Unveiling the mechanisms of cold sintering of ZnO at 250 °C by varying applied stress and characterizing grain boundaries by Kelvin Probe Force Microscopy

Abstract

The sintering behavior of nanocrystalline ZnO was investigated at only 250 °C. Densification was achieved by the combined effect of uniaxial pressure and the addition of water both in a Field Assisted Sintering Technology/Spark Plasma Sintering apparatus and a warm hand press with a heater holder. The final pure ZnO materials present high densities (> 90 % theoretical density) with nano-grain sizes. By measuring the shrinkage rate as a function of applied stress it was possible to identify the stress exponent related to the densification process. A value larger than one points to non-linear relationship going beyond single solid-state diffusion or liquid phase sintering. Only a low amount of water (1.7 wt.%) was needed since the process is dictated by the adsorption on the surface of the ZnO particles. Part of the adsorbed water dissociates into H + and OH -ions, which diffuse into the ZnO crystal structure, generating grain boundaries/interfaces with high defect chemistry. As characterized by Kelvin Probe Force Microscopy, and supported by impedance spectroscopy, this highly defective grain boundary area presents much higher surface energy than the bulk. This highly defective grain boundary area with high potential reduces the activation energy of the atomic diffusion, leading to sinter the compound at low temperature.

Thermosetting polymers in cold sintering: The fabrication of ZnO‐polydimethylsiloxane composites

“…Scanning electron microscopy images show the microstructural evolution of cold sintered (1 − x ) ZnO − x PDMS composites (Figure A‐D). When x = 0.00, the average grain size is 296 ± 97 nm, which is consistent with previous studies involving the CSP of ZnO and obviously higher than the initial particle size estimated at 79 ± 37 nm (Figure ). The addition of PDMS leads to a change in grain size distribution and average grain size, with 225 ± 94, 203 ± 82, and 149 ± 64 nm for x = 0.01, 0.03, and 0.05, respectively (Figure ).…”

“…This high “apparent” real permittivity can possibly be explained by charge accumulation at the ZnO grain boundaries after cold sintering. Similarly, recent studies of cold sintering mechanisms in ZnO report anomalously high concentrations of defects (eg, and ) in proximity to the grain boundary . Thus, the results of this study and those of the previously mentioned study suggest that a comprehensive study of CSP‐ZnO impedance may serve to shed light on the relation between cold sintering mechanisms and their implications on grain boundary chemistry and properties.…”

“…The calculated activation energies of the two relaxations (semicircles) at low and high frequency are 0.12 ± 0.001 eV and 0.16 ± 0.011 eV, respectively. These are smaller values than the one observed for grain boundary in the conventionally sintered ZnO varistor, but similar to those in cold sintered pure ZnO . None of the other characterizations performed in this study could distinguish these distinct interfaces so future work will be needed in order to explore the types of interfaces that can arise in such sintered ceramic‐dominated‐thermoset polymer composites.…”

Influence of processing parameters on the densification and the microstructure of pure zinc oxide ceramics prepared by spark plasma sintering

“…Recent work [9,11,26,27] focuses on the SPS sintering of ZnO na noparticles at low temperatures (less than 400 °C) by adding solvents (such as water, aqueous acetic acid, etc. …) to enhance densification.…”

Water‐Mediated Surface Diffusion Mechanism Enables the Cold Sintering Process: A Combined Computational and Experimental Study

“…Water is relatively reactive to drive dissolution, and the diffusion through GBs in the second stage thus augments grain growth. [1,6] However,the GB diffusion does not per se explain the previously observed extraordinary grain growth, since the growth substantially occurs during and toward the end of the solvent evaporation from the system (Figure 1c). [6a, 7] Ideally, the water present in the system hydroxylates the surface.The hydroxylation of the surface reduces the surface energy and thereby lowers the sinterability.B ut interestingly,t he reported CSP temperatures are not high enough for the surface dehydroxylation.…”