Browsing by Author "Karppinen, Maarit, Prof."

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Cation Ordering and Oxygen Stoichiometry in Double Perovskite Systems, (Sr,La)2FeTaO6−δ and LaBaCo2O5+δ
(2010) Rautama, Eeva-Leena
Aalto-yliopiston teknillinen korkeakoulu | Doctoral dissertation (article-based)
The present thesis consists of five publications, reviewed with relevant literature data. Different types of double perovskites were investigated and consequently, two new compounds are introduced accompanied by their extensive characterization. The B-site ordered double perovskite (Sr1-xLax)2FeTaO6 (x = 0..0.3) achieved by RIII-for-SrII substitution is described by its synthesis, crystallographic features and physical properties. The degree of order of Fe and Ta cations is determined from the X-ray diffraction data after determining the crystal structure. The degree of order is achieved to a practically full extent in the highest-doped samples with respect to the disordered parent phase. The decrease in the Fe oxidation state and its local coordination is analyzed by 57Fe Mössbauer spectroscopy and Fe and Ta L-edge XANES spectroscopy, combined with wet-chemical analysis. The physical properties are found to be similar to those of the disordered parent phase, indicating paramagnetism at room temperature with insulating behavior. The synthesis of the A-site and oxygen-vacancy ordered LaBaCo2O5+δ (δ = 0.5) compound is presented for the first time. The crystal and magnetic structure is determined by neutron diffraction and the microstructure probed by electron diffraction and high-resolution transmission microscopy. The properties resemble the previously known layered cobalt oxides but a partial A-cation and oxygen-vacancy disorder is evidenced. The compound undergoes a spontaneous magnetization at ∼320 K which is the highest measured compared to other similar compounds. In addition, a new polymorph of the δ = 1 phase is described. This structure consists of ordered, nanometer-sized domains affecting the crystal lattice and the physical properties. The chemical pressure effect was realized in both A- and B-site ordered perovskites. For the B-site ordered compounds, the band structures of halfmetallic A2FeMoO6 (A = Ca, Sr, Ba) were calculated and compared to the literature results. As a main result, a decrease of the partial density of states -comparable to the Fe and Mo valence states- is observed, supporting the reported experimental data. For the A-site ordered compounds, RIII cation substitution with precise oxygen engineering in RBaCo2O5+δ (δ = 0.5) system is demonstrated. A partial A-cation and oxygen-vacancy order is studied by crystal-chemical features and physical properties. While the disorder has an effect on the unit cell in addition to magnetic and transport properties, oxygen engineering plays a significant role in fine-tuning of the same properties.
Melting behaviour and quantification of low amorphous levels in sugars and sugar alcohols with DSC techniques
(2010) Lappalainen, Minna
Aalto-yliopiston teknillinen korkeakoulu | Doctoral dissertation (article-based)
The present thesis consists of six publications and a summary of the experimental results reviewed together with relevant literature data. The research was divided into three parts: the melting behaviour and the preparation of amorphous samples as well as the development of a method for quantification of low amorphous content. The melting behaviour of sucrose, glucose, fructose and xylose was investigated. The melting peaks were determined with DSC and the start of decomposition was studied with TG at different rates of heating. In addition, melting points were determined with a melting point apparatus and some StepScan DSC measurements were carried out. There were differences between the different samples of the same sugar and the rate of heating had a remarkable effect on the melting behaviour. The main objective of this thesis was to develop a method for quantification of low levels of amorphous content in sugars (sucrose) and sugar alcohols (maltitol). The techniques chosen were HyperDSC and StepScan DSC, some of the newest DSC techniques. The amorphous maltitol was prepared from crystalline material by melting followed by rapid cooling and the amorphous sucrose was prepared by spray drying. In DSC, detection of low concentrations of amorphous components is based on detection of changes in specific heat capacity (∆Cp) associated with the glass transition. The effects of time and temperature of the low-temperature annealing (below Tg) on the ∆Cp of amorphous samples was studied. The annealing had notable effect on the change in specific heat: as the annealing time was increased, the glass transition temperature moved to a higher temperature and the change of the specific heat increased. The ∆Cp was shown to be a good indicator for the degree of amorphicity in sugar and sugar alcohol samples that are mostly crystalline. With both HyperDSC and StepScan DSC methods experimental conditions were found under which the degree of amorphicity linearly depends on ∆Cp and low limit of detection (LOD) and limit of quantification (LOQ) values were achieved. In addition, the effect of mechanical treatment was tested for sucrose and a correlation between grinding time and amorphous content was found.