Weizhuo Zhong

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.

Twinning Morphologies and Mechanisms of ZnO Crystallites under Hydrothermal Conditions

Two kinds of twinning morphologies of ZnO crystallites prepared under hydrothermal conditions were obtained. The twin relations of the crystallites could be influenced by additives. The twinned crystallites in the pure H 2 O or weak basic solutions (1 N KOH) are bipyramidal and take (0001) as the twin and composition plane, whereas the twinning morphology of the crystallites obtained from 4 N KBr or 3 N NaNO 2 solutions is dumbbell-like and takes (0001) as the composition plane. Various twinning mechanisms based on the linkage of the growth units are suggested. The formation of twin morphologies of ZnO crystallites clearly demonstrate that twinning of crystals is either a result of differences between symmetrical and energetic most favourable structure arrangements or due to the consequence of oriented intergrowths.

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.

Hydrothermal synthesis and optical property of nano-sized CoAl2O4 pigment

Abstract Nano-sized CoAl 2 O 4 was synthesized by hydrothermal method. The powder was characterized by XRD, DTA–TG, TEM, BET, IR, XPS, and UV–Vis techniques. The particle size was ca. 70 nm, and the particle size distribution was narrow. The BET surface area was 29.22 m 2 g −1 . It was thermally stable. The maximum absorption was ∼600 nm.

Synthesis of mono-dispersed ZnAl2O4 powders under hydrothermal conditions

Abstract Mono-dispersed ZnAl 2 O 4 was successfully synthesized by a hydrothermal method at 260 °C for 20 h. The powder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), differential thermal analysis–thermal gravimetric (DTA–TG), and BET. The mean particle size was 482 nm with narrow distribution.

Structural defects of Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals grown by a Bridgman method

The types, formation mechanism and suppressing approaches of structural defects in PMNT crystals grown by a Bridgman method were investigated in this paper. The structural defects include composition non-uniformity, scattering particles, pores, negative crystal structures, cellular structures, fissure structures and point defects. They are formed due to one or several of the following: composition deviation, incomplete melting and diffusion, high-temperature volatilization, temperature fluctuation and instability of the position and shape of growth interfaces. On understanding the formation mechanism of structural defects, one can restrain them by means of adjusting stoichiometry of starting materials, using PMNT crystal bulks, suppressing composition volatilization and modifying growth parameters.

Hydrothermal synthesis of BaTiO3 nano/microcrystals

The results of the hydrothermal preparation of BaTiO 3 fine powders are reported. The effects of the reaction temperature, the molar ratio of Ba/Ti in the precursors, the chemical form of the precursors on the phase composition, and the size and morphology of the products have been determined. Perovskite-type BaTiO 3 crystallite forms more easily as the temperature, basicity, and Ba:Ti ratio in the precursor increase. BaTiO 3 microcrystals (150 to 300 nm) were synthesized through the hydrothermal reaction of commercial TiO 2 with a Ba(OH) 2 aqueous solution. The hydrothermal reaction of a newly prepared Ti(OH) 4 gel with the Ba(OH) 2 solution produced highly crystallized, well-dispersed perovskite-type BaTiO 3 crystallites with very fine (< 100 nm) particles. The newly prepared Ti(OH) 4 gel turned out to be a suitable precursor for the hydrothermal preparation of BaTiO 3 fine powders. X-ray diffraction (XRD) of the hydrothermal BaTiO 3 powders reveals a simple cubic perovskite structure. The lattice constant, a, decreased with an increase in the reaction temperature. These abnormal crystallographic features are assumed to result from lattice defects caused by OH incorporated in the perovskite lattice.

On the Morphological Changes and Twinning of ZnS (Sphalerite) Crystallites under Hydrothermal Conditions

Morphological characteristics and twinning mechanism of ZnS crystals under hydrothermal conditions have been investigated in this paper. It was shown that under hydrothermal conditions the morphology of ZnS crystallites changes along the four-fold axis directions, and the crystals are observed in a positive or negative tetrahedron, or in a combination of positive and negative tetrahedra depending on the growth conditions. The positive tetrahedral areas on the crystallites get larger with increase of the concentrations of OH - and S 2- in solutions, whereas the twinned crystallites of ZnS taking an elliptic shape with (111) as composition plane are easily formed in weak basic solutions. It can be found that the morphologies of ZnS crystals are in accordance with the crystallization orientations of positive or negative coordination tetrahedra ([S-Zn 4 ] 6+ , [Zn-S 4 ] 6- ) in the crystal although, in some cases, the practical morphology could be greatly affected by growth conditions, and the twinning mechanism can be suggested based on the linkage of growth units of positive and negative coordination tetrahedra, which were formed in the solution. The present investigations further indicated that the crystal chemistry approach based on the linkage/incorporation of growth units previously proposed by us can be sucessfully applied to interpret the growth mechanisms of the crystals and to control a desirable morphology.

Formation mechanism of barium titanate nanocrystals under hydrothermal conditions

Growth units and the crystallization habit of BaTiO3 nanocrystals have been investigated. It has been proposed that the growth units of BaTiO3 were surface hydroxylated Ti(OH)62− octahedra. The relationship between crystal morphology and the variation of the current intensity in the solution has been obtained through the measurement of the OH− in the solution and the hydrothermal experiment with superimposed direct electric field. Based on the computation of the stability energy of the growth units, the relationship between the crystallized morphology of crystallites and the most favorable growth units under different conditions has been derived. It has been found that there is structure similarity between anatase (TiO2) and BaTiO3 from the crystal chemistry viewpoint, so they are soluble in each other, which can serve as a reasonable explanation for the abnormal phenomenon of the retention of cubic substable phase at room temperature.

Morphology and defect structures of novel relaxor ferroelectric single crystals Pb(Mg1/3Nb2/3)O3−PbTiO3

High-performance relaxor ferroelectric single crystals Pb(Mg1/3Nb2/3)O3−PbTiO3 have been grown successfully by a modified Bridgman method. They have the size of 25×25×50 mm3 and are of pure perovakite phases with tetragonal or rhombohedral structures. Their {001} faces appear dominantly, which can be interpreted by the model of anionic coordination polyhedral growth units. Main macro defects observed under optical microscopes and SEM can be reduced or removed by improving growth parameters after understanding their formation mechanism. 70° or 109° macrodomains in rhombohedral PMNT 76/24 crystals and 90° macrodomains in tetragonal PMNT 65/35 crystals have been observed by optical microscopes. It has been found that the transition from microdomains to macrodomains can be induced by compositions. Both the imaging mechanism of non-180° domains and the relation between domain configurations and ferroelectric phase transition have been analyzed.