article posted 29 March 2016
Edgar Dutra Zanotto - Professor of Materials Science and Engineering at the Federal University of São Carlos, Brazil. Director of the Center for Glass Research, Education and Technology on Vitreous Materials (www.certev.ufscar.br). Editor of the Journal of Non-crystalline Solids. Chair elect of the Glass and Optical Materials Division of the American Ceramic Society. Fellow of the Society of Glass Technology, and of the Brazilian Ceramic Society, World Academy of Sciences, World Academy of Ceramics, Brazilian Academy of Sciences, National Academy of Engineering, and São Paulo Academy of Sciences.
The effect of nanoheterogeneities (liquid-liquid phase separation)
on crystal nucleation in glass-forming silicate liquids
Edgar Dutra Zanotto*1, Aldo F. Craievich2, P.F. James+,3
While the vast majority of glass forming substances only undergo surface (heterogeneous)
nucleation when sufficiently heated, a few oxide systems also show the thermodynamically
less favorable case of internal (homogeneous) nucleation on laboratory time/length scales
(Figure 1). Glass-in-glass or liquid-liquid phase separation (PS) is a widespread
phenomenon in supercooled liquids and has been liable to be the main cause of
internal nucleation in liquids that do not contain any nucleating agent.
In the 60's and 70's,
research on glass-ceramics was focused on the thermodynamic and kinetic aspects of PS
and its possible effects on crystallization, whereas more recently the metallic and
chalcogenide glass communities became interested in the relationships between
these two phenomena. A key question regarding this particular issue has always been:
What is the effect (if any) of PS on crystal nucleation? Interfacial? Compositional?
The objective of this talk is to shed light on the role of PS on crystal nucleation.
We will summarize a 35-year old, but still modern, systematic research work that made
use of carefully designed glass compositions and thermal treatments, as well as systematic
SAXS, TEM and OM (optical microscopy) characterisation about the morphology and
kinetics of PS and crystal nucleation occurring simultaneously in lithia-silica and
Experimental evidence for phase separating and for homogeneous glasses indicates
that the compositional shifts caused by PS significantly affect the crystal nucleation
kinetics. On the other hand, no correlation has been found between the surface area of
the liquid droplets and the crystal nucleation kinetics in both glass systems, which
shows that heterogeneous crystal nucleation does not take place on the interfaces of
the nanosize liquid regions [PS (liquid droplets):
= 1020 /m3
σ < 5 mJ/m2
; Crystal nucleation:
= 108 - 1010/m3
σ = 140 - 200 mJ/m2
= maximum nucleation rate;
σ = interfacial energy]. Therefore, PS does not promote internal crystal nucleation
in glasses; its main effect is to encourage compositional shifts that significantly
affect the crystal nucleation rates.
1 Federal University of São Carlos, Brazil
2 University of São Paulo, Brazil
+, 3 Sheffield University, UK, in memoriam