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: alarrea@unizar.es

- 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.