M.Sc Thesis


M.Sc StudentMirit Ram
SubjectChemically and Thermally Induced Expansion in Simple
Perovskites
DepartmentDepartment of Chemical Engineering
Supervisor Professor Tsur Yoed
Full Thesis textFull thesis text - English Version


Abstract

This work aims to explain the known phenomena of chemical expansion due to reduction in oxide perovskites, and to investigate the influence of oxygen partial pressure on the thermal expansion coefficient (TEC). The construction of a suitable experimental setup, including building an oxygen sensor and its characterization was an additional objective. A comparison between SrTiO3, SrZrO3 and Al2O3 was done by a single push-rod dilatometer. Two kinds of experiments were done. The first one designed to check the chemical expansion as a function of oxygen activity at a constant temperature. The degree of the expansion and contraction of the samples due to changing pO2 was examined. The second experiment designed to check the influence of pO2 on the TEC. At the end of each step (same steps as in the first experiment), when the sample was in equilibrium (chemically and thermally), small changes of temperature, around the constant temperature were done. The temperature was programmed to decrease and increase in 10 ºC from the constant temperature each direction, in rate of 5 Cº/min several times. Each slope of the dimension is due to change in temperature, so the slope (dx/dt) multiplied by (dT/dt)-1 is the TEC. By averaging the slopes that belong to the same temperature and oxygen partial pressure an average TEC is being received.

These experiments were done in three different temperatures: 722 ºC, 822 ºC and 1022 ºC. The pO2 was controlled by CO-CO2-Ar gas mixture with total flow rate of 55 cc/min, and monitored with an in house built zirconia sensor. The main results reveal that increasing the concentration of oxygen vacancies, leads to expansion of the unit cell, and decreasing  the concentration of oxygen vacancies leads to contraction. This is known for many perovskites, and is confirmed in this work for SrTiO3 and SrZrO3. Surprisingly enough, it does also happen in alumina.

We cannot suggest correlation between the material’s expansion and its ability to change valence at the B site. In addition we found that the TEC is influenced by the pO2. The TEC of SrTiO3 is more sensitive to changes in the pO2 than that of SrZrO3. Probably, the oxygen vacancies creation is easier in SrTiO3 than in SrZrO3. In addition we might suggest that there is a low limit temperature by which oxygen partial pressure influences the TEC.