of lead zirconate titanate (PZT) ceramics. Therefore, compressive strain and depolarization have been measured as a function of large-signal stress applied parallel to the poling direction. The nonlinear dependence of the strain and depolarization can clearly be explained by domain processes. Soft and hard PZT ceramics have been investigated. In hard PZT, domain switching appears at higher stresses than in soft PZT. Moreover, in hard PZT, the domains partly switch back during unloading. The critical stress (coercive stress) necessary for a domain-switching process shows a dependence on the Zr:Ti ratio that is quite similar to the dependence of the electric coercive field. The influence of an electric field applied parallel to the poling direction and superimposed on the compression experiment also has been examined. The coercive stress depends linearly on the electric field. The linear coefficient of this relation is given by the ratio of depolarization to compressive strain caused by domain switching.     

Electrophoretic Deposition of Lead Zirconate Titanate Ceramics

Electrophoretic deposition (EPD) of submicrometer lead zirconate titanate (PZT) powders in ethanol was conducted to form thick green PZT films up to 160 μm thick. The PZT colloid stability, as a function of pH, was studied by zeta-potential measurement. The electrical condition of the suspension was quantified by conductivity measurement at various pH values. The effect of different applied current densities on the deposition mass was investigated. A kinetic model for the constant-current-density EPD process, taking into account the particle concentration variation, was also examined, based on the experimental results obtained in the present work. An empirical relationship between the kinetic constant and the applied current density has been proposed.     

热压烧结对锆钛酸铅镧陶瓷电卡效应的影响

探讨热压烧结工艺对PLZT(锆钛酸铅镧)陶瓷材料介电击穿性能、饱和极化强度以及电卡性能的影响。通过X射线衍射和扫描电子显微镜,分析陶瓷样品的相组成和微观结构。结果表明,热压烧结法有助于控制陶瓷晶粒的生长,提高陶瓷的致密度并增大陶瓷的介电击穿场强,从而有效提高陶瓷的电卡性能以及电卡转换效率。在328 K(55℃)与478 K(205℃)附近,分别发生低温铁电三方相到高温铁电三方相的相变(FRL-FRH)以及高温铁电三方相到立方顺电相(FRH-Pc)的相变,展现了比较好的弛豫性,在室温下达到3.6 K的绝热温变与1.8×0-7(K·m)/V的电卡转换效率,具有良好的电卡性能。