Temperature-stable Y2.95Dy0.05MgAl3SiO12 garnet-type 5G millimeter-wave dielectric ceramic resonator antenna

BaLi1+xF3+x (x=0-0.01) were successfully mechanosynthesized by a simple ball-milling process. The effects of excessive LiF and sintering method and/or annealing atmosphere on its sintering behavior, microstructure, and microwave dielectric properties have been investigated in this paper. The mechanosynthesized powder can be densified with relative densities of ∼95% after sintering at 750-800 oC/2 h in N2. The obtained ceramics exhibit excellent optimized microwave dielectric properties with εr of ∼11.46±0.06, Q×f values of 83175±1839 GHz and τf of ∼ -70±3 ppm/oC at the x=0.006 composition. Its Q×f value could be improved to 94603±2037 GHz) by post-annealing in N2 after post annealing at 700 oC/2 h. The Q×f value could be further improved to (120098±2344 GHz) by hot-pressed sintering (HPS). Sintering in the ambient atmosphere or O2 leads to lower Q×f values than those of the counterparts sintered in N2 due to the introduction of F-vacancies by oxidation, while little variation in εr andτf. 

Preparation and microwave dielectric properties of BaLi1+xF3+x (x=0-0.01) ceramics

Recently, the cold sintering process (CSP) has gained worldwide recognition from the scientific community as a green and innovative fabrication technique due to the substantial reduction of processing energy, time, and cost. The present work reports the preparation, fabrication, and properties of (1 – x)Y3Fe5O12–xSnF2 (x = 0.2, 0.3, 0.4, 0.5 volume fraction) ceramic composites for the first time through the cold sintering process under 150 °C, by applying 300 MPa uniaxial pressure for 30 min. The evolved microstructures of the cold-sintered composites suggest enhanced densification with increasing SnF2 concentration. The evolution of the microstructure, consequent densification, and hardness of the composites can be due to the combined effect of water as a transient solvent and the relatively low melting point of SnF2. The main mechanism behind the densification in the SnF2-based system can be the liquid phase formation followed by pressure solution creep and plastic deformation. At the optimum additive concentration, the measured microhardness of the composite is around 4.7 GPa and its moisture absorption lies below 0.1%. The (1 – x)YIG–xSnF2 ceramics have relative permittivity (εr) in the range of 7.1–11.3 and loss tangent of the order of 10–2 at a frequency of 900 MHz. Further, the real part of permeability and the corresponding loss tangent lie below 2 and 10–1, respectively, at 900 MHz. Herein, a microstrip patch antenna (MPA) is designed, simulated, and fabricated using the 0.5YIG–0.5SnF2 composite, which shows excellent performance at 5.29 GHz with a return loss of −17.9 dB and an impedance bandwidth of 780 MHz. Hence, cold sintering using SnF2-based additives is considered a sustainable, energy-efficient, and cost-effective tool for the densification of magneto-dielectric materials. 

Water and SnF2 Assisted Ultralow-Temperature Sintering of Mechanically Tough Y3Fe5O12 Composites for Magneto-Dielectric Antennas

(1–x)BaV2O6-xLiF (x = 0.025, 0.05, 0.10, 0.15) ceramics with an ultra-low sintering temperature of 475 oC were synthesized using the standard solid-state reaction method. The ceramics demonstrated a remarkable enhancement in the quality factor (Qf), reaching a maximum value of 16,800 GHz at x = 0.10, which is 5.6 times higher than that of pure BaV2O6 ceramics obtained in this work. A near-zero temperature coefficient of resonant frequency (τf = 1.44 ppm/oC) was simultaneously achieved, together with a dielectric constant εr of 11.54. Additionally, the chemical compatibility between the present ceramics and the Al electrode was confirmed. Moreover, a millimeter-wave antenna with an excellent S11 value of –50.3 dB, and a large bandwidth of 3.44 GHz was fabricated using the 0.9BaV2O6-0.1LiF ceramic as the dielectric resonator. These results highlight the potential application prospects of (1–x)BaV2O6-xLiF ceramics in the ultra-low temperature co-fired ceramic technology and 5 G millimeter-wave antennas. 

Ultra-low temperature sintered (1–x)BaV2O6-xLiF ceramics for ULTCC and 5 G millimeter-wave antenna applications

Low temperature sintered Li8MgxTi3O9+xF2 microwave dielectric ceramics with x = 2−7 were developed based on a newly designed pseudo ternary phase diagram of the Li2TiO3–MgO–LiF system. Dense solid solution ceramics (of relative density >96 %) with cubic rock-salt structure, accompanied by a small amount of secondary phase MgO, were obtained in the temperature range of 800−925 °C. With increasing Mg2+ content, the value of εr decreased, whereas that of τf remained nearly constant, and the Q × f increased to a maximum at x = 5. The Li8Mg5Ti3O14F2 ceramic sintered at 875 °C exhibited superior microwave dielectric properties with εr = 16.8, Q × f = 119,700 GHz, and τf = −41.6 ppm/°C. The good compatibility with Ag electrodes highlights the promising prospects of this ceramic in low-temperature co-fired ceramic technology. Furthermore, a dielectric resonator antenna fabricated based on a Li8Mg5Ti3O14F2 ceramic exhibited an outstanding S11 of −34.7 dB and a broad bandwidth of 360 MHz at the desired resonant frequency of 5.98 GHz. 

Yu-Ze Jiang

Papers

Cold sintering optimized LiF microwave dielectric ceramics for the development of dielectric resonator antennas at 5G millimeter-wave band