Study On Surface Effect Of Ferroelectric Ceramics And Silicon Crystals

Ferroelectric materials possess a characteristic of large dielectric response,piezoelectric effect and pyroelectric effect because of spontaneous polarization,and have a wide range of applications in capacitors,sensors,actuators,transducers,memory and other devices.In addition,a larger flexoelectric response has been measured in ferroelectric materials,compared with that in other ordinary dielectric materials.However,the measured flexoelectric coefficients are several orders of magnitude larger than the theoretically predicted values in ferroelectric materials.For the issue,several mechanisms were proposed to explain the large difference between the measured flexoelectric coefficients and the theoretically predicted values in ferroelectric materials,but are still elusive.Recently,a spontaneously polarized layer in ferroelectric ceramics was found by our research group,and was demonstrated to have a large contribution to the large flexoelectric response of ferroelectric ceramics.However,it is not clear the extent to which the surface effect contributes to the measured flexoelectric response.Therefore,we studied the contribution of the surface effect to the flexoelectric response of ferroelectric ceramics in this thesis.We also further researched the surface effect on the weak-field dielectric constant,ferroelectricity and piezoelectric response of ferroelectric ceramics in this thesis.Besides the large flexoelectric response in insulating ferroelectric materials,a large flexoelectric response has also been measured in semiconductor materials,and was thought to be from a piezoelectric response of surface depletion layer,which also incompletely explains the large flexoelectric response in semiconductor materials.Therefore,in this thesis,we also studied the surface effect on the flexoelectric response of semiconductor materials in depth.Among the ferroelectric materials,ferroelectric ceramics are popular because of their high polarization and simple preparation process.In response to the environmental protection concept,the surface effect of lead-free ferroelectric ceramics on the flexoelectric response,weak-field dielectric constant,ferroelectricity and piezoelectric effect was studied.For the semiconductor materials,Si crystal,a typical semiconductor material,was studied for surface effect on the flexoelectric response.Firstly,in order to explore the contribution of spontaneously polarized surface layer to the measured flexoelectric response in ferroelectric ceramics,we measured the flexoelectric response of(Ba,Sr)TiO3 and Na0.5Bi0.5TiO3-based ferroelectric ceramics with sputtered or fired-on Au and Ag/Pd electrodes,and found that for all the ceramics,the flexoelectric response of the same material is strongly dependent on the methods used to fabricate the electrodes.It showed the samples with fired-on electrodes have a much smaller flexoelectric response than those with sputtered electrodes,and the flexoelectric response can be reduced by more than 99%in Ba0.75Sr0.25TiO3 ceramics.The reduction of flexoelectric response is attributed to the suppression of the contribution from the spontaneously polarized surface layers on ferroelectric ceramics to the flexoelectric response.Three reasons were proposed for the suppression of surface effect,namely,the diffusion of some substance of electrode pastes to the surface regions of ferroelectric ceramics,stress relief,and the generation of impurity phases caused by the evaporation of volatile element during the firing of electrode materials at a high temperature(impurity phase is easily produced in Na0.5Bi0.5TiO3-based ferroelectric ceramics).We also found that the flexoelectric response can approach the theoretically predicated response after the surface effect is reduced by fabricating the fired-on electrodes on ferroelectric ceramics,which indicates the polarized surface layer is the main contribution to the large flexoelectric response of ferroelectric ceramics,and is the primary mechanism leading to the deviation of measured flexoelectric response from the intrinsic response in ferroelectric ceramics.This work is helpful to resolve the mechanism controversy of the large difference between measured and theoretical flexoelectric response.We also studied the effect of spontaneously polarized surface layers(strain gradient surface layers)on weak-field dielectric constant,ferroelectricity and piezoelectric response in ferroelectric ceramics.We observed a similar scaling effect in BaTiO3 and Ba0.83Sr0.17TiO3 ceramics with a sub-millimeter thickness,as manifested by a reduction of weak-field dielectric constant near the Curie temperature,an increase of the coercive field,and a reduction of piezoelectric response with a reduction of sample thickness,especially for the ceramic with a thickness below 0.5 mm.We propose that the scaling effect could be explained by the presence of the spontaneously polarized surface layers with a stabilized ferroelectric phase and a higher coercive field.Because the thickness of the surface layers is typically?10 ?m,our results indicate the surface effect may not be negligible when the thickness of the ferroelectric materials is in millimeter or sub-millimeter scale.This study is important for the understanding of the scaling effect observed in ferroelectric ceramics.Lastly,we studied the surface effect on felxoelectric response of Si crystals.It is found in this work that Si crystals exhibit a large piezoelectric-like response under bending deformation.We show that the piezoelectric-like response is not from felxoelectric response of Si crystals and piezoelectric response of surface depletion layer,but rather originates from the piezoelectric response of surface natural oxide layers on Si crystals that are slightly oxidized upon their exposure to air.We also found the piezoelectric-like response is strongly dependent on the conduction type and the resistivity of the Si crystals,and on the metal electrode used for piezoelectric measurements,which affect the Schottky depletion layers between Si crystals and electrodes.In addition,the weak piezoelectric response of the surface layer is amplified by high conductivity of the bulk and bending deformation of Si plates.A multilayer model is proposed to explain the piezoelectric-like response of the Si crystals.This finding may be exploited to design piezoelectric devices using Si only,without the integration of additional piezoelectric materials,and is important for understanding the Si crystal surfaces.