- Email: [email protected]
Microwave dielectric properties of ZnO–B2O3–SiO2-doped Zn2SnO4 ceramics for application in triple bands inverted-U shaped monopole antenna
Journal of Alloys and Compounds 616 (2014) 356–362
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microwave dielectric properties of ZnO–B2O3–SiO2-doped Zn2SnO4 ceramics for application in triple bands inverted-U shaped monopole antenna Yih-Chien Chen ⇑, Hong-Mine You 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 27 May 2014 Received in revised form 5 July 2014 Accepted 7 July 2014 Available online 23 July 2014 Keywords: Zn2SnO4 Dielectric constant Quality factor Temperature coefficient of resonant frequency Monopole antenna
a b s t r a c t In this study, the microwave dielectric properties of Zn2SnO4 ceramics prepared by the conventional solid-state method were investigated for application in mobile communication. ZnO–B2O3–SiO2 was selected as a liquid sintering aid to reduce the sintering temperature of Zn2SnO4 ceramics. By adding 3.0 wt.% ZnO–B2O3–SiO2, a dielectric constant of 11.44 and a quality factor (Q f) of 33,000 GHz were obtained when Zn2SnO4 ceramics were sintered at 1075 °C for 4 h. The proposed CPW-fed inverted-U shaped monopole antenna has a 6 dB return loss with bandwidth 204 MHz (2371–2575 MHz) in the lower band, 1520 MHz (3130–4650 MHz) in the middle band, and 460 MHz (5250–5710 MHz) in the higher band. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The demand for novel microwave ceramics applications have been increasing rapidly in the mobile communication industries during the last decade. Recent studies have focused on developing good microwave dielectric materials to achieve device miniaturization and system stability. Many researches on the Mg2SnO4 ceramics have been investigated for resonators, filters, and antennas in the modern communication systems, such as radar and global positioning systems (GPS) operating at microwave frequencies. Mg2SnO4 ceramics sintered at 1550 °C for 4 h combines a dielectric constant of 8.41 and a quality factor (Q f) of 55,100 GHz [1]. The maximum values of dielectric constant and quality factor (Q f) for (Mg0.93Co0.07)2SnO4 ceramics sintered at 1550 °C for 4 h are 8.8 and 110,800 GHz, respectively [2]. A dielectric constant of 8.5 and a quality factor (Q f) of 186,100 GHz were obtained for (Mg0.93Zn0.07)2SnO4 ceramics sintered at 1550 °C for 4 h [3]. Additionally, a dielectric constant of 8.72 and a quality factor (Q f) of 103,100 GHz were obtained for (Mg0.95Ni0.05)2SnO4 ceramics sintered at 1550 °C for 4 h [4]. Since the ionic radius of Zn2+ (0.074 nm) is similar to that of Mg2+ (0.072 nm), it has certainly attracted our attention to
⇑ Corresponding author. Tel.: +886 2 8209 3211x5532; fax: +886 2 8209 9721. E-mail address: [email protected] (Y.-C. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.07.049 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
investigate the effect of substituting Mg2+ by Zn2+ to form a new Zn2SnO4 ceramics [5]. Moreover, the effects of the amounts of ZnO–B2O3–SiO2 aid and sintering temperature on the microwave dielectric properties of Zn2SnO4 ceramics have been studied. The microwave dielectric properties of the Zn2SnO4 ceramics have been found to vary with various amounts of ZnO–B2O3–SiO2 aid and sintering temperatures. For further understanding of these different microwave dielectric properties, they were analyzed on the basis of density, X-ray diffraction (XRD) patterns, and observation of the microstructures. Many commercial applications, including mobile radio and wireless communications, use monopole antenna. The monopole antenna is used extensively because it is reasonably compact, good efficiency, and is very simple [6,7]. Recently, monopole antennas for application in WLAN (wireless local area network, 2.4– 2.484 GHz) are implemented. Simultaneously, associating with the rapid development of WiMAX (worldwide interoperability for microwave access, 3.4–3.69, and 5.25–5.85 GHz), there is an increasing demand for antennas suitable for WLAN/WiMAX simultaneously. Monopole antenna has limitations in size, bandwidth, and efficiency, imposed by the dielectric substrate. Three properties of dielectric substrate must be considered for monopole used: a high dielectric constant, a high quality factor, and a near-zero temperature coefficient of resonant frequency. A high dielectric constant and a near-zero temperature coefficient of resonant frequency are required for small size and high-temperature stability,
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
357
respectively [8]. The quality factor is representative of the antenna losses. There are losses of radiation, conduction, dielectric, and surface wave. Therefore, the total quality factor is affected by all of these losses [9]. Since high dielectric constant substrate can be used as dielectric substrate to achieve compact size and low-profile monopole antenna, the development of antennas fabricated on high dielectric constant substrate for commercial mobile communication has been rapidly growing recently [10]. Combination the advantages of CPW feed line and monopole antenna, a CPW-fed inverted-U shaped monopole fabricated on high dielectric constant substrate operated in WLAN and WiMAX bands simultaneously, is proposed. The proposed antenna is simple in manufacturing because of single dielectric substrate, single metal layer, and without via holes. The proposed antenna is capable of operating in the WLAN and WiMAX bands. The design considerations and experimental results for the proposed antenna are presented and discussed. 2. Experimental procedure The starting raw chemicals were ZnO (99.0%) and SnO2 (99.9%) powders. The composition prepared was Zn2SnO4. Zn2SnO4 was then added 2, 3, and 4 wt.% ZnO–B2O3–SiO2. Specimens were prepared using the conventional mixed-oxide method. The raw material was weighed out in stoichiometric proportions, ballmilled in alcohol for 12 h, dried, and then calcined at 1100 °C for 4 h. The obtained powders were then crushed into a finer powder using a sieve with a 200 mesh. Calcined powders with different amounts of ZnO–B2O3–SiO2 aid were re-milled for 12 h with PVA solution as a binder. The fine powders obtained were then axially pressed at 2000 kg/cm2 into pellets with a diameter of 11 mm and a thickness of 6 mm, prior to sintering. The specimens obtained were then sintered in air for 4 h with different sintering temperature. Both the heating rate and cooling rate were set at 10 °C/min. After sintering, the phases of the samples were investigated by X-ray diffraction. The X-ray Rigaku D/MAX-2200 data were collected using Cu Ka radiation (at 30 KV and 20 mA) and a graphite monochromator in the 2h range of 10–80°. Scanning electron microscopy (SEM; JEOL JSM-6500F) and energy dispersive X-ray spectrometer (EDS) were employed to examine the microstructures of the specimens. The apparent densities of the specimens were measured by the Archimedes method using distilled water as the liquid and the microwave dielectric properties of the specimens were measured by the postresonator method developed by Hakki and Coleman [11]. 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 [12]. 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 into or out of the resonator. The other end of the antennas is connected to an Agilent N5230A network analyzer. The resonance characteristics are dependent on the size and dielectric properties of the specimen. The microwave energy was coupled using electric-field probes. The TE011 resonant mode was found to be optimal for obtaining the dielectric constant and the 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 sf is the same as that for dielectric constant measurement. The test cavity was placed in a chamber and the temperature was varied from 25 to 75 °C. The sf value (ppm/ °C) can be determined by noting the change in resonant frequency,
sf ¼
f2 f1 ; f1 ðT 2 T 1 Þ
ð1Þ
where f1 and f2 represent the resonant frequencies at T 1 and T 2 , respectively.
3. Results and discussion The X-ray diffraction patterns of Zn2SnO4 ceramics with 0, 2, 3, and 4 wt.% ZnO–B2O3–SiO2 aid under various sintering temperatures for 4 h are shown in Fig. 1. It is clear that Zn2SnO4 is the main crystalline phase. Zn2SnO4 accompanied by second phases of Zn2SiO2 and SnO2 when Zn2SnO4 ceramics added with ZnO– B2O3–SiO2 aid. The X-ray diffraction patterns of the Zn2SnO4 ceramics have no significant difference at different sintering temperature with same amount of ZnO–B2O3–SiO2 aid. The crystal structure of Zn2SnO4 has inverse-spinel structure belongs to Fd-3m space group. All the peaks were indexed based on the cubic
Fig. 1. The X-ray diffraction patterns of Zn2SnO4 specimens with (a) 0 wt.%, (b) 2 wt.%, (c) 3 wt.%, and (d) 4 wt.% ZnO–B2O3–SiO2 additives at different sintering temperatures.
unit cell. Zn2SiO2 with a rhombohedral crystal structure belongs to R-3 space group was identified. Formation of Zn2SiO4 was attributed to the partially substituted Sn4+ ions with the Si4+ ions, resulting in the Zn2SiO4 phase. Since the partially replacing Sn4+ with the Si4+, some of SnO2 phase have formed. The intensity demonstrated the difference in amounts of Zn2SnO4, Zn2SiO4, and SnO2 more clearly. The relative intensities of Zn2SnO4, Zn2SiO4, and SnO2 were
358
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
Table 1 Comparison the main phase (%) and second phases (%) of Zn2SnO4 with different amounts of ZnO–B2O3–SiO2 aid at optimum sintering temperature. ZnO–B2O3–SiO2 (wt.%)
S.T. (°C)
Zn2SnO4
Zn2SiO4
SnO2
2 3 4
1175 1075 1025
95.14 91.44 90.34
1.33 2.83 2.25
3.53 5.73 7.41
evaluated from most intensive line of each phase. The relative intensities of Zn2SnO4, Zn2SiO4, and SnO2 were calculated as follows:
Relative intensity of Zn2 SnO4 ¼
IZn2 SnO4 ð311Þ IZn2 SnO4 ð311Þ þ IZn2 SiO4 ð410Þ þ ISnO2 ð110Þ 100; ð2Þ
Relative intensity of Zn2 SiO4 ¼
IZn2 SiO4 ð410Þ IZn2 SnO4 ð311Þ þ IZn2 SiO4 ð410Þ þ ISnO2 ð110Þ 100; ð3Þ
Relative intensity of SnO2 ¼
ISnO2 ð110Þ IZn2 SnO4 ð311Þ þ IZn2 SiO4 ð410Þ þ ISnO2 ð110Þ 100; ð4Þ
Table 1 shows the amounts of main and second phases of the specimens sintered at optimum sintering temperatures for 4 h. The amount of the Zn2SnO4 decreased from 95.14% to 90.34% as the amount of ZnO–B2O3–SiO2 aid increased from 2 to 4 wt.%. The amount of the Zn2SiO4 ranged from 1.33% to 2.83% as the amount of ZnO–B2O3–SiO2 aid varied from 2 to 4 wt.%. The amount of the SnO2 increased from 3.53% to 7.41% as the amount of ZnO– B2O3–SiO2 aid increased from 2 to 4 wt.%. The formation of Zn2SiO4 and SnO2 affected the apparent density and microwave dielectric properties of Zn2SnO4 ceramics. The microstructures of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid and at different sintering temperatures for 4 h are shown in Fig. 2. Comparing the microstructures of Zn2SnO4 ceramics added with the 2 and 3 wt.% ZnO–B2O3–SiO2 aid sintered at optimum sintering temperature, the grain size did not vary with the amount of ZnO–B2O3–SiO2 obviously. The average
Fig. 2. The microstructures of the Zn2SnO4 (a) 2 wt.% ZnO–B2O3–SiO2/1150 °C, (b) 2 wt.% ZnO–B2O3–SiO2/1175 °C, (c) 2 wt.% ZnO–B2O3–SiO2/1200 °C, (d) 2 wt.% ZnO–B2O3– SiO2/1225 °C, (e) 3 wt.% ZnO–B2O3–SiO2/1075 °C and (f) 4 wt.% ZnO–B2O3–SiO2/1025 °C.
359
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
resulting in a decrease of relative density. At optimum sintering temperatures, the relative density remained stable as the amount of ZnO–B2O3–SiO2 aid increased from 0 to 3 wt.%, while decreased dramatically from 95.33% to 94.44% as the amount of ZnO–B2O3– SiO2 aid increased from 3 to 4 wt.%. Fig. 4 shows the dielectric constants of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid sintered at different temperatures for 4 h. The relationship between the dielectric constant and the sintering temperature revealed the same trend as that between the relative density and the sintering temperature. At optimum sintering temperatures, the dielectric constant increased from 10.2 to 11.4 as the amount of ZnO–B2O3–SiO2 aid increased from 0 to 3 wt.%, while decreased from 11.4 to 10.9 as the amount 12 11
Dielectric Constnat
grain size decreased dramatically as the amount of ZnO–B2O3–SiO2 aid increased from 3 to 4 wt.%. The significant decreasing of average grain size may affect the microwave dielectric properties of Zn2SnO4 ceramics. The inhibition of grain growth at high amount of ZnO–B2O3–SiO2 aid derived from impurities in the grain boundaries. In order to indentify the composition of the impurities in the grain boundaries, an EDS analysis was employed on the grain boundaries of Zn2SnO4 ceramics with 4 wt.% ZnO–B2O3–SiO2 sintered at 1025 °C for 4 h as shown in Fig. 2(f). According to the quantitative analysis as shown in Table 2, the compositions are sintering aid, ZnO–B2O3–SiO2. In order to indentify the composition of the second phase, an energy-disperse spectroscopy (EDS) analysis was employed on the grains of Zn2SnO4 ceramics with 2 wt.% ZnO–B2O3–SiO2 sintered at 1175 °C for 4 h as shown in Fig. 2(b). According to the quantitative analysis as shown in Table 3, it is evident that the grains of A and B are Zn2SnO4. The rod-like grains C and D are Zn2SiO4 and SnO2, respectively. The mixture phases observed in the microstructure supported the secondary phases detected in the X-ray diffraction patterns as shown in Fig. 1. The relative densities of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid after sintering from 1000 to 1250 °C for 4 h are shown in Fig. 3. The relative density was found to increase to a maximum value at an optimum sintering temperature and then decrease gradually. The relative density increased with increasing sintering temperature and this may be due to the decreasing number of pores as shown in Fig. 2. However, a high sintering temperature could also induce an abnormal grain growth,
10 9 8
0 wt% 2 wt% 3 wt% 4 wt%
7 6 975
1025
1075
1125
1175
1225
1275
o
Table 2 EDS data of grains of Zn2SnO4 ceramic with 4 wt.% ZnO–B2O3–SiO2 aid sintered at 1025 °C for 4 h. Zn (%)
B (%)
Si (%)
O (%)
33.47
18.96
1.42
46.12
Table 3 EDS data of grains of Zn2SnO4 ceramic with 2 wt.% ZnO–B2O3–SiO2 aid sintered at 1175 °C for 4 h. Atomic element
Zn (%)
Sn (%)
Si (%)
O (%)
A B C D
24.42 25.27 30.87 0
11.61 11.11 0 27.44
0 0 16.89 0
63.98 63.62 52.25 72.56
Sintering Temperature ( C) Fig. 4. Dielectric constants of Zn2SnO4 ceramics with different amounts of ZnO– B2O3–SiO2 aid sintered in the range of 1000–1250 °C for 4 h.
Table 4 The porosity, overall dielectric constant of second phases, and internal strain of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid at optimum sintering temperature. (*: pure Zn2SnO4). ZnO–B2O3–SiO2 (wt.%)
S.T. (°C)
Porosity (%)
er;ov erall
Internal strain
0 2 3 4
1225 1175 1075 1025
0.77 0.46 0.45 0.77
* 10.90 10.46 10.98
0.00130 0.00315 0.00425 0.20795
40000 96
Quality Factor (GHz)
Relative Density (%)
95.5 95 94.5 94 0 wt% 2 wt% 3 wt% 4 wt%
93.5 93 92.5 975
1025
1075
1125
1175
1225
30000
0 wt% 2 wt% 3 wt% 4 wt%
20000
10000
1275
o
Sintering Temperature ( C) Fig. 3. Relative densities of Zn2SnO4 ceramics with different amounts of ZnO–B2O3– SiO2 aid sintered in the range of 1000–1250 °C for 4 h.
0 975
1025
1075
1125
1175
1225
1275
o
Sintering Temperature ( C) Fig. 5. Quality factors (Q f) of Zn2SnO4 ceramics with different amounts of ZnO– B2O3–SiO2 aid sintered in the range of 1000–1250 °C for 4 h.
360
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
er ¼ 0 wt% 2 wt% 3 wt% 4 wt%
-20
o
Frequency (ppm/ C)
Temperature Coefficient of Resonan t
0
-40
3V m þ 8paD 3V m 4paD
ð5Þ
where Vm is the molar volume and aD is the sum of ionic polarizabilities of individual ions. The dielectric constant of the composites can be calculated by the mixture rule:
ln er ¼ v 1 ln er1 þ v 2 ln er2 þ v 3 ln er3 ;
ð6Þ
-60
-80
-100
-120 975
1025
1075
1125
1175
1225
1275
o
Sintering Temperature ( C) Fig. 6. sf Values of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid sintered in the range of 1000–1250 °C for 4 h.
of ZnO–B2O3–SiO2 aid increased from 3 to 4 wt.%. A maximum dielectric constant value of 11.4 was obtained for Zn2SnO4 ceramics with 3 wt.% ZnO–B2O3–SiO2 aid that were sintered at 1075 °C for 4 h. Table 4 shows the porosities of the specimens with different amounts of ZnO–B2O3–SiO2 aid that were sintered at optimum sintering temperatures for 4 h. Of the Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid, Zn2SnO4 ceramic with 3 wt.% ZnO–B2O3–SiO2 aid that was sintered at 1075 °C for 4 h had the lowest porosity of 0.45%. A lower the fraction porosity is, therefore, a higher dielectric constant. The second phase is another extrinsic factor in controlling the dielectric constant. The dielectric constant of ZnSiO4 is 6 [13]. The calculated dielectric constant of SnO2 is 13.1, using the Clausius-Mossotti equation, as suggested by Tohdo et al. [14]:
where er is the dielectric constant of the composite, v1, v2, and v3 are the volume fractions of Zn2SnO4, Zn2SiO4, and SnO2, respectively, and er1 , er2 , and er3 are the dielectric constants of Zn2SnO4, Zn2SiO4, and SnO2, respectively. Table 4 shows the overall dielectric constants (er;ov erall ) of Zn2SiO4 and SnO2. The overall dielectric constant spanned from 10.46 to 10.98 with various amounts of ZnO–B2O3– SiO2 aid at optimum sintering temperatures. Since the overall dielectric constants of Zn2SiO4 and SnO2 are close to the measured dielectric constant, which the influence of second phases on the dielectric constants of Zn2SnO4 ceramics, was small.The Q f of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid sintered at different temperatures for 4 h are shown in Fig. 5. The Q f decreased from 39,000 to 2,900 GHz as the amount of ZnO–B2O3–SiO2 aid increased from 0 to 4 wt.% for samples at optimum sintering temperature. A maximum Q f of 39,000 GHz was obtained for Zn2SnO4 ceramics that were sintered at 1225 °C for 4 h. The relationship between the Q f and the sintering temperature revealed the same trend as that between the relative density and the sintering temperature. This is caused by the microwave dielectric loss, which is affected by many factors that can be divided into the intrinsic and extrinsic loss. 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 [15,16]. Since the Q f of Zn2SnO4 ceramics was consistent with the variation of the relative density, it is suggested that the Q f of Zn2SnO4 ceramics is mainly controlled by the relative density. A significant decreased
Fig. 7. Configuration of the proposed CPW-fed inverted-U shaped monopole antenna on high permittivity ceramic substrate.
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
Q f at higher amounts of ZnO–B2O3–SiO2 aid could be related to the high internal strain. The internal strain g can be calculated using following equation, as suggested by Ohsato and Imaeda [17].
b ¼ 2g tan h;
ð7Þ
where b is the full width at half maximum (FWHM) of an X-ray diffraction peak and h is the corresponding diffraction angles. Eq. (7) was used for curve fitting to determine the internal strain g of the Zn2SnO4 ceramics with different degrees of ZnO–B2O3–SiO2 aid. The curve fitting was based on the least-squares method. In this method, the square of the deviation of the experimental value from the theoretical expectation was calculated while the fitted internal strain was varied. The fitting process was terminated at the fitted internal strain that minimized the deviation. As shown in Table 4, Zn2SnO4 ceramic with 4 wt.% ZnO–B2O3–SiO2 aid had the largest internal strain, 0.20795, of any of the Zn2SnO4 ceramics with various amounts of ZnO–B2O3–SiO2 aid. The grain size is a factor affecting the internal stain. Since the average grain size decreased significantly with the extent of ZnO–B2O3–SiO2 aid increased from 3 wt.% to 4 wt.%, the internal strain increased significantly. Accordingly, Q f decreased dramatically as the amounts of ZnO–B2O3– SiO2 additive increased from 3 wt.% to 4 wt.%. The sf of Zn2SnO4 ceramics with different amounts of ZnO– B2O3–SiO2 aid sintered at different temperatures for 4 h are shown in Fig. 6. In general, sf is related to the composition, the amount of additive, and the second phases that existed in the ceramics. No significant variation in sf was observed with a fixed amount of ZnO–B2O3–SiO2 aid at different sintering temperature in the entire sintering temperature range. Since there was no compositional variation in Zn2SnO4 ceramics with a fixed amount of ZnO–B2O3– SiO2 aid with different sintering temperature, sf was measured as a function of the amounts of ZnO–B2O3–SiO2 aid in this experiment. The sf of Zn2SiO4 ceramics is less negative compared with that of Zn2SnO4 ceramics, implying the presence of the Zn2SiO4 shifted the sf of the specimen to the positive direction. The amount of the Zn2SiO4 increased from 1.33% to 2.83% as the amount of ZnO–B2O3–SiO2 aid increased from 2 to 3 wt.%, the sf of specimen is inferred to be shifted to positive direction. However, the average value of sf decreased from 72.6 to 107.3 ppm/°C when the amount of ZnO–B2O3–SiO2 aid increased from 2 to 3 wt.%, the sf of SnO2 is inferred to be negative compared with that of Zn2SiO4, which is in agreement with Ref. [18]. The average sf increased from 107.3 to 8.1 ppm/°C as the amount of ZnO–B2O3–SiO2 aid increased significantly from 3 to 4 wt.%. This is associated with the formation of the grain boundary phase, ZnO–B2O3–SiO2. The geometry and parameters of CPW-fed inverted-U shaped monopole antenna are shown in Fig. 7. The CPW-fed inverted-U shaped monopole antenna was realized on a Zn2SnO4 with 3.0 wt.% ZnO–B2O3–SiO2 aid. The microwave ceramic substrate is 4.0 mm in thickness, 28 mm in diameter, 11.4 in dielectric constant, and 4.5 104 in loss factor. A 50 X CPW feed line is used to feed the inverted-U shaped monopole antenna, while without any metallization in the other side. The inverted-U shaped monopole antenna and CPW feed line are printed on the same metal layer of the substrate. Dimensions of the CPW feed line are calculated by close-form formulas given in Ref. [19], assuming infinite ground plane and finite dielectric thickness. The CPW feed line dimensions are chosen to be compatible with a subminiature version A (SMA) connector. The diameter of dielectric core of conventional SMA connector is about 4.5 mm. The CPW feed line dimensions are confirmed by AWR Microwave Office. Gap, width, and length of CPW are 0.5, 2.0, and 6.0 mm, respectively. Two finite ground planes with the same dimensions of length and width are designed to be 6.0 8.0 mm2. The two finite ground planes are designed symmetrically on each side of the CPW feed line. The proposed CPW-fed inverted-U shaped monopole antenna, which leads
361
to three resonant modes. The optimal parameters of CPW-fed inverted-U shaped monopole antenna on microwave ceramic substrate are set as follows: L1 = 5.8 mm, L2 = 4.4 mm, L3 = 4.0 mm, L4 = 5.0 mm, L5 = 6.4 mm, L6 = 6.0 mm, W1 = 2.0 mm, W2 = 19.5 mm, W3 = 1.0 mm, W4 = 10.5 mm, and W5 = 8.0 mm.
Fig. 8. Simulation current distribution of the proposed monopole antenna at (a) first, (b) second, and (c) third resonant frequencies.
362
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
4. Conclusions
Fig. 9. Measured and simulation return losses of the proposed monopole antenna.
Fig. 8 shows the simulation current field distribution of the proposed antenna at three resonant frequencies. The electric field distribution of the proposed antenna is studied to understand the behavior of the proposed antenna. The resonant frequencies can be obtained through the following formulas. The first resonant length can be calculated from the following formula.
L3 þ W 4 þ L2 þ W 2 þ L1 ¼ 3kg1 =4 The resonant length is nearly equal to three quarters of the guided wavelength excited in the radiating structure. The second and the third resonant frequencies can be obtained from the following formulas, respectively.
L3 þ W 4 þ L2 þ W 2 þ L1 ¼ 5kg2 =4 L3 þ W 4 þ L2 þ W 2 þ L1 ¼ 7kg3 =4 where kg1 , kg2 , and kg3 are the corresponding guided wavelength in the radiating structure at first, second, and third resonant frequency, respectively. Fig. 9 shows the measured and simulation return loss of the proposed antenna. The simulation resonant frequencies are 2.47, 3.54, and 5.61 GHz. The measured resonant frequencies are close to the simulation resonant frequencies. The return losses are 18.85, 15.73, and 18.39 dB at 2.44, 3.85, and 5.41 GHz, respectively. Furthermore, there is a 6 dB return loss bandwidth of 204 MHz (2371–2575 MHz) in the lower band, 1520 MHz (3130– 4650 MHz) in the middle band, and 460 MHz (5250–5710 MHz) in the higher band.
The effects of ZnO–B2O3–SiO2 aid and sintering temperature on the microwave dielectric properties of Zn2SnO4 ceramics were investigated. The X-ray diffraction patterns of the Zn2SnO4 ceramics have no significant difference at different sintering temperature. A relative density of 95.33%, a dielectric constant of 11.4, and a Q f of 33,000 GHz were obtained for Zn2SnO4 ceramics with 3 wt.% ZnO–B2O3–SiO2 aid sintered at 1075 °C for 4 h. A sintering temperature reduction about 150 °C was obtained by using ZnO– B2O3–SiO2 as a sintering aid to reduce the sintering temperature of Zn2SnO4 ceramics. The dielectric constant of Zn2SnO4 ceramics is strongly dependent on the relative density and microstructures. The decrease in the Q f at a high amount of ZnO–B2O3–SiO2 additive was due to low relative density as well as the internal strain. The structure of the proposed CPW-fed inverted-U shaped monopole antenna is simple and easily manufactured. The return loss is 18.58, 15.73, and 18.39 dB at 2.44, 3.85, and 5.41 GHz, respectively. The proposed antenna has a 6 dB return loss with bandwidth 204, 1520 and 460 MHz in the lower, middle, and higher bands, respectively. Acknowledgment The authors would like to thank the National Science Council in Taiwan, for financially supporting this research under Contract No. NSC 102-2622-E-269-009-CC3. References [1] Y.C. Chen, Y.N. Wang, C.H. Hsu, J. Alloys Comp. 509 (2011) 9650–9653. [2] Y.C. Chen, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 58 (2011) 2531– 2538. [3] Y.C. Chen, J. Alloys Comp. 527 (2012) 84–89. [4] Y.C. Chen, Y.N. Wang, C.H. Hsu, Mater. Chem. Phys. 133 (2012) 829–833. [5] R.D. Shannon, Acta Crystallogr. A32 (1976) 751–767. [6] H. Jiang, H. Arai, IEICE Trans. Commun. E85-B (2002) 2468–2475. [7] J. Itoh, T. Hung, H. Morishita, IEICE Electron. Express 7 (2010) 1359–1363. [8] Y.C. Chen, Y.W. Zeng, Microw. Opt. Technol. Lett. 51 (2009) 98–100. [9] E.A. Balanis, Antenna theory—Analysis and design, second ed., Wiley, New York, 1997. [10] Y.C. Chen, J.M. Tsai, Microw. Opt. Technol. Lett. 51 (2009) 715–717. [11] B.W. Hakki, P.D. Coleman, IEEE Trans. Microw. Theory Tech. 8 (1960) 402–410. [12] Y. Kobayashi, M. Katoh, IEEE Trans. Microw. Theory Tech. 33 (1985) 586–592. [13] Y. Guo, H. Ohsato, K.I. Kakimoto, J. Eur. Ceram. Soc. 26 (2006) 1827–1830. [14] Y. Tohdo, K. Kakimoto, H. Ohsato, H. Yamada, T. Okawa, J. Eur. Ceram. Soc. 26 (2006) 2039–2043. [15] B.D. Silverman, Phys. Rev. 125 (1962) 1921–1930. [16] W.S. Kim, T.H. Hong, E.S. Kim, K.H. Yoon, Jpn. J. Appl. Phys. 37 (1998) 3567– 3571. [17] H. Ohsato, M. Imaeda, Mater. Chem. Phys. 79 (2003) 208–212. [18] Y.C. Chen, M.D. Chen, J. Phys. Chem. Solids 72 (2011) 1447–1451. [19] K.C. Gupta, R. Garg, I. Bahl, P. Bhartia, Microstrip lines and slotlines, second ed., Artec House, Norwood, MA, 1996.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microwave dielectric properties of ZnO–B2O3–SiO2-doped Zn2SnO4 ceramics for application in triple bands inverted-U shaped monopole antenna Yih-Chien Chen ⇑, Hong-Mine You 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 27 May 2014 Received in revised form 5 July 2014 Accepted 7 July 2014 Available online 23 July 2014 Keywords: Zn2SnO4 Dielectric constant Quality factor Temperature coefficient of resonant frequency Monopole antenna
a b s t r a c t In this study, the microwave dielectric properties of Zn2SnO4 ceramics prepared by the conventional solid-state method were investigated for application in mobile communication. ZnO–B2O3–SiO2 was selected as a liquid sintering aid to reduce the sintering temperature of Zn2SnO4 ceramics. By adding 3.0 wt.% ZnO–B2O3–SiO2, a dielectric constant of 11.44 and a quality factor (Q f) of 33,000 GHz were obtained when Zn2SnO4 ceramics were sintered at 1075 °C for 4 h. The proposed CPW-fed inverted-U shaped monopole antenna has a 6 dB return loss with bandwidth 204 MHz (2371–2575 MHz) in the lower band, 1520 MHz (3130–4650 MHz) in the middle band, and 460 MHz (5250–5710 MHz) in the higher band. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The demand for novel microwave ceramics applications have been increasing rapidly in the mobile communication industries during the last decade. Recent studies have focused on developing good microwave dielectric materials to achieve device miniaturization and system stability. Many researches on the Mg2SnO4 ceramics have been investigated for resonators, filters, and antennas in the modern communication systems, such as radar and global positioning systems (GPS) operating at microwave frequencies. Mg2SnO4 ceramics sintered at 1550 °C for 4 h combines a dielectric constant of 8.41 and a quality factor (Q f) of 55,100 GHz [1]. The maximum values of dielectric constant and quality factor (Q f) for (Mg0.93Co0.07)2SnO4 ceramics sintered at 1550 °C for 4 h are 8.8 and 110,800 GHz, respectively [2]. A dielectric constant of 8.5 and a quality factor (Q f) of 186,100 GHz were obtained for (Mg0.93Zn0.07)2SnO4 ceramics sintered at 1550 °C for 4 h [3]. Additionally, a dielectric constant of 8.72 and a quality factor (Q f) of 103,100 GHz were obtained for (Mg0.95Ni0.05)2SnO4 ceramics sintered at 1550 °C for 4 h [4]. Since the ionic radius of Zn2+ (0.074 nm) is similar to that of Mg2+ (0.072 nm), it has certainly attracted our attention to
⇑ Corresponding author. Tel.: +886 2 8209 3211x5532; fax: +886 2 8209 9721. E-mail address: [email protected] (Y.-C. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.07.049 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
investigate the effect of substituting Mg2+ by Zn2+ to form a new Zn2SnO4 ceramics [5]. Moreover, the effects of the amounts of ZnO–B2O3–SiO2 aid and sintering temperature on the microwave dielectric properties of Zn2SnO4 ceramics have been studied. The microwave dielectric properties of the Zn2SnO4 ceramics have been found to vary with various amounts of ZnO–B2O3–SiO2 aid and sintering temperatures. For further understanding of these different microwave dielectric properties, they were analyzed on the basis of density, X-ray diffraction (XRD) patterns, and observation of the microstructures. Many commercial applications, including mobile radio and wireless communications, use monopole antenna. The monopole antenna is used extensively because it is reasonably compact, good efficiency, and is very simple [6,7]. Recently, monopole antennas for application in WLAN (wireless local area network, 2.4– 2.484 GHz) are implemented. Simultaneously, associating with the rapid development of WiMAX (worldwide interoperability for microwave access, 3.4–3.69, and 5.25–5.85 GHz), there is an increasing demand for antennas suitable for WLAN/WiMAX simultaneously. Monopole antenna has limitations in size, bandwidth, and efficiency, imposed by the dielectric substrate. Three properties of dielectric substrate must be considered for monopole used: a high dielectric constant, a high quality factor, and a near-zero temperature coefficient of resonant frequency. A high dielectric constant and a near-zero temperature coefficient of resonant frequency are required for small size and high-temperature stability,
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
357
respectively [8]. The quality factor is representative of the antenna losses. There are losses of radiation, conduction, dielectric, and surface wave. Therefore, the total quality factor is affected by all of these losses [9]. Since high dielectric constant substrate can be used as dielectric substrate to achieve compact size and low-profile monopole antenna, the development of antennas fabricated on high dielectric constant substrate for commercial mobile communication has been rapidly growing recently [10]. Combination the advantages of CPW feed line and monopole antenna, a CPW-fed inverted-U shaped monopole fabricated on high dielectric constant substrate operated in WLAN and WiMAX bands simultaneously, is proposed. The proposed antenna is simple in manufacturing because of single dielectric substrate, single metal layer, and without via holes. The proposed antenna is capable of operating in the WLAN and WiMAX bands. The design considerations and experimental results for the proposed antenna are presented and discussed. 2. Experimental procedure The starting raw chemicals were ZnO (99.0%) and SnO2 (99.9%) powders. The composition prepared was Zn2SnO4. Zn2SnO4 was then added 2, 3, and 4 wt.% ZnO–B2O3–SiO2. Specimens were prepared using the conventional mixed-oxide method. The raw material was weighed out in stoichiometric proportions, ballmilled in alcohol for 12 h, dried, and then calcined at 1100 °C for 4 h. The obtained powders were then crushed into a finer powder using a sieve with a 200 mesh. Calcined powders with different amounts of ZnO–B2O3–SiO2 aid were re-milled for 12 h with PVA solution as a binder. The fine powders obtained were then axially pressed at 2000 kg/cm2 into pellets with a diameter of 11 mm and a thickness of 6 mm, prior to sintering. The specimens obtained were then sintered in air for 4 h with different sintering temperature. Both the heating rate and cooling rate were set at 10 °C/min. After sintering, the phases of the samples were investigated by X-ray diffraction. The X-ray Rigaku D/MAX-2200 data were collected using Cu Ka radiation (at 30 KV and 20 mA) and a graphite monochromator in the 2h range of 10–80°. Scanning electron microscopy (SEM; JEOL JSM-6500F) and energy dispersive X-ray spectrometer (EDS) were employed to examine the microstructures of the specimens. The apparent densities of the specimens were measured by the Archimedes method using distilled water as the liquid and the microwave dielectric properties of the specimens were measured by the postresonator method developed by Hakki and Coleman [11]. 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 [12]. 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 into or out of the resonator. The other end of the antennas is connected to an Agilent N5230A network analyzer. The resonance characteristics are dependent on the size and dielectric properties of the specimen. The microwave energy was coupled using electric-field probes. The TE011 resonant mode was found to be optimal for obtaining the dielectric constant and the 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 sf is the same as that for dielectric constant measurement. The test cavity was placed in a chamber and the temperature was varied from 25 to 75 °C. The sf value (ppm/ °C) can be determined by noting the change in resonant frequency,
sf ¼
f2 f1 ; f1 ðT 2 T 1 Þ
ð1Þ
where f1 and f2 represent the resonant frequencies at T 1 and T 2 , respectively.
3. Results and discussion The X-ray diffraction patterns of Zn2SnO4 ceramics with 0, 2, 3, and 4 wt.% ZnO–B2O3–SiO2 aid under various sintering temperatures for 4 h are shown in Fig. 1. It is clear that Zn2SnO4 is the main crystalline phase. Zn2SnO4 accompanied by second phases of Zn2SiO2 and SnO2 when Zn2SnO4 ceramics added with ZnO– B2O3–SiO2 aid. The X-ray diffraction patterns of the Zn2SnO4 ceramics have no significant difference at different sintering temperature with same amount of ZnO–B2O3–SiO2 aid. The crystal structure of Zn2SnO4 has inverse-spinel structure belongs to Fd-3m space group. All the peaks were indexed based on the cubic
Fig. 1. The X-ray diffraction patterns of Zn2SnO4 specimens with (a) 0 wt.%, (b) 2 wt.%, (c) 3 wt.%, and (d) 4 wt.% ZnO–B2O3–SiO2 additives at different sintering temperatures.
unit cell. Zn2SiO2 with a rhombohedral crystal structure belongs to R-3 space group was identified. Formation of Zn2SiO4 was attributed to the partially substituted Sn4+ ions with the Si4+ ions, resulting in the Zn2SiO4 phase. Since the partially replacing Sn4+ with the Si4+, some of SnO2 phase have formed. The intensity demonstrated the difference in amounts of Zn2SnO4, Zn2SiO4, and SnO2 more clearly. The relative intensities of Zn2SnO4, Zn2SiO4, and SnO2 were
358
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
Table 1 Comparison the main phase (%) and second phases (%) of Zn2SnO4 with different amounts of ZnO–B2O3–SiO2 aid at optimum sintering temperature. ZnO–B2O3–SiO2 (wt.%)
S.T. (°C)
Zn2SnO4
Zn2SiO4
SnO2
2 3 4
1175 1075 1025
95.14 91.44 90.34
1.33 2.83 2.25
3.53 5.73 7.41
evaluated from most intensive line of each phase. The relative intensities of Zn2SnO4, Zn2SiO4, and SnO2 were calculated as follows:
Relative intensity of Zn2 SnO4 ¼
IZn2 SnO4 ð311Þ IZn2 SnO4 ð311Þ þ IZn2 SiO4 ð410Þ þ ISnO2 ð110Þ 100; ð2Þ
Relative intensity of Zn2 SiO4 ¼
IZn2 SiO4 ð410Þ IZn2 SnO4 ð311Þ þ IZn2 SiO4 ð410Þ þ ISnO2 ð110Þ 100; ð3Þ
Relative intensity of SnO2 ¼
ISnO2 ð110Þ IZn2 SnO4 ð311Þ þ IZn2 SiO4 ð410Þ þ ISnO2 ð110Þ 100; ð4Þ
Table 1 shows the amounts of main and second phases of the specimens sintered at optimum sintering temperatures for 4 h. The amount of the Zn2SnO4 decreased from 95.14% to 90.34% as the amount of ZnO–B2O3–SiO2 aid increased from 2 to 4 wt.%. The amount of the Zn2SiO4 ranged from 1.33% to 2.83% as the amount of ZnO–B2O3–SiO2 aid varied from 2 to 4 wt.%. The amount of the SnO2 increased from 3.53% to 7.41% as the amount of ZnO– B2O3–SiO2 aid increased from 2 to 4 wt.%. The formation of Zn2SiO4 and SnO2 affected the apparent density and microwave dielectric properties of Zn2SnO4 ceramics. The microstructures of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid and at different sintering temperatures for 4 h are shown in Fig. 2. Comparing the microstructures of Zn2SnO4 ceramics added with the 2 and 3 wt.% ZnO–B2O3–SiO2 aid sintered at optimum sintering temperature, the grain size did not vary with the amount of ZnO–B2O3–SiO2 obviously. The average
Fig. 2. The microstructures of the Zn2SnO4 (a) 2 wt.% ZnO–B2O3–SiO2/1150 °C, (b) 2 wt.% ZnO–B2O3–SiO2/1175 °C, (c) 2 wt.% ZnO–B2O3–SiO2/1200 °C, (d) 2 wt.% ZnO–B2O3– SiO2/1225 °C, (e) 3 wt.% ZnO–B2O3–SiO2/1075 °C and (f) 4 wt.% ZnO–B2O3–SiO2/1025 °C.
359
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
resulting in a decrease of relative density. At optimum sintering temperatures, the relative density remained stable as the amount of ZnO–B2O3–SiO2 aid increased from 0 to 3 wt.%, while decreased dramatically from 95.33% to 94.44% as the amount of ZnO–B2O3– SiO2 aid increased from 3 to 4 wt.%. Fig. 4 shows the dielectric constants of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid sintered at different temperatures for 4 h. The relationship between the dielectric constant and the sintering temperature revealed the same trend as that between the relative density and the sintering temperature. At optimum sintering temperatures, the dielectric constant increased from 10.2 to 11.4 as the amount of ZnO–B2O3–SiO2 aid increased from 0 to 3 wt.%, while decreased from 11.4 to 10.9 as the amount 12 11
Dielectric Constnat
grain size decreased dramatically as the amount of ZnO–B2O3–SiO2 aid increased from 3 to 4 wt.%. The significant decreasing of average grain size may affect the microwave dielectric properties of Zn2SnO4 ceramics. The inhibition of grain growth at high amount of ZnO–B2O3–SiO2 aid derived from impurities in the grain boundaries. In order to indentify the composition of the impurities in the grain boundaries, an EDS analysis was employed on the grain boundaries of Zn2SnO4 ceramics with 4 wt.% ZnO–B2O3–SiO2 sintered at 1025 °C for 4 h as shown in Fig. 2(f). According to the quantitative analysis as shown in Table 2, the compositions are sintering aid, ZnO–B2O3–SiO2. In order to indentify the composition of the second phase, an energy-disperse spectroscopy (EDS) analysis was employed on the grains of Zn2SnO4 ceramics with 2 wt.% ZnO–B2O3–SiO2 sintered at 1175 °C for 4 h as shown in Fig. 2(b). According to the quantitative analysis as shown in Table 3, it is evident that the grains of A and B are Zn2SnO4. The rod-like grains C and D are Zn2SiO4 and SnO2, respectively. The mixture phases observed in the microstructure supported the secondary phases detected in the X-ray diffraction patterns as shown in Fig. 1. The relative densities of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid after sintering from 1000 to 1250 °C for 4 h are shown in Fig. 3. The relative density was found to increase to a maximum value at an optimum sintering temperature and then decrease gradually. The relative density increased with increasing sintering temperature and this may be due to the decreasing number of pores as shown in Fig. 2. However, a high sintering temperature could also induce an abnormal grain growth,
10 9 8
0 wt% 2 wt% 3 wt% 4 wt%
7 6 975
1025
1075
1125
1175
1225
1275
o
Table 2 EDS data of grains of Zn2SnO4 ceramic with 4 wt.% ZnO–B2O3–SiO2 aid sintered at 1025 °C for 4 h. Zn (%)
B (%)
Si (%)
O (%)
33.47
18.96
1.42
46.12
Table 3 EDS data of grains of Zn2SnO4 ceramic with 2 wt.% ZnO–B2O3–SiO2 aid sintered at 1175 °C for 4 h. Atomic element
Zn (%)
Sn (%)
Si (%)
O (%)
A B C D
24.42 25.27 30.87 0
11.61 11.11 0 27.44
0 0 16.89 0
63.98 63.62 52.25 72.56
Sintering Temperature ( C) Fig. 4. Dielectric constants of Zn2SnO4 ceramics with different amounts of ZnO– B2O3–SiO2 aid sintered in the range of 1000–1250 °C for 4 h.
Table 4 The porosity, overall dielectric constant of second phases, and internal strain of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid at optimum sintering temperature. (*: pure Zn2SnO4). ZnO–B2O3–SiO2 (wt.%)
S.T. (°C)
Porosity (%)
er;ov erall
Internal strain
0 2 3 4
1225 1175 1075 1025
0.77 0.46 0.45 0.77
* 10.90 10.46 10.98
0.00130 0.00315 0.00425 0.20795
40000 96
Quality Factor (GHz)
Relative Density (%)
95.5 95 94.5 94 0 wt% 2 wt% 3 wt% 4 wt%
93.5 93 92.5 975
1025
1075
1125
1175
1225
30000
0 wt% 2 wt% 3 wt% 4 wt%
20000
10000
1275
o
Sintering Temperature ( C) Fig. 3. Relative densities of Zn2SnO4 ceramics with different amounts of ZnO–B2O3– SiO2 aid sintered in the range of 1000–1250 °C for 4 h.
0 975
1025
1075
1125
1175
1225
1275
o
Sintering Temperature ( C) Fig. 5. Quality factors (Q f) of Zn2SnO4 ceramics with different amounts of ZnO– B2O3–SiO2 aid sintered in the range of 1000–1250 °C for 4 h.
360
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
er ¼ 0 wt% 2 wt% 3 wt% 4 wt%
-20
o
Frequency (ppm/ C)
Temperature Coefficient of Resonan t
0
-40
3V m þ 8paD 3V m 4paD
ð5Þ
where Vm is the molar volume and aD is the sum of ionic polarizabilities of individual ions. The dielectric constant of the composites can be calculated by the mixture rule:
ln er ¼ v 1 ln er1 þ v 2 ln er2 þ v 3 ln er3 ;
ð6Þ
-60
-80
-100
-120 975
1025
1075
1125
1175
1225
1275
o
Sintering Temperature ( C) Fig. 6. sf Values of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid sintered in the range of 1000–1250 °C for 4 h.
of ZnO–B2O3–SiO2 aid increased from 3 to 4 wt.%. A maximum dielectric constant value of 11.4 was obtained for Zn2SnO4 ceramics with 3 wt.% ZnO–B2O3–SiO2 aid that were sintered at 1075 °C for 4 h. Table 4 shows the porosities of the specimens with different amounts of ZnO–B2O3–SiO2 aid that were sintered at optimum sintering temperatures for 4 h. Of the Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid, Zn2SnO4 ceramic with 3 wt.% ZnO–B2O3–SiO2 aid that was sintered at 1075 °C for 4 h had the lowest porosity of 0.45%. A lower the fraction porosity is, therefore, a higher dielectric constant. The second phase is another extrinsic factor in controlling the dielectric constant. The dielectric constant of ZnSiO4 is 6 [13]. The calculated dielectric constant of SnO2 is 13.1, using the Clausius-Mossotti equation, as suggested by Tohdo et al. [14]:
where er is the dielectric constant of the composite, v1, v2, and v3 are the volume fractions of Zn2SnO4, Zn2SiO4, and SnO2, respectively, and er1 , er2 , and er3 are the dielectric constants of Zn2SnO4, Zn2SiO4, and SnO2, respectively. Table 4 shows the overall dielectric constants (er;ov erall ) of Zn2SiO4 and SnO2. The overall dielectric constant spanned from 10.46 to 10.98 with various amounts of ZnO–B2O3– SiO2 aid at optimum sintering temperatures. Since the overall dielectric constants of Zn2SiO4 and SnO2 are close to the measured dielectric constant, which the influence of second phases on the dielectric constants of Zn2SnO4 ceramics, was small.The Q f of Zn2SnO4 ceramics with different amounts of ZnO–B2O3–SiO2 aid sintered at different temperatures for 4 h are shown in Fig. 5. The Q f decreased from 39,000 to 2,900 GHz as the amount of ZnO–B2O3–SiO2 aid increased from 0 to 4 wt.% for samples at optimum sintering temperature. A maximum Q f of 39,000 GHz was obtained for Zn2SnO4 ceramics that were sintered at 1225 °C for 4 h. The relationship between the Q f and the sintering temperature revealed the same trend as that between the relative density and the sintering temperature. This is caused by the microwave dielectric loss, which is affected by many factors that can be divided into the intrinsic and extrinsic loss. 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 [15,16]. Since the Q f of Zn2SnO4 ceramics was consistent with the variation of the relative density, it is suggested that the Q f of Zn2SnO4 ceramics is mainly controlled by the relative density. A significant decreased
Fig. 7. Configuration of the proposed CPW-fed inverted-U shaped monopole antenna on high permittivity ceramic substrate.
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
Q f at higher amounts of ZnO–B2O3–SiO2 aid could be related to the high internal strain. The internal strain g can be calculated using following equation, as suggested by Ohsato and Imaeda [17].
b ¼ 2g tan h;
ð7Þ
where b is the full width at half maximum (FWHM) of an X-ray diffraction peak and h is the corresponding diffraction angles. Eq. (7) was used for curve fitting to determine the internal strain g of the Zn2SnO4 ceramics with different degrees of ZnO–B2O3–SiO2 aid. The curve fitting was based on the least-squares method. In this method, the square of the deviation of the experimental value from the theoretical expectation was calculated while the fitted internal strain was varied. The fitting process was terminated at the fitted internal strain that minimized the deviation. As shown in Table 4, Zn2SnO4 ceramic with 4 wt.% ZnO–B2O3–SiO2 aid had the largest internal strain, 0.20795, of any of the Zn2SnO4 ceramics with various amounts of ZnO–B2O3–SiO2 aid. The grain size is a factor affecting the internal stain. Since the average grain size decreased significantly with the extent of ZnO–B2O3–SiO2 aid increased from 3 wt.% to 4 wt.%, the internal strain increased significantly. Accordingly, Q f decreased dramatically as the amounts of ZnO–B2O3– SiO2 additive increased from 3 wt.% to 4 wt.%. The sf of Zn2SnO4 ceramics with different amounts of ZnO– B2O3–SiO2 aid sintered at different temperatures for 4 h are shown in Fig. 6. In general, sf is related to the composition, the amount of additive, and the second phases that existed in the ceramics. No significant variation in sf was observed with a fixed amount of ZnO–B2O3–SiO2 aid at different sintering temperature in the entire sintering temperature range. Since there was no compositional variation in Zn2SnO4 ceramics with a fixed amount of ZnO–B2O3– SiO2 aid with different sintering temperature, sf was measured as a function of the amounts of ZnO–B2O3–SiO2 aid in this experiment. The sf of Zn2SiO4 ceramics is less negative compared with that of Zn2SnO4 ceramics, implying the presence of the Zn2SiO4 shifted the sf of the specimen to the positive direction. The amount of the Zn2SiO4 increased from 1.33% to 2.83% as the amount of ZnO–B2O3–SiO2 aid increased from 2 to 3 wt.%, the sf of specimen is inferred to be shifted to positive direction. However, the average value of sf decreased from 72.6 to 107.3 ppm/°C when the amount of ZnO–B2O3–SiO2 aid increased from 2 to 3 wt.%, the sf of SnO2 is inferred to be negative compared with that of Zn2SiO4, which is in agreement with Ref. [18]. The average sf increased from 107.3 to 8.1 ppm/°C as the amount of ZnO–B2O3–SiO2 aid increased significantly from 3 to 4 wt.%. This is associated with the formation of the grain boundary phase, ZnO–B2O3–SiO2. The geometry and parameters of CPW-fed inverted-U shaped monopole antenna are shown in Fig. 7. The CPW-fed inverted-U shaped monopole antenna was realized on a Zn2SnO4 with 3.0 wt.% ZnO–B2O3–SiO2 aid. The microwave ceramic substrate is 4.0 mm in thickness, 28 mm in diameter, 11.4 in dielectric constant, and 4.5 104 in loss factor. A 50 X CPW feed line is used to feed the inverted-U shaped monopole antenna, while without any metallization in the other side. The inverted-U shaped monopole antenna and CPW feed line are printed on the same metal layer of the substrate. Dimensions of the CPW feed line are calculated by close-form formulas given in Ref. [19], assuming infinite ground plane and finite dielectric thickness. The CPW feed line dimensions are chosen to be compatible with a subminiature version A (SMA) connector. The diameter of dielectric core of conventional SMA connector is about 4.5 mm. The CPW feed line dimensions are confirmed by AWR Microwave Office. Gap, width, and length of CPW are 0.5, 2.0, and 6.0 mm, respectively. Two finite ground planes with the same dimensions of length and width are designed to be 6.0 8.0 mm2. The two finite ground planes are designed symmetrically on each side of the CPW feed line. The proposed CPW-fed inverted-U shaped monopole antenna, which leads
361
to three resonant modes. The optimal parameters of CPW-fed inverted-U shaped monopole antenna on microwave ceramic substrate are set as follows: L1 = 5.8 mm, L2 = 4.4 mm, L3 = 4.0 mm, L4 = 5.0 mm, L5 = 6.4 mm, L6 = 6.0 mm, W1 = 2.0 mm, W2 = 19.5 mm, W3 = 1.0 mm, W4 = 10.5 mm, and W5 = 8.0 mm.
Fig. 8. Simulation current distribution of the proposed monopole antenna at (a) first, (b) second, and (c) third resonant frequencies.
362
Y.-C. Chen, H.-M. You / Journal of Alloys and Compounds 616 (2014) 356–362
4. Conclusions
Fig. 9. Measured and simulation return losses of the proposed monopole antenna.
Fig. 8 shows the simulation current field distribution of the proposed antenna at three resonant frequencies. The electric field distribution of the proposed antenna is studied to understand the behavior of the proposed antenna. The resonant frequencies can be obtained through the following formulas. The first resonant length can be calculated from the following formula.
L3 þ W 4 þ L2 þ W 2 þ L1 ¼ 3kg1 =4 The resonant length is nearly equal to three quarters of the guided wavelength excited in the radiating structure. The second and the third resonant frequencies can be obtained from the following formulas, respectively.
L3 þ W 4 þ L2 þ W 2 þ L1 ¼ 5kg2 =4 L3 þ W 4 þ L2 þ W 2 þ L1 ¼ 7kg3 =4 where kg1 , kg2 , and kg3 are the corresponding guided wavelength in the radiating structure at first, second, and third resonant frequency, respectively. Fig. 9 shows the measured and simulation return loss of the proposed antenna. The simulation resonant frequencies are 2.47, 3.54, and 5.61 GHz. The measured resonant frequencies are close to the simulation resonant frequencies. The return losses are 18.85, 15.73, and 18.39 dB at 2.44, 3.85, and 5.41 GHz, respectively. Furthermore, there is a 6 dB return loss bandwidth of 204 MHz (2371–2575 MHz) in the lower band, 1520 MHz (3130– 4650 MHz) in the middle band, and 460 MHz (5250–5710 MHz) in the higher band.
The effects of ZnO–B2O3–SiO2 aid and sintering temperature on the microwave dielectric properties of Zn2SnO4 ceramics were investigated. The X-ray diffraction patterns of the Zn2SnO4 ceramics have no significant difference at different sintering temperature. A relative density of 95.33%, a dielectric constant of 11.4, and a Q f of 33,000 GHz were obtained for Zn2SnO4 ceramics with 3 wt.% ZnO–B2O3–SiO2 aid sintered at 1075 °C for 4 h. A sintering temperature reduction about 150 °C was obtained by using ZnO– B2O3–SiO2 as a sintering aid to reduce the sintering temperature of Zn2SnO4 ceramics. The dielectric constant of Zn2SnO4 ceramics is strongly dependent on the relative density and microstructures. The decrease in the Q f at a high amount of ZnO–B2O3–SiO2 additive was due to low relative density as well as the internal strain. The structure of the proposed CPW-fed inverted-U shaped monopole antenna is simple and easily manufactured. The return loss is 18.58, 15.73, and 18.39 dB at 2.44, 3.85, and 5.41 GHz, respectively. The proposed antenna has a 6 dB return loss with bandwidth 204, 1520 and 460 MHz in the lower, middle, and higher bands, respectively. Acknowledgment The authors would like to thank the National Science Council in Taiwan, for financially supporting this research under Contract No. NSC 102-2622-E-269-009-CC3. References [1] Y.C. Chen, Y.N. Wang, C.H. Hsu, J. Alloys Comp. 509 (2011) 9650–9653. [2] Y.C. Chen, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 58 (2011) 2531– 2538. [3] Y.C. Chen, J. Alloys Comp. 527 (2012) 84–89. [4] Y.C. Chen, Y.N. Wang, C.H. Hsu, Mater. Chem. Phys. 133 (2012) 829–833. [5] R.D. Shannon, Acta Crystallogr. A32 (1976) 751–767. [6] H. Jiang, H. Arai, IEICE Trans. Commun. E85-B (2002) 2468–2475. [7] J. Itoh, T. Hung, H. Morishita, IEICE Electron. Express 7 (2010) 1359–1363. [8] Y.C. Chen, Y.W. Zeng, Microw. Opt. Technol. Lett. 51 (2009) 98–100. [9] E.A. Balanis, Antenna theory—Analysis and design, second ed., Wiley, New York, 1997. [10] Y.C. Chen, J.M. Tsai, Microw. Opt. Technol. Lett. 51 (2009) 715–717. [11] B.W. Hakki, P.D. Coleman, IEEE Trans. Microw. Theory Tech. 8 (1960) 402–410. [12] Y. Kobayashi, M. Katoh, IEEE Trans. Microw. Theory Tech. 33 (1985) 586–592. [13] Y. Guo, H. Ohsato, K.I. Kakimoto, J. Eur. Ceram. Soc. 26 (2006) 1827–1830. [14] Y. Tohdo, K. Kakimoto, H. Ohsato, H. Yamada, T. Okawa, J. Eur. Ceram. Soc. 26 (2006) 2039–2043. [15] B.D. Silverman, Phys. Rev. 125 (1962) 1921–1930. [16] W.S. Kim, T.H. Hong, E.S. Kim, K.H. Yoon, Jpn. J. Appl. Phys. 37 (1998) 3567– 3571. [17] H. Ohsato, M. Imaeda, Mater. Chem. Phys. 79 (2003) 208–212. [18] Y.C. Chen, M.D. Chen, J. Phys. Chem. Solids 72 (2011) 1447–1451. [19] K.C. Gupta, R. Garg, I. Bahl, P. Bhartia, Microstrip lines and slotlines, second ed., Artec House, Norwood, MA, 1996.