Recent Advances in Nanostructured Thermoelectric Half-Heusler Compounds

Half-Heusler (HH) alloys have attracted considerable interest as promising thermoelectric (TE) materials in the temperature range around 700 K and above, which is close to the temperature range of most industrial waste heat sources. The past few years have seen nanostructuing play an important role in significantly enhancing the TE performance of several HH alloys. In this article, we briefly review the recent progress and advances in these HH nanocomposites. We begin by presenting the structure of HH alloys and the different strategies that have been utilized for improving the TE properties of HH alloys. Next, we review the details of HH nanocomposites as obtained by different techniques. Finally, the review closes by highlighting several promising strategies for further research directions in these very promising TE materials.Comment: 34 pages, 22 figure

Nanostructured Nb-substituted CaMnO3 n-type thermoelectric material prepared in a continuous process by ultrasonic spray combustion

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 performanc

Influence of Thermal Aging Phenomena on Thermoelectric Properties of Al-Substituted ZnO

The thermoelectric properties and stability of Al-substituted ZnO as a potential high-temperature n-type material were studied in heating-cooling cycles. Zn1−x Al x O (x=0.02, 0.06) was prepared by soft chemistry and solid-state reaction synthesis methods. Cycling during the thermoelectric measurement leads to an increase of the electrical resistivity and Seebeck coefficient values. The reason for this aging phenomenon can be assigned to a change in composition due to oxygen uptake along with modification in the defect concentrations. The aging is enhanced if the cycling is performed in oxygen. ZT value of 0.21 is reached at 1275K for samples with 2% Al substitution made by soft chemistry synthesi

Thermoelectric oxide modules tested in a solar cavity-receiver

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 calculate

PdO x /Pd at Work in a Model Three-Way Catalyst for Methane Abatement Monitored by Operando XANES

The oxidation state of palladium in a model Pd/ACZ three-way catalyst was monitored by synchronous XANES and mass spectrometry during two consecutive heating (to 850°C) and cooling (to 100°C) cycles under stoichiometric conditions simulating exhaust after treatment of a natural gas engine. During heating in the first cycle, PdO reduction occurred around 500°C and the initial fully oxidized state of Pd was never recovered upon heating and cooling cycles. A mixed Pd2+/Pd oxidation state was at work in the second cycle. Hence, the operando XANES study reveals that the PdO x /Pd pair exists in a working catalyst but is less active than the catalyst in its initial state of fully oxidized palladium. It is also evident from XANES spectra that ceria-zirconia promotes re-oxidation of metallic Pd, thus reasonably sustaining catalytic activity after exposure to high temperature

Enzymatic Degradation of Fiber-Reinforced PLA Composite Material

Application of thermoplastic fiber-reinforced lightweight composite materials provides a wide range of advantages that are of particular importance for the mobility sector. UD tapes composed of unidirectionally (UD) oriented inorganic fibers embedded in a thermoplastic matrix represent light-weight materials with high tensile strength. This publication addresses recycling aspects of novel UD tape made of a combination of basalt fibers and different PLA (polylactic acid) formulations. The kinetics of enzyme-based separation of polymer from the fiber were investigated. Different types of UD tapes with a thickness of 270–290 µm reinforced with basalt fiber weight ratios ranging between 51 and 63% were incubated at 37 °C in buffer solution (pH 7.4) containing proteinase K. The influence of enzyme concentration, tape weight per incubation tube, proteinase K activators, and tape types on the rate of enzymatic decomposition was investigated. Enzyme activity was measured by analyzing lactate concentration with lactate dehydrogenase and by measuring weight loss of the composite material. The rate of lactate release increased in the first 30 min of incubation and remained stable for at least 90 min. Weight loss of 4% within 4 h was achieved for a tape with 56% (w/w) fiber content. For a sample with a surface area of 3 cm² in a buffer volume of 10 mL, the rate of lactate release as a function of enzyme concentration reached saturation at 300 µg enzyme/mL. With this enzyme concentration, the rate of lactate release increased in a linear manner for tape surface areas between 1 and 5 cm². Four tapes with different PLA types were treated with the enzyme for 17 h. Weight loss ranged between 7 and 24%. Urea at a concentration of 0.5% (w/v) increased lactate release by a factor of 9. Pretreatment of tapes in alkaline medium before enzymatic degradation increased weight loss to 14% compared to 5% without pretreatment. It is concluded that enzymatic PLA hydrolysis from UD tapes is a promising technology for the release of basalt fibers after alkaline pretreatment or for the final cleaning of basalt fibers

Seebeck coefficients of half-metallic ferromagnets

In this report the Co2 based Heusler compounds are discussed as potential materials for spin voltage generation. The compounds were synthesized by arcmelting and consequent annealing. Band structure calculations were performed and revealed the compounds to be half-metallic ferromagnets. Magnetometry was performed on the samples and the Curie temperatures and the magnetic moments were determined. The Seebeck coefficients were measured from low to ambient temperatures for all compounds. For selected compounds high temperature measurements up to 900 K were performed.Comment: accepted contribution o the Special Issue "Spin Caloritronics" of Solid State Communication

Thermoelectric properties of CaMnO3 films obtained by soft chemistry synthesis

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 behavio