Piezoelectric Properties And Mechanism Of Polar Ceramics Containing Sillenite Phase

Piezoelectric materials are able to convert between mechanical energy and electrical energy and are widely used in various electronic components,such as filters,motors,resonators,sensors,buzzers and ultrasonic transducers.Piezoelectric materials can comprise single crystals,ceramics,polymers and composites.Among these,ceramics have the greatest market share owing to their superior properties,relatively simple fabrication processes,low cost and multi-component.Traditionally,these materials are referred to as ferroelectric ceramics.To date,lead zirconate titanate(PZT)has been the most common piezoelectric compound,but the toxicity of the lead oxide that is used during the fabrication process is of increasing concern.Recently,the environments in which piezoelectric materials are employed have also become more challenging,as a result of the rapid development of new applications in the automotive,energy and aerospace industries.For these reasons,the fabrication of high-performance,lead-free replacements for PZT has received much attention.Our group recently discovered a new class of polar composite ceramics containing a sillenite phase that can be obtained using a standard preparation technique.These materials exhibit both direct and inverse piezoelectric effects without undergoing an electrical poling process.Previous work has shown that these sintered polar composites,including Bi12TiO20-SrTiO3,Bi12TiO20-BaTiO3,Bi12TiO20-BaSnO3,Bi12TiO20-CaTiO3 and Bi,2TiO20-Na0.5Bi4.5Ti4O15,do not contain obvious grain-oriented microstructures and that their piezoelectricity does not depend on ferroelectricity.The combined results from X-ray diffraction(XRD),differential scanning calorimetry,X-ray photoelectron spectroscopy and Raman spectroscopy have also confirmed that these substances contain amorphous phases and distorted local bonding units.Based on the theory of random networks of local bonding units in quasi-amorphous films,we proposed that the plastic flexoelectricity of the amorphous phases at grain boundaries,driven by temperature gradients,is likely the primary poling mechanism.Both on a theoretical and practical basis,it would be helpful to perform additional research of these polar ceramics,because they demonstrate low dielectric constants,high depoling temperatures,good thermal stability and multi-component.Due to limited relevant research,there are many challenges that remain regarding the development of polar composite ceramics containing sillenite phases.As an example,the piezoelectric properties and uniformity of these materials must be improved.In addition,work is need to experimentally verify the poling mechanism by which these compounds exhibit macroscopic polarity.The purpose of the work reported herein was to determine the origin of the piezoelectricity of polar composite ceramics containing sillenite compounds,to improve the piezoelectric properties of such ceramics,and to examine potential applications for these materials.The polar contributions of self-assembled impurity phases were eliminated by separately synthesizing the phases in the experimental ceramics using a solid-state reaction technique.The piezoelectric properties and uniformity of the specimens were improved by using polished sapphire crystals as the sintering substrates instead of the Al2O3 ceramic employed in our previous work.This substrate was selected because polished sapphire does not react with the materials used in this study.Sapphire also has other advantages,such as high thermal conductivity,good thermal stability and superior smoothness,which allowed improved thermal contact with the samples.The specimens were sintered using a thermal gradient method,applying a bottom-to-top axial thermal gradient field.After sintering at optimized temperatures,all the samples exhibited detectable piezoelectricity without undergoing an electrical poling process.The main focus and results of this work are as follows.(1)The effects of the Bi12TiO20 volume fraction on the structure and electrical properties of the Bi12TiO20-Na0.5Bi0.5TiO3 composite polar ceramics were systematically studied.With increases in the Bi12TiO20 content,the relative dielectric constant,?r,decreased,while the piezoelectric coefficient,d33,piezoelectric voltage coefficient,g33,and electromechanical coupling coefficient,k,increased.The corresponding maximum values of these variables were determined to be 21.5 pC/N,40×10-3 Vm/N and 10.1%,respecttively.The experimental ?r values were very close to the theoretical values calculated using the cube model,and so this model could possibly be employed as an alternate approach to predicting the dielectric constants of polar composite ceramics containing sillenite phases.In addition,these materials exhibit many useful characteristics,including zero frequency temperature coefficients near the Curie temperature,together with strong piezoelectricity,weak pyroelectricity and a high piezoelectric voltage constant.The Bi12TiO20-3Na0.5Bi0.5TiO3 samples having the largest full width at half maximum(FWHM)and I(310)/I(222)ratio obtained from XRD were found to exhibit superior performance characteristrics.The proportion of amorphous phase in each specimen was estimated based on DSC data.Data acquired from XRD,scanning electron microscopy(SEM)and Raman spectroscopy showed that the temperature gradient-driven plastic flexoelectricity observed at amorphous phase grain boundaries is likely the main poling mechanism.(2)The electrical properties of these ceramics were confirmed to vary with the Bi12TiO20 volume fraction,and the maximum values obtained for the piezoelectric coefficient,d33,and the piezoelectric voltage coefficient,g33,were 20.3 pC/N and 35.5×10-3 Vm/N,respectively.No obvious grain-oriented microstructures were identified in early studies of these materials,and so the contribution of the grain orientation to piezoelectricity has never been taken into consideration.However,in this work,grain orientation was present in samples with high proportions of Bi12TiO20,and the corresponding Lotgering factor was determined to be 59%.According to the results obtained from analyses by XRD and Raman spectroscopy,both the orientation of the grains and the partial alignment of grain boundary amorphous may contribute to the piezoelectricity of Bi12TiO20-K0.5Na0.5NbO3.phases(3)Single phase Bi12TiO20(BTO)and Bi12GeO20(BGO)ceramics were synthesized using a trial-and-error experimental process.Compared with the composite polar ceramics,these single phase ceramics were found to be more sensitive to variations in the sintering conditions,such that slight changes in the sintering temperature caused a significant decrease in piezoelectricity.Samples of BTO and BGO showing strong piezoelectricity could only be obtained by sintering at approximately 850 and 890 ?,respectively.These materials were highly homogeneous and had structures with preferentially-oriented and well-arrayed,nanometer-size subgrains.The Lotgering factors of the BTO and BGO were as high as 59%and 90%,respectively.The effects of grain orientation on the piezoelectric coefficient,d33,in crystals of these compounds were assessed,and values for the BTO and BGO ceramics were found to be higher than those of the corresponding crystals.Data acquired using XRD,SEM and Raman spectroscopy indicated that the oriented grains,together with the partial alignment of distorted Bi05 polyhedra at the grain boundaries,were responsible for the unusual piezoelectricity of the BTO and BGO ceramics.Samples of these two ceramics exhibited relatively strong piezoelectricity,low dielectric losses and low dielectric constants,along with increased electromechanical coupling coefficients and mechanical quality factors.The piezoelectric voltage coefficients,g33,of the BTO and BGO ceramics were 64× 10-3 and 50×10-3 Vm/N,respectively.(4)As a result of the effect of high temperature conductance,the piezoelectric coefficient,d33 of the BTO ceramic at low frequencies decreased at elevated temperatures.The incorporation of CaBi4Ti4O15 in Bi12TiO20(BTO)-based composite polar ceramics as a second phase is likely the best means of mitigating this effect,so as to maintain high piezoelectric and piezoelectric voltage constants.This approach is ppreferred because CaBi4Ti4O15 has relative high resistivity along with a low dielectric constant.Polar ceramics having a high Bi12TiO20 content were fabricated using a thermal gradient sintering method and theoretical dielectric constants were calculated,showing that the cube model is still applicable to such specimens.The Bi12TiO20-xCaBi4Ti4O15 polar ceramics were found to contain oriented grains,and samples exhibited relatively strong piezoelectricity(d33=24.3 pC/N),low dielectric constants(?r=52),high piezoelectric voltage coefficients(g33=52.6×10-3 Vm/N)and high mechanical quality factors(Qm=4156).The piezoelectric coefficient,d33,changed only slightly between ambient temperature and approximately 400 ?,and did not show a significant decrease even after annealing at 820 ?.The favorable piezoelectric properties and good thermal stability of Bi12TiO20-CaBi4Ti4O15 polar ceramics,which are comparable to those of the widely studied bismuth-layer structure ferroelectric ceramics,make these materials promising candidates for high-temperature piezoelectric applications.