Søren Linderoth

Enhancement of the Thermoelectric Performance of p‐Type Layered Oxide Ca3Co4O9+δ Through Heavy Doping and Metallic Nanoinclusions

By converting heat directly into electricity, thermoelectric (TE) generation offers a promising technology to recover waste heat emitted from industrial sectors and energy consumption processes. [ 1 ] The key to realize an effi cient TE generator lies, however, in fi nding good materials with high TE performance, a good durability at high temperature, and preferably robustness to operating in air. The performance of a TE material is evaluated by the dimensionless fi gure-of-merit ZT ( = S 2 T / ρ κ , where S , T , ρ , and κ are the Seebeck coeffi cient, absolute temperature, electrical resistivity, and thermal conductivity, respectively). By far the most widely used TE materials are alloys of Bi 2 Te 3 , PbTe, and SiGe, which often suffer from poor durability at high temperature, are harmful or scarce, and have costly constituting elements. Metal oxides have been considered as an alternative to overcome these problems. Metal oxide-based materials have been attracting continuous interest as TE materials over the years since the discovery of large TE power in p-type NaCo 2 O 4 single crystals by Terasaki et al. in 1997. [ 2 ] However, practical application of this oxide for power generation from waste heat has never been realized because of the volatility of Na and the instability of the compound against humidity. Another Co-based oxide p type material Ca 3 Co 4 O 9 + δ has also been intensively investigated because of its good TE performance ( ZT = 0.83 at 973 K for the single crystal) [ 3 ] and its high thermal and chemical stabilities even up to 1200 K in air. [ 4–7 ] An incommensurate character in the crystal structure of this compound is explicitly described as [Ca 2 CoO 3 ] b 1/ b 2 [CoO 2 ], where b 1 and b 2 are two different periodicities along the b axis for the rock salt-type Ca 2 CoO 3 subsystem and the CdI 2 -type CoO 2 subsystem, respectively. [ 4 ] Single crystals are less likely to be applied for fabricating practical TE devices, because they will be too expensive. It is hence highly desirable to achieve suffi cient TE properties in a polycrystalline form of these oxides. Although the diffi culty of discovering novel high performance

Metallic and Insulating Interfaces of Amorphous SrTiO3-Based Oxide Heterostructures

The conductance confined at the interface of complex oxide heterostructures provides new opportunities to explore nanoelectronic as well as nanoionic devices. Herein we show that metallic interfaces can be realized in SrTiO(3)-based heterostructures with various insulating overlayers of amorphous LaAlO(3), SrTiO(3), and yttria-stabilized zirconia films. On the other hand, samples of amorphous La(7/8)Sr(1/8)MnO(3) films on SrTiO(3) substrates remain insulating. The interfacial conductivity results from the formation of oxygen vacancies near the interface, suggesting that the redox reactions on the surface of SrTiO(3) substrates play an important role.

A high-mobility two-dimensional electron gas at the spinel/perovskite interface of γ-Al2O3/SrTiO3.

The discovery of two-dimensional electron gases at the heterointerface between two insulating perovskite-type oxides, such as LaAlO(3) and SrTiO(3), provides opportunities for a new generation of all-oxide electronic devices. Key challenges remain for achieving interfacial electron mobilities much beyond the current value of approximately 1,000 cm(2) V(-1) s(-1) (at low temperatures). Here we create a new type of two-dimensional electron gas at the heterointerface between SrTiO(3) and a spinel γ-Al(2)O(3) epitaxial film with compatible oxygen ions sublattices. Electron mobilities more than one order of magnitude higher than those of hitherto-investigated perovskite-type interfaces are obtained. The spinel/perovskite two-dimensional electron gas, where the two-dimensional conduction character is revealed by quantum magnetoresistance oscillations, is found to result from interface-stabilized oxygen vacancies confined within a layer of 0.9 nm in proximity to the interface. Our findings pave the way for studies of mesoscopic physics with complex oxides and design of high-mobility all-oxide electronic devices.

Investigations of metallic alloys for use as interconnects in solid oxide fuel cell stacks

Some high-temperature alloys have been investigated in order to determine whether they are suitable as metallic interconnect materials in solid oxide fuel cell stacks. The requirements for such alloys are formulated. Thermal dilatometry and oxidation tests, as well as theoretical calculations of the stresses that are induced by differences in thermal expansion of the individual materials, have been performed. The results show that a chromium-rich alloy, with dispersions of fine Y2O3 particles, perform best among the samples investigated. Improvements are still needed in order to make the alloy fully applicable in a solid oxide fuel cell stack. Some suggestions for improvements are put forward.

Ageing behaviour of zirconia stabilised by yttria and manganese oxide

The effect of Mn on the structure, lattice parameter and conductivity has been investigated for near-cubic YSZ with an yttrium content slightly under 8 mol% Y2O3. The structure and chemistry of the material were studied both as sintered, and also after prolonged heat treatments at 850 and 1000°C, using x-ray diffraction, electron microscopy, impedance spectroscopy and d.c. conductivity measurements. In the latter case, the measurements were made in-situ during ageing. At 1000°C and for Mn additions above about 2 mol% YSZ is found to be stabilised as the cubic structure, and thereby the conductivity is also stabilised. However, at all temperatures studied, the presence of Mn considerably reduces the ionic conductivity of YSZ.

Extreme mobility enhancement of two-dimensional electron gases at oxide interfaces by charge-transfer-induced modulation doping.

Two-dimensional electron gases (2DEGs) formed at the interface of insulating complex oxides promise the development of all-oxide electronic devices. These 2DEGs involve many-body interactions that give rise to a variety of physical phenomena such as superconductivity, magnetism, tunable metal-insulator transitions and phase separation. Increasing the mobility of the 2DEG, however, remains a major challenge. Here, we show that the electron mobility is enhanced by more than two orders of magnitude by inserting a single-unit-cell insulating layer of polar La(1-x)Sr(x)MnO3 (x = 0, 1/8, and 1/3) at the interface between disordered LaAlO3 and crystalline SrTiO3 produced at room temperature. Resonant X-ray spectroscopy and transmission electron microscopy show that the manganite layer undergoes unambiguous electronic reconstruction, leading to modulation doping of such atomically engineered complex oxide heterointerfaces. At low temperatures, the modulation-doped 2DEG exhibits Shubnikov-de Haas oscillations and fingerprints of the quantum Hall effect, demonstrating unprecedented high mobility and low electron density.

Magnetization and Mössbauer studies of ultrafine Fe-C particles

Abstract The magnetic properties of ultrafine amorphous Fe 1- x C x particles have been studied as a function of temperature and applied field. Magnetization and Mossbauer spectroscopy investigations were combined in order to study superparamagnetic relaxation phenomena. The particles were prepared by thermal decomposition of Fe(CO) 5 . This preparation method is known to produce almost mono-size particles. The mean particle diameter of the Fe-C particles in the present study was about 3.1 nm. The small size resulted in finite size effects, which, e.g. for the temperature dependence of the saturation magnetization gave a significant deviation from the T 3 2 law. The deviation is in accord with recent theoretical calculations of the behaviour of spin waves in ultrafine particles. From the superparamagnetic relaxation studies the minimum relaxation time was estimated to be about 2 × 10 −12 s.

Enhancement of the chemical stability in confined [delta]-Bi2O3

Bismuth-oxide-based materials are the building blocks for modern ferroelectrics, multiferroics, gas sensors, light photocatalysts and fuel cells. Although the cubic fluorite δ-phase of bismuth oxide (δ-Bi2O3) exhibits the highest conductivity of known solid-state oxygen ion conductors, its instability prevents use at low temperature. Here we demonstrate the possibility of stabilizing δ-Bi2O3 using highly coherent interfaces of alternating layers of Er2O3-stabilized δ-Bi2O3 and Gd2O3-doped CeO2. Remarkably, an exceptionally high chemical stability in reducing conditions and redox cycles at high temperature, usually unattainable for Bi2O3-based materials, is achieved. Even more interestingly, at low oxygen partial pressure the layered material shows anomalous high conductivity, equal or superior to pure δ-Bi2O3 in air. This suggests a strategy to design and stabilize new materials that are comprised of intrinsically unstable but high-performing component materials.

Amorphous to crystalline transformation of ultrafine Fe62B38 particles

Abstract Fe 62 B 38 particles, with sizes of 10–200 nm, have been produced by chemical reduction of Fe(II) ions in aqueous solution by KBH 4 . Electron and X-ray diffraction, X-ray absorption fine structure and Mossbauer spectroscopy results reveal that the particles are amorphous. The Mossbauer parameters and the estimates of temperatures for crystallization and for ferromagnetic to paramagnetic transition suggest that the amorphous structures are similar for the particles and ribbons or films produced by liquid-squench or sputtering. The increase of the magnetic hyperfine field with annealing temperature, as deduced from 57 Fe Mossbauer spectroscopy, is attributed to atomic rearrangement in the amorphous phase. Crystalline Fe 2 B and α-Fe are the products when the particles are annealed in Ar and H 2 at around 715 K. It is proposed that iron and boron partly separate during the initial stages of crystallization and subsequently form Fe 2 B. Passivation of the particles makes them oxidatively stable, even when heated in air at temperatures up to 650 K.

Indirect measurement of the magnetocaloric effect using a novel differential scanning calorimeter with magnetic field

A simple and high-sensitivity differential scanning calorimeter (DSC) unit operating under magnetic field has been built for indirect determination of the magnetocaloric effect. The principle of the measuring unit in the calorimeter is based on Peltier elements as heat flow sensors. The high sensitivity of the apparatus combined with a suitable calibration procedure allows very fast and accurate heat capacity measurements under magnetic field to be made. The device was validated from heat capacity measurements for the typical DSC reference material gallium (Ga) and a La(0.67)Ca(0.33)MnO(3) manganite system and the results were highly consistent with previous reported data for these materials. The DSC has a working range from 200 to 340 K and has been tested in magnetic fields reaching 1.8 T. The signal-to-noise ratio is in the range of 10(2)-10(3) for the described experiments. Finally the results have been compared to results from a Quantum Design(R) physical properties measuring system. The configuration of the system also has the advantage of being able to operate with other types of magnets, e.g., permanent magnets or superconducting coils, as well as the ability to be expanded to a wider temperature range.