Glass transition temperature studies of planetary ball milled glasses: Accessing the rapidly cooled glassy state in Na4P2S7-xOx, 0 ≤ x ≤ 7, Oxy-thio phosphate glasses

Abstract

This report details the very first in-depth study of the glass transition temperature (Tg) for planetary balled milled (PBM) Na4P2S7-xOx (NaPSO), where 0 ≤ x ≤ 7, glasses. For 0 ≤ x ≤ 5, fully amorphous, homogeneous, and chemically reacted compositions can be prepared. For samples with 5 < x ≤ 7, partially amorphous but fully chemically reacted glass-ceramics can be produced. The reproducible, thermally cyclable onset Tg and the onset crystallization temperature, Tc, were investigated as a function of the composition and of the short-range order (SRO) structures of the glasses. Additionally, we examined the extreme sub-Tg exothermic relaxations present due to the high energy milling processing and correspondingly high effective quenching rate. Using a differential scanning calorimeter (DSC) and scanning from well below the onset Tg, the glasses are observed to relax exothermally below Tg. The integrated relaxation enthalpy, ΔHrel, is ~ 0 for the x = 2 glass, but is significantly exothermic for lower and higher values of x. Once the high fictive state of these glasses has been erased by scanning to above the Tg, but below the Tc of the glass, the DSC scans of the glasses recover normal behavior for a normally cooled glass without any relaxation exotherm. The Tgs of these glasses remain relatively constant for 0 ≤ x ≤ 3, but for glasses with 3 < x ≤ 5, Tg increases sharply following the combined effects of the phosphate chain length increasing caused by the formation of bridging oxygens and the increasing concentration of more charge dense non-bridging oxygens (NBOs). For x > 5, both Tg and Tc decrease sharply, becoming nearly identical at x = 7 where glass formation essentially ceases. For smaller values of x, Tc increases with both chain length and the structural complexity of the composition but decreases sharply for glass-ceramics of x > 5.