Invited article: A round robin test of the uncertainty on the measurement of the thermoelectric dimensionless figure of merit of Co0.97Ni0.03Sb3.

A round robin test aiming at measuring the high-temperature thermoelectric properties was carried out by a group of European (mainly French) laboratories (labs). Polycrystalline skutterudite Co0.97Ni0.03Sb3 was characterized by Seebeck coefficient (8 labs), electrical resistivity (9 labs), thermal diffusivity (6 labs), mass volume density (6 labs), and specific heat (6 labs) measurements. These data were statistically processed to determine the uncertainty on all these measured quantities as a function of temperature and combined to obtain an overall uncertainty on the thermal conductivity (product of thermal diffusivity by density and by specific heat) and on the thermoelectric figure of merit ZT. An increase with temperature of all these uncertainties is observed, in agreement with growing difficulties to measure these quantities when temperature increases. The uncertainties on the electrical resistivity and thermal diffusivity are most likely dominated by the uncertainty on the sample dimensions. The temperature-averaged (300-700 K) relative standard uncertainties at the confidence level of 68% amount to 6%, 8%, 11%, and 19% for the Seebeck coefficient, electrical resistivity, thermal conductivity, and figure of merit ZT, respectively. Thermal conductivity measurements appear as the least accurate. The moderate value of the temperature-averaged relative expanded (confidence level of 95%) uncertainty of 17% on the mean of ZT is essential in establishing Co0.97Ni0.03Sb3 as a high temperature standard n-type thermoelectric material.

Evidence of a structural phase transition in superconducting SmFeAsO(1-x)F(x) from 19F NMR.

We report resistivity, magnetization and (19)F NMR results in a polycrystalline sample of SmFeAsO(0.86)F(0.14). The resistivity and magnetization data show a sharp drop at 48 K indicating a superconducting transition. The nuclear spin-lattice rate (1/T(1)) and spin-spin relaxation rate (1/T(2)) clearly show the existence of a structural phase transition near 163 K in the sample, which also undergoes a superconducting transition. This finding creates interest in exploring whether this is unique for Sm based systems or is also present in other rare-earth based 1111 superconductors.

Effects of Co doping on the transport properties and superconductivity in CeFe(1-x)Co(x)AsO.

A series of CeFe(1-x)Co(x)AsO oxyarsenide compounds with Co doping on iron sites (x = 0-0.2) have been synthesized by a solid state reaction method. The effects of Co doping on the electrical transport properties and superconductivity were analyzed with a special emphasis on the analysis of thermopower. Undoped CeFeAsO shows an electrical resistivity anomaly at about 150 K, which was ascribed to a spin-density-wave (SDW) instability. This anomaly is suppressed and a superconducting transition occurs at T(c) = 3.2 K in CeFe(0.95)Co(0.05)AsO, the maximum superconducting transition temperature (T(c)) of 12.5 K is observed in CeFe(0.90)Co(0.10)AsO, and the thermopower is increased by the Co doping. As has been previously suggested, the emergence of superconductivity seems to be closely linked to the thermopower, and there is a close correlation between T(c) and the thermopower, both showing a similar dome-like doping dependence.