Study On Morphology Regulation And Dispersion Of Tetragonal Phase Barium Titanate Powder By Hydrothermal Synthesis

With the trend of high capacity,lightweight and miniaturization of multilayer ceramic capacitor（MLCC）devices,the market has higher requirements for barium titanate（BT）powder.The tetragonality（c/a）,dispersion,particle size and morphology of barium titanate powder have an important influence on the sintering performance and dielectric performance of MLCC.The synthesis of barium titanate by hydrothermal method has the advantages of high purity,no secondary ball grinding and small particle size.However,most BT powder synthesized by hydrothermal method are mixed phases of cubic phase and tetragonal phase（the c/a value is between 1.0070 and 1.0090）,and the agglomeration of nanopowder results in uneven size distribution.These defects restrict the application and development of hydrothermal synthesis BT powder.Therefore,this paper use two-step hydrothermal method with Ba（OH）₂·8H₂O-Ti O₂-NH₄OH system as the basic raw material to explore the reaction temperature,reaction time,barium titanium ratio,titanium source type,titanium dioxide size and other conditions on the tetragonality of synthetic powder and compare the characteristics of the synthetic particles,initially optimize the optimal synthesisi conditions for high tetragonality BT powder.Then we explore and compare the effects of single dispersant Polyvinyl pyrooxinone（PVP）,hexadecyl trimethyl ammonium bromide（CTAB）and combined dispersants（PVP/CTAB）on BT the powder dispersion,particle size and morphology.Finally we proposed an combined optimization mechanism for the improved size and dispersion of BT powder.The study conclusions are as follows:1.In the two-step hydrothermal method,as the temperature increases from 200℃to 240℃and the time increases from 12 h to 36 h,the powder particles gradually change from hollow structure to solid structure,morphology becomes regular,the size and tetragonality increases.The increased barium titanium ratio conducive to competitive nucleation,leading to size reduction;Barium concentration increases,barium ion diffusion increases,barium vacancy reduces,favoring the synthesis of tetragonal phase BT powder with increasing c/a.Using large size of anatase phase Ti O₂ as titanium source can obtain high tetragonality BT powder,while small size Ti O₂ owns low crystallization,which is difficult to synthesize higher tetragonality BT.Rutile phase Ti O₂ inhibits to synthesize tetragonality BT powder.When metatitanic acid and tetrabutyl titanate use as titanium sources,these titanium sources contain more hydroxyl groups,which is not conducive to dehydrogenation process,and the tetragonality is small.Therefore,in the two-step method,the large size（191/88 nm）,anatase phase,TiO₂ is used as the titanium source,Ba（OH）₂·8H₂O is used as the barium source,alcohol water ratio is 1:1,barium titanium ratio is 4:1,ammonia addition amount is 142 m L/L,the first reaction conditions are 100℃,10 h and the second reaction conditions is 240℃,48 h.The obtained BT powder with high tetragonality c/a of 1.0101,which is close to the ideal tetragonality of 1.011.2.After the successful synthesis of the high tetragonal BT powder,the effects of single dispersant PVP,CTAB and combined dispersants PVP/CTAB on the tetragonality,dispersion and size control of synthetic BT powder are compared.The results show that in the absence of dispersants in the two step hydrothemal,there are defects including poor dispersion and big size.The dispersion performance of the powder is greatly improved when a single dispersant is used.The particle size distribution curve changes from bimodal to single peak,the size and tetragonality is also reduced.With the addition of combined dispersants,the pdi value of BT powder smaller,dispersion becomes better and the particles’size and tetragonality is further reduced.3.After the functional group,coating thickness and element distribution of the powder surface were analyzed by infrared characterization and transmission test,the advantages of the dispersion mechanism of the combined dispersants are revealed:macromolecule PVP relies on hydrogen bond to adsorb on the powder surface,and small molecule CTAB relies on electrostatic adsorption force cover the powder to achieve dispersion and inhibit the growth effect.However,the winding of macromolecules itself hinders the efficiency of hydrogen bond formation,this dispersion effect needs to be improved,the simple force of small molecules is easy to be destroyed in high temperature environment,the powder’s dispersion and size control are not ideal;When micromolecules and macromolecules are used together,small molecules,on the one hand,act as“lubricants”to reduce the winding of macromolecules and promote the formation of more hydrogen bonds of macromolecules pvp.On the other hand,small molecules can also locally coat the powder to strengthen the powder’s size and dispersion control effect.