The DTA spectra of the glasses are shown in Fig. 4. Typical glass
parameters: Tg (glass transition temperature), Tx (the onset of crystallization
temperature), and Tc (crystallization temperature) were extracted
from these curves and are summarized in Table 4. For the
A-(Na, K) samples, the DTA spectra reveals the existence of a single crystallization
at around 600 °C, while for the A-Li glass three broad exothermic
peaks are visible, related to the formation of at least, three crystalline
phases in its network by heating.
The values of the glass transition temperature of the samples are
very similar, being more detectable variations on the crystallization
temperature of the glasses, resulting from the different alkali oxides
used in the studied compositions. The relatively high glass transition
temperatures of these samples allow them, thermally, to be used as
electrolytes in intermediate temperatures fuel cells. For the evaluation
of the thermal stability of glasses, the ΔT = TX1 − Tg criterion was
applied. Higher values of this criterion usually correspond to a higher
thermal stability and glass-forming ability. The obtained values of ΔT
for the studied glasses are given in Table 4. The A-Li composition led
to the formation of a glass ceramic sample with an amorphous phase
besides the two crystalline phases, as previously discussed on the XRD results.
The existence of an amorphous phase on this sample, allows the determination
of its Tg in the same way as on the A-(Na, K) glasses. The
obtained value of ΔT=68 for the A-Li glass, is smaller than the ΔTmeasured
on the remaining samples. This means that the A-Li sample has
lower thermal stability, and a higher crystallization tendency that can
be achieved for instance, through heat treatments.