Study On The Composition Design,Piezoelectric And Strain Characteristics Of BaTiO₃-based Lead-free Ceramics

Ferroelectric ceramics,which can produce an obvious displacement under electric field loading,have been widely used in various precision positioning technologies as actuator materials.Generally,ceramics for actuator applications are required to exhibit large strains under an external electric field.So far,most of actuator materials have been based on various lead-based ferroelectric ceramics.However,due to the harm of lead to the ecological environment and human health,the development of lead-free alternatives has attracted more and more attention.Ba Ti O₃（BT）,which is a typical lead-free ferroelectric material with excellent ferroelectric and piezoelectric properties,is an ideal choice for preparing lead-free ferroelectric ceramic actuator.In this thesis,three kinds of BT based lead-free ferroelectric ceramics with different strain characteristics were constructed by using different element modification combined with different strain contribution mechanisms.The effects of composition modulation on phase structure,microstructure,piezoelectric properties,high field strain properties and the relevant structural mechanisms were investigated in detail.The main research contents of this paper are as follows:（1）The(Ba_0.9558Ca_0.0442)(Ti_0.9142-xSn_0.0858+x)O₃ system was constructed by co-doping Ca and Sn ions.The influence of Sn on the phase structure,microstructure,dielectric,ferroelectric,piezoelectric,and electrostrain of samples were investigated in detail.The results indicate that the phase structure changes from coexistence of orthorhombic and rhombohedral phases to single rhombohedral phase.The effect of changing Sn content on the phase structure,microstructure,dielectric,piezoelectric,ferroelectric and strain of ceramic materials were studied.With the increase of Sn content,the phase structure gradually changes from O-R phase coexistence to R phase.The x=0.006 sample is found to be located at the rhombohedral side of orthorhombic-rhombohedral phase boundary with weak dielectric relaxor characteristics.The combine effect of both dielectric relaxation and two-phase coexistence can obviously promote domain switching and decrease the coercive field of x=0.006 sample,leading to the optimal dynamic piezoelectric coefficient d₃₃*（=S/E）.The maximal d₃₃*value can reach up to 1422 pm/V at 0.2 k V/mm.This study provides new ideas for the design of BT-based lead-free ferroelectric materials with low driven electric field and high d₃₃*value.（2）The influence of Ba content on the phase structure,microstructure,dielectric,ferroelectric,piezoelectric and electrostrain of(Ba_0.835+xCa_0.165-x)(Ti_0.91Zr_0.09)O₃ was investigated in detail.The results indicate that the substitution of Ba for Ca is beneficial for the tetragonal-orthorhombic phase transition.The tetragonal-orthorhombic polymorphic phase boundary is found to be located the composition range of x=0.025-0.06.The x=0.025 sample,which is close to the tetragonal-rich side of tetragonal-orthorhombic polymorphic phase boundary,were reported to exhibit a field-insensitive giant dynamic piezoelectric response(d^*₃₃>1050 pm/V)over a wide electric field range of 0.4-2 k V/mm,resulting in the large strain value of～0.21%at 2 k V/mm.Detailed structural investigations by using synchrotron X-ray diffraction combined with various electrical properties measurements reveal that the observed superior dynamic piezoelectric response with the electric field far beyond the coercive field is attributed to the field-induced T-O phase transition at the high electric field range.（3）The composition of(Ba_0.856Ca_0.144)(Ti_0.896Zr_0.104)O₃（BCZT）was chosen as the matrix,the phase structure and relaxor of BCZT were tuned by La Fe O₃ substitution.The relation with composition and the microstructure,dielectric,piezoelectric,ferroelectric and electrostrictive properties were systematically studied.In(Ba_0.856Ca_0.144-xLa_x)(Ti_0.896-xFe_xZr_0.104)O₃ ceramics,the phase structure gradually changed from T phase to C phase,with the La Fe O₃ content increasing.Meanwhile,the dielectric constant gradually increased with T_m decreasing,and the degree of relaxor gradually increased.When x=0.0275,T_m was below room temperature,and the strain gradually transforms from the normal butterfly bipolar strain to the linear electrostrictive strain without hysteresis.It was found that with the increase of La Fe O₃ content,the electrostriction coefficient Q₃₃decreased gradually.At x=0.0275,the electrostrictive strain value is 0.026%,Q₃₃=0.03m⁴/C²,suggesting the great potential of these materials for the application of high-precision ceramic actuators.