Ionic Conduction in Nanoscale Films of Yttrium-Doped Barium Zirconate
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0972-AA01-07
Ionic Conduction in Nanoscale Films of Yttrium-Doped Barium Zirconate Joon Hyung Shim1, Turgut M. Gur2, and Fritz B. Prinz1,2 1 Mechanical Engineering, Stanford University, Stanford, CA, 94305 2 Material Science Engineering, Stanford University, Stanford, CA, 94305
ABSTRACT Proton conductivity of yttrium-doped barium zirconate grown on MgO(100) using pulsed laser deposition (PLD) has been explored as a function of film thickness (60~670nm) related to crystal and grain structure in the nano scale. Highly textured thin film (60nm) without significant grain separation showed high ionic conductivity close to the bulk BYZ value while thick polycrystalline samples showed lower values with clear grain and grain boundary formation. INTRODUCTION Acceptor-doped perovskites have been investigated for next generation protonic devices because of their high ionic conductivity [1-5]. However, instability of most of these materials in acidic gas environments such as carbon dioxide compromises their practical use. Only yttrium doped barium zirconate (Y:BaZrO3 or BYZ) has shown high proton conductivity combined with good chemical stability [4, 6]. To explore practicability of BYZ for technological applications, several studies attempted to prepare this material in thin film form with thickness in the range from 6µm to 1mm [4,7]. But, the ionic conductivity of these films was found to be significantly lower than the corresponding bulk conductivity of BYZ. This high impedance was identified to be due to grain separation resulting in a large ionic resistance in the grain boundary region [4, 68]. However, this phenomenon was only confirmed by macroscopic bulk samples without welldefined nano-scale structure. In this work, we have fabricated thin crystalline BYZ membranes using pulsed laser deposition (PLD) and investigated protonic conductivity related to structural characteristics. To investigate physical effects of grain structure in thin film format, BYZ have been grown on single crystalline oxide substrates. EXPERIMENTAL DETAILS For single crystal substrate, we chose MgO(100) whose lattice constants is 4.21Å, which is a good match for BYZ’s 4.19Å. A sintered BaZr0.8Y0.2O3-δ pellet (Praxair Inc.) was used as the target material to grow the BYZ films. A Lambda Physik 248nm KrF laser with an energy density of 3.0J/cm2 is used to ablate the target. Substrate temperature was set at 780°C during deposition and the pressure of the background oxygen was 300mTorr. After deposition, samples were cooled down at oxygen pressure of 300Torr for over 30 minutes. The resulting deposition rate was about 0.3 Å/pulse under these conditions. Thickness of the films was varied from 60nm to 670nm. Each sample had dense titanium electrodes deposited by DC sputtering on the surface of the BYZ films, and platinum was coated on the surface of titanium to prevent oxidation during
high temperature measurement. This setup was configured for planar conduction measurement in the direction parallel to film surface. AC impedance spectroscopy measurem
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