Journal of The Electrochemical Society, 153 �2� A354-A360 �2006�A354 Mixed Ionic–Electronic YSZ/Ni Composite for SOFC Anodes with High Electrical Conductivity Fabio C. Fonseca,a,z Daniel Z. de Florio,b Vincenzo Esposito,c,* Enrico Traversa,c,** Eliana N. S. Muccillo,a,** and Reginaldo Muccilloa,** aInstituto de Pesquisas Energéticas e Nucleares, 05508-170 São Paulo, SP, Brazil bInstituto de Química, UNESP, 14801-970 Araraquara, SP, Brazil cDipartimento di Scienze e Tecnologie Chimiche, Università degli Studi di Roma “Tor Vergata,” 00133 Roma, Italy The preparation of the ZrO2:8 mol % Y2O3/NiO �YSZ/NiO� composites by a modified liquid mixture technique is reported. Nanometric NiO particles dispersed over the yttria-stabilized zirconia �YSZ� were prepared, resulting in dense sintered specimens with no solid solution formation between the oxides. Such a feature allowed for the electrical characterization of the composites in a wide range of relative volume fraction, temperature, and oxygen partial pressure. The main results indicate that the composites have high electrical conductivity, and the transport properties in these mixed ionic-electronic �MIEC� composites are strongly dependent on the relative volume fraction of the phases, microstructure, and temperature. These parameters should hence be taken into consideration for the optimized design of MIEC composites for electrochemical applications. In this context, the composite was reduced under H2 for the preparation of high-conductivity YSZ/Ni cermets for use as solid oxide fuel cell anode material with relatively low metal content. © 2005 The Electrochemical Society. �DOI: 10.1149/1.2149312� All rights reserved. Manuscript submitted August 19, 2005; revised manuscript received October 3, 2005. Available electronically December 30, 2005. 0013-4651/2005/153�2�/A354/7/$20.00 © The Electrochemical Society Mixed ionic–electronic conductors �MIECs� have attracted a great deal of attention due to the variety of possible applications and interesting electrical properties.1-3 These materials are thought to be used in high-temperature electrochemical devices such as gas sepa- ration membranes and solid oxide fuel cell �SOFC� electrodes.2,3 The mixed conduction can be either an intrinsic property of single- phase materials �like cerium oxide and some rare-earth manganites� or a result of the mixture of ionic and electronic conductors �MIEC composite�.3 The main advantage of the MIEC composite is the possibility for tailoring the properties according to optimized param- eters for a specific application.3 In this context, the SOFC anode is the cermet ZrO2:Y2O3/Ni �YSZ/Ni�, produced by the reduction of the precursor composite ZrO2:Y2O3/NiO �YSZ/NiO�.4 Both the cer- met and the oxide precursor are MIEC composites and must fulfill several requirements for a high-performance SOFC. An important issue regarding this anode material is the reduction of the Ni volume fraction for better matching of the thermal expansion coefficient with the electrolyte, while both high electric conductivity and cata- lytic activity must be preserved.4 In fact, the final properties of the anode cermet are strongly dependent on the precursor composite, and various studies deal with the fabrication of the YSZ/NiO composite aiming for the production of SOFC anodes, but rather few papers report a more detailed mi- crostructural and electrical characterization of the precursor composite.5-7 The usually high temperatures ��1500°C� reported for the densification of the composite promote the YSZ destabiliza- tion or solid solution formation due to the reaction with NiO, result- ing in a considerable reduction of the electrical conductivity.8-10 It is well known that the electrical properties of composites depend not only on the electrical properties of each phase but also are strongly influenced by microstructural features such as grain size, distribu- tion, and morphology. The electrical properties of composite media are an important subject, and although several proposed models like percolation and effective media are usually applied, no analytical solution for this problem has been found so far.11,12 Moreover, the nature of the charge carriers and the dependence of the electrical transport properties on the microstructure, temperature, and oxygen partial pressure can be considered as important issues for electro- chemical applications of MIEC composites. Thus, in order to produce high-performance anodes for SOFCs, a * Electrochemical Society Student Member. ** Electrochemical Society Active Member. z E-mail: cfonseca@ipen.br address. Redistribution su200.145.3.46Downloaded on 2014-01-15 to IP detailed characterization of the electrical and microstructural prop- erties of the YSZ/NiO composite prepared by a liquid mixture tech- nique was performed. The observed high sinterability and high elec- trical conductivity of the studied composites can be considered as important advantages of the liquid mixture technique when com- pared to the ones usually reported. The main results show that the microstructure and the ionic/electronic transference numbers of high-conductivity composites can be tuned for high-temperature electrochemical applications. Moreover, an optimized microstruc- ture of the YSZ/NiO composite is achieved after reduction of YSZ/Ni cermets, with high electrical conductivity at relatively low Ni content. Experimental The precursor composite �1 − v� �ZrO2:8 mol % Y2O3�/vNiO �YSZ/vNiO� was prepared in the 0 � v�vol %� � 100 range and tailored to result, after reduction, in YSZ/vNi cermets with v within the range for SOFC anodes. These materials were prepared by a modified liquid mixture technique and further details can be found elsewhere.13 Briefly, this technique consists of the evaporation of a dispersion of YSZ �Tosoh� powder in a solution of nickel acetate tetrahydrate �Carlo Erba� and ethanol, followed by calcination at 450°C/5 h to eliminate the organic material. Scanning electron mi- croscopy �SEM� analysis of the as-produced powders revealed that the composite powders were made of a homogenous mixture of NiO nanoparticles �dNiO � 15 nm� with YSZ particles �dYSZ � 100 nm�.13 Pellets were formed by uniaxial pressing and sintering at 1350°C. X-ray diffraction �XRD� data were collected in a model D8 Advance Bruker-AXS diffractometer in a Bragg– Brentano �-2� configuration using Cu K� radiation in the 25–85° 2� range with step size 0.02° 2� and 10 s counting time. Refinement of the XRD patterns was carried out by the Rietveld method using GSAS software and the reported Inorganic Crystal Structure Data- base �ICSD� no. 75316, 9866, and 44767 for YSZ, NiO, and Ni, respectively. The microstructure of polished and fractured surfaces of sintered samples was observed by SEM. The electrical properties of the YSZ/vNiO composite were studied by electrochemical im- pedance spectroscopy measurements Z��,T� carried out using a 4192A LF impedance analyzer in the temperature range 100–800°C and frequency range 5 Hz–13 MHz with an applied excitation signal of 200 mV. The oxygen partial pressure �pO2 � was varied in the range of 1–10−6 atm using a YSZ electrochemical oxygen pump and sensor system connected to the impedance analyzer.14 The YSZ/vNi cermets were produced by reducing the precursor composite by   ecsdl.org/site/terms_usebject to ECS license or copyright; see http://ecsdl.org/site/terms_use http://ecsdl.org/site/terms_use A355Journal of The Electrochemical Society, 153 �2� A354-A360 �2006� A355 heat-treating at 550°C under H2 flow �100 mL/min� for 5 h. The microstructure of the cermets was characterized by XRD and SEM analysis. Standard four-probe dc electrical resistance ��T� measure- ments of bar-cut cermets were performed in the temperature range 100–800°C under He flow using a Keithley current source and volt- meter. In all electrical measurements, Pt wires were attached to Ag contact pads painted on the surface of the samples and cured at 400°C. Results and Discussion Figure 1 shows the XRD patterns of the YSZ/vNiO samples. All the diffraction peaks were indexed according to the cubic structure of both oxides, space group Fm3̄m, and no extra peaks due to any spurious phase were detected. Table I reports the main parameters of the calculated XRD patterns. The good quality of the Rietveld re- finements is evidenced by the low values of the reliability factors �2 � 2 and RBragg � 3%. The calculated volume fractions of the phases are within �3% of the nominal values, and the refined lattice parameters �a� of both oxides are in good agreement with previously reported data.15 The YSZ lattice parameter dependence on the NiO content, displayed on the inset of Fig. 1, shows no significant changes, indicating that no appreciable solid solution occurred at 1350°C. If Ni2+ substitutes either for Zr4+ or Y3+ ions in the YSZ structure, a pronounced decrease in the lattice parameter would oc- cur due to the smaller ionic radius of nickel.5,16 It was already re- ported that the NiO solubility limit in the cubic zirconia structure is �5 mol % ��8 vol %� for dense samples sintered at 1600°C for 4 h.9 In the present study, sintering the prepared powders at a rela- tively lower temperature resulted in dense composite samples, as inferred from the measured relative densities. The composite densi- ties were determined by the Archimedes method and the theoretical values were calculated by the rule of mixture using the reported density values of YSZ and NiO. The apparent density of the speci- mens was found to be �96% of the theoretical value. SEM images of polished and thermally etched surfaces shown in Fig. 2 indicate that both phases are homogenously distributed in the composites. Specimens sintered at 1350°C for 1 h �Fig. 2a and b� were found to have estimated average grain size �800 and �700 nm for YSZ and NiO, respectively. In addition, the average grain size of both phases was observed to be nearly independent of the relative composition. The SEM images show that the average size of the NiO particles heat-treated at 450°C exhibited a large increase in size when sintered at 1350°C, while pure YSZ samples sintered under similar conditions had an average grain size comparable to those found in the YSZ/NiO composites.17 The microstructural characterization of the YSZ/vNiO compos- ites revealed that the liquid mixture technique produced sinteractive powders, yielding dense samples formed by a homogeneous mixture of the oxides. The electrical properties were then investigated by impedance spectroscopy measurements. The Z��,T� data showed at least two semicircles in the whole frequency and temperature ranges studied.18 However, in this study the focus was on the total electrical resistance of the YSZ/vNiO composite, which was obtained by fit- ting the low-frequency end of the impedance diagrams. The electri- cal conductivity �T� of the composites was measured in a wide range of temperature, and Fig. 3 shows the �T� dependence on the NiO content. In addition, the transport mechanism of the composite was stud- ied with the Arrhenius plots, shown in Fig. 4. As far as the electrical transport is concerned, YSZ is known to be an ionic conductor �ionic transference number tI � 1� with thermally activated trans- port of oxygen vacancies with an activation energy E � 1 eV over a wide range of both temperature and oxygen partial pressure �pO2 �.4 NiO is a p-type semiconductor, the charge carriers being the electron holes due to Ni vacancies.5,19 NiO exhibits a discontinuity in the Arrhenius plots at a temperature close to the Néel temperature �TN � 250°C�, and the activation energies are E � 0.7 and 0.3 eV for T � T and T � T , respectively.5,19 The �T� data at N N address. Redistribution su200.145.3.46Downloaded on 2014-01-15 to IP low temperatures �T � 400°C� shown in Fig. 3 indicated that the NiO content had a large influence on the electrical conductivity of the composite. At low temperatures, YSZ behaved as insulating par- ticles and the �T� increased three orders of magnitude from v = 0–23 vol % at T � 240°C. This remarkable increase of �T� is due to the increase in the density of electronic charge carriers from NiO and indicates that the percolation threshold was attained at v � 20 vol %. The relatively low critical volume fraction may be attributed to the high dispersion and good connectivity of NiO par- Figure 1. XRD patterns of �1 − v� �ZrO2:8 mol % Y2O3�/v NiO samples with v = 40, 58, and 84 vol %. The * and � symbols indicate the YSZ and NiO main diffraction peaks, respectively. The experimental, calculated, and difference plots are shown. The inset shows the dependence of the calculated lattice parameter of cubic zirconia on the NiO content.   ecsdl.org/site/terms_usebject to ECS license or copyright; see http://ecsdl.org/site/terms_use http://ecsdl.org/site/terms_use A356 Journal of The Electrochemical Society, 153 �2� A354-A360 �2006�A356 ticles in the YSZ matrix, as well as to the average grain size ratio of the two phases.12 It is interesting that the usually observed decrease in the electrical conductivity at low NiO content �v � 10 vol %�, ascribed to solid solution formation of YSZ/NiO composites sin- tered at higher temperatures, is absent in the samples prepared by Table I. Rietveld refined parameters of the YSZ/NiO composite. The atomic positions and thermal factors were fixed according to the values reported in the crystallographic files. ICSD no. 75316 (YSZ) and 9866 (NiO). Nominal NiO �vol %� YSZ a �Å� NiO a �Å� �2 RBragg Calculated NiO �vol %� 40 5.1364�1� 4.1888�1� 2.7 4.0 41.5�2� 58 5.1375�1� 4.1886�1� 2.2 3.5 55.1�1� 84 5.1377�1� 4.1790�1� 2.3 3.4 84.9�1� Figure 2. Backscattered SEM images of the �1 − v� �ZrO2:8 mol % Y2O3�/v NiO samples with v = 40 �a� and 58 �b� sintered at 1350°C/1 h, and v = 40 �c� sintered at 1350°C/4 h. The darker grains cor- respond to the NiO. address. Redistribution su200.145.3.46Downloaded on 2014-01-15 to IP the liquid mixture technique.5 As the NiO percolation threshold was attained at v � 20 vol %, the electrical transport of the specimens with v � 20 vol % is dominated by the ionic charge carriers of YSZ. In fact, the Arrhenius plots show that samples with v � 20 vol % exhibited a single thermally activated process in the whole temperature range with E � 1 eV �Fig. 4�. However, samples with v � 16 vol % showed E � 0.7 eV, a value appre- ciably lower than the one of pure YSZ ionic conductor, as observed in Fig. 4.5 At v = 23 vol % the discontinuity of the Arrhenius plot at TN � 250°C was observed as a signature of the NiO transport be- havior, further confirming the percolation of the semiconductor phase in the ionic matrix and indicating that samples with v � 20 vol % have mixed conductivity. Samples with v � 20 vol % have E close to the values previously reported for NiO in the two temperature ranges �0.7 eV for T � TN and 0.3 eV for T � TN�, as shown in Fig. 4. Further increasing the NiO content up to v � 60 vol % resulted in increasing the electrical conductiv- ity; the electrical transport properties of the samples in the 20 � v � 60 vol % range are believed to be due to both the ionic Figure 3. Electrical conductivity dependence on NiO content of �1 − v� �ZrO2:8 mol % Y2O3�/v NiO composite measured at different temperatures. The lines are guide for the eye. Figure 4. Arrhenius plots of the �1 − v� �ZrO2:8 mol % Y2O3�/v NiO com- posites.   ecsdl.org/site/terms_usebject to ECS license or copyright; see http://ecsdl.org/site/terms_use http://ecsdl.org/site/terms_use A357Journal of The Electrochemical Society, 153 �2� A354-A360 �2006� A357 and electronic charge carriers, as observed at low temperatures �T � 400°C� in Fig. 3. The electrical transport data suggest that the enhanced NiO connectivity in the YSZ/NiO composites, prepared by the liquid mixture technique, lead to electronic pathways at rela- tively lower semiconductor content than in other reported samples.7 Specimens with NiO content v � 60 vol % exhibited a decrease in the electrical conductivity down to a minimum at v � 80 vol %. In these specimens with large NiO content the transport was mainly due to the electron holes, while YSZ grains acted as insulating in- clusions at low temperatures. However, the transport properties of the composites were strongly affected by the temperature due to the different activation energies of the electronic ��0.3 eV� and ionic ��1 eV� conductors at high temperatures. With increasing tempera- ture, the higher E value of YSZ promoted a high conductivity of the O2− ions, and �T � 500°C� of the composite samples were of the same order of magnitude in the whole NiO concentration range. In fact, samples with v � 20 vol % and v � 60 vol % exhibited the most pronounced increase in the electrical conductivity with increas- ing temperature, while samples in the mixed transport range were considerably less influenced by temperature �Fig. 3�. The combined results shown in Fig. 3 and 4 allowed us to esti- mate three different transport characteristics of the YSZ/vNiO com- posites: for v � 20 vol % the composites were essentially ionic conductors, for 20 � v � 60 vol % the composites were MIECs, and for v � 60 vol % the transport was predominately due to elec- tron holes.6,18 This behavior was further confirmed by the analysis of the electrical conductivity data taken under different oxygen par- tial pressures. Figure 5a-c shows the electrical conductivity depen- dence on the pO2 of v = 40, 58, and 84 vol % samples measured at 200, 300, and 600°C, respectively. These findings demonstrated that Figure 5. Electrical conductivity dependence on the oxygen partial pressure measured at 200°C �a�, 300°C �b�, and 600°C �c�, and electronic transfer n address. Redistribution su200.145.3.46Downloaded on 2014-01-15 to IP the samples exhibited the expected linear behavior of a p-type semi- conductor. In addition, by assuming that the phases are connected in parallel for NiO concentrations above the percolation threshold, the linear dependence of on the pO2 was fitted according to the rela- tion = A + B pO2 x , where A is the pO2 -independent ionic conduc- tion, B is the electronic conduction, and x is the exponent giving the − v� ZrO2:8 mol % Y2O3/v NiO composites with v = 40, 58, and 84 vol %, dependence on the NiO content �d�. Figure 6. Arrhenius plots of the �ZrO2:8 mol % Y2O3�/40 vol % NiO com- of �1 umber posite sintered at 1350°C for different times. The inset shows the depen- dence of the electrical conductivity measured at 400°C on the sintering time.   ecsdl.org/site/terms_usebject to ECS license or copyright; see http://ecsdl.org/site/terms_use http://ecsdl.org/site/terms_use A358 Journal of The Electrochemical Society, 153 �2� A354-A360 �2006�A358 pO2 dependence of the semiconductor phase. For measuring tem- peratures 200 and 300°C, both samples v = 40 and 58 vol % had a pO2 1/4-dependence and the v = 84 vol % sample exhibited a less pro- nounced pO2 dependence, being the fitted exponent x = 1/6. The electronic transport in NiO has been already studied, and both pO2 dependences 1/4 and 1/6 have been measured. It has been reported that the exponent x depends on the ionization state of the Ni vacancies.20 When the concentration of single-ionized Ni vacancies is larger than the double-ionized one, the pO2 dependence is 1/4, and when double-ionized vacancies predominate, the electron hole con- centration is proportional to pO2 1/6. In fact, high-purity NiO single crystals were found to have a pO2 1/6 dependence and for less pure NiO specimens a 1/4 power has been observed.20 Thus, the results here presented suggested that the samples with high NiO content have a higher content of doubly-charged Ni vacancies and the mixture with YSZ seems to yield a pO2 dependence close to the 1/4 power, in agreement with previously reported data.5 Increasing the measuring temperature up to 600°C caused a decrease in the electrical conduc- tivity dependence on the pO2 , and the fitted exponents were lower than those found at low temperatures �Fig. 5c�. Such an effect was more pronounced in samples with larger amounts of the ionic phase and indicates that the effective fraction of ionic charge carriers in- creased when the thermal energy activated the pO2 -independent transport of the YSZ oxygen ions, in agreement with the results of Fig. 3. The electronic transfer number tEl was estimated by using tEl = 1 − tI = BpO2 x / ; Fig. 5d shows the tEl values measured at 200°C. As expected, increasing the NiO content increased the elec- tronic transport from tEl � 45–98% for the samples v = 40 and 84 vol %, respectively, further indicating the mixed conduction of the samples in the 20 � v � 60 vol % range and the predominant Table II. Rietveld refined parameters of the reduced cermets. The atomic positions and thermal factors were fixed according to the values reported in the crystallographic files ICSD no. 75316 (YSZ) and 44767 (Ni). Nominal Ni �vol %� YSZ a �Å� Ni a �Å� �2 RBragg Calculated Ni �vol %� 28 5.1402�1� 3.5274�1� 2.1 3.1 28.2�3� 45 5.1405�1� 3.5277�1� 3.2 3.8 40.3�2� Figure 7. Experimental and calculated XRD patterns of the reduced cermet �ZrO2:8 mol % Y2O3�/45 vol % Ni and the difference plot. address. Redistribution su200.145.3.46Downloaded on 2014-01-15 to IP electronic conduction in the sample v = 84 vol %. In addition, for increasing temperatures, the electronic transfer number decreased, and at 600°C the tEl of the sample v = 84 vol % decreased from �98 to 80%, indicating that the transport due to O−2 in YSZ became relevant at high temperatures even for relatively low volume frac- tion �YSZ � 16 vol %�. The above results indicated that both the relative phase volume fraction and the temperature play important roles in the transport properties of the YSZ/NiO composites. The microstructure is also considered important for determining the transport properties of a composite. Thus, to further evaluate the electrical properties in the mixed conduction range, YSZ/NiO speci- mens were sintered at 1350°C for different sintering times, tS. Samples with v = 40 and 58 vol % were sintered at 1350°C for 0.2 h � tS � 6 h; the discussion is here focused on the v = 40 vol % sample, which is close to the percolation threshold and exhibited a more pronounced dependence on tS. The XRD data show that the YSZ lattice parameters did not depend on the sintering time, further indicating that no solid solution between the oxides occurred at 1350°C �see inset of Fig. 1�. The relative density was found to continuously increase from �82 to �97% of the theoret- ical value when the sintering time tS increased from 0.2 to 2 h, Figure 8. SEM of the fractured surface of the �ZrO2:8 mol % Y2O3�/28 vol % Ni sintered for 2 h. Figure 9. dc electrical resistivity dependence on temperature for the �ZrO2:8 mol % Y2O3�/v Ni cermets for v = 28 and 45 vol % sintered for 2 and 1 h, respectively.   ecsdl.org/site/terms_usebject to ECS license or copyright; see http://ecsdl.org/site/terms_use http://ecsdl.org/site/terms_use A359Journal of The Electrochemical Society, 153 �2� A354-A360 �2006� A359 while for tS � 2 h the relative density remained nearly constant. The SEM images revealed that increasing tS increased the average grain size of both YSZ and NiO, being �1.5 and �1.2 m, respec- tively, as displayed in Fig. 2c. It was already reported that NiO particles may inhibit YSZ grain growth; however, probably due to the initial average particle size of the powders produced by the liquid mixture technique, such an effect was not observed.21 By comparing the SEM images of the samples sintered for 1 and 4 h �Fig. 2a and c�, it is important to observe that increasing tS resulted in a larger separation between NiO grains. The microstructural fea- tures of the isothermal sintering at 1350°C have a strong influence on the electrical properties, as observed in Fig. 6. The Arrhenius plots of the samples with v = 40 vol % sintered for different times exhibited the same behavior previously discussed; however, at high temperatures �T � TN� a significant dependence on the sintering time was observed �see inset of Fig. 6�. The electrical conductivity �T = 400°C� increased with increasing tS up to 2 h, reaching a maximum due to the decrease in porosity. Further increasing tS � 2 h resulted in a decrease of of an order of magnitude, which may be correlated with the larger separation of NiO grains observed in Fig. 2. This finding evidenced that the percolation of charge car- riers in MIEC composites can also be controlled by the relative grain size of both phases. In order to investigate the suitability of the liquid mixture tech- nique and the influence of the precursor composite properties on the SOFC anode cermet, the composites in the mixed conduction region with 40 and 58 vol % NiO and sintered for tS = 2 and 1 h, respec- tively, were reduced under H2 flow at 550°C. The resulting cermets with nominal Ni content 28 and 45 vol % were analyzed by XRD, as shown in Fig. 7. The XRD data were refined by the Rietveld method, and Table II reports the main calculated parameters. The calculated lattice parameters of both phases are similar to the values reported in the literature, and the Ni volume fractions estimated were slightly smaller than the nominal values. This might be due to the relatively low temperature used in the reduction that resulted in a small fraction of remaining NiO ��3 wt %, as inferred from Ri- etveld refinements� in the cermet. Figure 8 shows, as an example of the microstructure of the reduced samples, a representative region of a typical fractured surface of the YSZ/28 Ni cermet sintered for 2 h. It is possible to observe that the reduction at 550°C did not cause any significant increase in the particle size of the metallic phase �brighter regions in Fig. 8� when compared to the NiO particles �darker regions in Fig. 2�. In addition, this micrograph suggests the presence of a larger porosity �cf. Fig. 2� resulting from the NiO reduction. However, a clear percolation path of the metallic phase was seen through the sample, as inferred from the electrical proper- ties of the cermet. Figure 9 shows the temperature dependence of the electrical resistivity of the samples with 28 and 45 vol % Ni sintered for 2 and 1 h, respectively. The electrical resistivity data exhibited essentially the same behavior as pure Ni, with the characteristic change of slope associated with the Curie temperature at Table III. Estimated values of NiO and Ni relative volume fractio YSZ/NiO (Ni) composites prepared by different methods reported in NiO �vol %� Ni �vol %� Preparation metho 23 — Modified liquid mix 25 — Aqueous coassemb 40 — Modified liquid mix 40 — Solid-state mixtur 48 — Modified liquid mix 52 — Modified Pechin — 28 Modified liquid mix — 30 Combustion synthe — �27 Polymer precurso — 40 Coat-mix address. Redistribution su200.145.3.46Downloaded on 2014-01-15 to IP T � 350°C, but shifted to higher resistivity values due to the pres- ence of both YSZ and pores.22 However, both samples showed larger values of electrical conductivity at lower Ni concentrations than data usually reported for this cermet.8,23,24 In addition, the cer- met with 28 vol % Ni �former v = 40 vol % NiO� had smaller elec- trical resistivity values, attributed to both the higher electrical con- ductivity of the precursor composite and to the lower porosity after reduction of samples with lower volume fraction of NiO. These results confirmed that the control of the microstructural properties of the precursor composite is an important factor to produce cermets with high electrical conductivity at low Ni contents. For comparison purposes, the present electrical conductivity val- ues along with reported ones for YSZ/NiO composites and YSZ/Ni cermets prepared by other techniques are displayed in Table III. The differences observed in the data referenced in Table III can be as- cribed to the strong dependence of the electrical conductivity on the preparation method. In order to attain high density, sintering tem- peratures higher than the ones used in this work are usually required. Sintering at higher temperatures is likely to promote the formation of solid solution between the oxides, which is expected to result in lower values of the electrical conductivity. Conclusions In summary, YSZ/NiO composites have been prepared by a liq- uid mixture method, which resulted in homogenous powders with high sinterability. The relatively low sintering temperature inhibited solid solution formation between the two oxides. The NiO concen- tration ranges where the main charge carriers are ionic, mixed, and electronic were estimated. The effectiveness of the described prepa- ration method is evidenced by the high values of both the relative density and the electrical conductivity of the composites at low NiO contents, indicating the good connectivity of the semiconductor par- ticles. The results show that the careful design of a mixed ionic– electronic composite for optimized transport properties must take into consideration the relative composition of the phases, the micro- structural features, and operation temperature of the electrochemical device. The suitable design of the YSZ/NiO composite allowed for the preparation of cermets with relatively low concentration of Ni and high electrical conductivity, which is an important feature con- cerning the application of this cermet as SOFC anodes. Acknowledgments This work was partially supported by the Brazilian agencies FAPESP �98/14324-0, 99/10798-0, and 03/08793-8� and CNPq �306496/88-7, 300934/94-7, 301661/2004-9�; and by the Italian Ministry of Education, University and Research �MIUR� under the framework of an FISR project. Thanks are also due to Dr. E. V. Spinacé �IPEN� and Dr. R. F. Jardim �IF-USP� who helped with the cermet fabrication and dc electrical measurements, respectively. Fundação de Amparo à Pesquisa do Estado de São Paulo assisted in meeting the publication costs of this article. ctrical conductivity, and the respective measuring temperatures of iterature. �� cm�−1 T �°C� Reference 3.3 � 10−3 450 This work 2.9 � 10−5 450 7 9.1 � 10−3 400 This work 6.3 � 10−5 400 5 1.3 � 10−2 450 This work 2.0 � 10−4 450 21 2.8 � 103 700 This work 77 700 8 3.4 � 102 700 23 from 78 to 4.0 � 103 800 24 n, ele the l d ture ly ture e ture i ture sis rs   ecsdl.org/site/terms_usebject to ECS license or copyright; see http://ecsdl.org/site/terms_use http://ecsdl.org/site/terms_use A360 Journal of The Electrochemical Society, 153 �2� A354-A360 �2006�A360 References 1. P. Knauth and H. L. Tuller, J. Am. Ceram. Soc., 85, 1654 �2002�. 2. A. Thursfield and I. S. Metcalfe, J. Mater. Chem., 14, 2475 �2004�. 3. I. Reiss, Solid State Ionics, 157, 1 �2003�. 4. N. Q. Minh, J. Am. Ceram. Soc., 76, 563 �1993�. 5. Y. M. Park and G. M. Choi, J. Electrochem. Soc., 146, 883 �1999�. 6. Y. M. Park and G. M. Choi, Solid State Ionics, 120, 265 �1999�. 7. M. Mamak, N. Coombs, and G. Ozin, Adv. Funct. Mater., 11, 59 �2001�. 8. U. Anselmi-Tamburini, G. Chiodelli, M. Arimondi, F. Maglia, G. Spinolo, and Z. A. Munir, Solid State Ionics, 110, 35 �1998�. 9. A. Kuzjukevics and S. Linderoth, Solid State Ionics, 93, 255 �1997�. 10. H. Kondo, T. Sekino, T. Kusunose, T. Nakayama, Y. Yamamoto, and K. Niihara, Mater. Lett., 57, 1624 �2003�. 11. N. F. Uvarov, Solid State Ionics, 136, 1267 �2000�. 12. D. S. McLachlan, M. Blaszkiewicz, and R. E. Newnham, J. Am. Ceram. Soc., 73, 2187 �1990�. 13. V. Esposito, C. D’Ottavi, S. Ferrari, S. Licoccia, and E. Traversa, in SOFC-VIII, S. C. Singhal and M. Dokiya, Editors, PV 2003–07, p. 643, The Electrochemical address. Redistribution su200.145.3.46Downloaded on 2014-01-15 to IP Society Proceedings Series, Pennington, NJ �2003�. 14. M. C. Steil, F. C. Fonseca, Y. V. França, J. F. Q. Rey, E. N. S. Muccillo, and R. Muccillo, Ceramica (Sao Paulo, Braz.), 48, 146 �2002�. 15. R. P. Ingel and D. Lewis III, J. Am. Ceram. Soc., 69, 325 �1986�. 16. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crys- tallogr., A32, 751 �1976�. 17. F. C. Fonseca and R. Muccillo, Solid State Ionics, 131, 301 �2000�. 18. V. Esposito, D. Z. de Florio, F. C. Fonseca, E. N. S. Muccillo, R. Muccillo, and E. Traversa, J. Eur. Ceram. Soc., 25, 2637 �2005�. 19. M. W. Vernon and M. C. Lovell, J. Phys. Chem. Solids, 27, 1125 �1966�. 20. C. M. Osburn and R. W. West, J. Phys. Chem. Solids, 32, 1331 �1971�. 21. P. Durán, J. Tartaj, F. Capel, and C. Moure, J. Eur. Ceram. Soc., 23, 2125 �2003�. 22. L. Gmelin, Gmelin Handbuch Der Anorganischen Chemie, Ni �A II�, p. 355, Springer-Verlag, New York �1974�. 23. V. Petrovsky, T. Suzuki, P. Jasinski, and H. U. Anderson, Electrochem. Solid-State Lett., 8, A341 �2005�. 24. F. Tietz, F. J. Dias, D. Simwonis, and D. Stöver, J. Eur. Ceram. Soc., 20, 1023 �2000�.   ecsdl.org/site/terms_usebject to ECS license or copyright; see http://ecsdl.org/site/terms_use http://ecsdl.org/site/terms_use