Feature articleHigh piezoelectricity in CuO-modified Ba(Ti0.90Sn0.10)O3 lead-free ceramics with modulated phase structure
Introduction
Pb(Zr1-xTix)O3 (PZT) based ceramics have been widely used for more than 50 years due to their excellent piezoelectric properties. But their usage will be gradually replaced by lead-free piezoelectric ceramics such as BaTiO3 [1], [2], (K,Na)NbO3 [3] or (Bi1/2Na1/2)TiO3 [4] for environment protecting concerns. BaTiO3 ceramics were first discovered as a piezoelectric ceramic but are now mostly used as dielectric materials rather than piezoelectric materials mainly because of their inferior piezoelectric properties (d33 = 191pC/N). Recently, high-performance BaTiO3-based ceramics such as (Ba,Ca)(Ti,Zr)O3 (BCTZ) [5], [6] and (Ba,Ca)(Ti,Sn)O3 (BCTS) [7], [8], [9], [10], [11] have been developed by co-doping Ca2+ and Zr4+ or Ca2+ and Sn4+ via establishing two or more phases coexistence. At present, the highest d33 of 697pC/N is obtained in Ba(Ti0.89Sn0.11)O3 ceramic in the vicinity of two converged triple points with coexisting multiphase [12]. However, d33 sharply deteriorates as the composition deviates from Ba(Ti0.89Sn0.11)O3. Meanwhile, d33 of Ba(Ti1-xSnx)O3 (BTS) (0 ≤ x ≤ 0.12) system at room temperature is still lower than 350pC/N even at the compositions with tetragonal-orthorhombic (T-O) (x = 0.02) and orthorhombic-rhombohedral (O-R) (x = 0.09) two-phase coexistence [12]. All those indicate that the piezoelectric property shows a strong dependence on the phase structure in BTS ceramics. The composition-stability of high performance BTS system still needs to be improved. The enhanced piezoelectric property in BCTS and BTS ceramics is at the expense of upping sintering temperature (Ts) over 1400 °C [7], [8], [9], [10], [11], [12] which impedes their applications. It is necessary to reduce the densification Ts of high-performance BTS piezoelectric ceramics. CuO as a sintering aid has been used to lower the Ts of Ba(Ti0.96Sn0.04)O3 [13], Ba(Ti0.9625Zr0.0375)O3 [14], (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 [15] and (Ba0.95Ca0.05)(Ti0.90Sn0.10)O3 [16] ceramics. Adding 1 mol% CuO reduced Ts to 1250 °C from 1350 °C and increased d33 to 390pC/N from 205pC/N for Ba(Ti0.96Sn0.04)O3 ceramics [13]. Decreased Ts from 1450 °C to 1300 °C and increased d33 from 260pC/N to 300pC/N were obtained in 1 mol% CuO doped Ba(Ti0.9625Zr0.0375)O3 ceramics [14]. The sinterability of (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 ceramics is also improved by the addition of CuO, which results in a reduction of Ts from 1540 °C to 1350 °C without sacrificing the high piezoelectric properties [15]. Our previous work also approved that the incorporation of 2 mol%CuO not only lowered Ts to 1250 °C from 1480 °C but also enhanced d33 to 683pC/N by structuring the coexisting multiphase for (Ba0.95Ca0.05)(Ti0.90Sn0.10)O3 ceramics at room temperature [16]. Besides the lowered Ts, the phase transition temperature of matrix ceramics can also be affected by adding CuO. For example, the phase transition temperatures TO-T and TC in Ba(Ti0.96Sn0.04)O3 ceramic decreased after adding CuO [13]. 1 mol% CuO-modified Ba(Ti0.9625Zr0.0375)O3 ceramics show lower TR-O and TC and higher TO-T than Ba(Ti0.9625Zr0.0375)O3 ceramics [14]. The varied TO-T, TR-O and TC would affect the piezoelectric properties. Since the phase structure of BaTiO3-based ceramics at room temperature is strongly affected by the introduced CuO and Ts, further enhanced piezoelectric properties can be expected in BTS ceramics by phase engineering via adding CuO and optimizing Ts.
In this work, Ba(Ti0.90Sn0.10)O3 ceramic with d33 of 300pC/N as sintered at 1400 °C [12] was chosen as target. We found that both the sinterability and piezoelectric properties of Ba(Ti0.90Sn0.10)O3 ceramic were improved as adding CuO. d33 of Ba(Ti0.90Sn0.10)O3 ceramic increased to 650pC/N by optimizing the CuO content (x) and Ts, which is two times higher than the undoped one. The effects of x and Ts on the phase structure and electrical properties in Ba(Ti0.90Sn0.10)O3 ceramics were also discussed concerning with BCTS, BCTZ and BTS systems.
Section snippets
Experimental procedure
A series of CuO-doped Ba(Ti0.90Sn0.10)O3 (abbreviated as BTSCux, x = 0, 0.5, 1.0, 1.5 and 2.0 mol%) ceramics were prepared by conventional solid-state synthesis using BaCO3, SnO2, TiO2 and CuO (>99%, all from Shantou Xilong Chemical Factory Guangdong, China) as the raw materials. The weighed Ba(Ti0.90Sn0.10)O3 powders were mixed by using a planetary ball mill with anhydrous ethanol under 300 rpm for 12 h in a nylon jar and then dried at 80 °C for 8 h in an oven. The dried powders were calcinated at
Effect of CuO content on microstructure, phase structure and electrical properties of BTS ceramics
Fig. 1a–e shows SEM images of fracture surface of BTSCux (x = 0.5–2.0) ceramics at Ts = 1300 °C. BTS ceramic (Fig. 1a) has a porous microstructure with small grains of 1–2 μm and low density ρr of 77% due to its high dense sintering temperature (Ts >1400 °C) [12]. Highly dense microstructures with grown grains and ρr > 95% (Fig. 1b–e) were developed after adding CuO. The enhanced sinterability is attributed to intermediately formed BaO-CuO liquid phase (with a low melting point of 925 °C) [16], [17]. The
Phase structure and piezoelectric property
Normally, BaTiO3 ceramics have three ferroelectric phases including R, O and T phase which has 8 〈111〉, 12 〈110〉 and 6 〈001〉 spontaneous polarization directions, respectively. The basic approach to achieving high piezoelectricity is to place the composition of the material to the proximity of a composition-induced phase transition between two ferroelectric phases or more. Such a transition has been known as the “morphotropic phase boundary (MPB)” or “polymorphic phase boundary (PPB)” in the
Conclusions
CuO-doped Ba(Ti0.90Sn0.10)O3 ceramics were fabricated by traditional solid state method at 1200–1550 °C. The phase structure of Ba(Ti0.90Sn0.10)O3 ceramics can be affected by CuO addition and sintering temperature. With increasing sintering temperature, the phase structure of BTSCu1.0 samples changed from coexisting R-T phases to R-O-T phases at room temperature. The piezoelectric property of BTSCu1.0 ceramics shows a dependence on phase structure: 250–360pC/N in R-T phase coexisting group and
Acknowledgments
This work was supported by Specialized Research Fund for the Doctoral Program of Higher Education (grant No. 20130006110006) and National Natural Science Foundation of China (grant No. 51472026 and 51332002).
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