Study on Fabrication of Several Kinds of Oxide Dielectric Materials and Their Photocatalytic and Electrical Properties
|Course||Materials Physics and Chemistry|
|Keywords||Photocatalytic citrate method BiNbO4 BiTaO4 phase transition ZrO2 ReRAM PSZT electrical tunability|
Materials, energy, information technologies are considered as the foundation of the modern civilization. In 21st century, the direction of materials development is to be multi-functional, complex, intelligent and environmental-friendly and so on. The research of new materials becomes more and more important. Among these new materials, oxide dielectric materials have attracted much attention due to their rich structural variety and excellent physical and chemical properties.With the industrialization process, human are facing increasing critical problems of environmental pollution. The photocatalytic materials, represented by TiO2, have been studied extensively, but TiO2 can only absorb ultraviolet light. In order to use more visible light, researchers focus the attention to seek new materials which can respond to visible light, with the modification of existing materials. BiNbO4 and BiTaO4 are microwave dielectric materials, they can also be used as photocatalytic materials. In the thesis, BiNbO4 and BiTaO4 powders and ceramics are prepared by a citrate method, using water-soluble Nb and Ta precursors as starting materials. The influence of preparation conditions on the structure, photocatalytic and dielectric performance is systematically studied. The photocatalytic mechanism of the two powders are compared, the phase transition mechanism of BiNbO4 and the thermodynamic stability ofβ-BiNbO4 obtained at low temperature are also deeply studied.With the development of semiconductor technologies, the device size is shrinking. The conventional flash memory is facing great challenges, so it’s important to seek new generation of high density, non-volatile memory technologies. Resistive memory records data based on reversible resistance change. it have many advantages, such as simple structure, small storage unit, rapid switching speed and so on, which make it has excellent prospect in next-generation memory. ZrO2 is an important high-k material, compatible with existing microelectronics processing, so it has attracted wide attention. Studies found that it has good resistive switching performance. In the thesis, ZrO2 films are prepared by sol-gel method, the influence of film crystallization, annealing atmosphere and electrodes on the resistive switching performance and resistive switching mechanism are investigated.Ferroelectric film has good permittivity nonlinear effect of the electric field and its dielectric loss is very low, which make it have advantages in the microwave phase shifter applications. PbZrxTi1-xO3(PZT) is one of the important ferroelectric systems. It has found that the dopant of Sr can effectively modify the Curie temperature of PZT films, which provide an opportunity for electrically tunable application. In the thesis, the influence of B-site modification on the Curie temperature and electrically tunable performance of PSZT thin films is systematically investigated and the electrically tunable performance is optimized.The main achievements made in this work are summarized as follows:1. The air-stable, water soluble pero-citrate-Nb and pero-citrate-Ta precursors are synthesized by the basic flux technique using Nb2O5 and Nb2O5. The purity is over 99% for both precursors and the yield is 74% and 65%, respectively. Using water-soluble Nb, Ta precursor as starting materials, pure p phase BiNbO4 and BiTaO4 powders are prepared at low temperature by citrate method. Compared with the conventional solid state reaction, the synthesis temperature decrease about 300℃and 550℃, respectively. BiNbO4 and BiTaO4 powders exhibit excellent visible-light photocatalytic performance, especially the BNO700 and BTO1000 powders. In the optimal process, both the BNO700 and BTO1000 show that 1g/L catalytic loading, basic condition (pH=8) and the addition of 2 mmol/L H2O2 are beneficial for the decomposition of MV. Compared with BTO700, BNO700 has better visible-light utilization efficiency, including direct absorption and self-photosensitized process of dyes, while for BTO700 powder; it can only absorb visible light through self-photosensitized process of dyes.2. it found thatβ-BiNbO4 exist at two different temperature ranges in the preparation process by citrate method, one is below 750℃and the other is above 1040℃. The formation ofβ-BiNbO4 at low temperature (Low-β) can be attributed to the appearance of the intermediate phase of Bi5Nb3O15. The structure similarity betweenβ-BiNbO4 and Bi5Nb3O15 reduce the reaction barrier, and the activity of Bi5Nb3O15 makes it decompose and causes the formation ofβ-BiNbO4 with the temperature increasing. It’s the first time to observe the phase transition ofβ→αin BiNbO4 powders, and it exist just inβphase prepared at low temperature. The reason may be that low-p is in thermodynamically metastable state, it’s small grain size and not fully evolution of crystal may also the reason for the occurrence of the phase transition. The sintering temperature, holding time and the stress existing in pellet samples can affect its stability. The presintering temperature of 700℃is beneficial to the formation of dense ceramic and the dielectric constant ofα-BiNb04 andβ-BiNbO4 ceramics obtained is 42 and 40, the dielectric loss is 0.002 and 0.003, respectively. At 7GHz, their corresponding microwave dielectric constant is～42 with dielectric loss of 4.67E-03 and 4.43E-03, respectively. The microwave dielectric properties need to be further optimized.3. ZrO2 films were prepared by sol-gel method. The XRD patterns show that the films are amorphous at 400℃and polycrystalline tetragonal phase at 500℃and above. No matter the films are amorphous or polycrystalline, sintering in N2 atmosphere leads to more O vacancies in ZrO2 films. The Pt/amorphous ZrO2/Pt and Cu/amorphous ZrO2/Pt structures exhibit unipolar switching behaviors. The conduction mechanism of the low resistance state of both structures can be attributed to the formation of filament; the conduction mechanism of the high resistance state of the former structure is Schottky emission, while the latter can be explained by electron trapping and detrapping at defective site in ZrO2 matrix. Compared with the Pt/amorphous ZrO2/Pt structure, Cu/amorphous ZrO2/Pt structures exhibit lower operation voltage, but its high resistance is more diffusive, and the widow between the high resistance and low resistance and the switching cycles are smaller. The Cu/crystal ZrO2/Pt structure shows unipolar switching behavior when the films sintered at N2 atmosphere, while shows bipolar switching behavior when sintered at O2 atmosphere.4. The content of Zr affects the structure and electrically tunable performance of PSZT thin films. When the Zr content≥0.2, the structure of PSZT transformed from tetragonal to cubic phase. With the increase of Zr content, the phase transition temperature decrease rapidly to about 200K (x=0.4). The dielectric constant, tunability and dielectric loss are strongly depend on the Zr content, they arrive maximum at x=0.1. In PSZT films, the ions at B site, that is Zr ion and Ti ion, have different roles. On one way, the tunability mainly comes from the anharmonic vibration of Ti ions and O ions in O octahedron; on the other way, the increase of Zr content can effectively reduce the dielectric loss. Optimal electrically tunable properties have been achieved for PSZT films with x=0.4. The tunability, dielectric loss, and FOM of PSZT films with x=0.4 at 1 MHz are 55.8%,0.023 and 24.3, respectively. The tunability of the Pb0.4Sr0.6Zr0.4Ti0.6O3 film shows weak temperature dependence, from 25℃to 145℃, the changes of tunability is less than 10%, which makes it suitable to work at different environmental temperatures.In summary, several oxide dielectric materials are prepared in the thesis, and we focus on the visible light photocatalytic performance, dielectric properties and phase transition mechanism of BiNbO4 and BiTaO4 powders. The resistive switching performance and resistive switching mechanism are investigated in ZrO2 films. In addition, we study the electrically tunable performance of PSZT thin films through B site doping. These works provide more opportunities for the applications of oxide dielectric materials.