Abstract
A tilted fiber Bragg grating (TFBG) hydrogen sensor coated with a palladium (Pd) membrane by the electroless plating method is proposed in this paper. A uniform layer of Pd metal is fabricated in aqueous solutions by the chemical coating method, which is used as the sensitive element to detect the change of the surrounding refractive index (SRI) caused by hydrogen absorption. The change in SRI causes an unsynchronized change of the cladding modes and the Bragg peak in the TFBG transmission spectrum, thereby eliminating the cross-sensitivity due to membrane expansion and is able to simultaneously monitor the presence of cracks in the pipe, as well as the hydrogen leakage. By subtracting the wavelength shift caused by fiber expansion, the change of SRI, i.e., the information from the H2 level, can be separately obtained. The drifted wavelength is measured for the H2 concentration below the hydrogen explosion limit between 1% and 4%. The chemical-based coating has the advantages of a low cost, a simple operation, and being suitable for coating on long fiber structures. The proposed sensor is able to detect the H2 signal in 5 min at a 1% H2 concentration. The proposed sensor is proved to be able to monitor the hydrogen level without the cross-sensitivity of temperature variation and expansion strains, so could be a good candidate for security applications in industry.
Highlights
Hydrogen is considered an ideal clean energy and efficient fuel
The higher the hydrogen concentration is, the faster and larger the surrounding refractive index (SRI) of the Pd film changes, which leads to a wavelength drift of the tilted fiber Bragg grating (TFBG) transmission spectrum
The measurements output spectrum, which are the refractive index change of the Pd film due to hydrogen absorption were carried out under constant temperature conditions, so there is no effect of temperature on the and the grating deformation caused by the Pd film expansion
Summary
Hydrogen is considered an ideal clean energy and efficient fuel. At present, liquid hydrogen has been widely used in the aerospace field. Once the Pd film absorbs hydrogen, the metal lattice experiences an α–β phase transition, which can be detected by various sensing structures. These types of hydrogen fiber sensors require special fibers or transform the sensing element into special forms in order to improve the sensitivity, which will introduce great difficulty to the production of the sensors. After the hydrogen absorption by the Pd layer, both the change of the refractive index and the expansion of the film can affect the output spectrum of FBG, which would cause inaccuracy of the sensing results. An optical sensor based on TFBG is proposed which uses a chemically coated Pd layer as the sensing material. Through experimental results combined with calculations, the proposed TFBG sensor can accurately monitor pipe cracks and hydrogen leakage at the same time at different temperatures
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