Preparation And Electrical Properties Of Na₃Zr₂Si₂PO₁₂ Ceramics

With the rapid development of steath technology,the fuction of microwave absorption materials in a weapon defense system has become more important.Good microwave absorption properties in a wide bandwidth have been already obtained in traditional microwave absorption materials,such as carbon-based materials,conductive polymers and ferrites.While their applications at high temperature are limited due to the poor oxidation resistance,undesirable high temperature stability and demagnetization.Therefore,it is necessary to develop new microwave absorption materials applied at high temperature.Na₃Zr₂Si₂PO₁₂ ceramic is a kind of solid electrolyte material which has been widely applied in the field of energy storage.It has high ionic conductivity and stable three-dimensional network.At present,the studies about Na₃Zr₂Si₂PO₁₂ ceramic mainly concentrated on the conductivity,but rarely involved in dielectric property.Considering the high conductivity and good high temperature stability of the ceramic and the technical advantages in microwave absorption materials of our group,the conductivity and dielectric property in the X-band（8.2-12.4GHz）of it were explored with the aim to develop a new kind of microwave absorption material.The main contents and results are as follows:Na₃Zr₂Si₂PO₁₂ ceramic was prepared using traditional solid-state reactions.The optimum power synthesis and ceramic sintering processes were explored.Relatively pure Na₃Zr₂Si₂PO₁₂powders with little ZrO₂ impurity were obtained at 1200°C for 6h with the molar ratio of Na₃PO₁₂·12H₂O:ZrO₂:SiO₂=1.15:1.8:2.Optimum electrical properties were obtained when the ceramic was sintered at 1250°C for 4h,and the conductivity was2.02×10^-4S·cm^-1.Then,the electrical properties from 50°C to 300°C were also explored.Results showed that the conductivity increased exponentially and the complex permittivity also increased dramatically.According to the result of simulated reflectivity,Na₃Zr₂Si₂PO₁₂ceramic had microwave absorption property at room temperature.The lowest reflection loss(RL_min)was-12.9dB at 10.74GHz and the bandwidth（RL<-5dB）was 3.42GHz with a thickness of 2.3mm.While the microwave absorption property became worse with elevated temperature,the reason might be that too high imaginary part increased the attenuation characteristics but caused impedance mismatch.The Zrsite of Na₃Zr₂Si₂PO₁₂ ceramic was substituted by doping strategy.The influences of ion valence,content and radius on the electrical properties were investigated,also the sintering properties,phase compositions,microstructures were studied.Due to the different melting point of the dopants,the synthesized temperatures of all the doped Na₃Zr₂Si₂PO₁₂powders decreased to 1100°C,the optimum sintering temperatures of them were different and decreased as the content of the dopants increased.The purity and density of the doped Na₃Zr₂Si₂PO₁₂ ceramics improved and the grain size decreased.The microstructures of them were composed of NASICON phase and glass phase.When doping with different valence of ions,such as Nb⁵⁺,Ti⁴⁺,Y³⁺and Zn²⁺,Na₃Zr_1.9Zn_0.1Si₂PO_11.9 ceramic had the highest conductivity,highest complex permittivity and optimum microwave absorption property.The grain conductivity represented the inherent property of the material,the improvement was mainly due to the less electrostatic interactions between dopant ions and Na⁺ions by low-valent doping;the grain boundary conductivity was the predominant contribution to the total conductivity,the enhancement was attributed to the improvement of purity and density,and the decrease of migration resistance of Na⁺ions through grain boundaries.The decreased activation energy caused the improvement of thermal ion relaxation polarization,then the real part of the permittivity increased.The augment of the imaginary part was the combined result of polarization and conduction loss.When doping with different content of ions,such as 0.1mol,0.2mol,0.3mol and 0.4mol,for Zn²⁺doped Na₃Zr₂Si₂PO₁₂ ceramics,the optimum electrical property was obtained when the content was 0.2mol;for other three doped Na₃Zr₂Si₂PO₁₂ ceramics,the optimum electrical property was obtained when the content was 0.1mol.Proper doping was beneficial to optimize the migration channel of Na⁺ions,reduce activation energy and increase the thermal ion relaxation polarization and conduction loss.Excessive doping led to serious lattice distortion,too wide or too narrow migration channel of Na⁺ions,precipitation of impurities,decrease of the Si/P ratio,which was not conducive to the electrical property.Among all the doped Na₃Zr₂Si₂PO₁₂ ceramics,Na₃Zr_1.8Zn_0.2Si₂PO_11.8 ceramic had the highest conductivity of1.44×10^-3S·cm^-1.And according to the results of DC-time polarization curve,the conductivity was ionic mobility rather than electronic mobility.In accordance with the results of simulated reflectivity,Na₃Zr_1.8Zn_0.2Si₂PO_11.8 ceramic had the optimum microwave absorption property with a bandwidth（RL<-5dB）of 4.14GHz and RL_min of-28.1dB at 9.88GHz in 2.1mm.When doping with different radius of ions,such as Ca²⁺（0.99（?））,Ni²⁺（0.69（?））,Mg²⁺（0.65（?））,Co²⁺（0.74（?））and Zn²⁺（0.74（?））,good electrical properties could be obtained with appropriate radius.When the radius was close to 0.74（?）,it was easy to form suitable size of Na⁺ions migration channel,decrease activation energy,improve ionic conductivity,complex permittivity and microwave absorption property.All the above results showed that proper doping could improve the electrical properties of Na₃Zr₂Si₂PO₁₂ ceramic.The Ti₃SiC₂/NASICON coating was fabricated by atmospheric plasma spraying.The effects of Ti₃SiC₂ additions on the phases,microstructures and electrical properties were investigated.The comparison between Na₃Zr₂Si₂PO₁₂ and Na₃Zr_1.8Zn_0.2Si₂PO_11.8 materials as matrixes were made,and the electrical properties at high temperatures were also measured.The matrix after doping had little effect on the phases,microstructures and electrical properties.Ti₃SiC₂ decomposed partially during the spraying process and a new phase TiC was generated.Ti₃SiC₂ was evenly distributed in NASICON matrix without obvious aggregation.The microstructure of the coating was composed of pores,microcracks,fully melted regions and partially melted regions.The melting degree of spraying particles and the density of coating improved as the concentration of Ti₃SiC₂ increased.The permittivity of Ti₃SiC₂/NASICON coating increased with the elevated Ti₃SiC₂addition because of the augment of space charge polarization and conduction loss.Moreover,due to the"percolation effect",both the conductivity and permittivity rose sharply when the content of Ti₃SiC₂ exceeded 20%.When the content of Ti₃SiC₂ was 30%,Ti₃SiC₂/Na₃Zr_1.8Zn_0.2Si₂PO_11.8 coating had an optimal microwave absorption property with a RL_min of-11.6dB at 9.25GHz and wide bandwidth which covered the whole X band in 1.4mm.Both the conductivity and permittivity increased as the temperature increased.The reason for the conductivity improvement was the increased migration rate of Na⁺ions at high temperature and the elevated contact conduction of Ti₃SiC₂/NASICON interfaces.The reason for the permittivity improvement was the enhancement of relaxation polarization.For the as-sprayed coating with 20%Ti₃SiC₂ addition,the optimal thickness became thinner with the elevated temperature,and they were all less than 2mm at different temperatures.As long as the appropriate thickness was selected,the coating had certain microwave absorption property from 200°C to 500°C,which indicated that Ti₃SiC₂/NASICON coating could be chosen as a potential candidate of high temperature microwave absorption material.