Investigation On Composite,Doping And Low-temperature Synthesis Of Na_0.5Bi_4.5Ti₄O₁₅ Ceramics

Aurivillius-type bismuth layer-structured ferroelectrics with high curie temperature Tc,large mechanical quality factor Q(2000?7200),low time and temperature coefficients of resonant frequency,good fatigue properties,have outstanding advantages in some high temperature and high frequency application fields.However,the unique layer-structure of this kind of materials restricts the two-dimensional plane rotation of the electric domains,which leads to the larger coercive electric field E,and poor piezoelectric activity,and the volatilization of bismuth at high temperatures produces a large number of oxygen vacancy.The intergrowth and solid solution of bismuth layer-structurd materials can improve the properties.The doping of A and B sites can increase lattice distortion,and reduce the concentration of oxygen vacancy,as a result,improve material performance.Nano-powders with homogeneous composition were prepared by a wet chemical method,and dense ceramics with fine crystal and stoichiometric ratio composition are obtained.In this paper,the crystal structure,microstructure and electrical properties of Na0.5Bi4.5Ti4O15 are investigated via three approaches by XRD,SEM,electronic energy spectrum and Raman spectroscopy.The main contents are as follows:1.2-4 Intergrowth material,0.5Na0.5Bi4.5Ti4O15-0.5Na0.5Bi2,5Nb2O9(0.5NBT-0.5NBNO),and its electrical properties,are investigated with equivalent molar ratio of NBT(m-4)and NBNO(m=2).The result indicated that obtained materials show increasing anisotropy of grain size with the grain length of>20?m and the grain thickness of<2?m,and the aspect ratio of grain(?10)is significantly increased.XRD analysis and electronic energy spectrum suggest that the intergrowth structure NaBi7Ti4Nb2O24 is obtained,alternating between NBT and NBNO along the c axis.The electrical properties reveal different characteristics with 2-3 and 3-4 layers bismuth intergrowth materials,with a lower curie point(?575?)than both NBT and NBNO,and a wide curie peak.In the mean time,grain impedance decreased obviously and high temperature conductance increased.The piezoelectric and ferroelectric properties of materials reduced,with a 2P,of 3.8?C/cm2 and d33 of 12 pC/N.2.Composite ceramics,(1-x)Na0.5Bi4.5Ti4O15-xNa0.5Bi2.5Nb2O9(NBT-NBNO-x),are investigated with various molar ratio of NBT and NBNO.With the change of x,multiphases coexist within the materials with various amount of NBNO.When x?0.4,the main crystalline phase of NBT-NBNO-x ceramics is Na0.5Bi4.5Ti4O15.When x?0.6,the pyrochlore phase Bi2Ti2O7 appears.The dielectric constant ?r of composite ceramics decreases significantly,the relaxation is enhanced,and Tc decreased with the increase of x.The electrical properties of composite ceramics at high temperature and high frequency decrease,with the increasing tan?and the reducing conductance activation energy Ea.When x=0.2?0.8,d33=12?13 pC/N,kp=7%.The result indicates that high temperature piezoelectric materials with good performance can not be obtained via combining bismuth layer-structured materials with psodo-perovskite layer of 2 with 4.3.The investigation on the Ta5+ substitution of B sites of Na0.5Bi4.5Ti4O15(NBT)ceramics,Na0.5Bi4.5Ti4-xTaxO15+x/2,x=0?0.2.Aproper amount of Ta5+ dopant does not change the crystal structure of the materials.Although the curie point T,decreased slightly,the concentration of oxygen vacancy is reduced effectively,which results in the decreasing tanb and electrical conductivity,and frequency dependence.Meanwhile,piezoelectric properties at high temperature and high frequency enhance.When x=0.04,the piezoelectric properties achieve the optimum value:d33=22pC/N,kp=7.9%,T,=670?.4.Investigation on low temperature synthesis of Na0.5Bi4.5Ti4O15(NBT).The optimum hydrothermal conditions were studied by changing the temperature,time and concentration of mineralizer using sol-gel hydrothermal method below 300?,and the nano ceramic powders with plate-like grain and the stoichiometric ratio composition were obtained.Compared with NBT samples prepared via a conventional solid state reaction route,the obtained nanocrystalline ceramics reveal a dense and uniform microstructure with fine grains.Although?r,decreased,the curie point T,does not change,and high temperature tan? and conductivity decreased obviously,showing an excellent temperature stability.