ULTCC post-annealing cold sintering densification process, microwave dielectric properties of Zn3B2O6 ceramics

Highly-density Zn 3 B 2 O 6 (ZBO) ceramics were firstly prepared via a two-step cold sintering and ultra-low temperature sintering process (CSP-plus ULTCC post annealing). In this study, pre-densified ZBO ceramics (relative density∼ 92 %) is initially obtained under the condition of 200 ℃/60 min/300 MPa via CSP using acetic acid as a liquid phase, yielding microwave properties of ε r = 5.05, Q × f = 6353 GHz and τ f = −69.51 ppm/℃. After annealing at 550 ℃, relative dielectric properties are significantly improved, resulting in the microwave dielectric properties of ε r = 5.95, Q × f = 20620 GHz, τ f = −67 ppm/℃ achieved in a ZBO ceramics with relative density> 95 %. Many fine grains are observed to precipitate between the larger grains of cold-sintered ZBO ceramics, as evident by SEM images, exhibiting disordered grain and porosity distribution. At contrast, a uniform microstructure with fine grain and less porosity are observed after annealing procedure. The densification process of ZBO ceramics prepared by this method is further investigated to validate cold sintering "dissolution-precipitation" mechanism, as a critical factor, for promoting the densification of ZBO ceramics.