| The lead-free piezoelectric ceramics have become an important research area due to the harm for environment of traditional lead-based ceramics.Compared with the perovskite structure ceramics,the BLSFs have been applied in high temperature and frequency devices because of high Curie temperature,high mechanical quality factor,low aging rate,excellent temperature frequency stability and so on.However,for the single-systematic BLSF,volatilization of the bismuth during sintering and difficulty to polarize will lead to the low density and bad properties.The SrBi4Ti4O15(SBTi)ceramic with four-layer structures is prepared at low temperatures.The as-prepared samples are characterized by X-ray diffraction,scanning electric microscope,dielectric and impedance testing systems.In this study,the microstructures,dielectric and impedance properties are analyzed after substituted Bi3+ on A site by Ce3+.And the composite and intergrowth ceramics are prepared by using BT and BIT.The Nb5+is employed to replace Ti4+on B site in BIT-SBTi intergrowth ceramics.Their structures and properties are also researched.The main studies and results are as following:Firstly,the SBTi poderws are prepared by adopting GNP,HT,MSS.Based on that,the SBTi-100xCe(x=0～0.08)with the substitution of Ce3+for Bi3+on A site were synthesized.The results indicate that the single-phase SBTi with small and uniform flake is obtained by GNP at 500℃,which is lower 300--400℃ than the one of solid reaction process.The crystalline structure is no affected by doping Ce3+ and the diameter of layer structure reduces from 12 to 2 μm.The εr-T character is perfected.While the Tc is improved,the dielectric loss at high temperature is degraded.When x=0.04,εr(35℃)=224,Tc=542℃,tanδ(35℃)=0.0004 and γ=1.71.For all Ce-doped samples,the TCPppm is about 0.012 and the η is about 0.01-0.04,which indicates the ceramics possess good stability for temperature and frequency.Secondly,taking layer-structured SBTi as matrix,the composite ceramics of(1-x)SBTi-xBT(x=0.2～0.8)are synthesized.The structure and properties are studied to analysis the effects of doping amount of BT.Results show that the good compound can be formed by using layer-structure SBTi with big size prepared by MSS and BT prepared by HT.The morphologies and XRD manifest the above-mentioned process is superior to two-step HT,and the composite powders have better homogeneity and dispersibility.When x<0.5,SBTi still keep the principal crystalline phase;and the composite structure of bismuth layer/perovskite can be obtained when x≥0.5.There are two characteristic peaks in the dielectric temperature spectras of the SBTi/BT composite ceramics.When x=0.2,Tm2 is 529℃ that gets close to the Tc of SBTi.When x=0.8,Tm1 is 97℃ that gets close to the Tc of BT.When x=0.5,y is 1.91 corresponding to the ideal relaxor ferroelectrics.At the room temperature,εr=704 and tanδ=0.068 for x=0.8,moreover,tanδ is less than 0.5 at 500℃,which shows excellent temperature stability.The impedance analysis indicates the main electrical conduction mechanism is grain effect and the σdc is below 10-4 for SBTi/BT composite when the temperature is up to 750℃,which shows good application prospect in high temperature fields.Furthermore,Nb-doped BIT-SBTi intergrowth ceramics are synthesised.The results indicate the phase is good accordance with the standard intercalation structure of PDF#31-1342.This structure improves the dielectric properties of single-phase SBTi and the addition of Nb5+increases εr at RT.When x=0.02,εr=8045 and 1726 for 1 and 100 kHz,even the average Tc of all intergrowth ceramics reaches 590℃.The varations of ferroelectric-paraelectric phase of BIT-SBTi undergo a first-order phase transformation,which can be explained by Curie-Weiss and Gibbs free energy models.The Nb-doped BIT-SBTi showes grain conduction and when x=0.08,Edc=1.99 eV that is twice as much as the one of oxygen vacancy.This result verifies there is some effect of inhibiting oxygen vacancy for donor ion doping of Nb5+. |
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