Quantum-mechanical relations in the calculation of heats of fusion for ionic crystals

A report is given on the quantum-mechanical relations in the calculation of heats of fusion for ionic crystals. By correlating the heats of fusion of the respective silver halides (AgCl, AgBr, and AgI) and the alkali nitrates (e.g. NaNO₃) with the frequency of the longitudinal optical phonons of the respective crystals, Planck's constant was found to be the proportionality constant. In the alkali halides, apart from the long range interaction, the energy for the formation of vacancies must also be taken into consideration. The calculated heats of fusion agree well with the experimental values given in the literature, deviations being found only in some alkali halides.     

Heat and mass transfer problems in solution growth of GaP crystals by the travelling solvent method (TSM)

The influence of the natural convection on the heat and mass transfer process during growth of GaP crystals from the solution is considered. The two-dimensional quasi-stationary thermodiffusive hydrodynamic problem for the GaP growth system by travelling solvent method (TSM), as a model, has been solved. Computation is given for the temperature and concentration field distribution in the solution. The picture of vortex configuration for different lengths of the liquid zone, the shape of the growing and dissolving interfaces and also the boundary layers thickness are obtained. The experimental dependence of the growth rate on the convection intensity observed mostly confirmed the calculation results.     

Influence of gravitation on the processes of heat and mass transfer in solution crystal growth by the travelling heater method (THM) (III)

The effect of the natural and thermocapillary convection on the vortex configuration in solution during growth of PbTe crystal by the travelling heater method is considered. The estimation of the parameters of growth process (i. e. axial temperature gradients, gravitational acceleration, degree of the solution's surface contact with ampoule), when the vortex configuration undergoes qualitative variation, is given. In terms of the one-dimensional thermodiffusive problem solution the effect produced by the convective stirring on the position of growing and dissolving interfaces is described.     

Effects of reactor type and mass transfer on the morphology of CuS and ZnS crystals

For the precipitation of CuS and ZnS, the effects of the reactor/precipitator type, mass transfer and process conditions on crystal morphology were studied. Either H₂S gas or a S^2– solution were applied. Three different types of reactors have been tested, namely a laminar jet, a bubble column and an MSMPR reactor. The choice of reactor type as well as mass transfer and metal concentration all have a considerable influence on the morphology of the produced crystals. A well mixed bubble column with H₂S containing gas as feed yields the coarsest crystals. Use is then made of the surface active properties of CuS‐particles, which induce agglomeration at the gas‐liquid interface, where as the low metal concentrations inside the reactor also contribute to the formation of coarser particles (especially for ZnS). (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of the conductivity and polarization processes in supported ionic liquid-like phases (SILLPs)

Different crosslinked polystyrene resins containing ionic liquid fragments covalently attached (supported ionic liquid-like phases: SILLPs) have been prepared and their conductivity has been studied by means of dielectric impedance spectroscopy. The main structural parameters considered in this study have been the presence of C2H bonds in the imidazolium subunits and the nature (level of hydrophobicity) of the N-substituents (methyl vs. butyl). The results obtained show that the presence of C2H fragments can have an influence on intrinsic conductivity. In the same way the role of the hydrophobicity of the N-substituents seems to play only a minor role. The main structural component associated with conductivity is the nature of the counteranion. The bulkiest, less coordinating anions, in particular the NTf₂^? anion, seem to be associated with the highest conductivities, with the values obtained being two orders of magnitude higher than those calculated for the same systems containing the Cl^? anions for each of the temperatures under study. The results obtained rule out the participation of some given conduction mechanisms such as the involvement of carbene formation. Instead, the formation of ion-pairs of higher or lower association (strong/weak ion pairs) could be responsible for the observed behavior.     

Ultra-micro-hardness measurements of non-metallic crystals and an attempt for their calculation

The ultra-micro-hardness of non metallic crystals has been measured for 16 substances. These are products usually produced in mass-crystallization; inorganic, organic, with hydrates and unhydrates. The hardness-force-dependency was examined and data are compared to literature. KNO₃ crystals showed a strong direction dependency (a factor of 2). In the end there has been made a first attempt to introduce a correlation of the measured micro-hardness and physical properties of the crystals, but so far this was only possible for face centred cubic lattices.     

Advances on potential-driven growth of metal crystals from ionic liquids

This article highlights the electrodeposition of metals, in crystalline or amorphous form, that are monentous in the present era of science and technology. Available literature related to nucleation and growth of metal crystals has been reviewed to gain insight into the mechanism and kinetics. The progress made in the electrodeposition technique, using an ionic liquid (IL) medium, has been detailed for selected metals using different ILs for achieving the controlled growth mechanism driven by electrochemical potential. Theoretical models for nucleation and growth of crystals by electrodeposition have been explained and the effect of crystallization overpotential on the growth of crystal growth has been discussed. Finally, the factors affecting the growth process and the mechanism have been identified and critically analyzed based on the available literature, fundamental knowledge-base, chemistry of ILs, and electrodeposition.     

Absorption and ionic conductivity of oxygen-containing SrF2 crystals

An absorption band at 3644 cm⁻¹ is caused by isolated OH⁻ ions. O⁻ ions cause an absorption band at 213 nm the oscillator strength of which is 0.020. Charge-compensation of O⁻ ions is effected by F⁻ ion vacancies (F). As for CaF₂ crystals there occur monomers and dimers of [O⁻ – F] complexes. The mass action constants of association of F with oxygen centres are K_AD = ⅓ exp (4.28 – 0.82 eV/kT) for [O⁻ – F], K_AT = 4 exp (17.4 – 1.25 eV/kT) for [2 O⁻ – F] and K_AQ = exp (4.2 – 0.89 eV/kT) for [2 O⁻ – 2 F].     

Heat transfer and mass transport in a multiwafer MOVPE reactor: modelling and experimental studies

An improved detailed model for the calculation of the temperature distribution in a multiwafer Planetary Reactor™ has been developed. The temperature field of the reactor has been calculated in dependence of the reactor parameters for (Al,Ga)As growth as well as on the kind and the thickness of the wall and susceptor deposits. The amount of parasitic wall deposits can be minimized by a proper tuning of the reactor temperature distribution. Calculated GaAs growth rate profiles on 3 inch wafers show a strong dependence on the temperature field in the reactor and the amount of parasitic deposits. These predicted relationships have been used to optimize the reactor temperature distribution in order to minimize parasitic wall depositions. By this procedure a growth rate uniformity of < 1% on 3 inch wafers can be reproducibly achieved.     

Glass foams: formation, transport properties, and heat, mass, and radiation transfer

Energy efficiency, environmental impact, and quality of the final product in glass manufacturing depend, to a large extent, on foams formed on the surface of the molten glass and of the batch due to entrapment of gas bubbles generated by the batch fusion and refining chemical reactions during the melting process. Hence, understanding the mechanisms of foam formation as well as development of theoretical models for thermophysical and transport properties and heat, mass, and radiation transfer in glass foams are not only a problem of significant fundamental interest but also of tremendous practical impact. In this paper, the review of the current state-of-the-art in our understanding of glass foams is provided, including some of our recent results in modeling the dynamics of the foam growth and its steady-state thickness, prediction of gas diffusion through glass foams, and thermal radiative properties of glass foams. In addition, the new results on simulation of combined conduction and radiation heat transfer in glass foams and radiative transfer in primary (batch) foams are presented and discussed in some detail. The paper also presents practical means available for reducing foaming in glass melting and concludes with the discussion of unresolved problems and summary of the directions for the future work in the area.     

Growth kinetics on the (100) and (001) faces of TGS crystals

The growth kinetics and mechanisms on the (001) and (100) faces of TGS crystals were investigated. A phase contrast microscope with a CCD camera was used to observe the growth of the crystal. We found the growth on the (001) and (100) faces at high supersaturation was mainly controlled by a BCF surface diffusion mechanism. The kinetic data for the (100) face were also fitted by the nucleation and layer growth model of two-dimension nucleation at high supersaturation. Some important growth parameters for TGS crystals, such as edge energy, activation energy, and so on, were estimated.     

Heterogeneous kinetics and mass transport in chemical vapour deposition processes: Part II application to silicon epitaxy

The results obtained for the epitaxial deposition of Si from SiC1₄ and H₂ in a rotating disc reactor are described by a single equation in terms of kinetics and transport. The kinetic parameters required to fit the experimental data suggest that the rate determining step for the CVD process is the homogeneous conversion of SiC1₄ to SiHCl₃. The kinetic analysis also shows that the commonly reported activation energy for the overall growth is too low as a result of measurements being made under conditions of combined transport and kinetics. Results of other workers obtained with reactors of very different geometries, when normalised, are in excellent quantitative agreement with both the experimental and theoretical curves found in this work.     

Propagation behavior of threading dislocations during physical vapor transport growth of silicon carbide (SiC) single crystals

The dislocation formation and propagation processes in physical vapor transport (PVT) grown 4H silicon carbide (4H–SiC) single crystals have been investigated using defect selective etching and transmission electron microscopy (TEM). It was found that while the growth initiation process generally increased the density of threading dislocations in the grown crystal, for certain areas of the crystal, threading dislocations were terminated at the growth initiation. Foreign polytype inclusions also introduced a high density of dislocations at the polytype boundary. In the polytype-transformed areas of the crystal, almost no medium size hexagonal etch pits due to threading screw dislocations were observed, indicating that the foreign polytype inclusions had ceased the propagation of threading screw dislocations. Based on these results, we argued the formation and propagation of the threading dislocations in PVT grown SiC crystals, and proposed the dislocation conversion process as a plausible cause of the density reduction of threading dislocations during the PVT growth of SiC single crystals.     

Growth and ionic conductivity of Li3 + x P1 − x GexO4 (x = 0.34) single crystals

Li_{3 + x} P_{1 ? x} Ge_xO₄ crystals (x = 0.34) with dimensions of about 3 × 3 × 5 mm³ were grown for the first time from flux. The conductivities of the crystals measured along three directions have close values and are equal to σ ≈ 1.8 × 10^?6 and 3.7 × 10^?2 Sm/cm at the temperatures 40 and 400°C, respectively (similar to the case of pure lithium phosphate, somewhat lower values of electric conductivity were measure along the b axis). The activation energy of conductivity is equal to 0.54 eV. A considerable increase in the conductivity of the solid solution in comparison with the conductivity of pure lithium phosphate is explained by the specific features of the lithium sublattice in the crystal structure of the λ-Li₃PO₄ type.     

Heterogeneous nucleation (and adhesion) of lysozyme crystals

The heterogeneous nucleation of hen-egg-white lysozyme (HEWL) crystals has been investigated using a double-(thermal)-pulse technique, thus detaching nucleation from the growth stage. n(t) dependencies of the nucleus number, n, vs. time, t, were plotted for surfaces rendered hydrophobic (by means of hexamethyl-disilazane) and for bare glass surfaces. Preferred crystallite orientation supplied additional information. The discussion is based on the classical Stranski–Kaischew theory of crystal nucleation. With a purposely developed new technique the adhesion energies have been measured for HEWL crystals, grown on different supports.     

Dewetting and transport property enhancement: antimonide crystals for high performance electronic devices

The dewetting technique has been applied to the growth of InSb and GaSb polycrystals. After optimization of the set up and growth parameters, four samples, 88 mm in length and 11 mm in diameter, have been obtained. Their electrical properties were investigated in relation to the type of solidification (attached, dewetted and so on). In spite of the polycrystalline structure of these samples, room temperature Hall mobilities obtained for dewetted antimonide samples have shown the highest values obtained till now. These results indicate that dewetting can be used to produce InSb and GaSb with high mobility for device applications.