Palladium Based Thin Film Hydrogen Sensor On Micro Hotplate

With the vigorous development of hydrogen energy,especially hydrogen fuel cell and hydrogen energy vehicle,hydrogen energy has been applied in more and more fields.To ensure the safety of hydrogen in the production,transportation,storage and use,rapid and sensitive hydrogen sensors are needed to detect the hydrogen content in the environment.There already exists many researches on hydrogen sensor,but the performances need to be improved further.Hydrogen sensors come in many varieties,among which the most commonly used is the resistive hydrogen sensor.Resistive hydrogen sensors include semiconductor metal oxide type and metal type.Semiconductor metal oxide has high operating temperature,poor selectivity,and can also respond to other combustible gases.By contrast,palladium(Pd)can react with hydrogen at room temperature with high selectivity,making it the preferred sensitive material for metallic hydrogen sensors.However,the Pd film hydrogen sensor has a slow response/recovery,the response time is typically several minutes long and the sensor cannot fully restore to the initial state.In a high concentration of hydrogen,excessive hydrogen absorption may cause the phase transition of Pd,which leads to mechanical deformation of the Pd film.Previous work was mostly based on the improvement of materials,alloying of Pd can effectively solve the problem of deformation and detachment caused by phase transition,however,the response speed and sensitivity cannot be significantly improved.Besides,increasing the buffer layer,surface modification of the bases and preparation of Pd nanoparticles can improve the performance of hydrogen sensor to a certain extent,but these methods can greatly increase the complexity of processes.Operating temperature is probably an important factor to affect the response performance,therefore,it is necessary to study hydrogen sensitive performance of Pd based thin film in the heating mode.In order to realize working in the heating mode,Pd based film hydrogen sensor on micro hotplate has been studied in this paper.First of all,several new designs of suspended micro hotplate are put forward,and the mechanical performance is studied by simulation and experiment,the results show that the shape of suspended membrane,especially the number and width of the suspended beams,affects the structural mechanics performance.The material of the suspended film also affects the structural strength,the Si O₂/Si₃N₄ composite film multi-beam suspended micro hotplate is the most reliable design.Subsequently,the shape,width and thickness of the platinum(Pt)heating resistance wire are further designed.The results show that the width of the heating resistance wire has the most significant influence on the temperature uniformity,and the width and thickness of the heating resistance wire have great influence on the power consumption.By contrast,the shape of the suspended film is only a secondary factor,although it also affects the thermal performance.Pt heating resistance wire and Pd/Ni alloy sensitive resistance wire are coplanar on the glass substrate,and the heating mode of the sensor is realized through electrifying of the Pt wire.The hydrogen sensitive temperature characteristics are studied to find that 75°C is the optimal operating temperature for such Pd based sensitive films.When the hydrogen concentration is 2%,the response/recovery time at room temperature and 75°C is 46 s/55 s and 7 s/6 s,respectively,and the response increases from 1.47%to 3.17%.The sensor has good repeatability and stability at this temperature,and its linearity and sensitivity are obviously improved than at room temperature.To reduce the power consumption of the sensor,the suspended micro hotplate and Pd sensitive film are integrated.In the low temperature zone,the responses are consistent with the sensor on the glass substrate,at this time,the power consumption is only 12.3 m W owe to the suspended micro hotplate.In the high temperature zone,Pd film on the suspended platform is prone to oxidation reduction reaction with the phenomenon of reverse resistance change.At 400°C,if 4000 ppm hydrogen gas is exposed,the response reaches to-3%within 10 s.Since the optimal working temperature of Pd based film is not high.In addition,the suspended platform has the problems of complex process and low yield.In this paper,a kind of solid micro hotplate with stacked heating resistance and sensitive resistance is designed,the main material of the insulation layer and the thermal insulation layer is polyimide,and the thermal conductivity can be changed by doping different nanoparticles.The simulation results indicate that the thermal conductivity of the insulation layer and the thermal insulation layer should be as low as possible,the insulation layer should be as thin as possible,and the thermal insulation layer should be as thick as possible for low power consumption.Furthermore,reducing the heating area also contributes to reducing the power consumption.The test results show that applying a layer of PI thermal insulation layer on the glass substrate can greatly reduce the power consumption,while doping nano-silica in the PI can further reduce the power consumption to 58%of undoped PI.At this point,the power consumption of the solid micro hotplate is roughly comparable to that of the suspended micro hotplate with the same heating area.Among all design factors,thermal conductivity of the thermal insulation layer is the most prominent.Therefore,doping nano-silica in PI as the thermal insulation layer is the core of the solid micro hotplate.The hydrogen sensitivity test results of the sensor based on the solid micro hotplate show that when the operating temperature is 70°C,the response time under the concentration of 2%H₂ is 6 s,and the power consumption is only 7.2 m W.Aiming at the shortcoming of low sensitivity in Pd based thin film hydrogen sensor,a bridge type hydrogen sensor with sensitive film,lead and electrode of bridge circuit integrated on PI or PI/nano-silica thermal insulation layer is fabricated.The applied supply voltage is partly used for increasing the operating temperature of the sensor through joule self-heating,and the other part is used for amplifying the signal of the bridge circuit.The optimal operating temperature range of this bridge type sensor is 40-45°C.When the temperature exceeds 50°C,there exists spike signals in the output voltage and the response time extends to 5 s.If the temperature continues to rise above80°C,the sensitivity declines instead.When the temperature is 45°C,the power consumption of the bridge type sensor based on undoped PI and nano-silica doped PI is 14.2 m W and 8.5 m W,respectively.The sensor response time is as fast as 2 s,and the sensitivity is greatly improved than that of a single resistance sensor,and the limit of detection is as low as 20 ppm.