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1Author    Piercarlo Mustarelli, Corrado Tomasi, Marco Villa, C.S T E, -CN R AndRequires cookie*
 Title    Heat Capacities of Thermally Treated Na 2 0-3B 2 0 3 Glasses Above and Below T g  
 Abstract    Various contributions have been obtained from the heat capacities of "as quenched" and annealed Na 2 0-3 B 2 0 3 glasses. It is shown that the enthalpic release gives a major contribution to the DSC curves obtained by reheating quenched glasses. Different relaxation processes are followed by comparing DSC scans of quenched and annealed samples in the vitreous state and around the glass transition temperature, 7^. As a function of the annealing time at T = T g — 30 "C, the specific heats at room temperature initially increase, due to the decrease of the exothermic contribution from quenched-in defects, which are absent in a fully annealed glass; over a month-long time scale these heat capacities drop, due to a partial ordering, which decreases the configurational contribution. Such a decrease is related with the "overshoot" which occurs above and which attains a limiting value AH g s 14 J moP 1 in a matter of days. The heat capacities above T g relax towards a value of C P of = 2.3 J mol" 1 K~ 1 after a few days of annealing, which is intermediate between the anomalously low C P (~ 1.8 J mol" 1 K _1) of a fast quenched (~10 3 Kmin _1) and that (~2.9J mol" 1 K" 1) of a sample annealed for one hour. These phenomena point to complex relationships among enthalpic relaxation, entropic relaxation and temperature in these glasses. 
  Reference    Z. Naturforsch. 51a, 187—191 (1996); received January 11 1996 
  Published    1996 
  Keywords    Glasses, borates, structural relaxation, annealing 
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 TEI-XML for    default:Reihe_A/51/ZNA-1996-51a-0187.pdf 
 Identifier    ZNA-1996-51a-0187 
 Volume    51 
2Author    PhilipJ. Bray, GaryL. PetersenRequires cookie*
 Title    NMR and NQR Studies of Borate Glasses  
 Abstract    Nuclear magnetic resonance (NMR) has been used for some 40 years to study atomic arrange-ments, chemical bonding, and structural groupings in borate glasses and crystalline compounds, and nuclear quadrupole resonance (NQR) has more recently increased the resolution and accuracy of the measurements. Examples are presented of the use of first-order and second-order quadrupo-lar effects in "B NMR spectra to obtain structural information, and n B and 10 B NQR spectra to obtain the quadrupolar parameters Q cc (the coupling constant) and 77 (the asymmetry parameter) with accuracies of 5 or 6 significant figures, and 3 figures, respectively. Q cc and 77 are extremely sensitive to changes in atomic rearrangements and chemical bonds, so they are excellent monitors and provide identification of bonding configurations and structural groupings in borates: Examples are also presented in which combinations of NMR and NQR data are used to extract the desired information. NQR detection of resonances at frequencies as low as 276 kHz is discussed. 
  Reference    Z. Naturforsch. 53a, 273—284 (1998); received December 30 1997 
  Published    1998 
  Keywords    NMR, NQR, Borates, Quadrupolar Interactions 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0273.pdf 
 Identifier    ZNA-1998-53a-0273 
 Volume    53 
3Author    Giorgio Concas, Giorgio Spano, Marzia Carrada, Marco Bettinelli, Adolfo SpeghiniRequires cookie*
 Title    Local Structure of Europium Sites in Oxide Glasses by Nuclear Gamma Resonance  
 Abstract    The symmetry and disorder of the Eu 3+ site was investigated in some phosphate and borate glasses by means of 151 Eu Mössbauer spectroscopy. The quadrupole interaction parameter, which is due to the distortion of the Eu site compared to a cubic symmetry, has been measured together with the asymmetry parameter, which points out the absence of a threefold or fourfold axis of symmetry at the rare earth site. The correlation of the isomer shift with the optical basicity of the glass indicates a covalent component with 6s character in the Eu-0 bond. The axial component of the electric field gradient at the Eu site is also correlated with the optical basicity. 
  Reference    Z. Naturforsch. 54a, 539—544 (1999); received August 5 1999 
  Published    1999 
  Keywords    Europium, Oxide Glasses, Phosphates, Borates, 15 'Eu Mössbauer Spectroscopy 
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 TEI-XML for    default:Reihe_A/54/ZNA-1999-54a-0539.pdf 
 Identifier    ZNA-1999-54a-0539 
 Volume    54