ZnO MPs powders (average size 0.2-1

ZnO MPs powders (average size 0.2-1 µm) from Asturiana de Cinc S.A.(Spain) and ZnO NPs powders (average size 20-100 nm) from Evonik Industries (Germany), manufactured via French process with a purity of 99 %, were employed for the preparation of the ceramics. These MPs and NPs powders were dispersed by a dry method in order to avoid the agglomeration of the ZnO particles remaining their morphology unaltered. CSP was performed using acetic acid as solvent with a 20 wt% 1 M aqueous solution. Prior to the cold sintering, powders were hand mixed in an agatemortar with the acid acetic solution to obtain a granulated powder. Granulates of powders were first pressed at 270 MPa for 5 min at room temperature (RT) in a cylindrical die with an inner diameter of 0.83 mm. Then, the pressed ZnO pellets were submitted to CSP in a Burkle D- 7290 press, heating at a target temperature for 1 h and applying a pressure of 750 MPa. The annealing rate of 20 ºC/min was used, and then cooled down in air. During the cold sintering method, some parameters were varied such as the height/diameter ratio of the pellets (from 0.17 to 0.44) and the target temperature (from 110 ºC to 190 ºC), in order to get the best ones. These processing parameters were studied to obtain the highest density values of the CSP ZnO pieces (see Supporting Information, SI). Both the pressure and the temperature values were not measured into the die, but are those registered in the pressing machine, that is, the nominal values. Once the CSP parameters were optimized, the effect of ZnO NPs in combination with MPs was evaluated. For that, the mixture of ZnO MPs and NPs powders was prepared at different percentages (0, 1, 5, 10, 30, 50 and 100 wt% of NPs) by a dry dispersion method for 10 min and the final product in each case was submitted to CSP. The resulting samples were compared with ZnO pellets obtained by a CN method, which were pressed at 270 MPa and subsequently sintered at 1300℃ for 5 h in air using the same starting MPs and NPs powders. Table I shows both, the samples selected for this work using CSP by varying the fraction of NPs and the ceramics prepared by CN route.