Research Trends in Contemporary Materials Science

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Authors: J.P. Šetrajčić, V.M. Zorić, S.M. Vučenović, D.Lj. Mirjanić, V.D. Sajfert, S.K. Jaćimovski, D.I. Ilić
Abstract: The dispersion law and density of states of phonons in ultrathin films was analyzed in this paper. It turned out that phonons in a thin film require activation energy for exciting. This leads to extremely low specific heat and specific conductivity at low temperatures. Consequences of quoted facts were discussed in detail and their influence on kinetic and thermodynamic properties of thin films is estimated.
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Authors: S.H. Shim, Hyoun Woo Kim, C. Lee, D.J. Chung, S.G. Park, S.G. Lee, B.H. O, J. Kim, S.P. Chang, S.H. Lee
Abstract: We have obtained one-dimensional (1D) nanomaterials of tin oxide (SnO2) on silicon nitride (Si3N4)-coated Si substrates by carrying out the thermal evaporation of solid Sn powders and varying the substrate temperature in an Ar/O2 ambient gas. We analyzed the samples with scanning electron microscopy, X-ray diffraction, transmission electron microscopy and photoluminescence (PL). Reactions at a lower substrate temperature gave rise to thinner 1D structures. The obtained 1D nanomaterials were single crystalline with a tetragonal rutile structure. We proposed a vapor-solid process as the growth mechanism for SnO2 nanorods. The PL spectrum exhibited visible light emission.
297
Authors: Ž. Bogdanov, N. Popović, M. Zlatanović, B. Goncić, Z. Rakočević, S. Zec
Abstract: The reactive sputter deposition of TiN thin films onto glass substrate at the ambient temperature using a homemade broad beam argon ion source was investigated in order to deposit the films with nanostructural characteristics. While constant Ar beam energy of 2 keV was used, the N2 partial pressure and the substrate current, adjusted by different accelerator grid potentials (Vacc) were varied. A negative substrate bias voltage (100 V) was additionally applied. The TiN film structure was investigated by XRD and STM methods. All deposited films exhibited (220) preferred orientation, and the change in normalized peak intensity (I220/d), lattice spacing (d220) and full-with at half-maximum (FWHM) were investigated. As a result of higher energy bombardment with 100 V negative substrate bias, compared to the substrate current change with Vacc, nearly constant (220) peak broadening with the increase of N2 partial pressure was obtained. The measured grain diameter (STM and XRD) confirms that the grain size is less than 12 nm, and the (220) preferred orientation was disturbed but not destructed.
303
Authors: A. Milutinović, Z. Dohčević-Mitrović, Diana Nesheva, M. Šćepanović, M. Grujić-Brojčin, Zoran V. Popović
Abstract: Silicon suboxide, SiOx thin films with different oxygen contents (1.15≤x≤1.70) were prepared by thermal evaporation of silicon monoxide at a residual oxygen pressure of 1·10-3 Pa and deposition rates of 0.2, 1.0, 3.0 and 6.0 nm/s. Rapid thermal annealing (RTA) of films was carried out at 1100°C in vacuum for 15 and 30 s and the films were analyzed by infrared (FTIR) and photoluminescence (PL) spectroscopy. In the FTIR spectra of SiOx annealed samples, a blue-shift of the stretching band with initial oxygen content, x, is observed. This band is shifted to a much lower frequency with prolonged RTA time. This behavior can be interpreted in terms of the partial decrease of oxygen content and film density upon annealing in vacuum. With annealing time increase a new band at 1106 cm-1 appears. Therefore, infrared spectra of SiOx films are significantly affected by the oxygen content. PL spectra of these films also change drastically with increasing annealing time. In the PL spectra of films annealed for 15 s two bands are easily visible: broad redorange band at 2.2 eV and a green band at 2.4 eV, while for 30 s annealing only low-energy band exists. Green band is connected with the defects in the SiOx matrix while red-orange band can be deconvoluted into several bands at 2.0, 2.3 and 2.5 eV. Two types of defects can be responsible for the PL band at about 2 eV: defects in a-Si (amorphous silicon) nanoparticles separated during RTA, and nonbridging oxygen hole recombination centers (NBOHC) formed by loosing of oxygen during RTA. The PL band at 2.3 eV is associated with the defects formed at a-Si/SiOx interfaces while a hardly visible band at 2.5 eV is related to the defects connected with the oxygen deficiency formed in the SiOx matrix during RTA.
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Authors: Z. Branković, G. Branković, K. Vojisavljević, M. Počuča, Tatjana Srećković, Dana Vasiljević-Radović
Abstract: The modified polymeric precursor method (Pechini method) was successfully used for the preparation of epitaxial and polycrystalline ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin films. Films were deposited on LaNiO3 (LNO) – coated silicium (1 0 0) and platinum substrates (Pt (1 1 1)/Ti/SiO2/Si) by spin coating technique. LNO electrodes were also prepared by the Pechini method and treated under different thermal treatment conditions to obtain films with different structural and microstructural properties. Investigation of PZT microstructure was performed as a function of orientation and morphology of the bottom electrode, as well as of thermal treatment conditions. Grain size and morphology were analyzed by AFM, while the quality and orientation of PZT films were determined by GIXRD analysis. It has been found that the proposed thermal treatment on a hot plate, with slow heating rate and long annealing time, can result in the formation of epitaxial PZT films on Si and LNO-coated Si substrates.
315
Authors: T. Novaković, Nenad Radić, Boško Grbić, Dimitar Stoychev, Plamen Stefanov, Ts. Marinova
Abstract: We have investigated the deposition of ZrO2 and Al2O3 films on a stainless steel (SS) substrate by spray pyrolysis method. The ZrO2 films were deposited from aqueous solution of zirconium dinitrate oxide. For Al2O3 films, a boehmite sol was used. The specific surface areas of samples were determined by the BET-method and sample surfaces were characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. The spray pyrolysis method permits the preparation of ZrO2 and Al2O3 thin films on SS with good adhesion. This method enables preparation of oxide layers with relatively high specific surface area, suitable for applications as catalysts supports.
321
Authors: R.A. Andrievski
Abstract: Features of hydrogen nanostructure synthesis are described as applied to metals (Mg and Pd) and intermetallics (Mg2Ni, FeTi and LaNi5). Attention is focused on the high-energy ball milling as a universal method for hydrogen nanostructure preparation. The effect of crystallite size, absorption/desorption properties of Pd - H2, Mg2Ni - H2, TiFe - H2 and Mg - H2 systems are characterized in detail. Structural features and some physical properties of nanohydrides studied by different independent characterization methods are considered.
327
Authors: Amelia Montone, Marco Vittori Antisari, N. Abazović, Jasna Grbović Novaković, Luca Pasquini, Ennio Bonetti, Anna Lisa Fiorini
Abstract: Hydrogen, being a regenerative and environmentally harmless fuel, can play a crucial role in the energetic scenario of the near future. In recent years several systems for solid-state hydrogen storage have been investigated, among which a few metals and metal alloys show the most promising properties. Mg and Mg-based micro and nanocomposites are widely studied for this application owing to the high gravimetric storage capacity, even if a proper microstructure, mainly at the material surface, has to be setup in order to overtake kinetic constraints often related to the high surface reactivity. In this paper, we present our approach to this problem and report the main findings. We have examined the influence on hydrogenation/dehydrogenation characteristics of different modifications of the bulk and surface microstructure of MgH2 powders processed by ball milling, which appears to be one the most useful methods for preparing powders suitable for hydrogen storage.
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Authors: Jasna Grbović Novaković, T. Brdarić, N. Novaković, Lj. Matović, Amelia Montone, S. Mentus
Abstract: Magnesium based composites MgH2 + X (X=Ti, Co) were synthesized by ball milling in an argon atmosphere using stainless steel vial and balls. The crystallographic behavior of the resulting powders was examined by XRD. Thermal stability and hydrogen desorption properties were investigated by thermal analysis methods. In order to obtain a deeper insight into bonding mechanisms of the transition metal in MgH2 relaxed structure, ab initio electronic structure calculation of MgH2 + X (X=Ti, Co) was performed using Full Potential Linearized Augmented Plane Wave method, implemented in WIEN2K code. DOS analysis, confirmed by DTA measurements, resulted in the conclusion that, in the composite, in comparison to MgH2, the bonding Mg-H was weakened, on account of the shortening of interatomic distances hydrogentransition metal.
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Authors: Fabrizio Cleri, Massimo Celino, Amelia Montone, Ennio Bonetti, Luca Pasquini
Abstract: Extensive electronic structure calculations, based on the density-functional theory, are used to characterize the equilibrium properties and the behaviour under pressure of MgH2. Moreover, substitutional solid solutions for several 3d metals (Fe, Ni, Ti, Al, Pd, Co, etc.) in MgH2 are characterized by computing both relaxed structures and solution energies. Theoretical results are combined with experimental results on samples synthesized by ball milling. MgH2 was milled with 10 wt.% of 3d metals followed by microstructural characterization and hydrogen desorption behaviour. Good correlation between theoretical solution energies and experimental desorption temperatures reveals a general trend in these systems. A few notable exceptions can be detected, which can be explained in terms of peculiarities in the experimental composite microstructure.
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