The effects of sintering atmosphere on microstructures and electrical properties of lead-free (Bi0.5Na0.5)TiO3-based ceramics

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CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 9591–9598 www.elsevier.com/locate/ceramint

The effects of sintering atmosphere on microstructures and electrical properties of lead-free (Bi0.5Na0.5)TiO3-based ceramics Cheng-Sao Chena, Pin-Yi Chenb,n, Chi-Shun Tuc, Ting-Lun Changb, Chih-Kang Chaib a

Department of Mechanical Engineering, Hwa-Hsia Institute of Technology, New Taipei City 23567, Taiwan Department of Mechanical Engineering, Ming-Chi University of Technology, New Taipei City 24301, Taiwan c Graduate Institute of Applied Science and Engineering, Fu Jen Catholic University, New Taipei City 24205, Taiwan b

Received 9 November 2013; received in revised form 24 January 2014; accepted 9 February 2014 Available online 20 February 2014

Abstract 0.93(Bi0.5Na0.5)TiO3–0.07BaTiO3 (BNT–7BT) piezoelectric ceramics sintered in O2 and N2 have been investigated to understand the effects of sintering atmosphere in defects, microstructures, and dielectric properties. The BNT–7BT ceramics sintered in O2 and N2 exhibit a major pseudocubic structure with slight distortion from the cubic cell. The specimen sintered in N2 atmosphere shows larger grain sizes than the specimen sintered in O2. The X-ray photoelectron spectroscopy (XPS), SEM–EDS, and TEM–EDS suggest that specimen sintered in N2 atmosphere exhibits more ion vacancies. Leakage current measurements suggest more oxygen vacancies for specimen sintered in N2. A higher remnant polarization and lower coercive field were observed for specimen sintered in O2. Strain vs. E field (S–E) loops display higher E-field induced strain up to 0.256% and higher dielectric permittivity for specimen sintered in O2. This work suggests that sintering in the O-rich atmosphere can reduce defects and enhances ferroelectric, dielectric, and piezoelectric properties. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Lead-free piezoceramics; Sintering atmosphere; Polarization hysteresis; E-field induced strain

1. Introduction Lead-based Pb(Zr, Ti)O3 materials have recently been restricted by legislations in many countries, because of the toxicity and high vapor pressure of lead oxide during processing. Therefore, lead-free piezoelectric materials have attracted considerable attentions as potential materials to replace leadbased piezoelectric ceramics. Perovskite lead-free piezoelectric materials, such as (K, Na)NbO3, BiFeO3, and (Bi0.5Na0.5) TiO3 (BNT)-based ceramics have been widely studied [1–6]. Among lead-free materials, BNT-based ceramics were found to be promising lead-free piezoelectric materials owing to their high ferroelectric responses. A large piezoelectricity is expected for the BNT-based solid solutions with a composition near the morphotropic phase boundary (MPB). To improve the piezoelectric properties of BNT ceramics, some investigators n

Corresponding author. E-mail address: [email protected] (P.-Y. Chen).

http://dx.doi.org/10.1016/j.ceramint.2014.02.034 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

have focused on binary or ternary BNT-based systems [7–15]. E-field induced strain is one of the most important parameters for electromechanical actuators. BNT-based materials are expected to be superior lead-free candidates, as they exhibit large E-field induced strains [16–26]. Defects can alter the leakage current and affect polarization switching as well as strain response in ferroelectric (FE)/ piezoelectric materials. Oxygen vacancies (VoUU ) in FE crystals play an active role in domain dynamics [27–29]. Noguchi et al. [27,28], based on the effects of different atmospheres in Bibased FE materials, proposed that the defects of oxygen vacancies and electron holes may result in a poor insulating property. Ren et al. [29] proposed a mechanism of dipole defect to promote E-field induced strain in FE crystals. In this work, the MPB piezoelectric ceramic 0.93(Bi0.5Na0.5) TiO3–0.07BaTiO3 (abbreviated as BNT–7BT) specimens were sintered in O2 and N2 atmospheres. The effects of atmospheres in ferroelectric polarization and E-field induced strain responses were studied to obtain insights of optimal sintering

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conditions and to reduce defects in the lead-free BNT–7BT ceramic. 2. Experimental procedure The solid state reaction technique was adopted to fabricate lead-free 0.93(Bi0.5Na0.5)TiO3–0.07BaTiO3 piezoelectric ceramics. The reagent grade oxides and carbonate powders of Bi2O3, Na2CO3, BaCO3, and TiO2 were mixed and milled with zirconia balls for more than 24 h. The mixed powders were calcined in air at 900 1C for 2 h. The calcined powders were pressed into disks of 10 mm diameter and sintered at 1150 1C for 2 h in oxygen and nitrogen atmospheres, respectively. The X-ray diffraction (XRD) analysis using a Rigaku Mulitplex with radiations of Kα1 (1.5406 Å) and Kα2 (1.5444 Å) was employed to determine structures of unit cells. Micrographs were obtained using a scanning electron microscope (SEM; HITCHI S-3400 N FE-SEM) and density was measured by the Archimedes method. The P–E hysteresis loops (polarization vs. E field) were measured by using the Sawyer–Tower circuit at measuring frequency f¼ 50 Hz. The S–E hysteresis loops (strain vs. E field) were measured using a Radiant Precision Workstation 2000 system connected with an optical fiber displacement meter of MTI 2000.

Fig. 1. XRD spectra of BNT–7BT ceramics sintered in (a) O2 and (b) N2 atmospheres.

Leakage current was measured using a Precision Workstation 2000 (Radiant Technologies) and samples were submerged in a silicone oil bath. The chemical compositions and bonding states of the specimens were investigated by X-ray photoelectron spectroscopy (XPS; ESCA PHI 1600). The XPS Mg Kα1 source operating at 250 W, provided non-monochromatic X-rays at 1253.6 eV. In order to take correct spectra, the specimen was cleaned by ion gun after surface polishing and the XPS chamber pressure is less than 5  10  10 Torr prior to data collection. The XPS depth profiling is performed with sputtering time for 10 min to confirm the uniformity. The scanning electron microscope with a field emission gun (FE-SEM; HITCHI S-3400N) equipped with an energydispersive spectrometer (EDS) and transmission electron microscopy (TEM; JEOL JEM-2100) equipped with a high resolution TEM and an energy-dispersive spectrometer (EDS) were used to observe internal distributions of compositions for specimens sintered in O2 and N2 atmospheres.

3. Results and discussion Fig. 1 shows the XRD spectra of BNT–7BT ceramics sintered in O2 and N2 atmospheres. XRD profiles exhibit a single perovskite structure without second phase. The (200) reflection exhibits a slight peak splitting and asymmetry without apparent tetragonal phase as shown in the insets of Fig. 1(a) and (b). Further analysis found a two-peak splitting in the (200) reflection from Kα1 and Kα2 radiations. This result is similar to the previous study by Jo et al. [30], suggesting a major pseudo-cubic structure with relaxor characteristic of polar nanoregions in BNT-based solid solutions. Based on the fitting analysis, BNT–7BT ceramic possesses a major pseudocubic structure with slight distortion from the cubic cell. There is no apparent structural difference between specimens sintered in O2 and N2 atmospheres. Fig. 2(a) and (b) shows SEM micrographs for BNT–7BT ceramics sintered in O2 and N2 atmospheres. The grain sizes of specimen sintered in O2 atmosphere are more uniform than the specimen sintered in N2. The average size of specimen sintered in O2 atmosphere is about 1.6 μm. Specimen sintered in N2 atmosphere has larger grains with irregular rectangular shapes and the average grain size is 2.0 μm. A lower oxygen pressure can easily cause oxygen vacancies. The presence of vacancies in oxide materials is beneficial for ion transport during sintering and thus generates larger grains for ceramic sintered in N2 atmosphere. BNT–7BT ceramic may partially favor the tetragonal structure [13], which leads to the cuboidal shaped morphology. As shown in Fig. 2(c), the inspection of surface color reveals that the specimen sintered in N2 atmosphere shows a brown color and the specimen sintered in O2 atmosphere exhibit a light yellow color. In general, specimens sintered in reducing atmosphere produce easily oxygen vacancy and display darker contrast color. Therefore, specimen sintered in N2 atmosphere suggests the evaporation of oxygen ions and possibly cause oxygen vacancies. Fig. 3 shows the P–E hysteresis loops at room temperature for BNT–7BT ceramics sintered in O2 and N2 atmospheres.

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Fig. 2. SEM micrographs of BNT–7BT ceramics sintered in (a) N2 and (b) O2 atmospheres, and (c) surface colors for specimens sintered in N2 and O2 atmospheres.

0.4

A sample sintered in O 2 atmosphere B sample sintered in N 2 atmosphere

strain (%)

0.3

0.2

0.1

0.0

-0.1 -60

Fig. 3. Polarization hysteresis loops in BNT–7BT ceramics sintered in O2 and N2 atmospheres.

The specimen sintered in O2 exhibits strong ferroelectric properties with coercive field of Ec  26.2 kV/cm and remnant polarization of Pr  29.5 μC/cm2. The specimen sintered in N2 atmospheres displays a larger coercive field Ec  31.9 kV/cm and a smaller remanent polarization of Pr  15.5 μC/cm2. The results suggest that sintering atmosphere can alter microstructures and physical dynamics of FE domains. Fig. 4 shows E-field induced strains (S–E curve), which show bipolar strains of 0.256%, and 0.193% for the specimens sintered in O2 and N2 atmospheres, respectively. The specimen sintered in O2 atmosphere exhibits a higher E-field induced strain. This result agrees well with the characteristic of P–E curves. Fig. 5 shows dielectric permittivity for specimens sintered in O2 and N2

-40

-20

0 20 E (kV/cm)

40

60

Fig. 4. Bipolar E-field induced strains of BNT–7BT ceramics sintered in O2 and N2 atmospheres.

atmospheres. The specimen sintered in O2 displays higher dielectric permittivity than that in N2. These results reveal that specimen sintered in the O-rich atmosphere can enhance ferroelectric, dielectric, and piezoelectric properties. In order to understand atmosphere effect on defects, XPS technique was performed as shown in Fig. 6 for ceramics sintered in O2 and N2 atmospheres. The XPS spectra confirm Bi, Na, Ba, Ti, O, and C elements in the BNT–7BT ceramics. The peak at 285.0 eV is assigned to C 1s from contaminated hydrocarbon, which has been used to rectify the binding energies. The spectral lines of elements agree well with the elements of BNT–7BT ceramic. It means that no extra element interacts with the specimens during the synthesis. The narrowscan spectra of O 1s peaks for BNT–7BT ceramics sintered in

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5000

1.8 1.6 1.4

4000

1.2 3000

1.0 0.8

2000

0.6

1000

dash line: BNT-7BT sintered in O2

0.4

solid line: BNT-7BT sintered in N 2

0.2

loss tangent(δ)

Dielectric permittivity (ε')

6000

0.0

0

0

100

200

300

400

500

600

o

Temperature ( C)

c/s

Fig. 5. Dielectric permittivity ε0 and loss tan (δ) of BNT–7BT ceramics sintered in O2 and N2 atmospheres.

800 600 400 Binding Energy (eV)

200

0

1000

800

200

0

c/s

1000

600

400

Binding Energy (eV)

Fig. 6. XPS and the fitted narrow-scan spectra of O 1s for BNT–7BT ceramics sintered in (a) O2 and (b) N2 atmospheres.

indicating that the O 1s spectra are contributed from O ions of lattice. To understand variation of oxygen in the lattice in different atmospheres, XPS data can be used for the quantitative analysis of compositions. A general expression for determining the atom fraction can be obtained by the equation [31] Cx ¼

I x =Sx ∑SI ii

ð1Þ

where Cx is the atom fraction (x¼ Bi, Na, Ti, and O), Ix is the integrated area of the specific peak, and Sx is the atomic sensitivity factor of the x element. Ii and Si (i¼ Ba, Bi, Na, Ti, and O) are respectively integrated area and sensitivity factor for each element. Table 1 shows the comparison of atomic concentration for BNT–7BT ceramics sintered in O2 and N2 atmospheres. The ratios of [O 1s]/[Ti 2p] for BNT–7BT ceramics sintered in O2 and N2 atmospheres are 2.87 and 2.76, respectively. It suggests that the concentration of oxygen vacancies for specimen sintered in N2 atmosphere is higher than the specimen sintered in O2 atmosphere. The ratios of ([Bi 4f] þ [Na 1s])/[Ti 2p] for BNT–7BT ceramics sintered in O2 and N2 atmospheres are 0.71 and 0.59, respectively. These results imply that the concentrations of Bi and Na vacancies for specimen sintered in N2 atmosphere are more than the specimen sintered in O2 atmosphere. The SEM–EDS and TEM–EDS analyses were also performed to observe the compositional distribution for the specimens sintered in O2 and N2 atmospheres. Fig. 7 shows SEM-backscattered electron image (BEI) and EDS spectra to explore the compositional distribution in BNT–7BT ceramics sintered in O2 and N2 atmospheres. The results show less oxygen content for the specimens sintered in N2 atmosphere. TEM micrographs for specimen sintered in N2 atmosphere show the dislocation loops with “coffee bean contrast” [32,33] as shown in Fig. 8(a) and (b). Metzmacher et al. [32] proposed that the formation of these dislocation loops is possibly caused by the ordering of oxygen vacancies in the lattice. The darker color feature of these dislocation loops were arisen from internal lattice distortion and strain. As shown in Fig. 8 (c) for specimen sintered in N2 atmosphere, the dislocation loops in “Area 2” apparently display less oxygen than other areas in the matrix. The dislocation loops are not observed in the specimen sintered in O2 atmosphere. These observed dislocations could be associated with aggregation of oxygen vacancies. Although ion vacancies can not be directly observed, we proposed a reasonable mechanism to explain the results as following. Vacancies of oxygen and volatile elements (such as bismuth and sodium) are easily generated in the specimen sintered in N2 atmosphere due to high vapor pressure. The reactions of defects chemistry can be described by the Kröger– Vink reaction equations as below: 2OnO -2VoUU þ 4e0 þ O2 ðgÞ

O2 and N2 atmospheres are given in insets of Fig. 6(a) and (b). Solid lines in the O 1s spectra are fitted of Gaussian and Lorentzian terms and are attributed to Ti–O, Bi–O, Ba–O, and Na–O bonds. The red dashed lines are the sums of fitted curves and are in good agreement with the measured O 1s spectra,

ð2Þ

0

BinBi þ NaNa -V‴Bi þ VNa þ 4 h 0

2OnO þ BinBi þ NanNa -V‴Bi þ VNa þ 2VoUU

ð3Þ ð4Þ

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Table 1 Comparison of atomic concentration percentage of [O 1s]/[Ti 2p] and ([Bi 4f] þ [Na 1s])/[Ti 2p] ratios for the BNT–7BT ceramics sintered in O2 and N2, respectively. ASFa

Bi 4f 9.848

Na 1s 1.102

Ba 3d 7.343

Ti 2p 2.077

O 1s 0.733

Total

½O1s ½Ti 2p

½Bi4f þ ½Na1s ½Ti 2p

O2 N2 Stoichiometry

7.65 6.83 9.30

7.58 6.52 9.30

1.42 1.38 1.40

21.52 22.68 20.00

61.83 62.59 60.00

100 100 100

2.87 2.76 3.00

0.71 0.59 0.93

a

ASF denote atomic sensitivity factor.

Fig. 7. SEM images at 5000 magnifications and EDS spectra in compositional distribution of BNT–7BT ceramics sintered in (a) O2 and (b) N2 atmospheres.

Eqs. (2)–(4) describe the evaporations of oxygen, bismuth, and sodium ions. BNT–7BT ceramics sintered in N2 atmosphere might have more vacancies of Bi, Na, and O elements. Fig. 5 also shows dielectric losses for BNT–7BTceramics sintered in O2 and N2 atmospheres. The dielectric loss begins to increase exponentially at higher temperatures for all frequencies. This phenomenon implies

the high-temperature dielectric loss to be associated with the ionic vacancies, which are mainly due to the thermally active ionic conductivity [34]. The dielectric loss tan δ (f¼ 10 kHz) of the specimen sintered in O2 begins to increase exponentially above 500 1C and that of the specimen sintered in N2 is above 450 1C. This difference may be attributed to more ions vacancies existence

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Fig. 8. (a) Bright-field TEM image and (b) HRTEM image with the circled areas of “coffee bean” images for specimen sintered in N2 atmosphere. (c) TEM–EDS analysis from the labeled “Area 1” in (b) represents the positions of non-dislocation loops. The labeled “Area 2” in (b) represents the positions of disloscation loops.

VoUU þ 1=2O2 ðgÞ-OnO þ 2 h:

ð5Þ

Noguchi et al. [27,28] proposed that electron–hole carriers cause higher leakage current at room temperature. Specimen sintered in N2 reveals more oxygen vacancies and thus oxide

1E-5

2

Current density(A/cm )

for specimen sintered in N2 atmospheres, which cause higher ionic conductivity and thus a rapid increase of dielectric loss takes place at lower temperature region. To further clarify defects in BNT–7BT ceramics, the leakage current densities (J) were measured at room temperature as a function of E field for specimens sintered in O2 and N2 as shown in Fig. 9. A higher leakage current density (J) (>10  6 A/cm2) was observed for specimen sintered in N2 with the post-annealed process. For the specimen sintered in N2, the post-annealed treatment might produce compensation of oxygen vacancies and electron–hole carriers [35,36]. The reaction can be expressed as below:

1E-6

1E-7

1E-8 o

sintered in N2 and post-annealed at 800 C for 10hrs o

sintered in O2 and post-annealed at 800 C for 10hrs

1E-9

sintered in N2 sintered in O2

1E-10 0

10

20

30

40

50

60

Electric field (kV/cm) Fig. 9. Leakage current density (J) as a function of E-field for BNT–7BT ceramics sintered in N2, O2, and post-annealed treatment.

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treatment generates more electron–hole carriers. That's why the specimen sintered in N2 and post-annealed treatment reveals higher leakage current. It has been demonstrated that oxygen vacancies are the main cause for reduction of domain-wall motion [37,38]. Oxygen vacancies can reduce the switching ability of dipoles, and thus make it more difficult to switch polarization vectors under an E field. Therefore, BNT–7BT ceramics sintered in N2 atmosphere exhibit weaker ferroelectric, dielectric, and piezoelectric properties. 4. Conclusions BNT–7BT ceramics sintered in O2 and N2 exhibit a similar pseudo-cubic structure with slight distortion from the cubic cell. The specimens sintered in N2 atmosphere exhibit larger grain sizes and cuboidal configurations. The XPS, SEM–EDS, and TEM–EDS analyses suggest that the specimens sintered in N2 atmosphere exhibit more defects including vacancies of Bi, Na, and O ions. Leakage current measurements reveal more oxygen vacancies for specimens sintered in N2 atmosphere. P–E loops measurements reveal higher remanent polarization of Pr  29.5 μC/cm2 and lower coercive field of Ec  26.2 kV/cm for specimens sintered in O2 atmosphere. S–E loops display a higher E-field induced strain up to 0.256% for specimens sintered in O2 atmosphere. The specimens sintered in O2 display higher dielectric permittivity as temperature increases. In brief, lead-free BNT-based ceramic sintered in O-rich atmosphere has demonstrated better ferroelectric, dielectric, and piezoelectric properties. Acknowledgment This work was supported by the National Science Council of Taiwan Grant Nos. 102-2221-E-131-006, 102-2221-E-146001, and 100–2112-M-030-002-MY3. References [1] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Lead-free piezoceramics, Nature 432 (2004) 84–87. [2] E. Cross, Lead-free at last, Nature 432 (2004) 24–25. [3] E. Hollenstein, M. Davis, D. Damjanovic, N. Setter, Piezoelectric properties of Li- and Ta-modified (K0.5Na0.5)NbO3 ceramics, Appl. Phys. Lett. 87 (2005) 1–3. [4] H. Birol, D. Damjanovic, N. Setter, Preparation and characterization of KNbO3 ceramics, J. Am. Ceram. Soc. 88 (2005) 1754–1759. [5] T. Karaki, K. Yan, T. Miyamoto, M. Adachi, Barium titanate piezoelectric ceramics manufactured by two-step sintering, Jpn. J. Appl. Phys. 46 (2007) 7035–7038. [6] J. Rödel, W. Jo, K.T.P. Seifert, E.M. Anton, T. Granzow, Perspective on the development of lead-free piezoceramics, J. Am. Ceram. Soc. 92 (2009) 1153–1177. [7] D. Lin, D. Xiao, J. Zhu, P. Yu, Piezoelectric and ferroelectric properties of [Bi0.5(Na1  x  yKxLiy)0.5]TiO3 lead-free piezoelectric ceramics, Appl. Phys. Lett. 88 (2006) 062901. [8] T. Takenaka, H. Nagata, Y. Hiruma, Current developments and prospective of lead-free piezoelectric ceramics, Jpn. J. Appl. Phys. (2008) 3787–3801.

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