Low permittivity cordierite-based microwave dielectric ceramics for 5G/6G telecommunications

Abstract

5G and forthcoming 6G communication systems require dielectric ceramics with low relative permittivity (ε_r) and near-zero temperature coefficient of resonant frequency (τ_f) for the lower part of the microwave (MW) band and at sub-Terahertz. Mg₂Al₄Si₅O₁₈ (MAS) ceramics are promising candidates due to their low ε_r (~ 6) and high-quality factor (Q×f > 40,000 GHz) but have a large τ_f. In this study, 5.5 wt% TiO₂ (MAS-T_5.5) was used to adjust τ_f of MAS to −2.8 ppm/℃ whilst retaining low ε_r (5.24) and good Q×f (33,400 GHz), properties consistent with those obtained by infrared reflectance. A demonstrator microstrip patch antenna with gain 4.92 dBi and 76.3% efficiency was fabricated from MAS-T_5.5.

Introduction

As 5G mobile communication technology develops, the trend is to move towards higher frequency bands, the so-called millimeter wave region, to enable faster data transmission speeds and larger information carrying capacity. 5G and its successor 6G will give birth to the ‘internet of things’ and enable ‘big-data’ to be transmitted through the mobile network for the development of smart energy efficient cities and autonomous transport systems. The construction of 5G and 6G base stations will guide the development of new materials, promote artificial intelligence, new concepts in electronics and provide strong support for sustainable growth of the global economy.

Microwave dielectric resonators are widely used in mobile base stations, satellite communications, military radar, global positioning systems (GPS), Bluetooth technology and other engineering fields [1], [2], [3], [4] The time delay in a resonator has the following relationship with the relative permittivity (ε_r) [5], [6]:

$$t_d=L\frac{\sqrt{{\epsilon }_r}}{c}$$

where $c$ represents the velocity of light and $L$ is signal transmission distance. The low time delay of 5G communication therefore requires the dielectric to have a low ε_r and silicate ceramics are consequently important for future millimeter wave technologies [7], [8], [9], [10]. Orthorhombic cordierite (Mg₂Al₄Si₅O₁₈, MAS) microwave ceramics have low relative permittivity and dielectric loss and were originally reported by Ohsato et al., ε_r ≈ 6，Q×f ≈ 40,000 GHz and τ_f ≈ −24 ppm/°C [11], [12]. However, its large τ_f and, to a lesser degree, its high sintering temperature (~1400 °C) provide potential barriers to commercialization. In previous studies, materials such as SrTiO₃ [13], CaTiO₃ [14], TiO₂ [15] and Mg₂SiO₄ [16] were added to MAS in an attempt to improve MW dielectric properties. The most promising results were achieved for composites of 0.75Mg₂Al₄Si₅O₁₈-0.25TiO₂, reported to have τ_f = −0.2 ppm/°C [15], and (1-x)Mg₂Al₄Si₅O₁₈-xMg₂SiO₄ [16] which reduced sintering temperature to 1340 ℃ for 50 wt% Mg₂SiO₄, improved Q×f (76,374 GHz) but had little impact on τ_f (−24 ppm/°C). Although improvements in Q×f and sintering temperature are important, the primary motivation to facilitate commercialization of MAS is to achieve close to zero τ_f and therefore our study will optimize TiO₂ concentration.

At present, both mobile phones and wireless network cards support a 2.4 GHz band which passes largely unhindered through masonry in cities. However, its limited bandwidth means that it rapidly becomes crowded, and signals are often dropped when connecting to multiple peripherals. 5.8 GHz transmission is more stable due to fewer consumer devices but requires antennas to work at higher frequencies and thus materials such as MAS with low ε_r are important for current micro- as well as future millimeter wave technology [17], [18], [19], [20], [21], [22], [23]. (1-x)Mg₂Al₄Si₅O₁₈-xTiO₂ (x = 0–5.5 wt%) microwave ceramics (Abbr. MAS-T_x=0–5.5) were therefore prepared by solid-state reaction. The effect of TiO₂ concentration on crystal structure, density, and microwave dielectric properties of MAS ceramic were investigated, and a demonstrator 5.8 GHz microstrip patch antenna was fabricated.