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Characterization of low temperature protonic conductivity in bulk nanocrystalline fully stabilized zirconia
Solid State Ionics 180 (2009) 297–301
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Characterization of low temperature protonic conductivity in bulk nanocrystalline fully stabilized zirconia Gaetano Chiodelli a, Filippo Maglia b, Umberto Anselmi-Tamburini b,c,⁎, Zuhair A. Munir c,⁎ a b c
C]NR–IENI, Department of Pavia, V.le Taramelli, 16, 27100 Pavia, Italy Department of Physical Chemistry, University of Pavia, V.le Taramelli 16, 27100 Pavia, Italy Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA
a r t i c l e
i n f o
Article history: Received 24 July 2008 Received in revised form 17 February 2009 Accepted 18 February 2009 Keywords: Nanostructure YFSZ Protonic conduction
a b s t r a c t We investigated the conductivity of samples of bulk nanometric cubic yttria stabilized zirconia (YSZ 8%) with a grain size of about 16 nm and a relative density above 98%. In an oxygen atmosphere the material showed a large grain boundary resistivity. However, when exposed to a moist atmosphere at temperatures below 150 °C it showed a high conductivity, several orders of magnitude higher than the corresponding extrapolated ionic conductivity. A fairly high conductivity was measured even at room temperature. The conductivity was strongly dependent on the water partial pressure and showed a distinct isotopic effect, suggesting a protonic conductivity mechanism. This effect was not observed in samples with grain size above 50 nm, suggesting the possibility that nanostructure can induce drastic modification in the conduction mechanism of ceramic electrolytes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the last decade considerable attention has been given to the role that nanostructure might play in defining the characteristics of solid ionic conductors. It has been suggested that the electrical properties of these materials are altered when the grain size falls in the nanometric range, particularly when it approaches 10 nm [1–5]. However, the challenges encountered in the preparation of bulk nanocrystalline materials with very low level of porosity have made it difficult to confirm the expectations of the nano-scale effect [6–9]. As a result, only very few examples of bulk fully dense ceramic materials with a grain size below 50 nm have been reported in the literature, and their electrochemical characterization has been quite limited [8]. However, it has been shown recently that the High Pressure Pulsed Electric Current Sintering method (HP-PECS) [10–14], which involves rapid sintering cycles and high pressure (up to 1 GPa), can be used to prepare near fully dense nanocrystalline materials with minimal grain growth. Through this approach the feasibility of consolidation of zirconia and ceria samples to relative densities exceeding 98% and with a grain size below 20 nm has been demonstrated [15]. In a recent short communication [16] we reported preliminary evidence of elevated protonic conductivity at temperatures as low as 200 °C in
⁎ Corresponding authors. Anselmi-Tamburini is to be contacted at the Department of Physical Chemistry, University of Pavia, V.le Taramelli 16, 27100 Pavia, Italy. Tel.: +39 0382 987208. Munir at the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA.. Tel.:+1 530 7524058. E-mail addresses: [email protected] (U. Anselmi-Tamburini), [email protected] (Z.A. Munir). 0167-2738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2009.02.031
dense bulk samples of fully stabilized zirconia with grain size below 20 nm. In a more recent paper it was shown that power generation is feasible at very low temperatures using water concentration cells when nanoscale fluorite-structured oxides are used as electrolytes [17]. In this paper we present a full characterization of the elevated protonic conductivity in nanometric YSZ demonstrating that, under appropriate conditions, these materials possess significant protonic conductivity even at room temperature. 2. Experimental The 8 mol% Y2O3-stabilized ZrO2 nanopowders used in this study were obtained by a co-precipitation method [16]. After precipitation the powders were dried, milled in a zirconia bowl with zirconia balls and annealed at 390 °C for 24 h in air. The average grain size of the resulting powder was 6.6 nm, as determined by line-broadening of the XRD peaks [18,19]. Some samples with micrometric grain size were obtained by appropriate annealing of commercial powders (Tosoh TZ8Y) having an initial grain size of 20 nm. The densification was performed using a high-pressure modification of a Pulsed Electric Current Sintering apparatus (HP-PECS) also known as Spark Plasma Sintering (SPS) (Sumitomo Dr. Sinter 1050) [15]. In this approach 0.15 g of powders were loaded in a double stage die. The low-pressure section of the die was made out of high density graphite, while the high pressure section was made out of silicon carbide and tungsten carbide, as described previosly [15]. The die was loaded in the SPS apparatus, which was evacuated to a pressure of 10 Pa. A moderate initial uniaxial pressure (150 MPa) was applied. The temperature was then increased at a heating rate of 200 °C.min− 1.
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Once the sample reached the designated temperature, the pressure was rapidly increased to the final value (typically about 750 MPa). The sample was held under these conditions for 5 min and then the pressure was quickly released and the power turned off. Temperatures were measured using a shielded K-type thermocouple inserted in the lateral wall of the die. At the end of the densification process the samples were very dark in color, probably due to oxygen substoichiometry induced by the SPS strongly reducing environment [20]. For this reason before performing any electrochemical characterization, the samples were annealed at 750 °C for 6 h in an atmosphere of dry oxygen. This heat treatment did not change the sample nanostructure, but removed the discoloration. Microstructural characterization of the samples was made on uncoated fracture surfaces, using a high-resolution SEM (Philips XL30s). The average grain size was determined measuring at least 100 grains on each HRSEM image using the software AnalySIS (Soft Imaging System Corp. Lakewood, CO). The density of the samples was measured using the Archimedes method and from geometric and gravimetric measurements. AC electrical characterization was performed using a Solartron 1260 frequency response analyzer (FRA) in the frequency range between 10− 3 Hz and 10 MHz. A voltage of 50–500 mV was used. A homemade high impedance adaptor (1013 Ω, 3 pF) with an active guard [21] was used in order to increase the sensitivity of the analyzer and reduce noise and capacitance effects of the cables and of the cell. Platinum electrodes were deposited on the sample by sputtering. The electrochemical characterization has been performed in oxygen and hydrogen gas at various temperatures under a constant gas flux of 8 l/h. All these measurements have been performed in quasi-equilibrium conditions, using a heating rate of 0.5 °C/min and allowing several hours for the stabilization at each temperature. The gases were used either dry or saturated with water at room temperature (PH2O = 32 mbar). Lower PH2O values were obtained by mixing dry and moist gases using two MKS mass/flow controllers. The water content in the gas was monitored at room temperature by a moisture sensor (Almemo 22904). 3. Results Fig. 1 shows a high- resolution SEM image of the fracture surface of a typical sample. The measured grain size is around 16 nm and the relative density is above 98%. Fig. 2a shows the impedance pattern of these samples as measured at 200 °C in dry oxygen after a heat treatment in air at 750 °C for 6 h. The pattern exhibits a single deformed semicircle. As reported recently [16,17,22–24], in nanometric bulk ceramics the contribution due to the grain boundaries becomes usually dominant, producing a very large low frequency semicircle that obscures the bulk component at higher frequencies.
Fig. 1. SEM image of a typical sample of YSZ densified with High Pressure Spark Plasma Sintering.
Fig. 2. Impedance patterns of: (a) a sample of nanometric zirconia measured at 200 °C after annealing in dry oxygen at 750 °C for 6 h; (b) the same sample measured at 30 °C after annealing for 10 h at 600 °C in moist hydrogen (PH2O = 32 mbar).
The presence of a bulk semicircle in these cases is evidenced only by a slight deformation at the high frequency region of the only visible semicircle. When this material is annealed in moist hydrogen (PH2O = 32 mbar) for 10 h at 600 °C and then equilibrated in the same atmosphere at temperatures close to room temperature (30 °C) the electrical properties change drastically (Fig. 2b). A single well-defined semicircle is now observed, corresponding to a resistance several orders of magnitude lower than that shown in Fig. 2a (note the difference in the scales of the abscissa between Fig. 2a and b). The tail at low frequencies observed in Fig. 2b is related to the electrode phenomena and does not involve the sample. The difference is even more striking considering that in Fig. 2a the measurement was performed at 200 °C, while in Fig. 2b it was performed near room temperature. The role played by water in defining the electrical properties of these samples is evidenced by the exponential dependence of the conductivity on the relative humidity, as shown at 30°C in Fig. 3. A change of more than five orders of magnitude is observed when the relative humidity (RH%) of the gas is increased from 0 to 95%. These measurements have been performed both with increasing and decreasing PH2O, with very reproducible results. In all of these measurements a dielectric constant value between 30 and 40 was evaluated (at high frequency), a value quite typical for dielectric materials like zirconia [25]. Despite the low temperatures the kinetics of the interaction with the moist environment appear to be very fast. Fig. 4 shows the real part of the sample impedance, measured at the fixed frequency of 1 Hz as a function of time for a sample exposed to a flux of moist hydrogen (PH2O = 32 mbar) at 30 °C. A drop of more than four orders of magnitude in resistivity is observed in the first few minutes, followed by a slower decrease that stabilizes in about 2 h. A behavior identical to the one displayed in Fig. 4 was observed even under moist (PH2O=32 mbar) oxygen atmosphere; the reducing atmosphere is therefore strictly required only during the high temperature (600 °C) annealing. When the samples equilibrated at 30 °C in moist environments are subsequently annealed at higher temperatures, always under a flux of
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Fig. 5. Change in the impedance patterns observed when the sample of Fig. 2b is exposed to increasing temperatures under a flux of moist hydrogen at 90 °C (a), 100 °C (b), 110 °C (c), 120 °C (d), and 130 °C (e). The pattern corresponding to the sample at room temperature (Fig. 2b) is not reported in this figure since its resistivity value would be to small on this scale. Fig. 3. Dependence of the conductivity at room temperature on water partial pressure for a sample of nanocrystalline zirconia. The maximum partial pressure of water was PH2O = 32 mbar.
moist hydrogen at constant water content (PH2O = 32 mbar), a gradual increase in the resistivity is observed (Fig. 5). The overall behavior of our zirconia samples in a moist environment as a function of temperature is summarized in Fig. 6 in which the logarithm of the conductivity is reported as a function of 1/T for a constant PH2O = 32 mbar. For comparison, data of a sample with a grain size of 3.0 μm are also reported. The typical dependence of the electrical conductivity on temperature in dry oxygen for a micrometric polycrystalline zirconia (YSZ) [25] is also included in this figure as a solid line. Note that when the materials showed two distinct semicircles for the bulk and grain boundary (for T N 150 °C), only the value of the bulk is reported. At temperatures between 150 and 500 °C the conductivity of our samples agrees well with the values reported in the literature for the ionic conductivity of bulk YSZ in dry oxygen [25]. However, for temperatures below 150 °C, in the nanometric sample a marked and unexpected deviation is observed; the conductivity begins to increase with decreasing temperature, showing an inversion in the sign of the slope. At room temperature, a value of conductivity about seven orders of magnitude higher than the value predicted by extrapolating the high temperature behavior is observed. It must be noted that this
Fig. 4. Change in the resistance as function of time, measured at 30 °C for a sample of nanocrystalline zirconia at the frequency of 1 Hz, following exposure to moisture (PH2O = 32 mbar).
trend is very reproducible and was observed in several different samples showing only a very limited hysteresis (triangles and open circles in Fig. 6) in the case of measurements performed with increasing or decreasing temperatures. Quite surprisingly, an increase in conductivity in moist environment is observed at low temperatures also in the case of samples of YSZ with micrometric grain size (see Fig. 6). In this case, however, the inversion in the slope begins at a lower temperature (below 50 °C) and the conductivity values are in any case considerably lower (see full circles in Fig. 6). More important, in the case of micrometric grain size the increase in conductivity is not observed if the lateral surfaces of the sample are protected by a polymeric paint, while in the case of nanometric zirconia the behavior reported in Fig. 6 is still observed, suggesting that in nanometric samples the conduction process involves the bulk of the material, while in the case of micrometric samples it involves only its external surface. A behavior very similar to the one observed on samples with micrometric grain size has been observed also in samples with a grain size just above 50 nm, suggesting that the enhancement in the low temperature bulk conductivity can be observed only in samples with extremely small grain size.
Fig. 6. Dependence of conductivity on temperature for two different samples of zirconia with a grain size below 20 nm (open symbols) and micrometric (full symbols) when exposed to a flux of moist hydrogen (PH2O = 32 mbar). The continuous line reproduces the temperature dependence of the conductivity for micrometric zirconia in dry oxygen [25]. The arrow indicates if the measurements have been performed with increasing or decreasing temperature.
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As a final remark, it is important to note that a preliminary high temperature treatment in hydrogen (10 h at 600 °C, PH2O = 32 mbar) seems to be essential in order to obtain the dramatic increase in the low temperature conductivity observed in the nanometric samples in moist atmosphere. At the end of this treatment the samples turn black in color due to the formation of color centers associated to the oxygen substoichiometry produced by the reducing environment. This phenomenon is well known in zirconia, although regular micrometric polycrystalline zirconia requires more drastic conditions (temperature well above 1000 °C or electrochemical reduction) to show a significant change in color [26–28]. In contrast, our fully dense nanocrystalline zirconia shows a change in color if exposed to moist hydrogen at temperatures as low as 600 °C. However, despite the change in color the material maintains its ionic characteristics. In fact, in a dry environment its resistance at room temperature is extremely high, very similar to the one observed in oxidized samples. Only when the sample is exposed to moist gas does a change in the conductivity occur. 4. Discussion The experimental results clearly indicate that the remarkable low temperature conductivity observed in nanocrystalline zirconia is associated with protonic conduction. In order to further demonstrate the protonic nature of the charge carriers we performed measurements also in gas saturated with D2O (Fig. 7). A large isotopic effect is clearly observed, as the resistance is more than tripled at 25 °C when deuterated water is used. Such extensive isotopic effect is somehow surprising; however it is not unusual for high temperature proton conducting oxides to observe a ratio of H/D conductivities between 2 and 3 (see for instance [29]). A further evidence of the protonic involvement in the conductivity mechanism of these samples is represented by their recently reported ability to produce EMF when used as electrolyte in a water concentration cell [17]. The possibility of protonic conduction in zirconia has been suggested before, although always in a high temperature regime [30]. Wagner was the first to propose such a possibility [31,32]. Although, the hydrogen diffusivity he measured in YSZ single crystals was quite high, the hydrogen solubility was extremely low, excluding the possibility that zirconia could act as a high temperature hydrogen conductor. These conclusions have been recently confirmed by Nigara et al. [33]. The possibility of high hydrogen mobility along the grain boundaries of polycrystalline YSZ has been recently proposed by Guo [34,35], suggesting that such elevated mobility might be responsible for the moisture-induced degradation in tetragonal zirconia. However, in cubic zirconia he observed only a modest (30%) increase in the conductivity as a result of the exposure to moisture at 250 °C for 24 months.
Fig. 7. Impedance patterns of nanometric samples exposed to moist hydrogen gas humidified with regular water (a) and deuterium enriched water (b).
The mechanism for water incorporation in the lattice of zirconia originally proposed by Wagner [31,32] involves the interaction between the water molecule and an oxygen vacancy: ••
X
•
H2 O þ VO þ OO →2ðOHÞO
ð1Þ
where the standard Kröger–Vink notation is used to identify oxygen X vacancies (V•• O), oxygen on regular crystallographic sites (OO), and the protonic defects ((OH)•O). The hydrogen transport is then due to hydrogen hopping between two neighboring oxygen atoms. This model has been largely used to interpret hydrogen conductivity in high temperature perovskitic protonic conductors [36]. The hydrogen conductivity that we observed in our dense nanometric zirconia does not seem to fit into this model. The mobility of point defects in bulk zirconia becomes extremely low at temperatures below 600 °C, so bulk diffusion cannot justify the conductivity values we observed at temperatures as low as room temperature. Furthermore, the temperature dependence of the low temperature conductivity in nanometric zirconia (Fig. 6) shows a strong negative activation energy for temperatures below 150 °C, an unusual feature for any solid-state ionic conductor. On the other hand, these results show a quite interesting similarity with the results by Riess et al. [37,38], obtained using cold-pressed pellets of YSZ micrometric powders exposed to moisture. Despite the lack of any sintering the authors observed in these pellets a significant lowtemperature conductivity which was strongly dependent on temperature and water partial pressure, with characteristics similar to those observed in our nanometric fully dense zirconia. They suggested an interpretation based on a conduction confined in chemically and physically adsorbed water molecules bound to the surface of the zirconia grains. They identified three different regimes: one at high temperature (T N 200 °C) where chemical adsorption prevails, one at intermediate temperatures (150 °C N T N 50 °C), characterized by a single layer physical adsorption, and one at low temperatures (T b 50 °C) where multilayer physical adsorption dominates. In the last two regimes an apparent negative activation energy is observed as a result of the strong dependence of the amount of absorbed water (and of charge carrier) on temperature, that reverses the intrinsic Arrhenius-like behavior. Despite these similarities it must be considered that Riess et al. [37,38] described the behavior of coldpressed powders, which had a relative density below 70%. Our samples, in contrast, are virtually fully dense, showing a relative density above 98% and a surface area (determined by BET) below 1 m2/g, a value that can be easily reconciled with the geometrical area of the sintered pellet and that excludes the presence of large amounts of open nanoporosity. The mechanism proposed by Riess et al. [37,38], on other hand, is probably responsible for the minimal level of conductivity observed in materials with micrometric grain size when exposed to a moist environment at temperatures below 50 °C. This conductivity, in fact, disappears if the lateral surface of the sample is coated with a polymeric paint, suggesting a conductivity confined to the external surface. At temperatures above 50 °C, however, a significant conductivity is observed only in samples with the grain size below 20 nm. This conductivity is retained also when the lateral surface of the sample is coated with the polymer, suggesting that in this case the conduction process involves the bulk of the sample. We suggest that this new conduction mechanism observed only in fully dense zirconia with grain size below 20 nm could be justified considering the possibility that in these samples the conduction is the result of an unusually fast movement of protons along the grain boundaries. In this respect the nanometric zirconia would behave very much like proton conducting polymers. It is important to note that the proposed mechanism is not in contradiction with the large literature that has demonstrated the blocking nature of grain boundary towards the ionic charge carrier in zirconia and perovskite oxides [36]. In this
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literature, which refers only to materials with micrometric grain size, the conduction is considered only across the grain boundary, while conduction along the grain boundary is not considered. However, in materials with grain size below 20 nm the density of grain boundaries is extremely high and transport along the grain boundaries might become dominant. Furthermore, in such materials a significant role might be played by the space charge regions adjacent to the grain boundaries. As the dimension of the space charge regions become comparable with the dimension of the grains, in fact, a relevant fraction of the material become characterized by defects distribution significantly different from the one characteristic of regular bulk materials. The concept of grain boundaries regions supporting a fast ionic transport mechanism is still poorly understood. Although no information on protonic diffusivity along the grain boundary is available, our results strongly suggest that grain boundaries can play a significant role in the protonic conductivity of nanocrystalline materials. Acknowledgments This work has been partially supported by a MIUR-FISR project of the Italian CNR, by the Fondazione Cariplo, and by the (US) National Science Foundation. References [1] [2] [3] [4] [5] [6] [7] [8]
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Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Characterization of low temperature protonic conductivity in bulk nanocrystalline fully stabilized zirconia Gaetano Chiodelli a, Filippo Maglia b, Umberto Anselmi-Tamburini b,c,⁎, Zuhair A. Munir c,⁎ a b c
C]NR–IENI, Department of Pavia, V.le Taramelli, 16, 27100 Pavia, Italy Department of Physical Chemistry, University of Pavia, V.le Taramelli 16, 27100 Pavia, Italy Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA
a r t i c l e
i n f o
Article history: Received 24 July 2008 Received in revised form 17 February 2009 Accepted 18 February 2009 Keywords: Nanostructure YFSZ Protonic conduction
a b s t r a c t We investigated the conductivity of samples of bulk nanometric cubic yttria stabilized zirconia (YSZ 8%) with a grain size of about 16 nm and a relative density above 98%. In an oxygen atmosphere the material showed a large grain boundary resistivity. However, when exposed to a moist atmosphere at temperatures below 150 °C it showed a high conductivity, several orders of magnitude higher than the corresponding extrapolated ionic conductivity. A fairly high conductivity was measured even at room temperature. The conductivity was strongly dependent on the water partial pressure and showed a distinct isotopic effect, suggesting a protonic conductivity mechanism. This effect was not observed in samples with grain size above 50 nm, suggesting the possibility that nanostructure can induce drastic modification in the conduction mechanism of ceramic electrolytes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the last decade considerable attention has been given to the role that nanostructure might play in defining the characteristics of solid ionic conductors. It has been suggested that the electrical properties of these materials are altered when the grain size falls in the nanometric range, particularly when it approaches 10 nm [1–5]. However, the challenges encountered in the preparation of bulk nanocrystalline materials with very low level of porosity have made it difficult to confirm the expectations of the nano-scale effect [6–9]. As a result, only very few examples of bulk fully dense ceramic materials with a grain size below 50 nm have been reported in the literature, and their electrochemical characterization has been quite limited [8]. However, it has been shown recently that the High Pressure Pulsed Electric Current Sintering method (HP-PECS) [10–14], which involves rapid sintering cycles and high pressure (up to 1 GPa), can be used to prepare near fully dense nanocrystalline materials with minimal grain growth. Through this approach the feasibility of consolidation of zirconia and ceria samples to relative densities exceeding 98% and with a grain size below 20 nm has been demonstrated [15]. In a recent short communication [16] we reported preliminary evidence of elevated protonic conductivity at temperatures as low as 200 °C in
⁎ Corresponding authors. Anselmi-Tamburini is to be contacted at the Department of Physical Chemistry, University of Pavia, V.le Taramelli 16, 27100 Pavia, Italy. Tel.: +39 0382 987208. Munir at the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA.. Tel.:+1 530 7524058. E-mail addresses: [email protected] (U. Anselmi-Tamburini), [email protected] (Z.A. Munir). 0167-2738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2009.02.031
dense bulk samples of fully stabilized zirconia with grain size below 20 nm. In a more recent paper it was shown that power generation is feasible at very low temperatures using water concentration cells when nanoscale fluorite-structured oxides are used as electrolytes [17]. In this paper we present a full characterization of the elevated protonic conductivity in nanometric YSZ demonstrating that, under appropriate conditions, these materials possess significant protonic conductivity even at room temperature. 2. Experimental The 8 mol% Y2O3-stabilized ZrO2 nanopowders used in this study were obtained by a co-precipitation method [16]. After precipitation the powders were dried, milled in a zirconia bowl with zirconia balls and annealed at 390 °C for 24 h in air. The average grain size of the resulting powder was 6.6 nm, as determined by line-broadening of the XRD peaks [18,19]. Some samples with micrometric grain size were obtained by appropriate annealing of commercial powders (Tosoh TZ8Y) having an initial grain size of 20 nm. The densification was performed using a high-pressure modification of a Pulsed Electric Current Sintering apparatus (HP-PECS) also known as Spark Plasma Sintering (SPS) (Sumitomo Dr. Sinter 1050) [15]. In this approach 0.15 g of powders were loaded in a double stage die. The low-pressure section of the die was made out of high density graphite, while the high pressure section was made out of silicon carbide and tungsten carbide, as described previosly [15]. The die was loaded in the SPS apparatus, which was evacuated to a pressure of 10 Pa. A moderate initial uniaxial pressure (150 MPa) was applied. The temperature was then increased at a heating rate of 200 °C.min− 1.
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Once the sample reached the designated temperature, the pressure was rapidly increased to the final value (typically about 750 MPa). The sample was held under these conditions for 5 min and then the pressure was quickly released and the power turned off. Temperatures were measured using a shielded K-type thermocouple inserted in the lateral wall of the die. At the end of the densification process the samples were very dark in color, probably due to oxygen substoichiometry induced by the SPS strongly reducing environment [20]. For this reason before performing any electrochemical characterization, the samples were annealed at 750 °C for 6 h in an atmosphere of dry oxygen. This heat treatment did not change the sample nanostructure, but removed the discoloration. Microstructural characterization of the samples was made on uncoated fracture surfaces, using a high-resolution SEM (Philips XL30s). The average grain size was determined measuring at least 100 grains on each HRSEM image using the software AnalySIS (Soft Imaging System Corp. Lakewood, CO). The density of the samples was measured using the Archimedes method and from geometric and gravimetric measurements. AC electrical characterization was performed using a Solartron 1260 frequency response analyzer (FRA) in the frequency range between 10− 3 Hz and 10 MHz. A voltage of 50–500 mV was used. A homemade high impedance adaptor (1013 Ω, 3 pF) with an active guard [21] was used in order to increase the sensitivity of the analyzer and reduce noise and capacitance effects of the cables and of the cell. Platinum electrodes were deposited on the sample by sputtering. The electrochemical characterization has been performed in oxygen and hydrogen gas at various temperatures under a constant gas flux of 8 l/h. All these measurements have been performed in quasi-equilibrium conditions, using a heating rate of 0.5 °C/min and allowing several hours for the stabilization at each temperature. The gases were used either dry or saturated with water at room temperature (PH2O = 32 mbar). Lower PH2O values were obtained by mixing dry and moist gases using two MKS mass/flow controllers. The water content in the gas was monitored at room temperature by a moisture sensor (Almemo 22904). 3. Results Fig. 1 shows a high- resolution SEM image of the fracture surface of a typical sample. The measured grain size is around 16 nm and the relative density is above 98%. Fig. 2a shows the impedance pattern of these samples as measured at 200 °C in dry oxygen after a heat treatment in air at 750 °C for 6 h. The pattern exhibits a single deformed semicircle. As reported recently [16,17,22–24], in nanometric bulk ceramics the contribution due to the grain boundaries becomes usually dominant, producing a very large low frequency semicircle that obscures the bulk component at higher frequencies.
Fig. 1. SEM image of a typical sample of YSZ densified with High Pressure Spark Plasma Sintering.
Fig. 2. Impedance patterns of: (a) a sample of nanometric zirconia measured at 200 °C after annealing in dry oxygen at 750 °C for 6 h; (b) the same sample measured at 30 °C after annealing for 10 h at 600 °C in moist hydrogen (PH2O = 32 mbar).
The presence of a bulk semicircle in these cases is evidenced only by a slight deformation at the high frequency region of the only visible semicircle. When this material is annealed in moist hydrogen (PH2O = 32 mbar) for 10 h at 600 °C and then equilibrated in the same atmosphere at temperatures close to room temperature (30 °C) the electrical properties change drastically (Fig. 2b). A single well-defined semicircle is now observed, corresponding to a resistance several orders of magnitude lower than that shown in Fig. 2a (note the difference in the scales of the abscissa between Fig. 2a and b). The tail at low frequencies observed in Fig. 2b is related to the electrode phenomena and does not involve the sample. The difference is even more striking considering that in Fig. 2a the measurement was performed at 200 °C, while in Fig. 2b it was performed near room temperature. The role played by water in defining the electrical properties of these samples is evidenced by the exponential dependence of the conductivity on the relative humidity, as shown at 30°C in Fig. 3. A change of more than five orders of magnitude is observed when the relative humidity (RH%) of the gas is increased from 0 to 95%. These measurements have been performed both with increasing and decreasing PH2O, with very reproducible results. In all of these measurements a dielectric constant value between 30 and 40 was evaluated (at high frequency), a value quite typical for dielectric materials like zirconia [25]. Despite the low temperatures the kinetics of the interaction with the moist environment appear to be very fast. Fig. 4 shows the real part of the sample impedance, measured at the fixed frequency of 1 Hz as a function of time for a sample exposed to a flux of moist hydrogen (PH2O = 32 mbar) at 30 °C. A drop of more than four orders of magnitude in resistivity is observed in the first few minutes, followed by a slower decrease that stabilizes in about 2 h. A behavior identical to the one displayed in Fig. 4 was observed even under moist (PH2O=32 mbar) oxygen atmosphere; the reducing atmosphere is therefore strictly required only during the high temperature (600 °C) annealing. When the samples equilibrated at 30 °C in moist environments are subsequently annealed at higher temperatures, always under a flux of
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Fig. 5. Change in the impedance patterns observed when the sample of Fig. 2b is exposed to increasing temperatures under a flux of moist hydrogen at 90 °C (a), 100 °C (b), 110 °C (c), 120 °C (d), and 130 °C (e). The pattern corresponding to the sample at room temperature (Fig. 2b) is not reported in this figure since its resistivity value would be to small on this scale. Fig. 3. Dependence of the conductivity at room temperature on water partial pressure for a sample of nanocrystalline zirconia. The maximum partial pressure of water was PH2O = 32 mbar.
moist hydrogen at constant water content (PH2O = 32 mbar), a gradual increase in the resistivity is observed (Fig. 5). The overall behavior of our zirconia samples in a moist environment as a function of temperature is summarized in Fig. 6 in which the logarithm of the conductivity is reported as a function of 1/T for a constant PH2O = 32 mbar. For comparison, data of a sample with a grain size of 3.0 μm are also reported. The typical dependence of the electrical conductivity on temperature in dry oxygen for a micrometric polycrystalline zirconia (YSZ) [25] is also included in this figure as a solid line. Note that when the materials showed two distinct semicircles for the bulk and grain boundary (for T N 150 °C), only the value of the bulk is reported. At temperatures between 150 and 500 °C the conductivity of our samples agrees well with the values reported in the literature for the ionic conductivity of bulk YSZ in dry oxygen [25]. However, for temperatures below 150 °C, in the nanometric sample a marked and unexpected deviation is observed; the conductivity begins to increase with decreasing temperature, showing an inversion in the sign of the slope. At room temperature, a value of conductivity about seven orders of magnitude higher than the value predicted by extrapolating the high temperature behavior is observed. It must be noted that this
Fig. 4. Change in the resistance as function of time, measured at 30 °C for a sample of nanocrystalline zirconia at the frequency of 1 Hz, following exposure to moisture (PH2O = 32 mbar).
trend is very reproducible and was observed in several different samples showing only a very limited hysteresis (triangles and open circles in Fig. 6) in the case of measurements performed with increasing or decreasing temperatures. Quite surprisingly, an increase in conductivity in moist environment is observed at low temperatures also in the case of samples of YSZ with micrometric grain size (see Fig. 6). In this case, however, the inversion in the slope begins at a lower temperature (below 50 °C) and the conductivity values are in any case considerably lower (see full circles in Fig. 6). More important, in the case of micrometric grain size the increase in conductivity is not observed if the lateral surfaces of the sample are protected by a polymeric paint, while in the case of nanometric zirconia the behavior reported in Fig. 6 is still observed, suggesting that in nanometric samples the conduction process involves the bulk of the material, while in the case of micrometric samples it involves only its external surface. A behavior very similar to the one observed on samples with micrometric grain size has been observed also in samples with a grain size just above 50 nm, suggesting that the enhancement in the low temperature bulk conductivity can be observed only in samples with extremely small grain size.
Fig. 6. Dependence of conductivity on temperature for two different samples of zirconia with a grain size below 20 nm (open symbols) and micrometric (full symbols) when exposed to a flux of moist hydrogen (PH2O = 32 mbar). The continuous line reproduces the temperature dependence of the conductivity for micrometric zirconia in dry oxygen [25]. The arrow indicates if the measurements have been performed with increasing or decreasing temperature.
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As a final remark, it is important to note that a preliminary high temperature treatment in hydrogen (10 h at 600 °C, PH2O = 32 mbar) seems to be essential in order to obtain the dramatic increase in the low temperature conductivity observed in the nanometric samples in moist atmosphere. At the end of this treatment the samples turn black in color due to the formation of color centers associated to the oxygen substoichiometry produced by the reducing environment. This phenomenon is well known in zirconia, although regular micrometric polycrystalline zirconia requires more drastic conditions (temperature well above 1000 °C or electrochemical reduction) to show a significant change in color [26–28]. In contrast, our fully dense nanocrystalline zirconia shows a change in color if exposed to moist hydrogen at temperatures as low as 600 °C. However, despite the change in color the material maintains its ionic characteristics. In fact, in a dry environment its resistance at room temperature is extremely high, very similar to the one observed in oxidized samples. Only when the sample is exposed to moist gas does a change in the conductivity occur. 4. Discussion The experimental results clearly indicate that the remarkable low temperature conductivity observed in nanocrystalline zirconia is associated with protonic conduction. In order to further demonstrate the protonic nature of the charge carriers we performed measurements also in gas saturated with D2O (Fig. 7). A large isotopic effect is clearly observed, as the resistance is more than tripled at 25 °C when deuterated water is used. Such extensive isotopic effect is somehow surprising; however it is not unusual for high temperature proton conducting oxides to observe a ratio of H/D conductivities between 2 and 3 (see for instance [29]). A further evidence of the protonic involvement in the conductivity mechanism of these samples is represented by their recently reported ability to produce EMF when used as electrolyte in a water concentration cell [17]. The possibility of protonic conduction in zirconia has been suggested before, although always in a high temperature regime [30]. Wagner was the first to propose such a possibility [31,32]. Although, the hydrogen diffusivity he measured in YSZ single crystals was quite high, the hydrogen solubility was extremely low, excluding the possibility that zirconia could act as a high temperature hydrogen conductor. These conclusions have been recently confirmed by Nigara et al. [33]. The possibility of high hydrogen mobility along the grain boundaries of polycrystalline YSZ has been recently proposed by Guo [34,35], suggesting that such elevated mobility might be responsible for the moisture-induced degradation in tetragonal zirconia. However, in cubic zirconia he observed only a modest (30%) increase in the conductivity as a result of the exposure to moisture at 250 °C for 24 months.
Fig. 7. Impedance patterns of nanometric samples exposed to moist hydrogen gas humidified with regular water (a) and deuterium enriched water (b).
The mechanism for water incorporation in the lattice of zirconia originally proposed by Wagner [31,32] involves the interaction between the water molecule and an oxygen vacancy: ••
X
•
H2 O þ VO þ OO →2ðOHÞO
ð1Þ
where the standard Kröger–Vink notation is used to identify oxygen X vacancies (V•• O), oxygen on regular crystallographic sites (OO), and the protonic defects ((OH)•O). The hydrogen transport is then due to hydrogen hopping between two neighboring oxygen atoms. This model has been largely used to interpret hydrogen conductivity in high temperature perovskitic protonic conductors [36]. The hydrogen conductivity that we observed in our dense nanometric zirconia does not seem to fit into this model. The mobility of point defects in bulk zirconia becomes extremely low at temperatures below 600 °C, so bulk diffusion cannot justify the conductivity values we observed at temperatures as low as room temperature. Furthermore, the temperature dependence of the low temperature conductivity in nanometric zirconia (Fig. 6) shows a strong negative activation energy for temperatures below 150 °C, an unusual feature for any solid-state ionic conductor. On the other hand, these results show a quite interesting similarity with the results by Riess et al. [37,38], obtained using cold-pressed pellets of YSZ micrometric powders exposed to moisture. Despite the lack of any sintering the authors observed in these pellets a significant lowtemperature conductivity which was strongly dependent on temperature and water partial pressure, with characteristics similar to those observed in our nanometric fully dense zirconia. They suggested an interpretation based on a conduction confined in chemically and physically adsorbed water molecules bound to the surface of the zirconia grains. They identified three different regimes: one at high temperature (T N 200 °C) where chemical adsorption prevails, one at intermediate temperatures (150 °C N T N 50 °C), characterized by a single layer physical adsorption, and one at low temperatures (T b 50 °C) where multilayer physical adsorption dominates. In the last two regimes an apparent negative activation energy is observed as a result of the strong dependence of the amount of absorbed water (and of charge carrier) on temperature, that reverses the intrinsic Arrhenius-like behavior. Despite these similarities it must be considered that Riess et al. [37,38] described the behavior of coldpressed powders, which had a relative density below 70%. Our samples, in contrast, are virtually fully dense, showing a relative density above 98% and a surface area (determined by BET) below 1 m2/g, a value that can be easily reconciled with the geometrical area of the sintered pellet and that excludes the presence of large amounts of open nanoporosity. The mechanism proposed by Riess et al. [37,38], on other hand, is probably responsible for the minimal level of conductivity observed in materials with micrometric grain size when exposed to a moist environment at temperatures below 50 °C. This conductivity, in fact, disappears if the lateral surface of the sample is coated with a polymeric paint, suggesting a conductivity confined to the external surface. At temperatures above 50 °C, however, a significant conductivity is observed only in samples with the grain size below 20 nm. This conductivity is retained also when the lateral surface of the sample is coated with the polymer, suggesting that in this case the conduction process involves the bulk of the sample. We suggest that this new conduction mechanism observed only in fully dense zirconia with grain size below 20 nm could be justified considering the possibility that in these samples the conduction is the result of an unusually fast movement of protons along the grain boundaries. In this respect the nanometric zirconia would behave very much like proton conducting polymers. It is important to note that the proposed mechanism is not in contradiction with the large literature that has demonstrated the blocking nature of grain boundary towards the ionic charge carrier in zirconia and perovskite oxides [36]. In this
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literature, which refers only to materials with micrometric grain size, the conduction is considered only across the grain boundary, while conduction along the grain boundary is not considered. However, in materials with grain size below 20 nm the density of grain boundaries is extremely high and transport along the grain boundaries might become dominant. Furthermore, in such materials a significant role might be played by the space charge regions adjacent to the grain boundaries. As the dimension of the space charge regions become comparable with the dimension of the grains, in fact, a relevant fraction of the material become characterized by defects distribution significantly different from the one characteristic of regular bulk materials. The concept of grain boundaries regions supporting a fast ionic transport mechanism is still poorly understood. Although no information on protonic diffusivity along the grain boundary is available, our results strongly suggest that grain boundaries can play a significant role in the protonic conductivity of nanocrystalline materials. Acknowledgments This work has been partially supported by a MIUR-FISR project of the Italian CNR, by the Fondazione Cariplo, and by the (US) National Science Foundation. References [1] [2] [3] [4] [5] [6] [7] [8]
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