Microstructure evolution and electric properties with addition amounts of dysprosium (DyO1.5) in (BaCa)(TiZr)O3 ceramics

Microstructure evolution and electric properties with addition amounts of dysprosium (DyO1.5) in (BaCa)(TiZr)O3 ceramics

Materials Science and Engineering B 123 (2005) 69–73 Microstructure evolution and electric properties with addition amounts of dysprosium (DyO1.5) in...

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Materials Science and Engineering B 123 (2005) 69–73

Microstructure evolution and electric properties with addition amounts of dysprosium (DyO1.5) in (BaCa)(TiZr)O3 ceramics Hsing-I Hsiang ∗ , Ga-Pon Lai Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan, ROC Received 20 October 2004; received in revised form 15 June 2005; accepted 2 July 2005

Abstract In this study, the effects of various amounts of added DyO1.5 on the substitution mechanism for the (BaCa)(TiZr)O3 (BCTZ) ceramic sintered in a reduction atmosphere were investigated using X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS).The Dy3+ ion occupation sites depended on the A/B ratio, whereas sites for the Ca2+ ions were affected by the DyO1.5 added amount and A/B ratio. For the specimens with A/B ratios below 1, Dy3+ ions mainly occupied the A-sites, which expelled some Ca2+ ions to occupy B-sites, based on XPS spectra and lattice parameter results. The Dy3+ ions occupied A-sites acting as donor, which resulted in a decrease in resistivity. For the samples containing 0.01 mol% DyO1.5 , the Dy2 Ti2 O7 phase was observed and the resistivity increased rapidly due to the incorporation of Dy3+ ions into B-sites at A/B ratios below 1. As the added DyO1.5 increased to 0.015 mol%, a certain number of Ca2+ ions were forced to shift from B-sites to A-sites, as suggested by XPS spectra and lattice parameter results, which resulted in a decrease in resistivity. © 2005 Elsevier B.V. All rights reserved. Keywords: BaTiO3 ; Rare earth; Dielectrics; Reducing atmosphere; XPS

1. Introduction The oxygen vacancies created by the addition of MnO or CaO to BaTiO3 could improve the insulation resistance degradation resulting from sintering in reducing atmospheres [1,2]. However, previous reports have found that resistance degradation measured in highly accelerated life tests (HALT) was caused by oxygen vacancies [3]. Lee et al. proposed that the addition of Dy3+ and Nb5+ could improve the resistance degradation of BME materials in HALT [4]. It is thought that Dy3+ entering Ba-sites as donors can improve the stability of life due to the reduction of oxygen vacancies and the formation of a barrier of grain boundaries that suppresses electromigration of oxygen vacancies [5]. The previous studies have mainly focused on the effects of Dy3+ on the dielectric and substitution mechanism of BaTiO3 or Ba(ZrTi)O3 systems[5]. However, the ∗

Corresponding author. Fax: +886 6 2380421. E-mail address: [email protected] (H.-I. Hsiang).

0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.07.003

substitution mechanism of Dy3+ for the BCTZ system has not been well understood. The purpose of the study is to investigate the effects of various added amounts of DyO1.5 on the substitution mechanism, microstructure, and electric resistance for the BCTZ ceramic sintered in reduction atmospheres.

2. Experimental The experimental material BCTZ with the standard composition (Ba0.96011 Ca0.03988 )1.0059 (Ti0.82089 Zr0.17911 )O3 (Kyoritsu chemical Co. Ltd., Japan) was used. The BCTZ powders were added with 0.002 mol% MnO, 0.0039 mol% NbO2.5 and various amounts (0.001, 0.003, 0.005, 0.010, and 0.015 mol%) of DyO1.5 and then ball-milled in ethanol for 24 h using YTZ balls. The powders were dried in an oven and then added with PVA for granulation. The powders were compacted using a cold isostatic press at 200 MPa. These specimens were then debindered at 500 ◦ C and sintered at

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1320 ◦ C for 2 h in a nitrogen reducing atmosphere. The sample crystalline phases were identified using the XRD powder method using Ni-filtered Cu K␣ radiation (D5000, Siemens, German). The lattice parameters were determined through (0 0 4) and (4 0 0) reflections. The sintered pellet microstructure development was observed by SEM (S-4100, Hitachi, Japan) and TEM (AEM-3010, Joel, Japan). The binding energies of 2p1/2 and 2p3/2 electrons for Ca2+ ion in BCTZ ceramics were analyzed by X-ray photoelectron spectroscopy (ESCALAB 210, VG Scientific, UK). The electrical resistivity of the samples was measured using the two-probe dc technique (Multimeter-2001, Keithley, USA). At least three samples for the sintered BCTZ ceramics were used to confirm the reproducibility.

3. Results and discussion

Fig. 1. The Ca(2p) spectra consisting of 2p1/2 and 2p3/2 electron lines from the samples containing various amount of DyO1.5 .

and then some Ca2+ ions occupied B-sites were expelled to occupy A-sites as the addition of DyO1.5 increased up to 0.015 mol%.

3.1. XPS 3.2. Microstructures The coordination number of Ca2+ ion occupying A-site of the perovskite structure is 12, and that of Ca2+ ion occupying B-site is 6. Thus, the binding energies of 2p1/2 and 2p3/2 electrons for Ca2+ ion occupying A-site are larger than that occupying B-site. The Ca(2p) spectra consisting of 2p1/2 and 2p3/2 electron lines from the samples containing various amount of DyO1.5 are shown in Fig. 1. It can be seen that the binding energies of 2p1/2 and 2p3/2 electrons are shifted toward lower values as the addition of DyO1.5 increases from 0 to 0.003, then show no change up to 0.01 mol%, and furthermore shifted toward higher values as the addition of DyO1.5 increases from 0.01 to 0.015 mol%. Therefore, a certain number of the Ca2+ ions were shifted from the A-sites to the B-sites as the addition of DyO1.5 increased from 0 to 0.01,

For all samples, only the cubic perovskite structure was observed. No other second phase was detected (Fig. 2) probably because the amount of precipitate was below the XRD (0.5 wt.%) detection limit. The specimen microstructures with various amount of DyO1.5 addition are shown in Fig. 3. A small TiO2 excess in BaTiO3 is well known to lead to liquid-phase formation above the eutectic temperature in a BaTiO3 –TiO2 system. The specimen without DyO1.5 addition showed abnormal grain growth, indicating the formation of a eutectic liquid due to TiO2 excess, probably induced by the wear of ZrO2 milling balls. In the case of the specimen with the addition of DyO1.5 , abnormal grain growth was still observed, meaning that TiO2 -excess was retained in the sample.

Fig. 2. XRD patterns of the sintered body with the addition of various amounts of DyO1.5 .

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Fig. 3. The microstructures for the specimen with various amount of DyO1.5 addition.

Fig. 4 shows the TEM micrograph and diffraction pattern for the specimen with the addition of 0.001 mol% DyO1.5 . The precipitate could be identified as Ba6 Ti7 O40 (phase with TiO2 excess) on the basis of its diffraction pattern, indicating Ba/Ti ratio below 1. Compared to the grain sizes of specimen with the addition of 0.001 mol% DyO1.5 , those of the specimen with the

addition of 0.003 and 0.005 mol% decreased from 46 to 27 and 3 ␮m, respectively. This suggested that the Ba/Ti ratio increased as the added DyO1.5 increased from 0.001 to 0.005 mol%, caused by the incorporation of Dy3+ ions into the A-sites. For the specimen containing 0.01 mol% DyO1.5 , Dy2 Ti2 O7 phase was observed as shown in Fig. 5. Lee et

Fig. 4. TEM micrograph and diffraction pattern for the specimen with the addition of 0.001 mol% DyO1.5 .

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Fig. 5. TEM micrograph and diffraction pattern for the Dy2 Ti2 O7 phase observed in the specimen with the addition of 0.01 mol% DyO1.5 .

al. [5] recently reported Dy2 Ti2 O7 phase was detected in the BaTiO3 with Ti-excess. Buscaglia et al. [6] investigated the incorporation of Er3+ into BaTiO3 ceramics and also observed Er2 Ti2 O7 crystallites existed inside the pockets of Ti-rich phase. Therefore, some Dy3+ ions are believed to enter into B-sites for the specimen containing 0.01 mol% DyO1.5 , which led to the occurrence TiO2 -excess and resulted in the formation of Dy2 Ti2 O7 . The samples containing above 0.01 mol% DyO1.5, with TiO2 -excess composition, had grain sizes of about 4–5 ␮m, which may be resulted from the formation of the Dy2 Ti2 O7 crystallites, suppressing grain growth due to the pinning effect as shown in Fig. 4. However, for the samples containing 0.015 mol% DyO1.5, the grain sizes are larger than those added 0.01 mol% DyO1.5 . It may be due to that as the added DyO1.5 was increased up 0.015 mol%, the amount of Dy3+ occupying B-sites increased, which

resulted in larger A/B ratio and grain size compared to 0.01 mol%. 3.3. Electric resistivity The electric resistivity of the samples with various amount of DyO1.5 measured at room temperature is shown in Fig. 6. Initially, the resistivity decreased rapidly with increasing the added DyO1.5 to 0.005 mol%. In the second stage, the increase in the resistivity was observed up to 0.01 mol% DyO1.5 . Finally, the resistivity decreased slightly as the added DyO1.5 was increased from 0.01 to 0.015 mol%. The change in the resistivity with the addition of DyO1.5 can be explained as follows. In the first stage (the addition of DyO1.5 from 0.001 to 0.003 mol%): Dy3+ ions mainly occupied the A-site and acted as donor dopant, which resulted in the decrease

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Fig. 6. The electric resistivity of the samples with various amount of DyO1.5 measured at room temperature.

Fig. 7. The variation in the lattice parameter as a function of the added DyO1.5 .

in resistivity. As the addition of DyO1.5 increased up to 0.05 mol%, a certain number of the Dy3+ ions started to occupy B-sites and acted as acceptor dopant. Therefore, the electron concentration is suppressed by Dy3+ ions at the B-sites, which would lead to little difference between the samples added 0.003 and 0.005 mol% DyO1.5 and a corresponding increase for the sample added 0.01 mol% DyO1.5 in the resistivity. As the added DyO1.5 was increased up to 0.015 mol%, a part of Dy3+ ions were shifted to occupy B-sites, which resulted in a decrease in the amount of Ca2+ ions occupying B-sites. Therefore, the resistivity decreased slightly as the added DyO1.5 increased from 0.01 to 0.015 mol%.

sion, the lattice parameter result is in agreement with the XPS spectra observation.

3.4. Lattice parameters The ionic radii of Ba2+ , Ca2+ , Ti4+ , Zr4+ , and Dy3+ are summarized as follows: A-site (12 coordinate): Ba2+ = 0.1610 nm, Ca2+ = 0.135 nm, Dy3+ = 0.117 nm; B-site (6 coordinate): Ti4+ = 0.0605 nm, Zr4+ = 0.072 nm, Dy3+ = 0.0908 nm, and Ca2+ = 0.10 nm. The substitution sites could be determined from the change in the lattice parameters. The variation in the lattice parameter as a function of the added DyO1.5 is shown in Fig. 7. The lattice parameter increased as the added DyO1.5 was increased from 0 to 0.01 mol%. This indicated that Dy3+ ions mainly occupied A-sites accompanied by B-sites occupation by a certain number of Ca2+ ions. Further increase in added DyO1.5 up to 0.015 mol%, the lattice parameter became smaller than that of samples containing 0.01 mol% DyO1.5 . Chan et al. [7] found that Ca2+ ions which are well known as A-site substitutions may also enter the B-sites of BaTiO3 . In the case of A/B ratio above 1, a certain number of the Ca2+ ions are shifted from A- to B-sites. As the added DyO1.5 was increased up 0.015 mol%, the A/B ratio would be smaller than 1, and then more Ca2+ ions were expelled to occupy A-sites, which led to the decrease in lattice parameters. From the above discus-

4. Conclusion Sites occupied by Dy3+ ions depend on the A/B ratio, whereas sites occupied by Ca2+ ions were affected by the amount of added DyO1.5 and the A/B ratio. For specimens with A/B ratios below 1, Dy3+ ions mainly occupied Asites, which expelled some Ca2+ ions to occupy B sites based on XPS spectra and lattice parameter results. The Dy3+ ions occupying A-sites acted as donors, which resulted in a decrease in resistivity. In samples containing 0.01 mol% DyO1.5 , the Dy2 Ti2 O7 phase was observed and the resistivity increased rapidly due to the incorporation of Dy3+ ions into B-sites and the A/B ratio falling below 1. As the amount of added DyO1.5 increased to 0.015 mol%, a certain number of Ca2+ ions were forced to shift from B-sites to A-sites, as suggested by the XPS spectra and lattice parameter results, which resulted in a decrease in resistivity.

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