Glass - Back to the Future!

Presenting Author:
Edgar Dutra Zanotto

article posted 09 May 2015

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 ( 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
1Federal University of São Carlos, Brazil
2University of São Paulo, Brazil
+,3Sheffield University, UK, in memoriam

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? Other effects?

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 baria-silica glasses.

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 tha t heterogeneous crystal nucleation does not take place on the interfaces of the nanosize liquid regions [PS (liquid droplets): Io = 1020 /m3.s, σ < 5 mJ/m2; Crystal nucleation: Io = 108 - 1010/m3.s, σ = 140 - 200 mJ/m2, where Io = 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.

Figure 1: Examples of oxide glasses with surface and internal crystals [1].

Figure 2: Number of crystals versus time plots for barium silicate glasses treated at 743oC (Tg ~ 700oC). Glasses: 28.3A and 28.3B (undergo liquid phase separation on heating); 28.3B PS (previously treated to phase separate) and 29.7H (homogeneous, free of phase separation) [2-3]. -------- previously phase separated at 821oC for 22min; _____ as quenched;

[1] E. D. Zanotto – Crystals in Glass. A Hidden Beauty, J. Wiley & Sons (2013).
[2] E.D. Zanotto, P.F. James, A. F. Craievich – J. Mat. Sci. 21, 9 (1986) 3050.
[3] E.D. Zanotto, A.F. Craievich, P.F. James – J. de Physique 43, C9 (1982) 110.