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Enhanced low temperature sintering of (Sr, Cu)-doped lanthanum ferrite SOFC cathodes
Solid State Ionics 175 (2004) 79 – 81 www.elsevier.com/locate/ssi
Enhanced low temperature sintering of (Sr, Cu)-doped lanthanum ferrite SOFC cathodes S. Simner*, M. Anderson, J. Bonnett, J. Stevenson Pacific Northwest National Laboratory, Materials Sciences Department, Richland, WA 99352, United States
Abstract This study details subtle compositional modifications (marginal A-site deficiencies and Cu B-site dopant additions) to a La0.8Sr0.2FeO3 (LSF-20) cathode to enhance sintering below 1000 8C. The interaction of the modified LSF-20 compounds with YSZ below 1000 8C was also investigated. Initial single cell studies utilizing a (La0.8Sr0.2)0.98Fe0.98Cu0.02O3 cathode on an anode-supported YSZ cell have indicated power densities in the range 1.35–1.75 W/cm2 at 750 8C and 0.7 V. D 2004 Elsevier B.V. All rights reserved. Keywords: SOFC cathodes; LSF-20; YSZ
1. Introduction Lanthanum ferrite cathodes are typically used on anodesupported YSZ solid oxide fuel cells (SOFCs) with a doped ceria (Ce0.8Sm0.2O1.9—SDC-20) interlayer between the YSZ and LSF since the latter configuration indicates ~50% improvement in power density [1]. There are, however, concerns with respect to the inability to sinter the ceria layer to full density via an economic processing route, such as screen-printing. The poor SDC densification (~30 vol.% porosity) is related to the fact that this layer cannot be co-fired (at ~1400 8C) with the anode–YSZ substrate, since above 1200 8C SDC and YSZ interact to form a poorly conducting solid solution [2]. Hence, the SDC layer must be applied to the post-sintered substrate and is subsequently subjected to constrainment in the x–y plane. The presence of a high pore volume in the SDC implies a reduced cathode-SDC contact area and subsequently less than optimized performance. A recent study [3] considered the role of SDC interlayer, and concluded that it acts as a barrier to the diffusion of Zr4+ cations onto the B-site of the LSF perovskite structure. The presence of Zr in LSF results in decreased electrical
* Corresponding author. 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.09.016
conductivity of the cathode. Note that the LSF–YSZ interaction does not cause La- and/or Sr-zirconate formation. As expected, the Zr diffusion exhibits thermal dependence and is typically observed at temperatures z1000 8C. The current cathode of choice at PNNL, La0.8Sr0.2FeO3 (LSF-20), has an optimized sintering temperature of ~1150 8C. This present study considers changes in the LSF-20 chemistry to facilitate enhanced cathode sintering b1000 8C in an attempt to avoid the LSF–YSZ reaction. Previous work on Sr-doped LaFeO3 [1] and LaCrO3 [4] has indicated that slight A-site deficiencies, and the addition of 2–5 mol.% transition metal cations to the perovskite B-site can significantly enhance sinterability.
2. Experimental All lanthanum ferrite based powders used in this investigation were synthesized via a glycine–nitrate combustion technique [5] and subsequently calcined at 975 8C/2 h. 8-YSZ powders were commercially supplied by DAIICHI KIGENSO KAGAKU KOGYO. Sintering shrinkage analysis was conducted using a Unitherm Model 1161 vertical pushrod dilatometer. Room temperature X-ray diffraction (XRD) analysis of reacted cathode/8-YSZ powder mixtures was recorded using a Philips XRG3100 X-ray Generator.
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S. Simner et al. / Solid State Ionics 175 (2004) 79–81
Fig. 1. Sintering shrinkage data for (La0.8Sr0.2)0.98Fe1 x Cux O3 compositions where x=0.01, 0.02, 0.05 and 0.2.
Anode-electrolyte substrate processing, and single cell SOFC preparation and testing is represented in detail in a previous publication [6]. Post-synthesized/calcined cathode powders were ball milled for 12–24 h to achieve a particle size distribution of d 50 b1 Am and d 90 b2 Am, and subsequently mixed with organic binderss, applied to the anode–electrolyte substrate by screen-printing, and fired at 950 8C/2 h (~40 Am post-sintered thickness). Current– voltage data was recorded at 750 8C using an Arbin BT2000 potentiostat–galvanostat electrochemical testing system cells were held at 0.7 V and periodically subjected to current sweeps from 0 to 8 A. The 97% H2–3% H2O was flowed to the anode at 200 sccm and air to the cathode at 300 sccm.
3. Results and discussion 3.1. Sintering shrinkage characteristics of Cu-doped LSF-20 Fig. 1 shows the effects of Cu-dopant level on the sinterability of (La0.8Sr0.2)0.98Fe1 x Cux O3 compositions (where x=0.01–0.2) compared to the base La0.8Sr0.2FeO3 material. The most important criterion is that the sintering
Fig. 2. XRD data showing the formation of ZrO2 for a 8-YSZ-2 mol.% CuO mixture calcined at 975 8C/2 h.
onset temperature be sufficiently below 1000 8C. A total of 1 mol.% Cu dopant appears insufficient to facilitate significantly improved densification at low temperatures, whereas increasing the level to 2 mol.% adequately lowers the sintering onset temperature. Further sintering improvements are achieved by increasing the Cu content to 5 and 20 mol.%. 3.2. LSFCu–YSZ interaction Table 1 indicates additional phases produced after reaction (900–1000 8C) of the LSFCu compounds with a commercially obtained 8-YSZ. (La 0.8 Sr 0.2 ) 0.98 Fe 0.98 Cu 0.02 O 3 —from the sintering shrinkage data in Fig. 1, it was surmised that the slightly A-site depleted composition with 2 mol.% Cu addition would not be readily sinterable below 925 8C. At 925 8C (Fig. 1 and Table 1), there were no apparent reaction products with YSZ. However, on increasing the cathode/ YSZ reaction temperature to 950–1000 8C, a monoclinic ZrO2 appeared and ~7 wt.% of this phase was detected at 1000 8C. In a previous study [3], ZrO2 was not detected for reactions between (La0.8Sr0.2)x FeO3 (x=0.95–1.05) and
Table 1 XRD data for LSFCu cathodes reacted with 8-YSZ from 900 to 1000 8C Composition
Impurities in LSFCu calcined at 975 8C/2 h
LSF-YSZ (50–50 vol.%) reaction temperature (wt.% of phases other than LSF and YSZ) 900 8C
925 8C
950 8C
975 8C
1000 8C
(La0.8Sr0.2)0.98Fe0.98Cu0.02O3 (La0.8Sr0.2)0.98Fe0.95Cu0.05O3
None None
None T ZrO2
Ma ZrO2 NA
T Fe2O3 ? T La2Sr6Cu8O17.64 ?
M La2Zr2O7 M CuO M SrZrO3 La2CuO4 ?
T ZrO2 Ma ZrO2 T La2CuO4 SrZrO3 ? M La2Zr2O7 M CuO T ZrO2 M SrZrO3 La2CuO4 ?
M ZrO2 NA
(La0.8Sr0.2)0.98Fe0.8Cu0.2O3
None M ZrO2 T La2CuO4 SrZrO3 ? M La2Zr2O7 M CuO M SrZrO3 La2CuO4 ?
NA
NA
T=trace impurity (b2 wt.%), M=minor impurity (2–5 wt.%), Ma=major impurity (5–10 wt.%), ?=phase questionable, NA=not analyzed.
S. Simner et al. / Solid State Ionics 175 (2004) 79–81
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Fe0.98Cu0.02O3 cathode (sintered at 950 8C/2 h) over a period of 1000 h. Most notable is the observed bburn-inQ effect, that is, the significant enhancement in performance over the first 200 h to reach a power density of 1.6 W/cm2 at 750 8C/0.7 V. It should be noted that in six samples tested, the maximum power density (at 750 8C/0.7 V) ranged 1.35–1.75 W/cm2. The bburn-inQ was followed by a period of stability lasting ~300 h, and then rapid degradation. The bburn-inQ and degradation phenomena are still under investigation.
4. Conclusion
Fig. 3. Long-term performance for a (La0.8Sr0.2)0.98Fe0.98Cu0.02O3 cathode sintered directly onto an anode-supported YSZ membrane.
8-YSZ. Hence, it appears likely that the precipitation of the monoclinic ZrO2 is related to the presence of Cu. Fig. 2 shows an XRD trace of an 8-YSZ sample mixed with 2 mol.% CuO and reacted at 975 8C that indicates ZrO2 formation (note that the CuO is not detected by XRD in the baseline trace since it is added in such a small proportion). Thus, it appears likely that the presence of Cu causes a shift in the phase boundaries in the ZrO2–Y2O3 system, resulting in ZrO2 precipitation. A particular concern with respect to the presence of the monoclinic zirconia phase is that undoped ZrO2 exhibits very poor ionic conductivity. However, as detailed later in the text, this cathode composition indicated very high power densities at 750 8C. (La0.8Sr0.2)0.98Fe0.95Cu0.05O3—the increased Cu content caused precipitation of the monoclinic ZrO2 at 900 8C and, from 925 to 950 8C, the proportion of this phase increased, in addition to the formation of another trace phase, which could not be positively identified. Only one peak (presumably the 100% peak unless other more prominent peaks were hidden by the LSF or YSZ traces) was visible for this unknown phase, and PDF matching indicated a possible match with La1.8Sr0.2CuO4. (La0.8Sr0.2)0.98Fe0.8Cu0.2O3—as expected increasing the Cu content further decreased the temperature at which LSF– YSZ chemically interacted to form secondary phases. Even as low as 900 8C, significant amounts of La2Zr2O7 and SrZrO3 were detected in addition to CuO. No ZrO2 was precipitated until 950 8C. Hence, although higher Cu proportions enhanced low temperature sintering, as little as 5 mol.% Cu induced significant instability with respect to chemical interaction with YSZ. 3.3. Initial cell performance studies Fig. 3 shows the long-term performance of a Ni/YSZ– YSZ anode-supported single cell with a (La0.8Sr0.2)0.98
A Sr-doped lanthanum ferrite composition (with 2 mol.% Cu-doping and 2 mol.% A-site deficiency) has indicated enhanced sinterability below 1000 8C, and as such can be sintered onto YSZ without significant Zr4+ diffusion into the perovskite. However, the Cu-doped composition does exhibit chemical interaction with YSZ (typically above 950 8C) resulting in the precipitation of monoclinic zirconia. Despite the observed presence of ZrO2, initial cell testing utilizing the (La0.8Sr0.2)0.98Fe0.98Cu0.02O3 cathode has indicated high power densities (1.35–1.75 W/cm2 at 750 8C/0.7 V). Studies to improve our understanding of the phenomena related to this performance, as well as the observed cell bburn-inQ, are in progress.
Acknowledgement The work summarized in this paper was funded as part of the Solid-State Energy Conversion Alliance (SECA) Core Technology Program by the U.S. Department of Energy’s National Energy Technology Laboratory (NETL). PNNL is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC0676RLO 1830.
References [1] S.P. Simner, J.W. Stevenson, K.D. Meinhardt, N.L. Canfield, SOFCVII, in: S.C. Singhal, M. Dokiya (Eds.), The Electrochemical Society Proceedings Series, 2001, p. 1051, PV 2001-16, Pennington, NJ. [2] A. Tsoga, A. Gupta, A. Naoumidis, P. Nikolopoulos, Acta Mater. 48 (2000) 4709. [3] S.P. Simner, J.P. Shelton, M.D. Anderson, J.W. Stevenson, Solid State Ionics (2003) (in press). [4] S.P. Simner, J.S. Hardy, J.W. Stevenson, T.R. Armstrong, in: S.C. Singhal, M. Dokiya (Eds.), SOFC-VI, The Electrochemical Society Proceedings Series, 1999, p. 696, PV 99-19, Pennington, NJ. [5] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6. [6] S.P. Simner, J.F. Bonnett, N.L. Canfield, K.D. Meinhardt, J.P. Shelton, V.L. Sprenkle, J.W. Stevenson, J. Power Sources 113 (2003) 1.
Enhanced low temperature sintering of (Sr, Cu)-doped lanthanum ferrite SOFC cathodes S. Simner*, M. Anderson, J. Bonnett, J. Stevenson Pacific Northwest National Laboratory, Materials Sciences Department, Richland, WA 99352, United States
Abstract This study details subtle compositional modifications (marginal A-site deficiencies and Cu B-site dopant additions) to a La0.8Sr0.2FeO3 (LSF-20) cathode to enhance sintering below 1000 8C. The interaction of the modified LSF-20 compounds with YSZ below 1000 8C was also investigated. Initial single cell studies utilizing a (La0.8Sr0.2)0.98Fe0.98Cu0.02O3 cathode on an anode-supported YSZ cell have indicated power densities in the range 1.35–1.75 W/cm2 at 750 8C and 0.7 V. D 2004 Elsevier B.V. All rights reserved. Keywords: SOFC cathodes; LSF-20; YSZ
1. Introduction Lanthanum ferrite cathodes are typically used on anodesupported YSZ solid oxide fuel cells (SOFCs) with a doped ceria (Ce0.8Sm0.2O1.9—SDC-20) interlayer between the YSZ and LSF since the latter configuration indicates ~50% improvement in power density [1]. There are, however, concerns with respect to the inability to sinter the ceria layer to full density via an economic processing route, such as screen-printing. The poor SDC densification (~30 vol.% porosity) is related to the fact that this layer cannot be co-fired (at ~1400 8C) with the anode–YSZ substrate, since above 1200 8C SDC and YSZ interact to form a poorly conducting solid solution [2]. Hence, the SDC layer must be applied to the post-sintered substrate and is subsequently subjected to constrainment in the x–y plane. The presence of a high pore volume in the SDC implies a reduced cathode-SDC contact area and subsequently less than optimized performance. A recent study [3] considered the role of SDC interlayer, and concluded that it acts as a barrier to the diffusion of Zr4+ cations onto the B-site of the LSF perovskite structure. The presence of Zr in LSF results in decreased electrical
* Corresponding author. 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.09.016
conductivity of the cathode. Note that the LSF–YSZ interaction does not cause La- and/or Sr-zirconate formation. As expected, the Zr diffusion exhibits thermal dependence and is typically observed at temperatures z1000 8C. The current cathode of choice at PNNL, La0.8Sr0.2FeO3 (LSF-20), has an optimized sintering temperature of ~1150 8C. This present study considers changes in the LSF-20 chemistry to facilitate enhanced cathode sintering b1000 8C in an attempt to avoid the LSF–YSZ reaction. Previous work on Sr-doped LaFeO3 [1] and LaCrO3 [4] has indicated that slight A-site deficiencies, and the addition of 2–5 mol.% transition metal cations to the perovskite B-site can significantly enhance sinterability.
2. Experimental All lanthanum ferrite based powders used in this investigation were synthesized via a glycine–nitrate combustion technique [5] and subsequently calcined at 975 8C/2 h. 8-YSZ powders were commercially supplied by DAIICHI KIGENSO KAGAKU KOGYO. Sintering shrinkage analysis was conducted using a Unitherm Model 1161 vertical pushrod dilatometer. Room temperature X-ray diffraction (XRD) analysis of reacted cathode/8-YSZ powder mixtures was recorded using a Philips XRG3100 X-ray Generator.
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S. Simner et al. / Solid State Ionics 175 (2004) 79–81
Fig. 1. Sintering shrinkage data for (La0.8Sr0.2)0.98Fe1 x Cux O3 compositions where x=0.01, 0.02, 0.05 and 0.2.
Anode-electrolyte substrate processing, and single cell SOFC preparation and testing is represented in detail in a previous publication [6]. Post-synthesized/calcined cathode powders were ball milled for 12–24 h to achieve a particle size distribution of d 50 b1 Am and d 90 b2 Am, and subsequently mixed with organic binderss, applied to the anode–electrolyte substrate by screen-printing, and fired at 950 8C/2 h (~40 Am post-sintered thickness). Current– voltage data was recorded at 750 8C using an Arbin BT2000 potentiostat–galvanostat electrochemical testing system cells were held at 0.7 V and periodically subjected to current sweeps from 0 to 8 A. The 97% H2–3% H2O was flowed to the anode at 200 sccm and air to the cathode at 300 sccm.
3. Results and discussion 3.1. Sintering shrinkage characteristics of Cu-doped LSF-20 Fig. 1 shows the effects of Cu-dopant level on the sinterability of (La0.8Sr0.2)0.98Fe1 x Cux O3 compositions (where x=0.01–0.2) compared to the base La0.8Sr0.2FeO3 material. The most important criterion is that the sintering
Fig. 2. XRD data showing the formation of ZrO2 for a 8-YSZ-2 mol.% CuO mixture calcined at 975 8C/2 h.
onset temperature be sufficiently below 1000 8C. A total of 1 mol.% Cu dopant appears insufficient to facilitate significantly improved densification at low temperatures, whereas increasing the level to 2 mol.% adequately lowers the sintering onset temperature. Further sintering improvements are achieved by increasing the Cu content to 5 and 20 mol.%. 3.2. LSFCu–YSZ interaction Table 1 indicates additional phases produced after reaction (900–1000 8C) of the LSFCu compounds with a commercially obtained 8-YSZ. (La 0.8 Sr 0.2 ) 0.98 Fe 0.98 Cu 0.02 O 3 —from the sintering shrinkage data in Fig. 1, it was surmised that the slightly A-site depleted composition with 2 mol.% Cu addition would not be readily sinterable below 925 8C. At 925 8C (Fig. 1 and Table 1), there were no apparent reaction products with YSZ. However, on increasing the cathode/ YSZ reaction temperature to 950–1000 8C, a monoclinic ZrO2 appeared and ~7 wt.% of this phase was detected at 1000 8C. In a previous study [3], ZrO2 was not detected for reactions between (La0.8Sr0.2)x FeO3 (x=0.95–1.05) and
Table 1 XRD data for LSFCu cathodes reacted with 8-YSZ from 900 to 1000 8C Composition
Impurities in LSFCu calcined at 975 8C/2 h
LSF-YSZ (50–50 vol.%) reaction temperature (wt.% of phases other than LSF and YSZ) 900 8C
925 8C
950 8C
975 8C
1000 8C
(La0.8Sr0.2)0.98Fe0.98Cu0.02O3 (La0.8Sr0.2)0.98Fe0.95Cu0.05O3
None None
None T ZrO2
Ma ZrO2 NA
T Fe2O3 ? T La2Sr6Cu8O17.64 ?
M La2Zr2O7 M CuO M SrZrO3 La2CuO4 ?
T ZrO2 Ma ZrO2 T La2CuO4 SrZrO3 ? M La2Zr2O7 M CuO T ZrO2 M SrZrO3 La2CuO4 ?
M ZrO2 NA
(La0.8Sr0.2)0.98Fe0.8Cu0.2O3
None M ZrO2 T La2CuO4 SrZrO3 ? M La2Zr2O7 M CuO M SrZrO3 La2CuO4 ?
NA
NA
T=trace impurity (b2 wt.%), M=minor impurity (2–5 wt.%), Ma=major impurity (5–10 wt.%), ?=phase questionable, NA=not analyzed.
S. Simner et al. / Solid State Ionics 175 (2004) 79–81
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Fe0.98Cu0.02O3 cathode (sintered at 950 8C/2 h) over a period of 1000 h. Most notable is the observed bburn-inQ effect, that is, the significant enhancement in performance over the first 200 h to reach a power density of 1.6 W/cm2 at 750 8C/0.7 V. It should be noted that in six samples tested, the maximum power density (at 750 8C/0.7 V) ranged 1.35–1.75 W/cm2. The bburn-inQ was followed by a period of stability lasting ~300 h, and then rapid degradation. The bburn-inQ and degradation phenomena are still under investigation.
4. Conclusion
Fig. 3. Long-term performance for a (La0.8Sr0.2)0.98Fe0.98Cu0.02O3 cathode sintered directly onto an anode-supported YSZ membrane.
8-YSZ. Hence, it appears likely that the precipitation of the monoclinic ZrO2 is related to the presence of Cu. Fig. 2 shows an XRD trace of an 8-YSZ sample mixed with 2 mol.% CuO and reacted at 975 8C that indicates ZrO2 formation (note that the CuO is not detected by XRD in the baseline trace since it is added in such a small proportion). Thus, it appears likely that the presence of Cu causes a shift in the phase boundaries in the ZrO2–Y2O3 system, resulting in ZrO2 precipitation. A particular concern with respect to the presence of the monoclinic zirconia phase is that undoped ZrO2 exhibits very poor ionic conductivity. However, as detailed later in the text, this cathode composition indicated very high power densities at 750 8C. (La0.8Sr0.2)0.98Fe0.95Cu0.05O3—the increased Cu content caused precipitation of the monoclinic ZrO2 at 900 8C and, from 925 to 950 8C, the proportion of this phase increased, in addition to the formation of another trace phase, which could not be positively identified. Only one peak (presumably the 100% peak unless other more prominent peaks were hidden by the LSF or YSZ traces) was visible for this unknown phase, and PDF matching indicated a possible match with La1.8Sr0.2CuO4. (La0.8Sr0.2)0.98Fe0.8Cu0.2O3—as expected increasing the Cu content further decreased the temperature at which LSF– YSZ chemically interacted to form secondary phases. Even as low as 900 8C, significant amounts of La2Zr2O7 and SrZrO3 were detected in addition to CuO. No ZrO2 was precipitated until 950 8C. Hence, although higher Cu proportions enhanced low temperature sintering, as little as 5 mol.% Cu induced significant instability with respect to chemical interaction with YSZ. 3.3. Initial cell performance studies Fig. 3 shows the long-term performance of a Ni/YSZ– YSZ anode-supported single cell with a (La0.8Sr0.2)0.98
A Sr-doped lanthanum ferrite composition (with 2 mol.% Cu-doping and 2 mol.% A-site deficiency) has indicated enhanced sinterability below 1000 8C, and as such can be sintered onto YSZ without significant Zr4+ diffusion into the perovskite. However, the Cu-doped composition does exhibit chemical interaction with YSZ (typically above 950 8C) resulting in the precipitation of monoclinic zirconia. Despite the observed presence of ZrO2, initial cell testing utilizing the (La0.8Sr0.2)0.98Fe0.98Cu0.02O3 cathode has indicated high power densities (1.35–1.75 W/cm2 at 750 8C/0.7 V). Studies to improve our understanding of the phenomena related to this performance, as well as the observed cell bburn-inQ, are in progress.
Acknowledgement The work summarized in this paper was funded as part of the Solid-State Energy Conversion Alliance (SECA) Core Technology Program by the U.S. Department of Energy’s National Energy Technology Laboratory (NETL). PNNL is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC0676RLO 1830.
References [1] S.P. Simner, J.W. Stevenson, K.D. Meinhardt, N.L. Canfield, SOFCVII, in: S.C. Singhal, M. Dokiya (Eds.), The Electrochemical Society Proceedings Series, 2001, p. 1051, PV 2001-16, Pennington, NJ. [2] A. Tsoga, A. Gupta, A. Naoumidis, P. Nikolopoulos, Acta Mater. 48 (2000) 4709. [3] S.P. Simner, J.P. Shelton, M.D. Anderson, J.W. Stevenson, Solid State Ionics (2003) (in press). [4] S.P. Simner, J.S. Hardy, J.W. Stevenson, T.R. Armstrong, in: S.C. Singhal, M. Dokiya (Eds.), SOFC-VI, The Electrochemical Society Proceedings Series, 1999, p. 696, PV 99-19, Pennington, NJ. [5] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6. [6] S.P. Simner, J.F. Bonnett, N.L. Canfield, K.D. Meinhardt, J.P. Shelton, V.L. Sprenkle, J.W. Stevenson, J. Power Sources 113 (2003) 1.