Microwave dielectric properties and microstructures of BaO modified CaO-Li2O-Sm2O3-TiO2 ceramics

Abstract

The proliferation of commercial wireless technologies, such as cellular phones and global positioning systems, has been making rapid progress due to the improved performance of dielectric resonators at microwave frequencies. Requirements for these dielectric resonators must be combined with a high dielectric constant for possible size miniaturization (size of a dielectric resonator ∼1/√er), a low dielectric loss for a stable resonant frequency and a near-zero temperature coefficient of resonant frequency (τf) for temperature stable circuits. Since the guided-wavelength in the UHF band is rather long as compared to other higher frequency bands, the requirement for high dielectric constant material becomes greater than ever in reducing the size of the device. With the combination of CaTiO3 [1] (τf ∼ +800 ppm/◦C) and (Li1/2Sm1/2)TiO3 [2] (τf ∼ −260 ppm/◦C), CaO-Li2O-Sm2O3-TiO2 ceramics system exhibits good dielectric properties at microwave frequencies [3]. A non-stoichiometric composition of CaO : Li2O : Sm2O3 : TiO2= 16 : 9 : 12 : 63 was proposed as a temperature stable dielectric material for the fabrication of dielectric resonator. In their study, CaO was further partially replaced by SrO. At the level of 1 mol% SrO substitution, a dielectric constant of 110, a Q× f value of 4500 GHz and a τf value of 7 ppm/◦C were obtained. Instead of SrO, a partial replacement of CaO by BaO for CaO-Li2O-Sm2O3-TiO2 (denoted as CLST) ceramics has been investigated in this paper. The microwave dielectric properties of BaO modified CLST ceramics were also studied with different compositions. High-purity raw material CaCO3, BaCO3, LiCO3, Sm2O3 and TiO2 (>99.9%) were weighed according to desired molar ratios. The powders were ground in distilled water for 16 h in a ball mill with agent balls. All mixtures were dried and calcined at 1000 ◦C for 3 h in air. The calcined powders were then remilled for 5 h with PVA solution as a binder. The sieved powders were then uniaxially pressed into pellets with 11 mm in diameter and 5 mm in height. After debinding, these pellets were sintered at temperatures of 1300–1400 ◦C for 3 h. The microstructure observation of the sintered ceramic surface was performed by means of SEM, EDS and WDS. The crystalline phases of sintered ceramics was identified by X-ray diffraction (XRD) pattern. The dielectric constant (er) and the quality factor values (Q) at microwave frequencies were measured using the Hakki-Coleman dielectric resonator method which had been modified and improved by Kobayashi [4, 5]. A system combined with a HP8757D network analyzer and a HP8350B sweep oscillator was employed in the measurement. The temperature coefficient of resonant frequency (τf) at microwave frequency was measured in the temperature range from 0 to 80 ◦C. The τf value is defined as follows.