EFFECT OF ANNEALING TREATMENTS ON ...

Journal of Science and Technology 50 (1B) (2012) 418-424

EFFECT OF ANNEALING TREATMENTS ON MICROSTRUCTURE AND PIEZOELECTRIC PROPERTIES OF ZnO-DOPED Pb(Zr,Ti)O3-Pb(Sb1/2Nb1/2)O3-Pb(Mn1/3Nb2/3)O3 LOW TEMPERATURE SINTERED CERAMICS Nguyen Dinh Tung Luan1,*, Truong Van Chuong2, Dang Anh Tuan2, Phan Thanh Ha2, Doan Nam Huu1 1 2

Hue Industry College, 77 Nguyen Hue, Hue City, Vietnam

Department of Physics, College of Sciences, Hue University, Vietnam *

Email: [email protected]

Received: 2 November 2011; Accepted for publication: 3 June 2012

ABSTRACT Polycrystalline ceramics of 0.9Pb(Zr0.49Ti0.51)O3-0.03Pb(Sb1/2Nb1/2)O3– 0.07Pb(Mn1/3Nb2/3)O3 (PZT-PMnN-PSbN) modified with a different amounts of ZnO sintering aid were prepared by Columbite and Wolframite prescuror method and thoroughly investigated in order to aim at lowering the sintering temperature. It showed that all samples with amount of ZnO upto 0.4 %wt were able to reduce the sintering temperature from 1200 0C to 950 0C due to forming the liquid phase along the grain boudary. The grain size, kp, d31 and Qm of the optimal amount of 0.25 %wt ZnO-doped PZT-PMnN-PSbN had values of 7.78 g/cm3, 7.55 m, 0.60, 217 pC/N and 2013, respectively, which were quite comparable properties of conventional undoped sample. The effect of annealing condition on the properties of PZT-PMnN-PSbN with ZnO-doped ceramic was also studied in detail. The results showed that the samples as annealed at 750 0C for 8 h exhibited: a decrease of amounts of the second phase, an increasing the values of grain size, density, kp and T33, and a strong decrease of the value of dielectric loss. Keywords: sintering aid, Columbite prescuror method, liquid phase, annealing condition.

1. INTRODUCTION A high electric field and high vibration level are required for high power piezoelectric devices associated with significant heat generation such as piezoelectric actuators, ultrasonic motors, and piezoelectric transformers. The materials for these applications should have

Effect of annealing treatments on microstructure and piezoelectric properties of ZnO-doped …

compromised characteristics between hard and soft piezoelectrics, implying high electromechanical coupling factor (k) and piezoelectric constant (d) with high mechanical quality factor (Qm) [1]. However, the sintering temperature of lead zirconate titanate (PZT)-based high power compositions is usually too high, approximately 1200 oC. Consequently, lowering of the sintering temperature of piezoelectric ceramics is essential for the fabrication of cost effective multilayer piezoelectric devices. Furthermore, low temperature sintering can provide advantages such as compatibility with low temperature cofired ceramics, the reduction of energy consumption, and the reduced PbO volatilization. Previously, various techniques were employed to obtain the low temperature sinterable PZT composition. The addition of dopants, which improves solid-state sintering, and the addition of oxides and compounds, which have low melting points for liquid phase sintering are the most popular methods [2]. The other processes such as: sintering in an inert atmosphere followed by hot pressing or use of fine starting powders, are not generally used due to their expensive, complicate and laborious procedure. Some of the oxides and compounds that have been used for assisting liquid-phase sintering are Li2CO3, CuO + Bi2O3, etc. Addition to, the electrical properties of these oxides are strongly influenced by their microstructure, defects, compositional inhomogeneity, external field and domain wall motion, so thermal annealing treatment is a very simple, economical, relatively new and effective technique to enhance the electrical properties of lead based complex perovskite. Normally, thermal annealing can be used to improve grain size and reduced oxygen vacancy of ceramics during lead volatilization in calcinations and sintering process. Improvement of dielectric and piezoelectric properties of ceramics was also observed in PZT – PZN after thermal annealing [3]. Previously, we successfully fabricated the PZT – PMnN – PSbN ceramics with high electro-mechanical coupling factor (k) and piezoelectric constant (d) with high mechanical quality factor (Qm) [4,5], but the temperature sintering was very high, approximately 1250 oC. Therefore, in this study, we has investigated and discussed the effect of ZnO amounts on the electrical properties of PZT – PMnN – PSbN ceramics sintered at low temperature and the effects of annealing treatments on the electrical properties of the optimal PZT – PMnN – PSbN + %wt ZnO ceramic. 2. EXPERIMENTAL 2.1. Samples preparation PZT – PMnN – PSbN + x %wt ZnO ceramics were prepared from reagent grade raw material oxides via the Columbite and Wolframite method in order to suppress the formation of pyrochlore phase. The processing of synthesis was through three steps: Step 1: Synthesize MnNb2O6 and Sb2Nb2O8 ;MnCO3 and Nb2O5; Sb2O3 and Nb2O5 were mixed and acetone- milled for 20 h in a zirconia ball mill and then calcined at 15000C for 3 h to form MnNb2O6 and Sb2Nb2O8. The material was acetone-ground for 10 h in the mill and dried again. Step 2: Synthesize PZT – PMnN – PSbN calcined powders

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Reagent grades PbO, ZrO2, TiO2 were mixed with MnNb2O6 and Sb2Nb2O8 powders by ball mill for 20 h in acetone. The mixed powders were dried and calcined at 850 0C for 2 h and then the calcined powders were ground by ball mill in acetone for 24 h. Step 3: Synthesize PZT – PMnN – PSbN + x %wt ZnO ceramics The PZT – PMnN – PSbN calcined powders were mixed with x %wt ZnO, x = 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5 symbol for Z05, Z10, Z15, Z20, Z25, Z30, Z40, Z50, respectively, and acetone-milled for 8 h in the zirconia ball mill and then dried. The ground materials were pressed into disk 12mm in diameter and 1.5 mm in thick under 100MPa. The samples were sintered in air at 950oC for 3 h in an alumina crucible to form PZT – PMnN – PSbN + ZnO ceramics. Finally, the best sample was chosen and carried out thermal annealing treatments at 750 oC for 4 h, 6 h, 8 h and 10 h. Symbol for ZA1, ZA2, ZA3, ZA4, respectively. The sintered and annealed samples were ground and cut to 1mm in thick. A silver electrode was fired at 680 0C for 10 minutes on the major surfaces of samples. Poling was done in the direction of thickness in a silicon oil bath under 30 kV/cm for 15 minutes at 120 0C. 2.2. Microstructure, dielectric and piezoelectric measurement The bulk densities of sintered specimens were measured by Archimedes technique. The crystalline phase was analyzed using an X-ray diffractometer (XRD). The microstructure of the sintered bodies was examined using a scanning electron microscope (SEM). The grain size was measured by using the line intercept method. The dielectric permittivity and dielectric dissipation of samples were measured by the highly automatized RLC HIOKI 3532 at 1 kHz. The electromechanical coupling factor, kp, mechanical quality factor, Qm, and several other piezoelectric constants were calculated by a resonant – anti resonant frequency method measured using an automatized impedance analyzer (HP-4193A). 3. RESULTS AND DISCUSSION 3.1. Microstructure and piezoelectric properties of PZT – PMnN – PSbN + x%wt ZnO low temperature sintered ceramics 3.1.1. Microstructure Figure 1 shows SEM micrographs of fractured surface of ZnO added PZT – PMnN – PSbN specimens sintered at 950 °C for 2 h. The sintering aid added PZT – PMnN – PSbN specimens showed uniform and densiﬁed structure. In the ZnO added PZT – PMnN – PSbN systems, the low-temperature sintering mechanism primarily originated from transition liquid phase sintering. In the early and middle stages of sintering process, ZnO additives with a low melting point forms a liquid phase, which wets and covers the surface of grains, and facilitates the dissolution and migration of the species.

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a)

b)

c)

d)

e)

f)

g)

h)

Figure 1. SEM micrographs of fractured surface of PZT – PMnN – PSbN specimens with different amounts of ZnO additive: a) 0.05 wt%, b) 0.1 wt%, c) 0.15 wt%, d) 0.2 wt%, e) 0.25 wt%, f) 0.3 wt%, g) 0.4 wt% and 0.5 wt% 400

250

(a )

(10 1 )

100 80

200 150

120

(1 1 1 )

(1 0 0)

(20 0 )

(2 10 )

x = 0 .5 0 x = 0 .4 0 x = 0 .3 0 x = 0 .2 5 x = 0 .2 0 x = 0 .1 5 x = 0 .1 0 x = 0 .0 5

100 50 0 -5 0 20

(2 11 )

60 40 20 0 -20

30

40

T h e ta (D e g .)

50

(b)

I(Cps)

300

I(Cps)

140 350

-40 40

60

x =0.50 x =0.40 x =0.30 x =0.25 x =0.20 x =0.15 x =0.10 x =0.05 42

44 Theta (Deg.)

46

48

Figure 2. XRD patterns of PZT– PMnN – PSbN specimens with different amount of ZnO additive

3.3.3. Piezoelectric properties Figure 3 shows the changes in the electromechanical coupling factor (kp) and the piezoelectric constant (d33) as a function of the amount of ZnO addition. As can be seen, both kp and d33 show a similar variation with increasing ZnO content. When the amount of ZnO is lower than 0.25 wt %, kp and d33 are rapidly increased with increasing ZnO content, respectively. The optimized values for kp of 0.60 and d31 of 217 m/V were obtained for 0.25 wt % ZnO content, which corresponds well with the composition near the solution limit. It is known that the substitutions of high valence ions of Zr4+, Ti4+, Nb5+ and Sb3+ by acceptor dopant Zn2+ ions will lead to the creation of oxygen vacancies for ionic charge compensation, which pin the movement of the ferroelectric domain walls and result in a increase of kp and d33.

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2500

0.61 0.60 0.59

200 160 120 80 -0.1

0.58

0.40 2400

0.36

tan

Qm tan d31 kp

kp

240

d31x 10

-12 (m/V)

280

0.32 2300

Qm

0.28

0.57

0.24

0.56

0.20

0.55

0.16

0.54

0.12

2200 2100

0.0

0.1 0.2 0.3 0.4 0.5 Amounts of ZnO additive, x(wt%)

2000

0.6

Figure 3. Electromechanical coupling factor (kp), the piezoelectric constant (d31), electromechanical quality factor (Qm) and dielectric loss (tan) of PZT - PMnN – PSbN ceramics with different amounts of ZnO additive

Table 1. Piezoelectric parameters of PZT–PMnN–PSbN + x wt% ZnO at different annealing time Sample kp

Z05

Z10

Z15

Z20

Z25

Z30

Z40

Z50

ZA1

ZA2

ZA3

ZA3

0.57

0.58

0.58

0.59

0.60

0.58

0.56

0.55

0.60

0.61

0.64

0.64

154

157

172

179

217

170

115

112

217

229

245

246

-12

d31×10 (m/V) Qm

2424 2395 2062 2017 2013 2032 2128 2133 2013 2011 2003 2002

The electromechanical quality factor (Qm) value gradually decreased with increasing of amount of ZnO. As the content of ZnO increased, the larger number of the Zn ions were able to substitute in the B sites in the perovskite structure of PZT – PMnN – PSbN systems. This substitution created the internal bias field which prohibits the domain wall motion under a small electric signal which was usually used for Qm measurement and thus reduced the Qm values. From this study, the optimum Qm found in the sample with 0.25 wt% ZnO at sintering temperature of 950 oC was 2013. This condition was considered suitable to for high power applications. 3.2. Microstructure and piezoelectric properties of PZT – PMnN – PSbN + 0.25%wt ZnO ceramics thermal annealed treatments 3.2.1. Microstructure Figure 4 shows the XRD patterns of the samples annealed at 750 oC for the different time. It can be seen in Fig. 6 that all samples exhibit a pure perovskite structure, and that there is no secondary phase. No distinction between the samples thermal annealed and non annealed.

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However, the SEM images in Fig. 4 indicated that the annealed samples were more homogeneous than non-annealed samples so much and the grain size also increased. 400

300 250

I (Cps)

350

a )

b )

c )

d )

200 150

ZA4 ZA3 ZA2 ZA1

100 50

Z25

0 20

30

40 Theta (Deg.)

50

60

Figure 4. XRD patterns and SEM micrographs of PZT – PMnN – PSbN specimens 0.25wt % ZnO additive annealed at 750oC for various time: a) 4 h, b) 6 h, c) 8 h, d) 10 h

3.2.3. Piezoelectric properties Figure 5 shows the changes in the electromechanical coupling factor (kp) and the piezoelectric constant (d31) as a function of time treatment. Both kp and d31 increased with increasing the time annealing up to 750 oC for 8 h. The optimal values of kp and d31 observed in this sample were significantly improved, approximately 0.64 and 245 m/V, respectively. The electromechanical quality factor (Qm) value gradually decreased with increasing of time treatment ( 2003) (table 1). 250

2014 0.110

0.64

kp 230

0.62

tan

Qm tan d31 kp

0.63

d31x 10

-12

(m/V)

2012 240

0.108

2010 Q

m

0.106

2008

0.104

2006

0.102

2004

0.100

2002

0.61

220

0.60

210 2

3

4

5

6 7 8 9 Annealing time,(h)

10

11

12

Figure 5. Electromechanical coupling factor (kp), the piezoelectric constant (d31), electromechanical quality factor (Qm) and dielectric loss (tan) of PZT - PMnN – PSbN + 0.25 wt% ZnO ceramics with different annealing time

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4. CONCLUDING REMARKS The results obtained from the experiment are as follows: The PZT – PMnN – PSbN ceramics with the addition of ZnO aids were fabricated by Columbite and Wolframite ceramic techniques. ZnO addition is an effective way to improve both the sintering ability and electric response of PZT – PMnN – PSbN relax ferroelectric ceramics. The transition liquid phase sintering mechanism induced by ZnO allowed the densiﬁcation of the ceramics at a sintering temperature of 300 °C lower than that of undoped specimens. Analysis of the microstructure evolution showed that the solubility limit of Zn ions in PZT – PMnN – PSbN systems was about 0.25 wt%. The piezoelectric behavior also depends on ZnO doping. The optimized values for kp of 0.61 and d33 of 457 pC/N were obtained at 0.25 wt % ZnO addition. By thermal annealing, most of the properties of all samples were significant improved. The optimal sample observed in PZT – PMnN – PSbN + 0.25 wt% ZnO sintered at 950 oC for 3 h and annealed at 750 oC for 8 h with kp of 0.64 and d31 of 457 pC/N, while Qm has no significant decreased, approximately 1988. It can be concluded that the optimal specimen with the solubility suitably of soft and hard compositions is best for piezoelectric transformer applications. Acknowledgments. This work is supported in part by the Pre-Doctor fund of Hue University.

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