Development and Characterization of Atmospheric Pressure Plasma Jets for Low Temperature Sintering

Plasma jet sintering is a low temperature material processing method that employs non-thermal plasma species to treat surfaces. It primarily aims to activate printed inks on a substrate so that conductive patterns can be utilized in circuits. Due to its non-thermal, low temperature nature, it offers a solution to the technological challenge of high temperature activation of inks in producing embedded electronics on temperature-sensitive surfaces. Instead of heat flux, the energetic plasma molecules and ions that propagate as ionization waves provide energy to initiate sintering. The development and characterization of plasma jets for effective sintering is the focus of the present work.

This dissertation first investigates the sintering of a silver nanoink on glass, paper, fruits, and flexible silicon at atmospheric pressure and at temperatures less than 50^◦C. The electrical, thermal, and optical characterizations of the plasma jet and the microscope imaging and chemical analysis of sintered silver thin films demonstrate that plasma jet sintering is mainly a non-thermal process that does not rely on elevated temperatures. Experiments conducted in a controlled environment also show that plasma jet sintering can provide comparable electrical conductivity values at milder conditions to high-temperature sintering. Following this initial study, the rate of sintering is investigated with varying plasma parameters (power and flow rate) that define specific energy input (SEI). Results show that the relation between conductivity and SEI follows an Arrhenius-like trend, which provides a way to tune plasma parameters as needed for higher conductivity. Lastly, plasma jet sintering is studied in situ to determine the required time to achieve fully sintered films. The detailed examination of current traces and optical signals of the plasma jet illustrates that the changing surface properties (being non-conductive to conductive) may not be captured by observing plasma.