Thermochemistry of glasses along the 2NdAlO3–3SiO2join

Abstract

Five Nd–aluminosilicate glasses along the 2NdAlO3–3SiO2join were synthesized using conventional drop-quenchtechniques. A sixth glass, with the end-member NdAlO3composition, required synthesis by containerless liquid-phase processingmethods to avoid crystallization. Enthalpies of drop solution (ΔHds) and formation(ΔHf)for the Nd–aluminosilicate glasses and the NdAlO3-compositionend-member glass were measured in molten 2PbO–B2O3at 1078 K in a twin Calvet type calorimeter. Values for ΔHdsfor the Nd–aluminosilicate glasses increase with decreasing silica content from 130.7 ± 1.5to 149.6 ± 0.6 kJ mol−1. Similarly,values of ΔHfincrease with decreasing silica content from 41.0 ± 2.0to 59.0 ± 1.6 kJ mol−1. Values ofΔHds and ΔHf for NdAlO3-composition glasswere measured as 99.3 ± 0.9and 139.2 ± 2.1 kJ mol−1,respectively. Using transposed temperature drop calorimetry, the enthalpy of vitrification for NdAlO3-compositionglass was measured as 69.5 ± 0.9 kJ mol−1relative to the stable crystalline neodymium aluminium perovskite (NdAlO3)phase. Enthalpies of mixing were calculated based on amorphous end members;the strongly negative values support the absence of immiscibility in this system.Differential scanning calorimetry was used to determine glass transition(Tg) andcrystallization (Tx)temperatures, as well as values for the configurational heat capacity (ΔCP(Tg))and the temperature range of the supercooled liquid interval (ΔT(SCL)). The NdAlO3-compositionglass showed no evidence of a glass transition prior to crystallization; only a single exothermwas observed, the onset of which occurred at 1045 K. For the Nd–aluminosilicates, valuesof Tg andΔT(SCL)increase with increasing silica content, from 1128 to 1139 K and from ∼95 to ∼175 K, respectively.Values of (ΔCP(Tg))increase with decreasing silica content, from ∼27 to ∼75 J/g fw ∗K,reflecting the increasing fragility and decreasing stabilityof the liquids as the end member composition, NdAlO3,is approached.