Study On Humidity Sensing Properties Of BaTiO₃ Nanofiber

Humidity is a physical quantity that expresses the dryness degree of the atmosphere. Humidity sensor is a class of widely used sensor in the large family of the chemical sensor. The operation principle of the humidity sensor was:the water molecules in the atmosphere were adsorbed by the sensing material and then the electricity properties of the sensing material changed, since this the change of the humidity could be detected. Recently, with the rapid development of the science and technology and the gradually improved living standards of people, the demand for a high-powered humidity sensor has become unprecedented. Nano-material has many characteristics different from traditional materials because of their special structure such as:high surface-to-volume ratio, quantum size effect, interface effect, quantum tunneling effect etc. Therefore, the preparation and application of nano-materials have been receiving wide attention from many researchers both in domestic and international. The nano-composite metal oxide could be designed both in structure and scale, and the properties also could be improved by controlling the chemical composition. Accordingly, the humidity sensor based on nano-composite metal oxide could exhibit its transnormal superior characteristics.In this dissertation, the traditional perovskite nano-composite metal oxide BaTiO3 was chosen as the humidity sensing material. And the resistive-type humidity sensor based on this material was studied. The material was designed on structure and scale by advanced electrospinning technical, and the BaTiO3 nanofiber with a large aspect ratio has ultra-fast response and recovery properties were obtained. The hysteresis characteristic of the material was improved by controlling the chemical composition. Many innovative achievements were obtained. The sensing mechanism was analyzed by combination of complex impedance spectra and dielectric loss method for the first time. The innovative results obtained in this dissertation were as following:1. The disordered one-dimensional BaTiO3 nanofiber with perovskite structure was prepared by combination of sol-gel and electrospinning method. The diameter of the one-dimensional BaTiO3 nanofiber was controlled by adjusting the parameters in the electrospinning process. The results show that, the BaTiO3 nanofiber prepared under 20 kV high voltages with a smaller diameter, when fixed other electrospinning parameter.2. The series resistive-type humidity sensors based on BaTiO3 nanofiber with different diameter were prepared. The resistive-type humidity sensor with ultra-fast response and recovery characteristics based on one-dimensional BaTiO3 nanofiber was produced for the first time. The response and recovery time of this sensor was less than 2 s and 3 s respectively, when the relative humidity change from 11% RH to 95% RH. The humidity sensing mechanism of ultra-fast response properties was researched by the combining of complex impedance spectra and dielectric loss method for the first time.3. BaTiO3 nanofibers were ordered by an innovative method of auxiliary electric field induced. The humidity sensing properties of the humidity sensor based on ordered BaTiO3 nanofiber array was studied. The results of test indicated that the humidity sensor based on ordered BaTiO3 nanofiber array also has ultra-fast response and recovery properties.4. The Sr element was introduced into BaTiO3 nanofiber by replacement change property method. The BaxSr1-xTiO3 nanofiber solid solution material with different x value was obtained by adjusting the molar ratio of the material. The results of humidity sensitivity test indicated that Ba0.8Sr0.2TiO3 nanofiber on the most sensitive to change of environment relative humidity. The complex impedance of resistive-type humidity sensor based on this material varying more than 3 orders of magnitude from 11% RH to 95% RH. The humidity hysteresis of this sensor was very small less than 1% RH. These results demonstrate that the added of the Sr element not only improved the sensitivity of BaTiO3 nanofiber to environment relative humidity, but also effectively decreased the humidity hysteresis of BaTiO3 nanofiber. The response and recovery time of this sensor was less than 9 s and 6 s respectively, when the relative humidity change from 11% RH to 95% RH.Through the above aspects of the work of this dissertation, we can conclude that the morphology of the material changes makes the humidity sensitive characteristics have been greatly improved when we make the traditional humidity sensitive material low-dimensional. This will open up a new way for develop and research high-performance humidity sensor, and also will find a new application direction of one-dimensional nano-materials.