Y. K. Kuo

Effects of Ru substitution for Mn on La0.7Sr0.3MnO3 perovskites

We report the investigations of crystal structure, electrical resistivity (ρ), magnetization (M), x-ray photoelectron spectroscopy (XPS), specific heat (CP), thermal conductivity (κ), and thermoelectric power (TEP) on La0.7Sr0.3(Mn1−xRux)O3 (LSMRO) compounds with x=0 to 0.90. From the analyzes of crystal structure and magnetization measurements, it is inferred that Ru should have a mixed valence of Ru3+ and Ru4+ for LSMRO with low level of Ru substitution, and an additional mixed valence of Ru4+ and Ru5+ with higher Ru substitution. Such a finding is further confirmed by the XPS measurements. Besides, it is found that all measured physical properties undergo pronounced anomalies due to the ferromagnetic-paramagnetic phase transition, and the observed transport properties of LSMRO can be reasonably understood from the viewpoint of polaronic transport. The Curie temperatures TC determined from the magnetization measurements are consistently higher than those of the metal-insulator transitions TMI determined...

Nonstoichiometric doping and Bi antisite defect in single crystal Bi2Se3

We studied the defects of Bi2Se3 generated from Bridgman growth of stoichiometric and nonstoichiometric self-fluxes. Growth habit, lattice size, and transport properties are strongly affected by the types of defects generated. Major defect types of the BiSe antisite and partial Bi2-layer intercalation are identified through combined studies of direct atomic-scale imaging with scanning transmission electron microscopy in conjunction with energy-dispersive x-ray spectroscopy, x-ray diffraction, and Hall effect measurements. We propose a consistent explanation to the origin of defect type, growth morphology, and transport property.

Great enhancements in the thermoelectric power factor of BiSbTe nanostructured films with well-ordered interfaces

An innovative concept of twin-enhanced thermoelectricity was proposed to fundamentally resolve the high electrical resistance while not degrading the phonon scattering of the thermoelectric nanoassemblies. Under this frame, a variety of highly oriented and twinned bismuth antimony telluride (BixSb2-xTe3) nanocrystals were successfully fabricated by a large-area pulsed-laser deposition (PLD) technique on insulated silicon substrates at various deposition temperatures. The significant presence of the nonbasal- and basal-plane twins across the hexagonal BiSbTe nanocrystals, which were experimentally and systematically observed for the first time, evidently contributes to the unusually high electrical conductivity of ~2700 S cm(-1) and the power factor of ~25 μW cm(-1) K(-2) as well as the relatively low thermal conductivity of ~1.1 W m(-1) K(-1) found in these nanostructured films.

Thermoelectric properties of the CoSi1−xGex alloys

The effects of Ge partial substitution for Si on the thermoelectric properties of CoSi1−xGex alloys were investigated by means of thermal and electrical transport measurements. Electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements were performed on a series of CoSi1−xGex alloys with x varying from 0 to 0.15. A substantial decrease in electrical resistivity and lattice thermal conductivity was noticed with increasing substitution level, whereas the Seebeck coefficient shows a weak variation with respect to Ge concentration. The thermoelectric efficiency was found to be enhanced by an order of magnitude in CoSi with Ge substitution. These observations were interpreted on the basis of the changes in the electronic band structure induced by Ge substitution.

Enhanced thermoelectric figure-of-merit ZT for hole-doped Bi2Sr2Co2Oy through Pb substitution

Single crystals of Bi2–xPbxSr2Co2Oy (0 ≤ x ≤ 0.55) have been grown using optical floating-zone method. The chemical compositions were determined using combined electron probe microanalysis and iodometric titration. Physical properties including electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were measured using single crystal specimens. Successful hole doping through Pb substitution is confirmed through combined iodometry titration, electrical transport, and Seebeck coefficient measurements. Significant reduction on both in-plane resistivity and thermal conductivity was found as a result of Pb substitution to the Bi site. The thermoelectric figure-of-merit ZT for x ∼ 0.55 is raised 20 folds from the undoped sample at room temperature.

Thermal and transport properties of the Heusler-type compounds Fe2−xTi1+xSn

We have studied the thermoelectric properties of the stoichiometric and off-stoichiometric Heusler compounds Fe2−xTi1+xSn between 10K and 400K. It is found that the electrical resistivity and Seebeck coefficient are very sensitive to the off-stoichiometry. Seebeck coefficient (S) measurements indicate that all studied materials are p-type materials with moderate S values of about 20–30μV∕K at room temperature. Broad maximums at around 330K are observed in S for Fe2TiSn and Fe2.05Ti0.95Sn, and this maximum shifts to higher temperatures with replacing more Ti for Fe. These features are consistent with other experimental results and are related to issues raised by band-structure calculations.

Thermal and electrical transport properties of ordered?FeAl2

The thermal and electrical transport properties of the ordered intermetallic FeAl2 have been measured as a function of temperature between 10 and 300 K. The electrical resistivity (ρ) is about 400μΩ cm over a broad temperature range. In character, it resembles those of semimetals, in spite of its anomalous low-temperature upturn. The thermal conductivity (κ) at room temperature is high, approximately 7 W m−1 K−1. The measured thermoelectric power is negative at low temperatures where there is also a positive phonon-drag effect, but changes to positive values above 220 K. This observation is ascribed to the positive carriers thermally excited across the pseudogap at higher temperatures. The electronic characteristics of FeAl2 are compared to those of other semimetallic materials.

Enhancement in the thermoelectric performance by Y substitution on SrSi2

We report the results of the Y substitution in Sr1−xYxSi2 with x≤0.15 via measuring the temperature-dependent electrical resistivity, thermal conductivity, as well as Seebeck coefficient. Upon substituting Y onto the Sr sites, the electrical resistivity exhibit semiconducting behavior and the room-temperature electrical resistivity tends to reduce for x≤0.08. The thermal conductivity also decreases with increasing the Y content. Moreover, the Seebeck coefficient has a substantial increase and a maximum of about 220 μV/K at around 80 K has been found for x=0.08. These promising effects lead to a significant enhancement in the thermoelectric performance characterized by the figure-of-merit, ZT. A room-temperature ZT value of approximately 0.4 is thus achieved for Sr0.92Y0.08Si2, about one order of magnitude larger than that of stoichiometric SrSi2.

Thermoelectric properties of binary Cd-Yb quasicrystals and Cd6Yb

The thermoelectric properties, including electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) on binary icosahedral Cd-Yb quasicrystals and Cd6Yb are investigated. The Cd-Yb quasicrystals have a room-temperature (RT) resistivity value of about 200 (μΩ cm), however, the temperature-dependent resistivity is sensitive to a small composition change in these compounds. For all studied samples, Seebeck coefficients are positive and small (∼10 μV/K at RT). Upon cooling, S decreases quasilinearly, indicative of a metallic diffusion behavior, and then develops a broad phonon drag peak at around 40 K. The temperature characteristic of thermal conductivity κ of Cd-Yb quasicrystals shows a most peculiar feature in all measured thermoelectric properties. The RT κ value ∼6 (W/m K) of Cd-Yb quasicrystals is considerably larger than that of conventional ternary counterparts. Such an observation is attributed to the substantial electrical contribution κe (∼50%) to their total thermal conductiv...

Magnetotransport, thermoelectric power, thermal conductivity and specific heat of Pr2/3Sr1/3MnO3 manganite

Magnetotransport and thermal studies of Pr2/3Sr1/3MnO3 polycrystalline sintered bulk sample are reported here. The resistivity ρ(T) and thermoelectric power S(T) data show an insulator to metal (I-M) phase transition at TP≈294 K and TS≈290 K, respectively. Magnetization measurement confirms that the sample undergoes a transition from paramagnetic to ferromagnetic phase at a defined Curie temperature TC=280 K. A substantial increase in magnetoresistance from 2.5% at 280 K to 5% at 77 K has been noticed in a low magnetic field 0.15 T. Small polaron hopping model is found to be operative above the transition temperature TP, whereas electron-electron and electron-magnon scattering processes govern the low temperature metallic behavior. A detailed analysis of thermoelectric power in the ferromagnetic regime suggests that the complicated temperature dependence of S may be understood on the basis of electron-magnon scattering. A transition from decreasing high temperature thermal conductivity (due to local anhar...