ISSN:
1662-8985
Source:
Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
Fluorine containing calcium aluminosilicate glasses are widely used for a number oftechnological applications including dental cements, mould fluxes in steel making and in a varietyof glass-ceramic systems. Despite of their importance these systems remain quite poorly understoodwith respect to their composition. To address this question a glass composition corresponding to theequimolar binary system anorthite−fluorite (Ca2Al2Si2O8−CaF2) was chosen as a base point for twoseries of compositions. One of the series is designed on the anorthite stoichiometry and consideredas classically charge balanced. Another series starts from the fluorine free composition of theanorthite−lime (Ca2Al2Si2O8−CaO) stoichiometry and, therefore, is characterized by a disruptednetwork with at least one non-bridging oxygen (NBO) attached to silicon. A multinuclear 19F, 27Al,29Si solid state NMR study of the glasses was undertaken. It is shown that in both series fluorine ispredominantly coordinated by calcium, F−Ca(n), and in addition interacts with aluminium formingAl−F−Ca(n) complexes, where n denotes the number of first neighbouring calcium cations. Smallamounts of high coordinated aluminium grows with increasing fluoride content in both glass series.However, the high coordinated aluminium may not be solely due to the formation of theAl−F−Ca(n) complexes. Glasses of the first series displayed systematic upfield shift of 29Si NMRresonance while substituting fluoride for oxide, starting from the fluorine free composition. Thisupfield shift is interpreted as the lack of cations in the network, due to formation of the F−Ca(n),which drives silicon network to polymerize toward a higher Qn structure. Contrary to the first series,the 29Si NMR resonance remains constant for fluorine containing compositions of the second seriesbut differs downfield from the initial anorthite glass. The latter is explained by the excess of cationsin the network due to addition of the fluorite resulting in formation of NBO on the silicon. Bindingof fluorine with silicon is considered negligible in these systems. Thus, fluorine and calcium bothdefine the degree of network polymerization and are considered as a cause for the changes in siliconand aluminium networks
Type of Medium:
Electronic Resource
URL:
http://www.tib-hannover.de/fulltexts/2011/0528/01/40/transtech_doi~10.4028%252Fwww.scientific.net%252FAMR.39-40.25.pdf
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