Publications

Homogeneous thin films have been obtained by UV-excimer pulsed laser deposition from a PMMA bulk material. The structure of films is at variance with the structure of the bulk polymer. The size of the ordered domains, as well as the structural features, evidenced by X-ray diffraction in the deposited films, depend on the fluence of the ultraviolet laser pulse used in the ablation process and on the temperature of the substrate. The radial distribution functions of the bulk PMMA material and thin films show differences in the short range order ascribed to the breaking of the polymer chains and monomer dissociation during deposition process.

Calcium orthophosphates (CaP) and hydroxyapatite (HA) were intensively studied in order to design and develop a new generation of bioactive and osteoconductive bone prostheses. The main drawback now in the CaP and HA thin films processing persists in their poor mechanical characteristics, namely hardness, tensile and cohesive strength, and adherence to the metallic substrate. We report here a critical comparison between the microstructure and mechanical properties of HA and CaP thin films grown by two methods. The films were grown by KrF* pulsed laser deposition (PLD) or KrF* pulsed laser deposition assisted by in situ ultraviolet radiation emitted by a low pressure Hg lamp (UV-assisted PLD). The PLD films were deposited at room temperature, in vacuum on Ti-5Al-2.5Fe alloy substrate previously coated with a TiN buffer layer. After deposition the films were annealed in ambient air at 500-600 C. The UV-assisted PLD films were grown in (10(-2)-10(-1) Pa) oxygen directly on Ti-5Al-2.5Fe substrates heated at 500-600 degreesC. The films grown by classical PLD are crystalline and stoichiometric. The films grown by UV-assisted PLD were crystalline and exhibit the best mechanical characteristics with values of hardness and Young modulus of 6-7 and 150-170 GPa, respectively, which are unusually high for the calcium phosphate ceramics. To the difference of PLD films, in the case of UV-assisted PLD, the GIXRD spectra show the decomposition of HA in Ca2P2O7, Ca2P2O9 and CaO. The UV lamp radiation enhanced the gas reactivity and atoms mobility during processing, increasing the tensile strength of the film, while the HA structure was destroyed. (C) 2002 Kluwer Academic Publishers.

A survey of the main NDT methods used to evaluate the quality and properties of wood and wooden structures is given. Among the most important ones there arc visual inspection, sonic and ultrasonic stress wave. Some other methods include: deflection, electrical, isotope and Xray methods. The ultrasonic methods seem to be the most suitable ones in many cases especially for quality control purposes, but these methods depend on the quality of the piezoelectric transducers. Consequently, the construction and characteristics of a new type of piezoelectric transducer are reported. The new transducer is a composite type transducer, consisting of a sandwich type piezoactive element and a steel cylinder glued together. The sandwich piezoactive element was made of five thinner discs of 20 mm diameter and 2 mm thickness. The whole construction has the resonant frequency at 60 kHz, proper to wood investigation. The transducer can be successfully used for NDT of wood, either in the trough transmission or pulse-echo arrangements. The transducers were tested for sound velocity measurements in different wood samples.

The structural configurations that govern the behavior of the amorphous chalcogenides under excitation are the nano-scale, disordered, layer-like clusters. The competition between breaking, formation and switching of bonds at the cluster boundaries, accompanied by precipitation-dissolution phenomena, gives rise to fractal morphology of these clusters and to the reversible scalar photo-structural effects. The photoinduced anisotropy phenomena are explained in the frame of a model with stimulation of chiral elements induced by polarized light in the disordered layers of the clusters.

High-resolution electron microscopy (HREM) studies of the microstructure and specific defects in hexagonal boron nitride (h-BN) precursors and cubic boron nitride (c-BN) crystals made under high-pressure high-temperature conditions revealed the presence of half-nanotubes at the edges of the h-BN particles. Their sp(3) bonding tendency could strongly influence the nucleation rates of c-BN. The atomic resolution at extended dislocations was insufficient to allow us to determine the stacking fault energy in the c-BN crystals. Its mean value of 191 +/- 15 mJ m(-2) is of the same order of magnitude as that of diamond. High-frequency (94 GHz) electron paramagnetic resonance studies on c-BN single crystals have produced new data on the D1 centres associated with the boron species. Ion-beam-induced luminescence measurements have indicated that c-BN is a very interesting luminescent material, which is characterized by four luminescence bands and exhibits a better resistance to ionizing radiation than CVD diamond.

A practical approach for in-plane angular spin distributions in layered systems, as obtained by Mossbauer spectroscopy, is discussed. The line intensity ratio R-23 of a Mossbauer pattern is expressed versus particular distribution parameters in unidirectional, step-shaped and ellipse-type models. The distribution parameters are deduced from experimental spectra taken by rotating the sample in its own plane. Three-dimensional spin distributions with small out-of-plane components can be analysed using the same method. The procedure is exemplified on four samples containing metallic nano-particles. The in-plane angular magnetic moment distributions derived with this method are compared with the results from bulk vector vibrating sample magnetometry in order to prove the accuracy of the described technique. (C) 2002 Elsevier Science B.V. All rights reserved.

We report the growth of TlBa2Ca2Cu3Oy superconducting thin film on CeO2 buffered r-cut sapphire substrate by amorphous phase epitaxy method, wherein an amorphous phase of a composition, TlBa2Ca3Cu4Oy was deposited by rf magnetron sputtering and subsequently crystallized by annealing at high temperatures in a closed system. There exists an optimum thickness of about 200 Angstrom of CeO2 buffer layer that gives a smooth buffer layer surface. At the temperature around 840 degreesC we get only Tl-2212 phase and for a longer annealing time it results in Tl-1212 phase. At higher temperatures Ba reacts with Ce to form BaCeO3. To avoid this reaction process we introduced another amorphous buffer layer, TlSr2CaCu2Oy onto CeO2 layer. By this method we could prepare a good quality TI-1223 thin film which showed a T-c of 104 K and a J(c) = 0.3 MA/cm(2) at 77 K and 0.1 T field. (C) 2002 Elsevier Science B.V. All rights reserved.

A 50 mum thick slice of (Cu,C)Ba2Ca3Cu4Oy (CU:1234) bulk superconductor grown by high-pressure synthesis technique was irradiated with high-energy (240 MeV) Au15+ ions (10(11) cm(-2)). Complimentary DC magnetization measurements up to 5 T showed an increase in critical current density by two orders of magnitude. AC susceptibility studies were performed in DC magnetic fields up to 14 T and at various temperatures, using various AC field amplitudes (up to 15 Oe) and frequencies (up to 10 kHz). Critical current densities at various temperatures and fields were estimated from critical state model. Using the third harmonic response of the sample we were able to separate different contributions to the dissipation: inter-grain regions, grains without columnar defects (heavy-ions penetration depth is about one quarter of sample's thickness), and grains with columnar defects. In irradiated grains, we could observe a peak-effect in the field dependence of critical current density at high DC fields and temperatures between 85 and 105 K, suggesting the possibility of the highest irreversibility field at liquid nitrogen temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

Superconducting samples have been synthesized from starting compositions CU0.5TlxBa2Ca3Cu4O12+y, x(Tl) = 0.35-1 in the sealed Au-tubes. Starting oxygen content was y = 0 and/or y = +/-0.225. Samples contain non-superconducting phases and only one main superconducting phase, 1234. The energy dispersive spectroscopy grain composition was Tl0.87-1.3Ba1.9-2.05Ca2.55-3.1Cu4Ox (normalized to Cu = 4). The starting Tl and oxygen content do not influence the occurrence of the phases, grain size, morphology and composition. On the other hand the phase content strongly depends on Tl content and almost does not depend on oxygen content. Both Tl and oxygen content are important for the transport properties of the superconducting grains and it seems that also of the non-superconducting matrix. T-c of the samples was in the range 119.1-111.3 K. (C) 2002 Elsevier Science B.V. All rights reserved.

Thin films of Cum-1Bam(Sr,Ca)(n)Cun+1O2m+2n+1, i.e., [BaCuO2](m)/[(Sr,Ca)CuO2](n) superlattices have been grown by layer-by-layer growth method with rf magnetron sputtering on NdGaO3(1 1 0) substrate. Two targets with the compositions BaCuO2 and (Sr,Ca)CuO2, which form the charge reservoir and superconducting layers, respectively, were used. [BaCuO2](m)/[(Sr,Ca)CuO2](n) superlattice was grown at 460-520 degreesC in a gas mixture of Ar and N2O by alternatively depositing the charge reservoir layer and superconducting layer. The superlattice modulation is close to that of high pressure synthesized bulk materials. By means of the layer-by-layer growth method, a system of Cu-based artificial lattice structures is formed. Electrical transport properties of these superlattices were also investigated. (C) 2002 Elsevier Science B.V. All rights reserved.