Improved high-q microwave dielectric resonator using CuO-doped MgNb 2 O 6 ceramics

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1 Materials Research Bulletin 38 (2003) Improved high-q microwave dielectric resonator using CuO-doped MgNb 2 O 6 ceramics Cheng-Shing Hsu a, Cheng-Liang Huang a,*, Jing-Fang Tseng a, Chi-Yuen Huang b a Department of Electrical Engineering, National Cheng Kung University, No. 1 University Rd., Tainan 70101, Taiwan b Department of Resources Engineering, National Cheng Kung University, No. 1 University Rd., Tainan 70101, Taiwan Received 6 December 2002; accepted 28 January 2003 Abstract The microwave dielectric properties and the microstructures of MgNb 2 O 6 ceramics with CuO additions (1 4 wt.%) prepared with conventional solid-state route have been investigated. The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. It is found that MgNb 2 O 6 ceramics can be sintered at C due to the liquid phase effect of CuO addition. At C, MgNb 2 O 6 ceramics with 2 wt.% CuO addition possesses a dielectric constant (e r ) of 19.9, a Q f value of 110,000 (at 10 GHz) and a temperature coefficient of resonant frequency (t f )of 44 ppm/8c. The CuO-doped MgNb 2 O 6 ceramics can find applications in microwave devices requiring low sintering temperature. # 2003 Published by Elsevier Science Ltd. Keywords: A. Ceramics; D. Dielectric properties 1. Introduction Recently, researchers have been focusing on developing dielectric materials with high quality factor (Q f ), high dielectric constant (e r ) and zero temperature coefficient of resonant frequency (t f ) for the use of dielectric resonator and microwave device substrate. High dielectric constant material can effectively reduce the size of resonators since that the wavelength (l) in dielectrics is inversely p proportional to ffiffiffiffi p e r (l ¼ l0 = ffiffiffiffi e r where l0 is the wavelength in vacuum). The inverse of the dielectric loss (Q ¼ 1/tand) is required to be high for achieving prominent frequency selectivity and stability in * Corresponding author. Tel.: þ ; fax: þ address: huangcl@mail.ncku.edu.tw (C.-L. Huang) /03/$ see front matter # 2003 Published by Elsevier Science Ltd. doi: /s (03)

2 1092 C.-S. Hsu et al. / Materials Research Bulletin 38 (2003) microwave transmitter components. Moreover, small temperature coefficient of resonant frequency ensures high stability of the microwave components at different working temperatures. Several compounds such as (Zr,Sn)TiO 4, Ba(Mg 1/3 Ta 1/3 )O 3 and (Mg,Ca)TiO 3 have therefore been developed [1 3]. The MgNb 2 O 6 ceramics having the columbite crystal structure, had been investigated by Lee et al. [4,5]. Based upon the microwave dielectric properties (e r 21:4, Q f 93,800 GHz and t f 70 ppm/8c at C; [5]), it proposed possible applications in dielectric resonators. However, a modification in t f is necessary due to the coefficient s large negative value. In addition, sintering temperatures also need to be lowered for applications with low-temperature co-fired ceramics. In the producing miniaturized devices, multilayer structures with low sintering temperatures are needed to co-fire with low melting point electrodes. Low melting glass addition, chemical processing and small particle sizes of the starting materials are generally advantageous to reduce the sintering temperature of dielectric materials [6 9]. Moreover, using low melting glass additions is generally the most effective and least expensive technique. Since CuO is one of the most popular sintering fluxes, it was chosen as a sintering aid to lower the firing temperature of the MgNb 2 O 6 ceramics in this paper. The microwave dielectric properties and the microstructures of CuO-doped MgNb 2 O 6 ceramics were also investigated. Fig. 1. X-ray diffraction patterns of 2 wt.% CuO-doped MgNb 2 O 6 ceramics at different sintering temperatures.

3 2. Experiment procedure C.-S. Hsu et al. / Materials Research Bulletin 38 (2003) Samples of MgNb 2 O 6 were synthesized by conventional solid-state method. The starting materials were mixed according to a stoichiometric ratio. High purity oxide powders MgO, Nb 2 O 5 and CuO were weighted and mixed for 24 h with distilled water. The mixture was dried at 100 8C, and thoroughly milled before it was calcined at C for 2 h. The calcined powder MgNb 2 O 6 was ground and sieved through 100-mesh screen. Phase formation of MgNb 2 O 6 was confirmed using X-ray diffraction. The calcined powder was then re-milled for 24 h with 2 wt.% of a 10% solution of PVA as a binder. The milled powders were pressed into disk 11 mm in diameter and 5 mm in thickness. A pressing pressure of 2000 kg/cm 2 was used for all samples. The pellets were sintered at temperatures C for 3 h in the air. The X-ray diffraction (XRD, Rigaku D/Max III. V) data of powder and bulk samples were collected using Cu Ka radiation (at 30 KV and 20 ma) and a graphite monochromator in the 2y range of The microstructural observations and analysis of sintered surface were performed using a scanning electron microscopy (SEM, Philips XL 40FEG) and an energy dispersive X-ray spectrometer (EDS). The dielectric constants (e r ) and Q f values at microwave frequencies were measured using the Hakki-Coleman dielectric resonator method, as modified and improved by Courtney. The dielectric resonator was positioned between two brass plates such like cavity. Microwave dielectric properties of sintered samples were measured by HP8757D network analyzer and HP8350B sweep oscillator. Fig. 2. X-ray diffraction patterns of MgNb 2 O 6 ceramics with different amount of CuO addition sintered at C.

4 1094 C.-S. Hsu et al. / Materials Research Bulletin 38 (2003) For temperature coefficient of resonant frequency (t f ), the technique is the same as that of quality factor measurement. The test cavity is placed over a thermostat and the temperature range used is þ25 to þ80 8C. The t f (ppm/8c) is calculated by noting the change in resonant frequency (Df) by, t f ¼ f 2 f 1 f 1 ðt 2 T 1 Þ where f 1 is resonant frequency at T 1 and f 2 is the resonant frequency at T 2. (1) 3. Results and discussions Fig. 1 shows the X-ray diffraction patterns of 2 wt.% CuO-doped MgNb 2 O 6 ceramics produced at different sintering temperatures ( C). Similar X-ray diffraction patterns were detected for the MgNb 2 O 6 ceramics with 2 wt.% CuO addition at sintering temperatures C. Second phase was not observed at the level of 2 wt.% CuO addition since detection of a minor phase by X-ray is extremely difficult. The X-ray diffraction patterns of MgNb 2 O 6 ceramics with different amount of Fig. 3. SEM photographs of MgNb 2 O 6 ceramics with (a) 1 wt.%, (b) 2 wt.% and (c) 4 wt.% CuO additions at different sintering temperatures.

5 C.-S. Hsu et al. / Materials Research Bulletin 38 (2003) Fig. 3. (Continued ). CuO addition sintered at C are illustrated in Fig. 2. Identical XRD patterns were observed for the ceramics irrespective of the amount of CuO additions. The surface microstructural photographs of CuO-doped MgNb 2 O 6 sintered at temperatures C are presented in Fig. 3. The grain size increased with the increase of sintering temperature as well as amount of CuO addition due to liquid phase effect. However, rapid grain growth appeared for MgNb 2 O 6 specimen with CuO additions at sintering temperatures higher than C. These may directly affect the microwave dielectric properties of the MgNb 2 O 6 samples. For liquid phase sintering of ceramics, the liquid would be either resident or disappear in the final stages. In the experiments, boundary phases were clearly observed at high CuO-doping levels (2 wt.%). Fig. 4 shows the EDS spectra of grain boundaries of CuO-doped MgNb 2 O 6 specimen. It revealed that the grain boundaries of the MgNb 2 O 6 ceramics with various amount of CuO additions exhibit Cu rich liquid phases. The formations of the liquid phases were attribute to the additions of CuO. In addition, high CuO additions also produce more liquid phase, which is believed to significantly affect the microwave dielectric properties of the ceramic samples. With 2 wt.% CuO addition, a Cu atomic percentage of was obtained for MgNb 2 O 6 ceramics sintered at C. The density of the CuO-doped MgNb 2 O 6 ceramics at different sintering temperatures is shown in Fig. 5. The density initially increased with increasing sintering temperature. After reaching the

6 1096 C.-S. Hsu et al. / Materials Research Bulletin 38 (2003) Fig. 4. EDS spectra of MgNb 2 O 6 ceramics with (a) 1 wt.%, (b) 2 wt.%, (c) 4 wt.% CuO additions at C and (d) 2 wt.% CuO addition at C.

7 C.-S. Hsu et al. / Materials Research Bulletin 38 (2003) Fig. 5. Dependence of sintering temperature of MgNb 2 O 6 ceramics on density with various CuO additions. maximum at C, it decreased thereby owing to the rapid grain growth as well as the liquid boundary phases. It seemed that CuO did contribute to the densification of MgNb 2 O 6 ceramics at low temperatures. The maximum density was found to be 4.85 g/cm 3 for specimen with 1 wt.% CuO addition at C. It is difficult to synthesize ceramics with ultra-high relative density by the conventional solid-state method unless it is augmented by other methods, such as chemical processing. The plots of dielectric constant of MgNb 2 O 6 ceramics with different amounts of CuO additions as functions of sintering temperature were illustrated in Fig. 6. The relationships between dielectric constant and sintering temperature reveal the same trend as that for density and sintering temperature since higher density represents lower porosity. Highest dielectric constant was obtained for CuO-doped MgNb 2 O 6 ceramics at C. Moreover, the dielectric constant slightly decreased with increasing CuO content. It varied from 20.5 to 19.4 as the amount of CuO addition increased from 1 to 4 wt.% at C. The variation of the E r value was mainly a result from the density of the specimen. Fig. 6. Dependence of sintering temperature of MgNb 2 O 6 ceramics on dielectric constant with various CuO additions.

8 1098 C.-S. Hsu et al. / Materials Research Bulletin 38 (2003) Fig. 7. Dependence of sintering temperature of MgNb 2 O 6 ceramics on quality factor value (Q f ) with various CuO additions. Fig. 7 shows the Q f value of CuO-doped MgNb 2 O 6 ceramics at different sintering temperatures. The microwave dielectric loss is mainly caused not only by the lattice vibrational modes, but also by the pores and the second phases. With increasing sintering temperature, the Q f increased to a maximum value at C and thereafter decreased. The decrease in Q f value was due to the rapid grain growth as observed in Fig. 3. However, highest Q f values were obtained for specimen at 2 wt.% CuO-doping level, which was not consistent with the variation of density. Density plays an important role in controlling the dielectric loss and has been shown for other microwave dielectric materials. The Q f value, however, was independent of the density for highly densified ceramics [10]. As observed in Fig. 3, the grain of MgNb 2 O 6 ceramics with 2 wt.% CuO addition was more uniform than others. Which revealed a reduction in lattice imperfection and dielectric loss was then reduced. Fig. 8. Dependence of sintering temperature of MgNb 2 O 6 ceramics on t f value with various CuO additions.

9 C.-S. Hsu et al. / Materials Research Bulletin 38 (2003) The decrease in Q f values for highly CuO-doped MgNb 2 O 6 ceramics could be a result from the grain morphology as well as the low Q f value of liquid phase. With 2 wt.% CuO addition, an excellent Q f value of 110,000 (at GHz) was obtained for MgNb 2 O 6 ceramics sintered at C for 3 h. The quality factors of CuO-doped MgNb 2 O 6 ceramics at low sintering temperature were relatively higher than that of pure MgNb 2 O 6 ceramics due to the addition of CuO. Fig. 8 shows the temperature coefficient of the resonant frequency (t f ) of the CuO-doped MgNb 2 O 6 ceramics at different sintering temperatures. The temperature coefficient of resonant frequency (t f ) was known related to the composition and the second phase of the ceramic samples. No significant change was observed in the t f value for specimen at different sintering temperatures. However, the t f value varied toward positive direction with increasing CuO content due to the formation of liquid phase. It implies that the t f value of MgNb 2 O 6 ceramics can be adjusted by controlling the amount of CuO addition. 4. Conclusion The dielectric properties of CuO-doped MgNb 2 O 6 ceramics have been investigated. The flux CuO was employed as a sintering aid to lower the firing temperature of the MgNb 2 O 6 ceramics. A large sintering temperature reduction ( C) can be achieved by adding CuO to the MgNb 2 O 6 ceramics. Significant improvements in the Q f and t f values have also been accomplished. With 2 wt.% CuO addition, a dielectric constant of 19.9, a Q f value of 110,000 (at 10 GHz) and a t f value of 44 ppm/8c were obtained for MgNb 2 O 6 ceramics sintered at C for 3 h. The dielectric properties of MgNb 2 O 6 ceramics with CuO additions are strongly dependent on the densifications and the microstructures. The decrease in Q f value at high CuO addition level (4 wt.%) was owing to the inhomogeneous grain growth as well as the low Q f value of liquid phase. Acknowledgements This work was supported by the National Science Council of the Republic of China under grant NSC E References [1] S. Nomura, K. Toyama, K. Kaneta, Jpn. J. Appl. Phys. 21 (1982) L624. [2] G. Wolfram, H.E. Gobel, Mater. Res. Bull. 16 (1981) [3] I. Burn, U. S. patent 4,845,062, [4] M. Maeda, T. Yamamura, T. Ikeda, Jpn. J. Appl. Phys. 26 (1987) 76. [5] H.J. Lee, K.S. Hong, S.J. Kim, I.T. Kim, Jpn. J. Appl. Phys. 36 (1997) L1318. [6] T. Kakada, S.F. Wang, Syoshikawa, S.T. Jang, R.E. Newnham, J. Am. Ceram. Soc. 77 (1994) [7] T. Kakada, S.F. Wang, Syoshikawa, S.T. Jang, R.E. Newnham, J. Am. Ceram. Soc. 77 (1994) [8] S.I. Hirno, Takashi, Hayashi, A. Hattori, J. Am. Ceram. Soc. 74 (1991) [9] V. Tolmer, G. Desqardin, J. Am. Ceram. Soc. 80 (1997) [10] W.S. Kim, T.H. Hong, E.S. Kim, K.H. Yoon, Jpn. J. Appl. Phys. 37 (1998) 5367.