Ultra–low–temperature sintering for giant permittivity in CuO ceramics via cold sintering process

Abstract

CuO ceramics were synthesized using the cold sintering process (CSP). CuO nanoparticles with an average size of ∼115 nm were employed as the starting powders. The CuO ceramic samples were fabricated under a pressure of 400 MPa at temperatures ranging from 150 to 300 °C for 30 min (CSP samples). A relative density greater than 81.9 % was achieved at temperatures ≥200 °C, which was significantly higher than that of samples prepared using the non−cold sintering process (non−CSP). By further annealing the CSP samples at ≥750 °C, the relative density increased to over 90 %. Additionally, the mean grain size of the CSP sample was slightly smaller compared to the non−CSP samples. Remarkably, at a sintering temperature of 200 °C, the CSP sample exhibited a giant dielectric permittivity of 17,000 at 1 kHz and room temperature, whereas the non−CSP samples did not exhibit such high dielectric permittivity. Furthermore, the dielectric permittivity of the CSP sample demonstrated slight frequency dependence across the range of 40–105 Hz. This research introduces a novel method for achieving giant dielectric materials at a low temperature of 200 °C. The dielectric permittivity increased further by annealing at 700–900 °C, which was attributed to the increase in relative density. The giant dielectric properties were explained using X−ray photoelectron spectroscopy, attributing them to charge hopping between Cu+ and Cu2+, the grain boundary barrier layer capacitor, and sample−electrode effects.