High-temperature thermoelectric properties of Na/sub x/CoO/sub 2-δ/ single crystals

Abstract

The thermoelectric properties of Na,CoO,, single crystal have been evaluated at high temperatures (300 < T < 800 K) for the first time. The Na,CoO,, single crystals are prepared by a flux technique, and the flaky single crystals are very thin on the c-axis. The in-plane electrical resistivity (p), the thermoelectric power (S) and the in-plane thermal conductivity ( K ) are measured in the range of 300 K to 800 K. The estimated power factor (PF = S'*p-') and the figure-ofmerit (Z = S'.g'K") are about 2.4 mWrn-'Ke2 and 0.12 mK-' at 300 K, respectively. These factors increase with temperature, and reach the value of PF = 7.7 mWm-'K2 and Z = 1.5 m K ' at 800 K, and the dimensionless value of ZT exceeds the criterion of 1. The PF and Z exceed the values of typical "high temperature thermoelectric materials". These results suggest that Na,CoO,, could be a very promising material for use in the high-temperature thermoelectric power generation devices.

Thermoelectric properties of copper selenide with ordered selenium layer and disordered copper layer

“…Another direction in reducing the lattice thermal conductivity is to use layered structures, both artificial ones such as superlattices [9,10], and naturally formed superlattices [11,12]. In the artificially grown superlattices, it was known that interface roughness reduces the thermal conductivity [13,14].…”

Semiconductors and Thermoelectric Materials

“…The material shows metallic-like transport properties as measured in single-crystal samples, with highly anisotropic behavior. 80 Because of the low electrical resistivity and large thermopower, a large power factor of 50 W cm À2 K À2 at room temperature was reported in the in-plane direction. 83 This power factor value exceeds that of Bi 2 Te 3 despite the much lower mobility estimated for NaCo 2 O 4 .…”

Size effects in thermoelectric cobaltate heterostructures

“…The observation of a high thermopower and a low resistivity in Na x CoO 2 single crystals [30,31] opened up the interest as a promising thermoelectric candidate material [34,[45][46][47][48] and ZT values exceeding unity have already been achieved. Thermoelectric power factor (S 2 ) values up to 50 µW/K 2 cm, which is comparable for Bi 2 Te 3 crystals [49] are reported, demonstrating that the electronic properties are comparable with traditional thermoelectric materials.…”

“…For single crystals, ZT values of about 0.03 at 300K are obtained, which increases to 1.2 at 800K, showing the high-temperature potential of oxide thermoelectrics. [45] Because of the large anisotropy observed in the resistivity for the di↵erent crystallographic orientations, polycrystalline samples of Na x CoO 2 are not performing as good as single crystals, due to the increased resistivity by about a factor of 10. The ZT value of polycrystalline Na x CoO 2 is limited to about 0.3 at 800K.…”

“…The ZT value of polycrystalline Na x CoO 2 is limited to about 0.3 at 800K. [45] In addition to changing the sodium content and thereby influencing the carrier concentration, attempts have been made to substitute sodium, however this has been very di cult and did not result in significant improvements of the thermoelectric performance. [70] Relatively comparable thermoelectric performance as for Na x CoO 2 is obtained for Ca 3 Co 4 O 9 single crystals.…”