| Piezoelectric materials are an important class of electronic materials that can realize the conversion between electrical energy and mechanical energy. Piezoelectric materials can be divided into single crystals, ceramics, polymers and composites. Among all these piezoelectric materials, piezoelectric ceramics are widely used owning to their relatively simple fabrication process, low cost as well as the easiness of being fabricated into different shapes and up to a large scale. Because of the excellent piezoelectric properties, Pb(Zr,Ti)O3(PZT) ceramics are currently taking the largest share of global market of piezoelectric materials. However, due to the toxicity of lead oxide which is widely used during the fabrication of PZT ceramics, there is an increasing demand to fabricate high-performance lead-free candidates to replace PZT.BaTiO3is one typical kind of lead-free piezoelectric materials and BaTiO3ceramics are also historically the first polycrystalline piezoelectric materials. BaTiO3ceramics were widely used during the early stage of piezoelectric ceramics. However, after the discovery of PZT, BaTiO3ceramics were rarely used as piezoelectric materials. Nevertheless, there have been several breakthroughs in the piezoelectric properties of BaTiO3and BaTiO3-based ceramics recently. All these progresses indicate that BaTiO3-based ceramics possess a high possibility of becoming good candidates of lead-free piezoelectric materials.Most of the BaTiO3ceramics with high piezoelectric properties show high density and fine grain size according to literature. Therefore, it can be speculated that, being similar to the dielectric properties, the piezoelectric properties of BaTiO3ceramics also show strong grain-size dependence. The dielectric grain-size effect in BaTiO3ceramics has been studied extensively and the domain wall model for dielectric grain-size effect has been widely accepted. However, the study of piezoelectric grain-size effect in BaTiO3ceramics is still not thorough. For one thing, the mechanism of the high piezoelectric properties in those fine-grained BaTiO3ceramics is still unclear; for another, although the high-performance BaTiO3ceramics usually show fine grain size, some coarse-grained BaTiO3ceramics were also found to exhibit high piezoelectric properties in our recent study. Therefore, it is of great interest to explore the piezoelectric grain-size effect not only for the underlying mechanisms of high piezoelectric properties but also for even higher piezoelectric properties in BaTiO3ceramics.In order to understand the piezoelectric grain-size effect in BaTiO3ceramics more thoroughly and accurately, three groups of dense BaTiO3ceramics were fabricated by conventional sintering and sparking plasma sintering (SPS) respectively using conventional micro-sized BaTiO3powders and hydrothermally synthesized nano-sized BaTiO3powders as raw materials. By comparison, it was found that three groups of BaTiO3ceramics show similar grain-size dependence of permittivity ε’, i.e., the permittivity increases with the decrease of average grain size and reaches the maximum value around1μm. However, three groups of BaTiO3ceramics show quite different grain-size dependences of piezoelectric constant d33, despite that the maximum d33of BaTiO3ceramics prepared from different powders and by different sintering methods are all over400pC/N. For BaTiO3ceramics prepared from micro-sized BaTiO3powder by conventional sintering,d33increases with the decrease of grain size and reaches the maximum value of410pC/N around1μm; for BaTiO3ceramics prepared from micro-sized BaTiO3powder by SPS, d/33first increases and then decreases with the increase of grain size and shows a peak value of432pC/N around4.5μm; for BaTiO3ceramics prepared from nano-sized BaTiO3powder by SPS, d33keeps increasing with the increase of grain size and reaches425pC/N around9.6μm. The differences between dielectric and piezoelectric grain-size effects in three groups of BaTiO3ceramics demonstrate that the mechanisms of high permittivity and high piezoelectricity in BaTiO3ceramics should be different. By comparing the piezoelectric properties, P-E hysteresis loops and domain wall density of BaTiO3ceramics prepared from different powders and by different sintering methods, it is concluded that the remnant polarization, instead of the domain wall density, determines the piezoelectric properties in BaTiO3ceramics. Large remnant polarization is essential for high d33in BaTiO3ceramics. Generally, the remnant polarization of BaTiO3ceramics increases with increasing grain size, but it can also be easily influenced by other factors such as defects. When sintered at high temperatures, BaTiO3ceramics can easily produce considerable amount of defects such as oxygen vacancies, which tend to aggregate along the domain boundaries or grain boundaries and could produce a strong pinning effect on the domain wall movement, thus decrease the remnant polarization significantly. Owing to their different sintering temperatures, BaTiO3ceramics prepared form different powders and by different sintering methods show different grain-size dependences of the remnant polarization. For conventionally sintered BaTiO3ceramics, the remnant polarization decreases with the increase of grain size because of their high sintering temperatures; while the relatively low sintering temperatures make the remnant polarization of SPSed BaTiO3increase with the increase of grain size. Thus it can be concluded that the different grain-size dependences of remnant polarization caused by different sintering temperatures is the cause of different piezoelectric grain size effects in BaTiO3ceramics.Besides piezoelectric grain-size effect, the grain-size dependence of electric field-induced strain of BaTiO3ceramics prepared from nano-sized powders by SPS was also systematically studied. Being similar to the permittivity and d33, the electric field-induced strain was also found to show strong grain-size dependence. However, being different from the dielectric and piezoelectric grain-size effects, the electric field-induced strain first increases with the increase of grain size and then decreases significantly after a peak value of0.28%around5μm. It is thought that the results are of great importance from the following two aspects:(1) Through grain size optimization, a large electric field-induced strain which is close to that of soft PZT ceramics was obtained successfully in poled BaTiO3ceramics;(2) It is made clear that high piezoelectricity and large electric field-induced strain have different mechanisms in BaTiO3ceramics. According to the experimental results, it is speculated that the recoverable non-1800domain reorientation plays an important role in electric field-induced strain. Generally, there are two kinds of non-1800domain reorientations in ferroelectric ceramics upon an electric field, i.e., the recoverable and unrecoverable domain reorientations. For BaTiO3ceramics, small grains have more recoverable non-1800domains but also more restriction from grain boundaries; coarse grains have less restriction from grain boundaries but less recoverable non-1800domains. Therefore, only BaTiO3ceramics with intermediate grain size, which have a better trade-off between recoverable non-180°domain reorientation and restriction from grain boundaries, can exhibit large electric field-induced strain.The grain-size effect of orthorhombic Ba(Ti,Sn)O3ceramics were also studied systematically. For orthorhombic Ba(Ti,Sn)O3ceramics, the permittivity decreases with the increase of grain size, the d33increases with the increase of grain size, and the electric field-induced strain first increases and then decreases with the increase of grain size. These results indicate that the grain-size effects in BaTiO3based ceramics are irrelevant of their ferroelectric phases, and once again verify that high permittivity, strong piezoelectricity and large electric field-induced strain in BaTiO3based ceramics have different mechanisms. For BaTiO3based ceramics, high permittivity mainly comes from the high domain wall density in fine grains, and strong piezoelectricity is closely related to the high remnant polarization, while the large electric field-induced strain is influenced by both recoverable non-1800domains and grain boundaries.In order to lower the sintering temperature and enhance the piezoelectric properties, Ba(Ti,Sn)O3ceramics were modified by a small amount of CuO. It was found that CuO-modified Ba(Ti,Sn)O3ceramics not only show lower sintering temperature and enhanced piezoelectric properties, but also exhibit much better temperature stability and thermal ageing stability. The planar electromechanical coupling factor kp keeps around50%in the temperature range of-10℃and50℃, moreover, the room temperature piezoelectric properties of CuO-modified Ba(Ti,Sn)O3ceramics are not influenced by thermal ageing below Curie temperature. In order to understand the mechanism of the good temperature stability and thermal ageing stability of CuO-modified Ba(Ti,Sn)O3ceramics, the ageing behaviour of CuO-modified BaTiO3ceramics were studied systematically. It was found that the P-E hysteresis loops of CuO-modified BaTiO3ceramics developed quickly with time from typical ferroelectric single hysteresis loops into double hysteresis loops which are commonly observed in antiferroelectrics. It is speculated that this kind of ageing behaviour is caused by that during sintering Ti4+may be partially replaced by Cu2+which will form a defect dipole with oxygen vacancy, these defect dipoles tend to align with spontaneous polarizations with time and the aligned defect dipoles can exert a strong restoring fore on domain wall movement. Therefore, CuO-modified BaTiO3ceramics can show stable domain structure after ageing. Accordingly, the good temperature stability and thermal ageing stability of CuO-modified Ba(Ti,Sn)O3ceramics were also thought to be closely related to their stable domain structure. |
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