The Study Of Fabrication And Oxygen-sensing Properties Of Double Perovskite LaBaCo₂O_5+δ Thin Films

The rapid development of technology promotes the applications of different gas sensors, which has been widely used to detect and monitor leaks of explosive, toxic and harmful gases and air pollution.Among them, oxygen sensor plays a very important role in many field. But the traditional oxygen sensors basing on ZrO2 and TiO2 are difficult to meet the requirements of high tech, which requires sensors with high sensitivity, high selectivity, high stability, small size, low power consumption and easy compatibility with integrated. Therefore, investigating new oxygen sensor basing on new materials is important in the development of modern technology.(LaBa)Co2O5+δ(LBCO) is a kind of mixed ionic-electronic conducting(MIEC) material with double layer perovskite structure, which is extremely promising materials for the applications as gas sensors due to the high sensitivity and selectivity to the specific gas species, long term stability and highly efficient operations over a wide range of temperatures. However, the applications of this material were be restricted by high cost for preparing high quality and large-scale thin films. In this thesis, an economical method called polymer assisted deposition(PAD) was used to prepare orientation preferred LBCO film on Si substrate. And the electrical transport properties and the oxygen-sensitive properties of the film were studied.1. The process for the preparation of LBCO thin films on Si substrates by PAD was studied. It was found that the in situ SiO2 buffer layer on Si substrate was benefit to grow LBCO thin film. And LBCO film on the Si substrate with good performance was prepared successfully.2. The effects of sintering process of LBCO film(different sintering temperatures and different sintering time) on the microstructure, surface morphology, electrochemical properties, electrical transport properties and oxygen-sensitive properties were studied. 1). XRD showed that the film begins to form crystals when the sintering temperature greater than 700 ° C, but other phase was formed when the sintering temperature greater than 1000 ° C. 2) From the morphology, the longer the holding time during sintering, the more uniform size of LBCO grains was observed. 3) The temperature dependent resistance showed that only the electrical transport properties of film sintered at 900 ° C for 150 min met the small polaron hopping mechanism in the whole testing temperature range. 4) The study of oxygen-sensitive showed that LBCO film sintered at 900 ° C for 150 min has the best oxygen-sensitive properties, which had maximum resistance change rate(Rhyd / Roxy) and minimum response time. In summary, the best sintering temperature for LBCO films was 900 ° C and sintering time was 150 min. And it was found that the properties of LBCO films have also been greatly improved compared to bulk LBCO crystal.3. In the oxygen-sensitive test for LBCO film material, when the reducing gas environment(6% H2, 94% N2) was switched to pure oxygen environment, the response time of resistivity was 1.9 seconds for the best sample, which showed that LBCO film has excellent gas sensing characteristics compared to other conventional oxygen-sensitive materials such as ZrO2, TiO2. And through repeated cycle tests, resistivity change curves of the sample did not change significantly, indicating the gas sensitivity properties of LBCO thin film has good repeatability and stability.4. Finally, this thesis discussed the oxygen-sensitive mechanism of LBCO thin film. The oxygen-sensitive of LBCO film should be related to transport of holes(or small polaron). Due to the double layer perovskite structure of LBCO film, a large number of oxygen vacancies present in the LaO layer, which making the oxygen ions move fast. Oxygen vacancies can change with the rapid change of oxygen partial pressure. Then the corresponding electron concentration is also changing rapidly, so that the resistivity changes rapidly caused by the change of concentration of major carriers(holes or small polaron) with oxygen partial pressure changes.In short, LBCO film with a high oxygen-sensitive performance on Si substrate was prepared by PAD method, and its oxygen-sensitive mechanism was investigate and explained. This work may be helpful to reduce costs and improve the performance of LBCO oxygen sensors.