Verteilungskoeffizienten und Konzentrationsanreicherungen an der Phasengrenzfläche bei ebenem und zellularem Erstarren von Zinn-Zink-Legierungen

The experimental results of KRUMNACKER on the effective distribution coefficient k_eff and the concentration-profil at the solidification interface were examined. On the basis of this examination and some new calculations the depencence of the segregation coefficient on the solidification rate R and on the concentration C₀ is studied in the cases of planar and cellular solidification. — In the case of planar interface the dependence of k_eff on the solidification rate is that of the Burton-Prim-Slichter-theorie. The interface distribution coefficient is not a function of R, or C₀. Thus, the result od BRICE , basing on the same experiments, is not correct. It is impossible, to improve the model of BRICE by these values. — In the case of cellular growth the distribution depends on the diffusion in the diffusion layer at the interface and on the segregation in the grooves of the cells. Under the applied experimental conditions of a large diffusion-boundary-layer the diffusion determines the dependence of k_eff on the solidification rate also under the condition of cellular growth.     

Zur Bestimmung des effektiven Verteilungskoeffizienten von Te bei der Züchtung von GaP-Einkristallen aus nichtstöchiometrischen schmelzen

An equation for determining the effective distribution coefficient, k_eff, under the conditions of growing single crystals from non-stoichiometric melts is proposed. This equation is used for the calculation of k_eff at Te concentrations in the melt from 3.5 × 10⁻³ up to 9.1 × 10⁻² at.%. From numerous measurements a k_eff value of 0.070 with the 1 σ boundaries at 0.116 and 0.043 is found. The relatively wide spread is accounted for by the heterogeneous distribution of doping material which is due to the technique applied and to the polar properties of the GaP lattice.     

Funktionelle Abhängigkeiten der Gleichgewichtsverteilungskoeffizienten in Molybdän

From the deeper investigation of the periodicity of the equilibrium distribution coefficient in molybdenum comes out that k_c values of admixtures (transition metals) in Mo depend on three factors of influence: a) the valence factor (the number of valence electrons), b) the dimension factor (the real or hypotetical melting point of the admixture phase, which is the same as that one of the basic metal Mo—KSC.) From this dependence it is possible to predict the value k_o of the elements in molybdenum, the diagrams of which are not known or inexact.     

Zum Wachstum von AIIIBV-Halbleitern aus nichtstöchiometrischen Schmelzen (II) Dotierung von GaAs und Ga1−xAlxAs mit Zink

Doping of GaAs and Ga_1−xAl_xAs with Zn in the LPE process was studied by a radioanalytical method. It is found that Zn diffuses from the Ga-(Al)-As-Zn-solution into the n-GaAs substrate before epitaxial growth starts. This “pre-diffusion” and the following diffusion of Zn out of the epitaxial layer into the substrate results in three regions with distinct Zn-graduation in the vicinity of the pn-junction. There are no striking differences for GaAs/GaAs and Ga_1−xAl_xAs/GaAs structures. The Zn concentration in GaAs epitaxial layers decreases exponentially from the substrate up to the layer surface. From this profile the temperature dependence of the Zn segregation coefficient is calculated. At 900°C a value k_eff = 5,2 · 10⁻² is found. The doping profile in Ga_1−xAl/As layers is more complex. It is influenced by the changings of temperature during the growth of the layer and by the nonuniform Aldistribution over the layer thickness.     

Effect of impurities on the optical properties of PbWO<Subscript>4</Subscript> crystals

The effect of excess W and La³⁺, Y³⁺, and Mo⁶⁺ impurities on the luminescence spectra and the thermally stimulated luminescence of PbWO₄ crystals is studied. A high tungsten content (up to 1 at %) in the charge mixture results in a high luminescence intensity in the green spectral region. Lanthanum impurity decreases the transmission in the short-wavelength (k > 29 000 cm^?1) spectral region. Lanthanum and yttrium impurities decrease the luminescence intensity. At low (10² ppm) concentrations, Mo impurity can somewhat increase the light yield. However, the presence of Mo in PbWO₄ shifts the absorption-band edge to longer wavelengths, while an increase in the Mo concentration above 10² ppm decreases the luminescence intensity.     

Kinetic of potash alum crystallisation

Metastable zone width of potash alum in aqueous solutions has been measured as function of the cooling rate in absence as well as in presence of seed crystals. From these data, the effective nucleation rate was calculated. Growth rate of potash alum crystals has been measured in a fluidized bed. Overall growth rate coefficient, k_G, was then used for determination of the mass transfer coefficient, k_D, and the rate constant of the intergration step k_r. The dependence of k_D on hydrodynamic conditions is discussed.     

Molybdenum and Tungsten Combined Oxidation by Dissociative Evaporation of Products during Refractory Oxides Crystal Growth from the Melt

The main chemical reactions and composition of gas and solid phases have been determined for the equimolar ratio Mo: W: O₂ = 1: 1: 2 at T = 2400 K in the pressure range of 1-1 x 10^-5 bar. It is established that the character of the main processes of combined oxidation depends significantly on the pressure and state of the oxidant (oxygen): at P > 7.52 x 10^-5 bar, oxidation reactions involve mainly molecular oxygen, whereas atomic oxygen dominates at lower pressures. At P ≥ 0.424 bar, the solid phase contains not only Mo but also MoO₂. At P = 1 x 10^-5 bar, the concentration of lower Mo and W oxides and elementary Mo and W in the gas phase sharply increases, which can negatively affect the main crystallization units.     

Spinodal decomposition during continuous cooling of a PbOB2O3Al2O3 glass

Spinodal decomposition during continuous cooling of the PbOB₂O₃Al₂O₃ quasi-binary glass system was analysed by numerical integration of Cook's differential equation (which includes the contribution of random density fluctuations) for small angle X-ray scattering (SAXS) intensity. The SAXS curves derived from the calculations have a wide range of k-Fourier components (0 < k < k_c) for which a positive amplification factor occurs and they show a “crossover” point at $\text{k}{}_{\mathrm{c}}\text{= 0.155}\text{A}\text{?}{}^{\mathrm{?1}}$. The wavenumber which receives maximal amplification, k_m, increases with the cooling rate, Q, as $\text{Q}{}^{\mathrm{<mtext>1</mtext><mtext>n</mtext>}}$, with n = 10.9. This Q dependence of k_m is similar to that predicted by Huston et al., however our results show a higher value of n. The dependence on Q and k_m of the SAXS intensity I(k_m) was also deduced. The measurements of SAXS curves were performed on glass samples prepared by the splat-cooling technique. Because of the difficulties which arise in the determination of the cooling rate of the samples, the only experimental results that could be compared with the theory are the k_m dependence of I and the value of k_c. These results are satisfactorily understood in terms of the present analysis.     

Axial macrosegregation in Ga‐doped germanium grown by the vertical gradient freeze technique with a rotating magnetic field

The impact of a rotating magnetic field (RMF) on the axial segregation in Vertical Gradient Freeze (VGF) grown, Ga doped germanium is investigated. Growth experiments were performed using the VGF‐RMF as well as the conventional VGF technique. Carrier concentration profiles characterising the Ga segregation were measured by the Spreading Resistance method and calibrated using Hall values of carrier concentration and mobility. The Ga concentration rises more gradually under RMF action, i.e., the dopant segregation is significantly reduced by the rotating field. This effect is attributed to a better mixing of the melt. Numerical results on the flow velocity confirm this explanation. The RMF induced flow is much more intense than the natural buoyant convection due to the radial temperature gradient and leads to a pronounced decrease of the effective partition coefficient k_eff. In the early stages of growth a k_eff value close to k₀ was obtained, i.e., the gallium was almost homogeneously distributed within the melt. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the dependence of the surface tension at the crystal-melt interface on the impurity concentration

The stationary task of impurity diffusion in a melt has been solved within a two-dimensional crystallization model in a second-order approximation with respect to the amplitude of deviation from a smooth crystallization front. The dependence of the surface tension Γ at the interface on the impurity concentration C is taken into account in the form Γ = Γ₀ + ζ _d C, where Γ₀ and ζ _d are constants. The variational method is used to obtain the condition for the transition from a smooth crystallization front to a cellular one. It is shown that calculated cell sizes are in agreement with the experimental data in the literature only when the parameter ζ _d ≠ 0. For binary systems with distribution coefficients k < 1 and k > 1, ζ _d should be positive and negative, respectively.

     

Some Aspects of Interactions in the Mo–W–Al2O3–H2 System: Formation of Polyoxides

The possibility of formation of molybdenum and tungsten polyoxides in the Mo–W–Al₂O₃–H₂ system at T = 2400 K and P = 1 bar in a controlled Ar + H₂ atmosphere has been investigated by the method of thermodynamic analysis. The formation of polyoxides is found to occur both due to the processes involving Al₂O₃ melt and in the absence of the latter. It is established that metals (Mo and W) and their mono-, di-, and even trioxides (in the latter case, mediated polymerization occurs) can be used as initial components to form polyoxides. It is shown that polyoxides themselves may interact with one of their main sources: Al₂O₃ melt.

     

A study of the relation between crystal growth kinetics and particle size distribution of crystals produced from solution

A new relation between crystal growth kinetics and particle size distribution based on statements in the literature is derived. It can be equally used for the determination of the characteristics of crystallization caused by cooling, evaporation or precipitation. Different kinetic constants (rate constants of nuclei formation k₁, k₂, k₃, and growth K₀, K₁, K₂) can be determined, and therefrom the particle size density curve. – The effect of kinetics characteristics on particle size distribution is summarized.     

Germanium Incorporation in Ga1–xInxAs LPE Layers and GaAs Bulk Crystals

Atomic absorption spectroscopy (AAS) is used to determine the Ge concentration in Ga_1–xIn_xAs and GaAs. The orientation dependence of Ge incorporation in (111)A-and (111)B oriented samples has been studied. The distribution coefficients for both the orientations were determined to be k_Ge^(111)A = 2.6 · 10⁻² and k_Ge^(111B = 1.4 · 10⁻² for Ga_1–xIn_xAs with 0 ≦ x ≦ 0.13. The differences between the two orientations are explained with the aid of the band bending model. Doping gradients in thick epitaxial layers and along crystal length of polycristalline TGS-grown GaAs ingots have been investigated too. In Ga_1–xIn_xAs layers any Ge concentration gradient couldn't be observed, but in TGS compact crystals Ge concentration increases with crystal length because the melt composition changes significantly during solidification. The results are compared with those of electrical measurements.     

Mechanisms of Interaction of Molybdenum and Tungsten Polyoxide with Aluminum Oxide Melt under Reducing Conditions

The main chemical reactions between Mo and W polyoxides and Al₂O₃ melt in a controlled Ar + H₂ atmosphere (T = 2400 K, P = 1 bar) during sapphire growth by horizontal directional solidification have been investigated. Under these thermodynamic conditions, the melt and products of its dissociative evaporation may actively react with the tungsten heater and molybdenum thermal screens of the crystallization system. It is shown that the polyoxides formed during evaporation do not directly interact with the melt; this interaction occurs only with participation of reagents exhibiting pronounced reducing properties (Al, H₂, H, WO, Al₂O, AlH, AlH₂, AlH₃). It is established that most of processes occur with participation of aluminum hydrides. A particular role of Mo(W) dioxide–W(Mo) polyoxide functional pairs in the interaction with the melt is determined.

     

Structure,microstructure and magnetic properties of mixed rare earth oxide (Dy1‐xErx)2O3

The mixed rare earth oxide (Dy_1‐xEr_x)₂O₃ (0.0 ≤ x ≤ 1.0) were synthesized by a sol–gel process. X‐ray and neutron diffraction data were collected and crystal structure and microstructure analyses were performed using Rietveld refinement method. All samples were found to have the same crystal structure and formed solid solutions over the whole range of x. Preferential cationic distribution is found for all samples but with different extent with Dy³⁺ preferring the 8b among the two non‐equivalent sites 8b and 24d of the space group Ia3. The lattice parameter is found to vary linearly with the composition x and a systematic variation is found in the r.m.s microstrain . Magnetization measurements were done in the temperature range 5‐300 K and a behavior in accordance with Curie‐Weiss law was found. Anomalous concentration dependence is found in magnetic susceptibility which is ascribed to the concentration dependence of effective crystal field combined with the contribution of ⁴I_15/2 and ⁶H_15/2 manifold at elevated temperature. The effective magnetic moments μ_eff is found to decrease linearly with composition parameter x, except for sample x=0.5 where the magnetization is enhanced. The Curie‐Weiss paramagnetic temperatures indicated antiferromagnetic interaction. These magnetic results are discussed in view of the cationic distribution. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effective distribution coefficient of Si in Fe—Si alloys at floating zone melting

Effective distribution coefficients of silicon in Fe—Si alloys with 1.0–12.1 at.% Si in floating zone melting at various solidification rates were determined. The dependence of k_ef on the solidification rate was compared with the Burton-Prim-Slichter equation. Effective distribution coefficients extrapolated to a zero solidification rate were compared with equilibrium distribution coefficients taken from the equilibrium phase diagram of Fe—Si.     

On the scandium distribution in indium arsenide crystals grown by the Czochralski method

In this paper the behaviour of scandium in n-type monocrystalline indium arsenide grown by Czochralski method in sealed ampoule is studied. The crystals have been grown in [111] direction under different conditions: crystallization rate, As-partial pressure and scandium initial concentration in the melt. The effect of the above factors on the scandium distribution along the crystals have been investigated. Using these results and by means of the equation of normal freezing the effective Sc distribution coefficient (k) in InAs under different technological conditions has been determined. It has been found that k < 1 in all experiments. In order to find the equilibrium coefficient (k₀) at fixed growth conditions the Burton-Prim-Slichter model has been used. On the basis of Hall measurements and atomic absorption analysis of Sc-doped InAs it is concluded that connected with Sc electrically active centers behave as shallow donors, most probably monovalent ones.     

Effective convection coefficient for porous interface and solute segregation

Convective heat and mass transfer coefficients are used to calculate the rate at which convection sweeps heat and mass away from the interface. Convection in melt growth is driven by various forces, and the resulting convoluted flows are laminar or turbulent. Furthermore, cross-flow through the “porous” interface has a profound effect on convection. Thus, a general effective coefficient (h_eff ), which accounts for: (i) uniform flow “suction” through the porous interface, (ii) forced and/or natural convection, (iii) laminar or turbulent flow, and (iv) finite Schmidt numbers, is derived. Focusing next on solute segregation, mass conservation is used to derive a simple equation for k_eff (effective segregation coefficient) as a function of h_eff. Here, h_eff is an input, which provides the rate at which convection sweeps the rejected solute away from the interface.From the general expression, even simpler expressions are developed for restricted range of conditions, e.g., Czochralski growth under forced laminar convection (no natural convection or turbulence). h_eff utilizes numerous established correlations, all developed for impermeable solids.     

Synthesis and tribological properties of Mo‐doped WSe2 Nanolamellars

The Mo‐doped WSe₂ nanolamellars have been successfully prepared via solid‐state thermal (750 °C) reaction between micro‐sized W, Mo with Se powders under inert atmosphere in a closed reactor and characterized by X‐ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the morphologies of the as‐prepared products changed from microplates to nanolamellars to aggregations composed of nanoparticles with the doping of Mo powders. And the sizes of crystallites evidently reduced while the contents of dopant increased within a certain limit (1 wt.%–7 wt.%). The tribological properties of the as‐prepared products as additives in HVI750 base oil were investigated by UMT‐2 multispecimen tribotester. The friction coefficient of the base oil containing Mo‐doped WSe₂ nanolamellars was lower and more stable than that of WSe₂ nanolamellars. A combination of rolling friction, sliding friction, and stable tribofilm on the rubbing surface could further explain the good friction and wear properties of Mo‐WSe₂ nanoparticles as additives than that of WSe₂.     

Determination of the Effective Vacancy Concentrations and the Vacancy Losses after the Quench of Al–Zn–Mg Alloys

On the basis of

• 1 the experimental observation of dislocation loops by means of TEM,
• 2 the conclusion from this that the concentration of vacancies in the dislocation loops exists at the cost of the vacancy concentration primarily present at T_q, and
• 3 the inversion of the decomposition kinetics of Al–Zn–Mg alloys found out by means of resistivity and microhardness measurements, the effective vacancy concentration c_v.eff being at disposal for the GP-zone formation was found out by means of the relation t_R,max (t_R,max being the time required to reach the maximum of resistivity) and the Arrhenius plot In t_R,max = f(1/T_q) of the isothermal resistivity measurements. From the comparison of c_v.eff with the theoretical V concentration at T_q it is possible to derive statements about the loss of vacancies during and after the quench through loop formation, building-in of vacancies in GP-zones and annealing of vacancies at lattice defects. From this comparison follows:
  • The ideal ZnV concentration at T_q (calculated by means of the Lomer equation) is sufficiently great to account for the effective vacancy concentration.
  • The part of c_MgV in the loss concentration is rising with increasing c_Mg.
  Both these statements support the hypothesis that the initial process of the decomposition in Al–Zn–Mg alloys is caused by the ZnV diffusion.