N-type Thermoelectric Properties Research Articles

熱電変換材料 コンビナトリアルケミストリーによる熱電酸化物の探索

It has been reported that single crystals of CoO2-based oxides with layered structures show good p-type thermoelectric properties. In order to realize thermoelectric power generation, n-type oxides possessing thermoelectric properties as good as CoO2-based oxides are indispensable. We are exploring new n-type oxides using a combinatorial technique, in which 1000 samples can be prepared and evaluated their thermoelectric properties a day. Aqueous solutions of metal nitrates were used as starting materials. In an hour, a single library of 100 samples can be prepared on a 100 × 100 mm alumina plate for heat treatment. Evaluation of coefficient was performed using a Seebeck tester at a rate of 600 samples/h. In the binary system, the Nd-Ni-O libraries are found out to show n-type thermoelectric properties.

Thermoelectric properties of n-type (Bi 2Se 3) x(Bi 2Te 3) 1− x prepared by bulk mechanical alloying and hot pressing

Starting from element Bi, Se and Te granules, n-type (Bi 2Se 3) x (Bi 2Te 3) 1− x with different chemical composition ( x=0.025, 0.05, 0.075, 0.10 and 0.15) were prepared via bulk mechanical alloying (BMA) and hot pressing (HP). Effects of chemical composition, doping content and annealing on thermoelectric properties were reported. The range x≤0.050 is more favorable than other chemical compositions for the thermoelectric properties of BMAed (Bi 2Se 3) x (Bi 2Te 3) 1− x . When doping with SbI 3, both the electrical resistivity and the Seebeck coefficient of (Bi 2Se 3) 0.05(Bi 2Te 3) 0.95 decrease, thus resulting in no improvement of the figure of merit. On the other hand, annealing the as-HPed (Bi 2Se 3) 0.05(Bi 2Te 3) 0.95 a sufficiently long time is helpful to improve thermoelectric properties, and the maximum figure of merit of (Bi 2Se 3) 0.05(Bi 2Te 3) 0.95 is 2.31×10 −3/K.

Effects of Mechanical Alloying and Copper Addition on Thermoelectric Properties of n-Type and p-Type β-FeSi<SUB>2</SUB>

The effects of mechanical alloying (MA) process by vibration ball milling and Cu addition on the thermoelectric properties of n-type Fe 1-x Co x Si 2 and p-type Fe 1-x Mn x Si 2 have been investigated. It has been found by X-ray diffraction analysis that mechanical alloying (MA) even after a short period is very effective to form the β-phase from the mixture of the α-Fe 2 Si 5 and e-FeSi phases during hot-pressing. Both of the hot-pressed n-type and p-type samples are composed of mostly the β-phase with dispersion of a small amount of e-phase particles. The amount of the e-phase increases with increasing Cu addition. EDX analysis of the n-type and p-type samples has shown that the β-phase contains no Cu, while the e-phase contains Cu and Co or Mn. Since the Co and Mn contents in the e-phase are higher than in the β-phase, the increment of the amount of the e-phase decreases the Co and Mn contents in the β-phase, and significantly changes the thermoelectric power. Cu addition to the n-type Fe 1-x Co x Si 2 significantly decreases the electrical resistivity, and a small amount of Cu addition increases the thermoelectric power, resulting in the improvement of the performance. It is considered that the decrease in the electrical resistivity results from the dispersion of the metallic e-phase in the semiconducting β-phase matrix. On the other hand, Cu addition to the p-type Fe 1-x Mn x Si 2 decreases the performance, which results from a significant decrease in the thermoelectric power due to the decrease in the Mn content of the β-phase as well as the dispersion of the metallic e-phase. The thermal conductivity of the n-type and p-type samples is slightly increased by Cu addition.

The maximum possible conversion efficiency of silicon-germanium thermoelectric generators

The thermoelectric properties of N-type and P-type Si-Ge alloys have been reviewed and detailed calculations for the efficiency of a thermoelectric generator made from a 70% Si-30% Ge alloy have been made over the temperature range from 300 to 1300 K. A model employing one valence band and two conduction bands has been used. A generator of standard material, optimally doped, and infinitely segmented will have an efficiency of 12.1% operating over this range. If the lattice thermal conductivity can be reduced to its minimum value without upsetting the electrical properties, then the efficiency can be raised to an ultimate maximum of 23.3%. A more modest increase in efficiency to 14.7% could be obtained by a 2.4 volume percent of finely dispersed second-phase precipitates which would act as phonon scatterers. The utility/futility of GaP additions and grain-boundary scattering as methods to increase the efficiency is discussed.