DSC of Al-O (512 kJ mol-1) bonds over


DSC
thermographs of Al2O3-TeO2 glasses are shown
in Fig. 4.2 and their thermal
properties are summarized in Table 4.1.
The Tg value increases
steadily from 311oC to 392oC on increasing Al2O3
concentration from 1 to 20-mol%. Tg
for alumino-tellurite glasses are in excellent agreement with the results from
the earlier study, for example Tg
for 1AlTe and 3AlTe glasses prepared in the present work are 311oC
and 323oC respectively and Taigara et al. (Tagiara, et al., 2017) reported Tg of 311.9oC
and 322.7oC  for glasses of
the same compositions. Taigara et. al.
and Lombson et.al. also reported that
the pure TeO2 glass prepared in alumina crucible shows Tg at 385oC and
380oC respectively while TeO2 glass prepared in Platinum
(Pt) crucible shows Tg at
303oC (Tagiara, et al., 2017; Lambson, 1984
#385). However in the present work, the
addition of 1-mol% Al2O3 in TeO2 glass
prepared in Pt crucible shows Tg
at 311oC, which is lower than the pure tellurite glass formed by
using alumina crucible. This indicates that the pure tellurite glass formed in
alumina crucible has high content of alumina impurity as compare to tellurite
glass containing 1-mol% of Al2O3. The high content of Al2O3
forms stronger Al-O-Te linkages over Te-O-Te linkages because of the higher
bond dissociation energy of Al-O (512 kJ mol-1) bonds over Te-O (391
kJ mol-1) bonds and results the high in Tg values. The increase in Tg of the alumino-tellurite glasses directly correlate
with the bond dissociation energy of the glasses which increases from 392 kJ
mol-1 to 415 kJ mol-1 with increasing Al2O3

Fig 4.2:
DSC patterns of aluminum tellurite glasses.

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concentration.
The average single bond enthalpy, EB
is calculated by the following formula (Kaur et al., 2015):

                                                   
(4.1)

where,
ETe-O and EAl-O are the single bond
dissociation energies of Te-O (391 kJ mol-1) and  Al-O bonds (512 kJ mol-1)
respectively (Cottrell, 1958; Darwent,
1970; Dean,
1999; Kaur,
et al., 2015). The increase in EB corresponds to the
replacement of weaker Te-O-Te linkages over stronger Al-O-Te/Al-O-Al linkages. Fig. 4.3 shows the linear variation of Tg with EB according to the following linear equation:

Fig 4.3:
Variation of Tg with bond dissociation energy of aluminum
tellurite glasses.

                                               
(4.2)

 

 

 

 

 

 

 

 

 

Table 4.1:  Compositional and DSC data of
alumino-tellurite glasses.

Sample Code

Composition
mol%

Tg ±1oC

Tc ±1 oC

Tm ±1 oC

?T =

EB
kJ mol-1

Al2O3

TeO2

Tc1

Tc2

Tc3

Tm1

Tm2

1AlTe

1

99

311

347

738

36

392.30

2AlTe

2

98

321

380

427

453

714

59

394.41

3AlTe

3

97

323

384

430

 

728

61

394.63

5AlTe

5

95

332

392

439

478

700

60

397.05

7AlTe

7

93

340

408

434

477

695

68

399.47

20AlTe

20

80

392

530

615

647

680

138

415.20

 

 

 

 

 

 

 

The addition of Al2O3
beyond 1-mol% in tellurite glasses shows number of exothermic peaks
(crystallization peak) and single endothermic peak (melting peak). These
multiple crystallization peaks reveals the existence of several crystalline
phases of Al2O3-TeO2 system which can be
formed by devitrification of the samples after annealing them at certain
temperature (> Tg).
Incorporation of Al2O3 in Al2O3-TeO2
glasses shifts the Tc1
from 347 oC to higher temperature, at 530 oC. The
crystallization temperatures Tc2
and Tc3 also get prominent
with increasing Al2O3 concentration from 1 to 20-mol%.
This increase in crystallization temperature reveals that the tendency of Al2O3-TeO2
glasses towards devitrification decreases with increasing Al2O3
content.

Aluminum
tellurite glass with 20-mol% of Al2O3 shows two glass
transitions phenomena, first a strong transition at 393 oC and the
second weak glass transition at 440 oC. This multiple transitions
phenomena indicate the presence of amorphous-amorphous phase separation in
alumino-tellurite glasses, which was also found by Kaur et. al. in case of alumino-borotellurite glasses containing 20-mol%
of Al2O3 (Kaur & Khanna, 2014). This implies that the higher
concentration of Al2O3 in tellurite and as well as in
borotellurite leads to the phase separation. The thermal stability

 
of Al2O3-TeO2 glasses increases from 36
oC to 138 oC on increasing Al2O3
concentration from 1 to 20-mol% (Table
4.1). 

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