Humidity Related Low Temperature Conductivity Hysteresis of Ce_1–xZr_xO₂ (0 ≤ x ≤ 1) Ceramics. Structural Disorder Relationship

Abstract

Ion‐electron mixed conductivity of the Ce1–xZrxO2 (0 < x < 1) series, prepared by solid‐state reaction, has been measured as a function of temperature, pO2 and pH2O relative pressures. The structural analysis of this series shows the formation of monoclinic, tetragonal, and cubic phases as the Ce content increases. The temperature used in samples preparation, 1,923 K, favors at intermediate compositions the homogeneous Ce and Zr mixing in the metastable tetragonal t′ phase. The variation of conductivity with the relative oxygen pressure indicates that electronic contribution increases, becoming preponderant, as the Ce‐content increases. The modifications in conductivity observed in Ce‐rich samples have been ascribed to the enhancement of the electronic contribution at the grain‐boundary of ceramics. The differences observed at low temperatures in electronic dc‐conductivity of Ce‐rich samples during heating and cooling treatments, have been ascribed to the incorporation of water as hydroxide defects at the grain boundary and interior of the particles. The surface hydroxylation is maximal in samples with improved OSC capacity, decreasing drastically in samples heated at 740 K. Finally, the potential application of prepared materials in intermediate temperature solid oxide fuel cells (SOFC) is discussed.

Crystal Structure and Oxide-Ion Diffusion of Nanocrystalline, Compositionally Homogeneous Ceria–Zirconia Ce0.5Zr0.5O2 up to 1176 K

“…The atomic displacement parameter of anions in nanocrystalline, compositionally homogeneous Ce 0.5 Zr 0.5 O 2 [ B (O) = 1.32(4) Å 2 from neutron data, see Table , B (O) = 1.58(11) Å 2 from synchrotron data] is higher than that in nanocrystalline CeO 2 [ B (O) = 0.502(17) Å 2 from neutron data, B (O) = 0.542(16) Å 2 from synchrotron data] at room temperature, which is consistent with the previous works of bulk samples at room temperature, at 900 K, and at 1832 K . The nuclear-density distributions of nanocrystalline Ce 0.5 Zr 0.5 O 2 and CeO 2 were obtained by the MEM analysis using neutron diffraction data (Figure ).…”

“…These results suggest that the oxygen diffusion coefficient increases with an increase of atomic displacement parameter of oxygen atom in nanocrystalline Ce x Zr 1– x O 2 as well as in bulk samples. The previous works ,, showed the correlation between the oxygen diffusivity and the B (O) for bulk Ce x Zr 1– x O 2 . In the present work, we have demonstrated this correlation for nanocrystalline Ce x Zr 1– x O 2 .…”

“…It is considered that the nonequilibrium tetragonal forms are important also in the ceria–zirconia catalysts for reforming ethanol and CH 4 at high temperatures to produce hydrogen for fuel cells. The existing phase and crystal structure of compositionally homogeneous, nonequilibrium, or metastable Ce x Zr 1– x O 2 solid solutions have been investigated by many researchers. − There exist three forms, t, t′, and t″, in the tetragonal phase. ,,,, All three forms have the distorted fluorite-type structure with the tetragonal P 4 2 / nmc space group (Figure ). The t-form is stable in the equilibrium state at high temperatures, and the solubility limit of CeO 2 in t-ZrO 2 is x = 0.18 in Ce x Zr 1– x O 2 .…”

Linking the Electrical Conductivity and Non-Stoichiometry of Thin Film Ce1−xZrxO2−δ by a Resonant Nanobalance Approach

“…Consistent with the literature, the sintered pellets of CZO-0 and CZO-20 were single phase polycrystalline materials with CaF 2 fluorite type cubic symmetry [ 77 , 78 ], whereas the co-existence of minor metastable ( t ’’ )-phase was found in CZO-33 [ 79 ]. The CZO-50 was composed of a single metastable tetragonal t ’ phase [ 80 ], and stable cubic and tetragonal phases ( c + t ) were found to co-exist with metastable t ’ phase [ 81 ] in CZO-67 ( Figure 3 a; the XRD peak at 2θ of 29.5°). We note the shift of the diffraction peaks towards larger 2θ in cubic CZO ( Figure 3 )), which indicates the decrease of their lattice parameter with increasing zirconia fraction, as expected from literature on Ce 1− x Zr x O 2 [ 9 ].…”

Resolving pseudosymmetry in tetragonal ZrO2 using electron backscatter diffraction with a modified dictionary indexing approach

“…The same lattice parameters were used for the Hough indexing and dictionary indexing, which were previously measured on the same sample by X-ray diffraction (Pang et al, 2019): a t = 3.631 Å and c t = 5.230 Å for the tetragonal phase, the same as for the simulated tests in Sections 2 and 3, and a m = 5.216, b m = 5.225, c m = 5.397 Å and m = 98.797 for the monoclinic phase. Atomic positions and Debye-Waller factors were obtained from ICSD #41579 (Yashima et al, 1995) and #183856 (Jimenez et al, 2011) for the tetragonal and monoclinic phases, respectively. Calculations were run on a Linux workstation, with an eight-core Intel i7-7820X CPU and an NVIDIA GeForce GTX 1080 GPU.…”