Effect of B2O3 Additives on the Sintering and Dielectric Behaviors of CaMgSi2O6 Ceramics

Effect of B2O3 Additives on the Sintering and Dielectric Behaviors of CaMgSi2O6 Ceramics

J. Mater. Sci. Technol., 2010, 26(4), 351-354. Effect of B2 O3 Additives on the Sintering and Dielectric Behaviors of CaMgSi2 O6 Ceramics Huanping Wan...

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J. Mater. Sci. Technol., 2010, 26(4), 351-354.

Effect of B2 O3 Additives on the Sintering and Dielectric Behaviors of CaMgSi2 O6 Ceramics Huanping Wang, Shiqing Xu† , Suya Zhai, Degang Deng and Haidong Ju College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China [Manuscript received January 16, 2009, in revised form June 12, 2009]

The effects of B2 O3 addition, as a sintering agent, on the sintering behavior, microstructure and dielectric properties of CaMgSi2 O6 ceramic have been investigated. The CaO-MgO-B2 O3 -SiO2 glassy phase came forth by adding B2 O3 into CaO-MgO-SiO2 ceramic powders, and it was advantageous to lower the synthesis temperature of CaMgSi2 O6 crystalline phase. Moreover, the emergence of CaO-MgO-B2 O3 -SiO2 glass phase could effectively lower the sintering temperature of CaMgSi2 O6 ceramic and promote the CaMgSi2 O6 grains growing. With 6 wt pct B2 O3 addition, the densification temperature of CaMgSi2 O6 ceramic could be effectively reduced from 1300 to 1100◦ C, and the dielectric constant (εr ) and dielectric loss (tanδ) were: εr =7.61 and tanδ=7.4×10−4 (1 MHz). KEY WORDS: Dielectric properties; Microstructure; CaMgSi2 O6 ceramic; B2 O3 addition

1. Introduction Low temperature co-fired ceramic (LTCC) technology has been playing an important role in modern wireless communication systems. In order to process ceramic with electrode material, such as 30Pd/70Ag (melting point, ∼1167◦ C) and silver (melting point, ∼961◦ C), it is required to sinter the dielectric materials at temperatures lower than the melting temperature of the co-fired electrode material. Research on the development of co-fired dielectrics with different dielectric constants has been carried out for years to fill the need for various radio frequency (RF) ranges[1–6] . However, low dielectric constant (εr ) material is still the most popularly used one, such as DuPont 951 (εr ∼7.8, Q×f ∼700 GHz at 3 GHz, τf ∼8×10−6 /◦ C), since it provides fast transmission in communication systems[7,8] . CaMgSi2 O6 ceramic was proved as a lowpermittivity dielectric ceramic system and possessed good microwave dielectric properties[9–13] . The dielectric constant, Q×f and τf of CaMgSi2 O6 ceramic sintered at 1290◦ C were 7.46, 59638 GHz and −4.6×10−5 /◦ C, respectively[9] . Zhang et al.[10] found † Corresponding author. Tel.: +86 571 86835781; Fax: +86 571 28889527; E-mail address: [email protected] (S.Q. Xu).

that the 0.88CaMgSi2 O6 -0.12CaTiO3 ceramic sintered at 1300◦ C showed a relatively low-permittivity (9.42), high Q×f value up to 52800 GHz, and near-zero temperature coefficients (5.6×10−6 /◦ C). The dielectric constants and dissipation factors of CaMgSi2 O6 were also determined at 1 MHz using a two-terminal method and empirically determined edge corrections. The results were: κ11 =9.69, tanδ=0.0016, κ22 =7.31, tanδ=0.0007, κ33 =7.29 and tanδ=0.00019[13] . Since B2 O3 was commonly used as a flux former and had been shown to decrease the sintering temperature[14–16] , it was chosen as a sintering aid in our experiments. In this work, B2 O3 was added into CaMgSi2 O6 ceramic to reduce its sintering temperature. The resultant dielectric properties were analyzed based on the densification, the X-ray diffraction patterns and the microstructures of the ceramics. 2. Experimental Powder samples were prepared by a conventional solid-state method. High-purity oxide powders (>99.5%): CaCO3 , MgO, SiO2 and B2 O3 were used as raw materials. The powders were weighed according to the composition of CaMgSi2 O6 , and were mixed

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in ethanol for 24 h in a balling mill with agate balls. The as-prepared powders were dried and calcined at 1100◦ C for 2 h in air. The calcined powders were mixed and milled as desired composition CaMgSi2 O6 with 0.5–8 wt pct B2 O3 of additions as sintering aids for 24 h. The fine powder together with the organic binder (5 wt pct polyvinyl alcohol) was pressed into pellets with dimensions of 15 mm in diameter and 5 mm in thickness. These pellets were sintered at temperatures of 1000–1200◦ C for 2 h in air with the heating rate of 5◦ C/min. The crystalline phases of calcined powder and sintered ceramics were identified by X-ray diffraction pattern (XRD, ARL XTRA, CuKα, USA). The bulk densities of the sintered pellets were measured by the Archimedes method. The microstructure observation of the sintered ceramics was performed by scanning electron microscopy (SEM, JSM-5601, Japan). The dielectric constant (εr ) and dielectric loss (tanδ) at room temperature were determined from capacitance measurements by an inductance, capacitance, and resistance (LCR) meter (HEWLETT PACKARD 4278A, USA) at 1 MHz. The dielectric constant was calculated from a parallel-plate capacitor equation, e.g., εr =Cd/ε0 A, where C was the capacitance of the specimens, d and A were the thickness and the area of the electrode, respectively, and ε0 was the dielectric permittivity of vacuum (8.854×10−12 F/m). Silver paste was used for the electrodes.

Fig. 1 X-ray diffraction pattern of CaCO3 -MgO-SiO2 powder calcined at 1100◦ C

3. Results and Discussion Figure 1 shows the X-ray diffraction pattern of CaCO3 -MgO-SiO2 powders calcined at 1100◦ C. It is obvious that the main crystal phases are Ca2 MgSi2 O7 and SiO2 accompanied with a little MgO phase, which suggests that it is difficult to obtain CaMgSi2 O6 crystal phase by calcining CaCO3 -MgO-SiO2 powders at 1100◦ C. The X-ray diffraction patterns of CaMgSi2 O6 ceramics sintered at 1100◦ C with different B2 O3 additions are shown in Fig. 2. With 0.5 wt pct B2 O3 addition, the single crystal phase of CaMgSi2 O6 is obtained, and no second phase is observed, which indicates that the addition of B2 O3 can decrease the synthesis temperature of CaMgSi2 O6 crystalline phase. Moreover, it is clear that the diffraction peaks of CaMgSi2 O6 are weakened with the increasing B2 O3 content. This result indicates that the reaction between B2 O3 and CaO-MgO-SiO2 results in the emergence of CaO-MgO-B2 O3 -SiO2 glass phase, and it is advantageous to lower the synthesis temperature of CaMgSi2 O6 crystalline phase. The density of B2 O3 -doped CaMgSi2 O6 ceramics at different sintering temperatures is shown in Fig. 3. With the increase of the sintering temperature, the apparent density is found to increase to a maximum value, and higher B2 O3 content shifts the obtainable maximum density to a lower temperature. The bulk densities of CaMgSi2 O6 ceramics sintered at 1100◦ C

Fig. 2 X-ray diffraction patterns of CaMgSi2 O6 ceramics sintered at 1100◦ C with 0.5 wt pct (a), 1 wt pct (b), 2 wt pct (c), 4 wt pct (d), 6 wt pct (e) and 8 wt pct (f) B2 O3 additions

Fig. 3 Dependence of volume density on sintering temperature for CaMgSi2 O6 ceramics with 0.5 wt pct (a), 1 wt pct (b), 2 wt pct (c), 4 wt pct (d), 6 wt pct (e) and 8 wt pct (f) B2 O3 additions

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Fig. 4 SEM photographs of CaMgSi2 O6 ceramics sintered at 1100◦ C with 0.5 wt pct (a), 1 wt pct (b), 2 wt pct (c), 4 wt pct (d), 6 wt pct (e) and 8 wt pct (f) B2 O3 additions

with 0.5, 1, 2, 4, 6 and 8 wt pct B2 O3 additions are 2.44, 2.76, 2.92, 3.05, 3.14 and 3.13 g·cm−3 , respectively. The optimal sintering temperatures of CaMgSi2 O6 ceramics with 0.5, 1–4 and 6–8 wt pct B2 O3 additions are 1200–1250, 1150–1200 and 1100– 1150◦ C, respectively. The decrease of the sintering temperature with the increase of B2 O3 addition is due to the augment of CaO-MgO-B2 O3 -SiO2 glass phase, which will transform into liquid phase and promote the sintering process. As it is obvious, the CaMgSi2 O6 ceramic sintered at 1100◦ C with 6–8 wt pct B2 O3 addition reaches above 96% of the theoretical density (the theoretical density of CaMgSi2 O6 ceramic is 3.27 g·cm−3 ), which can be co-fired with 30Pd/70Ag electrode material. Figure 4 shows the microstructures of CaMgSi2 O6 ceramics sintered at 1100◦ C with different B2 O3 additions. From Fig. 4(a), it can be seen clearly that the specimen is not dense and the grains does not grow with 0.5 wt pct B2 O3 addition, and the average grain

size of CaMgSi2 O6 ceramic is about 0.8–1 μm. Figure 4(b)–(d) show that the grain size increases with the increase of B2 O3 addition due to the liquid phase effect resulted from the addition of B2 O3 , and there are many grains growing up to 3–5 μm with 4 wt pct B2 O3 added in. When the B2 O3 content increases to 6 wt pct, the grains of CaMgSi2 O6 ceramic are in close contact, and the amount of pores decreases, which is consistent with the result of bulk densities. However, many pores are observed in the ceramic by adding 8 wt pct B2 O3 , which is due to the augment of CaO-MgO-B2 O3 -SiO2 glass phase. The dielectric properties of B2 O3 -doped CaMgSi2 O6 ceramics sintered at different temperatures are illustrated in Figs. 5 and 6. The relationships between dielectric properties and sintering temperature follow similar trend to those between density and sintering temperature, due to the fact that a higher density signifying a lower porosity. As the sintering temperature is less than 1150◦ C, the

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Fig. 5 Dependence of dielectric constant on sintering temperature for CaMgSi2 O6 ceramics with 0.5 wt pct (a), 1 wt pct (b), 2 wt pct (c), 4 wt pct (d), 6 wt pct (e) and 8 wt pct (f) B2 O3 additions

of B2 O3 -doped CaMgSi2 O6 ceramics have been investigated. A tremendous temperature reduction (>200◦ C) can be achieved by adding B2 O3 to the CaMgSi2 O6 specimens. B2 O3 is not detected in the ceramics but residual as a CaO-MgO-B2 O3 -SiO2 glass phase. The emergence of CaO-MgO-B2 O3 -SiO2 glass phase, acting as a sintering aid, can effectively lower the synthesis temperature of CaMgSi2 O6 crystalline phase and the sintering temperature of CaMgSi2 O6 ceramic. Sintered at 1100◦ C, the 6 wt pct B2 O3 doped CaMgSi2 O6 ceramic possesses good dielectric properties: εr =7.61 and tanδ=7.4×10−4 (1 MHz), which can be co-fired with 30Pd/70Ag electrode material.

Acknowledgements This work was supported by the Program for New Century Excellent Talents in University (No. NCET07-0786), the Nature Science Foundation of Zhejiang Province (No. R406007) and the Science Technology of Zhejiang Province (No. 2008C21054).

REFERENCES

Fig. 6 Dependence of dielectric loss on sintering temperature for CaMgSi2 O6 ceramics with 0.5 wt pct (a), 1 wt pct (b), 2 wt pct (c), 4 wt pct (d), 6 wt pct (e) and 8 wt pct (f) B2 O3 additions

dielectric constant is increased and the dielectric loss is decreased with the sintering temperature increasing. Sintered at 1100◦ C, the CaMgSi2 O6 ceramic with 6 wt pct B2 O3 addition possesses good dielectric properties: εr =7.61 and tanδ=7.4×10−4 (1 MHz), and the dielectric constant and dielectric loss of CaMgSi2 O6 ceramic with 8 wt pct B2 O3 addition are 7.31 and 8.6×10−4 (1 MHz), respectively, which indicates that it is disadvantageous to the dielectric properties of CaMgSi2 O6 ceramic by adding above critical amount B2 O3 . Moreover, increasing the sintering temperature higher than 1150◦ C, the dielectric constant decreases and the dielectric loss increases by adding 6–8 wt pct B2 O3 into CaMgSi2 O6 ceramics, which is due to the decrease of density and the augment of CaO-MgOB2 O3 -SiO2 glass phase. 4. Conclusion The sintering behavior and dielectric properties

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