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J Mater Sci: Mater Electron (2015) 26:1977–1981 DOI 10.1007/s10854-014-2637-0

Phase transition, microstructure and microwave dielectric properties of a-CaSiO3 ceramics with SiO2 addition Wei Hu • Hanxing Liu • Hua Hao • Zhonghua Yao • Minghe Cao • Zhijian Wang • Zhe Song • Haobo He

Received: 10 November 2014 / Accepted: 16 December 2014 / Published online: 25 December 2014 Ó Springer Science+Business Media New York 2014

Abstract The effect of SiO2 addition on phase transition, microstructure and microwave dielectric properties of aCaSiO3 ceramics was reported for the first time. The X-ray diffraction analysis confirmed that the phase transformation from cyclo-CaSiO3 to pseudo-CaSiO3 was inhibited with the increase of SiO2 content. It was observed that the average grain size decreased with the increasing addition of SiO2 by scanning electron microscope. The sf of the SiO2added a-CaSiO3 ceramics almost remained constant with the SiO2 content increased, while the er and Q 9 f changed obviously. The optimized properties were determined for SiO2 6 wt% additive a-CaSiO3 ceramics sintered at 1,350 °C: er = 7.37, Q 9 f = 33,714 GHz and sf = -11.08 ppm/°C.

1 Introduction Recently, the development of microwave dielectric materials for low-dielectric applications in communication systems such as wireless LAN, resonators, cellular phones and intelligent transport systems has been rapidly progressing in the last two decades [1–4]. The available frequencies have therefore been extended from microwave to millimeter-wave ranges because large quantity of information could be transported with high speed in the later range [5, 6]. Generally, these dielectric materials for microwave communication require a low dielectric

W. Hu H. Liu (&) H. Hao Z. Yao M. Cao Z. Wang Z. Song H. He State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, People’s Republic of China e-mail: [email protected]

constant (er \ 10), a high quality factor (Q 9 f) and a nearzero temperature coefficient of resonant frequency (sf B ±10 ppm/°C) [7–10]. CaSiO3 ceramic has been proved to be a good low dielectric material with low dielectric loss and excellent temperature coefficient of resonance frequency. As it is known, CaSiO3 has mainly two normal modifications, one is the low-temperature phase b-CaSiO3, which belongs to the triclinic syngony, and the other is the high-temperature phase a-CaSiO3, which also belongs to the triclinic syngony with a pseudo-hexagonal structure [11, 12]. However, the sintering temperature range of pure CaSiO3 ceramic is very narrow. Chakradhar et al. [13] pointed out that it was difficult to obtain dense CaSiO3 ceramic (the theoretic density of CaSiO3 ceramic is 2.91 g/cm3) since its grains grew exceptionally and the bulk CaSiO3 ceramic became more porous with the increase of the calcination temperature, thereby impacting its microwave dielectric properties. Considerable researches have been investigated to improve the microstructures, thus improving the microwave dielectric properties of b-CaSiO3 ceramic, Sun and co-workers [14] have used Mg2? to substitute Ca2? in the b-CaSiO3 host, (Ca0.9Mg0.1)SiO3 ceramic exhibits a high dense ceramics with a density of 2.77 g/cm3 and excellent dielectric properties of er = 6.49, Q 9 f = 62,420 GHz and sf = -43.3 ppm/°C. In addition, Wang and co-workers [15] have used Li2CO3 and CuO to improve the sintering characteristic of b-Casio wt% Also ceramic, and the derived b-Casio wt% Also ceramic with 1.5 wt% Li2CO3 and 0.2 wt% CuO addition exhibits a density of 2.79 g/cm3 and microwave dielectric properties of er = 7.15, Q 9 f = 21,950 GHz and sf = -46 ppm/°C. However, how to improve the microstructures and microwave dielectric properties of a-CaSiO3 ceramic are rarely reported, while the microwave dielectric properties of a-CaSiO3 ceramic are

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J Mater Sci: Mater Electron (2015) 26:1977–1981

better than that of b-CaSiO3 ceramic [16]. Furthermore, the research about phase transition of a-CaSiO3 ceramic has also not been investigated. In this work, SiO2 was selected as an additive to improve the sintering characteristics and microstructures of a-CaSiO3 ceramics, because SiO2 is an effective additive for microstructural modification of ceramics [17]. The phase transformation, microstructure and microwave dielectric properties of a-CaSiO3 with different amounts of SiO2 additives were investigated in detail.

2 Experimental CaSiO3 powders were prepared by a routine solid-state reaction process from reagent powders of CaCO3 ([99.0 %) and SiO2 ([99.0 %). The weighed raw materials were mixed by ball milling with zirconia media in ethanol for 24 h, and the mixtures were calcined at 1,200 °C in air for 2 h after drying. The CaCO3–SiO2 powders calcined at 1,200 °C were mixed with SiO2 powders ([99.0 %), and the content of SiO2 was 1, 2, 4, 6, 8, and 10 in weight percent. The mixed powders were remilled for 24 h and dried again to obtain homogeneous powders. Then the dried powders with 5 wt% of polyvinyl alcohol binders were pressed into disks 12 mm in diameter and 5–7 mm in thickness under a pressure of 200 MPa. After slow heating at 600 °C for 2 h to burn out the binder, these samples were sintered at the temperatures of 1,275–1,375 °C for 2 h in an air atmosphere. The bulk density of specimens with regular shape was calculated by Archimedes method. The crystalline structure and microstructure of the sintered samples were examined by X-ray diffraction (XED, Paralytical Expert PRO) and scanning electron microscopy (SEEM, JSM-7100F) measurements, respectively. The microwave dielectric constant er and Q 9 f values were measured by Hakki–Coleman dielectric resonator method using an Agilent 8722ET (50 MHz–40 GHz) Network Analyzer. The temperature coefficient of the resonant frequency sf was also measured by the same method and calculated by the following equation from 25 to 85 °C: sf ¼

f85 f25 106 ppm= C 60 f25

ð1Þ

3 Results and discussion Figure 1 shows the bulk densities of a-CaSiO3–SiO2 ceramics sintered at different temperatures with different amounts of SiO2 addition. It is obvious that with the increase of the sintering temperature, the densities of all bulks increase to a maximum value and then they are saturated. The optimal sintering temperature of a-CaSiO3

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Fig. 1 Bulk densities of a-CaSiO3–SiO2 ceramics sintered at different temperatures with a 1 wt%, b 2 wt%, c 4 wt%, d 6 wt%, e 8 wt%, and f 10 wt% SiO2 additions

ceramics with 1 and 2–10 wt% SiO2 additions are 1,375 and 1,350 °C, respectively. The bulk densities at these temperatures are 2.64, 2.68, 2.73, 2.78, 2.77, and 2.76 g/ cm3, respectively. Combining with the maximum bulk density of a-CaSiO3 ceramics prepared by the conventional solid-state process was 2.549 g/cm3 sintered at 1,375 °C [16], these results in the present work indicate that appropriate SiO2 addition benefits the sintering process of a-CaSiO3 ceramic. Figure 2 shows the X-ray diffraction patterns of a-CaSiO3–SiO2 ceramics sintered at 1,350 °C with different amounts of SiO2 addition and powder sintered at 1,200 °C. It can be seen that the compositions with SiO2 addition (B2 wt%) show the pseudo-CaSiO3 (according to JCPDS file 01-074-0874) structure at room temperature, while those with SiO2 addition (C4 wt%) are the cyclo-CaSiO3 (according to JCPDS file 00-010-0486) with trace amounts of SiO2. A careful examination of the XRD patterns are shown in Fig. 1b and reveal that three peaks merge into single peak, combining with the cyclo-CaSiO3 phase of powder, it indicates that the increase in SiO2 content inhibited the phase transformation from cyclo-CaSiO3 to pseudo-CaSiO3. This phenomenon was considered to be resulted from the internal stress, and considerable researches have been proved that the internal stress is depended on the variation of grain sizes, which can be observed by FE-SEM [18, 19]. The SEM micrographs of a-CaSiO3–SiO2 ceramics sintered at 1,350 °C with different amounts of SiO2 addition are presented in Fig. 3. It is observed that all samples sintered at 1,350 °C had a high density and exhibited uniform and homogeneous microstructures with almost no porosity, while the grain size was found to be strongly depend on the SiO2 addition. After the calculation on SEM


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Fig. 2 X-ray diffraction patterns of a-CaSiO3–SiO2 ceramics sintered at 1,350 °C with different amounts of SiO2 additions and powder sintered at 1,200 °C

Fig. 3 SEM micrographs of a-CaSiO3–SiO2 ceramics sintered at 1,350 °C with a 1 wt%, b 2 wt%, c 4 wt%, d 6 wt%, e 8 wt%, and f 10 wt% SiO2 additions

images by Image Pro Plus software, it was found that with increasing SiO2 content from 1 to 10 wt%, the average grain size changed from 5.53 to 3.35 lm. In order to calculate the internal stress with different grain sizes, we used the Hall-Williamson equation based on XRD patterns, as described by the following: ðb cos h=kÞ2 ¼ ð0:89=dÞ2 þ16e2 ðsin h=kÞ2

ð2Þ

where in b represents the FWHM (full width at half maximum), h, k and d are the diffraction, wavelength of X-ray and interplanar spacing, respectively. e2 is the slope of the fitting lines, corresponding to the value of internal stress. The (bcosh/k)2 versus (4sinh/k)2 plots of the XRD characteristics of a-CaSiO3–SiO2 ceramics show linear

behaviors, which are in good agreement with the HallWilliamson expression, as shown in Fig. 4. The slopes, e2, corresponding to the levels of internal stress, were calculated from the fitting lines, as listed in Table 1. With SiO2 added, e2 increases gradually from 1.64 9 10-6 to 2.28 9 10-6, indicating the continuous increase of internal stress. The addition of SiO2 is beneficial for the refinement of grain sizes, showing in Fig. 3. It is known that internal stress strongly depends on the varying of grain sizes, affecting the phase transition of a-CaSiO3–SiO2 ceramics [18, 19]. The microwave dielectric properties of a-CaSiO3–SiO2 ceramics sintered at 1,350 °C with different amounts of SiO2 addition are presented in Fig. 5. With the fixed SiO2 addition, it is obvious that the dielectric constant increases

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Fig. 4 (bcosh/k)2 versus (4sinh/k)2 plots of the XRD characteristics of a-CaSiO3–SiO2 ceramics with different SiO2 additions based on the Hall-Williamson expression

Table 1 The fitting results of the Hall-Williamson expression for aCaSiO3–SiO2 ceramics with different SiO2 additions SiO2 content (wt%)

0

1

2

4

6

8

10

e2 (10-6)

1.64

1.78

1.88

1.91

2.12

2.16

2.28

microwave devices. Several factors contributed to dielectric loss at microwave frequencies, including density, porosity, second phases and grain boundaries. The variation of Q 9 f was consistent with that of density suggesting the change of Q 9 f was mainly related to its corresponding density. The temperature coefficient of resonant frequency (sf) was known to be governed by the composition, additives, and the second phase of the material. It almost retained constant throughout the entire SiO2 content range in the experiment and showed only a small variance which implied it was not sensitive to the SiO2 content. With 6 wt% SiO2 addition, the a-CaSiO3–SiO2 ceramic sintered at 1,350 °C exhibits excellent microwave dielectric properties: er = 7.37, Q 9 f = 33,714 GHz and sf = -11.08 ppm/°C. Several low dielectric materials such as Mg2SiO4, Zn2SiO4, and Al2O3, exhibited a low er and high Q 9 f, but large sf restricted their further application. By contrast, a-CaSiO3–6 wt% SiO2 ceramic not only shows low er and high Q 9 f values, but also has near-zero sf, which indicates that it is a promising candidate of lowdielectric material for microwave applications.

4 Conclusions

to the maximum value (er = 7.37) and then decreased. The relationship between dielectric constant and SiO2 addition followed similar trend to those between density and SiO2 addition because a higher density is associated with a lower porosity and results in a higher dielectric constant. By increasing the SiO2 content, the Q 9 f value increases to a maximum value of 33,714 GHz and then decreases. As we know Q 9 f and dielectric loss had reciprocal relationship, consequently high Q 9 f led to low dielectric loss for

Fig. 5 Microwave dielectric properties of a-CaSiO3–SiO2 ceramics sintered at 1,350 °C with different amounts of SiO2 additions

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The SiO2 additive a-CaSiO3 ceramics were synthesized and their phase transition, microstructure and microwave dielectric properties were investigated. SiO2 addition inhibited the phase transformation of a-CaSiO3 from cycloCaSiO3 to pseudo-CaSiO3. With the increase of SiO2 content, the average grain size decreased, leading to enlarge the internal stress, which was the main cause of phase transition. The appropriate SiO2 addition (4–8 wt%) improved the densities of a-CaSiO3–SiO2 ceramics, and then enhanced the microwave dielectric properties. With


6 wt% SiO2 addition, the a-CaSiO3–SiO2 ceramic sintered at 1,350 °C exhibits excellent microwave dielectric properties: er = 7.37, Q 9 f = 33,714 GHz and sf = -11.08 ppm/°C. The features suggest that the ceramics are a potential candidate for applications as microwave devices in communication field. Acknowledgments This work was supported by the Key program of Natural Science Foundation of China (No. 50932004), International Science and Technology Cooperation Program of China (2011DFA52680), Natural Science Foundation of China (No. 51102189, No. 51372191), National Key Basic Research Program of China (No. 2015CB654601) and the program for New Century Excellent Talents in University (No. NCET-11-0685).

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