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[Department of Semiconductor Physics]
[Section of Physics, Faculty of Mathematics and Physics]
[Faculty of Mathematics and Physics]
[Charles University]
[CSCA - Czech and Slovak Crystallographic Association]
Texture, crystallinity and phase composition of ITO coatings prepared at different substrate temperatures were investigated in connection with their optoelectronic properties.
The project was supported by the Charles University Fund, grant No. GAUK-314, and by the Foundation of Czech Republic, grant No. 202/93/0508.
R. Kuzel, R. Cerny, V. Valvoda, M. Blomberg, M. Merisalo, V. Poulek, Complex XRD microstructural studies of hard coatings applied to PVD-deposited TiN films II. Thin Solid Films (1995), in press.
Several series of NiCo/Au multilayers were investigated to inspect an influence of different substrates, buffer thickness, buffer annealing, alloy composition and sublayer thickness on basic structural characteristics in this important system. Again, both the high- and low-angle XRD methods were used at the highest temporary level of data analysis available.
M. Chladek, C. Dorner, A. Buchal, V. Valvoda, H. Hoffmann, X-ray diffraction structure analysis of Ag/NiFe multilayers during annealing, J.Appl.Phys. supplied.
D. Rafaja, M. Chldek, V. Valvoda, M. Seddat, H. Lassri, R. Krishnan, Microstructure of NiCo/Au multilayers studied by XRD, Thin Solid Films, supplied.
Methodical studies of strongly textured thin films resulted in a new method for analysis of these materials called "Joint Texture Refinement". This method enables structure determination from diffraction data measured at several sample inclinations and fitted simultaneously by the extended least-squares program POWLS. The method is based on a description of the orientation distribution of crystallites by simple empirical functions with one or two variable parameters.
Forced orientation of yttrium-iron garnet crystallites in the magnetic field of 1.5 T was investigated in two different regimes. The method of pulsed magnetic field resulted in the orientation of crystallites with the width of 30o. The method of constant magnetic field using also rotating sample resulted in the width of the orientation distribution function equal to 12o.
V. Valvoda, M. Chladek, R. Cerny: Joint texture refinement, J.Appl.Cryst. 29 (1996)1-12, in press.
J. Kolovrat, D. Rafaja: Quasi-single crystal alignment of ferro(i)magnetic powder particles with negative cubic anisotropy. J. Magn. Magn. Mater., in press.
Two Round Robin tests were organized in 27 laboratories in Czech and Slovak Republic. Sintered and powder samples of corundum (Al2O3) were used in the first test to see an effect of sample preparation on intensities and line positions. It was found that the average intensities measured on different powder samples fit well with the values calculated for a sample with random orientation of crystallites. On the other hand, to fit the data measured on sintered corundum samples a texture correction procedure was needed to arrive at the same level of agreement as found in the case of powder samples. Accuracy in line position determination was the same for both kinds of samples. Powder samples of corundum and anglesite (PbSO4) were used in the second test devoted to the Rietveld program fitting procedures. It was found, among other results, that the Rietveld program is in some cases unable to correct for a severe errors in sample alignment or to utilize these correction abilities in cases of poor quality of the data (poor from the point of view of the Rietveld program). However, the intensities determined by the Rietveld program and the values determined by single line profile fitting agreed well.
The project was supported by the U.S. - Czechoslovak Science and Technology Joint Fund, in co-operation with the International Centre for Diffraction Data (USA) and Ministry of Economy (!R), under grant No. 93008.
Recent results: Structure of the hard sintered materials and ceramic-metal composites (CERMETS) was investigated from two complementary points of view. The first one was concerned on the study and characterisation of diffusion processes running in these materials. The diffusion processes influence significantly the formation of the phase and grain structure that determines the mechanical and chemical properties of materials in a large extent. The most important effect, which was observed and explained during the last year, was the variations in layer growth (phase boundary movement activated by reaction diffusion). The aberrations from the predicted layer growth occurred at a restricted diffusion flow through the gas/solid interface in diffusion couples. The observed effect helped us to extend our model of diffusion processes for reactive diffusion with variable activity of in-diffusing atoms. In addition, this extension of the diffusion model can improve the reliability of the diffusion coefficients obtained from the layer growth in finite diffusion couples.
The second aim of our study was the microstructure investigation of CERMETS. To be able to characterise also concentration gradients in small grains (< 1 (m), we improved the regular methods of data reduction used in powder diffraction, i.e. the deconvolution of diffraction profiles and the transformation of diffraction pattern to the form of lateral and/or depth concentration profiles.
The project was supported by the Fund of Charles University, grant No. GAUK - 4.
W. Lengauer, D. Rafaja, G. Zehetner, P. Ettmayer: The hafnium-nitrogen system: Phase equilibria and nitrogen diffusivities obtained from diffusion couples, Acta Metal. Mater., in press.
Rafaja, W. Lengauer, P. Ettmayer: Multiphase layer growth kinetics in finite gas/solid diffusion couples, Acta Metal. Mater., supplied.
Crystal structure of two high-Tc superconducting phases with transition temperatures Tc = 105K ((Bi1-x Pbx )2 Sr2 Ca2 Cu3 Oy , phase 2223) and Tc = 80K (Bi2 Sr2 Ca Cu2 Oy , phase 2212) have been studied. Structure changes in atomic configuration of high-Tc phase 2223 caused by different Pb concentration ( x = 0.15, 0.20, 0.25, 0.30, 0.40 ) and different preparation conditions (slowly cooled and quenched samples) were in the focus of our study. Rietveld refinement of x-ray powder diffraction data was used for structure change analysis. Our results can be summarized as folows: The correlations among transition temperatures and structure parameters, phase concentrations and atomic configuration variations of bulk samples were found. Using the bond valence sum method Cu valence value for 2223 phase of measured samples was investigated. Within the precision of our method no Cu valence changes were found. The temperature Tc of samples was closely related to the Bi-O bond distances. The precision of the used method was tested by the Rietveld structure analysis of CuO powder data for three samples with different oxygen stoichiometry.
Thin layer structure of superconducting phase 2212 prepared by laser ablation in the air were studied by x-ray powder diffraction technique. Quantitative phase and texture analysis was based on the profile analysis and precise intensity measurements of diffraction lines ( theta -2 theta scan, omega -scan ).Orientation distribution of c-axis of crystallites were characterized.
The project was supported by the Foundation of Charles University, grant GA UK - 13.
J. Wild, J. Macl, H. Sichova, P. Bohacek, J. Pracharova, Z. Janu, S. Civis, P. Kubat, 8O K superconducting Bi-Sr-Ca-Cu-O layers prepared by laser ablation in the air, J.Appl. Phys., supplied.
J. Wild, Modifikovana metoda pripravy tenkych supravodivych vrstev, Technik, supplied.
Recent results: By using X-ray diffraction methods we investigated changes in microstructure of fullerene C60 after neutron irradiation. The measurements carried out with non-irradiated samples confirmed that the molecules of fullerene C60 build a face-centered cubic elementary cell. The analysis of X-ray diffraction data verified also the expected rotation of fullerene molecules at the room temperature. The results obtained with the starting (non-irradiated) material were used as a reference set for further investigation of samples irradiated by 4(1016 n/cm2. As it follows from comparison of results, after irradiation the crystal structure is broken only at the grain boundaries. It was detected as a decrease of the coherent domain size in XRD experiments. An increase of the amorphous part contents was observed contemporary. Inside the crystallites, the crystal structure did not change. The increase of the amorphous amount agrees well with the generation of annihilating centres, which was found using the posi tron annihilation spectroscopy.
Structure and properties of intercalates has been studied using X-ray diffraction. The method of characterization of original and intercalated smectites has been developed, based on the detailed 00l profile analysis including the Lorentz-polarization and structure factor correction as a continuous function of diffraction angle. The application of this method led to the surprising conclusions concerning the lattice parameter and the stacking of layers in these layered structures.
Real time X-ray powder diffraction has been used to study the staging phase transitions in graphite intercalated compounds. Using position sensitive detector, the kinetics and mechanism of staging phase transitions has been found and the stability of these compounds was determined.
Molecular simulations using Cerius2 modeling environment have been used to study the structure of smectites, intercalated with anorganic complex cations. This study was focused to the structural parameters determining the porosity and consequently the sorption properties of these structures.
P. Capkova, R. Driessen, M. Numan, H. Schenk, Z. Weiss, Z. Klika, Molecular simulations: Montmorillonite intercalated with aluminium complex cation, Proc. of Royal Netherlands Academy of Arts and Science, supplied.
Aim: The intermetallic compounds based on the intermetallic phase Fe3Al are expected to be used in many technical and technological applications, where good mechanical properties together with a large chemical resistance are needed. The mechanical properties are significantly influenced by the stage of the long-ordering in the atomic structure. The aim of our study is to establish the exact course and temperature of the phase transformation and to estimate the diffusivities of atoms. Recent results: Non-ordered solid solution of Fe75Al25 forms cubic crystals crystallizing in the space group Im3m. Their structure is frequently reported as A2. At suitable temperatures, two basic stages of the long-order were observed. The first one is a structure with a particular long-ordering that is described by the space group Pm3m and reported as B2. The second one is a wholly ordered Fm3m structure (D03) that has an eight-times larger elementary cell than the structures A2 and B2. The preliminary X-ray diffraction measurements confirmed a continuous character of the phase transformations. Moreover, different diffusivities of atoms at the octahedral and at the tetrahedral sites in the structure D03 were found.
The long-range ordering was also investigated by the high-resolution transmission electron microscopy (TEM). The TEM study was focused mainly on visualization of anti-phase domains. By using the X-ray diffraction methods, we investigated the quality of Fe3Al single-crystals regarding the methods of surface treatment. Concurrently, the orientation of Fe3Al single-crystals was determined. From the oriented single crystals, sheets oriented (110) were prepared for the TEM investigations.
Another part of this project was concerned on the investigation of magnetic intermetallic phases. For UNiGa the lattice parameters were measured in the range 10-293 K. We observed slight anomalies in the temperature dependence of the lattice parameters a and c. That effect correlates well with the magnetostriction measurement and indicates an appearance of the anti-ferromagnetic phase bellow 50 K.
D. Rafaja, P. Kratochvil, J. Kopecek, On the ordering in Fe3Al, Scripta Metal. Mater., in press.
The evolution of the mean crystallite size and strain with annealing temperature (within the range 20-1000oC is studied for ultrafine-grained copper, germanium and silicon produced by plastic deformation. In as-prepared samples of germanium and silicon a minor high-pressure tetragonal phases were found which dissapeared after annealing above 300oC (for germanium). An increase of mean crystallite size and decrease of strain with increasing temperature is obvious. The greatest reduction of XRD line broadening was observed in the range of 400-800oC whereas significant change in eletrical transport properties was found after annealing above 800oC.
Different techniques (e.g. TEM) have revealed that the thermal stability of samples is not determined only by a mean grain size but also by the grain-boundary dislocations. For the correct interpretation of XRD profiles an application of classical (Williamson-Hall or Warren-Averbach) methods seems to be insufficient and more sophisticated model would be desireable.
R.K. Islamgaliev, F. Chmelik, R. Kuzel, Thermal Stability of Submicron Grained Copper and Nickel, Materials Science and Engineering, in preparation
The research of early stage of precipitation in Cu-Be alloys was completed with two papers describing a new simplified method for characterization of precipitate size and concentration and the results obtained for Cu-Be samples aged at 200oC. The method is based on the fitting of experimental diffraction profiles with three Pearson functions corresponding to Bragg peak, static diffuse scattering and quasiline (diffuse scattering). The three components are interconnected via the static Debye-Waller factor 2M which is refined by fitting.
Simultaneously, the factor and quasiline shifts are calculated using a simplified semi-analytical approximation of the precipitate displacement field including the effect of elastic anisotropy. The experimental and theoretical results are compared and dimensions of precipitates can be refined. Near surface preferred orientation of precipitate habit planes (100) leading to observed large anisotropy of 2M's and probable stair-step pairing of precipitates was found after aging. At 200oC, quantitative estimates of the diameters for disk-shaped stair-step pairs give 3.6 and 4.8 nm for 16 and 64 hrs aging, respectively, a height of one unit cell (0.29 nm), and a pair spacing which appears to increase with time.
C.R. Houska, R. Kuzel, Simplified Elastic Models for Disk-Shaped Precipitates, Journal of Materials Science, supplied
R. Kuzel, C.R. Houska, Baoping He, Characterization of Severe Matrix Distortions During Phase Separation from the Redistribution of Diffracted Intensities, Journal of Materials Science, supplied
The X-ray diffraction method was used to maintain a control over the air pollution. The mineralogical composition, crystalline or amorphous parts and mainly the content of substances insoluble in the body liquids (like SiO2, C, asbestos, etc.) were determined. The solid fraction of ambient aerosol from 2, 20, 40, 60 and 80 m heights above the surface of land was collected by a sampler during several hours. The 40 m level is the limit for soil mineral detection. Above this heigth the pollutants coming from the coal burning power plants are singnificant.
The project was supported by the Foundation of Charles University, grant No. GA UK - 96/95
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