Nanocrystallites of UO2 with a size of 3–5 nm were studied in situ with high temperature X-ray diffraction (HT-XRD), thermogravimetry (TGA), and differential thermal analysis. The evolution of the crystallite size, the lattice parameter, and the strain were determined from ambient temperature up to 1200 °C. Below 700 °C, a weak effect on the crystallite size occurs and it remains below 10 nm, while a strong expansion of the lattice parameter is measured. The strain decreases with temperature and is completely released at 700 °C. Above this temperature, begins the sintering of the nanocrystallites reaching a size of about 80 nm at 1200 °C. The weight loss curve observed in TGA is assigned to the desorption of water molecules and is correlated with the strain evolution observed by HT-XRD. The linear thermal expansion and the thermal expansion coefficient at 800 °C are 1.3% and 16.9 × 10−6 °C−1, respectively.
Crystallite Size Differential Thermal Analysis Curve Nanocrystalline Material Lattice Expansion Linear Thermal Expansion
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We are grateful to H. Hein for his technical support with TGA–DTA measurements. R. Jovani Abril acknowledges the European Commission for support in the frame of the program “Training and Mobility of Researchers.”