Indian Journal of Pure & Applied Physics
Vol. 43, May 2005, pp. 363-367
Specific heat studies in glassy Se78Ge22 and Se68Ge22M10 (M = Cd, In, Pb) alloys
R S Tiwari, N Mehta, R K Shukla, P Agarwal* & A Kumar†
Department of Physics, Harcourt Butler Technological Institute, Kanpur 208 002
*Department of Physics, D B S College, Kanpur 208 200
Received 19 April 2004; revised 3 February 2005; accepted 9 March 2005
The specific heat of glassy Se78Ge22 and Se68Ge22M10 (M = Cd, In, Pb) alloys using differential scanning calorimetry
(DSC) technique have been measured. An extremely large increase in the specific heat values has been observed at the glass
transition temperature. It has also been found that the values of Cp below glass transition temperature and the difference of
Cp values before and after glass transition (∆Cp) are highly composition dependent. This indicates that the additives used in
the present study influence the structure of the binary alloy. Specific heat of the additive element is found to be important for
the observed changes in the specific heat of the ternary alloys as compared to binary alloy.
Keywords: Specific heat measurement, Se78Ge22, Se68Ge22M10, Differential scanning calorimetry
IPC Code: G01N25/20
1 Introduction
A chalcogenide glass is one containing a large
amount of chalcogen elements belonging to VI group
of the Periodic table, i.e., sulphur, selenium and
tellurium. These glasses behave as semiconductor. A
variety of stable glasses have been prepared in bulk,
fiber, thin film and multilayer forms using melt
quenching, vaccum deposition and various other less
common techniques. These glasses are being used in
computer memories, erasable high density optical
memories1, photoconductive applications such as
photoreceptors in copying machines and X-ray
imaging plates2, I R optical lenses and windows3 and
high sensitivity ionic sensors4 using silver doped
chalcogenide glasses. Due to these technical
advantages of chalcogenide glasses, these materials
are being studied all over the world by scientists as
well as by engineers.
One of the most important problems in the area of
glasses is the understanding of glass transition
temperature and structural relaxation5-9. The glass
transition is exhibited as an endothermic peak or a
shift in the base line in the scan of differential
scanning calorimetry (DSC) due to change in specific
heat.
Specific heat is very sensitive to the way in which
atoms or molecules are dynamically bound in a
solid10. Thus measurement of such parameter like heat
capacity will lead to an effective test for
_________
†
E-mail: dr_ashok_kumar@yahoo.com
characterizing material as glassy substance. An abrupt
change in specific heat at the glass transition is
characteristic of the all chalcogenide glasses. The
parameter detects sensitively the change in the
microstructure of the glass which can be seen by the
jump of the specific heat close to the Dulong and the
Petit value Cp = 3R. Some attempts11-19 have been
made to measure the specific heat of chalcogenide
glasses. However, the explanations for the change in
specific heat before and after glass transition are of
diversified in nature. More experimental work is
required in this direction. .
For many years, it was believed that the physical
properties of chalcogenide glasses cannot be modified
by foreign atoms. The doping could not be achieved
by putting conventional impurities. However,
recently, p to n transition has been reported20-25 in
binary Ge-Se and In-Se chalcogenide alloys, when
third element is introduced in these glasses. The
electrical properties of these glasses have been studied
by various researchers20-25, but thermal properties
have not been studied in detail. The present paper
reports the effect of some additives (Cd, In, Pb),
having different specific heats, on the specific heat in
binary Se78Ge22 glassy alloy.
2 Experimental Details
Glassy alloys of Se78Ge22 and Se68Ge22M10 (M =
Cd, In, Pb) were prepared by quenching technique.
High purity materials (5N pure) were weighed
according to their atomic percentages and sealed in
INDIAN J PURE & APPL PHYS, VOL 43, MAY 2005
364
quartz ampoules under the vacuum of 10−5 torr. Each
ampoule was kept inside the furnace at 1000°C
(where the temperature was raised at a rate of
3-4°C/min.). The ampoules were rocked frequently
for 10 hrs at the maximum temperature to make the
melt homogeneous. Quenching was done in ice water
and the glassy nature of alloys was checked by X-ray
diffraction technique.
The glasses, thus prepared, were ground to make
fine powder for DSC studies. Constant heating rate of
20 K/min was used for DSC scans. First, we recorded
a blank run by putting sample and reference pans
empty inside the DSC cell. Then 5-10 mg of the
sample was kept inside in the pans and then
thermoscans were recorded under almost identical
conditions. A deflection from the initial equilibrium
point occurs between the two thermoscans with and
without the sample. This deflection on y-axis between
the two thermoscans is noted from the thermoscan
and specific heat Cp is calculated using the formula
given by26
Cp = [60 (E ∆qs/Hr)] . [∆Y/m]
… (1)
where E is the calibration coefficient, ∆qs is the y-axis
range setting, Hr the heating rate, ∆Y the y-axis
deflection, m is the mass of the sample.
Measurements were made under almost identical
conditions so that a comparison of specific heat Cp
could be made in order to understand the effect of
changing the additive element (M= In, Cd, Pb) in
ternary alloys Se68Ge22M10.
3 Results and Discussion
Figure 1 shows the typical DSC thermograms for
various binary and ternary alloy at the heating rate of
20 K/min.. The values of glass transition temperature
Tg obtained by DSC thermograms of these alloys at
the heating rate of 20 K/min are given in Table 1. The
values of Tg have been found to decrease in ternary
alloys Se68Ge22M10 (M = Cd, In, Pb) as compared to
binary alloy Se78Ge22.
The variation of Cp as a function of temperature at
the heating rate of 20 K/min for each glassy alloy is
shown in Fig. 2. It is clear from Fig. 2 that below
glass transition temperature, Cp is weakly temperature
dependent.
However,
near
glass
transition
temperature, Cp increases drastically with the increase
of temperature and shows maxima at glass transition
temperature. After glass transition temperature, Cp
attains a stable value which is slightly higher as
compared to Cp below glass transition temperature.
The sudden jump in Cp value for each alloy at glass
transition can be attributed27 to anharmonic
Fig. 1—DSC thermoscans of glassy Se78Ge22 and Se68Ge22M10 (M = Cd, In, Pb) alloys at the heating rate of 20 K/min
TIWARI et al.: SPECIFIC HEAT STUDIES IN GLASSY ALLOYS
contribution to the specific heat. The overshoot in the
value of Cp at the upper end of the “Cp jump” at glass
transition is due to the relaxation effects. The time
scale28 for structural relaxation is highly dependent
both on temperature and the instantaneous structure
itself. The observed peak in Cp at Tg may be due to the
fact that the structural relaxation times at this
temperature becomes of the same order as the time
scale of the experiment.
The difference of specific heat values (∆Cp) after
glass transition (i.e., equilibrium liquid specific heat
Cpe) and before glass transition (i.e., glass specific
heat Cpg) has been calculated for each glassy alloy and
Table 1—Values of Cpe, Cpg and ∆Cp (in m. cal / mg. °C) and Tg
(in °C) for glassy Se78Ge22 and Se68Ge22M10 (M = Cd, In, Pb)
alloys
the values of Cpe, Cpg and ∆Cp are given in Table 1.
From Table 1, it is observed that ∆Cp rises when third
element (Cd, In, Pb) is added to binary alloy Se78Ge22.
The value of glass specific heat Cpg is found to be
lower in ternary alloys Se68Ge22M10 (M = Cd, In, Pb)
as compared to binary alloy Se78Ge22 (see Table 1).
The additive elements (Cd, In, Pb) are added in Se-Ge
system at the cost of Se. The room temperature values
of Cp of additive elements Cd, In, Pb are smaller than
Cp of Se (Table 2). This is probably the reason of
lower Cpg values of ternary Se68Ge22M10 (M = Cd, In,
Pb). The decreasing order of Cpg in ternary alloys is
Se68Ge22In10 > Se68Ge22Cd10 > Se68Ge22Pb10, which is
explained in terms of decreasing sequence (In > Cd >
Table 2—Room temperature values of specific heat of elements
Element
Sample
Cpe
Cpg
∆Cp
Tg
Se78Ge22
Se68Ge22Pb10
Se68Ge22Cd10
Se68Ge22In10
4.30
2.80
3.38
3.84
3.65
1.77
1.89
2.10
0.65
1.03
1.49
1.74
330
236
300
303
365
Selenium (Se)
Lead (Pb)
Cadmium (Cd)
Indium (In)
Specific heat
(J/ gm °C)
0.318
0.129
0.226
0.233
Fig. 2—Temperature dependence of Cp in glassy Se78Ge22 and Se68Ge22M10 (M = Cd, In, Pb) alloys at the heating rate of 20 K/min
INDIAN J PURE & APPL PHYS, VOL 43, MAY 2005
366
Specific heat of additive element (J/gm °C)
Fig. 3—Plot of glass specific heat Cpg of ternary alloys versus room temperature value of Cp of additive elements
Pb) of room temperature values of Cp of additive
elements Cd, In, Pb. The plot of Cpg of ternary alloys
versus Cp of additive elements is shown in Fig. 3.
4 Conclusions
Calorimetric measurements have been performed in
binary Se78Ge22 and ternary Se68Ge22M10 (M = Cd, In,
Pb) glassy alloys to study the effect of additives (Cd,
In, Pb) on the specific heat in Se78Ge22 glassy system.
The values of Cp and Tg decrease in ternary alloys due
to incorporation of third element (Cd, In, Pb) in pure
Se-Ge system. This indicates that the additives (Cd,
In, Pb) drastically change the structure of the binary
Se78Ge22 glassy alloy. Specific heat of the additive
element is found to be important for the observed
changes in the specific heat of the ternary alloys as
compared to binary alloy.
Acknowledgement
One of the author, P Agarwal is grateful to
University Grants Commission (UGC) New Delhi for
providing a minor research project during the course
of the present work.
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