Ngo Van Nong

Enhancement of the Thermoelectric Performance of p‐Type Layered Oxide Ca3Co4O9+δ Through Heavy Doping and Metallic Nanoinclusions

By converting heat directly into electricity, thermoelectric (TE) generation offers a promising technology to recover waste heat emitted from industrial sectors and energy consumption processes. [ 1 ] The key to realize an effi cient TE generator lies, however, in fi nding good materials with high TE performance, a good durability at high temperature, and preferably robustness to operating in air. The performance of a TE material is evaluated by the dimensionless fi gure-of-merit ZT ( = S 2 T / ρ κ , where S , T , ρ , and κ are the Seebeck coeffi cient, absolute temperature, electrical resistivity, and thermal conductivity, respectively). By far the most widely used TE materials are alloys of Bi 2 Te 3 , PbTe, and SiGe, which often suffer from poor durability at high temperature, are harmful or scarce, and have costly constituting elements. Metal oxides have been considered as an alternative to overcome these problems. Metal oxide-based materials have been attracting continuous interest as TE materials over the years since the discovery of large TE power in p-type NaCo 2 O 4 single crystals by Terasaki et al. in 1997. [ 2 ] However, practical application of this oxide for power generation from waste heat has never been realized because of the volatility of Na and the instability of the compound against humidity. Another Co-based oxide p type material Ca 3 Co 4 O 9 + δ has also been intensively investigated because of its good TE performance ( ZT = 0.83 at 973 K for the single crystal) [ 3 ] and its high thermal and chemical stabilities even up to 1200 K in air. [ 4–7 ] An incommensurate character in the crystal structure of this compound is explicitly described as [Ca 2 CoO 3 ] b 1/ b 2 [CoO 2 ], where b 1 and b 2 are two different periodicities along the b axis for the rock salt-type Ca 2 CoO 3 subsystem and the CdI 2 -type CoO 2 subsystem, respectively. [ 4 ] Single crystals are less likely to be applied for fabricating practical TE devices, because they will be too expensive. It is hence highly desirable to achieve suffi cient TE properties in a polycrystalline form of these oxides. Although the diffi culty of discovering novel high performance

Low‐Cost High‐Performance Zinc Antimonide Thin Films for Thermoelectric Applications

Zinc antimonide thin films with high thermoelectric performance are produced by a simple sputtering method. The phase-pure Zn(4)Sb(3) and ZnSb thin films fulfill the key requirements for commercial TE power generation: cheap elements, cheap fabrication method, high performance and thermal stability. In addition, two completely new meta-stable crystalline phases of zinc antimonide have been discovered.

The Effect of (Ag, Ni, Zn)-Addition on the Thermoelectric Properties of Copper Aluminate

Polycrystalline bulk copper aluminate Cu1-x-yAgxByAlO2 with B = Ni or Zn were prepared by spark plasma sintering and subsequent thermal treatment. The influence of partial substitution of Ag, Ni and Zn for Cu-sites in CuAlO2 on the high temperature thermoelectric properties has been studied. The addition of Ag and Zn was found to enhance the formation of CuAlO2 phase and to increase the electrical conductivity. The addition of Ag or Ag and Ni on the other hand decreases the electrical conductivity. The highest power factor of 1.26 × 10-4 W/mK2 was obtained for the addition of Ag and Zn at 1,060 K, indicating a significant improvement compared with the non-doped CuAlO2 sample.

Anomalously high thermoelectric power factor in epitaxial ScN thin films

Thermoelectric properties of ScN thin films grown by reactive magnetron sputtering on Al2O3(0001) wafers are reported. X-ray diffraction and elastic recoil detection analyses show that the composition of the films is close to stoichiometry with trace amounts (∼1 at. % in total) of C, O, and F. We found that the ScN thin-film exhibits a rather low electrical resistivity of ∼2.94 μΩm, while its Seebeck coefficient is approximately ∼−86 μV/K at 800 K, yielding a power factor of ∼2.5 × 10−3 W/mK2. This value is anomalously high for common transition-metal nitrides.

Electronic-structure origin of the anisotropic thermopower of nanolaminated Ti3SiC2 determined by polarized x-ray spectroscopy and Seebeck measurements

Nanolaminated materials exhibit characteristic magnetic, mechanical, and thermoelectric properties, withlarge contemporary scientific and technological interest. Here we report on the anisotropic S ...

Effects of spark plasma sintering conditions on the anisotropic thermoelectric properties of bismuth antimony telluride

Bismuth antimony telluride (BixSb2−xTe3, 0.4 < x< 0.6) is one of the best and most-used p-type semiconductor materials for near-room-temperature thermoelectric power generation. In this work, p-type Bi0.4Sb1.6Te3 samples were prepared under various conditions (temperature, holding time, and ramp-rate) using spark plasma sintering (SPS). The effects of SPS conditions on the anisotropic thermoelectric properties and microstructure evolutions were systematically investigated. The change of sintering temperature showed stronger influence than other sintering parameters to the resulting thermoelectric properties. Samples sintered over the temperature range between 653 K and 773 K showed significant differences in the degrees of orientations. The change was mainly caused by grain growth and re-orientation. Despite of the anisotropy, zT value as high as 1.2 to 1.3 was achieved over the temperature range of 300 to 360 K by directly using commercial power sintered at 723 and 773 K. The sintering profiles and microstructure evolutions during SPS were illustrated and the thermoelectric properties as a function of the degrees of orientations were shown and discussed in detail.

Scandium-doped zinc cadmium oxide as a new stable n-type oxide thermoelectric material

Scandium-doped zinc cadmium oxide (Sc-doped ZnCdO) is proposed as a new n-type oxide thermoelectric material. The material is sintered in air to maintain the oxygen stoichiometry and avoid instability issues. The successful alloying of CdO with ZnO at a molar ratio of 1 : 9 significantly reduced the thermal conductivity by up to 7-fold at room temperature. By carefully selecting the Sc-dopant concentrations, a high power factor of 7.1 × 10−4 W m−1 K−2 at 1173 K could be obtained. Therefore, the highest ZT ∼ 0.3 at 1173 K was achieved for the Zn0.9Cd0.1Sc0.01O1.015 sample, and it has so far one of the highest ZT values among those reported for ZnO based thermoelectric materials over the temperature range, e.g., its ZT value at 300 K, which is 0.012, is over 1 order of magnitude higher than that of the state-of-the-art nanostructured Al-doped ZnO, which is 0.0013. It suggests that this material is a good candidate for improving the overall conversion efficiencies in oxide thermoelectric modules. Meanwhile, Sc-doped ZnCdO is robust in air at high temperatures, whereas other n-type materials, such as Al-doped ZnO, will experience rapid degradation of their electrical conductivity and ZT.

Experimental and theoretical investigation of Cr1-xScxN solid solutions for thermoelectrics

We investigate the trends in mixing thermodynamics of Cr1-xScxN solid solutions in the cubic B1 structure and their electronic density of state by first-principle calculations, and thin-film synthe ...

On the Challenges of Reducing Contact Resistances in Thermoelectric Generators Based on Half-Heusler Alloys

A method using fast hot pressing to join half-Heusler (HH) thermoelectric materials directly to an electrical current collector (Ag electrode) without using a third filler material is introduced. The compositions of the HH alloys used are Hf0.5Zr0.5CoSn0.2Sb0.8 and Ti0.6Hf0.4NiSn for p- and n-type, respectively. Using this method, the quality of the HH–electrode contacts is improved due to their low electrical contact resistance and less reaction–diffusion layer. The microstructure and chemical composition of the joints were examined using a scanning electron microscope equipped with energy-dispersive x-ray analysis. The electrical characteristics of the interfaces at the contacts were studied based on electrical contact resistance and Seebeck scanning microprobe measurements. In this paper, we show that joining the HH to a Ag electrode directly using fast hot pressing resulted in lower contact resistance and better performance compared with the method of using active brazing filler alloy.

Kinetics, Stability, and Thermal Contact Resistance of Nickel–Ca3Co4O9 Interfaces Formed by Spark Plasma Sintering

Incorporating oxide thermoelectric (TE) materials into TE power generation modules necessitates study of the interfaces between the oxide TE elements and the interconnect materials used to transfer current between them. In this study, interfaces between pure nickel and undoped calcium cobaltate (Ca3Co4O9) have been formed directly by spark plasma sintering (SPS). An intermediate NiO phase is formed during the SPS processes, which grows during post-heating with Co entering from the cobaltate side to form a graded Ni1−xCoxO interfacial layer. The electrical and thermal transport across these interfaces, as well as the long-term chemical stability of the intermediate layers, have been studied and are discussed.