160
K. Yukimitu et al. / Thermochimica Acta 426 (2005) 157–161
furnace for 5 min. As we can see, the as-quenched and heat
annealed glass at 310 ◦C shows a typical amorphous charac-
teristics. However, the glass state tends to disappear resulting
in a glass-ceramic when the glass is heat annealed at higher
temperatures. In glass annealed at 330 ◦C, some crystalliza-
tion peaks appear for 2θ at around 26.2◦, 29.9◦ and 48.6◦,
which were attributed to TeO2 crystalline phase permeating
glass matrix. Increasing the annealing temperature to 350 and
360 ◦C an unidentified phase and ␣-TeO3 crystalline phase
appears and coexists with the TeO2 phase, as indicated by
indexed peaks in Fig. 4E.
metastable phase was identified as a monoclinic phase perme-
ating the 30Li2O–70TeO2 glass matrix and can be interpreted
as a “link” between the unidentified phase and the orthorhom-
bic phase, this last one crystallized at high heat annealing.
The 30Li2O–70TeO2 glass crystallization was also stud-
ied from DSC and X-ray diffraction more recently [7]. In this
work, an unidentified phase was also observed and this crys-
talline phase converts to ␣-Li2Te2O5 while the vitreous TeO2
phase is clearly converted to ␣-TeO2 crystalline phase [7].
Apparently, there are contradictions between these both cited
works with respect to crystallization in the 30Li2O–70TeO2
glass matrix. However, the crystallization mechanisms in
these Li2O–TeO2 glasses are not completely understood and
must be investigated in detail. In contrast with these works in
30Li2O–70TeO2 glass, the hierarchy on crystallization was
evident in our studied 20Li2O–80TeO2 glass, the metastable
phase was not observed and the unidentified phase could not
be systematically identified up to this moment.
Using the (1 1 0), (1 1 1), (1 0 2), (2 0 0) and (2 1 2) hkl
planes of the XRD pattern in Fig. 4E, respectively, observed
at 26.19◦, 28.74◦, 29.94◦, 37.39◦ and 48.59◦, the lattice con-
stants a and c for TeO2 tetragonal phase were calculated
˚
˚
as a = 4.811 A and b = 7.618 A. These lattice parameters
agree with JCPDS (42-1365) data card, confirming the TeO2
this glass crystallization and the TeO2 phase crystallization
occurs prior to ␣-TeO3 and an unidentified phase. It is in-
teresting to observe that an expected lithium tellurite phase
was not properly identified in Fig. 4. Several possibilities
were tried including the LiTeO3 or Li2TeO3 phases in our
XRD refinements but these attempts were frustrated. In an-
other 30Li2O–70TeO2 glass matrix, whose crystallization
was studied using different techniques such as DSC and X-
but this crystalline phase converts to ␣-Li2Te2O5 at high
annealing temperatures. On the other hand, the present re-
sult for the studied glass composition disagrees if compared
with previous studies [5,6]. A possible explanation may be
founded if we consider that 30Li2O–70TeO2 composition is
a eutectic point on the phase diagram of the pseudo-binary
Li2O–TeO2 system [5]. Thus, based on this phase diagram
and the 20Li2O–80TeO2 glass composition, the unidentified
phase in the present work is most probably a kind of crys-
talline lithium–tellurium phase.
4. Conclusion
The crystallization kinetic on the 20Li2O–80TeO2 glass
was studied by simultaneously using XRD and DSC tech-
niques to understand the glass-ceramic formation from this
glass. A detailed DSC analysis as a function of different par-
ticle sizes was possible to predict a phase transformation on
this glass. Finally, the XRD analysis revealed a crystalliza-
tion hierarchy on this glass, starting with the TeO2 crystalline
phase permeating the glass matrix followed by the simul-
taneous crystallization of the ␣-TeO3 and an unidentified
phase, leading to two distinct activation energies at 301 and
488 kJ mol−1, respectively.
Acknowledgements
Kinetic studies are often performed by monitoring
changes in a specific physical parameter during crystalliza-
preted erroneously as possible finite size effects on crystal-
lization kinetics are neglected. These effects on the crystal-
lization kinetics have been considered by experimental [14]
and theoretical [15,16] studies and the interpretations of the
present work were based on the studies by these researchers.
Thus, considering particle size effects on DSC crystallization
peak (Fig. 2) it was possible to arrive at an interpretation re-
garding the two distinct phase transformations. With respect
to the hierarchy on crystallization of the 20Li2O–80TeO2
glass, some studies were carried out on similar matrix but
this hierarchy was not previously predicted. Using differen-
tial thermal analysis (DTA), early studies to construct the
phase diagram of the pseudo binary system Li2O:TeO2 in-
dicated a eutectic point to a 30Li2O–70TeO2 composition
and a metastable phase could be observed when this glass
was heat annealed at 410 ◦C [5]. In this work, the observed
We would like thank Conselho Nacional de Desenvolvi-
´
´
˜
mento Cientıfico e Tecnologico (CNPq), Coordenac¸ao de
´
Aperfeic¸oamento de Pessoal de Nıvel Superior (CAPES) and
˜
`
˜
Fundac¸ao de Amparo a Pesquisa do Estado de Sao Paulo
(FAPESP) for financial support.
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