The measurement and improvement of tensile strength in cold-sintered zinc oxide

Abstract

The cold sintering process (CSP) is a low-temperature densification technique for fabricating high-density ceramics, including zinc oxide (ZnO). Optimizing mechanical properties remains challenging due to weak grain boundaries leading to intergranular fracture. This study examines the effects of ZnO particle shapes and organic solvents on densification and tensile strength. ZnO powders with rod-like and isometric morphologies were cold-sintered at 250 °C and 530 MPa for 45 min using water, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methyl-2-pyrrolidone (NMP) mixed with 2 M acetic acid. Characterization techniques included SEM, BET, XRD, and UV–Vis spectroscopy. The isometric ZnO achieved a higher relative density (97.8 %) than rod-like ZnO (96.5 %) due to better packing. Among solvents, water yielded the highest density, while DMF resulted in the lowest. Despite polarity significantly impacting densification, solvent viscosity, flashpoint, and pH had negligible effects. Weibull analysis on Brazilian test data estimated the tensile strength of the densest ZnO at 23.9 MPa, with fractography confirming intergranular fracture. Fracture toughness, calculated via the Haberfield and Johnston equation, was 2.5 MPa m^0.5. The addition of MoS₂ nanoparticles (up to 1 wt%) slightly improved tensile strength (<3 %), while substituting 2 M formic acid for acetic acid led to a 21 % enhancement, emphasizing grain boundary reinforcement. These findings highlight the crucial role of solvent chemistry in improving sintering efficiency and mechanical integrity, suggesting future research should focus on optimizing solvent compositions for enhanced performance.