Effect Of Grain Size On Properties Of Bi₂o₃-ln₂o₃ (ln=y, Er, Dy) Oxygen Ion Conductors With Grain Size From Nano To Submicron

Bi₂O₃-based material is expected to be used as potential oxygen ion conductor due to its high oxygen ion conductivity at intermediate temperature. Especially,δ-Bi_2-xLn_xO₃ (Ln=Y, Er, Dy), which exhibits relatively excellent oxygen ion conductivity, is widely concerned. The study on nanocrystalline ceramic revealed that grain refinement could greatly improve the mechanical properties of structural ceramics, such as strength, toughness, ductility. It also has significant effects on other properties of functional ceramics, such as electrical, magnetic properties and so on. For the application of oxygen ion conductor materials at intermediate and high temperature, besides excellent oxygen ion conductivity, good mechanical properties are also required. Therefore, in order to satisfy the practical application, synthesis of ceramics which integrate the structural and functional characteristics is an important direction in the development of functional ceramics in future. In this work,δ-Bi_2-xLn_xO₃ oxygen ion conductor materials with grain size from nano to submicron were prepared, and the effects of grain size on mechanical properties and oxygen ion conductivity were investigated.The pH ranges of the related species were given by thermodynamic calculation of each reaction in the aqueous solution. The precursor of (Bi₂O₃)_0.75(Ln₂O₃)_0.25 were obtained by using reverse titration chemical co-precipitation method and controlling pH at 11.5. The composition of (Bi₂O₃)_0.75(Dy₂O₃)_0.25 precursor and its thermal decomposition behaviour were investigated by simultaneous thermogravimetric and mass spectrum (TG-MS), higher temperature XRD and Raman spectrum.After calcining the precursors at 500°C for 3h,βphase (Bi₂O₃)_0.75(Dy₂O₃)_0.25, (Bi₂O₃)_0.75(Y₂O₃)_0.25, (Bi₂O₃)_0.75(Er₂O₃)_0.25 and (Bi₂O₃)_0.75(Er₂O₃)_0.125(Y₂O₃)_0.125 nanopowders were obtained with the average grain sizes of 17nm, 12nm, 10nm and 11nm, respectively. The powders were well dispersed, and the particles were nearly spherical shape.The thermodynamic parameters state diagrams of (Bi₂O₃)_0.75(Ln₂O₃)_0.25 system at higher temperature and lower oxygen partial pressure were obtained by thermodynamics calculation. According to the diagrams, the temperature and oxygen partial pressure condition of spark plasma sintering (SPS) were optimized.δ-(Bi₂O₃)_0.75(Ln₂O₃)_0.25 oxygen ion conductor material with the smallest grain size were sintered by SPS in this work. The materials exhibited dense microstructure with the relative density higher than 95% and the average grain sizes of about 20nm.Moreover,δ-(Bi₂O₃)_0.75(Ln₂O₃)_0.25 oxygen ion conductor materials with different grain size were obtained by pressureless sintering, the relative densities were higher than 94%. The grain growth behavior and densification of (Bi₂O₃)_0.75(Ln₂O₃)_0.25 materials during the sintering process were also investigated. The results showed that the grain growth followed parabolic rule during the pressureless sintering process, which can be described as D² -D₀²=kt. The activation energies of grain growth for (Bi₂O₃)_0.75(Dy₂O₃)_0.25, (Bi₂O₃)_0.75(Er₂O₃)_0.25 and (Bi₂O₃)_0.75(Er₂O₃)_0.125(Y₂O₃)_0.125 were 280kJ/mol, 120.60kJ/mol and 88.30kJ/mol, respectively. The Vickers hardness and fracture toughness of (Bi₂O₃)_0.75(Ln₂O₃)_0.25 materials with different grain size obtained by pressureless sintering were also investigated. The results showed that both of the Vickers hardness and fracture toughness were greatly improved with the decrease of grain size.Oxygen ion conductivity of (Bi₂O₃)_0.75(Ln₂O₃)_0.25 materials with different grain size from room temperature to 575°C were investigated by impedance spectrum. It was found that in the grain size range of 20nm¹Î¼m, the total oxygen ion conductivity decreased with the decrease of grain size, especially when grain size down to 20nm, the decrease in conductivity was more significant. For (Bi₂O₃)_0.75(Ln₂O₃)_0.25 materials with smallest grain size, at 300°C and 500°C, the material doped with Y exhibited the highest total ion conductivity, then followed by materials doped with Dy, Er and Er+Y. The impedance spectrum results of (Bi₂O₃)_0.75(Dy₂O₃)_0.25 materials with different grain sizes showed that the response of grain and grain boundary can not be separated between 200⁵00°C.Structure model and equivalent circuit model for homogeneous polycrystalline solid state electrolyte materials were established. Based on these models, the effect of grain size on oxygen ion conductivity was discussed. For (Bi₂O₃)_0.75(Ln₂O₃)_0.25 materials, the conclusion obtained from these models was in good agreement with the experiment results.