Properties of HPT-Processed Large Bulks of p-Type Skutterudite DD0.7Fe3CoSb12 with ZT > 1.3

The influence of shear strain on the microstructural, physical, and mechanical properties was studied on large bulk samples (diameter: 30 mm, thickness: 1 or 8 mm), which were consolidated by high-pressure torsion (HPT) from a commercial powder DD0.7Fe3CoSb12. Particularly, the thick sample (mass similar to 53 g) allowed measuring the thermoelectric (TE) properties with respect to various orientations of the specimen in the sample. All data were compared with those of a hot-pressed (HP) reference sample, prepared with the same powder. Transmission electron microscopy, as well as X-ray powder diffraction profile analyses, Hall measurements, and positron annihilation spectroscopy, supported these investigations. Furthermore, synchrotron data for the temperature range from 300 to 825 K were used to evaluate the changes in the grain size and dislocation density as well as the thermal expansion coefficient via the change in the lattice parameter during heating. In addition, hardness and direct thermal expansion measurements of the HPT samples were performed and compared with the HP reference sample's values. With the increase of the shear strain from the center to the rim of the sample, the electrical resistivity becomes higher, whereas the thermal conductivity becomes lower, but the Seebeck coefficient remained almost unchanged. For the thin as well as thick samples, the enhanced electrical resistivity was balanced out by a decreased thermal conductivity such that the maximum ZT values (ZT = 1.3-1.35 at 856 K) do not vary much as a function of the shear strain throughout the sample, however, all ZTs are higher than that of the HP sample. The thermal-electric conversion efficiencies are in the range of 14-15% (for 423-823 K). With similar high ZT values for the n-type skutterudites, fabricated in the same fast and sustainable way, these p- and n-type skutterudites may serve as legs for TE generators, directly cut from the big HPT bulks.Peer reviewe

Boron site preference in ternary Ta and Nb boron suicides

X-ray single crystal (XSC) and neutron powder diffraction data (NPD) were used to elucidate boron site preference for five ternary phases. Ta3Si1-xBx (x=0.112(4)) crystallizes with the Ti3P-type (space group P4(2)/n) with B-atoms sharing the 8g site with Si atoms. Ta5Si3-x (x=0.03(1); Cr5B3- type) crystallizes with space group 14/mcm, exhibiting a small amount of vacancies on the 4 alpha site. Both, Ta-5(Si1-xBx)(3), X=0.568(3), and Nb-5(Si1-xBx)(3), x=0.59(2), are part of solid solutions of M5Si3 with Cr5B3-type into the ternary M-Si-B systems (M=Nb or Ta) with B replacing Si on the 8h site. The D8(8)-phase in the Nb-Si-B system crystallizes with the Ti5Ga4-type revealing the formula Nb5Si3B1-x (x=0.292(3)) with B partially filling the voids in the 2b site of the Mn5Si3 parent type. (C) 2012 Elsevier Inc. All rights reserved.Higher Education Commission of Pakistan (HEC)Austrian OEADFAPESP (Sao Paulo, Brazil)FAPESP (Sao Paulo, Brazil) [97/06348-4]European Commission [RII3-CT-2003-505925]European Commissio

On the constitution and thermodynamic modeling of the phase diagrams Nb-Mn and Ta-Mn

The constitution of the two phase diagrams Nb-Mn and Ta-Mn has been determined from light optical and transmission and scanning electron microscopy (LOM, TEM and SEM) with energy dispersive (EDX) as well as wavelength dispersive (WDX) X-ray spectroscopy, X-ray powder (XPD) and single crystal diffraction (XSCD), differential thermal analysis (DTA) and/or differential scanning calorimetry (DSC). The Laves phases NbMn2 and TaMn2 are the only binary compounds in these systems. High-temperature differential thermal analyses revealed congruent melting for NbMn2 with T,(NbMn2) = 1515 +/- 15 degrees C, whereas TaMn2 melts incongruently with T-m(TaMn2)= 1797 +/- 40 degrees C close to a depleted peritectic reaction. Both Laves phases engage in eutectic reactions l (Mn) + Nb(Ta)Mn-2 (T-eut = 1220 +/- 10 degrees C at 4.9 at% Nb and T-eut = 1234 +/- 10 degrees C at 0.7 at% Ta, respectively). NbMn2 also forms a eutectic with (Nb): l (Nb) + NbMn2 at T-eut = 1493 +/- 15 degrees C and 53.2 at% Nb. Mn shows remarkably large maximum solid solubilities of 19.4 at% Mn in (Nb) as well as of 21.3 at% Mn in (Ta). Detailed atom site distribution has been established for the Laves phases by means of temperature dependent X-ray single crystal data (both C14 - MgZn2-type). Combined data from XPD, EDX/WDX and SEM microstructure indicate that for both Laves phases extended homogeneity regions exist: Nb1+xMn2+x (62.5-73.0 at% Mn at 950 degrees C: -0.19 <= x <= 1.125) and Ta1+xMn2-x (59.5-68.5 at % Mn: -0.055 <= x <= 1.215). Density functional theory (DFT) calculations favor Nb(Ta)/Mn antisite occupation rather than defects. The phases, "NbMn" and "TaMn", adopted earlier in the literature as binary system inherent compounds, were shown (TEM, WDX electron microprobe data and X-ray Rietveld refinements) to be oxygen stabilized phases of the Ti4Ni2O type (so-called eta(eta)-phases) with modified Nb(Ta)/Mn site substitution to comply with the formula Nb(Ta)(3-x)Mn3+xO1-y (defect eta-W3Fe3C-type). From magnetic susceptibility and magnetization measurements, both oxide stabilized eta phases eta-Nb3Mn3O1-y and eta-Ta3Mn3O1-y were found to order ferromagnetically below T-c similar to 77 K, but the Laves phases NbMn2, TaMn2 reveal weakly temperature dependent paramagnetism. No trace of the rhombohedral kyphase (W6Fe7-type) has been encountered in our investigation of the two binary phase diagrams. Thermodynamic and transport properties (specific heat, electrical resistivity and magnetic susceptibility/magnetization) classify the Laves phases with metallic behavior whilst mechanical properties (elastic moduli from DFT and nanoindentation as well as hardness and thermal expansion) group both Laves phases among rather hard and brittle intermetallics. Based on (i) the experimentally derived constitution of the Nb-Mn and Ta-Mn systems, and (ii) on new own DFT data of the energy of formation of the Laves phases, a CALPHAD (CALculation of PHAse Diagrams) calculation of both systems was made providing a complete set of optimized thermodynamic data. Furthermore, the DFT calculations provided information on the instability of the eta-Ta3Mn3 structure and the atom-site specific stabilization effect of oxygen.Web of Science865art. no. 15871

Crystal chemistry of the G-phases in the systems Ti–{Fe, Co, Ni}–Al with a novel filled variant of the Th6Mn23-type

he crystal structures of the compounds Ti6M7Al17 with M=Fe,Co,Ni have been investigated by X-ray powder and single crystal, neutron powder and electron diffraction. These compounds crystallize with a new filled variant of the Mg6Cu16Si7-type (space group Image). A close structural relation is encountered for the series of crystal structures: Th6Mn16Mn7?, Mg6Cu16Si7?, Sc6Sc16Ir7Ir, Zr6Zn16Zn7Si, Ti6Al16M7Al, and Ti6Ni16Si7Si. Electrical resistivity of the Fe, Co-based compounds is typical metallic and temperature dependence follows the Bloch Grüneisen relation with a Debye temperature of about 300 K. No hydrogen was found to be absorbed in the Fe-, and Co-based compounds at room temperature under a H2 pressure of 5 MPa

The system Ce–Zn–Si for <33.3 at.% Ce: phase relations, crystal structures and physical properties

Phase equilibria of the system Ce–Zn–Si have been determined for the isothermal section at 600 °C for <33.3 at.% Ce by XRPD and EPMA. This partial section is characterized by the formation of five ternary compounds with homogeneity regions at constant Ce-content and partial substitution of Zn/Si: τ1-Ce7Zn21(Zn1−xSix)2 (unique type; 0.45 ≤ x ≤ 0.99), τ2-Ce(Si1−xZnx)2 (AlB2-type; 0.36 ≤ x ≤ 0.73), τ5-CeZn(Zn1−xSix)2 (CeNiSi2-type; 0.68 ≤ x ≤ 0.76), τ6-CeZn2(Si1−xZnx)2 (ThCr2Si2-type; 0.25 ≤ x ≤ 0.30) and τ7-Ce37Zn48Si15 (structure unknown). Whereas τ1, τ2 and τ5 are stable at 600 and 800 °C, the phases τ6, τ7 are unstable at 800 °C. Atom site distribution in the crystal structures of τ5, τ6 and {La,Ce}7Zn21(Zn1−xGex)2 have been elucidated from X-ray intensity refinements on single crystals. The small amounts of the stabilizing tetrel element in {La,Ce}7Zn21[Zn1−xSi(Ge)x]2 suggest a hypothetical binary phase “{La,Ce}7Zn23”. The stabilizing effect of Ge in Ce7Zn23−xGex has been elucidated from density functional theory (DFT) calculations discussing the electronic structure in terms of the density of states (DOS) and defining enthalpies of formation for Ce7Zn23−xGex (x = 0, 0.5, 2) as well as for several neighbouring binary Ce–Zn phases. A Schultz–Scheil diagram for the solidification behaviour in the (Zn,Si)-rich part of the diagram was constructed from DTA measurements in closed silica crucibles along with partial isothermal sections determined in the temperature range from 400 to 900 °C. The phases τ5 and τ6 both form in degenerate ternary peritectic reactions: L + CeSi2,β-Ce2Zn17 ⇔ τ5 at 865 ± 5 °C and L + τ5,CeZn11 ⇔ τ6 at 695 ± 5 °C, respectively. Magnetic susceptibility, specific heat and resistivity measurements of τ5-CeZn(Zn1−xSix)2 revealed Kondo lattice behavior with ferromagnetic ordering below TC = 4.4 K, whereas susceptibility and specific heat studies of τ6-CeZn2(Zn0.28Si0.72)2 revealed Curie–Weiss paramagnetic behaviour down to 3 K. The effective paramagnetic moments of Ce obtained from Curie–Weiss fits of τ5 (2.50μB) and τ6 (2.34μB) reveal a ground state close to trivalent Ce