Research On Sensitive Nanomaterials And Sensor Technology Of Ultra-low Concentration Hydrogen

Hydrogen has been widely applied in new energy,petrochemical engineering,nuclear power,aerospace and other fields.However,because of the flammability and explosivity,it is a great potential safety hazard once hydrogen leaks during storage and use.Therefore,it is necessary to monitor the concentration in the environment with hydrogen.In addition,because hydrogen is the most abundant element in the universe,and there are traces of hydrogen and hydrogen atoms in near space,the accurate measurement of hydrogen concentration is an important topic in the science of the near space environment,which has great scientific significance for revealing the creation of the universe and the evolution of the atmosphere.In the above applications,hydrogen detection technology with a wide detection range,low detection limit,high sensitivity,fast response,low power consumption,small size,strong selectivity and high repeatability is urgently needed.However,it is difficult for reported hydrogen sensors to meet the above requirements at the same time.Based on this,this dissertation systematically carried out researches about the designs,preparations and performances of four hydrogen sensors based on PdNi nanofilm,Pd nanoparticles loaded on foam Ni O,Pd-modified WO₃/foam Ni O and Pd-modified Bi-doped foam Ni O,and tried to clarify the hydrogen sensing mechanism of the four hydrogen sensors.The main research contents of this dissertation are as follows:1.Based on MEMS technology,a PdNi nano-film hydrogen sensor with a dimension of 1 mm×1 mm×0.3 mm,a structure of loop type（the line width of 30μm,the line length of 5 mm）was designed.The effects of annealing on the microstructure of PdNi nanofilm and sensor performance were studied.The results show that the vacuum annealing at250°C can improve the stability and uniformity of PdNi nanofilm,which help to effectively suppress the zero drift phenomenon and maintain high response value and response speed of the PdNi nanofilm hydrogen sensor.The effects of film thickness on the microstructure of PdNi film and the response of the hydrogen sensor were explored.It is found that reducing the film thickness can limit the crystallization and grain growth of the film,thereby improving the hydrogen adsorption capacity and hydrogen diffusion range of the film,which benefites to improve the sensor response.The relationship between the response of the sensor and the film thickness was fitted by a bivariate cross effect linear regression model.As results,the relationship between the sensor responses and the hydrogen concentrations conforms to Sieverts’law,and the Sieverts coefficient increases with the decrease of film thickness.As a result,the prepared PdNi nanofilm hydrogen sensor can detect hydrogen with a concentration as low as 50 ppb at room temperature and in nitrogen environment.Meanwhile,the sensor shows excellent gas selectivity for impurity gases like CO₂,CO,NH₃ and CH₄.And the response values are only attenuated by 3.74%after 180 days,reflecting reliable long-term stability.2.A hydrogen sensor with palladium nanoparticles as sensitive material which loaded on the surface of Ni O foam was designed and fabricated.The size of Pd nanoparticles/foam Ni O was 8 mm×3 mm×0.5 mm,and the substrate of the sensor was a flexible printed circuit board（FPCB）.The microstructure of Pd nanoparticles/foam-Ni O was studied.The results show that the grain and lattice size of Pd nanoparticles are increased after reacting with high concentration hydrogen.A method for activating the sensor by high-concentration hydrogen was proposed.It is found that the inreversible lattice distortion of Pd nanoparticles is the key to the activation of the sensor,which is caused by the generation ofβPd H_x in the reaction with high concentration hydrogen.A large number of defects are generated in Pd nanoparticles in the activation process,which provides a large number of active sites for hydrogen atoms to combine with Pd nanoparticles,so that the sensor had the ability to detect ppb concentration of hydrogen.The detection range of the Pd/foam-Ni O hydrogen sensor is 7 ppb～2%,and its response decreases monotonically with the decrease of hydrogen concentration.Meanwhile,the sensor shows excellent selectivity to CO₂,CO,He and other gases,and great humidity stability in high humidity environments.3.A hydrogen sensor based on the WO₃ nanolayer modified with Pd nanoparticles and loaded on Ni O foam was designed and fabricated.The micromorphology and crystal structure of Pd/WO₃/foam-Ni O were explored.The results shows that the WO₃ nanolayer was amorphous,containing oxygen vacancies.The Pd nanoparticles and WO₃ nanolayers are uniformly distributed on the surface of the foam Ni O.In addition,the Pd/WO₃/foam-Ni O sensor shows a unique n-p type conversion phenomenon,that is,in higher concentration of hydrogen,the resistance of the sensor is reduced,and the response is n-type;in lower concentration of hydrogen,the resistance is increased,and the response is p-type.The n-p type conversion phenomenon is because the Schottky contact between Pd/WO₃ and the n-p heterojunction between WO₃/Ni O deplete the electrons of the WO₃nanolayer,which results in the WO₃ nanolayer reversing from n-type to p-type with oxygen adsorption.In ppb-level hydrogen,the main carrier of the WO₃ nanolayer is holes,and the concentration is much lower than that of conventional p-type semiconductors,so it is more sensitive to carrier concentration changes,and even trace hydrogen can cause significant resistance changes of the sensor.Finally,the operating temperature of the Pd/WO₃/foam Ni O hydrogen sensor is 75°C,and the detection limit of the sensor is as low as 20 ppb in the air.The hydrogen sensor shows excellent gas selectivity to CO₂,CO,NH₃ and other gases,and demonstrates reliable repeatability.4.A hydrogen sensor based on the Pd-modified and Bi-doped Ni O foam was designed and fabricated.The crystal structure of Pd/Bi-Ni O foam were studied.The results shows that Bi-Ni O foam had a huge specific surface area due to a large number of scattered nanoparticles on the surface,and Bi³⁺in Bi-Ni O foam will self-doping to Bi⁵⁺after repeated hydrogen absorption and releasing.In addition,the hydrogen sensing mechanism of Pd/Bi-Ni O foam hydrogen sensor was explored.It is found that Bi⁵⁺self-doping significantly improved the response and stability of the Pd/Bi-Ni O-foam hydrogen sensor.This is because Bi³⁺doping reduced the thickness of the hole accumulation layer on the Ni O surface,and the Bi⁵⁺self-doping occurring during the repeated testing further promotes oxygen adsorption on the surface of the material,so that the hydrogen sensitivity of the sensor continues to increase.The working temperature of the Pd/Bi-Ni O foam hydrogen sensor is only 75°C,the hydrogen detection range is 20 ppb～1%,and the response values decreases monotonically with the decrease of hydrogen concentration.And the same time,the sensor shows great selectivity,humidity stability,repeatability and long-term stability.