Mari-Ann Einarsrud

Functionalized TiO2 nanoparticles by single-step hydrothermal synthesis: the role of the silane coupling agents

A simple, robust and versatile hydrothermal synthesis route to in situ functionalized TiO2 nanoparticles was developed using titanium(IV) isopropoxide as Ti-precursor and selected silane coupling agents (3-aminopropyltriethoxysilane (APTES), 3-(2-aminoethylamino)propyldimethoxymethylsilane (AEAPS), and n-decyltriethoxysilane (DTES)). Spherical nanoparticles (ca. 9 nm) with narrow size distribution were obtained by using DTES or by synthesis performed without silane coupling agents. Rod-like nanoparticles along with 9 nm spherical nanoparticles were formed using aminosilane coupling agents because of a combination of oriented attachment of nanoparticles and specific adsorption of the aminosilane on crystallographic faces of anatase nanoparticles. The nanoparticles were functionalized in situ and became hydrophobic as silanes reacted to form covalent bonds on the surface of TiO2. The versatility of the aqueous synthesis route was demonstrated, and by selecting the type of silane coupling agent the surface properties of the TiO2 nanoparticles could be tailored. This synthesis route has been further developed into a two-step synthesis to TiO2–SiO2 core–shell nanoparticles. Combustion of the silane coupling agents up to 700 °C leads to the formation of a nanometric amorphous SiO2 layer, preventing growth and phase transition of the in situ functionalized nanoparticles.

Toughening of Y-doped BaZrO3 proton conducting electrolytes by hydration

In BaZr1−xYxO3−x/2 (BZY), the state-of-the-art oxide proton conductors, the proton conductivity is facilitated by hydration of oxygen vacancies. Hydration induces lattice expansion, which may induce stress and thereby potentially reduce the mechanical integrity of fuel cells. Here, we report on the effect of hydration/dehydration on the mechanical properties of dense BZY-materials sintered by two different methods. The chemical expansion due to hydration was determined by X-ray diffraction, and the normalized chemical strain was calculated by combining these data with thermogravimetry. The mechanical properties were investigated by the Vickers-micro indentation technique. Hydration was demonstrated to enhance the fracture toughness of the materials with the change in fracture mode from intergranular to transgranular mechanisms. We demonstrate that the hydration/dehydration process is reversible and discuss the present findings with respect to the long-term stability of electrochemical devices based on BaZrO3.

Luminescent Eu3+-doped NaLa(WO4)(MoO4) and Ba2CaMoO6 prepared by the modified Pechini method

AbstractnModified Pechini synthesis routes were developed for synthesis of the novel red phosphor materials NaLa(WO4)(MoO4):Eu3+ and Ba2CaMoO6:Eu3+. Phase pure NaLa(WO4)(MoO4):Eu3+ was obtained at calcination temperatures ≥600xa0°C using malic acid or tartaric acid as complexing agents. Phase pure Ba2CaMoO6:Eu3+ was attained using EDTA and citric acid, at calcination temperatures ≥800xa0°C. Choice of complexing agents were discussed on the basis of the solubility of the precursors, metal complex stability constants and conformations of the complexes. The powder properties were characterised using X-ray diffraction, thermal analysis and electron microscopy. Photoluminescence emission intensity was studied as a function of the complexing agents used and calcination temperature of the powders. Maximum emission intensity for NaLa(WO4)(MoO4):Eu3+ was obtained at a calcination temperature of 600xa0°C, whereas the maximum for Ba2CaMoO6:Eu3+ was obtained after calcination at 1100xa0°C. Both materials displayed desirable optical properties for use as phosphors in white light-emitting diodes.Graphical AbstractPhase pure NaLa(WO4)(MoO4):Eu3+ and Ba2CaMoO6:Eu3+ red phosphor materials were synthesised by the modified Pechini route, thus permitting lower calcination temperatures which resulted in increased emission intensity for NaLa(WO4)(MoO4):Eu3+.

Effect of Cation Ordering on the Performance and Chemical Stability of Layered Double Perovskite Cathodes

The effect of A-site cation ordering on the cathode performance and chemical stability of A-site cation ordered LaBaCo2O5+δ and disordered La0.5Ba0.5CoO3−δ materials are reported. Symmetric half-cells with a proton-conducting BaZr0.9Y0.1O3−δ electrolyte were prepared by ceramic processing, and good chemical compatibility of the materials was demonstrated. Both A-site ordered LaBaCo2O5+δ and A-site disordered La0.5Ba0.5CoO3−δ yield excellent cathode performance with Area Specific Resistances as low as 7.4 and 11.5 Ω·cm2 at 400 °C and 0.16 and 0.32 Ω·cm2 at 600 °C in 3% humidified synthetic air respectively. The oxygen vacancy concentration, electrical conductivity, basicity of cations and crystal structure were evaluated to rationalize the electrochemical performance of the two materials. The combination of high-basicity elements and high electrical conductivity as well as sufficient oxygen vacancy concentration explains the excellent performance of both LaBaCo2O5+δ and La0.5Ba0.5CoO3−δ materials at high temperatures. At lower temperatures, oxygen-deficiency in both materials is greatly reduced, leading to decreased performance despite the high basicity and electrical conductivity. A-site cation ordering leads to a higher oxygen vacancy concentration, which explains the better performance of LaBaCo2O5+δ. Finally, the more pronounced oxygen deficiency of the cation ordered polymorph and the lower chemical stability at reducing conditions were confirmed by coulometric titration.

Thermal evolution of the crystal structure and phase transitions of KNbO3

The thermal evolution of the crystal structure and phase transitions of KNbO3 were investigated by high-temperature powder X-ray diffraction and Rietveld refinement of the diffraction data. Two phase transitions from orthorhombic (Amm2) to tetragonal (P4mm) and from tetragonal to cubic (Pm3¯m) were confirmed, both on heating and cooling. Both phase transitions are first order based on the observed hysteresis. The mixed displacive and order–disorder nature of the tetragonal to cubic transition is argued based on symmetry and apparent divergence of the atomic positions from pseudo-cubic values. The transition between the orthorhombic and tetragonal phase shows no temperature-dependence for atomic positions and only thermal expansion of the unit cell parameters and is thus discussed in relation to a lattice dynamical instability.

Structure and Optical Properties of Titania-PDMS Hybrid Nanocomposites Prepared by In Situ Non-Aqueous Synthesis

Organic-inorganic hybrid materials are attractive due to the combination of properties from the two distinct types of materials. In this work, transparent titania-polydimethylsiloxane hybrid materials with up to 15.5 vol. % TiO2 content were prepared by an in situ non-aqueous method using titanium (IV) isopropoxide and hydroxy-terminated polydimethylsiloxane as precursors. Spectroscopy (Fourier transform infrared, Raman, Ultraviolet-visible, ellipsometry) and small-angle X-ray scattering analysis allowed to describe in detail the structure and the optical properties of the nanocomposites. Titanium alkoxide was successfully used as a cross-linker and titania-like nanodomains with an average size of approximately 4 nm were shown to form during the process. The resulting hybrid nanocomposites exhibit high transparency and tunable refractive index from 1.42 up to 1.56, depending on the titania content.

Fabrication of Lead-Free Bi0.5Na0.5TiO3 Thin Films by Aqueous Chemical Solution Deposition

Piezoelectric ceramics are widely used in actuator applications, and currently the vast majority of these devices are based on Pb(Zr,Ti)O3, which constitutes environmental and health hazards due to the toxicity of lead. One of the most promising lead-free material systems for actuators is based on Bi0.5Na0.5TiO3 (BNT), and here we report on successful fabrication of BNT thin films by aqueous chemical solution deposition. The precursor solution used in the synthesis is based on bismuth citrate stabilized by ethanolamine, NaOH, and a Ti-citrate prepared from titanium tetraisopropoxide and citric acid. BNT thin films were deposited on SrTiO3 and platinized silicon substrates by spin-coating, and the films were pyrolized and annealed by rapid thermal processing. The BNT perovskite phase formed after calcination at 500 °C in air. The deposited thin films were single phase according to X-ray diffraction, and the microstructures of the films shown by electron microscopy were homogeneous and dense. Decomposition of the gel was thoroughly investigated, and the conditions resulting in phase pure materials were identified. This new aqueous deposition route is low cost, robust, and suitable for development of BNT based thin film for actuator applications.

134Ba diffusion in polycrystalline BaMO3 (M = Ti, Zr, Ce)

Cation diffusion in functional oxide materials is of fundamental interest, particularly in relation to interdiffusion of cations in thin film heterostructures and chemical stability of materials in high temperature electrochemical devices. Here we report on 134Ba tracer diffusion in polycrystalline BaMO3 (M = Ti, Zr, Ce) materials. The dense BaMO3 ceramics were prepared by solid state sintering, and thin films of 134BaO were deposited on the polished pellets by drop casting of an aqueous solution containing the Ba-tracer. The samples were subjected to thermal annealing and the resulting isotope distribution profiles were recorded by secondary ion mass spectrometry. The depth profiles exhibited two distinct regions reflecting lattice and grain boundary diffusion. The grain boundary diffusion was found to be 4-5 orders of magnitude faster than the lattice diffusion for all three materials. The temperature dependence of the lattice and grain boundary diffusion coefficients followed an Arrhenius type behaviou...

All-Oxide Thermoelectric Module with in Situ Formed Non-Rectifying Complex p–p–n Junction and Transverse Thermoelectric Effect

All-oxide thermoelectric modules for energy harvesting are attractive because of high-temperature stability, low cost, and the potential to use nonscarce and nontoxic elements. Thermoelectric modules are mostly fabricated in the conventional π-design, associated with the challenge of unstable metallic interconnects at high temperature. Here, we report on a novel approach for fabrication of a thermoelectric module with an in situ formed p–p–n junction made of state-of-the-art oxides Ca3Co4–xO9+δ (p-type) and CaMnO3–CaMn2O4 composite (n-type). The module was fabricated by spark plasma co-sintering of p- and n-type powders partly separated by insulating LaAlO3. Where the n- and p-type materials originally were in contact, a layer of p-type Ca3CoMnO6 was formed in situ. The hence formed p–p–n junction exhibited Ohmic behavior and a transverse thermoelectric effect, boosting the open-circuit voltage of the module. The performance of the module was characterized at 700–900 °C, with the highest power output of 5.7 mW (around 23 mW/cm2) at 900 °C and a temperature difference of 160 K. The thermoelectric properties of the p- and n-type materials were measured in the temperature range 100–900 °C, where the highest zT of 0.39 and 0.05 were obtained at 700 and 800 °C, respectively, for Ca3Co4–xO9+δ and the CaMnO3–CaMn2O4 composite.