Melted Synthetic Zirconolite-Based Matrices: Effect of Cooling Rate and Heat Treatment on Ceramic Microstructure and Che
- PDF / 1,142,443 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 86 Downloads / 192 Views
Melted Synthetic Zirconolite-Based Matrices: Effect of Cooling Rate and Heat Treatment on Ceramic Microstructure and Chemical Durability T. Advocat1, P.J. McGlinn2, C Fillet1, G. Leturcq2, S. Schuller1, A. Bonnetier1, K. Hart2 1 CEA/SCD-Marcoule, BP 171 30207 Bagnols-sur-Cèze Cedex, France 2 ANSTO, New Illawarra Road, Menai PMB1, NSW2234 Australia ABSTRACT The heat treatment conditions are a key factor in fabricating zirconolite ceramics and glassceramics following high-temperature melting. An oxide mixture melted at 1450°C and subsequently heat-treated at 1200°C yielded a glass-ceramic containing crystallized zirconolite–2M. The silicaenriched residual glass represented about 60-70 vol% of the total; the actinide surrogates (Nd, Ce) were equally distributed between the residual glass and the zirconolite crystals. Zirconolite ceramics obtained after melting an oxide mixture at 1600–1700°C consisted of zirconolite, perovskite and rutile. Rapid cooling rates (> 100°C·min-1) were obtained by pouring the melt into ingot molds; the resulting zirconolite ceramics were characterized by crystals of zirconolite-2M ranging from 1 to no more than 20 µm. Slow cooling (< 25°C·min-1) produced ceramics with crystals several hundred micrometers long. Despite the microstructural differences, the chemical durability of the zirconolite ceramics was identical. The initial alteration rates r0 were about two orders of magnitude lower than those measured for the residual aluminosilicate glass of the zirconolite glass-ceramics. Moreover, during long-term leach tests at high S/V ratios to obtain advanced degrees of reaction progress, the alteration rates of all the materials diminished by over 3 to 4 orders of magnitude below r0. INTRODUCTION Zirconolite CaZrxTi2-xO7 (0.8 < x 50°C·min-1) or slow cooling (< 25°C·min-1)
Precursors Batch size (kg) Melting temperature (°C) Crucible
ZIRCONOLITE GLASS-CERAMIC Physical and chemical characterization of the glass ceramics Two zirconolite glass-ceramics containing , respectively, Nd and Ce as surrogates for actinides, were obtained from aluminosilicate melts fabricated at 1450°C with an oxide mixture, then heattreated for 12 hours at 1200°C (Table I and Table II). These are the optimum conditions for obtaining the desired crystalline phase on cooling of the melt. Variants on this protocol are discussed by Loiseau et al. in another paper in these proceedings [9]. Previous tests, with only Nd-doped zirconolite glass-ceramics also showed that after heat treatment at a lower temperature (1050°C),zirconolite crystallized as fine micrometer-scale needles, but that minor phases such as sphene also appeared under these conditions [7,8]. Another advantage of the 1200°C heat treatment process is that it Figure 1. XRD spectra for the Nd-doped results in devitrification of the parent glass zirconolite glass-ceramic GC1. as the temperature diminishes. This feature could make the process more suitable for industrial implementation. When the glass is heat-treated at 1200°C, the zirconolite-2M (Figure 1) needles a
Data Loading...
