Mirko Stubičar

Mechanochemistry of zeolites: Part 1. Amorphization of zeolites A and X and synthetic mordenite by ball milling

Abstract X-ray diffractometry, FT i.r. analysis, cation-exchange capacity, and solubility were used to characterize the effect of ball milling on zeolite A, zeolite X, and synthetic mordenite. Changes of i.r. spectra during the ball milling show that a loss of crystallinity, a decrease of cation-exchange capacity, and an increase of solubility were caused by breaking of SiOSi and SiOAl bonds in the zeolite structure. Based on such a conclusion, the kinetic equation of the amorphization process was derived and evaluated.

Mechanochemistry of zeolites: Part 3. Amorphization of zeolite ZSM-5 by ball milling

Abstract The samples obtained during ball milling of zeolite ZSM-5 were characterized by different methods such as X-ray diffractometry, scanning-electron microscopy, FT i.r. spectrometry, and simultaneous thermal analysis. The milling caused considerable change of morphological and particulate properties followed by loss of crystallinity of the treated zeolite. The loss of crystallinity is caused by breaking of external TOT bonds of zeolite framework. Very slow rate of amorphization of “as-synthesized” zeolite as compared with the rate of amorphization of its “activated” form is explained by the stabilizing effect of TPA + ions present in the structure of the as-synthesized zeolite.

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

Study of structural transformations in potassium-exchanged zeolite A induced by thermal and mechanochemical treatments

Thermal transformations of potassium-exchanged zeolite A and the X-ray amorphous material obtained by ball milling the potassium-exchanged zeolite A were investigated by different methods, such as differential thermogravimetric analysis (DTA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Controlled heating of crystalline, potassium-exchanged zeolite A causes a phase transformation in the sequence: [0.22 Na2O, 0.78 K2O]·Al2O3·2SiO2·3.48 H2O⇒amorphous⇒kalsilite+kaliophilite, while the heating of mechanochemically amorphous potassium-exchanged zeolite A results in its transformation into a mixture of kalsilite and kaliophilite. The differences in the pathways of the transformation processes are discussed in terms of the structural properties of the starting materials.

Mechanochemistry of zeolites: Part 2. Change in particulate properties of zeolites during ball milling

Abstract Change of particulate and morphological properties of the samples obtained during the high-energy ball milling of zeolites A and X were studied by scanning-electron microscopy, X-ray diffraction, and determination of particle size distribution. It was found that decrease of crystallinity is followed by considerable change of particulate properties during the milling, but that the change of particulate properties and amorphization are two independent processes. Type of amorphization induced by ball milling was defined on the basis of the analysis of the change of effective crystallite size during the milling.

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.

Correlation between mechanical, thermal and electronic properties in Zr–Ni, Cu amorphous alloys

We show that mechanical properties (stiffness and hardness) of Zr–Ni, Cu amorphous alloys increase linearly with Ni, Cu content over a wide composition range (22 ≤ x Ni,Cu ≤ 65 at%). This correlates with the observed increase in the Debye temperatures and densities with x and shows that the strength of interatomic bonding increases with x in these alloys. Accordingly, the thermal stability (e.g. the crystallization and glass transition temperatures) of these alloys also increases with x. Since the electronic density of states at the Fermi level decreases linearly with x within the same x-range, a very simple relationship exists between the electronic structure and mechanical and thermal properties. We also deduce the mechanical properties of hypothetic amorphous Zr and briefly discuss the possibility of its preparation.

Crystal growth of lead fluoride using the constant composition method II. The effect of Pb/F activity ratio on the kinetics of crystal growth

Abstract A potentiometric pF-stat method has been used for the investigation the kinetics of lead fluoride crystal growth on seeds (α-PbF 2 ) at 25°C. The kinetics was studied as a function of ionic strength, pH and lead-to-fluoride ions activity ratio, which were changed only by changing Pb(NO 3 ) 2 and KF concentrations. Depending on the excess of lead(II) nitrate or potassium fluoride in supersaturated solution two different crystal phases grow: (a) In solutions with large excess of Pb(NO 3 ) 2 and corresponding pH =4.35−5.00, a large needle-like crystals with some sponge-like precipitate are grown. The effective order of crystallization of this mixture is 1.55, indicating preferentially one-dimensional growth of needles. (b) In solutions with KF in excess, and consequently with pH =5.0–7.0, however, voluminous bipyramids of orthorhomic α-PbF 2 crystals are grown. The effective order of crystallization of this known phase is 4.0, i.e. presumably an exponential rate law is valid.

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.

Surface properties of ripidolite and beidellite clays modified by high-energy ball milling

The effects of physical disintegration (high-energy ball-milling) on the surface properties (specific surface area and cation-exchange capacity) of beidellite and ripidolite are presented and discussed. The milling caused considerable changes in the morphology of the clay, as shown by SEM and surface properties, followed by loss of crystallinity as shown by X-ray diffraction of the treated clay minerals. The transformation of the crystalline into the amorphous phase takes place directly without the formation of intermediate solids different from the starting crystalline and the resulting amorphous phase. Major changes in specific surface area and cation-exchange capacity were produced after 6 min of milling in beidellite and ripidolite clays, with both samples becoming similar after 30 min and maintaining such a state for 300 min of milling.