Elsevier

Ceramics International

Volume 42, Issue 4, March 2016, Pages 5553-5557
Ceramics International

Short communication
Li2Zn2W2O9: A novel low-temperature sintering microwave dielectric ceramic with corundum structure

https://doi.org/10.1016/j.ceramint.2015.12.018Get rights and content

Abstract

A microwave dielectric ceramic Li2Zn2W2O9 with corundum structure was prepared through the solid state reaction method. The crystal structure was investigated using the Rietveld refinement. The sintering behavior, microwave dielectric properties, and chemical compatibility with silver electrodes of Li2Zn2W2O9 ceramic were studied in detail using X-ray diffraction, energy-dispersive spectrometer, and dielectric measurement. The Li2Zn2W2O9 ceramic could be well densified at relatively low sintering temperature range 790–830 °C. The optimum microwave dielectric performance, with a Q×f value ~15,710 GHz (at 9.7 GHz), a relative permittivity ~14.7, and a temperature coefficient of resonant frequency ~−76.8 ppm/°C, was obtained when sintered at 790 °C. XRD and EDS analysis of the co-fired sample confirmed the chemical compatibility of Li2Zn2W2O9 with silver electrodes. These merits make the Li2Zn2W2O9 ceramic a possible candidate for LTCC applications.

Introduction

With the rapid development of wireless communication technology and microwave devices, microwave dielectric ceramics have been widely studied in the past decades for practical applications [1], [2], [3], [4], [5], [6]. Recently, the low-temperature cofired ceramic (LTCC) technology has played an increasing important role in fabricating highly integrated microwave devices. The key issue of LTCC technology is the low sintering temperature, enabling its advantageous utilization in microelectronics and microsystems and in microwave modules. Because of the chemical stability and low-cost, silver is commonly selected as the inner electrodes in LTCC. Thus, the sintering temperatures of the ceramics should be lower than the melting point of Ag electrode (961 °C). In addition, the chemical compatibility between the ceramics and the electrodes is also a significant point to be taken into account [7], [8], [9], [10].

A series of corundum-type structured compounds were reported to have excellent microwave dielectric properties such as Al2O3 (εr~10, Q×f~335,000 GHz, τf~−60 ppm/°C and S.T.~1550 °C) [11], Mg4Nb2O9 (εr~11, Q×f~210,000 GHz, τf~−70 ppm/°C and S.T.~1300 °C) [12], Mg4NbTaO9 (εr~11.8, Q×f~281,600 GHz, τf~−66 ppm/°C and S.T.~1400 °C) etc [13]. In spite of the high microwave performance of these compounds, all of them need high sintering temperatures (>1300 °C) to be densified. Addition of sintering aids is usually used to lower sintering temperatures of the ceramics [14], [15], [16], [17]. Unfortunately, the addition usually causes the deterioration of quality factor of microwave dielectric ceramics. Hence, it is necessary to search for new microwave dielectric ceramics with intrinsic low sintering temperatures.

The formation of Li2Zn2W2O9 was firstly reported by Lv et al. [18] along with its corresponding crystal structure and thermal property. At room temperature, it belongs to a trigonal system with space group P3¯c1. It is worth noting that Li2Zn2W2O9 could be synthesized at relatively low-temperature (~660 °C). Therefore, it is worthwhile to investigate whether the Li2Zn2W2O9 could be a potential microwave dielectric ceramic with intrinsic low sintering temperature for LTCC application.

In the present work, Li2Zn2W2O9 ceramic was prepared using the solid-state reaction, and its sintering behavior, microwave dielectric properties and chemical compatibility with Ag powers were investigated in detail.

Section snippets

Experimental procedure

Specimens of the Li2Zn2W2O9 were prepared by a conventional solid-state reaction method using high-purity oxide powders (>99%): Li2CO3 (Guo-Yao Co., Ltd., Shanghai, China), ZnO (Guo-Yao Co., Ltd., Shanghai, China), and WO3 (Guo-Yao Co., Ltd., Shanghai, China). Stoichiometric proportions of the above pre-dried raw materials were milled for 4 h in a nylon jar with zirconia balls and alcohol as media. The mixtures were loaded in a corundum crucible and calcined at 750 °C for 4 h in air. The calcined

Results and discussion

The X-ray diffraction (XRD) patterns of Li2Zn2W2O9 ceramics sintered at 750–830 °C for 4 h are presented in Fig. 1. It is found that single phase with corundum-type structure was formed for all ceramics. All the observed peaks could be indexed according to the PDF card for Li2Zn2W2O9 (PDF files 04-016-5670). In addition, as shown in the inset of Fig. 1, with increasing sintering temperature, the enlarged (104) peak shifted towards higher angle and then to lower angle. This suggests that with

Conclusions

Li2Zn2W2O9 ceramics with corundum-type structure could be obtained in the whole sintering temperature range 750 – 830 °C. A well densified Li2Zn2W2O9 sample could be obtained when sintered at 790 °C and it exhibited promising microwave dielectric properties: Q×f~15,710 GHz (at 9.7 GHz), εr~14.7, τf~−76.8 ppm/°C. From XRD and EDS analysis of the co-fired sample, it was found that the Li2Zn2W2O9 ceramic is chemically compatible with Ag powder when sintered at 790 °C.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 21261007, 21561008, and 51502047), the Natural Science Foundation of Guangxi Zhuang Autonomous Region (No. 2015GXNSFBA139234, and 2015GXNSFFA139003), Project of Department of Science and Technology of Guangxi (No. 114122005-28), and Projects of Education Department of Guangxi Zhuang Autonomous Region (No. YB2014160, KY2015YB341, and KY2015YB122).

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