Chemical expansion in manganites

Şeren, Mehmet Hazar
Manganites are type of manganese oxides having mixed valence states with perovskite structure represented as Ln1-xAxMnO3 (Ln= rare-earth cation, A= alkaline earth cation) and they have been widely studied due to their potential applications in various important areas and their interesting properties. Numerous investigations have focused on their production, characterization of their properties and solution of problems encountered during their applications. Some of these applications are cathode and electrolyte materials for solid oxide fuel cells (SOFC), piezoelectric sensors or energy harvesting by thermoelectric materials. In some applications, materials can be in contact with highly oxidizing and/or reducing atmospheres. The possible variations in the stoichiometry of the materials under these conditions may also lead to changes in their lattice parameters. This effect is known as "chemical expansion". The component made of such a material may mulfunction when dimensional stabilities occur. For instance, dimensional changes cause residual stresses in thin film structures if the film is clamped to a rigid substrate. When these stresses are above a certain limit, some defects like cracks or hillocks are formed and the film loses its function. Therefore it is of cardinal importance to observe these dimensional variations and understand the governing mechanims. The aim of this study is to explore chemical expansion coefficients of La0.5Ca0.5MnO3-δ (LCMO) and La1-xSrxMnO3-δ (LSMO) (x= 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1) for the first time in literature. In addition, effect of Sr dopant on chemical expansion coefficient is another aim of this study. The chemical expansion in the structure during O loss was determined by combined study of dilatometry, in-situ X-ray diffraction and thermogravimetric analysis. Microstructural and chemical analysis were performed using room temperature X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. With increasing Sr dopant, chemical expansion coefficient and δ values have decreasing trend whereas the absence of La in the structure leads to increase in chemical expansion. Therefore, the lowest chemical expansion coefficient is observed in LSMO9 samples. Slope changes are observed in TGA curves and the reason for this can be explained with the effect of Jahn-Teller distortion due to change in Mn valence state.


Synthesis of zinc oxide nanoparticles by aqueous methods and effect of metal incorporation on the structural/functional properties of nanoparticles
Altıntaş Yıldırım, Özlem; Durucan, Caner; Department of Metallurgical and Materials Engineering (2014)
Zinc oxide (ZnO) nanostructures have attracted considerable attention in many electrical, optoelectronic and magnetic applications due to their unique properties originating form characteristic wide band gap and large exciton binding energy of ZnO. Electrical, optical and magnetic properties of ZnO nanostructures strongly depend on their size and morphology. Therefore, there has been a strong interest in the synthesis of ZnO nanostructures with well-controlled size and shape. These synthesis approaches shou...
Effect of fluorine substituted benzothiadiazole on electro-optical properties of donor-acceptor-donor type monomers and their polymers
Çakal, Deniz; Önal, Ahmet Muhtar (Elsevier BV, 2020-11-01)
A series of trimeric monomers bearing thiophene donor units and fluorinated analogues of benzothiadiazole acceptor units was synthesized via donor-acceptor-donor approach and polymerized electrochemically in order to investigate the effect of fluorine atom on electro-optical properties of both monomers and their corresponding polymers. All monomers exhibited solvatochromic and fluorescent properties. The introduction of fluorine atom into the conjugated backbone increased the oxidation potentials. Upon movi...
Dynamic and steady-state analysis of oxidative dehydrogenation of ethane
Karamullaoğlu, Gülsün; Doğu, Timur; Department of Chemical Engineering (2005)
In this research, oxidative dehydrogenation of ethane to ethylene was studied over Cr-O and Cr-V-O mixed oxide catalysts through steady-state and dynamic experiments. The catalysts were prepared by the complexation method. By XRD, presence of Cr2O3 phase in Cr-O; and the small Cr2O3 and V2O4 phases of Cr-V-O were revealed. In H2-TPR, both catalysts showed reduction behaviour. From XPS the likely presence of Cr+6 on fresh Cr-O was found. On Cr-V-O, the possible reduction of V+5 and Cr+6 forms of the fresh sa...
Chemical vapor deposition of boron carbide
Karaman, Mustafa; Özbelge, Önder; Department of Chemical Engineering (2007)
Boron carbide was produced on tungsten substrate in a dual impinging-jet CVD reactor from a gas mixture of BCl3, CH4, and H2. The experimental setup was designed to minimise the effect of mass transfer on reaction kinetics, which, together with the on-line analysis of the reactor effluent by FTIR, allowed a detailed kinetic investigation possible. The phase and morphology studies of the products were made by XPS, XRD,micro hardness and SEM methods. XPS analysis showed the existence of chemical states attrib...
Shipborne samples collected from the Black Sea atmosphere were analysed by instrumental neutron activation analysis, atomic absorption spectrometry and ion chromatography for approximately 40 elements and ions. Concentrations of most of the elements are found to be a factor of two higher in the western part of the Black Sea than corresponding concentrations in the eastern part. Main source regions for anthropogenic elements were in Europe. The WNW, WSW and SSW sectors together account for 70% of observed co...
Citation Formats
M. H. Şeren, “Chemical expansion in manganites,” M.S. - Master of Science, Middle East Technical University, 2015.