Angle-dependent THz tomography – characterization of thin ceramic oxide films for fuel cell applications

A time-resolved THz tomography system for the incidence-angle-dependent three-dimensional characterization of layered structures is presented. The capabilities of the developed system are demonstrated on multi-layer ceramic samples used for solid oxide fuel cells (SOFC). Appropriate methods for determining unknown refractive indices are discussed. It is shown how the angle of incidence of a THz imaging system has a significant influence on measured signals. This fact can be exploited especially in Brewster-angle configurations to enhance the capabilities of any THz tomography system. Data evaluation algorithms are presented. PACS: 42.30.Wb; 81.70.Fy; 78.47.+p Since the emergence of time-resolved THz imaging, many interesting applications have been demonstrated [1–6]. The imaging techniques developed to date rely on the spatially resolved analysis of either transmitted [1] or reflected [5] optically generated THz pulses. This paper focuses on the second alternative, namely the timeand spatially resolved detection of reflected THz pulses, which permits the threedimensional tomographic analysis of the internal structure of objects. A new THz tomography setup is presented, which permits incidence-angle-dependent measurements. It is shown how the angle of incidence of the THz pulses has a significant influence on the observability of the internal structure of analyzed samples. This dependency can be exploited, especially in Brewster-angle configurations, to enhance the general capabilities of any THz tomography system. The characterization capabilities of the developed THz tomography setup are demonstrated on multi-layered ceramic oxide structures used for solid oxide fuel cells (SOFC). SOFC have an enormous potential for power generation applications. Nevertheless, no non-destructive quality control system for their sensitive manufacturing process exists to date. We ∗Corresponding author. (E-mail: [email protected]) will show that the developed THz imaging system can fulfill this role and by that means aid the breakthrough of this attractive technology. The paper is structured as follows: Section 1 introduces SOFC and describes the samples investigated. Section 2 describes the THz tomography system developed and discusses appropriate methods for determining unknown refractive indices. Section 3 outlines the THz characterization of SOFC – analysis of single layer and multi-layered ceramic oxide samples are presented. Section 3 also discusses the data analysis algorithms employed. 1 Investigated samples – solid oxide fuel cells SOFC represent a promising future energy supply system due to their high overall efficiency and low emission of pollutants. SOFC technology is currently under development for a broad spectrum of power generation applications [7]. However, before SOFC can be introduced onto the market several technical challenges remain to be solved. Production costs are still not competitive and need to be reduced significantly. In this context it is therefore of primary importance to develop efficient characterization methods to control and optimize manufacturing processes. A schematic cross-section of a typical SOFC is depicted in Fig. 1. The ceramic fuel cell consists of two electrodes (the anode and the cathode) separated by a solid electrolyte such as yttria-stabilized zirconia (YSZ). Typical thickness are in the 10–100μm range. Fuel (e.g. H2) is fed to the anode, generally consisting of a Ni/YSZ cermet, and releases electrons in an electrochemical oxidation reaction to the external circuit. Oxidant (e.g. O2) is fed to the cathode, consisting of a perovskite-type material, where it undergoes a reduction reaction by accepting electrons from the external circuit. The solid electrolyte conducts ions between the two electrodes. Such a single cell typically produces a voltage of approximately 1 V at the operating temperature of 850–950 ◦C. Thus, for practical applications SOFC are connected in electrical series (stacks) to provide higher voltages. The bipolar plates