Electrical properties, structure, and defect chemistry of grain boundaries in n-conducting BaTiO3
Scientific supervisor: Wolfgang Preis
- Introduction:
The PTC effect is the rapid increase of the grain boundary resistance of n-conducting BaTiO3 ceramics by many orders of magnitude when the temperature is raised through the ferroelectric – paraelectric phase transition temperature (Curie-point). The formation of Schottky barriers at the grain boundaries gives rise to the enormous increase of the resistivity by 5 – 7 orders of magnitude. At the Chair of Physical Chemistry bulk and grain boundary resistances are measured in various atmospheres (different oxygen partial pressures). The composition and structure of grain boundaries in atomic resolution are investigated by HRTEM in cooperation with the Erich – Schmid – Institute
- Measurement set-up:
Chair of Physical Chemistry: Impedance spectroscopy and investigation of the microstructure by SEM/EDX Erich-Schmid-Institut: EELS/HRTEM
- Method:
The bulk and grain boundary conductivities are determined experimentally as a function of temperature by means of impedance spectroscopy. Segregation profiles of the dopants at the grain boundaries as well as the structure of the grain boundaries are investigated by means of EELS/HRTEM. Based on these experimental studies it is the aim to develop a comprehensive defect chemical model for the electrical properties of grain boundaries in BaTiO3 ceramics.
Modelling of oxygen diffusion in electroceramic materials
Scientific supervisor: Wolfgang Preis
- Introduction:
Diffusion processes play an essential role for both the manufacturing and function of electroceramics. In many cases diffusion in electroceramic materials is strongly affected by grain boundaries. Depending on the structure and composition, the transport can be blocked across the interface or grain boundaries can represent fast diffusion paths. Grain boundaries in oxygen ion conductors, such as doped ZrO2 and CeO2, are usually blocking for oxygen transport. On the contrary, electroceramic materials for varistors and PTC (positive temperature coefficient) - thermistors show fast grain boundary diffusion of oxygen. At the Chair of Physical Chemistry fast grain boundary diffusion is modeled, taking account of oxygen exchange reactions at the surface between the gas and the solid phase. Furthermore, the research activities have recently been focused on the simulation of diffusion and surface exchange reactions in ceramic composites.
- Measurement set-up:
Theoretical calculations
- Method:
Analytical solutions to the diffusion equations have been developed for the simulation of diffusion processes. In addition, numerical methods, e.g. finite element method and finite differences, are applied. The microstructure of the polycrystalline solids are described by various models, such as parallel grain boundaries, a spherical grain model, and a square grain model. It is the aim of these activities to provide a quantitative description as well as prediction of oxygen diffusion in PTC ceramics, cathode materials for solid oxide fuel cells (SOFCs) etc.
Grain boundary and bulk conductivities of SOFC-electrolytes and electroceramic materials
Scientific supervisor: Wolfgang Preis
- Introduction:
The electrical properties of solid electrolytes for SOFCs as well as numerous electroceramic materials, e.g. for PTC (positive temperature coefficient) resistors and varistors, are determined by both the bulk (grains) and the grain boundaries. The function of semiconducting PTC ceramics and varistors is predominantly given by high grain boundary resistivities which are caused by the formation of Schottky barriers. The ohmic resistance of oxygen ion conductors, such as solid electrolytes for SOFCs, consists of contributions of grains (bulk) as well as usually blocking grain boundaries. An essential aspect of the research activities at the Chair of Physical Chemistry is the determination of bulk and grain boundary conductivities of electrolyte materials (Gd-doped ceria and scandia-stabilized zirconia co-doped with Ce and Y) as well as n-conducting BaTiO3 ceramics as a function of temperature and oxygen partial pressure. These experimental data are an essential requirement for the development of defect chemical models for the bulk of solid electrolytes and grain boundary regions in PTC ceramics, respectively.
- Measurement set-up:
Measurement set-up: Impedance spectroscopy in a wide temperature (room temperature – 900°C) and oxygen partial pressure (1 – 10-25 bar) range. The oxygen partial pressure is varied by means of appropriate gas mixtures and an electrochemical oxygen pump. Besides ac measurements a 4-point dc technique according to van der Pauw is likewise applied.
- Method:
It is the aim to study the electrical properties as a function of temperature and oxygen partial pressure in order to gain a sound understanding of the defect chemistry of the investigated materials. The bulk and grain boundary resistances are determined by fitting appropriate equivalent circuits to the experimental impedance spectra (frequency range: usually 10 mHz – 10 MHz). The activation energies for the transport of electronic or ionic charge carriers in the grains and the grain boundaries are obtained from the temperature dependence of the respective conductivities. In addition, thermal analysis (thermogravimetry, dilatometry, and differential scanning calorimetry) is performed and the microstructure is investigated by SEM/EDX.