| Methane and other flammable and explosive gases exist in large quantities in industrial production and life,especially in coal mines.Once the leakage reaches the explosive concentration,it is easy to cause major accidents.The key to deal with methane risk is to realize rapid identification,early warning and long-term monitoring of methane gas.At present the traditional "blind hole structure" and "manual wire" technology under the catalytic and thermal conductivity of methane sensor performance has become the limit.In this paper,starting from improving the blind pore structure of sensor carrier,carbon nanotubes and alumina oriented porous materials were introduced to build a "transparent" carrier and a new catalytic system,which broke through the core indicators such as detection limit,response speed and working temperature,and realized the full range,rapid response and long-term stability of methane detection.In this paper,the diffusion and combustion reaction characteristics of methane gas in nanopore materials were investigated by numerical simulation to provide theoretical guidance for carrier design.The diffusion coefficient and thermal conductivity of methane in multi-walled carbon nanotubes(MWCNTs)and Al2O3 were calculated by molecular dynamics.It is proved that the doping of MWCNTs can improve the molecular transport capacity and thermal conductivity of the powders.Based on the standard κ-ε turbulence model,component transport model and thermal analysis model,the temperature field,mass distribution of species,reaction velocity and combustion rate of Al2O3 directional nanotube microporous reaction mode and surface reaction mode were simulated and analyzed,which provided guidance for the selection of characteristic parameters of Al2O3 directional nanotube membrane as carrier.Secondly,aiming at the full range(0-100%)detection,a full-range methane sensor was constructed based on the principle of catalytic and thermal conductance dual detection.A three-unit integrated detection structure was proposed and the working circuit of the sensor was designed.Based on the partial voltage technology,the catalytic and thermal conductivity dual detection voltage control is realized.The split three-unit integration method of traditional wire-wound ball sensor and MEMS three-unit integration design method of micro heat plate with porous alumina carrier were proposed.A simultaneous measurement system of sensor response and thermodynamic parameters was built independently,and the theoretical working temperature calculation model was established,which solved the problem of simultaneous measurement of gas sensor response and temperature characteristics.Then,the carrier material design of MWCNTs doped sensor and sensor performance measurement were carried out.MWCNTs were prepared experimentally,and their morphologies before and after hydroxylation and after doping were analyzed by SEM.The response performance of the sensors with different proportion doping is studied.The performance test of the catalytic unit showed that the optimal doping amount of MWCNTs was 1 wt%,the average operating temperature gradient was 11℃/1%CH4,the sensitivity change was less than±3.5%FS within 60 days,the output linearity decreased rapidly above 3%,and the oxidized of MWCNTs occurred.Thermal conductivity unit tests show that the T90 response time is up to 7 s in a 200 mesh metallurgical powder package,and the response speed is improved by 30%.Then,aiming at the problems of high temperature volatilization of MWCNTs and non-directional pore channels,the experimental study of large pore size ordered porous Al2O3 membrane was carried out to further improve the "permeability" and high temperature stability of the carrier.Directionally porous Al2O3 films with different pore sizes and thicknesses were prepared by secondary anodic oxidation in oxalic acid and oxalic acid-phosphoric acid composite electrolyte by controlling oxidation voltage,temperature.and time.Ultrasonic pore reaming was used to obtain 260 nm large pore size film.The pore reaming rate and pore growth rate were obtained by SEM characterization and ImageJ image analysis.Based on the analysis of film formation mechanism,a mechanical model of hexagonal rule nanopore formation was established,and the formation process of hexagonal pore structure was verified by experiments with 0.3mol oxalic acid at different oxidation times.The effect of heat treatment on membrane structure was studied.A new catalytic system consisting of noble metal co-catalyst and pore material was constructed based on the support method of double-sided micro-jet catalyst driven by negative pressure.EDS characterization indicated that the double-sided spraying process improved the distribution of catalyst in the pores,and the Pd loading mass fraction averaged over 16.0%,which improved the distribution uniformity of catalyst in the alumina double-pass channels.Finally,the performance of the full-range methane sensor based on Al2O3 directional nanotube membrane was investigated.The integrated methane sensor was fabricated by MEMS micro-machining process.The performance indexes and operating temperature characteristics of the sensor were measured by using the above synchronous test system.The experimental results show that the sensor has the characteristics of low voltage operation,the response recovery time of T90 is less than 8s and 10s at 2.6 V,respectively.The sensitivity of the catalytic and thermal conductivity units reached 9.5 mV/1%CH4 and 10.4 mV/10%CH4.The operating temperature and output characteristics of the sensor under the catalytic response were studied using a separate micro-hot plate.The experimental results showed that the initial response temperature of methane was reduced to 273℃,and the effective detection temperature was reduced from 400℃ to 350℃.Under the operating voltage of 2.5 V,the average temperature rise gradient of the sensor is 22℃/1%CH4.At 5%CH4 concentration,the operating temperature is only 530℃.This will help to improve overall sensor stability and high impact resistance. |
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