Effect of ZnO–WO3 additives on sintering behavior and microwave dielectric properties of 0.95MgTiO3–0.05CaTiO3 ceramics

Effect of ZnO–WO3 additives on sintering behavior and microwave dielectric properties of 0.95MgTiO3–0.05CaTiO3 ceramics

Available online at www.sciencedirect.com CERAMICS INTERNATIONAL Ceramics International 40 (2014) 6899–6902 www.elsevier.com/locate/ceramint Effect...

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Available online at www.sciencedirect.com

CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 6899–6902 www.elsevier.com/locate/ceramint

Effect of ZnO–WO3 additives on sintering behavior and microwave dielectric properties of 0.95MgTiO3–0.05CaTiO3 ceramics Wentao Xie, Hongqing Zhoun, Haikui Zhu, Jianxin Zhao, Guowei Tang, Xuan Yu College of Material Science and Engineering, Nanjing University of Technology, No. 5 New Mofan Road, Nanjing 210009, Jiangsu, China Received 23 October 2013; received in revised form 5 December 2013; accepted 5 December 2013 Available online 16 December 2013

Abstract The effect of WO3 addition on the phase formation, the microstructures and the microwave dielectric properties of 1 wt% ZnO doped 0.95MgTiO3–0.05CaTiO3 ceramics system were investigated. Formation of second phase MgTi2O5 could be effectively restrained through the addition of WO3, but should be in right amount. WO3 as additives could not only effectively lower the sintering temperature of the ceramics to 1310 1C, but also promote the densification. A dielectric constant εr of 20.02, a Q  f value of 62,000 (at 7 GHz), and a τf value of  5.1 ppm/1C were obtained for 1 wt% ZnO doped 0.95MgTiO3–0.05CaTiO3 ceramics with 0.5 wt% WO3 addition sintered at 1310 1C. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Microstructure; C. Dielectric properties; Densification

1. Introduction Development of microwave dielectric ceramics for applications in communication systems has been rapidly progressing in the past decade and this advancement also leads to a speedy growth market electro-ceramics industry. Nowadays, dielectric ceramics used in the miniaturization of microwave circuits must include two categories:low dielectric loss and low temperature coefficient of dielectric constant [1,2]. MgTiO3–CaTiO3 based ceramic is well known for the application of temperature compensating type capacitor and patch antenna. The material is made of a mixture of magnesium titanate (MgTiO3: εr  17 and τf   50 ppm/1C) and calcium titanate (CaTiO3: εr  170 and τf  800 ppm/1C) [3,4]. The composite of these with a ratio of Mg:Ca=95:5, (0.95MgTiO3–0.05CaTiO3) is popularly known as 95MCT. The dielectric properties of 95MCT are εr  20, Q  f  56,000 at 7 GHz, and a zero τf value. However, it required sintering temperatures as high as 1400–1450 1C. Many researchers made efforts to study the microstructures and dielectric properties of 95MCT via adding various additives n

Corresponding author. Tel.: þ86 25 86639976; fax: þ 86 25 86639976. E-mail address: [email protected] (H. Zhou).

[5–8]. In this paper, ZnO and WO3 were added to 95MCT ceramic system as a sintering aid to lower its sintering temperature. The effects of additions on the crystalline phases, the microstructures and the microwave dielectric properties of 95MCT ceramics were also investigated in this article. 2. Experimental procedure Samples of MgTiO3 and CaTiO3 were prepared using MgCO3, CaCO3 and TiO2 powders with purities higher than 99 wt%. The starting materials were mixed according to the stoichiometry: MgTiO3 and CaTiO3 and ground in distilled water for 12 h in a balling mill with agate balls. Both mixtures were dried and calcined at 1100 1C for 3 h. The calcined powders were mixed as desired composition 0.95MgTiO3– 0.05CaTiO3 with 1 wt% ZnO and various amount of WO3 (0.5–2 wt%) as sintering aids and re-milled for 8 h. After drying, the powders were add with PVA and pressed into disks with dimensions of 13 mm in diameter and 8 mm in thickness. The pellets were sintered at temperatures of 1280–1340 1C for 2 h. The densities of the sintered ceramics were measured using the Archimedes method. Crystalline phases were analyzed by

0272-8842/$ - see front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.12.010

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Fig. 1. X-ray diffraction patterns of 1 wt% ZnO doped 95MCT ceramics sintered at 1310 1C with (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt%, (d) 1.5 wt%, and (e) 2 wt% WO3 additions ((○)MgTiO3, (◇)CaTiO3, (*)MgTi2O5, (#)CaWO4).

means of the X-ray powder diffraction method using Cu Kα radiation from 201 to 801 in 2θ. The scanning rate was 10 1/ min. The microstructure was observed using a scanning electron microscope (SEM, Supra55). The dielectric constant and unloaded Q value at microwave frequencies were measured using the Hakki–Coleman dielectric resonator method. The temperature coefficient of resonant frequency (τf) was calculated in the temperature range of þ 25 1C to þ 80 1C [9,10]. The τf (ppm/1C) was obtained as follows: τf ¼

f 2 f 1 f 1 ðT 2  T 1 Þ

ð1Þ

where f1 is the resonant frequency at T1 and f2 is the resonant frequency at T2. 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of 1 wt% ZnO doped 95MCT ceramics with different amounts of WO3 addition sintered at 1310 1C for 2 h. The XRD diffraction patterns showed a mixture of a main phase MgTiO3 and a minor phase CaTiO3. The formation of MgTiO3 (ilmenite) and CaTiO3 (perovskite) was due to the structure and ionic size differences between Ca2 þ (0.1 nm) and Mg2 þ (0.046 nm). Moreover, a second phase MgTi2O5 was also detected, which would lead to a degradation in dielectric properties. Second phase MgTi2O5 decreased remarkably when 0.5 wt% WO3 added into 1 wt% ZnO doped 95MCT ceramics. However, the formation of MgTi2O5 seemed to be enhanced in 95MCT ceramics with further more WO3 addition. Although a very small amount of CaWO4 was identified, it became significant when the WO3 addition was increased to 2 wt%. The SEM photographs of 1 wt% ZnO doped 95MCT ceramics with various amounts of WO3 addition at 1310 1C are illustrated in Fig. 2 (a–d). The pores in the ceramics were almost eliminated and the grain sizes obviously increased as the addition of WO3 increases. However, inhomogeneous grain

growth could be clearly detected when WO3 content reached 1.5 wt%, which also aroused the increase of porosity and degradation of the density. The abnormal grain growth and porous microstructure could damage the microwave dielectric properties [11]. The SEM photographs of 1 wt% ZnO doped 95MCT ceramics with 0.5 wt% WO3 sintered at different sintering temperature are shown in Fig. 2 (e–f). Compared with the samples sintered at 1310 1C, some pores were observed in ceramics at 1280 1C and 1340 1C individually, which indicated that the compact ceramic could be sintered at 1310 1C. The apparent densities of 1 wt% ZnO doped 95MCT ceramics with various amounts of WO3 addition versus the sintering temperature are shown in Fig. 3. The density of 95MCT ceramics without any additive is 3.7 g cm  3 at sintering temperature above 1400 1C. The density of 1 wt% ZnO doped 95MCT was 3.74 g cm  3 at 1325 1C. The sintering temperature of 1 wt% ZnO doped 95MCT ceramics could be lowed to only 1310 1C despite of the amounts of WO3 addition and a maximum density of 3.82 g cm  3(98.9% of theoretical density) was obtained with 0.5 wt% WO3 addition. It obviously indicated that ZnO–WO3 as additives could not only effectively lower the sintering temperature of 95MCT ceramics, but also promote the densification. However, the decrease in the density with the increasing of WO3 addition was due to the abnormal grain growth observed in Fig. 2. Fig. 4 shows the dielectric constant of 1 wt% ZnO doped 95MCT ceramics with different amounts of WO3 addition. According to the Clausius–Mossotti equation, the dielectric constant of ceramic materials is predominantly dominated by the dipoles in unit cell volume and dielectric polarizabilities of ions. Higher density for ceramic materials means there are more dipoles in a unit cell volume which indicates that the ceramic is more apt to be polarized, thus exhibits higher dielectric constant. As seen in Fig. 4, the dielectric constant of ceramics decreased from 20.02 to 19.33 with increasing WO3 addition, which mainly attributed to the decrease in relative density of ceramics. The Q  f values of 1 wt% ZnO doped MCT ceramics with various amounts of WO3 addition are shown in Fig. 5. With increasing sintering temperature, the Q  f value was found to increase to a maximum value and thereafter decreased. A maximum Q  f value of 62,000 GHz was obtained for 1 wt% ZnO doped MCT ceramics with 0.5 wt% WO3 addition at 1310 1C. The microwave dielectric loss is mainly caused not only by the lattice vibrational modes, but also by the pores, the second phases, the impurities, or the lattice defect [12,13]. It was clearly that the Q  f values firstly increased with increasing WO3 content and then declined with further more addition. The phenomenon, on the one hand, was associated with the abnormal grain growth and the degradation of the density, which caused more pores and lattice defect. On the other hand, the decrease in Q  f value was due to the formation of second phase MgTi2O5 as well as the increase in WO3 content. The decrease in Q  f value also could be attributed to the secondary phase CaWO4, which were found range from 1 wt% to 2 wt% WO3 additions, but not with 0.5 wt% addition.

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Fig. 2. SEM micrographs of 1 wt% ZnO doped 95MCT ceramics with (a) 0.5 wt% WO3, (b) 1 wt% WO3, (c) 1.5 wt% WO3, (d) 2 wt% WO3 at sintering temperature 1310 1C, (e) 0.5 wt% WO3 at sintering temperature 1280 1C, (f) 0.5 wt% WO3 at sintering temperature 1340 1C.

Fig. 3. The relative densities of 1 wt% ZnO doped 95MCT ceramics with different amounts of WO3 addition vs. sintering temperature.

Fig. 6 illustrates the temperature coefficients of resonant frequency (τf) of 1 wt% ZnO doped 95MCT ceramics with various WO3 additions at different sintering temperatures. The τf value is basically related to the composition and the second

Fig. 4. εr value of 1 wt% ZnO doped 95MCT ceramics with various WO3 addition sintered at different temperatures.

phase of the material [14]. The τf values became more negative with increasing WO3 addition. It varied from  5.1 to  8.8 ppm/1C as the amount of WO3 addition increased from 0.5

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values firstly increased with increasing WO3 content and then declined with further more addition, which was associated with the abnormal grain growth and the change of second phase MgTi2O5. The decrease of Q  f values might also be related to the second phase CaWO4. At 1310 1C, 1 wt% ZnO doped 95MCT ceramics with 0.5 wt% WO3 addition gave excellent microwave dielectric properties: εr  20.02, Q  f value  62,000 (at 7 GHz) and τf value   5.1 ppm/1C. Acknowledgments

Fig. 5. Q  f values of 1 wt% ZnO doped 95MCT ceramics with various WO3 additions sintered at different temperatures.

This work has been supported by a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), IRT1146.

References

Fig. 6. τf values of 1 wt% ZnO doped 95MCT ceramics with various WO3 additions sintered at different temperatures.

wt% to 2 wt% at different sintered temperature. With 0.5 wt% WO3 addition, the 1 wt% ZnO doped 95MCT ceramics sintered at 1310 1C had excellent microwave dielectric properties with an εr value of 20.02, a Q  f value of 62,000 (at 7 GHz), and a τf value of  5.1 ppm/1C. 4. Conclusion The effect of WO3 addition on the microwave dielectric properties and the microstructures of 1 wt% ZnO doped 95MCT ceramics were investigated. WO3 addition could effectively inhibit the second phase MgTi2O5, but should be in right amount (only 0.5 wt%). While another phase CaWO4 was detected when WO3 addition reached 1 wt%, and became significant when increased to 2 wt%. WO3 as additives could not only effectively lower the sintering temperature of the 1 wt% ZnO doped 95MCT ceramics, but also promote the densification. The grain size increased as the increasing amount of WO3 addition, inhomogeneous grain growth was detected when WO3 addition increased to 1.5 wt%. The Q  f

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