Research on Preparation and Property Modification of New Niobate-Based Microwave Dielectric Ceramics

Author:Yang

Supervisor:zhang shu ren

Database:Doctor

Degree Year:2019

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Pages:134

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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 ofεr=15.7,Q×f=28,700 GHz,τf=-38.4 ppm/℃.By compounding CCN and TiO2,theτf of composite ceramic had been made to close to zero.The 0.91CCN-0.09TiO2 ceramic sintered at 920℃had excellent properties ofεr=16.9,Q×f=21,500 GHz,τf=-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:α-GaNbO4(monoclinic,space group P2/c(13),Z=2)andβ-GaNbO4(monoclinic,space group C2(5),Z=4).The contents ofαandβGaNbO4 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 ofεr=15.8,Q×f=101,000 GHz,τf=-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 ofεr=14.1,Q×f=17,640 GHz,a big positiveτf=+39.3 ppm/℃.While among the niobite-based ceramics,the low-firing Li3NbO4 exhibited excellent values ofεr=15.8,Q×f=55,000 GHz,a big negativeτf=-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 Li2+yZr1-4yNb3yO3(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 ofεr=21.3,Q×f=43,600 GHz,τf=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 ofεr=20.8,Q×f=50,200 GHz,τf=-2.2 ppm/℃.DyZrNbO6 and HoZrNbO6 in region 2 also exhibited excellent low-loss microwave dielectric properties.DyZrNbO6:εr=25.3,Q×f=56,100 GHz,τf=-11.3 ppm/℃.HoZrNbO6:εr=25.4,Q×f=62,300 GHz,τf=-24.5 ppm/℃.