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Crystallization and specific heat studies of Se100 − xSbx (x = 0, 2 and 4) glass
J. Phys. Chem Solids Vol58,
PII: SOO22-367!3(%)00148-5
Pergamon
CRYSTALLIZATION
Printed
AND SPECIFIC HEAT STUDIES (x = 0, 2 and 4) GLASS
P. PREDEEP,*
N. S. SAXENA*
No. 3, pp. 385-389,1997 Science Ltd in Great Britain. All rights reserved 00%3697/97 $17.00 + 0.00
Q 1997 Elsevier
OF Seloo_,Sb,
and A. KUMARt
*Condensed Matter Physics Laboratory, Department of Physics, University of Rajasthan, Jaipur 302 004, India tDepartment of Physics, HBTI, Kanpur, India (Received 5 June 1996; accepted 16 August 1996)
Abstract-Bulk glasses of compositions Se,,,&Sb, (x = 0, 2 and 4) are prepared by melt quenching technique. Differential scanning calorimetry (DSC) is employed to study the crystallization mechanism as well as specific heat of these glasses. Samples are subjected to thermal scanning in the DSC at various heating rates. All the compositions have shown well defined exothermic Peaks of crystallization. The crystallization data are examined in terms of modified Kisinger’s equation for the activation energy of crystallization. Also the results were analysed using the method suggested specifically for nonisothetmal crystallization by Matusita et al. and the activation energy for crystallization and the dimensionality of crystal growth are indicated for all the compositions. It has also been found that the thermal stability of the system is enhanced by the alloying of Se with small atomic percentages of antimony. Specific heat, Cr, of all the compositions were also evaluated and discussed. 0 1997 Elsevier Science Ltd. All rights reserved Keywords: C. differential scanning calorimetry, crystallization kinetics, glass transition, activation energy, D. specific heat.
1. INTRODUCTION Chalcogenide terms
of
Selenium
glasses physics
have evoked
as
well
thermal
to improve stability
[l] to be most stability
certain
by alloying. effective
and resistance of human
in
technology. poten-
and there have been a lot of of its properties
in increasing
the thermal
to crystallization. health.
like
Arsenic has been found
since As is toxic, it is not desirable viewpoint
much interest
device
has been found to have tremendous
tial in device technology attempts
as
However,
to use As from the
Alloying
elements
pro-
duce characteristic effects [2] depending on whether they are univalent or isoelectronic with Se or capable of producing chain branching. However, the effect of Sb doping in Se on the thermal stability and crystallization has not been given much attention. It has been found that the addition of a small atomic percentage of Sb in the Se-Te system drastically improved [3] the thermal stability of the system. In this paper we report the parameters of crystallization kinetics for amorphous SeloO_,Sb, (x = 0, 2 and 4) system together with its specific heat (C,).
2. EXPERIMENTAL
High purity
Se and Sb in appropriate
atomic per-
centage were weighed into quartz ampoules (length 5 cm and internal diameter 8 mm). The contents of the
ampoules were sealed in a vacuum of 10e6 Torr and heated in a rotary furnace to 800°C for lo-12 h to ensure the homogeneity. The molten samples were then rapidly quenched in ice cooled water. The glassy nature of the alloys was confirmed by X-ray diffraction (XRD). DSC scans of the samples were taken using a Rigaku-8230 DSC coupled with a thermal analysis station. The temperature precision of the equipment is 0.1 K with an average standard error of about 1 K in the measured values. The samples (typically 15 mg) in powder form were taken in standard aluminium pans and scanned over a temperature range of 200” at heating rates of 5, 10, 15 and 20 K/min. The fraction crystallized at any temperature, T is given by X = AT/A, where A is the total area of the exotherrn between
the temperature
at which crystallization
is
the area between T1 and T [4,5]. The values of the specific heat of the experimental samples has been determined using the formula
completed.
AT is
c, = mrAs ---Cc, msAr where m, is the mass of the reference material, m, that of the sample, A, and A, are the shifts for sample and reference materials with respect to base line, respectively, and C, is the specific heat of the reference material. To avoid any significant uncontrolled relaxation, the starting glass is always the one that 385
386
P. PREDEEP et al.
obtained from first heating to the end of glass transition and rapidly cooled to room temperature.
3. RESULTS AND DISCUSSION
ln[- ln(1 - X)] = - nln(a)
Figure 1 shows typical DSC thermograms of Seta,,, SessSbz and Se&b4 glasses at the heating rate of 5 K/ min. The glass transition temperatures (Ts) of these samples are not found to exhibit any discernible trend. At the heating rate of 5 K/min, Se has a Tg of 312.1 K while that of Se&b2 and Seg4Sb4 are 313.3 and 313.2 K, respectively. The interpretation of experimental data on crystallization is given on the basis of Kissinger’s and Matusita’s equations for nonisothermal crystallization. The activation energy (II,) for crystallization has been evaluated using the modified Kissinger’s equation [6-91. ln[a’/ Ti] = -mE,/RT,,
+ ln( C)
(1)
where C is a constant having factors dependent upon the thermal history of the samples. n and m are constants having values between 1 and 4 depending on the morphology of growth [lo]. Here (Y is the heating rate and Tp is the temperature corresponding to the crystallization peak. From the slope of the plot between In(o) and l/T,, the value of mE,ln can be calculated. Also from the equation suggested [l 1J by Matusita
SK/m
91.2
END0
END0
END0
I
100
et al., for nonisothermal crystallization, the value of activation energy for crystallization, EC can be evaluated.
2
Temperature C’CI Fig. 1. DSC thermograms of Seloo-$b, glass at the heating rate of 5 K/m.
+ constant
- 1.052mEJRT (2)
where X is the fraction of crystals precipitated in a glass heated at uniform rate and n is a numerical factor depending on the nucleation process. Here n = m + 1 is taken for a quenched glass containing no nuclei and n = m for a glass containing a sufhciently large number of nuclei. Also m has been assigned integer values of 1,2 and 3 for one-, two- and three-dimensional growth, respectively. The crystal nucleation rate [ 1l] in glass reaches the maximum at a temperature a little above the glass transition temperature and then decreases rapidly with increasing temperature. When a glass is heated at a constant rate [ 121,crystal nuclei are formed only at lower temperatures. Hence, when nuclei formed during the heating at constant rate are dominant, n is equal to m + 1 and when nuclei formed during any heat treatment prior to the thermal analysis are dominant, n = m. This can be decided by observing the change of n with reheating at the nucleation temperature. If n does not change with reheating, a large number of nuclei already exist and n can be taken as equal to m. A decreased value of n on reheating almost indicates the absence of pre-existing nuclei in the sample, which could influence the crystallization process and, in this case, n = m + 1. It is pointed out [lo] that in a crystallization process three types of activation energies has to be considered, activation energy for nucleation (E,), activation energy for crystal growth (E,) and that for the whole process of crystallization, (EC). It has been shown through various studies [13, 141that the values of EC evaluated using thermal analysis of the sample can be taken equal to the activation energy, Eg, for crystal growth. To calculate the activation energy for crystallization, E,, eqn (2) is used. From the slope of the plot between ln(cr) and 1000/T,, (Fig. 2), the value of mE,/n is deduced for various compositions and the values are listed in the Table 1. Figure 3 shows the plot of ln[- ln( 1 - X)] against 1000/T at different heating rates for the composition Se&bz. The break in linearity of these plots in high temperature regions seems to be due to the saturation of nucleation sites in the final stages of crystallization [ 15, 161or due to the restriction [ 171of crystal growth by the small size of the particles. The linear region covering a wide temperature range is used for analysis and the slope of these plots give the value of mE,. The values of mE, thus obtained for all the compositions are also listed in Table 1.
Crystallization and specificheat studies of Seloo_$b, glass
387
3.0-
2.7:z2.G
2.1 -
1.8 I 2.45
I 2.5
2.55
Fig. 2. ln(cY)vs lOOO/~,for Se9$bz glass. For evaluating the order parameter n, ln[- ln( 1 - X)] was plotted as a function of In(a). Plot for SessSb2 is shown in Fig. 4. Values of n obtained in a similar fashion for all compositions are also given in Table 1. A plot for reheated glass is also shown in Fig. 2. The value of n for reheated glass is found to be 1.3 which clearly shows a decrease from that of the quenched glass. As already discussed, the decreased value of n for reheated glass almost indicates the absence of the preexisting nuclei in the quenched glass and, hence, m is taken as equal to n - 1. Value of n is found to be about 2 for all the compositions which indicates bulk nucleation with one-dimensional growth for these glasses. The value of E, are calculated using these values of n and m and a difference has been observed in the values of EC obtained through the two different procedures.
It is interesting to note that considerable differences in values of E, calculated in a similar manner had also been reported in the case of Li20-2SiOz [18] and SeSb-Te glasses [19]. The values of EC calculated using eqn (2) indicate a fall in the value of EC for the composition Se&b2 and then again an increase for Ses6Sb4. Activation energy being an indication of the speed of the rate of crystallization, it can be concluded that SessSb2 glass has the slowest rate of crystallizaton. Addition of higher values of Sb in fact leads to an increase in crystallization rate thus reducing the thermal stability. The difference between T, and Tg is known to be a strong indication of thermal stability of the glasses. The higher the value of T, - Tg, the greater the thermal stability and ease of glass formation. Figure 5 shows the variation of T, - Tg with Sb
-4-
-5 23
I 2.35
I 24
I 2.45
I I 2.55 25 IU’l 7
I 2.6
I 2s
2‘.ir
Fig. 3. In[- In( I - X)] vs 1000/T for Se9sSb2 glass at three heating rates of 5( ), lO( ), and 15( ) K/min.
P. PREDEEP et al.
388
Table 1. Crystallization parameters of Se-Sb system from the heating rate data In(a) vs l/T
(m/n)-%
EC Wmolj
(kJ/mol)
Composition
Sk
100.4 88.8 65.8
Se&b2 SegsSb4
-41 1.5
ln(- ln( 1 - X)] vs In(o)
200.8 117.6
131.7
I
I
I
1.7
1.9
21
ln[-ln(1 -A’)] vs l/T
n
m
mEc (kJ/mol)
2.11 2.33 2.01
1 1 1
301.9 158.1 170.5
1
23 In(~)
I
I
I
2.5
27
2.9
EC Wmolj 301.9 158.1 170.5
:
Fig. 4.ln[- ln( 1 - X)] vs ln(cy)for Se&b2 glass at fixed temperatures. ( ) as quenched glass: 398 K, ( j reheated glass: 400 K. concentration and it can be seen that the addition of a small amount of Sb increases the thermal stability. Earlier reported studies [3] on the effect of addition of Sb on the thermal stability of Se-Te system indicates that the addition of only 0.1 at.% of antimony has a sufiicient effect on eliminating the exothermic peak due
to crystalliaztion thus improving the thermal stability drastically for the Se-Te system. Also higher concentrations of Sb in Se-Te system has found to be less effective in producing thermal stability in Se-Te alloys. The variation of Tc-Tg with Sb at.% indicates to a similar trend in the Se-Sb system.
__
70 -
at./. Sb Fig. 5. (T, - TB)as function of Sb content in Seloo-XSb, glasses.
Crystallization and specific heat studies of Sera&b,
glass
389
0.25
0.22 -
-
0.18
$o.,tS;;
0.13 -
0.$0
’
’
’
’
’ ’ 40
’
’
’
’ ’ 50
’
’
’
’
60
’
’
’
’
70
Temperature 1*Cl Fig. 6. Variation of specific heat with temperature at the heating rate of 5 K/m for Seroo_$b, glasses. ( )x=0,( ()x=4. The specific heat, Cp, for all the three compositions are evaluated
in the glass transition
region. It has been
that the variation of C, with temperature is more or less independent of heating rate. Figure 6 shows the variation of specific heat as a function of temperature at the heating rate of 5K/min. The sudden jump in C,, value at glass transition can be attributed [lo] to an harmonic contribution to the specific heat. Thus, glass transition temperature is usually defined as the temperature onset of the normally stepwise increase in heat capacity which occurs during heating of the glassy material. The overshoot in the value of C, at the upper end of the “CP jump” at glass transition is due to the relaxation effects. The time scale for structural relaxation [20] is highly dependent both on temperature and on the instantaneous structure itself. In the glass transition region, this time scale becomes of the order of lo2 s. The peak in C, is observed due to the fact the structural relaxation times of these glasses are of the same order as the time scale of the experiment. Also AC,, the difference between liquid and glass heat capacities, for these glasses, shows an increase with the addition of 2at.% of Sb and then remains almost the same for 4 at.% of Sb (mean values of AC, are found to be 0.0188 m cal/mg”C, O.O276mcal/ mg”C and 0.0262 m cal/mg”C for Seioo, SessSb2, and Ses6Sb4, respectively). This behaviour is similar to that of the Ts of these glasses. It can be concluded that configurational sources contributes to AC, in these glasses. found
)x=2,
REFERENCES 1. Myers M. B. and Fetty E. J., Mater. Res. Bull. 2,535 (1967). 2. Schottmiller J., TaBak N., Lucovsky G. and Ward A., J. Non-Cryst. Solids 4, 80 (1970). 3. Arata Onozuka and Osamu Gda, J. Non-Cryst. Solids. 103,289 (1988). 4. Savage J.‘A., j. Non-Cryst. Soli& 11, 121 (1972). 5. Rysava N., Spasov T. and Tichy L., J. Thermal Anal. 32, 1015 (1987). 6. MatusitaK.andSakkaS.,Phys.Chem.G1urses20,81(1979). I. MacFarlane D. R.. Mate&i. M. and Poulain M., J. NonCryst. Soli& 64, 351 (1984): 8. Matusita K. and Sakka S., J. Non-Cryst. SolirIs 38-39, 741 (1980). 9. Hafiz M. M., Ibrahim M. M., Dongl M. and Hammed F. H., J. Appl. Phys. 54, 1950 (1983). 10. Sudha Mahadevan, Giridhar A. and Singh A. K., J. Non-Cryst. Solids 88, 11 (1986). 11. Matusita K. and Sakka S., Bull. Inst. Chem. Res. Kyoto Univ. 59, 159 (1981). 12. Matusita K. and Tashiro M., Phys. Chem. Glasses 14,17 (1973). 13. Ranganathan S. and Von Heimendahl M., J. Mat. Sci. 16, 2401 (1981). 14. Von Heimendahl M. and Kuglstatter G., J. Mat. Sci. 16, 2405 (1981). 15. Colemenero J. and Barrandiaran J. M., J. Non-Cryst. Solidr 21,411 (1976). 16. Duhaj P., Baranock D. and Ondrejka A., J. Non-Cryst. Solids 21,411 (1976). 17 Speyer R. F. and Risbud S. H., Phys. Chem. Glasses 24, 26 (1983). 18 Matusita K., Konatsu T. and Yokota R., J. Mat. Sci. 19, 291 (1981). 19. Mehra R. M., Kaur G. and Mathur P. C., J. Non-Cryst. Solidr 26,3433-3437 (1991). 20. Ma H. L., Zhang X. H. and Lucas J., J. Non-Cryst. Solids 140,209-214 (1992). 21. Predeep P., Saxena N. S., Saksena N. P. and Kumar A., Phys. Status Solidi (a) 155 2 (1996).
PII: SOO22-367!3(%)00148-5
Pergamon
CRYSTALLIZATION
Printed
AND SPECIFIC HEAT STUDIES (x = 0, 2 and 4) GLASS
P. PREDEEP,*
N. S. SAXENA*
No. 3, pp. 385-389,1997 Science Ltd in Great Britain. All rights reserved 00%3697/97 $17.00 + 0.00
Q 1997 Elsevier
OF Seloo_,Sb,
and A. KUMARt
*Condensed Matter Physics Laboratory, Department of Physics, University of Rajasthan, Jaipur 302 004, India tDepartment of Physics, HBTI, Kanpur, India (Received 5 June 1996; accepted 16 August 1996)
Abstract-Bulk glasses of compositions Se,,,&Sb, (x = 0, 2 and 4) are prepared by melt quenching technique. Differential scanning calorimetry (DSC) is employed to study the crystallization mechanism as well as specific heat of these glasses. Samples are subjected to thermal scanning in the DSC at various heating rates. All the compositions have shown well defined exothermic Peaks of crystallization. The crystallization data are examined in terms of modified Kisinger’s equation for the activation energy of crystallization. Also the results were analysed using the method suggested specifically for nonisothetmal crystallization by Matusita et al. and the activation energy for crystallization and the dimensionality of crystal growth are indicated for all the compositions. It has also been found that the thermal stability of the system is enhanced by the alloying of Se with small atomic percentages of antimony. Specific heat, Cr, of all the compositions were also evaluated and discussed. 0 1997 Elsevier Science Ltd. All rights reserved Keywords: C. differential scanning calorimetry, crystallization kinetics, glass transition, activation energy, D. specific heat.
1. INTRODUCTION Chalcogenide terms
of
Selenium
glasses physics
have evoked
as
well
thermal
to improve stability
[l] to be most stability
certain
by alloying. effective
and resistance of human
in
technology. poten-
and there have been a lot of of its properties
in increasing
the thermal
to crystallization. health.
like
Arsenic has been found
since As is toxic, it is not desirable viewpoint
much interest
device
has been found to have tremendous
tial in device technology attempts
as
However,
to use As from the
Alloying
elements
pro-
duce characteristic effects [2] depending on whether they are univalent or isoelectronic with Se or capable of producing chain branching. However, the effect of Sb doping in Se on the thermal stability and crystallization has not been given much attention. It has been found that the addition of a small atomic percentage of Sb in the Se-Te system drastically improved [3] the thermal stability of the system. In this paper we report the parameters of crystallization kinetics for amorphous SeloO_,Sb, (x = 0, 2 and 4) system together with its specific heat (C,).
2. EXPERIMENTAL
High purity
Se and Sb in appropriate
atomic per-
centage were weighed into quartz ampoules (length 5 cm and internal diameter 8 mm). The contents of the
ampoules were sealed in a vacuum of 10e6 Torr and heated in a rotary furnace to 800°C for lo-12 h to ensure the homogeneity. The molten samples were then rapidly quenched in ice cooled water. The glassy nature of the alloys was confirmed by X-ray diffraction (XRD). DSC scans of the samples were taken using a Rigaku-8230 DSC coupled with a thermal analysis station. The temperature precision of the equipment is 0.1 K with an average standard error of about 1 K in the measured values. The samples (typically 15 mg) in powder form were taken in standard aluminium pans and scanned over a temperature range of 200” at heating rates of 5, 10, 15 and 20 K/min. The fraction crystallized at any temperature, T is given by X = AT/A, where A is the total area of the exotherrn between
the temperature
at which crystallization
is
the area between T1 and T [4,5]. The values of the specific heat of the experimental samples has been determined using the formula
completed.
AT is
c, = mrAs ---Cc, msAr where m, is the mass of the reference material, m, that of the sample, A, and A, are the shifts for sample and reference materials with respect to base line, respectively, and C, is the specific heat of the reference material. To avoid any significant uncontrolled relaxation, the starting glass is always the one that 385
386
P. PREDEEP et al.
obtained from first heating to the end of glass transition and rapidly cooled to room temperature.
3. RESULTS AND DISCUSSION
ln[- ln(1 - X)] = - nln(a)
Figure 1 shows typical DSC thermograms of Seta,,, SessSbz and Se&b4 glasses at the heating rate of 5 K/ min. The glass transition temperatures (Ts) of these samples are not found to exhibit any discernible trend. At the heating rate of 5 K/min, Se has a Tg of 312.1 K while that of Se&b2 and Seg4Sb4 are 313.3 and 313.2 K, respectively. The interpretation of experimental data on crystallization is given on the basis of Kissinger’s and Matusita’s equations for nonisothermal crystallization. The activation energy (II,) for crystallization has been evaluated using the modified Kissinger’s equation [6-91. ln[a’/ Ti] = -mE,/RT,,
+ ln( C)
(1)
where C is a constant having factors dependent upon the thermal history of the samples. n and m are constants having values between 1 and 4 depending on the morphology of growth [lo]. Here (Y is the heating rate and Tp is the temperature corresponding to the crystallization peak. From the slope of the plot between In(o) and l/T,, the value of mE,ln can be calculated. Also from the equation suggested [l 1J by Matusita
SK/m
91.2
END0
END0
END0
I
100
et al., for nonisothermal crystallization, the value of activation energy for crystallization, EC can be evaluated.
2
Temperature C’CI Fig. 1. DSC thermograms of Seloo-$b, glass at the heating rate of 5 K/m.
+ constant
- 1.052mEJRT (2)
where X is the fraction of crystals precipitated in a glass heated at uniform rate and n is a numerical factor depending on the nucleation process. Here n = m + 1 is taken for a quenched glass containing no nuclei and n = m for a glass containing a sufhciently large number of nuclei. Also m has been assigned integer values of 1,2 and 3 for one-, two- and three-dimensional growth, respectively. The crystal nucleation rate [ 1l] in glass reaches the maximum at a temperature a little above the glass transition temperature and then decreases rapidly with increasing temperature. When a glass is heated at a constant rate [ 121,crystal nuclei are formed only at lower temperatures. Hence, when nuclei formed during the heating at constant rate are dominant, n is equal to m + 1 and when nuclei formed during any heat treatment prior to the thermal analysis are dominant, n = m. This can be decided by observing the change of n with reheating at the nucleation temperature. If n does not change with reheating, a large number of nuclei already exist and n can be taken as equal to m. A decreased value of n on reheating almost indicates the absence of pre-existing nuclei in the sample, which could influence the crystallization process and, in this case, n = m + 1. It is pointed out [lo] that in a crystallization process three types of activation energies has to be considered, activation energy for nucleation (E,), activation energy for crystal growth (E,) and that for the whole process of crystallization, (EC). It has been shown through various studies [13, 141that the values of EC evaluated using thermal analysis of the sample can be taken equal to the activation energy, Eg, for crystal growth. To calculate the activation energy for crystallization, E,, eqn (2) is used. From the slope of the plot between ln(cr) and 1000/T,, (Fig. 2), the value of mE,/n is deduced for various compositions and the values are listed in the Table 1. Figure 3 shows the plot of ln[- ln( 1 - X)] against 1000/T at different heating rates for the composition Se&bz. The break in linearity of these plots in high temperature regions seems to be due to the saturation of nucleation sites in the final stages of crystallization [ 15, 161or due to the restriction [ 171of crystal growth by the small size of the particles. The linear region covering a wide temperature range is used for analysis and the slope of these plots give the value of mE,. The values of mE, thus obtained for all the compositions are also listed in Table 1.
Crystallization and specificheat studies of Seloo_$b, glass
387
3.0-
2.7:z2.G
2.1 -
1.8 I 2.45
I 2.5
2.55
Fig. 2. ln(cY)vs lOOO/~,for Se9$bz glass. For evaluating the order parameter n, ln[- ln( 1 - X)] was plotted as a function of In(a). Plot for SessSb2 is shown in Fig. 4. Values of n obtained in a similar fashion for all compositions are also given in Table 1. A plot for reheated glass is also shown in Fig. 2. The value of n for reheated glass is found to be 1.3 which clearly shows a decrease from that of the quenched glass. As already discussed, the decreased value of n for reheated glass almost indicates the absence of the preexisting nuclei in the quenched glass and, hence, m is taken as equal to n - 1. Value of n is found to be about 2 for all the compositions which indicates bulk nucleation with one-dimensional growth for these glasses. The value of E, are calculated using these values of n and m and a difference has been observed in the values of EC obtained through the two different procedures.
It is interesting to note that considerable differences in values of E, calculated in a similar manner had also been reported in the case of Li20-2SiOz [18] and SeSb-Te glasses [19]. The values of EC calculated using eqn (2) indicate a fall in the value of EC for the composition Se&b2 and then again an increase for Ses6Sb4. Activation energy being an indication of the speed of the rate of crystallization, it can be concluded that SessSb2 glass has the slowest rate of crystallizaton. Addition of higher values of Sb in fact leads to an increase in crystallization rate thus reducing the thermal stability. The difference between T, and Tg is known to be a strong indication of thermal stability of the glasses. The higher the value of T, - Tg, the greater the thermal stability and ease of glass formation. Figure 5 shows the variation of T, - Tg with Sb
-4-
-5 23
I 2.35
I 24
I 2.45
I I 2.55 25 IU’l 7
I 2.6
I 2s
2‘.ir
Fig. 3. In[- In( I - X)] vs 1000/T for Se9sSb2 glass at three heating rates of 5( ), lO( ), and 15( ) K/min.
P. PREDEEP et al.
388
Table 1. Crystallization parameters of Se-Sb system from the heating rate data In(a) vs l/T
(m/n)-%
EC Wmolj
(kJ/mol)
Composition
Sk
100.4 88.8 65.8
Se&b2 SegsSb4
-41 1.5
ln(- ln( 1 - X)] vs In(o)
200.8 117.6
131.7
I
I
I
1.7
1.9
21
ln[-ln(1 -A’)] vs l/T
n
m
mEc (kJ/mol)
2.11 2.33 2.01
1 1 1
301.9 158.1 170.5
1
23 In(~)
I
I
I
2.5
27
2.9
EC Wmolj 301.9 158.1 170.5
:
Fig. 4.ln[- ln( 1 - X)] vs ln(cy)for Se&b2 glass at fixed temperatures. ( ) as quenched glass: 398 K, ( j reheated glass: 400 K. concentration and it can be seen that the addition of a small amount of Sb increases the thermal stability. Earlier reported studies [3] on the effect of addition of Sb on the thermal stability of Se-Te system indicates that the addition of only 0.1 at.% of antimony has a sufiicient effect on eliminating the exothermic peak due
to crystalliaztion thus improving the thermal stability drastically for the Se-Te system. Also higher concentrations of Sb in Se-Te system has found to be less effective in producing thermal stability in Se-Te alloys. The variation of Tc-Tg with Sb at.% indicates to a similar trend in the Se-Sb system.
__
70 -
at./. Sb Fig. 5. (T, - TB)as function of Sb content in Seloo-XSb, glasses.
Crystallization and specific heat studies of Sera&b,
glass
389
0.25
0.22 -
-
0.18
$o.,tS;;
0.13 -
0.$0
’
’
’
’
’ ’ 40
’
’
’
’ ’ 50
’
’
’
’
60
’
’
’
’
70
Temperature 1*Cl Fig. 6. Variation of specific heat with temperature at the heating rate of 5 K/m for Seroo_$b, glasses. ( )x=0,( ()x=4. The specific heat, Cp, for all the three compositions are evaluated
in the glass transition
region. It has been
that the variation of C, with temperature is more or less independent of heating rate. Figure 6 shows the variation of specific heat as a function of temperature at the heating rate of 5K/min. The sudden jump in C,, value at glass transition can be attributed [lo] to an harmonic contribution to the specific heat. Thus, glass transition temperature is usually defined as the temperature onset of the normally stepwise increase in heat capacity which occurs during heating of the glassy material. The overshoot in the value of C, at the upper end of the “CP jump” at glass transition is due to the relaxation effects. The time scale for structural relaxation [20] is highly dependent both on temperature and on the instantaneous structure itself. In the glass transition region, this time scale becomes of the order of lo2 s. The peak in C, is observed due to the fact the structural relaxation times of these glasses are of the same order as the time scale of the experiment. Also AC,, the difference between liquid and glass heat capacities, for these glasses, shows an increase with the addition of 2at.% of Sb and then remains almost the same for 4 at.% of Sb (mean values of AC, are found to be 0.0188 m cal/mg”C, O.O276mcal/ mg”C and 0.0262 m cal/mg”C for Seioo, SessSb2, and Ses6Sb4, respectively). This behaviour is similar to that of the Ts of these glasses. It can be concluded that configurational sources contributes to AC, in these glasses. found
)x=2,
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