Microstructure and electrical conductivity of nanocrystalline nickeland nickel oxide/gadolinia-doped ceria thin films

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1 Available online at Acta Materialia 56 (2008) Microstructure and electrical conductivity of nanocrystalline nickeland nickel oxide/gadolinia-doped ceria thin films Ulrich P. Muecke, Silvio Graf, Urs Rhyner, Ludwig J. Gauckler ETH Zurich, Department of Materials, Nonmetallic Inorganic Materials, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland Received 13 April 2007; received in revised form 14 September 2007; accepted 18 September 2007 Available online 31 December 2007 Abstract NiO/Ce 0.8 Gd 0.2 O 1.9 x (NiO/CGO) films with thicknesses between 150 and 800 nm were prepared by spray pyrolysis. Average grain sizes were nm after annealing in air at C. After reduction, the Ni/CGO cermet microstructures were stable up to temperatures of 600 C for grain sizes of. Nickel coarsening was observed for films with smaller grains. The development of a rigid CGO grain network helped to prevent nickel growth. The electrical conductivities of the films were comparable to state-of-the-art Ni YSZ cermets and reached 3000 S cm 1 at 600 C. Films with stable microstructures showed no degradation in electrical conductivity over 1400 h at 570 C and upon thermal cycling. A transition from three-dimensional metallic percolation of cermets with small grains and large thickness to two-dimensional percolation for films with grain sizes in the range of the layer thickness was observed. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Thin films; Electrical conductivity; Cermets; Grain size; Nanocomposite 1. Introduction Miniaturized solid oxide fuel cells (l-sofcs) operating at temperatures of C are currently under investigation as a replacement for batteries in portable electronic devices [1 3]. Thin film deposition methods such as sol gel [4,5], sputtering [6,7], spray pyrolysis [8 10] or pulsed laser deposition [11,12] can be used for the preparation of the anode, electrolyte [13] or cathode layers. The thicknesses of the individual layers are usually <1 lm, reducing the ohmic resistance of the cell and compensating for the reduced conductivities at lower temperatures in the case of the cathode and electrolyte. Thin film electrolytes [13,14] and cathodes [15] have already been studied for use in traditional SOFCs or as model electrodes, respectively. However, little is known about the microstructure and electrical properties of thin film anodes with grain sizes in the nanometer range. Corresponding author. Tel.: ; fax address: ulrich.muecke@mat.ethz.ch (U.P. Muecke). State-of-the-art SOFC anodes are made of a mixture of nickel and yttria-stabilized zirconia (YSZ) or cerium gadolinium oxide (CGO), forming a two-phase cermet material with an electronically conductive metallic phase and an ionically or mixed conducting ceramic phase. The ceramic network provides a pathway for oxygen ions and the metal a pathway for electrons. The cermet itself is electronically conductive above a certain volume fraction of metallic phase (percolation limit) [16]. The percolation limit of Ni YSZ/CGO anodes sintered from powders is usually around 30 vol.% Ni in the solid phase [17]. The average grain size of NiO and YSZ/CGO in stateof-the-art anodes for operating temperatures between 800 and 950 C is in the micron range, with layer thicknesses of several hundred micrometers [18]. Thin film anodes are much thinner, with thicknesses of <1 lm and grain sizes below 100 nm. Coarsening of the metal grains in cermet anodes leads to a degradation of electrical conductivity and electrochemical performance over time [19,20]. The metal growth is expected to be more pronounced in thin film anodes because of the larger driving forces associated with smaller metal grains /$30.00 Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi: /j.actamat

2 678 U.P. Muecke et al. / Acta Materialia 56 (2008) The grain growth of pure nanocrystalline nickel films with starting grain sizes of nm is a widely investigated phenomenon. The growth of the metal grains usually occurs in two regimes: at temperatures below a certain threshold, the grain size is fairly stable and increases only gradually with temperature, whereas above the threshold, rapid growth to microcrystalline material is observed. The threshold temperature depends on the melting temperature and lies around times T melting [21]. For nickel, the threshold temperature was found to be around 500 K [22,23]. The grain size can be stabilized by introducing impurities or solutes such as sulphur [24] or silicon [25] into the metal, which results in slower grain growth due to the solute drag effect at the grain boundaries. However, the effect is relatively small with a deviation from the threshold temperature of 30 C in the case of Ni Si [25]. By alloying nickel with 15 50% iron, the threshold temperature increases by K [22]. The largest stabilization effect is expected from the addition of ceramic grains to the nickel matrix which leads to pinning of the metallic grain triple points. However, the thermal stability of nanocrystalline cermets remains a key issue. The aim of this paper was, therefore, to prepare Ni CGO cermets with grain sizes between 5 and 50 nm and to study the microstructure and electrical conductivity as a function of initial grain size, film thickness, film composition, operating temperature and operating time. 2. Experimental NiO CGO thin films with thicknesses between 150 and 800 nm were prepared by air blast spray pyrolysis. Spraypyrolyzed films are generally amorphous after deposition [26] and can be crystallized in a post-deposition annealing step. The average grain sizes after annealing in air were measured as a function of the annealing temperature and of the NiO/CGO ratio. The samples were then reduced to form Ni/CGO cermet thin films and the microstructures and electrical conductivities were determined for different Ni/CGO ratios, layer thicknesses, grain sizes and temperatures. Thermal cycling under reducing conditions and redox-cycling was performed while measuring the electrical conductivity Film preparation Grain size analysis samples A detailed description of the spray pyrolysis setup used to deposit films for the NiO/CGO grain size analysis, denoted spray setup 1 in the following, is given elsewhere [26,27]. The distance between the spray nozzle tip and substrate surface (working distance) was 45 cm, the air pressure 1.0 bar and the precursor flow rate 30 ml h 1 in this study. The majority of the sprayed precursor volume was transported to the substrate by droplets with diameters of lm [28]. The deposition time was 180 min, resulting in films with thicknesses of 400 ± 100 nm. All films were deposited on dense polycrystalline 8-YSZ substrates made by tape-casting (Kerafol, Stegenthumbach, Germany). The compositions of the films were chosen to result in a Ni/Ce 0.8 Gd 0.2 O 2 x volumetric ratio of 20/80, 40/60, 60/ 40, 80/20 and 100/0 (pure nickel) after reduction, corresponding to a NiO/CGO weight ratio of 28.2/71.8, 51.1/ 48.9,70.2/29.8, 86.3/13.7 and 100/0, respectively, in the oxidized state. For convenience, the composition of oxidized samples will be given as Ni(O)/CGO, e.g. 60/40 Ni(O)/ CGO denotes a sample where the NiO is not reduced yet but the composition corresponds to a cermet in the reduced state with a volumetric ratio of Ni to CGO of (70.2/ 29.8 wt.% NiO/CGO). The precursors were prepared by dissolving nickel(ii) nitrate hexahydrate (98% purity, Fluka, Buchs, Switzerland), cerium(iii) nitrate hexahydrate (99.5%, Alfa Aesar, Karlsruhe, Germany) and gadolinium(iii) chloride hexahydrate (99.9%, Alfa Aesar) in the corresponding stoichiometry in a mixture of 33:33:33 vol.% of ethanol (99.8%, Merck, Darmstadt, Germany), 1-methoxy-2-propanol (99%, Fluka) and diethylene glycol mono n-butyl ether acetate (98%, Fluka). The crystal water content of the salts was verified by thermogravimetry before weighing. The total salt concentration was 0.1 mol l Electrical conductivity samples The samples for electrical conductivity measurements were prepared with a different spray setup, denoted spray setup 2. This setup allowed a more precise control of the thickness of the films. Details of the setup were given previously [10]. The working distance was 39 cm, the air pressure was 1.0 bar and the precursor flow rate was 5 ml h 1 in this study. The majority of the sprayed precursor volume was transported by droplets with diameters between 10 and 50 lm. The deposition time was varied between 30 and 150 min, resulting in films with thicknesses between 150 and 800 nm. The same precursor as given above was used for the depositions except that the salts were dissolved in a mixture of 10:90 vol.% ethanol:tetraethylene glycol (99%, Aldrich, Steinheim, Germany). The changed composition of the organic solvents allows the preparation of films with a larger maximum crack-free film thickness [10]. All films were deposited on 1 mm thick, 35 mm diameter sapphire single crystals (Stettler, Lyss, Switzerland) with a ð1120þ orientation parallel to the surface. The substrates were cleaned with ethanol prior to deposition. The films were deposited through a molybdenum shadow mask with a thickness of 0.1 mm, resulting in a rectangular area of film approximately 14 mm wide and 28 mm long Temperature treatment All films were dense and amorphous after deposition. They were crystallized by annealing in air at 600, 800, 1000 or 1200 C for 10 h with a heating and cooling rate of 3 C min 1 after the deposition. The dwell time of 10 h

3 U.P. Muecke et al. / Acta Materialia 56 (2008) was chosen to establish a stable grain size and to fully crystallize the films [29] Conductivity measurements After annealing in air, four parallel platinum stripes of approximately 150 nm thickness and 1 mm width were sputtered (SCD 050, Balzers, FL, USA) across the film through a 0.1 mm thick molybdenum mask. The distance between the two inner stripes was 5 mm and between each of the two outer stripes 4 mm. Four flat-pressed platinum wires with a diameter of 0.25 mm (thermocouple grade, Alfa-Aesar) were attached to the stripes with Pt paste (C3605 P, Heraeus, Hanau, Germany). The wires were fixed to the sapphire substrate with two-component ceramic glue (Feuerfestkitt, Firag AG, Ebmatingen, Switzerland). For a schematic drawing of the geometry, see Ref. [30]. The as-prepared samples were heated in air to 650 C for 2 h with a heating and cooling rate of 3 C min 1 prior to testing to sinter the Pt paste and to harden the ceramic glue. The samples were then heated in a sealed furnace under a dry H 2 /N 2 atmosphere from room temperature to the measurement temperature at a heating rate of 3 C min 1. The gas flow rate of N 2 (quality 5.0, Pangas, Dagmarsellen, Switzerland) was 85 sccm and that of H 2 (quality 5.0, Pangas) 50 sccm, resulting in an oxygen partial pressure in the furnace of approximately Pa at 600 C. The total in-plane conductivity of the samples was measured with a multimeter (model 2700/7700, Keithley, Cleveland, OH, USA) in four-point mode. The reduction to Ni/CGO occurred between 310 and 350 C, and was marked by a sharp increase in conductivity. The temperature was measured with a type K thermocouple (MTS Messtechnik, Schaffhausen, Switzerland) that was in contact with the sample and a voltmeter (model 2182, Keithley). All conductivity values were recorded after 20 h at the operating temperature Characterization The surface morphologies and compositions of the films were analyzed by scanning electron microscopy (SEM) using a LEO 1530 microscope (Carl Zeiss SMT, Oberkochen, Germany) with an energy dispersive X-ray (EDX) detector (Thermo Noran Vantage and Tracor Northern, USA). Film thicknesses were measured after film deposition and before annealing by scratching off a small area of the film with a scalpel and measuring the step height between the substrate and an averaged height within about 1 mm scan length of the film with a surface profiler (Alpha Step 500, KLA Tencor, San Jose, CA, USA). Thermogravimetric measurements were performed in a mixture of 21 vol.% oxygen and 79 vol.% argon with a heating rate of 10 C min 1 (STA 449 C Jupiter, Netzsch, Selb, Germany). Table 1 Calculated and measured composition of films after annealing by EDX Ni(O)/CGO Calculated (mol.%) Measured (mol.%) Ni Ce Gd Ni Ce Gd 20/ / / The average grain size was determined from SEM topview micrographs by measuring 300 intersection lengths at two different spots on each sample with the program LINCE [31]. The interception lengths were multiplied by a grain size conversion factor of 1.56 [32]. The orientations of the intersection lines were chosen randomly. Reproducibility of the grain size measurements at different spots on the same sample was better than 10%. The reported grain sizes represent an averaged value of the NiO and CGO phase because a differentiation between the nanometer-sized NiO and CGO grains was not possible by EDX or SEM. The two-phase microstructure appeared homogeneous in the SEM images and no distinctly different size fractions were observable for all annealing temperatures. All error bars on grain size data represent the standard deviation of the grain size data of a single sample. Additionally, the grain sizes of 15 different samples with a composition of 60/40 vol.% Ni(O)/CGO after reduction, all annealed at 1000 C, were analyzed individually. The sample-to-sample grain sizes did not deviate by more than the standard deviation. The grain size of some samples was additionally verified by X-ray diffraction (XRD) measurements. The NiO grain size was calculated with the Scherrer equation from the (1 1 1) peak, similar to the technique described in Ref. [29]. The composition of the samples was evaluated by EDX and thermogravimetry. The measured molar atom ratios by EDX analysis corresponded well with the theory within the experimental limitations of the method (Table 1). For thermogravimetric analysis, a 60/40 Ni(O)/CGO film was scratched off the surface after deposition and heated in a 3 vol.% H 2 in Ar mixture at 10 K min 1. The reduction of NiO to metallic Ni started around 350 C and the sample was fully reduced at 550 C with a total mass change of 15.3%. This value corresponds well to a predicted mass change of 15.03%. It can, therefore, be concluded that the film compositions matched the theoretical values within experimental errors. 3. Results and discussion 3.1. Grain size analysis As mentioned earlier, anode performance can degrade through grain coarsening, which is highly dependent on grain size. The average grain sizes of oxidized NiO/ CGO composites were, therefore, measured by SEM for compositions of 20/80, 40/60, 60/40, 80/20 and 100/0

4 680 U.P. Muecke et al. / Acta Materialia 56 (2008) Ni(O)/CGO and annealing temperatures of 800, 1000 and 1200 C. Grain size data for pure CGO films prepared with the same setup as in this work were taken from Rupp et al. [29]. The average grain size at 800 C depended on the composition and decreased gradually from 30 nm for singlephase CGO to 15 nm for the 80/20 Ni(O)/CGO sample and then sharply increased again to 90 nm for the singlephase NiO (Fig. 1). The same trend was observed at 1000 C with a gradual decrease from 80 nm for pure CGO to 50 nm of 80/20 Ni(O)/CGO and an increase to 265 nm for pure NiO. At 1200 C, the grain size increased slightly from 220 nm for pure CGO to 350 at 80/20 and to 730 for pure NiO. The grain sizes of two 60/40 Ni(O)/CGO samples annealed at 800 and 1000 C were additionally measured by means of XRD and were found to be 21 and 51 nm, respectively. These values compare well to the SEM grain size analysis of 16 and. The grain size values larger than approximately 200 nm, e.g. the values for pure NiO and all compositions at 1200 C have to be considered with care because the average grain size approaches the layer thickness, possibly resulting in a non-isotropic grain growth. Grain size trends similar to those seen at 800 and 1000 C were reported by Min Park et al. for NiO YSZ cermets from micron-sized powders [33]. The average grain size decreased with increasing NiO content, went through a minimum and then reached a maximum for the singlephase NiO. As expected [34], the introduction of NiO as secondary phase into CGO or vice versa hinders the grain growth compared to the pure CGO or NiO phase and smaller average grain sizes are observed. All the above results are for NiO CGO film deposited with spray setup 1 on YSZ substrates. However, all electrical conductivity samples were sprayed on sapphire with setup 2 and a precursor with a different organic composition. To check if the grain size changed between setups and solvents, films with a composition of 60/40 Ni(O)/ CGO were sprayed on YSZ and sapphire with setup 2. The grain size of the films with setup 2 on YSZ was 12 ± 3 and 57 ± 18 nm for samples annealed at 800 and 1000 C, respectively. These values compare well to the 16 and for setup 1. Films deposited on sapphire with setup 2 and annealed at 1000 C showed an average grain size of 41 ± 6 nm. Although smaller than the 53 ± 14 nm measured for setup 1 on YSZ, the values are within the standard deviation. In the following it is, therefore, assumed that the grain sizes of the films on sapphire are the same as those on YSZ Percolation limit of Ni in the Ni/CGO cermet For application as thin film anodes it is important that the cermets are electronically conductive, which requires a percolating nickel phase. The conductivity of samples with different compositions and different average grain sizes was measured to determine the percolation limit of fine-grained cermet thin films. Grain sizes of 5 ± 5 (Fig. 4a), 18 ± 4 and 52 ± 10 nm (Fig. 1) were established by annealing in air at 600,800 and 1000 C, respectively. The grain sizes were assumed to be roughly constant within the compositional range from 25/75 to 60/40 Ni(O)/CGO. The film thickness of all samples was 550 ± 100 nm and the measurements were performed at 603 ± 2 C under reducing conditions. The total conductivities were approximately constant for compositions up to 25/75 Ni/CGO (Fig. 2). The conductivity of the films below the percolation limit was about one order of magnitude larger than that of micron sized state-of-the-art Ni YSZ [17,35,36]. At 600 C, the 1000 CGO content [vol.%] CGO [vol.%] average grain size [nm] C 1000 C 800 C Ni content after reduction [vol.%] Fig. 1. Average grain size of NiO/CGO thin films on YSZ as a function of annealing temperature and film composition. Dwell time was 10 h at the given temperatures. Data for pure CGO was taken from Rupp et al. [29]. total conductivity [S/m] T = 603±2ºC thickness = 550±100 nm this study, ~5nm (Ts=600 C) this study, 16nm (Ts=800 C) this study, 53nm (Ts=1000 C) CGO, Rupp et al. Ni-SDC, Yin et al. Ni-YSZ, Dees et al Ni [vol.%] Fig. 2. Percolation limit of Ni/CGO cermet thin films with different grain sizes [17,30,40].

5 U.P. Muecke et al. / Acta Materialia 56 (2008) conductivity of nanocrystalline CGO with a grain size of 78 nm was 52 S m 1 at an oxygen partial pressure of Pa, compared to 0.29 S m 1 in air [30]. The larger conductivity under reducing conditions was, therefore, due to the electronic conductivity of the CGO. At the percolation limit, the conductivities increased by 1 3 orders of magnitude for the 40/60 and 50/50 Ni/CGO films, and the films were electronically conductive with a percolating nickel phase. The metallic conductivity was indicated by an increase in conductivity of about half an order of magnitude upon cooling from 603 C to room temperature. At first glance the percolation limit would be expected to be independent of grain size in the case of an isotropic material. However, the conductivities of the 18 ± 4 nm grain size films were always orders of magnitude higher than those of the 52 ± 10 nm films for compositions of 40/60 and 50/50 Ni/CGO. At 60/ 40 Ni/CGO, the difference in conductivity for samples of all grain sizes vanished and the conductivity was comparable to that of a nickel film prepared by reducing a dense NiO film. When comparing the thickness of 550 ± 100 nm to the average grain size of 52 ± 10 nm for a film annealed at 1000 C it becomes obvious that a thin film cermet is only composed of about 8 10 grain layers compared to several hundreds for a bulk sample. Including porosity, the number of effective grain layers decreases even further. The percolation threshold value, therefore, approaches that of a two-dimensional (2D) situation rather than a three-dimensional (3D) one. Percolation theory predicts different percolation limits for 2D and 3D networks. The 2D percolation limit of randomly packed insulating and conducting spheres is vol.% of conducting spheres in the solids fraction [37,38] and the 3D limit vol.% [39]. The transition from 2D to 3D percolation, therefore, explains the different conductivities of samples with varying grain sizes around the percolation limit Electrical conductivity in a cermet: transition from 2D to 3D percolation as a function of film thickness If a transition from 2D to 3D percolation occurs in thin cermet films, it should also be possible to cross the percolation boundary by varying the film thickness of samples with a composition between the 2D and 3D percolation limit with constant grain sizes. Films with a thickness between 120 and 750 nm and a composition of 60/40, 50/ 50 and 60/40 vol.% Ni/CGO were prepared and the conductivity was measured at 603 C under reducing conditions. The grain sizes were 18 ± 4 and 52 ± 10 nm for each composition and thickness. As expected, the conductivity for all samples with grain sizes of 18 nm was constant and metallic (Fig. 3) for all thicknesses because the average grain size is always several times smaller than the layer thickness. However, for samples with a grain size of 52 nm, a percolation threshold was observed at a thickness of 150 nm for the 60/40 samples (Fig. 3a), at 200 nm for the 50/50 samples (Fig. 3b), and at nm for the 40/60 samples (Fig. 3c). The total conductivity of the metallically conductive films with 52 nm grain size increased by two orders of magnitude from the 40/60 Ni/CGO samples to the 60/40 Ni/ CGO samples. The nickel content of the 40/60 Ni/CGO films was close to the percolation limit and there were comparatively few percolating nickel pathways in the total cross-sectional area of the film, whereas for the 60/40 Ni/ CGO films, more of these pathways were formed and the total conductivity was higher. It can be concluded that the in-plane conductivity of cermet thin films depends not only on the composition but also on the ratio between grain size and layer thickness. A percolating metallic phase is formed at low metal volume ratios if the grain size is small compared to the film thickness (3D case). Larger metal volume ratios are needed for percolation when the grain size is comparable to the layer thickness and the percolation limit approaches the value of 2D percolation. conductivity [S/m] a) 40/ nm 52 nm T = 603±2 C b) 50/50 T = 603±2 C c) 60/40 T = 603±2 C film thickness [nm] film thickness [nm] film thickness [nm] Fig. 3. Conductivity as a function of film thickness for samples with 18 (T anneal = 800 C) and 52 nm (T anneal = 1000 C) average grain size and a composition of (a) 40/60, (b) 50/50 and (c) 60/40 vol.% Ni/CGO. Samples that were not metallically conductive upon cooling are marked with asterisks. 18 nm 52 nm 18 nm 52 nm

6 682 U.P. Muecke et al. / Acta Materialia 56 (2008) Fig. 4. SEM images of the morphology of 60/40 Ni(O)- and Ni/CGO layers with different grain sizes before reduction (top views a, b, c) and after reduction at 600 C for 72 h (top views d, e, f and cross-sections g, h). Some nickel agglomerates are marked with arrows Nickel coarsening in the Ni/CGO cermet under reducing conditions Coarsening of the metallic phase is a phenomenon sometimes observed in cermet thin films [7]. The growth of large grains at the expense of small grains is due to the minimization of the Gibbs free surface energy and mainly depends on the grain size and temperature. In order to study the microstructural development of the Ni/CGO cermet films and especially the coarsening of the metallic nickel phase, samples with different grain sizes were reduced and their microstructure studied by SEM. 60/40 Ni(O)/CGO films with average grain sizes of 5,16 and in the oxidized state were reduced and dwelled for 72 h at 600 C under reducing conditions. The layers were dense after annealing in air (Fig. 4a c) and became porous during the reduction (Fig. 4d h). Metallic nickel was agglomerating inside and on top of the 5 nm grain sized sample. The nickel agglomerates within the film formed a 2D nickel network (see arrows in Fig. 4d). Nickel particles grew predominantly on top of the film for samples with thicknesses below nm and nickel agglomerates within the film were formed in films with thicknesses between 400 and 800 nm. Disconnected CGO particles were distributed on the surfaces of the nickel agglomerates for the 5 nm sample, indicating that a stable CGO network was not formed under these conditions. The grain size of the CGO particles also increased during the testing period. This increase can be understood by comparing the CGO grain size of 5 nm in the NiO/CGO sample to the grain size of a pure CGO film of 15 nm (Fig. 5). Upon reduction, porosity is introduced in the film. The CGO grains in the cermet then grow unobstructed from NiO or nickel particles up to the grain grain size [nm] CGO, Rupp et al. NiO-CGO, this work sintering temperature [ C] Fig. 5. Comparison of the average grain sizes of a 60/40 Ni(O)/CGO film and a CGO film [29].

7 U.P. Muecke et al. / Acta Materialia 56 (2008) size of the pure CGO film. As a consequence, the grain size of the CGO phase in the cermet remains constant only if the NiO/CGO grain size before reduction is larger than the grain size of pure CGO at the operating temperature of the cermet. The microstructure of the sample with 16 nm grain size was more homogeneous after reduction than the 5 nm sample (Fig. 4e) and nickel grains were not found on the surface. However, nickel started to form agglomerates within the layer (Fig. 4g). The CGO network was well connected and stable. The most homogeneous and stable morphology was obtained from the sample (Fig. 4f and h). Nickel and CGO grains were not distinguishable after reduction and no agglomerates were found. The CGO grains were well sintered together with necks connecting the grains being clearly visible. Pronounced nickel coarsening was observed in the extreme case of heating a 60/40 Ni/CGO samples with a grain size of to 800 C under reducing conditions. Nickel grains with a diameter of nm were found on top and within the cross-section of the films (Fig. 6) and the electronic in-plane conductivity was lost between 700 and 800 C. All microstructural observations can be summarized in a schematic diagram, as shown in Fig. 7. During annealing of the NiO/CGO film in air, both NiO and CGO grains grew with increasing temperature (Fig. 7a c left). At low annealing temperatures, i.e. 600 C, the grains were 5 nm in size (Fig. 7a left) and after reduction at a temperature similar to the annealing temperature, the Ni and CGO grains coarsened rapidly (Fig. 7a middle). In this case, the CGO grains did not form a rigid network and were pushed away by Fig. 6. (a) Top view and (b) cross-section SEM images of a 60/40 Ni/CGO sample after reduction at 800 C for 10 hours with large nickel grains on top and within the film. The initial grain size was (T anneal = 1000 C). c) 1000 C reduction at 600 C t = 10 h 600 C t > 1000 h grain size / annealing temperature in air 16 nm 800 C 5 nm 600 C b) a) reduction at 600 C t = 10 h reduction at 600 C t = 10 h stable Ni + CGO network stable Ni + CGO network 600 C t > 1000 h stable CGO network stable CGO network Ni coarsening in pores Ni coarsening CGO NiO Ni CGO grain growth Ni coarsening time under reducing atmosphere Fig. 7. Schematic of the different morphologies of nanocrystalline NiO CGO films and Ni CGO cermets as a function of grain size and operating times. See text for details.

8 684 U.P. Muecke et al. / Acta Materialia 56 (2008) growing Ni grains. Additionally, the CGO grain size increased because the thermodynamically stable grain size of pure CGO without restraining Ni or NiO particles around it is larger than in the NiO/CGO composite or Ni/CGO cermet. The electrical conductivity of the cermets degraded rapidly over time in this regime. However, the situation changed when the grain size in the oxidized state was increased to 16 nm by annealing in air at 800 C (Fig. 7b left). A stable CGO network was formed during annealing in air, and, upon reduction, the nickel grains coarsened only slightly until restrained by the CGO framework (Fig. 7b middle), leading to a stable cermet. During long-term operation of more than 1400 h (see text below for details), unconstrained nickel grains grew on the film surface. Consequently, the underlying areas of the cermet were depleted of nickel and the conductivity decreased (Fig. 7b right). A stable Ni and CGO network was formed when the grain size was increased again to by annealing at 1000 C in air (Fig. 7c left). The size of Ni and CGO grains under reducing conditions was stable at operating temperatures up to 600 C under reducing conditions (Fig. 7c middle). The ageing of electrical conductivity over 1400 h was negligible and no nickel agglomerates were observed on top or in the cross-section of the film (Fig. 7c right). The grain size of the cermet can, therefore, be engineered to the operating temperature to yield a dimensionally stable microstructure during operation. The NiO CGO grains need to be annealed to a sufficiently large grain size in the oxidized state so that nickel coarsening after reduction is suppressed. This grain size depends on the operating temperature and can be chosen to be smaller by reducing the operating temperature Long-term stability of electrical conductivity Microstructural changes of the cermet thin films were also reflected in long-term conductivity measurements. Two 60/40 Ni/CGO samples with grain sizes of 16 and were measured simultaneously in the same test rig over a period of 1950 h under reducing conditions. The thicknesses were 438 and 830 nm, respectively. The samples were heated to 410 C for 10 h, then cooled to 310,210 and 105 C for 50 h each and then reheated to 410,460 and 510 for again 50 h each, followed by isothermal dwelling at 560 ± 1 C for 1630 h. During the cooling steps the conductivity increased, indicating metallic behavior after reduction at 410 C (Fig. 8). The conductivities reached the same level as immediately after reduction upon reheating to 410 C and then increased upon heating to 460,510 and 560 C. The conductivity increased by approximately half an order of magnitude for the grain size sample, whereas it stayed roughly constant for the 16 nm grain size sample. The increases upon heating indicate that changes in the microstructure due to the grain growth of metallic nickel occurred. The initial nickel coarsening improved the connectivity between the metal grains and increased the overall conductivity. The nickel growth depended on the grain size, and stable structures were reached at 560 C for the grain size sample and at 410 C for the 16 nm grain size sample. At 560 C, the conductivity of the 16 nm grain size sample degraded about one order of magnitude over 1630 h, whereas the degradation rate of the grain size sample was very low, going from Sm 1 at 500 h to Sm 1 at 1000 h ( 0.85%/1000 h) and from at 1300 h to Sm 1 at 1900 h nm 1200 conductivity [S/m] nm 500 nm temperature [ C] nm time [h] Fig. 8. Long-term conductivity measurement of two 60/40 Ni/CGO cermet thin films with grain sizes of 16 (T anneal = 800 C) and (T anneal = 1000 C) and SEM top views of samples after testing. The interruption of the curves at 1270 h was caused by a failure in the gas supply.

9 U.P. Muecke et al. / Acta Materialia 56 (2008) (+0.36%/1000 h). The 16 nm grain size sample was not metallically conductive after cooling, whereas the grain size sample retained metallic conductivity. The results can be explained by examining the microstructure after testing (Fig. 8). As expected, the degradation in conductivity of the 16 nm grain size sample was caused by the formation of nickel agglomerates on the surface, depleting the film of nickel. In contrast, the grain size sample exhibited a stable microstructure and no nickel agglomerates were observed. conductivity [S/m] nm 1μm m 3.6. Electrical conductivity during thermal cycling 1μm m Two 60/40 Ni/CGO samples with grain sizes of 16 and and thicknesses of 703 and 658 nm, respectively, were thermally cycled simultaneously over a period of 1450 h under reducing conditions. After an initial period of 50 h at 603 ± 2 C, the samples were cycled 130 times between 200 and 600 C with 2 h dwell time each and with heating and cooling rates of 3 C min 1. The peak conductivities of the 16 nm grain size sample decreased from to Sm 1, whereas the grain size sample with a conductivity of Sm 1 showed negligible degradation over the testing period (Fig. 9). Nickel particles were observed on the surface of the 16 nm sample, whereas the surface of the sample was free of nickel particles. However, nickel grains larger than the ceramic particles were found within and on top of the film (Fig. 9), indicating that slow nickel coarsening can also occur in samples with grain size Redox cycling In order to investigate the tolerance of the cermet composites towards cycling from reducing to oxidizing atmospheres at the operating temperature, the electrical time [h] Fig. 9. Conductivity during thermal cycling of two 60/40 Ni/CGO samples with 16 (T anneal = 800 C) and (T anneal = 1000 C) grain size and SEM top views of samples after testing. conductivity of two 60/40 Ni/CGO samples was measured at 570 ± 1 C while alternating the gas atmosphere between air and reducing conditions. The samples were heated to 570 C at3 C min 1 and kept under reducing conditions for 20 h. Fifty redox cycles were then performed by purging with nitrogen for 30 min, then flowing air for 60 min, nitrogen for 30 min and hydrogen/nitrogen for 60 min. The grain size of the layers was 16 and and the thickness 518 and 512 nm at the beginning of the experiments, respectively. The conductivity of the 16 nm grain size sample started to degrade immediately with the first cycle, whereas the grain size sample showed increased conductivity after the first cycle, roughly constant values during the following three or four cycles, and degradation afterwards. The conductivity of both samples leveled off after 25 cycles, T = 570 ± 1 C conductivi ty [S/m ] μm m conductivity [S/m] nm time [h] Fig. 10. Conductivity during redox cycling of two 60/40 Ni/CGO samples with 16 (T anneal = 800 C) and (T anneal = 1000 C) grain size and SEM top view of the sample after redox cycling.

10 686 U.P. Muecke et al. / Acta Materialia 56 (2008) alternating between the conductivity of pure CGO in air and under reducing conditions. Metallic conductivity was lost during thermocycling due to nickel segregation on the surface of both samples (Fig. 10). The mechanical stress induced during the expansion of oxidating nickel particles also disrupted the CGO network and the film could be wiped off the surface after the redox-cycling. 4. Summary and conclusion NiO/CGO thin films with thicknesses between 150 and 800 nm were deposited by spray pyrolysis. The average grain size of the NiO and CGO grains mainly depended on the annealing temperature and were found to be 5, 16, 53 and 260 nm for annealing temperatures of 600, 800, 1000 and 1200 C, respectively. The grains of the two-phase mixtures were always smaller than the grain size of the pure component and were roughly constant within the composition range from 20/80 to 80/20 Ni(O)/CGO. The percolation limit of metallic conductivity in the thin film cermets was found to be dependent not only on the film composition but also on the grain size. The percolation limit of samples with grain sizes in the range of the film thickness was higher than for samples with a small grain size to layer thickness ratio. In the first case, the percolation threshold approached the values of 2D percolation, whereas in the latter case the samples show 3D percolation and values of the percolation limit are found to be similar to isotropic bulk cermets. The morphology after reduction at 600 C depended strongly on the grain size of the thin films. Nickel grains or nickel structures within the film with dimensions exceeding the film thickness were found in samples with grain sizes up to 16 nm. The depletion of nickel from the film into large grains on the film surface was accompanied by a decrease in electrical conductivity. The nickel grain size in samples with grains of or larger was constant over time at 600 C. The conductivity of grain sized samples was stable over 1400 h at 570 C and during thermal cycling for 1400 h. The formation of a dimensionally stable microstructure of Ni/CGO cermet thin films can be influenced at two stages during the preparation process: in the oxidized state as NiO/CGO and in the reduced state as Ni/CGO. The average grain size of the NiO and CGO phase is set by the annealing temperature in air after preparation. This grain size determines the maximum temperature at which the reduced cermet can be operated without structural changes in the particle network, specifically in the metallic nickel phase. If the grain size is too small and the CGO particles are not well sintered together, forces from the metallic nickel originating from the minimization of the free nickel surface will disrupt the ceramic network and large nickel agglomerates will form within and on top of the film. The stability of the CGO network can be enhanced by annealing at higher temperatures. Nickel grains are then restrained by the rigid ceramic framework after reduction and during operation, and are hindered from coarsening. However, nickel agglomerates still form on the surface of the film after extended operating times and the conductivity of the cermets decreases due to nickel depletion from the film bulk. A stable CGO network and a stable nickel network are formed when the annealing temperature and the grain size in the oxidized state are further increased. At the optimum grain size, which depends on the desired operating temperature, the CGO network is well sintered, the nickel grains are large enough not to coarsen and the catalytic activity of the cermet anode is maintained over extended operation times. 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