Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10606-10613
Low Temperature Sintering Kinetics and Microwave Dielectric Properties of BaTi5O11 Ceramic Yawei Chen, Enzhu Li,* Shuxin Duan, and Shuren Zhang
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School of Microelectronics and Solid State Electronics, University of Electronic Science and Technology of China, Jianshe Road, Chengdu 610054, P. R. China ABSTRACT: The addition of BaO−ZnO−B2O3 (BZB) glass was proven to effectively reduce sintering temperature of BaTi5O11 ceramic from 1100 to 900 °C in the presence of the liquid phase while maintaining excellent microwave dielectric properties. Effects of BZB glass on the phase composition, wetting behavior, microstructure, activation energy, and microwave dielectric properties of BaTi5O11 ceramics were studied in detail to understand the basic mechanism of the lowtemperature sintering kinetics. The results revealed that the sintering process of BaTi5O11 was significantly promoted at a low temperature, owing to the existence of the liquid phase caused by BZB glass, which effectively lowers the activation energy, called liquid-phase sintering. KEYWORDS: BaTi5O11, Microwave dielectric ceramics, LTCC
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INTRODUCTION
The development of the low-temperature cofired ceramic (LTCC) is accelerated by the miniaturization and integration trends of wireless communication systems.1 Zhou et al.2 reported that the BaTi5O11 phase could be synthesized at 1100 °C with CuO addition and showed good microwave dielectric properties, εr = 40.5, Q × f = 44 530 GHz, and τf = 39 ppm/°C, which reflect an excellent material for the microwave device. However, in the LTCC industry, the sintering temperature of BaTi5O11 ceramics should be lowered to less than 900 °C for cofiring with Ag. It is proven that many kinds of additions with low melting temperature,3 such as B2O3, CuO,4 Zn−B−O glass,5 BaO−ZnO−B2O3 glass,6 B2O3−ZnO−La2O3 glass,7 Li2O−B2O3−SiO2 glass,8 CuB2O4, BaCu(B2O5),9−11 and BaO−ZnO−B2O3−SiO2 glass,12 could successfully lower the sintering temperature of the BaO−TiO2 system. Zhou et al.13 used BaCu(B2O5) to decrease the sintering temperature of BaTi5O11 ceramic from 1100 to 925 °C, and good dielectric properties were attained: εr = 37.4, Q × f = 25 502 GHz, and τf = 33.1 ppm/°C. However, the sintering temperature is still higher than 900 °C, and the sintering mechanism is not investigated. In this work, to further reduce the sintering temperature and explain the sintering mechanism, BaO−ZnO− B2O3 (BZB) glass (Ts ∼ 480 °C, εr ∼ 6.9, τf ∼ −53.7)6,14 was used as the sintering addition of BaTi5O11 ceramic. The wetting behaviors, activation energy, microstructures, phase composition, and microwave dielectric properties of BaTi5O11 ceramic have been researched systematically. © 2017 American Chemical Society
EXPERIMENTAL SECTION
BaTi5O11 ceramics were synthesized by a solid-state reaction method. The weighed raw materials of BaCO3 (>99%) and TiO2 (>99%) were mixed according to the formula of the BaTi5O11 and 1 wt % CuO, and then milled with ZrO2 media in deionized water for 3 h. After drying, the mixed powders were calcined at 900 °C for 3 h. 40 mol % BaCO3, 30 mol % ZnO, and 30 mol % H3BO3 were used to synthesize the BZB glass. The powders were milled for 2 h with ZrO2 and melted at 1400 °C for 2 h. Then, the melt material was quenched using a copper plate. The BaTi5O11 powders were further milled with BZB glass in alcohol for 3 h with ZrO2. The particle size of the mixed powder measured by using dynamic light scattering instrument (DLS, NanoBrook EliteSizer Omni, Brookhaven) was about 1.13 μm. Then, the mixed powders were pressed into pellets (8 mm in height and 15 mm in diameter). Subsequently, the samples were sintered at 875−900 °C for 1 h in air. The shrinkages of the samples were calculated with the dilatometer (NETZSCH, Germany). Wetting behaviors between BaTi5 O11 ceramic and BZB glass were studied by using material hightemperature character measurement instrument (XiangTan XiangYi Instrument Limited Company, China). A piece of BZB glass obtained by pressing the glass powder was placed on the surface of dense BaTi5O11 substrate. Then, the glass and the ceramic were heated from 30 to 900 °C, and the changes of BaTi5O11 and BZB were recorded by the camera system in the heating process. The phase components of the samples were confirmed by using X-ray diffraction analysis (XRD, Rigaku Industrial Corporation, Japan). The elemental and microstructure of the BaTi5O11 samples were measured by scanning electron microscopy (SEM) which had the EDS (JSM-6490LV, Japan). The bulk densities were determined by using the Archimedes method. Then, the εr and the Q × f values were measured by using an Received: July 29, 2017 Revised: October 11, 2017 Published: October 16, 2017 10606
DOI: 10.1021/acssuschemeng.7b02589 ACS Sustainable Chem. Eng. 2017, 5, 10606−10613
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Wetting behaviors between the BZB glass and BaTi5O11 ceramic at different temperatures. HP83752A network analyzer with the Hakki−Coleman dielectric resonator method. The τf values of the BaTi5O11 samples were determined by the equation
τf =
f85 − f25 f25 × 60
in Figure 4. The main crystal phase of BaTi5O11 (JCPDS 01074-0538), a small amount of BaTi4O9 (JCPDS 00-008-0367), and TiO2 (JCPDS 01-076-0322) phases were observed in the BaTi5O11 powders calcined at 900 °C. The same result was reported by Zhou et al.13 Furthermore, the phase compositions of BaTi5O11 ceramics were not changed after addition of the BZB glass, indicating that there is no reaction that occurred when BaTi5O11 ceramic with BZB glass was sintered at a low temperature. The SEM images of the surfaces of BaTi5O11 ceramic with 2−6 wt % BZB glass sintered at 900 °C were shown in Figure 5. The ceramic tended to be dense, accompanied by the reduction of pores and the grain growth with the increasing of BZB glass (2−6 wt %) (Figure 5a−e). The SEM results again demonstrate that the BZB glass effectively promotes the sintering process and densification of BaTi5O11 ceramic. The grain size is important for the dielectric properties of the polycrystalline ceramics. Therefore, the grain size distributions of the samples are measured and shown in Figure 6. For Figure 6a,b, although the grain size distributions are relatively concentrated, the average grain sizes are small. Also, the average grain sizes of the samples sintered at 900 °C increase from 0.15 to 0.28 μm with the increase of BZB glass. After 4 wt % BZB was added, although the average grain sizes increase, the distributions become nonuniform, and some abnormal large grains are observed. The grain sizes (
