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Improved dielectric properties of CaLa4Ti5O17 ceramics with Zr substitution at microwave frequency
Materials Chemistry and Physics 118 (2009) 161–164
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Improved dielectric properties of CaLa4 Ti5 O17 ceramics with Zr substitution at microwave frequency Yih-Chien Chen ∗ , Wei-Cheng Lee, Kuai-Cian Chen Department of Electrical Engineering, Lunghwa University of Science and Technology, No. 300, Sec. 1, Wanshou Rd., Gueishan Shiang, Taoyuan County, Taiwan
a r t i c l e
i n f o
Article history: Received 6 January 2009 Received in revised form 19 May 2009 Accepted 9 July 2009 Keywords: CaLa4 Ti5−x Zrx O17 Dielectric constant Quality factor Temperature coefficient of resonant frequency
a b s t r a c t In this paper, the effect of Zr4+ substitution on the dielectric properties of CaLa4 Ti5 O17 ceramics at microwave frequency by the conventional solid-state method has been studied for application in mobile communication. The diffraction peaks of CaLa4 Ti5−x Zrx O17 ceramics shifted to lower angle as x increasing from 0 to 0.05 and nearly unchanged with x increasing from 0.05 to 0.07. A maximum relative density of 97.5% TD can be obtained for CaLa4 Ti4.95 Zr0.05 O17 ceramic sintered at 1500 ◦ C for 4 h. A maximum dielectric constant and quality factor (Q × f) of CaLa4 Ti4.95 Zr0.05 O17 ceramic sintered at 1500 ◦ C for 4 h are 55.9 and 15,600 GHz, respectively. A temperature coefficient of resonant frequency ( f ) of 4.6 ppm ◦ C−1 can be obtained for CaLa4 Ti4.95 Zr0.05 O17 ceramic sintered at 1500 ◦ C for 4 h. © 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. Patch antennas however have limitations in size, gain, and efficiency, imposed by the dielectric substrate. Three dielectric properties of materials must be considered for patch antennas 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 representative of the antenna gain. Typically, there are radiation, conduction, dielectric, and surface wave losses. Therefore, the total quality factor is affected by all of these losses [1]. The microwave dielectric properties of (Ba,La)n Tin−1 O3n (n = 5, 6) have already been reported. BaLa4 Ti4 O15 and Ba2 La4 Ti5 O18 are characterized by high dielectric constant (εr ∼ 39–46), high quality factor (Q × f ∼ 11,600–31,800) and a small temperature coefficient of resonant frequency ( f ∼ −36 to 79 ppm ◦ C−1 ) [2]. MOLa2 O3 -TiO2 (M = Ca, Sr, Ba) is well known as microwave material for dielectric resonator and filter. Most of these ceramics combine a high dielectric constant (εr ∼ 42–54), high quality factor (Q × f ∼ 16,200–50,200) and an adjustable temperature coefficient of resonant frequency ( f ∼ −25 to 6 ppm ◦ C−1 ). Among the investigated orthorhombic phases, CaLa4 Ti5 O17 ceramic sintered at 1625 ◦ C gives dielectric constant ∼53.7, quality factor ∼17,300 GHz,
∗ Corresponding author. Tel.: +886 2 8209 3211x5537; fax: +886 2 8209 9721. E-mail address: [email protected] (Y.-C. Chen). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.07.028
and temperature coefficient of resonant frequency ∼−20 ppm ◦ C−1 [3]. CuO additives can effectively lower the sintering temperature of CaLa4 Ti5 O17 . A dielectric constant value of 57, a quality factor of 9000 GHz, and a temperature coefficient of resonant frequency of −10 ppm ◦ C−1 were obtained for CaLa4 Ti5 O17 ceramics with 0.5 wt% CuO sintered at 1500 ◦ C for 4 h [4]. By partial replacement of Ca with Zn, the dielectric properties of CaLa4 Ti5 O17 ceramics at microwave frequency are modified. A maximum dielectric constant of 57, a maximum quality factor of 15,000 GHz, and a near-zero temperature coefficient of resonant frequency ( f ) of −8.16 ppm ◦ C−1 were obtained for Ca0.99 Zn0.01 La4 Ti5 O17 ceramics with 0.5 wt% CuO additive sintered at 1450 ◦ C for 4 h [5]. Compositionally modified BaZrx Ti1−x O3 (BZT) received much attention due to the electrical properties to specific applications by the replacement of Ti4+ by Zr4+ [6,7]. It attracted our attention to investigate the effect of partial replacement of Ti4+ with Zr4+ on the microwave dielectric properties of CaLa4 Ti5 O17 ceramics. In this paper, investigations on CaLa4 Ti5−x Zrx O17 ceramics have been made to produce materials with better microwave dielectric properties for application in 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 obtained in order to realize an outstanding design of the patch antenna having a small size, high antenna gain, and high temperature stability. The microwave dielectric properties of CaLa4 Ti5−x Zrx O17 ceramics have been found to be different with different amounts of Zr4+ substitution and sintering temperature. For further understanding of these different microwave dielectric properties, they were analyzed on the basis of density and X-ray diffraction (XRD), and observation of microstructures.
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2. Experimental procedure The starting raw chemicals were high-purity CaCO3 , La2 O3 , TiO2 , and ZrO2 powders. The composition prepared was CaLa4 Ti5−x Zrx O17 , x = 0.01, 0.05, and 0.07. 0.5 wt% CuO was added to CaLa4 Ti5−x Zrx O17 for reducing 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 cm−2 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. Both the heating rate and cooling rate were set at 10 ◦ C min−1 . After sintering, the phases of the samples were investigated by X-ray diffraction. The X-ray Rigaku D/MAX-2200 data were collected using CuK˛ radiation (at 30 kV and 20 mA) and a graphite monochromator in the 2 range of 20–80◦ . Scanning electron microscopy (SEM; JEOL JSM-6500F) was employed to examine the microstructures of the specimens. The bulk densities of the specimens were measured by the Archimedes method using distilled water as the liquid. The microwave dielectric properties of the specimens were measured by the post-resonator method developed by Hakki and Coleman [8]. The post-resonator 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 quality 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−1 ) 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.
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of CaLa4 Ti5 O17 , CaLa4 Ti4.99 Zr0.01 O17 , CaLa4 Ti4.95 Zr0.05 O17 , and CaLa4 Ti4.93 Zr0.07 O17 ceramics in the range from 20◦ to 60◦ sintered at the temperatures of 1500 ◦ C for 4 h. The crystal structure of CaLa4 Ti5 O17 is known to have orthorhombic structure that belongs to Pnnm space group. The lattice parameters of CaLa4 Ti5 O17 are a = 0.5522 nm, b = 3.1256 nm, and c = 0.3899 nm [10]. The X-ray diffraction patterns of CaLa4 Ti5 O17 , CaLa4 Ti4.99 Zr0.01 O17 , CaLa4 Ti4.95 Zr0.05 O17 , and CaLa4 Ti4.93 Zr0.07 O17 ceramics in the range from 25◦ to 35◦ sintered at the temperatures of 1500 ◦ C for 4 h are shown in Fig. 2. It can be observed in Fig. 2, that the X-ray diffraction peaks shift to lower angles as x increases from 0 to 0.05 because the ionic radius of Zr4+ (0.083 nm) is larger than that of Ti4+ (0.064 nm) [11]. The X-ray diffraction peaks stop shifting with further increase in Zr4+ content from 0.05 to 0.07. This means
Fig. 1. X-ray diffraction patterns of CaLa4 Ti(5−x) Zrx O17 ceramics from 20 to 60◦ sintered at the temperatures of 1500 ◦ C for 4 h.
Fig. 2. X-ray diffraction patterns of CaLa4 Ti(5−x) Zrx O17 ceramics from 25 to 35◦ sintered at the temperatures of 1500 ◦ C for 4 h.
the Zr4+ solution in CaLa4 Ti(5−x) Zrx O17 ceramics is near saturation as the amounts increased to 0.05 molar. The microstructures of CaLa4 Ti4.95 Zr0.05 O17 after sintering from 1400 to 1550 ◦ C for 4 h are shown in Fig. 3. The CaLa4 Ti4.95 Zr0.05 O17 was not dense, and grains did not grow after it was sintered at 1400 ◦ C. It may affect the microwave dielectric properties of the ceramics. Comparing the microstructures of CaLa4 Ti4.95 Zr0.05 O17 sintered at different temperatures, the number of pores decreased and the rate of grain growth increased apparently. The pores almost disappeared in the specimen after sintering at 1500 ◦ C. The relative densities of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution after sintering from 1400 to 1550 ◦ C for 4 h are shown in Fig. 4. The theoretical density of CaLa4 Ti5 O17 is 5.46 g cm−3 [12]. The relative density of CaLa4 Ti(5−x) Zrx O17 ceramics exceeded 83% of its theoretical density (TD) in all cases. The relative density was found to increase to a maximum value when sintered at 1500 ◦ C and thereafter decreased. The relative density increased with increasing sintering temperature may be due to the decrease in the number of pores as shown in Fig. 3. The relative density increased from 83.3% TD to 97.5% TD as the sintering temperature increased from 1400 to 1500 ◦ C for CaLa4 Ti4.95 Zr0.05 O17 ceramics sintered at 1500 ◦ C for 4 h. A maximum relative density of 97.5% TD was obtained for CaLa4 Ti4.95 Zr0.05 O17 ceramics sintered at 1500 ◦ C for 4 h. The porosities of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution after sintering from 1400 to 1550 ◦ C for 4 h are shown in Fig. 4. The porosity of CaLa4 Ti(5−x) Zrx O17 ceramics is lower than 0.8% in all cases and spanned in the range from 0.4% to 0.8%. The dielectric constants of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution sintered at different temperatures for 4 h are illustrated in Fig. 5. The relationships between the dielectric constant and the sintering temperature revealed the same trend as that between the relative density and the sintering temperature. The decrease in dielectric constant was caused by the low densities of the ceramics. Higher density leads to lower porosity, thus, a higher dielectric constant can be achieved. The dielectric constant was found to increase to a maximum value at sintering temperature of 1500 ◦ C and thereafter decreased for CaLa4 Ti4.99 Zr0.01 O17 , CaLa4 Ti4.95 Zr0.05 O17 and CaLa4 Ti4.93 Zr0.07 O17 ceramics. Increasing sintering temperature is not necessary for getting a higher dielectric constant. A maximum value of dielectric constant of 55.9 was obtained for CaLa4 Ti4.95 Zr0.05 O17 sintered at 1500 ◦ C for 4 h. The difference between the dielectric constant of CaLa4 Ti4.95 Zr0.05 O17 and CaLa4 Ti4.93 Zr0.07 O17 is not obvious. This is due to the upper limit of Zr4+ solid solution in CaLa4 Ti(5−x) Zrx O17 ceramics is 0.05 molar as observed from the results of X-ray diffraction patterns. The quality factor (Q × f) of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution sintered at different temper-
Y.-C. Chen et al. / Materials Chemistry and Physics 118 (2009) 161–164
163
Fig. 3. Microstructures of the CaLa4 Ti4.95 Zr0.05 O17 ceramics sintered at different temperatures for 4 h.
atures for 4 h are also shown in Fig. 5. With increasing the sintering temperature, the quality factor increased to a maximum value at sintering temperature of 1500 ◦ C and thereafter decreased for CaLa4 Ti4.99 Zr0.01 O17 , CaLa4 Ti4.95 Zr0.05 O17 , and CaLa4 Ti4.93 Zr0.07 O17 . A maximum quality factor of 15,600 GHz was obtained for CaLa4 Ti4.95 Zr0.05 O17 sintered at 1500 ◦ C for 4 h. Compared with the measurement results of quality factor as shown in ref. 4, the value of quality factor was increased from 9000 to 15,600 GHz as the amounts of Zr4+ substitution increased from 0 to 0.005 molar. The deviation between the quality factor of CaLa4 Ti4.95 Zr0.05 O17 and CaLa4 Ti4.93 Zr0.07 O17 is not evident. Similar phenomena were also observed in dielectric constant. This is because the upper limit of Zr4+ solid solution in CaLa4 Ti(5−x) Zrx O17 ceramics is 0.05
molar. It also can be found that the maximum quality factor of the CaLa4 Ti5 O17 ceramic with partial replacing of Ti4+ with Zr4+ is larger than that with partial replacing of Ca2+ with Zn2+ as shown in ref. 5. The relationship between the quality factor 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 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 [13,14]. Because the quality factor of CaLa4 Ti(5−x) Zrx O17 ceramics was consistent with the variation of the relative density, it is suggested that the
Fig. 4. Relative densities and porosities of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution at different sintering temperatures.
Fig. 5. Dielectric constant and quality factor of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution at different sintering temperatures.
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lier reports, improvement on the microwave dielectric properties had been achieved by partially substituting Ti4+ ions with Zr4+ ions. A dielectric constant value of 55.9, a quality factor of 15,600 GHz, and a near-zero temperature coefficient of resonant frequency of 4.6 ppm ◦ C−1 were obtained for CaLa4 Ti4.95 Zr0.05 O17 ceramic sintered at 1500 ◦ C for 4 h. The microwave dielectric properties of CaLa4 Ti(5−x) Zrx O17 ceramics are strongly dependent on the density and microstructures. Acknowledgment
Fig. 6. Temperature coefficient of frequency ( f ) of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution at different sintering temperatures.
quality factor of CaLa4 Ti(5−x) Zrx O17 ceramics is mainly controlled by the density. The temperature coefficient of resonant frequency ( f ) values of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution sintered at different temperatures for 4 h can be found in Fig. 6. In general, the temperature coefficient of resonant frequency is related to the composition, the amount of additive, and the second phases that existed in the ceramics [15]. There was no second phase in CaLa4 Ti(5−x) Zrx O17 ceramics. The amount of CuO additive added in CaLa4 Ti(5−x) Zrx O17 ceramics is fixed at 0.5 wt% in all cases. So the effect of the second phase and amount of additive could be neglected. In this experiment, f was major influenced by the composition. A near-zero temperature coefficient of resonant frequency of 4.6 ppm ◦ C−1 was measured for CaLa4 Ti4.95 Zr0.05 O17 ceramic sintered at 1500 ◦ C for 4 h. 4. Conclusions The dielectric properties of CaLa4 Ti(5−x) Zrx O17 ceramics at microwave frequencies have been investigated. Compared with ear-
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, Japanese Journal of Applied Physics 47 (2008) 992. [2] C. Vineis, P.K. Davies, T. Negas, S. Bell, Material Research Bulletin 31 (1996) 431. [3] I.N. Jawahar, N.I. Santha, M.T. Sebastian, Journal of Material Research 17 (2002) 3084. [4] Y.C. Chen, Y.W. Zeng, Microwave Optical Technology Letter 51 (2009) 98. [5] Y.C. Chen, J.M. Tsai, Japanese Journal of Applied Physics 47 (2008) 7959. [6] S.M. Neirman, Journal of Material Science 23 (1988) 3973. [7] A. Outzourhit, M.A. El Idrissi Raghni, M.L. Hafid, F. Bensamka, A. utzourhit, Journal of Alloys and Compounds 340 (2002) 214. [8] B.W. Hakki, P.D. Coleman, IRE Transaction on Microwave Theory and Techniques 8 (1960) 402. [9] Y. Kobayashi, M. Katoh, IEEE Transaction on Microwave Theory and Techniques 33 (1985) 586. [10] M. Nanot, F. Queyroux, J.C. Gilles, Journal of Solid State Chemistry 11 (1974) 272. [11] R.D. Shannon, Acta Crystallographica A32 (1976) 751. [12] JCPDS Files: No. 25-1164, No. 27-1058, No. 27-1059, No. 28-0517, 1999. [13] B.D. Silverman, Physical Review 125 (1962) 1921. [14] W.S. Kim, T.H. Hong, E.S. Kim, K.H. Yoon, Japanese Journal of Applied Physics 37 (1998) 5367. [15] Y.C. Chen, Y.H. Chang, Journal of Alloys and Compound 477 (2009) 450.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Improved dielectric properties of CaLa4 Ti5 O17 ceramics with Zr substitution at microwave frequency Yih-Chien Chen ∗ , Wei-Cheng Lee, Kuai-Cian Chen Department of Electrical Engineering, Lunghwa University of Science and Technology, No. 300, Sec. 1, Wanshou Rd., Gueishan Shiang, Taoyuan County, Taiwan
a r t i c l e
i n f o
Article history: Received 6 January 2009 Received in revised form 19 May 2009 Accepted 9 July 2009 Keywords: CaLa4 Ti5−x Zrx O17 Dielectric constant Quality factor Temperature coefficient of resonant frequency
a b s t r a c t In this paper, the effect of Zr4+ substitution on the dielectric properties of CaLa4 Ti5 O17 ceramics at microwave frequency by the conventional solid-state method has been studied for application in mobile communication. The diffraction peaks of CaLa4 Ti5−x Zrx O17 ceramics shifted to lower angle as x increasing from 0 to 0.05 and nearly unchanged with x increasing from 0.05 to 0.07. A maximum relative density of 97.5% TD can be obtained for CaLa4 Ti4.95 Zr0.05 O17 ceramic sintered at 1500 ◦ C for 4 h. A maximum dielectric constant and quality factor (Q × f) of CaLa4 Ti4.95 Zr0.05 O17 ceramic sintered at 1500 ◦ C for 4 h are 55.9 and 15,600 GHz, respectively. A temperature coefficient of resonant frequency ( f ) of 4.6 ppm ◦ C−1 can be obtained for CaLa4 Ti4.95 Zr0.05 O17 ceramic sintered at 1500 ◦ C for 4 h. © 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. Patch antennas however have limitations in size, gain, and efficiency, imposed by the dielectric substrate. Three dielectric properties of materials must be considered for patch antennas 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 representative of the antenna gain. Typically, there are radiation, conduction, dielectric, and surface wave losses. Therefore, the total quality factor is affected by all of these losses [1]. The microwave dielectric properties of (Ba,La)n Tin−1 O3n (n = 5, 6) have already been reported. BaLa4 Ti4 O15 and Ba2 La4 Ti5 O18 are characterized by high dielectric constant (εr ∼ 39–46), high quality factor (Q × f ∼ 11,600–31,800) and a small temperature coefficient of resonant frequency ( f ∼ −36 to 79 ppm ◦ C−1 ) [2]. MOLa2 O3 -TiO2 (M = Ca, Sr, Ba) is well known as microwave material for dielectric resonator and filter. Most of these ceramics combine a high dielectric constant (εr ∼ 42–54), high quality factor (Q × f ∼ 16,200–50,200) and an adjustable temperature coefficient of resonant frequency ( f ∼ −25 to 6 ppm ◦ C−1 ). Among the investigated orthorhombic phases, CaLa4 Ti5 O17 ceramic sintered at 1625 ◦ C gives dielectric constant ∼53.7, quality factor ∼17,300 GHz,
∗ Corresponding author. Tel.: +886 2 8209 3211x5537; fax: +886 2 8209 9721. E-mail address: [email protected] (Y.-C. Chen). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.07.028
and temperature coefficient of resonant frequency ∼−20 ppm ◦ C−1 [3]. CuO additives can effectively lower the sintering temperature of CaLa4 Ti5 O17 . A dielectric constant value of 57, a quality factor of 9000 GHz, and a temperature coefficient of resonant frequency of −10 ppm ◦ C−1 were obtained for CaLa4 Ti5 O17 ceramics with 0.5 wt% CuO sintered at 1500 ◦ C for 4 h [4]. By partial replacement of Ca with Zn, the dielectric properties of CaLa4 Ti5 O17 ceramics at microwave frequency are modified. A maximum dielectric constant of 57, a maximum quality factor of 15,000 GHz, and a near-zero temperature coefficient of resonant frequency ( f ) of −8.16 ppm ◦ C−1 were obtained for Ca0.99 Zn0.01 La4 Ti5 O17 ceramics with 0.5 wt% CuO additive sintered at 1450 ◦ C for 4 h [5]. Compositionally modified BaZrx Ti1−x O3 (BZT) received much attention due to the electrical properties to specific applications by the replacement of Ti4+ by Zr4+ [6,7]. It attracted our attention to investigate the effect of partial replacement of Ti4+ with Zr4+ on the microwave dielectric properties of CaLa4 Ti5 O17 ceramics. In this paper, investigations on CaLa4 Ti5−x Zrx O17 ceramics have been made to produce materials with better microwave dielectric properties for application in 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 obtained in order to realize an outstanding design of the patch antenna having a small size, high antenna gain, and high temperature stability. The microwave dielectric properties of CaLa4 Ti5−x Zrx O17 ceramics have been found to be different with different amounts of Zr4+ substitution and sintering temperature. For further understanding of these different microwave dielectric properties, they were analyzed on the basis of density and X-ray diffraction (XRD), and observation of microstructures.
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2. Experimental procedure The starting raw chemicals were high-purity CaCO3 , La2 O3 , TiO2 , and ZrO2 powders. The composition prepared was CaLa4 Ti5−x Zrx O17 , x = 0.01, 0.05, and 0.07. 0.5 wt% CuO was added to CaLa4 Ti5−x Zrx O17 for reducing 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 cm−2 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. Both the heating rate and cooling rate were set at 10 ◦ C min−1 . After sintering, the phases of the samples were investigated by X-ray diffraction. The X-ray Rigaku D/MAX-2200 data were collected using CuK˛ radiation (at 30 kV and 20 mA) and a graphite monochromator in the 2 range of 20–80◦ . Scanning electron microscopy (SEM; JEOL JSM-6500F) was employed to examine the microstructures of the specimens. The bulk densities of the specimens were measured by the Archimedes method using distilled water as the liquid. The microwave dielectric properties of the specimens were measured by the post-resonator method developed by Hakki and Coleman [8]. The post-resonator 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 quality 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−1 ) 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.
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of CaLa4 Ti5 O17 , CaLa4 Ti4.99 Zr0.01 O17 , CaLa4 Ti4.95 Zr0.05 O17 , and CaLa4 Ti4.93 Zr0.07 O17 ceramics in the range from 20◦ to 60◦ sintered at the temperatures of 1500 ◦ C for 4 h. The crystal structure of CaLa4 Ti5 O17 is known to have orthorhombic structure that belongs to Pnnm space group. The lattice parameters of CaLa4 Ti5 O17 are a = 0.5522 nm, b = 3.1256 nm, and c = 0.3899 nm [10]. The X-ray diffraction patterns of CaLa4 Ti5 O17 , CaLa4 Ti4.99 Zr0.01 O17 , CaLa4 Ti4.95 Zr0.05 O17 , and CaLa4 Ti4.93 Zr0.07 O17 ceramics in the range from 25◦ to 35◦ sintered at the temperatures of 1500 ◦ C for 4 h are shown in Fig. 2. It can be observed in Fig. 2, that the X-ray diffraction peaks shift to lower angles as x increases from 0 to 0.05 because the ionic radius of Zr4+ (0.083 nm) is larger than that of Ti4+ (0.064 nm) [11]. The X-ray diffraction peaks stop shifting with further increase in Zr4+ content from 0.05 to 0.07. This means
Fig. 1. X-ray diffraction patterns of CaLa4 Ti(5−x) Zrx O17 ceramics from 20 to 60◦ sintered at the temperatures of 1500 ◦ C for 4 h.
Fig. 2. X-ray diffraction patterns of CaLa4 Ti(5−x) Zrx O17 ceramics from 25 to 35◦ sintered at the temperatures of 1500 ◦ C for 4 h.
the Zr4+ solution in CaLa4 Ti(5−x) Zrx O17 ceramics is near saturation as the amounts increased to 0.05 molar. The microstructures of CaLa4 Ti4.95 Zr0.05 O17 after sintering from 1400 to 1550 ◦ C for 4 h are shown in Fig. 3. The CaLa4 Ti4.95 Zr0.05 O17 was not dense, and grains did not grow after it was sintered at 1400 ◦ C. It may affect the microwave dielectric properties of the ceramics. Comparing the microstructures of CaLa4 Ti4.95 Zr0.05 O17 sintered at different temperatures, the number of pores decreased and the rate of grain growth increased apparently. The pores almost disappeared in the specimen after sintering at 1500 ◦ C. The relative densities of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution after sintering from 1400 to 1550 ◦ C for 4 h are shown in Fig. 4. The theoretical density of CaLa4 Ti5 O17 is 5.46 g cm−3 [12]. The relative density of CaLa4 Ti(5−x) Zrx O17 ceramics exceeded 83% of its theoretical density (TD) in all cases. The relative density was found to increase to a maximum value when sintered at 1500 ◦ C and thereafter decreased. The relative density increased with increasing sintering temperature may be due to the decrease in the number of pores as shown in Fig. 3. The relative density increased from 83.3% TD to 97.5% TD as the sintering temperature increased from 1400 to 1500 ◦ C for CaLa4 Ti4.95 Zr0.05 O17 ceramics sintered at 1500 ◦ C for 4 h. A maximum relative density of 97.5% TD was obtained for CaLa4 Ti4.95 Zr0.05 O17 ceramics sintered at 1500 ◦ C for 4 h. The porosities of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution after sintering from 1400 to 1550 ◦ C for 4 h are shown in Fig. 4. The porosity of CaLa4 Ti(5−x) Zrx O17 ceramics is lower than 0.8% in all cases and spanned in the range from 0.4% to 0.8%. The dielectric constants of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution sintered at different temperatures for 4 h are illustrated in Fig. 5. The relationships between the dielectric constant and the sintering temperature revealed the same trend as that between the relative density and the sintering temperature. The decrease in dielectric constant was caused by the low densities of the ceramics. Higher density leads to lower porosity, thus, a higher dielectric constant can be achieved. The dielectric constant was found to increase to a maximum value at sintering temperature of 1500 ◦ C and thereafter decreased for CaLa4 Ti4.99 Zr0.01 O17 , CaLa4 Ti4.95 Zr0.05 O17 and CaLa4 Ti4.93 Zr0.07 O17 ceramics. Increasing sintering temperature is not necessary for getting a higher dielectric constant. A maximum value of dielectric constant of 55.9 was obtained for CaLa4 Ti4.95 Zr0.05 O17 sintered at 1500 ◦ C for 4 h. The difference between the dielectric constant of CaLa4 Ti4.95 Zr0.05 O17 and CaLa4 Ti4.93 Zr0.07 O17 is not obvious. This is due to the upper limit of Zr4+ solid solution in CaLa4 Ti(5−x) Zrx O17 ceramics is 0.05 molar as observed from the results of X-ray diffraction patterns. The quality factor (Q × f) of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution sintered at different temper-
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Fig. 3. Microstructures of the CaLa4 Ti4.95 Zr0.05 O17 ceramics sintered at different temperatures for 4 h.
atures for 4 h are also shown in Fig. 5. With increasing the sintering temperature, the quality factor increased to a maximum value at sintering temperature of 1500 ◦ C and thereafter decreased for CaLa4 Ti4.99 Zr0.01 O17 , CaLa4 Ti4.95 Zr0.05 O17 , and CaLa4 Ti4.93 Zr0.07 O17 . A maximum quality factor of 15,600 GHz was obtained for CaLa4 Ti4.95 Zr0.05 O17 sintered at 1500 ◦ C for 4 h. Compared with the measurement results of quality factor as shown in ref. 4, the value of quality factor was increased from 9000 to 15,600 GHz as the amounts of Zr4+ substitution increased from 0 to 0.005 molar. The deviation between the quality factor of CaLa4 Ti4.95 Zr0.05 O17 and CaLa4 Ti4.93 Zr0.07 O17 is not evident. Similar phenomena were also observed in dielectric constant. This is because the upper limit of Zr4+ solid solution in CaLa4 Ti(5−x) Zrx O17 ceramics is 0.05
molar. It also can be found that the maximum quality factor of the CaLa4 Ti5 O17 ceramic with partial replacing of Ti4+ with Zr4+ is larger than that with partial replacing of Ca2+ with Zn2+ as shown in ref. 5. The relationship between the quality factor 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 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 [13,14]. Because the quality factor of CaLa4 Ti(5−x) Zrx O17 ceramics was consistent with the variation of the relative density, it is suggested that the
Fig. 4. Relative densities and porosities of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution at different sintering temperatures.
Fig. 5. Dielectric constant and quality factor of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution at different sintering temperatures.
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lier reports, improvement on the microwave dielectric properties had been achieved by partially substituting Ti4+ ions with Zr4+ ions. A dielectric constant value of 55.9, a quality factor of 15,600 GHz, and a near-zero temperature coefficient of resonant frequency of 4.6 ppm ◦ C−1 were obtained for CaLa4 Ti4.95 Zr0.05 O17 ceramic sintered at 1500 ◦ C for 4 h. The microwave dielectric properties of CaLa4 Ti(5−x) Zrx O17 ceramics are strongly dependent on the density and microstructures. Acknowledgment
Fig. 6. Temperature coefficient of frequency ( f ) of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution at different sintering temperatures.
quality factor of CaLa4 Ti(5−x) Zrx O17 ceramics is mainly controlled by the density. The temperature coefficient of resonant frequency ( f ) values of CaLa4 Ti(5−x) Zrx O17 ceramics with different amounts of Zr4+ substitution sintered at different temperatures for 4 h can be found in Fig. 6. In general, the temperature coefficient of resonant frequency is related to the composition, the amount of additive, and the second phases that existed in the ceramics [15]. There was no second phase in CaLa4 Ti(5−x) Zrx O17 ceramics. The amount of CuO additive added in CaLa4 Ti(5−x) Zrx O17 ceramics is fixed at 0.5 wt% in all cases. So the effect of the second phase and amount of additive could be neglected. In this experiment, f was major influenced by the composition. A near-zero temperature coefficient of resonant frequency of 4.6 ppm ◦ C−1 was measured for CaLa4 Ti4.95 Zr0.05 O17 ceramic sintered at 1500 ◦ C for 4 h. 4. Conclusions The dielectric properties of CaLa4 Ti(5−x) Zrx O17 ceramics at microwave frequencies have been investigated. Compared with ear-
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