ISBN 9783843940665

978-3-8439-4066-5, Reihe Materialwissenschaften

João Gustavo Pereira da Silva
Modelling Flash Sintering Using Systems of Nonlinear Differential Equations

123 Seiten, Dissertation Technische Universität Hamburg (2019), Hardcover, A5

Zusammenfassung / Abstract

Flash sintering is a non-linear phenomenon characterized by a sharp increase of the conductivity of the sample and concomitant rapid densification under an electric field in low temperatures in a matter of seconds. In this thesis, the densification of ceramics under flash sintering is modelled using an approach based on the heat equation coupled with an apparent activation energy kinetic equation for densification, leading to a system of nonlinear differential equations. Methods to analyse the system either by numerical solutions or via bifurcation theory are presented. Measurements needed to estimate sintering kinetic parameters are executed. The developed model explains the two main features of flash sintering: A characteristic electrical field threshold independent on the temperature, and an incubation time to flash sintering. The results are in agreement with experimental results obtained for a ceramic mixture of Yttria Stabilzed Zirconia (3YSZ) and Barium Titanate, and as well for Uranium Oxide in different stoichiometries. Those findings corroborate to the conclusion that the critical condition for the Flash phenomenon is the temperature dependent resistivity rather than the sintering kinetics. The influence of preexisting defects and the generation of new ones during flash sintering was studied in 3YSZ. Weibull statistics was used to analyse mechanical testing data from flash and conventionally sintered specimens with equal densification. The obtained values for Weibull modulus and characteristic strength are 6.09 and 371 MPa for the conventionally sintered samples, respectively, and 5.92 and 506 MPa for the flash sintered samples. In addition, preexisting defects were manufactured with rice starch as a pore forming agent, in 5, 10 and 15 vol%. The experiments show that there is no noticeable difference on the incubation time and the onset temperature for flash sintering. Therefore, it can be inferred that flash sintering does not alter significantly the defect distribution on the studied material.