Study On The Heterovalent Ion Doping And Dielectric Properties Of BaTiO₃ Ceramics

Barium titanate?BaTiO₃?ceramics have good ferroelectric and dielectric properties and are widely used in electronic components.However,the dielectric constant of pure BaTiO₃changes greatly,and the Curie temperature is high.It only reaches high dielectric constant at Curie point,and the dielectric constant is low at room temperature.Doping some ions in BaTiO3 can display dielectric relaxation behavior.Relaxor ferroelectrics have higher dielectric constant and better capacitance-temperature characteristics.Y³⁺,La³⁺,Bi³⁺,Na⁺,Nb⁵⁺were selected for single doping and donor-acceptor co-doping in BaTiO3 ceramics In this thesis.The effects of different ions on the structure and properties of ceramics were studied.GULP code molecular dynamics simulation method was used to calculate and analyze the doping mechanisms.The structure,dielectric properties and ferroelectric properties of various ceramic samples were analyzed,and the relationship between their microstructures and dielectric relaxation is discussed.The main results are as follows:?1?The doping mechanism of Y³⁺doped BaTiO₃ ceramics is electronic compensation when the Y³⁺doping amount x is less than 0.01,and the ceramics had turn into semiconducting ceramics.The ion defect is dominant when x<0.01.With the increase of doping amount,the tetragonal ratio of ceramic sample decreases,and the ceramics transform from tetragonal phase to the cubic phase at x=0.03.The dielectric peak shifts to lower temperature and was broadened with the increase of Y³⁺doping content.The dielectric constant of peak decreases gradually with the increase of Y³⁺doping amount.As the doping amount of Y³⁺increases,the ceramic samples exhibit the characteristics of relaxor ferroelectrics.,and the remnant polarization increases first and then decreases.When Y³⁺doping amount is large,the oxygen vacancy concentration is high,the coercive field increases and the residual polarization intensity decreases.?2?For BaTi_0.97Y_0.03O₃ ceramics doped with La³⁺,the ceramics are tetragonal phase at La³⁺doping amount x=0.02,and the ceramic undergo semiconducting.When x?0.03,the structure changes from tetragonal to cubic.La³⁺generally replaces Ba site in BaTiO₃ crystal.The increase of doping amount leads to the decrease of cell volume and surface grain size.La³⁺doping can broaden the dielectric peak and shift the dielectric peak to the low temperature direction.With the increase of doping amount,the degree of relaxation increases,causing the ceramics transforming into relaxor ferroelectrics.When x=0.02,the remnant polarization is the largest and the coercive field is the smallest.?3?The cell volume for Ba_0.99La_0.01TiO₃ ceramics doping with Y³⁺increases with the increase of doping amount.When x=0.04,the ceramic samples change from the tetragonal phase to the cubic phase.And when x<0.01,the second phase appears in the ceramic samples.Y³⁺replaces Ti site in ceramics,and Y³⁺compensate with La³⁺or oxygen vacancy.When x=0.02,the ceramic is semiconducting and has a large dielectric constant.The peak temperature of dielectric constant shifts to the low temperature with the increase of doping amount.As the doping amount increases,the dielectric peak is broadened and the degree of relaxation increases.The ceramics are gradually transformed into relaxor ferroelectrics.The coercive field decreases first and then increases with the increase of doping content.?4?Bi³⁺doped BaTi_0.97Y_0.03O₃ ceramics have tetragonal phase.With the increase of Bi³⁺doping content,the unit cell volume tends to decrease,and the grain size on the surface of ceramics sample becomes larger.Bi³⁺has the effect of assisting combustion.The main doping mechanism in ceramics is the mutual compensation of Bi³⁺with Y³⁺.When the doping amounts of Bi³⁺and Y³⁺are not equal,maybe there are[4Bi^·_Ba-V_T?_i???]or[Y^·_Ba-Y_T?_i]defect clusters.With the increase of Bi³⁺doping content,the dielectric peak temperature almost does not move,the relaxation degree of linear fitting is small,so the degree of relaxation is low.Both remnant polarization and coercive field increase first and then decrease with the increase of Bi³⁺doping content.?5?Na⁺and Nb⁵⁺are co-doped in BaTiO₃ ceramics,Na⁺substituting Ba²⁺at A site,and Nb⁵⁺for Ti⁴⁺at B site,and the equal co-doping of the two ions can compensate each other.The main crystalline phases of ceramics are cubic phase barium titanate.Co-doping of Na⁺and Nb⁵⁺can inhibit grain growth.As the doping amount increases,the dielectric peak shifts towards the low temperature direction,and the dielectric peak gradually becomes wider.The degree of relaxation increases first and then decreases with the increase of the doping amount,reaching the maximum values at x=0.04,and the ceramics are transformed into relaxation ferroelectrics.When x=0.02,remnant polarization is the largest and the coercive field is the smallest.?6?Bi³⁺doped Ba_0.96Na_0.04Ti_0.96Nb_0.04O₃ ceramics have cubic perovskite structure.Because of the sintering aid effect of Bi³⁺,the grain size increases with the increase of Bi³⁺doping amount.With the increase of Bi³⁺doping content,the dielectric peak shifts towards the high temperature direction and is broadened,showing the characteristics of diffuse phase transition.At Bi³⁺doping amount x=0.04,the ceramics exhibit the behavior of relaxor ferroelectrics,that is with the increase of frequency,the peak dielectric temperature T_m shifts to high temperature.With the increase of Bi³⁺doping content,the remnant polarization increases first and then decreases,reaching the maximum at x=0.03,and the coercive field reaches the minimum at x=0.03.