| The structure of film gas sensor has an important influence on its performance.In the micro aspect,films with different structures have different sensing performance.In the macro aspect,sensor architecture（sensing electrode,heating electrode and shape）also has an important impact on the sensing performance.The fractal geometry has the characteristics of large specific surface area and unique fractal dimension.The purpose of this dissertation is to explore the influence of the structure of gas sensors based on fractal geometry on the performance of gas sensors,and to obtain the design and fabrication methods of gas sensors with high sensitivity and low detection limit.The main researches include introducing fractal geometry into the design of the 3D substrate of the gas sensor and the characterization of the sensing film,which verified theoretically and experimentally that the design of the substrate structure of gas sensor and the microstructure of the film based on fractal geometry can improve the sensitivity and detection limit performance of the gas sensor.The main results are as follows:（1）The relationship between fractal substrate structure,specific surface area and sensitivity of gas sensor is revealed,and a design and fabrication method of gas sensor based on 3D fractal substrate is proposed.The effective contact area between the gas sensing film and the gas is one of the factors affecting the performance of the sensor.A design and fabrication method of gas sensing sensor based on fractal substrate is proposed by using the characteristic of large specific surface area of fractal geometry.Fractal geometry is introduced into the design of 3D substrate structure of gas sensor,the characteristic of the large specific surface area of the fractal substrate is utilized to increase the effective contact area between the gas sensing layer and the gas,so as to improve the sensitivity of gas sensor.Firstly,through mathematical derivation,it is theoretically elucidated that fractal geometry has a larger specific surface area than European geometry,and the design of substrate structure based on fractal geometry can increase the effective contact area.Then,through comparative analysis,the appropriate Koch snowflake cube and Menger sponge were selected as the model to design the substrates of sensors,and the relationship between the sensitivity of the fractal substrate sensor and the traditional（also widely used）cylinder substrate sensor was deduced theoretically,which demonstrates the advantages of the fractal substrate.Finally,the 3D printing technology was used to fabricate the substrates of the sensors,silver doped multi-walled carbon nanotubes was selected as the gas sensing material to fabricate the sensor,and NO2was selected as the target gas for experimental verification.Experimental results demonstrate that the sensitivity of sensor with fractal substrate is higher than that of sensor with cylinder substrate.The effectiveness of the proposed method in improving the sensitivity of gas sensor is demonstrated.（2）A microstructure characterization method of gas sensing films based on fractal theory is proposed.The preparation parameters of film will affect the microstructure of film,and the microstructure will affect its performance.A characterization method of gas sensing film based on fractal theory is proposed by using the characteristics that gas sensing performance can be influenced by the film microstructure and the fractal dimension can characterize the microstructure of the film,and the film with the best fractal dimension and gas sensing performance is prepared.Fractal box dimension is introduced into the characterization of the microstructure and gas sensing performance of films,and the film with the best fractal dimension and sensitivity is obtained by adjusting the preparation parameters of the film.Firstly,the relationship among the microstructure,fractal box dimension,specific surface area and gas sensing performance of films were analyzed.Then,silver doped multi-walled carbon nanotube films with different mass ratios were prepared,and the relationship between the microstructure,specific surface area and fractal dimension of films were analyzed.Finally,NO2 was selected as the target gas for experimental verification.Theoretical analysis and experimental results demonstrate that the fractal dimension can characterize the microstructure and gas sensing performance of the film,and the effectiveness of obtaining the film with the best fractal dimension and sensitivity by adjusting the preparation parameters of the film is demonstrated.（3）A design and fabrication method of gas sensor with 3D fractal substrate and fractal film is proposed.The fractal geometry is simultaneously introduced into the design of 3D substrate and film characterization of gas sensors to improve the sensitivity and detection limit of gas sensor by combining the advantages of both.A design and preparation method of gas sensors with 3D fractal substrate and fractal film is proposed.Firstly,the advantages of applying 3D fractal substrate and fractal film to sensors were theoretically analyzed.Next,the sensor substrates were designed based on Koch snowflake cube and Menger sponge,and the substrates were fabricated by 3D printing technology.Then,gold doped multi-walled carbon nanotube films with different mass ratios were prepared,and the relationship between the microstructure,specific surface area and fractal box dimension of the films were analyzed.Finally,NO2 was selected as the target gas for experimental verification.Experimental results demonstrate that the sensitivity of sensor with fractal substrate is higher than that of sensor with cylinder substrate,the film with higher box dimension has higher sensitivity,while the sensor with Menger sponge substrate and the highest box dimension film has the highest sensitivity,which is 9 times more than that with cylinder substrate and the lowest box dimension film,and the detection limit of the sensors can be as low as 30 ppb.The comparison between experimental results and the parameters of similar researches demonstrate the effectiveness and progressiveness of sensors with both fractal substrate and the film with the best fractal dimension in improving sensor sensitivity and lower detection limit. |
