Ni doping improving magnesium borate microwave dielectric ceramic for LTCC via cold sintering and post-annealing process

A cold sintering process (150 °C, 90 min, and 800 MPa) is employed to pre-densify (Mg 1 −  x Ni x ) 3 B 2 O 6 (0.00 ≤   x  < 0.10, MNBO) ceramics to 80% relative density, followed by a post-annealing treatment between 800 and 950 °C. The origin of optimizations of the sintering, microstructure, and dielectric properties of Ni 2+ -substituted Mg 3 B 2 O 6 ceramic are investigated in detail. All the samples are the single phase of magnesium borate, as confirmed by XRD and Raman spectra. The crystallographic lattice parameters of all compositions are obtained by full-pattern Rietveld refinement. It is found that the ε r of MNBO ceramics is related to the relative density and the ionic polarization of chemical bonds. The change of atomic filling fraction and half peak width is found to be an important effect on the Q  ×  f value. The optimum microwave dielectric properties ( ε r  = 6.92, Q  ×  f   =  24,336 GHz, τ f = − 53.43 ppm/°C) and the relative density of MNBO ceramics (92.4%) are obtained for composition with 4% Ni 2+ .