J O U R N A L O F C H E M I S T R Y Materials Chemical interactions between strontium-doped praseodymium manganite and 3 mol% yttria-zirconia Jin-Ping Zhang,*a San-Ping Jiang,b Jonathan G. Love,a Karl Fogera and Sukhvinder P. S. Badwalb aCeramic Fuel Cells Limited, 170 Browns Road, Noble Park, Vic. 3174, Australia bCSIRO, Manufacturing Science & Technology, Private Bag 33, Clayton South MDC, Clayton, Vic. 3169, Australia Received 27th July 1998, Accepted 14th September 1998 The interfacial reaction between (Pr0.8Sr0.2)yMnO3 (PSM, y=0.9, 1.0, 1.05) film and 3 mol% yttria tetragonal zirconia (TZ3Y) substrate has been studied at 1200 and 1400 °C in air. A diVusion layer of Pr and Mn in zirconia, which was identified to be a cubic phase of zirconia, was detected in all specimens.When the solubility limit of Pr ions in the cubic zirconia was reached, a pyrochlore phase, Pr2Zr2O7, was formed. A delay for the formation of pyrochlore phase was observed for the A-site sub-stoichiometric and stoichiometric PSM at 1200 °C. For the A-site over-stoichiometric PSM, a Pr-rich (Pr,Zr)Ox phase was detected at the interface besides the pyrochlore phase. At 1400 °C, a relatively thick layer of pyrochlore phase was formed after 24 hour heat treatment in all specimens.The amount of the pyrochlore phase formed at the interface depends on the A-site stoichiometry of perovskite in the initial stage. The growth of the pyrochlore layer after the initial stage, however, appears to be determined by contact area between PSM and the substrate.changes between porous (Pr0.8Sr0.2)yMnO3 ( y=0.9, 1.0, 1.05) 1 Introduction film and 3 mol% Y2O3-ZrO2 (TZ3Y) electrolyte have been Doped perovskite oxides and yttria-zirconia (YSZ) are investigated in air at 1200 and 1400 °C. Although not much commonly used as cathode and electrolyte materials respect- information is reported on interactions between TZ3Y and ively in solid oxide fuel cells (SOFCs) operating at tempera- LSM, this electrolyte composition, despite its low conductivity, tures of around 900–1000 °C.1,2 Interfacial reactions between is of considerable interest to many SOFC technology develthe cathode [especially in Sr-doped lanthanum manganite opers, because its mechanical strength is high and it is easy to (LSM)] and YSZ, have been studied extensively at high fabricate very thin (60–70 mm) sheets of this material (as temperatures, and the reaction products have been well charac- opposed to 150 mm for the 8 mol% Y2O3-ZrO2). terised and documented.3–7 The stoichiometric LSM reacts with YSZ extensively at temperatures above 1200 °C,6–10 form- 2 Experimental ing lanthanum zirconate La2Zr2O7 and/or strontium zirconate SrZrO3 phases at the interface depending on the La/Sr ratio The PSM powders with composition (Pr0.8Sr0.2)yMnO3 ( y= at the A-site.The interfacial reactions have also been reported 0.9, 1.0 and 1.05, coded hereafter PSM-A, PSM-B and PSMat lower temperatures by some authors (1150, 11004,11 and C respectively) were prepared by co-precipitation followed by 1000 °C12). It is generally known that an A-site deficient LSM calcination at 1000 °C for 4 h in air.The value of y is used to suppresses the formation of La2Zr2O7.4,6,11,13 The formation indicate the stoichiometry of the perovskite phase for conof zirconates at the interface is detrimental to the performance venience only and no assumption has been made that it is a of a solid oxide fuel cell system, causing substantial increase single phase material.The electrolyte substrate TZ3Y, 20 mm in the overpotential and resistivity at the cathode/electrolyte in diameter and 150 mm in thickness, was prepared from interface.14 3 mol% Y2O3-ZrO2 (Tosoh Corporation, Japan) by tape cast- Owing to severe corrosion of stack components, high cost ing and sintering at 1500 °C. The PSM was screen-printed (ca.and degradation of stack performance, it is necessary to lower 40 mm thick) on the TZ3Y substrate and sintered at 1200 °C the operating temperature of SOFCs from 900–1000 °C to the for 4, 24 or 168 h. Another batch was sintered at 1400 °C for intermediate range 700–800 °C.2 One of the requirements for 24 h. The heat treatment temperature of 1200 °C has been lowering the operating temperature is to develop a new cathode commonly used in the LSM/YSZ system for the investigation material which has reasonably low overpotential losses in the of the interfacial reaction.The 1400 °C temperature was used 700–800 °C temperature range. Ishihara et al.15 studied the to accelerate the solid state reaction for a more conspicuous electrochemical behaviour of the Sr-doped praseodymium observation of the interactions between PSM and TZ3Y.manganite (PSM) and found that the overpotential losses for PSM powders after calcination were characterised by X-ray PSM were significantly lower than those of other Sr-doped diVraction (XRD) for phase analysis and by scanning electron lanthanide manganites at intermediate operating temperatures. microscopy (SEM) for powder morphology.After heat treat- Therefore the material is a potential cathode for intermediate ments, the reaction couples were carefully fractured and in temperature SOFC operation. However, information on inter- some cases polished cross-sections were prepared. Both fracactions between PSM and YSZ is scarce. Wen et al.16 sintered tured face and polished cross-sections were examined with a PSM and YSZ powder mixture at 1000 °C for 100 h and did SEM.X-Ray energy dispersive spectroscopy (EDS) was used not detect any interfacial reactions. In order to examine to study the elemental distribution in the PSM/TZ3Y interface interfacial reactions between PSM and YSZ, a higher heat region. In some samples, PSM was carefully removed from the treatment temperature (1200 °C), similar to those used for TZ3Y substrate and the exposed TZ3Y surface was examined the LSM/YSZ system in most investigations, may be required. with XRD and SEM/EDS.A Siemens D500 X-ray diVractometer (Siemens, Germany) with Cu-Ka radiation and In the present study, the microchemical and microstructural J. Mater. Chem., 1998, 8, 2787–2794 2787Fig. 1 XRD patterns of the powders calcined at 1000 °C showing that PSM-A and PSM-B are relatively pure perovskite, while PSM-C contains the major phase perovskite and a small amount of praseodymium oxide. a Leica 360 field emission SEM (Cambridge, UK) equipped with an Oxford Link EDS system were used for specimen characterisation. Identification of phases from XRD patterns was based on the JCPDS-ICDD database. The Rietveld method,17 a technique for crystal structure refinement from powder diVraction data, was used in the current study to analyse zirconia phases at the PSM/TZ3Y interface.The Rietveld refinement was performed using the program LHPM1.18 3 Results XRD traces of powders calcined at 1000 °C are displayed in Fig. 1. The diVraction peaks marked ‘P’ in Fig. 1 are of perovskite, and those marked ‘O’ belong to praseodymium oxide Pr6O11. The stoichiometric PSM-B and the A-site substoichiometric PSM-A powders are relatively pure perovskite while A-site over-stoichiometric PSM-C powder contains a small amount of praseodymium oxide besides the major perovskite phase. A few minor unidentified reflections are also noticed in all the traces. The powder morphology of PSM-A, PSM-B and PSM-C is about the same after calcination at 1000 °C for 4 h.The PSM particle size was in the range 0.1–0.2 mm. Fig. 2 The backscattered electron micrographs of the polished cross (1) Reaction products after heat treatment at 1400 °C section of the interfaces after 24 h at 1400 °C: (a) PSM-A/TZ3Y; (b) PSM-B/TZ3Y; (c) PSM-C/TZ3Y. ‘L1’ and ‘L2’ refer to the Two reaction layers were identified between TZ3Y and PSM.pyrochlore layer and the Pr- and Mn-diVused zirconia layer Fig. 2 displays the backscattered electron micrographs taken respectively. from a polished cross section of the PSM/TZ3Y interface after heat treatment at 1400 °C for 24 h. The first reaction layer (marked L1 in Fig. 2) can be clearly observed in these micrographs.Pr and Zr were identified by EDS analysis as the major elements in the reaction layer. To identify the phase of the reaction layer, PSM was removed carefully by scraping and the exposed surface of the reaction layer was examined by XRD. A representative XRD trace from the reaction layer on TZ3Y substrate which was in contact with PSM-C is presented in Fig. 3 with the identification of each reflection.The major reflections (marked ‘X’ in Fig. 3) match with those of the pyrochlore phase, Pr2Zr2O7 (ICDD file No.: 19-1021), except that the intensity of the reflection (400) (2h=33.45°) is about twice that reported in 19-1021. This probably is due to Fig. 3 A representative XRD pattern from the substrate originally in contact with PSM-C showing that the pyrochlore phase was formed the preferred orientation. Thus both XRD and EDS analysis after 24 h at 1400 °C.results confirm that the major phase in the reaction layer ‘L1’ is Pr2Zr2O7, the pyrochlore phase. Some minor reflections in Fig. 3 arise from the substrate TZ3Y, and the perovskite when the layer was formed with PSM-A, 3.1 mm with PSM-B and 12.0 mm with PSM-C after heat treatment at 1400 °C powder left-over after scraping.The average thickness of the praseodymium zirconate layer for 24 h. In addition to the pyrochlore phase, a second reaction layer determined from backscattered electron micrographs varied with the compositions of PSM. It was about 9.5 mm thick was also observed from the fractured surface. Fig. 4 shows a 2788 J. Mater. Chem., 1998, 8, 2787–2794representative micrograph of the fractured surface of PSMA/ TZ3Y.In this micrograph a 28 mm thick distinct layer is obvious between PSM-A and TZ3Y. It contains the pyrochlore layer ‘L1’ that is only 9.5 mm thick, and another reaction layer ‘L2’. The technique of energy dispersive X-ray mapping was used to identify elements in the layer ‘L2’. Fig. 5 displays Xray maps recorded from the polished cross section of PSMA/ TZ3Y showing the distribution of related elements Zr, Y, Sr, Pr and Mn around the interface. It should be noted that the Mn Ka peak partly overlapped with Pr Lb2, the intensity of which has been subtracted from Mn Ka in Fig. 5. In order to show clearly the element distribution in layer ‘L2’, X-ray maps (g) and (h) do not include the PSM layer because the contrast between the Mn concentration in PSM and in ‘L2’ is so high that the Mn distribution in ‘L2’ can not be seen from Fig. 5(f ) when the PSM layer is included.From Fig. 5, it can Fig. 4 SEM micrograph of the fractured surface of PSM-A/TZ3Y be seen that the layer ‘L2’ consists of Pr, Mn and the elements showing a ca. 28 mm thick distinct reaction layer formed between of TZ3Y.The Pr ions and some Mn ions appeared to have PSM-A and TZ3Y after 24 h at 1400 °C, which consists actually of entered the TZ3Y lattice forming a solid solution. More results two layers of products, ‘L1’ (pyrochlore) and ‘L2’ (Pr- and Mn-diVused zirconia). about the nature of this diVusion layer will be presented in the following sections. The XRD, SEM and EDS observations thus far for Fig. 5 The EDS X-ray maps recorded from the polished cross section of PSM-A/TZ3Y after 24 h at 1400 °C showing the distribution of related elements in PSM and reaction layers L1 and L2: (a) backscattered electron image; (b) Zr La1; (c) Y Ka; (d) Sr Ka; (e) Pr La1; (f )Mn Ka. The Xray maps (g) Pr La1 and (h) Mn Ka do not include the PSM layer in order to show clearly the distribution of Pr and Mn in the diVusion layer ‘L2’, which can not be seen clearly from (e) and (f ) in which the PSM layer is included.J. Mater. Chem., 1998, 8, 2787–2794 2789Table 1 The thickness of the zirconate layer and the diVusion distance It was also noticed from SEM examination that the PSM-A of Pr and Mn in TZ3Y after 24 h at 1400 °C and PSM-C coatings were highly sintered compared with the PSM-B coating.Thickness of Pr diVusion Mn diVusion Specimen zirconate/mm distancea/mm distancea/mm (2) Reaction products after heat treatment at 1200 °C PSM-A/TZ3Y 9.5 28 55 Fig. 7 displays the backscattered electron micrographs of PSM-B/TZ3Y 3.1 6 14 polished cross sections of PSM-A/TZ3Y, PSM-B/TZ3Y and PSM-C/TZ3Y 12.0 30 70 PSM-C/TZ3Y after heat treatment at 1200 °C for 4, 24 and aThe distance was measured from the PSM/pyrochlore phase interface. 168 h respectively. The top part in each micrograph shows PSM, and the bottom section the substrate. Pyrochlore (PZ) formed at the interface is marked on the micrographs. From PSM-A/TZ3Y interface heat treated at 1400 °C can be summarthese micrographs it can be seen that the reaction products ised as below.between PSM and TZ3Y vary with the A-site stoichiometry 1 Pr ions have diVused into TZ3Y. The diVusion distance of PSM and the time of heat treatment. Substantial amounts is about 28 mm (from the PSM/pyrochlore interface), correof praseodymium zirconate were detected after heat treat- sponding to the thickness of the dense layer viewed from the ment of PSM-A/TZ3Y at 1200 °C for 168 h, PSM-B/TZ3Y for fractured surface. 24 h, and PSM-C/TZ3Y for 4 h. In PSM-B/TZ3Y and PSM- 2 Mn ions have also diVused into TZ3Y. The diVusion C/TZ3Y, praseodymium zirconate formed a continuous layer distance of Mn ions (ca. 55 mm from PSM) is much larger than at the interface, whereas in PSM-A/TZ3Y the zirconate formed that of Pr. However, Mn was not detected in the praseodymium islands at contact points between PSM-A and the substrate.zirconate layer. The thickness of the reaction layer grew with the time of heat 3 A small amount of Sr, estimated to be less than a few wt.%, treatment. was also detected in the zirconate layer (L1), but no strontium The inset in the micrograph of PSM-C/TZ3Y/24 h in Fig. 7 zirconate phase was formed.is an enlargement of the reaction layer, showing clearly two 4 Zr or Y was not found in the PSM phase. XRD study distinct layers of products between the PSM-C coating and showed that the perovskite phase of the PSM layer did not the substrate. Fig. 8 presents the EDS spectra (a) for the top change its structure after the reaction. layer (the brightest in contrast in the micrograph) and (b) for 5 The yttrium was detected in both reaction layers (L1 and the bottom layer, showing that both layers contain Pr and Zr L2) at about the same level as in the TZ3Y bulk phase (i.e. but with diVerent atomic ratios.The experimental conditions 3 mol% Y2O3). for the EDS X-ray analysis were kept the same in all cases so The same microstructure of the interface was also found in that a semi-quantitative comparison of elemental concentration the other two specimens PSM-B/TZ3Y and PSM-C/TZ3Y in diVerent specimens could be carried out.Trace (b) in Fig. 8 heat-treated at 1400 °C for 24 h. The thickness of the zirconate is a typical EDS spectrum of praseodymium zirconate. From and the diVusion distance (measured from the PSM/pyrochlore comparison of the two EDS traces it is known that the atomic interface) of the major elements Pr and Mn in TZ3Y in three ratio of Pr/Zr of the top layer is higher than that of the specimens are summarised in Table 1.It can be seen from pyrochlore layer which is about 151. The top layer, therefore, Table 1 that the degree of the reaction for PSM-A/TZ3Y is consists of a (Pr,Zr)Ox phase with the atomic ratio of Pr/Zr>1.slightly lower than that for PSM-C/TZ3Y interface whereas A very thin layer of (Pr,Zr)Ox phase was also detected in PSM-B/TZ3Y interface showed relatively higher stability. PSM-C/TZ3Y after 4 h at 1200 °C when it was examined at a The fact that Zr and Y were not detected in the PSM layer, higher magnification. After 168 h at 1200 °C the amount of clearly indicates that the growth of the zirconate layer is in (Pr,Zr)Ox was much less than that after 24 h, and did not form the direction of the abutting electrolyte.This was also obvious a distinct layer, as shown in Fig. 7. from Fig. 6, a micrograph taken from the cross section of After removal of most of the PSM-C coating from the PSM-A/TZ3Y after 24 h at 1400 °C. In the area where there specimen sintered for 24 h at 1200 °C, XRD analysis was was no PSM-A, the pyrochlore phase was not formed.It can carried out on the substrate and part of the trace is displayed be seen from the micrograph that the TZ3Y substrate and the in Fig. 9. There are a few extra peaks (marked ‘O’) in the XRD praseodymium zirconate top surfaces are almost level, indicattrace besides those of expected phases pyrochlore, TZ3Y and ing that the zirconate phase has grown into the TZ3Y substrate.perovskite ( left over from scraping). These extra reflections are The absence of PSM-A in such areas probably arose from the probably of the (Pr, Zr)Ox solid solution phase because they shrinkage of the coating at high temperature. match with the reflections of Pr6O11 with a systematic peak position shift (to larger angle) that is not a zero point error. The Pr and Mn diVusion layer observed in the specimens sintered at 1400 °C was also detected in all specimens heated at 1200 °C.Some representative EDS spectra of the diVusion layer are displayed in Fig. 10. Spectra (a) and (b) were recorded from the diVusion layers formed in PSM-A/TZ3Y after heat treatment for 4 and 24 h respectively at 1200 °C, showing clearly the presence of Pr and Mn in TZ3Y.For comparison the EDS spectrum of praseodymium zirconate formed in PSMA/ TZ3Y after 168 h is also displayed in Fig. 10(c). It can be seen from Fig. 10 that the concentration of Pr in TZ3Y increases with the time of heat treatment. The diVusion layer is not obvious from the contrast of SEM micrographs taken from polished cross sections (Fig. 7). However, the change of the substrate microstructure near the interface due to diVusion of Pr and Mn and the formation of the pyrochlore phase can be seen on the substrate surface by Fig. 6 SEM micrograph from the cross section of PSM-A/TZ3Y after removing the PSM coating carefully from the substrate. This 24 h at 1400 °C showing that the TZ3Y substrate and the praseodymis illustrated in the secondary electron micrographs shown in ium zirconate top surfaces are almost leveled, indicating that the zirconate has grown into the TZ3Y substrate.Fig. 11. Fig. 11(a) was taken from the unreacted TZ3Y for 2790 J. Mater. Chem., 1998, 8, 2787–2794Fig. 7 The backscattered electron micrographs of the polished cross sections of PSM-A/TZ3Y, PSM-B/TZ3Y and PSM-C/TZ3Y after heat treatment at 1200 °C for 4, 24 and 168 h respectively.The top part in each micrograph shows PSM, and bottom part the substrate. Pyrochlore (PZ) formed at the interface is marked in the micrographs. The inset in the micrograph of PSM-C/TZ3Y/24h is an enlargement of the reaction layer, showing clearly two distinct layers of products between the PSM-C coating and the substrate.Fig. 10 The EDS spectra recorded from the substrate near the interface of PSM-A/TZ3Y after sintering at 1200 °C for 4 h (a), 24 h (b) and Fig. 8 The EDS spectra recorded from the top layer (a) and the 168 h (c). The electron beam was located on the zirconate for bottom layer (b) shown as inset in the micrograph PSM-C/TZ3Y/24 h spectrum (c).in Fig. 7, indicating that both layers contain Pr and Zr but with diVerent atomic ratios. comparison; Fig. 11(b) from the Pr- and Mn-diVused zirconia in PSM-A/TZ3Y showing a dramatic increase of the grain size of zirconia near the interface after 24 h at 1200 °C; Fig. 11(c) from the same specimen as in Fig. 11(b) but in a diVerent area showing the trace of some contact points with PSM; Fig. 11(d) from the praseodymium zirconate surface formed in PSMA/ TZ3Y after heat treatment at 1200 °C for 168 h; and Fig. 11(e) from the substrate surface near the edge of the reaction layer in PSM-B/TZ3Y after heat treatment at 1200 °C for 168 h showing the microstructure of diVerent layers including the pyrochlore layer ‘L1’, the Pr- and Mn-diVused zirconia layer ‘L2’ and the unreacted TZ3Y.EDS analysis on the trace of the contact points shown in Fig. 11(c) showed a typical composition of the pyrochlore phase. This indicates that some crystal nuclei of praseodymium zirconate had been formed on the surface of Pr- and Mn-diVused zirconia in PSM-A/TZ3Y Fig. 9 The XRD pattern recorded from the substrate surface of after 24 h at 1200 °C.The crystal nuclei of praseodymium PSM-C/TZ3Y after 24 h at 1200 °C showing the formation of the zirconate were not observed from the cross-section in Fig. 7 praseodymia solid solution (Pr,Zr)Ox besides the expected phases pyrochlore, TZ3Y and perovskite. probably because the scale is too small. J. Mater. Chem., 1998, 8, 2787–2794 2791Fig. 11 SEM micrographs showing the morphology of the substrate surface: (a) unreacted TZ3Y; (b) the Pr- and Mn-diVused zirconia in PSM-A/TZ3Y showing a dramatic grain growth of zirconia at the Fig. 12 (a) The XRD patterns recorded from the substrate surface: interface after 24 h at 1200 °C; (c) the same specimen as (b) but in a (i) unreacted TZ3Y, (ii) reacted with PSM-A at 1200 °C for 24 h and diVerent area showing the formation of the pyrochlore crystal nuclei; (iii) reacted for 168 h.(b) Enlarged part of (a) showing that the (d) the praseodymium zirconate surface formed in PSM-A/TZ3Y after reflection marked * in (ii) consists of two peaks. sintering at 1200 °C for 168 h; and (e) the substrate surface near the edge of the reaction layer in PSM-B/TZ3Y after 168 h at 1200 °C showing the microstructure of diVerent layers including the pyrochlore layer ‘L1’, the Pr- and Mn-diVused zirconia layer ‘L2’ and the unreacted TZ3Y.Fig. 12(a) displays the XRD patterns recorded from the substrate surface: (i) unreacted TZ3Y, (ii ) reacted with PSMA at 1200 °C for 24 h and (iii ) reacted for 168 h. The TZ3Y has a tetragonal form of crystal structure, and the trace (i) in Fig. 12(a) is a typical XRD pattern of a tetragonal zirconia.A comparison between the trace (ii) and the trace (i) shows that the relative intensities of a number of reflections (marked with an asterisk) in trace (ii) are much higher than in trace (i). In fact, when the traces were enlarged, it was observed that each of the enhanced reflections consisted of two peaks, and one example is shown in Fig. 12(b). This suggests that another phase has been formed with the diVusion of Pr and/or Mn ions into the zirconia. Trace (iii ) in Fig. 12(a) is composed of the diVraction peaks belonging to the Pr- and Mn-diVused zirconia shown in the trace (ii) and those of pyrochlore phase. All XRD results are consistent with SEM observations. In order to characterise the phase of the Pr- and Mn diVused zirconia, the XRD data of trace (ii) in Fig. 12(a) were analysed using the Rietveld method. During the Rietveld refinement the unit cell parameters, zero point, scale factors, peak Fig. 13 The output from the Rietveld refinements of the XRD pattern of the Pr- and Mn-diVused TZ3Y in PSM-A/TZ3Y after 24 h at width/shape/asymmetry parameters and background 1200 °C using (a) tetragonal lattice, and (b) cubic as well as tetragonal coeYcients were refined simultaneously to convergence.The lattice. The observed data are indicated by crosses and the calculated atomic position parameters were fixed as reported in the by a continuous line overlying them, and the diVerence profile is the literature.19 When the structure parameters of tetragonal zir- lower curve in each figure.The short vertical lines show the positions conia were tested in the Rietveld refinement, the agreement of all possible Bragg reflections. index Rwp was 0.132. When the cubic as well as the tetragonal lattice of zirconia was used in the refinement, Rwp dropped significantly to 0.068. The output from the Rietveld refinements was improved significantly when the cubic lattice was included in the refinement, as shown in Fig. 13(b). This suggests that using (a) the tetragonal lattice, and (b) the cubic as well as the tetragonal lattice is displayed in Fig. 13. Large diVerence (lower the Pr- and Mn-diVused zirconia region has a cubic structure and is consistent with the observation of grain growth in the curve in each figure) in reflection intensities between the observed (+ markers) and the calculated (continuous line) diVusion layer because usually dopant-stabilised cubic zirconia has much larger grain size than tetragonal TZ3Y.XRD profiles was observed from Fig. 13(a) when only a tetragonal lattice was used for the refinement. The fit, however, The alteration of the crystal structure from tetragonal to 2792 J. Mater.Chem., 1998, 8, 2787–2794Table 2 A summary of reaction products between PSM and TZ3Y at 1200 °C Products Specimen 4 h 24 h 168 h PSM-A/TZ3Y DiVusion layera Pr2Zr2O7 nuclei Pr2Zr2O7 islands DiVusion layer DiVusion layer PSM-B/TZ3Y DiVusion layer Pr2Zr2O7 layer Pr2Zr2O7 layer DiVusion layer DiVusion layer PSM-C/TZ3Y (Pr,Zr)Ox layer (Pr,Zr)Ox layer (Pr,Zr)Ox islands Pr2Zr2O7 layer Pr2Zr2O7 layer Pr2Zr2O7 layer DiVusion layer DiVusion layer DiVusion layer aDiVusion layer refers to Pr- and Mn-diVused zirconia layer.cubic indicates that at least part of the Pr and Mn ions have In PSM-A and PSM-B, there was no free praseodymia in the coating. The atomic ratio of Pr/Zr>1 at the interface with entered the lattice of zirconia. The interfacial reaction products between TZ3Y and PSM TZ3Y was unlikely to occur, and therefore, the Pr-rich phase (Pr,Zr)Ox was not formed.of three compositions at 1200 °C are summarised in Table 2. The formation of pyrochlore phase was delayed in PSM-A/TZ3Y and PSM-B/TZ3Y. It has been reported before 4 Discussion that A-site deficiency in the perovskite may suppress or delay the formation of pyrochlore phase in the LSM/YSZ The above results can be summarised as follows. 1 A diVusion layer of Pr and Mn in zirconia was formed system.4,6,11,13 This was explained by the hypothesis that the diVusion of Mn into YSZ produced chemically active La2O3, in all specimens regardless of the A-site stoichiometry and the heat treatment temperature. The Pr- and Mn-diVused zirconia which formed pyrochlore phase with YSZ.Therefore, extra manganese in the perovskite should suppress the formation of is cubic and its grain size is much larger than that in TZ3Y. 2 The pyrochlore phase was formed at the interface of PSM free La2O3, and hence the pyrochlore phase. However, this hypothesis cannot explain why the formation of pyrochlore with Pr- and Mn-diVused zirconia after heat treatment at 1200 °C for diVerent times for all compositions of PSM studied.phase was delayed in PSM-B/TZ3Y with no excess Mn in the perovskite. Furthermore, the results of the current study Significant amounts of pyrochlore phase were detected after 168 h in PSM-A/TZ3Y, after 24 h in PSM-B/TZ3Y and after showed clearly that the Pr and Mn ions had diVused into zirconia, regardless of the stoichiometry of PSM.The chemi- 4 h in PSM-C/TZ3Y. 3 Besides the pyrochlore phase, a Pr-rich (Pr,Zr)Ox phase cally active praseodymia was unlikely to have been produced during the interaction. It seems that a diVerent explanation is was detected at the interface of the A-site over-stoichiometric PSM-C/TZ3Y. The (Pr,Zr)Ox phase formed a distinct layer required to understand the delay for the formation of the pyrochlore phase in PSM-A/TZ3Y and PSM-B/TZ3Y. after 24 h of heat treatment at 1200 °C, and was much less conspicuous after 168 h.From the phase relations, it is known that the pyrochlore phase is formed only when the local Pr concentration has 4 Relatively thick layers of pyrochlore phase were formed after 24 h at 1400 °C for all compositions of PSM in contact reached the solubility limit in cubic zirconia.When there was no extra praseodymia in the PSM coating as in PSM-A/TZ3Y with TZ3Y. The degree of the reaction for PSM-B/TZ3Y was much lower than that for PSM-A/TZ3Y and PSM-B/TZ3Y. and PSM-B/TZ3Y, the atomic ratio of Pr/Zr was initially very low in zirconia in the area near the interface. With time more The dissolution of Pr ions in TZ3Y can be understood from the phase relations between zirconia and praseodymia. Zirconia Pr ions diVused into the region near the interface.Some of it diVused out into an area in zirconia further away from the can react with praseodymia, forming solid solutions varying from tetragonal zirconia, cubic zirconia, pyrochlore phase to interface due to the gradation of Pr concentration. It would therefore take some time at 1200 °C for the Pr ions to reach praseodymia with the increase of the praseodymia content at 1600 °C.20 The solid state reactions between zirconia and the solubility limit in cubic zirconia in the area near the interface.Accordingly, the formation of the pyrochlore phase praseodymia at 900 and 1100 °C have also been reported.21 The current study has shown that Pr ions can enter the lattice was delayed in PSM-A/TZ3Y and PSM-B/TZ3Y. As the Pr content in PSM-A was lower than in PSM-B, the migration of zirconia at 1200 and 1400 °C and form a cubic phase of zirconia solid solution. The Pr concentration increased in the of Pr ions from PSM-A to TZ3Y is therefore expected to be slower than from the stoichiometric PSM-B during the initial cubic zirconia with the heating time.When the solubility limit of Pr ions in cubic zirconia was reached, the pyrochlore phase stage of heat treatment. Therefore the delay for forming pyrochlore phase with PSM-A was more pronounced than crystallised out. In the A-site over-stoichiometric PSM-C coating, a that with PSM-B at 1200 °C. At 1400 °C the whole process was accelerated.A complete considerable amount of free praseodymium oxide is present, which reacted with TZ3Y when heat treated at high tempera- layer of pyrochlore phase was formed for all specimens after 24 h. It needs to be noted that diVerent from the observations ture. A gradation of Pr concentration from the Pr6O11/TZ3Y interface (high) to the Pr-diVused zirconia (low) is expected.at 1200 °C, the amount of pyrochlore phase formed with PSMA at 1400 °C is much more than with PSM-B. This may be Therefore, various layers ranging from (Pr,Zr)Ox with atomic ratio of Pr/Zr>1, the pyrochlore with Pr/Zr$1, to the explained from the change of the migration rate of Pr ions at various stages of heat treatment and in particular, the diVerence diVusion layer with Pr/Zr<1, were formed between PSM and TZ3Y at 1200 °C.However, when the free Pr6O11 was con- in PSM/substrate contact areas between the two specimens. As the Mn content in PSM-A was higher than in PSM-B, the sumed, no further (Pr,Zr)Ox phase was formed. The Pr ions from the (Pr,Zr)Ox solid solution, which contains the highest migration of Mn ions from PSM-A to zirconia is expected to be faster than from PSM-B during the initial stage of heat Pr concentration in the specimen, would diVuse through the pyrochlore layer and react with more zirconia.Therefore, the treatment. On the other hand, the higher Pr content in PSMB resulted in a further migration of Pr ions from PSM-B to amount of (Pr,Zr)Ox solid solution after 168 h was less than that after 24 h at 1200 °C.It is expected to eventually disappear zirconia. This means that more Mn ions would leave from PSM-A than from PSM-B and more Pr ions would leave from completely. J. Mater. Chem., 1998, 8, 2787–2794 2793PSM-B than from PSM-A. It is quite likely that after some MnO3/TZ3Y were reached after much longer time compared with (Pr0.8Sr0.2)1.05MnO3/TZ3Y. The formation of the pyroch- time of heat treatment (the initial stage), the chemical composilore phase, therefore, was delayed in these two specimens at tions for PSM-A and PSM-B may become somewhat similar. 1200 °C. The amount of the pyrochlore phase formed at the After this initial stage, the amount of the pyrochlore formed interface was determined by the A-site stoichiometry of PSM between PSM and the substrate will be primarily determined in the initial stages of heat treatment.The growth of the by the contact area between the two materials. A large contact pyrochlore layer after the initial stage, however, appeared to area, as present in PSM-A (Fig. 2), will lead to enhanced Pr be controlled by the contact area between PSM and the migration from PSM to the substrate.This mechanism would substrate. It is believed that the initial stage is complete after explain the fact that the pyrochlore layer formed with PSM-A 24 h at 1400 °C but not even after 168 h at 1200 °C for all the after 24 h at 1400 °C is thicker than with PSM-B. compositions of PSM. The A-site deficient (Pr0.8Sr0.2)0.9MnO3 It appears that at the initial stage of heat treatment, the in contact with TZ3Y, following long term heat treatment, will amount of Pr ions migrating into TZ3Y was determined mainly form more pyrochlore phase than the stoichiometric by the A-site stoichiometry of the perovskite.The composition (Pr0.8Sr0.2)MnO3 due to the large contact area between PSM- diVerence between the diVerent perovskites may disappear A and TZ3Y. after the initial stage of the heat treatment is complete.The amount of Pr ions diVusing into the substrate was then determined mainly by the contact area between PSM and the Acknowledgements substrate. It seems that the initial stage is complete after 24 h The authors are grateful to Dr. R. Ratnaraj and Mr. K. at 1400 °C but not yet after 168 h at 1200 °C for all the Wilshier for reviewing this paper, Mr. D.Milosevic for compositions of PSM. synthesising the PSM powders, Mr. R. Donelson for supplying The relatively large contact area between PSM-A and the the TZ3Y substrates, Mr. H. Jaeger for assistance with substrate may be attributed to the high sinterability for the A- microscopy, Natasha Rockelmann with XRD, and Kristine site deficient perovskite. This has been observed previously by Giampietro and Kylie Chapman with specimen and microother authors.13 It was assumed that the vacancy within the graph preparation.structure of the A-site deficient perovskite enhanced the cation diVusion in perovskite itself and hence improved the References sinterability of the perovskite. 1 N. Q. Minh, J. Am. Ceram. Soc., 1993, 76, 563. The highest reaction degree for PSM-C at 1400 °C among 2 S.P. S. Badwal and K. Foger, Mater. Forum, 1997, 21, 183. all the diVusion couples can be attributed to the presence of 3 L. Kindermann, D. Das, D. Dahadur, R. Weiß, H. Nickel and free Pr6O11 at the initial stage as well as to the high contact K. Hilpert, J. Am. Ceram. Soc., 1997, 80, 909. area between PSM-C and the substrate. 4 A. Mitterdorfer, M.Cantoni and L. J. Gauckler, in Second European Solid Oxide Fuel Cells Forum, ed. B. Thorstensen, Mn was always detected in the diVusion layer in the current Druckerei J. Kinzel, Gottingen, 1996, p. 373. study. It appears that both Pr and Mn migrate out of the 5 D. Kuscer, J. Holc, M. Hrovat, S. Bernik, Z. Samardzija and PSM electrode (for all compositions) into the electrolyte.D. Kolar, Solid State Ionics, 1995, 78, 79. However, some Pr is trapped by the formation of the pyro- 6 G. Stchniol, E. Syskakis and A. Naoumidis, J. Am. Ceram. Soc., chlore phase, while Mn keeps on diVusing into zirconia. This 1995, 78, 929. may explain much deeper diVusion zone for Mn than that for 7 H. Taimatsu, K. Wada and H. Kaneko, J. Am. Ceram. Soc, 1992, 75, 401.Pr even after longer heat treatment times. There is no consist- 8 C. Brugnoni, U. Ducati and M. Scagliotti, Solid State Ionics, 1995, ency in the literature about the manganese diVusion in yttria 76, 177. stabilised zirconia. Some authors have reported manganese 9 G. Stochniol, H. Grubmeier, A. Naoumidis and H. Nickel, in diVusion in zirconia,3,6,22,23 whereas others did not detect any Proceedings of the Fourth International Symposium on Solid Oxide Mn in zirconia.4,8,12 Waller et al.recently studied the manga- Fuel Cells (SOFC-IV), ed. M. Dokiya, O. Yamamoto, H. Tagawa and S. C. Singhal, The Electrochemical Society, Pennington, NJ, nese diVusion in single crystal and polycrystalline yttria stabil- 1995, p. 995. ised zirconia using XRD and dynamic secondary ion mass 10 C.Clausen, C. Bagger, J. B. Bilde-Sorensen and A. Horsewell, spectrometry techniques, and showed convincingly that Mn Solid State Ionics, 1994, 70/71, 59. ions have diVused into both single crystal and polycrystalline 11 T. Tsepin and S. A. Barnett, Solid State Ionics, 1997, 93, 207. zirconia.24 The diVusion coeYcient of Mn in polycrystalline 12 J. A. M. van Roosmalen and E.H. P. Cordfunke, Solid State Ionics, zirconia was significantly higher than that in the single crystal. 1992, 52, 303. 13 J. W. Stevenson, T. R. Armstrong and W. J.Weber, in ref. 9, p. 454. They concluded that in polycrystalline zirconia, the grain 14 H. Y. Lee and S. M. Oh, Solid State Ionics, 1996, 90, 133. boundary diVusion of Mn dominates the mechanism for Mn 15 T. Ishihara, T.Kudo, H. Matsuda and Y. Takita, J. Am. Ceram. migration into zirconia. The substrate used in the current Soc, 1994, 77, 1682. study is polycrystalline TZ3Y that has a much higher density 16 T. L. Wen, H. Tu, Z. Xu and O. Yamamoto, in Extended Abstracts of grain boundaries to provide diVusion paths for manganese of 11th International Conference on Solid State Ionics, Honolulu, Hawaii, Nov. 16–21, 1997, p. 188. than the 7.5 mol% yttria stabilised zirconia used by Waller 17 H. M. Rietveld, J. Appl. Crystallogr., 1969, 2, 65. et al.24 Therefore, a considerable amount of manganese 18 R. J. Hill and C. J. Howard, Report No. M112, Australian Atomic diVusion into TZ3Y at both 1200 and 1400 °C is expected as Energy Commission (now ANSTO), Lucas Heights Research indeed was the case in the present study. Laboratories, New SouthWales, Australia, 1986. 19 C. J. Howard, R. J. Hill and B. E. Reichert, Acta Crystallogr., Sect. B, 1988, 44, 116. 5 Conclusions 20 R. L. Withers, J. G. Thompson and P. J. Barlow, J. Solid State Chem., 1991, 94, 89. The interfacial reactions between (Pr0.8Sr0.2)yMnO3 electrode, 21 M. K. Nasakar and D. Ganguli, J.Mater. Sci., 1996, 31, 6263. with diVerent A-site stoichiometry, and TZ3Y electrolyte at 22 A. Khandkar, S. Elangovan and M. Liu, Solid State Ionics, 1992, 1200 and 1400 °C in air have been studied. It has been found 52, 57. that both Pr and Mn ions diVused into TZ3Y forming a cubic 23 S. K. Lau and S. C. Singhal, in Corrosion 85, The National zirconia solid solution. When the solubility limit of Pr ions in Association of Corrosion Engineers (NACE) Meeting, Boston, MA, March 25–29, 1985, p. 345/1. the cubic zirconia was reached, a pyrochlore phase was formed. 24 D.Waller, J. D. Sirman and J. A. Kilner, in Proceedings of the Fifth When there was free praseodymia in the PSM coating, a International Symposium on Solid Oxide Fuel Cells (SOFC-V), ed. praseodymia–zirconia solid solution (Pr,Zr)Ox was formed, U. Stimming, S. C. Singhal, H. Tagawa and W. Lehnert, The which occurred only in the case of the (Pr0.8Sr0.2)1.05- Electrochemical Society, Pennington, NJ, 1997, p. 1140. MnO3/TZ3Y couple. The solubility limit of Pr in zirconia for specimens (Pr0.8Sr0.2)MnO3/TZ3Y and (Pr0.8Sr0.2)0.9- Paper 8/05835K 2794 J. Mater. Chem., 1998, 8, 2787–2794