Showing papers by "Silke Paschen published in 2003"

Electrochemical deposition of Bi2Te3 for thermoelectric microdevices

Abstract: The electrolyses of solutions of bismuth oxide and tellurium oxide in nitric acid with molar ratios of Bi:Te=3:3–4:3 lead to cathodic deposits of films of bismuth telluride (Bi2Te3), an n-type semiconductor. Current densities of 2–5 mA/cm2 were applied. Voltammetric investigations showed that Bi2Te3 deposition occurred at potentials more negative than −0.125 V (Ag/AgCl, 3 M KCl). The deposit was identified as bismuth telluride (γ-phase) by X-ray analysis. Hall-effect measurements verified the n-type semiconducting behaviour. The films can be deposited in microstructures for thermoelectric microdevices like thermoelectric batteries or thermoelectric sensors.

Are type-I clathrates Zintl phases and 'phonon glasses and electron single crystals'?

Abstract: We discuss to which extent the concepts of Zintl phases and of ‘phonon glasses and electron single crystals’ apply to type-I clathrates. In (β-) Eu 8 Ga 16 Ge 30 the presence of residual charge carriers appears to be related to a slight off-stoichiometry of the samples pointing to the validity of the Zintl concept in stoichiometric samples. The low and almost stoichiometry independent mobilities of (β-) Eu 8 Ga 16 Ge 30 , Sr 8 Ga 16 Ge 30 , and Ba 8 Ga 16 Ge 30 seriously question the validity of the ‘electron single crystal’ concept for type-I clathrates. The temperature dependence of the thermal conductivity of a Ba 8 Ga 16 Ge 30 single crystal indicates that tunneling states play a central role in producing ‘phonon glass’-like thermal conductivities.

Magnetic, thermodynamic, NMR, and transport properties of the heavy-fermion semiconductor U2Ru2Sn

Abstract: We have measured the magnetic susceptibility, specific heat. NMR, electrical resistivity, magnetoresistance, Hall effect, thermoelectric power, and thermal conductivity of polycrystalline U 2 Ru 2 Sn. Some of these properties are compared to those of Th 2 Ru 2 Sn. The experimental data indicate the formation of a narrow energy gap of approximately k B × 160 K in U 2 Ru 2 Sn. Similarities to the behavior of heavy-fermion semiconductors (Kondo insulators) are observed, in particular to CeNiSn. Thus, we believe that U 2 Ru 2 Sn may be classified as a heavy-fermion semiconductor.

Gap formation in the semimetal U2Ru2Sn: evidence from 119Sn NMR investigations

Abstract: We report measurements of the 119 Sn nuclear spin-lattice relaxation rate 1/T1 and the Knight shift K for polycrystalline U2Ru2Sn powder samples as a function of temperature. The T dependence of 1/T1 is very similar to that of CeNiSn with the only difference that the temperature scale is increased roughly by a factor of 10. Therefore a first estimate gives a (pseudo-) gap value of Δ/k B ≈140 K ( ≈14 K for CeNiSn). At low temperatures similar to CeNiSn a T1T=constant behaviour is observed which indicates the presence of a residual density of states at the Fermi level N(EF) in the pseudo-gap. Knight shift measurements on oriented polycrystalline powder samples show nearly no magnetic anisotropy, whereas the temperature dependence roughly follows the behaviour of the magnetic susceptibility. First investigations on the non-magnetic reference compound Th2Ru2Sn are also discussed.

Crystal structure of barium europium germanide, Ba6-xEuxGe25 (x = 0.6), a chiral clathrate

Abstract: Ba5.4oEuo.6oGe25, cubic, P4i32 (No. 213), a = 14.5271(2) Â, V= 3065.8Â, Ζ = 4, Rgt(F) = 0.055, wR^F) = 0.091, T= 295 K. Source of material The samples Ba6-.vEu.vGe25 (A = 0.1,0.2,0.4,0.5,0.6,0.8,2.0) were prepared by melting the elements in an open glassy carbon crucible (HF furnace, argon atmosphere) and annealing at 923 Κ (52 d). The materials are silvery metallic, brittle, and stable in air and moisture. The ICP-AES chemical analysis of the alloy with χ = 0.6 resulted in the composition Ba5.48(5)Euo.57(2)Ge24.9(2). Experimental details The lattice parameters of Bas.4Euo.6Ge25 were determined from the least-squares refinement of the 20 values of 140 reflections (powder data, 18°<26>< 100°, À(CuKai)= 1.540598 Â;LaB6 standard, a = 4.15695(6) Á). The rather large elongation of the displacement ellipsoid for the M2 site (Table 3) is typical for all members of the BaóIruGen family [1-10]. ForBa5.4Euo.6Ge25, the electron density distribution around the M2 position was additionally modelled using two split sites (M2' and M2\", Table 2). The lattice constant of Ba6-AEu.vGe25 alloys (x=0.1 to 2.0) depends on the composition and a phase range could be proven with the maximum solubility corresponding tox=0.6 [11]. Discussion Ba5.4Euo.öGe25 belongs to the Ba6ln4Ge2i structure type (Pearson symbol cP124) [ 1,2]. The structure is characterized by a 3D chiral framework of condensed Ge20 pentagondodecahedra (pdods) forming a 3D channel labyrinth (figure, top). Each pdod is centered by Ml . M2 and M3 are located in the cavities of the zeolite-like channels. The characteristic germanium environment around each of the metal sites is illustrated in the figure (bottom). The structure analysis indicates that Eu occupies partially the Ml and M3 sites. The refined Ba : Eu ratio at Ml (85.7(4) : 14.3) and at M3 (68.5(4) : 31.5) positions results in the chemical composition Ba5.40(4)Euo.6oGe25. The Ge—Ge bond lengths vary in the range 2.471(2) À to 2.565(3) Á [in particular d(Ge2—Ge2) = 2.494(5)  andí/(Gel—Gel) = 2.565(3) À], The Ge3 and Ge5 positions are three-fold bonded (3b), and the remaining Ge positions are four-fold bonded (4b). Magnetization and magnetic susceptibility measurements indicate an oxidation state for europium of 2+ [ 12]. In terms of the Zintl concept, the formula Bas.4Euo.6Ge25 might be written as [(Ba/Eu)]6[(3b)Gel8[(4b)Ge°]i7(4el. Thus, the title compound is a Zintl phase with few conduction electrons. This is confirmed by the electrical resistivity data, showing metall-like behavior above 230 K. The transport properties of Ba6-vEuAGe25 alloys are modified by europium insertion, and in comparison with BaéGe25, the two-step first-order phase transition at Tsi,s2 ~ 180 K, 220 Κ is quickly suppressed with increasing Eu content [11,12]. * Correspondence author (e-mail: carrillo@cpfs.mpg.de) 398 Ba6-.vEu.vGe25 (χ = 0.6) Table 1. Data collection and handling. silvery chunk, size 0.16 χ 0.23 χ 0.24 mm Ag Ka radiation (0.56087 Â) 168.65 cm\" Stoe IPDS, 268 exposures, Αφ = 0.6° 47.78° 31729, 1620 U s > 2 a(Iobs), 1386 56 SHELXL-97 [13], ATOMS [14] Table 3. Atomic coordinates and displacement parameters (in  ' ) .

First results on U2Ru2Sn single crystals

Abstract: The tetragonal compound U2Ru2Sn shares several properties with the strongly correlated semimetal CeNiSn. Here we present, for the first time, measurements of magnetic, thermal, and electrical transport properties of single-crystalline samples and compare them to results on polycrystals. The specific heat and the magnetic susceptibility provide evidence for the opening of an energy gap of approximately 150–160 K: Both the magnetic susceptibility and the electrical resistivity are anisotropic. The c-axis is the easy magnetic axis along which the resistivity is ‘metal-like’. The resistivity along the a-axis is ‘semiconductor-like’. r 2003 Elsevier Science B.V. All rights reserved.

High-temperature thermoelectric properties of /spl alpha/- and /spl beta/-Eu/sub 8/Ga/sub 16/Ge/sub 30/

Abstract: We present the thermoelectric properties of an /spl alpha/-Eu/sub 8/Ga/sub 16/Ge/sub 30/ and a /spl beta/-Eu/sub 8/Ga/sub 16/Ge/sub 30/ sample with a Ga/Ge ratio that deviates slightly from the ideal 16/30 ratio. Thermopower and resistivity have been measured up to 750 K. From an extrapolation of the thermal conductivity, measured up to 200 K, to higher temperatures we estimate the dimensionless figure of merit to have a maximum of 0.55 and 1.1 at 700 K and 750 K for the /spl alpha/ and /spl beta/ sample, respectively. The thermoelectric properties of the a sample are, at highest temperatures, degraded by the excitation of carriers into the conduction band.

High-temperature thermoelectric properties of a- and PEu,Ga,,Ge,

Abstract: We present the thermoelectric properties of an aEu,Ga,,Ge,, and a p Eu,Ga,,Ge,, sample with a GdGe ratio that deviates slightly from the ideal 16/30 ratio. Thermopower and resistivity have been measured up to 750 K. From an extrapolation of the thermal conductivity. measured up to 200 K, to higher temperatures we estimate the dimensionless figure of merit to have a maximum of 0.55 and 1.1 at 700 K and 750 K for the a and p sample, respectively. The thermoelectric properties of the a sample are, at highest temperatures, degraded by the excitation of carriers into the conduction band.

Physical properties of the skutterudites (Ce,La)Fe/sub 4-x/Ni/sub x/Sb/sub 12/

Abstract: We have explained the electrical transport experiments of the solid solution Ce/sub l-z/La/sub z/Fe/sub 4/Sb/sub 12/ with crystal electric field and Kondo effect. We also have shown that the transport and magnetic properties of the solid solutions R/sub y/Fe/sub 4-x/Ni/sub x/Sb/sub 12/ (R = and La) are strongly depending to the charge carrier concentration. At room temperature, the thermopower is essentially due to the transition metal density of states in both the Cc and the La containing compounds. Finally, we have investigated the thermoelectric properties of the solid solution Ce/sub y/Fe/sub 4-x/Ni/sub x/Sb/sub 12/ with y = 4/3-2/spl times//3 for which the best thermoelectric properties were found for y /spl ges/ 0.6 and also for CeFe/sub 4/Sb/sub 12/, at room temperature.