Study On Humidity Sensing Properties Of Composite Metal Oxides

In the past few decades, humidity sensor, as a kind of important chemical sensors, has caused the close attention due to its wide application in the fields of industry, agriculture, medicine, meteorology and aviation. Humidity sensor is composed of humidity component and conversion circuit, and makes the use of the physical and chemical effects of the material to detect the moisture in the air. The humidity sensor which owns practical application value should have the following sensitive characteristics: high sensitivity in a wide range of humidity, linear response, small hysteresis, rapid response speed, physical and chemical stability and so on. With the progress of modern science and technology and the emergence of various new technologies, new methods, new areas, especially the development of chemical materials, people began to develop various kinds of new preparation methods, and continue to explore the experimental conditions like temperature, time, material ratio, concentration to make the material microstructure better. The characteristics of the humidity sensor are closely related to the humidity sensing material, in the past decades, many kinds of materials have been explored to fabricate humidity sensing detectors. But a single kind of material sometimes can not fully meet all the outstanding performance of a sensor. Therefore, a variety of material modification methods have been used to improve humidity sensing properties. So, this paper studied the humidity sensitive characteristics of composite ceramic oxides CeO₂-NiO, MoO₃-NiO and SrTiO₃.In the second and third chapters, CeO₂-NiO nanoparticles and MoO₃-NiO nanofibers were synthesized by heat treatment and electrostatic spinning methods and then characterized. Then the samples were made into sensors. The materials of different mass ratio（CeO₂:NiO /MoO₃:NiO= 0.5,1,2,3） were synthesized, and the humidity sensing properties were tested, The optimal ratio of CeO₂:NiO（1:2） nanoparticles humidity sensor was selected, the impedance changes by five orders of magnitude over a relative humidity（RH） range from 11% to 95%RH. At the frequency of 100 Hz, response and recovery times are 3 s and 19 s, and the maximum hysteresis is <2% RH. MoO₃:NiO（1:2）nanofibers humidity sensor is the best among other mass doping ratios. The impedance varies by three orders of magnitude over a relative humidity（RH） range from 11% to 95%RH. At the frequency of 100 Hz, response and recovery times are 2 s and 10 s, and the maximum hysteresis is <2% RH.In the fourth chapter, SrTiO₃ nanospheres were synthesized by solvent thermal method. Perovskite-type materials with a general composition of ABO3 which has been explored to fabricate humidity sensing detectors owing to their unique crystal structure and large specific surface area, In this paper, the impedance of the SrTiO₃ humidity sensor varies by is 4 orders of magnitude in the whole humidity range of 11%-95%RH, response and recovery times are both 2 s, and the maximum hysteresis is only 1% RH. Finally, the complex impedance and ZSimpWin software simulation（including the resistance and capacitance equivalent circuit） is used to analyze the humidity sensing mechanism of SrTiO₃ humidity sensor.