Duisburg, Germany
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Kadas K.,Uppsala University | Kadas K.,Hungarian Academy of Sciences | Andersson M.,Uppsala University | Holmstrom E.,Austral University of Chile | And 8 more authors.
Acta Materialia | Year: 2012

By means of theoretical modeling and experimental synthesis and characterization, we investigate the structural properties of amorphous Zr-Si-C. Two chemical compositions are selected: Zr0.31Si0.29 C0.40 and Zr0.60Si0.33C0.07. Amorphous structures are generated in the theoretical part of our work by the stochastic quenching (SQ) method, and detailed comparison is made regarding the structure and density of the experimentally synthesized films. These films are analyzed experimentally using X-ray absorption spectroscopy, transmission electron microscopy and X-ray diffraction. Our results demonstrate a remarkable agreement between theory and experiment concerning bond distances and atomic coordination of this complex amorphous metal carbide. The demonstrated power of the SQ method opens up avenues for theoretical predictions of amorphous materials in general. © 2012 Acta Materialia Inc. Published by Elsevier Ltd.

Wieneke J.U.,Universitaat Duisburg Essen | Wieneke J.U.,Center for Nanointegration Duisburg Essen | Kommob B.,Universitaat Duisburg Essen | Kommob B.,Center for Nanointegration Duisburg Essen | And 5 more authors.
Industrial and Engineering Chemistry Research | Year: 2012

Dispersions of unmodified nanoparticles (titanium dioxide, hydroxyapatite) were prepared by redispersion of nanoparticle powders in organic solvents using an ultrasound treatment. The dispersion quality was judged by dynamic light scattering (DLS) measurements and visual evaluation. Whereas "bad"solvents led to no or unstable dispersions with large particle diameters, dispersions made from the "good" solvents consisted of particles with relatively small diameters and were stable for several days or longer. For titanium dioxide, mixtures from four of the "good" solvents identified after first screening of a large set of solvents were prepared and tested as dispersion agent. Thus obtained dispersions showed superior properties compared to the previous dispersions, with small particles sizes and good long-time stability. Based on a rating of solvent quality and by calculation using the software HSPiP v3, the Hansen solubility parameters of the particles were then determined. Subsequently, entirely new solvent mixtures that could best fit these parameters were selected and found to also exhibit suitable properties as dispersion agent for the nanoparticles. The same iterative and quantitative approach worked also for the preparation of good and stable dispersions of hydroxyapatite. All results show that this is a promising methodology to disperse inorganic nanoparticles into suited organic solvents, for instance for the preparation of new polymeric nanocomposites. Furthermore, the method can be used to indirectly characterize the surface chemistry of nanoparticles. © 2011 American Chemical Society.

Thiede T.B.,Ruhr University Bochum | Krasnopolski M.,Ruhr University Bochum | Milanov A.P.,Ruhr University Bochum | De Los Arcos T.,Ruhr University Bochum | And 6 more authors.
Chemistry of Materials | Year: 2011

Metal-organic chemical vapor deposition (MOCVD) of thin films of two representative rare-earth nitrides is reported here for the first time. Four homoleptic, all-nitrogen-coordinated, rare-earth (RE) complexes were evaluated as precursors for the respective nitride thin film materials. Two guanidinato complexes [RE{(iPrN)2C(NMe2)}3] [RE = Gd (1), Dy (2)] and two amidinato complexes [RE{(iPrN) 2CMe}3] [RE = Gd (3), Dy (4)] were compared and used either as single source precursors or together with ammonia for MOCVD of gadolinium nitride (GdN) and dysprosium nitride (DyN), respectively. The thermal properties of the precursors were studied and the fragmentation patterns were characterized by high-resolution electron impact-mass spectrometry (HR EI-MS). The obtained nitride films were investigated using a series of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), nuclear reaction analysis (NRA), Rutherford backscattering (RBS), and X-ray photoelectron spectroscopy (XPS). The films contain preferentially oriented grains of fcc-GdN and DyN and are contaminated with small amounts of carbon and oxygen (significantly below 10 at.-% in the best cases). The temperature-dependent magnetic properties of the films, as measured using a superconducting quantum interference device (SQUID), suggest the existence of small ferromagnetic grains of the rare-earth nitrides that exhibit superparamagnetism. Despite the chemical and structural similarity of the guanidinato and amidinato complexes (1-4), a distinctly different behavior as MOCVD precursors was found for 1 and 2, compared with that for 3 and 4. While the guanidinates operate well as single-source precursors (SSPs), the amidinates are not suited at all as SSPs, but give very good nitride films when used in the presence of ammonia. This characteristic behavior was correlated with the different fragmentation mechanisms, as revealed by EI-MS. © 2011 American Chemical Society.

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