Channeled Ni-YSZ and Co-YSZ Anodes produced from Directionally Solidified Eutectics:
Microstructural Stability
M. A. Laguna-Bercero, A. Larrea*, R. I. Merino, J. I. Peña and V. M. Orera
Instituto de Ciencia de Materiales de Aragón, C.S.I.C. – U. Zaragoza
c/ María de Luna 3, E-50.018 Spain
*Presenting author: [email protected]
- Introduction
- Material preparation
• Directional solidification of eutectics
- Cermet properties
• Composition & Microstructure
• Ni-YSZ interfaces
- Stability of the cermet microstructure
• Ageing experiments
- Summary
Outline
SEM & TEMElectrical conductivityHg porosimetry
Channeled Ni-YSZ cermet
Directionally Solidified Eutectics DSEself-organized lamellar microstructure
&Strong interphase bonding
YSZ electrolyte deposited by MOCVD on a Ni-YSZ channeled cermet
Fracture of a Ni-YSZ channeled cermet
YSZ
Ni
pore
DIRECTIONAL DIRECTIONAL SOLIDIFICATIONSOLIDIFICATION
Directional Solidification of Eutectics
Conventional NiO-YSZ ceramic Lamellar melt grown composite
Minimization of the interfacial energy
Diffusion at the solid-liquid interface
20 m
Minimization of the interfacial energy:self-organized lamellar microstructure if vol% minority phase x> 28%
NiO-YSZ (x = 0.43)CoO-YSZ (x = 0.39)
low energy interfaces (strong bonding)
Diffusion at the solid-liquid interface:lamellar thickness controlled by the growth
rate
R = 117 m2 mm/h (NiO-CaSZ)1
2 R = 116 m2 mm/h (CoO-YSZ)2
SV = 2/
2√2SV =
31/4√x
1 R.I. Merino et al. Recent Res. Devel. Mat. Sci. 4 (2003) 1-24.2 J-i Echigoya and S. Hayashi, J. Crystal Growth 129 (1993) 699-705.
L
1 cm
NiO-YSZ PlateNiO-YSZ Plate
Laser-Assisted directional solidification
laser beam
heater
Ceramic substrate
Rods
Samples for microstructural stability experiments produced bySamples for microstructural stability experiments produced by
The Laser Floating Zone technique
p
cvc
vp
Molten zone
Ø = 1.5 mm l = 10 cm
after the NiO Ni reduction
• Microstructural characterization
• Electronic conductivity
• Hg porosimetry
rod axis
(CTE: = 10.8 x 10-6 K-1)
Microstructure: YSZ channels & porous metal channels
red. H2
Co-YSZ: 40.7 YSZ + 59.3 porous Co
75 NiO – 25 8YSZ (mol%)
80 CoO – 20 8YSZ (mol%)
Ni-YSZ: 45.6 YSZ + 54.4 porous-Ni
43% vol pore
57% vol metal
41% vol pore
59% vol metal
Ni-YSZ, fracture Co-YSZ, polished cross section
(CTE: = 10.7 x 10-6 K-1)
Co
YSZ
pore
Matching (002) Ni // (002)YSZ
Christensen & Carter, J. Chem. Phys. 114 (2001) 5816.
Works of separation from ab initio DFT calculations:
WZr/N i= 5014 mJ/m2
WO/Ni = 5743 mJ/m2
J. I. Beltrán et al., Phys. Rev B 68 (2003) 075401.
2 Å
[110]
[001]
[100]
[110]
Zr - YSZ
= 2.94%
[100]
[001][010]
Ni
sep
sep
(002) Ni // (002)YSZ
4 types of Ni-YSZ interfaces
(111) Ni // (002)YSZ
[110]YSZ – [110]NiO
[100]YSZ – [110]NiO
[110]YSZ – [100]NiO
[100]YSZ – [100]NiO
Polycrystalline Ni film deposited on a (100) cubic-ZrO2 substrate by MBE
Ni-YSZ0 h
Co-YSZ300 h
Ni-YSZ300 h
Co-YSZ0 h
AGEING?: Microstructure
Treatment in 4%H2-N2 at 900 ºC
The lamellar microstructure and the low energy interfaces between metal and YSZ ensure a good microstructural stability of the cermets
SEM & TEM: No microstructural evolution during the treatment
Measurements: RT - 4 point DC -
[1] Simwonis, D., Tietz, F. & Stöver, D., Solid State Ionics, 2000, 132, 241-251.
[2] Skartmousos, D., Tsoga, A., Noumidis, A. & Nikolopoulos, P., Solid State Ionics, 2000, 135, 439-444.
Ni Co
CERMET
Ni-YSZ
Ni-YSZ
[1]
[2]
RESISTIVITY( in cm)
112±11
6.84
256 -
6.24
37
833 -
METAL Bulk
Cermet
Metal
-
--
-
16 6
37
122
110±11 39
0 hours
300 hours
0 h
300 h
300 h
2860 h
1041
AGEING?: Electrical conductivity
100 mA
Cermet
Metal
0 hours 300 hourss
0 hours 300 hours
0,01 0,1 10,00
0,01
0,02
0,03
0,04
Diámetro ( m)
Ni-YSZ (0h) Ni-YSZ (300h)
0,00
0,02
0,04
0,06
0,08
Co-YSZ (0h) Co-YSZ (300h)
PORE SIZE DISTRIBUTION
(open/theoret)
(open/theoret)
AGEING?: Hg porosimetry
Co-YSZ
Ni-YSZ
% open
% theoret.
% relative
15.2
26.2
58.0
13.8
26.2
52.3
% open
% theoret.
% relative
14.8
23.4
63.2
16.0
23.4
68.5 Diameter (µm)
Cermetporosity
• Channelled Ni-YSZ and Co-YSZ cermets for use in SOFC
•Alternating channels (400 nm wide) of YSZ and porous metal
• Strong Ni-YSZ interfacial bonding
• The lamellar microstructure and the strong bonding between the YSZ
and the metal prevent the coarsening of the metal particles in working
conditions
• After 300 h in 4% H2-N2 at 900 ºC
No microstructural evolution in SEM & TEM observations.
No drop in electronic conductivity.
No significant pore evolution.
Summary
Acknowledgments:
Ministerio de Ciencia y Tecnología (Spain),
Project MAT2003-1182.
I3P Program, financed by the European Union.