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'Borates' in keywords Facet   Publication Year 1996  [X]
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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