| Piezoelectric ceramic is one of the most important functional materials.It has been widely used in medicine,acoustics,machinery,electronics,automation,etc.Due to the urgent requirements of the environmentally friendly electronic devices,the research and development of lead-free piezoelectric ceramics become a hot research topic.Sodium bismuth titanate-based(BNT-based)lead-free piezoelectric ceramics is one of the most promising candidates.It presents unique advantages compared with other lead-free piezoelectric ceramics,such as high electric-field-induced strain,excellent stability under high vibration speed,high repeatability,and easy large-scale production,etc.However,the low depolarization temperature in BNT-based system limits its application.In this thesis,the strategies were explored to enhance the depolarization temperature,electric-field-induced strain and high-power application properties in perovskite-type BNT-based lead-free piezoelectric ceramics.The phase structure,ferroelectric/piezoelectric properties were systematically studied through introducing defects,phase structure,ion substitution,and adjusting grain size.The effects of heat treatment processes on the depolarization temperature in 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3(BNT-6BT)lead-free piezoelectric ceramics were studied.According to the temperature-dependent dielectric and temperature-dependent electric-field-induced strain measurements,it was found that the quenching process increases the depolarization temperature from 96? to 136?in BNT-6BT ceramic.The X-ray photoelectron spectroscopy(XPS)and electron paramagnetic resonance(EPR)results shown that the enhanced depolarization temperature is ascribed to the oxygen vacancy defects;the quenching process introduces oxygen vacancy defects into the ceramic,which exhibit a pinning effect on the ferroelectric domain walls,thus maintain the ferroelectricity over a wider temperature range,and give raise to enhance the depolarization temperature.Based on the above-mentioned oxygen vacancy strategy in enhancing depolarization temperature,the effect of oxygen defect contained BaInO2.5(BaIn)third component on the depolarization temperature,ferroelectric and piezoelectric properties in 0.8(Bi0.5Na0.5)TiO3-0.2(Bi0.5K0.5)TiO3(BNT-BKT)ceramics were studied.The temperature-dependent dielectric measurement indicates that the introduction of Baln enables increase the depolarization temperature,as high as 90 oC.Moreover it enhances the piezoelectric constant.The results of temperature-dependent complex impedance measurements illustrate the relationship between activation energy and the content of Baln.The ternary relaxor ferroelectrics BNT-BT-BNMN was successfully prepared through introducing the third component(Bi0.5Na0.5)(Mn1/3Nb2/3)O3(BNMN)into the typical ferroelectric BNT-BT.The morphotropic phase boundary(MPB)of the BNT-BT-xBNMN ternary system was determined to be between x=0.005-0.02.The highest piezoelectric constant of 150 pC/N was obtained at x=0.02.At x=0.02 75,an electric-field-induced strain of 0.28%was obtained,which is about twice as that of the BNT-BT.The non-stoichiometric method was used to optimize ferroelectric and piezoelectric properties of 0.88(Bi0.5Na0.5)TiO3-0.08(Bi0.5K0.5)TiO3-0.04(Bi0.5Li0.5)TiO3(BNT-BKT-BLT)ternary ceramic.The influence of excessive low melting point element of K,Na on the microstructure,high power characteristics and temperature stability of the ceramic were studied.It is found that the excess K,Na promote the growth of grain,and result in an increased grain size,which is more than 10 times that of the stoichiometric one.The results of impedance and dielectric indicate that the ceramics with larger grain size present higher mechanical quality factor and better temperature stability.Subsequently,the MnO2 was added in the non-stoichiometric component,forming defect dipoles.It further improves the mechanical quality factor and temperature stability of the ceramic. |
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