Thermoelectric energy conversion can be used to capture electric power from waste heat in a variety of applications. The materials that have been shown to have the best thermoelectric properties are compounds containing elements such as tellurium and antimony. These compounds can be oxidized if exposed to the high temperature air that may be present in heat recovery applications. Oxide materials have better stability in oxidizing environments, so their use enables the fabrication of more durable devices. Thus, although the thermoelectric properties of oxides are inferior to those of the compounds mentioned above, their superior stability may expand potential the high temperature application of thermoelectric energy conversion. In this paper, the thermoelectric properties of promising oxide materials are reviewed. The different types of oxides used for thermoelectric applications are compared and approaches for improving performance through doping are discussed.

Oxide materials for high temperature thermoelectric energy conversion

Transition metal oxides – Thermoelectric properties

We report a systematical investigation on the high temperature thermoelectric response of Ca1−xRxMnO3 (R = rare-earth) perovskites in the electron-doped range. The results reveal that electron concentration is the dominant factor for the high temperature electrical transport properties whereas the weight and size of R ions dominate the thermal transport properties. As the doping level varies, the best thermoelectric performance is observed at the relative electron concentration around 0.1. However, in the case of a fixed electron concentration, structural distortions become important since bandwidth has an observable influence on resistivity. By combining the three factors, electron concentration, crystal structure, and the weight/size of R ions, the largest thermoelectric figure of merit ZT for Ca1−xRxMnO3 reaches 0.2 at 1000 K. But this ZT value is still too far from the application criterion (ZT > 1). Using the dynamical mean field theory, we demonstrate that a ZT value larger than one in electron-dope...

High Temperature Thermoelectric Response of Electron-Doped CaMnO3

This paper reviews the usual techniques for producing YBCO-type single-domains and the microstructure of the as-obtained samples. The problems of seed dissolution and parasite nucleations are discussed in detail. The formation of microstructural defects, such as pores and cracks, is examined. An important part of this review is devoted to the study of the influence of RE-211 particles (RE2BaCuO5 where RE denotes Y, Yb, Nd, Sm, Dy, Gd, Eu or a mixture of them. Generally Nd4Ba2Cu2O10 is preferred to Nd2BaCuO5) for the microstructure and properties of RE?Ba?Cu?O single-domains. Pushing/trapping theory is described in order to explain the spatial distribution of RE-211 particles in the RE-123 ((RE)Ba2Cu3O7??) monoliths. The formation of RE-211-free regions is discussed. Different ways to limit the RE-211 coarsening are reviewed. Microstructural defects in the RE-123 matrix caused by the RE-211 particles are presented. It is also shown that RE-211 particles play a significant role in the mechanical properties of single-domain samples. We finish this review by discussing the infiltration and growth process as a good technique to control the microstructure.

From RE-211 to RE-123. How to control the final microstructure of superconducting single-domains

Electron-doped perovskite manganite Ca09R01MnO3 (R=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) polycrystalline samples were prepared and their transport and thermoelectric properties were studied from room temperature to 1000 K The transport behavior for all the samples is adiabatic small polaron hopping mechanism below 600 K but changes to metallic conductivity at higher temperature Above 600 K, more 3d electrons of Mn3+ ions will occupy eg orbitals, resulting in the variation in thermoelectric power values For all the samples, thermoelectric power is only determined by carrier concentration, but resistivity also rests with effective bandwidth The size matching between Ca2+ and R3+ ions together with heavier R3+ doping can improve thermoelectric performance evidently Combining these two factors, Ca09Dy01MnO3 and Ca09Yb01MnO3 reach ZT=02 at 1000 K, suggesting that they can be efficient high temperature n-type thermoelectric oxide materials

High temperature thermoelectric characteristics of Ca0.9R0.1MnO3 (R=La,Pr,…,Yb)

A process for the fabrication of high-quality and near-net-shaped superconducting specimens of (Y-123) with uniformly distributed fine (Y-211) is discussed. The process involves the fabrication of 211 preforms by conventional ceramic routes such as uniaxial and isostatic pressing, injection moulding and slip casting, and pressureless infiltration basically from a reservoir containing liquid phases. A compact of 123 or 123 rich in liquid phases(s), acting as a source of liquid phases, is placed in contact with the 211 preform, and heated above the peritectic temperature of 123. The liquid from the source compact infiltrates the 211 filler and the peritectic reaction occurring between the preform material (211) and the matrix (liquid phases) during slow cooling from the peritectic temperature results in the growth of 123 with uniformly distributed fine 211 particles. We have also demonstrated that the present process can be extended to the fabrication of other rare earth (RE) superconductors where a solid solution of the kind occurs due to comparable size of the RE and Ba atoms, by choosing the RE = Gd system as an example. Herein the advantages and bounds of the process are reviewed; the effect of various cooling rates on the growth kinetics of 123 and the resulting macro- and microstructures are discussed. Fabrication of composites containing fine and very uniformly distributed Ag is also demonstrated by the process. Comparisons are made between the characteristics of the samples produced in the present process and the existing melt texturing process. The utility of the process is demonstrated by the fabrication of a three-dimensional component - a hollow cylinder.

https://iopscience.iop.org/article/10.1088/0953-2048/11/5/014/pdf

Fabrication of textured (RE = Y, Gd) composites by infiltration and growth of preforms by liquid phases

Four-leg oxide thermoelectric modules consisting of two pairs of p type Ca3Co4O9 and n type Ca0.95Sm0.05MnO3 bulks of 4 × 4m m 2 cross-section and 5 and 10 mm heights were constructed and their performance discussed. The modules were assembled using silver paste wherein the thermoelectric elements are electrically connected in series by silver strips and placed between alumina plates that are in parallel thermally. The performance was evaluated up to a maximum temperature of 1000 K with thermal differences of up to 925 K between the hot and cold plates. An open circuit voltage of 400 mV was generated at an operating temperature of 1000 K with a temperature difference of 925 K and internal resistance of 1.27 � in the case of long legs module. A maximum output power of 31.5 mW was obtained and the power decreased to 24.5 mW in a module where the height of the thermoelectric elements is halved. The quality of the modules is discussed by introducing a manufacturing factor representing the cumulative influence of various factors of the fabrication process. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0022-3727/38/19/026/pdf

Fabrication and properties of four-leg oxide thermoelectric modules

Chemically stable, nanoscale (<50 nm) Y2Ba4CuMOy (M = Nb, Ta, W, Mo, Zr, Hf, etc,) phase inclusions, which potentially form a new type of artificial pinning centre, have been introduced successfully for the first time into large, single grain (RE)–Ba–Cu–O bulk superconductors.

https://iopscience.iop.org/article/10.1088/0953-2048/16/11/L03/pdf

New chemically stable, nano-size artificial flux pinning centres in (RE)-Ba-Cu-O superconductors

The effect of size, morphology and crystallinity of seed crystals on the nucleation and growth of melt processed Y–Ba–Cu–O (YBCO) during top seeded melt growth (TSMG) has been investigated Conventional seeding of bulk YBCO with a small seed crystal of square planar or irregular geometry leads to point nucleation of the YBa2Cu3O7−δ (Y-123) phase and a subsequent square planar c-sector growth habit The use of seeds of triangular and round geometries, however, results in a deviation from this growth habit symmetry The use of large surface area seed crystals of dimensions 20 mm × 10 mm, on the other hand, influences the growth of bulk YBCO in two significant ways Firstly, rapid epitaxial nucleation of the Y-123 phase occurs beneath the seed crystal, followed by grain growth via the usual peritectic solidification in the incongruent melt Secondly, a large area seed reduces the processing time of large single grains compared to that achieved by TSMG with small area seeds The present study is particularly relevant to the fabrication of single-grain samples with controlled growth sectors, reduced processing times and the multiple seeding of large area bulk materials containing good grain boundaries

https://iopscience.iop.org/article/10.1088/0953-2048/18/1/011/pdf

The effect of size, morphology and crystallinity of seed crystals on the nucleation and growth of Y–Ba–Cu–O single-grain superconductors

A novel process for the fabrication of a new form of YBa2Cu3Oy (123) superconducting material, with the dimensions of a thick film and the microstructure of a melt-textured single-domain bulk is described. The process allows the fabrication of 123 as a self-supporting fabric or as a thick film on various substrate materials. The process, which is generic and economical, uses commercially available Y2O3 fabrics as a precursor material. The Y2O3 cloth is infiltrated with barium cuprates and copper oxides from a liquid-phase source, then converted into Y2BaCuO5 (211) phase and eventually to 123. The nucleation and growth of the 123 phase is controlled by seeding the cloth with an oriented heterogeneous MgO or Nd123 seed. Interesting application areas for the new form of the 123 mono-domain fabric are discussed.

E Sudhakar Reddy

Papers

Fabrication of textured (RE = Y, Gd) composites by infiltration and growth of preforms by liquid phases

Fabrication and properties of four-leg oxide thermoelectric modules

New chemically stable, nano-size artificial flux pinning centres in (RE)-Ba-Cu-O superconductors

The effect of size, morphology and crystallinity of seed crystals on the nucleation and growth of Y–Ba–Cu–O single-grain superconductors

Mono-domain YBa 2 Cu 3 O y superconductor fabrics prepared by an infiltration process