| Sodium bismuth titanate(Na0.5Bi0.5TiO3)-based material is one of the most promising lead-free piezoelectric materials to replace the lead zirconate titanate piezoelectric ceramic.Na0.5Bi0.5TiO3-BaTiO3 system shows excellent piezoelectric and ferroelectric properties due to the morphotropic phase boundary(MPB)for some compolsitions.As complex A-site compound ABO3 perovskite structure,the effect of A-site ions on properties is very important.Here,(Na0.47Bi0.47Ba0.06)0.95A0.05TiO3(A=Ca2+/Sr2+)and(Bi0.5-x/0.94Erx/0.94Na0.5)0.94Ba0.06TiO3(x=0.0000,' 0.0025,0.0050,0.0025,0.0100,0.0200)lead-free piezoelectric ceramics were prepared by the conventional soild-state reaction method.Crystallite structure,microstructure,electrical properties and optical performance were analyzed systemically.In addition,effect of different preparation technology on the properties of the ceramics is explored.(Na0.47Bi0.47Ba0.06)0.95A0.05TiO3(A=Ca2+//Sr2+)(abbreviated as BNBT-Ca,and BNBT-Sr;(Na0.47Bi0.47Ba0.06)TiO3 is denoted as BNBT)lead-free piezoelectric ceramics were prepared via the conventional soild-state method.The radius of XX is 1.39 A.The radius of Ca2+ is smaller than the average size of the cations(Na+,Bi3+,Ba2+)in the A-site;while that of Sr2+ is larger than the average size.X-ray diffraction and Raman spectroscopy results reveal the lattice distortion due to the doping.With the increase in the mean radii of the A-site cations,lattice parameters,tolerance factor,band-gap energy,average size of grains,dielectric constant,and piezoelectric constant of the ceramics increase.The doping of Ca2+ and Sr2+ leads to a decrease of Td and TRE.BNBT-Sr has the largest d33,which is 180 pC/N.The chang of activation energy with r is consistent with the change of the lattice structure and microstructure parameters.By means of electrocaloric effect(ECE),it was obtained that the temperatures corresponding to the transtion from the coexistence state of ergodic phase and non-ergodic phasee to a single ergodic phase forBNBT-Ca,BNBT,and BNBT-Sr are 140,140,80 ?,respectively.The depolarization temperature obtained by the dielectric properties of poled samples,piezoelectric properties of annealing samples and the electrocaloric effect are different,but the change trend is consistent.The depolarization temperatures for BNBT-Ca and BNBT are larger than those of BNBT-Sr.The 0.94(Bi0.5-x/0.94Erx/0.94Na0.5)TiO3-0.06BaTiO3(x=0.0000,0.0025,0.0050,0.0075,0.0100,0.0200)lead-free piezoelectric ceramics were prepared.The XRD spectrum shows pure perovskite structure for all samples.The grain sizes of the samples with x=0.0000,0.0025,0.0050,0.0075,0.0100 and 0.0200 are 0.96,0.75,0.84,0.72,0.64 and 0.61 ?m,respectively.Within the measurement temperature range,all samples show frequency dispersion of dielectric constant.The Curie temperatures of the samples do not fluctuate dramatically with the change of the doping amount.The relaxation activation energy calculated via the electric modulus is in accordance to the activation energy of oxygen vacancy,and increases first and then decreases with increasing x.With the increase of the doping amount,the coercive field decreases significantly.The piezoelectric constant increases slowly in the case of small x,and reaches a maximum value of 137 pC/N at x=0.0025,and suddenly drops to 61 pC/N for x=0.0200.Under an excitation of 487 nm,the ceramics exhibit visible luminescent emissions at 528 nm(green),548 nm(green)and 670 nm(red).On CIE chromaticity coordinates,there is barely any change with different contents of Er3+.They are all belonging to typical green area.The transition slowly moves to the red area with the increase of doping amount.The Na0.47Bi0.47Ba0.06TiO3 lead-free piezoelectric ceramics were prepared via a sol-gel method and compared with the samples prepared via a conventional solid-state reaction method.The relative density of ceramic samples prepared by sol-gel method at different sintering temperatures was up to 96%,indicating good sintering of the ceramics.In addition,the mean grain sizes of the ceramics are about 0.3?0.5 ?m,while the ceramics prepared via the conventional solid-state reaction method exhibit the mean grain size of 1.6 ?m.The present samples have smaller grains.Their ferroelectric,piezoelectric and dielectric properties were not deteriorated.All ceramics show obvious dielectric dispersion behavior.The activation energy values were obtained via the impedance,electric modulus,and conductivity,respectively.The values are in the range of 0.60?1.06 eV.The conductance activation energy is significantly less than the activation energy calculated via the impedance and electric modulus.Compared to the ceramics synthesized via the solid-state reaction method,the present samples exhibit fine grains,higher depolarization temperature,and excellent temperature stability of piezoelectric constant(d33).The d33 value of the sample sintered at 1120 ? still keeps as high as 119 pC/N as annealing temperature rises to 115 ?,at which the reduced amplitude of d33 is only about 3%. |
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