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New dielectric material system of (Mg0.95Zn0.05)TiO3–Ca0.61Nd0.26TiO3 at microwave frequency
Journal of Alloys and Compounds 453 (2008) 337–340
New dielectric material system of (Mg0.95Zn0.05)TiO3–Ca0.61Nd0.26TiO3 at microwave frequency Cheng-Liang Huang a,∗ , Chia-Feng Tasi a , Yuan-Bin Chen a , Yao-Chung Cheng b a
Department of Electrical Engineering, National Cheng Kung University, No. 1 University Road, Tainan 70101, Taiwan b Universal Nano Tech Co., Ltd., Tainan, Taiwan Received 7 July 2006; received in revised form 15 November 2006; accepted 16 November 2006 Available online 18 December 2006
Abstract The microstructures and the microwave dielectric properties of the x(Mg0.95 Zn0.05 )TiO3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramic system prepared by conventional solid state method were investigated. In order to achieve a temperature-stable material, two compounds with negative and positive temperature coefficients were employed to form a mixed phases. With partial replacement of Mg by Zn, dielectric properties of the x(Mg0.95 Zn0.05 )TiO3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature and the composition. An excellent Q × f value of 130,000 GHz can be obtained for the system with x = 0.9 at 1300 ◦ C. For practical application, a dielectric constant (εr ) of 24.3, a Q × f value of 112,000 GHz and a temperature coefficient of resonant frequency (τ f ) of −10.1 ppm/◦ C for 0.85(Mg0.95 Zn0.05 Ti)O3 –0.15Ca0.61 Nd0.26 TiO3 at 1300 ◦ C is proposed in this paper. © 2006 Elsevier B.V. All rights reserved. Keywords: Ceramics; Oxides; X-ray diffraction; Solid state reactions; Dielectric properties
1. Introduction Due to rapid developments in mobile communication, mobile telephone systems, as well as in satellite broadcasting systems, the design of high-quality devices is very important. To miniaturize the devices and for the systems to work with high efficiency and stability, the materials for microwave resonators must be excellent in the following three dielectric characteristics. The first characteristic is a high dielectric constant; microwave devices such as resonators, filters, oscillators and antennas, are made of dielectric materials with high dielectric constant εr . The use of high dielectric constant materials can effectively reduce the size of resonators since the wavelength (λ) in dielectrics is √ inversely proportional to εr of the wavelength (λ0 ) in vac√ uum (λ = λ0 / εr ). The second is a high Q value; many studies have focused on developing dielectric materials with a highquality factor (Q × f). This is required to achieve high frequency selectivity and stability in microwave transmitters and receiver components. The third is a zero temperature coefficient of the
∗
Corresponding author. Tel.: +886 6 2757575x62390; fax: +886 6 2345482. E-mail address: [email protected] (C.-L. Huang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.084
resonant frequency (τ f ) for dielectric resonators and microwave device substrates. Small temperature coefficients of the resonant frequency ensure the stability of the microwave components at different working temperatures. To satisfy the demands of microwave circuit designs, each dielectric property requires precise control. Using two or more compounds with negative and positive temperature coefficients to form a solid solution or mixed phases is the most promising method of obtaining a zero temperature coefficient of the resonant frequency. Because most dielectric ceramics with high dielectric constants have positive τ f values, searching for materials with a high dielectric constant, a high Q and a negative τ f is necessary to achieve this goal. MgTiO3 -based ceramics are finding wide applications as dielectrics in resonators, filters and antennas for Wireless LAN and GPS in microwave regimes. The development of microwave dielectric components for communication systems such as cellular phones and global positioning systems has been growing rapidly in the past decade [1,2]. MgTiO3 –CaTiO3 (MCT) is well known as a material for temperature-compensating capacitors, dielectric resonators and patch antennas. The material is made of a mixture of modified ␣-Al2 O3 structured magnesium titanate (MgTiO3 : εr ∼ 17, Q × f value ∼ 160,000 GHz and τ f ∼ −50 ppm/◦ C) [3] and per-
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ovskite structured calcium titanate (CaTiO3 : εr ∼ 170, Q × f value ∼ 3600 GHz and τ f value ∼ 800 ppm/◦ C) [4]. With the ratio Mg:Ca = 95:5, 0.95 MgTiO3 –0.05CaTiO3 ceramics gives εr ∼ 21, Q × f ∼ 56,000 GHz, and a zero τ f value. Through partial replacement of Mg by Zn, the (Mg0.95 Zn0.05 )TiO3 ceramics having an ilmenite-type structure possesses excellent dielectric characteristics (εr ∼ 17.1, Q × f ∼ 264,000 GHz and τ f ∼ −40.3 ppm/◦ C) [5]. Moreover, Ca0.6 Nd0.8/3 TiO3 shows a positive τ f value of 213 ppm/◦ C associated with a high εr of 109 and a Q × f value of 17,600 GHz [6]. In order to achieve near zero τ f value, Ca0.6 Nd0.8/3 TiO3 was added to (Mg0.95 Zn0.05 )TiO3 as a ceramic system of (1 − x)(Mg0.95 Zn0.05 )TiO3 –xCa0.6 Nd0.8/3 TiO3 . The resultant microwave dielectric properties were analyzed based upon the densification, the X-ray diffraction (XRD) patterns and the microstructures of the ceramics. The correlation between the microstructure and the Q × f value was also investigated. 2. Experimental procedure The starting materials were mixed according to the stoichiometric ratio. High-purity oxide powders (>99.9%) CaCO3 , Nd2 O3 , MgO, TiO2 and ZnO were weighed and mixed for 24 h with distilled water. The powders were separately prepared according to the desired stoichiometry Ca0.61 Nd0.26 TiO3 and (Mg0.95 Zn0.05 )TiO3 , and ground in distilled water for 12 h in a ball mill with agate balls. The dried powders were calcined at 1100 ◦ C for 3 h and then mixed according to the molar fraction x(Mg0.95 Zn0.05 Ti)O3 –(1 − x)Ca0.61 Nd0.26 TiO3 . The calcined powders were mixed in the desired composition, re-milled for 5 h with PVA solution as a binder and pressed into pellets of 11 mm in diameter and 5 mm in thickness. A pressing pressure of 2000 kg/cm2 was used for all samples. These pellets were sintered at temperatures of 1250–1350 ◦ C for 4 h. The crystalline phases of the calcined powder and the sintered ceramics were identified by X-ray diffraction pattern analysis. Microstructure observations and analyses of sintered surfaces were performed by scanning electron microscopy (SEM, Philips XL-40FEG). Energy-dispersive spectroscopy (EDS) was also performed to identify the existence of second phases. The bulk densities of the sintered pellets were measured by the Archimedes method. The dielectric constant (εr ) and the quality factor values (Q) at microwave frequencies were measured by the Hakki–Coleman [7] dielectric resonator method under TE011 and TE01σ modes as modified and improved by Courtney [8]. The dielectric resonator was positioned between two brass plates. A system combined with an HP8757D network analyzer and an HP8350B sweep oscillator was employed in the measurement. An identical technique was applied to measure the temperature coefficient of resonant frequency (τ f ). The test set was placed over a thermostat in the temperature range from +25 to +80 ◦ C. The τ f value (ppm/◦ C) was calculated by noting the change in resonant frequency (f): τf =
f2 − f1 , f1 (T2 − T1 )
(1)
where f1 and f2 represent the resonant frequencies at T1 and T2 , respectively.
3. Results and discussion X-ray diffraction patterns of 0.85(Mg0.95 Zn0.05 Ti)O3 – 0.15Ca0.61 Nd0.26 TiO3 ceramics sintered at different temperatures for 4 h are shown in Fig. 1. 0.85(Mg0.95 Zn0.05 Ti) O3 –0.15Ca0.61 Nd0.26 TiO3 ceramics illustrated mixed phases of (Mg0.95 Zn0.05 Ti)O3 (can be indexed as MgTiO3 ) as the main phase associated with minor phases of Ca0.61 Nd0.26 TiO3 , MgTi2 O5 and Mg2 TiO4 . It is understood that crystals of (Mg0.95 Zn0.05 Ti)O3 and Ca0.61 Nd0.26 TiO3 are trigonal (JCPDS #06-0494) and orthorhombic (JCPDS #22-0153), respectively.
Fig. 1. X-ray diffraction patterns of 0.85(Mg0.95 Zn0.05 )TiO3 –0.15Ca0.61 Nd0.26 TiO3 ceramics sintered at different temperatures for 4 h.
MgTi2 O5 , usually formed as an intermediate phase, was identified, and was difficult to completely eliminate from the sample prepared by the mixed oxide route. The formation of mixed phases in the 0.85(Mg0.95 Zn0.05 Ti)O3 –0.15Ca0.61 Nd0.26 TiO3 ceramic system was due to structural differences and because the average ionic radii of Ca2+ (0.99 nm) and Nd3+ (1.15 nm) were larger than that of Mg2+ (0.65 nm); therefore, a solid solution could not be obtained. Moreover, Mg-rich second phase Mg2 TiO4 was only identified at 1325/1350 ◦ C. The excess Mg was resulted from the evaporation of Zn at high temperatures (>1300 ◦ C) [9]. Identical XRD patterns were also observed for other compositions. The SEM micrographs of 0.85(Mg0.95 Zn0.05 Ti)O3 – 0.15Ca0.61 Nd0.26 TiO3 ceramics sintered at different temperatures for 4 h are illustrated in Fig. 2. The grain size slightly increased with increasing temperature to 1300 ◦ C and the specimen was already dense with uniform grain morphology. However, rapid grain growth was observed at temperatures higher than 1300 ◦ C. Some rod-shaped grains appeared in the SEM micrographs were identified as MgTi2 O5 or Mg2 TiO4 by EDS. Fig. 3 shows the bulk density of x(Mg0.95 Zn0.05 Ti)O3 – (1 − x)Ca0.61 Nd0.26 TiO3 ceramics sintered at different temperatures for 4 h. With the increase of sintering temperature, the bulk density was found to increase to a maximum at 1300 ◦ C and thereafter slightly decreased. The increase in density was due to the grain growth and the specimen being denser. However, the degradation of the density at 1325 ◦ C for x = 0.7–0.9 was mainly the results from the rapid grain growth as observed in Fig. 2 and the evaporation of Zn at high temperatures (>1300 ◦ C). The figure revealed that densities of 3.71–3.86 (g/cm3 ) were obtained for 0.85(Mg0.95 Zn0.05 Ti)O3 –0.15Ca0.61 Nd0.26 TiO3 ceramics at sintering temperatures 1250–1350 ◦ C. Density was also influenced by the composition and increased with the increase of Ca0.61 Nd0.26 TiO3 content, which possesses a higher density than that of (Mg0.95 Zn0.05 Ti)O3 . Fig. 4 demonstrates the dielectric constant of the x(Mg0.95 Zn0.05 Ti)O3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics at different sintering temperatures for 4 h. Variation of the εr value
C.-L. Huang et al. / Journal of Alloys and Compounds 453 (2008) 337–340
339
Fig. 2. SEM photographs of 0.85(Mg0.95 Zn0.05 )TiO3 –0.15Ca0.61 Nd0.26 TiO3 ceramics at (a) 1250 ◦ C, (b) 1275 ◦ C, (c) 1300 ◦ C, (d) 1325 ◦ C and (e) 1350 ◦ C.
was consistent with that of density and started to saturate at 1275 ◦ C since higher density represents lower porosity for each composition. The dielectric constants declined from 32.8 to 22.1 as the x value increased from 0.7 to 0.9. It was mainly a result
from the variation of the composition since Ca0.61 Nd0.26 TiO3 shows a much higher εr value than that of (Mg0.95 Zn0.05 Ti)O3 . It also suggests an εr -tunable system through the control of the x value.
Fig. 3. Bulk density of x(Mg0.95 Zn0.05 )TiO3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics system sintered at different temperatures.
Fig. 4. εr value of x(Mg0.95 Zn0.05 )TiO3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics system sintered at different temperatures.
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Fig. 5. Q × f value of x(Mg0.95 Zn0.05 )TiO3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics system sintered at different temperatures.
The quality factor values (Q × f) of x(Mg0.95 Zn0.05 Ti)O3 – (1 − x)Ca0.61 Nd0.26 TiO3 ceramics at different sintering temperatures are shown in Fig. 5. Many factors are believed to affect the microwave dielectric loss and these factors can be divided into two areas: the intrinsic loss and the extrinsic loss. Intrinsic losses are mainly caused by lattice vibration modes while extrinsic losses are dominated by second phases, oxygen vacancies, grain sizes and densification or porosity. With increasing sintering temperature, the Q × f value increased to a maximum value at 1300 ◦ C and thereafter decreased. It is consistent with the variation of density, which plays an important role in controlling the dielectric loss. A maximum Q × f value of 130,000 GHz was obtained for 0.9(Mg0.95 Zn0.05 Ti)O3 –0.1Ca0.61 Nd0.26 TiO3 ceramics at 1300 ◦ C. Moreover, formation of Mg2 TiO4 and inhomogeneous grain growth at 1325/1350 ◦ C would further degrade the Q × f value of the specimen. As a result, the degradation in the Q × f value of the specimen was caused not only by its low density but also by the formation of Mg2 TiO4 .
Fig. 6 illustrates the temperature coefficient of the resonant frequency (τ f ) of x(Mg0.95 Zn0.05 Ti)O3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics at different sintering temperatures. In general, the temperature coefficient of the resonant frequency (τ f ) was related to the composition and the phases existing in the ceramics. The temperature coefficient of the resonant frequency (τ f ) was insensitive to the sintering temperature over the entire experiments. However, a higher Ca0.61 Nd0.26 TiO3 (τ f ∼ 213 ppm/◦ C) content would lead to a variation of the τ f value toward positive. The τ f value varied from −24.7 to 38.4 ppm/◦ C as the amount of added Ca0.61 Nd0.26 TiO3 increased from 0.1 to 0.3 at 1300 ◦ C. It gives a cross-zero line implying that zero τ f can be achieved through appropriate adjustment of the x value in the system. 4. Conclusions The dielectric characteristics of x(Mg0.95 Zn0.05 Ti)O3 – (1 − x)Ca0.61 Nd0.26 TiO3 ceramics were investigated. x(Mg0.95 Zn0.05 Ti)O3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics exhibited mixed phases of (Mg0.95 Zn0.05 Ti)O3 as the main phase and some minor phases of Ca0.61 Nd0.26 TiO3 , MgTi2 O5 and Mg2 TiO4 . Formation of Mg-rich second phase Mg2 TiO4 was attributed to the evaporation of Zn at high temperatures. At 1300 ◦ C, 0.85(Mg0.95 Zn0.05 Ti)O3 –0.15Ca0.61 Nd0.26 TiO3 ceramics demonstrated excellent microwave dielectric properties: εr ∼ 24.3, Q × f value ∼ 112,000 GHz (at 8 GHz) and τ f value ∼ −10.1 ppm/◦ C. Acknowledgement This work was supported by the National Science Council of the Republic of China under grant NSC-94-2213-E-006-045. References
Fig. 6. The τ f value of x(Mg0.95 Zn0.05 )TiO3 – (1 − x)Ca0.61 Nd0.26 TiO3 ceramics system sintered at different temperatures.
[1] W. Werising, in: H.B.C. Steele (Ed.), Electronic Ceramics, Elsevier, London, 1991, p. 67. [2] G. Wolfram, H.E. Gobel, Mater. Res. Bull. 16 (1981) 1455. [3] K. Wakino, Ferroelectrics 91 (1989) 69. [4] R.C. Kell, A.C. Greenham, G.C.E. Olds, J. Am. Ceram. Soc. 56 (1973) 352. [5] C.L. Huang, S.H. Liu, Jpn. J. Appl. Phys. 46 (1) (in press). [6] C.L. Huang, J.-T. Tasi, Y.B. Chen, Mater. Res. Bull. 36 (2001) 547. [7] B.W. Hakki, P.D. Coleman, IEEE Trans. Microwave Theory Tech. 8 (1960) 402. [8] W.E. Courtney, IEEE Trans. Microwave Theory Tech. 18 (1970) 476. [9] C.F. Tseng, C.L. Huang, W.R. Yang, C.H. Hsu, J. Am. Ceram. Soc. 89 (2006) 1465.
New dielectric material system of (Mg0.95Zn0.05)TiO3–Ca0.61Nd0.26TiO3 at microwave frequency Cheng-Liang Huang a,∗ , Chia-Feng Tasi a , Yuan-Bin Chen a , Yao-Chung Cheng b a
Department of Electrical Engineering, National Cheng Kung University, No. 1 University Road, Tainan 70101, Taiwan b Universal Nano Tech Co., Ltd., Tainan, Taiwan Received 7 July 2006; received in revised form 15 November 2006; accepted 16 November 2006 Available online 18 December 2006
Abstract The microstructures and the microwave dielectric properties of the x(Mg0.95 Zn0.05 )TiO3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramic system prepared by conventional solid state method were investigated. In order to achieve a temperature-stable material, two compounds with negative and positive temperature coefficients were employed to form a mixed phases. With partial replacement of Mg by Zn, dielectric properties of the x(Mg0.95 Zn0.05 )TiO3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature and the composition. An excellent Q × f value of 130,000 GHz can be obtained for the system with x = 0.9 at 1300 ◦ C. For practical application, a dielectric constant (εr ) of 24.3, a Q × f value of 112,000 GHz and a temperature coefficient of resonant frequency (τ f ) of −10.1 ppm/◦ C for 0.85(Mg0.95 Zn0.05 Ti)O3 –0.15Ca0.61 Nd0.26 TiO3 at 1300 ◦ C is proposed in this paper. © 2006 Elsevier B.V. All rights reserved. Keywords: Ceramics; Oxides; X-ray diffraction; Solid state reactions; Dielectric properties
1. Introduction Due to rapid developments in mobile communication, mobile telephone systems, as well as in satellite broadcasting systems, the design of high-quality devices is very important. To miniaturize the devices and for the systems to work with high efficiency and stability, the materials for microwave resonators must be excellent in the following three dielectric characteristics. The first characteristic is a high dielectric constant; microwave devices such as resonators, filters, oscillators and antennas, are made of dielectric materials with high dielectric constant εr . The use of high dielectric constant materials can effectively reduce the size of resonators since the wavelength (λ) in dielectrics is √ inversely proportional to εr of the wavelength (λ0 ) in vac√ uum (λ = λ0 / εr ). The second is a high Q value; many studies have focused on developing dielectric materials with a highquality factor (Q × f). This is required to achieve high frequency selectivity and stability in microwave transmitters and receiver components. The third is a zero temperature coefficient of the
∗
Corresponding author. Tel.: +886 6 2757575x62390; fax: +886 6 2345482. E-mail address: [email protected] (C.-L. Huang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.084
resonant frequency (τ f ) for dielectric resonators and microwave device substrates. Small temperature coefficients of the resonant frequency ensure the stability of the microwave components at different working temperatures. To satisfy the demands of microwave circuit designs, each dielectric property requires precise control. Using two or more compounds with negative and positive temperature coefficients to form a solid solution or mixed phases is the most promising method of obtaining a zero temperature coefficient of the resonant frequency. Because most dielectric ceramics with high dielectric constants have positive τ f values, searching for materials with a high dielectric constant, a high Q and a negative τ f is necessary to achieve this goal. MgTiO3 -based ceramics are finding wide applications as dielectrics in resonators, filters and antennas for Wireless LAN and GPS in microwave regimes. The development of microwave dielectric components for communication systems such as cellular phones and global positioning systems has been growing rapidly in the past decade [1,2]. MgTiO3 –CaTiO3 (MCT) is well known as a material for temperature-compensating capacitors, dielectric resonators and patch antennas. The material is made of a mixture of modified ␣-Al2 O3 structured magnesium titanate (MgTiO3 : εr ∼ 17, Q × f value ∼ 160,000 GHz and τ f ∼ −50 ppm/◦ C) [3] and per-
338
C.-L. Huang et al. / Journal of Alloys and Compounds 453 (2008) 337–340
ovskite structured calcium titanate (CaTiO3 : εr ∼ 170, Q × f value ∼ 3600 GHz and τ f value ∼ 800 ppm/◦ C) [4]. With the ratio Mg:Ca = 95:5, 0.95 MgTiO3 –0.05CaTiO3 ceramics gives εr ∼ 21, Q × f ∼ 56,000 GHz, and a zero τ f value. Through partial replacement of Mg by Zn, the (Mg0.95 Zn0.05 )TiO3 ceramics having an ilmenite-type structure possesses excellent dielectric characteristics (εr ∼ 17.1, Q × f ∼ 264,000 GHz and τ f ∼ −40.3 ppm/◦ C) [5]. Moreover, Ca0.6 Nd0.8/3 TiO3 shows a positive τ f value of 213 ppm/◦ C associated with a high εr of 109 and a Q × f value of 17,600 GHz [6]. In order to achieve near zero τ f value, Ca0.6 Nd0.8/3 TiO3 was added to (Mg0.95 Zn0.05 )TiO3 as a ceramic system of (1 − x)(Mg0.95 Zn0.05 )TiO3 –xCa0.6 Nd0.8/3 TiO3 . The resultant microwave dielectric properties were analyzed based upon the densification, the X-ray diffraction (XRD) patterns and the microstructures of the ceramics. The correlation between the microstructure and the Q × f value was also investigated. 2. Experimental procedure The starting materials were mixed according to the stoichiometric ratio. High-purity oxide powders (>99.9%) CaCO3 , Nd2 O3 , MgO, TiO2 and ZnO were weighed and mixed for 24 h with distilled water. The powders were separately prepared according to the desired stoichiometry Ca0.61 Nd0.26 TiO3 and (Mg0.95 Zn0.05 )TiO3 , and ground in distilled water for 12 h in a ball mill with agate balls. The dried powders were calcined at 1100 ◦ C for 3 h and then mixed according to the molar fraction x(Mg0.95 Zn0.05 Ti)O3 –(1 − x)Ca0.61 Nd0.26 TiO3 . The calcined powders were mixed in the desired composition, re-milled for 5 h with PVA solution as a binder and pressed into pellets of 11 mm in diameter and 5 mm in thickness. A pressing pressure of 2000 kg/cm2 was used for all samples. These pellets were sintered at temperatures of 1250–1350 ◦ C for 4 h. The crystalline phases of the calcined powder and the sintered ceramics were identified by X-ray diffraction pattern analysis. Microstructure observations and analyses of sintered surfaces were performed by scanning electron microscopy (SEM, Philips XL-40FEG). Energy-dispersive spectroscopy (EDS) was also performed to identify the existence of second phases. The bulk densities of the sintered pellets were measured by the Archimedes method. The dielectric constant (εr ) and the quality factor values (Q) at microwave frequencies were measured by the Hakki–Coleman [7] dielectric resonator method under TE011 and TE01σ modes as modified and improved by Courtney [8]. The dielectric resonator was positioned between two brass plates. A system combined with an HP8757D network analyzer and an HP8350B sweep oscillator was employed in the measurement. An identical technique was applied to measure the temperature coefficient of resonant frequency (τ f ). The test set was placed over a thermostat in the temperature range from +25 to +80 ◦ C. The τ f value (ppm/◦ C) was calculated by noting the change in resonant frequency (f): τf =
f2 − f1 , f1 (T2 − T1 )
(1)
where f1 and f2 represent the resonant frequencies at T1 and T2 , respectively.
3. Results and discussion X-ray diffraction patterns of 0.85(Mg0.95 Zn0.05 Ti)O3 – 0.15Ca0.61 Nd0.26 TiO3 ceramics sintered at different temperatures for 4 h are shown in Fig. 1. 0.85(Mg0.95 Zn0.05 Ti) O3 –0.15Ca0.61 Nd0.26 TiO3 ceramics illustrated mixed phases of (Mg0.95 Zn0.05 Ti)O3 (can be indexed as MgTiO3 ) as the main phase associated with minor phases of Ca0.61 Nd0.26 TiO3 , MgTi2 O5 and Mg2 TiO4 . It is understood that crystals of (Mg0.95 Zn0.05 Ti)O3 and Ca0.61 Nd0.26 TiO3 are trigonal (JCPDS #06-0494) and orthorhombic (JCPDS #22-0153), respectively.
Fig. 1. X-ray diffraction patterns of 0.85(Mg0.95 Zn0.05 )TiO3 –0.15Ca0.61 Nd0.26 TiO3 ceramics sintered at different temperatures for 4 h.
MgTi2 O5 , usually formed as an intermediate phase, was identified, and was difficult to completely eliminate from the sample prepared by the mixed oxide route. The formation of mixed phases in the 0.85(Mg0.95 Zn0.05 Ti)O3 –0.15Ca0.61 Nd0.26 TiO3 ceramic system was due to structural differences and because the average ionic radii of Ca2+ (0.99 nm) and Nd3+ (1.15 nm) were larger than that of Mg2+ (0.65 nm); therefore, a solid solution could not be obtained. Moreover, Mg-rich second phase Mg2 TiO4 was only identified at 1325/1350 ◦ C. The excess Mg was resulted from the evaporation of Zn at high temperatures (>1300 ◦ C) [9]. Identical XRD patterns were also observed for other compositions. The SEM micrographs of 0.85(Mg0.95 Zn0.05 Ti)O3 – 0.15Ca0.61 Nd0.26 TiO3 ceramics sintered at different temperatures for 4 h are illustrated in Fig. 2. The grain size slightly increased with increasing temperature to 1300 ◦ C and the specimen was already dense with uniform grain morphology. However, rapid grain growth was observed at temperatures higher than 1300 ◦ C. Some rod-shaped grains appeared in the SEM micrographs were identified as MgTi2 O5 or Mg2 TiO4 by EDS. Fig. 3 shows the bulk density of x(Mg0.95 Zn0.05 Ti)O3 – (1 − x)Ca0.61 Nd0.26 TiO3 ceramics sintered at different temperatures for 4 h. With the increase of sintering temperature, the bulk density was found to increase to a maximum at 1300 ◦ C and thereafter slightly decreased. The increase in density was due to the grain growth and the specimen being denser. However, the degradation of the density at 1325 ◦ C for x = 0.7–0.9 was mainly the results from the rapid grain growth as observed in Fig. 2 and the evaporation of Zn at high temperatures (>1300 ◦ C). The figure revealed that densities of 3.71–3.86 (g/cm3 ) were obtained for 0.85(Mg0.95 Zn0.05 Ti)O3 –0.15Ca0.61 Nd0.26 TiO3 ceramics at sintering temperatures 1250–1350 ◦ C. Density was also influenced by the composition and increased with the increase of Ca0.61 Nd0.26 TiO3 content, which possesses a higher density than that of (Mg0.95 Zn0.05 Ti)O3 . Fig. 4 demonstrates the dielectric constant of the x(Mg0.95 Zn0.05 Ti)O3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics at different sintering temperatures for 4 h. Variation of the εr value
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339
Fig. 2. SEM photographs of 0.85(Mg0.95 Zn0.05 )TiO3 –0.15Ca0.61 Nd0.26 TiO3 ceramics at (a) 1250 ◦ C, (b) 1275 ◦ C, (c) 1300 ◦ C, (d) 1325 ◦ C and (e) 1350 ◦ C.
was consistent with that of density and started to saturate at 1275 ◦ C since higher density represents lower porosity for each composition. The dielectric constants declined from 32.8 to 22.1 as the x value increased from 0.7 to 0.9. It was mainly a result
from the variation of the composition since Ca0.61 Nd0.26 TiO3 shows a much higher εr value than that of (Mg0.95 Zn0.05 Ti)O3 . It also suggests an εr -tunable system through the control of the x value.
Fig. 3. Bulk density of x(Mg0.95 Zn0.05 )TiO3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics system sintered at different temperatures.
Fig. 4. εr value of x(Mg0.95 Zn0.05 )TiO3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics system sintered at different temperatures.
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Fig. 5. Q × f value of x(Mg0.95 Zn0.05 )TiO3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics system sintered at different temperatures.
The quality factor values (Q × f) of x(Mg0.95 Zn0.05 Ti)O3 – (1 − x)Ca0.61 Nd0.26 TiO3 ceramics at different sintering temperatures are shown in Fig. 5. Many factors are believed to affect the microwave dielectric loss and these factors can be divided into two areas: the intrinsic loss and the extrinsic loss. Intrinsic losses are mainly caused by lattice vibration modes while extrinsic losses are dominated by second phases, oxygen vacancies, grain sizes and densification or porosity. With increasing sintering temperature, the Q × f value increased to a maximum value at 1300 ◦ C and thereafter decreased. It is consistent with the variation of density, which plays an important role in controlling the dielectric loss. A maximum Q × f value of 130,000 GHz was obtained for 0.9(Mg0.95 Zn0.05 Ti)O3 –0.1Ca0.61 Nd0.26 TiO3 ceramics at 1300 ◦ C. Moreover, formation of Mg2 TiO4 and inhomogeneous grain growth at 1325/1350 ◦ C would further degrade the Q × f value of the specimen. As a result, the degradation in the Q × f value of the specimen was caused not only by its low density but also by the formation of Mg2 TiO4 .
Fig. 6 illustrates the temperature coefficient of the resonant frequency (τ f ) of x(Mg0.95 Zn0.05 Ti)O3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics at different sintering temperatures. In general, the temperature coefficient of the resonant frequency (τ f ) was related to the composition and the phases existing in the ceramics. The temperature coefficient of the resonant frequency (τ f ) was insensitive to the sintering temperature over the entire experiments. However, a higher Ca0.61 Nd0.26 TiO3 (τ f ∼ 213 ppm/◦ C) content would lead to a variation of the τ f value toward positive. The τ f value varied from −24.7 to 38.4 ppm/◦ C as the amount of added Ca0.61 Nd0.26 TiO3 increased from 0.1 to 0.3 at 1300 ◦ C. It gives a cross-zero line implying that zero τ f can be achieved through appropriate adjustment of the x value in the system. 4. Conclusions The dielectric characteristics of x(Mg0.95 Zn0.05 Ti)O3 – (1 − x)Ca0.61 Nd0.26 TiO3 ceramics were investigated. x(Mg0.95 Zn0.05 Ti)O3 –(1 − x)Ca0.61 Nd0.26 TiO3 ceramics exhibited mixed phases of (Mg0.95 Zn0.05 Ti)O3 as the main phase and some minor phases of Ca0.61 Nd0.26 TiO3 , MgTi2 O5 and Mg2 TiO4 . Formation of Mg-rich second phase Mg2 TiO4 was attributed to the evaporation of Zn at high temperatures. At 1300 ◦ C, 0.85(Mg0.95 Zn0.05 Ti)O3 –0.15Ca0.61 Nd0.26 TiO3 ceramics demonstrated excellent microwave dielectric properties: εr ∼ 24.3, Q × f value ∼ 112,000 GHz (at 8 GHz) and τ f value ∼ −10.1 ppm/◦ C. Acknowledgement This work was supported by the National Science Council of the Republic of China under grant NSC-94-2213-E-006-045. References
Fig. 6. The τ f value of x(Mg0.95 Zn0.05 )TiO3 – (1 − x)Ca0.61 Nd0.26 TiO3 ceramics system sintered at different temperatures.
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