Study on oxygen vacancy of rare earth elements (cerium, scandium) and zirconia and its application

The formation mechanism of oxygen vacancy in zirconia doped cerium oxide (ZDC) and scandium oxide stabilized zirconia (ScSZ) was studied. At present, ZDC materials have been used in the manufacture of automobile exhaust catalysts, but there is a lack of systematic research on the formation of oxygen vacancy and thermodynamic mechanism and quantitative analysis of related parameters. In this paper, the effects of zirconia doping ratio on oxygen vacancy of ZDC materials at different temperatures and oxygen partial pressures were quantitatively analyzed, and the thermodynamic parameters of different oxygen vacancy states were fitted. On the basis of thermodynamic parameters, the experiment of simulating hydrogen generation by solar thermal cycle and the feasibility study of its application were carried out on ZDC materials, which further expanded the application field of ZDC materials. For ScSZ material research at present mainly concentrated in the state of high temperature (800-1000 ℃), the study of the ionic conductivity and as a solid oxide fuel cell (SOFC) application base of dielectric materials. There are few studies on the relationship between the change of oxygen vacancy concentration on the grain boundary of ScSZ material and the specific conductivity of grain boundary and the potential of space charge. Secondly, the application of ScSZ material as a dielectric material is limited due to the excessively high temperature working state. Based on the above shortcomings, the relationship between the oxygen vacancy concentration on the grain boundary of ScSZ materials with different grain sizes, the conductivity of grain boundary ratio and the potential of space charge was analyzed in this paper. Moreover, doping holmium oxide (H0203) in ScSZ materials made the ionic conductivity of this new material lower than 6880C was significantly higher than that of the substrate ScSZ. The main research contents and conclusions are as follows:

In this paper, two ZDC materials with different formulations were prepared by sol-gel method (s01-gel). The first one was ZDC03(Ce0.97Zr0.0302), which was composed of 97m01 % cerium oxide and 3m01 % zirconia. The second is ZDC20(Ceo). 8 zro. 202), consisting of 80mol % cerium oxide and 20mol % zirconia. The XRD results show that the crystal structures of these two materials and pure cerium oxide are cubic fluorite structures. The experimental results of TG show that with the increase of the amount of zirconia added to cerium oxide, the reduction of ZDC becomes easier and the number of oxygen vacancies becomes larger at the same temperature and partial pressure of oxygen. Thermodynamic parameters of ZDC materials were obtained after fitting and calculation of TG experimental results. Comparing the thermodynamic parameters of ZDC material and water (gas), the results show that the entropy of ZDC material determines the temperature range of decomposed water, and the enthalpy determines the lowest temperature of decomposed water. Based on the study of the thermodynamic parameters of ZDC materials, the author of this paper independently designed a device to simulate the decomposition of water by ZDC materials using solar heat, and compared and analyzed the capacity of ZDC materials with different zirconia doped components to decompose water, including the ability to release oxygen and generate hydrogen. The experimental results show that the temperature of 1500 ℃ ZDC materials of all components are the best reduction temperature. At this temperature, the amount of oxygen released by pure cerium oxide is significantly lower than that of ZDC doped with zirconia at the same time. However, did not increase with the increase of the content of zirconia oxygen release quantity, ZDC03 and ZDC20 at 1500 ℃, and the same time release is almost the same amount of oxygen. At the same time, at the low temperature oxidation stage, the increase of the doping amount of zirconia reduced the hydrogen production rate. At the same temperature of 8000C, the hydrogen production of ZDC03 and pure ceria was 61% and 58% higher than that of ZDC20 in the first 2 minutes, respectively. Based on the above experimental results, this paper calculated the energy conversion rate of ZDC materials in the process of simulating solar thermal hydrogen generation. Both pure cerium oxide and ZDC03 have solar conversion rates of more than 13 percent, with ZDC03 reaching 14 percent. 1%.

In this paper, several ScSZ powders with different scandium oxide doping ratios were prepared by hydrothermal method, respectively 97/3 (3mol % Sc203 doped in Zr02, 3ScSZ), 95/5 (5mol % Sc203 doped in Zr02, 5ScSZ), 92: / 8(8mol % Sc203 doped in Zr02, 85c5Z), 90/10 (10mol % Sc203 doped in Zr02, 10ScSZ), 88/12 (12m01 % Sc203 doped in Zr02, 12ScSZ), 85/15 (15mol % Sc203 doped in Zr02, 15ScSZ). XRD analysis results show that the Sc203 doping ratios of 5mol %, 8mol % and 10mol % can stabilize the high-temperature cubic structure of zirconia at room temperature, while ScSZ powders prepared with Sc203 doping ratios less than 5mol % and greater than 10mol % will show other asymmetry, such as monoclonal and rhombic. Solid ScSZ materials were prepared by pressing and sintering ZDC powder materials of the above three formulations. XRD analysis results show that the crystal structure of 5ScSZ and 8ScSZ powders remains cubic structure after sintering, but the original 100% cubic structure part of 10ScSZ powders is transformed into a diamond square structure after sintering. In this paper, impedance spectrum analysis of three formulations of ScSZ materials was carried out by using impedance analyzer. The results show that the ionic conductivity of 8ScSZ material is higher than that of other ScSZ materials. In order to further improve the ionic conductivity of zirconia, Hozo doped ScSZ material was studied in this paper. The results of impedance analysis showed that the conductivity of 7SclHoSZ material formed by replacing Sc203 with l mol % H0203 was higher than that of 8ScSZ material when its conductivity was below 6880C. In this paper, TEM analysis is combined to analyze, calculate and fit the grain boundary conductivity, grain boundary specific conductivity, space charge potential and grain boundary thickness of ScSZ materials. The results show that there is almost no amorphous phase and component segregation in the grain boundary of 8ScSZ solid material. When the grain size of the material increases, its total grain boundary conductivity increases, while the grain boundary conductivity decreases. The space charge potential increases with the increase of grain boundary, while the oxygen vacancy concentration decreases. The space charge potential of 8ScSZ is larger than that of HoScSZ material with Ho203 added. The larger the doping ratio of Ho203, the lower the space charge potential. However, the oxygen vacancy concentration on the grain boundary of 8ScSZ is lower than that of HoScSZ material with Ho203 added. The higher the doping ratio of Ho203, the higher the oxygen vacancy concentration on the grain boundary.

Keywords: zirconia doped cerium oxide (ZDC); Zirconia stabilized by scandium oxide (ScSZ); Stable zirconia (HoScSZ); Oxygen vacancy; Decomposition of water. Charge potential at grain boundary; Specific conductivity of grain boundary

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