Research On Gas Sensor Based On Fractal Geometry Structure

The use of gas sensor is becoming more and more widespread,and at the same time the various applications also put higher demands on the sensitivity performance of gas sensors.This dissertation combines fractal geometry theory with microelectromechanical systems(MEMS)technology and micro hot plate process to study a new gas sensor electrode structure which optimized by fractal geometry,and proposes the design and fabrication method of high performance gas sensor based on fractal structure electrode,and verifies the correctness and superiority of the proposed architecture through theoretical analysis,simulation analysis and actual fabrication of the sensor experimental test together.The main research work and innovations of the dissertation are as follows.① Proposing the electrode structures based on fractal geometry and design concepts and methods for new gas sensorBased on the self-similarity and scale-invariant properties of fractal geometry,the three advantages of fractal geometry have been concluded in this dissertation,which are of large specific surface area,large field strength and simple rules to generate complex structures.The concept of combining fractal geometry with structural optimisation of gas sensors is also provided based on the bionic basis of fractal characteristics in biological olfactory system.For the current application of fractal geometry in sensors mainly for microscopic analysis of sensitive materials,which has the problems of limited effect on sensitivity improvement of sensor.The methoddology of using fractal geometry theory to design the electrode structure of gas sensors is proposed in this dissertation,and the principle of this method for improving the performance of the sensor is analyzed theoretically,that are,the large specific surface area characteristic can improve the contact area between the target gases and sensor electrodes as well as gas-sensitive materials,the strong electric field strength characteristic can increase the strength of the sensor’s gas response signal,and the simple rules to generate complex pattern characteristics can reduce the difficulty and cost of sensor manufacture.② Exploring the design method and performance analysis of fractal structure electrodes based on space-filling curvesThe Peano and Hilbert space filling curves were reproduced using recursive algorithms and grammatical composition algorithms,and then two sensor electrodes with fractal structures were designed.The dimensionality and specific surface area of the fractal electrode and the traditional interdigital electrode were calculated,and it was demonstrated that the fractal structure have a higher degree of irregularity and a larger specific surface area.The electric field intensity distribution of the fractal and interdigital electrodes was analyzed theoretically,the magnitude of field intensity at the parallel point and tip of electrode plate was deduced,the principle of enhancement of electric field strength by fractal structure was analyzed according to Gauss’s electric field law,and the large field strength property of fractal structure is proved.Finally,the electric field intensity of electrodes of different structures was checked using Maxwell electromagnetic simulation software.The results show that the electric field strength of the fractal electrode is 1.6 times higher than that of the interdigital electrode at the same scale,and the Hilbert electrode has a better effect on the increase of electric field than that of the Peano electrode.③ Verifying that fractal electrodes can improve sensor sensitivity compared to conventional forked finger electrodesThe electrode structure of the gas sensor was designed based on the Hilbert space filling curve.To verify the validity of this type of fractal curve electrode structure,the carbon nanotubes were chosen as the gas-sensitive material.A resistive-capacitive gas sensor with a three-layer planar thin-film structure has been built using a printed circuit board process.The sensor was then characterised using Raman spectroscopy and scanning electron microscopy,and the resistive-capacitance circuit model was analysed.Finally,the sensor test rig was constructed and gas experiments were carried out using NO as the target gas.The results show that the sensor fabricated has good gas response for concentrations of NO in the range of 2-10 ppm,and the sensor has different response characteristics at different frequency signals.It is also found that the fractal electrode sensor has a 76%increase in response sensitivity compared to the conventional interdigital electrode sensors.However,the sensor still suffers from a long response recovery time.④Proving that MEMS-based fractal electrodes can improve the comprehensive performance of existing gas sensorsAccording to the authors’ theoretical derivation,reducing the width and gap of the electrode plate has an electric field strength enhancement effect and thus improves the sensor sensitivity.So the width and gap of the electrode plate were reduced to 10μm and 5μm using MEMS technology.A multilayer planar structured gas sensor was fabricated based on micro-thermal plate technology to reduce sensor response recovery time.Carbon nanotubes doped with silver nanoparticles were prepared as gas-sensitive materials and gas-sensitive films were prepared using spraying method.Experimental results with low concentrations of NO standard gases show that:the combination of micro hot plate technology and the sensor can reduce its response recovery time from more than 1 hour to 4 minutes;the fractal structure reduces the minimum detection limit of the sensor from 2 ppm to 50 ppb,and the sensitivity of the fractal electrode sensor is higher than that of the interdigital electrode sensor,and the sensor has good repeatability and stability performance.It is confirmed that the sensor based on fractal geometry electrode structure can further improve its sensing performance based on the reduction of the width and gap of the electrode plate,the addition of micro hot plate and the improvement of the sensitive material.