Anomalous lattice expansion in yttria stabilized zirconia under simultaneous applied electric and thermal fields: A time-resolved in situ energy dispersive x-ray diffractometry study with an ultrahigh energy synchrotron probe

摘要

Nonisothermal densification in 8% yttria doped zirconia (8YSZ) particulate matter of 250?nm median particle size was studied under 215?V/cm dc electric field and 9?°C/min heating rate, using time-resolved in-situ high temperature energy dispersive x-ray diffractometry with a polychromatic 200?keV synchrotron probe. Densification occurred in the 876–905?°C range, which resulted in 97% of the theoretical density. No local melting at particle-particle contacts was observed in scanning electron micrographs, implying densification was due to solid state mass transport processes. The maximum current draw at 905?°C was 3 A, corresponding to instantaneous absorbed power density of 570?W/cm3. Densification of 8YSZ was accompanied by anomalous elastic volume expansions of the unit cell by 0.45% and 2.80% at 847?°C and 905?°C, respectively. The anomalous expansion at 905?°C at which maximum densification was observed is characterized by three stages: (I) linear stage, (II) anomalous stage, and (III) anelastic recovery stage. The densification in stage I (184?s) and II (15?s) was completed in 199?s, while anelastic relaxation in stage III lasted 130?s. The residual strains (ε) at room temperature, as computed from tetragonal (112) and (211) reflections, are ε(112)?=?0.05% and ε(211)?=?0.13%, respectively. Time dependence of (211) and (112) peak widths (β) show a decrease with both exhibiting a singularity at 905?°C. An anisotropy in (112) and (211) peak widths of {β(112)/β(211)}?=?(3:1) magnitude was observed. No phase transformation occurred at 905?°C as verified from diffraction spectra on both sides of the singularity, i.e., the unit cell symmetry rem- ins tetragonal. We attribute the reduction in densification temperature and time to ultrafast ambipolar diffusion of species arising from the superposition of mass fluxes due to Fickian diffusion, thermodiffusion (Soret effect), and electromigration, which in turn are a consequence of a superposition of chemical, temperature, and electrical potential gradients. On the other hand, we propose defect pile-up at particle-particle contacts and subsequent tunneling as a mechanism creating the “burst-mode” discontinuous densification at the singularities observed at 847 and 905?°C.