Kayla Chuong

Advisor: Prof. Lauren M. Garten

will propose a doctoral thesis entitled,

Developing ZnO-based MOVs using Cold Sintering for DC Circuit Breakers

On

Friday, May 2, 2025

2pm – 4pm

Pettit Microelectronics Building, Rm102A
and

Virtually via MS Teams

Meeting Link

Committee:

Professor Lauren M. Garten, Primary Co-Advisor, MSE

Professor Lukas Graber, Co-Advisor, ECE

Professor Mark Losego, MSE

Professor Rosario Gerhardt, MSE

Dr. Eric Neuman, Sandia National Laboratory

Abstract

ZnO-based Metal oxide varistors (MOVs) are currently used for surge arrestors but show great potential for high voltage DC circuit breakers (HV DCCBs) due to their ability to quickly dissipate energy in an overcurrent event. However, under higher voltage conditions and a different circuit configuration, the performance, reliability, and lifetime of MOVs must be reevaluated and improved. Under high voltages, effective current dissipation is paramount for device reliability and lifetime, which is controlled by the microstructure. Cold sintering is proposed as a processing route to improve microstructural homogeneity and reduce grain size in ZnO-based MOVs. Furthermore, by adjusting the composition of the MOV and tailoring the grain boundary interface, the rate of degradation can be decreased and lifetime improved. This research will provide the critical missing insight into cold sintering processing and intergranular phase control, which will enable MOVs to be used in HV DCCBs due to their improved microstructural homogeneity and electrical response.

To improve MOV reliability by achieving a homogenous microstructure and reduced grain size, cold sintering is proposed as an alternative sintering method that uses high pressure coupled with a transient liquid phase to densify the ceramic rather than high temperatures. The transient liquid phase, such as acetic acid or zinc acetate, is used as a medium to promote the diffusion of zinc ions at low temperatures. ZnO samples were cold sintered with different liquid phases to observe the effects of the choice of the transient liquid phase on the grain size. Overall, cold sintering is shown to minimize grain growth while achieving similar densities to commercial varistors.

Degradation is a critical issue for MOVs used in DCCBs. Over time, the gradual electromigration of Zn interstitials towards the GB and device electrodes leads to a decrease in insulation resistance of the MOV. Multiple routes have been suggested to mitigate the degradation of the double Schottky barrier (DSB) height, but the most straightforward solutions are decreasing the concentration of Zn interstitials in the dielectric and interfering with the migration of Zn interstitials. If doping with MnO2, TiO2, and Co2O3 causes changes in the ZnO lattice that must be compensated with Zn vacancies (or oxygen interstitials), then the formation (or migration) of Zn interstitials is limited, which will increase MOV lifetime by slowing the decrease in DSB height at the GB. Alternatively, doping with MnO2, TiO2, Mn2O3, and Co2O3 may be electrically compensated, making ZnO further n-type, which would increase the potential gradient between the p-type intergranular phase and the ZnO grains thus increasing barrier height. This work proposes to use cold sintering to improve microstructural homogeneity while adding MnO2, TiO2, and Co2O3 to suppress Zn interstitials to improve cold sintered MOV lifetime for DCCBs.