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Ilyas Savkliyildiz1 Enver Koray Akdoğan2 Hülya Biçer3 Zhong Zhong4 Thomas Tsakalakos2

1, Selçuk University, Konya, , Turkey
2, Rutgers, The State University of New Jersey, Piscataway, New Jersey, United States
3, Dumlupinar University, Kutahya, , Turkey
4, Brookhaven National Lab, Upton, New York, United States

Effects of superimposed thermal and electric fields on dense 8% yttria doped zirconia (8YSZ) ceramics, which were sintered Flash Sintered at 900 oC (FS900) and Sinter Forged at 1400 oC (SF1400), were studied with in situ energy dispersive x-ray diffractometry using a polychromatic synchrotron probe with photon energies up to 200 keV. The initial grain size of the FS900 and SF1400 systems were ~200 nm and ~1 μm, respectively, while their initial sintered density was ~97 % and, ~99 %, respectively. No discernible increase in the grain size and sintered density were observed when both systems were heated at 20 oC/min to 700 oC under an applied dc electric field intensity of 100 V/cm. No local melting at grain boundaries was observed in scanning electron micrographs. Moreover, no theoretical density improvement was measured on both samples. However, a singularity in the unit volume of the FS900 system was observed at 614 oC at which point the peak current draw (jmax) was 1.5 A corresponding to 2.14 kW/cm3 instantaneous maximum power absorption density (Pmax). The tetragonal unit cell expansion associated with power absorption is 1.64% in addition to the volumetric expansion due to thermal expansion at 614 oC. In the SF1400 sample, on the other hand, two singularities were observed at 630 oC and 643 oC for which jmax is 0.6 and 3.0 A with corresponding Pmax of 0.375 kW/cm3 at and 1.36 kW/cm3, respectively. The tetragonal unit cell expansions associated with power absorptions are 0.53 % and 2.61 % in additional to the volumetric expansion due to thermal expansion at 630 oC and 643 oC, respectively. Neither system exhibit temperature dependent peak broadening under 100 V/cm which suggests no long range vacancy re-arrangement has taken place in the FS900 and SF1400 sintered systems that would otherwise affect both coherently diffracting domain size and d-spacing variation (microstrain). The observed singularities in unit cell expansion are attributed to oxygen vacancy localized rearrangement at the unit scale as no direct electric field strain coupling is possible in an ionic conductor such as 8YSZ. The results reveal that the effects of superimposed temperature and electric fields, which are present in burst mode densification (a.k.a. flash sintering), can indeed be decoupled once grain boundaries are formed upon densification in a given system.

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