A. Borshchevsky

Preparation and thermoelectric properties of semiconducting Zn4Sb3

Hot-pressed samples of the semiconducting compound β-Zn4Sb3 were prepared and characterized by X-ray and microprobe analysis. Some physical properties of β-Zn4Sb3 were determined and its thermoelectric properties measured between room temperature and 650 K. Exceptionally low thermal conductivity values were obtained in the 300–650 K temperature range and the room temperature lattice thermal conductivity was estimated at 6.5 W cm−1 K−1. High thermoelectric figures of merit (ZTs) were obtained between 450 and 670 K and a maximum of about 1.3 was obtained at a temperature of 670 K, the highest known at this temperature. The stability of the compound was investigated by several techniques, including thermogravimetric studies. The results showed that the samples were stable under argon atmosphere and static vacuum up to about 670 K and up to 520 K in dynamic vacuum. The high thermoelectric performance of β-Zn4Sb3 in the 300–670 K temperature range fills the existing gap in the ZT spectrum of p-type state-of-the-art thermoelectric materials between Bi2Te3-based alloys and PbTe-based alloys. This material, relatively inexpensive, could be used in more efficient thermoelectric generators for waste heat recovery and automobile industry applications, for example.

Properties of Single Crystalline Semiconducting CoSb3

A study of the thermoelectric properties of the skutterudite compound CoSb3 was carried out on single crystals grown by the Bridgman gradient freeze technique. p‐ and n‐type samples were obtained over a wide range of carrier concentration. Undoped As‐grown crystals show p‐type conductivity while n‐type samples were obtained by addition of Te or Pd. Samples were characterized by x‐ray diffractometry, electron microprobe analysis, and density measurements. The physical properties of CoSb3 such as linear thermal expansion coefficient, sound velocity, and Debye temperature were also determined and are presented. Seebeck coefficient, electrical resistivity, thermal conductivity, and Hall effect measurements were performed between room temperature and about 900 K. Exceptionally high Hall mobilities were obtained on p‐type samples with a maximum room‐temperature Hall mobility of 3300 cm2 V−1 s−1 at a carrier concentration of 1×1017 cm−3. The results of the transport property measurements are discussed and are in...

Bridgman-solution crystal growth and characterization of the skutterudite compounds CoSb3 and RhSb3

Abstract Compounds with the skutterudite structure have recently been identified as advanced thermoelectric materials. We report on the crystal growth and characterization of the skutterudite compounds CoSb 3 and RhSb 3 which form peritectically at 873 and 900°C, respectively. Large single crystals were obtained by the vertical gradient freeze technique from solutions rich in antimony. The samples were characterized by high-temperature Hall-effect and electrical resistivity measurements. Bandgaps of 0.56 and 0.80 eV were estimated from these measurements for CoSb 3 and RhSb 3 , respectively. N-type CoSb 3 samples were obtained by doping with Te. Exceptionally high p-type Hall-mobility values have been measured and a room-temperature value of 3445 cm 2 V −1 s −1 was obtained for CoSb 3 at a carrier concentration of 4 × 10 17 cm −3 and 8000 cm 2 V −1 s −1 was obtained for RhSb 3 at a carrier concentration of 3.5 × 10 18 cm −3 .

Preparation and thermoelectric properties of the skutterudite‐related phase Ru0.5Pd0.5Sb3

A new skutterudite phase Ru0.5Pd0.5Sb3 was prepared. This new phase adds to a large number of already known materials with the skutterudite structure which have shown good potential for thermoelectric applications. Single phase, polycrystalline samples were prepared and characterized by x‐ray analysis, electron probe microanalysis, density, sound velocity, thermal‐expansion coefficient, and differential thermal analysis measurements. Ru0.5Pd0.5Sb3 has a cubic lattice, space group Im3 (T5h), with a=9.298 A and decomposes at about 920 K. The Seebeck coefficient, the electrical resistivity, the Hall effect, and the thermal conductivity were measured on hot‐pressed samples over a wide range of temperatures. Preliminary results show that Ru0.5Pd0.5Sb3 behaves as a heavily doped semiconductor with an estimated band gap of about 0.6 eV. The lattice thermal conductivity of Ru0.5Pd0.5Sb3 is substantially lower than that of the binary isostructural compounds CoSb3 and IrSb3. The unusually low thermal conductivity m...

Thermoelectric microcoolers for thermal management applications

Due to the combined increase in circuit integration and chip power dissipation, there is a rapidly growing demand for solving the thermal management issues of power microelectronics. We are pursuing a novel thermal management approach that actively cools only the key high power devices by using a novel thermoelectric microcooler located under each of these power devices. In this way the device can operate at temperatures at or even below the ambient temperature of the heat sink, resulting in increased reliability and efficiency. To successfully handle the high heat flux densities generated at the back of the power chips, a microcooler with thin legs and low thermal resistance at the interfaces must be built. We are currently developing a thermoelectric microcooler combining thick films of Bi/sub 2/Te/sub 3/-based alloys and very high thermal conductivity substrates, such as CVD diamond or AlN. Electrochemical deposition is a very attractive process for depositing thick films of compound semiconductors on metallic surfaces. This paper presents recent results on the deposition of Bi/sub 2/Te/sub 3/ and related ternary solid solutions on a variety of metallic substrates. We also report on the development of Cu diffusion barriers for Bi/sub 2/Te/sub 3/ and stable metallizations and diffusion barriers for diamond and AlN substrates.

New materials and devices for thermoelectric applications

New high efficiency thermoelectric materials with ZT values substantially larger than 1.0 have been recently developed. The successful development of a segmented thermoelectric generator utilizing a combination of state-of-the-art thermoelectric materials and these novel thermoelectric materials could result in a high thermal-to-electric materials conversion efficiency over 19%. This is because the new thermoelectric materials have a higher average ZT value and the generator can be operated in a temperature range wider than possible with state-of-the-art (SOA) materials. Such improved power generators could be used for a variety of applications including waste heat recovery. This could also lead to the development of high performance thermoelectric generators operating in a relatively narrow temperature range for specific applications. However, there are also new low power and thermal management applications for small thermoelectric devices using SOA materials. If more efficient thermoelectric materials are incorporated, the performance of these new devices will be even higher.

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.

Development of a high efficiency thermoelectric unicouple for power generation applications

To achieve high thermal-to-electric energy conversion efficiency, it is desirable to operate thermoelectric generator devices over large temperature gradients and also to maximize the performance of the thermoelectric materials used to build the devices. However, no single thermoelectric material is suitable for use over a very wide range of temperatures (/spl sim/300-1000 K). It is therefore necessary to use different materials in each temperature range where they possess optimum performance. This can be achieved in two ways: (1) multistage thermoelectric generators where each stage operates over a fixed temperature difference and is electrically insulated but thermally in contact with the other stages; and (2) segmented generators where the p- and n-legs are formed of different segments joined in series. The concept of integrating new thermoelectric materials into a segmented thermoelectric unicouple has been introduced in earlier publications. This new unicouple is expected to operate over a 300-973 K temperature difference and will use novel segmented legs based on a combination of state-of-the-art thermoelectric materials and novel p-type Zn/sub 4/Sb/sub 3/, p-type CeFeSb/sub 12/-based alloys and n-type CoSb/sub 3/-based alloys. A conversion efficiency of about 15% is predicted for this new unicouple. We present the latest experimental results from the fabrication of this unicouple, including bonding studies between the different segments of the p-legs, n-legs, and p-leg to n-leg interconnect. Thermal and electrical tests of the unicouple are in progress and are briefly described.

Skutterudites: an update

Materials with the skutterudite crystal structure possess attractive transport properties and have a good potential for achieving ZT values substantially larger than for state-of-the-art thermoelectric materials. Studies conducted at JPL on CoAs/sub 3/, RhAs/sub 3/, CoSb/sub 3/, RhSb/sub 3/ and IrSb/sub 3/ have shown that p-type conductivity samples are characterized by carriers with low effective masses and very high mobilities, low electrical resistivities and moderate Seebeck coefficients. The carrier mobilities of n-type samples are about an order of magnitude lower, but low electrical resistivities and relatively large Seebeck coefficients can still be obtained at high doping levels. The room temperature lattice thermal conductivities of these binary skutterudites was found to be 7 to 10 times larger than that of Bi/sub 2/Te/sub 3/. This results in low ZT values at 300K, though very heavily doped n-type CoSb/sub 3/ samples can achieve ZT/spl sim/1 at 600/spl deg/C. Several research groups, mostly in the US, are now working on understanding and optimizing the transport properties of skutterudites. Most of the efforts are focusing on reducing the lattice thermal conductivity by filling the empty octant cages in the skutterudite structure with rare earth atoms. Additional approaches have also been pursued at JPL, in particular the formation of solid solutions and alloys, and the study of novel ternary skutterudite compounds. Recent experiments have demonstrated that ternary compounds such as Ru/sub 0.5/Pd/sub 0.5/Sb/sub 3/ and filled skutterudites such as CeFe/sub 4/Sb/sub 12/ had much lower lattice thermal conductivity. High ZT values have been obtained for several filled skutterudites in the 500-700/spl deg/C temperature range, but figures of merit at 300K are still low. This paper reviews recent experimental and theoretical results on skutterudites with a particular emphasis on the transport properties of ternary compounds and filled compositions. The latest results obtained at JPL are presented and the possibility of obtaining high ZT values near room temperature is discussed.

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.