Παρακαλώ χρησιμοποιήστε αυτό το αναγνωριστικό για να παραπέμψετε ή να δημιουργήσετε σύνδεσμο προς αυτό το τεκμήριο:
|Εξειδίκευση τύπου : ||Ανακοίνωση σε συνέδριο|
|Τίτλος: ||Preferential bonding in low alkali borosilicate glasses|
|Δημιουργός/Συγγραφέας: ||Möncke D.|
[EL] Καμίτσος, Ευστράτιος Ι.[EN] Kamitsos, Efstratios I.
|Εκδότης: ||Society of Glass Technology|
|Περίληψη: ||In an earlier review(1) we discussed the connectivity in borosilicate glasses and compared our experimental findings by NMR, infrared and Raman spectroscopies with older structural models. We could show, contrary to the often cited reedmergnerite type model, that a significant preference exists in low alkali borosilicate glasses for trigonal rather than tetrahedral borate groups to link to silicate entities. Another often cited misconception is the application of the Loewenstein rule to borate tetrahedra. While linking of two "[AlØ4]-tetrahedra is disadvantageous compared to higher coordinated aluminate polyhedra, the borate tetrahedral units represents already the alternate higher coordination state and accordingly, many examples of linked [BØ4]-tetrahedra are known to exist in glasses as well as in crystalline compounds of boron at normal pressure conditions. We now present more NMR data on three different low alkali borosilicate glasses with Na2O:B2O3=0.2 to 0.35 and decreasing SiO2 fractions (74 to 43 mol% SiO2), for which we compare variations in the near and intermediate range structure of quenched and slowly annealed samples. None of the studied glasses showed a significantly higher fraction of trigonal BØ3 groups in the quenched than in the annealed samples, even though borate in the three-coordinated state is the preferred metaborate unit in the melt. However, for the two silicate rich glasses (including NBS 2), we observe at low temperatures a deviation of the viscosity-temperature plot from the ideal VFT-fit, that is from Tg at circa 440 to the expected 600°C. For samples prepared at any annealing temperature below 600°C, structural variations with thermal history are apparent, and are also reflected in many glass properties including density, fracture probability, or refractive index. Even though the glass NBS 2 shows no visible phase separation, DSC measurements indicate the presence of two different Tg values corresponding to the borate and the silica rich subnetworks. The values of the two Tg events shift with different cooling rates: for fast quenched glasses the two Tg values are closer together than for slowly annealed glasses. The study of these low alkali borosilicate glasses is ongoing, as we understand better how structural variations with changing thermal history impact the glasses' properties. Analogously, we can apply the same techniques to follow structural variations under external forces, such as irradiation, laser modification, pressure and mechanical impact.|
|Τίτλος πηγής δημοσίευσης: ||Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B|
|Θεματική Κατηγορία: ||[EL] Φυσική και θεωρητική χημεία[EN] Physical and theoretical chemistry|
[EL] Φασματοσκοπία[EN] Spectroscopy
|Αξιολόγηση από ομότιμους (peer reviewed): ||Ναι|
|Σημειώσεις: ||Financial support by the priority program SPP 1594 of the German Science Foundation (DFG) under grant no. MO1375/3-2 is gratefully acknowledged, as well as from the programme IKYDA 2015 for the promotion of the exchange and scientific cooperation between Greece and Germany. The glass compositions NBS C and NBS D have been part of a round robin test of the TC03 of the ICG. We thank Rita Cic-coni and Dominque de Ligny for preliminary tests regarding the optical properties and backscattering experiments.|
|Εμφανίζεται στις συλλογές:||Ινστιτούτο Θεωρητικής και Φυσικής Χημείας (ΙΘΦΧ) - Επιστημονικό έργο|
Αρχεία σε αυτό το τεκμήριο:
Το πλήρες κείμενο αυτού του τεκμηρίου δεν διατίθεται προς το παρόν από τον ΗΛΙΟ.