Structure and chemical composition of LiB3O5 surfaces

The electronic and structural properties of LiB₃O₅ (LBO) surfaces have been studied by X‐ray photoemission spectroscopy (XPS) and reflectance high‐energy electron diffraction (RHEED). The as‐grown (110) crystal face and mechanically polished (001) surfaces have been investigated comparatively. Electronic structure of LBO has been determined on as‐grown (110) crystal face previously cleaned by chemical etching with RHEED control. The correlation of valence band structure and measured binding energies with earlier reported results has been discussed. Core‐level spectroscopy reveals strong enriching of mechanically polished LBO surface with carbon, when nanodiamond powder is used as an abrasive. So high carbon level as C:B = 0.7 has been observed at the surface while the ratio Li:B:O remains according to LBO chemical composition. The association of LBO Kikuchi‐lines with strong background has been shown by RHEED analysis of the surface. Thus, the polished LBO surface constitutes a high structure quality LBO with the inclusions of some amorphous carbon compound. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of chemical composition and homogeneity of thin magnetic layers using microprobe

Thin layers (0.7–1.0 μm) of permalloy on berylium bronze wires of 100 μm in diameter were analysed using microprobe CAMECA MS 46. The layers were deposited by electrolytic method. A calibrating curve for 12 standards of similar composition as analysed layers was made. A special method of analysing was worked out, with regard to very thin layers. Point and line analyses were made for each wire to determine the contents of Fe, Ni and Co. Homogeneity of layers of different wires were compared.     

Single step synthesis of CdSeS nanorods with chemical composition gradients

We report a simple method of fast synthesis of CdSeS nanorods with chemical composition gradients. The CdSeS nanorods were prepared by single-step addition of Se and S source mixture into Cd precursor solution at elevated temperatures. The size of the nanorods can be tuned by changing the S/Se feed molar ratio. As a result of the reactivity difference between Se and S precursors, the nanorods have a core/shell structure with chemical composition gradients, leading to enhancements of their photoluminescence quantum yield and photostability, which make them attractive for use in optoelectronic devices and bio-labeling.     

Influence of chemical composition on structural and selected physical properties of natural natrolites

The relation among changes in contents of Si and Al atoms, molecules of water and symmetry of the unit cell of crystal natural Natrolites was determined. It was found that with the change of the relations Si/Al and Na/H₂O the symmetry of the Natrolite structure changes from Fdd2 through Fd(Cc) to F4 d2 (I4 d2). With the change in the distribution of these atoms in this structural position also changes of optical (refractive index, IR spectra) and thermodynamic (S_conf) properties and thermal stability are closely connected.     

Thermodynamic analysis of gas phase chemistry in hot wire chemical vapor deposition of a-Si:H and μc-Si:H

Gas phase reactions amongst filament-generated radicals play a crucial role in growth and properties of films deposited by hot wire chemical vapor deposition (HWCVD) technology. Gas phase species of interest are SiH₄, H₂, Si, H, SiH₃, SiH₂ and SiH. Partial pressures of these species for different sets of deposition conditions have been determined from the standard Gibbs free energy data. Equilibrium concentrations of the film forming precursors have been determined. The effect of the various process parameters on the equilibrium concentration of the precursors has been studied. H, Si and SiH are found to be the dominant species in gas phase above a filament temperature of 2300 K. However SiH₃ and SiH₂ concentration peaks are between 1900 and 2300 K, of the filament temperature.     

Comparative crystal chemical analysis of borophosphates and borosilicates

A comparative crystal chemical analysis of mixed borophosphate and borosilicate anionic radicals revealed the main difference between them: the condensation of [PO₄] tetrahedra does not proceed via common vertices, whereas the sharing of vertices is possible for the [SiO₄] tetrahedra. In all the borophosphates considered, phosphorus is found only in the formally isolated tetrahedra. The central part of the heterogeneous radicals is formed by tetrahedra of lower charge ions (B⁺³), and the tetrahedra that incrust the central part have higher valences (Si⁺⁴, P⁺⁵). In borosilicates, silicon forms its own constructions up to complex layers and frameworks because its charge (Si⁺⁴) is lower than that of phosphorus (P⁺⁵). The higher charge of phosphorus impedes two-dimensional and three-dimensional condensation. As a result, borophosphates form a variety of soro and chain radicals and only few layer and framework constructions.     

X-ray diffraction phase analysis of the crystalline phase of polytetrafluoroethylene

Samples of ultradispersed polytetrafluoroethylene, which were prepared with the use of different technological approaches, were studied by X-ray diffraction. The samples were found to belong to the monoclinic system. For some samples, basal reflections, which can be related to paraffins, were found for the first time. This indicates that polymer chains can form yet another type of monoclinic structure.     

Interrelation between the parameters of equations of viscous flow and chemical composition of glassforming melts

The number of free parameters of viscosity equations are reduced to two by assuming that the glass transition temperature T_g is the temperature at which viscosity is 10^13.5 [dPa s]. In this way the Avramov and Milchev AM equation is becoming while the VFT equation is transformed to the form: . It is demonstrated that the dimensionless fragility parameter a of the AM equation depends on composition, x, as α = α_ο + 6x, where α_ο = 1 − 2 for silicates and α_ο = 2.75 for borates. We show that the fitting parameters of AM and of VFT equations are correlated. Thus the relationship between the pre-exponential constants is lg η_V ≈ lg η_A − 2. There is also relationship between the fragility parameter a and the divergence temperature T_o of the VFT equation.We develop an alternative approach to consider the vibration frequency of building units of the system. The superposition of the independent vibrations of the atoms constituting the building unit causes a “beat” of the latter.     

Effect of surface treatments on the chemical durability and surface composition of soda-lime glass bottles

Chemical analyses show that water and 1% KHCO₃ solutions extract material that resembles the bulk glass composition for both untreated and treated soda-lime bottles although much less material is extracted from treated bottles. Also, fluorine appears to be deeply dispersed at perhaps 1% concentration in a fluorocarbon-treated glass surface. Electron spectra obtained before and after ion milling detected no fluoride and indicated calcium and some sodium depletion for fluorocarbon-treated surfaces, and a depletion of calcium as well as sodium for sulfur-treated surfaces. There also is evidence from electron spectra of some sodium and calcium depletion in untreated glass surfaces.     

Development of the DIFFRACALC program for analyzing the phase composition of alloys

Crystallography Reports - The DiffraCalc software package is developed for automatic indexing of electron diffraction patterns. The program can operate with the existing base of X-ray diffraction...     

The structure and phase composition of sinters obtained from siderite concentrate containing BaSO4

To obtain a full diagnostic characteristic of mineral components of investigated sinters, microscopic methods used to supplement X-ray diffraction studies and X-ray spectral analysis in microvolume. All obtained results provide an adequate basis for phase identification. In silicate phase of the siderite sinters occur the following minerals with Ba admixture: in acid sinters: olivines and pyroxenes. in basic sinters: α′ – Ca₂SiO₄, melilites and 3 CaO · SiO₂. In no sinter occurs β – Ca₂SiO₄.     

Dependence of chemical composition and bonding of amorphous SiC on deposition temperature and the choice of substrate

Amorphous SiC has superior mechanical, chemical, electrical, and optical properties which are process dependent. In this study, the impact of deposition temperature and substrate choice on the chemical composition and bonding of deposited amorphous SiC is investigated, both 6 in. single-crystalline Si and oxide covered Si wafers were used as substrates. The deposition was performed in a standard low-pressure chemical vapour deposition reactor, methylsilane was used as the single precursor, and deposition temperature was set at 600 and 650 °C. XPS analyses were employed to investigate the chemical composition, Si/C ratio, and chemical bonding of deposited amorphous SiC. The results demonstrate that these properties varied with deposition temperature, and the impact of substrate on them became minor when deposition temperature was raised up from 600 °C to 650 °C. Nearly stoichiometric amorphous SiC with higher impurity concentration was deposited on crystalline Si substrate at 600 °C. Slightly carbon rich amorphous SiC films with much lower impurity concentration were prepared at 650 °C on both kinds of substrates. Tetrahedral Si-C bonds were found to be the dominant bonds in all deposited amorphous SiC. No contribution from Si-H/Si-Si but from sp² and sp³ C-C/C-H bonds was identified.     

On the driving force in phase transitions: Calculation of chemical potential differences

Chemical potential differences for phase transitions of monocomponent and two-component systems are calculated by Taylor expansions at phase equilibrium. The derived expressions (put in frames in the text) are governed by meaningfull physico-chemical entities.     

Dependence of the parameters of equations of viscous flow on chemical composition of silicate melts

The temperature dependence of viscosity of silicate melts is discussed in the framework of the Avramov–Milchev (AM) equation. The composition is described by means of two parameters: the molar fraction, x, and the “lubricant fraction”, l. The molar fraction is the sum of the molar parts x_i of all oxides dissolved in SiO₂, the molar fraction of the latter being 1 ? x. It is shown that, with sufficient precision, two of the parameters of the AM equation can be presented as unique functions of the molar fraction. On the other hand, x is not sufficient to determine properly the reference temperature T_r , at which viscosity is η_r = 10¹³ [dPa.s]. Therefore, additional parameter, “lubricant fraction” l, is introduced. For each of the components, l_i is a product of molar part x_i and a specific dimensionless coefficient 0 ≤ k_i ≤ 1 accounting for the specific contribution of this component to the increased mobility of the system. It is demonstrated that, for l > 0, the reference temperature is related to the “lubricant fraction” l through the reference temperature T_r,SiO2 of pure SiO₂.     

Microstructure and composition analysis of group III nitrides by X-ray scattering

The importance of Group III-nitride structures for both light-emitting devices and high-power field effect transistors is well known (J.W. Orton, C.T. Foxon, Rep. Prog. Phys. 61 (1998) 1). In both cases, different alloy composition and doping levels or type are utilised and the device performance also depends critically on the interface quality and defect density. We have used high resolution X-ray scattering to measure the state of strain in the individual layers on an absolute scale to derive the alloy composition, i.e. we have avoided the conventional method of using the substrate as an internal reference since it could be strained. The composition and individual layer thickness are derived through simulation of the profile with this additional strain information and the best-fit profile is obtained with an automatic procedure. These structures are laterally inhomogeneous arising from defects breaking up the structure into narrow vertical columns of nearly perfect material and this produces significant broadening of the diffraction pattern. This broadening in the diffraction pattern has been modelled using an extended dynamical scattering model (P.F. Fewster, X-Ray Scattering from Semiconductors, Imperial College Press, World Scientific, Singapore, 2000) to yield the size distribution of perfect crystal regions. The measurement of the rotation about an axis defined by the growth direction of the GaN with respect to the sapphire is determined and is found to be small. However, a poor quality sample indicates that a large range of rotations is possible in these structures.     

Investigations on mechanical properties and chemical composition of thermally treated TiCx coatings on steel (CVD technique)

The described technology allows the production of dense TiC_x coatings onto steel at standard pressure. The residual stress in the coatings is a compressive one with a level of some GPa. It is lowered with an increase of the coating thickness and/or a substrate hardening. Both the residual stress level and the constitution of the contact region substrate/coating mainly influence the mechanical behaviour under external load. An occasional formation of cementite islands on the substrate surface in the initial stage of the deposition process reduces the adhesion of the coating. The coating failure of substratehardened samples has not to occur as a necessity at the contact region, but is possible also by breaking out inside the coating. Besides, the existence of voids along the contact region was observed in this case.     

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.     

Quantitative AFM analysis of phase separated borosilicate glass surfaces

Phase separated borosilicate glass samples were prepared by applying various heat treatments. Using selective chemical etching we performed AFM measurement on the phase separated glass surfaces. A quantitative roughness analysis allowed us to measure precisely the dependence of the characteristic size of the phase domains on heating time and temperature. The experimental measurements are very well described by the theoretically expected scaling laws. Interdiffusion coefficients and activation energy are estimated from this analysis and are consistent with literature data.     

A comparative analysis of quantitative ratios in formulas of chemical compounds and twins

A comparative analysis is performed for the distribution and the frequency of occurrence of the quantitative ratios between the chemical coefficients of binary chemical compounds, symbols of twinning faces, etc. Comparison is carried out using the modified Weiss-Goldschmidt prognostic ranked scheme. It is demonstrated that the Pauling parsimony law is combined with the Goldschmidt complication principle.     

Investigations on the homogeneity of single crystals of materials with variable composition grown by chemical transport reactions

To investigate the problem of homogeneity in crystals grown by chemical transport reactions (CTR) solid solutions Fe_1-yCo_yS, Fe_1-yNi_yS, and Zn_1-yCo_yO were choosen as model substances. Relative concentration profiles of Co and Ni, respectively, were measured by electron-probe microanalysis and light absorption (in Zn_1-yCo_yO only). No significant deviations from homogeneity were found for Fe_1-yMe_yS, whereas in Zn_1-yCo_yO the Co contents increase in the growth direction. A qualitative explanation due to different diffusivities in the solids can be given.