Search

The crystal structure of the novel Al-rich intermetallic phase Ce4Pt9Al13 was determined from single-crystal X-ray diffraction data and confirmed by powder X-ray diffraction. The compound crystallizes with its own structure type, which is a “site exchange” variant of the Ho4Ir13Ge9-type (Pearson symbol oP52, space group Pmmn, No. 59, Z = 2, lattice parameters: a = 4.1826(1) A, b = 11.4424(3) A, c = 19.8475(5) A, V = 949.88(7) A3). The magnetic properties of Ce4Pt9Al13 were determined in the temperature range 1.72–400 K. Down to the lowest temperatures studied, the compound shows a Curie-Weiss paramagnetic behavior with strong crystalline electric field contribution observed below about 200 K. The electrical conductivity in Ce4Pt9Al13 has metallic character, and at low temperatures it is governed by an interplay of strong crystal field and Kondo interactions.

Skutterudites are known as excellent thermoelectric p- and n-type materials and have already achieved good efficiencies for the conversion of heat to electricity, but nevertheless researchers try to further enhance the figure of merit, ZT. In the present work the aim was to mix p- and n-type skutterudite powders with additives in order to produce an evenly dispersed distribution of sub micron grains, preferable small nano-particles, which enhance the scattering of the heat carrying phonons of different wavelengths and reduce thermal conductivity without changing electrical resistivity and Seebeck coefficient. Various quantities of three groups of materials (a) nonmetallic oxides (Al 2 O 3 ), (b) metallic oxides (Cu 2 O and La 1.85 Sr 0.15 CuO 4 ) and (c) metallic borides (Fe 2.25 Co 0.75 B and Ta 0.8 Zr 0.2 B) were added to industrially produced p-type (DD y Fe 3 CoSb 12 ) and n-type ((Mm,Sm) y Co 4 Sb 12 ) skutterudite powders. First the influence of pre-sieving and various ball milling conditions before hot pressing were studied, using Al 2 O 3 as additive. As a consequence of these studies pre-sieved powders and high-energy ball milling were used for all following experiments. The goal, to enhance ZT was not reached with Al 2 O 3 and Cu 2 O. La 1.85 Sr 0.15 CuO 4 was successful for the n-type, Fe 2.25 Co 0.75 B for the p-type skutterudites, although ZT-enhancement was small, but with Fe 2.25 Co 0.75 B the maximum ZT could be shifted to lower temperatures, a valuable information for device production. Much better results in respect to ZT values were gained with adding 0.5, 1.0 and 1.5 wt.% Ta 0.8 Zr 0.2 B to p-type DD y Fe 3 CoSb 12 . In this series it was possible to enhance ZT (from ZT ∼ 1.2 to ZT ∼ 1.3) as well as to significantly increase the thermal-electrical conversion efficiency η. In addition, we found that all boride additives enhanced the hardness, elastic moduli and fracture resistance.

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

After our successful new and energy-saving production route to prepare bulk skutterudites with high ZTs from commercial powders via cold pressing (CP) and high-pressure torsion (HPT), we present in this paper a detailed study of the influence of shear strain and the preparation parameters, especially the temperature, on the micro-structural, thermoelectric and mechanical properties by analyzing specimens cut from various positions of the HPT disks of (Sm,Mm)0.15Co4Sb12. Across these large discs (30 mm in diameter and about 1 mm thickness) the shear strain γ rises linearly from the center of the sample to the rim reaching γ ∼ 450 (for 5 revolutions), a value first time realized for skutterudites. HPT-straining is essential for the introduction of high densities of crystal lattice defects, particularly dislocations, which result in a significant refining of the grains. Whereas the processing temperature mainly influences the density of the HPT consolidated samples and as a consequence, physical and mechanical properties, the applied shear strain rules the thermoelectric parameters. As an important feature, the Seebeck coefficient remained roughly unchanged under strain. Although the increasing shear strain caused the thermal conductivity to significantly decrease, this beneficial effect is accompanied by a concomitant significant increase of the electrical resistivity. Consequently, the interplay of the two controversial influences results in a maximum and record high thermoelectric figure of merit ZT ∼2.1.

This paper documents that consolidation and subsequent severe plastic deformation of commercial skutterudite powders via high pressure torsion (HPT) serves for an easy, fast, cheap and sustainable production method for excellent n-type skutterudite bulks, (Sm,Mm)0.15Co4Sb12, with high ZT values. Large cylindrical samples with 30 mm in diameter and 8 mm thickness (∼55 g) were successfully prepared in one step. These large samples enabled for the first time to (i) study the influence of shear strain on the micro-structural, thermoelectric and mechanical properties by analysing specimens cut from various positions of the HPT disks and (ii) to evaluate these properties orientation sensitive. These investigations were supported by electron probe microanalyses, transmission electron microscopy as well as XPD full profile analyses, XPS, Raman and Hall measurements. Whilst the orientation dependence on the electrical and thermal conductivity is found to be rather weak, the applied shear strain has a pronounced influence, as with increasing strains the thermal conductivity was reduced to the targeted levels. This beneficial effect, however, is accompanied by a massive increase of the electrical resistivity. ZT values turned out to be rather homogeneous over the entire sample with ZT∼1.3 at 850 K.

Phase relations have been established by electron probe microanalysis (EPMA) and X-ray powder diffraction (XPD) for the Sr-poor part of four ternary systems: Sr–{Pd,Pt}–Si at 900 °C and Sr–{Pd,Pt}–Ge at 700 °C. In the Sr–Pd–Si system the formation of the silicide SrPdSi 3 (BaNiSn 3 -type) was confirmed and a small homogeneity region was found. Furthermore, two novel compounds were detected and their crystal structure was refined from X-ray powder patterns: SrPd 0.3 Si 1.7 (AlB 2 -type) and SrPd 5.9 Si 6.1 (own-type). In the Sr–Pt–Si ternary system a novel compound with AlB 2 -type was discovered (SrPt 0.3 Si 1.7 ), whereas SrPtSi 3 with the BaNiSn 3 -type was confirmed. Two more compounds were detected by EPMA, but their crystal structure remains unknown. In the Sr–{Pd,Pt}–Ge systems no new compounds were observed, but the existence of SrPdGe 3 and SrPtGe 3 (both adopt the BaNiSn 3 structure type), and SrPt 4 Ge 12 , crystallizing in the LaFe 4 Sb 12 structure type, was corroborated. For selected ternary silicides the electronic structure was evaluated by DFT calculations.

Phase relations for the Ba-poor part of the ternary system Ba–Zn–Sn at 500 °C were established by electron probe microanalysis (EPMA) and X-ray powder diffraction (XPD). Besides the already known compounds BaZnSn (ZrBeSi type) and BaZn 2 Sn 2 (CaBe 2 Ge 2 type), a novel ternary compound BaZnSn 2 was found to crystallize with the CeNiSi 2 -type. The precise determination (EPMA) of the homogeneity region at 500 °C for the compound BaZn 1− x Sn 1+ x with the ZrBeSi type (0.22 ⩽ x ⩽ 0.35) excludes the stoichiometric composition. The stannide BaZn 2− x Sn 2+ x ( x = 0.05) is characterized by mixed occupancy of Zn and Sn atoms in the 2 c site; accordingly the composition is slightly shifted from stoichiometry 1:2:2. Structural effects in the Ba–Zn–Sn compounds were explained on the basis of crystal chemistry analyses and electronic structure calculations employing the electron localization function ELF. The solubility of Sn in the clathrate phase Ba 8 Zn 7 Si 39 was found to be insignificant (around 1 at.%; 800 °C).

The crystal structure of the novel intermetallic Yb ∼3 Pt ∼4 Si 6− x was determined by single crystal X-ray diffraction: space group P 2 1 / m (No. 11); a = 8.4560(3) A, b = 4.2109(2) A, c = 12.7864(5) A, β = 99.537(2)°, R F = 0.046, wR2 = 0.132. The title compound is a disordered variant of the Y 3 Pt 4 Ge 6 type. It is characterized by the intergrowth of slabs cut from the ThCr 2 Si 2 and YIrGe 2 types of structures.

X-ray powder data for YbCu 2 Ge 2 , YbAg 2 Si 2 and YbAg 2 Ge 2 confirmed the atom order of the ThCr 2 Si 2 type (ordered BaAl 4 type) as reported earlier. YbCu 2 Ge 2 is an intermediate valence system with the Yb ions valence close to +2 at low temperatures, whereas the other two compounds are weakly temperature-dependent paramagnets. All these ternaries show metallic character of their electrical conductivity. The structure of Yb(Cu 1− x Zn x ) 2 Si 2 ( x = 0.65, 0.77) was found to correspond to the BaAl 4 type, assuming a random distribution of Cu and Zn atoms over the positions 4d (0, 1/2, 1/4). The Si atoms were found in the sites 4e (0, 0, z ). These two pseudoternary compounds are diamagnetic. A novel phase with the composition Yb(Ag 0.18 Si 0.82 ) 2 crystallizes in the ThSi 2 -type on the verge to a symmetry reduction towards orthorhombic GdSi 2 -type. This phase is a Pauli paramagnet and exhibits metallic behavior of the resistivity.

The crystal structure of Mn6Ni16Si7 was reinvestigated by X-ray powder, single crystal and neutron powder diffraction data in form of a combined refinement. In contrast to the various structure variants observed for titanium aluminium-based G-phases Ti6({Fe,Co,Ni}1−xAlx)23+1 as well as for Ti6Ni16+xSi7, Mn6Ni16Si7 was confirmed to crystallize as a simple ordered variant of the Th6Mn23 structure type (a = 1.11674(6) nm, Mg6Cu16Si7-type; space group F m 3 ¯ m ): Ni atoms adopt two 32f sites, Si atoms occupy 4a and 24d sites and Mn-atoms are located at 24e sites. A Fourier difference map proved the absence of atoms in the 4b site (1/2, 1/2, 1/2). Thus the crystal structure of Mn6Ni16Si7 is a true representative of the Mg6Cu16Si7-type.

The ternary intermetallic Ce 16 Ru 8 In 37 was synthesized by arc-melting from elemental Ce, Ru, and In under an argon atmosphere. The crystal structure of Ce 16 Ru 8 In 37 was determined from the single-crystal X-ray diffraction data: a = 4.7451(1) A, b = 9.3518(2) A, c = 32.6162(7) A; space group Immm , Z = 1. The architecture of the crystal structure of Ce 16 Ru 8 In 37 is based on CeIn 3 units with cubic Cu 3 Au-type and can be presented as consisting of two different slabs: CeIn 3 -like slab of composition Ce 4 In 14.5 and a slab of composition Ce 4 Ru 4 In 4 .

Phase relations in the entire ternary system Mo–Si–B were studied at subsolidus temperatures on alloys prepared by arc-melting and employing Pirani–Althertum melting point data, differential thermal analysis, light and electron microscopy, X-ray diffraction, and electron probe microanalysis including the light element boron. All isothermal reaction temperatures measured were used to construct two vertical sections and a Schulz–Scheil flow diagram monitoring solidification (crystallization) in thermodynamic equilibrium over the whole concentration range. The ternary compound Mo 5 SiB 2 exhibits a solubility range from 8 to 13 at.% Si at solidus temperatures. Stoichiometric Mo 5 SiB 2 forms from a pseudobinary peritectic reaction with a maximum tie line at 2130 °C. The phase diagram is presented as a melting diagram projection.