Incorporation Of Growth Units Research Articles

Kinetic Roughening of Glucose Isomerase Crystals

Protein crystals grow by the incorporation of growth units from solution into the crystal bulk. At low supersaturation, growth sites are kinks at step edges that are generated through the formation of spiral dislocations and/or two-dimensional (2D) nucleation. At conditions farther away from equilibrium, a critical supersaturation σR can be reached where the critical 2D nucleus equals a single growth unit. As a result, each surface site becomes a potential growth site, making growth fast and the surface rough. This transition from layer-by-layer growth to a faster random addition of solute molecules to the surface, that is, kinetic roughening, was visualized using laser confocal microscopy combined with differential interference contrast microscopy for the case of glucose isomerase. Evidence for continuous growth is presented at σR: the critical nucleus is predicted to be the size of 2 glucose isomerase tetramers and normal face growth velocities switch from a nonlinear to a linear relationship with the s...

Mean-field theory of habit change phenomenon during crystal growth

A description of a growth morphology diagram (morphodrom) of a two-dimensional crystal growing without impurities that shows a possibility of habit change is presented. By taking into account an additional channel of incorporation of growth units into kink sites by edge diffusion, a detailed analysis of the structures and growth rates of a two-dimensional island on a (001) surface of a Kossel crystal is performed. The change in habit from the squarelike shape near equilibrium to the diamondlike one at higher supersaturation conditions is predicted based on a consideration of growth rates.

Growth mechanism and electrical properties of Pb[(Zn 1/3Nb 2/3) 0.91Ti 0.09]O 3 single crystals by a modified Bridgman method

Relaxor-based piezoelectric Pb[(Zn 1/3Nb 2/3) 0.91Ti 0.09]O 3 (PZNT 91/9) single crystals 28 mm in diameter and 30 mm in length were grown by a modified Bridgman technique with PbO flux using an allomeric seed crystal. The crystals were grown in sealed platinum crucibles at about 1250°C. The obtained crystals are of pure perovskite structure within the sensitivity of X-ray diffraction measurement and the as-grown crystals have three appearing faces with slight deviation in angle from the pseudocubic (0 0 1) cub face. The growth mechanism of the PZNT crystals could be explained from the viewpoint of the formation of growth units and the incorporation of growth units into different interfaces of the crystal lattice. The growth rate of various crystal faces is related to the assembling modes of the growth units into different interfaces. Assembling of [BO 6] octahedral growth unit into (1 1 1) cub interface has the strongest bonding force, and this crystal direction has the fastest growth rate; therefore, (1 1 1) cub crystal face disappears, assembling of [BO 6] into (0 0 1) cub interface has the smallest bonding force, and this crystal face has the smallest growth rate; therefore, (0 0 1) cub crystal face appears. The growth steps aligning approximately along the 〈0 0 1〉 cub direction reveal that (0 0 1) cub face proceeds by a layer growth mechanism controlled by two-dimensional nucleation starting at crystal corners or edges. Dielectric measurement proves that the quality of the obtained PZNT crystals is good. Domain observation shows that both the rhombohedral and tetragonal orientation state domain coexist in the morphotropic PZNT 91/9 single crystals.

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.

Kinetic smoothening and habit change during crystal growth.

Crystal faces with no strong bonds along them are atomistically rough under equilibrium conditions. The growth of such faces has been considered to be governed by an adhesive growth mechanism. In the present study, the growth of such a face of a two-dimensional crystal is investigated based on a concept of crystal growth where, under certain conditions, the incorporation of growth units onto kink sites by a series process via step sites can be faster than the direct incorporation. This concept reveals a new growth mechanism similar to the two-dimensional nucleation-growth. The new mechanism predicts that surface structure can change from rough under equilibrium conditions to smooth in growth and dissolution processes, resulting in kinetic smoothening. The new mechanism also offers a general explanation of growth habit change from one polyhedral shape to another, something which could not be explained by the published theories of growth in pure systems.

Morphological characteristics of α-Al2O3 crystallites obtained direct1y from hydro /solvothermal solvents

The morphological characteristics of α-Al2O3 crystallites obtained directly from hydro/solv othermal solvents are reported and the formation mechanisms of corundum morphology are discussed from crystal growth and crystal chemistry principles. The crystal growth process is considered as a process of incorporation of growth units on the growth interfaces, and the crystal morphology is determined by the linkage of the coordinated polyhedra.

The mechanism of phenytoin crystal growth

Phenytoin crystal growth kinetics have been measured as a function of pH, supersaturation and temperature in phosphate buffer. Incorporation of growth units into the crystal lattice was not influenced by diffusion from the bulk to the solid surface. Thus, the rate limiting step for phenytoin growth is surface integration. Phenytoin crystal growth is via a screw-dislocation mechanism; this mechanism explains the observed dependence of growth rate on supersaturation and the increase in growth rate with pH.

Growth mechanism and morphology of phenytoin and their relationship with crystallographic structure

Phenytoin crystal growth kinetics have been measured in phosphate buffer as a function of pH, supersaturation, and temperature. Incorporation of growth units into the crystal lattice is controlled by surface integration and follows a screw-dislocation mechanism. This mechanism explains the observed dependence of growth rate on supersaturation and pH. A prediction of phenytoin crystal morphology was made by two methods: the Bravais-Friedel-Donnay-Harker (BFDH) model and attachment energy calculations. The results from both methods compare well with observed phenytoin morphology resulting from growth in certain supersaturation ranges. Observed phenytoin crystal morphology is dominated by the (020) and (011) faces. Both faces were predicted to be morphologically important with the BFDH and attachment energy models. The (002) and (200) faces were also predicted to be morphologically important. However, the (002) was not observed and the (200) was only observed on crystals grown at low supersaturation ( sigma <or=0.4). Solvent-solvent interaction at the growing crystal faces and the types of intermolecular bond formed are proposed as the source of deviation between predicted and observed morphology.

The kinetics of crystallization of hopeite

The kinetics of crystallization of α-trizinc phosphate tetrahydrate (hopeite) has been investigated over a range of supersaturation under conditions of constant solution composition. Following inoculation of supersaturated solutions with well characterized seed crystals, the activities of ion species in the supersaturated solutions were maintained constant by the potentiometrically controlled addition of solutions containing the lattice ions. Hopeite was precipitated, without participation of precursor phases, at a pH of 5.0, with a rate proportional to the square of the relative supersaturation. The effective activation energy for growth was 26 kJ mol -1. The results suggest a surface controlled growth mechanism involving factors such as exchange of a water molecule at a zinc ion or incorporation of growth units at spiral dislocations on the crystal surfaces.