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 (ε_r), near zero temperature coefficient of resonant frequency (τ_f), 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, ε_r = 25.6, and τ_f = −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.

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 τ_f 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.

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,

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 ε_r 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 ε_r 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 ε_r. 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 τ_f 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 τ_f 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

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.

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 ε_r = 25.5, Qf = 22,400 GHz, and τ_f = −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.

The authors declare no conflicts of interest regarding the publication of this paper.

Zhu, J.H., Liu, J.Y., Lu, S.J. and Zeng, Y. (2019) Effects of V₂O₅ Addition on Microwave Dielectric Properties of Li₂ZnTi₃O₈ Ceramics for LTCC Applications. New Journal of Glass and Ceramics, 9, 25-32. https://doi.org/10.4236/njgc.2019.93003