Fluctuations of impurity composition and their role in dislocation cross slip in crystals

The influence of impurities on the kinetics of split-dislocation cross slip caused by a change in the stacking-fault energy is studied theoretically. It is shown that the fluctuations in the impurity composition of a crystal make a considerable contribution to the kinetics of dislocation cross slip. The activation-energy spectrum and the average frequency of the processes of dislocation cross slip are calculated for a model of random impurity distribution in a crystal. The calculation shows that the fluctuations in impurity concentration, reducing the stacking-fault energy, play an important role in the low-temperature region.     

Influence of impurity atmosphere on the deformation of silicon crystals

The Alexander–Haasen theory, which describes the deformation kinetics of silicon crystals, has been generalized for impurity crystals. The deformation kinetics of an impurity sample is calculated in a wide range of parameters, including the cases of partial and complete entrainment of impurities by moving dislocations. The developed model, despite its simplicity, adequately describes the qualitative transformation of the stress–strain curves of impurity silicon crystals in dependence of the impurity concentration and other material parameters. The manifestation of negative velocity dependence of the yield stress, observed in natural experiments, is analyzed.     

Influence of sol-gel coatings on crack initiation by vickers indentation in soda-lime glass

In this work an indentation technique was used to study the effect of the sol-gel coatings of soda-lime glasses on crack formation. The tendency of samples to develop radial cracks has been evaluated by measuring their number in correspondence with the Vickers impressions as a function of the applied load. Silica coatings of different thickness were obtained from alcoholic solutions of TEOS by the dipping technique on glass sheets with different degrees of surface hydration.

All the coated samples show a lower crackability compared with the uncoated ones. It was found that this parameter is influenced by layer thickness, heat treatment temperature and by the degree of galss surface hydration.     

Influence of impurity on rhythmic crystallization of calcium carbonate in agar gel

During the gel growth of calcite crystals in test tubes, for a narrow concentration of inner and outer electrolytes we observed rhythmic crystallization of calcium carbonate. In this paper the effect of adding impurities on the pattern of periodic crystallization of calcium carbonate has been presented. The addition of impurities either increases or decreases the solubility of reaction product and hence may affect the rate of nucleation of calcium carbonate crystals. The variation of spacing coefficient and velocity constant due to the addition of impurities has also been studied.     

Influence of Coriolis force on thermal convection and impurity segregation during crystal growth under microgravity

In contrast with the generally accepted viewpoint, it is shown that the Coriolis force caused by rotation of an orbital station can appreciably affect natural convection and impurity distribution during the growth of crystals from a melt in orbital flight conditions. 2D and 3D steady and oscillatory convection in a rectangular enclosure is considered. The resonance phenomenon arising due to the interaction of the Coriolis force and harmonic oscillations of the gravity force is demonstrated. It is shown that for moderate values of the Ekman number the Coriolis force suppresses convection in one direction and amplifies it in the other, which in turn results in deformation of the impurity distribution over the cross-section of the crystal.     

Finite element analysis of indentation on glass

The finite element method was applied to the analysis of the indentation process on glass. The calculated values of hardness for ideal elastic-plastic materials showed reasonable agreement with those obtained experimentally by other authors, but the calculated shapes of indentation were somewhat different from the real ones. When densification was taken into account, the calculated shapes of indentation showed better agreement with the real ones. The parameters for the calculation were estimated. The contribution of densification to the total deformation was estimated to be less than 45%, using the yield criterion as a basis for this estimation. A structure similar to the rosette-like structure was also calculated by this method. The finite element method was found to be one of the most effective methods for analysing indentations on glass, and it was shown that the mechanical properties of some glasses may be determined by precise observation of their indentations.     

Influence of SiO32‐ impurity on growth habit of potassium dihydrogen phosphate crystal

By altering the concentration of silicate (SiO₃^2‐) impurity in the solution, a series of potassium dihydrogen phosphate (KDP) crystals was obtained by the conventional temperature cooling and the rapid growth methods, respectively. It was observed that the presence of SiO₃^2‐ made KDP crystals tapering in conventional cooling method, while more SiO₃^2‐ induced inclusions at prismatic sectors in the rapid growth method. Laser‐polarization‐interference results showed that SiO₃^2‐ extended the dead zone and reduced the growth rate of (100) face of KDP crystals. The negative influence of SiO₃^2‐on the growth was considered absolutely similar to the effect of cations. It was also suggested that the stability of solution doped with SiO₃^2‐ was improved without seed crystals, while it was destructed with seed crystals. The inhibition mechanism was analyzed in terms of SiO₃^2‐ absorption on (100) face. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Eutectic crystallization of aluminium copper alloys (II). Influence of impurity elements

The directional crystallization of eutectic Al–Cu alloys with Fe additions under molten salts has been studied. According to Tiller's theory the specific interphase boundary energy σ_αβ was estimated for eutectic Al–Cu alloys with three concentrations of additive. Values of σ_αβ depend on Fe-concentration. Small additions of Fe modify the eutectic structure in two directions: a) enlargement of interlamellar spacings. b) formation of rod-like structure.     

Effect of temperature on hardness and indentation cracking of fused silica

The fracture behavior induced by Vickers indentations in fused silica was investigated as a function of temperature. Indentations were performed from room temperature to 400 °C in air. The indentations and the crack pattern formed were analyzed by optical and scanning electron microscopy. The hardness at room temperature was 7.3 ± 0.3 GPa and decreased to 4.2 ± 0.1 GPa at 400 °C. Cone and radial cracks were observed at all temperatures. The radial crack length increased with temperature for a constant load. Lateral and median cracks were present under the indenter, and their expansion was constrained by cone cracks nucleated during the loading-unloading cycle. The threshold loads for cone and radial crack nucleation increased with temperature. The results are discussed in terms of the elastic modulus/hardness ratio variation with temperature.     

Influence of chemical composition on dislocation structure and its change by plastic deformation of V3Si single crystals

The dislocation structure and its change by plastic deformation of V₃Si single crystals has been studied by an etch technique. The solution hardening effect established elsewhere is interpreted in terms of the observed dependence of the dislocation density ϱ(r) and its increment with the plastic deformation on the chemical composition within the range of homogeneity.     

The effect of crystal surface roughness on impurity adsorption

The effect of crystal surface roughness on impurity adsorption was investigated in a fluidized bed crystallizer and in a batch crystallizer. The crystallisation of sucrose in pure and impure systems was the study subject. Calcium chloride was utilized as impurity in this work. The results show that the impurity adsorption is growth rate dependent and is strongly influenced by the crystal surface properties. Crystals with high surface roughness have lower impurity adsorption. Based on experimental evidences, a new theoretical model is proposed to quantify the surface roughness influence on the impurity adsorption, allowing, by operating at the more adequate supersaturation, to control the impurity transfer into crystals. The used impurity does not have a significant influence on the growth rates at the studied temperatures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of subsurface damage on indentation behavior of ground ULE™ glass

The subsurface damage in ground optical glasses was characterized using the ball-dimple technique. At the same lapping conditions, larger abrasive size introduced deeper subsurface damage, and the surface damage was proportional to the surface roughness. Surface degradation in the ground glasses due to the formation and propagation of the subsurface damage was evaluated using microindentation method. A two parameters empirical equation was used to describe the indentation behavior of the ground glasses. It was found that the nominal near-surface elastic constant decreases with the indentation load, suggesting the propagation and formation of surface cracks during the indentation.     

New gaseous impurity and its effect on the purity of LPE GaAs

The present work describes a new gaseous impurity source of GaAs grown by the conventional liquid phase epitaxy. A large quantity of impurity gas of mass number 16 was detected in the LPE atmosphere even when H₂O vapour and O₂ residual gas were exhausted almost completely. The impurity gas was only detected in the presence of H₂ and a graphite boat at high temperature, and above 850°C this was exaggerated. From comparison of the mass-spectrum of standard gas, the impurity gas was identified as methane. An amount of 130 ppm of methane in H₂ resulted in a carrier concentration of LPE GaAs higher than 10¹⁶ cm^-3.     

Crack-free high load Vickers indentation of silica glass

The surface of pure silica glass was prepared to be resistant to cracking from application of Vickers indentation loads of up to one kilogram force (1 kgf). Sample preparation, testing environment, and water content of the glass sample were found to be critical in producing crack-free indentations. Sample preparation consisted of a careful polishing and etching to remove the densified surface layer. Indentation was performed under a nitrogen environment which was also necessary to produce crack-free specimens. Water introduced into the silica glass sample mechanically as well as water introduced into the glass during melting or heat-treatment, were found to be detrimental to crack resistance.     

Anelastic indentation recovery of bioglass at room temperature

Using the microindentation technique, micro-indentations were made on bioglass 45S5 by a spherical diamond indenter for the indentation loads in the range of 50-500 mN at room temperature. Complete anelastic recoveries of the indentation depths and the impression marks were observed for the first time. The recovery rate of the indentation depth decreased with increasing the indentation load. A phenomenological model was proposed to explain the indentation recovery. It found that the recovery rate of the indentation depth was a power-function of the indentation load as supported by the experimental results.     

Dependence of refractive indexes dispersion for LiNbO3 on impurity

The effect of the impurities of the transition metals Co, Mn, Cr, Fe, Ni, Cu on the dispersion of the refractive indexes of LiNbO₃ is studied. The dispersion curves for LiNbO₃ are compared to those for the pure crystals. The relation between the difference of the refractive indexes of the pure and doped LiNbO₃ and the ionic radius of the impurity is established.     

Effects of impurity ions on solubility and metastable zone width of phosphoric acid

Effects of two typical impurity ions as Fe²⁺ and SO₄^2– in phosphoric acid on solubility and metastable zone width (MZW) had been studied. It was shown that polythermal method was suitable for solubility measurement in this study. The solubility experimental data was correlated with a polynomial equation. Being compared with Fe²⁺, SO₄^2– had a greater impact on the phosphoric acid solubility; and the variation in phosphoric acid MZW was just opposite. Phosphoric acid apparent secondary nucleation order was calculated to be 3.50 by means of modified regression method. And also the influence of impurity ions on apparent secondary nucleation order was investigated in this paper. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Direct AFM observations of impurity effects on a lysozyme crystal

Impurity effects on the growth of tetragonal lysozyme crystals have been studied using in situ atomic force microscopy. Commercially available hen egg white lysozyme was characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis with silver staining, and purified by re-crystallization and successive high pressure liquid chromatography. On the (1 1 0) crystal surface, there was no significant difference in morphology between crystals grown in commercial and in purified solutions. On the (1 0 1) surface, however, a large number of small particles were found when the crystal was grown in the commercial solution, while the surface grown in the purified solution was quite smooth. Among the typical residual impurities contained in commercial lysozyme, only covalently bound lysozyme dimer yielded such particles. From measurements of particle separation and an estimate of the critical nucleation size, we infer that the particles reduced the step velocity according to the mechanism described by Cabrera et al. [N. Cabrera, D.A. Vermilyea, in: R.H. Doremus et al. (Eds.), Growth and Perfection of Crystals, 1958, P. 393].