Saori Urata

Ca2.7Bi0.3Co4O9∕La0.9Bi0.1NiO3 thermoelectric deviceswith high output power density

Different versions of a thermoelectric unicouple composed of p-type Ca2.7Bi0.3Co4O9 (Co-349) and n-type La0.9Bi0.1NiO3 (Ni-113) bulks were constructed using Ag paste containing p- and n-type oxide powders, prepared from the same bulks, for connection of the p and n legs, respectively. Internal resistance (RI) of the unicouple corrected using Ag paste containing 6 wt. % of the oxide powders is 26.2mΩ at 1073K in air and decreases with increasing temperature. Maximum output power (Pmax), evaluated using the formula Pmax=VO2∕4RI, (VO is open-circuit voltage), is 94mW at 1073K (ΔT=500K) and increases with temperature. This value corresponds to a volume power density of 0.66W∕cm3.

A portable thermoelectric-power-generating module composed of oxide devices

High power density is a strong point of thermoelectric generation. Exploitation of this salient characteristic would make thermoelectric modules promising candidates for mobile power applications. Here we show how power can be generated using a small thermoelectric module composed of 140 pairs of oxide thermoelectric unicouples. The module weighs 19.8g and its dimensions are 53mm long, 32mm wide, and 5.0mm thick. The hot-pressed thermoelectric oxide bulk materials used were connected with a Ag paste, incorporating oxide powder, and Ag electrodes. The module’s open circuit voltage increases with increasing hot-side temperature (TH) and reaches 4.5V at a TH of 1072K in air. No deterioration in output power was seen when power generation was carried out ten times at a TH of 723K with intermediate cooling to room temperature. The module was successfully used to charge a lithium-ion battery in a mobile phone.

Enhancement of thermoelectric figure of merit by incorporation of large single crystals in Ca 3 Co 4 O 9 bulk materials

Having recently succeeded in synthesizing large single crystals of (Ca 2 CoO 3 )CoO 2 (Co-349) with superior thermoelectric properties using a modified flux method, we have prepared a composite material of Co-349 powder and single crystals and examined its thermoelectric properties. The electrical conductivity σ of this composite, which contained 20 wt.% single crystals, was higher than that of a sample without the single crystals. While the achievable effect has yet to be fully realized, improved grain alignment and the effect of current bypassing grain boundaries through the large single crystals in the composite are thought to cause the increasing σ, which consequently results in an enhanced thermoelectric figure of merit of about 0.56 at 973 K in air.

Thermoelectric module made of perovskite cobalt oxides with large thermopower

We have fabricated a trial product of an oxide thermoelectric module using the perovskite cobalt oxides. The thermoelectric properties of the p- and n-leg materials are carefully controlled, and the room temperature thermopower is set to be larger than 200 μV/K. This module generates an open circuit voltage of 1.0 V with a small temperature difference of 170 K. At a large temperature difference of 399 K, it generates a substantial power of 40 mW, and the generated energy density is comparable with that of commercial solar cells.

Mechanical Properties of Ca0.9Yb0.1MnO3/Ag Composites for n-Type Legs of Thermoelectric Oxide Devices

CaMnO3 systems are known to exhibit excellent thermoelectric (TE) performance at high temperature in air and are, therefore, good candidates for the n-type legs of TE oxide devices. In previous studies, however, several CaMnO3 system legs have fractured easily after the power generation test. The fractures were probably caused by the inherently low mechanical strength of CaMnO3, due to which it developed a great thermal stress arising from the big difference in thermal expansion coefficient between the Ag electrode and the CaMnO3 leg. In the present study, Ag particles were dispersed in a Ca0.9Yb0.1MnO3 matrix to mitigate the problems mentioned above. The incorporation of Ag particles into the matrix led to both improvement of the mechanical strength of Ca0.9Yb0.1MnO3 and to reduction in the thermal stress at the junction between the Ag electrode and the Ca0.9Yb0.1MnO3/Ag composite leg. Additionally, the ability of TE oxides to withstand application of a maximum temperature gradient without impairment of their mechanical strength could be significantly improved as a result.

Preparation and properties of thermoelectric pipe-type modules

Different versions of thermoelectric modules composed of p-type Ca 3Co4O9 (Co-349) and n-type CaMnO3 (Mn-113) bulks, which have a pipe shape, were constructed using Ag electrodes, Ag paste and stainless steel tubes. Power generation was carried out in flame by combustion of natural gas. Water flowed inside the stainless steel tube for cooling. The number of pairs of oxide thermoelectric legs was 10-60. The devices were connected with the stainless steel tube coated with ZrO2 by thermal spray using a dielectric paste composed of silica glass and iron oxide. The module consisting of 10 pairs of legs can generate about 20mW in a condition of 1053K of temperature around it, 293K and 16cm3/min of water temperature and flow rate inside the stainless steel tube, respectively. The power generation was carried out three times with intermediate cooling and kept on for 1h each time. Deterioration in power and internal resistance of the module were not observed after the third combustion. The module consisting of 54 pairs of legs can generate 1.5V, 0.28W and steam simultaneously by installing in an instantaneous water heater

Power Generation Using Oxide Thermoelectric Modules

Different versions of thermoelectric unicouples composed of p-type Ca3Co4O9 (Co-349) and n-type LaNiO3 (Ni-113) or CaMnO3 (Mn-113) bulk materials were prepared. In the unicouples p- and n-type legs were connected with Ag electrodes using Ag paste including various oxide powders with various ratios. For the Co-349/Ni-113 unicouples, maximum output power (Pmax) reaches 177mW at a hot side temperature (TH) of 1073K and a temperature differential (ΔT) between TH and cold side temperature of 500K at 6wt% of Co-349 powder. On the other hand, the lowest internal resistance (RI) is observed in a Co-349/Mn-113 unicouple prepared using Ag paste including 3wt% of Mn-113 powder. Thermoelectric modules consisting of 8 pairs of oxide legs were fabricated using the same method with the unicouples. The open circuit voltage (VO) and Pmax increase with increasing TH and reach 0.392 V and 0.314 W, and 0.911 V and 0.233 W at a TH of 1273 K in air for the Co-349/Ni-113 and Co-349/Mn-113 modules, respectively.

Power generation of thermoelectric oxide modules

Different versions of a thermoelectric unicouple composed of p-type Ca/sub 2.7/Bi/sub 0.3/Co/sub 4/O/sub 9/ (Co-349) and n-type La/sub 0.9/Bi/sub 0.1/NiO/sub 3/ (Ni-113) bulks were constructed using Ag paste containing p- and n-type oxide powders, for connection between p- or n-legs and Ag electrodes, respectively. Open-circuit voltage V/sub 0/ of the unicouple connected using Ag paste containing 6wt% of the oxide powders reaches 100 mV at a hot-side temperature T/sub H/ of 1073 K and a temperature difference AT of 500 K in air. Internal resistance R/sub I/ of this unicouple is 26.2 m/spl Omega/ at 1073 K in air and decreases with increasing temperature. Maximum output power P/sub max/, evaluated using the formula P/sub max/ = V/sub 0//sup 2//4R/sub I/, is 94 mW at 1073 K (/spl Delta/T= 500 K) and increases with temperature. This value corresponds to a volume power density of 0.66 W/cm/sup 3/. The incorporation of oxide powders in Ag paste is shown to be effective to reduce the contact resistance and the thermal hysteresis effect at oxide/metal junctions. High power density is a strong point of thermoelectric generation. Exploitation of this salient characteristic would make thermoelectric modules promising candidates for mobile power applications. Here we show how power can be generated using a small thermoelectric module composed of 140 pairs of oxide thermoelectric unicouples. The module weighs 19.8 g and its dimensions are 53 mm long, 32 mm wide, and 5.0 mm thick. The hot-pressed thermoelectric oxide bulk materials used were connected with an Ag paste, incorporating 6wt% of oxide powder, and Ag electrodes. The modules V/sub 0/ increases with increasing hot-side temperature (T/sub H/) and reaches 4.5 V at a T/sub H/ of 1072 K in air. No deterioration in output power was seen when power generation was carried out ten times at a T/sub H/ of 723 K with intermediate cooling to room temperature. The module was successfully used to charge a lithium-ion battery in a mobile phone.

Mechanical properties of oxide materials

Thermoelectric modules composed of eight pairs of p-type Ca3Co4O9 (Co-349) and n-type CaMnO3 (Mn-113) or LaNiO3 (Ni-113) bulks were constructed using Ag electrodes, paste, and alumina substrate. In these modules, it was reported that after power generation at 1273K of the hot side heater temperature the Mn-113 legs were broken near the alumina substrate. To make the mechanism of the destruction clear, coefficient of thermal expansion (CTE), three-point bending strength, and microstructure were investigated. The CTE of the Co-349 and Ni-113 bulks is closer than that of the Mn-113 one to the alumina substrate. The three-point bending strength of the Co-349 and Ni-113 bulks are higher than that of Mn-113 one. These are the reasons of the destruction of the Mn-113 legs near the alumina substrate in the modules by heating. Lower relative density of the Mn-113 bulks than the other oxide ones leads low bending strength. The Mn-113 grains, moreover, are grown much more than other oxides. This allows cracks to run easily in the Mn-113 bulks. The stronger bending strength, thermal expansion closer to the substrate, higher relative density, and smaller grains for the Ni-113 and Co-349 bulks than those for the Mn-113 bulks are the reasons of destruction of the latter bulks.

Thermoelectric Modules For High Temperature Waste Heat

A new adhesive material has been developed in order to obtain practically usable thermoelectric modules composed of oxide thermoelectric legs. The thermoelectric module composed of 8-pair oxide legs has been fabricated. Both hot- and cold-sides of the module were covered by alumina plates. Open circuit voltage V O and maximum power P max reach 0.38 V and 0.30 W, respectively at 803 K of a hot-side temperature T H and 362 K of a temperature differential Δ T between T H and cold-side temperature T C . Generating power was repeated 11 times at 873-993 K of T H and at 200-290 K of Δ T . The module was cooled down to room temperature after each generation. At third measurement internal resistance R I of the module increased by 30 %. This is due to destruction of junctions because of thermal strain. No deterioration, however, was observed in thermoelectric properties for the oxide legs.