1. Network Formation in nano-composite RuO 2 -Glass Systems: Effect on the Electrical Conductivity Network Formation in nano-composite RuO 2 -Glass Systems: Effect on the Electrical Conductivity Ruthenium-glass systems are widely used in micro-electronics because of their unique electrical properties. They are also formed in the vitrification of nuclear waste since ruthenium belongs to the fission products. The presence of ruthenium is known to significantly affect the viscosity and the thermal and electrical properties of the glass melt [1]. However, the interaction of this element with the glass matrix remains poorly understood. Recent studies have pointed out mixed ionic and electronic conduction from molten state to glassy state [2] in these heterogeneous materials, whose microstructure can be described as a dispersion of conducting particles in the continuous isolating glass matrix. Indeed, above a critical volume fraction of RuO 2, an electronic contribution is added to the ionic contribution of the matrix and the electrical conductivity increases significantly with the RuO 2 content. This effect has been described in terms of electrical percolation of the particle network [2-4]. Although the electronic conduction can be interpreted using the percolation theory, the very low percolation threshold (of the order of 1 vol%, to be compared with the 16 vol% necessary for a geometrical percolation) suggests that the metal oxide particles build a network. However, they actually do not touch: along a conducting chain, they are separated from their neighbors by a thin glassy layer, as was suggested by Pike and Seager [5], and later brought to light by Chiang et al. [6] who measured its thickness to be about 1 nm. Electrical conduction through these layers can be described by the generally accepted resonance tunneling conduction model of Pike and Seager [5]. Rachel Pflieger 1, Mohammed Malki 2, Mathieu Allix 2 and Agnès Grandjean 1 1 ICSM: UMR5257, 30207 Bagnols sur Cèze, France; SCDV – LEBV, CEA Valrhô, Centre de Marcoule, 30207 Bagnols sur Cèze, France. 2 CNRS-CEMHTI, 1D, Av de la Recherche Scientifiques, 45071 Orléans cedex 2, France. [1]: B. Luckscheiter, in: Proc. 1993 Int. Conf. Nucl. Waste Management and Environ. Remediation, Prague, Czech Republic, 1993, 1:427 [2]: C. Simonnet, A. Grandjean, J. Non-Cryst. Solids, 2005, 351:1611 [3]: T. Yamaguchi, Y. Nakamura, J. Am. Ceram. Soc., 78(5):1372, 1995 [4]: N. Nicoloso, A. LeCorre-Frisch, J. Maier, R.J. Brook, Solid State Ionics 75:211, 1995 [5]: G.E. Pike and C.H. Seager, J. Appl. Phys. 48(12):5152, 1977 [6]: Y.-M. Chiang, L.A. Silverman, R.H. French, R.M. Cannon, J. Am. Ceram. Soc. 77(5):1143, 1994 [7]: J. Mukerji, S.R. Biswas, Glass. Ceram. Bull. 14(2):30, 1967 Solid-state electrical conductivity measurements (<600°C) Measurement by two-electrode impedance spectroscopy. Ionic transport (displacement of ions within the ‘frozen’ glass matrix) Electronic transport: based on electron-carrying species (e.g. Fe 2+ /Fe 3+ ) or on electron conducting particles (e.g. RuO 2 ). RuO 2 -glass systems: combination of both transport types Spent nuclear fuel reprocessing is performed in order to recover energy producing elements such as uranium and plutonium. These processes generate also high level waste flows containing fission products. Today the best accepted treatment of these wastes is immobilization through vitrification. Immobilization of nuclear waste is followed by storage and ultimately disposal in geological repositories. Among the fission products are the platinoids Ru, Rh and Pd, whose solubilities in glass melts are very limited. Moreover they significantly affect the viscosity and the thermal and electrical properties of the melt during vitrification processes [1]. The electrical conductivity of a glass melt is an important processing parameter in the Joule-heated melters used for the vitrification of nuclear wastes. It depends on the type of mobile species (ions or electrons), on their concentrations and on their mobilities. Despite their technical importance in many processes, electrical transport mechanisms in glass containing RuO 2 particles have still not been fully elucidated. In this work, the particular system RuO 2 - nuclear glass FNOC57 composites is studied. The importance of the building of a network by the RuO 2 nanoparticles in the appearance of a low-rate electronic conductivity is investigated. Studied samples Composition of FNOC57 glass frit (wt%): 58.84 SiO 2, 18.15 B 2 O 3, 7.00 Na 2 O, 5.23 CaO, 4.28 Al 2 O 3, 3.24 ZnO, 2.56 Li 2 O, 0.70 ZrO 2 RuO 2 amounts: 0-4 wt% (0-1.4 vol%) Composites are obtained by fusion at 1100 or 1200°C, under static conditions or under mechanical stirring. They are heterogeneous materials. Optical microscope and SEM pictures of a FNOC57-4wt%RuO 2 composite synthesised at 1200°C under static conditions. RuO 2 TEM pictures of a FNOC57-4wt%RuO 2 composite synthesized at 1100°C under static conditions; zone rich in RuO 2 particles and zone of pure glass. Conductivity: with e the sample thickness and S the metalized area. The ionic conductivity follows Arrhenius law: Verre Cationic transport (Na + ) Electronic transport Glas RuO 2 Results Electrical conductivity of FNOC57-xRuO 2 composites, static synthesis, 2h at 1100°C The electrical percolation threshold is around 0.52-0.69 vol% RuO 2., i.e. much less than the geometrical percolation threshold (16 vol%). Are the particles rearranging to build a network? What are the electronic relays between the RuO 2 particles? Evolution of the electrical conductivity at 400°C with the synthesis temperature. The 1400°C data were obtained on the upper part of samples refined during 8 hours. Optical microscopy pictures of FNOC57-1.04 vol% RuO 2 conductivity pellets from samples prepared under stirring at 1200°C (a) or under static conditions at 1100°C (b) (x20 magnification). Comparison of the conductivities of FNOC57-xRuO 2 composites synthesised under static conditions and under continuous stirring. Comparison of the electrical conductivity of FNOC57-1.4vol%RuO 2 composites synthesised at T and quenched with the electronic conductivity measured by Simonnet et al. [2] in the glass melt at T, and with Ru solubility [7]. All properties were normalised to their values at 1200°C. Electrical conductivity of composites synthesised under static conditions Evolution of the electrical conductivity with the synthesis temperature (static conditions) Effect of mechanical stirring This confirms the hypothesis [5,6] that dissolved Ru serves as electron relays. a.b. Although in both cases the particle sizes vary over a large interval, an homogeneous dispersion of small particles can be observed when stirring is applied during the synthesis. Besides, under static synthesis conditions RuO 2 particles tend to regroup in millimetric aggregates. Under stirring the percolation threshold is increased by approximately a factor of 2. Rearranging of RuO 2 particles to form a network is hindered by mechanical stirring. In RuO 2 -borosilicate glass systems, an electronic conductivity can appear at RuO 2 rates lower than 1 vol%. An explanation of this very low percolation threshold (to be compared with the 16 vol% RuO 2 that would be necessary for a geometrical percolation) can be the formation of RuO 2 particle chains to build a percolating network. Continuous mechanical stirring was applied during the synthesis in order to hinder this network formation, and this resulted in a clear increase of the percolation threshold. However, as Chiang et al. [6] brought to light, the RuO 2 particles constituting these percolating chains do not touch but are separated by a glass layer of 1 nm thickness. Therefore, electronic relays are needed in this glass layer. Pike and Seager [5] and later Chiang et al. [6] made the hypothesis that dissolved ruthenium may play this role of electron relay. Indeed, present measurements and molten-state measurements of Simonnet et al. [2] show a very good correlation between the temperature evolution of the electronic conductivity and of ruthenium solubility in borosilicate glasses, which confirms this hypothesis.