Ca2MnO4 nanoparticles were prepared by the Pechini method and acid treated to extract Ca2+ ions. Structural, morphological and spectroscopic analyses by XRD, SEM/EDX, TEM/EDS and Raman revealed the formation of an amorphous outer layer at the particles surface with a preserved inner crystalline bulk. Thanks to the outer layer, which is electrochemically active, the acid-treated compounds showed capacity up to 150 Ah/kg. The crystalline bulk improved cycling stability, allowing reaching capacity retention up to 70% after 30 cycles.

In 1913, physicist Heike Kamerlingh-Onnes received the Nobel Prize for liquifying He and his discovery of superconductivity two years prior. It would be over 76 years later until K. Alexander Müller, together with Johannes Georg Bednorz, would be honored as Nobel laureates for their discovery of high-temperature superconductivity (HTS), grounded in their research with metal oxides. When we asked Müller, recently, what he would advise young materials scientists in regards to research for energy, he said, “I’ve always been a fan of oxides, therefore to work in oxide would not be bad.”

Nanostructuring has been the foremost approach to the manufacture of high-performance thermoelectric materials for nearly a decade. This study explores a novel nanostructuring technique, attrition-enhanced nanocomposite synthesis, in maximum indium-filled, iron-substituted cobalt antimonide skutterudites. In0.3Fe0.8Co3.2Sb12 was synthesized and subjected to varying degrees of mechanical attrition (via ball milling). These samples exhibited increased indium precipitation coincident with the duration of mechanical attrition. Indium readily diffused through the skutterudite crystal structure and rapidly precipitated forming 20-50 nm-sized indium-rich inclusions during sintering.

The thermoelectric properties of W-substituted CaMn1-xWxO3-δ (x = 0.01, 0.03; 0.05) samples, prepared by soft chemistry, were investigated from 300 K to 1000 K and compared to Nb-substituted CaMn0.98Nb0.02O3-δ. All compositions exhibit both an increase in absolute Seebeck coefficient and electrical resistivity with temperature. Moreover, compared to the Nb-substituted sample, the thermal conductivity of the W-substituted samples was strongly reduced. This reduction is attributed to the nearly two times greater mass of tungsten. Consequently, a ZT of 0.19 was found in CaMn0.97W0.03O3-δ at 1000 K, which was larger than ZT exhibited by the 2% Nb-doped sample.

As an approach to improve the thermoelectric properties of the polycrystalline Ca3Co4O9 misfit-layered oxide, substitutions of Co2+…4+ with the heavier cations Ru3+/4+ and In3+ were tested. Polycrystalline samples Ca3Co4-xRuxO9 and Ca3Co4-xInxO9 (0 < x < 0.21) were prepared via a solid-state-reaction route. For each sample the crystal structure was analyzed and a complete thermoelectric characterization was done within a temperature range of 300 K < T < 1125 K.

Both substitution strategies resulted in a significant decrease of the thermal conductivity (κ). For the In3+-substituted samples the decrease of the Seebeck coefficient (α) balanced the κ reduction so that no overall enhancement of the figure of merit (ZT) was found. The Ru3+/4+ substitution reduced the p-type carrier concentration and thus increases the electrical resistivity (ρel), while α became larger at low temperatures. Despite the reduction of the power factor, a small enhancement in ZT was observed in the case of x = 0.1 Ru substitution, due to the strong κ reduction. Considering the observed preferred orientation of the Ru-substituted crystallites, a maximum value of ZT = 0.14 perpendicular to the pressing direction is found at T = 1125 K, indicating that Ru substitution is a promising strategy for a further ZT improvement.

Polycrystalline randomly oriented CaMnO3 films were successfully deposited on sapphire substrates by soft chemistry methods. The precursor solutions were obtained from a mixture of metal acetates dissolved in acids. The Seebeck coefficient and the electrical resistivity were measured in the temperature range of 300 K < T < 1000 K. Modifications of thermal annealing procedures during the deposition of precursor layers resulted in different power factor values. Thermal annealing of CaMnO3 films at 900 °C for 48 h after four-layer depositions (route A) resulted in a pure perovskite phase with higher power factor and electrical resistivity than four-layer depositions of films annealed layer by layer at 900 °C for 48 h (route B). The studied films have negative Seebeck coefficients indicative of n-type conduction and electrical resistivities showing semiconducting behavior.

One way to further optimize the thermoelectric properties toward a higher ZT is a temperature stable nanoengineering of materials, where the thermal conductivity is reduced by increasing the phonon scattering at the grain boundaries. To study this, Nb-substituted CaMnO3 perovskite-type material was synthesized by ultrasonic spray combustion (USC). The grain growth has been characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Finally, the thermoelectric properties of compacted and sintered bulk samples from powder prepared by a continuous scalable USC process were measured up to 1050 K. The thermoelectric legs were prepared by an adapted sintering process. Here, a compromise between enhanced porosity to reduce the thermal conductivity and securing of mechanical stability and low resistivity should be obtained. Based on the grain growth mechanisms, an advanced sintering process for additional interconnection of the particles without particle growth is needed to further increase the thermoelectric performance.

Four-leg thermoelectric oxide modules (TOMs) consisting of two p-type (La1.98Sr0.02CuO4) and two n-type (CaMn0.98Nb0.02O3) thermoelectric (TE) legs were produced with a manufacturing quality factor between 30 and 60%. The pressed sintered TE legs revealed 90% of the theoretical density to ensure a sufficient mechanical stability of the TE modules. The legs were connected electrically in series and sandwiched thermally in parallel between two Al2O3 plates serving as absorber and cooler, respectively. A solar cavity-receiver packed with an array of TOMs was subjected to concentrated thermal radiation with peak solar radiative flux intensities exceeding 600 kW/m2. Temperature distributions in the cavity, open-circuit voltage (VOC), and maximum output power (Pmax) were measured for different external loads and solar radiative fluxes (qin). Finally, the solar-to-electricity conversion efficiency (η) was calculated.

Transition metal oxides and oxynitrides with perovskite-type structures are potential high temperature stable thermoelectric materials. With innovative soft chemistry synthesis procedures the crystallite size in complex oxide ceramics with perovskite-type structure can be decreased compared to classical synthesis routes and allows to lower the heat conductivity dramatically while a large power factor can be maintained at the same time. Submicron crystallites are beneficial to lower the lattice thermal conductivity without influencing the electron mobility.

In this work, perovskite-type cobaltates, molybdates and manganates are studied. The influence of heterostructures and nanocrystalline domains in perovskite oxides/oxynitrides and the correlation with transport properties and Seebeck coefficient will be discussed.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

In this paper we describe the synthesis and characterisation of La1-xAxCoO3 (A=Ca, Sr) (0 < x < 0,5) of different morphologies using pulsed laser deposition, ceramic methods and alternative soft-chemistry techniques. Furthermore the potential use of a La1-xCaxCoO3/carbon nanotube composite material for oxygen electrodes is discussed.