Nathan D. Lowhorn

A high-throughput thermoelectric power-factor screening tool for rapid construction of thermoelectric property diagrams

The authors have developed a high-throughput screening tool that maps out thermoelectric power factors of combinatorial composition-spread film libraries. The screening tool allows one to measure the electrical conductivity and Seebeck coefficient of over 1000 sample points within 6h. Seebeck coefficients of standard films measured with the screening tool are in good agreement with those measured by traditional thermoelectric measurement apparatus. The rapid construction of thermoelectric property diagrams is illustrated for two systems: (Zn, Al)–O binary composition-spread film on Al2O3 (0001) and (Ca,Sr,La)3Co4O9 ternary composition-spread film on Si (100).

Phase compatibility and thermoelectric properties of compounds in the Sr–Ca–Co–O system

Two low-dimensional cobaltite series in the Sr–Ca–Co–O system have been investigated for their solid solution limit, structure, and compatibility phase relationships (850u2009°C in air). Thermoelectric properties have been measured for selected members of these solid solutions. In (Ca,Sr)3Co4O9, which has a misfit layered structure, Sr was found to substitute in the Ca site to a limit of (Ca0.8Sr0.2)3Co4O9. Compounds in the homologous series, An+2ConCo′O3n+3 [where A=Sr, Ca, (Ca,Sr), or (Sr,Ca)], consist of one-dimensional parallel Co2O66− chains that are built from successive alternating face-sharing CoO6 trigonal prisms and “n” units of CoO6 octahedra along the hexagonal c-axis. In the Can+2ConCo′O3n+3 series, only the n=1 phase (Ca3Co2O6) could be prepared under the present synthesis conditions. Sr substitutes in the Ca site of Ca3Co2O6 to a limit of (Ca0.9Sr0.1)3Co2O6. In the Srn+2ConCo′O3n+3 series, Ca substitutes in the Sr site of the n=2, 3, and 4 members to a limit of (Sr0.7Ca0.3)4Co3O9, (Sr0.67Ca0.33...

Texture and phase analysis of a Ca3Co4O9∕Si (100) thermoelectric film

This paper reports the texture analysis as well as the identification of two crystalline phases between a thin film of monoclinic Ca3Co4O9 and a cubic (100) Si substrate, using a diffractometer equipped with a two-dimensional area detector. No reflections other than 00l were observed in the symmetric configuration using an x-ray powder diffraction scan (Bragg-Brentano geometry). Pole figures collected for six reflections using asymmetric configurations did not show ab-plane epitaxial relationships between the film and the substrate. These results establish the Ca3Co4O9 fiber texture of the film with the (001) pole parallel to the surface normal. Single-crystal-like second phases, CaCoSi2O6 and CoO, presumably the interface reaction products of Ca3Co4O9 with the substrate Si, were identified. The near four-fold symmetry and the similar intensity displayed by the 220 reflection from the CaCoSi2O6 structure indicated an epitaxial relation between CaCoSi2O6 and Si, with four symmetry-induced variants being ge...

Enhancement of the power factor of the transition metal pentatelluride HfTe5 by rare-earth doping

The transition metal pentatellurides HfTe5 and ZrTe5 have been observed to possess interesting electrical transport properties with high thermopower and low resistivity values leading to high thermoelectric power factors. We have investigated the effect of doping HfTe5 with rare-earth elements by measuring the power factor data from about 10K to room temperature on single crystals of Hf1−xRxTe5, where R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, and Ho. Samples that have been doped with Nd (Hf1−xNdxTe5) possess power factors more than a factor of 2 larger than that of the commonly used thermoelectric material Bi2Te3.

Statistical Analysis of a Round-Robin Measurement Survey of Two Candidate Materials for a Seebeck Coefficient Standard Reference Material.

In an effort to develop a Standard Reference Material (SRM™) for Seebeck coefficient, we have conducted a round-robin measurement survey of two candidate materials—undoped Bi2Te3 and Constantan (55 % Cu and 45 % Ni alloy). Measurements were performed in two rounds by twelve laboratories involved in active thermoelectric research using a number of different commercial and custom-built measurement systems and techniques. In this paper we report the detailed statistical analyses on the interlaboratory measurement results and the statistical methodology for analysis of irregularly sampled measurement curves in the interlaboratory study setting. Based on these results, we have selected Bi2Te3 as the prototype standard material. Once available, this SRM will be useful for future interlaboratory data comparison and instrument calibrations.

Thermoelectric properties of doped titanium disulfides

We report herein the doping of titanium disulfide (TiS2) with the pnictides (Pn): P, As, and Sb. The incorporation of these pnictides into titanium disulfide (TiS2−xPnx) is performed at extremely low concentrations (x∼0.2%). The effects on the electronic transport of titanium disulfide by doping with arsenic is quite profound, reducing the resistivity and thermopower to 0.2mΩcm and −35μV∕K at 300K, respectively, from 1.8mΩcm and −170μV∕K at 300K for the parent compound TiS2. For a wide range of thermopower values we find that the thermopower (α) of these doped titanium disulfides is linearly related to the infrared reflectivity minimum and can be correlated by the experimentally determined proportionality of λ=−0.0457α, where λ is the wavelength of the minimum.

A High-Throughput Screening System for Thermoelectric Material Exploration Based on a Combinatorial Film Approach

A high-throughput system that consists of a combinatorial tool (a sputtering deposition tool and a pulsed laser deposition tool) and two developed property screening devices was used for thermoelectric material exploration. The thermoelectric power factor (S2?, S = Seebeck coefficient, ? = electrical conductivity) screening device allows us to measure electrical conductivity and Seebeck coefficient of over 1000 sample-points within 6 h. The thermal effusivity measurement system using the frequency domain thermoreflectance technique allows us to screen thermal conductivity of combinatorial/conventional films. Illustrations of these applications are provided with a Co?Sn?Ce/Si(100) film for power factor determination and with a Ba2YCu3O7/SrTiO3(100) film for thermal conductivity derivation.

Thermoelectric Properties of TiS 2 type materials

TiS 2 belongs to a family of layered compounds that displays promise as a thermoelectric material. At room temperature the thermopower (a) of TiS 2 displays an n-type behavior, with a magnitude of ≈ -200 μV/K. The electrical resistivity (ρ), is on the order of 1 mΩ-cm at room temperature and displays a “metallic-like” behavior with dR/dT > 0 from 300 K to 10 K. Thus, these compounds exhibit relatively large power factors (PF = α 2 /ρ) with a PF ∼30 μW/K 2 cm at T = 300 K, which are comparable to the state-of-the art Bi 2 Te 3 type materials, which have a PF ∼40 μW/K 2 , at T = 330K. These values suggest that further investigations of these systems could be profitable. Thin plate-like crystals of TiS 2 are grown by the iodine vapor transport method with planar dimensions of 1 cm and thicknesses of 20 μm or more. In this synthetic approach some dopants can be integrated into the parent compound, effectively providing a route for the tuning of electronic properties. We present here some effects of elemental doping on the electronic properties in these TiS 2 based materials.

Thermoelectric and Structural Characterization of Ba2Ho(Cu3-xCox)O6+y

The search for thermoelectric materials for power generation and for solid-state cooling has led to increased interest of layered cobalt-containing oxides because of their thermal stability at high temperature and their desirable thermoelectric properties. This paper examines the effect of substitution of Co in the layered pervoskite Ba2Ho(Cu3−xCox)O6+y (x=0.3, 0.4, 0.5, 0.6, and 1.0). Structural analysis using the neutron Rietveld refinement technique reveals that when x≤0.4, Co substitutes mainly for Cu in the “chain sites” of the Ba2Ho(Cu3−xCox)O6+y structure. As x>0.4, Co also enters in the Cu-O “plane sites” as well. The thermoelectric properties of polycrystalline Ba2Ho(Cu3−xCox)O6+y samples were studied in the temperature range of 10–390 K. In general, as the cobalt content x increases, the resistivity and Seebeck coefficient of these samples increase while the thermal conductivity decreases. Among the five Ba2Ho(Cu3−xCox)O6+y compositions, the x=0.4 member gives the highest figure of merit ZT of ≈...

Raman scattering in doped transition metal pentatellurides

We have measured the Raman spectra of Sb doped (ZrTe5−xSbx; 0<x<0.25) and Hf doped (ZryHf1−yTe5; 0<y<1) pentatellurides in the 90–300 cm−1 range, and compared them to the corresponding spectrum for the parent material ZrTe5. X-ray diffraction data revealed that the pentatelluride structure prevails at all doping concentrations. With increasing Sb concentration, the linewidths for three of the four Raman peaks observed at 115, 120, and 147 cm−1 in the parent material broadened with insignificant change in the linewidth for the fourth peak at 180 cm−1. Interestingly, the peak intensity for the mode at 180 cm−1 gradually diminishes in ZrTe5−xSbx up to x=0.15, and completely vanishes in the x=0.20 compound. On the other hand, the electrical and Raman properties of ZryHf1−yTe5 exhibit a systematic shift from the corresponding properties of ZrTe5 to that of HfTe5 for increasing Hf concentration in the sample. Implications of the disappearance of the resistivity anomaly with concomitant vanishing of 180 cm−1 Ram...