Armin Reller

Colossal dielectric constants in transition-metal oxides

Abstract. Many transition-metal oxides show very large (“colossal”) magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energy-storage devices. In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation. In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is today’s most investigated material with colossal dielectric constant. Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2−xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant.

Solar-Processed Metals as Clean Energy Carriers and Water-Splitters

Abstract Two-step solar thermochemical cycles and processes for the production of hydrogen, hydrocarbons, and synthesis gas are considered. The first step is based on the thermal, electrothermal, or carbothermal reduction of metal oxides, producing metals, metal nitrides, metal carbides, or lower-valence metal oxides. These are hightemperature highly endothermic reactions that can be driven by concentrated solar energy, reducing the consumption of fossil fuels and their concomitant emissions. The second step involves hydrolysis reactions. The thermodynamics of both reaction steps are examined and relevant experimental studies conducted using solar energy are reviewed.

Selective catalytic reduction of nitric oxide with ammonia: I. Monolayer and Multilayers of Vanadia Supported on Titania

Abstract Titania supported vanadium oxide layers have been prepared and tested as catalysts for the selective catalytic reduction (SCR) of nitric oxide with ammonia. SCR was performed in a continuous tubular fixed-bed microreactor in the temperature range 400–600 K and at atmospheric pressure. Highly active and selective catalysts for SCR were obtained by successive impregnations using the specific reaction of vanadyl triisobutoxide with surface hydroxyl groups followed by subsequent calcination. One such impregnation resulted in a catalyst covered with an incomplete monolayer of strongly disordered vanadium oxide species. Upon successive impregnations the vanadium oxide content increased, and after four to five impregnations, an additional vanadium oxide phase was formed, which exhibited a significantly different reduction behavior, as was indicated by the appearance of a second reduction peak at higher temperature in the temperature programmed reduction profile. High-resolution electron microscopy showed that the vanadia was preferentially deposited at the edges of the titania and that even after several successive impregnations the amorphous vanadia does not form a compact layer. The activity expressed as moles nitric oxide converted to nitrogen per moles vanadium (V) species and per time increased markedly upon a second impregnation and was highest for catalysts covered with three and four layers of vanadium oxide.

Influence of the A-site cation in ACoO3 (A = La, Pr, Nd, and Gd) perovskite-type oxides on catalytic activity for methane combustion

The effect of rare-earth ions (La, Pr, Nd, and Gd) in ACoO3 perovskites on the thermal behavior and on the catalytic activity for methane oxidation has been studied. The thermal reduction, as investigated by oxygen desorption analysis, revealed for all perovskites one evolution step occurring above 1100 K. Transmission electron micrographs at the beginning of the desorption step revealed the presence of superstructures, which are attributed to the formation of oxygen-deficient domains (ACoO2.5 ≤ ACoO3-x ≤ ACoO3) adopting perovskite-related brownmillerite-type structures. A model for the sequence of the structural changes of the perovskites during the reduction is proposed. The amount of oxygen which evolved above 1100 K increased with decreasing size of the lanthanide ion. Comparative catalytic studies were carried out in a fixed-bed microreactor at atmospheric pressure in the temperature range 600–1200 K. The activities at 830 K, expressed as reaction rates referred to the BET surface area, increased in the order PrCoO3 ⪡ LaCoO3 < NdCoO3 < GdCoO3. However, no significant influence of the A-site cations on the catalytic activity was found, with exception of praseodymium. PrCoO3 showed a 30 times lower activity than the other samples. The catalytic activity of ACoO3 perovskites can be markedly affected by the presence of Co3O4 impurity in the perovskites.

The route to resource-efficient novel materials

Combining the efforts of physicists, materials scientists, economists and resource-strategy researchers opens up an interdisciplinary route enabling the substitution of rare elements by more abundant ones, serving as a guideline for the development of novel materials.

Selective catalytic reduction of nitric oxide with ammonia: II. Monolayers of Vanadia Immobilized on Titania—Silica Mixed Gels

Abstract Monolayers of vanadia were immobilized on pure titania and silica, and on mixed gel carriers of titania and silica containing 1, 10, 20 and 50 mol% of titania using the selective reaction of vanadyl triisobutoxide with surface hydroxyl groups of the carriers. The catalysts were investigated with regard to their structural properties and their activity in selective catalytic reduction (SCR) of nitric oxide with ammonia. The textural properties of both pure and impregnated carriers depended strongly on their chemical composition. Low titania content (1 mol%) led to a marked increase of the BET surface area and the pore volume, whereas with a higher titania content (>10 mol%) these properties decreased drastically. X-ray diffraction and high-resolution electron microscopy indicated that silica was present as an amorphous phase in all carriers, whereas crystalline domains of titania (anatase) were found in carriers containing 10 mol% and more of titania. On all carriers the immobilized vanadia species were well dispersed and disordered. Temperature programmed reduction showed for all samples only a single peak for the reduction of the immobilized vanadia layer. The temperature of maximum reduction rate which reflects the ease of reduction of the supported vanadia layer decreased with increasing titania content of the carrier. It was highest for vanadia supported on pure silica (790 K) and lowest for vanadia supported on titania (700 K). This behavior was attributed to the markedly stronger support interaction of titania compared with silica. The highest activity for SCR was found for vanadia supported on mixed gels containing 20 and 50 mol% titania. As a result of the weak support interaction, the vanadia species supported on pure silica tended to agglomerate when exposed to higher temperatures under SCR conditions. This agglomeration was suppressed when titania was added to the silica matrix of the carrier. Of all the catalyst preparations only the vanadia layer supported on equimolar titania-silica and on pure titania maintained stable activity for SCR when exposed to sulfur dioxide-containing feed.

DIRECT SOLAR THERMAL DISSOCIATION OF ZINC OXIDE: CONDENSATION AND CRYSTALLISATION OF ZINC IN THE PRESENCE OF OXYGEN

Abstract The solar thermal production of zinc from zinc oxide is part of a two-step water splitting cycle. Zinc oxide thermally dissociates into zinc vapour and oxygen at elevated temperatures; ZnO(s)=Zn(g)+0.5O 2 In practice, the yield of zinc depends on the kinetics of dissociation and the technical feasibility of quenching its gaseous products fast enough to avoid reoxidation. As the gaseous products cool, the rate of zinc oxidation varies as the zinc changes its phase. The condensation of zinc in the presence of oxygen was studied by fractional crystallisation in a temperature gradient tube furnace. It was observed that the oxidation of zinc vapour is a heterogeneous process. In the absence of nucleation sites, zinc vapour and oxygen can coexist in a meta-stable state. The different stages and forms of nucleation, crystal growth and oxidation were deduced from the morphology of the quenched products, as determined by scanning and high-resolution transmission electron microscopy. Zinc morphology depends on whether it crystallised at above or below the melting point.

Phase transformation and grain growth of doped nanosized titania

The influence of altervalent cation doping of TiO 2 on its phase transition and grain growth has been investigated. It is shown that dopants like Fe 3+ , Si 4+ , V 5+ , Ru 3+ and Ni 2+ affect the phase transition temperature of the titania host, and that significant variation is observed for silicon doping. Iron titanium oxide and nickel titanium oxide phases were found in the iron-doped and nickel-doped system, respectively, at elevated calcination temperatures, while other doped systems only show the modifications of anatase and rutile at the observed range of calcination temperature and dopant content. Compared with the pure TiO 2, grain growth is arrested for the iron-doped and silicon-doped systems, and this tendency is especially distinct in the silicon-doped system.

Dielectric behavior of copper tantalum oxide

A thorough investigation of the dielectric properties of Cu2Ta4O12, a material crystallizing in a pseudocubic, perovskite-derived structure is presented. We measured the dielectric constant and conductivity of single crystals in an exceptionally broad frequency range up to gigahertz frequencies and at temperatures from 25to500K. The detected dielectric constant is unusually high (reaching values up to 105) and almost constant in a broad frequency and temperature range. Cu2Ta4O12 possesses a crystal structure similar to CaCu3Ti4O12, the compound for which such an unusually high dielectric constant was first observed. An analysis of the results using a simple equivalent circuit and measurements with different types of contact revealed that extrinsic interfacial polarization effects, derived from surface barrier capacitors are the origin of the observed giant dielectric constants. The intrinsic properties of Cu2Ta4O12 are characterized by a (still relatively high) dielectric constant in the order of 100 and ...

STRUCTURE AND THERMOCHEMICAL REACTIVITY OF CARUO3 AND SRRUO3

Abstract Single crystals of CaRuO3 (1) and SrRuO3 (2) have been synthesized by flux method and their structures have been determined by means of single crystal X-ray diffraction techniques. The crystallographic data are (1): formula weight fw = 189.15, orthorhombic, space group Pnma (No. 62), a = 5.524(1), b = 7.649(2), c = 5.354(1) A , V = 226.2 A 3 , Z = 4, Dx = 5.554 g cm−3, μ = 81.97 cm−1, final R = 0.031 for 297 observed reflections. (2): formula weight fw = 236.69, cubic, space group Pm 3 (No. 200), a = b = c =3.910(1) A , V = 59.8 A 3 , Z = 1, Dx = 6.572 g cm−3, μ = 275.28 cm−1, final R = 0.032 for 33 observed reflections. The thermochemical reactivity of the two phases has been investigated as function of temperature and ambient atmosphere. In oxidizing or inert gas atmospheres both compounds prove to be stable up to 1200 K. In reducing atmospheres such as molecular hydrogen, however, elementary ruthenium and the corresponding earth alkali metal oxides are obtained as microcrystalline mixtures at relatively low temperatures. The mechanisms of the reductions prove to be different.