Synthesis of Na_xCo₂O₄thermoelectric oxide with crystallographic anisotropy by chemical solution process

Abstract

The synthesis of the Na x Co 2 O 4 with a layered crystal structure was attempted by using the polymerized complex (PC) method or citric acid complex (CAC) method, both of which are a kind of chemical solution process. The effects of these chemical solution processes on the crystallographic anisotropy, microstructure and thermoelectric properties of the sintered Na x Co 2 O 4 were investigated and compared to those of the conventional SSR method. In the case of the PC method, the sintered body with higher chemical homogeneity and finer microstructure than those of the SSR sample was obtained, resulting in the high thermoelectric performance. However, the crystallographic orientation was almost the same as that of the SSR sample. On the other hand, in the case of the CAC method, a flaky powder precursor was formed by grain growth of the Na x Co 2 O 4 phase along the c-plane. The Na x Co 2 O 4 sample sintered from this powder precursor showed a high crystallographic anisotropy and good chemical homogeneity, so the CAC process was found to be quite effective for synthesizing a crystallographically oriented Na x Co 2 O 4 polycrystal with high thermoelectric performance. q

Fabrication and thermoelectric properties of highly textured NaCo2O4 ceramic

“…The Lotgering factor f values of the pellet and its powder were calculated to be 0.94 and 0.71, respectively, according to the integrated intensity of the diffraction peaks in their XRD profiles. The value of 0.71 is very close to 0.70 for the sample prepared with traditional solid-state reaction method alone [11]. The f value above 0.9 has been found in the samples prepared by the reactive templated grain growth method [10] and critic acid complex method [11], respectively.…”

“…The value of 0.71 is very close to 0.70 for the sample prepared with traditional solid-state reaction method alone [11]. The f value above 0.9 has been found in the samples prepared by the reactive templated grain growth method [10] and critic acid complex method [11], respectively. The f value of 0.94 for our sample indicates that a polycrystalline sample with a large crystallographic anisotropy can be fabricated by adopting a cold high-pressure compacting process before sintering.…”

“…27-0682 as in Ref. [11]. When the c-axis directions of grains are completely aligned along the pressing direction, the f value is calculated to be 1.…”

“…Recently, the correlation between thermoelectric properties and texture strength was demonstrated within the framework of Ca 3 Co 4 O 9 textured ceramics based on an innovative method of X-ray diffraction analysis [9]. With the above guiding principle in mind, many attempts, such as the reactive templated grain growth method [10], chemical solution process [11] and magnetic alignment technique [7], have been made to prepare highly textured ceramics.…”

Thermoelectric properties of SrTiO3 synthesized by the polymerized complex method

“…[1][2][3][4][5][6] These thermoelectric oxides can be used at high temperatures without deterioration of their performance due to oxidation, and their production costs are comparatively low. SrTiO 3 is one of the potential candidates for n-type thermoelectric materials.…”