Phase structure and electrical properties of PSN–PMN–PZ–PT quaternary piezoelectric ceramics near the morphotropic phase boundary

ARTICLE IN PRESS Physica B 405 (2010) 1941–1945

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Phase structure and electrical properties of PSN–PMN–PZ–PT quaternary piezoelectric ceramics near the morphotropic phase boundary Haiyan Chen , Chunhua Fan Institute of Marine Materials Science and Engineering, Shanghai Maritime University, Shanghai 201306, China

a r t i c l e in fo

abstract

Article history: Received 7 October 2009 Received in revised form 15 January 2010 Accepted 15 January 2010

0.06Pb(Sb1/2Nb1/2)O3–0.06Pb(Mn1/3Nb2/3)O3–0.88Pb(ZrxTi1  x)O3 (PSN–PMN–PZ–PT) quaternary piezoelectric ceramics with varying Zr/Ti ratios located near the morphotropic phase boundary (MPB) were prepared by powder solid-state reaction. The phase structure, dielectric and piezoelectric properties and temperature stability of the systems were investigated. In the present system the MPB, in which the tetragonal and rhombohedral phases coexist, is in a composition range of 0.49 o xo 0.52. The relative permittivity, dielectric dissipation, piezoelectric coefficient and electromechanical coupling factor reach maximum values, while the mechanical quality factor is lowest when x=0.50. These properties include eT33/e0 =1730, tan d = 0.007, d33 = 365 pC/N, d31 =  151 pC/N, kp =0.62, k31 =0.37 and Qm = 1170. A Curie temperature of 308 1C was achieved when x =0.50. The resonant frequency changes from a positive to a negative value as the Zr/Ti ratio increases. The smallest temperature coefficient (Dfr/ DTfr25 1C =3.08  10  5/1C) was obtained between  50 and 120 1C in the sample when x =0.50. In the temperature range of 20–80 1C, the piezoelectric coefficient and electromechanical coupling factor show high temperature stability and the mechanical quality factor maintained a comparatively high value close to room temperature when x=0.50. The properties of this type of ceramics make it a very promising piezoelectric material for application in ultrasonic motors. & 2010 Elsevier B.V. All rights reserved.

Keywords: Piezoelectric ceramics Phase transition Morphotropic phase boundary Dielectric properties Piezoelectric properties

1. Introduction Lead zirconate titanate Pb(Zr,Ti)O3 (PZT), a solid solution of perovskite ferroelectric PbTiO3 and anti-ferroelectric PbZrO3 with different Zr/Ti ratios, is an important material that is widely used in electronic sensors, actuators, resonators and filters [1–3]. For use in high power devices such as ultrasonic motors, it is desirable for piezoelectric ceramics to combine a high mechanical quality factor (Qm) with high piezoelectric constants (d33, d31), high electromechanical coupling factors (kp, k31), high piezoelectric constant (eT33/e0) and low dielectric loss (tan d), as well as excellent temperature stability [4–8]. To meet above requirements, a number of polynary systems have been developed from PZT binary systems, such as Pb(Mn1/ 3Nb2/3)O3–Pb(Mg1/3Nb2/3)O3–PbZrO3–PbTiO3 [9], Pb(Mn1/3Nb2/ 3)O3–Pb(Zn1/3Nb2/3)O3–PbZrO3–PbTiO3 [10], Pb(Mg1/3Nb2/3)O3– Pb(Zn1/3Nb2/3)O3–PbZrO3–PbTiO3[11], Pb(Ni1/3Nb2/3)O3–Pb(Zn1/ and Pb(Ni1/2W1/2)O3–Pb(Mn1/ 3Nb2/3)O3–PbZrO3–PbTiO3[12] 3Nb2/3)O3–PbZrO3–PbTiO3 [13]. All of the compositions of these systems are close to the morphotropic phase boundary (MPB).

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E-mail address: [email protected] (H. Chen). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.01.092

Previous researchers have reported that the Pb(Mn1/3Nb2/3) O3–PbZrO3–PbTiO3 ternary system exhibits a very high mechanical quality factor [14,15]; on the other hand, Pb(Sb1/2Nb1/2)O3– PbZrO3–PbTiO3 has a high electromechanical coupling factor [16]. It is believed that Pb(Sb1/2Nb1/2)O3–Pb(Mn1/3Nb2/3)O3–PbZrO3– PbTiO3 (PSN–PMN–PZ–PT) quaternary piezoelectric ceramics, obtained by combining Pb(Mn1/3Nb2/3)O3–PbZrO3–yPbTiO3 with Pb(Sb1/2Nb1/2)O3–PbZrO3–PbTiO3, could possess many desirable properties. In this study, PSN–PMN–PZ–PT quaternary piezoelectric ceramics were investigated near the MPB by varying the Zr/Ti ratio. The purpose of this work was to study phase structure, dielectric and piezoelectric properties and temperature stability of these ceramics near the MPB in detail.

2. Experimental 2.1. Sample preparation The compositions of the PSN–PMN–PZ–PT system were Pb0.98Sr0.02[(Sb1/2Nb1/2)0.06(Mn1/3Nb2/3)0.06(ZrxTi1  x)0.88]O3 + 0.2 wt% CeO2 containing different Zr/Ti ratios (0.48 rxr0.53) close to the MPB. It is evident that cerium (Ce) has almost equal probability to replace Pb2 + at A site or Zr4 + /Ti4 + ions at B sites

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H. Chen, C. Fan / Physica B 405 (2010) 1941–1945

x = 0.53 x = 0.52

x = 0.51

x = 0.50

45

40

T (211)

T (112)

T (210)

T (102)

T (002)

T (200)

x = 0.49

50

x = 0.48

55

60

2θ (°)

2.2. Measurements

100

100 MPB (T+R)

T 80

R 80

60

60

40

40

20

20

0

Rhombohedral relative fraction (%)

Fig. 1. XRD patterns of the PSN–PMN–PZ–PT samples with different Zr/Ti ratios.

Tetragonal relative fraction (%)

The sintered samples were ground and polished to remove the surface layer for X-ray diffraction (XRD, D/MAX-RB, Rigaku, Japan) to identify the phase structure. Cu Ka radiation with a step of 0.011 was used. The piezoelectric constant (d33) was measured using a quasi-static method (Model ZJ-2, China). The capacitance and dielectric dissipation at 1 kHz were measured directly using an impedance analyzer (Agilent 4294A Precision Impedance Analyzer, Japan). The capacitance at a frequency of 1 kHz was measured over a temperature range of 25–350 1C using a computer-controlled measurement system (TH2617, Tonghui Electron, China), then the dielectric constant eT33 and relative permittivity eT33/e0 were calculated from the capacitance and dimension of the samples. The resonant–antiresonant frequencies (RAF) were also measured using an impedance analyzer (Agilent 4294A Precision Impedance Analyzer, Japan) in an oven over a temperature range of  100 to 175 1C. The electromechanical coupling coefficient (kp and k31), the elastic compliance (sE11) and the mechanical quality factor (Qm) were calculated from the RAF, and then the transverse piezoelectric constant (d31) was calculated from k31, sE11 and eT33.

R(210)

R(200)

has both ‘‘donor’’ and ‘‘acceptor’’ abilities, improving kp and Qm. The modification of CeO2 near the MPB composition of PZT exhibits a large effect on the electrical properties [18]. Commercially available PbO, SrCO3, Sb2O3, Nb2O5, ZrO2, CeO2, MnCO3 and TiO2 were used as the raw materials. Each mixture of the starting powders was mixed in a centrifugal mill with absolute alcohol using an agate ball for 3 h. The powders were then calcined at 850 1C for 3 h. The PSN–PMN–PZ–PT perovskite powders were mixed with 6 wt% polyvinyl alcohol (PVA) solution and then pressed into disks 12 mm in diameter at 100 MPa. The samples were sintered at 1260 1C for 3 h in a covered alumina crucible. To prevent PbO evaporating from the pellets, PbZrO3 was used as a packing powder. The pellets were polished to a thickness of 0.5 mm. Electrodes were made by applying a silver paste (Kunming Institute of Precious Metals, China) on the two major faces of the disks followed by heat treatment at 830 1C for 8 min and subsequent poling in a DC electric field of 30–40 kV/cm in a silicon oil bath at 120 1C.

R(211)

of ABO3 perovskite lattice. This is feasible because Ce can exist in ˚ by Ce3 + both + 3 and + 4 valence states and replace Pb2 + (1.22 A) ˚ and/or Zr4 + (0.79 A) ˚ by Ce4 + (0.96 A) ˚ [17]. Moreover, CeO2 (1.15 A)

0 0.48

0.49

0.50

0.51

0.52

0.53

Zr/(Zr+Ti) Fig. 2. Variation of the relative content of the tetragonal and rhombohedral phases with different Zr/Ti ratios in the PSN–PMN–PZ–PT samples.

3. Results and discussion

equations [19]:

3.1. Phase structure

MR ¼

IRð200Þ IRð200Þ þ ITð002Þ þ ITð200Þ

ð1Þ

MT ¼

ITð200Þ þITð002Þ IRð200Þ þ ITð002Þ þ ITð200Þ

ð2Þ

Fig. 1 shows the room temperature XRD patterns of the sintered PSN–PMN–PZ–PT samples with different Zr/Ti ratios. It is observed that a pure perovskite phase is obtained. The tetragonal, rhombohedral and tetragonal–rhombohedral phases are identified by an analysis of the peaks [002 (tetragonal), 200 (tetragonal), 200 (rhombohedral)] in the 2y range of 43–461. The splitting of the (0 0 2) and (2 0 0) peaks indicates that they are the ferroelectric tetragonal phase (FT), while the single (2 0 0) peak is consistent with the ferroelectric rhombohedral phase (FR). A transition from the tetragonal to the rhombohedral phase is observed as Zr/Ti ratio increases. The multiple peak separation method was used to estimate the relative fraction of coexisting phases. The relative phase fraction (MR) was then calculated using the following

where IR(2 0 0) is the integral intensity of the (2 0 0) reflection of the rhombohedral phase and IT(2 0 0) and IT(0 0 2) are the integral intensities of the (2 0 0) and (0 0 2) reflections of the tetragonal phase, respectively. Analysis of the relative phase fractions in the PSN–PMN–PZ–PT ceramics indicates that the tetragonal and rhombohedral phases coexist in the composition range of 0.49 ox o0.51 as shown in Fig. 2. The MPB shifts to higher Ti concentration compared with the well-known MPB of pure PZT. It can also be seen clearly that the MPB is over a broad composition range. The width of the MPB in PZT systems has been extensively investigated and found to be

ARTICLE IN PRESS H. Chen, C. Fan / Physica B 405 (2010) 1941–1945

related to the heterogeneous distribution of Zr4 + and Ti4 + cations on the B-site of the perovskite lattice (ABO3) [20]. Fig. 3 shows the lattice constants of the a and c axes for the samples calculated from the XRD patterns. The lattice structure changes from the tetragonal to the rhombohedral phase as the ratio of Zr to Ti increases. In the rhombohedral phase, the lattice constant increases as the Zr/Ti ratio increases, and in the tetragonal phase, the c/a ratio decreases with increasing Zr/Ti ratio.

4.16 MPB (T+R)

T

R

4.10

aR

cT

4.08 4.06

3.2. Dielectric and piezoelectric properties

aT

4.04 4.02 4.00 0.48

0.49

0.50 0.51 Zr/(Zr+Ti)

0.52

0.53

Fig. 3. Variation in the lattice parameters with changes in the Zr/Ti ratio (aT: a axis of the tetragonal phase; cT: c axis of the tetragonal phase; aR: a axis of the rhombohedral phase).

1800

0.025

1600

0.020 0.015

1200

0.010

tanδ

εT33/ε0

1400

1000 0.005 800 0.000 600 0.48

0.49

0.50

0.51

0.52

0.53

Zr/(Zr+Ti) Fig. 4. Dielectric properties of PSN–PMN–PZ–PT as a function of the Zr/Ti ratio (f= 1 kHz).

The dielectric properties at room temperature are plotted as a function of the Zi/Ti ratio in Fig. 4. eT33/e0 increases with increasing Zr/Ti ratio, achieving a maximum value of 1730 when x =0.50, and then decreases significantly as the Zr/Ti ratio increases further. The tan d shows a similar trend and reaches a maximum value of 0.007 when x = 0.50. Fig. 5 shows the piezoelectric coefficient (d33, d31), planar coupling factor (kp, k31) and mechanical quality factor (Qm) as a function of the Zr/Ti ratio. PSN–PMN–PZ–PT exhibits desirable d33, d31, kp and k31 values around the MPB. From the trend of the piezoelectricity, the sample reaches the maximum values of d33 = 365 pC/N, d31 = 151 pC/N, kp = 0.62, k31 = 0.37 when x = 0.50. In the MPB region, it is common that piezoelectric ceramics show high piezoelectric and dielectric activity [1]. The dielectric and piezoelectric properties of the samples also confirm that the MPB in PSN–PMN–PZ–PT quaternary piezoelectric ceramics is close to x= 0.50. A minimum value of Qm of 1165 is obtained when x = 0.50 (Fig. 5), in contrast to the dielectric and piezoelectric properties. Because the tetragonal and rhombohedral phases coexist near the MPB and can move easily, the increase in internal friction results in a minimum of Qm, which reflects the mechanical loss. In spite of that, the Qm of the sample with x =0.50 is high enough to allow its use in high power devices such as ultrasonic motors. Because the Curie temperature of PbZrO3 (Tc =230 1C) is lower than PbTiO3 (Tc = 490 1C), increasing the Zr/Ti ratio causes the Tc of the PSN–PMN–PZ–PT ceramics to decrease. Consequently the peak in the dielectric spectrum corresponding to the Tc moves towards room temperature, as shown in Fig. 6. The dielectric constant peaks are in the range of 13,000–21,000. The highest dielectric constant peak is achieved in the sample with x =0.50, which gives a Tc of 308 1C.

2000

0.60

400

1800

350

1600

300

Qm

0.65

450

1400 d31 d33 Kp K31 Qm

1200

1000

250 200

0.55 0.50 0.45 0.40 0.35

150 0.30 100

800 0.48

0.49

0.50

0.51

0.52

0.53

Zr/(Zr+Ti) Fig. 5. d33, d31, kp, k31 and Qm of PSN–PMN–PZ–PT as a function of the Zr/Ti ratio.

Kp and K31

Lattice constant (Å)

4.12

d33 and -d31 (pC/N)

4.14

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3.3. Temperature stability Fig. 7 shows the variation in the temperature coefficient (Dfr/ fr25 1C) of the resonant frequency (fr) of the samples with different Zr/Ti ratios. The temperature coefficient changes from a positive to a negative value as the Zr/Ti ratio increases and obtains a minimum value (Dfr/DTfr25 1C = 3.08  10  5/1C) from  50 to 120 1C when x = 0.50. It is a general trend that with increasing ambient temperature, fr decreases because the elastic compliance coefficient increases [21]. This is because the equilibrium distance between the ions increases, weakening their interaction in a thermal field. Hence, larger strain is created under smaller stress, that is, the elastic compliance coefficient increases, resulting in a decrease in the resonant frequency. In contrast, the temperature coefficient is related to the domain structure and movement of the domain walls. Motion of the whole crystal grain induced by motion of the non-1801 domain walls generates elastic deformation, which creates an internal stress field in ceramics, which reduces the elastic compliance coefficient. The effect of the stress field dominated that of the thermal field in the tetragonal phase, which led to the positive temperature coefficient of the resonant frequency. The negative temperature coefficient for the rhombohedral branch phase was

because of the contribution of the thermal field [22]. As a result, the temperature dependence of fr shows complicated behavior near the MPB. Because the temperature coefficient changes from a positive to a negative value with increasing Zr/Ti ratio, there exists a point where the temperature coefficient of fr reaches a minimum near the MPB [23]. Fig. 8 shows the temperature dependence of k31 for three typical samples where x = 0.48, 0.50 and 0.53. It is observed that k31 for the sample with x= 0.48 is in the tetragonal phase region and decreases slowly as the temperature increases until 80 1C and then decreases sharply when the temperature is over 80 1C. In the MPB (when x = 0.5), k31 shows similar behavior to that in the tetragonal phase region. In the rhombohedral phase region (when x= 0.53), k31 increases steadily as the temperature increases. The results agree with that of a previous study [24]. The internal stress that was developed in the structure may have been caused by the mismatches in thermal expansion coefficients between domains and grains [25]. Further research is being conducted into the mechanism of the varying k31 in different phase structures. We find that the sample with x = 0.50, which is within the MPB, has a high temperature stability (Dk31/DT= 6.33/1C) in the temperature

0.42

24000 0.40

x = 0.48, Tc = 323°C 20000

0.38

x = 0.50, Tc = 308°C

0.36

x = 0.51, Tc = 302°C

0.34

x = 0.52, Tc = 292°C

K31

33

εT /ε0

16000

x = 0.49, Tc = 316°C

x = 0.53, Tc = 287°C

12000

0.32 0.30

8000

0.28

x = 0.48 x = 0.50 x = 0.53

0.26

4000

0.24 0.22 -150

0 50

100

150

200

250

300

350

-100

-50

0

T (°C) Fig. 6. Temperature dependence of the dielectric constants of PSN–PMN–PZ–PT with different Zr/Ti ratios (f =1 KHz).

1.5

150

200

Fig. 8. Temperature dependence of k31 for PSN–PMN–PZ–PT samples with x= 0.48, 0.50 and 0.53.

x = 0.48 x = 0.50 x = 0.53

150 140 -d31 (pC/N)

Δfr/fr25° C (%)

0.5

100

160

x = 0.48 x = 0.49 x = 0.50 x = 0.51 x = 0.52 x = 0.53

1.0

50 T (°C)

0.0

130 120 110

-0.5

100 90

-1.0

80 -1.5 -50

-25

0

25

50

75

100

T (°C) Fig. 7. Temperature dependence of the resonant frequency of PSN–PMN–PZ–PT with different Zr/Ti ratios.

-150

-100

-50

0

50

100

150

200

T (°C) Fig. 9. Temperature dependence of d31 for PSN–PMN–PZ–PT samples with x= 0.48, 0.50 and 0.53.

ARTICLE IN PRESS H. Chen, C. Fan / Physica B 405 (2010) 1941–1945

2400 x = 0.48 x = 0.50 x = 0.53

2200 2000

Qm

1800 1600 1400 1200 1000 800 600 -100

-50

0

50 T (°C)

100

150

200

Fig. 10. Temperature dependence of Qm for PSN–PMN–PZ–PT samples with x= 0.48, 0.50 and 0.53.

range of 20–80 1C. This is significant for the practical application of PSN–PMN–PZ–PT in high power devices. Increasing the temperature enhances the transverse piezoelectric constant d31 steadily for the samples where x= 0.48, 0.50 and 0.53 (Fig. 9). This is because d31 is derived from the equation qffiffiffiffiffiffiffiffiffiffiffiffiffi d31 ¼ k31 eT33 sE11 . The increase in d31 is because of the increase in T 33,

e

T 33

and e is the dominant factor in controlling the trend of d31, more than k31 and sE11. Fig. 9 shows the temperature dependence of d31 for tetragonal PSN–PMN–PZ–PT ceramics when x= 0.48 is less than that for rhombohedral ceramics when x = 0.53 because the increase in eT33 is compensated by decreases in k31 and sE11. On the other hand, the dependence of d31 for rhombohedral PSN– PMN–PZ–PT ceramics is large because eT33, k31 and sE11 increase with temperature [24]. Fig. 10 shows the Qm for three typical samples with x= 0.48, 0.50 and 0.53 as a function of temperature. In the tetragonal phase and MPB regions, Qm has maximum values of 1650 at 25 1C when x =0.48 and 1175 at 30 1C when x= 0.50, respectively. In the rhombohedral phase region, Qm steadily decreases as the temperature increases. Qm maintains a comparatively high value close to room temperature.

4. Conclusions From the results presented, PSN–PMN–PZ–PT quaternary ceramics exhibit a MPB over a broad composition region of 0.49ox o0.52 where the tetragonal and rhombohedral phases

1945

coexist. When x= 0.5 in the sample, maximum values of eT33/e0, tan d, d33, d31, kp, k31 are achieved. The mechanical quality factor is at its lowest when x= 0.5, but remains high enough to allow the material to still be useful for high power applications. As the Zr/Ti ratio increases, the Tc of PSN–PMN–PZ–PT ceramics decreases and consequently the peak in the dielectric spectrum corresponding to the Tc moves towards room temperature. A Tc of 308 1C is obtained when x= 0.50. The resonant frequency changes from a positive to a negative value as the Zr/Ti ratio increases. The smallest temperature coefficient (Dfr/DTfr25 1C = 3.08  10  5/1C) was obtained between  50 and 120 1C in the sample with x= 0.50. Over the temperature range of 20–80 1C, where high power devices such as ultrasonic motors are usually operated, d31 and k31 show high temperature stability. Qm maintains a comparatively high value close to room temperature when x= 0.50.

Acknowledgment This work was supported by Shanghai Maritime University Foundation.

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