Preparation of Dense Lead Magnesium Niobate–Lead Titanate (Pb(Mg1/3Nb2/3)O3–PbTiO3) Ceramics by Spark Plasma Sintering

The effect of spark plasma sintering (SPS) on the densification behavior of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ ceramics has been investigated. Specimens with a density of >99% of the theoretical density (TD) were obtained using SPS treatment at 900°C. Through normal sintering at 1200°C, however, the density of the specimen was only ∼92% of TD.     

Microstructure Characterization of the (1−x)La2/3TiO3·xLaAlO3 System

Microstructural characterizations on the (1− x )La_2/3TiO₃· x LaAlO₃ (LTLA) system were conducted using transmission electron microscopy. The presence of La₂Ti₂O₇ and La₄Ti₉O₂₄ phases in pure La_2/3TiO₃ is confirmed by the electron diffraction pattern. When x = 0.1, the ordering due to the A-site vacancies could be confirmed by the presence of antiphase boundaries (APBs) and return ½(100) superlattice reflection. As x increases, the ordering decreases and finally disappears when x = 0.6. The tilting of oxygen octahedra could be demonstrated by the presence of the ferroelastic domains in the matrix and return ½(111) and return ½(110) superlattice reflections in selected area electron diffraction patterns. In pure LaAlO₃, only the antiphase tilting of oxygen octahedra is present due to the presence of return ½(111) superlattice reflection. In the LTLA system of x = 0.1, both the antiphase and in-phase tiltings of the oxygen octahedra are involved; however, in the range of x from 0.3 to 0.9, the antiphase tilting of oxygen octahedra has appeared. The growth of the ferroelastic domains is influenced by the APBs in the matrix.     

Fabrication of Optically Transparent Lead Lanthanum Zirconate Titanate ((Pb,La)(Zr,Ti)O3) Ceramics by a Three-Stage-Atmosphere-Sintering Technique

An easy technique has been developed to fabricate optically transparent lanthanum-modified lead zirconate titanate (PLZT) ceramics. This technique consists of three stages: (1) sintering in an oxygen atmosphere, (2) elimination of pores in a carbon dioxide atmosphere, and (3) elimination of oxygen vacancies in an oxygen atmosphere. The carbon dioxide atmosphere enhances the diffusion of oxygen from the pores to outside the sintered body. The experimental results reveal that use of a carbon dioxide atmosphere effectively decreases residual pores and improves optical transmittance. From commercially available raw powders, an optical transmittance of 51% (wavelength of 550 nm) can be achieved for 0.7 mm thick polished PLZT9/65/35 ceramics using a carbon dioxide atmosphere, whereas the value is only 34% without a carbon dioxide atmosphere. The advantage of this technique is that PLZT ceramics having high optical quality can be obtained using conventional sintering tools.     

Microstructures and Pyroelectric Properties of Multicomposition 0.9PbZrO3·xPbTiO3·(0.1−x)Pb(Zn1/3Nb2/3)O3 Ceramics

The microstructures and pyroelectric properties of multicomposition 0.9PbZrO₃· x PbTiO₃·(0.1− x )Pb(Zn_1/3Nb_2/3)O₃ (PZ–PT–PZN) ceramics were investigated. The PZ–PT–PZN ceramics with nearly theoretical density were prepared by spark plasma sintering at low temperature (800°C) for a very short time (10 min) from two original compositions with x = 0.025 and x = 0.050. The heat treatment was successfully used to control the diffusion between the different compositions in such ceramics. For ceramics heat-treated at 900°C, two pyroelectric peaks corresponding to the original compositions were observed. When the heat-treatment temperature was increased to 1200°C, these two pyroelectric peaks combined into one sharp pyroelectric peak, which corresponds to average composition. When the spark-plasma-sintered ceramics were heat-treated at 950°C, a high and stable pyroelectric coefficient (>100 nC·cm⁻²·K⁻¹) over a wide temperature range (23°–47°C) was obtained. It was found that the pyroelectric properties strongly depended on the microstructures.     

Crystal Structure Refinement of La0.683(Ti0.95Al0.05)O3 Perovskite by the Rietveld Method

The structural parameters of orthorhombic and tetragonal phases (space group P / mmm and P 4/ mmm ) of A-site deficient La_0.683(Ti_0.95Al_0.05)O₃ perovskite have been refined by Rietveld analysis through the X-ray powder diffraction patterns measured in the temperature range from 15° to 500°C. With an increase in temperature the unit-cell parameters a , b , and c increased, while the b / a ratio decreased and became unity between 200° and 400°C. No significant changes were observed for atomic coordinates throughout the temperature studied. These results strongly suggest that the phase transition is induced by lattice distortion.     

Pressure-Induced Phase Transformation of Controlled-Porosity Pb(Zr0.95Ti0.05)O3 Ceramics

Chemically prepared Pb(Zr_0.95Ti_0.05)O₃ (PZT 95/5) ceramics were fabricated with a range of different porosity levels, while grain size was held constant, by systematic additions of added organic pore former (Avicel). Use of Avicel in amounts ranging from 0 to 4.0 wt% resulted in fired ceramic densities that ranged from 97.3% to 82.3%. Hydrostatic-pressure-induced ferroelectric (FE) to antiferroelectric (AFE) phase transformations were substantially more diffuse and occurred at lower hydrostatic pressures with increasing porosity. An ∼12 MPa decrease in hydrostatic transformation pressure per volume percent added porosity was observed. The decrease in transformation pressure with decreasing density was quantitatively consistent with the calculated macroscopic stress required to achieve a specific volumetric macrostrain (0.40%). This strain was equivalent to experimentally measured macrostrain for FE-to-AFE transformation. The macroscopic stress levels were calculated using measured bulk modulus values that decreased from 84 to 46 GPa as density decreased from 97.3% to 82.3%. Good agreement between calculated and measured values of FE-to-AFE transformation stress was obtained for ceramics fired at 1275° and 1345°C.     

Perovskite Formation and Dielectric Characteristics of Pb(Zn1/3Ta2/3)O3 with PbTiO3 Substitution

Perovskite developments in the Pb(Zn_1/3Ta_2/3)O₃–PbTiO₃ system were explored. Formation yields and lattice parameters of the perovskite were determined from X-ray diffractometry results. Weak-field low-frequency dielectric properties of the system ceramics were investigated, followed by microstructure examination. Perovskite started to develop in Pb(Zn_1/3Ta_2/3)O₃ after the introduction of 30 mol% PbTiO₃, whereas complete stabilization was accomplished at 60% substitution. Dielectric relaxation behavior was not substantial across the entire composition range, whereas phase transition modes changed from diffuse to sharp with increased PbTiO₃ fraction.     

Robocast Pb(Zr0.95Ti0.05)O3 Ceramic Monoliths and Composites

Robocasting, which is a computer-controlled slurry-deposition technique, was used to fabricate ceramic monoliths and composites of chemically prepared Pb(Zr_0.95Ti_0.05)O₃ ceramics. The densities and electrical properties of the robocast samples were equivalent to those obtained for cold isostatically pressed parts formed under a pressure of 200 MPa. Three-layer robocast composites that consisted of alternating layers of different sintered densities—93.9%/96.1%/93.9%—were fabricated using different levels of organic pore-former additions. Modification from a single-material to a multiple-material deposition robocaster was essential for the fabrication of composites that could withstand repeated cycles of saturated polarization switching under fields of 30 kV/cm. Furthermore, these composites withstood a poled ferroelectric-to-antiferroelectric phase transformation that was induced by a hydrostatic pressure of 500 MPa, during which strain differences on the order of 0.8% occurred between the composite elements.     

Morphotropic Phase Boundary in the Pb(Zn1/3Nb2/3O3)-BaTiO3-PbTiO3 System

The morphotropic phase boundary (MPB) in the relaxor ferroelectric system Pb(Zn_1/3Nb_2/3O₃)-BaTiO₃-PbTiO₃ (PZN-BT-PT) with 15 mol% BT was investigated through dielectric permittivity and high-temperature X-ray diffraction measurements. It was revealed that MPB is a broad composition region extending from 12 to 18 mol% PT, within which the temperatures of the permittivity maximum are close to the ending temperatures for the phase transformation from coexisting rhombohedral and tetragonal phases to cubic phase on heating. When the specimen is cooled, the starting temperatures for the rhombohedral-to-tetragonal phase transition increase with increasing PT content. The large thermal hysteresis observed by X-ray diffraction is caused by the phase transformation between rhombohedral and tetragonal phases. On cooling, the MPB curves toward the PT-rich side, so that ceramics within this composition range undergoe successive phase transitions from cubic to rhombohedral and from rhombohedral to tetragonal phase. The diffuseness of the paraelectric-to-ferroelectric phase transition is remarkably decreased by the addition of PT. The enhanced dielectric permittivity peak values for the MPB compositions are correlated with the reduced lattice distortion and phase coexistence.     

Structural Analysis of a New Layered Compound: La0.05Sr0.95MnO3

We have found a new phase of La_0.05Sr_0.95MnO₃ with a 30-layer rhombohedral structure by using electron microscopy. The lattice constants were hexagonal axes of a = 0.5444 nm and c = 6.7582 nm. Both weak and strong intensities appeared in selected area diffraction (SAD) patterns. The strong intensities were caused by the periodicity of 15 (Sr,La)O₃ layers that had a new stacking sequence of (cchch)₃. However, the weak intensities indicated that the 15-layer structure has modulation along the c -direction that is twice as long as that of the structure indicated by the strong intensities. We concluded that the modulation of the 30-layer structure was produced by the introduction of two kinds of oxygen octahedra, Mn³⁺O₆ and Mn⁴⁺O₆.     

Fatigue-Free Lead Zirconate Titanate (Pb(Zr,Ti)O3)-Based Capacitors Co-modified by Lanthanum and Lithium for Nonvolatile Memories

Pb_0.98(La_{1− x} Li _x )_0.02(Zr_0.55Ti_0.45)O₃(PLLZT with 0.1 ≤ x ≤ 0.7) thin films were sol-gel-grown on Pt(111)/Ti/SiO₂/Si substrates, employing a thin lead zirconate titanate (PZT) template layer. Films annealed at >550°C showed a highly (111)-oriented preferential growth. Typical values of the switchable remanent polarization (2 P _r) and the coercive field ( E _c) of the PLLZT/PZT/Pt film capacitor for x = 0.3 were 50 μC/cm² and 39 kV/cm, respectively, at 5 V. All the PLLZT/PZT/Pt capacitors (for 0.1 ≤ x ≤ 0.7) exhibited fatigue-free behavior up to 6.5 × 10¹⁰ switching cycles, a quite stable charge retention profile with time, and high 2 P _rvalues, all which assure their suitability for nonvolatile ferroelectric memories.     

Vaporization Studies of the La2O3–Ga2O3 System

The vaporization of the samples of the compositions Ga₂O₃+ LaGaO₃, LaGaO₃+ La₄Ga₂O₉, and La₄Ga₂O₉+ La₂O₃ was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O₂, Ga, GaO, Ga₂O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO₃( s ) and La₄Ga₂O₉( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol⁻¹, respectively. The results were compared with the literature data obtained using other methods.     

Dielectric Properties of Pb(Fe2/3W1/3)O3–PbTiO3 Solid-Solution Ceramics

The dielectric properties of (1− x )Pb(Fe_2/3W_1/3)O₃· x PbTiO₃ solid solutions were investigated from 10² to 10⁶ Hz in the temperature range 150–600 K. The phase transition of Pb(Fe_2/3W_1/3)O₃ (PFW) was shifted by PbTiO₃ (PT) additions to higher temperatures at a rate of 6.3 K/mol% of PT. The temperature dependence of dielectric permittivity showed a sharper transition as the PT content increased. Dielectric measurements in a wide temperature range showed the presence of a second set of dielectric peaks at higher temperatures (350–600 K), besides the ferroelectric–paraelectric phase transition. This second set of peaks vanished when the samples were annealed in nitrogen. The activation energy values for the second relaxation varied between 0.50 and 0.63 eV, in agreement with the conduction activation energy determined for each sample. This relaxation is apparently related to electron holes.     

(Bi1/2Na1/2)TiO3–Ba(Cu1/2W1/2)O3 Lead-Free Piezoelectric Ceramics

(Bi_1/2Na_1/2)TiO₃ with 0–6 mol% Ba(Cu_1/2W_1/2)O₃ (BNT-BCW), a new member of the BNT-based group, has been prepared following the conventional mixed oxide route. The compacted bodies were sintered at 1130°C for 2 h to get dense ceramics. The addition of BCW into BNT ceramics facilitated the poling process because of a reduction in leakage current. 0.995BNT·0.005BCW ceramics exhibit a relatively high piezoelectric constant ( d ₃₃= 80 × 10⁻¹² C/N) and a relatively low dielectric loss (tan δ= 1.5%). Increased amount of BCW was found to increase the dielectric constant and loss of BNT-BCW ceramics and to suppress the grain growth. During sintering, some BCW diffuses into the lattice of BNT to form a solid solution and some remains on the grain boundaries.     

Structure and Magnetic Properties of La0.7Ca0.3MnO3−δ for (3 −δ) < 3.0

Annealing studies were conducted on bulk La_0.7Ca_0.3MnO_3−δ to determine the sensitivity of its structural and magnetic properties to oxygenation conditions. Standard bulk sintering conditions, thin-film annealing conditions for obtaining good magnetoresistive properties, and a reducing anneal, which corresponded to the onset of phase decomposition, were conducted. The main phase formed was a face-centered (fcc) pseudocubic double-perovskite structure, with cell parameters of a ∼ 2 a _p∼ 0.772 nm, where a _p is the single-perovskite cubic cell parameter. A minor superstructure—body-centered pseudotetragonal, with lattice parameters of c = 4 a _p and a =√2 a _p—was observed in samples with (3 −δ) < 3. A maximum of 20% of the superstructure was formed using the most-reducing conditions. The superstructure had a lower critical temperature than the main phase and depressed ferromagnetic order.     

Dielectric-State Analysis in Solid-Solution Pb(Yb1/2Ta1/2)O3–Pb(Lu1/2Nb1/2)O3

The structure and temperature dependence of complex lead perovskite dielectrics were investigated for the system (1 − x )Pb(Yb_1/2Ta_1/2)O₃– x Pb(Lu_1/2Nb_1/2)O₃. Superlattice reflections for the compositions 0.8 < x < 1.0 were observed by X-ray diffractometry, and the temperature-composition dielectric-state diagram was determined. In the present study, the disordered middle composition, with 0.2 < x < 0.8, showed a diffuse paraelectric–ferroelectric phase transition, whereas the ordered end compositions, with 0 ≤ x < 0.2 and 0.8 < x ≤ 1.0, revealed successive sharp paraelectric–antiferroelectric and weak antiferroelectric–ferroelectric phase transitions. The dielectric state was confirmed by examining the variation of polarization ( P ) with electric field ( E ).     

Electromechanical Properties of Pb(Yb1/2Nb1/2)O3-PbZrO3-PbTiO3 Ceramics

The electromechanical and electric-field-induced strain properties of x Pb(Yb_1/2Nb_1/2)O₃· y PbZrO₃·(1− x − y )PbTiO₃ ( x = 0.12, 0.25, 0.37; y = 0.10–0.40) ceramics have been studied systematically as a function of Pb(Yb_1/2Nb_1/2)O₃ (PYN) content and PbZrO₃/PbTiO₃ (PZ/PT) ratio. In addition, the effect of MnO₂ on the electromechanical properties of 0.12Pb(Yb_1/2Nb_1/2)O₃·0.40PbZrO₃·0.48PbTiO₃ was also investigated. The maximum transverse strain values of 1.6 × 10⁻³ for x = 0.12, 1.45 × 10⁻³ for x = 0.25, and 1.36 × 10⁻³ for x = 0.37 were obtained at the compositions which were regarded as the morphotropic phase boundary (MPB). The transverse strain was maximized at the MPB composition. The value of the maximum electromechanical coupling coefficient was 0.69 for y = 0.40 and x = 0.12 composition. In the 0.12Pb(Yb_1/2Nb_1/2)O₃·0.40PbZrO₃·0.48PbTiO₃ composition, the temperature of the maximum dielectric constant decreased and the grain size increased with an addition of MnO₂. The electromechanical coupling coefficient decreased while the mechanical quality factor rapidly increased with an addition of MnO₂. These resulted mainly from the acceptor effect of manganese ions that were produced by doping MnO₂ into the perovskite structure.     

Preparation of 0.95Bi1/2Na1/2TiO3·0.05BaTiO3 Ceramics by an Aqueous Citrate-Gel Route

This report details development of a route to solution-derived (1− x )Bi_1/2Na_1/2TiO₃· x BaTiO₃ powders. The method developed was the citrate-gel method—an evaporative, aqueous technique. When applied to 0.95Bi_1/2Na_1/2TiO₃·0.05BaTiO₃ (BNBT-5), the method produced perovskite phase powders that readily densified in the temperature range of 1000°C. The grain size of the sintered materials was on the order of 1 μm, and the weak-field dielectric properties at 1 MHz were similar to those reported for conventionally prepared materials sintered at higher temperatures (e.g., 1200°C).     

Mechanosynthesis and Thermal Stability of Piezoelectric Perovskite 0.92Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 Powders

The mechanosynthesis of piezoelectric perovskite 0.92Pb(Zn_1/3Nb_2/3)O₃–0.08PbTiO₃ (PZN–PT) by direct mechanochemical activation of the constituent oxides has been studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). This and the PbO flux method are the only two procedures that have succeeded in synthesizing this phase, which has recently been shown to present very high electromechanical response. The thermal stability of the single perovskite phase powders has been studied by differential thermal analysis/thermogravimetry and by high-temperature XRD as a function of mechanical activation time and pressure. The phase was found to transform into a pyrochlore type structure at temperatures above 400°C. The transformation presented a significant time dependence, and it was slowed down by increasing mechanical activation time and by the application of pressures by hot pressing. Sintering experiments were accomplished and 85% density, 77% perovskite-phase ceramics were obtained after heating at 1000°C for 1 h. Hot pressing at this temperature failed to increase the percentage of perovskite phase. Results are discussed, and procedures for obtaining dense single-phase PZN–PT-based ceramics with ultrahigh piezoelectricity are proposed.     

Dielectric Properties of the Fluorite-like Bi2O3–Nb2O5 Solid Solution and the Tetragonal Bi3NbO7

Our analysis of the microwave dielectric properties of the δ-Bi₂O₃–Nb₂O₅ solid solution (δ-BN_ss) showed a continuous increase in permittivity and dielectric losses with an increasing concentration of Nb₂O₅. The only discontinuity was found for the temperature coefficient of resonant frequency, which is negative throughout the entire homogeneity range but reaches a minimum value for the sample with 20 mol% Nb₂O₅. At the same composition there is a discontinuity in the grain size of the δ-BN_ss ceramics. For the sample containing 25 mol% Nb₂O₅ two structural modifications were observed. A single-phase tetragonal Bi₃NbO₇, in the literature referred to as a Type-III phase, is formed in a very narrow temperature range from 850° to 880°C. A synthesis performed below or above this temperature range resulted in the formation of the end member of the δ-BN_ss homogeneity range. Compared with the δ-BN_ss the Bi₃NbO₇ ceramics exhibit lower microwave dielectric losses, an increased conductivity, and a positive temperature coefficient of resonant frequency.