| Although PZT-based ceramics are widely used,people have focused on developing alternative lead-free piezoceramics considering environmental concern.Among lead-free piezoelectric materials,potassium sodium niobate(KNN)has been widely studied.KNN ceramics exhibit high Curie temperature and good piezoelectric properties.However,it is hard to obtain dense KNN-based ceramics by means of conventional sintering methods.In this work,KNN-based ceramics were prepared by low pO2 sintering method,hot pressing sintering method,and spark plasma sintering method.Structure,dielectric,ferroelectric,and piezoelectric properties of obtained ceramics were studied.In addition,the grain-size effect in electrical properties of KNN ceramics was investigated.Ba(Ti0.99Fe0.01)O3-? ceramics were prepared by solid-state reaction method,the electrical conduction of Fe3+-doped BT ceramics were studied.(1)Pure K0.5Na0.5NbO3 lead-free piezoelectric ceramics without any dopants/additives were sintered at various temperatures(950-1125?)in low pO2 atmosphere(pO2?10-6 atm).All ceramics exhibit high relative densities(>94%)and low weight loss(<0.6%).Compared to the ceramics sintered in air,the ceramics sintered in low pO2 exhibit improved electrical properties.The piezoelectric constant d33 and converse piezoelectric constant d33*are 112 pC/N and 119 pm/V,respectively.The ceramics show typical ferroelectric behavior with the remnant polarization of 21.6 pC/cm2 and coercive field of 15.5 kV/cm under measurement electric field of 70 kV/cm.The good electrical properties of the present samples are related to the suppression of volatilization of the alkali cations during the sintering process in low pO2 atmosphere.(2)(Na0.5K0.5)NbO3-2mol%LiF lead-free piezoelectric ceramics were sintered in the atmospheres of air and low pO2(pO2?10-12 atm)at 1065? for 3 h,respectively.The pO2 sintered ceramics exhibit high relative densities(-96%)and grain size of 1.4 ?m.The mean size of grains is 1.2 ?m for air sintered ceramics and relative densities of?92%.Compared to the pure KNN ceramics sintered in pO2,the ceramics added LiF show decreased TO-T and increased Tc and ?r.Via pO2 sintering as well as the addition of 2mol%LiF into K0.5Na0.5NbO3,piezoelectric and ferroelectric properties of the ceramics were obviously enhanced.The piezoelectric constant d33 and converse piezoelectric constant d33' are 125 pC/N and 186 pm/V for the ceramics sintered in the atmosphere of pO2.For the ceramics sintered in air,d33?114 pC/N and d33*=161 pm/V.Both the air and pO2 fired ceramics show typical ferroelectric behavior and butterfly-shaped bipolar strain curves.The remnant polarization Pr and coercive field Ec are of 23.0 ?C/cm2,and 15.6 kV/cm at 70 kV/cm for the ceramics sintered in air.The corresponding values are 25.9?C/cm 2,and 13.9 kV/cm for ceramics sintered in pO2.Under E=65 kV/cm,the Spos=0.093%and Sneg=-0.049%for air sintered ceramics;the pO2 sintered ceramics exhibit Spos=0.092%and Sneg=-0.027%.(3)Lead-free K0.5Na0.5NbO3 ceramics were successfully obtained via a hot-press sintering method.The ceramics have dense microstructure,fine grains,and pure orthorhombic structure.All ceramics exhibit high relative densities(>96%).The mean size of grains are 0.16 ?m(975?/1h),0.49 ?m(1100?/2h),and 0.72?m(1075?/6h),respectively.The ceramics sintered at 975? for 1 h exhibit high piezoelectric coefficient d33*=178 pm/V positive strain Spos=0.149%,negative strain Sneg=-0.033%,high ?r=838 and low tan8=0.02.The piezoelectric constant d33 is 132 pC/N for the ceramics sintered at 1100? for 2 h.The ceramics sintered at 1100? for 2 h exhibit high coercive field Ec=17.6 kV/cm under electric field E=80 kV/cm.Pure K0.5Na0.5NbO3 ceramics with high relative densities(99.1%)and fine grains(0.53 ?m)were successfully obtained by means of spark plasma sintering.At room temperature and 1 kHz,?r is 765 and tan? is 0.02.The ceramics exhibit high piezoelectric coefficient d33 is 141 pC/N.Under E=80 kV/cm,the remnant polarization Pr is 28.8 ?C/cm2,coercive field Ec is 14.9 kV/cm,positive strain Spos is 0.145%and negative strain Sneg is-0.072%,respectively.The maximum unipolar strain is 0.153%and the corresponding converse piezoelectric coefficient d33*is 191 pm/V under electric field E=80 kV/cm.The grain-size effect in dielectric,ferroelectric and piezoelectric properties of K0.5Na0.5NbO3 ceramics prepared by HP and SPS was investigated.?r decreases with increasing in grain size.d33 and Pr increase with increasing grain size and reache the maxmium value around 0.53 ?m.Compared to Tc,TO-T shows large changes with varying grain size.The change of Tc with increasing grain size is smaller than 10?.(4)BaTiO3 and Ba(Ti0.99Fe0.01)O3-? ceramics with dense microstructure have been synthesized by a solid-state reaction method.Compared with BaTiO3,the real part of the permittivity ?' and the Curie temperature Tc of Ba(Ti0.99Fe0.01)O3-?decreased.Relaxation peaks appeared in the curves of the imaginary part of the permittivity as a function of frequency.With the increase in frequency,the peaks gradually shifted towards higher frequency and their height increased.Conductivity was closely related to frequency and temperature.Frequency-dependent conductivity was analyzed using the Jonscher double power law.Compared with BaTO3,Ba(Ti0.99Fe0.01)O3-? exhibited high impedance at given frequency and temperature.Impedance Cole-Cole plots displayed two semicircles,which could be well fitted using two parallel RC equivalent circuit models.The conductivity activation energy was found to be around 1 eV.For Ba(Ti0.99Fe0.01)O3-?,the electrical modulus curve versus frequency displayed two peaks. |
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