Yu. G. Nosov

Faceting of sapphire crystals grown from a melt by the Stepanov method

The appearance of faces on cylindrical sapphire single crystals grown from a melt by the Stepanov method is studied by photogoniography and optical microscopy. The crystallographic indices of the detected faces are established, and the microstructure of the growth layers is investigated. A relationship between the faceting and the growth conditions is found. The experimental results are compared with data on the faceting of the sapphire crystals grown from a solution-melt and with the calculated specific free surface energies of the faces.

Single crystals of 2223 phase in the BiPbSrCaCuO system: Characterization and Knoop microhardness

Abstract Single crystals of 2223 phase have been prepared by flux growth method from KCl. Their crystal structure is tetragonal (I4/mmm), a=3.82 A , c=37.06 A . Chemical composition of crystals as determined by EDAX was Bi2.00Pb0.27Sr1.62Ca1.92Cu3.18O10+δ. Microhardness measurements with Knoop indenter carried out on (001) pinacoid face show polar hardness anisotropy of between 0.9 and 1.33 GPa.

The influence of thermal screens on the temperature distribution, thermal stress, and defect structure during growth of shaped sapphire crystals

The ways in which a block structure is formed in shaped sapphire single crystals grown from melt by the Stepanov method are considered. The measured temperature distributions and results of a mathematical modeling of the heat exchange in the growth zones, as well as the calculated thermoelastic fields and measured residual stresses, are reported. The possibility of effectively controlling the thermal fields and growth of block-free crystals by choosing optimal screening is shown for single crystals in the form of tubes and basal-plane-faceted ribbons.

Effect of lead doping in the system Bi-Sr-Ca-Cu-O on the formation of 2212 crystals, grown from a KCl flux using pre-synthesized compounds (precursors)

A combination of the solution-melt method and solid-phase synthesis of complex oxides has been used to obtain crystals of Bi2Sr2CaCu2O8 (2212). Partial substitution of bismuth by lead leads to a change in the orientation of the growth layers in the (001) plane of the 2212 crystal which has an orthorhombic lattice. An increase in the dimensions is noted along with an enhancement of the quality of the crystals when lead is introduced into the composition of the precursors.

Residual stresses in sapphire rods grown by the Stepanov method

The residual stresses in cylindrical [0001] sapphire crystals have been studied using the polarization-optical method. The angle between the optical axes 2V and the difference in the normal components of the tensor of elastic residual stresses (σφ − σr) have been determined from the isogyre divergence. It has been found that a tangential tensile stress of no more than 20 MPa acts on the ingot surface. The residual stresses have been compared with the calculated thermoelastic stresses generated during the crystal growth in a given heating zone. It has been shown that the determined pattern of residual stresses can be caused by thermoelastic stresses developing in the immediate vicinity of the crystallization front.

Investigation of the heat exchange processes during growth of basal-plane-faceted sapphire ribbons by the Stepanov method

A three-dimensional model is proposed for the global heat exchange during growth of basal-plane-faceted sapphire ribbons. The model is based on the method for solving three-dimensional problems of radiative heat transfer in complex regions with diffusely and specularly reflecting boundaries. The temperature distributions in the ribbon and elements of the thermal zone are investigated. It is shown that the developed model correctly reflects the main features of the processes of heat exchange in the crystallization setup and makes it possible to purposefully modify the thermal zone.

Blocks and residual stresses in sapphire rods of different crystallographic orientations grown by the Stepanov method

The formation of blocks in shaped sapphire rods of two crystallographic orientations has been investigated. It is shown that, when growth occurs in the direction of the optical c axis, blocks are formed with a higher probability than in the case of growth in the a direction. A model of formation of blocks in rods of different orientations is proposed. The distribution of residual stresses over sapphire rod cross sections is measured by conoscopy. It is found that stresses increase from the middle of a rod to its periphery and reach 20 MPa.

Formation of the block structure during growth of basal-plane-faceted sapphire ribbons with different widths

The defect structure of basal-plane-faceted sapphire ribbons 10, 30, and 50 mm wide is investigated. It is shown that the high density of blocks in these crystals is associated with the specific features of the plastic deformation of the crystals with this orientation during crystal growth. In the basal-plane-faceted ribbons, the plastic deformation begins with the slip in the prismatic system. Dislocations of this system form characteristic low-angle boundaries, which then transform into block boundaries. It is revealed that the slip in the easiest basal system, which is characteristic of ribbons with all other orientations, is impossible because of the geometric constraints. Block-free basal-plane-faceted ribbons 50 mm wide are produced by decreasing thermal stresses in growing crystals.

Block structure and residual stresses in tubular sapphire single crystals grown by the Stepanov method

The formation of the block structure and residual stresses in sapphire single-crystal tubes grown from the melt by the Stepanov method has been studied. The distribution of the difference in the residual stress tensor components (σφ–σr) in a thick-walled [0001] sapphire tube has been measured by the conoscopy method. It has been shown that circumferential tensile σφ and compressive stresses act on the outer and inner tube surfaces, respectively. The maximum stress is ∼20 MPa. It has been demonstrated that the development of the block structure begins from the outer tube surface; as the crystal length increases during growth, blocks propagate over the entire cross section.

Specific features of the morphology and distribution of gas inclusions in single-crystal sapphire ribbons grown by the Stepanov method

The distribution of gas inclusions in single-crystal sapphire ribbons with different crystallographic orientations, grown by the Stepanov method, is analyzed. In all ribbons gas inclusions are observed only in thin surface layers located at a depth of about 100 μm. One characteristic feature is the presence of several zones with different distributions and morphologies of inclusions. We observed inclusions of the two types: cylindrical pores and microbubbles. The differences in the morphology and zonality of the gas-inclusion distribution are consistent with the α-Al2O3 crystallography.