**3.1 Effect of sintering stage on the real structure of SOFC materials**

### **Electrolytes**

Fresh (after synthesis) powders of GDC and BE are characterized by cubic fluorite –like structure, with some tetragonal distortions for YSZ. After sintering stage (radiation-thermal

Advanced Sintering Techniques in Design of

Planar IT SOFC and Supported Oxygen Separation Membranes 127

Fig. 3 presents high resolution TEM images of GDC sample -fresh and after RTS or MH. Fresh sample consists of disorderly stacked nanosized (10-20 nm) domains. Sintering increases domains size up to 50-100 nm and provides their coherent stacking. EDX analysis revealed that the chemical composition of domains in sintered oxide is close to that in fresh sample Ce0.86-0.91Gd0.14-0.09. Similar variation of particle size and the mode of stacking after

TEM images for BYS electrolyte are presented in Fig. 4. The 111 lattice spacing observed (3.172 Å) is close to that in Bi1.5Y0.5O3 (PDF#33-223 (Fm3m). The chemical composition of fresh sample at particles edges is characterizes by a higher bismuth content (Bi1Y0.9Sm0.6 ) as compared with the core (Bi0.4Y1Sm0.3). This segregation disappears after sintering being accompanied by rotational disordering of nanosized domains stacking. Hence, cooperative rhombohedral distortion of fluorite-like BYS structure in fresh sample can be caused by

a b

c

Fig. 3. TEM images of GDC after synthesis (a), radiation-thermal sintering at 1200 C

(b) or microwave heating at 1200C (c).

radiation-thermal treatment was observed by TEM for YSZ powder

spatial nonuniformity of dopants distribution in particles.

treatment, microwave or conventional heating) new phases were not revealed and no significant changes in diffraction pattern could be observed (Fig. 1). For fresh BYS sample, diffraction pattern corresponds to rhombohedral Bi0.775Sm0.225O1.5 phase [89-4391, 44-0043] (Fig. 2). After sintering, it transforms into cubic fluorite phase of Bi1.5Y0.5O3 type [33-0223] with lattice parameter a =5.511 Ǻ (CH) - 5.507 Ǻ (RTS). The lattice parameters of GDC and YSZ calculated for fresh or sintered samples vary only slightly (for 1-2%). Diffraction peaks are narrowed after sintering reflecting grain growth. Thus, X-ray particle sizes calculated by using the Scherrer equation change from 10.8 nm for fresh YSZ sample to 59.8 nm after radiation treatment. For BYS, variation is smaller –from 66.0 nm for fresh sample to 80 nm for sample after RTS.

Fig. 1. Effect of sintering on XRD patterns of GDC electrolyte fresh (a), after microwave heating (b) and after radiation-thermal treatment (c).

Fig. 2. Effect of sintering on XRD patterns of BYS electrolyte fresh (a), after conventional heating (b) or microwave heating (c).

treatment, microwave or conventional heating) new phases were not revealed and no significant changes in diffraction pattern could be observed (Fig. 1). For fresh BYS sample, diffraction pattern corresponds to rhombohedral Bi0.775Sm0.225O1.5 phase [89-4391, 44-0043] (Fig. 2). After sintering, it transforms into cubic fluorite phase of Bi1.5Y0.5O3 type [33-0223] with lattice parameter a =5.511 Ǻ (CH) - 5.507 Ǻ (RTS). The lattice parameters of GDC and YSZ calculated for fresh or sintered samples vary only slightly (for 1-2%). Diffraction peaks are narrowed after sintering reflecting grain growth. Thus, X-ray particle sizes calculated by using the Scherrer equation change from 10.8 nm for fresh YSZ sample to 59.8 nm after radiation treatment. For BYS, variation is smaller –from 66.0 nm for fresh sample to 80 nm

20 30 40 50 60

c b a

c

b

a

2 Theta

20 30 40 50 60 70 80

2 Theta

Fig. 2. Effect of sintering on XRD patterns of BYS electrolyte fresh (a), after conventional

F F

F

Fig. 1. Effect of sintering on XRD patterns of GDC electrolyte fresh (a), after microwave

for sample after RTS.

Intensity, a.u.

Intensity, a.u.

heating (b) or microwave heating (c).

heating (b) and after radiation-thermal treatment (c).

F

F F

F

Fig. 3 presents high resolution TEM images of GDC sample -fresh and after RTS or MH. Fresh sample consists of disorderly stacked nanosized (10-20 nm) domains. Sintering increases domains size up to 50-100 nm and provides their coherent stacking. EDX analysis revealed that the chemical composition of domains in sintered oxide is close to that in fresh sample Ce0.86-0.91Gd0.14-0.09. Similar variation of particle size and the mode of stacking after radiation-thermal treatment was observed by TEM for YSZ powder

TEM images for BYS electrolyte are presented in Fig. 4. The 111 lattice spacing observed (3.172 Å) is close to that in Bi1.5Y0.5O3 (PDF#33-223 (Fm3m). The chemical composition of fresh sample at particles edges is characterizes by a higher bismuth content (Bi1Y0.9Sm0.6 ) as compared with the core (Bi0.4Y1Sm0.3). This segregation disappears after sintering being accompanied by rotational disordering of nanosized domains stacking. Hence, cooperative rhombohedral distortion of fluorite-like BYS structure in fresh sample can be caused by spatial nonuniformity of dopants distribution in particles.

Fig. 3. TEM images of GDC after synthesis (a), radiation-thermal sintering at 1200 C (b) or microwave heating at 1200C (c).

Advanced Sintering Techniques in Design of

domain.

Typical domain size for both samples is about 10-50 nm.

**3.2 Microstructure of sintered SOFC materials** 

**Solid electrolytes (GDC, BE)** 

porosity in observed in BYS (Fig. 8).

**Cathode materials (LFN-GDC, LFC-GDC)** 

radiation treatment.

Planar IT SOFC and Supported Oxygen Separation Membranes 129

Fig 6. presents high- resolution TEM images for LFN-GDC cathode composite after radiation-thermal treatment. As judged by TEM data, reasonably uniform spatial distribution of perovskite and fluorite domains is observed. As follows from EDX data, pronounced redistribution of elements between P and F domains takes place. La and Fe from perovskite phase move to fluorite-type oxide while cerium and gadolinium ions migrate to perovskite structure. Nickel has not demonstrated significant mobility. Domain boundaries appear to be rather coherent providing a good epitaxy between P and F phases.

Fig. 6. TEM images of LFN-GDC composite after RTS at 1100 C. 1- LFC domain, 2- GDC

Microstructure of sintered pellets of solid electrolytes was studied by SEM. Figs. 7. and 8 present typical microstructure of samples after radiation-thermal treatment and microwave heating. Both electrolytes demonstrate a high density without visible cracks. Some closed

The average grain size of GDC oxide after RTS (0.2-0.5 μm) seems bigger than domains of GDC sintered by microwave heating (0.1-0.4 μm). This fact can be explained by very fast diffusion in the solid under e-beam. Thus, microwave heating stimulates local radiation absorption, and only part of this energy dissipates into heat that stimulates grain growth. Moreover, RTS was held for 150 min vs 90 min of MH. For conventional sintering it is a

Sintering of bulk cathode composites was studied using radiation-thermal sintering and conventional heating techniques. In Figs. 9 and 10 values of density estimated by Archimedes method are presented in dependence on sintering temperature and duration of

common knowledge that increasing sintering time increases grain size.

Fig. 4. TEM images of BYS after synthesis (a) and microwave heating at 900С (b).

Hence, advanced sintering procedures resulted in substantial grain growth even for rather refractory electrolytes (GDC, YSZ) at moderate temperatures.

#### **Cathode nanocomposite materials**

After synthesis, complex perovskite oxides LFN and LFC (P) have the rhombohedral –type structure (space group R3-c) , and GDC - fluorite-type (F) cubic structure. The same structures are observed after sintering by conventional heating as well as by radiation treatment (Fig. 5). Some shift of reflections is observed after sintering due to redistribution of cations between P and F phases (Sadykov , 2010). X-ray particle sizes of both phases in composites increase with the temperature of sintering. Thus, for perovskite phases, after radiation-thermal sintering of composites at 1100C, X-ray particle sizes increase from 20-30 nm for fresh samples to 90-100 nm. Variation of more refractory GDC phase particle size is less pronounced -from 10-20 nm to ~ 40-50 nm.

Fig. 5. Effect of sintering on XRD patterns of LFN-GDC composite (a-GDC fresh, b-LFN fresh, c- LFN-GDC after CH 1300C, d - LFN-GDC, after RTS). P – perovskite-type phase, F – fluorite-type phase.

Fig. 4. TEM images of BYS after synthesis (a) and microwave heating at 900С (b).

refractory electrolytes (GDC, YSZ) at moderate temperatures.

**Cathode nanocomposite materials** 

less pronounced -from 10-20 nm to ~ 40-50 nm.

P

Intensity, a.u.

F – fluorite-type phase.

Hence, advanced sintering procedures resulted in substantial grain growth even for rather

After synthesis, complex perovskite oxides LFN and LFC (P) have the rhombohedral –type structure (space group R3-c) , and GDC - fluorite-type (F) cubic structure. The same structures are observed after sintering by conventional heating as well as by radiation treatment (Fig. 5). Some shift of reflections is observed after sintering due to redistribution of cations between P and F phases (Sadykov , 2010). X-ray particle sizes of both phases in composites increase with the temperature of sintering. Thus, for perovskite phases, after radiation-thermal sintering of composites at 1100C, X-ray particle sizes increase from 20-30 nm for fresh samples to 90-100 nm. Variation of more refractory GDC phase particle size is

20 30 40 50 60

2 Theta

Fig. 5. Effect of sintering on XRD patterns of LFN-GDC composite (a-GDC fresh, b-LFN fresh, c- LFN-GDC after CH 1300C, d - LFN-GDC, after RTS). P – perovskite-type phase,

P

P F

d

a b c

F

P

P

F

P

F

P

F

Fig 6. presents high- resolution TEM images for LFN-GDC cathode composite after radiation-thermal treatment. As judged by TEM data, reasonably uniform spatial distribution of perovskite and fluorite domains is observed. As follows from EDX data, pronounced redistribution of elements between P and F domains takes place. La and Fe from perovskite phase move to fluorite-type oxide while cerium and gadolinium ions migrate to perovskite structure. Nickel has not demonstrated significant mobility. Domain boundaries appear to be rather coherent providing a good epitaxy between P and F phases. Typical domain size for both samples is about 10-50 nm.

Fig. 6. TEM images of LFN-GDC composite after RTS at 1100 C. 1- LFC domain, 2- GDC domain.
