E. Bauer

Improved thermoelectric properties in double-filled Cey∕2Yby∕2Fe4−x(Co∕Ni)xSb12 skutterudites

We have investigated the thermal and electrical properties of filled skutterudites belonging to the series Ce1−zYbzFe4Sb12 and Cey∕2Yby∕2Fe4−x(Co∕Ni)xSb12, as well as their potential for thermoelectric power generation. In the first series, increasing the Yb fraction decreases the resistivity and thermopower. These effects are related to the variations of valence of Yb with the Yb fraction (from 2.2 in Yb0.92Fe4Sb12 to 2.7 in Ce0.85Yb0.05Fe4Sb12). The power factor is increased in Ce0.40Yb0.53Fe4Sb12 while the thermal conductivity is reduced and this leads to an improved figure of merit in this compound when compared to Ce0.85Fe4Sb12. Co or Ni substitution was used to tune the carrier concentration and improve the power factor: we measure a ZT=0.5 at 500K in Ce0.44Yb0.32Fe3.02Co0.98Sb12 and an extrapolation leads to ZT=0.95 at 800K, which is close to state-of-the-art Ce0.28Fe1.52Co2.48Sb12 although the composition is far from being optimized. Double filling happens to be a promising path to provide good th...

Atomic ordering and thermoelectric properties of the n-type clathrate Ba8Ni3.5Ge42.1□0.4

Single crystals of the type-I clathrate Ba(8)Ni(3.5)Ge(42.1)square(0.4) (space group Pm3n, no. 223, a = 10.798(2) A, l = 30 mm, slashed circle = 8 mm) were grown from the melt using the Bridgman technique. Their composition, determined by microprobe analysis, reveals a distinctly lower Ni content than previously reported for the lower limit (x = 5.4) of the homogeneity range of the clathrate-I phase Ba(8)Ni(x)Ge(46-x). From single crystal X-ray diffraction data we introduce a crystal structure model that takes point defects (vacancies) square in the Ge network into account. It reveals that both Ni and square accumulate at a single site (6c) and that, as a consequence, the Ge network distorts considerably. Ba(8)Ni(3.5)Ge(42.1)square(0.4) shows metal-like behaviour (drho/dT > 0) albeit with high resistivity at room temperature (rho(300 K) approximately 1 mOmega cm). Together with the low charge carrier concentration of 2.3 e(-)/unit cell at 300 K this is typical of a degenerate semiconductor. The lattice thermal conductivity is distinctly smaller than that of Ba(8)Ge(43)square(3), where the vacancies partially order, and smaller than those of Ba-Ni-Ge type-I clathrates without vacancies, suggesting that disordered vacancies efficiently scatter heat-transporting phonons. We provide evidence that the maximum value of the thermoelectric figure of merit reached in Ba(8)Ni(3.5)Ge(42.1)square(0.4), ZT(680 K) congruent with 0.21, can be further improved by adjusting the charge carrier concentration.

High thermoelectric performance of triple-filled n-type skutterudites (Sr,Ba,Yb)yCo4Sb12

The influences of single, double and triple filling of Yb, Ba and Sr in CoSb3 on thermoelectric performance were evaluated. A significant suppression of lattice thermal conductivity κl is achieved by the increase in the number of filler elements. The highest power factor appears in triple-filled (Ba0.1Yb0.1)0.25(Sr0.1Yb0.1)0.75Co4Sb12, whereas single-filled Ba0.23Co4Sb12 has higher S2/ρ than double-filled (Ba,Yb)0.2Co4Sb12 forming a sequence S2/ρ(triple filling) > S2/ρ(single filling) > S2/ρ(double filling). However, the respective figures of merit follow a sequence ZT(triple filling) > ZT(double filling) > ZT(single filling) proving a strong correlation with κl that appears to be a predominant factor in the efficiency of filled skutterudites. Thus, κ and ZT increase with increasing lattice thermal resistivity (1/κl) and maximum efficiencies may be obtained for 1/κl > 0.2 cm K mW−1.

The transport properties of RCo2 compounds

The transport properties of intermetallic compounds RCo2 (R=Pr, Nd, Sm, Gd-Lu, Sc and Y) in the temperature range from 4.2 up to 1000 K were studied and different contributions were determined. In the non-magnetic compounds the spin fluctuation contribution to the electrical resistivity, rho sf varies as (T/Tsf)2, and thermal resistivity, Wsf varies as (T/Tsf), were found to follow the theoretical predictions. The spin disorder contribution arising from the scattering of conduction electrons on 4f moments was deduced and found to be proportional to the de Gennes factor. It is shown that the spin fluctuations are responsible for the low-temperature features of the temperature-dependent thermopower, whereas the high-temperature behaviour is mainly determined by the density of states function. In the paramagnetic region near the Curie temperature, the critical fluctuations cause a considerable enhancement of the electrical resistivity of the magnetic RCo2 compounds, which is manifested by pronounced upturns in rho versus T dependences.

Structure and physical properties of type-I clathrate solid-solution Ba 8 Pt x Ge 46 − x − y ◻ y ( ◻ = vacancy )

Formation, crystal chemistry, and physical properties were investigated for the solid-solution Ba{sub 8}Pt{sub x}Ge{sub 46-x-y}{open_square}{sub y} ({open_square} is a vacancy) deriving from binary clathrate Ba{sub 8}Ge{sub 43}{open_square}{sub 3} with a solubility limit of {approx}3.5 Pt atoms/f.u. at T=800 deg. C. Structural investigations throughout the homogeneity region confirm isotypism with the cubic primitive clathrate type-I structure (space group type Pm3n) and lattice parameters ranging from a=1.0657(2) nm for Ba{sub 8}Ge{sub 43}{open_square}{sub 3} to a=1.0752(2) nm for Ba{sub 8}Pt{sub 3.5}Ge{sub 41.5}{open_square}{sub 1.0}. Phase relations for the region concerning the clathrate solution were derived at subsolidus temperatures as well as at 800 deg. C. Transport properties evidence electrons as the majority charge carriers in the system with a slight dependency on the Pt content. The system is located close to a semiconducting regime with a gap in the electronic density of states of a few thousand K. No low temperature maximum is obvious from thermal conductivity which is dominated by the lattice contribution. Thermal conductivity furthermore documents a high efficiency of phonon scattering on vacancies.

Chemical properties and thermopower of the new series of skutterudite Ce1-pYbpFe4Sb12

Abstract We report on sample preparation and electron microprobe analysis in the series Ce 1− p Yb p Fe 4 Sb 12 that shows that a solid solution exists between CeFe 4 Sb 12 and YbFe 4 Sb 12 for all compositions in the series. The valence states of Ce and Yb have been studied using X-ray absorption spectroscopy. Ce is trivalent throughout the series, whereas the Yb valence linearly changes from 2.16 for Yb 0.93 Fe 4 Sb 12.08 to 2.71 for Ce 0.85 Yb 0.05 Fe 4 Sb 12.06 . The Seebeck coefficient was measured from 4 K to room temperature. It is enhanced by more than 20% at room temperature in Ce–Yb mixed compounds as compared to CeFe 4 Sb 12 or YbFe 4 Sb 12 .

Structural chemistry, magnetism and thermodynamic properties of R2Pd2In

We report on magnetisation, resistivity and specific heat measurements of R2Pd2In compounds synthesised with the nominal composition R40Pd41In19 (R=La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y). Magnetic and thermodynamic measurements reveal antiferromagnetic order below 32 K for this series except for Y, La, Yb and Lu. An appreciably wide homogeneity range is found for Ce2Pd2+xIn1−x where ferro- or antiferromagnetic order or both occur with typical features of a Kondo lattice. Yb2Pd2In exhibits intermediate valent behaviour and no magnetic order could be detected down to 0.3 K.

CeCu4Ga: A high γ heavy fermion compound

Abstract In the search for new Ce intermetallics which display unusual physical properties we have identified CeCu4Ga as a new example of a heavy fermion compound. CeCu4Ga is characterized by an extremely enhanced electronic contribution to the specific heat which exceeds 3000 mJ/mol K2 at 0.9 K. The Ce ion in this hexagonal compound is nearly trivalent. The Kondo temperature estimated from low temperature susceptibility and specific heat data is of the order of a few kelvin.

Structural, magnetic, electronic and transport properties of NdCu2

NdCu2 crystallizes in the orthorhombic CeCu2 structure. Using temperature dependent X-ray diffraction the variations of the lattice parameters with temperature have been measured and compared with those of YCu2. A magnetic ordering temperature at TN=6.5+or-0.1 K and a spin-reorientation temperature (TR=4.1+or-0.1 K) have been observed in the susceptibility, specific heat and resistivity. Further field-induced magnetic transitions around 1, 3 and 15 T (depending on the measuring temperature) were found in the magnetization and magnetoresistance. A preliminary magnetic phase diagram of NdCu2 is presented. The crystal field excitation temperatures determined by inelastic neutron scattering experiments are E1/kB=34 K, E2/kB=58 K, E3/kB=84 K and E4/kB=164 K. For the determination of the nine crystal field parameters Vlm the superposition model has been applied. The deduced set of parameters was used to calculate the crystal field influence on the magnetization, specific heat, thermal expansion and electrical resistivity. From an analysis of the X-ray powder diffraction data the lattice constants a=4.3844 AA, b=7.0350 AA and c=7.4186 AA and the atomic positional parameters (yCu=0.0506, zCu=0.1659, zNd=0.5383) inside the unit cell were obtained.

A crossover from the magnetic Kondo compound CeCu5 to the heavy Fermion compounds CeCu4Al and CeCu3Al2

Abstract We present measurements on the isostructural series CeCu 5 , CeCu 4 Al and CeCu 3 Al 2 , which demonstrate the supression of the antiferromagnetic phase transition at 4 K in CeCu 5 , due to the substitution of Cu by Al in CeCu 4 Al and CeCu 3 Al 2 . This substitution, however, is linked to a dramatic enhancement of properties reflecting the electronic density of states.