The Effects Of Grain Size On Multi-field Properties Of Ferroelectrics

Due to their distinguished multi-field coupling properties,ferroelectric(FE)polycrystals are widely employed in different functional devices such as sensors,memories and transducers.The size of grains in FE polycrystals has a great influence not only on the microstructures of the materials but also on their multi-field coupling properties.In this thesis,the effects of grain size on the domain structures,electromechanical responses,electrocaloric effect of FE polycrystals and the magnetoelectric effect of ferroelectric-ferromagnetic(FE-FM)composites are investigated by real-space phase field models.Firstly,the phase field simulations show that the different types of domains within grains play an important role in the size-dependent electromechanical properties of FE polycrystals.The remnant polarization,coercive field and dielectric coefficient increase monotonously with the increase of grain size.However,the piezoelectric coefficient increases first and then decreases as the grain size increases.The decrease of the number of vortex domains is responsible for the increase of piezoelectric coefficient in the range of small grain size,the decrease of the density of 90° domain walls results in the decrease of piezoelectric coefficient in the range of large grain size.Secondly,the effect of grain size on electrocaloric effect(ECE)of FE polycrystal is also investigated in this thesis.It is found that the adiabatic temperature change(ATC)AT increases as the grain size increases within a given range of grain size.The temperature,at which the value of ATC is the maximum,shifts to room temperature as the grain size decreases.To explain the size-dependent ECE property,the evolution of domain structures with temperature is discussed for the polycrystal FE with different sizes.Finally,the effect of grain size on the magnetoelectric properties is investigated in FE-FM composites.The phase field simulations show that the magnetoelectric coupling coefficient increases first and then decreases when the grain size increases.The maximum value of magnetoelectric coupling coefficient is found in a specific grain size of FE-FM composites.The results in this thesis suggest optimal grain sizes for ferroelectric polycrystals that exhibit high performance of multi-field coupling properties.