Single crystal growth of actinide dioxides

Abstract

High temperature solution growth (HTSG) is a very successful method to grow single crystals of refractory oxides. Crystallisation is obtained from solutions of oxides in molten salts by slow cooling, solvent evaporation or by transport in a temperature gradient. High temperature solution growth produces small single crystals (1–100 mm 3) with well formed natural faces and a high degree of perfection. In order to prepare actinide dioxide single crystals the thermal gradient method was adopted. In this process, relatively small solvent quantities are necessary. The yield is high (up to 80%) and a detailed knowledge of the solvent-actinide oxide system is not necessary, the temperature remaining constant during the process. The 2PbO-V 2O 5 flux has been chosen because of its [1, 2]. 1. low vapour pressure even at high temperature (1300 °C) 2. compatibility with platinum 3. viscosity. The apparatus used to grow actinide dioxide single crystals is shown in Fig. 1. ▪ The platinum crucible containing the flux and the nutrient (oxide powder or pressed pellets) is heated in t001 Experimental Conditions and Results. Compound Pb 2V 2O 7 (g) AnO 2 powder (g) Crucible temp. (°C) Temp. gradient ΔT/1 (°C/cm) Reaction time (days) Single crystals (mm 3) ThO 2 36 1.8 1250 50 7 pale-yellow octahedrals 2 × 2 × 2 NpO 2-doped ThO 2 pink octahedrals a) 0.1% 20 5.0 1200 50 7 1.5 × 1 × 1 b) 1.0% 20 5.0 1200 50 15 2 × 1 × 1 NpO 2 20 11.0 1100 25 10 black cubes 2.5 × 2 × 1.5 PuO 2 20 6.4 1100 25 15 black cubes 0.8 × 1 × .05 a resistance furnace. Cold gas (nitrogen) is injected into a platinum tube to obtain a suitable temperature gradient. The single crystals are growing on the platinum tube. Single crystals of thorium, neptunium, and plutonium dioxide have been grown as well as Np doped thorium. The experimental conditions and results are summarized in Table I. The average growth rate for all the actinide dioxides is about 1 mm/week. The single crystals have been characterized by X-ray diffraction (DebyeScherrer, Gandolfi, Laue). A more complete characterization including the determination of the oxygen/ metal ration and a panoramic analysis of the impurities and the detection of inclusions is in progress.