Independent but Additive Effects of Fluorine and Nitrogen Substitution on Properties of a Calcium Aluminosilicate Glass

Abstract

The effects of fluorine and nitrogen substitution for oxygen in a calcium aluminosilicate glass have been examined. Twelve glasses in the Ca–Si–Al–O–N–F system, with constant cation ratio, have been prepared and characterized with respect to density, molar volume (MV), fractional glass compactness (C), Young's modulus (E), microhardness (μHv), and glass transition temperature (Tg). Linear regression analysis of the property data with respect to nitrogen and fluorine substitution for oxygen strongly indicates that each affects property values independent of each other. Nitrogen substitution decreases MV and increases C, E, μHv, and Tg values due to the greater network connectivity associated with tri‐coordinated nitrogen. Fluorine substitution, which reduces network connectivity, decreases Tg values as might be expected. However, MV, C, E, and μHv are little affected by increases in fluorine content. The vastly different effects of nitrogen and fluorine on E and μHv are due to the manner in which their substitution for oxygen affects glass free volume and thus fractional glass compactness (i.e. atomic packing density). Differences in the effects of N and F substitution on Tg reflect their different effects on glass crosslink density. A method for calculating crosslink density is presented here, and the need for this method to take into account the ratio of network forming to network modifying aluminum is discussed.

Compositional effects on the crosslink density of Ca–(Mg)–(Y)–Si–Al–oxyfluoronitride glasses

“…The latter technique was also used to determine dilatometric softening temperature (T DS ) and the thermal expansion coefficient (α), in the range 300‐600°C. Young's modulus (E) was determined by an ultrasonic pulse‐echo‐overlap technique and microhardness (μHv) by the Vickers indentation method.…”

“…The cross‐link density (CLD) was calculated as the ratio of potential crosslinks ( N PCL ) less nonbridging crosslinks or network terminations ( N T ), to the number of glass‐forming cations ( N FC ), as shown in Equation . In this sense, the number of network terminations ( N T ) is due to nonbridging oxygens and fluorines less the charge compensation requirement for each aluminum.…”

“…In this sense, the number of network terminations ( N T ) is due to nonbridging oxygens and fluorines less the charge compensation requirement for each aluminum. Assuming that all aluminum is in IV‐fold coordination (all aluminum fulfils a glass former role), the following relationships, in which square brackets represent molar fractions, can be written for N PCL (Equation ), N T (Equation ) and N F (Equation ) …”

“…Hanifi et al also studied the effect of the systematic replacement of oxygen by both nitrogen and fluorine in oxyfluoronitride glasses with a cation ratio (eq.%) of Ca:Si:Al = 28:57:15. The results showed that fluorine introduction had no effect on the molar volume, glass compactness, Young's modulus or micro‐hardness.…”

“…Regarding nitrogen substitution for oxygen, the results were found to be consistent with those previously observed for oxynitride glasses: increase in glass compactness, Young's modulus, microhardness, glass transition, and dilatometric softening temperatures with N. While the changes in glass transition temperature could be related to changes in crosslink density resulting from N or F substitution, Young's modulus and microhardness could not. However, these latter properties exhibited a linear relationship with fractional glass compactness (C) or atomic packing …”