Research Of Grain Size Effect In Nano/Submicron Crystalline Barium Titanate Ceramic System

In this thesis, the properties of the nano / submicron crystalline barium titanate ceramics are analyzed, and some theoretical models have been built to explain the experimental phenomenon.The phase transitions of nanocrystalline barium titnante ceramics could be understood in two ways. One side, the charges in the grain boundaries will affect the spontaneous polarizations in the grains, especially in the regions near the boundaries. The polarization will deviate from the ideal value, which make the total free energy different from the single crystal. The volume fractions of these regions becomes bigger as decreasing the grain sizes, which causes the change of free energy, then affects the phase transitions. On the other side, the broken bonds on the surfaces of grains will bring surface bond contraction, which will deduce the compressive and shear stress because of the anomalous shapes of grains. The two kinds of stresses will bring additional elastic energy into total free energy, and the elastic energy will become bigger as decreasing grain sizes. It is derived that the size of grain which makes the ferroelectric property disappear is 2.5 nm. From the two models, results can be derived out to fit the experiments: as decreasing the grain sizes, Curie's temperature decreases and the other two transition temperatures increase.Theoretical models of size effects in dielectric response according to DC (direct current) bias field in submicron barium titanate ceramic system are discussed. The dielectric response contains two typical parts: intrinsic ferroelectric and the vibration of domain walls, then the total dielectric response is the summation of the both. In this work, the size effects of the two parts are analyzed separately. The displacement of domain walls will be changed by DC bias field, then the domain walls will be clamped near the boundaries, which can be discussed with a mechanic model. The volume fraction of the clamped parts will become bigger as decreasing the grain sizes, which reduce the dielectric response of the domain walls. The dielectric response of domains will be clamped in the field, and can be calculated with Landau - Ginsburg - Devonshire phenomenological theories. With the combination of the two parts and the boundaries, a theoretical model on size effect in dielectric response in DC bias field is derived. The model shows that finer grains are helpful to decrease permittivity loss when a DC bias field is applied.Size effect on uniaxial stress affecting dielectric response in barium titanate system is also studied. In theory, the decreasing of dielectric constant can be derived out with phenomenological theories. The volume fraction of the grain boundaries will increase, and they will bear more part of the total loading when decreasing grain size, then the loading on the ferroelectric parts becomes weaker. At the same time, the dielectric constant of grain boundaries changes less with stress, which will also dilute the change rate of total dielectric response. Considering the two sides, the change rates of dielectric constants for different grain sizes are different, samples with finer grains have smaller change rate.