1. Introduction
Low temperature co-fired ceramics (LTCC) technology has drawn worldwide attention for more than thirty years, since its advantages in miniaturizing and integrating electronic components and modules. Silver is usually used as inner electrode for LTCC technologies, due to its relatively low cost and high conductivity. However, the low melting point of silver (961˚C) prevents it from co-firing with most of ceramic materials. To match with silver inner paste, lowering down the sintering temperature of the used ceramics to around 900˚C is very necessary. What’s more, for the fabrication of electronic components, the ceramic materials should have suitable permittivity (εr), near zero temperature coefficient of resonant frequency (τf), and low dielectric loss (usually replaced by Q*f value for microwave dielectric ceramics) [1] .
Much attention has been paid to the LTCC applications of Li-containing compounds, such as Li2TiO3, Li2MgSiO4, Li3Mg2NbO6, Li3NbO4, and Li2O-Nb2O5-TiO2 [2] [3] [4] [5] [6] . In 2010, George and Sebastian [7] firstly reported Li2ZnTi3O8 ceramics with good microwave dielectric properties (Qf = 72,000 GHz, εr = 25.6, and τf = −11.2 ppm/˚C). However, its relatively high sintering temperature (1075˚C) limits its application for LTCC components. Several sintering aids, such as H3BO3, Bi2O3, B2O3, and glass, have been successfully used to lower down the sintering temperature of Li2ZnTi3O8 ceramic because of the low melting point or softening point of these sintering aids [8] - [16] . V2O5 has also been used as sintering aid for some Li-containing compounds [16] [17] [18] , but its effectiveness on Li2ZnTi3O8 ceramics has not been reported. Other than that, the co-firing ability of those materials with silver inner paste was seldom discussed by using multilayer component technologies. In this work, the sintering and microwave dielectric properties of Li2ZnTi3O8 ceramics after V2O5 doping, as well as its co-firing behavior with Ag inner electrodes were investigated.
2. Experimental
Reagent grade Li2CO3 (99 wt%, Aladdin, China), V2O5 (99 wt%, Aladdin, China), TiO2 (99.9 wt%, Yaxing, China), and ZnO (99.7 wt%, Maixin, China) powder were used. According to the stoichiometries of Li2ZnTi3O8, those oxides were weighed and then ball milled in planetary ball mill machine for 3 h with alcohol and zirconia balls as the medium. After the milling, the mixtures were dried at 75˚C and then calcination process was performed at 850˚C for 3 h. Following the calcination process, V2O5 with different amounts were added to the Li2ZnTi3O8 powder. The mixture were re-milled for 3 h. Polyvinyl alcohol (PVA) binders were added into dried powder and then sieved. The sieved powders were pressed into disks under a pressure of 150 MPa. Finally, the samples were sintered at temperature range from 850˚C to 925˚C for 3 h in the air.
The densities of the ceramic disks were measured based on the Archimedes’ method. Crystal structures of the ceramics were analyzed using X-Ray diffraction (XRD) (XRD-7000 diffractometer). Scanning electron microscope (SEM) (JEOL JSM-64) and energy dispersive spectrometer (EDS) (Oxford X-max N50) were used to observe the morphologies and material compositions of the as-sintered disk surfaces. Microwave dielectric properties of the ceramics were measured according to Hakki and Coleman’s methods [19] [20] , and the same method was also used to measure τf values of the samples at temperature from 25˚C to 75˚C.
The calcined Li2ZnTi3O8 powder was mixed with V2O5, solvent, binder, plasticizer, and dispersant, and then ball milled for approximately 36 h as the slurry. Traditional LTCC procedures, including tape casting, printing, lamination, isostatic pressing, cutting, and sintering, were performed for cofiring ability test. Elements distribution was carried out by energy dispersive spectrometer using line scan analysis.
3. Results and Discussions
The sintered densities of Li2ZnTi3O8 ceramics with various V2O5 proportions and sintered at various temperatures are shown in Figure 1. The addition of V2O5 effectively increased the densities of Li2ZnTi3O8 ceramics, even with a small V2O5 amount of 0.25 wt%. The sintered densities of Li2ZnTi3O8 ceramics increased with the sintering temperatures when the addition amounts of V2O5 are less than 0.25 wt%, while there are no obvious changes when the addition amounts of V2O5 are over 0.5 wt%, which means that the 0.5 wt% addition amount of V2O5 is sufficient for the low temperature sintering of Li2ZnTi3O8. The density of Li2ZnTi3O8 ceramics with 0.5 wt% V2O5 addition and sintered at 900˚C can reach 3.75 g/cm3, which was approximately 94.4% of the theoretical value of Li2ZnTi3O8 ceramic (3.974 g/cm3) [7] . The densification effect was due to the low melting point of V2O5 (about 650˚C) and the consequent appearance of liquid phase. The appearance of liquid phase promoted the rearrangement and growth of grains and therefore increased the densities of Li2ZnTi3O8 ceramic, while further increasing of sintering temperature caused the abnormal grain growth and therefore decreased the densities of the samples.
Figure 2 shows the XRD spectrum of sintered Li2ZnTi3O8 ceramics with various V2O5 doping amounts at sintering temperature of 900˚C. All the spectrum matched well with Li2ZnTi3O8 phase (JCPDS#86-1512), consistent with the reports of George [7] and Fang [21] . This suggested that the phase composition of Li2ZnTi3O8 were insensitive to V2O5 doping. The morphologies of V2O5-doped Li2ZnTi3O8 ceramics with various addition amounts and sintered at 900˚C are shown in Figure 3. Those sintered samples exhibited relatively dense microstructure, and both small- and large-sized grains existed. As shown in Figure 4,
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Figure 1. Densities of Li2ZnTi3O8 ceramics as a function of sintering temperatures and the V2O5 addition contents.
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Figure 2. XRD patterns of Li2ZnTi3O8 ceramics doped with different amounts of V2O5 and sintered at 900˚C; (a) 0.25 wt%; (b) 0.5 wt%; (c) 0.75 wt%; and (d) 1 wt%.
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Figure 3. SEM micrographs of Li2ZnTi3O8 ceramics doped with different amounts of V2O5 sintered at different temperatures; (a) 0 wt%, 900˚C; (b) 0.25 wt%, 900˚C; (c) 0.5 wt%, 900˚C; (d) 0.75 wt%, 900˚C; (e) 1 wt%, 900˚C.
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Figure 4. EDS analysis of Li2ZnTi3O8 ceramics added with 0.5 wt% V2O5 and sintered at 900˚C for 3 h.
for large- (spot A) and small-sized grains (spot B), the atomic ratio of Ti and Zn elements were detected to be approximately the same value of 3. These results agreed well with the molecular formula of Li2ZnTi3O8, indicating that the Li2ZnTi3O8 phase was the matrix phase and no secondary phase existed, which was also consistent with the XRD spectrum in Figure 2.
The microwave dielectric properties of Li2ZnTi3O8 with different V2O5 doping amounts (sintered at 900˚C) were then detected as shown in Figure 5. With the increase of V2O5 addition, the εr of the Li2ZnTi3O8 ceramics raised up to a maximum value of 25.5 at a V2O5 content of 0.5 wt% and then decreased afterwards. The change of εr had similar trend to that of density, as shown in Figure 1. High density usually results in the presence of considerable number of dipoles per unit volume, and consequently large εr. The Qf value of Li2ZnTi3O8 ceramics was enhanced significantly with increased V2O5 doping amount at the range below 0.5 wt%, due to the increased density and the decreased defects and grain boundaries. Further addition of V2O5 led to excess liquid phase and consequently decreased Qf value. The τf value of the V2O5-doped Li2ZnTi3O8 ceramics slightly changed from −12 ppm/˚C to around −10 ppm/˚C when 1 wt% amount was added, manifesting that V2O5 had little influence on the τf of the Li2ZnTi3O8 ceramic.
Figure 6 shows the optical image of Li2ZnTi3O8 ceramics with the addition of 0.5 wt% V2O5 and cofired with Ag at 900˚C for 3 h. No obvious cracks and distortion were observed between Li2ZnTi3O8 ceramics and Ag inner electrode interfaces. EDS line scanning was performed to further investigate the cofiring properties of the ceramic and Ag inner electrode. Figure 7 shows the element
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Figure 5. Microwave dielectric properties of Li2ZnTi3O8 ceramics as a function of V2O5 addition content; (a) Relative permittivity; (b) Qf values; and (c) τf values.
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Figure 6. Optical microscopy of Li2ZnTi3O8 ceramics cofired with Ag.
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Figure 7. EDS line scanning of the Li2ZnTi3O8 ceramics cofired with Ag.
analysis and the corresponding morphology of the line region which cross the ceramic and the Ag electrode. The Ag profile shows a platform in the middle, and both the Zn and Ti profiles show two platforms in the side regions. All the sharp transitions to near-zero level happened at the two interfaces, indicating that reaction and diffusion did not occur between the low temperature sintered Li2ZnTi3O8 ceramics and the Ag inner electrodes.
4. Conclusion
Li2ZnTi3O8 ceramic was densified at about 900˚C with the addition of V2O5. Li2ZnTi3O8 ceramic with small amounts (less than 1 wt%) of V2O5 had single phase with a spinel crystal structure. When 0.5 wt% V2O5 was doped, microwave dielectric properties of εr = 25.5, Qf = 22,400 GHz, and τf = −10.8 ppm/˚C was obtained. EDS line scanning results showed that Li2ZnTi3O8 ceramics could be cofired with Ag inner paste without cracks and diffusion, making it very potential for LTCC applications.