Annika Pohl

Preparation of La0.67Ca0.33MnO3 Nanophase Powders and Films from Alkoxide Precursors

The development of more advanced materials forms the basis of technological progress. One group of fascinating compounds with many potential applications in spintronic devices are the mixed-valence perovskite manganites. These have attracted considerable interest during the last decade through their very large magnetoresistance near the Curie Temperature. Although the properties of a material determinie any application, the development of reliable and flexible synthesis methods is crucial, as is the understanding of these methods. Knowledge of how different materials are formed is also of general importance in tailoring new materials. The aim of this project has therefore been not only to develop a new synthesis route, but also to understand the mechanisms involved.This thesis describes the synthesis and characterization of a novel manganese alkoxide and its use in sol–gel processing of magnetoresistive perovskite manganites. In searching for a soluble manganese alkoxide for sol–gel processing, we found that the methoxy-ethoxide, [Mn19O12(moe)14(moeH)10]·moeH, has a high solubility in appropriate organic solvents. Being 1.65 nm across, it is one of the largest alkoxides reported; it is also of interest because of its (for oxo-alkoxides) rare planar structure. After mixing with La, Nd, Ca, Sr, and Ba methoxy-ethoxides, [Mn19O12(moe)14(moeH)10]·moeH was used in the first purely alkoxide based sol–gel processing of perovskites manganites. The phase evolution on heating xerogel powders to 1000°C was studied, and thin films were prepared by spin-coating.It was found that the easily oxidised Mn-alkoxide facilitates the formation of high oxygen-excess modifications of the perovskites. The reactive precursor system yields fully hydrolysed gels almost without organic residues, but the gel absorbs CO2 from the air, leading to carbonate formation. The carbonate decomposition is the limiting step in oxide formation. Transport measurements of La0.67Ca0.33MnO3 films on LaAlO3 substrate show that all-alkoxide sol–gel derived films can compete with PLD films in terms of quality of epitaxy and transport. The somewhat different behaviour of the sol–gel derived films compared to PLD films is attributed to differences in morphology and oxygen stoichiometry.

Preparation of La0.75Sr0.25MnO3 Nano-Phase Powders and Films from Alkoxide Precursors

The first purely alkoxide-based sol-gel route to nano-phase powders and thin films of perovskite La0.75Sr0.25MnO3 is described. The phase and microstructure evolution on heat treatment of free gel films to form the target nano-phase oxide were investigated by TGA, IR spectroscopy, powder XRD, SEM and TEM-EDS. The xerogel consisted of a hydrated oxo-carbonate, without remaining alkoxo groups or solvent. Heating at 5°C·min−1 decomposed the carbonate groups and yielded the pure perovskite La0.75Sr0.25MnO3 at 760°C. The cell dimensions were virtually unchanged from the first observation of perovskite at 680°C, to 1000°C, 4 h. The monoclinic cell of La0.75Sr0.25MnO3 obtained at 1000°C, 4 h, had the dimensions a = 5.475(1), b = 5.504(2), c = 7.771(1) Å, β = 90.50(2), fitting the literature data quite well. Crack-free, homogenous, 150 nm thick La0.75Sr0.25MnO3 films were prepared by spin-coating Si/SiO2/TiO2/Pt and polycrystalline α-Al2O3 substrates with a 0.6 M alkoxide solution, followed by heating at 5°C·min−1 to 800°C, 30 min.

Solution processing of conducting dense and porous films; doped ZnO and TiO2, and perovskites

Solution based processing of porous and dense films by all-alkoxide based precursor systems have been investigated and compared, with a focus on the connection between the precursors and heat-treatment, and the product oxide quality and structure. Three systems were investigated typically with TGA, DSC, XPS, IR spectroscopy, X-ray diffraction, TEM-EDS and SEM-EDS, but also with other advanced characterisation techniques: (i) manganate and cobaltate perovskites of the LCMO (La0.67Ca0.33MnO3), LSMO (La0.75Sr0.25MnO3), LNCMO (La0.33Nd0.33Ca0.33MnO3), LBSM (La0.75Ba0.125Sr0.125MnO3) and LSCO (La0.50Sr0.50CoO3) compositions were investigated. Highly epitaxial films of LCMO and LSCO were prepared, and the CMR properties of the LCMO were comparable to those of PVD derived films. Polycrystalline films were prepared for all perovskites. (ii) Three routes to ZnO : Co/Al were investigated and compared; an acetate based route and two alkoxide based routes with different heat-treatments. With the acetate based and the alkoxide based route using hydrolysis in air, a maximum Co doping of 6% was obtained, while in absence of CO2 and O2, the doping range could be extended to 20%Co. Both dense films and porous nano-structured films were prepared. The magnetic properties of the Co-doped films did not show any room-temperature ferro-magnetism. (iii) ZrO2 and NbO2.5 doped anatase TiO2 were prepared by heat-treatment or hydrothermal routes. Alio-valent doping of up to more than 35% of NbO2.5 could be achieved in the anatase. It is believed that the pentavalent metal doping is compensated by metal vacancies, and as expected from this assumption, the materials have a low density. ZrO2 doping in titania led to an increase in the dye-sensitised solar cell efficiency and highly active and stable photo-catalysts.

Photoinduced reduction of surface states in Fe:ZnO.

We report on the electronic structure of nano-crystalline Fe:ZnO, which has recently been found to be an efficient photocatalyst. Using resonant photoemission spectroscopy, we determine the binding energy of Fe 3d states corresponding to different valencies and coordination of the Fe atoms. The photo-activity of ZnO reduces Fe from 3+ to 2+ in the surface region of the nano-crystalline material due to the formation of oxygen vacancies. Electronic states corresponding to low-spin Fe(2+) are observed and attributed to crystal field modification at the surface. These states are potentially important for the photocatalytic sensitivity to visible light due to their location deep in the ZnO bandgap. X-ray absorption and x-ray photoemission spectroscopy suggest that Fe is only homogeneously distributed for concentrations up to 3%. Increased concentrations does not result in a higher concentration of Fe ions in the surface region. This is limiting the photocatalytic functionality of ZnO, where the most efficient Fe doping concentration has been shown to be 1%-4%.

Sol-gel derived Nd1/3La1/3Ca1/3MnO3 : Phase evolution and preparation of films and nanopowders

An all-alkoxide sol-gel route to the formation of Nd1/3La1/3Ca1/3MnO3 thin films and powders has been developed. The microstructural evolution on heat treatment of the gel to yield the perovskite oxide was monitored by means of thermogravimetric analysis-differential scanning calorimetry, powder x-ray diffraction (XRD), Fourier transform infrared spectroscopy, and transmission electron microscopy (TEM)-energy dispersive spectroscopy (EDS). It was found that the amorphous gel consists of hydrated oxo-carbonate without organic residues, and on heating it decomposes in several steps, finally forming single-phase perovskite at 680 degrees C. Further heating results in only slight changes in the cell parameters and crystal growth. Films were prepared by spin coating, followed by heat treatment in air to a temperature of 800 degrees C, and studied by scanning electron microscopy, TEM-EDS, and XRD. Films on Al2O3 were more porous, while films on Pt-TiO2-SiO2-Si were rather dense and consisted of areas with different crystal orientations.

Comparison of sol-gel derived and pulsed laser deposited epitaxial La0.67Ca0.33MnO3 films for IR bolometer

Epitaxial La0.67Ca0.33MnO3 films have been prepared on LaAlO3 crystals by pulsed laser deposition (PLD) and by a novel all-alkoxide sol-gel technique. Different out-of-plane lattice parameters are found for the as-prepared films, and scanning electron microscopy shows a more porous structure for sol-gel films as compared to PLD films. These differences are largely removed by post-annealing at 1000 degreesC. Transport measurements show maximum temperature coefficient of resistivity of 8.2 % K-1 at 258 K (PLD) and 6.1% K-1 at 241 K (sol-gel) and colossal magnetoresistance at 7 kOe of 35% at 263 K (PLD) and 32% at 246 K (sol-gel).

Preparation and Characterization of Alumina Based TiNn and SiCn Composites

Abstract : Ceramic composites containing 2 and 5vol % of nanosized commercially available TiN and SiC particles in alumina were prepared via a water based slurry processing route followed by spark plasma sintering (SPS) at 75 MPa in the temperature range 1200-1600 deg. C. Some of the samples could be fully densified by use of SPS already after five minutes at 1200 deg. C and 75 MPa. The aim was to control the alumina grain growth and thus obtain different nano-structure types. The microstructures have been correlated to some mechanical properties; e.g. hardness and fracture toughness.