Zhiwen Yin

Growth mechanism and growth habit of oxide crystals

In this paper, the growth mechanism and growth habit of oxide crystals are investigated. Firstly, from the kinetics viewpoint, the growth mechanism of ZnO powders under hydrothermal condition is disclosed starting from the hypothesis of growth unit. It is concluded that the growth mechanism of oxide crystals contains the formation of growth units and the incorporation of growth units into the crystal lattice by a dehydration reaction. Then, a new growth interface model of oxide crystals in solution is established on the basis of an ideal growth mechanism of oxide crystals, which considers the interface structure of the crystal as the stacking order of coordination polyhedrons with OH− ligands. Finally, a new rule concerning the growth habit is deduced considering the relation between the growth rate and the orientation of the coordination polyhedron at the interface. It is concluded that the direction of the crystal face with the corner of the coordination polyhedron occurring at the interface has the fastest growth rate; the direction of the crystal face with the edge of the coordination polyhedron occurring at the interface has the second fastest growth rate; the direction of the crystal face with the face of the coordination polyhedron occurring at the interface has the slowest growth rate. In terms of this rule, the growth habit of ZnO crystal particles and AlO(OH) crystal particles, and the effect of reaction medium on the growth habit are successfully explained.

Phase transitional behavior and piezoelectric properties of the orthorhombic phase of Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

We report on the observation of an orthorhombic ferroelectric phase in 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystals, whose polarization is along 〈011〉 direction and stability can be altered by poling conditions. We studied the piezoelectric properties on poled 〈011〉 crystals, in both monodomain and polydomain states, and found that the piezoelectric d32 coefficient, which is the piezoelectric response in perpendicular to the poling direction, is positive in both cases. Based on the phenomenological theory, we show that this is possible in a crystal with the electrostrictive coefficients Q11>Q44–Q12.

GROWTH AND CHARACTERIZATION OF RELAXOR FERROELECTRIC PMNT SINGLE CRYSTALS

Abstract The results of relaxor ferroelectric PMNT single crystals grown by a modified Bridgman method with the size more than φ25×40 mm are reported in this paper. The crystal habit and growth mechanism of PMNT has been investigated to optimize the growth conditions. The reason that the PMNT crystals will grow along [111] direction under the condition of no seed crystals has been illustrated. It was found that the domain configuration in PMNT is much complicated and they will dominate the piezoelectric properties of PMNT single crystals. The characterization of PMNT single crystals shows that PMNT 67/33 single crystals have d33>2000pC/N. ϵ∼5500, k33-0.93.

GROWTH, CHARACTERIZATION AND PROPERTIES OF RELAXOR FERROELECTRIC PMN-PT SINGLE CRYSTALS

Abstract The present paper reviews the recent progress of studies on growth, characterization and properties of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals in Shanghai Institute of Ceramics. Their chemical compositions were selected near the rhombohedral-tetragonal morphotropic phase boundary, more favorable in the rhombohedral phase. The dominant method used for growing the single crystals was the modified Bridgman technique, by which the single crystal boules with the size larger than 25mm diameter and 50 mm long were grown. Different orientation thin plates with thickness less than 1mm were cut from the boules and their domain structures, morphology and properties were characterized. Their phase transition were discussed as well.

The phase transition sequence and the location of the morphotropic phase boundary region in (1 − x)[Pb (Mg1/3 Nb2/3)O3 ]–xPbTiO3 single crystal

Piezoelectric constant and temperature-dependent dielectric constant measurements have been performed on 110-oriented (1 − x)Pb (Mg1/3 Nb2/3)O3–xPbTiO3 crystals with different compositions under different poling fields. The width of the morphotropic phase boundary region (0.30 < x < 0.35) is determined on the basis of two abnormal regions of the dielectric and piezoelectric properties. An irreversible rhombohedral–monoclinic MA–monoclinic MC–tetragonal phase transition sequence was observed directly from the dielectric constant versus temperature results for 001-poled rhombohedral crystals with compositions near the rhombohedral–monoclinic phase boundary. The structure of the morphotropic phase is shown to be monoclinic with space group Pm.

Single crystal PMN-0.33PT/epoxy 1-3 composites for ultrasonic transducer applications

Lead magnesium niobate-lead titanate 0.67Pb (Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-0.33PbTiO/sub 3/ (PMN-0.33PT, abbreviated as PMN-PT) single crystals were used to fabricate PMN-PT/epoxy 1-3 composites with different volume fractions of PMN-PT ranging from 0.4 to 0.8. The electromechanical properties of the 1-3 composites were determined by the resonance technique. Theoretical modeling of the 1-3 composites matched quite well with the measured material properties. It was demonstrated that the thickness electromechanical coupling coefficients of the composites could reach as high as 0.8. A 2.4 MHz plane ultrasonic transducer was fabricated using a PMN-PT/epoxy 1-3 composite with 0.37 volume fraction of PMN-PT. It shows a -6 dB bandwidth of /spl sim/61% and an insertion loss of -14 dB.

Growth and piezoelectric properties of Pb(Mg1/3Nb2/3)O3–PbTiO3 crystals by the modified Bridgman technique

Abstract The Bridgman technique has been used to grow PMNT single crystals based on the understanding of the features of PMN–PT system and the thermal stability of PMNT crystals. The technique has some advantages for the control of spontaneous nucleation, parasitic growth, crystal size and perfection compared to conventional methods. In order to suppress the leaking of crucibles, improve the compositional uniformity and enhance the piezoelectric performance, some modifications were adopted in starting materials and crystal growth procedure. The PMNT crystals grown using this technique were large in size and excellent in piezoelectric properties.

Effect of composition and poling field on the properties and ferroelectric phase-stability of Pb(Mg1/3Nb2/3)O3–PbTiO3 crystals

Piezoelectric constant and temperature vs dielectric constant measurements have been performed on the 〈001〉 and 〈011〉 oriented Pb(Mg1/3Nb2/3)O3–PbTiO3 crystals in the rhombohedral phase with compositions near the morphotropic phase boundary under different poling fields. The investigations have revealed that the dielectric and piezoelectric properties strongly depend on composition (PbTiO3 content), and on poling field for crystals in a narrow composition region. The influence of composition on the stability of orthorhombic ferroelectric phase has been identified by x-ray diffraction on the 〈011〉 oriented crystals. And the origin of high piezoelectric response for crystals in different composition regions was proposed.

Hydrothermal preparation of nanometer ZnO powders

Zinc oxide powder is an important material for applications such as sensors, varistors, pigments, electrography, medical materials, etc. [1]. Various methods adopted for the preparation of zinc oxide crystallites include the sol-gel method [2], evaporative decomposition of solution (EDS) [3], wet chemical synthesis [4] and gas-phase reaction [5], etc. The hydrothermal synthesis is an ideal method which is widely used to synthesize the oxide powders with high quality. However, it is difficult to synthesize nanometer ZnO powders by the traditional hydrothermal route. In 1999, our groups first reported the hydrothermal discharging-gas method [6] which was used to synthesize the acicular ZnO particles. The letter describes our successful attempt in the preparation of nanometer ZnO powders using Zn(CH3COO)2 solution as precursor by the hydrothermal discharging-gas method. The reaction vessel adopted in the experiments is a silver-lined tube-type stainless steel autoclaves with 30 mm inner diameter and about 215 ml capacity. In the upper part of the autoclave are located the discharge valve for discharging gas and a gas manometer. The Zn(CH3COO)2 solution with the concentration ranges of 0.5–1.5 mol/l is used as precursor. The percentage of fill is 70% in the experiment. The reaction temperature ranges from 160 ◦C–250 ◦C, and no temperature gradient is applied. After the temperature of autoclave is elevated to a desired value and remains stable for 1 h in a furnace with a constant temperature-controlling device, the discharge valve is immediately opened to complete reaction. Meanwhile, the power is cut off to make the autoclave cool to room temperature. After the preparation process is completed, the obtained solid products are washed out with water into a glass beaker, then each is filtered. The as-dried products are studied by X-ray diffractometer, and electron microscope. The results are shown in Table I. From Table I, it can be seen that the crystallite size of ZnO powders, which are prepared using 1 mol/l Zn(CH3COO)2 solution as precursor at different temperatures is greatly different; when reaction tempera-

Ferroelectric and piezoelectric properties of novel relaxor ferroelectric single crystals PMNT

Relaxor ferroelectric single crystals PMNT with the size of ϕ40 mm × 80 mm have been grown by a modified Bridgman method and their ferroelectric and piezoelectric properties have been characterized. The properties varied with the compositions and cut types. On the (001) cut, PMNT76/24 single crystals exhibited a dielectric constant ∈ of about 3 400, a dielectric loss of tanδ <0.7%, a piezoelectric constantd33 of 980 pC/N, an electromechanical coupling factorkt of 0.55 andTc of about 110°C. Whereas the properties of PMNT67/33 single crystals on (001) cut were better:∈ of about 5 300, tanδ <0.6%, d33 up to 3 000 pC/N,kt 0.64, k33 0.93 and Tc of about 150°C. The piezoelectric properties on other cuts such as (110) and (111) were much lower than those on the (001) cut. The rhombohedral PMNT crystals grown by this method showed more excellent piezoelectric properties than those grown by high temperature solution method and higher value of kt than the rhombohedral PZNT single crystals. It has also been found that the fluctuation in ferroelectric and piezoelectric properties was related to such factors as composition uniformity and poling degree.