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.
In this study, a new zeolite thin film-coated long-period fiber grating (LPFG) sensor was developed and
evaluated for chemical vapor detection. The sensor was fabricated by growing nanoporous MFI-type zeolite (pore
size ~0.55nm) thin film on fiber grating using in situ hydrothermal crystallization method. The hydrothermal
synthesis process was controlled by continuously monitoring the LPFG transmission spectrum evolution, which
indicated the zeolite film formation and growth process. The zeolite-LPFG sensor was activated by calcination in air
to remove the structural directing agent from the zeolite pores and then demonstrated for sensitive detection of
chemical vapor in gas phases.
We report in this paper the fabrication of high performance thermal LPFGs by point-by-point CO2 laser irradiations.
These thermal LPFGs have shown much better temperature tolerance and promised applications in high temperature
harsh environments. The computer-controlled fabrication system with in situ signal monitoring capability is described.
The fabricated LPFGs survived high temperatures up to 800°C. Long term stability test at 550°C for 200 hours indicated
that thermal shock at a higher temperature could significantly reduce the drift.
Recently, we discovered that the nanoporous zeolite materials possess the unique combination of optical and
chemical properties suitable for developing highly sensitive chemical sensors. This paper summarizes our recent
work in developing such highly sensitive chemical sensors by functionally integrating zeolite thin films with optical
fiber devices. These include the zeolite Fabry-Perot interferometric sensor and the zeolite thin film-coated thermal
long period fiber grating sensor. Both types of sensors operate by monitoring the adsorption-induced optical
refractive index changes in the zeolite thin film. The sensors were tested using various organic chemicals with
different molecular sizes and in both vapor and liquid phases.
A zeolite-fiber integrated chemical sensor was developed for in situ point detection of chemical warfare agents. The sensor was made by fine-polishing the MFI polycrystalline zeolite thin film synthesized on the endface of the single mode optical fiber. The sensor device operates by measuring the optical thickness changes of the zeolite thin film caused by the adsorption of analytes into the zeolite channels. The sensor was demonstrated for sensitive detection of toluene and dimethyl methylphosphonate (DMMP).
A new type of optical chemical sensor recently developed in our lab has been demonstrated for highly sensitive, in-situ detection of explosives. The sensor is comprised of a dense silica thin film grown on the straight-cut endface of a standard, 125μm telecommunication optical fiber. Silicalite is an all-silica MFI-type zeolite with an effective pore size of 0.55nm. MFI zeolite is highly hydrophobic and selectively adsorbs organics of appropriate molecular size. The sensor device operates through measuring the optical refractive index or optical thickness of the coated zeolite film which changes in response to the adsorption of molecular species in its crystalline structure. In this work, the sensor exhibited different responses to simulants including pxylene, o-xylene, and triisopropylbenzene and trinitrotoluene (TNT) trace vapor in helium carrier gas.
Molecular sieve zeolites are capable of selectively adsorbing molecular species into their nanoporous structures. Using whitelight interferometry, the changes in optical thickness of b-oriented MFI zeolite thin films have been measured as a function of organic vapor partial pressure in the surrounding environment. The adsorption induced optical thickness changes of the oriented MFI zeolite films were found to be reversible and monotonically dependent on the organic concentration level. The quantitative results of this study are useful for designing optical fiber-based chemical sensors for in-situ detections.
Polycrystalline MFI zeolite thin films have been grown on the surface of optical fibers by in-situ crystallization method. The zeolite-coated optical fibers were investigated for detection of trace organic compounds in gas and liquid phases. The reflectivity of the zeolite-coated fibers responded to the change in organic concentration monotonically and reversibly. The effect of zeolite material chemistry of the zeolite crystals on the sensor behavior was also studied.
As a clean, reliable, and comparatively inexpensive alternative to fossil fuels, geothermal electricity generation could provide energy for fifty years or more if properly managed. Real-time and remote measurement of some key parameters of the geothermal well as well as their temporal and spatial variations can provide critical information to improve plant efficiency and optimize plant operation to accommodate a resource that is declining with time. A hermetically sealed single mode fiber extrinsic Fabry-Perot interferometric (EFPI) sensor was developed for in-situ monitoring the temperature of geothermal wells. The amplified spontaneous emission of an erbium doped fiber amplifier was investigated as the broadband source to interrogate the fiber EFPI temperature sensor. Real-time compensation of the source spectrum distortion was investigated to improve the measurement accuracy and extend the sensor lifetime.
Low-cost, reliable, miniaturized gas sensors capable of fast, accurate, and in-situ monitoring of gas compositions in harsh environment are essential in developing high-efficiency, clean energy technologies. These sensors permit intelligent process control and optimization of power plant operations, which improves the energy efficiency and system reliability, reduces emissions, and minimizes the maintenance cost. In the past few decades, significant progress has been made in developing physical sensors for measuring various physical quantities under high temperature and high pressure conditions. However, currently available gas chemical sensors cannot withstand the hostile environment found in fossil fuel energy systems. In this paper, we present our exploratory research on nanomaterial enabled fiber optic gas sensors for in-situ chemical monitoring under high temperatures.
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