Low permittivity cordierite-based microwave dielectric ceramics for 5G/6G telecommunications
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 (εr) [5], [6]:where represents the velocity of light and is signal transmission distance. The low time delay of 5G communication therefore requires the dielectric to have a low εr 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., εr ≈ 6,Q×f ≈ 40,000 GHz and τf ≈ −24 ppm/°C [11], [12]. However, its large τf 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 τf = −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 τf (−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 τf 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 εr 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-Tx=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.
Section snippets
Preparation of MAS-T0–5.5 ceramics
High purity raw materials of MgO (99.99%), Al2O3 (99.99%), SiO2 (99.99%) were weighed based on the stoichiometric formula of MAS. Raw materials were ball milled 6 h with ethanol. After drying, mixtures were calcined 4 h at 1400 °C to synthesize MAS. TiO2 (99.99%) with different mass fraction were added into MAS powders and then ball milled. Dried MAS-T slurries were added with 8% PVA as binder, the powders were ground and pressed into cylindrical samples in a steel die at 100 MPa. Finally, the
Results and discussion
The bulk density of MAS-T0–5.5 ceramics at different sintering temperatures are shown in Fig. 1. The optimal sintering temperature decreases with the increase in fraction of TiO2. The maximum bulk density of MAS-T5.5 is 2.481 g/cm3 at 1320 °C which corresponded to a relative density of 95%.
Fig. 2 displays the XRD patterns of MAS-xT ceramics. When x = 0, the diffraction peaks correspond to the Mg2Al4Si5O18 cordierite phase (JCPDS No.82–1541) with a space group of Cccm (66). As x increases,
Conclusions
In this work, the τf of MAS based ceramics is optimized by forming a composite with TiO2. The XRD, SEM and Raman spectra results illustrate that the MAS and TiO2 coexist but Rietveld refinement indicates partial interaction in which ~1%wt of TiO2 diffuses into the MAS lattice, with Ti preferentially occupying Si3 positions. Temperature stable composites are achieved for x = 5.5 with εr = 5.24, Q×f = 33,400 GHz and τf = −2.8 ppm/°C. Patch antennas based on MAS-T5.5 ceramic substrates were
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China under Grant Nos. 52161145401 & 51672063. Professor Reaney would like to acknowledge the support of Engineering and Physical Science Research Council under Grant No. EP/V026402/1. S. Sun also acknowledges the Guangdong Key Platform & Programs of the Education Department of Guangdong Province for funding under Grant No. 2021ZDZX1003.
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