Abstract
This work presents a systematic investigation of scandia alumina stabilized zirconia (ScAlSZ, composition: ZrO2:Sc2O3:Al2O3 93:6:1 wt.%) thin films (~2 μm). Thin films were formed by the e-beam evaporation method on 450 °C substrates. The influence of Al concentration on thin film microstructure, structure, and electrochemical properties was characterized by EDS, XRD, Raman, and EIS methods. It was found that the aluminum concentration in the deposited thin films decreased with an increase in the deposition rate. The concentration of Al changed from 15.9 to 3.8 at.% when the deposition rates were 0.2 and 1.6 nm/s, respectively. The crystallinity of the thin films depended strongly on the concentration of Al, resulting in an amorphous phase when Al concentration was 22.2 at.% and a crystalline phase when Al concentration was lower. ScAlSZ thin films containing 15.9 at.% of Al had monoclinic and tetragonal phases, while thin films with 1.6 and 3.8 at.% of Al had a mixture of cubic, tetragonal, and monoclinic phases. The phase transition was observed during the thermal annealing process. Cubic and rhombohedral phases formed in addition to monoclinic and tetragonal phases appeared after annealing ScAlSZ thin films containing 15.9 and 22.2 at.% of aluminum. The highest total ionic conductivity (σbulk = 2.89 Sm−1 at 800 °C) was achieved for ScAlSZ thin films containing 3.8 at.% of Al. However, thin films containing a higher concentration of aluminum had more than 10 times lower total conductivity and demonstrated changes in activation energy at high temperatures (>560 °C). Activation energies changed from ~1.10 to ~1.85 eV.
Highlights
Doped ZrO2 is one of the most widely investigated fluorite-structured electrolytes.Pure ZrO2 does not exhibit high oxygen ion conductivity due to a low number of oxygen vacancies and polymorphic nature
All scandia alumina stabilized zirconia (ScAlSZ) thin films were deposited on 450 ◦ C substrates using 0.2, 0.4, 1.2, and 1.6 nm/s deposition rates, which were controlled with a crystal sensor (Inficon, Bad Ragaz, Switzerland)
The energy-dispersive X-ray spectroscope “BrukerXFlash QUAD 5040” (EDS) analysis of ScAlSZ thin films was performed after deposition (Table 1), and it was determined that the chemical composition of thin films depended on the deposition rate
Summary
Doped ZrO2 is one of the most widely investigated fluorite-structured electrolytes.Pure ZrO2 does not exhibit high oxygen ion conductivity due to a low number of oxygen vacancies and polymorphic nature. The ionic conductivity of electrolytes depends on dopant atomic radii, dopant concentration, electrostatic interactions between dopants and oxygen vacancies, and phase composition [1,2]. The conductivity decreases at higher or lower dopant concentrations due to dopant clustering or a low number of oxygen vacancies [4]. Zirconia doped with 10 mol.% of Sc (10ScSZ) exhibit the highest conductivity at 1000 ◦ C among all ZrO2 based electrolytes due to low association enthalpy of the defect reactions and similar ionic radii (0.075 nm—Sc3+ , 0.072 nm—Zr4+ ) [2,5]. The oxygen ion conductivity and cubic phase stability should be increased even more. Authors suggest that a low concentration of alumina co-dopant can stabilize the cubic phase [13,14].
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