Rheological design of

ceramic pastes for the co-

extrusion of solid oxide

fuel cells

Jonathan Powell1 and Stuart Blackburn2 1. Faculty of Engineering, Chulalongkorn University, Thailand

2. Department of Chemical Engineering, The University of Birmingham, UK

Project summary

Binder rheological design

Paste rheological design

Co-extruder design

Pressure modelling

Co-extrusion

Motivation

Sustainable energy production.

Wide range of fuels

Rapid start up

Thermally and mechanically stable

High power densities

Opportunity to reduce production

costs

Objectives and scope Micro tubular

solid oxide fuel cell

Co-extrusion

Liang & Blackburn, J. Mat. Sci., 37, 2002

Stage 1 Stage 2

Stage 3 Stage 4

Building of

repeat units Multi-billet extrusion

Building of

laminated tube

Liang & Blackburn, J. Mat. Sci., 37, 2002

Phase migration

Preferential flow of liquid – Unstable extrudate composition

– Initially low extrusion pressures,

followed by very high pressures

(due to depletion of liquid from

barrel)

l

PRum

2

Paste rheology

Yield stress and shear thinning behaviour – Retain shape after extrusion

– Low extrusion pressures required

Main controlling factors – Liquid volume fraction

– Rheology of liquid phase

– Particle properties and behaviour

Shape and size

Particle wetting

Particle dispersion and stability

Paste rheology unification

The pastes must flow in a uniform manner

The pastes must therefore have unified rheological properties

Binder properties

High apparent viscosity – Reduce tendency for phase migration

Shear thinning – Lower apparent viscosity at high shear rates where less tendency

for phase migration

– Increased apparent yield stress

Very high polymer content stabilises powder

dispersion

Low volatility liquid carrier – Prevent localised loss of liquid

Binder rheology

Controlled stress rotational cone and plate

rheometer

Time sweep, change in viscosity with time

Flow curve

Dynamic flow curve

Time sweep

Steady shear and complex viscosity

Paste rheological characterisation

nm VD

L

D

DVPPP

0

0021 4ln)(2

σo Bulk yield stress

α Velocity factor convergent flow

n Velocity exponent convergent

flow

τo Die wall shear stress

Die wall velocity factor

m Die wall velocity exponent

Binder volume fraction – twin roll

mill mixed

Binder volume fraction – twin roll

mill mixed

Binder volume fractions – z-blade

mixed

Binder volume fractions – z-blade

mixed

Bulk shear and plug flow

Acknowledgements

Thanks goes to;

EPSRC for the funding of this research

S Blackburn 2004

0 m 0 nPaste

Benbow Bridgwater parameters

1 0.20 19.08 0.37 0.04 12.85 0.45

2 0.75 53.69 0.50 0.02 9.55 0.41

3 0.71 40.68 0.48 0.00 6.27 0.35

4 0.74 61.15 0.55 0.08 11.05 0.44

5 0.58 34.92 0.48 0.14 15.25 0.54

42

3

1

3

41

8 w

i

w

im

l

PRu