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  • 1990-1994  (8)
  • 1970-1974
  • 1994  (8)
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  • 1990-1994  (8)
  • 1970-1974
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  • 1
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    Silica fracture (1994)
    Springer
    Details
    Publikationsdatum: 1994-07-01
    Print ISSN: 0022-2461
    Digitale ISSN: 1573-4803
    Thema: Maschinenbau , Physik
    Publiziert von Springer
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
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    Unbekannt
    Silica fracture (1994)
    Springer
    Details
    Publikationsdatum: 1994-11-01
    Print ISSN: 0022-2461
    Digitale ISSN: 1573-4803
    Thema: Maschinenbau , Physik
    Publiziert von Springer
    Standort Signatur Erwartet Verfügbarkeit
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    AKTUELLE ARTIKEL
  • 3
    Digitale Medien
    Digitale Medien
    Silica fracture (1994)
    Springer
    Journal of materials science 29 (1994), S. 5808-5816 
    Details
    ISSN: 1573-4803
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau
    Notizen: Abstract A quantitative model of environmentally sensitive crack growth of amorphous silica (a-silica) is based upon semiempirical molecular orbital (MO) calculations (AM-1 method) of a water molecule interacting with strained three- and four-fold silica rings and a five-fold ring-chain structure. The energy barrier for hydrolysis of strained 3-fold rings is only 7 kcal mol−1, the energy barrier for hydrolysis of strained four-fold rings is 29 kcal mol−1; for a five-fold ring-chain it is 39 kcal mol−1. Thus, the MO model predicts that the energetics of Region 1 slow crack growth is controlled primarily by the distribution and hydrolysis of three-membered silica rings in the a-silica structure, and Region III is controlled by the distribution and energy of contraction of four and four + membered rings.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00366861
    Standort Signatur Erwartet Verfügbarkeit
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  • 4
    Digitale Medien
    Digitale Medien
    Laser densification of medium pore size gel-silica glass (1994)
    Springer
    Journal of sol gel science and technology 2 (1994), S. 723-728 
    Details
    ISSN: 1573-4846
    Schlagwort(e): gel-silica ; laser densification ; microlenses
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Type VI sol-gel silica monoliths with 32 Å and 45 Å pore radii were made using tetramethylsiloxane (TMOS) with HF as a catalyst. The pore texture was characterized using nitrogen absorption/desorption isotherms and BET analysis. Monoliths thermally stabilized to specific bulk densities of 1.67 g/cc to 2.00 g/cc were irradiated with a CO2 laser to create microlenses. The densified spots were characterized using microscopic Fourier transform infrared (FTIR) spectroscopy. Full density microlenses were achieved; the peak position of the Si-O-Si vibrational mode was at 1122 cm−1, equivalent to commercial optical silicas. The refractive index in the center of the densified spots was 1.46 at a wavelength of 632.8 nm with a gradient in refractive index of 0.07–0.08. The combinations of laser power and substrate bulk density that yield full density microlenses is expanded when compared with conditions previously established for small pore, 12 Å, gel-silica substrates. Dimensions of the laser densified region, magnitude of the index gradient, and shape of the GRIN profile are easier to control with the medium pore size gel-silica substrates.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00486338
    Standort Signatur Erwartet Verfügbarkeit
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  • 5
    Digitale Medien
    Digitale Medien
    Laser densification of channel waveguides in gel-silica substrates (1994)
    Springer
    Journal of sol gel science and technology 2 (1994), S. 729-735 
    Details
    ISSN: 1573-4846
    Schlagwort(e): laser densification ; optical waveguides ; gel: silica
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Channel waveguides are important components in optical signal processing. A new method is described for producing such waveguides with high design flexibility. The channel waveguides are produced using CO2 laser densification of partially densified gel-silica matrices (Type VI optical silica). Critical processing conditions include pore size and initial density of the matrix, laser power, translational speed of the sample, distance between sample and focusing lens, and ambient humidity. Channel waveguides less than 500 µm wide were produced in gel-silica substrates of different pore sizes and bulk densities throgh laser densification. Optically transparent waveguides were obtained for speeds of the sample over 1.4 cm/s and laser power settings ranging between 12 and 16 mA. Substrates with three different pore sizes were analyzed, i.e, 12, 30 and 45 Å, with densities varying from 1.1 g/cc to 2.1 g/cc. Fourier transform infrared microspectrometry of the densified regions showed that IR shifts ranging from 1 to 38 cm−1 in the peak position of the Si-O-Si stretching vibrational mode were achieved. This corresponds to changes of index of refraction ranging from 0.01 to 0.20. The experiments show also that the larger the pore size the wider is the range of parameters for producing effective waveguides.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00486339
    Standort Signatur Erwartet Verfügbarkeit
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    Artikel (Nationallizenzen)
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  • 6
    Digitale Medien
    Digitale Medien
    Gel-silica optics: Theory and application (1994)
    Springer
    Journal of sol gel science and technology 2 (1994), S. 647-655 
    Details
    ISSN: 1573-4846
    Schlagwort(e): gel silica optics ; silica ; porosity ; pore distributions
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract A new generation of silica optics has resulted from alkoxide-based sol-gel processing of silica including: net shape transmissive optical elements, surface diffractive optics, inorganic doped GRIN optics, organic impregnated optical composites such as dye lasers and scintillators, optics with internal diffraction gratings, laser densified microoptical lenses and arrays, and laser densified waveguides. Processing control of the ultrastructure of the monolith at the time of gelation and during aging is essential to producing the optical devices together with chemical and thermal stabilization of the surface of the pore network prior to densification or impregnation. The process control variables for gel-silica optics are summarized together with spectroscopic analysis and molecular orbital calculations that explain how and why the thermal-chemical processing controls work at a molecular level.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00486326
    Standort Signatur Erwartet Verfügbarkeit
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  • 7
    Digitale Medien
    Digitale Medien
    Silica fracture (1994)
    Springer
    Journal of materials science 29 (1994), S. 3601-3606 
    Details
    ISSN: 1573-4803
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau
    Notizen: Abstract A quantitative ring contraction model for the fracture of amorphous silica is described based upon AM-1 semiempirical molecular orbital calculations of strained three- and four-fold silica rings and a five-fold ring-chain structure. The fracture barrier for five-fold ring-chain structures is 103 kcal mol−1. The barrier for fracture of a three-fold ring is 96 kcal mol−1. Fracture by contraction of four-fold rings has a lower energy barrier of 77 kcal mol−1 due to formation of pentacoordinate silicon transition states which produce trisiloxane rings and a broken siloxane bond. Thus, the ring contraction model predicts that a crack will follow a path which depends on the distribution of four-fold (or larger) rings in vacuum or fast fracture.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00357324
    Standort Signatur Erwartet Verfügbarkeit
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    Artikel (Nationallizenzen)
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  • 8
    Digitale Medien
    Digitale Medien
    Calcium phosphate formation on sol-gel-derived bioactive glasses in vitro (1994)
    Hoboken, NJ : Wiley-Blackwell
    Journal of Biomedical Materials Research 28 (1994), S. 693-698 
    Details
    ISSN: 0021-9304
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Medizin , Technik allgemein
    Notizen: When soluble salts are used in the sol-gel processing of bioactive glasses, the resulting materials are not homogeneous. To produce homogeneous gel-glasses, a sol-gel method using all alkoxide precursors was developed. In this work the in vitro bioactivity of the new all-alkoxide gel-glasses is investigated. Three compositions in the system, CaO—P2O5—SiO2, were studied. Solid samples were soaked in Tris buffer and simulated body fluid (SBF), and the rate of formation of hydroxy carbonate apatite (HCA) on the surface of the glasses was measured by Fourier transform infrared spectroscopy. The solutions were analyzed before and after reaction by inductively coupled plasma. All compositions studied formed a hydroxy carbonate apatite (HCA) layer within 8 h in both test solutions. The HCA layer grew rapidly when SBF was used. © 1994 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jbm.820280606
    Standort Signatur Erwartet Verfügbarkeit
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