Hydrogen sensors based on palladium micro- and nanostructures for fuel cell applications

Hydrogen (H2) has raised considerable interest as a future energy carrier in its application to fuel cell driven vehicles, as it is absolutely free of polluting emissions. However, it is flammable in air and difficult to contain, for which reason innovative sensors for leak detection are required. To this end, this thesis explores novel concepts of hydrogen sensors, based on micro- and nanostructured palladium (Pd), aiming both for industrially compatible manufacturing processes as well as a deeper insight into physical processes that are involved in the sensing mechanism. The rapidly growing fuel cell market drives the development of hydrogen sensors to lower costs and lower power consumption while maintaining the high performances that are required by e.g. the automotive industry. Nanogaps in palladium offer an interesting approach for hydrogen detection and promise to fulfill many of those needs. However, many aspects that are important for an industrialization have not been touched yet. Basically, hydrogen sensors based on nanogaps make use of the fact that palladium is subject to a considerable reversible volume expansion when it absorbs H2. This volume expansion can be exploited to mechanically close nanoscopic discontinuities (nanogaps), which allows an electrical current to switch. To address different aspects of this principle, three different sensor concepts were developed in this work. Potentials for mass fabrication and some fundamental understanding of the physical mechanisms involved are of major interest. The deposition of the H2-sensitive material via electron-beam evaporation, which is a common physical vapor deposition (PVD) method, ensures the process compatibility with standard microfabrication technology. Further, the focus turned to the use of top-down fabrication techniques, aiming at good structural control of large nanogap assemblies by design. For each of the sensors, the design, their fabrication and characterization are described, and finally a comparison of their performances and properties are presented. The first sensor concept consists of a single lateral nanotrench, created by focused ion beam (FIB) milling in a Pd microwire. It can be used as a binary switch, actuated by hydrogen and serves as a model structure for the investigation of the mechanical nanogap-closing mechanism. The second concept makes use of a multitude of vertical nanoswitches, arranged in highly ordered arrays on large scales. Therein, three generic mechanisms are combined: (i) the hydrogen induced bending of a trimorph structure, (ii) the reversible closing of a nanogap and (iii) percolation in an interconnected array. Finally, a sensor based on dis- and semicontinuous, nanocrystalline palladium films on polymeric substrates is developed. The sensor characteristics as a function of the film properties are discussed. The sensors presented in this work show extremely low power consumption, good performance and are manufacturable using simple process steps. Further, some theoretical aspects of nanogap sensors are addressed, which are related to conduction by percolation transport. In this frame, an extension to percolation theory, in particular related to conduction in finite random resistor networks with rectangular geometries is presented. In conclusion, this thesis presents novel practical approaches and theoretical aspects for nanogap based hydrogen sensors, which are of interest for both industrial applications and fundamental physics. It provides new insights in potentials of the underlying principle and aims to stimulate further research in this field.


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