| In this dessertation, the properties of the doped p-Ca3Co4O9 and n-CaMnO3 based thermoelectric materials and the relevant devices were systematically investigated. Both p- and n-type oxide based thermoelectric with optimal properties were obtained by improving the preparation techniques combined with the optimization of doping methods. The thermoelectric devices made up of the preceding optimal p- and n type oxide thermoelectric were fabricated and characterized. The results suggested that the thermoelectric generation performance was dramatically promoted by optimizing the device preparation process. The work demonstrates the practical value of doped p-Ca3Co4O9 and n-CaMnO3 based thermoelectric materials and has the benefit for their commercial application. The results are following:1. Self-ignition method was employed to fabricate powders of pure-phase p-Ca3Co4-xAgxO9, p-Ca3-xBixCo4O9 (x=0, 0.1, 0.2, 0.3) and n-Ca1-ySmyMnO3 (y=0,0.02,0.05,0.1) high temperature oxide thermoelectric materials. The results show that the resultant powders are beneficial for the preparation of high performance thermoelectric materials, because of their fine particle size, homogeneous size distribution. Furthermore, the self-ignition method also possesses its intrinsic advantages of easy operation and low cost, thus making it suitable for industrialized large-scale production. In the aspect of preparation of p- and n-type materials, we systematically studied a series of preparation technologies including powder calcinations, pressing process, material sintering, etc, and confirmed the optimal fabrication conditions, yielding pure-phase and compact thermoelectric materials with excellent performance.2. The Ag-doped p-Ca3Co4-xAgxO9 materials was prepared by using of Powder metallurgy process. The results show that with the increasing content of Ag the electric conductivity increases gradually whereas both the Seebeck coefficient and thermal conductivity show a converse tendency. As for the Bi-doped p-Ca3-xBixCo4O9 (x=0, 0.1, 0.2, 0.3), the electric conductivity and the thermal conductivity were reduced simultaneously with the increasing content of Bi, accompanied with a rise in the Seebeck coefficient. The results suggested that the best thermoelectric properties was obtained at x=0.2 for p-Co349/Ag0.2, yielding electrical resistivity, Seebeck coefficient, thermal conductivity and ZT of 89.99μ?m , 130.31μVK-1 , 0.954Wm-1K , 0.197, respectively, at 973 K. Meanwhile, the best thermoelectric properties was obtained at x=0.3 for p-Co349/Bi0.3, resulting electrical resistivity, Seebeck coefficient, thermal conductivity and the maximum ZT of 199.14μ?m, 189.42μVK-1, 0.643Wm-1K, 0.27, respectively, all of which meet the demands of high temperature thermoelectric devices.3. The Sm-doped n-Ca1-ySmyMnO3 materials was prepared by using of Powder metallurgy process also. With the increase of Sm content, the electrical conductivity was promoted with a concomitant decline in both the Seebeck coefficient and thermal conductivity. The results indicate that the best thermoelectric properties was obtained at y=0.02 for n-Mn113/Sm0.02, corresponding to electrical resistivity, Seebeck coefficient, thermal conductivity and ZT of 178.2μ?m, 202.75μVK-1, 1.539Wm-1K, 0.15, respectively, satisfying the requirements of practical application of high temperature oxide thermoelectric materials.4. The thermoelectric devices were constructed with the optimal materials including p-Co349/Ag0.2, p-Co349/Bi0.3 and n-Mn113/Sm0.02. Also the related thermoelectric generation performance was examined. The four-leg devices based on Co349/Ag0.2-Mn113/Sm0.02 was denoted as Device 1, while that based on p-Co349/Bi0.3 and n-Mn113/Sm0.02 was denoted as Device 2. For Device 1, the Pmax and volume power density for the four legs devices reached 36.8 mW and 81.9 mWcm-3 withΔT and Thot of 523 K and 873 K, respectively, together with a contact resistance of 0.43 ? and a manufacturing factor of 42.5 %. The devices show good high temperature durability. As for Device 2, the device performance was enhanced evidently by optimization of the preparation processes. Consequently, a high MF of 66.3 % and a comparatively low contact resistance of 0.21? were obtained simultaneously, leading to the maximum output power and volume power density of 51.4 mW and 127.6 mWcm-3 in the condition ofΔT of 525 K and Thot of 873 K. The devices also show well high temperature durability, achieving the standard of practical applications. Our work may fill in the gaps in the fields of the preparation of high temperature thermoelectric devices, and consequently pave the way for their commercial production and application. |
PDF Full Text Request