Preparation And Mechanism Research Of Sr²⁺ Doping BaTiO₃ Ferroelectric Thin Films By Micro Arc Oxidation

Ba_xSr_1-xTiO₃?0?x?1??BST?is a kind of ABO₃ perovskite ferroelectric thin film widely used in various applications such as dynamic random access memory,microwave tuner,phase shifter,pyroelectric infrared sensor,and H₂ detector due to the benefits of high dielectric constant,low dielectric loss and high dielectric tenability.Micro-arc oxidation?MAO?is expected to become a kind of low cost,easily scaled-up technology to prepare high quality BST ferroelectric thin films,because of its simple operation,pollution-free,good adhesion and process without post heat treatment.In this dissertation,BST ferroelectric thin films were prepared by constant voltage DC MAO on Ti?TA2?substrate in the solutions containing Ba?OH?₂·8H₂O and Sr?OH?₂·8H₂O.Technological parameters of MAO were optimized through orthogonal experiments and single factor experiments.Electrolyte was optimized by screening of four additives,and the effects and mechanism of additives on the BST thin films were analyzed.BST thin films with different ratio of Ba²⁺/Sr²⁺ were prepared.The influences of Sr²⁺ doping on film surface morphology,phase composition,film growth and electrical properties were discussed.Finally,the growth and formation mechanism of MAO BST ferroelectric thin films were investigated based on the experimental phenomena and results.In comprehensive consideration of good film microstructure,high dielectric constant,low dielectric loss and good ferroelectric properties,the optimal preparation process of BST thin films by MAO were determined as the following:the electrolyte with 0.6mol/L Ba?OH?₂·8H₂O,0.4mol/L Sr?OH?₂·8H₂O and 0.05mol/L EDTA,current density of 0.500 A/cm2,current frequency of 150 Hz,the duty ratio of 85%and the reaction time of 15 min.The BST thin film obtained under the optimized condition was mainly composed of Ba_xSr_1-xTiO₃,while a tiny amount of?Ba,Sr?CO₃ and TiO2 was also observed.At the frequency of 1 KHz,the film was found to possess a dielectric constant of 282 and a dielectric loss?tan??of 0.164.The film has a positive residual polarization?Pr?about 7.8 ?C/cm2,and the reverse residual polarization?-Pr?about 7.5 ?C/cm2 under the voltage of 500 V and the frequency of 100 Hz.Compared with the BaTiO₃?BT?films prepared early in our group,the BST film showed around 100%increase in dielectric constant.All of the performance indexes,except the dielectric constant of our MAO-prepared BST films were also superior to that of the material prepared by the sol-gel method.The dielectric constant of MAO-prepared BST thin films was similar to that of the BST thin films prepared by magnetron sputtering method?Under the condition of the same micro-arc oxidation process parameters,the thickness of the BST film prepared with 0.05 mol/L EDTA additives was reduced by about 80%when compared with the one prepared without EDTA,leading the BST film more attractive to meet the requirements for practical application.Usually,the dielectric constant of the BST thin films was proportional to the film thickness.So when the dielectric constant of the BST film prepared with and without EDTA additives was almost the same,it meant that the capacitor of the BST film was increased by adding EDTA to the electrolytic liquid.It was found that the BST thin films had smoother surface and denser structure by adding EDTA,which was helpful to increase the capacitance value and reduce the dielectric loss.The Raman characteristic peaks' strength of the BST film became lower and wider with EDTA additives,it showed that the grain size of the BST thin film became smaller.And the effect of grain refinement was enhanced with the increase of the amount of EDTA.When the EDTA was increased to 0.1 mol/L,the BST thin film's ferroelectric property disappeared.The composition of the BST thin film was Ba²⁺ doping SrTiO₃ when the content of Ba²⁺was less than 0.6 mol/L,and it had no ferroelectricity at room temperature.But when the content of Ba²⁺ was greater than 0.6 mol/L,the composition of the BST thin film was Sr²⁺doping BaTiO₃,and it had obvious nonlinear dielectric characteristics at room temperature.HRTEM analysis showed the BST thin film was mainly composed of tetragonal phase when the content of Ba²⁺ was 0.7 mol/L,with small amounts of carbonate as well as amorphous component in the BST film.Nano sized crystals?5-20 nm?and polycrystalline growth of the BST film were observed in the HRTEM.Both the BST film and Ti substrate exhibited the same grain stripe direction in the high resolution transmission electron microscopy,indicating that the BST film was in situ grown on the Ti substrate.The BST thin films growth toward the reaction duration could be divided into three stages:1)when response time was less than 3 min,the film thickness was rapidly increased with an average growth rate of about 3.50?m/min;2)when reaction time was between 3?15 min,the increase of film thickness was slowed down,with an average growth rate of about 0.25?m/min;3)the growth rate of film thickness was further decreased to about 0.08?m/min when the reaction time was longer than 15 min.It can be reduced that the film thickness will change little when the reaction was lasted for certain duration.The growth mechanism of MAO-prepared BST ferroelectric thin films was revealed,according to the observed phenomenon in MAO experimental process and testing analysis.Reactions happened in the MAO process was put forward and the kinetics model of the BST ferroelectric thin films were established.It was found that the content of Ba,Sr and Ti elements in BST thin films increased very slowly and tended to be stable when the reaction time was greater than 5 min.The atomic percentage of Ba,Sr and Ti elements was basically in accordance with the stoichiometric ratio.Additionally,the distribution of Ba,Sr,Ti and O elements along the section direction of the BST film were uniform when the reaction time was more than 5 min.Furthermore,to avoid the polarization decreased with the increasing of electric field intensity,micro-arc oxidation reaction time should be greater than 5 min.