Search for possible structures of cubic approximants of icosahedral quasicrystals

Admissible structures of the approximants are obtained within the model of the dodecahedral local order (DLO) with due regard for the constraints imposed by the space symmetry. It is shown that the number of cubic approximants of icosahedral quasicrystals having a certain order and dodecahedral local order is finite and that the number of positions that can be occupied by the atoms from the unit cells of an approximant of each order is also finite. The corresponding estimates from above are made. In particular, it is shown that there exists only one structure for the 1/?1 approximant. It is also shown that it is possible to determine all the structures for any approximant of any order. The corresponding algorithm of the exhaustive search for these structures is suggested. The implementation of this algorithm provided the determination of all the structures of the 0/1 approximants and also some possible structures of the 0/1 and 1/1 approximants. The tables of possible approximant structures can be useful in the studies of new phases having approximant structures.     

On the mechanical stresses caused by growth centers

The question of the development of mechanical stresses in crystals during their growth is investigated. The mechanical stress field is calculated which can be caused by a radially inhomogeneous impurity concentration distribution existing around growth centers. For the case of KCl: Pb crystals it is shown that depending on growth condition these stresses can be very considerable.     

Modelling tracer diffusion and mobility of interstitials in disordered materials

A disordered system is described by a model in which interstitials can occupy sites of different energy. Tracer diffusion occurs in the absence of a concentration gradient and by jumps over energy barriers with the same saddle point energy. Using statistical mechanics and simple random walk considerations the tracer diffusion coefficient is D^* = γ⁰D⁰ where D⁰ is the known or unknown tracer diffusion coefficient of an arbitrary reference state, and γ⁰ is the activity coefficient with respect to the same reference state. The activity coefficient γ⁰ can be calculated for a given energy distribution. The results are in agreement with rigorous treatments of diffusion for the same potential trace. It is shown that Henry's law (γ⁰ → constant for c → 0) is valid for small interstitial concentrations c and for all types of energy distributions. Thus the diffusivity also becomes independent of concentration for c → 0.

Measurements of the hydrogen diffusion coefficient in liquid-quenched and vapour-quenched Pd₈₀Si₂₀ and its dependence upon temperature and hydrogen concentration are presented. These are in excellent agreement with the model if a Guassian distribution of energies is assumed. The width of the energy distribution is different for the two materials, but in each case agrees with results from solubility measurements. For alkali ions in oxide glasses two energy levels, one for sites adjacent to the immobile anions and one for sites far away from them, describes the so-called “weak electrolyte behaviour”, which determines diffusivity and conductivity at low alkali contents. With increasing alkali content the energy distribution changes, and predictions were made, which are in agreement with experimental findings in borate-silicate and GeO₂-glasses.     

Mathematical modelling of the SSD-process

In order to achieve a better agreement between the calculated and experimental results of crystal production by the SSD-process, it was essential to extend the application of the modell presented earlier (Weinert et al.). The thermal and geometrical conditions, which vary strongly during the course of the process and influence the GaP crystallisation, have never been taken into consideration. It has been attempted, in the present work, to consider these changes by dividing the total time of crystallisation into so small durations (Δt), that the evaluation model presented earlier for instant times can be successfully applied. The results of crystallisation for the total period can then be obtained through a mathematical analysis named as sequence correction of process parameters. It has been shown that an exact knowledge of the temperature distribution in the furnace will yield results which are in very good agreement with the experimental values. Moreover, the changes in the process parameters during the whole time of crystallisation have been represented graphically. — Application of this model to other systems, for example to GaAs, is also possible.     

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.     

On the Chemical Reaction Mode of Polysilicon Deposition from Silane

Silane decomposition and silicon layer growth will be described by way of theory taking into account heterogeneous as well as homogeneous reaction mode and 1st as well as 0.5th order of the chemical reaction. Comparing axial layer growth rate distribution and total substrate surface area effect on the latter with respect to theoretical and experimental results (for the process conditions investigated), it will be shown that the deposition of undoped poly silicon is characterized by a heterogeneous reaction mode, whereas chemical reaction is of first reaction order in the temperature range above 973 K and of 0.5th reaction order below 973 K. The deposition kinetics of strongly in-situ phosphorus doped poly silicon is shown to be in agreement with a homogeneous reaction mode of 0.56^th reaction order.     

Formation of atomic solution in liquid eutectic alloys

The influence of Pb, In, Bi and Ge on structure of tin within 0–12.5 at.% concentration range has been studied by means of X-ray diffraction method. Samples were prepared by melting in vacuum furnace filled with pure argon from ingots of Sn(99.999%), Pb(99.99), In(99.999), Bi(99.99), Ge(99.999) and checked by means of X-ray microanalysis The structure factors and pair correlation functions were analyzed. Concentration dependencies of structure parameters for liquid Sn-enriched melts in binaries with eutectic point are described. It is shown that atomic arrangement in tin-based molten alloy deviates from random atomic distribution within concentration range where these alloys can be considered as diluted solutions. Limited solubility in solid state in eutectic Sn-based systems is a precursor to deviation from random atomic distribution in liquid state.     

Perfection and homogeneity of space-grown GaSb:Te crystals

A Te-doped GaSb crystal grown by the method of directed crystallization under microgravitation has been studied by X-ray topography. It is shown that the structural perfection of the crystal part grown without contact with the ampule walls is substantially higher than the structural perfection of the crystals grown under terrestrial conditions. The distribution of a Te dopant in the crystal is studied by quantitative X-ray topography, and it is shown that, if the gap between the growing crystal and the ampule is small, the Marangoni convection in the melt can be less intense than the convection provided by residual microgravitation. The relation between the formation of a flat face on the crystallization front (faceting) and the formation of twins is also established.     

On the dependence of the properties of glasses on cooling and heating rates II: Prigogine-Defay ratio, fictive temperature and fictive pressure

In a previous analysis (J. Chem. Phys. 125 (2006) 094505) it was shown by us that - in contrast to earlier believe - a satisfactory theoretical interpretation of the experimentally measured values of the so-called Prigogine-Defay ratio, Π, can be given employing only one structural order parameter. According to this analysis, the value of this ratio has to be, in accordance with experimental findings, larger than one (Π > 1). This analysis is extended here and, in particular, the dependence of the value of Π on cooling and heating rates is studied. Finally, employing the general model-independent definition of internal (fictive) pressure and fictive temperature, developed by us (J. Non-Crystalline Solids 355 (2009) 653), it is shown how these parameters behave in dependence on temperature for different sets of cooling and heating rates. Some further consequences and possible extensions are discussed briefly.     

Molecular beam epitaxy (MBE) growth and structural properties of GaN and AlN on 3C-SiC(0 0 1) substrates

AlN and GaN was deposited by molecular beam epitaxy (MBE) on 3C-SiC(0 0 1) substrates on low-temperature (LT) GaN and AlN buffer layers. It is shown that not only GaN but also epitaxial AlN can be stabilized in the metastable zincblende phase. The zincblende AlN is only obtained on a zincblende LT-GaN buffer layer; on the other hand, AlN crystallizes in the wurtzite phase if it is grown directly on a 3C-SiC(0 0 1) substrate or on a LT-AlN buffer layer. The structural properties of the layers and in particular the orientation relationship of the wurtzite AlN on the 3C-SiC(0 0 1) were analyzed by conventional and high-resolution transmission electron microscopy.     

Preparation of zirconia mullite flakes via a plasma rapid solidification process using starting materials derived from a sol–gel technique

Flakes of zirconia–mullite with different zirconia contents varying from 3 to 24 wt% were produced from sol–gel derived raw materials via a plasma melting method followed by a rapid solidification process using a rotating copper roll. The morphology, phase constitution and microstructure development of the as-prepared flakes and of the flakes after various heat treatments were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). It was found that the starting materials could be transformed from the initial irregular-shaped powders into flakes which consisted of mullite, zirconia phase, a relatively large amount of glassy phase and pores. Using TEM, it was shown that the crystalline phases consisted of zirconia polymorphs and mullite. The glassy phases consisted of Al₂O₃–SiO₂ solid solution supersaturated with zirconia. Firing at 1500 °C or 1700 °C resulted in full crystallisation of the flakes and a fairly homogeneous distribution of zirconia particles in terms of size and shape dispersed in the mullite matrix, which contained both intra-granular and inter-granular precipitates. The microstructural characteristics of the flakes may provide promising physical properties for applications in high temperature thermal insulation materials.     

Processing of intensities of electron scattering from amorphous silicon by methods of continuous wavelet analysis

It is shown that intensities of electron scattering from amorphous silicon can be processed by the methods of wavelet analysis. The coordination radii for amorphous silicon at various annealing temperatures are calculated using the intensity distribution of coefficients of the wavelet transform.     

Multiple bonding as a screening phenomenon

The elimination of a ligand pair from contiguous atoms in a molecule increases the bond order between them by unity as the remnant electrons rearrange into a suitable molecular orbital. Starting from experimental single-bond Morse curves, it is shown that screening of the internuclear repulsion by these electrons accounts for almost the total increase in bond strength during the formation of double and triple bonds. Reverse screening has a similar though smaller effect on adjacent single bonds, and observed trends can be accounted for adequately by the same mechanism.     

Sol–gel simulation—II: Mechanical response

A novel computational procedure is proposed to predict the outstanding mechanical properties of sol–gel structures. An aggregation algorithm incorporating Brownian motion and chemical reactions is used to recreate the sol–gel structures at molecular scale. Just like in the physical colloidal aggregation process, the computational aggregation process produces structures with fractal features. Such fractal character leads to a recursion algorithm for calculating mechanical properties at any scale using a recursive multiscale approach. The mechanical properties are then predicted at each scale by calculating the effective properties using the Finite Element Method. It is shown that Young's modulus naturally follows a power law relationship with density, whereas Poisson's ratio displays more complicated behavior. Also, it is shown that Young's modulus and Poisson's ratio depend on a) the mass distribution of the structure, which is influenced by the Brownian motion and chemical reactivity during the aggregation process, and b) the connectivity, which is also influenced by additional processes as sintering and/or aging. Finally, it is shown that the Young's modulus and Poisson's ratio can be correlated to scattering intensity of sintered and/or aged structures.     

Determination of the distribution coefficient of Pb in NaCl crystals by estimation of the lead concentration from the optical absorption in aqueous solutions

It is shown that the intensity of a UV absorption band of lead ions in aqueous solution of NaCl is linear dependent on the concentration of lead. This can be developed to a simple determination method of lead in NaCl crystals even in the ppm region. Its application on melt grown NaCl:PbCl₂ crystals was used for a calculation of the distribution coefficient, for which was found k = 0.24. Furthermore it could be shown that an essential part of lead is evaporated from the melt before the growth process starts so that the real initial concentrations are drastically lower than the nominal concentrations of the melts.     

Time-of-flight measurements in inhomogeneous electric fields

Time-of-flight (TOF) measurements are commonly accepted as one of the most reliable experimental technique to determine the mobility and density-of-states (DOS) distribution in disordered organic and inorganic materials. However, interpretation of TOF data is bound to several a priori assumptions one of which is the assumption of an essentially constant and homogeneous field distribution in the bulk of a sample. This condition can be fulfilled in a wide-band-gap intrinsic semiconductor supplied with blocking contacts. Dark charge injection from contacts and, notably, background doping immediately invalidate this assumption. In the present work we analyze the effects of inhomogeneous field distribution on the TOF photocurrent transients. In particular, it will be shown that the post-transit photocurrent analysis of the data, obtained with a steeply decreasing field, can yield an overestimated value of the carrier mobility and an apparent DOS distribution strongly deviating from the true one.     

Characterization of the free volume in a Zr45.0Cu39.3Al7.0Ag8.7 bulk metallic glass by reverse Monte Carlo simulation and density measurements

The absolute contents of free volume in the as-cast and annealed Zr_45.0Cu_39.3Al_7.0Ag_8.7 bulk metallic glass (BMG) samples were quantified by density measurements and reverse Monte Carlo (RMC) simulation using the total structural factors F(Q) determined by X-ray diffraction (XRD) experiments as fitting constraints. The densities of the as-cast, annealed and crystallized samples measured by Archimedes method are 7.319, 7.327 and 7.342 g/cm³ (precision ± 0.003 g/cm³), respectively. A new approach was used in the RMC simulation to define the free volume as the difference between the volume of the Voronoi polyhedron and the volume of the Wigner-Seitz cell of the constituent atoms. Two types of initial configurations were constructed: (1) in configuration A, all types of potential atomic pairs are allowed; (2) in configuration B, Al-Al and Ag-Ag atomic pairs are excluded. The contents of free volume in the as-cast and annealed samples were found to be 0.59% and 0.39% (configuration A), 0.55% and 0.34% (configuration B), respectively (precision ± 0.03%). The probability distribution of the as-calculated free volume can be well-fitted by the equation proposed by Turnbull and Cohen. Finally, it is shown that the contents of free volume determined by density measurements and RMC are comparable, while the discrepancy of the results is discussed.     

TEM investigation on the structure of amorphous silicon monoxide

Commercially available powder samples of silicon monoxide have been investigated by methods of transmission electron microscopy: electron scattering, electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Pair distribution functions (PDFs) as well as EEL spectra can be shown to be a composition of the PDF and EEL spectra of elemental silicon and amorphous SiO₂. The distribution of the elements silicon and oxygen calculated from ESI images proof the silicon monoxide to be inhomogeneous, i.e. it consists of amorphous silicon and amorphous SiO₂. The phase separated regions measure ≈3–4 nm. One maximum in the PDF at 2.95 Å does not stem from either a-Si or a-SiO₂, and it is assigned to atomic configurations at the interphase boundary layer between Si and SiO₂. The portion of the interphase domain in the total composite material is estimated to be between 20% and 25%.     

Computer simulation of dispersive transport in amorphous materials: Time-of-flight-signals

A method of computer simulation of charge transport in amorphous materials, based on the Scher-Montroll hopping transport model is presented. The method can be applied to a broader range of problems than the original analytical solution because it allows the introduction of an arbitrary distribution of hopping times, space distribution of charge carriers etc.Results of the simulation of time-of-flight experiments with the same initial conditions as in Scher-Montroll's work are found to agree with the analytical solution in asymptotic behaviour (the slope in bilogarithmic coordinates changes from ?1 ?α to ?1 +α) and in the thickness dependence of the transit time. This is considered to be an argument that the simulation method reflects the properties of the model itself.It is further shown, that the electric field dependence introduced in the model by an asymmetry factor ν does not lead to the field dependence of transit time consistent with experiments, because it imposes both upper and lower limits on its values.     

Polyvinyl alcohol cross-linked macroporous polymeric hydrogels: Structure formation and regularity investigation

A series of novel polyvinyl alcohol (PVA) hydrogels were synthesized by cross-linking of acrylate-modified PVA in aqueous solutions. Hydrogels were prepared at a temperature range −7.5 to −25 °С, macromer concentration 4-12 wt.%, and initiator concentration 0.4 to 1.6 mg/ml. The swelling behavior of polymeric hydrogels in aqueous media with different pH and ionic strength values was investigated. It was shown that they possess a high level of water absorption. The influence of different factors (porosity, pore size, and pore size distribution) and reaction conditions on the hydrogel structure was studied. The interior morphology of the hydrogel networks exhibits a complicated structure filled by fibrillar, lamellar and dendritic formations consisting of cross-linked polymer. The dispersed pores which are randomly distributed can be observed inside these formations and between them.