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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Discrete & computational geometry 21 (1999), S. 205-216 
    ISSN: 1432-0444
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract. We present a hierarchy of covering properties of rational convex cones with respect to the unimodular subcones spanned by the Hilbert basis. For two of the concepts from the hierarchy we derive characterizations: a description of partitions that leads to a natural integer programming formulation for the HILBERT PARTITION problem, and a characterization of ``binary covers'' that admits a linear algebra test over GF(2) for the existence of BINARY HILBERT COVERS. Implementation of our test leads to interesting new examples, among them: cones that have a HILBERT PARTITION but no REGULAR one; a four-dimensional cone with unimodular facets that has no HILBERT PARTITION; and two five-dimensional cones that do not have any BINARY HILBERT COVER.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Discrete & computational geometry 17 (1997), S. 439-448 
    ISSN: 1432-0444
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract. We provide lower and upper bounds for the maximal number of facets of a d-dimensional 0/1-polytope, and for the maximal number of vertices that can appear in a two-dimensional projection (``shadow'') of such a polytope.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Discrete & computational geometry 19 (1998), S. 159-174 
    ISSN: 1432-0444
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract. There is a long history of constructions of nonshellable triangulations of three-dimensional (topological) balls. This paper gives a survey of these constructions, including Furch's 1924 construction using knotted curves, which also appears in Bing's 1962 survey of combinatorial approaches to the Poincaré conjecture, Newman's 1926 explicit example, and M. E. Rudin's 1958 nonshellable triangulation of a tetrahedron with only 14 vertices (all on the boundary) and 41 facets. Here an (extremely simple) new example is presented: a nonshellable simplicial three-dimensional ball with only 10 vertices and 21 facets. It is further shown that shellings of simplicial 3 -balls and 4 -polytopes can ``get stuck'': simplicial 4 -polytopes are not in general ``extendably shellable.'' Our constructions imply that a Delaunay triangulation algorithm of Beichl and Sullivan, which proceeds along a shelling of a Delaunay triangulation, can get stuck in the three-dimensional version: for example, this may happen if the shelling follows a knotted curve.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Journal of materials science 16 (1997), S. 1124-1129 
    ISSN: 1573-4811
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Abstracts are not published in this journal
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Applied Organometallic Chemistry 11 (1997), S. 181-194 
    ISSN: 0268-2605
    Keywords: synthesis ; silazanes ; polymer pyrolysis ; non-oxide ceramics ; Si3N4 ; SiC ; crystallization ; ceramic matrix composites ; Chemistry ; Industrial Chemistry and Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The goal of this investigation was to optimize the synthesis of silazane-based polymers for processing fibre-reinforced ceramic matrix composites (CMCs). Liquid oligomeric silazanes were synthesized by ammonolysis of chlorosilanes and characterized spectroscopi- cally (FTIR, NMR) as well as by elemental analysis. The silazanes were obtained in high yield and purity. Different functional groups (system S1: Si - H, Si - CH3, Si - CH=CH2) and different degrees of branching in the Si - N backbone [system S2; Si(NH)3, Si(NH)2] were realized in order to study the properties of the silazanes that are dependent on the molecular structure.For processing ceramics via pyrolysis of pre-ceramic oligomers, molecular weight, rheological behaviour, thermosetting and ceramic yield were investigated systematically and correlated with the molecular structure of the silazanes. Low molecular weights (500-1000 g mol-1) as well as low viscosity values (0.1-20 Pa s) enable processing of the silazanes in the liquid phase without any solvent. Due to the latent reactivity of the functional groups, curing of the polymers via hydrosilylation is achieved.Structural changes and weight loss during polymer curing as well as the organic/inorganic transition were monitored by FTIR spectroscopy and differential thermogravimetric analysis. With increasing temperature (room temperature to 800 °C) the hydrogen content decreases from 7 to 〈 0.5 wt% due to the formation of gaseous molecules (NH3, CH4, H2). High ceramic yields up to 80% were reached by branching the oligomers, thus reducing the amount of volatile precursor fragments.Up to 1300 °C, ceramic materials remained amorphous to X-rays. At higher temperatures (1400-1800 °C) either SiC or SiC/Si3N4 composites were selectively crystallized, depending on the pyrolysis conditions. The utility of the optimized precursors for CMCs has been demonstrated by infiltration of fibre preforms and subsequent pyrolysis. © 1997 by John Wiley & Sons, Ltd.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
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