Preparation And Characterization Of P Type Na_xCo₂O₄Based Thermoelectric Ceramics

NaxCo2O4, a P-type thermoelectric ceramic, is a typical representative of thermo-electric oxide materials. There have been many explorations about the NaxCo2O4thermoelectric properties at present, but most concentrated in doping research. In this paper, NaxCo2O4thermoelectric materials were studied object, and the research to seek to improve further thermoelectric properties were done by composited and co-doped with other compound. The co-doped powders were prepared used the method of combining self-propagating with solid phase reaction, while the Na1.7Co2O4/La0.6Sr0.4CoO3, a composite thermoelectric ceramic, by powder metallurgy. The preparation of the corresponding powders and NaxCo2O4based thermoelectric ceramics materials had been rounded analysed. The effects of composite and co-doping on electrical conductivity, Seebeck coefficient and power factor were investigated.The main achievements of this paper are listed as followings:1. The Na1.7-xCax(Co0.85Cu0.15)2O4(x=0,0.05,0.1,0.15,0.2) powders and Na1.7-xCaxCo2O4-yFy(x=0,0.05,0.1,0.15,0.2,0.25) powders were prepared by the method of combining self-propagating with solid phase reaction. The results showed that the average size of those powders was about16.45μm and28.543μm individually. First, the powder precursor of Na1.7-xCax(Co0.85)2O4and the Na1.7Co2O4powder with layered structure whose the average size was about12.534μm, were synthesized by self-propagating; Second, these were doped with CuO and CaF2by solid phase reaction. The Na1.7-xCax(Co0.85Cu0.15)2O4and Na1.7-xCaxCo2O4-yFy were obtained.2. The thermoelectric properties of Na1.7Co2O4/La0.6Sr0.4CoO3composite material prepared by PM process were studied. La0.6Sr0.4CoO3powder was synthesized by the glycine-nitrate process. The results of the XRD displayed that the composite might be mechanical impurity when the volume of La0.6Sr0.4CoO3was a small amount, and that some impurity phase would be generated in material internally when the volume was larger than10%(wt). The SEM showed that the particle size did not change significantly along with the increase of La0.6Sr0.4CoO3, the layered structure of this material was no longer apparent, and that the structural changes caused the pore increasing. Compared with Na1.7Co2O4, the electrical conductivity of the composite material increased obviously along with temperature change, but the Seebeck coefficient was no significant decreased or increased, and was stable. The most power factor of Na1.7Co2O4/Lao.6Sro.4CoO3composite arrived5.33×10-4W/mK2when the amount of La0.6Sr0.4CoO3was8%(wt).3. The results of the XRD displayed that there were small amount other impurities in the powders of Na1.7.xCax(Co0.85Cu0.15)2O4. The optimal compacting pressure of powders was600MPa, and its sintered density could reach4.126g/cm3. The SEM showed the crystal grain of sintered body became large with increasing doping level. The electrical conductivity and Seebeck coefficient of Na1.7-xCax(Co0.85Cu0.15)204were improved apparently compared with Na1.7Co2O4.When x=0.15, the maximum value of power factor was9.45×10-4W/mK2at873K. When x=0.1, the thermoelectric properties changes was the smallest on the measured temperature range, the maximum value was9.22×10-4W/mK2at823K.4. The SEM displayed that the prepared powder particles had apparent orientation growth. The size analysis showed the size increased in a certain extent with adding cation Ca2+and anion F-to material. The XRD showed that Na1.7-xCaxCo2O4-yFy would be accompanied by a small amount of NaO2generation. Except x=0.05, the electrical conductivity of Na1.7-xCaxCo2O4-yFy were significantly reduced, but the Seebeck coefficient of all doped samples could be improved. Although the maximum power factor of Na1.7-xCaxCo2O4-yFy was smaller than that of Na1.7Co2O4, but the test results demonstrated the power factor was relatively stable on the measured temperature range. The average value arrived2.96×10-4W/mK2when x was0.05.