Studies On The Low-temperature Sinterable ZnO-TiO₂-based Microwave Dielectric Ceramics

Three zinc titanate compounds have been confirmed to exist in ZnO-TiO₂ system,namely,Zn₂TiO₄ (cubic),ZnTiO₃ (hexagonal)and Zn₂Ti₃O₈ (cubic).Due to its relativelylow sintering temperature and excellent microwave dielectric properties,moreover,for thelow cost of its raw materials,ZnO-TiO₂ ceramics has been considered to be a promisingcandidate for low temperature co-fired ceramics (LTCC),and then more and moreinteresting has been paid in the investigation of its structure,sintering behavior andmicrowave dielectric properties in recent years.However,the sintering aids ever used inZnO-TiO₂ ceramics have some shortcomings,in addition,ZnTiO₃,which has high Qfvalue at microwave frequencies,has unsuccessfully been prepared by solid state methodso far.Thus,the main purposes of this dissertation were to investigate the correspondingquestions of the aforementioned two aspects.In this dissertation,some basic questions about microwave dielectric ceramics(MWDC)were summarized,as well as the low temperature sintered MWDC and itsnewest development.A comprehensive survey of the synthesis,phase composition,structure and microwave dielectric properties of ZnO-TiO₂ ceramics were also made.ZnO-TiO₂ ceramics doped separately with V₂O₅,MoO₃ and 0.25CuO-0.75MoO₃ (inmole)were prepared by traditional fixed oxides method (i.e.solid state method),thesintering,phase composition,microstructure and microwave dielectric properties of theceramics prepared were characterized or measured by means of XRD,SEM,EDS and anet analyzer.As a result,pure ZnO-TiO₂ ceramics sintered at 1150℃for 4 h could get arelative density value of 94%,and exhibited microwave dielectric properties as follows:∈_r=25.17,Qf=7577 GHz,Ï„_f=+61 pprn/℃;The addition of V₂O₅ could effectivelyenhance the densification of ZnO-TiO₂ ceramics at lower sintering temperatures.ZnO-TiO₂ ceramics doped with 2 wt% addition of V₂O₅ sintered at 900℃for 4 h couldreach a relative density value of 95.3%,however,the microwave dielectric properties were seriously degraded (ε_r=23.32,Qf=6528GHz);MoO₃ was less effective than V₂O₅ in theenhancement of the densification of ZnO-TiO₂ ceramics,the addition of MoO₃,however,could improve the microwave dielectric properties of the ceramics matrix.With 4 wt%MoO₃ addition,the ZnO-TiO₂ ceramics sintered at 1050℃for 4 h could reach a relativedensity value of 91%,andε_r=25.65,Qf=9881GHz,Ï„_f=+4 pprn/℃;The addition of0.25CuO-0.75MoO₃ could improve both the densification and microwave dielectricproperties of ZnO-TiO₂ ceramics.With 4 wt% 0.25CuO-0.75MoO₃ addition,the ZnO-TiO₂ceramics sintered at 975℃for 4 h could get a relative density value of 94%,and exhibitbetter microwave dielectric properties:∈_r=28.60,Qf=12150 GHz,Ï„_f=+17.8 ppm/℃;Zinc metatitanate,ZnTiO₃,starts to decompose into zinc orthotitanate (Zn₂TiO₄)andrutile at a sintering temperature of around 900℃,thereby leading to a difficulty in thesolid state synthesis of pure ZnTiO₃.There always exist considerable amounts of Zn₂TiO₄and rutile in the zinc titanate powders prepared.In this dissertation,the preparationprocessing and technique were investigated to synthesize pure ZnTiO₃ powder or at least azinc titanate powder with a higher mass fraction of ZnTiO₃.In details,commercial rutileand ZnO powders were used as precursors to synthesize zinc titanate powder via loweringthe calcining temperature,prolonging the soaking time and a multi-step calcining.As aresult,a zinc titanate powder with a 95.71% mass fraction of ZnTiO₃ was obtained.Incomparison,the mass fraction of ZnTiO₃ was only 66.15 % in the zinc titanate powdersintered at 800℃for 4 h with the same precursors.Anatase TiO₂ powders with different grain size,were employed to synthesize zinctitanate powders in a mole ratio ofZnO:TiO₂=1:1.The experimental results demonstratedthat,the existence of ZnO could lower the onset temperature of anatase to rutile transition.Meanwhile,a grain size effect in ZnO-TiO₂ system was found out for the first time,i.e.thelarger the mean grain size of precursor anatase TiO₂ powder,the less the mass content ofZnTiO₃ in the zinc titanate powder prepared.Conversely,the smaller the mean grain sizeof precursor anatase TiO₂ powder,the higher the mass content of ZnTiO₃ in the zinc titanate powder prepared.The above grain size effect was explained in this dissertation,onthe basis of which,an inference was drawn as follows:if the mean grain size of aprecursor anatase TiO₂ powder was small enough,very likely,ZnTiO₃ would not occur inthe zinc titanate powder prepared.Three rutile and ZnO mixed powders with different particle sizes in the same moleratio of ZnO:TiO₂=1:1 (symbolize by M0,M1 and M2,respectively.The size of M0 isthe smallest,the one of M2 is the largest)were used as precursors to synthesize zinctitanate powders.As a result,in the three zinc titanate powders individually derived fromM0 sintered at 630℃for 30 h,M1 sintered at 645℃for 30 h and M2 sintered at 675℃for 30 h,no Zn₂TiO₄ was detected.However,in the three zinc titanate powdersindividually derived from M0 sintered at 640℃for 4 h,M1 sintered at 655℃for 4 h andM2 sintered at 685℃for 4 h,Zn₂TiO₄ was detected.This demonstrated that,at the samesynthesis temperature,the larger the particle size of the precursor mixed powder,thesmaller the possibility of the occurrence of Zn₂TiO₄ in the zinc titanate powder prepared.In other words,under the same reaction conditions,the larger the particle size of theprecursor mixed powder,the higher the temperature at which Zn₂TiO₄ starts to appear.There are structure similarity phenomena in ZnO-TiO₂ system,i.e.,the connectionmanner of TiO6 octahedra in the structure of ZnTiO₃ is similar with the one of rutile,andthe connection manner of TiO6 octahedra in the structure of Zn₂TiO₄ and Zn₂Ti₃O₈ issimilar with the one of anatase.Accordingly,a structure similarity effect theory was putforward that,the formation of Zn₂TiO₄ or Zn₂Ti₃O₈ is limited by the presence of anatasewhereas ZnTiO₃ is only formed in the presence of rutile.On the basis of the experimentalresults,the structure similarity phenomena and its mechanism were investigated,and somedisagreements between theory and experimental results were also interpreted.