Properties And Temperature Stabilities Of Lead-free Piezoelectric Ceramics

In recent decades, there is an increasing demand in searching for materials that are benign to the environment and human health. Many government regulations have been enacted in response to this demand. The directive of "the Restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS)" issued by the European Parliament and the European Council has been implemented since 2008. The European RoHS directive bans electrical and electronic equipment containing any more than trace amounts of hazardous substances, including lead (Pb). Other related legislative acts passed by the European Union include End-of-life Vehicles (ELV) in 2003 and Waste from Electrical and Electronic Equipment (WEEE) in 2004.For the past 50 years, Pb(Zr_1-xTi_x)O₃ (PZT) and related compositions have been the mainstay for high performance actuators and transducers because of their excellent piezoelectric and dielectric properties. However, the content of PbO in PZT is as high as 60wt%. The environmental and health hazards of lead are well known and recycling and disposal of devices containing lead-based piezoelectric materials is of great concern, especially those used in consumer products such as cars, mobile phones, sound systems and medical devices. With the EU's issuing of the WEEE/RoHS/ELV, a lead-free revolution in electronic products is pushed throughout the world. Thus, it is urgent to develop environmental friendly lead-free piezoelectric materials and products to replace PZT-based ceramics and products.This thesis work will be divided into the following chapters:In chapter 1, the author first introduced the history, the characteristics and the applications of piezoelectric materials, and described the mechanism of piezoelectric effect in details. The article then reviewed the development of lead-free piezoelectric ceramics and classified them into the following five classifications: barium titanate based lead-free piezoceramics, sodium bismuth titanate system lead-free piezoceramics, bismuth-layered structure lead-free piezoceramics, niobate system lead-free piezoceramics and tungsten bronze structure system lead-free piezoceramics. The article briefly summed up their advantages, disadvantages and fields of applications. Among these systems, K_0.5Na_0.5NbO₃ (KNN) based system, attracted wide attention for its high Curie temperature and enhanced piezoelectric response. However, pure KNN ceramics had low piezoelectric activity due to its poor densification induced by high volatilization of potassium during sintering, moreover it would deliquesce once exposed to humidity. Sintering aids as well as hot pressing, cold isostatic pressing and spark plasma sintering had been used as alternatives to obtain high electrical properties. However, such techniques were not suitable for industrial use for their high costs. With traditional solid-state sintering process, Li, Ta and Sb were added into the KNN compositions to form new solid solutions which exhibited properties comparable to those of PZT. Researchers had done a lot of research work in this aspect and confirmed that the KNN based ceramics had the potential of practical application as substitues for lead-based piezoceramics. Finally, the preparation methods of lead-free piezoceramics were described and the formulas of piezoelectric factors were given.Over the past few years, significant attention had been paid to the KNN family since the report of (Li, Ta, Sb) modified KNN ceramics with a high d₃₃ values of 300pC/N and a Curie temperature (T_c) of 253℃. However, the low T_c (253℃) limits the applications of this system to a narrow temperature range. In chapter 2, lead-free system (Na_0.52K_0.44Li_0.04)Nb_0.9-xSb_xTa_0.1O₃ (NKLNST) were prepared by a solid-state method. The effects of Sb content on piezoelectric and dielectric properties of NKLNST ceramics were investigated. The experimental studies showed that the substitution of Sb⁵⁺ for B-site ion Nb⁵⁺ and Ta⁵⁺ decreased the paraelectric cubic-ferroelectric tetragonal phase transition temperature (T_c). The polarization-electric field hysteresis behaviors for NKLNST ceramics were also studied in this chapter. It was found that the remnant polarization (P_r) decreased and coercive electric field (E_c) increased with increasing antimony amount. High performances with d₃₃=306pC/N, k_p=48%, k_t=50%, and a relatively high Curie temperature T_c=320℃were obtained for the 3.7mol% Sb doped composition. This was the best result for the ceramics prepared by the solid-state sintering techniques at that time. The results showed that adding an appropriate amount of Sb into NKLNT, both the piezoelectric properties and T_c were greatly improved. The appearance of tetragonal polymorphic phase around room temperature may be responsible for the improved piezoelectric properties.Considering the doping amount of Li was usually higher than 4mol% in previous reports about Li, Ta or Sb modified KNN-based piezoceramics, there have been few investigations on low Li substituted KNN-based one. In chapter 3, the composition (Na_0.5K_0.5)_0.975Li_0.025Nb_0.93-xSb_0.07Ta_xO₃ (NKLNST_x) were prepared with traditional solid-state process to study the effect of Ta doping on phase transition behavior, dielectric and piezoelectric properties of the low Li modified KNN-based ceramics. The experiment results showed that NKLNS doped with 18mol% Ta had excellent performances with d₃₃=330pC/N, mass densityρ=4.755g/cm³, dielectric losstanδ=1.9%. Excellent piezoelectric properties showed that the NKLNST_0.18 lead-free piezoelectric material was another potential substitute of PZT piezoceramics. Subsequently, the effects of Sb doping on the piezoelectric and dielectric properties of (Na_0.5K_0.5)_0.975Li_0.025Nb_0.82-xSb_xTa_0.18O₃(NKLNS_xT) lead-free piezoceramics had been investigated. It was found that T_c shifted toward lower temperature region while T_O-T changed little with increasing Sb content. The composition of NKLNS_0.06T possessed excellent piezoelectric properties with d₃₃ as high as 352pC/N. In addition to its high d₃₃ value, the ceramics also maintained desirable electromechanical coupling factors of k_p=47% and k_t=38%. In the end, the author analyzed the reasons for the high piezoelectric activity of the NKLNS_0.06T ceramics and pointed out that the NKLNS_0.06T ceramics was a promising candidate for lead-free piezoceramics.From the previous chapters, we had learnt that (Li, Ta, Sb) modified KNN based ceramics exhibited excellent piezoelectric properties comparable to those of PZT. The enhancement in piezoelectric properties of the modified KNN-based ceramics was generally attributed to the T_O-T near or at room temperature. However, the sharp decrease of piezoelectric and dielectric properties on both sides of T_O-T around room temperature was disadvantageous for usage. That is to say, the performance stability of the KNN-based ceramics with T_O-T around room temperature would become a "bottleneck" for their practical applications. Thereupon the key approach to enhance the temperature stability was to lower the T_O-T below room temperature. To enhance the thermal stability of KNN based ceramics, Zhang et al have prepared CaTiO₃ modified KNN based ceramics with high performances and a broad temperature usage range. Considering that SrTiO₃ is similar to CaTiO₃ in phase structure, it is expected that the addition of SrTiO₃ into KNN could be able to shift the T_O-T well down. In chapter 4, (1-x)(Na_0.53K_0.404Li_0.066)Nb_0.92Sb_0.08O₃-xSrTiO₃ [ abbreviated as (1-x)NKLNS-xST] ceramics with single perovskite phase had been synthesized by conventional solid-state sintering technique. Compared to the unmodified ceramics, the T_O-T of the ST modified ceramics was shifted to below room temperature. Meanwhile, relatively high piezoelectric properties were obtained. The author did a detailed analysis for the physical mechanism of the enhanced thermal stability of the ST modified ceramics. Subsequently, Ca_0.5Sr_0.5TiO₃ (CST) was doped into (Na_0.53K_0.407Li_0.063)Nb_0.937Sb_0.063O₃ (NKLNS) ceramics. The temperature dependences of piezoelectric constant d₃₃, dielectric constantε_r, dielectric loss tanδ, elastic compliance constant s₁₁^E and electromechanical coupling factor k₃₁ had beeninvestigated. It was found that the T_O-T and T_c decreased monotonously with increasing CST content, and the tetragonal phase regions were expended by the addition of CST. The changes of s₁₁^E and k₃₁ of CST doped NKLNS in thetemperature range of -50℃to 200℃were much less than that of the pure NKLNS ceramics, indicating the temperature stability of CST doped NKLNS ceramics was better than that of the undoped NKLNS ceramics. The ceramics with 1.5mol% CST exhibited high piezoelectric performances (d₃₃=202pC/N, k_p=44%) and low dielectric loss (tanδ=2%) at room temperature. The improved temperature stability associated with the excellent piezoelectric properties comparable to conventional PZT ceramics indicated that these ceramics could be applied over a wide temperature usage range.In chapter 5, another kind of promising lead-free candidate material Bi_0.5Na_0.5TiO₃ (NBT) was studied. NBT has been considered to be a promising candidate material for lead-free piezoelectric ceramics for its strong ferroelectrics at room temperature. However, pure NBT is hard to be poled for its relatively large conductivity and large coercive field. To solve the problems, some solid solutions have been added into NBT ceramics and remarkable results have been achieved. Based on the achievement in the past, lead-free piezoelectric ceramics 0.94(Na_0.5Bi_0.5)TiO₃-0.06Ba(Zr_0.055Ti_0.945)O₃ with d₃₃ as high as 185pC/N were successfully fabricated using the traditional solid-state process. By means of compensating the volatilization of Na, extra Na₂CO₃ was added into the above composition of NBT based ceramics. In order to reduce the dielectric loss, the effect of (Ce,Mn) dopants on the properties of0.94(Na_0.5+0.0008Bi_0.5)TiO₃-0.06Ba(Zr_0.055Ti_0.945)O₃ were studied. Subsequently, extra Bi₂O₃ was added into the above 0.94(Na_0.5Bi_0.5)TiO₃-0.06Ba(Zr_0.055Ti_0.945)O₃ composition to compensate the volatilization of Bi. The piezoelectric constant d₃₃ of the samples doped with extra 0.08mol% Bi₂O₃ wasas high as 218pC/N. Furthermore, the effects of Mn dopant on the piezoelectric and dielectric properties of 0.94(Na_0.5Bi_0.5016)TiO₃-0.06Ba(Zr_0.055Ti_0.945)O₃ were investigated, and one optimal composition, 0.2wt% Mn doped NBT-based piezoceramics with d₃₃=214pC/N, k_t=44%, tanδ=2.2% was obtained.