**3.3 Microstructure of functional layers on metallic anodic substrates**

Typical SEM images of different electrolyte layers supported on anodic substrate NiO/YSZ/Ni-Al foam are shown in Fig. 12. GDC layer was sintered to complete density by radiation –thermal treatment at 1200C (Fig. 12a), which is ~ 200C lower than for conventional sintering. Bi-Er and BYS layer were sintered by this method to a similar density even at much lower (900C) temperature. Though small (less than 0.5 μm diameter) closed pores were observed, their share is estimated to be below 5%.

For Sr-doped lanthanum silicate electrolyte La9SrSi6O26.5 known for its poor sinterability (Sadykov et al, 2010), porosity is apparently bigger (Fig. 12d), though the layer is also rather dense. Hence, for the latter type of electrolyte, combination of radiation-thermal treatment with sintering aids could be applied to provide complete layer densification.

Sintering of cathode composites should provide reasonably porous (porosity ~ 30-40%) mechanically strong layers strongly attached to the layer of electrolyte. As follows from Fig. 13a, radiation-thermal treatment (as well as microwave radiation) provides required characteristics combining porosity with developed triple-phase boundary to ensure both developed surface area for oxygen molecules activation and diffusion paths for oxygen ions to enter the electrolyte layer. At the same temperatures cathode layer sintered by conventional heating was only weakly attached to electrolyte.

On the other hand, for nanocomposite mixed ionic-electronic conducting layers supported on the surface of oxygen separation membrane, complete density/absence of porosity is required to provide selective oxygen permeation. As shown in Fig. 13b, Bi-containing composites are sintered to complete density by radiation-thermal treatment at moderate (900C) temperatures, thus meeting requirement.

Fig. 11. Internal microstructure by SEM images in back-scattered electrons of LFN-GDC

Typical SEM images of different electrolyte layers supported on anodic substrate NiO/YSZ/Ni-Al foam are shown in Fig. 12. GDC layer was sintered to complete density by radiation –thermal treatment at 1200C (Fig. 12a), which is ~ 200C lower than for conventional sintering. Bi-Er and BYS layer were sintered by this method to a similar density even at much lower (900C) temperature. Though small (less than 0.5 μm diameter)

For Sr-doped lanthanum silicate electrolyte La9SrSi6O26.5 known for its poor sinterability (Sadykov et al, 2010), porosity is apparently bigger (Fig. 12d), though the layer is also rather dense. Hence, for the latter type of electrolyte, combination of radiation-thermal treatment

Sintering of cathode composites should provide reasonably porous (porosity ~ 30-40%) mechanically strong layers strongly attached to the layer of electrolyte. As follows from Fig. 13a, radiation-thermal treatment (as well as microwave radiation) provides required characteristics combining porosity with developed triple-phase boundary to ensure both developed surface area for oxygen molecules activation and diffusion paths for oxygen ions to enter the electrolyte layer. At the same temperatures cathode layer sintered by

On the other hand, for nanocomposite mixed ionic-electronic conducting layers supported on the surface of oxygen separation membrane, complete density/absence of porosity is required to provide selective oxygen permeation. As shown in Fig. 13b, Bi-containing composites are sintered to complete density by radiation-thermal treatment at moderate

**3.3 Microstructure of functional layers on metallic anodic substrates** 

closed pores were observed, their share is estimated to be below 5%.

conventional heating was only weakly attached to electrolyte.

(900C) temperatures, thus meeting requirement.

with sintering aids could be applied to provide complete layer densification.

nanocomposite, CH 1300 C.

Fig. 12. SEM images of thin (10 microns) layers of electrolytes supported on anodic substrate NiO/YSZ/Ni-Al foam and sintered by radiation-thermal treatment at different temperatures. a- GDC, 1200C; b - BE , 900C, c-BYS, 900C, d- La9SrSi6O26.5 with apatite structure, 1200C.

Fig. 13. SEM image demonstrating typical structure of LFN-GDC (a) and LaBiMn –BiEr layers (b) on Ni-Al foam-based substrates after radiation-thermal sintering at 900C.

Advanced Sintering Techniques in Design of

procedures are properly optimized.


(Sadykov, Pavlova et al, 2010).

**sintered functional layers** 

**3.5.1 Fuel cells** 

log σ, Ω−1

⋅cm

−1

Planar IT SOFC and Supported Oxygen Separation Membranes 135

composition in a static reactor under contact with the gas phase containing initially only 18O2 . This value can be expressed either in oxygen monolayers (XS) or as a fraction of all oxygen contained in the bulk of sample (XV). Fig. 16 presents such dependence for LFN-GDC composites with different ratio of perovskite/fluorite phases. As follows from these results, for composites with a higher content of GDC, already at 700C more than half of overall amount of bulk oxygen is exchanged, which is clearly possible only in the case of high oxygen mobility in the bulk of composite. The increase of oxygen mobility with the amount of GDC phase, and, hence, the length of perovskite-fluorite interface, agrees with suggestion that such an interface provides a path for fast oxygen diffusion due to its specific structure and composition (Sadykov et al, 2011). The values of Xv for LFN-GDC composites with a high GDC content are close to those for Sr-containing LSFC-GDC composites known for their very high oxygen mobility (Sadykov et al, 2008). Hence, even composites without Sr and Co in complex perovskites could provide a high bulk oxygen mobility combined with a low reactivity with respect to interaction with YSZ if their composition and sintering

1,0 1,5 2,0 2,5 3,0 3,5

LFC-GDC RT 11000

LFC-GDC CH 13000

GDC CH 13000

LFC lit CH 13000

C

C

C

C

1000/T

Fig. 15. Effect of sintering mode on specific conductivity of LFC-GD composite after radiation-thermal sintering and conventional heating compared with literature data

**3.5 Some characteristics of SOFC and oxygen separation membranes with co-**

For assembling cells, a home-made Ni/yttria-doped zirconia (YSZ) anode substrate (the exposed cell surface area 1 cm2 ) as well as Ni-Al foam supported NiO/YSZ layers with 10 micron thick 8YSZ layers supported by CVD were used. Cathode slurries made from

#### **3.4 Electrochemical and transport properties of cathode composite material**

#### **Conductivity**

Fig. 14 demonstrates that conventional sintering provides a lower specific conductivity even at higher sintering temperature than RTS. While the main factor in controlling conductivity is certainly residual porosity of pellets, note that for the radiation-thermal treatment somewhat higher conductivity was observed after sintering at lower (900C) temperatures. This can be explained by a loss of oxygen from LFN at higher temperatures leading to conductivity decrease (Sadykov, 2011).

Fig. 14. Effect of sintering mode on specific conductivity of LFN-GDC samples after radiation-thermal sintering and conventional heating compared with literature data (Sadykov, Pavlova et al, 2010).

For LFC-GDC composite (Fig.15) sintering by e-beam at 1100C for 1 h provides the same level of specific conductivity as prolonged (5 hours) conventional sintering at 1300C. Surprisingly dwelling under e-beam for 2 hours or more has not improved conductivity. This can be explained by more pronounced interaction between phases in this nanocomposite as compared with LFN-GDC. Indeed, the difference between specific conductivity of LFC and LFC-GDC composite is ~ 3 order of magnitude, which is much bigger than that observed for LFN and LFN-GDC, respectively (Fig. 14).

#### **Oxygen isotope heteroexchange**

As a measure of oxygen mobility in perovskite-fluorite nanocomposites, so called dynamic extent of oxygen isotope exchange was shown to be simple and efficient characteristic varying in parallel with the oxygen diffusion coefficient (Sadykov et al, 2009-2010). It is defined as the number of oxygen atoms exchanged up to a given temperature in the temperature –programmed mode of heating the sample with the natural oxygen isotope

Fig. 14 demonstrates that conventional sintering provides a lower specific conductivity even at higher sintering temperature than RTS. While the main factor in controlling conductivity is certainly residual porosity of pellets, note that for the radiation-thermal treatment somewhat higher conductivity was observed after sintering at lower (900C) temperatures. This can be explained by a loss of oxygen from LFN at higher temperatures leading to

1,0 1,5 2,0 2,5 3,0 3,5

LFN-GDC RT 9000

LFN-GDC RT 10000

LFN-GDC RT 11000

LFN-GDC CH 13000

GDC CH 13000

LFN lit CH 13000

C

C

C

C

C

C

1000/T

For LFC-GDC composite (Fig.15) sintering by e-beam at 1100C for 1 h provides the same level of specific conductivity as prolonged (5 hours) conventional sintering at 1300C. Surprisingly dwelling under e-beam for 2 hours or more has not improved conductivity. This can be explained by more pronounced interaction between phases in this nanocomposite as compared with LFN-GDC. Indeed, the difference between specific conductivity of LFC and LFC-GDC composite is ~ 3 order of magnitude, which is much

As a measure of oxygen mobility in perovskite-fluorite nanocomposites, so called dynamic extent of oxygen isotope exchange was shown to be simple and efficient characteristic varying in parallel with the oxygen diffusion coefficient (Sadykov et al, 2009-2010). It is defined as the number of oxygen atoms exchanged up to a given temperature in the temperature –programmed mode of heating the sample with the natural oxygen isotope

Fig. 14. Effect of sintering mode on specific conductivity of LFN-GDC samples after radiation-thermal sintering and conventional heating compared with literature data

bigger than that observed for LFN and LFN-GDC, respectively (Fig. 14).

**3.4 Electrochemical and transport properties of cathode composite material** 

**Conductivity** 

conductivity decrease (Sadykov, 2011).


(Sadykov, Pavlova et al, 2010).

**Oxygen isotope heteroexchange** 

log σ, Ω−1

⋅cm

−1

composition in a static reactor under contact with the gas phase containing initially only 18O2 . This value can be expressed either in oxygen monolayers (XS) or as a fraction of all oxygen contained in the bulk of sample (XV). Fig. 16 presents such dependence for LFN-GDC composites with different ratio of perovskite/fluorite phases. As follows from these results, for composites with a higher content of GDC, already at 700C more than half of overall amount of bulk oxygen is exchanged, which is clearly possible only in the case of high oxygen mobility in the bulk of composite. The increase of oxygen mobility with the amount of GDC phase, and, hence, the length of perovskite-fluorite interface, agrees with suggestion that such an interface provides a path for fast oxygen diffusion due to its specific structure and composition (Sadykov et al, 2011). The values of Xv for LFN-GDC composites with a high GDC content are close to those for Sr-containing LSFC-GDC composites known for their very high oxygen mobility (Sadykov et al, 2008). Hence, even composites without Sr and Co in complex perovskites could provide a high bulk oxygen mobility combined with a low reactivity with respect to interaction with YSZ if their composition and sintering procedures are properly optimized.

Fig. 15. Effect of sintering mode on specific conductivity of LFC-GD composite after radiation-thermal sintering and conventional heating compared with literature data (Sadykov, Pavlova et al, 2010).

#### **3.5 Some characteristics of SOFC and oxygen separation membranes with cosintered functional layers**

#### **3.5.1 Fuel cells**

For assembling cells, a home-made Ni/yttria-doped zirconia (YSZ) anode substrate (the exposed cell surface area 1 cm2 ) as well as Ni-Al foam supported NiO/YSZ layers with 10 micron thick 8YSZ layers supported by CVD were used. Cathode slurries made from

Advanced Sintering Techniques in Design of

0

O

promising for the practical applications.

**5. Acknowledgements** 

**6. References** 

substrate.

**4. Conclusions** 

2 flux, mlO2/cm2min

1

2

3

4

5

Planar IT SOFC and Supported Oxygen Separation Membranes 137

650 700 750 800 850 900 950

Advanced sintering techniques based upon radiation-thermal sintering by e-beam action and microwave heating allow to provide required density and consolidation of thin functional layers in design of intermediate temperature solid oxide fuel cells and oxygen separation membranes. Due to decreased temperature and duration of sintering as compared with conventional sintering methods, such negative phenomena as variation of functional layers phase composition, their cracking and damage of metallic substrates were prevented. For oxide mixed ionic-electronic conducting composites advanced sintering provides developed interfaces which act as paths for fast oxygen diffusion required for considered applications. As the results, fuel cells and oxygen separation membranes manufactured using advanced sintering techniques demonstrate performance characteristics

The authors gratefully acknowledge support from by Integration Project 57 (T09CO-003) SB RAS- NAS of Belarus, Russian Federation Government Grant N 11.G34.31.0033, OCMOL FP7 EC Project, Project 57 of RAS Presidium Program No. 27 and Contract 02.740.11.0852 of

Annenkov, Yu. (1996). Physical foundations high-temperature electron-beam of ceramics,

Alexeff, I. & Meek, T. (2011). The effect of electric field intensity on the microwave sintering

*Russian Physics Journal*, 39, No. 11, 1146-1159, ISSN: 1573-9228

of zirconia, *Materials Letter*, 65, 2111-2113, ISSN: 0167-577X

the Federal Program "Scientific and Educational Cadres of Russia.

Fig. 17. Temperature dependence of oxygen flux under air/He gradient for supported asymmetric membrane comprised of La-BiMnO-YSmBi layers on binary Ni-Al foam

Temperature, o

C

nano-powders ultrasonically dispersed in isopropanol with addition of butyral resin were deposited on half cells by air spray (perovskite + fluorite 10 microns thick nanocomposite functional layer, such as LSM-ScCeSZ or LSFN-GDC) and by painting (porous thick LSFN cathode layer) followed by drying and sintering at 900-1100C for 2 h using microwave or e-beam heating (Sadykov et al, 2011). The cell performance was evaluated using air at cathode side and humidified H2 at anode side with Pt current collectors adding Pt or Ar pastes on the cathode side. For these cells, the typical level of power density at 700C was in the range of 500 mW/cm2, which is promising for the practical application. Performance stability was demonstrated for short-term (~ 100 h) testing. No cracking or layers spallation was observed after testing. Decreased sintering temperature allowed to prevent any damages to anodic substrates during cells manufacturing as well as any undesired interaction between YSZ electrolyte and cathode layers leading to formation of isolating pyrochlore layers.

Fig. 16. Temperature dependence of the dynamic extent of exchange XV for LFN-GDC composites sintered by microwave heating at 1100C. PO2 = 4 Torr, heating ramp 5o/min.

#### **3.5.2 Membrane performance**

Fig. 17 presents the temperature dependence of oxygen flux through asymmetric supported oxygen separation membranes with Bi-containing perovskite-fluorite functional layers sintered by using microwave radiation (vide supra). As follows from these results, the values of oxygen flux are close to best results obtained with supported membranes in these conditions (Sadykov et al, 2010).

Fig. 17. Temperature dependence of oxygen flux under air/He gradient for supported asymmetric membrane comprised of La-BiMnO-YSmBi layers on binary Ni-Al foam substrate.
