| Piezoelectric materials have an important position in the field of informationtechnology which are widely used in various types of transducers in water, ultrasonic andelectroacoustic ones. Lead zirconate titanate(PZT) based piezoelectric ceramics are themost successful piezoceramic material in practicality and they are widely used in allkinds of fields. Although the PZT piezoelectric ceramics have excellent piezoelectricproperties, but because of its toxic components of Pb, it may cause environmentalpollution in the process of production, use and disposal. Therefore, how to improve theperformance of the lead-free piezoelectric ceramics to replace the lead basedpiezoelectric ceramics gradually is one of the important research topic in the current fieldof piezoelectric ceramic materials.Na0.5Bi0.5TiO3(NBT) is a kind of perovskite-type A-site ion co-substitutingferroelectric and its Curie point Tcis320℃, and it has strong ferroelectric in roomtemperature and is regarded as one of most promising candidate material of lead-freepiezoelectric ceramics. Potassium bismuth titanate K0.5Bi0.5TiO3(KBT) is another ABO3type lead-free piezoelectric material, has the similar structure as Na0.5Bi0.5TiO3which isbeing studied mostly at present. The structure at room temperature of KBT is tetragonalperovskite phase, KBT has more wide temperature-using interval and lower difficulty ofpolarization, thus it has broad prospect in application. NBT and KBT are regarded asmost promising candidate materials of lead-free piezoelectric ceramics. On the otherhands, The preparation method and preparation process of NBT and KBT ceramicpowder is closely related with its composition, structure and properties. Therefore,appropriate preparation method and process is the premise and basis of preparation of theexcellent piezoelectric ceramic powder, which has an important significance to improveNBT and KBT lead-free piezoelectric ceramics electrical properties.There are two preparation methods of piezoelectric ceramic materials–solid statemethod, and liquid phase method. Most of the present research choosed the solid statemethod to prepare NBT, KBT piezoelectric ceramic powder, and the research on liquidphase method of NBT, KBT ceramic powder is not deep enough. Therefore this paperstudied on process of the hydrothermal synthesis of NBT,KBT by using Bi(NO3)35H2O,Ti(OC4H9)4, NaOH, KOH as materials, The prepared ceramic powder was characterizedby XRD, SEM, FTIR, XRF to analyze composition, structure and morphology, Theprocess parameter of hydrothermal synthesis was optimized. NBT was synthesized usingthe citrate method and prepared NBT-BT6powder by substituting the instrinsic cations at the A sites of preovskite with Ba2+. One the basis of these NBT-BT6ceramic wasprepared. We studied the relationship between its piezoelectricity and ferroelectricity andits preparation method and structure.(1) Using Bi(NO3)35H2O, Ti(OC4H9)4as materials, NaOH as mineralizing agentand sodium source to synthetize NBT powder of different sizes and morphologiesthrough controlling hydrothermal reaction process parameters. The results showed thatthe process of hydrothermal synthesis NBT powder was simple, high temperaturecalcination was not necessary; the chemical composition of the obtained powder wasbasically in accordance with the theoretical phase composition of Na0.5Bi0.5TiO3, thepowder was spherical shape, diameter of about0.2-0.5μm, or a cube shape, particle sizeof about3-5μm; In addition to the influence of parameters such as hydrothermaltemperature, hydrothermal time and concentration of mineralize agent, Bi/Ti mole ratioalso has a significant influence on composition, structure and morphology of the NBTpowder. The result showed that slightly excessive Bi in precursor(the Bi/Ti ratio iscontrolled at0.5~0.6) would benefit the formation of single-phase perovskite structure.(2) Achieving of different morphologies of NBT power is the result of combinedworking of "in-situ crystallization" and "dissolution-crystallization" mechanism, and thesaturation level of reactant is the critical point for working of the two mechanisms. In thecase of reactant saturation level lower than the critical point, in-situ crystallizationmechanism takes a dominant role, and the products are mainly spherical particles oragglomerating particles, while in the case of reactant saturation level higher than thecritical point, dissolution-crystallization mechanism takes a dominant role, and theproducts are cubic particles.(3) Using Bi(NO3)35H2O, Ti(OC4H9)4as materials, KOH as mineralizing agent andpotassium source to synthetize KBT piezoelectric ceramic powder through hydrothermalmethod. The results showed that the hydrothermal temperature, hydrothermal time andwash medium had a significant influence on composition, structure and size distributionof the KBT powder. Under the condition of [Ti]=0.5mol/L, Bi/Ti=0.5, mineralizer KOHstaying at a density of12mol/L, hydrothermal temperature200℃for lasting36hours, asingle phase KBT could be achieved, ultrasonic water cleaning reduced agglomeration ofthe powder and its particle size was the smallest.(4) Using NaNO3, Bi(NO3)35H2O, Ti(OC4H9)4as materials to synthetize NBTpiezoelectric ceramic powder through the citrate method. The results showed that the pHvalue of the precursor solution had a great influence on structure and the micro structureof powder and the distribution of particle size. Under conditions of n(citricacid)/n(nitrates)=1.25/1, environment temperature with80℃，calcinations with600℃for1h, the reaction was incomplete and the production could hardly form the perovskitestructure, rod titanate crystal which rich in Na、Ti、O was obtained while pH=2.5；largerparticle with main Na0.5Bi0.5TiO3phase was gained under pH=6.5, the powder wasspherical crystallites of about500~600nm with a small amount of large particle, the particle size distribution was not uniform；the homogeneous Na0.5Bi0.5TiO3powder wasobtained under pH=8.5and the spherical particle size was about100~200nm；with pHincreasing to10.5, Bi3Ti4O12phase was appeared in Na0.5Bi0.5TiO3crystal as impurity, theparticle size distribution was not uniform.(5) Studied the relationship between the electrical properties and the sinteringtemperature of NBT-BT6piezoelectric ceramics prepared by citrate method. Whilesintering temperature between1120-1160℃, crystal structure analysis of ceramicsindicated that there was the morphotropic phase boundary (MPB) with rhombohedral andtetragonal co-existed crystalline structure in the sample. NBT-BT6ceramic sampleshrinked uniformly and densely while sintered at1140℃for2hours, The density of theceramic reached to maximum value5.53g/cm3; Its relative dielectric constant reached tomaximum value509; Its planar electromechanical coupling coefficient Kpreached to0.255; Its piezoelectric constant d33reached to105pC/N; And it had the least dielectricloss and best comprehensive performance. Incorporation of BT effectively reducesceramic’s coercive field, the minimum value of coercive field is42.9kV/cm. it is muchlower than that of the pure NBT ceramic’s coercive field73kV/cm. The results show thatthe variation between ferroelectric properties and piezoelectric properties of theNBT-based ceramics has intrinsic link. |
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