Available online at www.sciencedirect.com
Journal of the European Ceramic Society 32 (2012) 291–298
Effect of K
2
O on structure–property relationships and phase transformations
in Li
2
O–SiO
2
glasses
Hugo R. Fernandes
a,∗
, Dilshat U. Tulyaganov
a,b
, Ashutosh Goel
c
, José M.F. Ferreira
a
a
Department of Ceramics and Glass Engineering, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal
b
Turin Polytechnic University in Tashkent, 17, Niyazova str., 100174 Tashkent, Uzbekistan
c
Glass Processing Group, Radiological and Nuclear Science and Technology Division, Pacific Northwest National Laboratory, Richland, WA 99354, United States
Received 3 August 2011; accepted 18 September 2011
Available online 8 October 2011
Abstract
Glass compositions with formula (71.78 - x)SiO
2
–2.63Al
2
O
3
–(2.63 + x)K
2
O–23.7Li
2
O (mol.%, x = 0–10) and SiO
2
/Li
2
O molar ratios far beyond
that of stoichiometric lithium disilicate (Li
2
Si
2
O
5
) were prepared by conventional melt-quenching technique to investigate the influence of K
2
O
content on structural transformations and devitrification behaviour of glasses in the Li
2
O–SiO
2
system. The scanning electron microscopy (SEM)
examination of as cast non-annealed glasses revealed the presence of nanosized droplets in glassy matrices suggesting occurrence of liquid–liquid
phase separation. An overall trend towards depolymerization of the silicate glass network with increasing K
2
O content was demonstrated by
employing magic angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy. The distribution of structural units in the experimental
glasses was estimated using
29
Si MAS-NMR spectroscopy suggesting the appearance of Q
2
, enhancement of Q
3
and diminishing of Q
4
groups
with increasing K
2
O contents. X-ray diffraction (XRD) and differential thermal analysis (DTA) were used to assess the influence of K
2
O on
devitrification process and formation of lithium disilicate (Li
2
Si
2
O
5
) and/or lithium metasilicate (Li
2
SiO
3
) crystalline phases.
© 2011 Elsevier Ltd. All rights reserved.
Keywords: Glass; Glass ceramics; Lithium disilicate; Thermo-physical properties
1. Introduction
The immiscible region between the Li
2
O–2SiO
2
and SiO
2
end members is an important feature in the Li
2
O–SiO
2
sys-
tem. The synthesis of glass-ceramic (GC) materials in the
Li
2
O–SiO
2
system is based on controlled nucleation and crystal-
lization of lithium metasilicate and/or lithium disilicate phases
which govern the properties for the final product. The glasses
with SiO
2
contents higher than the stoichiometric Li
2
O·2SiO
2
(33.33 mol.% Li
2
O–66.66 mol.% SiO
2
) tend to separate into a
matrix phase with a composition almost similar to that of lithium
disilicate along with an isolated droplet SiO
2
rich phase,
1
while
glasses with Li
2
O contents <30 mol.% usually turn out to be
opalescent or opaque on cooling owing to phase separation.
1–3
Although, nucleation of base glass with stoichiometric com-
position of lithium disilicate has been widely investigated for
∗
Corresponding author.
E-mail address: h.r.fernandes@ua.pt (H.R. Fernandes).
GC manufacture,
4
the GCs derived from this parent binary sys-
tem exhibit some unfavourable characteristics in terms of their
mechanical and chemical properties which hinder their potential
applications in several technological areas.
On the other hand, lithium disilicate GCs derived from
non-stoichiometric compositions have proven themselves to
be potential candidates for different functional applications,
for example: dental restorations,
5–8
metal-glass seals,
9,10
etc.
Fundamental research on certain non-stoichiometric lithium dis-
ilicate based glass compositions was carried out by Stookey
(1959).
11
It is noteworthy that according to Höland and Beal,
4
the term ‘non-stoichiometric’ implies that SiO
2
/Li
2
O molar ratio
deviates greatly from 2:1 and the system in rendered consid-
erably more complex with numerous additional components
and nucleating agents. However, the present investigation aims
towards investigating a relatively simpler non-stoichiometric
lithium disilicate based GC system in the glass forming region of
Li
2
O–K
2
O–Al
2
O
3
–SiO
2
with its SiO
2
/Li
2
O molar ratio vary-
ing between 2.69 and 3.13. The simultaneous incorporating of
K
2
O and Al
2
O
3
is known to significantly improve the chemical
0955-2219/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jeurceramsoc.2011.09.017