So. Kasap et D. Tonchev, Glass transformation in vitreous As2Se3 studied by conventional and temperature-modulated differential scanning calorimetry, J MATER RES, 16(8), 2001, pp. 2399-2407
We have studied the glass transition behavior of vitreous As2Se3 by carryin
g out temperature-modulated differential scanning calorimetry (TMDSC) and c
onventional differential scanning calorimetry (DSC) experiments to measure
the glass transition temperature T-g In TMDSC experiments we have examined
the reversing heat flow (RHF), that is the complex heat capacity C-P in the
glass transition region as the glass is cooled from a temperature above th
e glass transition temperature (from a liquidlike state) and also as the gl
ass is heated starting from room temperature (from a solidlike state). The
RHF, or C-P versus T, in TMDSC changes sigmoidally through the glass transi
tion region without evincing an enthalpic peak which is one of its distinct
advantages for studying the glass transformations. The T-g measurements by
TMDSC were unaffected by the amplitude of the temperature modulation. We h
ave determined apparent activation energies by using T-g-shift methods base
d on the T-g-shift with the frequency (omega) of temperature modulation in
the TMDSC mode and T-g-shift with heating and cooling rates, r and q, respe
ctively, in the DSC mode. It is shown that the apparent activation energies
Deltah* obtained from In omega versus 1/T-g and In q versus 1/T-g plots ar
e not the same, but nonetheless, they are approximately the same as the app
arent activation energy Deltah(eta) of the viscosity over the same temperat
ure range where the empirical Vogel expression of Henderson and Ast, eta =
12.9 exp[2940/(T - 335)], was used for the viscosity. The latter observatio
n is in agreement with the assertion that the structural relaxation time ta
u is proportional to the viscosity eta. The apparent activation energy Delt
ah(r) obtained from the In r versus 1/T-g plot during heating DSC scans is
lower than Deltah* observed during cooling scans. The results are discussed
in terms of a phenomenological Narayanaswamy type relaxation time. It was
observed that T-g obtained from TMDSC cooling experiments did not depend on
the underlying cooling rate for q less than or equal to 1 degreesC min(-1)
and for temperature amplitudes 0.5-5 degreesC. The transition due to the t
emperature modulation was well separated from the transition due to the und
erlying cooling rate. Further, the apparent activation energies obtained fr
om In omega versus 1/T-g during cooling and heating scans for q and r less
than or equal to 1 degreesC min(-1) are approximately the same as expected
from Hutchison's calculations using a single relaxation time model of TMDSC
experiments.