Crystal structure, lattice dynamics and thermodynamic properties of thermoelectric orthorhombic BaCu2Se2 compound

International audienceIn the present work, we report a combined experimental and ab-initio study of the crystal structure, lattice dynamics and thermodynamic properties of the thermoelectric orthorhombic BaCu2Se2 compound. The thermal and pressure evolution of the crystal structure is obtained using synchrotron XRD experiments. The lattice dynamics is studied through Raman scattering experiments and ab-initio supercell calculations and reveals that the lowest Raman-active and infrared-active optical modes are at about 50 cm-1 (7 meV) and the presence of an optically silent low-energy optical mode at about 5 meV. From the determination of the heat capacity, the thermal expansion and the bulk modulus, a Grüneisen parameter of about 2.03 is estimated. This rather large value together with the presence of low energy optical modes and rather low Debye temperature in orthorhombic BaCu2Se2 can explain its low thermal conductivity. Additionally, we have also studied the lattice dynamics, the thermodynamic properties and the stability of the metastable tetragonal BaCu2Se2 against the stable orthorhombic phase and found that it might be stabilized at high temperature or high pressure

Layered oxychalcogenide in the Bi–Cu–O–Se system as good thermoelectric materials

International audienc

Metal organic precursor effect on the properties of SnO₂ thin films deposited by MOCVD technique for electrochemical applications

International audienceTin dioxide (SnO2) thin films are deposited by metal organic chemical vapor deposition (MOCVD) on Si substrates from three commercial metal organic precursors. The morphology and the microstructure of the films are observed by field emission gun-scanning electron microscopy (FEG-SEM). The films are dense and made of crystallites with a nanometer size. The structural properties of the films are assessed by grazing incidence X-ray diffraction (GIXRD). A tetragonal SnO2 phase is observed in the deposited thin films with various texturations as a function of the metal organic precursor used for the deposition. Chemical characterizations of the thin films are also carried out by Fourier transformed infrared spectroscopy (FT-IR) and by X-ray photoelectron spectroscopy (XPS). Finally, the observed morphological, chemical and structural modifications induce changes in the electrical properties of the film. Results are discussed and indicate that the electrical properties of the synthesized SnO2 thin films differ as a function of the MOCVD precursor used for the deposition. Therefore, their electrochemical behavior is modified which influences the grafting of organic molecules to synthesize and develop novel hybrid materials sensors

High-Entropy Metal Diborides: A New Class of High-Entropy Materials and a New Type of Ultrahigh Temperature Ceramics

Seven equimolar, five-component, metal diborides were fabricated via high-energy ball milling and spark plasma sintering. Six of them, including (Hf(0.2)Zr(0.2)Ta(0.2)Nb(0.2)Ti(0.2))B(2), (Hf(0.2)Zr(0.2)Ta(0.2)Mo(0.2)Ti(0.2))B(2), (Hf(0.2)Zr(0.2)Mo(0.2)Nb(0.2)Ti(0.2))B(2), (Hf(0.2)Mo(0.2)Ta(0.2)Nb(0.2)Ti(0.2))B(2), (Mo(0.2)Zr(0.2)Ta(0.2)Nb(0.2)Ti(0.2))B(2), and (Hf(0.2)Zr(0.2)Ta(0.2)Cr(0.2)Ti(0.2))B(2), possess virtually one solid-solution boride phase of the hexagonal AlB(2) structure. Revised Hume-Rothery size-difference factors are used to rationalize the formation of high-entropy solid solutions in these metal diborides. Greater than 92% of the theoretical densities have been generally achieved with largely uniform compositions from nanoscale to microscale. Aberration-corrected scanning transmission electron microscopy (AC STEM), with high-angle annular dark-field and annular bright-field (HAADF and ABF) imaging and nanoscale compositional mapping, has been conducted to confirm the formation of 2-D high-entropy metal layers, separated by rigid 2-D boron nets, without any detectable layered segregation along the c-axis. These materials represent a new type of ultra-high temperature ceramics (UHTCs) as well as a new class of high-entropy materials, which not only exemplify the first high-entropy non-oxide ceramics (borides) fabricated but also possess a unique non-cubic (hexagonal) and layered (quasi-2D) high-entropy crystal structure that markedly differs from all those reported in prior studies. Initial property assessments show that both the hardness and the oxidation resistance of these high-entropy metal diborides are generally higher/better than the average performances of five individual metal diborides made by identical fabrication processing

Microstructural Evolution and Mechanical Properties of (Mg,Co,Ni,Cu,Zn)O High-Entropy Ceramics

The reaction sequence and mechanical properties were studied for (Mg,Co,Ni,Cu,Zn)O high-entropy ceramics that were synthesized using field-assisted sintering technology. The evolution from binary oxide starting powders to a single-phase rock salt structure exhibited a distinct incorporation sequence. For the rock salt oxides, MgO and CoO had the lowest vacancy formation energies and were the first to be incorporated into the high-entropy ceramic followed by NiO, which had a higher vacancy formation energy. Both CuO and ZnO had different crystal structures, and were incorporated into the single phase structure after the rock salt oxides due to the additional energy barrier associated with the transformations from their original structures to the rock salt structure. Distinctive morphological features including Cu-rich regions and lattice distortion were observed in the high entropy ceramic. In addition, a trade-off between densification and grain growth resulted in a maximum in strength (323 MPa) and elastic modulus (108 GPa) after densification at 900°C. This study has revealed new information that can be used to design other high-entropy ceramics including selection criteria for constituent compounds based on crystal structure and defect formation energy as well as the effects of grain size and porosity on control strength and elastic modulus