A feasibility study on the scalability of the cold sintering process through ultrasonic nondestructive evaluation
Open Access
- Author:
- Wheatley, Christopher
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- October 18, 2023
- Committee Members:
- Andrea P. Arguelles, Thesis Advisor/Co-Advisor
Clive A Randall, Committee Member
John Mauro, Program Head/Chair
Mike Lanagan, Committee Member - Keywords:
- cold sintering
ultrasound
dielectrics
flaw characterization - Abstract:
- The cold sintering process (CSP) provides an energy-efficient alternative for synthesiz- ing bulk materials from constituent powders. However, large-scale CSP ceramics are more likely to contain microstructural inhomogeneities, yielding lower mechanical strength and dielectric performance. The process control of large-scale CSP needs to be better under- stood in order to limit the presence of these flaws. In this study, cold sintered sodium molybdate (Na2Mo2O7) bulk dielectrics were manufactured at two different diameters and four different thicknesses while maintaining high relative densities (>95%), with some samples measuring over 99%. Ultrasonic nondestructive testing using a 20 MHz transducer was used to assess the microstructures of these samples, and these results were correlated with low dielectric loss ranging from 5E-04 to 2E-03 and relative permittivities ranging from 11.5 to 14.5. The heating rate was found to be inconsequential for the smaller 13-mm diameter samples, but decreasing the heating rate to 5◦C per minute decreased the likelihood of flaws in the 25-mm diameter samples. While larger-sized ceramics had comparable ultrasonic and electrical measurements, these samples suffered from areas of the back surface with no discernable ultrasonic signal. As low energy ultrasound was used, these unmeasurable areas suggested higher concentrations of flaws around the edges of these larger samples. The ultrasonic analysis of these flaws revealed them to be delamination, cracks, and porosity at length scales ranging from millimeters to micrometers. The presence of these flaws were then confirmed using optical and electron microscopy. These results show that CSP can create homogeneous microstructures at various scales, further supporting CSP as an energy-efficient manufacturing technique for industrial applications.