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Investigation of the microwave dielectric properties of Ca1−xMgxLa4Ti5O17 ceramics for application in coplanar patch antenna
Journal of Alloys and Compounds 486 (2009) 410–414
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Investigation of the microwave dielectric properties of Ca1−x Mgx La4 Ti5 O17 ceramics for application in coplanar patch antenna Yih-Chien Chen ∗ , Shi-Li Yao, Ren-Jie Tsai, Kuei-Chien Chen Department of Electrical Engineering, Lunghwa University of Science and Technology, Gueishan Shiang, Taoyuan County, Taiwan
a r t i c l e
i n f o
Article history: Received 12 January 2009 Received in revised form 25 June 2009 Accepted 28 June 2009 Available online 5 July 2009 Keywords: Ca1−x Mgx La4 Ti5 O17 X-ray diffraction pattern Dielectric constant Quality factor Temperature coefficient of resonant frequency
a b s t r a c t In this paper, the dielectric properties of Ca1−x Mgx La4 Ti5 O17 ceramics at microwave frequency have been studied. The diffraction peaks of Ca(1−x) Mgx La4 Ti5 O17 ceramics nearly unchanged with x increasing from 0 to 0.03. Similar X-ray diffraction peaks of Ca0.99 Mg0.01 La4 Ti5 O17 ceramic were observed at different sintering temperatures. A maximum density of 5.3 g/cm3 can be obtained for Ca0.99 Mg0.01 La4 Ti5 O17 ceramic sintered at 1500 ◦ C for 4 h. A maximum dielectric constant (εr ) and quality factor (Q × f) of Ca0.99 Mg0.01 La4 Ti5 O17 ceramic sintered at 1500 ◦ C for 4 h are 56.3 and 12,300 GHz (at 6.4 GHz), respectively. A near-zero temperature coefficient of resonant frequency ( f ) of −9.6 ppm/◦ C can be obtained for Ca0.99 Mg0.01 La4 Ti5 O17 ceramic sintered at 1500 ◦ C for 4 h. The measurement results for the aperturecoupled coplanar patch antenna at 2.5 GHz are presented. With this technique, a 3.33% bandwidth (return loss <−10 dB) with a center frequency at approximately 2.5 GHz has been successfully achieved. © 2009 Elsevier B.V. All rights reserved.
1. Introduction There are many commercial applications, such as mobile radio and wireless communications that use patch antennas. However, patch antennas have limitations in size, gain, and efficiency, imposed by the dielectric substrate. Three dielectric properties of materials must be considered when patch antennas are used: a high dielectric constant, a high quality factor, and a near-zero temperature coefficient of resonant frequency. High dielectric constant and a near-zero temperature coefficient of resonant frequency are required for small size and high temperature stability, respectively. The quality factor is a representation of the antenna gain. Typically, there are radiations, conduction, dielectric, and surface wave losses, thus the total quality factor is affected by all of these losses [1,2]. The microwave dielectric properties of (Ba,La)n Tin−1 O3n (n = 5, 6) have already been reported. Both BaLa4 Ti4 O15 and Ba2 La4 Ti5 O18 are characterized by high dielectric constant (εr ∼ 39–46), high quality factor (Q × f ∼ 11,583–31,839) and small temperature coefficient of resonant frequency ( f ∼ −36 to 79 ppm/◦ C) [3]. On the other hand, MO–La2 O3 –TiO2 (M = Ca, Sr, Ba) is well known as microwave material for dielectric resonator and filter. Generally speaking, most of these ceramics combines a high dielectric constant (εr ∼ 42–54), high quality factor (Q × f ∼ 16,222–50,215) and an adjustable temperature coefficient of resonant frequency
∗ Corresponding author. Tel.: +886 2 8209 3211; fax: +886 2 8209 9728. E-mail address: [email protected] (Y.-C. Chen). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.189
( f ∼ −25 to 6 ppm/◦ C). Among the investigated orthorhombic phases, CaLa4 Ti5 O17 ceramic sintered at 1625 ◦ C give εr ∼ 53.7, Q × f ∼ 17,359 GHz, and f ∼ −20 ppm/◦ C [4]. Also, CuO additives can effectively lower the sintering temperature of CaLa4 Ti5 O17 . A dielectric constant value of 56.6, a quality factor of 8700 GHz, and a temperature coefficient of resonant frequency of −15 ppm/◦ C were obtained for CaLa4 Ti5 O17 ceramics with 0.5 wt% CuO sintered at 1500 ◦ C for 4 h [5]. By the partial replacement of Ca with Zn, the dielectric properties of CaLa4 Ti5 O17 ceramics at microwave frequency are modified. A dielectric constant of 57, a quality factor of 15,000 GHz, and a temperature coefficient of resonant frequency ( f ) of −8.16 ppm/◦ C were obtained for Ca0.99 Zn0.01 La4 Ti5 O17 ceramics with 0.5 wt% CuO additive sintered at 1450 ◦ C for 4 h [6]. Because of the ionic radii of Mg2+ are similar to that of Ca2+ [7], it has attracted our attention to investigate the effect of partial replacement of Ca with Mg for Ca(1−x) Mgx La4 Ti5 O17 ceramics. In this paper, investigations on Ca(1−x) Mgx La4 Ti5 O17 ceramics have been made to produce materials with better microwave dielectric properties for application in aperture-coupled coplanar patch antennas. The microwave dielectric properties, including a high dielectric constant, a high quality factor, and a near-zero temperature coefficient of resonant frequency have been considered in order to realize an outstanding design of the aperture-coupled coplanar patch antenna that have small size, high antenna gain, and high temperature stability. The microwave dielectric properties of Ca(1−x) Mgx La4 Ti5 O17 ceramics have been found to be different with different amounts of Mg substitution and sintering temperature.
Y.-C. Chen et al. / Journal of Alloys and Compounds 486 (2009) 410–414
Fig. 1. XRD patterns of Ca(1−x) Mgx La4 Ti5 O17 ceramics with sintered at 1500 ◦ C.
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Fig. 2. XRD patterns of Ca0.99 Mg0.01 La4 Ti5 O17 ceramics with at different sintering temperatures.
2. Experimental procedure The starting raw chemicals were high-purity CaCO3 , MgO, La2 O3 , and TiO2 powders. The composition prepared was Ca(1−x) Mgx La4 Ti5 O17 , x = 0, 0.01 and 0.03. Ca(1−x) Mgx La4 Ti5 O17 was added 0.5 wt% CuO to lower the sintering temperature. Specimens were prepared using the conventional mixed-oxide method. The raw material was weighed out in stoichiometric proportions, ball-milled in alcohol, dried, and then calcined at 1200 ◦ C for 4 h. The obtained powders were then crushed into a fine powder of less than 200 mesh size. The fine powders obtained were then axially pressed into pellets at 2000 kg/cm2 with a diameter of 11 mm and thickness of 6 mm, prior to sintering. The specimens obtained were then sintered at 1400, 1450, 1500, and 1550 ◦ C for 4 h. After sintering, the phases of the samples were investigated by XRD. The bulk densities of the specimens were measured by the Archimedes method. The microwave dielectric properties of the specimens were measured by the postresonator method developed by Hakki and Coleman [8]. The postresonator method employs a specimen in the form of a cylinder of diameter D and length L. The specimens used for microwave dielectric property measurements had an aspect ratio D/L of about 1.6, which is in the permitted range reported by Kobayashi and Katoh [9]. The cylindrical resonator is sandwiched between two conducting plates. Two small antennas are positioned in the vicinity of the specimen to couple the microwave signal power in or out of the resonator. The other end of the antennas is connected to the Agilent N5230A network analyzer. The resonance characteristics are dependent
on the geometrical size and dielectric properties of the specimen. The microwave energy was coupled using E-field probes. The TE011 resonant mode was found to be most suitable for measuring the dielectric constant and loss factor of the specimen. Using the Agilent N5230A network analyzer, the TE011 resonant frequency of the dielectric resonator was identified, and the dielectric constant and quality factor were calculated. The technique for measuring f is the same as that of dielectric constant measurement. The test cavity was placed in a chamber in the temperature range from 25 to 75 ◦ C. The f value (ppm/◦ C) can be determined by noting the change in resonant frequency, f =
f2 − f1 × 106 f1 (T2 − T1 )
(1)
where f1 and f2 represent the resonant frequencies at T1 and T2 , respectively. The aperture-coupled coplanar patch antenna designed consists of a highdielectric-constant Ca0.99 Mg0.01 La4 Ti5 O17 ceramic substrate with a coplanar rectangular patch, and a feed substrate with a microstrip feed-line and rectangular aperture on back-side and top-side, respectively. The microstrip feed-line and rectangular aperture were fabricated by wet etching. The coplanar rectangular patch was fabricated using thick-film technology. The conductor material used was silver, and was in the form of a paste. The mesh was designed in accordance with the pattern to be fabricated. The past was passed through the designed mesh in the screen-printing
Fig. 3. Microstructures of the Ca0.99 Mg0.01 La4 Ti5 O17 ceramics with 0.5 wt% CuO additives sintered for 4 h at different sintering temperatures.
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Table 1 The microwave dielectric properties of Ca(1−x) Mgx La4 Ti5 O17 . Material
S.T.
Density (g/cm3 )
εr
Q × f (GHz)
f (ppm/◦ C)
CaLa4 Ti5 O17 CaLa4 Ti5 O17 Ca0.99 Mg0.01 La4 Ti5 O17 Ca0.99 Mg0.01 La4 Ti5 O17 Ca0.99 Mg0.01 La4 Ti5 O17 Ca0.99 Mg0.01 La4 Ti5 O17 Ca0.97 Mg0.03 La4 Ti5 O17 Ca0.97 Mg0.03 La4 Ti5 O17 Ca0.97 Mg0.03 La4 Ti5 O17 Ca0.97 Mg0.03 La4 Ti5 O17
1450 1500 1400 1450 1500 1550 1400 1450 1500 1550
5.321 5.319 4.663 5.196 5.318 5.247 4.790 5.295 5.262 5.242
57 56.6 47.0 53.8 56.3 56.0 48.5 56.7 55.9 55.3
9,000 8,700 6,600 8,200 12,300 8,800 9,900 10,800 8,600 6,100
−10 −15 −19 −8 −10 −13 −6 −11 −16 −10
process. After screen printing, conductors were dried at approximately 150 ◦ C for 30 min. The reflection coefficients and input impedances were measured using the Agilent network analyzer (N5230A).
3. Results and discussion Fig. 1 showed the X-ray diffraction patterns of CaLa4 Ti5 O17 , Ca0.99 Mg0.01 La4 Ti5 O17 , and Ca0.97 Mg0.03 La4 Ti5 O17 ceramics in the range from 20◦ to 60◦ sintered at the temperatures of 1500 ◦ C for 4 h. As we can see from the results, the diffraction peaks of Ca(1−x) Mgx La4 Ti5 O17 ceramics nearly unchanged with x increasing from 0 to 0.03. The crystal structure of CaLa4 Ti5 O17 is known to have orthorhombic structure belongs to Pnnm space group. The lattice parameters of CaLa4 Ti5 O17 are a0 = 0.5522 nm, b0 = 3.1256 nm, and c0 = 0.3899 nm [10]. The X-ray diffraction patterns of the Ca0.99 Mg0.01 La4 Ti5 O17 specimens after sintering at 1450, 1500, and 1550 ◦ C for 4 h are shown in Fig. 2. Similar spectral angles of X-ray diffraction peaks were observed at different sintering temperatures. The microstructures of Ca0.99 Mg0.01 La4 Ti5 O17 with 0.5 wt% CuO additive sintered in the range of 1400–1550 ◦ C for 4 h are shown in Fig. 3. The Ca0.99 Mg0.01 La4 Ti5 O17 with 0.5 wt% CuO additives was not dense, and the grains did not grow after it was sintered at 1400 ◦ C. Comparing the microstructures of Ca0.99 Mg0.01 La4 Ti5 O17 with 0.5 wt% CuO additives sintered at different temperatures, the number of pores decreased and the rate of grain growth increased apparently. The pores almost disappeared in the specimen sintered at 1500 ◦ C. The apparent densities of Ca(1−x) Mgx La4 Ti5 O17 ceramics with different amounts of Mg substitution sintered at 1400, 1450, 1500, and 1550 ◦ C for 4 h are shown in Table 1. The bulk density was found to increase to a maximum value at sintering temperature of 1500 ◦ C and thereafter decreased for Ca0.99 Mg0.01 La4 Ti5 O17 . The bulk density increased with increasing sintering temperature, but this may be due to the decrease in the number of pores. The bulk density was found to decrease as the sintering temperature increases from 1450 to 1550 ◦ C for Ca0.97 Mg0.03 La4 Ti5 O17 . The theoretical density of CaLa4 Ti5 O17 is 5.46 g/cm3 [11] while a maximum density of 5.3 g/cm3 was obtained for Ca0.99 Mg0.01 La4 Ti5 O17 ceramics sintered at 1500 ◦ C for 4 h. The dielectric constants of Ca(1−x) Mgx La4 Ti5 O17 ceramics with different amounts of Mg substitution sintered at different temperatures for 4 h can be found in Table 1. The maximum value of dielectric constant in the ceramic of Ca0.97 Mg0.03 La4 Ti5 O17 is 56.7 sintered at 1450 ◦ C for 4 h. The dielectric constant was found to increase to a maximum value at sintering temperature of 1500 ◦ C and thereafter decreased for Ca0.99 Mg0.01 La4 Ti5 O17 . The value of dielectric constant is 56.3 in the ceramic of Ca0.99 Mg0.01 La4 Ti5 O17 that was sintered at 1500 ◦ C for 4 h. The dielectric constant decreased from 56.3 to 55.9 as the amounts of Mg substitution increased from 0.01 to 0.03 molar when the ceramics were sintered at 1500 ◦ C. We also found increasing sintering temperature is
not necessary for getting a higher dielectric constant. The decrease in dielectric constant was due to low densities and since higher density means lower porosity, a higher dielectric constant can be achieved. The quality factor (Q × f) of Ca(1−x) Mgx La4 Ti5 O17 ceramics with different amounts of Mg substitution sintered at different temperatures for 4 h are illustrated in Table 1. With increasing the sintering temperature, the quality factor increased to a maximum value at sintering temperature of 1500 ◦ C and thereafter decreased for Ca0.99 Mg0.01 La4 Ti5 O17 . A maximum quality factor of 12,300 GHz (at 6.4 GHz) was obtained for Ca0.99 Mg0.01 La4 Ti5 O17 sintered at 1500 ◦ C for 4 h. Compared with the measurement results of quality factor as shown in Table 1, the value of quality factor was increased from 8700 to 12,300 GHz as the amounts of Mg substitution increased from 0 to 0.01 molar sintered at 1500 ◦ C. The quality factor decreased from 12,300 to 8600 GHz as the amount of Mg substitutions increased from 0.01 to 0.03 sintered at 1500 ◦ C. The relationship between the quality factor and the sintering temperature revealed the same trend of that between the apparent density and the sintering temperature. This is caused by the microwave dielectric loss, which is affected by many factors that can be divided into intrinsic and extrinsic losses. The intrinsic loss is caused by the lattice vibrational modes. The extrinsic loss is induced by the density, porosity, the second phases, the impurities, the oxygen vacancies, the grain size, and the lattice defects [12,13]. Because the quality factor of Ca(1−x) Mgx La4 Ti5 O17 ceramics was consistent with the variation of the apparent density, it is suggested that the quality factor of Ca(1−x) Mgx La4 Ti5 O17 ceramics is mainly controlled by the apparent density. The temperature coefficient of resonant frequency ( f ) values of Ca(1−x) Mgx La4 Ti5 O17 ceramics with different amounts of Mg substitution sintered at different temperatures for 4 h can be observed in Table 1. In general, the temperature coefficient of resonant frequency is related to the composition, the additives, and the second phases that existed in the ceramics. The temperature coefficient of resonant frequency was insensitive to the sintering temperature in the entire sintering temperature range. A near-zero temperature coefficient of resonant frequency of −9.6 ppm/◦ C was measured for Ca0.99 Mg0.01 La4 Ti5 O17 ceramic sintered at 1500 ◦ C for 4 h. The geometry of the aperture-coupled coplanar patch antenna fed with a microstrip feed-line is shown in Fig. 4. The energy resulting from the fringing fields of microstip feed-line is used to excite the coplanar rectangular patch. The coplanar rectangular patch is located centrally on the FR4 substrate. The microstrip feed-line was printed on a FR4 substrate. The rectangular aperture was wet-etched centrally on the FR4 substrate. The parameters of the FR4 substrate are εr1 = 4.4, tan ı = 0.014, thickness of the substrate h1 = 1.6 mm, and size of the substrate = 50 mm × 50 mm. The dimensions of the rectangular aperture are 12.6 mm × 13.6 mm. The parameters of the Ca0.99 Mg0.01 La4 Ti5 O17 substrate are εr2 = 56.3, tan ı = 0.0002, thickness of the substrate h2 = 5.5 mm, and diameter of the substrate = 27 mm. The dimensions of the microstrip
Y.-C. Chen et al. / Journal of Alloys and Compounds 486 (2009) 410–414
413
Fig. 4. Configuration of aperture-coupled coplanar rectangular patch antenna. (a) Microstrip feed-line and rectangular aperture fabricated on FR4 substrate, (b) coplanar rectangular patch printed on high-dielectric-constant Ca0.99 Mg0.01 La4 Ti5 O17 ceramic substrate, and (c) top and side views of the proposed antenna.
feed-line were calculated using closed-form formulas given in ref. 10, assuming an infinite ground plane and a finite dielectric thickness. The microstrip feed-line dimensions were confirmed by AWR Microwave Office. And, the length and width of the microstrip feedline are 36 and 5 mm, respectively. The design process started with the design of a coplanar rectangular patch with a resonant frequency of 2.5 GHz, by choosing the length of the non-radiating edges to be g /2 at 2.5 GHz, where g is the guided wavelength [11]. The coplanar rectangular patch was then fed with a microstrip feed-line. The resonance frequencies could be adjusted by modifying the length of the coplanar rectangular patch and impedance matching could be realized by adjusting the width of the coplanar rectangular patch. Therefore, the bandwidth could also be adjusted by modifying the width of the coplanar rectangular patch. The following set of dimensions of the coplanar rectangular patch could achieve the resonant frequency of 2.5 GHz:
Fig. 5. Return loss of the aperture-coupled coplanar rectangular patch antenna from 2.2 to 2.8 GHz.
length of the coplanar rectangular patch: Lp = 12.0 mm, width of the coplanar rectangular patch: Wp = 16.0 mm. slot of the coplanar rectangular patch: Sp = 1.0 mm. The measured return loss of the aperture-coupled coplanar patch antenna fed with the CPW is shown in Fig. 5. The measured frequency range is from 2.2 to 2.8 GHz. The resonance point is near
Fig. 6. Input impedance of the aperture-coupled coplanar rectangular patch antenna from 2.2 to 2.8 GHz.
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Y.-C. Chen et al. / Journal of Alloys and Compounds 486 (2009) 410–414
the frequency of 2.5 GHz. The return loss of the aperture-coupled coplanar patch antenna is −20 dB at 2.5 GHz. As illustrated from the measurement results, the aperture-coupled coplanar patch antenna has a −10 dB S11 bandwidth of 3.33 %, which is larger than that of the antenna with a similar structure discussed in Ref. [12]. The corresponding Smith Chart representation of the S11 from 2.2 to 2.8 GHz is shown in Fig. 6. To match an antenna, the impedance locus should be shifted as close as possible to the center of the Smith Chart to obtain a low return loss at resonant frequency. As seen from the measurement results, the input impedance is 49.53 + j9.0 at the frequency of 2.5 GHz. A matching point is near the point of 2.5 GHz, which is close to the center of the Smith Chart. However, the reactance of input impedance is inductive, leading to an impact on the impedance matching and a decrease in return loss. 4. Conclusions Our study showed that the dielectric properties of Ca(1−x) Mgx La4 Ti5 O17 ceramics at microwave frequencies is influenced by the amounts of Mg substitution and sintering temperature. The diffraction peaks of Ca(1−x) Mgx La4 Ti5 O17 ceramics nearly unchanged with x increasing from 0 to 0.03. Similar X-ray diffraction peaks of Ca0.99 Mg0.01 La4 Ti5 O17 ceramic were observed at different sintering temperatures. A dielectric constant value of 56.3, a quality factor of 12,300 GHz (at 6.4 GHz) and a temperature coefficient of resonant frequency of −9.6 ppm/◦ C were obtained
for Ca0.99 Mg0.01 La4 Ti5 O17 ceramic sintered at 1500 ◦ C for 4 h. A successful design of an aperture-coupled coplanar patch antenna with a microstrip feed-line on high dielectric constant substrate has been presented. The return loss is −20 dB at 2.5 GHz, which corresponds to a −10 dB S11 bandwidth of 3.3%. Acknowledgment This work was supported by the National Science Council of the Republic of China under Grant NSC 97-2221-E-262-001. References [1] Y.C. Chen, C.W. Wang, K.H. Chen, Y.D. Huang, Y.C. Chen, Jpn. J. Appl. Phys. 47 (2008) 992–997. [2] Y.C. Chen, Y.W. Zeng, Microw. Opt. Technol. Lett. 51 (2009) 98–100. [3] C. Vineis, P.K. Davies, T. Negas, S. Bell, Mater. Res. Bull. 31 (1996) 431–437. [4] I.N. Jawahar, N.I. Santha, M.T. Sebastian, J. Mater. Res. 17 (2002) 3084–3089. [5] Y.C. Chen, Y.W. Zeng, Proc. 2007 ‘International Symposium on Nano Science and Technology’, Tainan, Taiwan, October, Southern Taiwan University of Technology, 2007, p. 173. [6] Y.C. Chen, J.M. Tsai, Jpn. J. Appl. Phys. 47 (2008) 7959–7962. [7] R.D. Shannon, Acta Crystallogr. A32 (1979) 751–767. [8] B.W. Hakki, P.D. Coleman, IEEE Trans. Microw. Theory Tech. 8 (1960) 402–410. [9] Y. Kobayashi, M. Katoh, IEEE Trans. Microw. Theory Tech. 33 (1985) 586–592. [10] M. Nanot, F. Queyroux, J.C. Gilles, J. Solid State Chem. 11 (1974) 272–283. [11] JCPDS Files: No. 25-1164, No. 27-1058, No. 27-1059, No. 28-0517, 1999. [12] B.D. Silverman, Phys. Rev. 125 (1991) 1921–1930. [13] W.S. Kim, T.H. Hong, E.S. Kim, K.H. Yoon, Jpn. J. Appl. Phys. 37 (1998) 3567–3571.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Investigation of the microwave dielectric properties of Ca1−x Mgx La4 Ti5 O17 ceramics for application in coplanar patch antenna Yih-Chien Chen ∗ , Shi-Li Yao, Ren-Jie Tsai, Kuei-Chien Chen Department of Electrical Engineering, Lunghwa University of Science and Technology, Gueishan Shiang, Taoyuan County, Taiwan
a r t i c l e
i n f o
Article history: Received 12 January 2009 Received in revised form 25 June 2009 Accepted 28 June 2009 Available online 5 July 2009 Keywords: Ca1−x Mgx La4 Ti5 O17 X-ray diffraction pattern Dielectric constant Quality factor Temperature coefficient of resonant frequency
a b s t r a c t In this paper, the dielectric properties of Ca1−x Mgx La4 Ti5 O17 ceramics at microwave frequency have been studied. The diffraction peaks of Ca(1−x) Mgx La4 Ti5 O17 ceramics nearly unchanged with x increasing from 0 to 0.03. Similar X-ray diffraction peaks of Ca0.99 Mg0.01 La4 Ti5 O17 ceramic were observed at different sintering temperatures. A maximum density of 5.3 g/cm3 can be obtained for Ca0.99 Mg0.01 La4 Ti5 O17 ceramic sintered at 1500 ◦ C for 4 h. A maximum dielectric constant (εr ) and quality factor (Q × f) of Ca0.99 Mg0.01 La4 Ti5 O17 ceramic sintered at 1500 ◦ C for 4 h are 56.3 and 12,300 GHz (at 6.4 GHz), respectively. A near-zero temperature coefficient of resonant frequency ( f ) of −9.6 ppm/◦ C can be obtained for Ca0.99 Mg0.01 La4 Ti5 O17 ceramic sintered at 1500 ◦ C for 4 h. The measurement results for the aperturecoupled coplanar patch antenna at 2.5 GHz are presented. With this technique, a 3.33% bandwidth (return loss <−10 dB) with a center frequency at approximately 2.5 GHz has been successfully achieved. © 2009 Elsevier B.V. All rights reserved.
1. Introduction There are many commercial applications, such as mobile radio and wireless communications that use patch antennas. However, patch antennas have limitations in size, gain, and efficiency, imposed by the dielectric substrate. Three dielectric properties of materials must be considered when patch antennas are used: a high dielectric constant, a high quality factor, and a near-zero temperature coefficient of resonant frequency. High dielectric constant and a near-zero temperature coefficient of resonant frequency are required for small size and high temperature stability, respectively. The quality factor is a representation of the antenna gain. Typically, there are radiations, conduction, dielectric, and surface wave losses, thus the total quality factor is affected by all of these losses [1,2]. The microwave dielectric properties of (Ba,La)n Tin−1 O3n (n = 5, 6) have already been reported. Both BaLa4 Ti4 O15 and Ba2 La4 Ti5 O18 are characterized by high dielectric constant (εr ∼ 39–46), high quality factor (Q × f ∼ 11,583–31,839) and small temperature coefficient of resonant frequency ( f ∼ −36 to 79 ppm/◦ C) [3]. On the other hand, MO–La2 O3 –TiO2 (M = Ca, Sr, Ba) is well known as microwave material for dielectric resonator and filter. Generally speaking, most of these ceramics combines a high dielectric constant (εr ∼ 42–54), high quality factor (Q × f ∼ 16,222–50,215) and an adjustable temperature coefficient of resonant frequency
∗ Corresponding author. Tel.: +886 2 8209 3211; fax: +886 2 8209 9728. E-mail address: [email protected] (Y.-C. Chen). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.189
( f ∼ −25 to 6 ppm/◦ C). Among the investigated orthorhombic phases, CaLa4 Ti5 O17 ceramic sintered at 1625 ◦ C give εr ∼ 53.7, Q × f ∼ 17,359 GHz, and f ∼ −20 ppm/◦ C [4]. Also, CuO additives can effectively lower the sintering temperature of CaLa4 Ti5 O17 . A dielectric constant value of 56.6, a quality factor of 8700 GHz, and a temperature coefficient of resonant frequency of −15 ppm/◦ C were obtained for CaLa4 Ti5 O17 ceramics with 0.5 wt% CuO sintered at 1500 ◦ C for 4 h [5]. By the partial replacement of Ca with Zn, the dielectric properties of CaLa4 Ti5 O17 ceramics at microwave frequency are modified. A dielectric constant of 57, a quality factor of 15,000 GHz, and a temperature coefficient of resonant frequency ( f ) of −8.16 ppm/◦ C were obtained for Ca0.99 Zn0.01 La4 Ti5 O17 ceramics with 0.5 wt% CuO additive sintered at 1450 ◦ C for 4 h [6]. Because of the ionic radii of Mg2+ are similar to that of Ca2+ [7], it has attracted our attention to investigate the effect of partial replacement of Ca with Mg for Ca(1−x) Mgx La4 Ti5 O17 ceramics. In this paper, investigations on Ca(1−x) Mgx La4 Ti5 O17 ceramics have been made to produce materials with better microwave dielectric properties for application in aperture-coupled coplanar patch antennas. The microwave dielectric properties, including a high dielectric constant, a high quality factor, and a near-zero temperature coefficient of resonant frequency have been considered in order to realize an outstanding design of the aperture-coupled coplanar patch antenna that have small size, high antenna gain, and high temperature stability. The microwave dielectric properties of Ca(1−x) Mgx La4 Ti5 O17 ceramics have been found to be different with different amounts of Mg substitution and sintering temperature.
Y.-C. Chen et al. / Journal of Alloys and Compounds 486 (2009) 410–414
Fig. 1. XRD patterns of Ca(1−x) Mgx La4 Ti5 O17 ceramics with sintered at 1500 ◦ C.
411
Fig. 2. XRD patterns of Ca0.99 Mg0.01 La4 Ti5 O17 ceramics with at different sintering temperatures.
2. Experimental procedure The starting raw chemicals were high-purity CaCO3 , MgO, La2 O3 , and TiO2 powders. The composition prepared was Ca(1−x) Mgx La4 Ti5 O17 , x = 0, 0.01 and 0.03. Ca(1−x) Mgx La4 Ti5 O17 was added 0.5 wt% CuO to lower the sintering temperature. Specimens were prepared using the conventional mixed-oxide method. The raw material was weighed out in stoichiometric proportions, ball-milled in alcohol, dried, and then calcined at 1200 ◦ C for 4 h. The obtained powders were then crushed into a fine powder of less than 200 mesh size. The fine powders obtained were then axially pressed into pellets at 2000 kg/cm2 with a diameter of 11 mm and thickness of 6 mm, prior to sintering. The specimens obtained were then sintered at 1400, 1450, 1500, and 1550 ◦ C for 4 h. After sintering, the phases of the samples were investigated by XRD. The bulk densities of the specimens were measured by the Archimedes method. The microwave dielectric properties of the specimens were measured by the postresonator method developed by Hakki and Coleman [8]. The postresonator method employs a specimen in the form of a cylinder of diameter D and length L. The specimens used for microwave dielectric property measurements had an aspect ratio D/L of about 1.6, which is in the permitted range reported by Kobayashi and Katoh [9]. The cylindrical resonator is sandwiched between two conducting plates. Two small antennas are positioned in the vicinity of the specimen to couple the microwave signal power in or out of the resonator. The other end of the antennas is connected to the Agilent N5230A network analyzer. The resonance characteristics are dependent
on the geometrical size and dielectric properties of the specimen. The microwave energy was coupled using E-field probes. The TE011 resonant mode was found to be most suitable for measuring the dielectric constant and loss factor of the specimen. Using the Agilent N5230A network analyzer, the TE011 resonant frequency of the dielectric resonator was identified, and the dielectric constant and quality factor were calculated. The technique for measuring f is the same as that of dielectric constant measurement. The test cavity was placed in a chamber in the temperature range from 25 to 75 ◦ C. The f value (ppm/◦ C) can be determined by noting the change in resonant frequency, f =
f2 − f1 × 106 f1 (T2 − T1 )
(1)
where f1 and f2 represent the resonant frequencies at T1 and T2 , respectively. The aperture-coupled coplanar patch antenna designed consists of a highdielectric-constant Ca0.99 Mg0.01 La4 Ti5 O17 ceramic substrate with a coplanar rectangular patch, and a feed substrate with a microstrip feed-line and rectangular aperture on back-side and top-side, respectively. The microstrip feed-line and rectangular aperture were fabricated by wet etching. The coplanar rectangular patch was fabricated using thick-film technology. The conductor material used was silver, and was in the form of a paste. The mesh was designed in accordance with the pattern to be fabricated. The past was passed through the designed mesh in the screen-printing
Fig. 3. Microstructures of the Ca0.99 Mg0.01 La4 Ti5 O17 ceramics with 0.5 wt% CuO additives sintered for 4 h at different sintering temperatures.
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Table 1 The microwave dielectric properties of Ca(1−x) Mgx La4 Ti5 O17 . Material
S.T.
Density (g/cm3 )
εr
Q × f (GHz)
f (ppm/◦ C)
CaLa4 Ti5 O17 CaLa4 Ti5 O17 Ca0.99 Mg0.01 La4 Ti5 O17 Ca0.99 Mg0.01 La4 Ti5 O17 Ca0.99 Mg0.01 La4 Ti5 O17 Ca0.99 Mg0.01 La4 Ti5 O17 Ca0.97 Mg0.03 La4 Ti5 O17 Ca0.97 Mg0.03 La4 Ti5 O17 Ca0.97 Mg0.03 La4 Ti5 O17 Ca0.97 Mg0.03 La4 Ti5 O17
1450 1500 1400 1450 1500 1550 1400 1450 1500 1550
5.321 5.319 4.663 5.196 5.318 5.247 4.790 5.295 5.262 5.242
57 56.6 47.0 53.8 56.3 56.0 48.5 56.7 55.9 55.3
9,000 8,700 6,600 8,200 12,300 8,800 9,900 10,800 8,600 6,100
−10 −15 −19 −8 −10 −13 −6 −11 −16 −10
process. After screen printing, conductors were dried at approximately 150 ◦ C for 30 min. The reflection coefficients and input impedances were measured using the Agilent network analyzer (N5230A).
3. Results and discussion Fig. 1 showed the X-ray diffraction patterns of CaLa4 Ti5 O17 , Ca0.99 Mg0.01 La4 Ti5 O17 , and Ca0.97 Mg0.03 La4 Ti5 O17 ceramics in the range from 20◦ to 60◦ sintered at the temperatures of 1500 ◦ C for 4 h. As we can see from the results, the diffraction peaks of Ca(1−x) Mgx La4 Ti5 O17 ceramics nearly unchanged with x increasing from 0 to 0.03. The crystal structure of CaLa4 Ti5 O17 is known to have orthorhombic structure belongs to Pnnm space group. The lattice parameters of CaLa4 Ti5 O17 are a0 = 0.5522 nm, b0 = 3.1256 nm, and c0 = 0.3899 nm [10]. The X-ray diffraction patterns of the Ca0.99 Mg0.01 La4 Ti5 O17 specimens after sintering at 1450, 1500, and 1550 ◦ C for 4 h are shown in Fig. 2. Similar spectral angles of X-ray diffraction peaks were observed at different sintering temperatures. The microstructures of Ca0.99 Mg0.01 La4 Ti5 O17 with 0.5 wt% CuO additive sintered in the range of 1400–1550 ◦ C for 4 h are shown in Fig. 3. The Ca0.99 Mg0.01 La4 Ti5 O17 with 0.5 wt% CuO additives was not dense, and the grains did not grow after it was sintered at 1400 ◦ C. Comparing the microstructures of Ca0.99 Mg0.01 La4 Ti5 O17 with 0.5 wt% CuO additives sintered at different temperatures, the number of pores decreased and the rate of grain growth increased apparently. The pores almost disappeared in the specimen sintered at 1500 ◦ C. The apparent densities of Ca(1−x) Mgx La4 Ti5 O17 ceramics with different amounts of Mg substitution sintered at 1400, 1450, 1500, and 1550 ◦ C for 4 h are shown in Table 1. The bulk density was found to increase to a maximum value at sintering temperature of 1500 ◦ C and thereafter decreased for Ca0.99 Mg0.01 La4 Ti5 O17 . The bulk density increased with increasing sintering temperature, but this may be due to the decrease in the number of pores. The bulk density was found to decrease as the sintering temperature increases from 1450 to 1550 ◦ C for Ca0.97 Mg0.03 La4 Ti5 O17 . The theoretical density of CaLa4 Ti5 O17 is 5.46 g/cm3 [11] while a maximum density of 5.3 g/cm3 was obtained for Ca0.99 Mg0.01 La4 Ti5 O17 ceramics sintered at 1500 ◦ C for 4 h. The dielectric constants of Ca(1−x) Mgx La4 Ti5 O17 ceramics with different amounts of Mg substitution sintered at different temperatures for 4 h can be found in Table 1. The maximum value of dielectric constant in the ceramic of Ca0.97 Mg0.03 La4 Ti5 O17 is 56.7 sintered at 1450 ◦ C for 4 h. The dielectric constant was found to increase to a maximum value at sintering temperature of 1500 ◦ C and thereafter decreased for Ca0.99 Mg0.01 La4 Ti5 O17 . The value of dielectric constant is 56.3 in the ceramic of Ca0.99 Mg0.01 La4 Ti5 O17 that was sintered at 1500 ◦ C for 4 h. The dielectric constant decreased from 56.3 to 55.9 as the amounts of Mg substitution increased from 0.01 to 0.03 molar when the ceramics were sintered at 1500 ◦ C. We also found increasing sintering temperature is
not necessary for getting a higher dielectric constant. The decrease in dielectric constant was due to low densities and since higher density means lower porosity, a higher dielectric constant can be achieved. The quality factor (Q × f) of Ca(1−x) Mgx La4 Ti5 O17 ceramics with different amounts of Mg substitution sintered at different temperatures for 4 h are illustrated in Table 1. With increasing the sintering temperature, the quality factor increased to a maximum value at sintering temperature of 1500 ◦ C and thereafter decreased for Ca0.99 Mg0.01 La4 Ti5 O17 . A maximum quality factor of 12,300 GHz (at 6.4 GHz) was obtained for Ca0.99 Mg0.01 La4 Ti5 O17 sintered at 1500 ◦ C for 4 h. Compared with the measurement results of quality factor as shown in Table 1, the value of quality factor was increased from 8700 to 12,300 GHz as the amounts of Mg substitution increased from 0 to 0.01 molar sintered at 1500 ◦ C. The quality factor decreased from 12,300 to 8600 GHz as the amount of Mg substitutions increased from 0.01 to 0.03 sintered at 1500 ◦ C. The relationship between the quality factor and the sintering temperature revealed the same trend of that between the apparent density and the sintering temperature. This is caused by the microwave dielectric loss, which is affected by many factors that can be divided into intrinsic and extrinsic losses. The intrinsic loss is caused by the lattice vibrational modes. The extrinsic loss is induced by the density, porosity, the second phases, the impurities, the oxygen vacancies, the grain size, and the lattice defects [12,13]. Because the quality factor of Ca(1−x) Mgx La4 Ti5 O17 ceramics was consistent with the variation of the apparent density, it is suggested that the quality factor of Ca(1−x) Mgx La4 Ti5 O17 ceramics is mainly controlled by the apparent density. The temperature coefficient of resonant frequency ( f ) values of Ca(1−x) Mgx La4 Ti5 O17 ceramics with different amounts of Mg substitution sintered at different temperatures for 4 h can be observed in Table 1. In general, the temperature coefficient of resonant frequency is related to the composition, the additives, and the second phases that existed in the ceramics. The temperature coefficient of resonant frequency was insensitive to the sintering temperature in the entire sintering temperature range. A near-zero temperature coefficient of resonant frequency of −9.6 ppm/◦ C was measured for Ca0.99 Mg0.01 La4 Ti5 O17 ceramic sintered at 1500 ◦ C for 4 h. The geometry of the aperture-coupled coplanar patch antenna fed with a microstrip feed-line is shown in Fig. 4. The energy resulting from the fringing fields of microstip feed-line is used to excite the coplanar rectangular patch. The coplanar rectangular patch is located centrally on the FR4 substrate. The microstrip feed-line was printed on a FR4 substrate. The rectangular aperture was wet-etched centrally on the FR4 substrate. The parameters of the FR4 substrate are εr1 = 4.4, tan ı = 0.014, thickness of the substrate h1 = 1.6 mm, and size of the substrate = 50 mm × 50 mm. The dimensions of the rectangular aperture are 12.6 mm × 13.6 mm. The parameters of the Ca0.99 Mg0.01 La4 Ti5 O17 substrate are εr2 = 56.3, tan ı = 0.0002, thickness of the substrate h2 = 5.5 mm, and diameter of the substrate = 27 mm. The dimensions of the microstrip
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Fig. 4. Configuration of aperture-coupled coplanar rectangular patch antenna. (a) Microstrip feed-line and rectangular aperture fabricated on FR4 substrate, (b) coplanar rectangular patch printed on high-dielectric-constant Ca0.99 Mg0.01 La4 Ti5 O17 ceramic substrate, and (c) top and side views of the proposed antenna.
feed-line were calculated using closed-form formulas given in ref. 10, assuming an infinite ground plane and a finite dielectric thickness. The microstrip feed-line dimensions were confirmed by AWR Microwave Office. And, the length and width of the microstrip feedline are 36 and 5 mm, respectively. The design process started with the design of a coplanar rectangular patch with a resonant frequency of 2.5 GHz, by choosing the length of the non-radiating edges to be g /2 at 2.5 GHz, where g is the guided wavelength [11]. The coplanar rectangular patch was then fed with a microstrip feed-line. The resonance frequencies could be adjusted by modifying the length of the coplanar rectangular patch and impedance matching could be realized by adjusting the width of the coplanar rectangular patch. Therefore, the bandwidth could also be adjusted by modifying the width of the coplanar rectangular patch. The following set of dimensions of the coplanar rectangular patch could achieve the resonant frequency of 2.5 GHz:
Fig. 5. Return loss of the aperture-coupled coplanar rectangular patch antenna from 2.2 to 2.8 GHz.
length of the coplanar rectangular patch: Lp = 12.0 mm, width of the coplanar rectangular patch: Wp = 16.0 mm. slot of the coplanar rectangular patch: Sp = 1.0 mm. The measured return loss of the aperture-coupled coplanar patch antenna fed with the CPW is shown in Fig. 5. The measured frequency range is from 2.2 to 2.8 GHz. The resonance point is near
Fig. 6. Input impedance of the aperture-coupled coplanar rectangular patch antenna from 2.2 to 2.8 GHz.
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the frequency of 2.5 GHz. The return loss of the aperture-coupled coplanar patch antenna is −20 dB at 2.5 GHz. As illustrated from the measurement results, the aperture-coupled coplanar patch antenna has a −10 dB S11 bandwidth of 3.33 %, which is larger than that of the antenna with a similar structure discussed in Ref. [12]. The corresponding Smith Chart representation of the S11 from 2.2 to 2.8 GHz is shown in Fig. 6. To match an antenna, the impedance locus should be shifted as close as possible to the center of the Smith Chart to obtain a low return loss at resonant frequency. As seen from the measurement results, the input impedance is 49.53 + j9.0 at the frequency of 2.5 GHz. A matching point is near the point of 2.5 GHz, which is close to the center of the Smith Chart. However, the reactance of input impedance is inductive, leading to an impact on the impedance matching and a decrease in return loss. 4. Conclusions Our study showed that the dielectric properties of Ca(1−x) Mgx La4 Ti5 O17 ceramics at microwave frequencies is influenced by the amounts of Mg substitution and sintering temperature. The diffraction peaks of Ca(1−x) Mgx La4 Ti5 O17 ceramics nearly unchanged with x increasing from 0 to 0.03. Similar X-ray diffraction peaks of Ca0.99 Mg0.01 La4 Ti5 O17 ceramic were observed at different sintering temperatures. A dielectric constant value of 56.3, a quality factor of 12,300 GHz (at 6.4 GHz) and a temperature coefficient of resonant frequency of −9.6 ppm/◦ C were obtained
for Ca0.99 Mg0.01 La4 Ti5 O17 ceramic sintered at 1500 ◦ C for 4 h. A successful design of an aperture-coupled coplanar patch antenna with a microstrip feed-line on high dielectric constant substrate has been presented. The return loss is −20 dB at 2.5 GHz, which corresponds to a −10 dB S11 bandwidth of 3.3%. Acknowledgment This work was supported by the National Science Council of the Republic of China under Grant NSC 97-2221-E-262-001. References [1] Y.C. Chen, C.W. Wang, K.H. Chen, Y.D. Huang, Y.C. Chen, Jpn. J. Appl. Phys. 47 (2008) 992–997. [2] Y.C. Chen, Y.W. Zeng, Microw. Opt. Technol. Lett. 51 (2009) 98–100. [3] C. Vineis, P.K. Davies, T. Negas, S. Bell, Mater. Res. Bull. 31 (1996) 431–437. [4] I.N. Jawahar, N.I. Santha, M.T. Sebastian, J. Mater. Res. 17 (2002) 3084–3089. [5] Y.C. Chen, Y.W. Zeng, Proc. 2007 ‘International Symposium on Nano Science and Technology’, Tainan, Taiwan, October, Southern Taiwan University of Technology, 2007, p. 173. [6] Y.C. Chen, J.M. Tsai, Jpn. J. Appl. Phys. 47 (2008) 7959–7962. [7] R.D. Shannon, Acta Crystallogr. A32 (1979) 751–767. [8] B.W. Hakki, P.D. Coleman, IEEE Trans. Microw. Theory Tech. 8 (1960) 402–410. [9] Y. Kobayashi, M. Katoh, IEEE Trans. Microw. Theory Tech. 33 (1985) 586–592. [10] M. Nanot, F. Queyroux, J.C. Gilles, J. Solid State Chem. 11 (1974) 272–283. [11] JCPDS Files: No. 25-1164, No. 27-1058, No. 27-1059, No. 28-0517, 1999. [12] B.D. Silverman, Phys. Rev. 125 (1991) 1921–1930. [13] W.S. Kim, T.H. Hong, E.S. Kim, K.H. Yoon, Jpn. J. Appl. Phys. 37 (1998) 3567–3571.