The Preparation Of WO₃-Pd Composite Film And Its Application In Hydrogen Leakage Monitoring

Hydrogen is a clean energy and an important chemical raw material, and it has been widely used in many field such as aerospace, fuel cells, metal smelting and chemical synthesis. Hydrogen is so easy to leak because of its smallest molecule, which may explode when its concentration reach to certain value. Therefore, it is extremely important to monitor hydrogen leakage in these fields.Firstly, the recent developments in optical fiber hydrogen sensor are introduced systematically. To solve the current problem in this field, side-polished fiber Bragg grating (SP-FBG) hydrogen sensor based on WO3-Pd composite films is proposed in this paper. Then the strain and temperature sensing mechanism of fiber Bragg grating are analyzed. The theoretical model of fiber Bragg grating hydrogen sensor is also illustrated. By magnetic sputtering method WO3-Pd composite film was deposited on SP-FBG to prepare optical fiber hydrogen sensor. The durability and sensitivity of the hydrogen sensor are improved by optimizing the structure of WO3-Pd composite film which was deposited on SP-FBG.WO3-Pd composite film consists of WO3, WO3/Pd composite film and Pd film. WO3 is deposited the basal layer for it has good adhesion toward fiber, and Pd film is used as hydrogen sensitive film duo to its high sensitivity and selectivity to hydrogen. WO3/Pd composite film is set as intermediate layer to overcome material properties' mismatch of WO3 and Pd. When the thickness of pure Pd film is more than 40nm, it is easy to crack because of the accumulation of lattice dislocations caused by its volume expansion. By adding the intermediate layer, the accumulation of lattice dislocations of single Pd layer can be reduced, therefore the repeatability of WO3-Pd composite film can be enhanced. During the side-polishing process, the FBG fiber was connected to light source and optic power meter to enable in-line monitoring of the polishing process.XRD results demonstrate the WO3-Pd composite films are mainly amorphous. SEM results indicate that the WO3-Pd composite films are uniform and dense. After exposed to hydrogen for many times, the WO3-Pd composite film still has good surface morphology, which manifest WO3-Pd composite film has good mechanical property. FBG coated with thicker WO3-Pd composite films has higher sensitivity but slower response rate. Compared to standard FBG, SP-FBG can significantly improve the sensor's sensitivity. And SP-FBG with more side-polishing content has higher hydrogen sensitivity. By annealing WO3-Pd composite films under certain temperature for 1 h, the hydrogen sensor's repeatability can be improved, but its sensitivity will be reduced. By optimizing the structure and preparation technology of the sensor, the SP-FBG will be very promising for distributed measurement of hydrogen concentration.