Optical Fiber Sensors Detect Cryogenic Hydrogen

Lauren I. Rugani

The low flammability limit and ignition energy of molecular hydrogen make it an ideal fuel source for rocket engines. However, these combustion properties also are a risk to human safety and call for a mechanism to detect low concentrations of hydrogen at cryogenic temperatures. Optical fiber sensors work well in such conditions because they are highly sensitive and nonflammable.

To this end, researchers from the University of Sannio in Benevento, the National Agency for Atomic Energy in Brindisi and the Institute for Composite and Biomedical Materials in Portici, all in Italy, and the University of Madrid in Spain have fabricated silica optical fiber sensors coated with two types of single-walled carbon nanotube-based materials.

The group coated the distal ends of two silica optical fibers with two and six monolayers of close-end nanotubes, respectively, and one with two monolayers of open-end nanotubes, and placed the sensors into a chamber at 113 K. Pulses of argon gas with decreasing concentrations of hydrogen flowed through the chamber to evaluate the sensors’ responses. Adsorption of hydrogen molecules altered the thickness and dielectric constant of the coating, affecting the optical reflectance at the fiber/film interface and, therefore, the reflected signal at the photodetector.

For the probe with two close-end nanotube monolayers, the reflectance did not differ significantly for varying concentrations of hydrogen. The probe demonstrated a response time of four minutes and a complete recovery time of nine minutes.

When the number of monolayers was increased to six, the reflectance revealed a marked sensitivity to the hydrogen concentration. While also having a response time of four minutes, the increased number of monolayers delayed the recovery time to 11 minutes. The probe with two monolayers of open-end nanotubes showed a response time of five minutes but never returned to its steady state. These characteristics have been attributed to the various hot guest interactions between the single nanotubes and the hydrogen molecules.

The researchers are further investigating the relationships between the optical, morphological and geometric properties of the sensitive overlay and the hydrogen concentration to tailor and control the sensing performances of the proposed devices.

The techniques that were used by the scientists to produce fiber optic nanosensors coated with engineered single-walled carbon nanotube overlays for hydrogen detection at cryogenic temperatures are patent pending.

Applied Physics Letters, Nov. 13, 2006, 201106.