Microwave dielectric properties of Li2TiO3 ceramics sintered at low temperatures
Introduction
Microwave dielectric ceramics are widely used in fields such as telecommunication, radar, and navigation. Nowadays increased attention has been paid to develop microwave dielectric ceramics with low sintering temperatures (Ts) for LTCC applications. Several ways have been adopted for this purpose: (1) adding sintering aids into some established microwave dielectric ceramic systems [1], [2], (2) using ultrafine powders as raw materials or synthesizing a compound by wet chemical methods [3], and (3) searching for new ceramic systems with an intrinsic low sintering temperature [4]. Of these methods adding sintering aids is the easiest and cheapest way to decrease the Ts of ceramics.
The Li2TiO3 ceramic system is a promising candidate for LTCC applications at microwave frequencies. In our previous study, good microwave dielectric properties were obtained for the ZnO-B2O3 frit-doped Li2TiO3 ceramics [5], but the properties of pure Li2TiO3 ceramics were needed to be investigated and the microwave dielectric properties may be improved by the variety of the Ts. B2O3 (in the form of H3BO3) was chosen as the sintering aid because of its low melting point and it is easy to form a liquid phase at the Ts of 900 °C. It has been successfully used to decrease the Ts of some other ceramics. This paper investigates the microwave dielectric properties of pure Li2TiO3 and that of the H3BO3-doped Li2TiO3 ceramics sintered at low temperatures. The novel result that the microwave dielectric properties of H3BO3-doped Li2TiO3 ceramics sintered at 920 °C were much better than that of pure Li2TiO3 ceramics confirms the effect the control of the volatilization of Li2O at a low Ts on the microwave dielectric properties of Li2TiO3 ceramics and may inspire us to find some new microwave dielectric ceramic systems for LTCC applications.
Section snippets
Experiment
High-purity powders of TiO2 (99.5%) and Li2CO3 (99.0%) were mixed stoichiometrically according the formula of Li2TiO3 (Li2O in the form of Li2CO3) in ethanol with zirconia balls in a planetary ball milling system for 3 h. After drying and sieving, the mixtures were calcined at 820 °C for 4 h. The calcined powders were ball milled, dried, sieved again and then homogeneous Li2TiO3 powders were obtained. One part of the Li2TiO3 powder was mixed directly with 10 wt.% polyvinybutyral (PVB), the
Results and discussion
Fig. 1 shows XRD patterns of (a) the Li2TiO3 powder calcined at 820 °C for 4 h, (b) the Li2TiO3 ceramic sintered at 1200 °C for 3 h, (c) the 2.0 wt.% H3BO3-doped Li2TiO3 ceramic sintered at 900 °C for 3 h. All diffraction patterns could be indexed with C2/c monoclinic Li2TiO3 structure, a = 5.0690 Å, b = 8.7990 Å, c = 9.7590 Å and β = 100.2000°, no secondary phase was detected. Fig. 2 shows the SEM micrographs of the polished and thermal etched surface of Li2TiO3 ceramics sintered at 1200 °C for 3 h. The grain
Conclusions
The best Ts of Li2TiO3 ceramics was 1200 °C and the microwave dielectric properties of pure Li2TiO3 ceramics were not ideal for the volatilization of Li during sintering. A little addition of H3BO3 decreased the Ts of Li2TiO3 ceramics to about 900 °C and the microwave dielectric properties of H3BO3-doped Li2TiO3 ceramics were much better than that of pure Li2TiO3 ceramics. Only monoclinic Li2TiO3 was found in these samples. The best microwave dielectric properties were ɛr = 23.28, Q × f = 37,110 GHz (6.3
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