ARTICLE pubs.acs.org/JPCC
Dopant Segregation and Space Charge Effects in Proton-Conducting BaZrO3 Perovskites Mona Shirpour,*,†,§ Behnaz Rahmati,‡ Wilfried Sigle,‡ Peter A. van Aken,‡ Rotraut Merkle,† and Joachim Maier† † ‡
Max Planck Institute for Solid State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, D-70569, Stuttgart, Germany ABSTRACT: In a humidified atmosphere, acceptor-doped BaZrO3 perovskites exhibit a high bulk proton conductivity, but the total conductivity is severely decreased by the blocking character of the grain boundaries. In our study, we compare rapidly densified Y- and Sc-doped BaZrO3 ceramics (Spark Plasma Sintering, 5 min at 1600 °C) with samples after extended annealing at high temperature (20 h at 1700 °C). Under these conditions, the dopants become mobile, resulting in a strong grain boundary conductivity enhancement, although no grain growth occurs. This increase is accompanied by a significant increase in dopant concentration in the grain boundary region, as evidenced by transmission electron microscopy. The correlation between the electrical properties of grain boundaries and their chemical composition is consistent with the interpretation in terms of the space charge model with a positive excess charge in the grain boundary core and adjacent proton depletion zones.
1. INTRODUCTION Blocking grain boundaries in many polycrystalline oxides (typically with large band gaps, e.g., SrTiO3, ZrO2, TiO2, and CeO2)19 have been explained by the presence of an excess charge in the grain boundary (GB) core and resulting space charge depletion layers. The existence of proton depletion due to a positive GB core charge has been discussed for Y-doped BaZrO3 based on changes of GB conductivity under strongly reducing conditions,10,11 changes in impedance spectra observed for high AC amplitudes,12 and the characteristic variation of depletion zone width with dopant concentration.13 The presence of space charge layers has been recently confirmed by applying DC-bias over single-grain boundaries in Y-doped BaZrO3.14,15 The redistribution of the defects at the grain boundaries is governed by the energy balance of each individual species (in terms of preference of GB core occupation versus bulk occupation) along with possible interactions with the gas phase.16,17 A variety of recent results, in particular, for perovskites, points toward oxide ion removal from the GB core as charge-dominating process, which can also be understood in terms of segregation of oxygen vacancies to the GB core. The core charge can be influenced or even determined by dopant effects as soon as the dopants are sufficiently mobile (during high-temperature sintering and/or annealing). Typically oversized substitutional impurities experience a driving force to segregate to the core, provided the accommodation is easier there owing to the core structure differing from bulk. For this as well as for the resulting charge it matters whether the impurity r 2012 American Chemical Society
can be accommodated substitutionally or interstitially in the core. When the acceptor is accommodated substitutionally in the GB core, segregation will decrease the positive core charge or even eventually turn it negative. In addition, dopant redistribution also occurs within the space charge zones as the consequence of the excess core charge. In the case of a positive core charge, which might be established by excess oxygen vacancies or interstitially segregated cations, this leads to an accumulation of acceptor dopants M0 . Unless the recently developed aberration corrected microscopes are the TEM resolution is not good enough to allow for a direct distinction of the different cases and the analysis has to rely on indirect conclusions. Iguchi et al. found an increased Y concentration in the GB region of BaZr1xYxO3δ ceramics.18 This accumulation was more pronounced for samples with high dopant content, which also showed a higher GB conductivity, but the study does not permit to deconvolute effects of nominal dopant content and of GB dopant accumulation. High-temperature annealing (2200 °C) of BaZr0.9Y0.1O3δ was found to increase the GB conductivity much stronger than the increase in grain size (2),19 but because the cation distribution in the GB region was not further analyzed the origin of this improvement could not be elucidated. In the present work, we investigate the electrical properties and the GB composition of polycrystalline BaZrO3 samples Received: August 25, 2011 Revised: December 14, 2011 Published: January 17, 2012 2453
dx.doi.org/10.1021/jp208213x | J. Phys. Chem. C 2012, 116, 2453–2461
The Journal of Physical Chemistry C
ARTICLE
doped with Y3+ and Sc3+ acceptors having a different size mismatch to Zr4+. In particular, we compare samples that were only briefly exposed to high temperature during Spark Plasma Sintering (immobile dopants) with samples after extended hightemperature annealing (mobile dopants, profile later frozen-in) and correlate the changes in dopant distribution with the modified electrical properties.
2. EXPERIMENTAL SECTION High-purity Y-stabilized zirconia ((Zr0.94,Y0.06)O1.97, Tosoh TZ3), Sc-stabilized zirconia ((Zr0.82,Sc0.18)O1.91, FCM Sc10SZ-TC), and BaCO3 (Merck, 99%) were used to prepare Y-doped and Sc-doped BaZrO3. Calcination was performed in three cycles at 1300 °C for 3 h with intermittent ball milling (2 h). “Spark Plasma Sintering” (HP D 5, FCT Systems) was used to produce dense polycrystalline samples with minimized Ba loss under a pressure of 50 MPa at 1600 °C (heating rate = 100 °C/min, holding time = 5 min, cooling rate (16001000 °C) ≈ 400 °C/min, and cooling rate (1000400 °C) ≈ 240 °C/min). The graphite layer remaining after the sintering was removed by heating to 1000 °C for 8 h in air. The density of as-sintered SPS samples measured by the Archimedes method was ∼98%. For a number of as-sintered samples, an additional annealing step in air was performed, in most cases for 20 h at 1700 °C. The samples were covered with sacrificial powder of the same composition to minimize barium loss during annealing. Phase analysis and unit cell size determination was done by XRD (Philips PW 3710, Cu Kα radiation, BraggBrentano geometry). A ZEISS 1540 EsB Cross Beam was used to take scanning electron microscopy (SEM) images from thermally etched surfaces (polished surface treated at 1350 °C for 30 min (heating/cooling rate +600 °C/h and 300 °C/h)). Mean grain size and grain size distribution were estimated from the SEM images by the linear intercept method. AC impedance spectroscopy (High-Resolution Dielectric Analyzer Novocontrol, Germany) with an amplitude of 100 mV in the frequency range from 106 to 0.1 Hz was used for conductivity measurements. Platinum electrodes about 400 nm thick were sputtered on both sides of the samples. The measurements were carried out in humidified N2 (pH2O = 20 mbar adjusted by the water temperature in the evaporator). The impedance spectra were fitted by equivalent circuits consisting of serially connected RC (parallel) pairs using the Z-View software (Scribner). Where necessary the capacitors (C) in the equivalent circuit were replaced by constant phase elements (Q) describing depressed semicircles, and the capacitance was calculated from C = (R1nQ)1/n with the nonideality parameter n.20 At higher temperatures, the bulk semicircle is not visible any more in the accessible frequency range, and the intercept on the real axis has been used to estimate the bulk resistance. At lower temperatures, bulk resistance as well as capacitance can be obtained, but the electrode semicircle appears at very low frequencies and is not completely recorded. The local GB composition of polycrystalline ceramics was investigated by transmission electron microscopy (TEM) with spatially resolved energy dispersive X-ray analysis (EDXS) and electron energy loss spectroscopy (EELS). TEM specimens were prepared in an automatic tripod polisher (Allied MultiPrep). Final ion polishing at low Ar-ion energy (0.5 keV) was performed using a LINDA ion mill (TECHNOORG). The specimens were analyzed in a dedicated scanning TEM (STEM - VG HB_501UX,
Figure 1. X-ray diffraction pattern of as-sintered and annealed Y-doped and Sc-doped BaZrO3.
Vacuum Generator) by EDXS (Thermo Fischer Scientific, Noran SIX) and EELS (Gatan UHV Enfina). The STEM is equipped with a cold-field emission gun and is operated at 100 kV. The composition of edge-on grain boundaries (i.e., GB plane parallel to the electron beam) and neighboring bulk area were measured by EDXS on an area of 2 by 3 nm. The width of the image contrast was assigned as the thickness of the GB core. It was below 2 nm for the edge-on GBs. The bulk composition was recorded from the center of the grains. To obtain a realistic picture of the GB composition, EDXS analysis was performed on 1520 grain boundaries. The wedge-shaped specimens offer a relatively long thin edge to find a sufficient number of grain boundaries parallel to the electron beam. A similar distance from the edge of the TEM specimen was always chosen to avoid thickness variations. The thickness was measured using EELS. If I0 is the integrated zero-loss intensity and I is the total integrated intensity in the energy loss spectrum, then the specimen thickness t is given in terms of the total inelastic mean free path λ by t/λ = ln(I/I0).21 The measured t/λ was always below 0.5, which corresponds to specimen thickness of
