Vol.1 No.2 2008

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Research paper : Development of a small-size cogeneration system using thermoelectric power generation (R. Funahashi et al.)−88 Synthesiology - English edition Vol.1 No.2 (2008) However, we believe that we will be able to construct a heat recovery system by utilizing high-temperature durability, which is the greatest advantage of oxide materials, and we are continuously working on the thermoelectric generation system for efficient use of high-temperature energy.6 Intermediate integration technology6.1 Joining technologyTo obtain good thermoelectric module, it is necessary to form junctions with excellent heat durability, high mechanical strength, and low electric contact resistance between thermoelectric and electrode (generally metal) materials. However, for joining metal (Ag in this research) and oxides, problems of high contact resistance and detachment occur due to differences in Fermi energy and thermal expansion (Figure 5). More work is needed on joining materials to solve these problems.6.1.1 Manufacture of elementThe element was created by joining a pair of sintered Co-349 and Ni-113 onto alumina substrate of which the surface was metalized with Ag, and the electric contact resistance and the heat resistance were assessed[6]. The joining material was Ag paste containing powder of Co-349 or Ni-113 at 0~10 wt.%. Normally, it would seem better to use the same powder for both p- and n-type elements, but since application of Ag paste using screen printing required a “2-color printing” technology, in this research, we created an element using one of the composite Ag paste with either p- or n-type powder. The oxide composite Ag paste was applied on the surface of a sintered oxide compact, and placed on a metalized surface of alumina substrate. The Ag paste was solidified by heat treatment at 1123 K while applying uniaxial pressure 65kg/cm2 vertical to the junction surface, to create a thermoelectric element composed of a pair of p- and n-type sintered compacts. The compositions of sintered compact were Ca2.7Bi0.3Co4O9 and La0.9Bi0.1NiO3, where part of Ca and La of Co-349 and Ni-113 were replaced with Bi. The sintered compact was made by hot pressing the powders. The reasons for replacing Ca and La with Bi were: for p-type, S, ρ and κ [Equation(1)] were improved[7]; and for n-type, ρ only could be reduced while maintaining S constant[8].6.1.2 Property assessmentCompositing Co-349 powder into Ag paste was found to be effective for reducing internal resistance (RI) of the element[6]. Lowest RI was obtained when Co-349 powder content was 6 wt.%. This reduction was caused by a decreased contact resistance between Ag paste and sintered oxide compact. Although the mechanism is not clear yet, it is thought to be due to a reduction of the effect of Schottky barrier and an improvement of contact by increasing wettability between the Ag paste and the surface of the sintered oxide compact.The smoothness of the joining surface of sintered compact was important to strongly and closely connect the oxide material and the Ag electrode. The surface of sintered oxide was buffed before applying the Ag paste, and was joined to the alumina substrate using Ag paste composited with Co-349 powder at 6 wt.% and at the same condition as above. The smoothed surface of sintered oxide was effective in forming good junction.Next we shall describe the durability of thermoelectric element against the heating and cooling cycle. The thermoelectric element was placed in an electric furnace, temperature raised to 1073 K over 3 h in air, kept in that state for 1 h, removed directly out of the furnace at high temperature, and cooled rapidly to room temperature. This procedure was repeated 5 times, and RI before and after the heating/cooling cycles were measured and variations were calculated. In elements connected only with Ag paste, RI increased significantly after the cycle at 600 K or less. On the other hand, in elements using oxide composite paste, RI increase due to heating/cooling cycles became extremely small[6]. It was found that compositing of oxides into Ag paste was effective in improving durability against heating/cooling cycles. As a result of observation under scanning electron microscope (SEM), large cavities were found in the Ag paste in the element made with Ag paste only. On the other hand, in the thermoelectric element using Ag paste composited with 6 wt.% Co-349 powder, it was found that alumina substrate and sintered oxide were joined closely, although some fine holes were observed. The improvement of fine structure was the reason for controlling the RI increase in heating/cooling cycles. The reasons for production of cavities are thought to be: contraction due to sintering of Ag, difference in heat expansion bet dispersal ween Ag and oxides, and exfoliation due to poor wettability.Fig. 5 Issues in joining technology.Joining technology is mandatory for creating module with high durability and power generation performance. Particularly, developments of joining material and method to realize strong junction strength and low contact resistance are important for practical application.p-type thermoelectric materialJoining materialHeat expansion ・・・DetachmentSchottky barrier ・・・High contact resistancen-type thermoelectric materialElectrodematerialCondition of joining material・High junction strength・Low contact resistance・High heat transfer(16)−

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