| As the "Industry 4.0" strategy in the domestic high-end manufacturing industry continues to be promoted,meteorological monitoring,national defense technology,biopharmaceuticals,chemical production,and other industries have increasingly high requirements for accurate and reliable humidity measurement.Compared with the traditional electronic humidity sensor,the fiber Bragg grating humidity sensor has the advantages of anti-electromagnetic interference,corrosion resistance,compact structure,can be connected in series into a network,and long-distance quasi-distributed measurement.The performance of fiber optic humidity sensors depends on the combined humidity-sensitive materials.Compared with other humidity-sensitive materials,polyimide has the advantages of a wide measurement range,good linearity,corrosion resistance,and good stability,but its humidity sensitivity is not outstanding.Based on the aforementioned background,the main research of this paper is as follows.(1)A novel fiber Bragg grating temperature and humidity sensor using graphene quantum dot sensitized polyimide was proposed.The sensor consists of a temperature grating and a humidity grating,in which the temperature grating area is coated with a protective film of metallic titanium on the fiber surface by magnetron sputtering.The protective film has been proven to effectively improve the mechanical strength of the bare grating,which basically does not affect the temperature test characteristics of the reference grating.A composite moisture-sensitive film of graphene quantum dots doped with polyimide was prepared in the humidity grating area by lifting the coating method.The sensors can achieve accurate measurements of both temperature and humidity.(2)The effects of different mass ratios of doped graphene quantum dots on the comprehensive moisture-sensitive performance of the composite films were investigated,and the humidity sensitivity,linearity,response speed,dynamic repeatability,long-term stability,temperature crossover,and temperature sensitivity of the four sensors were tested.The results show that the sensitivity of the humidity sensor increases with graphene quantum dot doping,and the sensitivities of the probes with 0.2% and 0.3% doping mass ratios are similar,but the 0.3% doping mass ratio sensor has a longer response time.Therefore,the optimal doping mass ratio of graphene quantum dots is 0.2%.It is also found that the doping of graphene quantum dots has no significant effect on the humidity linearity,stability,and temperature cross-sensitivity of the sensor.(3)In addition,a stainless steel encapsulation structure was designed to protect the fragile gate region for the fiber optic sensor in the test scenario for practical engineering applications.The encapsulated sensor was tested before and after water immersion,which proved the composite thin-film fiber optic temperature.The humidity sensor prepared in this paper has excellent water resistance and can be directly applied in harsh environments with high humidity and even condensation for a long time.Finally,the fiber optic sensor and the electronic sensor were compared for 18 consecutive days of actual environmental temperature and humidity tests.The results proved that most of the temperature deviations of both sensors were controlled within ±0.5°C and humidity deviations within ±2% RH.The special case of short-term fluctuations in temperature and humidity data was also analyzed in detail,showing that the humidity testing performance of the fiber optic sensor prepared in this paper has reached the standard of the technically mature electronic sensors available in the market. |
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