Recent advances in study of high-temperature behavior of non-stoichiometric TaCX, HfCXand ZrCX in the domain of their congruent melting point

Melting behavior of nonstoichiometric carbides of tantalum and hafnium remains one of the most challenging tasks in the high-temperature materials science despite the fact that the first studies of melting of these compounds are dated back to the beginning of 60’s. However, the data on melting points of tantalum and hafnium carbides in the vicinity of their homogeneity domain are still very contradictory. Zirconium carbide is a more studied compound, which makes him a sort of the reference material for testing the method of investigation, since its melting point is substantially lower. But even for this material, the data on its solidus and liquidus parameters look very incomplete and need to be clarified. In present study the parameters of the solidus and liquidus line for TaCx, HfCx and ZrCx are studied using laser heating technique similar to what described in [1]. A special emphasis is given on the accurate measurement of temperature and determination of spectral emissivity in course of the experiment using advanced multichannel pyrometry. The phase transitions corresponding to solidus and liquidus are determined by means of the laser-probe reflectometry, peculiarities in spectral emissivity behavior in the vicinity of the phase transitions and by high-speed video recording of the melt formation and freezing. New data on solidus and liquidus parameters for the Zr-C system in the homogeneity domain are presented. The temperatures of congruent melting of super-refractory carbides TaC and HfC are determined along with the data on their solidus and liquidus lines close to the congruent composition. The measured melting point of HfC was found to be close to the value given in a recent paper [2], whereas the melting point of TaC, according to the data of the present study, exceeds the value given in [2] by more than 150 K. The second part of the study concerns evaporation of UHTC carbides and focused at significant extension of the temperature domain limited earlier to ca. 3000 K. In the present study the method and apparatus used for determination of the composition of carbon vapor presented in [3] were significantly improved in order to make a further considerable extension towards extremely high temperatures. Due to some major improvements in the design of the TOF mass spectrometer, time-shape of the laser pulse and pyrometer time resolution the vapor composition at laser-induced evaporation of zirconium carbide samples of various starting composition within the homogeneity domain ranging from ZrC0.65 to ZrC1.0 were investigated up to 4500 K. Thus, molecular composition in vapor during evaporation of liquid ZrCx was obtained for the first time. The conditions of congruent evaporation of liquid zirconium carbide are estimated using the experimental data. References 1. Manara, D., Sheindlin, M., Heinz, W. & Ronchi, C. New techniques for high-temperature melting measurements in volatile refractory materials via laser surface heating. The Review of scientific instruments 79, 113901, (2008). 2. Cedillos-Barraza, O. et al. Investigating the highest melting temperature materials: A laser melting study of the TaC-HfC system. Sci. Rep. 6, 37962, (2016). 3. Pflieger R., Sheindlin M., Colle J.-Y. Advances in the mass spectrometric study of the laser vaporization of Graphite. J. Appl. Phys. 104, 054902 (2008). This work was supported by Russian Science Foundation under Grant No. 14-50-0012

Etude par spectrométrie de masse de la vaporisation par laser du graphite et du dioxyde d'uranium jusqu'à 4000k

Une nouvelle méthode de spectrométrie de masse (TOF MS) à haute température a été développée. La surface de l échantillon y est chauffée par laser pendant environ 20 ms, et température et spectres de masse sont mesurés en fonction du temps. Chaque expérience couvre tout un intervalle de température. Cette méthode a été appliquée au graphite pyrolytique et au dioxyde d uranium.L étude du graphite a clairement montré que la sublimation est de type Langmuir (ou surface libre), malgré les très hautes températures et pressions. Les pressions partielles relatives de C1, C2, C3, C4 et C5 ont été mesurées jusqu à 4100 K, ainsi que les enthalpies de sublimation des trois espèces principales de la vapeur. Les coefficients d évaporation relatifs de C1-C5 ont été estimés par comparaison des pressions partielles obtenues ici à 4000 K avec celles à l équilibre thermodynamique de la littérature.La courbe de pression de vapeur de UO2 au-dessus du dioxyde d uranium a été mesurée entre 2800 et 3400 K. Des enthalpies de sublimation et de vaporisation sont proposées pour UO2, ainsi qu une première valeur expérimentale de l enthalpie de vaporisation de UO3. Les rapports de pressions partielles p(UO2)/p(UO), p(UO2)/p(UO3) et p(UO2+)/p(UO+) ont été mesurés aux alentours de 3300 K et indiquent que l évaporation se fait dans des conditions proches de l équilibre thermodynamique.La méthode développée ici est adaptée à l étude par spectrométrie de masse jusqu à de très hautes températures de la vaporisation de matériaux réfractaires, et pourrait être utilisée pour l étude de matériaux chimiquement instables comme le dioxyde d uranium hyperstœchiométrique ou des carbures et nitrures.A new method of high-temperature mass spectrometry (TOF MS) was developed, where the specimen surface is heated by a laser pulse of approx. 20 ms. During it, time-resolved measurements of mass spectra and of the temperature are performed. Each experiment covers an entire temperature interval. The method was applied to pyrolytic graphite and uranium dioxide.In graphite study, it was clearly shown that the sublimation occurs in a Langmuir-like mode (free surface vaporisation), despite the very high temperatures and thus pressures. Relative partial pressures of C1, C2, C3, C4 and C5 were measured up to 4100 K. Obtained sublimation enthalpies of the main three vapour species are in a good agreement with literature values. Relative vaporisation coefficients of C1-C5 were estimated by comparison of the present partial pressures at 4000 K with equilibrium ones given in the literature.The vapour pressure curve of UO2 over stoichiometric uranium dioxide was measured between 2800 and 3400 K. Obtained sublimation and vaporisation enthalpies are in agreement with the literature. The vaporisation enthalpy of UO3 was measured for the first time. Relative partial pressure ratios p(UO2)/p(UO), p(UO2)/p(UO3) and p(UO2+)/p(UO+) were measured at around 3300 K and indicate that the vaporisation occurs in a regime close to thermodynamic equilibrium.This method is suitable for the fast and time-resolved mass spectrometric measurements of refractory materials up to very high temperatures, and could now be applied to the study of chemically unstable materials such as hyperstoichiometric urania and some carbides and nitrides.Key words: pyrolytic graphite, HOPG, uranium dioxide, laser vaporisation, TOF MS, vaporisation coefficients, Langmuir evaporation.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

High Temperatures ^ High Pressures, 2003/2004, volume 35/36, pages 25 ^ 33

Solidus and liquidus of UO 2 ‡ x from high-pressure melting experiment

New High-Power Laser Facility for Testing Properties and Behaviour of Irradiated Fuel

see attachmentJRC.E.3-Materials researc

On the Thermal Conductivity of UO2 Nuclear Fuel at a High Burn-up of around 100 MWd/kgHM

A study of the thermal conductivity of a commercial PWR fuel with an average pellet burn-up of 102 MWd/kgHM is described. The thermal conductivity data reported were derived from the thermal diffusivity measured by the laser flash method. The factors determining the fuel thermal conductivity at high burn-up were elucidated by investigating the recovery that occurred during thermal annealing. It was found that the thermal conductivity in the outer region of the fuel was much higher than it would have been if the high burn-up structure were not present. The increase in thermal conductivity is a consequence of the removal of fission products and radiation defects from the fuel lattice during recrystallisation of the fuel grains (an integral part of the formation process of the high burn-up structure). The gas porosity in the high burn-up structure lowered the increase in thermal conductivity caused by recrystallisation.JRC.E.2-Hot cell

Melting of Stoichiometric and Hyperstoichiometric Uranium Dioxide

The solid–liquid transition in stoichiometric and hyperstoichiometric UO2 was investigated by means of advanced techniques. Laser heating enabling fast melting and freezing processes was used under container-less conditions and buffer gas pressures up to 250 MPa, making non-congruent evaporation ineffective. Pulse thermograms of UO2 x with 0 6 x 6 0.21 were recorded with fast pyrometers and interpreted with computer simulations. In addition, a novel method for identification of phase transitions was implemented, based on the detection of surface reflectivity variations. The melting line of UO2.00 was for the first time determined at pressures between 10 and 250 MPa, and the melting temperature of the stoichiometric oxide was measured to be 3147 20 K, in fair agreement to previous measurements reported in the literature. The liquidus and solidus lines of UO2 x differ from the currently recommended data, which substantially underrate the effect of oxygen on melting.JRC.E.3-Materials researc

Experimental Determination of the Thermal Conductivity of Liquid UO2 near the Melting Point

The paper gives an account of new measurements of the thermal conductivity of liquid UO2. The sample was heated up to above the melting point by a laser pulse of a controlled shape, and the produced thermogram of temperature history was measured by a fast and accurate pyrometer with a time resolution of 10 μs.JRC.DG.E.3-Materials researc

Impact of Auto-irradiation on the Thermophysical Properties of Oxide Nuclear Reactor Fuels

The nuclear fuel assemblies are used in the reactor for 3 to 6 years, and then are stored underwater at relatively low temperature. During this cooling phase, radioactive decay damage and helium begin to accumulate in spent fuel. After a few years of cooling, when the radioactivity level has sufficiently decreased samples of spent fuel are extracted for post-irradiation examination. In order to assess the in-pile value of thermophysical properties using out-of pile measurements, the effect of decay damage has to be distinguished from that caused by fission during reactor operation. This characterisation is also necessary in order to provide information on the evolution of the state of spent fuel under final or interim storage conditions. The characterisations performed on fuel that has accumulated damage during years of storage reveal that the damage has significant effects on the thermophysical properties and that some of these effects can be progressively annealed-out by heat treatments. This makes the interpretation of the experiments difficult, and specific methods and verifications have to be applied. The effects of -damage on the thermophysical properties of the fuel were investigated at JRC/ITU using samples of UO2 doped with ~0.1 and ~10 wt% 238Pu. These samples were stored for different times, under conditions where -damage dose resulting from the decay of 238Pu could be accurately calculated. Some specimens were annealed to as-fabricated state (t = 0) and prepared for periodical X-ray diffraction to monitor the lattice parameter evolution. Knudsen-cell helium release experiments and transmission electron microscopy examinations were performed after four years of damage accumulation. After five years, the degradation and recovery of the thermal diffusivity was measured with a laser flash technique during thermal annealing programs. Six months later, the apparent heat capacity, Cp*, was measured by differential scanning calorimetry. The deviation of the measured Cp*(T) from the real heat capacity, Cp(T), is related to the recovery of the latent heat of the lattice defects during thermal healing. Each recovery stage observed on the Cp*(T) curve was analysed and attributed to a certain kind of defect (oxygen Frenkel pair recombination, uranium vacancy/interstitial clusters recombination, dislocation loop growth, void growth). It was observed that the recovery process in the apparent heat capacity did not affect individual laser flash heat transport measurements, since these are characterized by very short laser pulse duration (and very small temperature increase) compared to the kinetics of the Cp* recovery. Repeated laser flash measurements did not induce any change in the apparent heat capacity. The thermal diffusivity, measured during annealing cycles, displayed three annealing stages. Comparison between the 0.1 and 10 wt% 238Pu samples shows that the degradation of the diffusivity with increasing -dose is not linear, and that saturation occurs at relatively low doses. As a result, the thermal conductivity degradation of stored spent-fuel can be expected to level off after a few years of storage. A correlation quantifying this degradation is proposed.JRC.E.3-Materials researc

The Radial Dependence of the Thermal Diffusivity of High Burn-up UO2 from Commercial PWR Fuel Rods

see attachmentJRC.DG.E.2-Hot cell

New Laser-flash Experiments for Irradiated Fuels Under Contrainted Conditions

A new group of facilities dedicated to the characterisation of irradiated nuclear fuel under conditions reproducing a variety of in-pile events is being constructed at JRC/ITU. The first planned experiment is the measurement of the local thermophysical properties (local thermal diffusivity, gap condductance)of the fuel. The set-up is used both for steady state and perturbative experiments (heating power variations or other excursions simulating in-pile transients or accidents like RIA).JRC.E.3-Materials researc