טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentNeta Shomrat
SubjectThe Influence of Non-Stoichiometry and Doping on Flash
Sintering of Oxide Perovskite
DepartmentDepartment of Energy
Supervisor Professor Tsur Yoed
Full Thesis textFull thesis text - English Version


Abstract

The ternary oxide perovskites, , exhibit many interesting theoretical aspects and applications. Their properties are closely related to, and can sometimes be controlled by non-stoichiometry, which can result from oxygen loss and/or when the ions A/B ratio deviates from unity. The latter can be caused by inaccuracy of initial compositions or vaporization of different compounds during high temperature processes such as calcination and sintering.

This work includes both theoretical aspects regarding non-stoichiometry in perovskites (section ?3) and a practical solution, flash sintering technique, to the unstable nature and stoichiometric deviation of alkali- niobate (section ?4). In section 3 it was shown that despite the well-known fact that frozen-in metal vacancies behave as acceptors, in some perovskite systems there is a finite range in the A/B ratio where an increase in metal vacancy concentration results in donor-like behavior. In section 4, a new field-assisted sintering technique, flash sintering, was proven as a process which overcomes the main challenge of sintering ceramics with highly volatile species. This was a breakthrough in the synthesis of dense stoichiometric KNbO3.

While flash sintering is considered a promising technique that was already conducted on several other ceramic systems, the mechanism of this process remain uncertain. Thus, a dynamic heat balance simulation predicting the onset conditions for the flash was compared to flash sintering experimental results of iron doped strontium titanate (STFO; section?5). The contribution of point defects to flash mechanism in STFO was studied by electrical measurements at different oxygen partial pressures (sections ?6) and impedance spectroscopy analysis of the green body (section ?7). Reduction of onset temperature with increasing doping is shown with high compatibility of model and experimental results. However, the green sample resistance is linked to the charged defects concentration. To study the contribution of defects on the flash sintering mechanism, this link should be untied. In STFO system, point defect concentration can be pinned while the resistance is influenced by the oxygen partial pressure (pO2). SrTi0.97Fe0.03O2.985 onset temperature at different pO2 was examined at constant oxygen vacancy concentration. The furnace onset temperature decreases with increasing pO2. Ex-situ impedance measurements of green samples reveal an overlapping Nyquist plots close to the sample onset temperature. This indicates that the onset is determined by the green body resistance regardless how it has been achieved.