Ordered Micro-/Nanostructure Based Humidity Sensor for Fuel Cell Application
Humidity sensors are one of the most widely used sensors in commercial and industrial applications for environmental monitoring and controlling. Although related technology have been studied intensively, humidity sensing in harsh environments still remains a challenge. The inability of current humidity sensors to operate in high temperature environments is generally due to the degradation of the sensing films caused by high temperature, high humidity level, and/or contamination. Our goal is the design and fabrication of a humidity sensor that is capable of working under high temperatures and in a condensing environment. The targeted application of this sensor is in the polymer electrolyte membrane (PEM) fuel cell, where humidity control is crucial for performance optimization. In this work, ordered macroporous silicon is thoroughly studied as a humidity sensing layer. In addition to the advantages of traditional porous silicon for gas sensing (high resistance to high temperature and good compatibility with current IC fabrication process), the ordered macroporous silicon used in these experiment has uniform pore size, pore shape and distribution. All the vertical aligned pores can be opened to the environment at both ends, which can significantly increase the efficiency of gas diffusion and adsorption. Moreover, this special structure opens the door to uniform surface modifications for sensing enhancement. Both ordered macroporous silicon based heterostructure and self-supporting membrane are fabricated and investigated as a humidity sensor. Heterostructure sensors with different thin film surface coatings including bare Si, thermally grown SiO2, atom layer deposited ZnO, HfO2, and Ta2O5 are characterized. Post micro-fabrication is achieved on this ordered porous structure without affecting the material and its sensing properties. It has been proven that the ordered macroporous silicon with Ta2O5 surface coating shows the best sensing property due to its ultra-hydrophilic surface. The sensor shows high sensitivity, fast response times, small hysteresis, and extraordinary stability and repeatability under high temperatures and in condensing environment. It demonstrates great potential and advantages over existing commercial humidity sensors in the fuel cell application field. In addition to ordered macroporous silicon, well aligned 1D ZnO nanorods/nanowires -another widely used nanostructure in gas sensing- is also investigated as humidity sensing materials. Both vertically and laterally aligned nanorods/nanowires are fabricated and tested against humidity changes. The sensors shows increasing resistance to increasing relative humidity, which is contrary to most published works so far. Possible mechanisms have been proposed in this thesis and future work has been suggested for further study. To the best of our knowledge, this work is the first to use ordered macroporous silicon and well aligned 1D ZnO nanorods/nanowires for humidity sensing.