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Low-temperature sintered Zn2TiO4:TiO2 with near-zero temperature coefficient of resonant frequency at microwave frequency
Journal of Alloys and Compounds 485 (2009) 408–412
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Low-temperature sintered Zn2 TiO4 :TiO2 with near-zero temperature coefficient of resonant frequency at microwave frequency Chuan-Feng Shih a,b,∗ , Wei-Min Li a , Ming-Min Lin a , Chu-Yun Hsiao a , Kuang-Teng Hung a a b
Department of Electrical Engineering, National Cheng Kung University, Tainan, 70101, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 22 February 2009 Received in revised form 19 May 2009 Accepted 24 May 2009 Available online 17 June 2009 Keywords: Zinc titanate Microwave dielectrics LTCC Nanowires
a b s t r a c t This work presents the microwave dielectric properties of TiO2 incorporated Zn2 TiO4 sintered at lowtemperatures. The Zn2 TiO4 was synthesized using ZnO and TiO2 nanowires as starting materials. Within the interim studied (TiO2 = 0–12%), the bulk density, the dielectric constant, and the quality factor markedly increased with sintering temperature. When the TiO2 content (x) was 8% (970 ◦ C), the value of quality factor multiples its resonant frequency of the Zn2 TiO4 :8% TiO2 achieved a maximum of ∼35,000 GHz. From XRD patterns, the phase stability of TiO2 added Zn2 TiO4 changed when the TiO2 content exceeded 10 wt%. Further addition of TiO2 up to 12% approached zero, with high quality factor and k values of 30,000 GHz and 22, respectively. The high quality factor was attributed to the good cyrstallinity of Zn2 TiO4 . The fabricated Zn2 TiO4 :12% TiO2 ceramic is suitable for microwave dielectric applications. © 2009 Elsevier B.V. All rights reserved.
1. Introduction ZnO–TiO2 alloy system has been shown to have great potential for use in low-temperature co-fired ceramics (LTCCs), microwave dielectrics, phosphors, and catalysts [1–5]. Three compounds are known to exist in the ZnO–TiO2 system. Cubic Zn2 Ti3 O8 has been regarded as a low-temperature phase of hexagonal zinc metatitanate (h-ZnTiO3 ), stabilizing in ∼600–800 ◦ C, transforming to h-ZnTiO3 at ∼820 ◦ C [6]. The h-ZnTiO3 decomposes into rutile and zinc orthotitanate (Zn2 TiO4 ) when the temperature exceeds 945 ◦ C. Particularly, the ilmentite ZnTiO3 draw the most attention among this alloy system due to it potential applications to the LTCCs and microwave dielectrics. As a good microwave dielectric, however, single-phase ZnTiO3 ceramic is rarely obtained solely from the conventional solid-state reaction method because it decomposes at high temperature and poor sinterability at low-temperature (<945 ◦ C) [7]. Generally, the microwave dielectric properties of hZnTiO3 sintered below 945 ◦ C were: dielectric constant (εr ) = 22, temperature coefficient of resonant frequency ( f ) = −60 ppm/◦ C, and quality factor (Q × f) = 40,000 GHz. Increasing the sintering temperature to exceed 945 ◦ C always degraded the Q × f (<20,000 GHz) [1,2].
∗ Corresponding author at: Department of Electrical Engineering, National Cheng Kung University, Tainan, 70101, Taiwan. Tel.: +886 6 2757575x62398; fax: +886 6 2080687. E-mail address: [email protected] (C.-F. Shih). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.153
In the same alloy system, Zn2 TiO4 (εr = 21, f = −60 ppm/◦ C, Q × f = 20,000 GHz) [8] is also a good candidate for microwave dielectric applications. Compared with ZnTiO3 , Zn2 TiO4 has several advantages. For instance, it can be easily formed via solid-state sintering of the 2ZnO:1TiO2 at elevated temperature. However, the Zn2 TiO4 shows similar dielectric constant but much poor quality factor at microwave frequencies than the ZnTiO3 . It is known that the quality factor is related not only to the crystal structure of the dielectrics, but also the material imperfections. Accordingly, the sintering temperature of Zn2 TiO4 -based microwave dielectrics should be high enough to overcome the low quality factor problem. Recently, Kim reported the microwave dielectric properties of the titanium incorporated Zn2 TiO4 [2]. Accordingly, the TiO2 forms solid solution within the Zn2 TiO4 matrix that improves the dielectric properties of Zn2 TiO4 . However, the required temperature (∼1100 ◦ C) is still high to obtain satisfying dielectric properties. More recently, we reported a method to synthesize the high quality Zn2 TiO4 with promising microwave properties at low-temperature (<1000 ◦ C) [9,10]. Taking advantages of the high specific surface area of the TiO2 and ZnO nanowires, the Zn2 TiO4 was sintered via a calcine and additives-free process. Further, we found that the Zn2 TiO4 showed negative f value in a wide temperature range (900–1000 ◦ C). However, from the view point of practical application, the near-zero f is desired to prevent the disturbance from temperature variation. In this paper, an attempt was made to achieve the near-zero f Zn2 TiO4 by incorporating the TiO2 . Due to the high crystallinity of the Zn2 TiO4 prepared by nano-scaled starting materials, the
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2. Experimental ZnO and TiO2 nanopowders were prepared separately by hydrothermal processes as reported previously [9,10]. TiO2 :ZnO (1:1 molar ratio) nanowires were mixed and ball-milled for 24 h with zirconia beads and distilled water. The milled mixture was dried at 80 ◦ C, ground, and sieved through a 100 mesh screen. The powders were calcined at 850 ◦ C for 2 h to form the spinel Zn2 TiO4 . After the calcination, powders were ground and sieved. 2 wt% polyvinyl alcohol (PVA) solution was added as a binder and the additional anatase TiO2 nanowires (2–12 wt%) were added at this stage. A disk with a diameter of 11 mm and a thickness of 5 mm was formed using uniaxial pressing. The compacts were sintered for 4 h at elevated temperatures (900, 930, 970, and 1000 ◦ C). The structures of the ceramic compacts were examined by X-ray diffractometry (XRD; Siemens D500). Field-emission scanning electron microscope (FESEM; Philips XL-40FEG) was used to examine the morphology of the samples. High-resolution transmission electron microscopy (HR-TEM, JEOL 2100) was used to determine the presented phase. The apparent densities (d) of the sintered compacts were determined by the Archimedes method. The relative dielectric constant (εr ) and quality factor at microwave frequencies were measured using the Hakki–Coleman dielectric resonator method [11]. f at microwave frequencies was measured in the temperature range from 25 to 80 ◦ C, and was defined as f = Fig. 1. HR-TEM image of Zn2 TiO4 sintered at 970 ◦ C for 4 h. Inset shows selection-area diffraction pattern of Zn2 TiO4 .
f0 (ppm/◦ C), f0 T
(1)
where f0 is the shift in the central frequency caused by a temperature change (T) in the range 20–80 ◦ C.
3. Results and discussion Zn2 TiO4 :xTiO2 (x = 0.02–0.12) showed good dielectric properties even at low sintering temperatures. When x = 0.08 and 0.12, the Q × f value reached a maximum of ∼35,000 GHz and the value of f approached zero, respectively.
Fig. 1 shows the HR-TEM image of the pure Zn2 TiO4 sintered at 970 ◦ C. The lattice image indicates a good lattice arrangement. Only few dislocations and defects were observed, revealing good interdiffusion and sinterability between the ZnO and TiO2 nanowires.
Fig. 2. SEM images of Zn2 TiO4 :xTiO2 sintered at 970 ◦ C, where x = (a) 0.02, (b) 0.04, (c) 0.06 (d) 0.08, (e) 0.1, and (f) 0.12.
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Fig. 3. SEM images of Zn2 TiO4 :12%TiO2 sintered at (a) 850, (b) 900, (c) 930, (d) 970, and (d) 1000 ◦ C.
The good crystal quality implies a possible high quality factor of the sintered Zn2 TiO4 . From the diffraction pattern shown in inset of Fig. 1, the as-sintered sample was identified to be spinel. No rutile was observed. SEM images of the Zn2 TiO4 with different TiO2 content (2–12%) and fixed sintering temperature (970 ◦ C) were shown in Fig. 2(a)–(f). Independent of the TiO2 , the grains were densely arranged and uniformly distributed. Noted that the sample with 12% TiO2 (Fig. 2(f)) showed a zero f that will be discussed later. SEM images of samples (Zn2 TiO4 :12 wt% TiO2 ) with different sintering temperature (850–1000 ◦ C) were presented in Fig. 3(a)–(e). It was found that the grain size increased while the pores between the grain boundaries decreased as the sintering temperature increased. XRD patterns give further information about the structural change due to altering the TiO2 content and sintering temperature. Fig. 4(a) demonstrates the XRD patterns of the Zn2 TiO4 :xTiO2 sintered at 970 ◦ C with x varies from 2 to 12 wt%. The spinel structure sustained when x ≤ 0.1, while an additional diffraction peak related to the ZnTiO3 emerged when x = 0.12, indicating the TiO2 changed the phase stability of Zn2 TiO4 when x > 0.1 at 970 ◦ C. Fig. 4(b) shows the XRD profiles of the Zn2 TiO4 :12 wt% TiO2 sintered at different temperatures (850–1000 ◦ C). Peak intensity related to the ZnTiO3 phase decreased with increased sintering temperature that confirms the known ZnO–TiO2 phase diagram. We note that there is no rutile phase found throughout the samples, indicating the TiO2 content is still lower than its solubility limit in Zn2 TiO4 . Fig. 5(a) shows the apparent density of Zn2 TiO4 :xTiO2 with various x (2–12 wt%) and sintering temperatures (850–1000 ◦ C). The bulk density markedly increased with sintering temperature. It could be explained by two reasons: First, the higher sintering
temperature provided more energy for grain growth. SEM images (Fig. 3) showed that the grain grew up since 850 ◦ C and sustained when achieved 970 ◦ C, confirming this suggestion. No grain overgrowth was observed when the sintering temperature increased to 1000 ◦ C. Thus, SEM observations explained why the density of the 1000 ◦ C sintered samples look similar to the 970 ◦ C sintered ones. Second, ZnTiO3 phase (density ∼5.171 g/cm3 , JCPDS # 01-073-0578) transferred to the denser Zn2 TiO4 phase (density ∼5.331 g/cm3 , JCPDS # 00-014-0033) when sintering temperature increased. Fig. 6(a)–(c) show the dielectric properties measured at microwave frequency. Fig. 6(a) shows the dielectric constant as a function of the TiO2 content. It was observed that the dielectric constant increased with sintering temperature. This variation of dielectric constant was associated with the bulk density. The dielectric constants also increased gradually when the TiO2 content increased due to the high dielectric constant nature of the TiO2 . An obvious jump of dielectric constant from 900 to 930 ◦ C was found. It was attributed to the markedly increases of the bulk density as shown in Fig. 5. Besides, drop of the dielectric constants of the 0.06 TiO2 samples sintered at 900 and 850 ◦ C might be associated with the low bulk density. From SEM observation (not shown), increased the TiO2 from 4% to 6% also introduced many pores. Because the relative dielectric constant of air is 1, the dielectric constant thus dropped. Fig. 6(b) shows the relation between Q × f and TiO2 content. It was found that the Q × f reached a maximum (∼30,000 GHz) when the TiO2 was 8%. When the TiO2 exceeded 8%, the Q × f value decreased because of the loosy TiO2 (Q × f ∼ 5000 GHz) formation. Additionally, the Q × f values increased with the sintering temperature. However, further increase of the sintering temperature
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Fig. 4. (a) XRD profiles for Zn2 TiO4 :xTiO2 sintered at 970 ◦ C, where x = 0.2–0.12. (b) XRD profiles for Zn2 TiO4 :12%TiO2 sintered at various temperatures, from 850 to 1000 ◦ C.
(≥1000 ◦ C) decreased the Q × f. The observed high Q × f value is abnormal for Zn2 TiO4 sintered at such a low-temperature. Because the Q × f is known to be limited mainly by the crystal structure and imperfections, we ascribed the obtained high Q × f value to the usage of nanometer size starting materials. Fig. 6(c) shows the relation between f and TiO2 content for samples sintered at 970 ◦ C. As expected, the f increased with TiO2 content, approaching zero when TiO2 was 12 wt%. Kim reported [2] the dielectric properties
Fig. 6. (a) Dielectric constant, (b) Q × f, and (c) f of Zn2 TiO4 :xTiO2 , where x = 0.2–0.12.
Fig. 5. Apparent bulk density of the Zn2 TiO4 :xTiO2 with different x (0.2–0.12) and sintering temperatures.
of the 1100 ◦ C sintered Zn2 TiO4 :50%TiO2 : Q × f ∼ 20,000, k ∼ 25, and f ∼ 0. In this case, the dielectric properties were similar but the sintering temperature was much low. When the TiO2 content was 8%, we obtained a maximum Q × f and k of ∼35,000 GHz and 24, respectively. When TiO2 content was 12%, the f approached zero and the Q × f and εr were 30,000 GHz and 22, respectively. The dielectric properties shown were superior to those ever been reported in literature and were compatible to the pure ZnTiO3 phase, suggesting potential applications to the microwave dielectrics. The improved dielectric properties were attributed to the good cyrstallinity of Zn2 TiO4 prepared from the ZnO and TiO2 nanowires.
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4. Conclusions
References
This work presented the microwave properties of the TiO2 incorporated Zn2 TiO4 . The Zn2 TiO4 phase was synthesized using ZnO and TiO2 nanowires as starting materials. From the XRD pattern, phase stability of the Zn2 TiO4 changed when x exceeded 0.1. XRD intensity related to the ZnTiO3 peak decreased as sintering temperature increased. The bulk density, the dielectric constant, and the quality factor markedly increased with sintering temperature. When TiO2 content was 12%, the f approached zero and the Q × f and εr were as high as 30,000 GHz and 22, respectively. These were superior to those ever been reported in literature and compatible to the pure ZnTiO3 phase.
[1] H.T. Kim, S. Nahm, J.D. Byun, J. Am. Ceram. Soc. 82 (1999) 3476. [2] H.T. Kim, Y. Kim, M. Valant, D. Suvorov, J. Am. Ceram. Soc. 84 (2001) 1081. [3] H.T. Kim, K.H. Kim, S. Nahm, J.D. Byun, Y. Kim, J. Am. Ceram. Soc. 82 (1999) 3043. [4] A. Chaouchi, M. Aliouat, S. Marinel, S. d’Asorg, H. Bourahla, Ceram. Int. 33 (2007) 245. [5] Y.R. Wang, S.F. Wang, Y.M. Lin, Ceram. Int. 31 (2005) 905. [6] O. Yamaguchi, M. Morimi, H. Kawabata, K. Shimizu, J. Am. Ceram. Soc. 70 (1987) C97. [7] F.H. Dulin, D.E. Rase, J. Am. Ceram. Soc. 43 (1960) 125. [8] A. Templeton, X. Wang, S.J. Penn, S.J. Webb, L.F. Cohen, N. McN Alford, J. Am. Ceram. Soc. 83 (2000) 95. [9] C.F. Shih, W.M. Li, M.M. Lin, K.T. Hung, J. Electrochem. Soc. 156 (2009) E03. [10] C.F. Shih, W.M. Li, M.M. Lin, K.T. Hung, C.Y. Hsiao, C.L. Lee, Electrochem. Solid State Lett. 11 (2008) K105. [11] B.W. Hakki, P.D. Coleman, IEEE Trans. Microwave Theory Technol. 8 (1960) 402.
Acknowledgements This work was supported by the National Science Council under contract no. NSC-96-2221-E-006-288-MY2 and the Center for Micro/Nano Science and Technology.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Low-temperature sintered Zn2 TiO4 :TiO2 with near-zero temperature coefficient of resonant frequency at microwave frequency Chuan-Feng Shih a,b,∗ , Wei-Min Li a , Ming-Min Lin a , Chu-Yun Hsiao a , Kuang-Teng Hung a a b
Department of Electrical Engineering, National Cheng Kung University, Tainan, 70101, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 22 February 2009 Received in revised form 19 May 2009 Accepted 24 May 2009 Available online 17 June 2009 Keywords: Zinc titanate Microwave dielectrics LTCC Nanowires
a b s t r a c t This work presents the microwave dielectric properties of TiO2 incorporated Zn2 TiO4 sintered at lowtemperatures. The Zn2 TiO4 was synthesized using ZnO and TiO2 nanowires as starting materials. Within the interim studied (TiO2 = 0–12%), the bulk density, the dielectric constant, and the quality factor markedly increased with sintering temperature. When the TiO2 content (x) was 8% (970 ◦ C), the value of quality factor multiples its resonant frequency of the Zn2 TiO4 :8% TiO2 achieved a maximum of ∼35,000 GHz. From XRD patterns, the phase stability of TiO2 added Zn2 TiO4 changed when the TiO2 content exceeded 10 wt%. Further addition of TiO2 up to 12% approached zero, with high quality factor and k values of 30,000 GHz and 22, respectively. The high quality factor was attributed to the good cyrstallinity of Zn2 TiO4 . The fabricated Zn2 TiO4 :12% TiO2 ceramic is suitable for microwave dielectric applications. © 2009 Elsevier B.V. All rights reserved.
1. Introduction ZnO–TiO2 alloy system has been shown to have great potential for use in low-temperature co-fired ceramics (LTCCs), microwave dielectrics, phosphors, and catalysts [1–5]. Three compounds are known to exist in the ZnO–TiO2 system. Cubic Zn2 Ti3 O8 has been regarded as a low-temperature phase of hexagonal zinc metatitanate (h-ZnTiO3 ), stabilizing in ∼600–800 ◦ C, transforming to h-ZnTiO3 at ∼820 ◦ C [6]. The h-ZnTiO3 decomposes into rutile and zinc orthotitanate (Zn2 TiO4 ) when the temperature exceeds 945 ◦ C. Particularly, the ilmentite ZnTiO3 draw the most attention among this alloy system due to it potential applications to the LTCCs and microwave dielectrics. As a good microwave dielectric, however, single-phase ZnTiO3 ceramic is rarely obtained solely from the conventional solid-state reaction method because it decomposes at high temperature and poor sinterability at low-temperature (<945 ◦ C) [7]. Generally, the microwave dielectric properties of hZnTiO3 sintered below 945 ◦ C were: dielectric constant (εr ) = 22, temperature coefficient of resonant frequency ( f ) = −60 ppm/◦ C, and quality factor (Q × f) = 40,000 GHz. Increasing the sintering temperature to exceed 945 ◦ C always degraded the Q × f (<20,000 GHz) [1,2].
∗ Corresponding author at: Department of Electrical Engineering, National Cheng Kung University, Tainan, 70101, Taiwan. Tel.: +886 6 2757575x62398; fax: +886 6 2080687. E-mail address: [email protected] (C.-F. Shih). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.153
In the same alloy system, Zn2 TiO4 (εr = 21, f = −60 ppm/◦ C, Q × f = 20,000 GHz) [8] is also a good candidate for microwave dielectric applications. Compared with ZnTiO3 , Zn2 TiO4 has several advantages. For instance, it can be easily formed via solid-state sintering of the 2ZnO:1TiO2 at elevated temperature. However, the Zn2 TiO4 shows similar dielectric constant but much poor quality factor at microwave frequencies than the ZnTiO3 . It is known that the quality factor is related not only to the crystal structure of the dielectrics, but also the material imperfections. Accordingly, the sintering temperature of Zn2 TiO4 -based microwave dielectrics should be high enough to overcome the low quality factor problem. Recently, Kim reported the microwave dielectric properties of the titanium incorporated Zn2 TiO4 [2]. Accordingly, the TiO2 forms solid solution within the Zn2 TiO4 matrix that improves the dielectric properties of Zn2 TiO4 . However, the required temperature (∼1100 ◦ C) is still high to obtain satisfying dielectric properties. More recently, we reported a method to synthesize the high quality Zn2 TiO4 with promising microwave properties at low-temperature (<1000 ◦ C) [9,10]. Taking advantages of the high specific surface area of the TiO2 and ZnO nanowires, the Zn2 TiO4 was sintered via a calcine and additives-free process. Further, we found that the Zn2 TiO4 showed negative f value in a wide temperature range (900–1000 ◦ C). However, from the view point of practical application, the near-zero f is desired to prevent the disturbance from temperature variation. In this paper, an attempt was made to achieve the near-zero f Zn2 TiO4 by incorporating the TiO2 . Due to the high crystallinity of the Zn2 TiO4 prepared by nano-scaled starting materials, the
C.-F. Shih et al. / Journal of Alloys and Compounds 485 (2009) 408–412
409
2. Experimental ZnO and TiO2 nanopowders were prepared separately by hydrothermal processes as reported previously [9,10]. TiO2 :ZnO (1:1 molar ratio) nanowires were mixed and ball-milled for 24 h with zirconia beads and distilled water. The milled mixture was dried at 80 ◦ C, ground, and sieved through a 100 mesh screen. The powders were calcined at 850 ◦ C for 2 h to form the spinel Zn2 TiO4 . After the calcination, powders were ground and sieved. 2 wt% polyvinyl alcohol (PVA) solution was added as a binder and the additional anatase TiO2 nanowires (2–12 wt%) were added at this stage. A disk with a diameter of 11 mm and a thickness of 5 mm was formed using uniaxial pressing. The compacts were sintered for 4 h at elevated temperatures (900, 930, 970, and 1000 ◦ C). The structures of the ceramic compacts were examined by X-ray diffractometry (XRD; Siemens D500). Field-emission scanning electron microscope (FESEM; Philips XL-40FEG) was used to examine the morphology of the samples. High-resolution transmission electron microscopy (HR-TEM, JEOL 2100) was used to determine the presented phase. The apparent densities (d) of the sintered compacts were determined by the Archimedes method. The relative dielectric constant (εr ) and quality factor at microwave frequencies were measured using the Hakki–Coleman dielectric resonator method [11]. f at microwave frequencies was measured in the temperature range from 25 to 80 ◦ C, and was defined as f = Fig. 1. HR-TEM image of Zn2 TiO4 sintered at 970 ◦ C for 4 h. Inset shows selection-area diffraction pattern of Zn2 TiO4 .
f0 (ppm/◦ C), f0 T
(1)
where f0 is the shift in the central frequency caused by a temperature change (T) in the range 20–80 ◦ C.
3. Results and discussion Zn2 TiO4 :xTiO2 (x = 0.02–0.12) showed good dielectric properties even at low sintering temperatures. When x = 0.08 and 0.12, the Q × f value reached a maximum of ∼35,000 GHz and the value of f approached zero, respectively.
Fig. 1 shows the HR-TEM image of the pure Zn2 TiO4 sintered at 970 ◦ C. The lattice image indicates a good lattice arrangement. Only few dislocations and defects were observed, revealing good interdiffusion and sinterability between the ZnO and TiO2 nanowires.
Fig. 2. SEM images of Zn2 TiO4 :xTiO2 sintered at 970 ◦ C, where x = (a) 0.02, (b) 0.04, (c) 0.06 (d) 0.08, (e) 0.1, and (f) 0.12.
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Fig. 3. SEM images of Zn2 TiO4 :12%TiO2 sintered at (a) 850, (b) 900, (c) 930, (d) 970, and (d) 1000 ◦ C.
The good crystal quality implies a possible high quality factor of the sintered Zn2 TiO4 . From the diffraction pattern shown in inset of Fig. 1, the as-sintered sample was identified to be spinel. No rutile was observed. SEM images of the Zn2 TiO4 with different TiO2 content (2–12%) and fixed sintering temperature (970 ◦ C) were shown in Fig. 2(a)–(f). Independent of the TiO2 , the grains were densely arranged and uniformly distributed. Noted that the sample with 12% TiO2 (Fig. 2(f)) showed a zero f that will be discussed later. SEM images of samples (Zn2 TiO4 :12 wt% TiO2 ) with different sintering temperature (850–1000 ◦ C) were presented in Fig. 3(a)–(e). It was found that the grain size increased while the pores between the grain boundaries decreased as the sintering temperature increased. XRD patterns give further information about the structural change due to altering the TiO2 content and sintering temperature. Fig. 4(a) demonstrates the XRD patterns of the Zn2 TiO4 :xTiO2 sintered at 970 ◦ C with x varies from 2 to 12 wt%. The spinel structure sustained when x ≤ 0.1, while an additional diffraction peak related to the ZnTiO3 emerged when x = 0.12, indicating the TiO2 changed the phase stability of Zn2 TiO4 when x > 0.1 at 970 ◦ C. Fig. 4(b) shows the XRD profiles of the Zn2 TiO4 :12 wt% TiO2 sintered at different temperatures (850–1000 ◦ C). Peak intensity related to the ZnTiO3 phase decreased with increased sintering temperature that confirms the known ZnO–TiO2 phase diagram. We note that there is no rutile phase found throughout the samples, indicating the TiO2 content is still lower than its solubility limit in Zn2 TiO4 . Fig. 5(a) shows the apparent density of Zn2 TiO4 :xTiO2 with various x (2–12 wt%) and sintering temperatures (850–1000 ◦ C). The bulk density markedly increased with sintering temperature. It could be explained by two reasons: First, the higher sintering
temperature provided more energy for grain growth. SEM images (Fig. 3) showed that the grain grew up since 850 ◦ C and sustained when achieved 970 ◦ C, confirming this suggestion. No grain overgrowth was observed when the sintering temperature increased to 1000 ◦ C. Thus, SEM observations explained why the density of the 1000 ◦ C sintered samples look similar to the 970 ◦ C sintered ones. Second, ZnTiO3 phase (density ∼5.171 g/cm3 , JCPDS # 01-073-0578) transferred to the denser Zn2 TiO4 phase (density ∼5.331 g/cm3 , JCPDS # 00-014-0033) when sintering temperature increased. Fig. 6(a)–(c) show the dielectric properties measured at microwave frequency. Fig. 6(a) shows the dielectric constant as a function of the TiO2 content. It was observed that the dielectric constant increased with sintering temperature. This variation of dielectric constant was associated with the bulk density. The dielectric constants also increased gradually when the TiO2 content increased due to the high dielectric constant nature of the TiO2 . An obvious jump of dielectric constant from 900 to 930 ◦ C was found. It was attributed to the markedly increases of the bulk density as shown in Fig. 5. Besides, drop of the dielectric constants of the 0.06 TiO2 samples sintered at 900 and 850 ◦ C might be associated with the low bulk density. From SEM observation (not shown), increased the TiO2 from 4% to 6% also introduced many pores. Because the relative dielectric constant of air is 1, the dielectric constant thus dropped. Fig. 6(b) shows the relation between Q × f and TiO2 content. It was found that the Q × f reached a maximum (∼30,000 GHz) when the TiO2 was 8%. When the TiO2 exceeded 8%, the Q × f value decreased because of the loosy TiO2 (Q × f ∼ 5000 GHz) formation. Additionally, the Q × f values increased with the sintering temperature. However, further increase of the sintering temperature
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Fig. 4. (a) XRD profiles for Zn2 TiO4 :xTiO2 sintered at 970 ◦ C, where x = 0.2–0.12. (b) XRD profiles for Zn2 TiO4 :12%TiO2 sintered at various temperatures, from 850 to 1000 ◦ C.
(≥1000 ◦ C) decreased the Q × f. The observed high Q × f value is abnormal for Zn2 TiO4 sintered at such a low-temperature. Because the Q × f is known to be limited mainly by the crystal structure and imperfections, we ascribed the obtained high Q × f value to the usage of nanometer size starting materials. Fig. 6(c) shows the relation between f and TiO2 content for samples sintered at 970 ◦ C. As expected, the f increased with TiO2 content, approaching zero when TiO2 was 12 wt%. Kim reported [2] the dielectric properties
Fig. 6. (a) Dielectric constant, (b) Q × f, and (c) f of Zn2 TiO4 :xTiO2 , where x = 0.2–0.12.
Fig. 5. Apparent bulk density of the Zn2 TiO4 :xTiO2 with different x (0.2–0.12) and sintering temperatures.
of the 1100 ◦ C sintered Zn2 TiO4 :50%TiO2 : Q × f ∼ 20,000, k ∼ 25, and f ∼ 0. In this case, the dielectric properties were similar but the sintering temperature was much low. When the TiO2 content was 8%, we obtained a maximum Q × f and k of ∼35,000 GHz and 24, respectively. When TiO2 content was 12%, the f approached zero and the Q × f and εr were 30,000 GHz and 22, respectively. The dielectric properties shown were superior to those ever been reported in literature and were compatible to the pure ZnTiO3 phase, suggesting potential applications to the microwave dielectrics. The improved dielectric properties were attributed to the good cyrstallinity of Zn2 TiO4 prepared from the ZnO and TiO2 nanowires.
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4. Conclusions
References
This work presented the microwave properties of the TiO2 incorporated Zn2 TiO4 . The Zn2 TiO4 phase was synthesized using ZnO and TiO2 nanowires as starting materials. From the XRD pattern, phase stability of the Zn2 TiO4 changed when x exceeded 0.1. XRD intensity related to the ZnTiO3 peak decreased as sintering temperature increased. The bulk density, the dielectric constant, and the quality factor markedly increased with sintering temperature. When TiO2 content was 12%, the f approached zero and the Q × f and εr were as high as 30,000 GHz and 22, respectively. These were superior to those ever been reported in literature and compatible to the pure ZnTiO3 phase.
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Acknowledgements This work was supported by the National Science Council under contract no. NSC-96-2221-E-006-288-MY2 and the Center for Micro/Nano Science and Technology.