Christian Stiewe

Macroscopic thermoelectric inhomogeneities in (AgSbTe 2) x (PbTe) 1-x

Exceptionally high thermoelectric figure of merit (zT>2), has been reported for (Ag1−ySbTe2)0.05(PbTe)0.95, which may involve the nanoscale microstructure. However, conflicting reports on the same materials claim only zT of 1 or less. Here we show that (Ag1−ySbTe2)0.05(PbTe)0.95 materials are multiphase on the scale of millimeters despite appearing homogeneous by x-ray diffraction and routine electron microscopy. Using a scanning Seebeck microprobe, we find significant variation of Seebeck coefficient (including both n-type and p-type behavior in the same sample) that can explain the discrepancy in reported zT. More homogeneous samples can be prepared with faster cooling rates.

Large thermoelectric figure of merit at high temperature in Czochralski-grown clathrate Ba8Ga16Ge30

The Czochralski method was used to grow a 46-mm-long crystal of the Ba8Ga16Ge30 clathrate, which was cut into disks that were evaluated for thermoelectric performance. The Seebeck coefficient and electrical and thermal conductivities all showed evidence of a transition from extrinsic to intrinsic behavior in the range of 600–900K. The corresponding figure of merit (ZT) was found to be a record high of 1.35 at 900K and with an extrapolated maximum of 1.63 at 1100K. This makes the Ba8Ga16Ge30 clathrate an exceptionally strong candidate for medium and high-temperature thermoelectric applications.

High temperature transport properties of partially filled CaxCo4Sb12 skutterudites

Partially filled CoSb3 skutterudite compounds are emerging materials for thermoelectric energy conversion at high temperature. CaxCo4Sb12 with different Ca contents has been prepared by the conventional metallurgical route. The temperature dependences of the electrical resistivity, Seebeck coefficient, and thermal conductivity have been measured on these compounds in the 300–800 K temperature range. These measurements have identified Ca as being a true n-type filler atom and offer, for this family of skutterudite, several valuable insights into the potential of Ca to provide good thermoelectric performance.

Nanostructured Co1−xNixSb3 skutterudites: Synthesis, thermoelectric properties, and theoretical modeling

Nanostructured skutterudite Co1-xNixSb3 has been synthesized by chemical alloying with Ni substitution for Co up to 27.5 at. %. High concentration of grain boundaries provided by nanostructuring is ...

Effect of ceramic dispersion on thermoelectric properties of nano-ZrO2∕CoSb3 composites

In the present work, nano-ZrO2∕CoSb3 composites were fabricated by milling ZrO2 and CoSb3 powders and hot pressing at different sintering temperatures. For the prepared compacts, the phase purity, microstructure, and temperature-dependent thermoelectric properties were characterized. The effect of nano-ZrO2 dispersion on composite electrical conductivity and thermal conductivity is strictly clarified by comparing the transport properties of the nondispersed and dispersed CoSb3 at identical porosity, so that the effect of porosity on thermoelectric parameters could be eliminated. The effect of the insulating inclusion itself on transport properties is also considered and eliminated using effective media theories. It is clearly verified that charge carrier scattering and phonon scattering occur simultaneously to lower the electrical conductivity and the thermal conductivity of CoSb3 due to the introduction of nano-ZrO2 inclusions. The investigated composites show higher electrical conductivity due to existe...

Nanostructured Co1-xNix(Sb1-yTey)(3) skutterudites : Theoretical modeling, synthesis and thermoelectric properties

The properties of Te-doped Co(Sb1-yTey)(3) and Te-Ni double-doped Co1-xNix(Sb1-yTey)(3) nanostructured skutterudites were evaluated by means of x-ray powder diffraction, and transport properties me ...

Macroscopic thermoelectric inhomogeneities in (AgSbTe2)x(PbTe)1−x

Exceptionally high thermoelectric figure of merit (zT>2), has been reported for (Ag1−ySbTe2)0.05(PbTe)0.95, which may involve the nanoscale microstructure. However, conflicting reports on the same materials claim only zT of 1 or less. Here we show that (Ag1−ySbTe2)0.05(PbTe)0.95 materials are multiphase on the scale of millimeters despite appearing homogeneous by x-ray diffraction and routine electron microscopy. Using a scanning Seebeck microprobe, we find significant variation of Seebeck coefficient (including both n-type and p-type behavior in the same sample) that can explain the discrepancy in reported zT. More homogeneous samples can be prepared with faster cooling rates.

From phase-change materials to thermoelectrics?

Abstract Metastable tellurides play an important role as phase-change materials in data storage media and non-vol atile RAM devices. The corresponding crystalline phases with very simple basic structures are not stable as bulk materials at ambient conditions, however, for a broad range of compositions they represent stable high-temperature phases. In the system Ge/Sb/Te, rocksalt-type high-temperature phases are characterized by a large number of vacancies randomly distributed over the cation position, which order as 2D vacancy layers upon cooling. Short-range order in quenched samples produces pronounced nanostructures by the formation of twin domains and finite intersecting vacancy layers. As phase-change materials are usually semimetals or small-bandgap semiconductors and efficient data storage requires low thermal conductivity, bulk materials with similar compositions and properties can be expected to exhibit promising thermoelectric characteristics. Nanostructuring by phase transitions that involve partial vacany ordering may enhance the efficiency of such thermoelectrics. We have shown that germanium antimony tellurides with compositions close to those used as phase-change materials in rewritable Blu-Ray Discs, e.g. (GeTe)12Sb2Te3, exhibit thermoelectric figures of merit of up to ZT = 1.3 at 450 °C if a nanodomain structure is induced by rapidly quenching the cubic high-temperature phase. Structural changes have been elucidated by X-ray diffraction and high-resolution electron microscopy.

Nano ZrO2/CoSb3 composites with improved thermoelectric figure of merit

Nano ZrO2/CoSb3 composites with different ZrO2 contents were prepared using hot pressing. The phase purity, the microstructure and the temperature-dependent transport parameters of the composites were investigated. The dimensionless figure of merit (ZT) of 0.18 of the non-dispersed CoSb3 preponderates the maximal value (0.17) of pure CoSb3 reported in the literature, which is attributed to the prepared sample having higher electrical conductivity due to the existence of a small amount of metallic Sb and lower thermal conductivity due to the fine-grained structure. Compared to non-dispersed CoSb3, a further improvement of 11% on ZT (0.20) was achieved in the composite with 0.05ZrO2 inclusions, which resulted from the enhanced ratio of electrical conductivity to thermal conductivity and the Seebeck coefficient. The nanodispersion method provides an effective approach to improving a material’s thermoelectric properties and performance.

Beneficial effect of Ni substitution on the thermoelectric properties in partially filled CayCo4−xNixSb12 skutterudites

We have investigated the influence of nickel doping in a series of n-type CoSb{sub 3} skutterudite materials partially filled with Ca. The electrical resistivity, thermopower, and thermal conductivity have been measured in the 300-800 K temperature range on Ca{sub y}Co{sub 1-x}Ni{sub x}Sb{sub 12} compounds prepared by a powder metallurgy route. The chemical composition and structure have been analyzed by electron probe microanalyses and x-ray diffraction, respectively. It was found that the presence of nickel substantially decreases the electrical resistivity without any detrimental effect on the thermopower, resulting in an improvement of the power factor. The dimensionless thermoelectric figure ZT increases with increasing temperature and reaches a maximum value of 1 at 800 K.