Takeshi Souma

Low-temperature thermoelectric properties of α- and β-Zn4Sb3 bulk crystals prepared by a gradient freeze method and a spark plasma sintering method

Abstract The low temperature thermoelectric properties of Zn 4 Sb 3 samples prepared by the gradient freeze (GF) method and sintering have been characterized. With decreasing temperature a dramatic rise in the thermal expansion is observed associated with the structural transition from β- to α-phase; Δ l / l =2.8×10 −4 at T s GF =257.4 K for GF and Δ l / l =1.6×10 −4 at T s S =236.5 K for sintered samples. Enhancement is observed in electrical conductivity and p -type thermopower at T s GF and T s S , while a reduction is observed in the magnetic susceptibility. The GF sample exhibits higher thermoelectric performance than the sintered sample. The power factor of the α-phase in the GF sample is twice as large as that of the β -phase; it exceeds 20 μW/cm·K 2 between 120 and 240 K, indicating that the α-phase Zn 4 Sb 3 is one of the prime candidates for thermoelectric materials for cryogenic use.

Power generation characteristics of oxide thermoelectric modules incorporating nanostructured ZnO sintered materials

A practical oxide thermoelectric module consisting of 12 pairs of p-NaCo2O4 and nanostructured n-ZnO sintered materials was successfully fabricated by using a diffusion welding technique. By applying a nanostructure using a void forming agency (VFA), the electrical conductivity of the n-ZnO sintered materials were drastically enhanced, leading that the power factor of the materials were dramatically improved. It is revealed that the oxide module can be operated up to 930 K on the hot side temperature. A maximum power output of 52.5 mW was achieved at the temperature condition of TH/TL=934/479 K, where TH and TL are temperatures at the hot and cold sides of the module, respectively. The details of the power generation characteristics of the present oxide module will be presented and the performance will be discussed with those of our previous and other oxide modules.

Fabrication and power generation characteristics of p-NaCo2O4/n-ZnO oxide thermoelectric modules

A prototype of thermoelectric module using oxide materials has been successfully fabricated, and the power generation characteristics have been examined in the high temperature region around 773 K (500 degC). Twelve couples of sintered p-NaCo2O4 and n-ZnO materials were connected via silver conducting strips with a planer arrangement jointed by using a diffusion welding technique under 16 MPa at 1023 K in Ar. A maximum power output of 58 mW was achieved at a temperature condition of TH/TL = 839/377 K. The details of the power generation characteristics of the oxide module will be presented and the performance will be discussed with other oxide modules

Synthesis and rietveld analysis for CoSb/sub 3/ compounds prepared by Sb self-flux method

High purity CoSb3 bulk crystals have been successfully synthesized by Sb self-flux method and a relation between reaction conditions and chemical composition on the method has been systematically analyzed by powder XRD study using the Rietveld analysis. Sb self flux method at 923 K using 100 mesh Co elements can directly provide a single phase CoSb3 bulk crystal in a brief time of 10 h. Advantages of Sb self-flux methods will be discussed compared with other preparation methods.

Evaluation of durability of thermoelectric properties for Zn/sub 4/Sb/sub 3/ compounds

Durability of the Zn/sub 4/Sb/sub 3/ compounds was evaluated in term of their thermoelectric properties by applying repeated heat cycles to the vacuum-casted and hot-pressed samples. The power factor of both samples decreased by applying 20 cycle heat load. The major reason of the deterioration was formation of the ZnCd compounds due to evaporating Zn from the Zn/sub 4/Sb/sub 3/ compounds. The degree of the deterioration of the thermoelectric performance and a solution to enhanced durability of the compounds are discussed.

Optimization of synthesis conditions for CoSb3 compounds prepared by Sb self-flux method

CoSb3 bulk materials were successfully prepared by Sb self-flux technique, and the synthesis conditions were widely investigated to obtain the high purity compounds utilizing a quantitative powder XRD study with the Rietveld analysis applying a multi-phase-fitting condition. The reaction temperature range for the Sb self-flux technique was extended up to 1123 K to promote the reaction in the Co-Sb system. The relation between the reaction temperature and the chemical composition is discussed to optimize the preparation conditions in the Sb self-flux technique

Relation between zn content and thermoelectric properties of zn 4+x sb 3 (-0.12⩽x⩽0.12)

A series of seven bulk crystals of Zn_4+xSb₃ (-0.12 ⩽ x ⩽ 0.12) are successfully prepared, and the relation between the Zn content and the thermoelectric properties is investigated. The sample with the nominal composition of Zn_3.96Sb₃ (x = -0.04) contain the maximum mass fraction of Zn ₄Sb₃. The shapes of the S(T) and ρ(T) curves are drastically changed with varying x in the temperature range from 300 K to 700 K. Whereas the lower x leads to lower ρ, the magnitude of S unchanges, resulting in the fact that the power factor of the Zn-poor compositions in the Zn_4+xSb₃ system is lower than that of the Zn-rich compositions. The details of the influence of the Zn content and the scope of the improvement of the durability are discussed.

Preparation and thermoelectric properties of Co/sub x/Fe/sub y/Ni/sub 2/Sb/sub 3/ (x+y+z=1, x=0 to 1, y=z) sintered samples

Eleven sintered samples of the CoxFeyNizSb3 (x+y+z=1, x=0 to 1, y=z) compounds have been successfully prepared using hot-pressing technique and the thermoelectric properties of the samples have been measured between 300 and 700 K. The purities and densities for the sintered samples exceed 93 mass% and 80 % of XRD density, respectively. The thermopower of the compounds is in the range of -150 to 200 µW/K and shifts from p-type to n-type with increasing the value of y+z. The electrical conductivity for the samples shows a semiconductor like behavior. The power factors for the present samples show several µW/K 2 cm. The possibility for improvement in the thermoelectric performance will be dicussed by optimizing the Fe/Ni ratio in the compounds.

Comparison of structural parameters for Zn/sub 4-x/Cd/sub x/Sb/sub 3/ compounds analyzed by the Rietveld method using two crystallographic models

A comparison of detailed structural parameters between the 3IS and Mayer models for the Zn/sub 4-x/Cd/sub x/Sb/sub 3/ system in the Zn-rich region has been successfully performed by the powder XRD study with the Rietveld method using 9 bulk crystals synthesized by vacuum casting method without annealing. The 3IS model gives a better fitting on the Rietveld analysis not only for the undoped Zn/sub 4/Sb/sub 3/ but also for the whole compositional range of the Zn-rich region of the Zn/sub 4-x/Cd/sub x/Sb/sub 3/ compounds. The purities and densities of the bulk crystals exceeded approximately 96 mass% and 95% of XRD density, respectively.

Low-temperature transport properties of Zn/sub 4/Sb/sub 3/ bulk crystal grown by gradient freeze method

The low temperature thermoelectric properties have been investigated on Zn/sub 4/Sb/sub 3/ compound between 4 K and 300 K. A single phase Zn/sub 4/Sb/sub 3/ bulk crystal with the density of 95.4% of the theoretical has been grown by a gradient freeze method. The resistivity exhibits a metallic conduction with an anomaly associated with a structural transformation from /spl alpha/ to /spl beta/ phase at T/sub S//sup GF/=257.4 K. The thermopower is positive and larger in /spl alpha/ than in /spl beta/ phase. The elongation of Zn/sub 4/Sb/sub 3/ is small (1/spl times/10/sup -4/) at T/sub S//sup GF/. The magnetic measurement shows that Zn/sub 4/Sb/sub 3/ exhibits diamagnetism and /spl chi/(T) curve has an appreciable increase at T/sub S//sup GF/. The power factor of a phase is two times as large as that of /spl beta/ phase at low-temperature region (below 300 K).