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Effects of WO3 additions on the phase structure and transition of zinc titanate ceramics
Journal of Alloys and Compounds 450 (2008) 440–445
Effects of WO3 additions on the phase structure and transition of zinc titanate ceramics Xiangchun Liu ∗ , Ming Zhao, Feng Gao, Lili Zhao, Changsheng Tian School of Materials Science and Engineering, Northwestern Polytechnical University, 127 Youyi West Road, Xi’an 710072, People’s Republic of China Received 7 August 2006; received in revised form 12 October 2006; accepted 29 October 2006 Available online 1 December 2006
Abstract WO3 -doped zinc titanate ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. The effects of WO3 additions on the phase structure and phase transitions of zinc titanate ceramics were investigated by high-temperature X-ray diffractometry (HTXRD) and transmission electron microscopy (TEM). The results showed that the major phase of zinc titanate ceramics transformed from zinc orthotitanate phase to zinc metatitanate phase with the amounts of WO3 additions increasing. Small WO3 (<1.00 mass%) addition accelerated the transition of ZnTiO3 to Zn2 TiO4 , while excessive WO3 addition restrained the transition. HTXRD showed that WO3 enhanced the stability of Zn2 Ti3 O8 and weakened the stability of ZnTiO3 . A precipitate within the Zn2 TiO4 matrix was observed. Viewed along the [0 2¯ 1] orientation of Zn2 TiO4 , the precipitate was found to have a rectangular shape and to be nanometer level in size; its composition was concluded to be Zn2 Ti3 O8 . The dielectric properties of WO3 -doped zinc titanate ceramics were measured at different frequencies. The results showed the decreasing tendency with the increasing measuring frequencies for both the dielectric constants and the loss tangents, and there existed maximum values when the amount of WO3 was 0.50 mass%. © 2006 Elsevier B.V. All rights reserved. Keywords: Electronic materials; Ceramics; Phase transitions; Dielectric properties
1. Introduction Zinc titanate ceramics are candidates for low-temperature sintering dielectrics, since they can be sintered at 1150 ◦ C without sintering aids and can be sintered at even lower temperatures than 900 ◦ C if a small amount of addition is added, but the unstable dielectric properties coming from complex phase transitions restrict their practical applications. It is significant to investigate the phase transition mechanism of zinc titanate ceramics with sintering aids. Three compounds are known to exist in the ZnO–TiO2 system: Zn2 TiO4 (zinc orthotitanate) with cubic spinel crystal structure, ZnTiO3 (zinc metatitanate) with hexagonal illuminate structure, and Zn2 Ti3 O8 with cubic defect spinel structure. Zn2 TiO4 is stable from room temperature to its liquid temperature (1418 ◦ C), while ZnTiO3 is stable from room temperature to about 945 ◦ C. At temperatures near 945 ◦ C, ZnTiO3 was reported to separate into Zn2 TiO4 and TiO2 . Yamaguchi
∗
Corresponding author. Tel.: +86 1389 180 1847. E-mail address: [email protected] (X. Liu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.120
et al. [1] clarified that Zn2 Ti3 O8 is a low-temperature form of ZnTiO3. It is stable at temperatures lower than 820 ◦ C. The phase transitions of ZnO–TiO2 system are relatively complex and sensitive to the start material, additions, and preparing process. Only a few investigators have been found to research the phase structure of dielectric ceramics using HTXRD and TEM. In the present work, the effects of WO3 additions on the phase structure and phase transitions of zinc titanate ceramics were investigated by high-temperature X-ray diffractometry (HTXRD) and transmission electron microscopy (TEM). The phase transition mechanism of zinc titanate ceramics with WO3 additions was discussed. 2. Experimental procedure ZnTiO3 powders were prepared via a chemical route with A.R zinc hydroxide carbonate (Zn5 (CO3 )2 ·(OH)6 ) and anatase (TiO2 ; 10–30 nm; >99.0%). Zinc hydroxide carbonate was heat-treated at 350 ◦ C for 2 h in air to obtain ZnO with high active energy as a starting material. Anatase nanopowders were mixed with ZnO powders (50–70 nm) using planetary milling with zirconium balls in ethanol for 24 h. The mixture was dried and calcined at 750 ◦ C for 2 h. After 1.00 mass%, different ratio of WO3 addition were added in the mixture, the calcined powders were milled for 12 h and dried. The resultant powders were granulated using a
X. Liu et al. / Journal of Alloys and Compounds 450 (2008) 440–445
Fig. 1. XRD patterns of the zinc titanate ceramics with various amounts of WO3 addition sintered at 900 ◦ C for 4 h. 2 mass% poly vinyl alcohol (PVA) solution and pelleted to 12 mm diameter and 1.2 mm thick disks. The ceramics were prepared by sintering at temperatures from 850 to 1000 ◦ C for 4 h with a heating rate of 10 ◦ C/min and cooled to room temperature in the furnace. The crystalline structure and phase identification of the samples were investigated using XRD (X’ Pert MPD PRO, Holland). High-temperature X-ray diffracting was performed from 25 to 1000 ◦ C with a heating rate of 10 ◦ C/min. The specimens were polished and acid-etched for microstructure observation which was carried out by scanning electron microscope (SEM, JEOL JSM5800, Japan) with an energy dispersive spectroscopy (EDS). Specimens for TEM observation were mechanically thinned to about 0.03 mm thickness and ion beam milled after being mounted onto the grids.
3. Result and discussion Fig. 1 shows the XRD patterns of the zinc titanate ceramics with 0.0, 0.10, 0.20, 0.50, 1.00, and 3.00 mass% WO3 additions sintered at 900 ◦ C for 4 h, separately. According to ZnO–TiO2 binary system phase diagram [2] and the correlative literature [3], the phase transition in ZnO–TiO2 system can be expressed simply as follows: ∼700 ◦ C
700−820 ◦ C
ZnO + TiO2 −−−→ZnTiO3 −−−−−→ 820−900 ◦ C
ZnTiO3 + Zn2 TiO4 + Zn2 Ti3 O8 −−−−−→ ≥950 ◦ C
ZnTiO3 +Zn2 TiO4 + Zn2 Ti3 O8 +TiO2 −−−→Zn2 TiO4 + TiO2 (1) As shown in formula (1), the ZnO: TiO2 = 1:1 powders show various phase at different calcining temperatures. It is difficult to prepare pure ZnTiO3 from the mixtures of ZnO: TiO2 = 1:1 because some satellite reactions may occur. ZnTiO3 forms at about 700 ◦ C, but at 820 ◦ C, some satellite reactions occur strongly, cubic Zn2 TiO4 and cubic Zn2 Ti3 O8 are formed. At 900 ◦ C, ZnTiO3 start to decompose to Zn2 TiO4 and rutile. When the calcining temperatures are higher than 950 ◦ C, the decomposition of ZnTiO3 phase completes, Zn2 Ti3 O8 also separates into Zn2 TiO4 and rutile completely. From Fig. 1, it is clear that the major phase of zinc titanate ceramics transformed from hexago-
441
Fig. 2. High-temperature XRD patterns of the zinc titanate with 1.00 mass% WO3 additions.
nal zinc metatitanate (ZnTiO3 ) phase to cubic zinc orthotitanate (Zn2 TiO4 ) phase when small WO3 (<1.00 mass%) additions were added, and transformed from Zn2 TiO4 phase to ZnTiO3 phase when the amount of WO3 additions increased from 1.00 to 3.00 mass%. Moreover, the relative intensity of ZnTiO3 diffraction peak weakened gradually with WO3 content increasing from 0.1 to 1.0 mass% (as shown in the inset of Fig. 1, it was plotted according to the relative intensity of ZnTiO3 diffraction peak). The above results indicate that small WO3 (<1.00 mass%) addition accelerated the decomposition of hexagonal ZnTiO3 phase to cubic Zn2 TiO4 phase, while excessive addition (for example, 3.00 mass%) restrained the decomposition. A new phase ZnWO4 appeared in the 3.00 mass% WO3 -doped zinc titanate ceramics, implying that a solid-phase reaction of WO3 and zinc titanate happened. Compared with 1.00 mass% WO3 -added samples, the strongest peaks of ZnTiO3 phase and Zn2 TiO4 phase in the 3.00 mass% WO3 added samples moved to higher degree (from 32.7778 to 32.8110 ◦ C for ZnTiO3 phase, and from 35.1506 to 35.3312 ◦ C for Zn2 TiO4 phase), as shown in the figure in the inset of Fig. 1, suggesting that excessive WO3 addition resulted in crystal lattice distortion. From above results, it can be concluded that the phase transitions of zinc titanate were sensitive to the amount of WO3 additions. The high-temperature X-ray patterns of the zinc titanate with 1.00 mass% WO3 addition calcined from 750 to 1000 ◦ C are indicated in Fig. 2. As shown in Fig. 2, Zn2 Ti3 O8 , a lowtemperature form of ZnTiO3 , existed as a stable phase at the calcining temperature from 750 to 940 ◦ C. The diffractive peak of Zn2 Ti3 O8 weakened as the sintering temperature was higher than 910 ◦ C, at the same time, ZnTiO3 phase and rutile phase appeared in powder, which implies that Zn2 Ti3 O8 decomposed to ZnTiO3 and rutile phase at the calcining temperature from 910 to 960 ◦ C. At 960 ◦ C, Zn2 Ti3 O8 decomposed completely, and at the same time, hexagonal ZnTiO3 phase also disappeared. This result shows that small WO3 enhanced the stability of Zn2 Ti3 O8 and weakened the stability of ZnTiO3 . In ZnO–TiO2 system without any other additions, Zn2 Ti3 O8 phase exists at temper-
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X. Liu et al. / Journal of Alloys and Compounds 450 (2008) 440–445
Table 1 Ionic radii of Zn2+ , Ti4+ , and W6+ and experience expression of solid solution Ionic
˚ Ionic radii (A) Experience expression of solid solution 1 − RW 6+ /M (M = Zn2+ or Ti4+ )
atures less than or equal to 820 ◦ C, and ZnTiO3 phase exists at temperatures from 700 to 950 ◦ C. With WO3 added, Zn2 Ti3 O8 phase disappeared at about 940 ◦ C, while ZnTiO3 phase presented at about 910 ◦ C. Table 1 shows the ionic radii of Zn2+ , Ti4+ , and W6+ . According to experience expression of solid solution, as in Table 1, W6+ can substitute the cation of A site and/or B site of the AB2 O4 -type (A = Zn, B = Zn and Ti) spinel structure. This would result in crystal lattice distortion and solid-phase reaction. The reaction can be expressed as follows: 900−950 ◦ C
ZnTiO3 ←−−−−−−→Zn2 TiO4 + TiO2 900 ◦ C
Zn2 TiO4 + WO3 −−−−→ZnWO4 + ZnTiO3
(2) (3)
From formula (3), excessive WO3 addition (for example, 3.00 mass%) accelerated the decomposition of cubic Zn2 TiO4 to hexagonal ZnTiO3 , while small WO3 (<1.00 mass%) addition accelerated the reaction (2) going on right, because hexagonal ZnTiO3 is more stable than cubic Zn2 TiO4 at temperatures 900–950 ◦ C. According to Kim’s report [6], the compound Zn2 Ti3 O8 is formed only on the basis of the Zn2 TiO4 phase, because the crystal structure of Zn2 Ti3 O8 is a defect spinel exhibiting ordered cation vacancies within the structure, this ordering leading to the decrease of the space group symmetry ¯ to P43 32. The increase of the amount of Zn2 TiO4 from Fd 3m phase in the ceramics with small WO3 additions enhanced the stability of Zn2 Ti3 O8. TEM micrograph and the electron diffraction patterns of 1.00 mass% WO3 -added samples sintered at 930 ◦ C were performed, as shown in Figs. 3 and 4, respectively. From Fig. 3, it can be seen that three different grains existed in the sample: the small white grains were symbolized as A, and the black grains were symbolized as B. Some tiny black precipitates (the arrow in Fig. 3) presented in the third large grains. Four phases, Zn2 TiO4 , ZnTiO3 , TiO2 , and ZnWO4, coexisted in the samples from XRD patterns, and the major phase is Zn2 TiO4 . According to the crystal color and the appearance, and combined with the EDS analysis (as shown in Fig. 5), the white grain symbolized as A in Fig. 3 was constituted by ZnWO4 phase. The black grain B was considered to be rutile according to the electron diffraction patterns, as shown by Fig. 4a. Fig. 3 shows that the large grains precipitated some tiny black grains in them presented as a major phase of the ceramics. It can be concluded that the large grains were composed of Zn2 TiO4 but the ZnTiO3 cannot be observed in the micrograph. It can be attributed to the similar crystal color and appearance of ZnTiO3 compared to those of the Zn2 TiO4 . The precipitates have a rectangular shape, and the crystal color and the appearance are different from the other present phases. Electron diffraction pattern of the matrix taken with the inci-
Zn2+
Ti4+
W6+
0.74 16.22%
0.68 8.82%
0.620 –
dent electron beam parallel to the [0 2¯ 1] orientation is shown in Fig. 4b. It can be seen that the electron diffraction patterns of matrix take on multiphase structure. Strong spots are deduced to correspond to spinel-type Zn2 TiO4 . In addition to these common spots, some extra weak spots (expressed as arrows) can be observed. Considering the fact that the composition of the matrix was approximately the same as Zn2 TiO4 and that of the precipitate was different, it can be concluded that these extra spots are from the precipitate area. Analyzing the electron diffraction patterns of matrix, the precipitate phase was considered as a cube structure. Excluding hexagonal crystal symmetry ZnTiO3 , it is supposed that the precipitate was Zn2 Ti3 O8 . To prove the above supposition, EDS analysis of zinc titanate ceramics with 1.00 mass% WO3 sintered at 900 ◦ C was performed. The results are shown in Fig. 5 and Table 2. From Fig. 5, it is clear that Ti enriches in the black grains, so they
Fig. 3. TEM micrographs of zinc titanate ceramics with 1.00 mass% WO3 additions sintered at 930 ◦ C.
X. Liu et al. / Journal of Alloys and Compounds 450 (2008) 440–445
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Fig. 5. EDS analysis of zinc titanate ceramics with 1.00 mass% WO3 sintered at 900 ◦ C.
Fig. 4. Electron diffraction patterns of (a) B grain in Fig. 3 and (b) matrix of the sample.
corresponded to rutile phase. ZnWO4 was believed to exist in the W and Zn-rich white grains, but as the concentration of Ti element in them is also high, it is deduced that W6+ came into the crystal lattice of ZnTiO3 , thus the integrality of crystal lattice was destroyed, the phase stability of ZnTiO3 was decreased, and the decomposition of ZnTiO3 was accelerated. However, it cannot be ensured that the gray grains are ZnTiO3 or Zn2 TiO4 . The spectrum 2 in Fig. 5 shows the EDS spectra of the matrix with precipitate and the EDS quantum analysis is shown in Table 2 and it is clear that the Ti content is higher than Zn. Considering the fact that the composition of the matrix was approximately the same as Zn2 TiO4 , while that of the precipitate was different, it is very reasonable to assume that the Ti content is higher than Zn in the precipitate. It can be concluded that the precipitate was rutile, Zn2 Ti3 O8 or ZnTiO3 . In view of an ilmenite-type phase (hexagonal symmetry) of the structure of ZnTiO3 that has been reported by Dulin and Rase [4], it can be excluded from the candidacy of precipitate. Excluding rutile because of the same reason as ZnTiO3 , it is believed that precipitate is likely to consist of Zn2 Ti3 O8 . Li et al. [5] found a precipitate in Zn2 TiO4 matrix in the ceramics prepared by ZnO and TiO2 in a molar ratio of 3:2. By powder X-ray diffraction and energy-dispersive X-ray spectroscopy, they concluded that the precipitate is ZnTiO3 . The similar precipitate was found by Kim et al. [6] in (ZnNi)TiO3 ceramics, but they believed that the precipitate was Zn2 Ti3 O8 , and formed on the cooling stage. When the ceramics prepared by ZnO and TiO2 in a molar ratio of 1:1 were cooled to the temperature less than 820 ◦ C, Zn2 Ti3 O8 formed because it is a
Table 2 EDS quantum analysis of zinc titanate ceramics with 1.00 mass% WO3 sintered at 900 ◦ C Spectrum
O (mass%)
Ti (mass%)
Zn (mass%)
W (mass%)
Total
Spectrum 1 Spectrum 2 Spectrum 3
35.79 29.62 20.17
45.26 37.78 38.88
14.73 32.60 32.31
4.22 – 8.64
100.00 100.00 100.00
Maximum Minimum
35.79 20.17
45.26 37.78
32.60 14.73
8.64 0.00
– –
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X. Liu et al. / Journal of Alloys and Compounds 450 (2008) 440–445
Fig. 7. Dielectric constant of WO3 -doped zinc titanate ceramics measured at 100 and 400 kHz and at 1, 4, and 10 MHz.
Fig. 6. Bulk density and shrinkage of zinc titanate ceramics with different amounts of WO3 added vs. the sintering temperatures.
low-temperature form, and existed steadily at temperatures less than 820 ◦ C. Fig. 6 shows the bulk density and shrinkage of zinc titanate ceramics with different amount of WO3 added versus the sintering temperatures. The density and shrinkage increased with the sintering temperature increasing, and reached saturated value at 900 ◦ C. The density of ceramics with 3.0 mass% WO3 additions is higher than those of ceramics with 1.0 and 0.5 mass% WO3 additions, but the relative density of the former is lower than the latter’s, which can be attributed to the higher density of the ZnWO3 phase (7.81 g/cm3 ) than to the Zn2 TiO4 (5.33 g/cm3 ) and ZnTiO3 phase (5.17 g/cm3 ): 0.5 mass% WO3 -doped samples have the highest relative density at 900 ◦ C. The dielectric constant (εr ) and the loss tangent (tan δ) properties of WO3 -doped zinc titanate ceramics sintered at 900 ◦ C were measured at 10 and 100 kHz and at 1, 10, and 100 MHz at ambient temperature; the results are shown in Figs. 7 and 8, respectively. The dielectric constants of samples increased with the amount of WO3 addition at the same frequency, and had a maximum value at 0.50 mass% as shown in Fig. 6, then gradually decreased with the increasing amount of tungsten. This could be ascribed to the higher relative density of 0.5 mass%doped samples compared to the other ones. Fig. 7 also indicates a decreasing tendency of all the dielectric constants with increasing measuring frequency. Even at a microwave frequency, the εr value of samples is presumed to be about 21–24, higher than that of single-phase hexagonal ZnTiO3 ceramics (19) [7]. Similarly, the evolution of dielectric loss tangent value of WO3 -doped zinc titanate ceramics became a maximum at 0.50 mass%, and then
Fig. 8. Dielectric loss of WO3 -doped zinc titanate ceramics measured at 100 and 400 kHz and at 1, 4, and 10 MHz.
decreased gradually with the increasing amount of tungsten (as shown in Fig. 8). It might be ascribed to the formation of more stable hexagonal ilmenite phase with the amount of WO3 addition increasing from 0.5 to 3.0 mass%, for the cubic spinel phase would strongly deteriorate the tan δ and induce a high value. Compared with 0.5 mass%-doped samples, 1.0 mass%-doped samples had more stable hexagonal ilmenite phase and smaller cubic spinel phase, so 1.0 mass%-doped samples exhibited lower dielectric loss. Much smaller tan δ value of WO3 -doped zinc titanate ceramics at a microwave frequency can be presumed from the decreasing tendency of tan δ with the increasing value of measuring frequency [8]. 4. Conclusions The phase structure and transition of zinc titanate ceramics were sensitive to WO3 additions. Small WO3 (<1.00 mass%) addition accelerated the decomposition of hexagonal ZnTiO3 to cubic Zn2 TiO4 , while excessive WO3 addition restrained
X. Liu et al. / Journal of Alloys and Compounds 450 (2008) 440–445
the decomposition and leaded to a ZnWO4 phase formation. The major phase of zinc titanate ceramics transformed from cubic zinc orthotitanate phase to hexagonal zinc metatitanate phase with the amounts of WO3 additions increased from 1.00 to 3.00 mass%. Moreover, WO3 addition affected the formation of ZnTiO3 phase; WO3 enhanced the stability of Zn2 Ti3 O8 and weakened the stability of ZnTiO3 . A new phase, ZnWO4 , formed in the 1.00 mass% WO3 -added zinc titanate ceramics sintered at 930 ◦ C and a precipitate within the Zn2 TiO4 matrix was observed. The size of the precipitate is nanometer level, and its composition was Zn2 Ti3 O8 . The dielectric properties of WO3 -doped zinc titanate ceramics were measured at different frequencies. The results showed the decreasing tendency with the increasing measuring frequencies for both the dielectric constants and the loss tangents, and there existed maximum values when the amount of tungsten was 0.50 mass%. The best dielectric properties for WO3 -doped ceramics was obtained at 1.00 mass%.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Project 60501015) and the Doctorate Foundation of Northwestern Polytechnical University under Grant CX200408. References [1] O. Yamaguchi, M. Morimi, H. Kawabata, et al., J. Am. Ceram. Soc. 70 (1987) C97. [2] J. Yang, J.H. Swisher, Mater. Charact. 37 (1996) 153. [3] H.T. Kim, S.H. Kim, S. Nahm, et al., J. Am. Ceram. Soc. 82 (1999) 3043. [4] F.H. Dulin, D.E. Rase, J. Am. Ceram. Soc. 43 (1960) 125. [5] C.F. Li, Y.S. Bando, M. Nakamuraa, et al., Mater. Res. Bull. 35 (2000) 351. [6] H.T. Kim, J.C. Hwang, J.H. Nam, et al., J. Mater. Res. 18 (2003) 1067. [7] A. Golovchansk, H.T. Kim, Y.H. Kim, J. Korean Phys. Soc. 32 (1998) S1167. [8] J. Luo, X.X. Xing, R.B. Yu, et al., J. Alloys Compd. 402 (2005) 263.
Effects of WO3 additions on the phase structure and transition of zinc titanate ceramics Xiangchun Liu ∗ , Ming Zhao, Feng Gao, Lili Zhao, Changsheng Tian School of Materials Science and Engineering, Northwestern Polytechnical University, 127 Youyi West Road, Xi’an 710072, People’s Republic of China Received 7 August 2006; received in revised form 12 October 2006; accepted 29 October 2006 Available online 1 December 2006
Abstract WO3 -doped zinc titanate ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. The effects of WO3 additions on the phase structure and phase transitions of zinc titanate ceramics were investigated by high-temperature X-ray diffractometry (HTXRD) and transmission electron microscopy (TEM). The results showed that the major phase of zinc titanate ceramics transformed from zinc orthotitanate phase to zinc metatitanate phase with the amounts of WO3 additions increasing. Small WO3 (<1.00 mass%) addition accelerated the transition of ZnTiO3 to Zn2 TiO4 , while excessive WO3 addition restrained the transition. HTXRD showed that WO3 enhanced the stability of Zn2 Ti3 O8 and weakened the stability of ZnTiO3 . A precipitate within the Zn2 TiO4 matrix was observed. Viewed along the [0 2¯ 1] orientation of Zn2 TiO4 , the precipitate was found to have a rectangular shape and to be nanometer level in size; its composition was concluded to be Zn2 Ti3 O8 . The dielectric properties of WO3 -doped zinc titanate ceramics were measured at different frequencies. The results showed the decreasing tendency with the increasing measuring frequencies for both the dielectric constants and the loss tangents, and there existed maximum values when the amount of WO3 was 0.50 mass%. © 2006 Elsevier B.V. All rights reserved. Keywords: Electronic materials; Ceramics; Phase transitions; Dielectric properties
1. Introduction Zinc titanate ceramics are candidates for low-temperature sintering dielectrics, since they can be sintered at 1150 ◦ C without sintering aids and can be sintered at even lower temperatures than 900 ◦ C if a small amount of addition is added, but the unstable dielectric properties coming from complex phase transitions restrict their practical applications. It is significant to investigate the phase transition mechanism of zinc titanate ceramics with sintering aids. Three compounds are known to exist in the ZnO–TiO2 system: Zn2 TiO4 (zinc orthotitanate) with cubic spinel crystal structure, ZnTiO3 (zinc metatitanate) with hexagonal illuminate structure, and Zn2 Ti3 O8 with cubic defect spinel structure. Zn2 TiO4 is stable from room temperature to its liquid temperature (1418 ◦ C), while ZnTiO3 is stable from room temperature to about 945 ◦ C. At temperatures near 945 ◦ C, ZnTiO3 was reported to separate into Zn2 TiO4 and TiO2 . Yamaguchi
∗
Corresponding author. Tel.: +86 1389 180 1847. E-mail address: [email protected] (X. Liu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.120
et al. [1] clarified that Zn2 Ti3 O8 is a low-temperature form of ZnTiO3. It is stable at temperatures lower than 820 ◦ C. The phase transitions of ZnO–TiO2 system are relatively complex and sensitive to the start material, additions, and preparing process. Only a few investigators have been found to research the phase structure of dielectric ceramics using HTXRD and TEM. In the present work, the effects of WO3 additions on the phase structure and phase transitions of zinc titanate ceramics were investigated by high-temperature X-ray diffractometry (HTXRD) and transmission electron microscopy (TEM). The phase transition mechanism of zinc titanate ceramics with WO3 additions was discussed. 2. Experimental procedure ZnTiO3 powders were prepared via a chemical route with A.R zinc hydroxide carbonate (Zn5 (CO3 )2 ·(OH)6 ) and anatase (TiO2 ; 10–30 nm; >99.0%). Zinc hydroxide carbonate was heat-treated at 350 ◦ C for 2 h in air to obtain ZnO with high active energy as a starting material. Anatase nanopowders were mixed with ZnO powders (50–70 nm) using planetary milling with zirconium balls in ethanol for 24 h. The mixture was dried and calcined at 750 ◦ C for 2 h. After 1.00 mass%, different ratio of WO3 addition were added in the mixture, the calcined powders were milled for 12 h and dried. The resultant powders were granulated using a
X. Liu et al. / Journal of Alloys and Compounds 450 (2008) 440–445
Fig. 1. XRD patterns of the zinc titanate ceramics with various amounts of WO3 addition sintered at 900 ◦ C for 4 h. 2 mass% poly vinyl alcohol (PVA) solution and pelleted to 12 mm diameter and 1.2 mm thick disks. The ceramics were prepared by sintering at temperatures from 850 to 1000 ◦ C for 4 h with a heating rate of 10 ◦ C/min and cooled to room temperature in the furnace. The crystalline structure and phase identification of the samples were investigated using XRD (X’ Pert MPD PRO, Holland). High-temperature X-ray diffracting was performed from 25 to 1000 ◦ C with a heating rate of 10 ◦ C/min. The specimens were polished and acid-etched for microstructure observation which was carried out by scanning electron microscope (SEM, JEOL JSM5800, Japan) with an energy dispersive spectroscopy (EDS). Specimens for TEM observation were mechanically thinned to about 0.03 mm thickness and ion beam milled after being mounted onto the grids.
3. Result and discussion Fig. 1 shows the XRD patterns of the zinc titanate ceramics with 0.0, 0.10, 0.20, 0.50, 1.00, and 3.00 mass% WO3 additions sintered at 900 ◦ C for 4 h, separately. According to ZnO–TiO2 binary system phase diagram [2] and the correlative literature [3], the phase transition in ZnO–TiO2 system can be expressed simply as follows: ∼700 ◦ C
700−820 ◦ C
ZnO + TiO2 −−−→ZnTiO3 −−−−−→ 820−900 ◦ C
ZnTiO3 + Zn2 TiO4 + Zn2 Ti3 O8 −−−−−→ ≥950 ◦ C
ZnTiO3 +Zn2 TiO4 + Zn2 Ti3 O8 +TiO2 −−−→Zn2 TiO4 + TiO2 (1) As shown in formula (1), the ZnO: TiO2 = 1:1 powders show various phase at different calcining temperatures. It is difficult to prepare pure ZnTiO3 from the mixtures of ZnO: TiO2 = 1:1 because some satellite reactions may occur. ZnTiO3 forms at about 700 ◦ C, but at 820 ◦ C, some satellite reactions occur strongly, cubic Zn2 TiO4 and cubic Zn2 Ti3 O8 are formed. At 900 ◦ C, ZnTiO3 start to decompose to Zn2 TiO4 and rutile. When the calcining temperatures are higher than 950 ◦ C, the decomposition of ZnTiO3 phase completes, Zn2 Ti3 O8 also separates into Zn2 TiO4 and rutile completely. From Fig. 1, it is clear that the major phase of zinc titanate ceramics transformed from hexago-
441
Fig. 2. High-temperature XRD patterns of the zinc titanate with 1.00 mass% WO3 additions.
nal zinc metatitanate (ZnTiO3 ) phase to cubic zinc orthotitanate (Zn2 TiO4 ) phase when small WO3 (<1.00 mass%) additions were added, and transformed from Zn2 TiO4 phase to ZnTiO3 phase when the amount of WO3 additions increased from 1.00 to 3.00 mass%. Moreover, the relative intensity of ZnTiO3 diffraction peak weakened gradually with WO3 content increasing from 0.1 to 1.0 mass% (as shown in the inset of Fig. 1, it was plotted according to the relative intensity of ZnTiO3 diffraction peak). The above results indicate that small WO3 (<1.00 mass%) addition accelerated the decomposition of hexagonal ZnTiO3 phase to cubic Zn2 TiO4 phase, while excessive addition (for example, 3.00 mass%) restrained the decomposition. A new phase ZnWO4 appeared in the 3.00 mass% WO3 -doped zinc titanate ceramics, implying that a solid-phase reaction of WO3 and zinc titanate happened. Compared with 1.00 mass% WO3 -added samples, the strongest peaks of ZnTiO3 phase and Zn2 TiO4 phase in the 3.00 mass% WO3 added samples moved to higher degree (from 32.7778 to 32.8110 ◦ C for ZnTiO3 phase, and from 35.1506 to 35.3312 ◦ C for Zn2 TiO4 phase), as shown in the figure in the inset of Fig. 1, suggesting that excessive WO3 addition resulted in crystal lattice distortion. From above results, it can be concluded that the phase transitions of zinc titanate were sensitive to the amount of WO3 additions. The high-temperature X-ray patterns of the zinc titanate with 1.00 mass% WO3 addition calcined from 750 to 1000 ◦ C are indicated in Fig. 2. As shown in Fig. 2, Zn2 Ti3 O8 , a lowtemperature form of ZnTiO3 , existed as a stable phase at the calcining temperature from 750 to 940 ◦ C. The diffractive peak of Zn2 Ti3 O8 weakened as the sintering temperature was higher than 910 ◦ C, at the same time, ZnTiO3 phase and rutile phase appeared in powder, which implies that Zn2 Ti3 O8 decomposed to ZnTiO3 and rutile phase at the calcining temperature from 910 to 960 ◦ C. At 960 ◦ C, Zn2 Ti3 O8 decomposed completely, and at the same time, hexagonal ZnTiO3 phase also disappeared. This result shows that small WO3 enhanced the stability of Zn2 Ti3 O8 and weakened the stability of ZnTiO3 . In ZnO–TiO2 system without any other additions, Zn2 Ti3 O8 phase exists at temper-
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X. Liu et al. / Journal of Alloys and Compounds 450 (2008) 440–445
Table 1 Ionic radii of Zn2+ , Ti4+ , and W6+ and experience expression of solid solution Ionic
˚ Ionic radii (A) Experience expression of solid solution 1 − RW 6+ /M (M = Zn2+ or Ti4+ )
atures less than or equal to 820 ◦ C, and ZnTiO3 phase exists at temperatures from 700 to 950 ◦ C. With WO3 added, Zn2 Ti3 O8 phase disappeared at about 940 ◦ C, while ZnTiO3 phase presented at about 910 ◦ C. Table 1 shows the ionic radii of Zn2+ , Ti4+ , and W6+ . According to experience expression of solid solution, as in Table 1, W6+ can substitute the cation of A site and/or B site of the AB2 O4 -type (A = Zn, B = Zn and Ti) spinel structure. This would result in crystal lattice distortion and solid-phase reaction. The reaction can be expressed as follows: 900−950 ◦ C
ZnTiO3 ←−−−−−−→Zn2 TiO4 + TiO2 900 ◦ C
Zn2 TiO4 + WO3 −−−−→ZnWO4 + ZnTiO3
(2) (3)
From formula (3), excessive WO3 addition (for example, 3.00 mass%) accelerated the decomposition of cubic Zn2 TiO4 to hexagonal ZnTiO3 , while small WO3 (<1.00 mass%) addition accelerated the reaction (2) going on right, because hexagonal ZnTiO3 is more stable than cubic Zn2 TiO4 at temperatures 900–950 ◦ C. According to Kim’s report [6], the compound Zn2 Ti3 O8 is formed only on the basis of the Zn2 TiO4 phase, because the crystal structure of Zn2 Ti3 O8 is a defect spinel exhibiting ordered cation vacancies within the structure, this ordering leading to the decrease of the space group symmetry ¯ to P43 32. The increase of the amount of Zn2 TiO4 from Fd 3m phase in the ceramics with small WO3 additions enhanced the stability of Zn2 Ti3 O8. TEM micrograph and the electron diffraction patterns of 1.00 mass% WO3 -added samples sintered at 930 ◦ C were performed, as shown in Figs. 3 and 4, respectively. From Fig. 3, it can be seen that three different grains existed in the sample: the small white grains were symbolized as A, and the black grains were symbolized as B. Some tiny black precipitates (the arrow in Fig. 3) presented in the third large grains. Four phases, Zn2 TiO4 , ZnTiO3 , TiO2 , and ZnWO4, coexisted in the samples from XRD patterns, and the major phase is Zn2 TiO4 . According to the crystal color and the appearance, and combined with the EDS analysis (as shown in Fig. 5), the white grain symbolized as A in Fig. 3 was constituted by ZnWO4 phase. The black grain B was considered to be rutile according to the electron diffraction patterns, as shown by Fig. 4a. Fig. 3 shows that the large grains precipitated some tiny black grains in them presented as a major phase of the ceramics. It can be concluded that the large grains were composed of Zn2 TiO4 but the ZnTiO3 cannot be observed in the micrograph. It can be attributed to the similar crystal color and appearance of ZnTiO3 compared to those of the Zn2 TiO4 . The precipitates have a rectangular shape, and the crystal color and the appearance are different from the other present phases. Electron diffraction pattern of the matrix taken with the inci-
Zn2+
Ti4+
W6+
0.74 16.22%
0.68 8.82%
0.620 –
dent electron beam parallel to the [0 2¯ 1] orientation is shown in Fig. 4b. It can be seen that the electron diffraction patterns of matrix take on multiphase structure. Strong spots are deduced to correspond to spinel-type Zn2 TiO4 . In addition to these common spots, some extra weak spots (expressed as arrows) can be observed. Considering the fact that the composition of the matrix was approximately the same as Zn2 TiO4 and that of the precipitate was different, it can be concluded that these extra spots are from the precipitate area. Analyzing the electron diffraction patterns of matrix, the precipitate phase was considered as a cube structure. Excluding hexagonal crystal symmetry ZnTiO3 , it is supposed that the precipitate was Zn2 Ti3 O8 . To prove the above supposition, EDS analysis of zinc titanate ceramics with 1.00 mass% WO3 sintered at 900 ◦ C was performed. The results are shown in Fig. 5 and Table 2. From Fig. 5, it is clear that Ti enriches in the black grains, so they
Fig. 3. TEM micrographs of zinc titanate ceramics with 1.00 mass% WO3 additions sintered at 930 ◦ C.
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Fig. 5. EDS analysis of zinc titanate ceramics with 1.00 mass% WO3 sintered at 900 ◦ C.
Fig. 4. Electron diffraction patterns of (a) B grain in Fig. 3 and (b) matrix of the sample.
corresponded to rutile phase. ZnWO4 was believed to exist in the W and Zn-rich white grains, but as the concentration of Ti element in them is also high, it is deduced that W6+ came into the crystal lattice of ZnTiO3 , thus the integrality of crystal lattice was destroyed, the phase stability of ZnTiO3 was decreased, and the decomposition of ZnTiO3 was accelerated. However, it cannot be ensured that the gray grains are ZnTiO3 or Zn2 TiO4 . The spectrum 2 in Fig. 5 shows the EDS spectra of the matrix with precipitate and the EDS quantum analysis is shown in Table 2 and it is clear that the Ti content is higher than Zn. Considering the fact that the composition of the matrix was approximately the same as Zn2 TiO4 , while that of the precipitate was different, it is very reasonable to assume that the Ti content is higher than Zn in the precipitate. It can be concluded that the precipitate was rutile, Zn2 Ti3 O8 or ZnTiO3 . In view of an ilmenite-type phase (hexagonal symmetry) of the structure of ZnTiO3 that has been reported by Dulin and Rase [4], it can be excluded from the candidacy of precipitate. Excluding rutile because of the same reason as ZnTiO3 , it is believed that precipitate is likely to consist of Zn2 Ti3 O8 . Li et al. [5] found a precipitate in Zn2 TiO4 matrix in the ceramics prepared by ZnO and TiO2 in a molar ratio of 3:2. By powder X-ray diffraction and energy-dispersive X-ray spectroscopy, they concluded that the precipitate is ZnTiO3 . The similar precipitate was found by Kim et al. [6] in (ZnNi)TiO3 ceramics, but they believed that the precipitate was Zn2 Ti3 O8 , and formed on the cooling stage. When the ceramics prepared by ZnO and TiO2 in a molar ratio of 1:1 were cooled to the temperature less than 820 ◦ C, Zn2 Ti3 O8 formed because it is a
Table 2 EDS quantum analysis of zinc titanate ceramics with 1.00 mass% WO3 sintered at 900 ◦ C Spectrum
O (mass%)
Ti (mass%)
Zn (mass%)
W (mass%)
Total
Spectrum 1 Spectrum 2 Spectrum 3
35.79 29.62 20.17
45.26 37.78 38.88
14.73 32.60 32.31
4.22 – 8.64
100.00 100.00 100.00
Maximum Minimum
35.79 20.17
45.26 37.78
32.60 14.73
8.64 0.00
– –
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Fig. 7. Dielectric constant of WO3 -doped zinc titanate ceramics measured at 100 and 400 kHz and at 1, 4, and 10 MHz.
Fig. 6. Bulk density and shrinkage of zinc titanate ceramics with different amounts of WO3 added vs. the sintering temperatures.
low-temperature form, and existed steadily at temperatures less than 820 ◦ C. Fig. 6 shows the bulk density and shrinkage of zinc titanate ceramics with different amount of WO3 added versus the sintering temperatures. The density and shrinkage increased with the sintering temperature increasing, and reached saturated value at 900 ◦ C. The density of ceramics with 3.0 mass% WO3 additions is higher than those of ceramics with 1.0 and 0.5 mass% WO3 additions, but the relative density of the former is lower than the latter’s, which can be attributed to the higher density of the ZnWO3 phase (7.81 g/cm3 ) than to the Zn2 TiO4 (5.33 g/cm3 ) and ZnTiO3 phase (5.17 g/cm3 ): 0.5 mass% WO3 -doped samples have the highest relative density at 900 ◦ C. The dielectric constant (εr ) and the loss tangent (tan δ) properties of WO3 -doped zinc titanate ceramics sintered at 900 ◦ C were measured at 10 and 100 kHz and at 1, 10, and 100 MHz at ambient temperature; the results are shown in Figs. 7 and 8, respectively. The dielectric constants of samples increased with the amount of WO3 addition at the same frequency, and had a maximum value at 0.50 mass% as shown in Fig. 6, then gradually decreased with the increasing amount of tungsten. This could be ascribed to the higher relative density of 0.5 mass%doped samples compared to the other ones. Fig. 7 also indicates a decreasing tendency of all the dielectric constants with increasing measuring frequency. Even at a microwave frequency, the εr value of samples is presumed to be about 21–24, higher than that of single-phase hexagonal ZnTiO3 ceramics (19) [7]. Similarly, the evolution of dielectric loss tangent value of WO3 -doped zinc titanate ceramics became a maximum at 0.50 mass%, and then
Fig. 8. Dielectric loss of WO3 -doped zinc titanate ceramics measured at 100 and 400 kHz and at 1, 4, and 10 MHz.
decreased gradually with the increasing amount of tungsten (as shown in Fig. 8). It might be ascribed to the formation of more stable hexagonal ilmenite phase with the amount of WO3 addition increasing from 0.5 to 3.0 mass%, for the cubic spinel phase would strongly deteriorate the tan δ and induce a high value. Compared with 0.5 mass%-doped samples, 1.0 mass%-doped samples had more stable hexagonal ilmenite phase and smaller cubic spinel phase, so 1.0 mass%-doped samples exhibited lower dielectric loss. Much smaller tan δ value of WO3 -doped zinc titanate ceramics at a microwave frequency can be presumed from the decreasing tendency of tan δ with the increasing value of measuring frequency [8]. 4. Conclusions The phase structure and transition of zinc titanate ceramics were sensitive to WO3 additions. Small WO3 (<1.00 mass%) addition accelerated the decomposition of hexagonal ZnTiO3 to cubic Zn2 TiO4 , while excessive WO3 addition restrained
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the decomposition and leaded to a ZnWO4 phase formation. The major phase of zinc titanate ceramics transformed from cubic zinc orthotitanate phase to hexagonal zinc metatitanate phase with the amounts of WO3 additions increased from 1.00 to 3.00 mass%. Moreover, WO3 addition affected the formation of ZnTiO3 phase; WO3 enhanced the stability of Zn2 Ti3 O8 and weakened the stability of ZnTiO3 . A new phase, ZnWO4 , formed in the 1.00 mass% WO3 -added zinc titanate ceramics sintered at 930 ◦ C and a precipitate within the Zn2 TiO4 matrix was observed. The size of the precipitate is nanometer level, and its composition was Zn2 Ti3 O8 . The dielectric properties of WO3 -doped zinc titanate ceramics were measured at different frequencies. The results showed the decreasing tendency with the increasing measuring frequencies for both the dielectric constants and the loss tangents, and there existed maximum values when the amount of tungsten was 0.50 mass%. The best dielectric properties for WO3 -doped ceramics was obtained at 1.00 mass%.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Project 60501015) and the Doctorate Foundation of Northwestern Polytechnical University under Grant CX200408. References [1] O. Yamaguchi, M. Morimi, H. Kawabata, et al., J. Am. Ceram. Soc. 70 (1987) C97. [2] J. Yang, J.H. Swisher, Mater. Charact. 37 (1996) 153. [3] H.T. Kim, S.H. Kim, S. Nahm, et al., J. Am. Ceram. Soc. 82 (1999) 3043. [4] F.H. Dulin, D.E. Rase, J. Am. Ceram. Soc. 43 (1960) 125. [5] C.F. Li, Y.S. Bando, M. Nakamuraa, et al., Mater. Res. Bull. 35 (2000) 351. [6] H.T. Kim, J.C. Hwang, J.H. Nam, et al., J. Mater. Res. 18 (2003) 1067. [7] A. Golovchansk, H.T. Kim, Y.H. Kim, J. Korean Phys. Soc. 32 (1998) S1167. [8] J. Luo, X.X. Xing, R.B. Yu, et al., J. Alloys Compd. 402 (2005) 263.