Microwave dielectric properties and microstructures of MgTa 2 O 6 ceramics with CuO addition

Transcription

1 Materials Chemistry and Physics 90 (2005) Microwave dielectric properties and microstructures of MgTa 2 O 6 ceramics with CuO addition Cheng-Liang Huang a,, Kuo-Hau Chiang a, Chi-Yuen Huang b a Department of Electrical Engineering, National Cheng Kung University, No. 1 University Road, Tainan 70101, Taiwan b Department of Resources Engineering, National Cheng Kung University, No. 1 University Road, Tainan 70101, Taiwan Received 18 June 2004; received in revised form 11 October 2004; accepted 19 October 2004 Abstract The effect of CuO addition on the microstructures and the microwave dielectric properties of MgTa 2 O 6 ceramics has been investigated. It is found that low level-doping of CuO (up to 1 wt.%) can significantly improve the density of the specimens and their microwave dielectric properties. Tremendous sintering temperature reduction can be achieved due to the liquid phase effect of CuO addition observed by scanning electronic microscopy (SEM). The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. With 0.5 wt.% CuO addition, MgTa 2 O 6 ceramic can be sintered at 1400 C and possesses a dielectric constant (ε r ) of 28, a Q f value of GHz and a temperature coefficient of resonant frequency (τ f ) of 18 ppm/ C Elsevier B.V. All rights reserved. Keywords: A. Sintering; C. Dielectric Properties; E. Functional applications 1. Introduction Development of a microwave dielectric resonator for applications in communication systems such as cellular phone, direct broadcasting satellite (DBS) and global positioning systems has been rapidly progressing in the past decade. An advantage of using dielectric resonators is that they make the size reduction of microwave components possible. Requirements for these dielectric resonators include the combined dielectric properties of a high dielectric constant (ε r > 25), a low dielectric loss (Q > 5000, where Q = 1/tan δ) and a nearzero temperature coefficient of resonant frequency (τ f ) which is as small as possible [1]. These three parameters are related 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. Most of the known dielectric materi- Corresponding author. Tel.: x62390; fax: address: huangcl@mail.ncku.edu.tw (C.-L. Huang). als for high frequency applications show high quality factors and dielectric constants. However, they also require high sintering temperature. Chemical processing and small particle sizes of the starting materials are generally advantageous to reduce the sintering temperature of dielectric materials. However, they require a flexible procedure, which in not only expensive but also time-consuming. Liquid phase sintering by adding glass or other low melting point material was found to effectively lower the firing temperature of ceramics [1 5]. MgTa 2 O 6 compound, reported by Lee et al., possesses the microwave dielectric properties as follows: ε r 30.3, Q f (GHz), and τ f 30 ppm/ C [6]. However, it also requires a sintering temperature as high as 1550 C. The purpose of this paper is to introduce CuO additive that could produce liquid-phase sintering to lower the firing temperature of MgTa 2 O 6 dielectric material. Moreover, good densification could be achieved by the liquid phase effect of CuO addition [7 9]. The effect of CuO addition on the microstructures and the microwave dielectric properties of the MgTa 2 O 6 ceramics were also investigated /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.matchemphys

2 374 C.-L. Huang et al. / Materials Chemistry and Physics 90 (2005) Experimental procedure Specimens of the MgTa 2 O 6 ceramics were synthesized by conventional solid-state method from individual oxide powders: MgO, Ta 2 O 5 (>99.9%). The starting materials were weighted in stoichiometric proportions with CuO as sintering aids and ball-milled for 12 h in de-ionized water using resin pots with agate balls. The mixtures were dried and calcined at 1100 C for 3 h. The calcined powders were remilled for 6 h with PVA solution as a binder and pressed at 2000 g/cm 3 to form pellets with 11 mm in diameter and 5 mm thickness. The samples were sintered at C for 3 h with heating and cooling rates of 10 C/min. The quantitative analysis of the sintered ceramics was completed using X-ray powder diffraction spectroscopy with Cu K radiation. The microstructures observation of the sintered specimens was performed using scanning electron microscopy. Fig. 1. X-ray diffraction patterns of MgTa 2 O 6 ceramics with 0.5 wt.% CuO addition at different sintering temperatures. Fig. 2. SEM micrographies of MgTa 2 O 6 ceramics with (a) 0.25 (b) 0.5 and (c) 1 wt.% CuO addition at different sintering temperatures.

3 C.-L. Huang et al. / Materials Chemistry and Physics 90 (2005) Results and discussions Fig. 2. (Continued ). Fig. 1 shows the X-ray diffraction patterns of the 0.5 wt.% CuO-doped MgTa 2 O 6 ceramics at different sintering temperatures. The diffraction peaks indicated single MgTa 2 O 6 phase with trirutile structure. With the increase of sintering temperature, the intensity of MgTa 2 O 6 phase was slightly enhanced. No second phase was found in the XRD spectrum since identification of the minor phase by X-ray diffraction is extremely difficult. The SEM photographs of CuO-doped MgTa 2 O 6 ceramics at different sintering temperatures ( C) were illustrated in Fig. 2. Many pores were observed at 1310 C and eliminated at higher temperatures in all cases. The grain size increased with increasing sintering temperature. The liquid phase effect was observed in the grain morphology. For MgTa 2 O 6 ceramics with 0.25 and 0.5 wt.% CuO additions, the grain size was also enhanced with the increase of CuO content. It suggested that the liquid phase did benefit to the densification of the specimen. Although the grain size of the 1 wt.% CuO-doped sample also increased with the increase of sintering temperature, it seemed to saturate and its size remained almost unchanged at temperatures higher than 1340 C. It implied exceed liquid phase would inhibit the grain growth of the ceramics. Similar phenomenon was observed and reported for different materials [7]. Fig. 3 illustrates the apparent densities of the MgTa 2 O 6 ceramics as a function of sintering temperature with different amount of CuO additions. The densities increased with increasing sintering temperature and saturated at 1400 C for samples with 0.25 wt.% CuO addition. The saturated temperature of the density was even lower (1370 C) for 0.5 wt.% CuO-doped ceramics. The improved densification was caused by the liquid phase effect of CuO as discussed in Fig. 2. However, the density decreased with the increase of sintering temperature for 1 wt.% CuO-doped MgTa 2 O 6 ceramics, which was due to exceed liquid phase. It could be concluded that better densification of ceramics can be achieved with proper amount of liquid phase. The bulk densities of the sintered samples were measured by Archimedes method. The dielectric constant and the quality factor at microwave frequency were measured using Hakki- Coleman dielectric resonant method as modified and improved by Courtney [10,11]. The temperature coefficient of resonant frequency was obtained by measuring the TE 01 resonant frequency at temperatures C calculated by the following equation. f 80 f ppm/ C (1) 60f 20 where f T is the resonant frequency of the dielectric resonator at temperature T ( C). The instrument of the HP8757D network analyzer and the HP8350 sweep oscillator were applied in the measurement. Fig. 3. The apparent density of CuO-doped MgTa 2 O 6 ceramics as a function

4 376 C.-L. Huang et al. / Materials Chemistry and Physics 90 (2005) Fig. 4. The dielectric constant of CuO-doped MgTa 2 O 6 ceramics as a function Fig. 4 plots the dielectric constant of the CuO-doped MgTa 2 O 6 ceramics as a function The relationship between ε r value and sintering temperature revealed the same trend with those between density and sintering temperature since higher density represents lower porosity. Decrease of the ε r value for specimen with 1 wt.% CuO addition could be explained owing to lower densities as well as exceed liquid phase. However, the ε r value increased from 26.7 to 27.4 as the sintering temperature increased from 1310 to 1440 C for MgTa 2 O 6 ceramics with 0.5 wt.% CuO addition since appropriate amount of liquid phase would benefit to the densification of the ceramics. The plots of the Q f values of the CuO-doped MgTa 2 O 6 ceramics versus its sintering temperature are illustrated in Fig. 5. The microwave dielectric loss was mainly caused not only by the lattice vibration but also by the densification and the porosity. Relative density is also recognized as an important factor in controlling the dielectric loss and has been shown for other microwave dielectric materials. Variation of the Q f value of the CuO-doped MgTa 2 O 6 ceramics was similar with that of the apparent density. It suggested the density dominated the dielectric loss of the samples. However, one exception appeared for specimen with 0.5 wt.% Fig. 6. The temperature coefficient of resonant frequency of CuO-doped MgTa 2 O 6 ceramics as a function CuO addition at 1400 C and it possessed a maximum Q f value of 58,000 GHz. As observed in Fig. 2, a rapid grain growth appeared at 1440 C, which might caused an increase in the dielectric loss resulting in a degradation in its Q f value. Fig. 6 shows the temperature coefficient of resonant frequency of the CuO-doped MgTa 2 O 6 ceramics at different sintering temperatures. The temperature coefficient of resonant frequency is related to the composition and the second phase of the materials. Since the composition (MgTa 2 O 6 ) remained unchanged, no significant difference was observed in the τ f value with a fixed addition level at different sintering temperatures. However, the τ f value was a function of CuO content and varied from 23.9 to 10 ppm/ C as the amount of CuO addition increased from 0.25 to 1 wt.% at 1440 C. Since the ceramics are compositionally fixed, it is believed that liquid phase should be responsible for the variation of the τ f value. 4. Conclusion The microwave dielectric properties and the microstructures of MgTa 2 O 6 ceramics with CuO additions were investigated. The liquid phase was found to dominate the densification and the dielectric loss of the specimen. Appropriate amount of liquid phase could not only lower the sintering temperature but also possess compatible microwave dielectric properties of the MgTa 2 O 6 ceramics. With 0.5 wt.% CuO addition, MgTa 2 O 6 ceramics can be sintered at 1400 C and shows excellent microwave dielectric properties: ε r 28, Q f 5 8,000 GHz and τ f 18 ppm/ C. Acknowledgement Fig. 5. The quality factor of CuO-doped MgTa 2 O 6 ceramics as a function This work was supported by the National Science Council of the Republic of China under grant NSC E

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