KCl Concentration Measurement Based On Polyimide Film FBG Sensor

The nuclear industry,as a national high-tech strategic industry,is an important cornerstone to ensure national security and economic development.As the working medium to carry the heat released from the core out of the nuclear reactor,the concentration of its substance determines the current cooling state of the pipeline and directly affects the safe operation of the nuclear industry system.At present,the traditional concentration meter has problems of electromagnetic interference,large size,easy to deposit,difficult to distribute measurement,etc.,which is difficult to meet the demand of safe,efficient and accurate measurement in nuclear industry.This thesis adopts Fiber Bragg Grating as the sensing element,and designs a fiber grating concentration sensor with Polyimide film coating to address the problems of low sensitivity,complex structure and cumbersome fabrication process of the current grating type concentration sensor.Theoretical derivation,simulation analysis and experimental verification are used to build a theoretical model of fiber grating concentration sensing based on the mechanism of water-sensitive materials and fiber grating sensing mechanism;various coating methods are compared,and a prototype of fiber grating concentration sensor with polyimide coating is fabricated;relevant concentration experiments are carried out to analyze the sensing characteristics of various types of concentration sensors;to improve the sensitivity of the sensor,a prototype of fiber grating concentration sensor with polyimide coating is fabricated.To improve the sensitivity of the sensor,the sensitization method of polyimide thin film fiber grating concentration sensor was studied and analyzed.The specific research contents of the article are as follows.(1)Based on the mechanism of water-sensitive materials,the adsorption theory of polymer materials was analyzed,the adsorption mechanism between polymer materials and water molecules was studied,and the selection criteria of coating materials for fiber grating concentration sensors were determined;based on the sensing principle of fiber grating,the mapping relationship between the wavelength drift of fiber grating and temperature and strain was elaborated,the strain and temperature sensing model of fiber grating was constructed,and the sensing mechanism of corrosion-type fiber grating concentration sensors and polyimide film-plated concentration sensors was deduced.(2)analyzed and compared the performance parameters of various water-sensitive materials,and selected polyimide as the sensitive film material according to the material properties;studied the processing and coating process of polyimide film based on the sensing principle in Chapter 2,and introduced in detail the key aspects of the fabrication of the fiber grating concentration sensor with polyimide film plating;processed and fabricated the sensor prototype,and studied the sensing characteristics of the fiber grating concentration sensor with polyimide film plating through numerical simulation analysis.(3)The concentration and temperature sensing experimental platform was built for the fiber optic grating concentration sensor,and the concentration measurement and temperature calibration experiments of the fiber optic grating concentration sensor were carried out.The performance characteristics of the fiber grating concentration sensors were analyzed.The sensitivities of the four concentration sensors were 3.03pm/(mol/L),20.1pm/(mol/L),124.272pm/(mol/L),157.6pm/(mol/L)for KCl solution,respectively;the maximum difference between the forward and reverse stroke outputs of the FBG concentration sensor with laminated polyimide film was 35 pm,and the temperature sensitivities during the temperature rise and temperature fall were 23.5pm and 23.5pm,respectively.The temperature sensitivity of the process was 23.2pm/°C and 23.12pm/°C,respectively.(4)In order to improve the sensitivity of the sensor,the sensitization study of the fiber optic grating concentration sensor with polyimide film plating was carried out,analyzing the principle of each sensitization method from the perspective of both film and grating,studying the sensitization process and results,and finally selecting the form of etching grating and then plating polyimide film,and designing and producing the corrosion-type plated polyimide film fiber optic grating concentration sensor.The experimental results show that the resolution of the sensor is 0.2 mol/L,the equation of the concentration characteristic curve fitting in the potassium chloride concentration measurement experiment is y=1.156x+1559.355,the correlation coefficient is 0.99934,the sensitivity of the sensor is 1156.1 pm/(mol/L),and the repeatability error is 4.31%.To sum up,this thesis focuses on the problems of complex structure,easy flow obstruction,easy media deposition and difficult distributed measurement of concentration sensors at the present stage,takes fiber grating as the research object,carries out the research on sensing mechanism,sensor design and experimental verification of fiber grating concentration sensor,and lays a certain foundation for the application of the sensor to the field of coolant concentration measurement in nuclear industry cooling pipes.