Effect of Segregation on Surface and Near-Surface Chemistry of Yttria

Mar 6, 2012 - The present work reports the effect of segregation on surface and near-surface composition of yttria-stabilized zirconia, YSZ. The study...
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Effect of Segregation on Surface and Near-Surface Chemistry of Yttria-Stabilized Zirconia M. Asri Idris,†,# T. Bak,‡ S. Li,† and J. Nowotny*,‡ †

School of Materials Science & Engineering, The University of New South Wales, Sydney, NSW 2052, Australia Solar Energy Technologies, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia # University of Malaysia Perlis, 02600 Arau, Perlis, Malaysia ‡

ABSTRACT: The present work reports the effect of segregation on surface and near-surface composition of yttria-stabilized zirconia, YSZ. The study included the determination of the effect of both temperature (1073−1673 K) and oxygen activity (10−10 Pa < p(O2) < 75 kPa) on segregation-induced concentration profiles of yttrium. The secondary ion mass spectrometry, SIMS, was applied to determine the chemical composition of the outermost surface layer as well as the layers beneath the surface. It is shown that the effect of oxygen activity on the segregation-induced enrichment of yttrium is substantial in the temperature range 1073−1473 K. The maximum of enrichment in yttrium is observed at 1273 K. The segregation-induced enrichment of YSZ is considered in terms of a low-dimensional surface structure that is formed due to surface enrichment in both yttrium (dopant) and silicon (impurity). The effect of the surface and near-surface chemistry on the performance of YSZ-based energy conversion devices is discussed.

1. INTRODUCTION Yttria-stabilized zirconia (YSZ) is a well-known oxygen ion conductor, which has been commonly applied as a solid electrolyte of electrochemical energy conversion devices, such as solid oxide fuel cells (SOFCs)1 and chemical gas sensors.2 It has been a general perception that the performance of the YSZbased electrochemical devices is determined by the kinetics of oxygen diffusion in the bulk phase. It has also been commonly assumed that the presence of an electronic conductor attached to the surface of YSZ, such as platinum, is essential for oxygen incorporation from the gas phase into the YSZ lattice. However, the studies of surface semiconducting properties of YSZ indicate that (i) oxygen may react with the surface of YSZ without the presence of an electronic conductor and (ii) the charge transport within the YSZ-based electrochemical devices is determined by the electric field induced by gradients in chemical composition in the outermost surface layer rather than bulk diffusion.3−8 In other words, it has been documented that, while YSZ is a poor electronic conductor, the local concentration of electronic charge carriers at the surface is high enough to allow the charge transfer during oxidation of YSZ in the absence of an electronic conductor. Awareness is growing that the performance of electrochemical devices based on YSZ is determined by the local chemical composition of solid interfaces.9−13 The established effect of the surface on the reactivity of YSZ with oxygen is a prominent example indicating that the charge transfer within YSZ-based SOFCs is determined by interfaces. On the other hand, it is relatively well-known that the chemical composition of interfaces is different from that of the bulk phase as a result of segregation. This is the reason why there is an increasingly important need to enhance the present state of understanding © 2012 American Chemical Society

on the effect of segregation on the surface properties of YSZ.14−16 The studies reported so far on segregation in YSZ, which are based on X-ray photoelectron spectroscopy, XPS, inform about the average concentration of yttrium in the surface layer of a certain thickness (approximately 6 nm).14,17−19 At the same time, little is known about the segregation-induced concentration gradients within the surface layer. Moreover, little is known on the effect of gas phase composition, especially oxygen activity, on segregation. Therefore, there is a need to assess the effect of segregation on concentration gradients within the surface and near-surface layer using the technique that allows the determination of depth profiles with a high sensitivity. The technique, which may be used to perform such analysis is secondary ion mass spectrometry, SIMS. The aim of the present work is to use SIMS analysis to determine the effect of segregation of yttrium in YSZ on the chemical composition of the surface layer and the layers beneath the surface.

2. POSTULATION OF THE PROBLEM The YSZ-based SOFC is an electrochemical device, which may be used for the conversion of chemical energy into electrical energy. Its electrical circuit is represented schematically in Figure 1. As seen, the SOFC involves the YSZ as a solid electrolyte and two electrodes, the air electrode (cathode) and the fuel electrode (anode). At elevated temperatures (1073− 1273 K), the YSZ becomes an excellent oxygen ion conductor. Received: December 15, 2011 Revised: March 5, 2012 Published: March 6, 2012 10950

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oxides as well; however, there is no simple analogy between metals and oxides. YSZ is not an exception. The phenomenon of segregation that is induced by the excess in interface energy is termed equilibrium segregation or thermodynamic segregation.20,23 As shown in Figure 2, certain

Figure 1. Schematic representation of the electrochemical reaction in YSZ-based SOFC.

The current in the SOFC circuit is induced by the supply of oxygen to the air electrode and its removal at the fuel electrode where oxygen reacts with the fuel, e.g., hydrogen. Since YSZ is a poor electronic conductor, the electrons formed at the fuel electrode must be transported over the external circuit to the air electrode to complete the cycle. According to the present perception, the performance of SOFCs including YSZ as a solid electrolyte is based on the following two basic assumptions: (1) The rate of the charge transport within the SOFC circuit is determined by the bulk diffusion of oxygen. (2) Effective charge transfer at the oxygen/YSZ interface requires the presence of a contact made of an electronic conductor. The studies of Nowotny et al.3−8 have shown that these two assumptions are in question. First, it has been documented that oxidation of YSZ does not require the presence of an electronic conductor.11 These studies indicate that, while the bulk phase of YSZ is an ionic conductor, its surface layer exhibits mixed ionic−electronic conduction. The latter is high enough to allow efficient oxygen incorporation without the presence of an electronic conductor. The derived theoretical model indicates that the outermost surface layer, which exhibits entirely different properties than those of the bulk phase as a result of segregation, is a mixed conductor.7,8 In consequence, this finding indicates that the phenomenon of segregation may be used to develop bifunctional YSZ, involving the functions of an oxygen conductor and the function of an electrode that exhibits mixed charge transport and, therefore, is able to provide electrons at the cathode and remove these at the anode. However, the progress in using the phenomenon of segregation as the technology in the imposition of desired local surface properties indicates the need to undertake in-depth studies of the effect of segregation on surface and near-surface chemistry of YSZ. This is the aim of the present work. The experimental part of the present work is preceded by a short overview of the literature reports on segregation in YSZ and definitions of basic terms that will be used in the present work.

Figure 2. Schematic representation of phenomena of adsorption and segregation and related free energy changes (large and small circles represent the anions and cations in the lattice, respectively, and solid squares represent the species in the gas phase).

defects result in reduction of the interfacial energy. Then, these defects have the tendency to be transported from the bulk to the surface. Both adsorption and segregation can be considered in terms of the same concepts and laws. Therefore, segregation can also be considered as adsorption from the solid phase. The main difference between adsorption and segregation is the temperature at which the thermodynamic equilibrium can be established. Adsorption equilibrium may be established at relatively low temperatures, such as room temperature. However, segregation kinetics is rate-controlled by the lattice diffusion of segregating species. Therefore, due to the kinetic reasons, segregation equilibrium may be reached at elevated temperatures at which the lattice is sufficiently mobile to enable fast ionic transport. Consequently, segregation-induced concentration profiles imposed at temperatures below the level required for equilibrium may not be considered as equilibrium segregation and, therefore, are not well-defined. The recent studies using work function measurements for monitoring the oxidation kinetics of YSZ indicate that, even at high temperatures, corresponding to segregation equilibrium, the surface layer of YSZ is still covered by oxygen chemisorbed species, forming a surface electrical barrier preventing oxygen incorporation into the YSZ lattice.23 Thermodynamic equilibrium of the gas/solid system requires that the activity of all species is the same across the interface. However, the concentrations of the surface and the layers beneath the surface may differ. A simple model representing equilibrium segregation for the YSZ−oxygen system is shown in Figure 3. As seen, while the activity of oxygen is the same across the gas/solid interface, the concentrations may differ. The relationship between the two depends on the activity coefficient, f, (where a = cf).1,23

3. DEFINITION OF BASIC TERMS 3.1. The Segregation Driving Force. The chemical composition of interfaces is different from that of the bulk phase as a result of segregation.20−22 The theory of segregation, which has been derived initially for metals and alloys,21 may be used to explain certain segregation-induced effects in metal 10951

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where ΔHseg is the heat of segregation, R is the gas constant, and T is the absolute temperature. When the enthalpy of segregation assumes a negative value (viz., exothermic), the segregation-induced enrichment decreases with increasing temperatures. Consequently, the surface concentration of the impurities will have the tendency to increase with decreasing temperature. However, for kinetic reasons, lowering of the temperature will result in an increase of the time required to assume segregation equilibrium. Segregation results in a stable modification of the surface composition and the related properties if in equilibrium. However, practical use of the phenomenon of segregation to engineer interface properties requires a better understanding of the segregation thermodynamics. In particular, it is essential to distinguish the difference between equilibrium and nonequilibrium segregation as only the data for equilibrium segregation may be considered in terms of thermodynamics.

Figure 3. Schematic representation of the relation between concentration and activity as a function of distance.

An important consideration of equilibrium segregation is that the segregation-related data are determined by the conditions of the equilibrium. In this case, the segregation-induced enrichment is independent of the applied experimental procedure. On the other hand, however, a difference between the surface and bulk properties may also be achieved as a result of an impact or a shock. For example, the chemical, thermal, mechanical, and radiation shock applied to the surface also results in segregation. Segregation produced in this way is termed nonequilibrium segregation. The related concentration gradients are determined by the nature of the applied shock and its intensity rather than by specific material properties.23 Consequently, it is essential to distinguish these two effects. A wide range of experimental data confirms that segregationinduced concentration gradients within the surface layer of metal oxides are determined by oxygen activity.17−19 Up to now, however, the data for the effect of oxygen on segregation in zirconia and its solid solutions have been limited only to the confirmation of this effect.23 The data reported by Hughes indicate an important role of oxygen activity in the gas phase on segregation for metal oxides. It is not clear, however, if the data reported by Hughes are related to the gas/solid equilibrium. Therefore, these data require verification. This is one of the aims of the present work. Oxygen is the lattice component of oxides. Therefore, the oxygen activity, p(O2), of the gas phase surrounding the oxide specimen during high-temperature treatment has a substantial effect on the segregation-induced enrichment of the surface layer. In consequence, the segregation-induced concentration gradients may be considered to be well-defined material properties only when the gas/solid equilibrium has been established. So far, however, little is known on the effect of oxygen partial pressure on segregation in metal oxides, such as zirconia, in equilibrium. The aim of the present work is to address this problem. 3.2. Effect of Temperature. According to the regular solution approximation,11 the following relation between the concentration of solute (1) and solvent (2) at the surface (s) and in the bulk phase (b), expressed in mole fraction (X) applies: ⎛ ΔHseg ⎞ Xb1 ⎟ ⎜− exp = RT ⎠ ⎝ Xs2 Xb2

4. SEGREGATION IN YTTRIA-STABILIZED ZIRCONIA Studies on segregation in solid solutions of zirconia aim mainly at the determination of segregation-induced concentration gradients of a wide range of cations.8−10,17−24 These include: (i) cations used as structure stabilizers, such as yttrium and calcium, (ii) cations added intentionally (dopants) in order to modify the level of electronic or ionic conductivity component, (iii) additions aimed to enhance the oxygen exchange kinetics, such as Bi, and (iv) unintentionally added cations (impurities), such as Si. The effect of segregation on electrical properties of zirconia was first reported by Burggraaf et al.10−12 They observed that the local ionic conductivity of the grain boundaries is lower than that of the bulk phase by 2 orders of magnitude. This effect has been confirmed by Miyayama and Yanagida.13 Extensive studies on segregation in YSZ were reported by Winnubst et al.14,16 and Theunissen et al.15 They studied yttrium segregation using both AES and X-ray photoelectron spectroscopy (XPS). Their quantitative analysis has revealed the following effects: (1) The segregation-induced surface concentration of yttrium is practically constant and independent of its content in the bulk phase. They observed that the specimens including the bulk content of yttrium in the range 2−21 mol % exhibit a surface concentration which is limited to a narrow range between ∼19 and 23 mol % Y2O3 (∼38−46 at % Y) regardless of the bulk content (Figure 4). (2) The surface layer enriched in yttrium is approximately 2 nm thick. (3) YSZ exhibits silicon segregation (present as an impurity), which seems to be associated with yttrium. The studies of Axelsson et al.24 observed that segregation in YSZ under reduced conditions results in the formation of a surface layer structure. Its composition is similar to that of ZrSiO4. The conclusion of Axelsson et al. has been confirmed by Chaim et al.25 Using computer simulation, Bingham et al.26 studied the electrical properties of the grain boundary layer in YSZ. Their studies essentially support the experiments reported earlier by Burggraaf et al.11,12,14,16 The results of the X-ray photoelectron spectroscopy (XPS) data of Hughes,19 showing the Y/Zr ratio determined in air and in argon for YSZ, are in Figure 5 (the related oxygen activities,

Xs1

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Figure 6. Surface vs bulk concentration of Y in YSZ (10 atom %) according to Hughes et al.17−19

Figure 4. Surface vs bulk composition for yttria-doped zirconia according to Burggraaf et al.10,11,15

Figure 5. The effect of temperature on the Y/Zr atomic ratio for YSZ annealed in air and in argon according to Hughes.19

reported by Hughes19 have not been verified and the effect of oxygen activity on the segregation-induced surface chemistry has not been studied extensively. Specifically, there is a need to determine the effect of segregation on the concentration gradients of yttrium within the surface layer and also nearsurface layers in order to understand the composition of the sublayer. The disadvantage of the available XPS data is that the XPS analysis provides only an average concentration within approximately 6 nm, while the specific gradients are unknown. The gradients may be determined using the depth profile analysis using secondary ion mass spectrometry, SIMS. The aim of the present work is to verify the effect of oxygen activity on surface segregation of yttrium using SIMS. It is expected that comparison of the present data obtained by SIMS and the XPS data reported by Hughes17−19 will provide an interesting insight into the effect of segregation on the surface and near-surface chemistry of YSZ.

p(O2), are 21 kPa and 10 Pa, respectively). These data indicate that the segregation-induced surface enrichment of yttrium is sensitive to the p(O2). As seen, in the range 1573−1973 K, the Y/Zr ratio in air decreases monotonously with temperature. However, the picture is entirely different at lower p(O2). Then, in the range 1573−1723 K, the surface enrichment in yttrium is practically independent of p(O2). However, increase of temperature above 1723 K in argon results in a sudden increase of the Y/Zr ratio. The latter effect suggests that a p(O2)-induced structural transition takes place within the low-dimensional surface layer.17,19,23 The extensive studies of Hughes et al.,17−19 performed for both single crystal and polycrystalline specimens of YSZ with different contents of yttrium, resulted in a surface layer model that is shown schematically in Figure 6. According to this model proposed by Hughes, segregation of yttrium results in the formation of an outermost surface layer enriched mainly in Si and the sublayer (∼7 nm thick) is enriched in yttrium. The effect of segregation can be considered in terms of the segregation-induced low dimensional surface structures.7 The formation of such surface structures has been initially reported by Hughes.17−19 The study of Hughes that was based on X-ray photoelectron spectroscopy suggested that cosegregation of Y and Si in YSZ results in the formation of a distinct local Si−Y− Zr−O interface structure.17 This study also shows that the extent of yttrium segregation depends on oxygen activity in the gas phase during annealing. To date, however, the effects

5. EFFECT OF SEGREGATION ON PROPERTIES Badwal et al.27 have observed that silicon segregation results in a decrease in the oxygen conductivity of YSZ. They also observed that the silicon segregation-induced grain boundary phase may be removed by the addition of alumina. In this case, Al acts as a scavenger for the Si-rich glassy phase. A similar effect was reported by Mori et al.28 They observed that addition of a small amount of alumina to YSZ results in an increase in the electrical conductivity. According to Guo,29,30 the space charge at the grain boundary of YSZ that is enriched by interstitials of Zr and Y is compensated by a negative charge in the space charge layer owing to the presence of Y3+ ions located in the Zr4+ sites. Aoki et al.31 reported cosegregation of Si along with Ca in the grain boundaries of relatively pure Ca-stabilized ZrO2 (CSZ), although the bulk Si content was below 80 ppm. Burggraaf et al.32 reported that some transition metal ions, such as Fe and Ti, in the surface layer of YSZ result in an increase of the local electrical conductivity of this layer. Zhang et al.33 show the effect of a long-term annealing (up to 500 h) at 1473 K on surface concentrations of YSZ implanted with Ca, La, Sr, Nb, Fe, and Ti. Bernasik et al.34 reported that the enrichment factor of titanium in Ti-doped YSZ was 3.4 and 2.4 at 1273 and 1673 K, respectively. Surface studies using work function and SIMS analysis show that prolonged annealing of YSZ results in extensive segregation of calcium despite that its bulk content is extremely 10953

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• YSZ powder was pressed isostatically at 200 MPa into a disk (8 mm diameter × 1 mm thickness) • The discs were sintered in air at 1673 K for 2 h and cooled down to room temperature. • The discs were polished to a finish of 0.25 μm diamond powder, cleaned in acetone, annealed at 1673 K for 2 h in air, and cooled to room temperature. All heating and cooling rates were 400 K/h in air. The aim of annealing was to restore a stable surface microstructure, which is shown in Figure 7.

low (below 10 ppm).35 This effect indicates that the segregation driving force of this cation is high. The studies of the electrical effects accompanying oxidation of YSZ by Nowotny et al. indicate that oxygen may be incorporated into the YSZ lattice in the absence of an electronic conductor.6 The results of theoretical studies of Lei et al.36 confirm that the segregation of defects in YSZ results in an increase in the concentration of electrons within the interface layer enriched with yttrium. This study indicates that segregation may lead to the formation of the surface structure, which exhibits enhanced electronic charge transport. The overview of the literature allows the following points to be made: (1) Yttrium segregates to the surface of YSZ. The enrichment factor depends on bulk yttrium content. (2) Most of the data on segregation in YSZ were done in air. However, the data of Hughes19 indicate the segregationinduced enrichment of yttrium depends on the gas phase composition during the thermal treatment. The observed effect of the gas phase seems to be closely related to oxygen activity; however, such data have not been reported so far. Therefore, there is a need to perform surface analysis for YSZ annealed in the gas phase of well-defined oxygen activity. Such studies are of interest especially from the viewpoint of using segregation as the technology in surface processing. (3) The XPS data reported by Hughes17−19 are reflective of the average concentration within a surface layer that is approximately 6 nm thick. It is of interest to understand the specific concentration gradient of yttrium vs distance from the surface. Determination of such a gradient requires the layer-by-layer chemical analysis. The secondary ion mass spectrometry (SIMS) offers such analysis. The aim of the present work is to perform a surface analysis of YSZ using SIMS. The present overview is reflective of the present state of understanding on the effect of segregation on the surface properties of YSZ. This overview also allows formulation of several questions that still should be addressed: (1) What is the effect of segregation on the concentration gradients of yttrium within the surface and near-surface layers of YSZ? (2) What is the effect of oxygen activity of the segregationinduced enrichment of yttrium in the surface and nearsurface layers of YSZ? (3) What is the effect of temperature on the above effects? (4) To what extent is the picture of segregation-induced chemistry of YSZ determined by SIMS similar to that obtained by XPS? The significance of these questions for surface science is clear. Specifically, it is important to understand the effect of oxygen activity on segregation. The established effect is also important for application. Namely, the effect of segregation may be used as a technology in the modification of surface composition in a controlled manner.

Figure 7. SEM micrograph of the YSZ specimen after standardization, polishing, and subsequent annealing.

The processing procedure of the YSZ specimens used in the present study, which are described as polished and annealed or standard, is reported elsewhere.4 Identical specimens were used for annealing in the gas phase of controlled oxygen activity (10−10 Pa < p(O2) < 75 kPa) in the temperature range 1073− 1673 K. After annealing, the specimens were cooled down to room temperature in the same gas phase. Then, the specimens were transferred to the vacuum chamber of the SIMS unit. Depth profiles were determined using SIMS. 6.2. Secondary Ion Mass Spectrometry (SIMS). The SIMS measurements were performed using a Cameca IMS 5f dynamic SIMS instrument. Samples were gold coated as a means of preventing charging of the sample during the analysis. A primary ion beam of 10 kV Cs was used to sputter craters of approximately 200 μm × 200 μm area and 7.19 μm depth. The crater is represented in Figure 8.

Figure 8. The crater caused by sputtering of primary ion (Cs+) of SIMS on the YSZ specimen.

6. EXPERIMENTAL SECTION 6.1. Specimens. Polycrystalline specimens of YSZ (10YZrO2), in the form of small discs required for SIMS analysis, were made according to the following procedure:

The SIMS analysis included the following elements: • Cesium. This ion was used to bombard the YSZ lattice. In the initial period, the Cs ions implant into the surface 10954

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Figure 9. The raw data of SIMS intensity profiles vs time of sputtering at (a) 1073 K, p(O2) = 75 kPa, (b) 1673 K, p(O2) = 75 kPa, (c) 1073 K, p(O2) = 10−10 Pa, and (d) 1673 K, p(O2) = 10−10 Pa, concluding the ideal starting surface lattice.

• • • •

primary ion energy, etc.), it can take several nanometers for equilibrium sputtering to be achieved. This process is a complex one, involving interaction and implantation of the primary beam species with the sample surface, while interacting with any pre-existing sample surface topography. This process is even more complex in multiphase and polycrystalline samples with varying crystallographic orientation (such as those studied in this work). Equilibrium sputtering is achieved once all of these interacting phenomena have equalized/normalized and a constant secondary ion yield is achieved. Due to these complications, it can be difficult to determine conclusively if there is a real (i.e., sample related) change in the secondary ion being monitored. For the purpose of quantitative assessment of data, the following parameters have been used to establish the point at which equilibrium sputtering has been assumed: • the time of sputtering and the related depth at which the intensity profile for Au (the surface coating applied to minimize charging) has reduced to background levels • the time/depth at which the intensity profile for cesium (the primary ion beam applied during the analysis) exhibits a constant secondary ion yield • the time at which bulk species assume a constant secondary yield From these indicators, the erosion time can be deduced to be the “sample surface”. The dashed and solid lines in Figure 9 illustrate the times at which the surface effects have been

layer, so their content is elevated. After segregation equilibrium is reached, the same amount of Cs is implanted and removed. As a consequence, the Cs content is reduced. Therefore, the change of the Cs intensity profile may be used to assess the time after which the sputter equilibrium is reached. Gold. This metal was used to prevent surface charge. The gold depth profile informs about its penetration profile. Oxygen - the host lattice element. Yttrium - the dopant (structure stabilizer) Selected impurities (Si, Al, Ca, Na)

7. RESULTS The SIMS spectra for YSZ, in terms of the secondary ion intensity vs erosion time, are shown in Figure 9. These include the spectra for YSZ annealed under oxidized conditions, p(O2) = 75 kPa, at 1073 and 1673 K (Figure 9a and b, respectively) and reduced conditions, p(O2) = 10−10 Pa, at 1073 and 1673 K (Figure 9c and d, respectively). Several effects at the surface or in the near surface region must be considered in order to correctly interpret the SIMS data at the beginning of the depth profile.37 A native oxide layer is usually present of most samples. This oxide can cause anomalous ion yields for other species being monitored in this region. In addition, most surfaces are not completely clean, and so contaminants, especially carbon (notably advantageous carbon contamination,38 are also common in this region. In addition, depending on the nature of the specific sample surface and the instrument parameters being used (sputtering rate, 10955

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concluded to be at a minimum and the profiles are reflective of the equilibrium sputtering. The isothermal depth profiles of yttrium at p(O2) = 75 kPa and p(O2) = 10−10 Pa at different distances from the surface are shown in Figure 10a and b, respectively. These profiles are limited to the time scale corresponding to sputtering equilibrium.

Figure 12. Effect of temperature on the Y/Zr surface ratio for YSZ annealed at p(O2) = 75 kPa and p(O2) = 10−10 Pa.

enrichment of Y above 1673 K is enhanced under reduced conditions. According to the present work, the Y/Zr ratio is enhanced below 1473 K.

8. DISCUSSION Comparison of the Y/Zr intensity ratios at different depths for both reduced and oxidized specimens, which are shown in Figure 10, indicates the following: (1) The segregation-induced enrichment factor of yttrium exhibits a maximum at 1273 K at all depths. The effect of temperature on segregation may be considered in terms of a competition between the thermodynamic factor, which requires that the segregation-induced enrichment decreases with temperature, and the kinetic factor, which leads to an increase of segregation kinetics with increase of temperature. (2) It is interesting to note that a weak minimum of enrichment is observed at 1473 K. This minimum seems to be determined by the thermodynamic factor, since the kinetics in the range 1373−1673 K is high. (3) The maximum at 1273 K decreases with the distance from the surface. However, the enrichment is substantially larger at higher oxygen activity. As seen, the enrichment of the surface in yttrium is still high at p(O2) = 75 kPa at 5.72 nm. On the other hand, the enrichment at p(O2) = 10−10 Pa becomes negligibly low already at 1 nm. Figure 11 shows the enrichment vs depth dependences under both oxidized and reduced conditions. These data allow us to make the following points: (1) The enrichment of the surface in yttrium is substantially larger under oxidized conditions (see Figure 12). (2) The enrichment under reduced conditions is practically limited to 1 nm. On the other hand, the enrichment depth under oxidized conditions is 10 times larger (10 nm). This effect seems to be related to the effect of oxygen activity on the screening depth and the related Debye depth. The present work considers surface segregation resulting in surface vs depth concentration profiles. Similar segregationinduced concentration gradients are expected for all interfaces, such as grain boundaries. In the latter case, segregation results in an enrichment of grain boundaries and concentration gradients vs distance from grain boundaries. The data reported in this work are representative of polycrystalline specimens, which are formed of crystallites of different crystallographic planes exposed to the surface. One

Figure 10. The Y/Zr SIMS intensity profiles at the surface and near surface as a function of temperature for the YSZ specimen annealed at (a) p(O2) = 75 kPa and (b) p(O2) = 10−10 Pa.

The data in Figure 10a and b related to the same distance from the surface (ranging between 0 and 5.7 nm) were plotted in Figure 11a and b as a function of temperature. As seen, these data exhibit the following specific features:

Figure 11. Y/Zr SIMS depth profiles of the YSZ specimen annealed at (a) p(O2) = 75 kPa and (b) p(O2) = 10−10 Pa at temperatures of T = 1073−1673 K.

• The specimen annealed under oxidized conditions, p(O2) = 75 kPa, exhibits the maximum of the Y/Zr ratio at 1273 K. This ratio becomes substantially reduced in the range 1473−1673 K and also at 1073 K. • The specimen annealed under reduced conditions, p(O2) = 10−10 Pa, exhibits substantially reduced enrichment of Y; however, the maximum of the Y/Zr enrichment is at 1273 K as well. The depth profiles representing the effect of p(O2) on the Y/ Zr intensity profiles at the surface are shown in Figure 12. As seen, the maximum of the enrichment is at 1273 K under both oxidized and reduced conditions. Comparison of the data obtained in the present work with that reported by Hughes19 is possible only at 1673 K at which there an an overlap of data. However, in both cases, the effect of oxygen activity on segregation at this temperature is very low, if any. According to Hughes, the segregation-induced 10956

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should expect, however, that the ratio of the planes of different orientations exposed to the outermost surface layer is a characteristic feature of the specimens. Therefore, even if a substantial effect of segregation-induced enrichment may be expected for different crystallographic planes, the enrichment is the quantity that is representative of the studied YSZ specimens. The obtained data indicate that the phenomenon of segregation may be used for engineering of surface and nearsurface chemical composition. As seen, desired chemical composition may be achieved by the manipulation with the temperature and the time of annealing as well as oxygen activity.

9. CONCLUSIONS The present work determined the effect of oxygen activity on surface segregation of yttrium for YSZ (10 at % Y). The experimental data obtained indicate the following: Comparison of the Y/Zr intensity ratios for both reduced and oxidized specimens of YSZ indicates that the segregationinduced enrichment of Y in the range 1073−1473 K is larger for the specimens annealed under oxidized conditions. The data of Hughes indicates that annealing under reduced conditions above 1673 K results in an enhanced segregation. The segregation-induced enrichment factor of Y exhibits a maximum at 1273 K. At this temperature, the Y/Zr intensity ratio at p(O2) = 75 kPa is larger than that at p(O2) = 10−10 Pa by a factor of 3. Data in the range 1473−1673 K indicate that p(O2) has little effect on segregation of Y. This essentially is in agreement with the XPS data reported by Hughes8 who did not observe the effect of p(O2) on Y segregation in the same temperature range.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The help provided by L. Sheppard in annealing the specimens as well as K. Prince and A. Atanacio in surface analysis is sincerely appreciated. The present work was supported by the Ministry of Higher Eduation in Malaysia through the School of Materials Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia. This support is sincerely appreciated.

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REFERENCES

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