Study On Properties Of Textured Lead-free Piezoelectric Ceramics By Aqueous Tape-casting Process

In this dissertation, KxNa1-xNbO3(abbreviated as KNN) ceramics and(Na0.8K0.2)0.5Bi0.5TiO3(abbreviated as NKBT) were selected as theinvestigation object， Firstly, The Sr2Bi4Ti5O18（abbreviated as S2BT）plate-like particles and Nb2O5plate-like particles were synthesized by moltensalt method and proton replace method,respectively,and then slurries wereobtained by using S2BT and Nb2O5plate-like powder as template particles，KNN and NKBT prepared by solid state reaction method as base materials，deionized water as the solvent and adding all sorts of function reagents，Finally,The KNN and NKBT textured lead-free piezoelectric ceramics wereobtained by green body which was fabricated by tape casting processing afteraminating hot-pressing and sintering.The S2BT plate-like particles and Nb2O5plate-like particles weresynthesized by molten salt method and proton replace method，respectively:Firstly, High-anisotropic plate-like Sr2Bi4Ti5O18micro-crystals were preparedby molten salt method. The X-ray diffraction (XRD) and scanning electronmicroscopy (SEM) were employed to characterize the crystal structure andmicroscopic morphology of the reaction products. It was found that the pureSr2Bi4Ti5O18crystal can be prepared at1100oC for3h when the weight ratioof the molten salt to total reactants is1:1. The SEM results show theanisotropic Sr2Bi4Ti5O18crystals are plate-like with5-25μm in edge lengthand0.2μm in thickness. The XRD results show the plate-like Sr2Bi4Ti5O18crystal are prefer to grow on (00l) plane,Secondly, Using reagent grade oxideor carbonate powders of K2CO3and Nb2O5as raw materials,NaCl as moltensalt,The K4Nb6O17powder were prepared by molten salt method when theweight ratio of the molten salt to total reactants is1:1,and then putting theK4Nb6O17powder into6M HCl proton exchanging for50h,after thenexcellent plate-like Nb2O5particles which possesses a length of35~60μm, awidth of15~40μm and a thickness of about1μm can be fabricated at450oCfor3h.Green body was fabricated by aqueous tape-casting process: Using Nb2O5plate-like particles as template, potassium sodium niobate KNN and Sodiumbismuth titanate NKBT particles prepared by solid state reaction method asbase materials, the slurry was obtained by adding moderate dispersingagent,binder,plasticizer and defoamer,and then Green body was fabricated by aqueous tape-casting process. The influence of dispersing agent,binder andplasticizer content on the properties of the slurry was investigatedsystematically. The results showed that uniform, diversified, moderateviscosity KNN slurries with55wt%solid content were obtained with5.5wt%PVP,13.5wt%styrene-acrylic emulsion and5.5wt%glycerol,and the betterplastic, smooth surface and indehiscent green body were obtained when usingaqueous tape-casting process,and when the content of triethanolamine,styrene-acrylic emulsion and glycerol were5wt%,14wt%,0.4(R: the ratio ofbinder and plasticizer dosage) respectively, the uniform, diversified, moderateviscosity NKBT slurries and the better plastic, smooth surface and indehiscentgreen body can be obtain.Study on Properties of KNN Textured Lead-free PiezoelectricCeramics:Using S2BT plate-like particles as template, KNN textured ceramicwere fabricated by template grain growth technology. The results showed thatWith the increase of temperature, the ceramic line shrinkage and bulk densityfirst increases then decreases, The maximum line shrinkage and bulk densityof KNN textured ceramic can be obtained sintering at1120oC with5hsoaking and The same rule in orientation degree of the KNN textured ceramic(f=0.55) can be found.Piezoelectric properties and dielectric properties ofthe textured ceramic can be obtained when sintering temperature and soakingtime were,1120oC and5h, respectively. On parallel tape-casting direction thepiezoelectric constant d33reaches103pC/N, dielectric constant εT33/ε0is486,planar electromechanical coupling factor kpis39.4%, remanentpolarization P2ris41.46μC/cmand coercive field Ecis1.96kV/mm,and onperpendicular tape-casting direction the d33=72pC/N, εT33/ε0=456andkp=35.5%, Pr=30.15μC/cm2，Ec=1.89kV/mm,respectively.Using Nb2O5plate-like particles as template, potassium sodium niobateK0.5Na0.5NbO3(KNN) particles prepared by solid state reaction method asbase materials, KNN textured ceramic with high orientation degrees werefabricated by tape casting process. The influence of template content,sintering temperature and soaking time on the structure and piezoelectricproperties and dielectric properties of the ceramic was investigatedsystematically. The results showed that the optimized orientation degree(orientation degree f=0.78), piezoelectric properties and dielectric propertiesof the textured ceramic can be obtained when the template content, sinteringtemperature and soaking time were10wt%,1120oC and5h, respectively. On parallel tape-casting direction the piezoelectric constant d33reaches141pC/N,dielectric constant εT33/ε0is503and planar electromechanical coupling factorkpis39.7%, remanent polarization Pris32.30μC/cm2and coercive field Ecis1.57kV/mm and on perpendicular tape-casting direction the d33=112pC/N,εT33/ε0=454and kp=37.5%, Pr=28.95μC/cm2，Ec=1.86kV/mm,respectively.Study on Properties of NKBT Textured Lead-free Piezoelectric Ceramics:Using the Nb2O5plate-like particles synthesized by proton replace method andmolten salt method as template, sodium-bismuth titanate(Na0.8K0.2)0.5Bi0.5TiO3(NKBT) particles prepared by solid state reactionmethod as base materials，the high orientation degrees textured ceramic werefabricated by solid phase forming technology (dry method) and tape castingforming technology (wet method), respectively; The influence of dry and wetmethod on the microstructure, dielectric and piezoelectric properties of theceramic was studied. The results showed that the piezoelectric properties anddielectric properties of the textured ceramic by wet method were better thanthe textured ceramic by the dry method, optimized orientation degree(orientation degree f=0.66) can be obtained sintering at1150oC with5hsoaking by wet method, and it has better piezoelectric properties anddielectric properties as follows: with wet method, the piezoelectric constantd33reaches149pC/N, dielectric constant εT33/ε0is912, electromechanicalcoupling factor kpis29.4%, remanent polarization Pris18.18μC/cm2andcoercive field Ecis2.87kV/mm; with dry method, the d33=129pC/N,εT33/ε0=746，k2p=28.7%，Pr=15.96μC/cm，Ec=2.91kV/mm.