Studies on the (Mg,Zn)TiO3-CaTiO3 microwave dielectric ceramics

Materials Letters 59 (2005) 2337 – 2341 www.elsevier.com/locate/matlet

Studies on the (Mg,Zn)TiO3-CaTiO3 microwave dielectric ceramics Hao Su*, Shunhua Wu School of Electronic Information Engineering, Tianjin University, Tianjin, 300072 P.R. China Received 10 December 2004; accepted 25 January 2005 Available online 7 April 2005

Abstract In this paper, the microwave dielectric characteristics and microstructures of (1 x)(Mg1 y,Zny )TiO3 – xCaTiO3 (x = 0.07 ¨ 0.11, y = 0.2 ¨ 0.4) ceramic system were studied. The replacement of Mg2 + ion with Zn2 + ion increases the dielectric constant and quality factor. The phase composition of the system changes from Mg2TiO4 to MgTiO3 during the sintering, which improves the dielectric properties. CaTiO3 increases the dielectric constant, reduce the loss and also compensate the temperature coefficient of (Mg,Zn)TiO3. The existing area of the different compositions in the ceramics was observed by means of energy dispersive spectroscopy, which reveals that CaTiO3 coexists with (Mg,Zn)TiO3 in minor crystal grain. And the existing of CaTiO3 phase can inhibit abnormal grain growth of the phase of (Mg,Zn)TiO3. A dielectric constant e r of 22.5, a Q  f value of 86,000 (at 7.5 GHz) and a s f value of + 3 ppm/-C were obtained for 0.91(Mg0.7,Zn0.3)TiO3 – 0.09CaTiO3 ceramics sintered at 1310 -C for 3 h. D 2005 Elsevier B.V. All rights reserved. Keywords: Ceramic; Microstructure; Dielectric properties

1. Introduction In the past decade, microwave dielectric resonators and antennas have been developed for applications in communication systems such as cellular phone, direct broadcasting satellite and global positioning systems. These microwave dielectric devices require the combined dielectric properties of a high dielectric constant, a low dielectric loss and a nearzero temperature coefficient of resonant frequency (s f ). These three parameters are correlated to the size, frequency selectivity and temperature stability of the system, respectively. To satisfy the demands of microwave circuit designs, each dielectric property should be precisely controlled. MgTiO3-based ceramics are well known as microwave dielectric with high Q value and low s f value. But the sintering temperature to densify MgTiO3-based ceramics is 1400 -C at least. Since the last 10 years in 20th century, some studies concerned with (Zn,Mg)TiO3 focused on lowering the sintering temperature, but the dielectric proper-

* Corresponding author. E-mail address: [email protected] (H. Su). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.01.087

ties gained were not perfect to satisfy the practical application in the microwave devices [1,2]. This study focused on finding the joint of perfect dielectric characteristics and low sintering temperature of (Mg,Zn)TiO3 (noted as MZT), and investigated the microstructures of the phases. The ratio of Mg2 + ion to Zn2 + ion here was much different with previous studies mentioned above [1,2]. CaTiO3 was added to increase the dielectric constant, decrease the loss value and compensate the temperature coefficient.

2. Experimental procedure The raw materials (MgCO3)4IMg(OH)2I5H2O, ZnO and TiO 2 were mixed according to the composition of (Mg1 x ,Znx )TiO3(x = 0.2 ¨ 0.4). The purity of these powders was higher than 99.9%. They were milled with ZrO2 balls in distilled water for 5 h, then dried and calcined in air at 1170 -C for 3 h. CaCO3 and TiO2 were mixed aside according to the stoichiometry of CaTiO3. They were milled with ZrO2 balls in distilled water for 7 h, then dried and calcined in air at 1300 -C for 3 h. After that, the two kinds of calcined powders were mixed together according to the

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H. Su, S. Wu / Materials Letters 59 (2005) 2337 – 2341

3. Results and discussion

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3.1. Crystalline phase and discussion

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2θ Fig. 1. XRD pattern for calcined (Mg,Zn)TiO3 powders g: MgTiO3 r: Mg2TiO4 n: ZnTiO3 ?: Zn2TiO4.

The XRD patterns for the specimens of (Mg,Zn)TiO3 powders calcined at 1170 -C for 3 h and MZTC ceramics sintered at 1310 -C for 3 h are shown in Figs. 1 and 2, respectively. It’s clearly observed that the main crystal phase of the calcined (Mg,Zn)TiO3 powders is MgTiO3 accompanied by minor phases Mg2TiO4 ZnTiO3 and Zn2TiO4. On the other hand, the main crystal phase of the MZTC ceramics is MgTiO3 accompanied by minor phases CaTiO3, ZnTiO3 and Zn2TiO4. Comparing the results, it can be found that the phase of Mg2TiO4 in the calcined powders almost disappeared after

composition of (1 x)(Mg1 1y,Zny )TiO3 – yCaTiO3 (x = 0.07 ¨ 0.11, y = 0.2 ¨ 0.4, noted as MZTC), milled again for 9 h and dried. Then they were sieved with an 80 mesh screen and pressed homogeneously into pellets at 75 MPa. The pellets were sintered in air at 1270 –1330 -C for 3 h at a heating rate of 5 -C/min. Finally they were covered with Ag electrodes for measurement by decomposing AgO slurry under 800 -C. The crystalline phases of sintered ceramics were examined with an X-ray diffractometer (Model 2038X, Rigaku). The surface microstructure was observed with Scanning Electronic Microscope (SEM) and Energy Dispersive Spectroscopy (EDS).The dielectric constants and the Q values were measured by employing the Hakki and Coleman method [3]. The apparatus consisted of parallel conducting brass plates and coaxial probes connected to a HP8720ES S-parameter network analyzer. The temperature coefficient of resonant frequency (s f ) was measured with the test set which was placed over a thermostat in the temperature range from 25 to 85 -C. The s f value was calculated using the equation s f = ( f 85 f 25) / ( f 25I60 -C), where f 85 and f 25 are the resonant frequency of the samples at 85 and 25 -C, respectively.

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2θ Fig. 2. XRD pattern for MZTC ceramics g: MgTiO3 n: ZnTiO3 ?: Zn2TiO4 >: CaTiO3.

Fig. 3. The EDS results of 0.91(Mg0.7,Zn0.3)TiO3 – 0.09CaTiO3 ceramics.

H. Su, S. Wu / Materials Letters 59 (2005) 2337 – 2341 23.5

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rules, adding CaTiO3 to the (Mg,Zn)TiO3 system can enhance the dielectric constant and adjust the temperature coefficient of dielectric constant to near zero. Electronic and ionic polarizations are the main polarization mechanisms of MgTiO3. They are rapid polarizations with low dielectric loss in microwave range, so MZTC system with a main phase of MgTiO3 has a low dielectric loss. The existing areas of the different compositions in the ceramics were observed by means of EDS (Fig. 3). The grain morphology of dense MZTC ceramics exhibited two different types of grains: larger circular grain of 15 – 25 Am, small grain of 2– 6 Am. Spot 1 showed Mg Zn and Ti rich but Ca almost free. Spot 2 showed Mg Zn Ca and Ti rich. Combining these results with the XRD analysis of the ceramics, it can be confirmed that the large grains were (Mg,Zn)TiO3 and the small grains were (Mg,Zn)TiO3 and CaTiO3 coexisted. Extra analysis of spot 3 demonstrated the same composition as that of spot 2.

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Zn (y) Fig. 4. Dielectric properties of MZTC ceramics as a function of the amount of Zn.

the powders were sintered to ceramics, which illustrates that Mg2TiO4 and rutile synthesized MgTiO3 during the sintering process. The crystal structure of MgTiO3 is known as hexagonal structure with lattice parameter a 0 = b 0 = 0.50787 nm, c 0 = 1.3898 nm. MgTiO3 has a dielectric constant e r of 17 approximately, a Q  f value of 160,000 at 7 GHz and a negative temperature coefficient of resonant frequency ( 50 ppm/-C) [4]. ZnTiO3 is also hexagonal structure. MgTiO3 and ZnTiO3 form the solid solutions of the zinc magnesium titanates easily because of their same ilmenite structure and similar ionic radius (Mg2 + 0.066 nm, Zn2 + 0.074 nm). The formation of the solid solutions helps to decrease the sintering temperature needed to densify the ceramics. When sintered above 945 -C, ZnTiO3 decomposes to Zn2TiO4 and rutile. Therefore it’s unsuccessful to prepare pure ZnTiO3 without a companion of Zn2TiO4 from a mixture of 1ZnOI1TiO2 by conventional solid-state reaction [5]. Zn2TiO4 is cubic structure with the lattice parameter of 0.84602 nm. Except the studies concerned about the ZnO –TiO2 system, the microwave dielectric characteristics of pure ZnTiO3 and Zn2TiO4 are rarely measured. CaTiO3 is cubic structure. The dielectric constant of CaTiO3 (e r = 170) is much higher than that of (Mg,Zn)TiO3, and CaTiO3 has a low Q  f value of 3600 at 7 GHz, a large positive temperature coefficient of resonant frequency (+ 800 ppm/-C) [6]. According to the well-known mixing

Fig. 5. The SEM micrograph of the surface of 0.91(Mg1-y,Zny )TiO3 – 0.09CaTiO3 ceramics with different amount of Zn (a) y = 0.2, (b) y = 0.3, (c) y = 0.4.

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H. Su, S. Wu / Materials Letters 59 (2005) 2337 – 2341 100000

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factor. The s f value isn’t affected obviously by the change of y.

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3.3. The effects of Ca on the dielectric properties of the MZTC system Fig. 6 shows the dielectric properties of (1 x) (Mg0.7,Zn0.3)TiO3 xCaTiO3 ceramics with different x value sintered at 1310 -C for 3 h. Fig. 7 is the SEM micrograph of the surface of MZTC ceramics with different x values. The figures show that the dielectric constant of the system increased from 21.8 to 23.2 and the temperature coefficient of resonant frequency varied from 22 to + 17 ppm/-C with the increase of the mole ratio of CaTiO3. CaTiO3 has a much higher e r and a larger positive s f value than those of (Mg,Zn)TiO3 [6]. According to Lichtenecker’s mixing rules, the system with higher content of CaTiO3 gives a higher dielectric constant and a trend of s f value to be positive.

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CaTiO3 (x) Fig. 6. Dielectric properties of MZTC ceramics as a function of the amount of Ca.

3.2. The effects of Zn on the dielectric properties of the MZTC system Fig. 4 shows the dielectric properties of 0.91(Mg1 y,Zny ) TiO3 0.09CaTiO3 ceramics with different Mg/Zn mole ratio sintered at 1310 -C for 3 h. Fig. 5 is the SEM micrograph of the surface of MZTC ceramics with different y values. Both of the dielectric constant and Q  f value reach the peak values when the mole ratio of Zn is 0.3. The amount of Zn added decides the amount of unreacted ZnO as a kind of sintering agent in the system during the sintering process. The existing of ZnO is helpful to the densification of the ceramics, which can be confirmed from the SEM micrograph in Fig. 5. The specimen of y = 0.2 (5a) is not dense enough with some pores in the ceramic. But the pores were almost eliminated in the specimen of y = 0.3 (5b). The liquid phase sintering effect by ZnO was clearly observed in the grain morphology. More uniform grain size was observed when y = 0.3 than y = 0.2, which is due to more liquid phase wetting the grain of MZTC ceramics. The dielectric constant and quality factor are mainly affected by the grain size and density of the ceramic here. For liquid sintering of ceramics, the liquid phase would be resident or disappeared in final stages. In this study, the liquid phase is observed to be resident in the ceramic among the grains according to y = 0.4 in Fig. 5(c), which results in the decreases of the dielectric constant and quality

Fig. 7. The SEM micrograph of the surface of MZTC ceramics with different amount of Ca (a) x = 0.07, (b) x = 0.09, (c) x = 0.11.

H. Su, S. Wu / Materials Letters 59 (2005) 2337 – 2341

The quality factor was mainly affected by the grain phase. When the amount of CaTiO3 is appropriate (x = 0.09), the average grain sizes of the large and small grains were approximate 20 and 4 Am, respectively, which is shown in Fig. 7(b). When the mole ratio of CaTiO3 reduced to 0.07, the grain sizes of large grains have a remarkable increase, which is shown in Fig. 7(a). This phenomenon reveals that the existing of CaTiO3 phase can inhibit abnormal grain growth of the main phases, which is helpful to gain excellent dielectric properties. But excess amount of CaTiO3 (x = 0.11) increased the loss value of the system because of its low Q  f value of 3600. The SEM micrograph of the surface of MZTC ceramics with x = 0.11 is shown in Fig. 7(c).

4. Conclusion The main crystal phase of the (1 x) (Mg1 y,Zny )TiO3 – xCaTiO3 systems are MgTiO3 accompanied by minor phases CaTiO3, ZnTiO3 and Zn2TiO4. (Mg,Zn)TiO3 forms large grains. And (Mg,Zn)TiO3 and CaTiO3 coexisted in minor grains. CaTiO3 phase can inhibit abnormal grain

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growth of the main phases. The replacement of Mg2 + ion with Zn2 + ion increase the dielectric constant and quality factor of the MZTC ceramics. CaTiO3 improved the dielectric constant, reduce the loss value and compensate the temperature coefficient of (Mg,Zn)TiO3. By appropriately adjusting the content of CaTiO3 (x value), a near zero temperature coefficient can be gained.

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