Study Of Combined Synthesis Methods And Structure For Producing Rare-Earth Orthoniobates Ceramics

The researches about low-temperature sintering of the rare-earth ortho-niobates ceramics（RENbO₄）usually concentrated in the low-temperature co-fired ceramics（LTCCs）and the chemical processes.The addition of those LTCCs could greatly reduce the sintering temperature,while LTCCs always led to negative effects in the microwave dielectric properties due to the introduction of impurities.The chemical processes（such as sol-gel process）normally have a low yield and were not suitable for mass production.Therefore,our work focused on the improvement of the microwave dielectric properties by adding nano-sized ceramic powders and the amelioration of the sintering process in order to reduce the sintering temperature without introducing impurities,as detailed below:1.Firstly,the preparation process of the nano-sized RENbO₄ ceramic powders by sol-gel method was studied.The niobium oxalate was used as raw material for the preparation of La NbO₄,Nd NbO₄ and Sm NbO₄ nano-sized powders.Subsequently,the nanopowders were mixed in a certain mass ratio with the corresponding micro-sized ceramic powders.Thus the RENbO₄ ceramics were successfully obtained at lower temperature by more than 100℃comparing with the ceramics prepared by conventional solid-state method.2.The lattice parameters of La NbO₄ and Nd NbO₄ ceramics were obtained by Rietveld refinement,and the bond ionicity,lattice energy as well as bond energy were calculated by the complex crystal bond theories.Through the calculation and actural tests,we found that the dielectric constantε_r was positively correlated with the bond ionicity fi,the quality factor Q×f₀ was positively correlated with the lattice energy U,and the absolute value of the temperature coefficient of the resonance frequency|τ_f|was inversely related to the bond energy E.In addition,the effect of the microwave dielectric properties by the variation of relative densitiesρ_relativewas analyzed after the testing of Sm NbO₄ ceramics.3.The densities of the RENbO₄ ceramics decreased under the condition of high sintering temperature or large amount of nanopowders.With the experimental experience of the La NbO₄ ceramics,the densities of Nd NbO₄ ceramics were initially improved by extending the holding time and adding process for the excretion of paraffin.Subsequently,Sm NbO₄ ceramics were prepared by a new two-step sintering process,which better promoted the grain growth with the addition of the nanopowders,and obtained densely grown Sm NbO₄ ceramics at lower temperatures.