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  Thermoelectric Materials and Devices
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I. High Performance Thermoelectric Materials

1. Phonon-liquid Electron-crystal materials (liquid-like materials)

A new mechanism to tune TE transport properties by “engineering” the crystal structure has also been realized in cupper ion conducting compounds. In the Cu2-δX (X: VI elements) binary system, the M atoms form a rigid face-centered cubic lattice providing a crystalline pathway for semiconducting electrons, while the copper ions are highly disordered around the M sublattice and are superionic with liquid-like mobility. The extraordinary “liquid-like” behavior of Cu ions could eliminate some of the vibrational modes in Cu2-δX as well as strongly scatter heat transferred phonons. As a result, intrinsically very low lattice thermal conductivity and high zT value of 1.5-2.1 in this otherwise simple semiconductor family have be achieved. This unusual combination of properties leads to an ideal thermoelectric material within the new concept of “Phonon-Liquid Electron-Crystal”.

Crystal structure of liquid-like material Cu2Se

2. Phase transition and thermoelectrics (critical phenomenon):

Phase transition and thermoelectric figure of merit of Cu2Se

Phase transformations are common occurrences observed in nature and are usually accompanied by dramatic changes in many of the material’s properties at temperatures close to the critical point. Typical examples are ferroelectricity, ferromagnetism, and super- conductivity. Intentional use of phase transitions and investigations of phase-transition- induced thermoelectric (TE) effects have only reported in metal or magnetic alloys with negligible thermoelectric performance. Recently, we found abnormally huge thermoelectric effects around the critical point in Cu2Se with large ZT. Electronic industry focuses specially on the potential to rapidly cool microprocessors and sensors by the technology within a relatively narrow temperature range around or slightly above room temperature. This calls the interests of thermoelectrics during the phase transitions.

3. Diamond-like compounds:

Thermoelectric figure of merit as a function of calculated CF values in non-cubic tetragonal chalcopyrites at 700 K.

Current TE materials usually possess high symmetry crystal structures that support the presence of highly degenerate, multi-valley electronic bands yielding good electronic properties characterized by a large power factor. As a consequence, most of the state-of-the- art TE materials have cubic structures with degenerate band edges and symmetry-related multi-valley carrier pockets. Typical examples are SiGe alloys, PbTe, skutterudites, half- Heusler alloys, Mg2Si, and liquid-like Cu2Se. A notable exception is Bi2Te3 and compounds based on it that have comparatively high hexagonal symmetry. This severely restricts the exploration of TE materials to a small percentage of semiconductors that possess high- symmetry cubic structures, and thus excludes a large number of low-symmetry non-cubic materials even though they might manifest ideal band gaps and low thermal conductivities. It remains a key challenge to discover or design novel high-performance TE compounds among non-cubic materials. Taking a hint from the recently emerging chalcopyrite TE materials with reasonable zT values, a new strategy is proposed to design high performance non-cubic thermoelectric materials through the utilization of a rational pseudocubic structure that assures cubic-like degenerate electronic bands via the coexistence of long-range cubic framework with short-range non-cubic lattice distortions. The pseudocubic approach leads to a simple selection rule, that is to maintain the distortion parameter near unity, to identify a series of novel chalcopyrites with zT values enhanced significantly above the unity.

4. Caged compounds:

A proven approach to improve figure-of-merit is to “decorate” the crystal voids in cage-structured compounds such as the skutterudites and clathrates. As for the CoSb3-based skutterudites, the lattice thermal conductivity has been greatly reduced by filling multiple types of atoms into the Sb-voids. The distributions in vibrational frequency and valence state among fillers provide the possibilities of simultaneously optimizing both the electrical and thermal transports.

Localized vibration frequencies of different fillers in Skutterudites.

5. Organic thermoelectric materials

Organic materials have attracted increasing attentions for applications as thermoelectric materials due to their abundant raw materials, light weight, flexibility, low cost and low thermal conductivity. However, great challenges are faced in improving ZT of organic-based materials. Currently, investigation on improving thermoelectric performance of conducting polymers and the composite are focused mainly on design organic molecular structure, tuning molecular chain arrangement, doping approach, and engineering organic/inorganic interface. Especially, it has been found that the ordered alignment of polymer molecular chain induced by epitaxial growth on small molecular particle as well as by the inorganic/organic interface is effective to improve electrical properties of the polymer-based materials.

High performance SWNT/PANI composite TE materials

Highly orderly P3HT film with high TE performance by small organic molecule epitaxy

6. Thermoelectric thin films and micro devices:

Electronic industry has raised growing demand in the thermal management such as to rapidly cool microprocessors. Thermoelectric thin-film devices could meet this demand due to their high cooling power density and high cooling/heating rate. The thin film TE devices are also promising for the application as micro power supply. Development of thermoelectric thin-film devices with high performance involves high performance thin films, optimal structure design, effective manufacturing process focusing on high performance electrode technology especially with low interfacial resistance. Currently, study on the fabrication and thermoelectric properties of the nanostructured bismuth-telluride-based films and copper selenide films, the advanced manufacturing process and integrated technology of device are being conducted.

II. Thermoelectric devices and applications

1. Thermoelectric devices

The highly expected applications of thermoelectric power generation for harvesting thermal energy have made increasing demands on the TE devices with all-round advances including high energy conversion efficiency and power density, excellent service reliability, competitive cost, and less environment impact as well. For realizing high conversion efficiency, the optimization of structural geometry and interfacial microstructure as well as the joining technology is important as developing high ZT materials. The segment device, containing two or more kinds of TE materials coping with different temperature range, is a promising approach for obtaining high conversion efficiency, but new challenges are simultaneously raised such as realizing stable bonding between the different TE materials with low interfacial electrical/thermal resistance and low thermal stress caused by the different CTEs (coefficient of thermal expansion). Further more, to improve the service behavior, which mainly regards the structural stability and the performance degradation under the harsh environment (such as high temperature and large temperature gradient, electrical field, vibration, oxidation or erosive atmosphere, etc.), besides using the intrinsically stable TE materials, the interfacial structure, sealing and insulating components, and the fabricating process are also among the key technical issues.

Several types of TE devices for power generation with high efficiency and long life duration have been developed. The Bi2Te3-based TEG devices are fabricated using arc-spray method for fabricating electrode without solder and can be used at the temperatures up to 270 oC. These TEG devices with hard structure and can be applied by the pressure of 100MPa. The maximum conversion efficiency is 4~5% under temperature difference about 250 oC. As for the high temperature TE devices, Mo-Cu alloys have been developed as good electrode materials with tunable CTE that matches skutterudite (SKD) materials. The Ti-Al alloys are used as the barrier layer, which can effectively depresses the interfacial diffusion between the Ti-Al barrier layer and SKD during high temperature service keeping low contact resistivity. The SKD devices can continuously working at the hot-side temperature of 550 oC, and the maximum efficiency can be reached to larger than 8%. High temperature (700~1000 oC) TEG devices (including half-Heusler and SiGe) have also been developed. 

Bi2Te3-based(left)、CoSb3-based SKD(middle)、half-Heusler(right) TEG devices

2. Development of TEC/TEG demonstrator systems

A TE cooling/heating demonstrator system has been developed and it can be used in the passenger’s seats in vehicle. This demonstrator system can quickly adjust the surface temperature of the passenger’s seat to a comfortable level within 2 minutes. We also developed 100W~300W TEG demonstrator systems for the applications in exhaust heat recovery of vehicle as well.

Temperature distribution on the seat surface under cooling (up) and heating (down) modes

3. Evaluation technology for TE materials and devices

New evaluation method and technology for both thermoelectric materials and devices are also under development in SICCAS. SICCAS jointed in the International Energy Agency (IEA) under the Implementing Agreement on Advanced Materials for Transportation (AMT), in which the Annex VIII is focusing on the measurement of transport properties of thermoelectric materials. The round-robin test shows good consistence in the measurements of Seebeck and electrical resistivity among different organizations. The new testing methods and equipment for the evaluation of energy conversion efficiency, power output, and long- term / thermal-circle durability of TE devices have also been developed.

Round-robin test: good agreements in Seebeck coefficient and resistivity

 
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