Heat capacity, thermal expansion, and thermal diffusivity of NaUO2BO3

Journal of Thermal Analysis and Calorimetry, Dec 2017

In the present studies, the thermal behaviour of NaUO2BO3 has been investigated. This compound is a potential product of interaction between the coolant (Na), control rods (B4C), and the oxide fuel, which could form under accidental conditions in sodium-cooled fast reactors. The thermal expansion, the heat capacity, and thermal diffusivity of NaUO2BO3 have been measured. The thermal conductivity of the material is derived from these results and presented here for the first time.

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Heat capacity, thermal expansion, and thermal diffusivity of NaUO2BO3

Journal of Thermal Analysis and Calorimetry https://doi.org/10.1007/s10973 Heat capacity, thermal expansion, and thermal diffusivity of NaUO2BO3 Karin Popa 0 1 Ondrej Benesˇ 0 1 Dragos¸ Staicu 0 1 Jean-Christophe Griveau 0 1 Eric Colineau 0 1 Alice Seibert 0 1 Jean-Yves Colle 0 1 Sarah Stohr 0 1 Philippe E. Raison 0 1 Joseph Somers 0 1 Rudy J. M. Konings 0 1 0 European Commission, Joint Research Centre , P.O. Box 2340, 76125 Karlsruhe , Germany 1 & Karin Popa In the present studies, the thermal behaviour of NaUO2BO3 has been investigated. This compound is a potential product of interaction between the coolant (Na), control rods (B4C), and the oxide fuel, which could form under accidental conditions in sodium-cooled fast reactors. The thermal expansion, the heat capacity, and thermal diffusivity of NaUO2BO3 have been measured. The thermal conductivity of the material is derived from these results and presented here for the first time. Sodium uranyl borate; Solid state reactions; Thermal expansion; Calorimetry; Thermal analysis; Thermodynamic properties Introduction Sodium uranyl borate NaUO2BO3 (or NaUBO5) is the only anhydrous borate that forms in the Na–U–B–O phase diagram [ 1–3 ]. This compound has been reported for the first time by Hoekstra [ 4 ], and its crystal structure was determined by Gasparin [ 5 ] from X-ray diffraction data measurements on single crystal. NaUO2BO3 crystallises in an orthorhombic unit cell in the space group Pcam (N 57). In this structure, uranium atoms are sevenfold coordinated, the boron atoms have three coplanar surrounding oxygen and sodium atoms are sixfold coordinated and linked together by three oxygen atoms along the c axis as shown in Fig. 1. An extensive thermodynamic characterisation has been performed by Chernurukov et al. [ 6, 7 ]. The latter authors measured the enthalpy of formation and the low-temperature heat capacity, from which the Gibbs energy was derived as DfG (298.15 K) = - (2122.5 ± 5.0) kJ mol-1. This results in a reaction energy from the oxides of Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10973-017-6923-y) contains supplementary material, which is available to authorized users. DrG (298.15 K) = - (190.5 ± 5.1) kJ mol-1, indicating that the compound can form under oxidising conditions when the oxides of sodium, uranium, and boron are mixed together. This could occur for instance in nuclear borosilicate waste glasses (containing about 10 wt% sodium oxide) or in the debris of damaged nuclear reactor cores containing oxidised fuel and B4C control rod material, such as in Fukushima case (reaction with the sodium ions from the cooling sea water). In the present study, we have extended the thermodynamic characterisation of this compound to higher temperatures and determined the thermal expansion, the enthalpy increment, and the thermal diffusivity, from which the heat capacity and thermal conductivity are derived. In addition, we re-measured the low-temperature heat capacity in order to verify the change in the slope observed by Karyakin et al. [ 6 ] in their measurements. Materials and methods Synthesis and material processing NaUO2BO3 was produced using a simplified solid state reaction procedure. Thus, stoichiometric amounts of commercial NaHCO3 (Alfa Aesar, 99.7%), H3BO3 (SigmaAldrich, 99.999%), and UO2.10 (COGEMA powder, stoichiometry calculated from the value of the lattice parameter) were reacted for 5 h at 1173 K under an air atmosphere. Pellets of 3.5 mm diameter and 1–2 mm in height were produced by pressing and re-heated at 1173 K under air for another 5 h. XRD analysis NaUO2BO3 was characterised at room temperature by X-ray powder diffraction (XRD) using a Bruker D8 diffractometer mounted in a Bragg–Brentano configuration with a curved Ge (1, 1, 1) monochromator, a ceramic copper tube (40 kV, 40 mA) and equipped with a LynxEye position sensitive detector. The data were collected by step scanning in the angle range 10 B 2h B 120 at a 2h step size of 0.0092 . For the measurements, the powder was deposited on a silicon wafer to minimise the background and dispersed on the surface with several drops of isopropanol. The structural refinement was performed using the Fullprof2k suite [ 8 ]. The shape of the peaks was described by a Pseudo-Voigt function, and the background was fitted by a linear interpolation between a set of about 50 background points. The thermal expansion of NaUO2BO3 was measured by high-temperature X-ray diffraction. The data were collected on a second Bruker D8 X-ray diffractometer mounted with a curved Ge (1, 1, 1) monochromator, a copper ceramic X-ray tube (40 kV, 40 mA), a LynxEye position sensitive detector and equipped with an Anton Paar HTK 2000 chamber. Measurements were conducted up to 1073 K under helium, in the angle range 16 B 2h B 90 with a 2h step size of 0.017 . Raman and IR spectroscopy NaUO2BO3 was characterised in the temperature range from 6 to 290 K by Raman spectroscopy using a Jobin– Yvon T 64000 equipped with a macro-inlet. To obtain the Raman spectra, a laser excitation source was used to illuminate the pressed powder sample fixed with indium inside the cold finger of a closed cycle He cryostat. The scattered radiation from the surface was detected with a low-noise LN2-cooled CCD detector after separation of the Raman signal from the Rayleigh component. The excitation source was a Kr? Coherent cw laser radiating at 647 nm with a power of 50 mW. Spectra were obtained with an acquisition time of 10 s. FT-IR spectra were measured using Bruker Alpha spectrometer employing the attenuated total reflectance (ATR) technique with a resolution of 0.01 cm-1 in a range 375–4000 cm-1. No special preparation for powder samples was necessary for this technique. DTA/TG measurements The thermal behaviour was investigated using a Netzsch STA 449C DTA/TG, using an alumina crucible and different atmospheres (Ar and air). The temperature was controlled by a Pt-PtRh (10%) thermocouple. The applied heating and cooling rates were 10 K min-1. Heat capacity measurements The low-temperature heat capacity of NaUO2BO3 was measured on a pellet of 51.9 mg from 293.4 to 1.82 K at 0 T with a PPMS-9T instrument (quantum design) using the hybrid adiabatic relaxation method, in the absence and the presence of the magnetic field. The sample was fixed on the sapphire platform by a small amount of thermal conductive grease (Apiezon-N). The heat capacity contributions of the puck and of the layer of grease were determined separately through the addenda protocol determination. A complete description of the technique and the method has been reported by Lashley et al. [ 9 ], whereas details of the instrument used in this work can be found in Javorsky´ et al. [ 10 ]. Based on comparison with standard materials and experience for other compounds, we estimate the uncertainty of the measurements to be better than 3%, depending on the temperature range. Solid pieces of 53.7–62.9 mg were further used to measure the enthalpy increments using a Setaram multidetector high-temperature calorimeter (MDHTC-96) using a drop detector. For more details about the technique, we refer to our previous studies [ 11, 12 ]. The measurements were carried out under an argon atmosphere. The temperature range of the experiment was from 482.5 to 1190.5 K. Each isothermal run consisted of three drops of analysed material. Before and after each sample, a reference material (platinum ingots of 99.95 at.% purity) was dropped to determine the sensitivity (signal vs. heat ratio) of the detector. The drops were separated by time intervals of 20 min, long enough to re-stabilise the monitored heat flow signal. All evaluations of the background subtraction and peak integration were done using commercially available software for data processing. The reported temperatures were corrected (ITS-90) in accordance with the calibration curve obtained prior to measurement using several highpurity standard metals (Sn, Pb, Zn, Al, Ag, Ni) with various melting temperatures in order to cover the whole temperature range of the measurement. Thermal diffusivity measurements The thermal diffusivity measurements were carried out using a laser flash device, designed and constructed in house [ 13 ]. The curves were measured with pyrometers. The samples were heated at the measurement temperatures in a high-frequency furnace, in the temperature range from 500 to about 1130 K. The samples were disc fragments with a thickness of about 1 mm. The faces were checked to ensure that they were plane and parallel, without defects. Results and discussions Structural characterisation and thermal expansion The synthesis resulted in pure phase NaUO2BO3 (space group Pcam, Z = 4), as confirmed by room temperature XRD and vibrational spectroscopy. The results of our refinement are in good agreement with the previous literature reports. Table 1 summarises the main experimental crystallographic data of the present studies. The DTA/TG and measurements performed under air and argon indicated that NaUO2BO3 is stable up to 1470 K (supplementary material S1). However, a slight change in colour may be observed at about 1200 K, indicating that a transformation is occurring already at this temperature. Moreover, the HTXRD measurements performed under inert atmosphere indicated the appearance of U3O8 as a secondary phase at 1273 K. This is the reason of limiting our heat capacity and thermal expansion measurement around 1200 K. The relative variation of the cell parameters of NaUO2BO3 with temperature is presented in Fig. 2. The 0.015 K 3 0 /3 y )K0.010 3 0 3 y – (y 0.005 0.000 400 600 800 Temperature/K 1000 1200 linear thermal expansion is anisotropic and varies more along the c axis. This can be explained by the fact that NaO6 units are made of weak Na–O bonds and are linearly connected along the c axis as shown in Fig. 1. As often observed, thermal expansion varies inversely with the bond strength. Heat capacity The low-temperature heat capacity measurements data on NaUO2BO3 are presented in Table 2. The heat capacity data have been fitted to a series of overlapping polynomials, and we derive for the Cp(298.15 K) = (143.2 ± 2.9) J K-1 mol-1, S (298.15 K) = (174.9 ± 3.5) J K-1 mol-1, and {H (298.15 K) H (0 K)} = (26.37 ± 0.53) kJ mol-1. The errors were IR Raman Fig. 3 a IR and Raman spectra of NaUO2BO3 at room temperature; b series of Raman spectra as a function of temperature in the range 6 to 290 K. The inset shows the full sizes of the most intense peak at 788 cm-1. No significant change in position is observed. The slight asymmetry at lower wavenumbers is increasing with increasing temperature estimated from the accuracy of the equipment, which is in the order of 2%. The low-temperature heat capacity measurements indicate an anomalous change in the slope of the curve at about 175 K. Our results agree satisfactory with the measurements reported by Karyakin et al. [ 6 ] (supplementary material S2), including the anomaly that was not identified as such by them. It should be noted that our measurements were performed from room temperature downwards, whereas Karyakin et al. measured from low temperature upwards. This phenomenon was further analysed by performing X-ray diffraction and IR, respectively, and Raman spectroscopic measurements in the low-temperature range. Figure 3a, b summarises the IR and Raman spectra at room temperature and the temperature-dependent Raman spectra in the range from 6 to 290 K. The IR spectrum corresponded to earlier investigations by Hoekstra [ 4 ]. To our knowledge, there is no Raman spectrum of sodium uranyl borate reported in the literature. Anyhow, the temperaturedependent series showed no significant differences in the Tm/K 482.5 533.9 585.8 636.5 687.5 738.0 788.5 838.7 889.0 938.7 989.0 1039.3 1089.6 1139.7 1190.5 300 250 1 200 –1K –lo150 m J / Cp100 50 0 increment data were fitted using a combined linear regression together with the room temperature heat capacity data obtained from the PPMS measurements performed as part of this study. A Maier–Kelly-type second-order polynomial equation was used, similar to our previous studies for alkaline and alkaline-earths uranates [ 14, 15 ]. The following heat capacity equations were obtained: (a) 3.0 × 10–7 1 2.5 × 10–7 2–/sm2.0 × 10–7 y iit v fsu 1.5 × 10–7 if d l am1.0 × 10–7 r e h T 5.0 × 10–8 Thermal diffusivity and conductivity The results of the thermal diffusivity measurements are listed in Table 4, and are shown in Fig. 5a. The relative uncertainty on the thermal diffusivity measurements is of 5%, mainly due to sample thickness variations. The sample thickness change due to thermal dilatation was corrected using the thermal expansion data obtained in this work. The results show very moderate temperature dependence and can be described by the following equation: a= m s 1 2 ¼ 2:398 The thermal conductivity of the compound was calculated from the measured thermal diffusivity, heat capacity, and density using the formula: k ¼ q a Cp: ð3Þ The density of the samples, measured by the Archimedes technique at ambient temperature, was of 5.37 g cm-3 (98% of the theoretical density, with TD = 5.50 g cm-3). The temperature dependence of the density was calculated using the thermal dilatation data obtained in this work. The obtained sample thermal conductivity (Fig. 5b) showed no significant temperature dependence and was therefore considered to be temperature independent, with a value of k/(W m1 K-1) = 0.634, which is very low for a ceramic material [ 16, 17 ]. The relative uncertainty on the calculated thermal conductivity is estimated to be of 10%, resulting from the sum of the uncertainties on the thermal diffusivity (5%), specific heat (3%), and density (2%). Conclusions • • • The linear thermal expansion of NaUO2BO3 is very anisotropic along all the crystallographic axes. The anomalous change in the curve of the lowtemperature heat capacity at about 175 K found by Karyakin et al. [ 6 ] was reproduced, indicating that it is a genuine effect. The thermal conductivity of NaUO2BO3 was found to be very low for a ceramic material. Acknowledgements The authors acknowledge Daniel Bouexie`re, Co Boshoven, Herwin Hein, Michael Holzha¨user, Davide Robba, and Marc Sierig, for technical support. 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Karin Popa, Ondrej Beneš, Dragoş Staicu, Jean-Christophe Griveau, Eric Colineau, Alice Seibert, Jean-Yves Colle, Sarah Stohr, Philippe E. Raison, Joseph Somers, Rudy J. M. Konings. Heat capacity, thermal expansion, and thermal diffusivity of NaUO2BO3, Journal of Thermal Analysis and Calorimetry, 2017, 1-9, DOI: 10.1007/s10973-017-6923-y