Bismuth Doping in Nanostructured Tetrahedrite: Scalable Synthesis and Thermoelectric Performance

In this study, we demonstrate the feasibility of Bi-doped tetrahedrite Cu12Sb4−xBixS13 (x = 0.02–0.20) synthesis in an industrial eccentric vibratory mill using Cu, Sb, Bi and S elemental precursors. High-energy milling was followed by spark plasma sintering. In all the samples, the prevailing content of tetrahedrite Cu12Sb4S13 (71–87%) and famatinite Cu3SbS4 (13–21%), together with small amounts of skinnerite Cu3SbS3, have been detected. The occurrence of the individual Cu-Sb-S phases and oxidation states of bismuth identified as Bi0 and Bi3+ are correlated. The most prominent effect of the simultaneous milling and doping on the thermoelectric properties is a decrease in the total thermal conductivity (κ) with increasing Bi content, in relation with the increasing amount of famatinite and skinnerite contents. The lowest value of κ was achieved for x = 0.2 (1.1 W m−1 K −1 at 675 K). However, this sample also manifests the lowest electrical conductivity σ, combined with relatively unchanged values for the Seebeck coefficient (S) compared with the un-doped sample. Overall, the lowered electrical performances outweigh the benefits from the decrease in thermal conductivity and the resulting figure-of-merit values illustrate a degradation effect of Bi doping on the thermoelectric properties of tetrahedrite in these synthesis conditions

Atomic and electronic structure of twin growth defects in magnetite

We report the existence of a stable twin defect in Fe3O4 thin films. By using aberration corrected scanning transmission electron microscopy and spectroscopy the atomic structure of the twin boundary has been determined. The boundary is confined to the (111) growth plane and it is non-stoichiometric due to a missing Fe octahedral plane. By first principles calculations we show that the local atomic structural configuration of the twin boundary does not change the nature of the superexchange interactions between the two Fe sublattices across the twin grain boundary. Besides decreasing the half-metallic band gap at the boundary the altered atomic stacking at the boundary does not change the overall ferromagnetic (FM) coupling between the grains

Atomic and electronic structure of twin growth defects in magnetite

We report the existence of a stable twin defect in Fe3O4 thin films. By using aberration corrected scanning transmission electron microscopy and spectroscopy the atomic structure of the twin boundary has been determined. The boundary is confined to the (111) growth plane and it is non-stoichiometric due to a missing Fe octahedral plane. By first principles calculations we show that the local atomic structural configuration of the twin boundary does not change the nature of the superexchange interactions between the two Fe sublattices across the twin grain boundary. Besides decreasing the half-metallic band gap at the boundary the altered atomic stacking at the boundary does not change the overall ferromagnetic (FM) coupling between the grains

How to best measure atomic sergregation to grain boundaries by analytical transmission electron microscopy

This study provides an overview of the recent experiments employing methods that analyse, systematically, series of analytical spectra acquired either in nanobeam mode in a transmission electron microscope or using elemental mapping in a scanning transmission electron microscope. A general framework is presented that describes how best to analyse series of such spectra to quantify the areal density of atoms contained within a very thin layer of a matrix material, as, for example, appropriate to measure grain boundary segregation. We show that a systematic quantification of spectra as a function of area size illuminated by the electron beam eliminates the large systematic errors inherent in simpler approaches based on spatial difference methods, integration of compositional profiles acquired with highly focused nanoprobes or simple repeats of such measurements. Our method has been successfully applied to study dopant segregation to inversion domain boundaries in ZnO, to quantify the thicknesses of sub-nm thin layers during epitaxial growth by molecular beam epitaxy of (In)GaAs and to prove the absence of gettering of dopants at Σ = 3{111} grain boundaries in Si, with a precision <1 atom/nm2 in all these cases

Spectroscopy investigation of nanostructured nickel-zinc ferrite obtained by mechanochemical synthesis

Nano crystalline samples of nickel zinc ferrite, Ni0.5Zn0.5Fe2O4 were prepared by mechanochemical route in a planetary ball mill starting from two mixtures of the appropriate quantities of the powders: case (1) oxide powders: NiO, ZnO and alpha-Fe2O3 in one case, and in the second case (2) hydroxide powders: Ni(OH)(2), Zn(OH)(2) and Fe(OH)(3). In order to monitor the progress of chemical reaction and confirm phase formation, powder samples obtained after 5 h and 10 h of milling were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman, IR and Mossbauer spectroscopy. It is shown that the soft mechanochemical method, i.e. mechanochemical activation of hydroxides, produces high quality single phase Ni0.5Zn0.5Fe2O4 samples in much more efficient way. From the IR spectroscopy of single phase samples it is obvious that energy of modes depends on the ratio of cations. The deconvolution of Raman spectra allows to separate contributions of different cations to a particular type of vibration and to estimate the degree of inversion. (C) 2014 Elsevier Ltd. All rights reserved

A new complex ternary phase in the Al-Cr-Sc push-pull alloy

International audienceThe purpose of this study is to find new ternary intermetallic compounds within the Al-Cr-Sc system, which is typical of a push-pull system in which two constituents (Cr and Sc) are immiscible, whilst they form compounds, respectively, with the third constituent (Al). By arc-melting of the three components aluminium (Al), chromium (Cr) and scandium (Sc) under inert atmosphere, a range of different alloys was produced. The microstructure was observed using scanning electron microscopy (SEM) and the phase compositions were analysed by electron dispersive X-ray spectroscopy (EDS). The samples were measured with X-ray diffraction (XRD) and transmission electron microscopy. The results were confirmed by Rietveld refinement. Three different phases were found in the microstructure of the samples, the two known binary phases Al 16 Cr 10 and Al 3 Sc and a new ternary phase coined f-Al 8 Cr 4 Sc. f-Al 8 Cr 4 Sc showed great similarities to other Al 8 Cr 4 RE compounds with tetragonal I4/mmm crystal structure. Starting from the structure of already known Al 8 Cr 4 RE crystals, ab initio calculations were performed to determine the crystallographic parameters of f-Al 8 Cr 4 Sc and also to investigate its electronic structure, which identified the energy band factor as the key factor that determines the stability of this compound. The lattice parameters and atomic positions were found in good agreement with the ones obtained by Rietveld refinement. Experimental atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image analysis of the f-Al 8 Cr 4 Sc phase confirmed the predicted atomic model

Abnormal grain growth of lead zirconium titanate (PZT) ceramics induced by the penetration twin

Lead zirconium titanate (PZT) ceramic specimens were prepared by liquid phase sintering with excess PbO. By the addition of a small amount of MgO, the grain shape was changed from spherical to angular. When SiO2 was further added, twin was induced in a few grains, which grew abnormally during heat treatment. Through the electron backscatter diffraction analysis and the observation of three-dimensional grain morphology, the abnormally grown large PZT grains were determined to be penetration twinned. Abnormal grain growth was suggested to be because of reentrant edges formed at the twinned grains.close3