| In recent years,the global ecological environment has suffered serious deterioration and extreme weather has occurred frequently.Countries around the world have adopted many corresponding policies and countermeasures,China has also put forward the goals of "emission peak" and "carbon neutrality" accordingly.Gas sensors play a very important role in online monitoring of air quality and industrial waste gas,which is very important for improving ecological environment.Compared with the traditional gas sensors,optical fiber gas sensors based on optical principles have gradually become a popular research topic in the field of gas sensors due to the advantages of intrinsic safety,anti-electromagnetic interference,small size,corrosion resistance and long-distance monitoring.There are many types of optical fiber gas sensors with different optical principles,and their detection sensitivity and limits for some gases(such as H2,C2H2,etc.)are even better than those of traditional gas sensors.However,the industrial development and application of those optical fiber gas sensors are limited by their high manufacturing cost and sophisticated coating technology.To address the technical challenges of low-cost,self-networking and real-time online monitoring of atmospheric and industrial waste gases,the most mature commercial fiber Bragg grating(FBG)was used as the basic sensing element,and two types of gas sensors were designed and fabricated by changing the grating period and the effective refractive index of the fiber core.The corresponding coating technology was also developed in this paper.The gas-sensitive performance of the sensors,the interference of environmental factors,and the corresponding solutions were systematically investigated.Moreover,the gas-sensitive mechanism of the sensors was revealed by a combination of molecular dynamics simulation and finite element simulation.The FBG gas sensors developed in this paper provide a potential solution for online monitoring of industrial waste gases due to its inexpensive manufacturing cost,easy industrial manufacturing process,easy self-networking and excellent gas-sensitive performance.The main research contents and conclusions of this paper are summarized as follows.(1)The polyimide-coated FBG CO2 gas sensor was successfully developed by combining the polyimide coating with FBG,which was based on the mechanism of CO2-induced volume dilation effect of polyimide coating to change the FBG grating period.Two types of soluble polyimide raw materials(Matrimid?5218 and P84)were selected in this paper.To obtain a uniform and controllable polyimide coating,a low-speed rotating coating device and a precision mold with corresponding coating process of polyimide were designed and developed.The developed sensor exhibited excellent reversibility and repeatability towards CO2 gas.The effect of polyimide coating thickness on the sensor performance was investigated.A series of FBG gas sensors with different coating thicknesses were obtained by varying the concentration of the polyimide solution.The test results of the gas-sensitive performance indicated that the response value of the sensor was linearly increased with the increase of the polyimide coating thickness.In addition,the response/recovery time of the sensors with different coating thicknesses and different CO2 concentrations were investigated,and the results indicated that the response/recovery time of the sensors gradually increased with the increase of coating thickness and gas concentration.Furthermore,the effect of ambient temperature and relative humidity(RH)on the sensor performance was investigated,and the response characteristics of the sensor were tested at 2.2～85.0%RH and15～55℃,respectively.The response value of the sensor increased and then decreased with the increase of temperature and reached the maximum response value at 5℃.The polyimide/FBG CO2 sensor developed in this paper further broadens the application range of FBG gas sensors.(2)Molecular dynamics simulation was carried out to reveal the microscopic mechanism of CO2-induced volume dilation of polyimide coating.Including the dissolution and diffusion behavior of CO2 in polyimide,the interaction between CO2 molecules and typical atoms on polyimide molecular chain,and the effect of CO2 concentration on volume dilation of polyimide.The results of adsorption isotherms and RDF indicated that there is a stronger interaction and affinity between CO2 molecules and polyimide molecular chains compared to CH4 and N2,and the O atom in the imide of polyimide molecular chains is the preferential adsorption site for CO2 molecules.The free volume distribution in different pore size ranges,the volume dilation percentage of the simulated cells and their internal free volume fraction(FFV)with different CO2 concentrations were further extracted and analyzed.It was found that the percentage of pores with radius less than 2.0 A in the simulated polyimide unit decreased,while the percentage of pores with radius greater than 2.0 A increased,and the total FFV increased gradually.Finally,the maximum volume dilation percentages of the two polyimide cells reached approximately 1.04%(Matrimid? 5218)and 1.13%(P84)after equilibration in 100 vol%CO2,respectively.Therefore,the corresponding gas-sensitive expansion coefficients were derived as 3.47×10-5/vol%CO2(Matrimid? 5218)and 3.77×105/vol%CO2(P84),respectively.In order to further understand the gas-sensitive response process of the sensor,the gas-sensitive expansion coefficients obtained from the above molecular dynamics simulation were applied to the finite element simulation of the volume dilation process of the polyimide/FBG composite structure.The effect of polyimide coating thickness and fiber radius on the axial strain of the fiber core was investigated.The finite element simulation results indicated that with the increase of polyimide coating thickness and the decrease of fiber radius,the axial strain of the fiber core shows a trend of regular increase,and the error between the simulated and the experimental results is small.(3)Considering that the polyimide/FBG CO2 sensor is also sensitive to environmental humidity,it is necessary to eliminate the interference of relative humidity.In addition,on the basis of the above results,reducing the fiber diameter is an effective way to significantly enhance the response value and sensitivity of the polyimide/FBG.Therefore,the chemical etching method was used to thin the cladding of the grating region,and the polyimide/FBG sensor with significantly enhanced sensitivity was successfully developed.Then,the response characteristics of the sensor to CO2 and humidity were systematically investigated.The effect of etching time on the sensitivity of the sensor was investigated,while the results indicated that the sensor with the etching time of 20 min in 40 wt%hydrofluoric acids has the highest sensitivity under the premise of satisfying the mechanical strength of the sensor.Subsequently,two sensors with different sensitivities were selected for a combination to detect a series of CO2+RH gas mixtures with different concentrations.It was found that the detection error of CO2 concentration was less than 3.55%while the detection error of relative humidity was less than 1.09%.This detection method of combining FBG gas sensors with different sensitivities provides a new method for the real-time online monitoring of high-concentration CO2 gas in environments with large variations in humidity.(4)All the above three sections are based on the principle of changing the FBG grating period to realize gas detection.However,the volume dilation effect of the polymer coating induced by small gas molecules is weak at normal temperature and pressure.Therefore,the FBG grating period can be only changed by higherconcentration gases,which means that the above method is suitable for the detection of high concentration gas.However,in atmospheric environments and most industrial production processes,the concentration of hazardous gases tends to be ppm level and varies over a wide range.Therefore,there is a great demand for developing a gas sensor with a wide dynamic response range and fast response time.In response to the above requirements,the grating region was further etched to the level close to the fiber core.Based on the refractive index sensitivity of deep etched FBG(eFBG),the excellent conductivity of multi-walled carbon nanotubes(MWCNTs)and the reversible reaction between polyaniline(PANI)and NH3 molecule,a PANI-MWCNTs/eFBG ammonia sensor with wide dynamic response range and rapid response was developed.The results from SEM,FTIR and XRD indicated that the MWCNTs were uniformly distributed on the surface of the grating region of eFBG,and the PANI was successfully grown on the surface of the MWCNTs.The response characteristics of the sensor to NH3 were systematically studied,including the gas selectivity,repeatability,response/recovery time,sensitivity,limits of detection and stability of the sensor.The results indicated that the PANI-MWCNTs/eFBG NH3 sensor can realize the sensitive detection of NH3 in the concentration range of 50～2000 ppm with the sensitivity of 0.1 pm/ppm,the LOD of 10 ppm,and the response/recovery time of 13/183 s in 150 ppm NH3 gas.Moreover,the sensor also shows excellent reversibility and stability.Compared with the traditional PANI-based NH3 sensors in the literature,the PANI-MWCNTs/eFBG NH3 sensor developed in this paper has a wider dynamic response range and faster response time,which provides a new method for real-time online monitoring of lowconcentration NH3 in complex and variable industrial environments. |
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