G. J. Snyder

High-temperature electrical and thermal transport properties of fully filled skutterudites RFe4Sb12 (R = Ca, Sr, Ba, La, Ce, Pr, Nd, Eu, and Yb)

Fully filled skutterudites RFe_(4)Sb_(12) (R = Ca, Sr, Ba, La, Ce, Pr, Nd, Eu, and Yb) have been prepared and the high-temperature electrical and thermal transport properties are investigated systematically. Lattice constants of RFe_(4)Sb_(12) increase almost linearly with increasing the ionic radii of the fillers, while the lattice expansion in filled structure is weakly influenced by the filler valence charge states. Using simple charge counting, the hole concentration in RFe_(4)Sb_(12) with divalent fillers (R = Ca, Sr, Ba, Eu, and Yb) is much higher than that in RFe4Sb12 with trivalent fillers (R = La, Ce, Pr, and Nd), resulting in relatively high electrical conductivity and low Seebeck coefficient. It is also found that RFe_(4)Sb_(12) filled skutterudites having similar filler valence charge states exhibit comparable electrical conductivity and Seebeck coefficient, and the behavior of the temperature dependence, thereby leading to comparable power factor values in the temperature range from 300 to 800 K. All RFe_(4)Sb_(12) samples possess low lattice thermal conductivity. The correlation between the lattice thermal resistivity WL and ionic radii of the fillers is discussed and a good relationship of W_L ~ (r_(cage)−r_(ion))^3 is observed in lanthanide metal filled skutterudites. CeFe_(4)Sb_(12), PrFe_(4)Sb_(12), and NdFe_(4)Sb_(12) show the highest thermoelectric figure of merit around 0.87 at 750 K among all the filled skutterudites studied in this work.

Potential of Chevrel Phases for Thermoelectric Applications

Abstract A low lattice thermal conductivity is one of the requirements to achieve high thermoelectric figures of merit. Several low thermal conductivity materials were identified and developed over the past few years at the Jet Propulsion Laboratory (JPL), including filled skutterudites and Zn 4 Sb 3 -based materials. A study of the mechanisms responsible for the high phonon scattering rates in these compounds has demonstrated that materials with structures that can accommodate additional atoms in their lattice are likely to possess low lattice thermal conductivity values. Chevrel phases (Mo 6 Se 8 -type) are just such materials and are currently being investigated at JPL for thermoelectric applications. The crystal structures of the Chevrel phases present cavities which can greatly vary in size and can contain a large variety of atoms ranging from large ones such as Pb to small ones such as Cu. In these materials, small inserted atoms usually show large thermal parameters which indicate that they move around and can significantly scatter the phonons. The electronic and thermal properties of these materials can potentially be controlled by a careful selection of the filling element(s). We have synthesized (Cu, Cu/Fe, Ti) x Mo 6 Se 8 samples and report in this paper on their thermoelectric properties. Approaches to optimize the properties of these materials for thermoelectric applications are discussed. Solid State Sciences, 1293-2558/99/7-8/© 1999 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Development of high efficiency segmented thermoelectric unicouples

Highly efficient, segmented thermoelectric unicouples incorporating advanced thermoelectric materials with superior thermoelectric figures of merit are currently being developed at the Jet Propulsion Laboratory (JPL). These segmented unicouples include a combination of state-of-the-art thermoelectric materials based on Bi/sub 2/Te/sub 3/ and novel p-type Zn/sub 4/Sb/sub 3/, p-type CeFe/sub 4/Sb/sub 12/-based alloys and n-type CoSb/sub 3/-based alloys developed at JPL. The maximum predicted thermal to electrical efficiency is about 15% for a hot-side temperature of 975K and a cold-side temperature of about 300K. Various segmentations have been explored and several unicouples have been fabricated and tested. The set-up for testing these unicouples is described in this paper and some of the tests results reported. I-V curves have been generated for selected unicouples. The results show that experimental thermal to electrical efficiency values close to theoretical predicted values have been measured.

Thermoelectric microdevice fabrication process and evaluation at the Jet Propulsion Laboratory (JPL)

In the Materials and Device Technology Group at JPL, we have developed a unique fabrication method for a thermoelectric microdevice that utilizes standard integrated circuit techniques in combination with electrochemical deposition of compound semiconductors (Bi/sub 2/Te/sub 3//Bi/sub 2-x/Sb/sub x/Te/sub 3/). Our fabrication process is innovative in the sense that we are able to electrochemically micro mold different thermoelectric elements, with the flexibility of adjusting geometry, materials composition or batch scalability. Successive layers of photoresist were patterned and electrochemically filled with compound semiconductor materials or metal interconnects (Au or Ni). A thermoelectric microdevice was built on either glass or an oxidized silicon substrate containing 63 couples (63 n-legs/63 p-legs) at approximately 20 microns in structure height and with a device area close to 1700 /spl mu/m x 1700 /spl mu/m. In cooling mode, we evaluated device performance using an IR camera and differential thermal imaging software.

Preparation and thermoelectric properties of some phosphide skutterudite compounds

Thermoelectric properties of CoP3 and CeFe4P12 have been measured. These compounds were synthesized by a flux technique using Sn as the solvent. The samples were characterized by x-ray diffractometry and electron microprobe analyses. The Seebeck coefficient, the electrical resistivity, the Hall effect, and the thermal conductivity were measured over a wide range of temperatures. The results indicate that CoP3 and CeFe4P12 are semiconductors, in agreement with theoretical predictions. The thermal conductivity of CeFe4P12 is about 10 times larger than that for CeFe4Sb12 which is primarily due to both reduced motion of the Ce ions in smaller voids and lower hole–phonon scattering. The results are analyzed and discussed to provide guidelines for optimization of the thermoelectric properties of these materials.

Thermal stability and thermoelectric properties of p-type Ba8Ga16Ge30 clathrates

The thermal stability of p-type Ba(8)Ga(16)Ge(3)0 clathrates grown from gallium flux has been tested by heat treatment in low pressure Ar atmosphere at 400, 600, and 800 degrees C. Significant gallium loss was observed for all samples during heat treatment. The treatment at 400 degrees C does not significantly change the sample properties, and the samples remain p-type and comparable to the untreated, as-prepared, sample. At 600 degrees C the sample switches from extrinsic p-type to extrinsic n-type, presumably due to significant loss of Ga, and shows a high thermopower but a reduced electrical conductivity compared to as-made n-type samples. Surprisingly, after a thermal treatment at 800 degrees C, the crystal structure seemingly loses less Ga, only reducing the hole concentration to near intrinsic levels and thus has a negative impact on ZT. Regardless of the heat treatment temperature of the p-type samples the thermal conductivity remained exceptionally low, for some samples 0.9 W/m K. Heat treatment can thus greatly affect the thermoelectric properties of p-type Ba(8)Ga(16)Ge(3)0, but the crystal structure remains intact

Compatibility of segmented thermoelectric generators

It is well known that power generation efficiency improves when materials with appropriate properties are combined either in a cascaded or segmented fashion across a temperature gradient. Past methods for determining materials used in segmentation were mainly concerned with materials that have the highest figure of merit in the temperature range. However, the example of SiGe segmented with Bi/sub 2/Te/sub 3/ and/or various skutterudites shows a marked decline in device efficiency even though SiGe has the highest figure of merit in the temperature range. The origin of the incompatibility of SiGe with other thermoelectric materials leads to a general definition of compatibility and intrinsic efficiency. The compatibility factor derived as s = ( /spl radic/(1+ZT-1))//spl alpha//sub T/ is a function of only intrinsic material properties and temperature, which is represented by a ratio of current to conduction heat. For maximum efficiency the compatibility factor should not change much with temperature both within a single material, and in the segmented leg as a whole. This leads to a measure of compatibility not only between segments, but also within a segment. General temperature trends show that materials are more self compatible at higher temperatures, and segmentation is more difficult across a larger /spl Delta/T. The compatibility factor can be used as a quantitative guide for deciding whether a material is better suited for segmentation or cascading. Analysis of compatibility factors and intrinsic efficiency for optimal segmentation are discussed, with intent to predict optimal material properties, temperature interfaces, and/or current/heat ratios.

A study of heat sink performance in air and soil for use in a thermoelectric energy harvesting device

A suggested application of a thermoelectric generator is to exploit the natural temperature difference between the air and the soil to generate small amounts of electrical energy. Since the conversion efficiency of even the best thermoelectric generators available is very low, the performance of the heat sinks providing the heat flow is critical. By providing a constant heat input to various heat sinks, field tests of their thermal conductances in soil and in air were performed. A prototype device without a thermoelectric generator was constructed, buried, and monitored to experimentally measure the heat flow achievable in such a system. Theoretical considerations for design and selection of improved heat sinks are also presented. In particular, the method of shape factor analysis is used to give rough estimates and upper bounds for the thermal conductance of a passive heat sink buried in soil.

Progress in the development of high efficiency segmented thermoelectric generators

The integration of new more efficient thermoelectric materials developed at the Jet Propulsion Laboratory into a new high performance segmented thermoelectric generator has been reported earlier. Progress in the development of this new segmented thermoelectric generator is reported in this paper. This generator would operate over a large temperature difference (300–973 K) and uses novel segmented legs based on a combination of state-of-the-art thermoelectric materials and p-type Zn4−xCdxSb3 alloys, p-type CeFe4Sb12-based alloys and n-type CoSb3-based alloys. An increase in the thermoelectric materials conversion efficiency of about 60% is expected compared to Bi2Te3- and PbTe-based generators. A computer program was written to optimize the thermal efficiency of the device. The optimal geometry, power output, efficiency and other properties of the generator were calculated and are presented. In addition results of bonding studies between Zn4Sb3 and Bi0.4Sb1.6Te3 are reported and discussed.

Reduction of lattice thermal conductivity from planar faults in the layered Zintl compound SrZnSb2

phonon scattering. 12 In the present study we perform transmission electron microscopy (TEM) investigations of the SrZnSb2 compound. We show that planar defects are present, but that these are not due to twinning. Instead, the defects are out-of-phase boundaries created by rigid shifts in the b–c plane of the material. These defects are discussed in light of the similar SrZnBi2 compound, and we report on calculations of the energetics of the observed defects. Furthermore, the effect of FIG. 1. (Color online) The crystal structure of (a) SrZnSb2 and (b) SrZnBi2. The arrangement of Zn and Sb/Bi atoms is very similar in the two structures, while the sequence of Sr atoms is different. A rigid shift of half the unit cell by the vector [0 1=2 1=2] would give similar arrangement of Sr atoms, while