Thermoelectric Oxide Ceramics and Devices: Comparison
Please note this is a comparison between Version 2 by Lindsay Dong and Version 3 by Lindsay Dong.

Thermoelectric materials have gained wide attention to realize multilevel efficient energy management to alleviate the increasingly severe energy crisis. Oxide ceramics were well-explored as potential thermoelectric candidates because of their outstanding merits, including abundance, eco-friendliness, high-temperature stability, and chemical stability. AThis entry aims to provide a comprehensive summary of the diversified state-of-the-art oxide ceramics and establish the links between composition designing, preparation process, structural characteristics, and properties to summarize the underlying chemistry and physics mechanism of band engineering, doping, composited with the second phase, defects engineering, and entropy engineering is provided. Furthermore, advanced device design and applications such as thermoelectric modules, miniature generators, sensors, and coolers were summarized. Ultimately, the challenges and future perspective of oxides ceramics for the device design and thermoelectric applications in the development of energy harvesting technology have been prospected.

  • thermoelectrics
  • oxides ceramics
  • ZT
  • electrical conductivity
  • phonon scattering

1. Introduction

Thermoelectric materials (TEs) have been used as a potential energy harvesting technology because they can convert heat into electricity and have no requirements for waste heat temperature [1][2][3]. Thermoelectric devices generally consist of n-type and p-type TEs wired electrically in series (or partly parallel) and thermally in parallel. Furthermore, there are also thermoelectric devices with single n-type and p-type TEs. They have the advantages of no moving parts, no noise, small size, etc., and have significant application merits in the military, aerospace, and high-tech energy fields [4][5].

It has been more than 200 years since the thermoelectric effect was discovered, and people have been constantly exploring and developing its industrial applications. In the early 1920s, Altenkirch, a German physicist, developed the fundamentals of thermoelectric power generation and refrigeration and summarized the performance evaluation parameters of TEs [6]: electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ). Dimensionless thermoelectric merit (ZT = S2σT/κ, S is Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, κ is thermal conductivity) is usually used as an indicator to measure the thermoelectric performance [7]. TEs with large ZT values must meet the requirements of a high Seebeck coefficient to ensure the generation of the obvious thermoelectric effect—high electrical conductivity leading to small Joule heat, large output power, as well as low thermal conductivity, are required to generate a substantial temperature difference. The above three thermoelectric parameters have a strong coupled relationship because they are dependent on the carrier concentration in a conflicting manner that restricts and influences each other, making how to optimize thermoelectric performance a huge challenge. Therefore, the coordinated regulation of S, σ, and κ to improve ZT has become the key point to realize the industrial application of thermoelectric materials.

2. Thermoelectric Fundamentals

Thermoelectric materials utilize the thermoelectric effect to achieve direct heat-to-electricity conversion. As shown in the schematic diagrams in Figure 1, the thermoelectric effect includes three effects: (i) the Seebeck effect, which transforms heat into electricity; (ii) the Peltier effect, absorption or release of heat at a junction in which there is an electric current; and (iii) the Thomson effect [1], the evolution or absorption of heat when an electric current passes through a circuit composed of a single material that has a temperature difference along its length. The most common application of the Seebeck effect is the widely existing thermocouple, which can be used in thermometers, thermoelectric power generation, and other thermal cycle fields. Static cooling is the major application of the Peltier effect. The Thomson effect establishes a connection between the previous two and reflects their differential influence.

Figure 1. Schematic diagram of thermoelectric effects. (a) Seebeck effect for power generation, (b) Peltier effect for refrigeration, (c) Thomson effect for reversible cooling or heating.

3. Fabrication of Thermoelectric Oxide Ceramics

3.1. Lattice Structures of Thermoelectric Oxide Ceramics

3.1.1. n-Type Thermoelectric Oxides

As an n-type thermoelectric oxide, strontium titanate (SrTiO3) has attracted widespread interest due to its high effective mass of carriers, chemical and thermal stability at high temperatures, and high structural tolerance. SrTiO3 has a cubic perovskite structure and Pm3m space group at room temperature, and its lattice constant is a = b =c = 3.905 Å [8]. In a unit cell unit connected by solid lines in the figure, Ti4+ ions occupy the central position in the unit cell, Sr2+ ions occupy the eight vertex positions, O2- ions form an oxygen octahedron at the center of six faces of the cubic unit cell, and Ti4+ ions occupied the octahedral gaps. Therefore, the coordination number of Sr2+ ions is 12, and the coordination number of Ti4+ ions is 6. From another perspective, there is a cubic unit cell structure composed of eight Ti-O octahedrons in strontium titanate. Eight Ti-O octahedrons are located at the eight top corners of the cubic structure, while Sr2+ ions occupy the center of the cubic structure. The direct band gap of SrTiO3 is 3.2 eV, and its σ is very low. Its phase transitions from cubic to tetragonal will occur at temperatures below 105 K [9].

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