Thermoelastic and structural properties of ionically conducting cerate perovskites: (II) SrCeO₃ between 1273 K and 1723 K

Thermodynamic grüneisen parameter and bond valence parameters for strontium cerate perovskite.</p

Thermal performance of two heat exchangers for thermoelectric generators

Thermal performance of heat exchanger is important for potential application in integrated solar cell/module and thermoelectric generator (TEG) system. Usually, thermal performance of a heat exchanger for TEGs is analysed by using a 1D heat conduction theory which ignores the detailed phenomena associated with thermo-hydraulics. In this paper, thermal and mass transports in two different exchangers are simulated by means of a steady-state, 3D turbulent flow k -e model with a heat conduction module under various flow rates. In order to simulate an actual working situation of the heat exchangers, hot block with an electric heater is included in the model. TEG model is simplified by using a 1D heat conduction theory, so its thermal performance is equivalent to a real TEG. Natural convection effect on the outside surfaces of the computational model is considered. Computational models and methods used are validated under transient thermal and electrical experimental conditions of a TEG. It is turned out that the two heat exchangers designed have a better thermal performance compared with an existing heat exchanger for TEGs, and more importantly, the fin heat exchanger is more compact and has nearly half temperature rise compared with the tube heat exchanger

Enhancement of thermoelectric properties of CuFeS2 through formation of spinel-type microprecipitates

CuFeS2 (chalcopyrite) is a promising n-type thermoelectric candidate for low-grade waste heat recovery. In this work, chromium-containing CuFeS2 materials of general formula Cu1 xCrxFeS2 (0.0 ≤ x ≤ 0.1) were prepared via solid-state synthesis. Efforts to substitute chromium in CuFeS2 leads to the preferential formation of a composite, in which lamellar precipitates of a Cr-rich, spinel-type [Cu,Fe,Cr]3S4 phase, are embedded in the unsubstituted CuFeS2 matrix. X-ray absorption near-edge spectroscopy (XANES) reveals that the electronic structure of copper, iron and sulfur in the principal CuFeS2 phase remains unaltered by chromium incorporation. However, the formation of [Cu,Fe,Cr]3S4 precipitates alters the Cu:Fe ratio of the CuFeS2 phase, producing a change in the net carrier concentration through reduction of a portion of Fe3+ ions to Fe2+. The chromium content of the spinel precipitates determines the extent of the change in the Cu:Fe ratio of the main CuFeS2 phase, and hence, indirectly affects the electrical properties. The micro/nanometre-sized [Cu,Fe,Cr]3S4 precipitates and nanoscale dislocations enable a broad spectrum of heat-carrying acoustic phonons to be scattered, resulting in a significantly reduced lattice thermal conductivity. Combined with an enhanced power factor, a maximum thermoelectric figure-of-merit, zT of 0.31 at 673 K is achieved for the x = 0.08 sample; a three-fold increase over that of the pristine phase

Self-Nano-Structuring in SrTiO₃: A Novel Strategy for Enhancement of Thermoelectric Response in Oxides

Nanostructuring is recognized as an efficient route for enhancing thermoelectric response. Here, we report a new synthesis strategy for nanostructuring oxide ceramics and demonstrate its effectiveness on an important n-type thermoelectric SrTiO3. Ceramics of Sr0.9La0.1TiO3 with additions of B2O3 were synthesized by the mixed oxide route. Samples were sintered in air followed by annealing in a reducing atmosphere. Crystallographic data from X-ray and electron diffraction showed Pm3̅m cubic symmetry for all the samples. High-resolution transmission electron microscopy (HRTEM) showed the formation of a core–shell type structure within the grains for the annealed ceramics. The cores contain nanosize features comprising pairs of nanosize voids and particles; the feature sizes depend on annealing time. Atomic-resolution, high-angle annular-dark-field imaging and electron energy loss spectroscopy in the scanning transmission electron microscopy (STEM-HAADF-EELS) showed the particles to be rich in Ti and the areas around the voids to contain high concentrations of Ti3+. Additionally, dislocations were observed, with significantly higher densities in the shell areas. The observed dislocations are combined (100) and (110) edge dislocations. The major impact of the core–shell type microstructures, with nanosize inclusions, is the reduction of the thermal conductivity. Sr0.9La0.1TiO3 ceramics containing grain boundary shells of size ≈ 1 μm and inclusions in the core of 60–80 nm exhibit a peak power factor of 1600 μW/m·K2 at 540 K; at 1000 K, they exhibit a low thermal conductivity (2.75 W/m·K) and a power factor of 1050 μW/m·K2 leading to a high of ZT of 0.39 ± 0.03. This is the highest ZT reported so far for Sr0.9La0.1TiO3 based-compositions. This nanostructuring strategy should be readily applicable to other functional oxides

Managing dose-, damage- and data-rates in multi-frame spectrum-imaging

As an instrument, the scanning transmission electron microscope is unique in being able to simultaneously explore both local structural and chemical variations in materials at the atomic scale. This is made possible as both types of data are acquired serially, originating simultaneously from sample interactions with a sharply focused electron probe. Unfortunately, such scanned data can be distorted by environmental factors, though recently fast-scanned multi-frame imaging approaches have been shown to mitigate these effects. Here, we demonstrate the same approach but optimized for spectroscopic data; we offer some perspectives on the new potential of multi-frame spectrum-imaging (MFSI) and show how dose-sharing approaches can reduce sample damage, improve crystallographic fidelity, increase data signal-to-noise, or maximize usable field of view. Further, we discuss the potential issue of excessive data-rates in MFSI, and demonstrate a file-compression approach to significantly reduce data storage and transmission burdens

Prospects for Engineering Thermoelectric Properties in La1/3NbO3 Ceramics Revealed via Atomic-Level Characterization and Modeling

A combination of experimental and computational techniques has been employed to explore the crystal structure and thermoelectric properties of A-site-deficient perovskite La1/3NbO3 ceramics. Crystallographic data from X-ray and electron diffraction confirmed that the room temperature structure is orthorhombic with Cmmm as a space group. Atomically resolved imaging and analysis showed that there are two distinct A sites: one is occupied with La and vacancies, and the second site is fully unoccupied. The diffuse superstructure reflections observed through diffraction techniques are shown to originate from La vacancy ordering. La1/3NbO3 ceramics sintered in air showed promising high-temperature thermoelectric properties with a high Seebeck coefficient of S1 = −650 to −700 μV/K and a low and temperature-stable thermal conductivity of k = 2–2.2 W/m·K in the temperature range of 300–1000 K. First-principles electronic structure calculations are used to link the temperature dependence of the Seebeck coefficient measured experimentally to the evolution of the density of states with temperature and indicate possible avenues for further optimization through electron doping and control of the A-site occupancies. Moreover, lattice thermal conductivity calculations give insights into the dependence of the thermal conductivity on specific crystallographic directions of the material, which could be exploited via nanostructuring to create high-efficiency compound thermoelectrics

The Structure and Thermoelectric Properties of Tungsten Bronze Ba6Ti2Nb8O30

Tungsten bronze structured materials have attracted attention as possible thermoelectrics because of their complex crystal structure. In this work, a new thermoelectric ceramic with tetragonal tungsten bronze (TB) structure, Ba6Ti2Nb8O30 (BTN) was prepared by the conventional mixed oxide (MO) route with some samples processed by Spark Plasma Sintering (SPS). Additions of MnO enabled fabrication of high density BTN ceramics at the low sintering temperature of 1580 K in air and by SPS. All samples were annealed in a reducing atmosphere after sintering. X-ray diffraction showed that Ba6Ti2Nb8O30 crystallises with tetragonal symmetry (P4bm space group). HAADF-EELS analysis confirmed the proposed crystal structure and provided exact elemental distributions in the lattice, showing higher concentrations of Ti in the 2b lattice sites compared to the 8d lattice sites. XPS showed the presence of two spin-orbit double peaks at 207.7eV in the reduced BTN samples, confirming the presence of Nb4+ ions. By the use of a sintering aid and optimisation of the processing parameters the ceramics achieved a high power factor of 280 μW/m∙K2 at 873 K. The BTN ceramics showed phonon glass type thermal conduction behaviour with low thermal conductivity of 1.7 to 1.65 W/m.K at 300 to 873 K. A thermoelectric figure of merit (ZT) of 0.14 was achieved at 873 K. This ZT value is comparable with results for many TB thermoelectrics. However, BTN has the advantage of much easier processing