The Effect of Nb2O5 Addition on the Structural, Dielectric and ...

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ScienceDirect Energy Procedia 74 (2015) 198 – 204

International Conference on Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES15

The effect of Nb2O5 addition on the structural, dielectric and piezoelectric properties of Pb0,98 Ba0,02 [(Zr0.52Ti0.48)0,98(Cr3+0.5, Ta5+0.5)0,02] ceramics Louanes Hamziouia,b, Fares Kahoula,b, Ahmed Boutarfaiaa,b,* a

Université Kasdi Merbah Ouargla, Laboratoire de génie des procédés, Faculté des sciences appliquées, Ouargla 30000 Algérie b Département de chimie, Laboratoire de chimie apliquées, Université de Biskra B. P. 145, RP-Biskra 07000, Algérie

Abstract Specimens of Pb0,98 Ba0,02[((Zr0.52Ti0.48)0,98(Cr3+0.5, Ta5+0.5)0,02)] + x wt.% Nb2O5 ( x = 0, 0.1, 0.2, 0.3 and 0.4 wt.%) ceramics were prepared by a conventional mixed-oxide route. In the present study, the effect of Nb doping was investigated on the structural, microstructural, dielectric and piezoelectric properties of the ceramics. X-ray diffraction analysis indicated that the phase of the material changed from an rhombohedral structure to a MPB structure and then a tetragonal structure when the Nb 2O5 content was increased from 0.00 to 0.40 wt.%. The PBZT–PBCT ceramics doped with 0.20 wt.% of Nb2O5 exhibited denser and finer microstructures, which produced a high relative density of ≥ 97%. Scanning electron microscopy (SEM) observations revealed that Nb2O5 (x) addition enhanced the sintering density through the formation of a liquid phase. The maximum dielectric constant (εr = 17100) and the piezoelectric charge coefficient (d 31 = 203 pC/N) were observed for 0.20 wt.% of Nb2O5. It has been observed that the dielectric and piezoelectric constants increased gradually with increasing x up to 0.20 and was found optimum, which could be ideal for electromechanical and energy harvesting applications. © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD). Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) Keywords:Ceramics, grain saze, piezoelectrics proprieties, X-ray diffraction, SEM

* Corresponding author. Tel.: +21329717070; fax: +21329715161 E-mail address: [email protected]; [email protected]; [email protected]

1876-6102 © 2015 Published by Elsevier Ltd. 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 the Euro-Mediterranean Institute for Sustainable Development (EUMISD) doi:10.1016/j.egypro.2015.07.577

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1. Introduction Today the Perovskite ceramics with high dielectric constant have been used in various fields such as dynamic random access memory (DRAM), multi-capacitors, transducers, actuators, etc. [1–5]. Among these ceramics, PZT is the most efficient and widely applied material [6,7]. These ceramics properties are strongly influenced by the density and microstructure which depend upon the procedure of synthesis and the powder processing [8]. Furthermore, the morphotropic phase boundary (MPB) [9] is an essential parameter to be considered because in this region tetragonal and rhombohedral phases coexist, and consequently the properties of these materials are improved (e.g., piezoelectric properties). A PZT perovskite structure is distorted below 350 °C [10] (Curie temperature). When this structure has Zr4+ in position B (the center of the unit cell), the PbZrO 3 rhombohedral phase is evident. However, when Ti4+ occupies the B site of a structure, the tetragonal phase of PbTiO 3 appears [11]. Both phases are ferroelectric and consequently exhibit spontaneous polarization [8,12]. Dopants can replace the ions in the lattice and, depending on ion radii, they can substitute the A+2 (lead) and B+4 (titanium/ zirconium) sites, creating the A or B site vacancies for charge compensation. High valence dopants (La +3, Nb+5, Ta+5) are donors due to electrons contribution, and their substitution to A and B sites, respectively, create the A vacancies. Conversely, ions like Na+ and Cr+3 act as an acceptor, and their substitution into the A and B sites, respectively, create the O-vacancies [13–15]. The kinetics of oxygen vacancies has been already investigated and described [16]. Doping with different element changes the physical properties of PZT ceramics. Namely, the electromechanical properties of PZT ceramics can be improved by the use of additives of rare earth elements, e.g. the incorporation of Nb ions into PZT reduced the creative field, lowers aging kinetic and elasticity modulus and increase dielectric constant. If compared with pure PZT, the addition of Nb atom can additionally increase the piezoelectric coupling factor (kp) and decreased the mechanic quality factor (Qm) [17,18]. Based on the influences of each dopant on piezoelectric ceramic properties, this work reported the observation of phenomena in Nb-doped Pb0,98 Ba0,02 [(Zr0.52Ti0.48)0,98(Cr3+0.5, Ta5+0.5)0,02] ceramics and the resulting effects on the electromechanical and dielectric properties of the ceramics.

2. Experimental procedure The specimens were manufactured using a conventional mixed oxide process. The composition used in this study was as follows: Pb0,98 Ba0,02[(Zr0.52Ti0.48)0,98(Cr3+0.5, Ta5+0.5)0,02] + x wt.% Nb2O5 ( x = 0, 0.1, 0.2, 0.3 and 0.4 wt.%). The raw materials, such as PbO, BaO, ZrO2, TiO2, Ta2O5, Cr2O3 and Nb2O5 among the given composition were weighted by weight ratio and the powders were ball-milled in ethanol for 12 h using Zirconia balls. After drying, they were calcined at 800 ◦C for 2 h. The powders were molded by the pressure of 130 MPa in 11mm Φ mold. The pressed disks were covered with alumina crucible and then sintered at 1180 ◦C for 2 h. To limit PbO loss from the pellets, a PbO-rich atmosphere was maintained by placing a PbZrO3 inside the crucible. For measuring the piezoelectric characteristics, the specimens were polished to 2 mm thickness and then electrodeposited with Ag paste. The pellets are carried out at 110 ◦C in a silicone oil bath by applying fields of 2.6 kV cm−1 for 45 min. All samples were aged for 24 h prior to measuring the piezoelectric and dielectric properties. The microstructure and crystal structure of specimens were analyzed through SEM and X-ray diffractometry (XRD using Cu K_radiation), respectively. For investigating the dielectric properties, the capacitance was measured at 1 kHz using an LCR meter (TH2617, China) and dielectric constant was calculated. For investigating the piezoelectric properties, the resonant and antiresonant frequencies were measured by an Impedance Analyzer (Agilent 4294A) according to IRE standard and then the piezoelectric charge coefficient was calculated by the following equations [19,20]. ஼

ᖡ௥ ൌ ஼ 

(1)

బ

ௌ ௘

‫ܥ‬଴ ൌ ᖡ଴ 

(2)

200

Louanes Hamzioui et al. / Energy Procedia 74 (2015) 198 – 204

ሺଵିఈሻᖡೝ ᖡబ

݀ଷଵ ൌ ‫ܭ‬௣ ට

ଶ௒

௄ಳ ௙ಲ

ᖡಲ

೛ ೝ

ೝ

 (C/N) at 25 °C

஻ ஺ ೛ ೝ ට ೝ ݀ଷଵ ൌ ݀ଷଵ ௄ಲ ௙ಳ ᖡಳ

(3)

(4)

Where: Superscript A denotes room temperature and Superscript B refers to the elevated temperature. e: disk diameter (m); S: disk surface (m2); DE: Poisson ratio. H0: permittivity of vacuum; Y: Young's modulus. Hr: dielectric constant at temperature T(25°C) and at frequency below all resonances. fr : resonant frequency, fa: antiresonant frequency; C: capacity (f ); CO: capacity of vacuum (f ).

3. Results and discussion 3.1. Structural and microstructural properties Fig. 1 displays XRD patterns of Nb2O5 doped PBZT–PBCT ceramics sintered at 1180 ◦C. apparently; a pure perovskite structure without any secondary impurity phases could be confirmed. It can be seen that the phase structure of the specimens change from rhombohedral to tetragonal by the addition of Nb 2O5. The PBZT–PBCT ceramics exist as rhombohedral phase which is indicated by the single (200)R peak at x = 0.00 wt.%. As Nb2O5 content increases from 0.20 wt.%, the ceramics coexist as tetragonal and rhombohedral phase revealed by the coexistence of (002)T, (200)R and (200)T peaks in the 2θ range from 43.5° to 46°. The ceramics with x = 0.30 wt.% and x = 0.40 wt.% exist as tetragonal phase revealed by the splitting of (002)T and (200)T peaks.

Fig. 1. XRD patterns of sintered PBZT–PBCT ceramics with a varying Nb 2O5 addition: (a) 0.00; (b) 0.10; (c) 0.20; (c) 0.30 and (e) 0.40 wt.% sintered at 1180°C.

201

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The change could be understood as follows: the perovskite structure is stable, when the tolerance factor: ‫ݔ‬ൌ

୰ఽ ା୰ో

(5)

ξଶሺ୰ా ା୰ో

Here, rA is the average ionic radius of A site cation (Pb and Ba), rB is the average ionic radius of B site cation (Zr and Ti) and ro is the ionic radius of O anions. Fig. 2 shows the porosity and bulk density of PBZT–PBCT according to the amount of Nb2O5 sintered at 1180 ◦C for 2 h and without Nb2O5 addition. The porosity decreased as the Nb 2O5 addition and then increased, due to the process of sintering densification. Meanwhile, the density of specimens increased towards, shows the maximum value of 7.7 at 0.2 wt.% Nb2O5 addition (near the phase boundary) and then decreased. This result clearly indicates that Nb2O5 enhances sinterability and the density of ceramics. On the one hand, this may be attributed to the liquid phase sintering. On the other hand, since Nb5+ are considered to enter the B sites of the ceramics, and a large amount of oxygen vacancies could be formed in the samples, which expedites the lattice diffusion, and leads to the enhancement of the bulk density of the ceramics. Up to 0.3 wt% Nb2O5, excess Nb2O5 content beyond the solubility limit will be segregated at the grain boundary and make the density of ceramics decrease. The density of a piezoelectric ceramic used for a hydrophone or transducer was very important in that efficient energy transfer could be achieved only through excellent impedance matching.

7,8

7,6

0,14 0,12

7,4

0,10

7,2

0,08

Porosity

Bulk density

0,16

Density Porosity ° 1180 C

7,0 0,06 6,8

0,04 0,0

0,1

0,2

0,3

0,4

Nb2O5 Wt %.

Fig. 2. Change in porosity and bulk density as functions of Nb2O5.

Micrographs of all the sintered pellets of the Nb-modified PBZT–PBCT ceramics are presented in Fig. 3. A uniform microstructure is observed in the present study for Nb-modified PBZT–PBCT ceramics. These micrographs suggest that the sintered pellets of Nb-modified PBZT–PBCT are fully dense compared to the undoped PBZT– PBCT ceramic, and reveal that the pores are free and the tightly bound grains promote densification in the ceramics. However, the grain size increases when the excess of Nb 2O5 is added. It is demonstrated that the Nb2O5 addition is helpful to improve the sinterbility, owing to the fact that there are some oxygen vacancies in the Nb 2O5-doped ceramics, making the diffusion easier. Grain size of Nb 2O5 added specimens sintered at 1180 °C are 1.75, 2.21, 2.56 and 1.24 μm at Nb2O5 content of 0.00, 0.10, 0.20 and 0.30 wt.%, respectively. The ceramics with 0.20 wt.% Nb2O5 additive present a homogeneous microstructure and well-grown grains.

202

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Fig. 3. SEM micrographs of Nb-modified PBZT–PBCT ceramics: (a) 0.00; (b) 0.10; (c) 0.20 and (c) 0.30 wt.% sintered at 1180°C .

3.2. Dielectric and piezoelectric properties Fig.4 shows the temperature dependence of the dielectric constant of PBZT–PBCT ceramics with different amounts of Nb2O5 at 1 kHz. The maximum dielectric constant εmax was observed to increase from 13053 for the undoped sample to around 17100 for the sample doped with 0.20 wt.% of Nb2O5. This can be explained by the increasing grain-size effect [23,24] (the densification of ceramics and the uniform microstructure with fewer pores). A similar observation was also reported by Randell et al. [25]. They explained that the maximum dielectric constant decreased systematically with decreasing grain size.

18000 16000

Dielectric constant (Hr)

0.0 0.1 0.2 0.3 0.4

1180 °C

14000 12000

wt.% wt.% wt.% wt.% wt.%

Nb205 Nb205 Nb205 Nb205 Nb205

10000 8000 6000 4000 2000 0

100

200

300

400

500

°

Temperature ( C)

Fig. 4. Variation of dielectric constant with temperature at 1 kHz for different amounts of Nb 2O5.

Louanes Hamzioui et al. / Energy Procedia 74 (2015) 198 – 204

220

203

°

1180 C

d31 (pC/N)

200

180

160

140

120 0,0

0,1

0,2

0,3

0,4

Nb2O5 Wt.%

Fig. 5. Variation of d31 of the PBZT–PBCT ceramics as functions of Nb2O5 sintered at 1180 °C for 2 h.

The variation of d31 value of the PBZT–PBCT ceramics with different amounts of Nb 2O5 sintered at 1180 °C for 2 h is shown in Fig. 5. With the increase of Nb content, the piezoelectric properties are improved rapidly at first, and then tend to decrease. The maximum d31 value of 203 pC/N is observed for the specimen with x = 0.2 wt.%. This phenomenon confirmed that the specimens with x = 0.2 wt.% correspond to the MPB composition of Pb0,98 Ba0,02[((Zr0.52Ti0.48)0,98(Cr3+0.5, Ta5+0.5)0,02)] ceramics.

4. Conclusion In this study, Pb0,98 Ba0,02[((Zr0.52Ti0.48)0,98(Cr3+0.5, Ta5+0.5)0,02)] ceramics with the addition of Nb2O5 were synthesized by conventional ceramic techniques. X-ray diffraction analyses indicated a pure solid solution of PBZT–PBCT with Nb2O5 without any second phase. Moreover, X-ray diffraction studies revealed that PBZT–PBCT with 0.1 and 0.2 wt.% of Nb2O5 showed a MPB region. SEM micrographs of the sintered and the fractured surfaces indicated uniform, fine-grained, and almost pore-free microstructures. The relative densities of the specimens doped with 0.20 wt.% of Nb2O5 was higher than 97% of the theoretical density, indicating the development of a dense microstructure due to the formation of a liquid phase. The maximum dielectric constant (εr = 17100) and the piezoelectric charge coefficient (d31 = 203 pC/N) were observed for 0.20 wt.% of Nb2O5. It has been observed that the dielectric and piezoelectric constants increased gradually with increasing x up to 0.20 and was found optimum, which could be ideal for electromechanical and energy harvesting applications. References [1] E. Andrich, Electron. Appl. 26 (1965) 123–144. [2] A.J. Moulson, J.M. Herbert, Electroceramics, second ed., John Wiley and Sons, New York, 2003. [3] G.H. Haertling, J. Am. Ceram. Soc. 82 (1999) 797–818. [4] Xu Yuhan, Ferroelectric Materials and their Applications, Elsevier, Amsterdam, 1991. [5] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic, London, 1971. [6] X.Y. Zhang, X. Zhao, C.W. Lai, J. Wang, X.G. Tang, J.Y. Dai, Synthesis and piezoresponse of highly ordered Pb(Zr 0.53Ti0.47)O3 nano wire arrays, Applied Physics Letters 85 (2004) 4190–4192. [7] G. Xu, W.J. Weng, H.X. Yao, P.Y. Du, G.R. Han, Low temperature synthesis of lead zirconate titanate powder by hydroxid ecoprecipitation, Microelectronic Engineering 66 (2003) 568–573.

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