Scholarly article on topic 'Properties of Ce-doped Bi0.5Na0.5TiO3 Synthesized using the Soft Combustion Method'

Properties of Ce-doped Bi0.5Na0.5TiO3 Synthesized using the Soft Combustion Method Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Khairunisak Abdul Razak, Wai Chen Song, Chai Yan Ng

Abstract In this work, bismuth sodium titanate (BNT) and cerium (Ce)-doped BNTwere successfully synthesized using the soft combustion method. The effect of 3, 5, and 7 mol% Ce, respectively added as dopant on stoichiometry, microstructure, density and dielectric properties were studied. Pure BNT phase was obtained in the sample containing 3 mol% Ce after calcination at 800°C for 3h. The calcined powders were then pressed into pellets and sintered at 1100°C for 3h. The grain size of the pellets decreased with the addition of Ce3+ because Ce acted as a grain growth inhibitor. Maximum density was obtained in 3 mol% Ce-doped BNT, and decreased with increasing amount of Ce dopant. In addition, the maximum dielectric constant of 468.35 was obtained in 3 mol% Ce-doped BNT and decreased at higher amount of Ce doping. The addition of Ce as a dopant in BNT also decreased the dielectric loss.

Academic research paper on topic "Properties of Ce-doped Bi0.5Na0.5TiO3 Synthesized using the Soft Combustion Method"

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Procedia Chemistry 19 (2016) 816 - 821

5th International Conference on Recent Advances in Materials, Minerals and Environment (RAMM) & 2nd International Postgraduate Conference on Materials, Mineral and Polymer

(MAMIP), 4-6 August 2015

Properties of Ce-doped Bi05Na05TiO3 Synthesized using the Soft

Combustion Method

Khairunisak Abdul Razak*, Wai Chen Song, Chai Yan Ng

School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

Abstract

In this work, bismuth sodium titanate (BNT) and cerium (Ce)-doped BNTwere successfully synthesized using the soft combustion method. The effect of 3, 5, and 7 mol% Ce, respectively added as dopant on stoichiometry, microstructure, density and dielectric properties were studied. Pure BNT phase was obtained in the sample containing 3 mol% Ce after calcination at 800°C for 3 h. The calcined powders were then pressed into pellets and sintered at 1100°C for 3 h. The grain size of the pellets decreased with the addition of Ce3+ because Ce acted as a grain growth inhibitor. Maximum density was obtained in 3 mol% Ce-doped BNT, and decreased with increasing amount of Ce dopant. In addition, the maximum dielectric constant of 468.35 was obtained in 3 mol% Ce-doped BNT and decreased at higher amount of Ce doping. The addition of Ce as a dopant in BNT also decreased the dielectric loss.

© 2016 The Authors.Publishedby Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia

Keywords:(Bio.Nao.5)TiO3; soft combustion; dielectric

1. Introduction

Bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT) is a widely used lead-free piezoelectric material with its relatively large remnant polarization (38 ^C cm-2) and coercive field (73 kV cm-1) at room temperature1. However, BNT has

* Corresponding author. Tel.: +604-5996126; fax: +604-5941011. E-mail address: khairunisak@usm.my

1876-6196 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia doi:10.1016/j.proche.2016.03.107

several drawbacks such as large coercive field (7.3 kV mm and high current leakage, causing poling of the structure difficult2. Thus, doping was performed on BNT to improve its properties.Praseodymium dopant acted as a grain growth inhibitor and produced small crystallite and grain sizes in BNTprepared by using the soft combustion method3.Lanthanum dopantsin BNT produced by solid state reaction were found to be beneficial in obtaining dense ceramics (>95% of theoretical density) and suppressing grain growth4, 5.Niobium pentoxide in BNT synthesized by the solid state reactionsuppressed the grain growth and improved the densification and piezoelectric properties6, 7.Similarly, cerium (Ce) dopants in BNT and barium titanate prepared by the solid state reaction suppressed the grain growth and improved the densification and dielectric properties (relatively high permittivity and low loss tangent)8,9.BNT was largely synthesizedusing the solid state reaction because of its large amount production. However, a low cost soft combustion method with the ability to produce fine particle size powder (nanometer scale) could be beneficial for BNT production.To the authors' knowledge, Ce has never been doped in BNT using soft combustion method.

In this work, BNT was synthesized using the soft combustion method and Ce (3, 5, and 7 mol%) were doped into BNT for the first time to observe its structural, morphological, and density changes. Dielectric characterization was then performed on the undoped and doped BNT to study the effect of Ce dopant on the dielectric properties.

2. Experimental details

First, bismuth (III) nitrate pentahydrate[Bi(NO3)3-5H2O] and sodium nitrate (NaNO3) were dissolved in 25 ml 2-methoxyethanol with continuous stirring at 40°C. In a separate beaker, the titanium (IV) isopropoxide{Ti[OCH(CH3)2]4} was dissolved in 25 ml 2-methoxyethanol, with 5 ml acetylacetone added as a chelating agent. The titanium solution was then added to the bismuth-sodium solution with continuous stirring at room temperature for 2 h. Upon completion, the mixture was heated to 130°C with continuous stirring. Evaporation occurred causingthe mixture to turn into sticky gel, and followed by a soft combustion process.

Soft combustion reaction transformed the sticky gel into foam, which was crushed using an agate mortar to obtain the fine powder. This synthesized powder was then calcined at 800°C for 3 h. After calcination, the powder was crushed again and pressed into 12 mm diameter pellets with a pressure of 5.4 MPa. Lastly, the pellets were sintered at 1100°C for 3 h.For the preparation ofCe-doped BNT[(Bi05Na05)(1_x)CexTiO3, BNCT], cerium (III) nitrate hexahydrate(CeN3O9-6H2O)was dissolved in 2-methoxyethanol along with Bi(NO3)3-5H2O and NaNO3 in the first step of preparation. The subsequent steps were similar to those for the preparation of BNT powder.

The phases present in the powders and pellets were analyzed usingan X-ray diffractometer(XRD) (Bruker AXS D8 ADVANCE) equipped with Cu Karadiation. The surface morphology of the samples was observed usingfield emission scanning electron microscope (FESEM) (Zeiss SUPRA 35). The density of pellets was measured using the Archimedesmethod. The dielectric properties of the pellets were measured using a LCR meter(GW INSTEK LCR-817) at 1 kHz and 1 V. Prior to dielectric measurement, silver paste wasapplied on both surfaces of the pellets for ohmic contact.

3. Results and discussion

The XRD patterns of the calcined BNT and BNCT powders are shown in Fig. 1. The BNT and BNCT (x = 0.03) powders contained only single phase of tetragonal sodium bismuth titanate (ICSD No. 98-005-5573), indicates total doping and dissolution of Ce3+ in the BNT perovskite lattice. Meanwhile, the excessive addition of Ce3+ in BNCT (x = 0.05 and 0.07) lead to the formation of Bi4Ti3O12 (ICSD No.98-010-2668) and Bi2O3 (ICSD No. 98-001-5604) secondary phases. BNCT was formed by Bi3+, Na+, Ti4+ and Ce3+, with ionic radii of 1.03 A, 1.02 A, 0.605 A and 1.01 A, respectively10. Similar ionic radius is preferable for a dopant to dope into the structure of the component3. Therefore, Ce3+ with ionic radius of 1.01 A is preferable to replace into the Bi3+ and Na+ sites with ionic radii of 1.03 A and 1.02 A, respectively. With that, the bondless Bi3+ thenreacted with Ti4+ and O2-in air and formed secondary phases of Bi4Ti3O12 and Bi2O3n.

The calcined powders were then pressed into pellets and sintered. The XRD patterns of the sintered pellets are shown in Fig. 2. The peaks of the host matrix for the BNCT pellets matched with tetragonal sodium bismuth titanate (ICSD No. 98-005-5573), whereas the undoped BNT pellet matched with hexagonal sodium bismuth titanate (ICSD No. 98-006-3231). The addition of Ce into BNT altered the crystal symmetry of BNT from hexagonal to tetragonal

due to the lattice distortion during the substitution of smaller Ce3+(1.01 A)into the larger Bi3+ (1.03 A)and Na+ (1.02 A)sites. In addition, the XRD patterns of BNCT pellets with x = 0, 0.05, and 0.07 show the presence of Bi2O3 secondary phase. BNCT (x = 0.03) pellet does not contain Bi2O3 and a nearly pure perovskite phase of BNT was obtained.

(ar b. uni

20 30 40 50 60 70 80 90

26 (°)

Fig. 1. XRD patterns of the calcined BNCT powders with varying x: (a) 0, (b) 0.03, (c) 0.05, and (d) 0.07.

A ■ BNT *Bi:Oj \ A A , A T *

Int ens ity (ar b. uni t) <e) \. . k J . , A k

.1 A * , (a) l A l « A * A

20 30 40 50 00 70 SO 9C

26 (°)

Fig.2. XRD patterns of the sintered BNCT pellets with varying x: (a) 0, (b) 0.03, (c) 0.05, and (d) 0.07.

The FESEM images of the sintered pellets are shown in Fig. 3. The grain sizes of the BNCT pellets decreased from x = 0 to x = 0.03, followed by increasing from x = 0.03 to x = 0.05, and slightly decreased from x = 0.05 to x = 0.07. The grain sizes of the BNCT were smaller than the undoped BNT. The undoped BNT had the largest average grain size (~2.23 ^m), whereas the BNCT (x = 0.03) had the smallest average grain size (~421 nm). Ce dopant acts as a grain growth inhibitor and decreases the average grain size of the 3 mol% BNCT. By doping with a small amount of Ce dopant, the grain size can be reduced due to lower diffusivity of Ce ion in comparison to the Bi ion. With further increase of the Ce dopant, the average grain size increased. Similarly, McLaughlin12 reported that grain growth during sintering process was suppressed with small amount of additives, but larger amount of additives increased the grain size. Herabut and Safari4found that the average grain size of BNT samples increased from 3.1 to 7.8 ^m with the addition of 1 at% lanthanum. However, an addition of more than 1 at% lanthanum resulted in a decrease in the average grain size, except for the sample with 6 at% lanthanum. Moreover, Razaket al.13found that the undoped BNT had a large grain size of> 10^m, whereas BNT with 5 mol% praseodymium doping showed a much smaller grain size of 683.152 nm. Meanwhile, this work shows doping of 3 mol% Ce on BNT is able to produce rather small grain size of 421 nmas compared to the reportedworks.

■ ■ BNT Bi4Ti3Ol2 * Bi,o3 : r - : - - №

(c) 1 I _ _»

i. * ^ ' *-----

Fig.3. FESEM images of sintered BNCT pellets with varying x: (a) 0, (b) 0.03, (c) 0.05, and (d) 0.07.

The plot of density of the sintered pellets against the amount of dopant (x) is shown in Fig. 4. The densities of the pellets increased from x = 0 (5.328 g cm-3) to x = 0.03 (5.588 g cm-3), and then decreased for x = 0.05 (4.506 g cm-3) and x = 0.07 (4.439 g cm-3). The densities of the pellets were correlated to the average grain sizes of the pellets. The density increased with decreasing grain size and vice versa. The addition of small amount of Ceresulted in the formation of closed packed micro structure, suppressed the grain growth and thus increased the density. Moreover, smaller grain size caused high density of grain boundaries and increased the density of domain walls14. However, with further addition of Ce, the density started to decrease from x = 0.03 due to increasing of grain size. With larger grain size, the samples tend to have more pores among the grains (as shown in Fig. 3) and thus resulted in lower density. The porosities presence in the ceramic body plays an important role in determining the density of the sample given that the porosities could lower the compactness of grains within the ceramic body.

The plot of dielectric constant (er) and loss tangent (tan 8) against the amount of dopant (x) is shown in Fig 5. The er increased from x = 0 (275.32) to x = 0.03 (468.35), and decreased beyond x = 0.03 (310.91 for x = 0.05 and 227.34 for x = 0.07). The results are in agreement with the results reported by Yasmin et al.9, where the value of erincreased as the Ce content in barium titanate increased up to x = 0.03, and decreased beyond x = 0.03. This result could be due to the solubility limit of Ce in barium titanate ceramics. At room temperature, beyond the solubility limit, the Ce substitutions lead to a small compression of the unit cell and thus resulted in a decrease in net polarization.

4.2 4 -

0 0.03 0.05 0.07

Amount of dopant (x)

Fig. 4. Density of sintered BNCT pellets with varying x.

The changing trend of the dielectric constant is also related to the average grain size of the sintered pellets. The presence of domains in the grains of the sintered samples influences the dielectric properties. For the undoped BNT, large grain causes the presence of many 90° and 180° domains in various directions, which suppressed the dielectric properties of the sintered pellets. Whereas, for the 3 mol% Ce-doped BNT, the smaller grain size reduced the number of domains in the grains. Therefore, the 3 mol% Ce-doped BNThad better dielectric properties than the undoped BNT. Moreover, the increase in the er was due to the domain size effect. Smaller grain size corresponds to higher domain density, which enhanced the orientation and ionic polarizability due to the domain-wall and lattice vibration of BNT and BNCT grains15. Meanwhile, erdecreasedbeyond x=0.07 because of the poor densities of the sintered BNCT pellets (high amount of pores). On the other hand, tan 8 shows opposite trend to the er, where it decreased from x = 0 (0.1278) to x = 0.03 (0.0499), increased at x = 0.05 (0.2058) and decreased at x = 0.07

(0.1369). The 3 mol% Ce-doped BNT had the lowest tan 8 due to its smallest grain size and highest density. Fine grain size has low dielectric loss since there is minimum domain reorientation in the sample16.

500 0.25

Di 400 <- 0.2 Lo

ele ct ric co 350 300 250 -> 0.15 ss ta ng en

ns 200 0.1 t

ta 150 (ta

nt 100 0.05 n

a 003 0.05 0.07

Amount of dopant (x)

Fig. 5.sr and tan 8 of sintered BNCT pellets with varying x.

4. Conclusions

BNT and BNCT were successfully synthesized using the soft combustion method.BNCT (x = 0.03) exhibited the highest sr (468.35) and the lowest tan 8 (0.0499)owing to the presence of single phase sodium bismuth titanate, small grain size (~421 nm),and high density (5.588 g cm-3).

Acknowledgements

The authors are grateful to the financial support provided by Research University grant 1001/PBAHAN/811069 from UniversitiSains Malaysia.

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