Preparation And Properties Of Beta"-Al₂O₃ Electrolyte Ceramics With Oriented Structure

The sodium-sulfur(Na/S)battery system has gained widespread attention and been studied by scholars at home and abroad due to its high theoretical energy density,high charge/discharge efficiency,long life,environmental-friendliness and recyclability.As a core component of the Na/S battery system,beta"-Al2O3 electrolyte material serves as both a solid electrolyte and a separator and therefore plays a vital role in increasing the service life of the system and enhancing its charge/discharge efficiency.The Na+ conductivity of polycrystalline beta"-Al2O3 prepared by traditional solid-phase reaction sintering approach is quite low because of and the tortuous migration paths of Na+carriers,a result of the existence of the boundaries and pores,and the low content of beta" phase.In this thesis,the hydrothermal reaction method was used to prepare boehmites(y-AlOOH)with different morphologies as the precursor powder,and traditional wet method and the liquid nitrogen freeze-drying process were combined to prepare oriented beta"-Al2O3 ceramics.The work consists of the following three sections:1.The preparation process for beta"-Al2O3 material with excellent comprehensive performance regarding the orientation degree(LF),beta" phase content,density,and ion conductivity was explored.The impact on beta" phase content,density and electrical properties of beta"-Al2O3 materials prepared from Al sources of different Al2O3 crystal forms,which are obtained by calcining boehmite prepared by hydrothermal method at different temperatures(500??1300?);the results show that beta" phase content of beta"-Al2O3 material after sintered at high temperature is up to 96.12%when the calcination temperature is 1100?.Then,in order to improve the density of beta"-Al2O3 material,calcined power was ball milled;as a result,the beta" phase content of beta"-Al2O3 material stays above 90%and the relative density is increased up to 78%.Next,the effect of the sintering temperature and holding time during the high temperature sintering process on beta"-Al2O3 material was studied;the results show that the electrical conductivity of beta"-Al2O3 material reaches the peak(6.59E-2 S·cm-1)when the sintering temperature is within 1550??1600? and the holding time within 3 h to 4 h.In addition,MgO/Li2O was doped in the preparation process as a beta"-Al2O3 phase stabilizer,and the mechanism that why the two stabilizers show different effects on beta"-Al2O3 material properties was explained.2.The hydrothermal reaction method was used to prepare rod/flake-like boehmite precursors,which were then wet-mixed with other raw material;the mixture was then subject to high temperature sintering and formed oriented beta"-Al2O3 and material.The formation mechanism of beta/beta"-Al2O3 from the perspective of crystal structure and the influence of the two morphologies of alumina on the orientation degree of beta"-Al2O3 material were explained.The rod-shaped beta"-Al2O3 sample provided the maximum degree of orientation(LF),i.e.,0.21.Also,the electrical measurement results show a great difference of ion conductivity between the surface and cross-section directions of each sample at the test temperature of 350? due to the existence of oriented structure:the surface conductivity parallel to the uniaxial pressure direction(1.634E-1 S·cm-1)is 1.5 times that of the surface conductivity perpendicular to the direction.3.Oriented beta"-Al2O3 was prepared by the freeze-drying process with liquid nitrogen to increase the degree of orientation.First,the effect of sequence of the two processes,i.e.,calcination of mixed powder and freeze drying,on the beta"-Al2O3 phase content,the degree of orientation,and the electrical conductivity was investigated.The results showed that it is better to freeze dry the ?-AlOOH powder calcined at 1100? and other raw materials such as Na2CO3 before they are mixed.Second,the effect of freeze-drying process with liquid nitrogen on the orientation degree of beta"-Al2O3 material was studied,the results show that this process has a positive effect on both the orientation,with the LF value increased to 0.38,and the electrical conductivity,with the conductivity up to 1.852E-1 S·cm-1.Third,rod/flake-shaped beta"-Al2O3 samples were prepared by the freeze-drying process with liquid nitrogen with their properties studied by comparison.