PbO Flux Method Research Articles

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

The mechanosynthesis of piezoelectric perovskite 0.92Pb(Zn1/3Nb2/3)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.

High Orientation of PZT Single Crystal Grains and Piezoelectric Properties in 1-3 Type Composite Sheet

Since it is very difficult to grow PZT single crystal in a satisfying size in the composition near the morphotropic phase boundary (MPB, PZ/PT=52/48), there is few crystallography data on the ferroelectric and piezoelectric properties. According to our experience, the MPB PZT crystals obtained by a PbO flux method, though no more than several 10 μ m in size, exhibited a habit well-developing the six planes 100. From this cube-habit, it is suggested that the PZT grains can be arranged to the 100 plane. Such the situation is possible in a composite sheet of 1-3 type [1]. Though 1-3 composites made of PZT ceramics are widely studied for ultrasonic transducer application [2, 3], the one consisting of PZT single crystal grains is of quite new style and allow us to meet interesting physical properties, possibly like of single crystal.

Growth and characterization of PMNT relaxor-based ferroelectric single crystals by flux method

Relaxor ferroelectric single crystals of lead magnesium niobate-lead titanate (PMNT), in the vicinity of the morphotropic phase boundary, were prepared by the PbO flux method. The microstructure and the growth mechanism of the as-grown single crystals were investigated by scanning electronic microscopy and X-ray diffraction. The growth morphology of PMNT crystals shows the presence of a quasi-cubic {1 0 0} perovskite habit. The growth of the {1 0 0} faces proceeded by means of a layer mechanism and was found to be controlled by 2D nucleation starting at the crystal corners or edges. The perovskite stabilization and phase development were also investigated by thermal analysis. Significant phase transformation from the perovskite to the pyrochlore occurred when PMNT crystals were heated at a relatively high temperature. This suggested that the soaking temperature seriously affects the crystal growth behavior in the PMNT system.

Preparation and properties of single crystals in the system of pb{(Mg1-xZnx)1/3Nb2/3}O3 by flux method

Single crystals of perovskite type Pb{(Mg1-xZnx)1/3Nb2/3}O3 (PMZN) solid solutions were prepared by a PbO flux method with excess additions of MgO and/or ZnO. The optimum condition to obtain relatively large single crystals in this system was determined. As a result, rapid quenching is essential for the formation of the perovskite PMZN single crystals, which showed the successive change of the lattice parameters. However, the lattice parameter did not change in proportion to the bath composition (Fig. 1). These results exhibited the difficulty to prepare the single crystals with relatively high content ZnO more than 40%.

Thermodynamic Stability and Mechanisms of Formation and Decomposition of Perovskite Pb(Zn1/3Nb2/3)O3 Prepared by the PbO Flux Method

The mechanisms of formation and decomposition of the ferroelectric perovskite PZN [Pb(Zn1/3Nb2/3)O3] were examined. The formation of perovskite PZN from the excess molten PbO environment is characterized by an inititial rapid formation of pyrochlore phase, followed by a subsequent reaction between the intermediate pyrochlore phase, ZnO, and liquid PbO to produce perovskite phase. The pure perovskite PZN crystal prepared by the PbO flux method is thermodynamically unstable over a wide range of temperature (600° to 1400°C), yielding pyrochlore phase and PbO as the decomposition products. The decomposition reaction of perovskite PZN proceeded uniformly with a spatial homogeneity throughout the specimen. The estimated activation energy of the decomposition reaction is approximatley 18 kcal/mol.

Preparation of [formula omitted] powder by molten salt method

The preparation of single-phase perovskite Pb( Mg 1 3 Nb 2 3 ) O 3 powder or ceramics has been known to be quite difficult by means of the conventional mixed oxide method. Its high dielectric constant, however, makes Pb( Mg 1 3 Nb 2 3 ) O 3 one of the most promising perovskite materials. In the present paper, a conventional mixed oxide method was reinvestigated and non-conventional ceramic processing methods such as the PbO flux method and a KCl molten salt method have been performed to prepare single-phase Pb( Mg 1 3 Nb 2 3 ) O 3 powder. It was found that the KCl molten salt method is extremely successful only when excess PbO is present in the salt, because pyrochlore Pb 1·83Nb 1·71Mg 0·29O 6·39, which inevitably appears upon synthesizing Pb( Mg 1 3 Nb 2 3 ) O 3 , can be completely transformed to perovskite.

Preparation and Several Properties of Ferroelectric Powder Pb{(Mg, Zn)<sub>1/3</sub>Nb<sub>2/3</sub>}O<sub>3</sub> (Part 1)

Powders containing perovskite type double oxides of the series Pb(Mg1/3Nb2/3)O3-Pb(Zn1/3Nb2/3)O3 as a main constituent have been prepared by firing the batch mixture according to the chemical formulae. It is interesting to know the substitutional solid solubility of ZnO for MgO in this series because the latter end member Pb(Zn1/3Nb2/3)O3 is known not to be prepared under ordinary firing conditions. The substituted ratio was estimated by the precise determination of lattice constant and its comparison with the Vegard's line. It was shown that MgO in the Pb(Mg1/3Nb2/3)O3 was able to be substituted by ZnO up to 35 or 40mol% depending on the firing temperature (950° or 1100°C). Peak temperature of capacitance (corresponding to Curie point) of the fired disc varied from 0° to 40°C depending on the substitutional ratio of this series. Parallelism between the observed data and the predicted line connecting the Curie points of the end members suggested that the substitution of ZnO for MgO proceeded up to 30-35mol% in this series. However, a certain unbiguity still remained as the observed data at the higher substitutional ratios of the batch mixture were shown to be considerably high. Single crystals of this series were prepared by the “PbO-flux method”. According to the variation of the lattice constant of perovskite type crystal along with the feed composition, it was judged that the homogeneous substitution of ZnO for MgO went up to 30-35mol% in this series. Summarizing the above-mentioned results it has been concluded that the solid solubility of ZnO for MgO in this series is 35±5mol%.

Magnetic Anisotropy of Vanadium-Indium-Substituted YIG

Crystals of the general composition {Y3−2xCa2x} [Fe2−yIny] (Fe3−xVx)O12 were grown by the PbO flux method. Electron microprobe studies revealed zoning patterns of varying x within these crystals, while the indium content was found uniform among all crystals from the same crucible. Average x and y was determined by measuring the saturation magnetization Ms and the Curie temperature. Three series of crystals were produced with y=0, 0.11 and 0.35, and x ranging from 0.19 to 0.82. Ferrimagnetic resonance between 4 and 300 K yielded the anisotropy constants K1 and K2. These single-crystal data were supplemented by the results of line-shape analysis of polycrystalline samples with low Ms. At T=0, | K1 (x) | is nonlinear with a maximum at x≈0.2. The reduced temperature dependence of the reduced anisotropy field was found independent of composition.

The Crystal Structure and Piezoelectricity of The System Pb(Zn1/3Nb2/3)O3-PbTiO3

Crystal structure and piezoelectric property of the system Pb(Zn1/3Nb2/3)O3-PbTiO3 were studied with single crystals and ceramic polycrystals. A PbO-flux method was used for growing the single crystals. Pure Pb(Zn1/3Nb2/3)O3 single crystal has perovskite structure of rhombohedral system. The crystal structure changes to tetragonal when dissolved PbTiO3 exceeds 10 mol %. Piezoelectric activity increases remarkably in the composition range where the phase change is observed i.e., the electromechanical coupling factor k33 amounts to 75% and the dielectric constant e33 is 4200. These tendencies are similar to those in the solid solution PbZrO3-PbTiO3.Pb(Zn1/3Nb2/3)O3 ceramic polycrystal has a pyrochlore structure and shows undetectable piezoelectricity. A compound of perovskite structure and that of pyrochlore structure coexist in specimens containing less than 40 mol% PbTiO3. The coexistence gives rise to extremely low piezoelectric activity.

Dependence on Heat-treatment of the Ferromagnetic Resonance Width in Ni Ferrite

The ferromagnetic resonance absorption in Ni ferrite is observed at 9300 MC. The single crystal specimen is prepared by PbO flux method, and has the composition Ni 0.96 Fe 2.04 O 4 . The experiments are made repeatedly on one and the same specimen after various heat-treatments. The width, anisotropy constants, and g -factor change appreciably by the treatments. The results can be accounted for qualitatively to some extent, as caused by the change in the spinel phase itself or by the effect of precipitation. The minimum width obtained of 15 Oe is far narrower than that reported previously for the ferrites.