The effect of production regime and crucible materials on the thermal properties of sodium phosphate glasses produced from salts

Abstract

Changes in the thermal properties of sodium phosphate glasses during melt production have been investigated using Pt/Au and fused alumina crucibles. Glasses were produced from NaH(2)PO(4) as a starting material, providing an intrinsic Na(2)O:P(2)O(5) ratio of 1:1 and giving an O/P = 3, that is, a metaphosphate. In Pt/Au crucibles, glass transition temperatures rose to a plateau value of 295 degrees C at a rate determined by melt temperature. No contamination of the glass by platinum or gold was detected or indicated in the results. E(a) for the reaction was found to be 66.4 kJ mol(-1). In fused alumina crucibles, glass transition temperatures rose to over 450 degrees C, with these values showing some convergence at higher furnace temperatures. Extensive erosion of the alumina crucibles was observed. The amount of alumina incorporation within the glasses correlated well with the rise in glass transition temperature up to a maximum of 15.5 mol % Al(2)O(3) content. Al(2)O(3) incorporation above this value caused a reduction in the value of the T(g).

Glass forming region and physical properties in the system P2O5-Na2O–Fe2O3

“…It is not inconceivable that Al 2 O 3 contamination could also account for the 120°C difference in T g seen between the lower iron content glasses. This difference would require in the region of 10% Al 2 O 3 contamination in an iron-free sodium phosphate, although this level of contamination would not be expected within 30 minutes at 1000°C [21]. This contamination is of course dependent on the shape, size and porosity of the crucible used as the action of corrosion works through surface tension erosion.…”

“…2.49 g cm À3 in our study for the 50% P 2 O 5 , 50% Na 2 O glass) but a very significant effect on T g (755°C and 274°C for the aforementioned glasses, respectively c.f. 295°C in our study for the 50% P 2 O 5 , 50% Na 2 O glass, or 278°C using perceived similar production regime of 800°C for 1 h melt [21]). A small inclusion of Al 2 O 3 could easily account for the discrepancy between the higher iron content glasses (481°C vs. 500°C and 498°C vs. 502°C).…”

“…This contamination is of course dependent on the shape, size and porosity of the crucible used as the action of corrosion works through surface tension erosion. Additionally, Mogus-Milankovic et al melted their glass formulations at around 1000°C for only 30 min; previously published data suggests that the maximum contribution to T g due to phosphate polymerisation would not have been achieved by this stage [21].…”

“…Alumina crucibles have been shown to cause significant contamination of the resultant glasses, which greatly affects their properties. Whilst some argue that the small quantities involved (usually <2% when measured) are insignificant, even small quantities of Alumina contamination can have a large effect on the properties of phosphate glasses, as previously reported [21].…”

Modification of silicophosphate glass composition, structure, and properties via crucible material and melting conditions

“…The addition of Al 2 O 3 to phosphate glasses not only affects the glass structure but also promotes chemical durability and improves the resistance to moisture invasion, and the effect of Al 3+ ions has been described as strengthening the glass . In general, Al 2 O 3 can be introduced into the glass network (a) by direct addition of either alumina to the batch or through adding other aluminous materials which require less energy and, thus, a lower temperature for melting, for example, Al(OH) 3 or AlO(OH), or (b) by diffusion from alumina crucibles owing to crucible corrosion by the glass melt . In the latter case, the amount of dissolved Al 3+ and O 2− depends on the melting conditions and the type of alumina crucible used.…”