High purity commercially available ZnO (99.9%, Aladdin) with an average grain size of 113 nm and 336 nm powders were utilized to fabricate the ZnO ceramics, designed as samples A and B, respectively. The powders were firstly high-pressure treated (HPT) at 4.4 GPa for 5 min as described elsewhere [11], then the ceramics were sintered at 1250 C for 2 h in a conventional
electric furnace and then cooled down to room temperature in air. The bulk density was obtained using the Archimedes method. The crystalline phase identification was determined using X-ray
diffractometry (X’pert Pro PANalytical) with Cu Ka radiation. The microstructures of the ZnO ceramic pieces were characterized by scanning electron microscopy (SEM, FEI Quanta 250).
After polishing and cleaning, silver paste was coated on both sides of the ZnO pieces and then cured at 100 C for 3 h. Finally, the dielectric responses of the samples were measured using an
Agilent 4294A Precision LCR Meter over a frequency range of 40–100 MHz. The AC impedance spectra were measured at room temperature in a frequency range from 0.1 Hz to 10 kHz by
electrochemistry station (CHI660E, Shanghai) with test signal level of 0.5 V.