Preparation And Properties Of Alumina-based Functional Ceramic

Beta-alumina solid electrolyte is an excellent ionic conductor used in energy storage and conversion system.Sodium-sulphur battery,sodium-metal halide（zero emission battery research activities,ZEBRA）battery and alkali-metal thermoelectric converter（AMTEC）,are promising energy storage and conversion system based on beta-alumina solid electrolytes for their high specific power,long cycle life and high efficiency.Na-S battery and ZEBERA battery have attracted much attention because of their applications in medium-and large-scale energy storage,and AMTEC is an attractive power generation device used in space nuclear reactor power system.However,there is a large distance before these systems can be widely produced and used in practical application.The reason is mainly laid in the insufficient conductivity and stability of SBA.Na⁺ionic conductivity and stability of the SBA are depended on the nature and production process of the SBA.SBA is sensitive to water vapor in atmosphere which leads to fluctuation of electrical and mechanical properties of the SBA ceramic.The main problems in the production process are the sluggish transformation of high conductive beta’’-alumina,loss of Na₂O and exaggerate grain growth,which make the preparation of high performance and high reliable sodium beta alumina solid electrolyte（BASE）become a challenge.This paper focuses on the improvement of production process of SBA.Influence of water vapor on the electrical properties of SBA is discussed.The influence of parameters in preparation process on the phase constitution and microstructure of the prepared ceramics are investigated,too.New methods are explored for preparation of high reliable BASE.The results will help to improve the performance of the BASE,and promote a further application of BASE in energy storage and conversion systems.The detail contents are as follows:（1）Phase-pure lithium stabilized sodiumβ’’-alumina（Li SB2A）,which synthesized by solid phase reaction using phase control method,is investigated as a humidity sensing material in order to explore the stability of BASE exposed to water vapor in the atmosphere.Humidity properties of Li SB2A powder are investigated in detail through AC complex impedance and DC transient property analysis.Features of the Li SB2A sensor are obtained,including sensitivity,humidity hysteresis,response and recovery time,relationship of impedance modulus with temperature and time period.The variation in DC charge and discharge current versus time curve,AC dielectric loss versus frequency properties and AC complex impedance properties under different RH conditions are explored on the Li SB2A sensor.It is found that Li SB2A sensor is highly sensitive to humidity.The humidity hysteresis,response and recovery time of the sensor are a little big and long at high RH condition.It is also found that the response of the sensor is contributed by polarization,migration of electrons and ions,which are related not only to the absorption of water molecules on the particle surface but also related to the exchange reaction between water and Na⁺ions in the conduction slab of the Li SB2A crystal lattice.Under medium and high humidity condition,the exchange and migration of Na⁺ions in the conduction slab join the electric responses,which are slow processes and can lead to sluggish recovery of the electric response.BASE ceramics should be used in an environment with humidity less than 11.3%RH.Li SB2A sensor is a promising humidity sensitive material used in low RH environment because of its high humidity sensitivity and quick response.（2）Phase transformation and sintering behavior are investigated on Li SBA ceramics with and without YSZ addition prepared by double zeta method.Changes of phase constitution,linear shrinkage,relative density,and grain size in the ceramics sintered at different temperature are analyzed.The dependence of grain size on relative density and dwelling time is discussed.It is shown that phase transformation in sintered ceramics is controlled by a relationship between Na₂O evaporation loss and Li⁺ions stabilization,and microstructure evolution is controlled by pore-boundary interaction.Surface diffusion is potentially the mechanism of grain growth in SBA grains.Due to second phase effect and O^2-ionic conductive properties of YSZ,Zr-Li SBA ceramics show a lowβ’’-phase fraction and a high densification.（3）BASEs with uniformly distributed nano-3YSZ grains are prepared by surface modification of precursor particles with EDTA-Zr⁴⁺/Y³⁺ions complex and spray drying process.Systematic investigation is performed on relationships of the nano-3YSZ concentration with phase composition,density,microstructure,bending strength,and conductivity of the prepared BASEs.The mechanism of nano-YSZ formation and performance variation of BASE after nano-YSZ addition are discussed.It is found that EDTA-Zr⁴⁺/Y³⁺ions complex leads to in-situ formation of nano-3YSZ grains on SBA particles,and spray drying process maintains the homogeneity of components in mixing suspension to the drying powders.Thus,SBA powders with in-situ generated and uniformly distributed nano-YSZ grains are obtained.Compare to BASE without YSZ addition,BASEs with nano-YSZ addition show similarβ’’-phase faction,higher densification,more homogeneous microstructure,higher Weibull’s modulus,higher characteristic strength and higher conductive resistance.The enhancement of mechanical strength is originated from the improved microstructure,higher densification and more fully exertion of mechanical toughening effect by homogeneously distributed YSZ grains in BASE.The increased conductive resistance is ascribed to the unconductive YSZ on the grain boundary and the small grain size in the Zr-BASEs.Fortunately,BASEs with small amount of YSZ addition show good conductivity and good mechanical strength,which indicates homogeneous distribution of a small amount of nano-YSZ can not only effectively improve mechanical strength of BASE but also maintain good ionic conductivity.In-site nano-YSZ formation by EDTA-Zr⁴⁺/Y³⁺ions complex absorption on SBA precursor particles is a good choice for preparing Zr-BASE with better performance.（4）A novel gelcasting system based on the polymerization of 2-hydroxyethyl methacrylate（HEMA）with low toxicity is applied to the fabrication of Li-BASE.Effects of dispersant concentration,solids loading and plasticizer concentration on the rheological behavior of the suspensions are investigated and optimized.Gelcast from the optimized suspension shows homogeneous microstructure and excellent appearance without cracking and warpage.Properties of Li-BASEs sintered from green body formed by gelcasting and cold isostatic press are evaluated,including relative density,phase composition,microstructure,bending strength and ionic conductivity.Results show that gelcasting forming technique performs well in preparing BASE ceramics with high densification,high Weibull modulus and ionic conductivity,which indicates that gelcasting forming method effectively decrease the defects and improve the homogeneity in the BASE.Therefore,Gelcasting will be highly recommended to serve as a promising forming technique for fabrication of reliable Li-BASE.