Hans-Peter Buchkremer

Non-destructive characterization of plasma-sprayed ZrO2 coatings by ultrasonic surface waves

A surface wave method has been used to determine the elastic modulus of ZrO2 coatings plasma-sprayed onto a nickel-base superalloy and aluminium. The phase velocity depending upon the frequency of laser induced surface waves has been measured. The inverse problem of the surface wave dispersion relation has been solved to calculate the elastic modulus of the coatings from the measuring data. Modulus values in the range 11.2-46 GPa were obtained for the coatings applied on a nickel base superalloy by atmospheric plasma spraying. These moduli are lower by up to one order of magnitude than those measured on compact ZrO2 samples (240 GPa) owing to a network of long stretched-out pores which intersperse the coating material. An elastic modulus of 90 GPa has been determined for the ZrO2 coating vacuum-plasma-sprayed on aluminium. The micrograph of this coating showed a structure with more isolated pores having almost equiaxial form. The nature of the pore structure can be predicted from the measured elastic modulus using the theory of elastic modulus of heterogeneous solids.

Sintering Behavior of (La,Sr)MnO3 Type Cathodes for Planar Anode-Supported SOFCs

One of the main targets in the development of anode-supported solid oxide fuel cell (SOFCs) is to improve the electrochemical performance. This can be achieved by optimizing processing and microstructural parameters of the SOFCs. Variations of the thickness of the cathode functional layer and the cathode current collector layer, the grain size of the powders used for applying these layers, and the sintering temperature, can influence the electrochemical performance as such that lower operation temperatures become possible without detrimentally affecting the power output to a great extent. In this study the effect of variations of the sintering temperature of the cathode on (I) the microstructure, (2) the gas diffusivity and permeability in the cathode, and (3) electrochemical performance of FZJ-type anode-supported single cells, was investigated. The FZ-Julich cell design is based on anode-supported type cells, which are characterized by a relatively thick anode (thickness: 1.0-1.5 mm) consisting of a NiO/8YSZ cermet, a thin 8YSZ electrolyte, and a bi-layered cathode. The cathode distinguished two separated layers: first a cathode functional layer consisting of La 0.65 Sr 0.3 MnO 3 (LSM)/Y 2 O 3 -stabilized ZrO 2 (8YSZ) and a cathode current collector layer of pure La 0.65 Sr 0.3 MnO 3 (LSM). This study can be considered as a follow-up of that (Journal of Power Sources 141 (2005) 216-226) describing the improvement of the cell performance by a systematic variation of the microstructure. The experiments described in this paper and the corresponding results are part of a more extensive study to investigate in more detail the effect of the sintering temperature on the electrochemical performance of LSM-type SOFCs. Since research is still going on, conclusions, drawn in this contribution, are yet not definitive.