Research On Preparation And Property Modification Of New Niobate-Based Microwave Dielectric Ceramics

Since Dr.Robert proposed dielectric materials into microwave communication technology in 1939,microwave dielectric ceramics have rapidly developed for about 80years.Over the past decade,the number of papers published by Chinese researchers in more than 30 world mainstream journals in this field has exceeded that of the traditional leading countries such as United States,Japan,Germany and so on.And a number of high-quality research results have emerged.With the changing requirements of microwave communication technology for materials,Chinese researchers have mainly carried out researches in following directions:1.Expanding dielectric constant to meet design requirements of various types and sizes of microwave components.2.Reducing dielectric loss to enhance frequency selectivity of materials.3.Making temperature coefficient of resonant frequency close to zero,so as to improve temperature stability of materials.Following on above directions,and on the basis of the concern,analysis and summary of various existing niobite-based ceramics,this paper focused on developing new materials.Finally,a series of new niobite-based ceramics with excellent microwave dielectric properties were developed:1.A new niobate-based microwave dielectric ceramic CaO-2CuO-Nb₂O₅?CCN?was developed.In previous studies,it was found that the doping of CuO had effectively reduced the sintering temperature of ZnNb₂O₆ ceramic and resulted in the formation of a low-firing mesophase Zn?Cu?₂NbO₈.Inspired by the mesophase M²⁺?Cu?₂NbO₈,a low-cost and low-firing niobate-based ceramic Ca?Cu?₂NbO₈?CCN?for LTCC applications was sought.Hence,for the first time,CCN ceramic was prepared by solid state reaction method,and its phase composition,micro-morphology and microwave dielectric properties were investigated.The CCN ceramic sintered at 870?had good microwave dielectric properties of?_r=15.7,Q×f=28,700 GHz,?_f=-38.4 ppm/?.By compounding CCN and TiO₂,the?_f of composite ceramic had been made to close to zero.The 0.91CCN-0.09TiO₂ ceramic sintered at 920?had excellent properties of?_r=16.9,Q×f=21,500 GHz,?_f=-1.6 ppm/?.2.A new low-loss niobate-based microwave dielectric ceramic GaNbO₄ was developed.It was found that,neither in niobate-based ceramics(RE³⁺NbO₄,M²⁺Nb₂O₆,(M²⁺)₃Nb₂O₈,(M¹⁺)₃NbO₄,M²⁺M⁴⁺Nb₂O₈,etc.)nor in gallium-based ceramics?ZnGaO₄,MgGaO₄,etc.?,had GaNbO₄ been involved and reported.It might be that Ga³⁺didn't belong to lanthanide rare earth RE³⁺,nor to common trivalent metals such as Al³⁺and Fe³⁺,which lead to the neglect or oblivion of GaNbO₄ ceramic.Hence,for the first time,GaNbO₄ ceramic was synthesized by solid state reaction method,and its sintering temperature,bulk density,relative density,phase composition,phase content,atomic packing fraction,crystal structure,cell parameters,micro-morphology and microwave dielectric properties were investigated.XRD results showed that GaNbO₄ ceramic existed as a composite of two allomorphic GaNbO₄ phases:?-GaNbO₄?monoclinic,space group P2/c?13?,Z=2?and?-GaNbO₄?monoclinic,space group C2?5?,Z=4?.The contents of?and?GaNbO₄ phases changed with the change of sintering temperature.Micro-morphology also showed that GaNbO₄ ceramic had two kinds of grains with different shapes and sizes.The GaNbO₄ sintered at 1060?exhibited superior properties of?_r=15.8,Q×f=101,000 GHz,?_f=-63.4 ppm/?.3.A new low-firing,low-loss and temperature-stable niobite-based microwave dielectric ceramic Li₉Zr₃NbO₁₃ was developed.Among the series of existing lithium-based ceramics?Li₃AlB₂O₆,Li₂WO₄,Li₂ZrO₃,etc.?,Li₂ZrO₃ had good properties of?_r=14.1,Q×f=17,640 GHz,a big positive?_f=+39.3 ppm/?.While among the niobite-based ceramics,the low-firing Li₃NbO₄ exhibited excellent values of?_r=15.8,Q×f=55,000 GHz,a big negative?_f=-49 ppm/?.Therefore,it was speculated that there should exist a low-firing,low-loss and temperature-stable ceramic between Li₂ZrO₃ and Li₃NbO₄ in Li₂O–ZrO₂–Nb₂O₅ system.With the purpose of searching for this ceramic,the composite ceramic system?1-x?Li₃NbO₄-xLi₂ZrO₃?0?x?1?was prepared for the first time.And its preparation process,bulk density,relative density,atomic packing fraction,phase composition,phase content,crystal structure,cell parameters,micro-morphology and microwave dielectric properties were systematically investigated.This ceramic system had high selectivity and sensitivity to sintering temperature.The XRD results showed that while Li₂ZrO₃ formed an extensive range of solid solutions,Li₃NbO₄ formed little or no solid solutions.The expression of the solid solution was Li_2+yZr_1-4yNb_3yO₃?0<y?0.16?.The mechanism of ion substitution in solid solution was 4Zr⁴⁺?Li¹⁺+3Nb⁵⁺.In?1-x?Li₃NbO₄-xLi₂ZrO₃ ceramic system,the desirable ceramic Li₉Zr₃NbO₁₃ was found at x=0.75.The Li₉Zr₃NbO₁₃ formed as a Li₂ZrO₃-type solid solution at 880-900?,with monoclinic structure in C2/c?15?space group and Z=4.Typically,the Li₉Zr₃NbO₁₃ sintered at 900?exhibited excellent microwave dielectric properties of?_r=21.3,Q×f=43,600 GHz,?_f=7.3 ppm/?.4.A new niobite-based microwave dielectric ceramic system REZrNbO₆ was developed.In extensive studies of an existing niobite-based ceramic system RETiNbO₆,main focus was always on ion substitution of RE³⁺and Nb⁵⁺,and REZrNbO₆ had never been involved or reported.REZrNbO₆ had blank information on phase composition,crystal structure,micro-morphology and microwave dielectric properties in a wide range of databases.Hence,for the first time,REZrNbO₆ ceramics were synthesized by solid state reaction method.The sintering temperature,bulk density,relative density,phase composition,phase content,crystal structure,cell parameters,micro-morphology and microwave dielectric properties of REZrNbO₆ ceramics were systematically investigated.The results showed that the Hume-Rothery law of the formation of solid solution had been vividly reflected and verified in REZrNbO₆ ceramic system.According to the radius of RE³⁺,REZrNbO₆ ceramics could be divided into two regions.Region 1:?.Region 2:<.REZrNbO₆?RE=La,Nd,Sm,Gd?in region 1 formed as two-phase composite ceramic of RENbO₄ and ZrO₂.While REZrNbO₆?RE=Gy,Ho?in region 2 formed as pure phase ceramic of RENbO₄ type solid solution.The ion substitution mechanism in solid solution was RE³⁺+Nb⁵⁺?2Zr⁴⁺.A new temperature-stable and low-loss niobite-based ceramic GdZrNbO₆ had been found in region I,it had superior microwave dielectric properties of?_r=20.8,Q×f=50,200 GHz,?_f=-2.2 ppm/?.DyZrNbO₆ and HoZrNbO₆ in region 2 also exhibited excellent low-loss microwave dielectric properties.DyZrNbO₆:?_r=25.3,Q×f=56,100 GHz,?_f=-11.3 ppm/?.HoZrNbO₆:?_r=25.4,Q×f=62,300 GHz,?_f=-24.5 ppm/?.