Structural characterisation and thermo-physical properties of glasses in the Li 2 O–SiO 2 –Al 2 O 3 –K 2 O system Hugo R. Fernandes • Dilshat U. Tulyaganov • Ashutosh Goel • Jose ´ M. F. Ferreira Received: 4 August 2010 / Accepted: 9 September 2010 / Published online: 25 September 2010 Ó Akade ´miai Kiado ´, Budapest, Hungary 2010 Abstract This article aims to shed some light on the structure and thermo-physical properties of lithium disili- cate glasses in the system Li 2 O–SiO 2 –Al 2 O 3 –K 2 O. A glass with nominal composition 23Li 2 O–77SiO 2 (mol%) (label- led as L 23 S 77 ) and glasses containing Al 2 O 3 and K 2 O with SiO 2 /Li 2 O molar ratios (3.13–4.88) were produced by conventional melt-quenching technique in bulk and frit forms. The glass-ceramics (GCs) were obtained from nucleation and crystallisation of monolithic bulk glasses as well as via sintering and crystallisation of glass powder compacts. The structure of glasses as investigated by magic angle spinning-nuclear magnetic resonance (MAS-NMR) depict the role of Al 2 O 3 as glass network former with four- fold coordination, i.e., Al(IV) species while silicon exists predominantly as a mixture of Q 3 and Q 4 (Si) structural units. The qualitative as well as quantitative crystalline phase evolution in glasses was followed by differential thermal analysis (DTA), X-ray diffraction (XRD) adjoined with Rietveld-reference intensity ratio (R.I.R.) method, Fourier transform infrared spectroscopy (FTIR) and scan- ning electron microscopy (SEM). The possible correlation amongst structural features of glasses, phase composition and thermo-physical properties of GCs has been discussed. Keywords Sintering Á Thermo-physical properties Á Glass Á Glass ceramics Á Lithium disilicate Introduction Glass-ceramic (GC) materials are obtained by the con- trolled nucleation and crystallisation of glasses. The choice of glass composition is crucial to ensure that a high rate of internal, rather than surface, nucleation occurs. An exces- sively high crystal growth rate is to be avoided since such materials do not develop fine-grained microstructure, nec- essary for the achievement of high mechanical strength [1, 2]. The binary alkali silicate systems show liquid–liquid phase separation or immiscibility at temperatures below the liquidus temperature of crystallisation. This type of phase separation is often called metastable because crystalline phases are more stable than liquid at the temperature of phase separation [1, 3]. The presence of metastable immiscibility region is the main cause of S-like course of the liquidus curve and binary Li 2 O–SiO 2 system is a typical example in this regard which demonstrates S-like course of the liquidus curve in silica-rich region. According to Vogel [3], Li 2 O–SiO 2 liquids containing less than 30 mol% Li 2 O lead to opalescent or opaque glasses on cooling owing to phase separation. However, mechanical properties and chemical durability of these glasses after devitrification are low. Study of nucleation and crystallisation processes in parent glasses is essential, enabling to produce final materials of desired properties. In the previous study [4], we observed that glasses with composition in the Li 2 O– K 2 O–Al 2 O 3 –SiO 2 system comprising equimolar amount of Al 2 O 3 and K 2 O were prone to volume nucleation and crystallisation, resulting in formation of fine Li 2 Si 2 O 5 (LD) crystals within the temperature interval of 650–900 °C. Also, it has been demonstrated that Al 2 O 3 and K 2 O might also improve chemical durability of Li 2 O–SiO 2 glasses [3, 5, 6]. H. R. Fernandes Á D. U. Tulyaganov Á A. Goel Á J. M. F. Ferreira (&) Department of Ceramics and Glass Engineering, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal e-mail: jmf@ua.pt D. U. Tulyaganov Turin Polytechnic University in Tashkent, 17, Niyazova Str., Tashkent, Uzbekistan 100174 123 J Therm Anal Calorim (2011) 103:827–834 DOI 10.1007/s10973-010-1049-5