Synthesis,Structure And Properties Of Ba₈B’B’’₆O₂₄ Hexagonal Perovskites

Microwave dielectric ceramics are widely used in fields such as dielectric resonators,attenuators,and microwave integrated circuit substrates.Researchers have been working on improving formulas and manufacturing processes for obtaining high dielectric constant,low microwave loss,and low resonant frequency temperature coefficient.The microwave dielectric performance is different in shifted and twinned 8-layer hexagonal perovskite result from the different ordering types between AO₃ and BO₆ units.In order to research the effect of isovalent substitution on structure and electric properties of 8-Layered hexagonal perovskites,In this work,Ba₈CoNb_6-xTa_x O₂₄（x=1-5）,Ba₈Zn_1-xNi_xNb₆O₂₄（x=0.2-0.8）and Ba₈Zn_1-xCu_xNb₆O₂₄（x=0.2-0.8）were prepared using the conventional solid state method,the microwave dielectric properties and the conductivities were characterized for dense ceramics.What’s more,the（1-x）Ba₈CoNb₆O₂₄-xBa₃CoNb₂O₉ compositions were synthesized to adjust the temperature coefficient of resonant frequencyτ_f by adjusting the ratio of Ba₃CoNb₂O₉ and Ba₈CoNb₆O₂₄.The main research results obtained are as follows:1)For the Ba₈CoNb_6-xTa_xO₂₄（x=1-5）hexagonal perovskite compositions.The results from X-ray diffraction,microwave dielectric properties and conductivities were abtained.a)It suggested that it’s shifted structure for x≤2 compositions,the twinned structure are synthesized for x≤2 compositions.b)the parameter of Q×f for Ba₈CoNb_6-xTa_x O₂₄4 materials is 3080～37895 GHz,the dielectric constant?is 29.7～37.2.and temperature coefficient of resonant frequencyτ_f is 5.6～63.3,showing the lowest value（～5.6）in Ba₈CoNb₄Ta₂O₂₄.c)All compositions are confirmed the electronic conductor thorugh AC impedance spectroscopy,the conductivity of Ba₈CoNbTa₅O₂₄ is highest,which is 2.1×10^-8～8.1×10^-5S/cm at 400-800℃,the lowest conductivity is measured for Ba₈CoNb₄Ta₂O₂₄(2.6×10^-9～6.7×10^-6 S/cm).2)For（1-x）Ba₈CoNb₆O₂₄-xBa₃CoNb₂O₉（x=0-1）materials,a)The precise ratio of Ba₃CoNb₂O₉ and Ba₈CoNb₆O₂₄ in each component was determined by Rietveld structural refinement of the XRD data.b)The resonant frequency temperature coefficientτ_f can be effectively adjusted by adjusting the ratio of the cubic and the hexagonal phase,the value ofτ_f,?and Q×f are-6.82～20.7ppm/℃,21.6～35.1 and 3769.2～57171.6GHz,respectively.c)The bulk response of two-phase depends on the content of Ba₃CoNb₂O₉,For x=0.49 composition,the bulk conductivity is 3.1×10^-8～1.4×10^-4 S/cm at 300℃-800℃,significantly higher 1～2orders of magnitude than the conductivity of the x=0.2 component sample.3)For Ba₈Zn_1-xM_xNb₆O₂₄（M=Ni,Cu,x=0.2-0.8）.a)only the sturcture of Ba₈Zn_0.8Ni_0.2NbO₂₄is pure 8-layered shifted hexagonal perovskite,for x=0.4-0.6 materials,the main phase is matched well with the 8-layered shifted hexagonal structure and contains some impurity of Ba₅Nb₄O₁₅.b)The bulk conductivity for Ba₈Zn_0.8Ni_0.2NbO₂₄ pure shifted hexagonal perovskite ceramic is 4.93×10^-9～2.72×10^-5 S/cm at 350℃-800℃.c)For Ba₈Zn_1-xCu_x NbO₂₄,It is found that all compositions are 8-layered twinned structure including a small amount of Ba₅Nb₄O₁₅5 phase,and the amount of impurity Ba₅Nb₄O₁₅ increases with increasing of x.