National Institute Of Materials Physics - Romania

In the last years oxide materials for electronics show significant progress. However, many details regarding technology control of the properties have to be solved. For electronics, thin films and heterestructures are important taking advantage of integration and synergetic concepts leading to new types of devices and functionalities. It is notable that, while fabrication of new devices and materials showing new phenomena are booming, the growth principles and concepts are somehow developing slowly within this general trend. This is because in many cases, growth of materials is very personalized. Understanding of the bi-directional relationship between the general and particular principles deserves attention. The immediate benefit is that knowledge on growth for one material can be transferred to another one. In our work we have analyzed such relationships for some oxide multicomponent perovskites. Materials used in our examples are Bi-Sr-Ca-Cu-O and YBa2Cu3O7, (Ca, Sr)CuO2, (Ca, Ba)CuO2 and Bi4Ti3O12. Presented thin films or heterostructures are with c-axis and non-c-axis orientations and based on these examples we discuss some of the growth principles.

The half-metallic properties of Ti2CoSn full-Heusler compound is studied within the framework of the density of states theory with the Perdew Burke Ernzerhof generalized gradient approximation (GGA). Structural optimization was performed and the calculated equilibrium lattice constant is 6.340 angstrom. The spin up band of compound has metallic character and spin down band is semiconducting with an indirect gap of 0.598 eV at equilibrium lattice constant. For the lattice parameter, ranging from 6.193 to 6.884 angstrom the compound presents 100% spin polarization and a total magnetic moment of 3 mu(B). (C) 2012 Elsevier Ltd. All rights reserved.

We were successful in synthesizing non-linear YBa(2)Cu(3)Ox whiskers, i.e. half loops or kinked shapes, which are promising candidates for solid-state devices based on the intrinsic Josephson effect and with improved electrical connections. We report on a complete characterization of their structural properties via a synchrotron nanoprobe as well as laboratory single-crystal diffraction techniques. This investigation allowed us to fully disclose the growth mechanism, which leads to the formation of curved whiskers. The superconducting properties are evaluated in comparison with their straight counterpart, revealing a strong functional analogy and confirming their potential applicability in superconducting electronic devices.

We review, based on our results, the problems and solutions for the growth of thin films and composite heterostructures emphasizing the general growth aspects and principles vs specifics for each material or heterostructure. The materials used in our examples are Bi2Sr2Ca2Cu3O10, Bi2Sr2CaCu2O8, YBa2Cu3O7, (Sr, Ca)CuO2, (Ba, Ca) CuO2, and Bi4Ti3O12. The growth method was metal organic chemical vapor deposition (MOCVD). The presented thin films or heterostructures have c- and non-c-axis orientations. We discuss the implications of the film-substrate lattice relationships, paying attention to film-substrate lattice mismatch anisotropy and to film-film lattice mismatch, which has a significant influence on the quality of the non-c-axis heterostructures. We also present growth control through the use of vicinal substrates and two-temperature (template) and interrupted growth routes allowing significant quality improvements or optimization. Other key aspects of the growth mechanism, that is, roughness, morphology, and interdiffusion, are addressed. It is concluded that the requirements for the growth of non-c-axis heterostructures are more severe than those for the c-axis ones. (C) 2012 The Japan Society of Applied Physics

Mixtures of MgB2 and metal or oxide additions with starting compositions of (MgB2)(M2O3)(x), x = 0.0025, 0.005, 0.015, and (MgB2)(M)(y), y = 0.01, M = Bi, Sb, were processed by spark plasma sintering (SPS). As-obtained samples are composites with high density exceeding 94% of the theoretical values. Secondary phases indicate similar reactions for samples with Bi- or Sb-based additions. However, samples show very different superconducting characteristics depending on the addition type and amount. A direct correlation with the melting temperature of the addition could not be revealed, although some aspects will be discussed. From the critical current density (J(c)) and irreversibility field (H-irr) enhancement viewpoints, optimum additions are oxides for x = 0.0025; 0.005. Both oxides improve Jc at high fields, but Sb2O3 is effective up to 10 K, whereas Bi2O3 is effective up to 30 K. Metal additions decrease J(c) and H-irr when compared with a pristine MgB2 sample. (C) 2012 The Japan Society of Applied Physics

Magnetite alternating copolymers divinylbenzene-maleic anhydride (DVB-MA) composites were prepared by dispersion polymerization. Because magnetite is used as a complex with oleic acid (Fe(3)O(4)OLA), the final hybrids show good dispersion of inorganic nanofillers in the polymer matrix. The obtained composites were analyzed by infrared absorption spectrometry, diffuse reflectance in visible light, thermogravimetry, X-ray fluorescence, X-ray diffraction, dynamic light scattering, scanning electron microscopy and vibrating sample magnetometry. The obtained results indicate the successful preparation of magnetite nanoparticles with an average size of about 23 nm dispersed in micrometer size copolymer spherical particles, which relative content can be controlled via the processing parameters. A relationship between the relative content of magnetite nanoparticles and the size of the polymer particles, with direct influence on the diffuse reflectance in the visible domain, was observed. A superparamagnetic behavior was evidenced at room temperature with a blocking temperature lower than as expected from the bulk anisotropy constant and the average size of the magnetite nanoparticles. Both the unexpected low blocking temperature and the observed low specific magnetizations were explained by a defected and poor crystalline structure of the magnetite nanoparticles, giving rise to spin disorder and diminished crystalline anisotropy constant. (C) 2012 Elsevier Ltd. All rights reserved.

High-temperature superconducting objects of Bi2Sr2CaCu2O8 and YBa2Cu3O7 highly curved in the ab-plane, such as curved/ kinked whiskers, bows and ring-like structures, were obtained within a solid-liquid-solid (SLS) grass-like growth mechanism. As-grown objects are crystals with three-dimensional epitaxy similar to conventional single crystals: they can be viewed as crystal parts 'cut' from a conventional rectangular crystal. Between our curved objects and conventional crystals, whiskers or thin films there are some differences in the superconducting properties induced only by the shape factors and no new physics is observed. Some details of the growth mechanism are discussed, emphasizing curved-line formation.

Mesoporous carbon frameworks were synthesized using the soft-template method. Ca(BH4)(2) was incorporated into activated mesoporous carbon by the incipient wetness method. The activation of mesoporous carbon was necessary to optimize the surface area and pore size. Thermal programmed absorption measurements showed that the confinement of this borohydride into carbon nanoscaffolds improved its reversible capacity (relative to the reactive portion) and performance of hydrogen storage compared to unsupported borohydride. Hydrogen release from the supported hydride started at a temperature as low as 100 degrees C and the dehydrogenation rate was fast compared to the bulk borohydride. In addition, the hydrogen pressure necessary to regenerate the borohydride from the dehydrogenation products was reduced.

Intermetallic Fe-Sn and nanocrystalline metallic Sn nanoparticles have been successfully synthesized from organic precursors using the laser pyrolysis technique with ethylene as sensitizer. Nano-structured Sn (single phase) was prepared by the pyrolysis of Sn(CH3)(4) (TMT) vapors. Controlled Fe/Sn atomic ratios, ranging from 0.69 to 1.64 were obtained for the prepared Fe-Sn nanopowders by the control of Fe(CO)(5) and TMT flows, respectively. XRD studies evidence three main phases: the tetragonal metallic Sn phase and the intermetallic FeSn2 phase and, to a much lesser extent, the cubic ternary carbide Fe3SnC. Complex core-shell structural characteristics were found by HRTEM analysis. More complete information about the Fe phase distributions in the new intermetallic Fe-Sn nanomaterial is provided by temperature dependent Fe-57 Mossbauer spectroscopy. (c) 2012 Elsevier B.V. All rights reserved.