Preparation, Microstructure And Dielectric Properties Of Barium Titanate-Based Ceramics

Barium titanate is one of the important ferroelectric materials with perovskite structure, which has been used as multi-layer ceramic capacitor (MLCC), ferroelectric random access memories (FRAM), pyroelectric detector, frequency multiplier, dielectric phase shifter and voltage-controlled filter, and so forth because of its excellent piezoelectric, pyroelectric and ferroelectric properties. There are still some disadvantages in barium titanate materials. In order to further improve the dielectric nonlinearity and reduce dielectric loss at the low frequency, Sr2 + are doped into barium titanate to substitute for Ba2+ on A-site to form barium strontium titanate (Ba_1-xSr_xTiO₃, short for BST) material. However, when the applied electric field exceeds a few hundred kV/cm, the leakage current density of barium strontium titanate increases by an order of magnitude, and the breakdown occurs at about the 2MV/cm. The important problem has not yet been solved by improving the preparation process and doping. Therefore, other materials have been developed to replace barium strontium titanate. Barium zirconate titanate (BaZr_xTi_1-xO₃, short for BZT) is a possible good substitute because of low leakage current density and high breakdown strength. Barium stannate titanate (BaSn_xTi_1-xO₃, short for BTS) is a typical barium titanate-based relaxor ferroelectrics, which have been attracted much attention. Further research on doping and modification of these barium titanate-based materials is needed.In this paper, the influence of the doping type and concentration on microstructures, dielectric and ferroelectric properties of barium titanate-based ceramics has been investigated systematically and the doping mechanism has been discussed. The electronic structure of barium titanate-based materials has been studied by first-principle calculation. The results are as follows:â‘ Grain size of barium zirconate titanate ceramics can affect the type of its domain structure. There is no 90°domain in the BZT ceramics with the larger grain size. Domains with herribone, lamellar and water-mark characters are observed in BZT ceramics. As the grain size increases, the coercive field of BZT ceramics decreases, while the remnant polarization increases.â‘¡Mn⁴⁺, Al3+, Hf⁴⁺, Ce⁴⁺ and Ni³⁺ (Ni₂O₃ content<1 at.%) enter into the cell of BZT ceramics to substitute for Ti⁴⁺ and Zr⁴⁺ on the B sites. When content of Ni₂O₃ is more than 2 at.%, there is the second phase Ni₂O₃. MnO₂, Al₂O₃, CeO₂, Ni₂O₃ and HfO₂ have grain refinement.MnO₂, CeO₂, HfO₂ and Ni₂O₃ can reduce dielectric loss, but Al₂O₃ increases the dielectric loss. The diffuseness of phase transition of BZT ceramics weakens with the increasing of MnO₂ and Al₂O₃ content, and the diffuseness of phase transition initially increases and then decreases with the increasing of Ni₂O₃ and HfO₂ content. There is no obvious frequency dispersion phenomenon in barium zirconnate titante doped with MnO₂, Al₂O₃, Ni₂O₃ and HfO₂. The dielectric peak of BZT ceramics doped with 1 at.% CeO₂ broadens and there is apparent frequency dispersion phenomenon. The remnant polarization and coercive field of BZT ceramics decrease with increasing of MnO₂ and CeO₂ content. As Ni₂O₃ content increases, the remnant polarization decreases and the coercive field initially decreases and then increases. The coercive field of Al-doped BZT ceramics increases with the increase of Al₂O₃ content. As HfO₂ content increases, the remnant polarization initially increases and then decreases, while the coercive field initially decreases and then increases. In conclusion, variable-valent MnO₂ on B sites is an ideal modifier, because that it can lead to the larger dielectric constant and lower dielectric loss of barium zirconate titanate ceramics at room temperature.â‘¢When x is 0⁰.30, BaSn_xTi_1-xO₃ ceramics transit from the tetragonal phase to cubic phase as x increases. Mn⁴⁺ and Zn²⁺ enter into the cell to substitute for Ti⁴⁺ and Sn⁴⁺ on B sites. Sn⁴⁺, Mn⁴⁺ and Zn²⁺ have grain refinement. The dielectric loss of BaSn_xTi_1-xO₃ ceramics at room temperature is lower than that of barium titanate ceramics. The diffuseness of phase transition of BaSn_xTi_1-xO₃ ceramics enhances gradually with the increase of tin content. The diffuseness of phase transition of BaTi0.9Sn0.1O3 ceramics weakens with the increasing of MnO₂ content and addition of ZnO can enhance the diffuseness of phase transition. The remnant polarization of BaSn_xTi_1-xO₃ ceramics decreases with the increasing of tin content. When x is 0.10⁰.20, the coercive field increases with the increase of tin content.â‘£In doped barium titanate ceramics, V⁵⁺ and La³⁺ enter into the cell to substitute for Ti⁴⁺ on B sites and Ba2+ on sites, respectively. When MgO content is less than 1.5 at.%, Mg²⁺ enters into the cell to replace Ti⁴⁺ on B sites, and there is second phase (MgO) when MgO content is above 1.5 at.%. MgO and La₂O₃ have grain refinement. Grain size of barium titanate ceramics increases with the increasing of V₂O₅ content.V₂O₅ can reduce dielectric loss, and MgO increases the dielectric loss. When La₂O₃ content is above the critical value, it can reduce the dielectric loss of barium titanate ceramics. The dielectric peak of MgO-doped and La₂O₃-doped BZT ceramics broadens. As V₂O₅ content increases, the remnant polarization of barium titanate ceramics initially increases the maximum and then decreases. The coercive field of V₂O₅-doped barium titanate ceramics is much lower than that of pure barium titanate ceramics. In conclusion, variable-valent V₂O₅ on B sites is an ideal modifier, because that it can lead to the lower dielectric loss and larger remnant polarization of barium titanate ceramics.⑤Electronic structure by first-principles calculations shows that Ba-O and Ti-O of barium titanate is ionic bonds and covalent bonds, respectively. The phase transition is due to the change in the relative position of Ti atoms. Hybridization of Ti and O is mainly from the O 2p states and Ti 3d states. Hybridization of ferroelectric phase of barium titanate is lower than that of paraelectric phase.Band gap of Zr-doped barium titanate increases with the increasing of zirconium content. When Zr⁴⁺ is doped, the total density of states and partial density of states changes significantly, which is due to hybridization between Zr-4d electron and the O-2p. As hafnium content increases, band gap of Hf-doped barium titanate initially increases and then decreases, and the degeneracy of band structure increases. Ferroelectricity of Hf-doped barium titanate is stronger than pure barium titanate. When hafnium is doped, the populations of O-Ti bond increase and covalent bond enhances, while the absolute value of populations of O-Ba bond decreases and ionic bond weakens. There is strong hybridization between O-Ti and O-Hf, which is the cause of formation of ferroelectricity.