| The exploration and development of deep oil-gas has become an important way to solve the shortage of petroleum resources.The acoustic wave transducer is an important tool for deep oil-gas exploration,which can be used to diagnose the working station of oil-gas wells.However,the temperature of piezoelectric materials used in acoustic wave transducer cannot reach the requirements of deep exploration.To develop a new high-temperature transducer material based on the piezoelectric and magnetic properties of single-phase multiferroic materials is a possible way to solve the problem.Ca3Ti2O7 with layered perovskite structure is a novel ferroelectric material with high Curie temperature.It can be made it to become a novel single-phase multiferroic material by doping.In the paper,the effects of self-doping,A-,B-doping on microstructure,electrical and magnetic properties of Ca3Ti2O7-based ceramics were studied systematically.Ca3Ti2O7-based multiferroic materials with high Curie temperature,excellent electrical and magnetic properties was obtained.Otherwise,the electronic structure of Ca3Ti2O7-based materials was studied by first-principle calculations.The results are as follows:?1?The optimization of preparation process of Ca3Ti2O7 ceramicsCa3Ti2O7 ceramics were prepared by solid state reaction method.The microstructure and electric properties were characterized by XRD,SEM,impedance analyzer,and ferroelectric test system.The effects of sintering process on microstructure,dielectric and ferroelectric properties were studied.The results show that two-step continuous sintering process?1400°C/12h+1550°C/12 h?is the best method to prepare Ca3Ti2O7 ceramics.Ca3Ti2O7 ceramics fabricated by two-step continuous sintering process show the densest surface,a large average grain size,the best dielectric and ferroelectric properties?when the frequency is 1 kHz,the room-temperature dielectric constant?is 70,the Curie temperature is above 480°C,and the remnant polarization Pr is 1.56?C/cm2?.?2?The effects of self-doping on microstructure,electrical and magnetic properties of Ca3Ti2O7-based ceramicsAfter Ca3Ti2O7 ceramics were prepared by solid-state reaction method,Ca3Ti2O7-?ceramics were obtained by air annealing and vacuum sealing of elemental metal oxygen annealing.The effects of oxygen vacancy on microstructure,electrical and magnetic properties were studied.The results show that annealing in air can reduce the oxygen vacancy content of Ca3Ti2O7-?ceramics,while vacuum sealing and elemental metal oxygen annealing can significantly increase oxygen vacancy content of Ca3Ti2O7-?ceramics.For Ca3Ti2O7-?ceramics with more oxygen vacancy,the Curie temperature is higher than 500°C,and the room temperature ferroelectricity?Pr=0.61?C/cm2?and magneticity?Mr=0.51 emu/g?are enhanced.The Ca3-xTi1.5Mn0.5O7 ceramics were prepared by controlling the molar ratio of Ca raw material.The effects of Ca vacancy on microstructure,electrical and magnetic properties were studied.The results show that:the average grain size of Ca3-xTi1.5Mn0.5O7 ceramics is affected by Ca vacancy and oxygen vacancy.Due to the introduction of Ca vacancy,the dielectric relaxation behavior is transformed from the Debye relaxation behavior of Ca3Ti1.5Mn0.5O7ceramics to the Maxwell-Wagner relaxation behavior of Ca3-x-x Ti1.5Mn0.5O7 ceramics.There are no room temperature magnetic properties observed in Ca3-xTi1.5Mn0.5O7 ceramics.?3?The effects of A-,B-doping on microstructure,electrical and magnetic properties of Ca3Ti2O7 based ceramics(Ca1-x-x Srx)3Ti2O7,Ca3(Ti1-xNbx)2O7,(Ca1-xSrx)3(Ti1-yMny)2O7 ceramics were prepared by conventional solid-state method.The effects of A-,B-doping on microstructure,electrical and magnetic properties of Ca3Ti2O7 based ceramics were investigated.For(Ca1-x-x Srx)3Ti2O7ceramics,the lattice constant increases,the grain size decreases,the dielectric constant becomes large and the remnant polarization decreases with the increase of Sr2+contents.And the room temperature magnetic is not observed.For Ca3(Ti1-xNbx)2O7 ceramics,Ca3(Ti0.94Nb0.06)2O7ceramics with larger average grain size,the maximum dielectric constant,remnant polarization and remnant magnetization are obtained by optimizing the composition.For(Ca1-xSrx)3(Ti1-yMny)2O7 ceramics,the remnant polarization decreases with the increase of Sr2+and Mn4+content.By optimizing the composition,(Ca0.8Sr0.2)3(Ti0.95Mn0.05)2O7 ceramics with magnetization of Mr=0.046 emu/g are obtained.?4?First-principles study of Ca3Ti2O7-based materialsThe first-principles were used to calculate the sites of oxygen vacancy,the sites of Ca vacancy,and the amount of Nb5+for Ca3Ti2O7-?,Ca3-x-x Ti1.5Mn0.5O7,and Ca3(Ti1-xNbx)2O7respectively.For Ca3Ti2O7-?,the oxygen vacancy on the salt-rock layer has more significant influence on the electronic structure than the oxygen vacancy on the perovskite layer.When oxygen vacancy is on the salt-rock layer,the band gap is the smallest?Eg=1.007 eV?,and the density of states value is the smallest.Compared with Ca3-xTi1.5Mn0.5O7 without Ca vacancy,Ca1-site vacancy in the perovskite layer makes its cell volume decrease.And the difference of the sites of Ca vacancy has little effect on its energy gap.For Ca3(Ti1-xNbx)2O7,the unit cell volume increases with the increase of Nb5+amount.When x<2/8,the band gap decreases with the increase of Nb5+amount.When x>2/8,the band gap increases with the increase of Nb5+amount. |
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