It is difficult to get pure‐phase Mg3B2O6 (abbreviated as MBO) ceramics by the traditional high‐temperature solid‐state reaction method. In this paper, pure‐phase MBO ceramics were successfully densified and obtained by combining the cold sintering and post‐annealing process. The relative density of MBO ceramics was ∼80% cold sintered at 150°C/90 min/800 MPa, which was further improved to ∼91% by post‐annealing at 900°C, 400°C lower than that of the traditional high‐temperature sintering process (∼1300°C). X‐ray powder diffraction (XRD), scanning electron microscopy (SEM), and Raman results demonstrated that the secondary phase of MgO was effectively eliminated, and dense microstructure was observed by the cold‐sintering process plus post‐annealing treatment. Finally, the microwave dielectric properties of MBO were evaluated with εr: 5.15–6.37, Q×f: 5942–16 686 GHz, τf: −48.45–69.72 ppm/°C.

In this work, Y2.95R0.05MgAl3SiO12 (R = Yb, Y, Dy, Eu, Sm) microwave single-phase dielectric ceramics were successfully prepared via a conventional ceramic sintering technology by doping a series of rare earth elements (Yb, Y, Dy, Eu, Sm) with different ionic radii for the first time. The effects of A-sites occupied by rare earth elements on the microwave dielectric properties of Y2.95R0.05MgAl3SiO12 were studied using crystal structure refinement, a scanning electron microscope (SEM), bond valence theory, P-V-L theory, and infrared reflection spectroscopy. It was found that the ionicity of the Y-O bond, the lattice energy, the bond energy, and the bond valance of the Al(Tet)-O bond had important effects on the microwave dielectric properties. Particularly, the optimum microwave dielectric properties, εr = 9.68, Q × f = 68,866 GHz, and τf = −35.8 ppm/°C, were obtained for Y2.95Dy0.05MgAl3SiO12 when sintered at 1575 °C for 6 h, displaying its potential for 5G communication.

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