Microwave dielectric properties of Li₂TiO₃ ceramics doped with LiF for LTCC applications

Abstract

Sintering characteristics and microwave dielectric properties of LiF (≤4.0 wt.%) doped Li₂TiO₃ ceramics were studied in this paper. The phase presence, surface morphology and according energy dispersive spectrometer (EDS) analysis were determined by X-ray diffractometer (XRD) and scanning electron microscope (SEM) techniques, respectively. The addition of LiF as sintering aid can effectively reduce the sintering temperature of Li₂TiO₃ ceramics, and well-densified microwave ceramics with uniform grains could be obtained under 950 °C. Continuous solid solution formed for all doped samples in which LiF was uniformly distributed in the matrix. In addition, it is found that 1.0–3.0 wt.% amount of LiF can significantly improve microwave dielectric properties of Li₂TiO₃ ceramics. The temperature coefficient of resonant frequency (τ_f) decreased with increasing the amount of LiF addition. Typically, optimum dielectric microwave of ε_r = 24.01, Q × f = 75,500 GHz, and τ_f = 36.2 ppm/°C was achieved for 2.5 wt.% LiF doped samples sintered at 950 °C for 3 h. The chemical compatibilities of 2.5 wt.% LiF doped ceramics with silver were also investigated. The low-temperature sintering ceramics are compatible with Ag electrodes, and therefore, suitable for low-temperature co-fired ceramics (LTCC) application.

Introduction

With the rapid evolvement of modern wireless telecommunication and satellite network, resonators, filters and all kinds of microwave ceramic components are widely investigated [1], [2], [3], [4], [5]. Nowadays, considerable attention has been paid to develop the low-temperature co-fired ceramics (LTCC) for the advantages of providing fabrication of miniature multilayer devices. In order to fulfill the requirement of integrating many kinds of related electronic components and devices in a compact multiplayer ceramic structure, in addition to the appropriate microwave dielectric properties, a sintering temperature lower than 950 °C so that the ceramic materials could be co-fired with Ag electrode is desirable [6], [7], [8], [9], [10], [11], [12], [13]. Several methods have been developed for this purpose: (i) using ultrafine nanopowders synthesized by a wet-chemistry routine as raw materials, (ii) developing new ideal low-temperature sintered ceramic material with good microwave properties, (iii) adding sintering aids into some well-defined microwave dielectric ceramic systems. Among them, adding sintering aids is the most effective and convenient method to improve the sintering properties of ceramics [14], [15], [16].

Over recent years, many new materials have been developed and modified such as BiNbO₄, MTiO₃ (M = Mg, Zn, Ca), BaO–TiO₂, ZnNb₂O₆, BaO–R₂O₃–TiO₂ (R = Nd, Sm) and Li_1+x−yNb_1−x−3yTi_x+4yO₃, etc., according to the concrete needs of the specific applications [17], [18], [19], [20], [21], [22], [23]. Among them, an ordered rock salt superstructure crystallized Li₂TiO₃ is a relatively new promising candidate with high quality factor (Q × f > 15,000 GHz for pure Li₂TiO₃) and medium dielectric constant [24]. The Li₂TiO₃ compound with edge-sharing cation oxygen octahedra in rock salt superstructure has been reported with a monoclinic structure in a space group C2/c [25], [26]. In this model, the compound undergoes an order–disorder phase transition at 1150–1215 °C [27]. Since the porous microstructure caused by evaporation of lithium and phase transition under high sintering temperature can be detrimental for the sintering at temperature above 1150 °C, it was difficult to obtain a pure Li₂TiO₃ with high densification [28]. Recently, several approaches have been adopted for overcoming the issue. Bian et al. conducted a detailed investigation on the dielectric properties of (1 − x)Li₂TiO₃ + xMgO system [29]. It indicated that with increasing MgO addition (x ≈ 0.2), the cleavage on cation layers perpendicular to [0 0 1] due to high temperature sintering disappeared and the Q × f value was improved significantly. However, the relatively high sintering temperature of 1050 °C and a mass of porous microstructures in (1 − x)Li₂TiO₃ + xMgO system considerably impaired the use as LTCC material. Liang et al. have successfully reduced the sintering temperature of Li₂TiO₃ by adding ZnO–B₂O₃ glass frit as sintering aid, 2.5 wt.% ZnO–B₂O₃ frit doped Li₂TiO₃ ceramic exhibits the best Q × f value of 32,300 GHz, dielectric constant ε_r = 23.06 and τ_f = 35.79 ppm/°C [30]. Liang et al. also observed a similar result by adding the H₃BO₃ addition [24]. Nonetheless, the glass-derived compounds such as B₂O₃ which have lower ε_r value and higher dielectric loss significantly hindered the improvement of microwave dielectric properties.

In this paper, we investigated the structural evolution, sintering characteristics and microwave dielectric properties of the LiF-doped Li₂TiO₃ ceramics. The LTCC Li₂TiO₃ ceramics with the high Q × f value, low-cost material, and the compatibility with Ag electrodes, was obtained.