The Thermo-electrical Transitions Based On Flexoelectric Effect

As entering 21 century,the whole world begin to recognize the urgency from the energy crisis.Seeking for new energy resources has been an important issue,and the researches on solar energy,wind energy,geothermal energy,and biomass energy have experienced rapid expansion.Meanwhile,the thermo-electrical transition based on waste heat recovery to improve the efficiency of energy usage,also attracts a lot of attentions.The thermo-electrical generation is mainly based on the Seebeck effect,and pyroelectric effect is also involved.Generally,both of these two thermo-electric coupling effects need special thermal environments that with temperature gradient or varying temperature.Though the thermal environments with these requirements are the direct driving forces for the thermo-electrical output,but they also limit the applicability of these two thermo-electric coupling effects in reality to some extent.To make the thermo-electrical transition more controllable,this dissertation focuses on the process of thermo-electrical coupling,and seeks for the separation of thermo-input and electrical output during it.Obviously,the involvement of other effects is inevitable.Flexoelectric effect describes the mechanical-electric coupling between polarization and strain gradient over the material,which can be written as:Pl=?ijkl((?)sij/(?)xk).In analogy with the formation process of polarization induced by electrical field,we can extract the "flexoelectric electrical field" Ejlexo from the flexoelectric effect.Likely,the Eflexo could drive the carriers into current.So,when putting the materials under bending deformation into an elevated temperature environment,the induced flexoelectric electrical field over it could drive the activated carriers into current.From this point,this dissertation firstly tries to elucidate this phenomenon;after that,an improved system for this flexoelectric thermo-electrical transition will be proposed.(1)The pyroelectric effect of oxide ferroelectric ceramic and Seebeck effect of composition-modified oxide ferroelectric ceramic were characterized experimently,the temperature environments required and the characteristics of their thermo-electrical responses can be clearly got.Based on it,we can effectively analyze the origin of the measured thermo-electrical current.From the estimation based on i=?·Eflexo,we found that the measured thermo-electrical current in the bent ceramic plate is mainly from the directed migration process of activated carriers drived by the flexoelectric electrical field.This is the prototypical exploration on the flexoelectric thermo-electrical transition,which can progress in the homogenous temperature environment.In addition,the preparation processes of ceramic materials could inevitably lead to a stable macroscopic polarity over it.However,we found that this polarity can be rewritten by the flexoelectric effect.It suggests that the formation process of macroscopic polarity may be closely related to flexoelectric effect.This phenomenon enlightens the design for materials used in flexoelectric thermo-electrical transition in the future.(2)From the function process of flexoelectric effect,it is clear that the strain gradient over the material dominates the flexoelectric thermo-electrical transition process.The asymmetrical reduction on the ceramic system induces an obvious compositional gradient across the thickness,which causes a macroscopic strain gradient over the system below the reducing temperature.Furthermore,the reduction also improved the conductivity significantly.By estimation,we verified that it is flexoelectric effect that governs the apparent voltage over asymmetrically reduced ceramic.Which suggested that the large thermo-electrical response of asymmetrically reduced ceramic in the elevated temperature environment is an enhanced flexoelectricthermo-electric transition compared with the prototypical research.(3)The significantly improved conductivity in the asymmetrically reduced ceramic system makes it behaves like a semiconductor.According to the theory of energy band,we qualitatively analysized the macroscopic potential barrier across the asymmetrically reduced ceramic.Combining the macroscopic potential barrier and the induced current,we described the presented rectification behavior of the asymmetrically reduced ceramic under the external applied voltages.The simple preparation process of the ceramic sample with rectification behavior provides a feasible way to product rectification devices for the electronic industry.