Pure and Ba-Zr co-doped multiferroic BiFeO 3 nanopowders are synthesized via sol-gel method. X-ray diffraction demonstrates the transformation of crystal structure (from R3c to R3c + P4mm) and the absence of impurity phase. Scanning electron microscopy and transmission electron microscopy indicate that the co-doping leads to a decrease in particle size and the Bi 0.85 Ba 0.15 Fe 0.95 Zr 0.05 sample has a particle size of 30–60 nm. Surprisingly, the room-temperature magnetization hysteresis loops change to be soft ferromagnetic with co-doping. The maximum and remnant magnetizations of the as-prepared nanopowders first increase and then decrease with increasing dopant concentration with the Bi 0.85 Ba 0.15 Fe 0.95 Zr 0.05 sample having the best magnetic properties. The enhanced magnetization is due to the crystal lattice distortion caused by the ionic-radius difference between the substituent and host elements and the suppression of the antiferromagnetic spiral spin structure induced by the size effect of the nanopowders.
Hui Guo,Yifeng Li,Lulu Jiang,Yenan Sha,Shihang Guo,Donglin Han
Proton conducting oxides are attracting increasing attention due to their promising application as electrolytes in solid-state electrochemical devices for energy conversion and chemical production, including protonic ceramic fuel cells, electrolyzers and membrane reactors. BaZrO3 and Ba(Zr,Ce)O3 substituted by acceptor dopants are currently the most promising proton conductors. In principle, replacing Zr or Ce with one kind of trivalent element (e.g., Y or Yb) can generate oxygen vacancies, which are necessary for the hydration reaction to introduce charge carriers of protons. However, it is commonly recognized that substituting Zr or Ce with Y and Yb simultaneously leads to high performance of the devices, but the reason is not clear. In this work, we studied the transport properties of four different compositional classes: BaZr0.8−xCexY0.2O3−δ (BZCY20), BaZr0.8Y0.2−xYbxO3−δ (BZYYb), BaZr0.1Ce0.7Y0.2−xYbxO3−δ (BZCYYb) and BaCe0.8Y0.2−xYbxO3−δ (BCYYb), at 600, 650 and 700 °C. For BZCY20, the ionic conductivity decreases on replacing Zr in BZY20 with x = 0.1 of Ce, increases with increasing Ce content within x = 0.1–0.5, and decreases on further increasing Ce content from x = 0.5 to 0.8. The ionic transport number increases with increasing Ce content. The ionic conductivity, partial conductivity of oxide ions and hole conductivity of BZCYYb and BCYYb increase with increasing Yb content. Since the increment of hole conductivity is larger, the ionic transport number decreases. For BZYYb, the ionic conductivity decreases and the hole conductivity increases with increasing Yb content, leading to decreasing ionic transport number. Both the high ionic conductivity and transport number are vital factors governing the performance of protonic ceramic cells (PCCs). Therefore, the improved performance of PCCs by adding both Y and Yb into the electrolyte cannot be explained from the viewpoint of electrical properties of the electrolytes. Thereby, we summarized our recent series of studies on the chemical compatibility between the electrolyte and NiO negatrode substrate, and suggested that the loss of dopant was suppressed by partly replacing Y with Yb. The high ionic conduction of the electrolyte can be maintained after co-sintering at high temperature to fabricate PCCs. This might be the real reason for the high performance of the devices using Y and Yb co-substituted electrolytes, prepared by the co-sintering method.
Y-doped Ba(Zr,Ce)O3 (BZCY) proton conducting ceramics have attracted extensive attention because of their promising applications as electrolytes for intermediate-temperature proton ceramic electrochemical cells. The proton conductivity of BZCY is governed by their lattice volume and crystal symmetry; however, systematic studies of the competition between these two factors are lacking, hindering further improvement of their proton conductivity at low and intermediate temperatures. In this work, the crystal symmetry of BZCY epitaxial films deposited on NdGaO3 (110) is compared to that of polycrystalline bulk BZCY based on Raman spectra. The Raman signal of BZCY epitaxial films is amplified using surface-enhanced Raman spectroscopy and subsequently extracted utilizing the nonnegative matrix factorization method. The relative Raman intensity ratio of the BO6 octahedra motion to the Ba-BO6 relative motion in the ABO3 structure indicates that as the Ce content increases, the crystal symmetry of polycrystalline bulk BZCY decreases, whereas that of the epitaxial films on NdGaO3 remains consistently lower compared to bulk BZCY. The relationship between the activation energy and relative Raman intensity of epitaxial films and bulk BZCY indicates quantitatively that higher activation energy is observed in the crystal structure with lower symmetry. This work suggests that crystal symmetry has a predominant effect on the activation energy of BZCY. The results provide a deeper insight for developing proton conducting electrolyte membranes with high proton conductivity and low activation energy.
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