Modulated differential scanning calorimetry in the glass transition region

1997 ◽  
Vol 49 (1) ◽  
pp. 57-70 ◽  
Author(s):  
B. Wunderlich ◽  
I. Okazaki
Keyword(s):  
Differential Scanning Calorimetry ◽  
Glass Transition ◽  
Transition Region ◽  
Glass Transition Region ◽  
Scanning Calorimetry ◽  
Modulated Differential Scanning Calorimetry

Glass transformation in vitreous As2Se3 studied by conventional and temperature-modulated differential scanning calorimetry

2001 ◽  
Vol 16 (8) ◽  
pp. 2399-2407 ◽  
Author(s):  
S. O. Kasap ◽  
D. Tonchev
Keyword(s):  
Activation Energy ◽  
Differential Scanning Calorimetry ◽  
Relaxation Time ◽  
Glass Transition ◽  
Activation Energies ◽  
Temperature Modulation ◽  
Glass Transition Region ◽  
Scanning Calorimetry ◽  
Modulated Differential Scanning Calorimetry ◽  
Apparent Activation Energies

We have studied the glass transition behavior of vitreous As2Se3 by carrying out temperature-modulated differential scanning calorimetry (TMDSC) and conventional differential scanning calorimetry (DSC) experiments to measure the glass transition temperature Tg. In TMDSC experiments we have examined the reversing heat flow (RHF), that is the complex heat capacity CP in the glass transition region as the glass is cooled from a temperature above the glass transition temperature (from a liquidlike state) and also as the glass is heated starting from room temperature (from a solidlike state). The RHF, or CP versus T, in TMDSC changes sigmoidally through the glass transition region without evincing an enthalpic peak which is one of its distinct advantages for studying the glass transformations. The Tg measurements by TMDSC were unaffected by the amplitude of the temperature modulation. We have determined apparent activation energies by using Tg-shift methods based on the Tg-shift with the frequency (ω) of temperature modulation in the TMDSC mode and Tg-shift with heating and cooling rates, r and q, respectively, in the DSC mode. It is shown that the apparent activation energies ∆h* obtained from ln ω versus 1/Tg and ln q versus 1/Tg plots are not the same, but nonetheless, they are approximately the same as the apparent activation energy ∆hn of the viscosity over the same temperature range where the empirical Vogel expression of Henderson and Ast, η = 12.9 exp[2940/(T - 335)], was used for the viscosity. The latter observation is in agreement with the assertion that the structural relaxation time Ʈ is proportional to the viscosity h. The apparent activation energy ∆hr obtained from the ln r versus 1/Tg plot during heating DSC scans is lower than ∆h* observed during cooling scans. The results are discussed in terms of a phenomenological Narayanaswamy type relaxation time. It was observed that Tg obtained from TMDSC cooling experiments did not depend on the underlying cooling rate for q ≤ 1 °C min-1; and for temperature amplitudes 0.5–5 °C. The transition due to the temperature modulation was well separated from the transition due to the underlying cooling rate. Further, the apparent activation energies obtained from ln ω versus 1/Tg during cooling and heating scans for q and r ≤ 1 °C min−1 are approximately the same as expected from Hutchison's calculations using a single relaxation time model of TMDSC experiments.

Glass transformation, heat capacity, and structure of GexSe100−x glasses studied by temperature-modulated differential scanning calorimetry experiments

1997 ◽  
Vol 12 (7) ◽  
pp. 1892-1899 ◽  
Author(s):  
T. Wagner ◽  
S. O. Kasap ◽  
Kouji Maeda
Keyword(s):  
Thermal Analysis ◽  
Differential Scanning Calorimetry ◽  
Glass Transition ◽  
Heat Flow ◽  
Chalcogenide Glasses ◽  
Glass Transition Region ◽  
Scanning Calorimetry ◽  
Modulated Differential Scanning Calorimetry ◽  
Heat Flows ◽  
Glass Transformation

The recent novel temperature-modulated differential scanning calorimetry (DSC) (MDSCTM TA Instruments) technique has been applied to characterize the thermal properties of Ge–Se chalcogenide glasses in the glass transition region. All samples in this work were given the same thermal history by heating to a temperature above the glass transition, equilibrating, and then cooling at a rate of 5 °C/min to a temperature of 20 °C. The reversing and nonreversing heat flows through the glass transformation region during both heating and cooling schedules were measured, and the values of the parameters Tg, ΔH, Cp, and ΔCp, which characterize the thermal events in the glass transition region, were determined. The ability of determining the reversible heat flow in MDSC enables an accurate measurement of the true heat capacity (that normally associated with reversible heat flow), which could not be done hitherto in conventional thermal analysis where the detected heat flow is the total heat flow, the sum of reversing and nonreversing heat flows. The structurally controlled parameters Tg, ΔH, Cp, and ΔCp reveal extrema when the Ge–Se glass system reaches the average coordination number 〈r〉 = 2.67 at 33.3 at.% Ge which corresponds to the stoichiometric composition GeSe2. We also observed extrema in the composition dependence of the above thermal parameters at 20.0 and 40.0 at.% Ge which correspond to stoichiometric compositions GeSe4 and Ge2Se3 with average coordination numbers 2.40 and 2.80, respectively. No such clear local maxima below and above the 33.3 at.% Ge composition could be observed previously in thermal analysis. We compare our MDSC results with previously published works on glass transition in Ge–Se glasses and discuss the results in terms of recent structural models for chalcogenide glasses.

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