A fiber optical sensor has been developed by coating proton conducting perovskite oxide (Sr(Ce0.8Zr0.1)Y0.1O2.95, SCZY) thin film on the long-period fiber grating (LPFG) for high temperature in situ measurement of bulk hydrogen in gas mixtures relevant to the fossil- and biomass-derived syngas. In this paper, we investigate in the H2-sensing mechanism of the SCZY-LPFG sensor. The high temperature H2 adsorbance in the SCZY, the SCZY electric conductivity in H2, and the resonant wavelength shift of the SCZY-LPFG (ΔλR,H2) have been experimentally studied to understand the effect of operation temperature on the sensor's sensitivity to H2. Because of the activation process of the H2 reaction with the perovskite oxide, increasing temperature benefits the H2 uptake in the SCZY phase and the sensitivity of the SCZY-LPFG sensor. However, the thermal stability of the LPFG and the microstructure of the SCZY nanocrystalline film limit the application temperature of the fiber optic sensor.
This paper reports the development of surface modified ZSM-5 zeolite thin-film coated long-period fiber grating
(LPFG) sensors for in situ detection of ammonia (NH3). The sensor was fabricated by growing MFI-type zeolite thin film
(i.e. ZSM-5 with Si/Al ratio of 15) on the optical fiber grating by in situ hydrothermal crystallization. The sensor
measures ammonia concentration by monitoring the molecular adsorption-induced shift of LPFG resonant wavelength
(λR) in near infrared (IR) region. Upon loading the analyte (NH3) molecules, the refractive index of the zeolite film
changes in the close vicinity of the fiber index where the LPFG has a large response to achieve high sensitivity. High
sensitivity of this sensor also comes from the ability of the nanoporous zeolite to effectively concentrate the target
molecules by selective adsorption. The sensor was capable of sensitive detection of ammonia at lower ppm level. The
zeolite's internal surface was modified by ion exchange with NH4+ followed by thermal treatments to enhance the
surface acidity. The acidic ZSM-5 (i.e. H-ZSM-5) film exhibited higher sensitivity and improved selectivity for NH3.
There is a range of ways to couple light in a single mode fiber (SMF) from core mode to cladding modes, which can be
applied in some fiber sensors. Recently, a very simple method using CO2 laser irradiation is put forward. By coupling
core mode to cladding mode in the first irritation point and re-coupling in the second one, in-line Mach-Zehnder
interferometer (MFI) and Michelson interferometer(MI) sensors have be demonstrated.
To understand the mechanism underneath this coupling phenomenon, several parameters (laser power, laser lasting time,
etc) tests are investigated. With bigger laser power and longer lasting time, one can obtain higher mode coupling, which
is potential for greater sensitivity sensor. Combined with a long period fiber grating (LPFG), the cladding modes
promoted in fiber cladding are studied.
In some big power conditions, permanent deformation can be met on the irradiation points of the fiber. Although higher
loss is induced, there is also other advantage, such like high temperature stability. The sensitivity and stability of
temperature are discussed based on these sensors' configuration and mechanism. The experiences verify our laser
irritation sensors can survive in very high temperature. When coating with some gas absorption film such like zeolite
film, one reliable high sensitivity gas sensor is successfully demonstrated in low ppm vapor level.
Dense nanocrystalline copper-doped zirconia (CDZ, Cu:Zr=16:84) thin film was coated on the surface of a 125 μm-diameter
long-period fiber grating (LPFG) by a facile synthesis route involving polymeric precursor coating and
subsequent thermal treatments. The CDZ film had a uniform thickness of ~100 nm and grain size of 20 to 35 nm after a
brief annealing step at 700°C for 1 hour. This CDZ thin film coated LPFG (CDZ-LPFG) was evaluated at a high
temperature of 550°C for its change of resonant wavelength (λR) in response to the variation of carbon monoxide (CO) concentration in nitrogen (N2). The λR was found to shift toward longer wavelength when increasing the CO concentration. The CDZ-LPFG sensor response was found to be reproducible and reversible at low level CO
concentrations (<1,000 ppm) but became irreversible when the CO concentration was high (e.g. at 10,000 ppm). The
high temperature stability of the CDZ material in CO-containing atmospheres was studied to understand the limit of CO
measurement range.
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