Investigation On The Preparation,performance Optimization And Mechanism Of Barium Titanate Based Dielectric Fibers And Their Composites

High dielectric fiber materials show greater potential in dielectric and energy storage applications due to their high aspect ratio and anisotropy properties.In the construction of high performance composite dielectric materials,electrospinning fiber structure has high functional properties,flexible and adjustable properties,and is considered as a potential material to improve advanced functional devices.BaTiO3(BT)is one of the most widely studied ferroelectric materials,currently widely used in a variety of fields,including multilayer capacitors,sensors,actuators,optoelectronic devices,electromechanical devices,dynamic random access memory,field-effect transistors,and thermistor,etc.BaTiO3 fiber has larger specific surface area and aspect ratio and better dielectric properties.The main content of this paper is to prepare barium titanate fiber with excellent structure and performance by using polyacetylacetonatotitantium(PAT)as titanium source through electrospinning technology,and improve the dielectric properties of barium titanate fiber by lanthanum ion doping.In order to make the fiber ceramics have better flexibility and tensile strength,the barium titanate based fiber Ba0.7Sr0.29La0.01TiO3 was compounded with the polymer polyvinylidene fluoride(PVDF)by papermaking technology.The composite film with high fiber concentration had good dielectric properties and mechanical strength.CoFe2O4 fibers with good flexibility and magnetic properties were prepared by electrospinning technology,and mixed with different ratios of Ba0.7Sr0.29La0.01TiO3 and then compounded with PVDF.Magnetoelectric composite fiber membranes with good dielectric properties and excellent ferromagnetic properties were prepared,and the mechanism of improvement was studied.The main work of the dissertation is as follows:I.Preparation of BaTiO3 fibers with excellent dielectric properties by electrospinning1.Preparation,characterization and evolution of BaTiO3(BT)fibers.BT fibers were prepared by electrospinning and heat treatment.The obtained BT fibers were tetragonal perovskite,and tetragonal BT had better dielectric properties than cubic BT fibers.The crystallinity of fibers increased with increasing temperature,and the grain size increased with increasing temperature.BT fiber prepared by electrospinning method with PAT as titanium source could still maintain good fiber morphology after heat treatment at 1000℃,and its morphology was superior to other reported fibers at high temperatures.2.Performance comparison of fiber ceramics and powder ceramics.BaTiO3 fiber(BTF)and BaTiO3 powder(BTP)were pressed under the same pressure to formed thin sheets.It was found that BTF had better formability than BTP.For BTF and BTP ceramics obtained at the same pressure and sintering temperature,the frequency variation curves of dielectric constant(ε)and dielectric loss at room temperature were tested.The dielectric constant of BTF ceramics was more dependent on frequency.The dielectric constants of BTF and BTP decreased with increasing frequency.The decreasing rate of dielectric constant of BTF was more obvious.The temperature dependence of dielectric constants of BTF and BTP ceramics was measured.It was found that the Curie temperatures of BTP and BTF ferroelectric tetragonal to paraelectric cubic phase transition were 122℃ and 125℃,respectively.Ⅱ.Preparation and dielectric properties of Ba(1-X)LaxTiO3(BLxT)(x=0.003～0.012)The Ba(1-X)LaxTiO3(BLXT)(x=0.003～0.012)fibers were prepared by electrospinning method.The addition of La3+significantly inhibited the growth of BTF grains.With the increase of La3+ doping concentration,the grain size decreased gradually.The obtained BLxT fiber was tetragonal.The fibers with different doping concentrations were pressed and sintered into ceramics and their dielectric properties were tested.When the doping concentration of La3+was less than 0.009,the dielectric constant and dielectric loss both increased with the increase of La3+doping amount.When the doping concentration was 0.009,the dielectric constant of BTF ceramics reached the maximum value,i.e.5100(25℃,f=100 Hz).When the doping concentration of La3+ increased from x=0 to x=0.012,Curie temperature(Tc)decreased from 122℃ to 98℃.The dispersion phase transition occurred when the dielectric peak gradually widened with the increase of La3+ doping concentration.Fiber ceramics doped with La still maintained very low dielectric loss.Therefore,they may be a better choice for enhancing temperature control capacitors.III.Preparation and dielectric properties of Ba0.7Sr0.29La0.01TiO3 fibers and organic composite materials1.Preparation of Ba0.7Sr0.29La0.01TiO3(BSLT)fibers.Using PAT as titanium source,BSLT precursor fibers were prepared by electrospinning technology,and BSLT fibers with good flexibility were obtained by heat treatment.Composited fiber membranes with different proportions of BSLT/PVDF(PVDF content ratio of 5%,10%,15%,20%)were prepared by papermaking technology.Compared with pure BSLT fiber film,the composite has higher fracture strength and elastic constant.The fracture strength of the composite with PVDF content of 5%was 50%higher than that of the pure BSLT fiber membrane,and the fracture strength of the composite with PVDF content of 20%was more than two times higher than that of the pure BSLT fiber membrane.This was because that the mechanical properties of the ceramics were greatly improved when the polymer penetrated into the pores of the BSLT fiber membrane.2.Effect of PVDF content on dielectric constant and dielectric loss of BSLT/PVDF composite fiber membrane.The dielectric constants of all samples decreased with increasing frequency,because the mobility of the dipole decreased when the frequency of applied electric field exceeded the relaxation frequency,and the dielectric constant of the composites decreased with increasing PVDF content.The dielectric constants of BSLT/PVDF membrane with PVDF 20%at frequency of 1kHz decreased to about 49.5.In the low frequency range,dielectric loss changed slowly,and in the high frequency range,dielectric loss increased sharply.Compared with pure BSLT films,the dielectric loss tanδ of all BSLT/PVDF membrane increased significantly.3.Effect of PVDF on dielectric properties of BSLT/PVDF films with temperature.No Curie temperature point was observed in the temperature range of 26℃-160℃ for all composite films,but the dielectric constant decreased after Curie temperature,which also indicated that the Curie temperature of BSLT fiber film samples was lower than 26℃.When the temperature was over 170℃,with the further increase of polymer content,the hindrance effect of fiber on the ionic conductivity of polymer matrix decreased and the dielectric response increased.The dielectric constant and dielectric loss of all samples in the range of 42℃-160℃ had relatively good temperature stability.The polymer ceramic composite film with high inorganic fiber ratio had good temperature stability and small frequency dispersion.Therefore,the composite ceramic is an excellent temperature-stable dielectric material.Ⅳ.Preparation of CoFe204 fiber and magnetoelectric performance research of xCoFe2O4-(1-x)Ba0.7Sr0.29La0.01TiO3/PVDF(CFO-BSLT/PVDF)composite fiber membrane1.Preparation of CoFe2O4(CFO)fiber.Fluffy and flexible CoFe2O4 fibers were prepared by using electrospinning and heat treatment method.It was found that the precursor fibers of cobalt ferrite prepared from cobalt acetate as cobalt source and ferric nitrate as iron source were treated to a lower temperature(400)and the fibers with good crystallinity were formed.Through the observation of SEM,it could be obtained that with the increase of heat treatment temperature,the grain size of fiber surface gradually increased.The hysteresis loops of the fibers treated at different temperatures revealed the soft magnetic properties of CoFe2O4 ferrite,indicating that these composites had magnetic order.Ms of CoFe2O4 fibers increased from 23.35 to 38.48 emu/g with the heat treatment temperature increased from 500℃ to 800℃,which may be due to the reduction of crystal defects and increase of net magnetic moment for the improvement of crystallization and grain growth.2.Preparation and magnetoelectric performance research of xCoFe2O4-(1-x)Ba0.7Sr0.29La0.01TiO3/PVDF composite fiber membrane.The paper making technique was used to prepare the xCoFe2O4-(1-x)Ba0.7Sr0.29La0.01TiO3/PVDF composite fiber membrane(x=0.2,0.4,0.6 and 0.8).The variation characteristics of dielectric constant and dielectric loss of these composite fiber membranes with different CoFe2O4 content were measured.The dielectric constant of composite fiber membranes decreased with increasing frequency.However,with the increase of CoFe2O4 content,the dielectric constant changed more obviously with the frequency,and the dielectric loss increased significantly,that is,with the increase of magnetic fiber content,the dielectric stability of the composite fiber membrane became worse at high frequencies.The dielectric constant and dielectric loss of the composite fiber membranes with different CoFe2O4 content were tested to study the temperature stability of the dielectric performance of the composite fiber membranes.The dielectric constant decreased with the increase of temperature in the range from room temperature to 50℃,and the dielectric constant had good stability in the range of 50℃-160℃.The temperature stability of the dielectric loss was significantly worse with the increase of CoFe2O4 content,but the dielectric loss was still lower than 0.16,keeping a low dielectric loss.The saturation magnetization value of composite fiber membranes increased with the increase of CoFe2O4 concentration.