Publications (12)22.89 Total impact
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Article: Kinetic effect of boron on the crystallization of Si3N4 in Si–B–C–N polymer-derived ceramics
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ABSTRACT: The amorphous Si–B–C–N ceramics with a similar Si/C/N atomic ratio and various boron contents of 3.7 and 6.0 at.% B were synthesized and then isothermally annealed at temperatures ranging from 1550 to 1775 °C. The course of crystallization for the modifications of Si3N4 was examined by quantitative analysis of the corresponding x-ray diffraction patterns. Additionally, recent results of similar investigations on the ceramic with 8.3 at.% B were also considered. The kinetic analysis demonstrates that the controlling mechanisms of the Si3N4 crystallization, continuous nucleation and diffusion-controlled growth, are independent of the boron content. Nevertheless, the estimated activation energy of the crystallization significantly increases from 7.8 to 11.5 eV with the amount of boron ranging from 3.7 to 8.3 at.%. It is concluded that the role of boron in the crystallization kinetics is mainly due to the effect of boron on the nucleation process. Beside the kinetic analysis, the correlation between the boron content and the Si3N4 crystallite size has been discussed.Journal of Materials Research. 02/2011; 26(04):600 - 608. -
Article: Quantitative X‐ray Diffraction Analysis and Modeling of the Crystallization Process in Amorphous Si–B–C–N Polymer‐Derived Ceramics
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ABSTRACT: Amorphous Si–B–C–N polymer-derived ceramics (PDCs) with 8.3 at.% of boron were synthesized by thermolysis of the boron-modified poly(methylvinylsilazane). The isochronal crystallization process was quantitatively studied by X-ray diffraction (XRD) measurements using variable heating rates. Crystalline structures form within the amorphous Si–B–C–N PDCs at two stages including the formation of nanocrystalline SiC (NC-SiC) at the first stage followed by the formation of nanocrystalline Si3N4 (NC-Si3N4) and additional NC-SiC at the second stage. The change of the SiC crystallite size with temperature determined from the XRD analysis was used as a part of input data for the modeling. The metastable phase fraction diagrams were computed using an available model of metastable phase equilibria including amorphous and nanocrystalline phases for various modeling parameters and variable heating rates as well. The modeling performed is consistent with the experimental results to a large extent. The impact of modeling free parameters is discussed in order to explain the discrepancies observed between the experimental and computational results. The extended study of the NC-SiC formation at the first stage of crystallization justifies that this process is not purely controlled by kinetics and proves a crucial role of the metastable phase equilibrium between the amorphous Si–C–N domains and NC-SiC.Journal of the American Ceramic Society 04/2010; 93(5):1470 - 1478. · 2.27 Impact Factor -
Article: Templated self-assembly of ZnO films on monolayer patterns with nanoscale resolution.
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ABSTRACT: Lithographically defined self-growing ZnO films were prepared by a bioinspired chemical bath deposition technique (CBD). We observed a high selectivity of ZnO deposition: Teflon-like per-fluoro-decyl-trichlorosilane (FDTS) monolayers repelled ZnO primary particles, whereas amino-functionalized areas of the substrate were selectively covered by a highly anisotropic, oriented, and compact ZnO film with a thickness of 50 nm. The size of the primary particles in our methanol-based solution was approximately 2.5 nm. On the amino substrate they formed agglomerates not larger than 30 nm. Monolayer patterns made by polymer blend lithography were templated with an edge resolution of 30 nm. By using a specialized derivative of microcontact printing, we prepared layout-defined silane templates, which reliably determined the growth of a layout-defined, patterned oxide film with submicrometer lateral resolution.Langmuir 02/2010; 26(6):3774-8. · 4.19 Impact Factor -
Article: DNA-templated synthesis of ZnO thin layers and nanowires.
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ABSTRACT: In this paper, we report a novel synthetic approach towards electrically conductive ZnO nanowires close to ambient conditions using lambda-DNA as a template. Initially, the suitability of DNA to assemble ZnO nanocrystals into thin coatings was investigated. The ZnO nanowires formed on stretched and aligned lambda-DNA molecules were prepared via chemical bath deposition (CBD) of zinc acetate in methanol solution in the presence of polyvinylpyrrolidone (PVP). After 10 deposition cycles, the nanowires exceed 10 microm in length and the height can be varied from 12 to around 40 nm. The nanocrystalline structure of the ZnO wires was confirmed by high-resolution transmission electron microscopy (HRTEM). The electrical conductivity was found to be of the order of several Omega cm at room temperature in two terminal measurements.Nanotechnology 10/2009; 20(36):365302. · 3.98 Impact Factor -
Article: Aqueous Deposition of Ultraviolet Luminescent Columnar Tin‐Doped Indium Hydroxide Films
Advanced Functional Materials 08/2008; 18(17):2572 - 2583. · 10.18 Impact Factor -
Article: Chemical Modification of Single-Walled Carbon Nanotubes for the Reinforcement of Precursor-Derived Ceramics
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ABSTRACT: Single-walled carbon nanotubes (SWNTs) were incorporated into precursor-derived ceramics made from a polysilazane. To improve the dispersion of the nanotubes in the liquid precursor and finally in the ceramic matrix, the SWNTs were chemically modified by (iodomethyl)trimethylsilane via a radical reaction. The functionalization degree of the modified SWNTs was determined to be 3 atom %. Microscopic investigation combined with viscosity measurements and thixotropy tests demonstrated that the functionalized SWNTs are more homogeneously dispersed in the liquid SWNT/polymer mixtures and the solid cross-linked precursor, as compared to pristine nanotubes. SWNT/Si−C−N ceramics with nanotube contents of up to 1 wt % were obtained through pyrolysis of cross-linked SWNT/polymer composites at 1000 °C. The presence of intact nanotubes in these composites could be verified by scanning transmission electron microscopy. The high viscosity of the SWNT/polysilazane mixtures was identified as an important prerequisite for attaining good nanotube dispersion in the Si−C−N matrix.08/2008; -
Article: Novel precursor-derived Al–C–N–(O)-based ceramic additive for the low-temperature pressureless sintering of silicon nitride
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ABSTRACT: A polymer, which by pyrolysis transforms into Al–C–N–(O) ceramics, was synthesized from trimethylamine alane and cyan amide, and its applicability as a sintering additive for Si3N4 was investigated. Si3N4 powders were mixed with the precursor by treatment with organic slurries of the precursor to induce the homogeneous distribution of the additive. The green-bodies were pretreated in air or NH3 at 800 °C to control the chemical composition of the additive, through which the densification of Si3N4 could be improved. Dense samples with very fine grains (<2 μm) were obtained after sintering at 1600 °C in 0.1 MPa N2. Besides silicon nitride, submicrometer silicon carbide particles were observed in the samples, indicating that this procedure (i.e., the use of this novel sintering additive) also allows for the fabrication of SiC–Si3N4 composites.Journal of Materials Research. 05/2008; 23(06):1713 - 1721. -
Article: Mineralization from Aqueous Solutions of Zinc Salts Directed by Amino Acids and Peptides
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ABSTRACT: The suitability of amino acids and dipeptides as structure-directing agents for the deposition of coatings from aqueous solutions of zinc salts is discussed. According to a bio-inspired approach, the influence of these biomolecules was investigated with respect to the evolution of architectures based on zinc oxide and basic zinc salts. The small molecules were able to trigger the morphology of these materials ranging from grainlike to two-dimensional up to three-dimensional features. Besides morphological aspects, the structural characterization of these solids by means of electron and atomic force microscopies, photoelectron and infrared spectroscopies, and X-ray diffraction are discussed in order to extract the function of the biomolecules with regard to the formation of the inorganic phases.12/2005; -
Article: Synthesis of Ultrahigh‐Temperature Si–B–C–N Ceramic from Polymeric Waste Gas
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ABSTRACT: A “green” route to ultrahigh-temperature Si–B–C–N ceramic from vacuum-degassing waste gas of polyborosilazane {B[C2H4Si(CH3)NH]3}n (T2-1) has been developed. After gas-to-gel transformation, an amorphous ceramic Si5.3B1.0C19N3.7 was derived from the gel by dehydrocoupling and polymer-to-ceramic transformation. The ceramic started to form a nanostructure at 1700°C and resisted thermal degradation up to 2200°C in argon. This suggests that vacuum-degassing waste gases of polymer precursors may be perfect raw materials for various advanced ceramics.Journal of the American Ceramic Society 09/2005; 88(10):2709 - 2712. · 2.27 Impact Factor -
Article: Si−B−C−N Ceramic Precursors Derived from Dichlorodivinylsilane and Chlorotrivinylsilane. 1. Precursor Synthesis
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ABSTRACT: Dichlorodivinylsilane and chlorotrivinylsilane were used as starting materials for the synthesis of Si−B−C−N−H polymers. They were either reacted with ammonia to yield silazanes or with cyanamide to form silylcarbodiimides. The reactions occurred almost quantitatively. Starting from dichlorodivinylsilane, oligomeric or polymeric compounds were obtained, whereas reaction of chlorotrivinylsilane delivered monomers. The products were polymerized into solid Si−B−C−N−H polymers by the addition of borane dimethyl sulfide leading to an almost complete hydroboration of the vinyl groups. The compounds were characterized by chemical analysis and infrared spectroscopy. Boron-free compounds were further analyzed by nuclear magnetic resonance spectroscopy and partly by gas chromatography−mass spectroscopy studies.07/2002; -
Article: Boron-modified poly(propenylsilazane)-derived Si–B–C–N ceramics: preparation and high temperature properties
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ABSTRACT: Boron-modified poly(propenylsilazanes) [B{C3H6Si(CH3)NH}3]n are synthesized via both ammonolysis of B(C3H6Si(CH3)Cl2)3 (monomer route) and hydroboration of [(H2CCHCH2)Si(CH3)NH]n (n=3, 4) (polymer route) and subsequently transformed into ceramic materials by pyrolysis up to 1400 °C. Thermogravimetric analysis of the as-obtained materials as well as X-ray diffraction of annealed samples were performed to investigate high temperature properties, revealing the stability of the materials toward thermal degradation up to 1800 °C and the presence of silicon nitride at 1800/2000 °C. The crystallization is additionally investigated by transmission electron microscopy. SiC crystallization is observed first, followed by the evolution of a composite material which is composed of a turbostratic BNC matrix embedding SiC and Si3N4 nanocrystals.Journal of the European Ceramic Society. -
Article: Correlation of boron content and high temperature stability in Si–B–C–N ceramics
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ABSTRACT: The preparation and characterization of precursor derived Si–B–C–N ceramics with similar Si/C/N ratios but variable boron content are reported. The polymeric precursors were prepared via hydroboration of poly(methylvinylsilazane) using different BH3·SMe2/polymer stoichiometries. High temperature thermogravimetric analysis of as-pyrolysed ceramics as well as XRD studies of post-annealed samples display a retarding effect of boron on both crystallization of SiC and Si3N4 and stabilization of crystalline β-Si3N4.Journal of the European Ceramic Society.
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Institutions
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2005–2011
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Universität Stuttgart
- Institute of Material Science
Stuttgart, Baden-Wuerttemberg, Germany
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