Low‐temperature Sintering of Bi 2 O 3 ‐B 2 O 3 Glass Doped Li 2 ZnTi 3 O 8 ‐ Li 2 TiO 3 Ceramic a

www.ijm‐me.org International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016 doi: 10.14355/ijmme.2016.05.006

Low‐temperature Sintering of Bi2O3‐B2O3 Glass Doped Li2ZnTi3O8 ‐ Li2TiO3 Ceramic and Its Dielectric Properties1 Ying‐xiang Li1, Bin Tang1,*, Shu‐ren Zhang1, Hao Li1 State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, China 1

Corresponding author: E‐mail: tangbin@uestc.edu.cn; 408960349@qq.com; Tel: +86‐28‐83208048; Fax: +86‐28‐83202139 *

Abstract The effects of Bi2O3‐B2O3 (BBO) glass addition on phase composition, microstructure and microwave dielectric properties of 0.65Li2ZnTi3O8‐0.35Li2TiO3 (LZT) ceramics were investigated in this paper. It was found that a small amount of BBO addition could not only effectively lower the sintering temperatures from 1100 ℃ to 840 ℃, but also reduce the temperature coefficient of resonant frequency (τƒ) and the Q×f value of LZT ceramic. When BBO content exceeded 3 wt%, an unknown phase was detected. Typically, the 0.5 wt% BBO doped LZT ceramic sintered at 840 ℃ for 4 h had good microwave dielectric properties of εr = 26.6, Q×f = 40,683GHz and τƒ = +1.0ppm/℃, it a promising ceramic for LTCC technology application.

Keywords Li2ZnTi3O8; Dielectric Properties; Microstructure; LTCC

Introduction In the past decades, with the rapid development of wireless communication technology, to meet the requirements of the miniaturization of the device size, low‐temperature co‐fired ceramic (LTCC) technology has become an important approach to meet the requirements of device miniaturization owing to its predominance in integrating various electronic components and devices into a compact multilayer ceramic structure.1‐4 Silver is a widely used metallic electrode in LTCC technology due to its high conductivity and relatively low cost. However, the melting temperature of Ag is low (about 961℃), and it is necessary to search potential ceramics with low sintering temperature that can be co‐fired with Ag.5 Besides, for application in LTCC, microwave dielectric ceramics are generally required to have a high permittivity (εr) for miniaturization, a high‐quality factor (Q×f) for better selectivity and a near‐zero temperature coefficient of resonant frequency (τƒ) for stability.6 Most commercial microwave dielectric ceramics such as BaO‐Nd2O3‐TiO2, (Zr,Sn)TiO4 and CaTiO3‐NdAlO3 usually have a good microwave dielectric properties, but their sintering temperature may always be above 1300℃, which is too high to be applied to LTCC. To reduce the sintering temperature of dielectric ceramics, three methods are widely used, including low‐melting glass addition, chemical processing, and particle size reduction of starting materials. Among these methods, low‐melting temperature glass addition for liquid phase sintering is lower in cost and easier to handle than the other two.6,7 Recently, many studies have been focused on the Li2O‐MOx (A = Zn, Mg, Co)‐TiO2 compounds. The Li2ZnTi3O8 material was first reported as a low loss dielectric for microwave applications by George and Sebastian.8 This kind of materials has low sintering temperature and good microwave dielectric properties. For example, Li2ZnTi3O8 ceramic can be well sintered below 1100 ℃, and meanwhile exhibits good microwave dielectric properties of εr = 26.2, Q×f = 62,000 GHz, and τƒ = −15 ppm/℃.8‐13 Furthermore, the Li2ZnTi3O8 ceramic is advantageous in low cost of raw materials. Therefore, this ceramic has been widely considered as a promising candidate for LTCC in the last 1

This paper was selected from the 2nd International Conference on Advance Materials Research and Application (AMRA 2015).

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decade.9‐19 George et al. lowered the sintering temperature of the Li2ZnTi3O8 materials by adding LMZBS glass.10 Chen et al. investigated the microwave dielectric properties of BaCu(B2O5)‐added Li2ZnTi3O8 ceramics.16 Lu et al. studied the sintering behaviors and microwave dielectric properties of Li2ZnTi3O8 ceramic doped with ZnO‐B2O3‐SiO2 glass.17 Although the sintering temperature of Li2ZnTi3O8 ceramic can be reduced to a large extent, the τƒ value is still too large for commercial dielectric materials. In order to tune the τƒ value to zero ppm/℃, TiO2‐added Li2ZnTi3O8 ceramics with glass addition was investigated by Hou et al. and Chen et al..18,19 Moreover, the effects of TiO2 ratio on the phase composition and microwave dielectric properties of Li2ZnTi3+xO8+2x ceramics were also studied by Li et al., and the τƒ value could be modified to near zero ppm/℃.20 More recently, Li2ZnTi3O8‐Li2TiO3 ceramic was investigated by Wu et al..21 However, few studies on glass‐doped Li2ZnTi3O8‐Li2TiO3 ceramics have been reported.

In the past few years, Bi2O3‐B2O3 glass has been chosen as a candidate sintering aid due to its low melting point (about 660℃). It can be successfully applied in lowering the sintering temperatures of various ceramics, such as Ca[(Li1/3Nb2/3),Ti]O3‐δ, BaTiO3, and Ca2MgSi2O7.22‐26 However, to the best of our knowledge, BBO glass addition has not been reported in Li2ZnTi3O8‐Li2TiO3 system so far. In this work, the phase composition, the microstructure, and the microwave dielectric properties of 0.65Li2ZnTi3O8‐0.35Li2TiO3 (LZT) ceramic doped with BBO glass will be investigated in detail.

Experimental 0.65Li2ZnTi3O8‐0.35Li2TiO3 was synthesized by the solid‐state ceramic route. High‐purity powders of Li2CO3 (99.6%) (Ni‐Co Guorun New Materials Co. Ltd., Sichuan, China), ZnO (99.9%) (Kelong Chemical Co.,Ltd., Sichuan, China), and TiO2 (99.9%) (Zhongxing Electronic Materials Co., Ltd., Hubei, China) were used as the starting materials. The raw oxide materials were weighed according to the stoichiometry proportions and then ball‐milled in an alcohol medium for 6 h in a plastic bottle using zirconia balls. The mixture was dried and calcined at 870℃ for 4 h. To synthesize the BBO (7Bi2O3:3B2O3) glass powder, Bi2O3 (99.5%) (Kelong Chemical Co.,Ltd., Sichuan, China) and B2O3 (99.8%) (Kelong Chemical Co.,Ltd., Sichuan, China) were weighed stoichiometrically and mixed for 4 h using distilled water as medium. Then, the mixture was dried, and melted in a Pt crucible at 1100 ℃/1 h.

Afterwards, the melt was quenched and powdered.

The calcined LZT powders were re‐milled together with different weight percentages of BBO glass additives (0.5–5 wt%) in alcohol medium for 4 h. After drying, the dried powder was mixed with 6 wt% polyvinyl alcohol as a binder. Then, the mixture was dried again and ground well. The resultant fine powder was axially pressed under a pressure of 25 MPa into cylindrical disks of 11 mm in diameter with thickness of about 4–6 mm. These pellets were sintered at different temperatures from 810 to 900℃ for 4 h in air.

After sintering, the bulk density of the samples was measured using the Archimedes method. The phase composition and crystal structure of the ceramics were examined by X‐ray diffraction technique using CuKa radiation (DX–1000 CSC, Japan). Scanning electron microscopy (SEM) (JSM‐6460LV, Jeol, Tokyo, Japan) was employed to observe the surface morphology of the specimens. The microwave dielectric properties were determined by a Vector Network Analyzer (E5071C, Agilent Technologies) and a temperature chamber (DELTA 9023, Delta Design, USA). The temperature coefficients of resonant frequency τƒ values were calculated by the equation:  f  ( ft2  ft1 ) ( ft1  (t2  t1 )) , where ft1 and ft2 are the resonant frequencies at the measuring temperatures of t1 (25℃) and t2 (85℃), respectively.

Results and Discussion Fig.1 shows the room‐temperature XRD patterns of LZT ceramic with BBO (a = 0.5wt%, b = 1wt%, c = 3wt% and d = 5wt%) glass sintered at 840℃for 4 h. All diffraction peaks of LZT samples with 0.5–3.0 wt% BBO can be indexed as a cubic structure Li2ZnTi3O8 (JCPDS file No. 86–1512) phase and Li2TiO3 (JCPDS file No. 33–0831) phase. These two phases can coexist to form a stable Li2ZnTi3O8–Li2TiO3 system. However, when the addition of BBO content exceeds 3.0 wt%, a small amount of unknown secondary phase can be detected besides the main Li2ZnTi3O8 and Li2TiO3 phases, as shown in Figures 1(d).

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FIG. 1 XRD PATTERNS OF LZT CERAMIC WITH DIFFERENT BBO CONTENTS (A = 0.5WT%, B = 1WT%, C = 3WT% AND D = 5WT%) FIRED AT 840 ℃ IN AIR FOR 4 H.

FIG. 2 SEM IMAGES OF THE LZT CERAMIC WITH DIFFERENT BBO CONTENTS (A) X = 0.5WT%, (B) X = 1WT%, (C) X = 3WT% AND (D) X = 5WT% SINTERED AT 840℃ FOR 4 H.

Fig. 2 shows the SEM images of LZT ceramic with different BBO contents sintered at 840℃ for 4 h. As can be seen, large amounts of tiny grains appear in the microstructure of LZT ceramic with 0.5 wt% BBO, but the growth of these grains is limited just around the molten phases because of the insufficient liquid phase. In contrast, the LZT ceramic doped with 1wt% BBO glass presents a large grain size, as illustrated in Fig. 2(a) and 2(b), indicating that the BBO glass may accelerate the grain growth through a liquid phase sintering mechanism. However, when the content of BBO glass exceeds 1.0 wt%, the grain size becomes smaller again, as shown in Fig. 2c–d. As we know, an increase of glass content can increase the fluidity of matrix as well as the grain size through a liquid phase sintering mechanism, but an excess of liquid phase during sintering may inhibit grain growth of materials, which produces more pores in the ceramics.5, 27 That is to say, an excess of BBO glass during sintering can inhibit grain growth of LZT ceramic. What is more, a large amount of flake‐like grains are detected when the content of BBO glass exceeds 3.0 wt%. This may also inhibit grain growth of LZT ceramics.

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FIG. 3 TEMPERATURE COEFFICIENTS OF RESONANT FREQUENCY OF BBO‐DOPED LZT CERAMIC SINTERED AT 840℃ FOR 4 H AS A FUNCTION OF BBO CONTENT (X).

FIG. 4 (A) BULK DENSITY, (B) DIELECTRIC CONSTANT AND (C) QUALITY FACTOR OF LZT + X BBO (X = 0.5, 1, 3, 5 WT% ) CERAMICS AS A FUNCTION OF SINTERING TEMPERATURE.

The temperature coefficients of resonant frequency of BBO‐doped LZT ceramic sintered at 840℃ for 4 h as a function of BBO content (x) are shown in Fig. 3. With the increase of BBO content from 0.5 to 5wt%, the τƒ value gradually decreases from +1.0 ppm/℃ to ‐7.3 ppm/℃. It is well known that the τƒ value of composite ceramic is greatly related to its composition.20 According to previous studies, BBO glass possesses a negative τƒ value about −41～−203 ppm/℃.22‐24 Therefore, the τƒ value in this system can be tuned from positive to negative by changing the

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content of BBO glass.

Figure 4 illustrates the (a) bulk density, (b) dielectric constant and (c) quality factor of LZT + x BBO (x=0.5, 1, 3, 5 wt%) ceramics as a function of sintering temperature. As sintering temperature increases from 810 to 900℃, the bulk density firstly increases because of the elimination of pores at the beginning of sintering, and then slightly decreases with further increase of sintering temperature, which may be attributed to the abnormal grain growth and a small amount of trapped porosity.5,6 When the content of BBO glass increases from 0.5 to 5wt%, bulk density of the samples sintered at 840℃ decreases from 3.803 to 3.74 g/cm3. The dependency of dielectric constant on sintering temperature presents the same trend as that of bulk density, since a high bulk density means a high relative permittivity owing to lower porosity. The dielectric constant reaches a saturated value of 26.6 when x = 0.5wt% at 840℃, and then decreases with increasing BBO content due to the increase of BBO liquid phase with lower εr value. From Figure 4(c), it can be found that Q×f value of the ceramic increases with the increase of sintering temperature, and that the maximum Q×f value of 40,683GHz can be obtained when the LZT + 0.5 wt% BBO specimen is sintered at 840℃. Besides, increasing BBO content can induce the decrease of Q×f value. The microwave dielectric loss can be attributed to not only intrinsic factors but also extrinsic factors such as second phases, oxygen vacancies, grain sizes and densification or porosity.18‐20 Therefore, the decrease of Q×f value is probably caused by the amount of trapped porosity and abnormal grain growth, which can be demonstrated by the SEM images, shown in Fig. 2. In addition, the decrease of Q×f value may be related the relatively lower Q×f value of BBO (about 2000～10,800GHz) relative to that of LZT (more than 50,000 GHz).21, 24 Notably, when the BBO content added in LZT ceramic exceeds 5wt%, the excessive liquid phase and the unknown second phase may deteriorate the microwave dielectric properties of the ceramic, which leads to a low Q×f value. To sum up, the LZT ceramic with 0.5wt% BBO addition sintered at 840℃ shows good microwave dielectric properties of εr = 26.6, Q×f = 40,683GHz and τƒ = +1.0ppm/℃.

Conclusions The phase composition, microstructure, sintering behavior and microwave dielectric properties of BBO‐doped LZT ceramic were investigated. The addition of BBO glass could significantly reduce the sintering temperature of LZT ceramics from 1100 to 840℃. With the increase of BBO content up to 5 wt%, the dielectric constant (εr), the quality factor (Q×f) and the temperature coefficient of resonant frequency (τƒ) all decreased relative to those of LZT. Typically, by adding 0.5wt% BBO in LZT ceramic, the microwave dielectric properties of εr = 26.6, Q×f = 40,683GHz and τƒ = +1.0ppm/℃ were obtained after sintering at 840℃ for 4 h, which meant that this ceramic could be a promising candidate for LTCC application.

ACKNOWLEDGMENTS

This work is supported by National Natural Science Funds of China (Grant No. 51402039) REFERENCES

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