Xiling Tang

Zeolite thin film-coated long period fiber grating sensor for measuring trace chemical.

This paper reports the development of a new zeolite thin film-coated long period fiber grating (LPFG) sensor for direct measurement of trace organic vapors. The sensor was fabricated by growing pure silica MFI-type zeolite thin film on the optical fiber grating by in situ hydrothermal crystallization. The sensor measures chemical vapor concentration by monitoring the molecular adsorption-induced shift of LPFG resonant wavelength (lambda(R)) in near infrared (IR) region. Upon loading analyte molecules, the zeolites refractive index changes in the close vicinity of the fiber index where the LPFG has a large response to achieve high sensitivity.

Measurement of CO2-laser-irradiation-induced refractive index modulation in single-mode fiber toward long-period fiber grating design and fabrication.

We report a new method to measure the CO(2)-laser-irradiation-induced refractive index modulation in the core of a single-mode optical fiber for the purpose of design and fabrication of long-period fiber gratings (LPFGs) without applying tension. Using an optical fiber Fabry-Perot interferometer, the laser-induced axial refractive index perturbation was measured. We found that the CO(2)-laser-irradiation-induced refractive index change in the fiber core had a negative value and that the magnitude was a sensitive function of the laser exposure time following almost a linear relation. Under the assumption of a Gaussian-shaped refractive index modulation profile and based on the first two terms of Fourier series approximation, the measured refractive index perturbations were used to simulate the LPFG transmission spectra. LPFGs with the same laser exposure parameters were fabricated without applying tension, and their spectra were compared with those obtained by simulations.

Perovskite-type oxide thin film integrated fiber optic sensor for high-temperature hydrogen measurement.

Small size fiber optic devices integrated with chemically sensitive photonic materials are emerging as a new class of high-performance optical chemical sensor that have the potential to meet many analytical challenges in future clean energy systems and environmental management. Here, we report the integration of a proton conducting perovskite oxide thin film with a long-period fiber grating (LPFG) device for high-temperature in situ measurement of bulk hydrogen in fossil- and biomass-derived syngas. The perovskite-type Sr(Ce(0.8)Zr(0.1))Y(0.1)O(2.95) (SCZY) nanocrystalline thin film is coated on the 125 microm diameter LPFG by a facile polymeric precursor route. This fiber optic sensor (FOS) operates by monitoring the LPFG resonant wavelength (lambda(R)), which is a function of the refractive index of the perovskite oxide overcoat. At high temperature, the types and population of the ionic and electronic defects in the SCZY structure depend on the surrounding hydrogen partial pressure. Thus, varying the H(2) concentration changes the SCZY film refractive index and light absorbing characteristics that in turn shifts the lambda(R) of the LPFG. The SCZY-coated LPFG sensor has been demonstrated for bulk hydrogen measurement at 500 degrees C for its sensitivity, stability/reversibility, and H(2)-selectivity over other relevant small gases including CO, CH(4), CO(2), H(2)O, and H(2)S, etc.

Acidic ZSM-5 zeolite-coated long period fiber grating for optical sensing of ammonia

A ZSM-5 zeolite thin film with Si/Al ratio of about 23 has been grown on a long period fiber grating (LPFG) for optical gas sensing by monitoring its resonance wavelength (λR) shift caused by molecular sorption into the zeolite cavity. The sensing selectivity, sensitivity and speed of response of the zeolite-coated LPFG (Z-LPFG) are determined by the adsorption equilibria and transport properties of the analyte molecules in the zeolite pores. The ZSM-5 zeolite was modified through ammonium ion exchange and subsequent calcination to form acidic ZSM-5 (H-ZSM-5). The surface acidified Z-LPFG (HZ-LPFG) achieved dramatically improved sensitivity and selectivity for detecting ammonia gas. Also, increasing operation temperature improves sensing selectivity for the strongly adsorbing ammonia over weakly adsorbing gases and enhances response speed but compromises detection sensitivity due to reduced amount of adsorption for ammonia.

Nanocrystalline Cu-doped zirconia film-coated long-period fiber grating for CO monitoring at high temperature

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.

Proton conducting perovskite-type ceramics for fiber optic sensors for hydrogen monitoring at high temperature

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 sensors 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.

Modified ZSM-5 zeolite film-integrated fiber optic sensors for ammonia detection

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 zeolites 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.

DEVELOPMENT OF NOVEL CERAMIC NANOFILM-FIBER INTEGRATED OPTICAL SENSORS FOR RAPID DETECTION OF COAL DERIVED SYNTHESIS GAS

The overall goal of this project is to conduct fundamental studies on advanced ceramic materials and fiber optic devices for developing new types of high temperature (>500{degree}C) fiber optic chemical sensors (FOCS) for monitoring fossil (mainly coal) and biomass derived gases in power plants. The primary technical objective is to investigate and demonstrate the nanocrystalline doped-ceramic thin film enabled FOCS that possess desired stability, sensitivity and selectivity for in-situ, rapid gas detection in the syngas streams from gasification and combustion flue gases. This report summarizes research works of two integrated parts: (1) development of metal oxide solid thin films as sensing materials for detection and measurement of important gas components relevant to the coal- and biomass-derived syngas and combustion gas streams at high temperatures; and (2) development of fiber optic devices that are potentially useful for constructing FOCS in combination with the solid oxide thin films identified in this program.