2, Hokkaido University, Sapporo, Hokkaido, Japan
3, Akita Industrial Technology Center, Akita, Akita, Japan
Thermoelectric oxides are considered as promising materials because of their durability against high temperature in air, low cost for producing and non-toxicity etc. Thermoelectric modules using p-type Ca2.7Bi0.3Co4O9 (Co-349) and n-type CaMn0.98Mo0.02O3 (Mn-113) have been produced using Ag paste to form junctions. In order to enhance the conversion efficiency of the modules, repetition of hot-forging was attempted to prepare the Co-349 bulks. The power factor of the sample prepared by three repetitions of hot-forging is 1.4-2.9 fold higher than one-time hot-forging. The out-put power of the thermoelectric module composed of Co-349 and Mn-113 devices is enhanced by twice. The maximum power density of the module was increased to 0.72 W/cm2 against the total cross-sectional area of the devices at 1073 K of the heat source temperature (TH) by water cooling at 293 K (Tc).
The durability against high temperature, heat cycling, and vibration of the oxide modules was investigated quantitatively. Life time tests have been carried out for the oxide modules up to 1073 K of TH by water circulation at 293 K under the air atmosphere. No degradations in both generated power are observed up to 1073 K of TH. The durability against heat cycling was investigated between 873 and 373 K of TH in air. The maximum out-put power is kept constant during 1000 times of the heat cycling. The vibration test assumed to be used on automobiles was carried out for the oxide thermoelectric module at room temperature. The change of contact resistance at the junctions between before and after the vibration test was measured. Great changes tend to be observed near the four corners of the module.
The air-cooled thermoelectric units have been developed using heat pipes. The maximum out-put power reaches 2.2 W at 823 K of the heat source temperature. The power generation can be shown by lighting LED lamps, charging the smart phone and portable TV, and wireless transmission of data and moving images by the temperature sensor and web camera, respectively using the combustion of natural gas or firewood as the heat sources.