Journal of Sol-Gel Science and Technology 14, 87–94 (1999) c  1999 Kluwer Academic Publishers. Manufactured in The Netherlands. High Temperature Behavior of a Gel-Derived SiOC Glass: Elasticity and Viscosity TANGUY ROUXEL ∗ Laboratoire “Verres et C´ eramiques”, UMR-CNRS 6512, Universit´ e de Rennes 1, Campus Beaulieu, 35042 Rennes Cedex, France tanguy.rouxel@univ-rennes1.fr GEORGES MASSOURAS ENSCI, LMCTS, UPRESA-CNRS 6015, 47 av. A. Thomas, 87065 Limoges Cedex, France GIAN-DOMENICO SORAR ` U Dipartimento di Ingegneria dei Materiali, Universit` a di Trento, via Mesiano 77, 38050 Trento, Italy Received April 6, 1998; Accepted August 11, 1998 Abstract. The high temperature behavior of a sol-gel derived silicon oxycarbide glass containing 12 at.% carbon has been characterized by means of creep and in-situ ultrasonic echography measurements. Temperature induced changes include structural relaxation and densification from 1000 to 1200 ◦ C, and crystallization to form a fine and homogeneous β -SiC/glass-matrix nanocomposite with 2.5 nm large crystals above 1200 ◦ C. Young’s modulus measurements clearly reveal a consolidation of the material upon annealing below 1200 ◦ C. Crystallization is almost complete after few hours at 1300 ◦ C and results in a significant increase in Young’s modulus. The viscosity of the oxycarbide glass is much higher than that of fused silica, with two orders of magnitude difference at 1200 ◦ C, and the glass transition temperature ranges from 1320 to 1370 ◦ C. Keywords: oxycarbide glasses, sol-gel, mechanical properties, elasticity, viscosity 1. Introduction The substitution in traditional oxide glasses of some of the divalent oxygen atoms by trivalent nitrogen ones results in a significant improvement of both mechanical properties and refractoriness [1–5]. For instance, the replacement of one oxygen anion upon five increases the elastic modulus of 30% and the viscosity coefficient of two orders of magnitude at a given temperature (or a 50 ◦ C shift for a given viscosity) [5]. These changes clearly correlate to the increase of the average coordi- nation number in the glass network [6, 7]. Interest in oxycarbide glasses is hence obvious, as far as carbon ∗ To whom all correspondence should be addressed. atoms form C Si covalent bonds, as it is mainly the case for the presently studied glass in which most car- bon atoms get into C(Si) 4 fourfold coordinated sites [8]. However, an increase of the cross-linking degree of the network, gives rise to a rapid enhancement of the glass melting temperature. It follows that common melting techniques cannot be used anymore, because at the tem- peratures required, most of the carbon escape from the melt through the volatilization of CO gas [9]. In this preliminary work, a sol-gel derived silicon- oxycarbide glass was studied, with a special focus on its thermal stability, including thermal expansion, crys- tallization, softening or stiffening (elasticity measure- ments) and viscous flow (creep). Each technique gives a particular view of the observed phenomena; but,