Nondestructive acoustic mapping of cold sintered ZnO: Effects of density, porosity, and transient phase

The cold sintering process (CSP) is a low-temperature densification method for ceramics, metals, and composites. CSP enables densification at 70–500°C, halving processing times compared to standard firing temperatures greater than 1000°C. A transient liquid phase drives densification by facilitating mass transport, influencing final density and microstructure. This study explores how transient phase affects longitudinal wave speed and attenuation of CSP ZnO. A single-element 20-MHz transducer in a water immersion pulse-echo system provides thickness-averaged spatial maps of longitudinal wave speed and attenuation. Sample sets include a range of densities (∼80%–95% of theoretical density) as well as heat-treated conditions to evaluate the impact of residual amorphous phases on elastic properties. Scanning Electron Microscopy characterizes microstructural features, complementing acoustic data. Comparisons with Hashin–Shtrikman bounds assess alignment between measured wave speeds, theoretical elastic moduli, and microstructural effects. Attenuation data further highlight porosity and phase differences across samples. These comparisons aid tuning of CSP parameters and solvent selection. Nondestructive acoustic analysis provides insights into CSP ZnO microstructure and elastic properties. Findings may guide future optimization of transient phase chemistry and processing variables in functional ceramics, leveraging acoustic techniques for rapid feedback on materials prepared through innovative processes.