Heat capacity of liquid transition metals obtained with aerodynamic levitation

The development of contactless measurement methods has allowed investigating the properties of molten materials at high temperatures in a controlled environment. As the sample is not in contact with any container walls while being heated, levitation techniques have an edge over traditional contact methods at elevated temperatures. Among the various thermophysical properties of interest, it has been challenging to measure heat capacity with levitation techniques because it is directly related to the emissivity of the sample. Previous studies on heat capacity measurement with various levitation techniques have produced results with large deviations, especially at elevated temperatures. In addition, there is a general lack of information on the heat capacity of liquid transition metals at temperatures exceeding 2000 K, especially using conventional calorimetry methods. In this study, we successfully obtained the isobaric heat capacity of liquid transition metals such as Co, Hf, Ir, Mo, Nb, Rh, Ru, Ti, V, and Zr with aerodynamic levitation using the newly developed “multiple-gas cooling” method. A comparison between our reported values and reference data enabled us to assess the accuracy of previous experiments and provide much needed heat capacity data for high-temperature liquid metals. This study highlights the applicability and reliability of the multiple-gas cooling method for measuring the heat capacity of liquid non-noble metals at temperatures approaching 3000 K

Phase behavior of oxidized Ce and Gd-doped (U,Zr)O₂

Re-criticality analysis of the fuel debris at the Fukushima Dai-ichi Nuclear Power Plant is the key step to ensure the safe retrieval and storage of the fuel debris. Knowledge of the amount and distribution of Pu and Gd within the fuel debris greatly contributes to such analysis as they directly affect the fission-chain reaction. However, little is known about how Pu-doped and Gd-doped (U, Zr)O₂ solid solutions oxidize and whether phases concentrated in Pu or Gd form. In this study, CeO₂ is used as a surrogate material for PuO₂ because of the similarities in their crystal structures and valence states. (U₀.₉-xZr₀.₁Cex)O₂ and (U₀.₉-xZr₀.₁Gdx)O₂ solid solutions are prepared by sintering under an argon atmosphere and oxidized at 1073 K in air for 2 hours to simulate heavily oxidized fuel debris. Samples doped with 5 at% Ce and Gd contain only an orthorhombic-U3O8-x phase after oxidation, but its diffraction peaks’ intensities decrease as the amount of dopant increases. The phase transformation of (U₀.₉-xZr₀.₁Gdx)O₂, with further oxidation, is found to be cubic-(U, Zr, Gd)O₂+x → orthorhombic-(U, Zr, Gd)₃O₇±x → orthorhombic-(U, Zr, Gd)₃O₈-x. SEM/EDS analysis reveals that Ce and Gd are uniformly distributed in the (U₀.₉-xZr₀.₁REx)O₂ (RE = Ce, Gd) samples after oxidation

Chalcopyrite ZnSnSb_2: A Promising Thermoelectric Material

Ternary compounds with a tetragonal chalcopyrite structure, such as CuGaTe2, are promising thermoelectric (TE) materials. It has been demonstrated in various chalcopyrite systems, including compounds with quaternary chalcopyrite-like structures, that the lattice parameter ratio, c/a, being exactly 2.00 to have a pseudo-cubic structure is key to increase the degeneracy at the valence band edge and ultimately achieve high TE performance. Considering the fact that ZnSnSb_2 with a chalcopyrite structure is reported to have c/a close to 2.00, it is expected to have multiple valence bands leading to a high p-type zT. However, there are no complete investigations on the high temperature TE properties of ZnSnSb_2 mainly because of the difficulty of obtaining a single-phase ZnSnSb_2. In the present study, pure ZnSnSb_2 samples with no impurities are synthesized successfully using a Sn flux-based method and TE properties are characterized up to 585 K. Transport properties and thermal analysis indicate that the structure of ZnSnSb_2 remains chalcopyrite with no order–disorder transition and clearly show that ZnSnSb_2 can be made to exhibit a high zT in the low-to-mid temperature range through further optimization