Microstructure And Property Modulation In Piezoelectric Ceramic For Energy Harvesting

As a new emerging energy technology, piezoelectric energy harvesting has attracted much attention, since its wide application prospect. Researching the key materials to satisfy the requirement of piezoelectric energy harvesting system, including energy conversion and energy storage, is of great significance to further promote the application of piezoelectric energy harvesters. Controlling the crystal structure and composition to obtain high piezoelectric constant and low dielectric constant is the key factor for achieving high electromechanical conversion performance. In this work, the component selection and design rules of multicomponent system ceramic materials for piezoelectric energy harvesting devices have been put forward. Based on the PZN–PZT system, through different material modification methods in the morphotropic phase boundary(MPB) and tetragonal phase region, the high piezoelectric constant and lower dielectric constant have been obtained simultaneously. Furthermore, inorganic-metal dielectric composites and antiferroelectric energy storage materials, which have the similar composition with energy harvesting materials, have also been studied in detail. It shows important theory value and practical significance to build integrated energy harvesting system(including both energy conversion unit and energy storage unit).Firstly, the effect of composition control and sintering behavior on crystal structures and electric properties of PZN–PZT ceramics has been investigated. The perovskite type 0.2Pb(Zn1/3Nb2/3)O3–0.8Pb(Zr1/2Ti1/2)O3 ceramics with different grain size were prepared at different sintering temperatures. The results showed that the volatilization of Pb O cannot be inevitable during high temperature sintering process. A corresponding amount of lead vacancies and oxygen vacancies should exsit in the ceramic samples, forming the defect dipoles. As a result, the interaction of defect dipoles with spontaneous polarization Ps inside each domain creates an internal bias field. Research also found that with decrease of the internal bias field, the ferroelectric and piezoelectric properties increased. In the further study, columbite precursor method combined with high-energy ball milling technology has been proposed to prepare the x Pb(Zn1/3Nb2/3)O3–(1-x)Pb(Zr0.47Ti0.53)O3 powders with size at submicron scale(~200 nm). And the fine-grained ceramics have been prepared from the submicron precursor powders. With the decrease of grain size, the internal stress increased, which was conducive to stabilize the rhombohedral phase, leading the location of MPB shifted to low PZN content side. Through the modification of crystal structure and grain size, 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb(Zr0.47Ti0.53)O3 system with grain size of 0.65 ?m in the MPB region, presented the good electromechanical conversion and mechanical properties, which is promising in application as multilayer energy harvesting devices.Secondly, the effect of group VIII metal ions on electromechanical properties of PZN–PZT ceramics has been investigated in detail. Introducing the group VIII metal ions(Fe, Co, Ni) into 0.2Pb(Zn1/3Nb2/3)O3–0.8Pb(Zr0.5Ti0.5)O3 system favors to obtain a series of piezoelectric ceramic systems meeting the requirment of energy harvesting. The addition of VIII metal ions induced a phase transformation from MPB to the tetragonal phase side. The liquid-phase sintering process and formation mechanism of ilmenite secondary phase above the solubility limit, have been analysized by TEM technique in detail. It should pay attention that a new kind of substitution mechanism–“equivalence substitution” has been put forward. On the tetragonal phase side, based on tailoring the different variation tendency between piezoelectric constant and dielectric constant, it is successful to achieve the good electromechanical conversion performance. Optimum electric properties are obtained for the 0.8 wt.% doped ceramic system: d33 = 440 p C/N, g33 = 32×10-3 Vm/N, kp = 73%, d33?g33 = 14083×10-15 m2/N.Furthermore, the PZN–PZT based composites for energy storage have been constructed. Novel dielectric composites with intragranular and core–shell nanostructure were prepared with submicro-0.2Pb(Zn1/3Nb2/3)O3–0.8Pb(Zr0.5Ti0.5)O3 powders and nano-Ag particles. The core–shell nanostructures formed by the nano-Ag particles and Pb O reinforced the MWS polarization between the nano-Ag particles and ceramic matrix, reduced the tunnel current between Ag particles, and endowed the low dielectric loss. The 0.2Pb(Zn1/3Nb2/3)O3–0.8Pb(Zr0.5Ti0.5)O3/16.6 vol.% Ag system has the optimum energy storage properties with a high dielectric constant(~ 16600) and low dielectric loss(<0.056) at room temperature.At last, the antiferroelectric x Pb(Zn1/3Nb2/3)O3–(1-x)Pb(Zr0.95Ti0.05)O3 system has been designed to investigate the effect of PZN content on crystal structure and energy storage property. The results showed that with increase of x, the phase structure changed from antiferrelectric(AFE) through the coexsitence of antiferrelectric and ferrelectric(AFE-FE), finally to the ferrelectric phase. The optimum electric properties are obtained in 0.15Pb(Zn1/3Nb2/3)O3–0.85Pb(Zr0.95Ti0.05)O3 system with FE phase, and the ferroelectric and piezoelectric properties of the sample showed a wide temperature stability.This work deepens the cognition of piezoelectric materials desighed for energy harvesting. Equivalence substitution mechanism of VIII ions and the grain size driven MPB migration behavior have been clarified. Building intragranular and core–shell nanostructures is also a new research progress for energy storage materials. The above works have an important scientific significance and application value.