Vaporization of Lead Zirconate-Lead Titanate Materials

A thermogravimetric investigation of the vaporization of cold-pressed Pb(Zr_0.65Ti_0.35)O₃ in vacuum from 690° to 1130°C shows that the vaporization occurs in two steps. An initial loss of 1 to 7 wt% proceeds by a mechanism with a logarithmic time dependence and represents the vaporization of the unreacted PbO that remains after the initial calcining. The second step proceeds by a slower, diffusion-controlled mechanism with a parabolic time dependence. It is shown that this rate-determining step is bulk diffusion across a thickening lead-depleted layer in the cold-pressed pellet, despite pore volumes of 24 to 42%. The rate constant, g/cm² sec½, for the parabolic weight loss process is expressed by: log K = (3.37 ± 0.14) - (8.46 ± 0.16) (10³/ T _K). The activation energy is 38.7 ± 2.0 kcal/mole.     

Reaction Sequence in the Formation of Lead Zirconate-Lead Titanate Solid Solution: Role of Raw Materials

The reaction sequence in the formation of Pb(Zr_0.6Ti_0.4)O₃ (PZT 60/40) was investigated as a function of PbO and ZrO₂ raw material variations, using powder X-ray diffraction. Particular emphasis was placed on the final stages of reaction in the formation of PZT solid solution. Based on the present work, a more detailed reaction sequence is being proposed for Pb(Zr_0.6Ti_0.4)O₃ and similar compositions. This reaction sequence is believed to account for many of the apparent discrepancies which have appeared in the literature.     

Lead Zirconate-Lead Titanate Piezoelectric Ceramics with Iron Oxide Additions

An investigation of the effects of small additions of Fe₂O₃ to lead zirconate-lead titanate ceramics with compositions between 45 and 60 mol% lead zirconate has shown that, with 2 moles of PbO to balance each mole of Fe₂O₃, the solubility of Fe₂O₃ was about 0.8 wt% in compositions near the tetragonal-rhombohedra1 boundary. The dielectric constant and dissipation factor for both rhombohedra1 and tetragonal materials were decreased by addition of iron oxide, whereas the mechanical quality factor and frequency constant were increased. The dependence of the electromechanical properties on grain size was qualitatively similar for both undoped and iron oxide-doped materials; the presence of iron oxide inhibited grain growth and lowered the limiting grain size below which the electromechanical properties change rapidly with grain size.     

Reaction Mechanisms in the Formation of Lead Zirconate Titanate Solid Solutions under Hydrothermal Conditions

Reaction mechanisms in the formation of PZT solid solution were studied under hydrothermal conditions (Pb/(Zr+Ti) = 1.0 to 1.9, Zr/Ti = 0/10 to 10/0, 1 M to 5 M KOH, 100° to 220°C, 2 h). A yellow tabular crystallite with tetragonal symmetry and Pb/Ti ∼ 2 was formed at 100° to 130°C. A PZT crystallite was formed just above 150°C. The crystallite was a mixture of Ti-rich PZT and Zr-rich PZT phases. When the temperature and KOH concentration were increased, the composition of the PZT product tended to be homogeneous. The PZT in the morphotropic phase boundary zone was formed at Zr/Ti = 5/5, 5M KOH, 220°C, 2 h. Neither PbTiO₃ nor PbZrO₃ was detected as a separate phase under the above hydrothermal conditions.     

Grain Growth and Densification of Hot-Pressed Lead Zirconate-Lead Titanate Ceramics Containing Bismuth

Grain growth and densification were studied in a hot-pressed ferroelectric composition of 65/35 (Zr/Ti molar ratio) lead zirconate-lead titanate containing 2 at.% bismuth. The grain growth and densification rate processes were measurable from 1050° to 1300°C and from 700° to 1100°C, respectively. Grain growth as a function of time followed a ⅓ power law ( D (grain size) = kt ^⅓). An activation energy of 95 kcal/mole was calculated. Densification was a two-stage process as a function of time: a rapid initial stage and a slower final stage. The initial stage of densification behaved viscously with an activation energy of 36.7 kcal/mole. Grain boundary sliding and the Nabarro-Herring mechanism of stress-directed movement of vacancies are suggested as the densification mechanisms. Results indicated that densification is sensitive to stoichiometry and to additives.     

Barium Zirconate-Lead Titanate Ferroelectric Ceramics

Hot-pressed barium zirconate-lead titanate ceramics have been examined to determine crystal-line symmetry and dielectric, ferroelectric, and piezoelectric properties. Barium zirconate additions to lead titanate form solid solutions with a decreasing tetragonal c/a axial ratio until at 60PbTiO₃–40BaZrO₃ to 75PbTiO₃–25BaZrO₃ the ceramics have coercive forces low enough to permit polarization. High-coercive-force piezo-electric ceramics are formed with k_p up to 0.30 and d₃₃ up to 110 × 10⁻¹² coulombs per newton.     

Positions of the Morphotropic Phase Boundaries in Solid Solutions of Lead Titanate

The morphotropic phase boundary, which, separates the tetragonal and rhombohedral ferroelectric solid solutions in the (1 - x)PbTiO₃ +xPbBO₃ and (1 - x)PbTiO₃ + xPb(B', B')O₃ systems, is almost always close to x = 0.5, when the relation of numbers of the smaller Ti and larger B or (B', B') ions is near to 1:1. Then, the smaller and larger ions can be ordered chequerwise. As the main cause of the morphotropic boundary existence, the paper considers growth of the B, B', B' ion sizes at increase of x. Nevertheless, it is assumed that its appearance can be triggered by the ion ordering, expected at x ≈ 0.5. Possibly, such an ordering could assist also to formation of some intermediate monoclinic ferroelectric phase.     

Synthesis of Lead Nickel Niobate–Lead Zirconate Titanate Solid Solutions by a B-site Precursor Method

A modified processing method for lead nickel niobate–lead zirconate titanate (Pb(Ni_1/3Nb_2/3)O₃–Pb(Zr,Ti)O₃, PNN–PZT) solid solutions is presented. This method is based on the high-temperature synthesis of a precursor that contains all the B-site cations (Ti, Zr, Ni, and Nb). This synthesis yields a diphasic mixture that contains a ZrTiO₄-like phase and a rutile-like phase. Both phases exhibit a cationic valence of 4; thus, it is concluded that the mixing of Ni and Nb cations is adequate for the preparation of PNN–PZT solid solutions. Indeed, a pure perovskite phase has been obtained after calcination with lead oxide for compositions that contain 40 and 50 mol% PNN. Moreover, their electromechanical properties have been shown to be superior to values reported for standard columbite routes. This conclusion has been interpreted in terms of enhanced chemical homogeneity.     

Field and Frequency Dependence of the Dynamic Hysteresis in Lead Zirconate Titanate Solid Solutions

Dynamic hysteresis of Nb‐doped Pb(Zr_1?xTi_x)O₃ (PZT, 0.20 ≤ x ≤ 0.60) ceramics were studied systemically at different field (E) and frequency (f). The hysteresis loops were strongly dependent on E and f. The measured coercive fields (E_c0) were far lower than the calculated values based on the Landau–Ginzburg theory, and increased dramatically from the rhombohedral phase to the tetragonal phase while had less variation with composition in the same phase. With increasing E or f for each composition, three types of loops were observed: linear, minor, and saturated loops. The cross fields (E₀), cross polarizations (P₀), and hysteresis areas (<A>) showed different variation regularities with f. Similar varying curves were observed for all PZT ceramic samples by normalizing E₀, P₀, and <A> with E/E_c0, which indicated the same polarization switching for different domain structures. Further analyses revealed that the switching processes can be divided into three stages: space charge polarization, domain switching, and steady state. The first and second stage occurred at ~0.5 and ~1.5 E/E_c0, respectively. These results would be very helpful to further understand the polarization switching of ferroelectric ceramics.     

Formation and Transformation of Cubic Lead Niobate Pyrochlore Solid Solutions

Formation of lead niobate pyrochlores by the reaction of PbO with Nb₂O₅ in various molar ratios and at various temperatures has been investigated. It has been shown that for a PbO/Nb₂O₅ molar ratio between 1.5 and 3.0, a pyrochlore solid solution having cubic symmetry is formed at low temperatures (∼500-600°C). X-ray diffraction data show evidence for the existence of compositional inhomogeneity for this phase which contains part of the Pb in the Nb site leading to the formula Pb_{1.5+ x} (Nb_{2- y} Pb _y )O_7-delta (0.0 < x < 0.5; 0.0 < y < 0.5). A phase-pure Pb-deficient pyrochlore (Pb_1.5Nb₂O_6.5) and Pb-excess pyrochlores (Pb_2.5Nb₂-O₈, Pb₃Nb₂O₈) are formed by the decomposition of this cubic solid solution and subsequent reaction at high temperatures. These results indicate that the mechanism of the structural transformation from the low-temperature (LT) cubic solid solution to the corresponding high-temperature (HT) phases is reconstructive.     

Vaporization of Lead Zirconate-Lead Titanate Materials: II,Hot-Pressed Compositions at Near Theoretical Density*

The vaporization of lead monoxide from lead zirconate-lead titanate materials was studied by thermogravimetric techniques in vacuum with hot-pressed pellets of near theoretical density. The initial step of the complex, two-step vaporization observed in cold-pressed pellets in an earlier investigation is absent. The rate-determining step for both the porous, cold-pressed, and pore-free, hot-pressed materials is the bulk diffusion of lead across a thickening lead-depleted layer at the surface of the pellet and results in a parabolic time dependence. Although parabolic rate constants are slightly lower overall for the dense materials, the activation energies for the weight loss process remain approximately 38.5 and 35 kcal/mole for the pure and Bi-doped Pb(Zr_0.65Ti_0.35)O₃ compositions, respectively. Vaporization mechanisms and rates were unchanged under nitrogen or oxygen pressures up to 50 torr at 1000°C. Alumina Knudsen cells were used to study the equilibrium vapor above a powdered, hot-pressed Pb(Zr_0.65.Ti_0.35)O₃ composition. Mass spectrometric analysis of the vapor at 950°C showed that the vapor species and their relative abundances were the same as the equilibrium vapor species above pure lead monoxide. Continuous weight loss data from these cells gave an apparent vapor pressure that was 2% of the vapor pressure of pure lead monoxide at a given temperature, but gave the same second law enthalpy of vaporization. The overall vaporization process can be represented by the reversible equilibria: the diffusion of Pb²⁺ and O²- to the surface through a lead-depleted region whose phases and compositions are determined by the subsolidus phase relations in the PbO-ZrO₂-TiO₂ ternary system, the reaction at or near the surface to form PbO, and the subsequent vaporization of PbO.     

Hydrothermal Precipitation of Lead Zirconate Titanate Solid Solutions: Thermodynamic Modeling and Experimental Synthesis

A previously developed thermodynamic model of hydro-thermal synthesis of ceramic powders has been extended to include cases when solid solutions are formed. The model has been applied to the synthesis of a series of lead titanate zirconate solid solutions PbZr _x Ti_{1– x} O₃ (PZT, 0.46 < x ≤ 0.75). It predicts the optimum conditions (i.e., reagent, concentration, pH, and temperature) for the precipitation of phase-pure homogeneous PZT, provided that the reactants are well mixed. The predictions have been experimentally corroborated using coprecipitated hydrous oxide Zr _x Ti_{1– x} C₂ n H₂O (0.46 < x ≤ 0.75), as a precursor for Ti and Zr and water-soluble lead acetate or nitrate salts as a source for Pb. When mixtures of hydrous oxides ZrO₂· n H₂O and TiO₂· n H₂O were employed as Ti and Zr precursors, independent PbTiO₃ and PbZrO₃ precipitates rather than the PZT solid solutions formed. These results can be rationalized on the basis of reaction kinetics where thermodynamic modeling includes or excludes the possibility of solid-solution formation.     

Dielectric Aging in Tetragonal Solid Solutions of Calcium Titanate in Barium Titanate

The dielectric aging phenomenon was observed in tetragonal polycrystalline solid solutions of calcium titanate in barium titanate between 30° and 90°C, and the 90° ferroelectric domain microstructures were analyzed using replica electron microscopy and statistical procedures. The results show a correlation between aging rate and 90° domains that may be satisfactorily interpreted in terms of the internal residual stress relaxation theory of aging.     

Formation of Zirconia Titanate Solid Solution from Alkoxides

In the system ZrO₂–TiO₂, ZrTiO₄ solid solutions prepared by the simultaneous hydrolysis of zirconium and titanium alkoxides crystallize at low temperatures from amorphous materials between 30 and 70 mol% TiO₂. As zirconium is substituted for titanium, the solid solutions can be indexed in an orthorhombic unit cell with a and c decreasing linearly from 0.4832 to 0.4778 nm and from 0.5063 to 0.5002 nm, respectively, and b increasing linearly from 0.5401 to 0.5478 nm. The volume of the unit cell decreases continuously with increasing TiO₂ content. At higher temperatures the solid solutions decompose into ZrTiO₄ and either ZrO₂ (monoclinic) or TiO₂ (rutile), depeanding on the starting composition.     

Synthesis and Thermal Stability of Aluminum Titanate Solid Solutions

Aluminum titanate solid solutions with empirical formulas of Al₂Ti_1-xZr_xO₅, Al_6(2-x)(6+x)Si_6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ were synthesized by reaction sintering and annealed at 900° to 1300°C in air to evaluate the thermal stability. Substitution of Al in Al₂TiO₅ by Si and 2Al by Mg and Ti ions to form solid solutions such as AI_{6(2-x)/(6+x)l-Si}6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ was effective in controlling the thermal decomposition, but substitution of Ti by Zr had little effect.     

Dielectric Properties of Strontium Titanate Solid Solutions Containing Niobia

SrTiO₃ solid solutions containing Sr_0.5NbO₃ were studied by X-ray diffraction and dielectric measurements. Single-phase perovskite solid solutions were formed up to the composition containing 30 mole % Sr_0.5 NbO₃. The addition of niobium resulted in a symmetry change from cubic (SrTiO₃) to tetragonal (solid solution) and the c/a ratio increased with increasing niobium content. The dielectric constant for the solid solutions was lower than that for SrTiO₃. Pronounced dielectric relaxation peaks and dispersion in permittivity were observed for the composition 7SrTiO₃:3Sr_0.5NbO₃. The activation energy for the relaxation process is about 0.42 ev. It is suggested that the existence of the A-site vacancy in the perovskite ABO₃ lattice introduced by the niobium substitution distorts the oxygen octahedra, producing more than one possible "noncentral" site for the Ti⁴⁺ ion. The relaxation arises from the thermal motion over potential barriers separating these alternative sites.     

Formation of Lead Zirconate Titanate Powders by Spray Pyrolysis

Twin-fluid atomization spray pyrolysis (SP) has been investigated for the production of lead zirconate titanate (PZT) powders, using aqueous solutions of lead acetate and zirconium and titanium alkoxide precursor reagents. The particle size distribution of the PZT powder showed a d ₅₀ value of 0.3 μm, but with a small fraction of relatively large particles, several micrometers in size. Most particles were spherical but many of the largest particles, in the size range ca. 1–5 μm, were irregular. It was demonstrated that the morphology of the final PZT powder was controlled by decomposition processes occurring during the initial drying stages, at ≤200°C. A pyrochlore or fluorite-type intermediate crystalline phase was present in the final powders, but when the maximum reactor temperature was raised, and/or when the levels of excess lead in the starting solutions were increased, the proportion of the desired perovskite phase increased. However, at the highest process temperatures studied, ∼900°C, small crystallites of another phase formed on the surface of the PZT particles; these were probably lead oxide carbonate particles. Overall, a starting solution composition containing around 5 mol% excess Pb, and a maximum reactor temperature of 800°C, were selected as offering the most suitable conditions for producing PZT (52/48) powder, with minimal secondary phases(s). Preliminary densification studies showed that the powders could be sintered at 1150°–1200°C to give pellets of 95%–96% theoretical density.     

Particle Formation During Spray Pyrolysis of Lead Zirconate Titanate

A twin-fluid atomization spray pyrolysis technique has been used to produce lead zirconate titanate (PZT) powders from a sol–gel precursor solution. Samples were removed from ports sited along the reactor in order to examine particle development at progressive stages of pyrolysis. The total time in the reactor was 2.6 s. The size and shape of the particles showed no change between the first port (190°C) and the hottest part of the reactor (820°C), indicating that the physical structure of the particles was established during the initial drying stages. The powders were mainly composed of spherical particles, but irregular forms were also present, which were thought to result from the inward collapse of hollow gelatinous particles. Crystallization of PZT commenced at around 700°C, initially to a pyrochlore or fluorite intermediate phase, with the desired perovskite phase developing between 790°C and 815°C. However, a minor amount of the pyrochlore/fluorite phase persisted in the final powder. The final powders also contained basic lead carbonate, 2PbCO₃·Pb(OH)₂, which existed in the form of elongated crystallites on the surface of the PZT particles.     

Polymorphic Phase Transformation of Lead Monoxide and Its Influence on Lead Zirconate Titanate Formation

The polymorphic phase transformation of PbO from massicot to litharge phase is easily produced by mechanical force. The transformation is relatively quick by wet ball-milling. The additions of TiO₂ and ZrO₂ tend to retard it. A second-order exponential relation is observed between the fractional conversion and the milling time for pure PbO, PbO-TiO₂, and PbO-ZrO₂-TiO₂ systems with a rutile constituent. This exponential relation is changed into a first-order logarithmic relation when anatase instead of rutile is used. Although the PbO _ss phase is observed irrespective of the phase of the PbO in the raw materials, this PbO _ss peak is more obvious when massicot PbO or anatase TiO₂ is used. The formation of PbO _ss phase is also dependent on the reactivity of the ZrO₂, and its presence can affect the formation and piezoelectric properties of PZT ceramics.