Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Electrocaloric Properties of the Zr-Substituted Batio3 – Na0.5bi0.5tio3 Ceramics(2023) Akkaşoğlu, Oğuz; Adem, UmutThe aim of this study is to investigate electrocaloric properties and thermodynamic behaviour, obtaining high adiabatic temperature change (ΔT) values with a broad temperature span of zirconium doped barium titanate-sodium bismuth titanate by substitution of Zr into B-site (titanium). Ceramics are synthesized in a pellet form by solid-state reactions. Chemical composition was Ba0.7Na0.15Bi0.15TixZr1-xO3 (abbreviated as BT-NBT) where x= 0.00, 0.01, 0.02, 0.03, 0.035, 0.04 and 0.05. Phase analysis was conducted by X-ray diffraction method. Microstructural analysis and average grain size determination was performed by Scanning Electron Microscopy. To understand phase transitions and physical behaviours, dielectric measurements are performed. Ferroelectric properties are investigated by using temperature dependent polarization, strain and current-electric field relationships. Electrocaloric measurements are done by using temperature dependent polarization-electric field data. Maxwell relations are used to calculate temperature dependent electrocaloric temperature change, ΔT, and from this data, temperature span, Tspan, was calculated. It was observed even though Zr substitution into the Ti-site rapidly decreases the Curie temperature and introduces relaxor ferroelectric character to the samples. However, the 1st order like nature of the ferroelectric-paraelectric phase transition at the Curie temperature and, consequently significant ΔT is maintained even for 4 % Zr substituted sample close to room temperature. Temperature span, on the other hand, has a lower value compared to previous works related to barium titanate systems. Electrocaloric efficiency is comparable to other works on Pb-free sytems and these results showed that BT-NBT systems have promising features for electrocaloric cooling technologies.Master Thesis Formation of Srtio3[tio2] Ceramic Composites at Low Temperatures(01. Izmir Institute of Technology, 2021) Karataş, Esin; Adem, Umut; Ahmetoğlu, Çekdar VakıfStrontium titanate ceramics are materials belonging to the perovskite material group with the formula ABO3. Strontium titanate ceramics have been a preferred material in many areas, such as the electroceramics industry, due to its high dielectric constant and high chemical stability and generally produced by manufacturing processes such as solid-state synthesis, hydrothermal method, and sol-gel. SrTiO3 ceramics have been produced in the literature with different temperatures, times, and starting materials. For ceramic products, a sintering step is required after the powder production step. Traditional sintering methods, which have been used for many years, are used to densify powders with high temperatures. Recently developed low-temperature densification methods enable the sintering process to be carried out at relatively lower temperatures. For this purpose, in this thesis, SrTiO3-TiO2 ceramic composites were produced at different reaction temperatures and times using the rHLPD method, which combines powder production and sintering steps in a single process. In the studies, the reaction temperature, time, and the effect of adding mineralizer to the prepared solution for the hydrothermal reaction on the final product were investigated. The aim of the thesis is to produce SrTiO3 from TiO2 green body using the rHLPD method. In addition, the production of SrTiO3-TiO2 ceramic composites with as high mole conversion and relative density values as possible was targeted with different parameters such as reaction temperature, reaction time, and addition of NaOH to the solution. As a result of the studies carried out with different parameters, SrTiO3-TiO2 ceramic composites with a final relative density value of approximately 81 % were produced. Consequently, in the XRD and Rietveld analysis, it was seen that the main phase was SrTiO3. In addition, there was a TiO2 phase in the structure. Finally, the SrCO3 phase was not detected in the structure.