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Chair of Physical Chemistry

Current topics

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 ( Prof. Gerhard Dehm): 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 BaTiO 3 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 ZrO 2 and CeO 2, 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.

Defect chemical modelling of complex oxides

Scientific supervisor: Edith Bucher

  • Introduction:

While the theoretical "ideal crystal" has a perfectly ordered crystal lattice at 0 K, real crystals always exhibit lattice defects. The defect chemistry describes these deviations from perfect order of inorganic crystalline materials, as well as material properties induced by defects. At the Chair of Physical Chemistry defect chemical properties of complex oxides, especially of perovskites, K2NiF4- and fluorite type oxides, are investigated.

  • Experimental:

Theoretical calculations

  • Method:

Defect chemical equations (models) are proposed and evaluated based on experimental data (oxygen nonstoichiometry, electronic and ionic conductivities, Seebeck-coefficients) as a function of T and pO 2. The aim is the theoretical description and prediction of defect concentrations as well as various mass and charge transport properties for given values of T and pO 2.

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 BaTiO 3 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.

Long-term stability of electrolyte materials for SOFCs

Scientific supervisor: Wolfgang Preis

  • Introduction:

The long-term stability of solid oxide fuel cells (SOFCs) is affected by various factors, such as poisoning of cathodes by chromium from the interconnectors and sulfur poisoning as well as coking of nickel-cermet based anodes. An additional aspect is the conductivity degradation of solid electrolytes, leading to an increase of the ohmic cell resistance. At the Chair of Physical Chemistry the degradation of the ionic conductivity of relevant electrolyte materials (Sc-stabilized zirconia) is studied in oxidizing and reducing atmospheres

  • Measurement set-up:

Impedance spectroscopy as well as four-point dc-measurements according to van der Pauw at 600 and 700°C in air and reducing atmospheres (e.g. 1%-H 2/Ar).

  • Method:

The bulk and grain boundary conductivities are investigated in oxidizing (air) and reducing atmospheres by means of impedance spectroscopy over a long period of time (more than 3000 h). Additionally, the dc conductivities are determined by van der Pauw measurements. It is the aim to optimize the composition of electrolyte materials with respect to their long-term stability.

Long-term stability of SOFC cathode materials

Scientific supervisor: Edith Bucher

  • Introduction:

Besides excellent oxygen exchange kinetics the long-term stability over the targeted life time of the stack is a crucial issue for the successful technical implementation of promising SOFC cathode materials. Therefore, systematic investigations on the degradation of the oxygen exchange kinetics are performed.

  • Experimental:

Various test facilities for long-term investigations by electrical conductivity relaxation method at temperatures up to 900°C in reference atmospheres (O2-Ar-mixtures) as well as real fuel cell conditions (H2O- and CO2-containing gases) are available. Pre- and post-test analysis by REM/EDX, XPS, RFA etc.

  • Method:

Measurements of the kinetic parameters versus time allow for in-situ characterisation of the long-term degradation of selected SOFC cathode materials. The data are combined with pre- and post-test analysis of the microstructure and chemical composition of the samples on various length scales (few nanometers to bulk) for obtaining an in-depth understanding of the degradation mechanisms.

Determination of the kinetic parameters for oxygen exchange of SOFC cathode materials

Scientific supervisor: Edith Bucher

  • Introduction:

The oxygen exchange kinetics of mixed conducting SOFC cathode materials, e.g. the perovskites (La,Sr,Ba)(Co,Fe)O 3, is determined by the oxygen exchange reaction at the gas-solid interface as well as by the oxygen diffusion process in the bulk.

  • Experimental:

Various test facilities for electrical conductivity relaxation measurements with in-situ oxygen sensor for measurements up to 900°C in defined atmospheres (O 2-Ar-mixtures, H 2O- and CO 2-containing gases) are available.

  • Method:

After a step-wise change of the oxygen partial pressure of the surrounding gas atmosphere the equilibration kinetics are monitored via the electrical conductivity of a dense oxide sample. By fits (nonlinear regression) of the adequate diffusion equation to the experimental relaxation curve, the kinetic parameters (chemical surface exchange coefficient kchem and chemical diffusion coefficient Dchem) are obtained as a function of T and pO2.

Determination of the electrical conductivity of complex oxides

Scientific supervisor: Edith Bucher

  • Experimental:

Test facilities for electrical conductivity measurements at high temperatures (20-900°C) in air or in defined atmospheres (O 2-Ar- and H 2-Ar-mixtures, H 2O- and CO 2-containing gases); Cryostat for electrical conductivity measurements at low temperatures (11-300 K).

  • Method:

By pressing of oxide powders, sintering, cutting and polishing, dense bar- or disc-shaped samples are obtained. Electrical contacts are realised by noble metal pastes and wires or measurement tips (Pt, Au etc.). For the determination of the electronic conductivity 4-point dc-measurements are applied in linear or van der Pauw geometry.

Determination of the oxygen nonstoichiometry of complex oxides

Scientific supervisor: Edith Bucher

  • Introduction:

The occupation of the oxygen sub-lattice of complex transition metal oxides, e.g. the perovskites ABO 3-d where A=(La,Sr,Ba) and B=(Co,Fe,Ni,Mn), is controlled by the oxygen exchange reaction with the gas phase, i.e. oxygen vacancies are generated/annihilated as a function of T and pO 2. High oxygen vacancy concentrations together with adequate electronic conductivities may result in high ionic conductivities as well as oxygen diffusion and surface exchange coefficients which make these materials attractive for applications in SOFCs, membrane reactors and sensors.

  • Experimental:

Precision thermobalance with mass flow controllers and in-situ oxygen sensor for measurements at 20-900°C in defined atmospheres (O 2-Ar- and H 2-Ar-mixtures, H 2O- and CO 2-containing gases).

  • Method:

The changes of the oxygen nonstoichiometry are calculated from relative mass changes of powder or solid oxide samples as a function of T and pO 2 and the given elemental composition. Absolute values of the oxygen nonstoichiometry are available from total reduction of the sample in hydrogen.

Data acquisition and device control

Operator: Peter Gsaxner

For research work data acquisition applications and device control are applied. Most of the applications are developed with LabView.

According to requirements the user is able to customize the applications. Measurement data are written to a file and afterwards analized using Origin and/or MS-Excel.

  • All applications are showing measurement data vs. time.
  • All applications are designed to control the measurement on workstations over the network.

Thermodynamic Modeling of Metal-Sulfur Systems

Scientific supervisor: Peter Waldner

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