Large Burgers Vectors Research Articles

On the origin of growth spirals having large step heights

A new model for the formation of dislocations having large Burgers vectors during crystal growth is proposed. According to this model, a dislocation having a large Burgers vector is introduced by a misarrangement in the combination of the block crystals. The characteristics of such dislocations are described and the conditions for their formation are discussed.

Observations of the influence of stress fields on the shape of growth and dissolution spirals

Examples are presented which show the influence of stress fields around screw dislocations upon the shape of growth and dissolution spirals. When a growth spiral originates from an independent single screw dislocation with a large Burgers vector, the spiral step shows a change of curvature from negative to positive as it escapes from the centre of the spiral. The degree of change of curvature varies depending on the type of stress field. In case of a strong stress field a stable hollow core is formed in the spiral centre, whose diameter depends on the stress field. When a composite spiral is formed from a group of screw dislocations, a depressed area may occur in the spiral centre due to stress. In the case of dissolution spirals, although a hollow core appears, a change of curvature is not observed. The relation between the hollow cores thus formed and the outcrops of the hollow tubes in crystals observed in SiC or YIG crystals is also discussed.

Influence of orientation, temperature, and strain-rate on the yield strength of Fe2B single crystals

The deformation behaviour of Fe2B single crystals with different orientations has been studied by the compression test over the temperature range 800 to 1100° C. Three slip systems (1 1 0) [0 0 1], (1 1 0) [1¯1 1], and (hkl) [1¯11] have been found to be active in the present work. The operation of (h k l) [1 ¯1 1] slip system appears to require about ten times higher stress than those of (1 1 0) [0 0 1] and (1 1 0) [1 ¯1 1] slip systems. The temperature and strain-rate dependence of the yield stress suggests that the Peierls mechanism controls the plastic deformation of Fe2B crystals. The activation energy for the thermally activated deformation of Fe2B varies between 6 and 10 eV and the activation volume between 10b3 and 160b3. The remarkably high activation energies have been discussed in connection with the large Burgers vectors of Fe2B.

The core configuration of dislocations with large Burgers vectors

The core configuration of dislocations with large Burgers vectors

Growth spiral patterns in electrodeposited silver

To study the mechanism of crystal growth, silver electrodeposition on silver single crystal (100) surface was investigated by means of a differential interference microscope. The deposition was performed from an acid silver nitrate bath containing a small amount of the additives at various current densities. It was found that poly-amideamine played an important role in the conditions under which spirals grew. The screw dislocation groups of large Burgers vectors are produced by adding additives to the electrolytes, which indicates the importance of the impurities in the formation of screw dislocation groups. A mechanism for the formation of macro-steps by drawing together monatomic steps is discussed in terms of “step-bunching”. Several patterns of spiral growth, which were very useful to know the mechanism of basic crystal growth, were observed in electrodeposited silver. The growth patterns for two or more spirals on the deposit are also reported in relation to the theory of crystal growth.

The growth of potassium chloride crystals in the presence of lead

Potassium chloride seeds (cleaved from pure, melt grown crystals) were grown from aqueous solutions with supersaturation ratios in the range 1.005 to 1.050 and with concentrations of lead chloride ⪖ 6 × 10 M. While the seeds were growing the {100} cleavage faces were studied by using reflection microscopy and two beam interference microscopy.On the initially flat cleavage faces many growth hills developed during the early stages of growth. The hills were approximately conical and their surfaces were inclined at angles ⪝ 3° to the close-packed {100} pinnes. The most steeply inclined hills grew most rapidly, both parallel and perpendicular to the crystal surface and soon overran their neighbours. Crystal growth occured solely through the continuing development of these hills.Profiles of growth hills were determined at time intervals and it was found that the development of the profiles could be described in terms of the kinematic theory of crystal growth. Furthermore, on the otherwise smooth surfaces of the hills, kinematic shock waves of sub-microscopic steps were observed. In many instances, the shock waves had the forms of spirals centered on the apices of the hills. These observations are consistent with the thesis that the hills originate at the points of emergence in the surface of groups of dislocations with large Burgers vectors.

On giant screw dislocations in ZnS polytype crystals

Abstract Vapour-grown platelet crystals of ZnS containing long-period polytypes have been found by x-ray topography to contain a single screw dislocation with a very large Burgers vector parallel to the c axis. Individual polytype regions can have highly perfect lattices; the few dislocations observed in them in addition to the giant screw have Burgers vectors in the basal plane. A divergent-beam section topograph arrangement has been used to measure giant Burgers vectors to an accuracy of about 30%. Within this uncertainty, the Burgers vector magnitudes match the polytype repeat period in the five crystals upon which measurements have been made, the repeat periods ranging from 50 Å to 106 Å. The x-ray topographic studies provide good evidence in support of the periodic slip process for polytype formation.

Dislocation etching of GaSe single crystals

Dislocation etching of GaSe single crystals was investigated by using a dilute chromic-sulphuric acid mixture, when conical etch pits were revealed on the (0001) surfaces. At the apices of spiral growth hills, bunches of spiral dislocations were revealed, proving that it is not a single screw dislocation of large Burgers vector, but a bunch of co-operating screw dislocations that were responsible for the spiral growth formations of large step-height. In the case of GaSe crystals, grown by vapour transport methods, dislocation densities of 102 to 106 cm−2 were found. The Bridgman crystals investigated were completely free from non-basal dislocations.

The thermal and electrical conductivity of silver-palladium and silver-cadmium alloys at low temperatures

Measurements have been made from 2 to 300° K of the thermal and electrical conductivities of a wide range of silver-palladium and silver-cadmium alloys. The thermal conductivity is resolved into its electronic and lattice components. It is shown that in annealed alloys below 10°K the lattice waves are scattered mainly by free electrons, that the conduction electrons interact with waves of all polarizations, and that this scattering is particularly strong for alloys of high palladium content, where one expects holes in the d -band. Above 40 % palladium , s - d scattering increases the electrical resistance and serious departures from Matthiessen’s rule are observed. The lattice conductivity of strained specimens is much lower, and the additional thermal resistance varies as T -2 , as does the resistance due to interactions with conduction electrons. It is suggested that the additional scattering is due to dislocations of large Burgers vector.

Dislocations in Germanium

A light microscope study of etched germanium crystals has shown the dislocation distribution in them. The dislocations form a network in the crystal. The formation of small angle grain boundaries is discussed. Screw dislocations with large Burgers vectors have been found in some germanium crystals.