Infotech Oulu Doctoral Program
Functional materials for high temperature energy and aerospace applications
Lecturer: Dr. Bilge Saruhan-Brings, German Aerospace Center, Institute of Materials Research, Cologne, Germany
Monday, 16th September
13:15 – 15:00
Thermal barrier coatings (45 min),
Integrated Gas Sensors for Aeronautic Applications (45min)
Tuesday, 17th September
09:15 – 12:00 Chemiresistive type of NO2-Gas Sensors (3 x 45 min)
13:15 - 14:00 Supercapacitors (45 min + discussion)
Chemiresistive type of NO2-Gas Sensors based on nanostructured and doped Semi-conductive Metal Oxides
One of the challenges in developing gas sensors for high temperature emission detection is the total hold of sensitivity at operating temperatures greater than 500°C. NO2 is a common air pollutant that is associated even with poor air quality at the ppb level. Its emission is mostly related to combustion processes where gas temperatures exceed far beyond 500°C. Combustion and exhaust gases contains next to NO/NO2 also CO, O2, water vapour and unburned hydrocarbons, requiring higher NO2-selectivity and less cross-sensitivity to reducing gases such as CO for detection of NO2 at relatively high temperatures. Selective NO2-detection is important for applications related to exhaust and combustion gases, most of which contain oxidising gases influencing the conductivity of the sensor material.
Semiconducting metal oxides such as SnO2 and TiO2 can detect NO/NO2 if doped with trivalent elements (e.g. Cr3+, Al3+, etc.). Selectivity of TiO2-sensors towards NO2 in oxygen containing environments increases by doping with aluminium at operation temperatures as high as 500°C. Al-doping inhibits the grain growth and retards phase transformation from anatase to rutile by stabilizing the surface state of TiO2-particles. Moreover, the substitution of Ti by Al in cationic positions results in a charge imbalance, creating oxygen vacancies. Incorporation of up to 10 at% Cr into TiO2 yields p-type conductivity resulting in sensor stability and increased selectivity towards NO2.
This work presents the results obtained from the sensor electrodes prepared by reactive sputtering of Al-doped TiO2 layers. Sensor characteristics towards NO2 were tested at the temperature range of 300°-900°C. Al-doping of TiO2 alters the sensing properties at elevated temperatures. The factors affecting the changes are attributed to the sensing mechanism. Moreover, nanotubular structuring and doping of TiO2 with Cr yields favourable properties regarding sensors selectivity and sensitivity. In this work, the methods are described for the synthesis of nano tubular TiO2 layers by anodic oxidation of titanium metal. As prepared nano-tubular TiO2 structures were doped with Cr3+ by wet-chemistry method. On heat-treatment at temperatures up to 700°C, the titania films were converted mainly to rutile. The sensor performance of the nano-tubular TiO2 sensors were investigated at temperatures up to 500°C towards NO2 concentration varying from 10 to 50 ppm and CO concentration from 25 to 75 ppm. Cr-doping of nano-tubular TiO2 sensors increases the NO2-selectivity significantly. The high-temperature capability of the sensors is investigated by altering the test conditions and the material characteristics are determined by XRD and spectroscopic methods.
Effect of Pseudocapacitance on Performance of Supercapacitors
Batteries require long time to charge but store greater energy densities, while capacitors can be charged very rapidly within seconds, but suffer from lower energy densities. Commercial electric double layer capacitors store energy in an electric field. This is caused by charged particles arranged on two electrodes consisting mostly of carbon, in the form of a double layer. Such double layer capacitors exhibit a low energy density, so that components with large capacity according to large electrode areas are required. With new electrodes which introduce pseudo-capacitance and higher surface area, super-capacitors charge rapidly and also have higher electrical energy densities and supply this longer. These features are desirable for a range of applications, in electric vehicles and for storage of energy from renewable energy supplies such as solar and wind power which can come in short bursts.
In this talk, an introduction to the area of super-capacitors and its applications to various areas of energy management will be given. The effect of electrode materials will be described to realize electrical energy storage systems with high energy density and high power density. Metal oxide based electrodes increase the capacitance by addition of pseudo capacitance to the static capacitance present by the double layer super-capacitor electrodes. The so-called hybrid capacitors combine both types of energy storage in a single component. The electrochemical testing of the electrodes in half-cells indicates improvement of charge storage behaviour relying on combination of atomic- and nano-scale structures in films. The redox pseudo-capacitive behaviour of the films is analysed by means of cyclic voltammeter measurements. Capacitive charge-storage properties of mesoporous films made of complex metal-oxides are superior to those of non-porous and crystalline metal-oxides. Preliminary the Mn-based mixed redox oxides are investigated using various aqueous electrolytes.
TEM and SANS-Analysis of Pore Morphology of EB-PVD processed Thermal Barrier Coatings and Correlation with Thermal Conductivity
Thermal barrier coatings (TBCs), typically comprised of a ceramic coating deposited onto a bond-coated superalloy substrate, are used to increase lifetime and efficiency of highly loaded turbine blades and vanes in aero-engines and land-based gas turbines by reducing the average metal temperature and mitigating the detrimental effects of hot spots. Electron-beam physical vapor deposition EB-PVD of the standard partially yttria-stabilized zirconia (PYSZ) produces various anisotropic pores which contribute in the reduction of thermal conductivity of TBCs. However, increases in combustion temperature to obtain improved turbine efficiency cause greater thermal loading at the thermal barrier coatings (TBCs) resulting in alteration of the pore morphology and thus, increase of thermal conductivity. Long-term dignity of the ceramic top coat of TBCs is required und can be provided through better hold of low thermal conductivity and phase stability, improved sintering-resistance.
Standard TBC Material which has found a large application is the 7 wt. / Yttria doped ZrO2 (PYSZ-Partially Yttria Stabilised Zirconia). Studies over the past decades carried out in our laboratories showed that the different morphologies obtained by altering the process parameters may influence the thermal conductivity of TBCs. It is possible to optimize the TBC properties by tailoring its microstructure, since a significant relationship between thermal conductivity and processing conditions of EB-PVD TBCs exists. Alternative TBCs such as 14 wt. % Yttria stabilised ZrO2 (FYSZ - Fully Yttria Stabilised Zirconia-) and Pyrochlores yield intrinsically lower thermal conductivity. Pore morphology and microstructure of EB-PVD pyrochlore for instance differ significantly from those of zirconia based TBCs. Its intrinsically low thermal conductivity is of importance. The influence of the pore morphology on reduction of thermal conductivity and resistance to thermal loading in the new TBC compositions are not well established. Our recent studies carried out by analysis methods such as small angle neutron scattering (SANS), ATEM and Laser Flash Technique allowed us to define this relation for various systems. In order to differentiate the 3D closed and open pores in 400 µm thick coatings, a contrast matching SANS technique were employed. This work describes and compares the thermal driven changes in crystal structure as well as pore size and morphology of the anisotropic nano-size pores in PYSZ, FYSZ and Pyrochlore (La2Zr2O7) based TBCs and correlates with thermal conductivity values.
High-Temperature Component-Integrated Gas Sensors for Aeronautic Applications
High-temperature combustion in engines of vehicles and aeroplanes, energy production turbines, power plants and at industrial processes is the main reason for the release of pollutants. NOx is one of the green-house emissions, produced during combustion processes and cannot be avoided by improving the fuel-quality and is associated even at the ppb level, with poor air quality. Sensing of NO2 at high and low temperatures is essential for human health and protection of nature. Gas sensors provide the possibility of the precise determination of the quantity and chemistry of gas emissions to control of combustion processes taking place in aeronautic and stationary applications. Real-time signals from combustion and emission gases (e.g. NOx) are required to establish a basis for more sophisticated control procedures.
Component integration of the sensing layers can be realized by means of sensor arrays and/or by incorporation of sensing layers into the high-temperature turbine components. For that, a concept is developed for detection of total NOx by means of turbine blade integrated gas sensors. The sensing layers are embedded in EB-PVD deposited thermal barrier coating (TBCs) to serve as electrolyte in impedance-metric based gas sensors. The sensing layers were selected from Ni-based oxides and spinels. This multifunctional layer system is proved to yield high sensitivity for total NOx-sensing at temperatures up to 650°C under high oxygen containing gas atmospheres. Response time measurements showed a stable signal and an acceptable time of about 60 seconds in the presence of O2. Cross-sensitivity in gas mixtures containing CO and hydrocarbons are also tested.
Bilge Saruhan-Brings has received her BSc in Metallurgical Engineering and her MSc in Materials Science from the Faculty of Chemical and Metallurgical Engineering of Istanbul Technical University in Turkey. Her PhD is from Materials Research Center at the University of Limerick in Ireland. She has been working at the Institute of Materials Research of German Aerospace Center since completing her PhD. In 2002, she has completed her lectureship qualification and received the habilitation degree from Technical University of Freiberg in Germany. Previously she lectured on topics such as “Basics of Materials Science”, “Nanomaterials and Nanotechnologies” and “Nanomaterials for Microsystem-Technology” at the Applied Universities of Cologne and of Rhein-Sieg in Germany. Her research interests are in the fields of thermal barrier coatings, thin films and nanostructured materials for gas sensor devices and catalytic applications as well as for energy storage systems. She has authored and coauthored more than 100 publications and 1 book. 7 patents have been registered from which 3 of them are pending.
More information: Jari Juuti
Last updated: 10.9.2013