Research On Paper-Based LC Resonant Humidity Sensor

Inductor-Capacitor(LC)resonant humidity sensor utilizes the coupling principle between inductive coils to realize wireless and passive humidity detection,and has developed rapidly in recent years.There are two types of LC resonant humidity sensors: rigid substrates and flexible substrates.The fabrication process of rigid substrate LC sensors is relatively complex,high cost and poor biocompatibility.LC sensors based on flexible polymer substrates cannot be degraded naturally and easily cause environmental pollution.As a kind of flexible material,paper has the advantages of low cost,greenness,and natural degradability.This paper takes the paper substrate as the research object,and focuses on the structural design of the paper-based LC resonant humidity sensor,the research on the humidity sensitivity of the sensor and the portable humidity measuring instrument.The research content is mainly divided into the following parts:The LC resonator structure is optimized by HFSS electromagnetic simulation software,and the LC resonator model with high quality factor at the resonant frequency of 13.56 MHz is obtained.The paper and polyimide flexible substrate sensors are simulated,and the resonant frequency decreases with the increase of humidity,and the resonant frequency offset is determined as the sensor sensitivity index.The overall sensor measurement circuit is designed and simulated by software such as Comsol Multiphysics and Miltisim,and the measurement circuit is made for the actual measurement of the sensor.Four kinds of paper-based LC resonant humidity sensors with different materials are fabricated by screen printing process.The effect of paper type on the sensitivity of LC resonant humidity sensor is investigated.Under the premise of ensuring the flatness of the printed electrodes,paper with a smooth surface and a high surface microcrack rate should be selected as the sensor substrate.Among the four types of paper,Kodak photo paper has the best humidity sensitivity: the sensitivity is 2.32KHz/%RH,the maximum humidity hysteresis is 2.9%,and the screen printing ensures good process consistency.In order to further improve the humidity sensitivity of the Kodak photo paper humidity sensor,graphene oxide(GO)films with different mass percentages are prepared by spraying on the surface of the interdigital capacitor.The effects of film concentration,film thickness and reading distance on the humidity-sensitive characteristics of the sensor are studied.When the reading distance is 1mm,the humidity sensor sprayed with 1m L 5% GO concentration has the best performance,and the sensitivity reaches 3.65KHz/%RH,which is about 1.6 times that of the sensor without humidity sensitive material;the maximum humidity hysteresis is 3.3%;the response time is 45s;recovery time 140 s.The experimental results show that the GO moisture-sensing material and the Kodak photo paper substrate synergistically sense moisture,which can significantly improve the sensitivity of the sensor without changing other properties of the sensor.A portable humidity measuring instrument for LC resonant humidity sensor is designed and manufactured,which is composed of paper-based LC humidity sensor,main controller,Colpitts oscillation circuit,buffer circuit,humidity measurement and display circuit.The tester is used to test the sensitivity of the developed Kodak photo paper humidity sensor.The results show that the humidity measuring instrument can measure the humidity of the current environment in real time,the deviation of the measurement results of each humidity point is within 5%RH,and the maximum error is 13.3%.The paper-based LC resonant humidity sensor studied in this paper has the advantages of high sensitivity,good process consistency,low cost,small hysteresis,and fast response speed.In the future,it is expected to be popularized and used in the fields of logistics transportation and intelligent packaging.