Chemical Sensors

Photograph of a microsensor chip showing the fine lines of several palladium-alloy chemical resistors. These chemical resistors are candidates for hydrogen sensors in the extreme process environments of steam forming.

Chemical manufacturers and petroleum refiners use steam reforming to extract hydrogen (H₂) from methane, refinery off-gases, and other feedstocks. The process requires vast amounts of high-pressure steam, electricity, and natural gas. Sandia is developing solid-state devices to rapidly and cost effectively monitor H₂ content in various process streams, a necessary precursor to the implementation of control strategies to improve plant productivity and reduce environmental impact.

This project aims to produce inexpensive, robust, sensing elements capable of surviving the chemical and thermal extremes of the industrial reforming environment. The devices would be capable of measuring H₂ over a wide range of concentrations, ranging from parts per million to 100%, at various absolute pressures.

Response of palladium-based metal-insulator-semiconductor (MIS) microsensor to small concentrations of hydrogen at ambient pressure. MIS devices have extremely low detection limits to hydrogen.

The electrical properties of thin films of palladium-metal alloys change in the presence of hydrogen, allowing them to operate as hydrogen sensors. However, surface-mediated processes, such as competitive reactions among adsorbates and catalyst poisoning, interfere with hydrogen uptake by the palladium-metal alloy and adversely affect sensor performance. Work at Sandia is focused on understanding the failure modes of these devices under various chemical and physical stresses in order to design more process-compatible structures. In-house research and development capabilities include advanced microelectronics design and manufacturing, as well as environmental chambers that simulate realistic process conditions for characterizing dose-response behavior.

Response of palladium-based chemical resistor to hydrogen pressure illustrating the effects of Ni composition in the film. The presence of Ni increases sensor durability at high absolute hydrogen pressures.

So far, researchers find that ultimate sensitivity and response depends on the operating condition as well as the physical characteristics of the films themselves. Device structure, alloy composition, film morphology, and protective overlayers all influence performance. The overlayers are placed on the devices to help them withstand the extreme operating conditions of industrial reforming environments.

Contact:
Anthony McDaniel
amcdani@sandia.gov
(925) 294-1440