E. Hartmann

The role of impurities in the quality of paratellurite single crystals

Abstract Paratellurite crystals grown by the Czochralski technique often contain Pt inclusions unless extrapure source material is used. Using analytical data obtained on comparable uniform paratellurite crystal samples we have shown that there is a joint incorporation of Pt and the added Fe. In the absence of Fe in the melt no incorporation of Pt occurs. The undesirable interactions of Pt inclusions with dislocations are pointed out.

The dissolution forms of YIG single crystal spheres

Abstract Single crystal spheres of yttrium iron garnet have been dissolved in solvents of H 3 PO 4 and HBr at 140 and 126°C, respectively. The final dissolution form was a tetrahexahedron in H 3 PO 4 and an octahedron in HBr. In the latter case the faces were built up of terraces. The observed dissolution forms could be explained using the theory of Lacmann, Franke and Heimann.

Diffuse interface model of nucleation

A diffuse interface theory of nucleation proposed recently for vapor condensation and crystal nucleation is compared with experiments and field-theoretical models of nucleation. Without adjustable parameters the theory gives an improved description of condensation experiments on non-polar and weakly polar substances. Cluster dynamics calculations are performed for homogeneous and heterogeneous crystal nucleation. Experiments on homogeneously nucleating oxide glasses indicate that a temperature-independent interface thickness is a reasonable assumption for even deep undercoolings. This offers a unique possibility for distinguishing homogeneous and bulk heterogeneous nucleation processes.

Dissolution of GGG single crystal spheres in acids

Abstract Single crystal spheres of gadolinium gallium garnet have been dissolved in acids. The activation energies for dissolution of Czochralski-grown and flux-grown crystals of GGG in H 3 PO 4 between 100–200°C were 0.75 and 0.88 eV respectively. The final dissolution form was a tetrakishexahedron in H 3 PO 4 and an octahedron in HBr. In the latter case the faces were built up of terraces. The relationship between the growth and dissolution forms is discussed.

Unidirectional solidification and properties of shaped halide eutectic composites

Unidirectionally solidified eutectic composites (EC) NaCl-LiF, NaCl-NaF, LiF-NaF, NaF-CaF2 and CaF2-MgF2 have been prepared by the Stepanov method using shapers wetted and nonwetted by the melt. The composites obtained display a large variety of microstructures due to the thermodynamic peculiarities of structure formation in various eutectic systems. The use of wetted Ni shapers with drilled capillaries results in a distortion of the microstructure of EC in the areas corresponding to the capillary outlets. The use of the porous sintered shapers allows one to avoid this effect. The stability of crystal growth is greatly influenced by the melt composition when preparing EC by EFG method. A considerable anisotropy of electrical and optical properties of the EC is related to their anistropic microstructure. In the reflection spectra of the EC crystals, some new maxima are observed suggesting the occurence of surface polaritons localized at the interfaces.

Electrical conductivity of alkali halide eutectic composites

Abstract The electrical conductivity of undirectionally solidified NaClNaF eutectic composites has been measured by a dc method in the temperature range between 40 and 600°C. The considerable anisotropy is related to the fibrous microstructure of the eutectic crystals.

Face statistics on the dissolution forms of garnet crystals

Abstract Statistics based on 390 crystal faces show that octahedron and tetrahexahedron faces occur most frequently on the dissolution forms of single crystal garnet spheres. The results obtained on dissolution forms are compared with those obtained on growth forms of natural and synthetic garnets.

Comparative study of dissolution forms of some rare-earth iron garnet single crystal spheres

Abstract Single crystal spheres of rare-earth iron garnets (R 3 Fe 5 O 12 where R = Yb, Er, Ho, Y, Gd, Sm) have been dissolved in acids. The dependence of the dissolution forms on the chemical composition is discussed.

Dissolution forms of rare-earth gallium-iron garnets

Abstract Single crystal spheres of rare-earth gallium-iron garnet crystals (R 3 Fe 5− x Ga x O 12 , where R = Lu, Yb, Er, Ho, Y, Gd, Eu, Sm and (O ⩽ x ⩽ 5) have been dissolved in acids. It was observed that on dissolution in H 3 PO 4 , the combination of faces {100} D and {111} D changed into the combination of faces {hk0} D and { hhl } D (where h > l), while on dissolution in HBr the combination of faces {111} D and {111} D was reduced 0 to {111} D when the size of the rare-earth ion and the Ga concentration were increased.

The relaxation curve of a crystal-vapour system using BCF theory

Abstract The relaxation behaviour of a crystal-vapour system is described using the theory of Burton, Cabrera and Frank. Two parameters occurring in the BCF growth rate function were determined for the growth of GeS crystal form the vapour.