Antun Tonejc

An analysis of evolution of grain size-lattice parameters dependence in nanocrystalline TiO2 anatase

Abstract Due to the specific properties of nanocrystalline materials in comparison with crystalline materials, it is essential to investigate their structural parameters like lattice parameters and grain sizes. We used the Rietveld method and refined electron powder data (recorded with selected area electron diffraction—SAED) on nanocrystalline (nc) TiO2-anatase prepared by sol–gel route. We correlated refined lattice parameters with average grain size obtained from transmission electron microscopy (TEM) images. We give preliminary results on relative changes of lattice parameters a and c vs. the mean grain size in nc TiO2-anatase.

Evidence from HRTEM image processing, XRD and EDS on nanocrystalline iron-doped titanium oxide powders

We investigated nanosized iron-doped TiO2 in the presence of poly(ethylene glycol) (PEG), synthesised by modified sol–gel method, by transmission electron microscopy (TEM), selected area electron diffraction (SAED), high-resolution electron microscopy (HRTEM), HRTEM image processing, energy dispersive X-ray analysis (EDS) and X-ray diffraction (XRD). XRD of as prepared sample showed very broad diffraction maxima of anatase as the dominant phase and an additional maxima ascribed to PEG. The SAED pattern reveals nanocrystalline rings of anatase whereas PEG was barely observed. The presence of PEG in HRTEM image was not observed, while in the Fourier transforms (FT) of HRTEM images the PEG spots were indexed. The Fourier filtered images reveal lattice images of PEG and anatase that constitute the observed grain. The elongation of unit-cell parameter c was found and ascribed to infiltration of PEG and iron into TiO2 anatase lattice. XRD pattern of annealed sample showed again that the anatase is the dominant phase, but the faint lines of rutile and brookite were also detected. PEG was not observed. However, SAED patterns and FT of HRTEM images revealed the Fe2O3 maghemite-q that precipitated during thermal treatment of the as-prepared sample. In both samples, in as-prepared and in annealed, an amorphous phase was observed and its amount determined from HRTEM images.

Determination of the monoclinic, tetragonal and cubic phases in mechanically alloyed ZrO2-Y2O3 and ZrO2-CoO powder mixtures by Raman spectroscopy

Abstracts are not published in this journal

An X-ray diffraction study of the crystalline to amorphous phase change in cellulose during high-energy dry ball milling

In methods used widely for determining crystallinity of cellulose substrates by X-ray wide angle diffraction the scattering characteristics of truly amorphous cellulose is needed. So far, such scattering curves have not been available. It was the aim of this study to arrive at an experimentally determined scattering curve of amorphous cellulose. For this purpose, two cellulose samples (a pine wood pulp and a microcrystalline powder) were ball milled in a high-energy equipment in order to destroy the crystalline structure. The structural changes in the cellulose samples and the kinetics of amorphization were followed by taking X-ray diffractograms. After 30min of milling, no more change in the structure could be observed and the diffraction curve of amorphous cellulose with two diffuse maxima at spacings of 0.427nm (strong) and 0.247nm (weak) was obtained. The amorphization was found to follow a first order kinetics, the rate constant of k=(0.0033±0.0004)s -1 holding for both samples. However, while the amorphization of the microcrystalline cellulose started immediately, with the wood pulp sample an induction period of about 300 seconds was observed, which is probably due to residual morphological structures present in this sample.

Preparation and structure of AlW thin films

Abstract Thin films of AlW alloys were prepared by co-deposition of pure aluminum and pure tungsten, each sputtered by an independently controlled magnetron source. The deposition rate at the substrate (glass, fused quartz, and alumina ceramic), positioned 5 cm away from the target surface was 0.1–0.2 nm/s for pure metals, and the final film thickness was a few μ m. Completely amorphous films were obtained in the Al 80 W 20 –Al 67 W 33 composition range. At higher tungsten content, the W(Al) solid solution and pure tungsten phases appeared. The amorphous alloys exhibit a high negative temperature coefficient of the electric resistivity, increasing with the aluminum content up to −5.5·10 −4 K −1 . Finally, the amorphous AlW alloys exhibit a remarkable microhardness (6–7 GPa), and are structurally stable up to at least 400°C.

Structural relaxation of Al–W amorphous thin films

Abstract The pronounced variation of the electrical resistivity of the amorphous Al–W thin films observed during initial heating above room temperature was examined. Both isochronal and isothermal treatments were performed in order to investigate the effects of the film composition, substrate material, and substrate temperature, on the magnitude of the relaxation phenomena. Regarding the isochronal heating, it was observed that the relaxation effects decreased with an increase of the heating rate, and decreased with the aluminum content in the film. The Al 78 W 22 amorphous thin films were subjected to isothermal annealing for 6 h at a temperature of 515 °C. The effects of the substrate material (alumina ceramic, glass and sapphire), and the deposition temperature (LNT, RT, 200 and 400 °C) were examined. The relaxation decreased in a sequence of: alumina ceramic–glass–sapphire substrates, as well as with an increase of the substrate temperature. An assumed dominant role of the aluminum in the effects observed, was tested by the corresponding investigation of Al–Ti and Cu–Ti amorphous thin films.

Crystallization of glassy selenium during its plastic deformation

Crystallization of glassy selenium (Se) during its plastic deformation has been investigated in the temperature interval between 60°C and 95°C. We found that plastic reshaping of small Se pellets into thin discs greatly enhances the notoriously sluggish transition from glassy to crystalline state. However, the content of the crystalline phase in the deformed samples is found to be strongly dependent upon the temperature of the process. While the 30-min long reshaping at 60°C and 95°C gives only partially crystallized samples (6% and 10%, respectively), an equally long process at 80°C gives 100% crystallized samples. Average crystallite size in the samples prepared at 60°C and 80°C is very similar (25 nm and 15 nm, respectively), but in the sample prepared at 95°C the grains are much larger: about 80 nm. The results are discussed taking into account the possibility that the mechanism of plastic deformation enhances both the nucleation frequency and the rate of crystal growth.

Structural (XRD and HRTEM) investigations of fullerite C60 and C70 samples

Abstract Since the carbon states on the basis of C 60 and C 70 exhibit very interesting physical properties such as hardness exceeding diamond, we focused our investigations on the comparison of structures of the C 60 and C 70 hard phases synthesized at high pressure and high temperatures. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) investigations have revealed that two states (amorphous and nanocrystalline) co-exist in samples of both kinds of fullerites. The difference between the diffraction patterns of C 60 and C 70 has been ascribed mainly to the grade of disorder. The XRD and HRTEM results have been correlated with the transport properties (electrical and thermal conductivity) of the samples.

Effects of ball milling on pure antimony, on GaSb alloy and on Ga+Sb powder mixture; oxidation, glass formation and crystallization

Elemental polycrystalline antimony powder, a mixture of antimony and gallium powder, and a Ga-88at.%Sb alloy were mechanically alloyed or ground by high-energy ball milling in an air atmosphere. An excessive oxidation of antimony was observed and this gave us the possibility to estimate the temperature of the milling process in our case. The mechanical grinding of the Ga-88at.%Sb alloy first produced an amorphous oxide, which subsequently crystallized on further milling.

Atomic volume expansion of Ag + 52wt.% Cd alloy

Abstract The lattice constant and atomic volume expansion of an Ag + 52wt.% Cd alloy have been measured from 17° to 560°C using the X-ray technique. The atomic volume shows a discontinuity in transition from ordered bcc (B2) phase to hcp (A3) phase and a continuity in transition from hcp (A3) to disordered bcc (A2).