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Slip-casting of alumina bodies with differential porosities
Ceramics hlternational 19 (1993) 121-124
Slip-Casting of Alumina Bodies w i t h Differential Porosities A. G. Lamas, Margarida Almeida & H. M. M. Diz Departamento de Engenharia Cer~mica e do Vidro, Universidade de Aveiro, 3800 Aveiro, Portugal (Received 20 January 1992; accepted 30 March 1992)
Abstract: Rheological and sedimentation studies were used to determine ftoc
structure in aqueous alumina suspensions in which the degree of flocculation was varied by modifying the pH of the suspensions. A slip-casting technique was developed to allow fabrication of alumina bodies with differential porosities by varying the degree of flocculation of the suspensions during the forming period. The porosity and density of the pre-sintered bodies were determined. The results show that the conditions of the starting suspensions influence the density of the bodies and that their variation during the slip-casting process enable samples to be fabricated with differential porosity.
1 INTRODUCTION
isoelectric point, there is no structure in suspension. At the isoelectric point, there is no repulsion between the particles, van der Waals forces bring them together, and flocculation occurs. If the particles combine to form flocs and floc networks (aggregates), a dynamic equilibrium is established between the aggregate growth and destruction at any given shear rate. Assuming that the flocs are the basic flow units, the plastic and pseudo-plastic behaviour of suspensions can be explained in terms of an elastic-floc model. 2 According to this model, the Bingham-yield value is a measure of the degree of coagulation of the suspension. It is dependent on: the volume fraction of flocs, Or; its radius; the number of bonds between particles per floc; and the energy required to separate a doublet of flow units. Plastic viscosity can be related to ~F by using empirical equations (Thomas's equation2), and, this being so, one can say that plastic viscosity is a measure of the concentration of flow units. Floc density can be estimated from another parameter:
The main objective of this investigation was the production, by slip-casting, of alumina bodies with differential porosities, so that impregnation with molten metal could be carried out. It was then necessary to study the factors that influence the porosity of cast walls, namely the degree of deflocculation of the starting suspension, the water/plaster ratio, and the casting time. The first factor studied was the degree of deflocculation of the alumina suspension. The main thought was that, whereas deflocculated suspensions lead to dense cast walls, flocculated ones give rise to porous cast walls. Oxide particles in aqueous suspension carry a charge that is dependent on the pH of the suspension and adsorption of ions. This charge results in the formation of an electrical double layer around the particles, and an isoelectric point may be defined at a certain pH value. Interaction between the particles in suspension is dependent on the nature and thickness of the electrical double layers associated with them. When the ionic strength of the suspending medium is low, particles with similar electrical double layers repel each other. Thus, at pH values away from the
OF. -- CF/~.
which means the relation between the floc-volume fraction, Jr, and the particle-volume fraction, ~p. 121
122
A. G. Lamas, Margarida Almeida, H. M. M. Diz
Firth and Hunter 3 have been able to explain the flow behaviour of several systems of oxide particles, based on this model. They estimated the distance of closest approach of colliding flow units. Calculations of the hydrodynamic force which arises from the field acting on the doublet have shown that it is adequate to separate the flocs.
1.4/1. Cylindrical pellets with differential porosities were obtained by first pouring a focculated suspension into the mould, this being followed by a deflocculated one. Moulds were filled with suspensions. Drainage of excess of suspension was carried out after formation of the wall. After mould release, the samples were dried at 40°C for two hours and then dried at 110°C to constant weight. Pre-sintering at 1450°C for three hours was carried out and was followed by cutting off the samples and analyzing them by mercury porosimetry.
2 EXPERIMENTAL
2. 1 Material The starting material was an alumina powder from Reynolds (RC 172 DBM), which had already been characterized. 4 Hydrochloric acid and sodium hydroxide solutions (10-1 m o l d m -3) were used to vary the degree of flocculation/deflocculation of the particles in suspension.
3 RESULTS
DISCUSSION
3. 1 Rheological properties Variation of the Bingham-yield value with the pH of the suspensions is shown in Fig. 1. From this graph, one can see that alumina suspensions are deflocculated at low pH values (4-4.5) and also at high pH values (9,5). These variations may be related to variations in the surface potential of the particles. The results point out to an isoelectric point around a pH of 8.0. This conclusion is in agreement with other authors. 5,6 Variation of the plastic viscosity of the suspensions with pH confirms these results, as can be seen from Fig. 2.
2.2 Equipment and procedure The theological properties of aqueous suspensions with a 90% (wt) concentration were measured with a Ferranti-Shirley cone-and-plate viscometer, reaching a maximum speed of, 1000r/min, followed by mechanical stirring, for a period of 30 min. Particle-size analysis for the determination of flow units was carried out in dilute suspensions (0.01% wt) with a sedimentograph (Lumosed-RETCH). Electrophoretic mobilities were determined in dilute suspensions (0.01% wt) by using a Rank Brothers microelectrophoresis apparatus and flat cells. All dispersions that were used in the electrophoresis experiments were prepared in 10 - 3 m o l d m - 3 KC1 solution in order to maintain the electrical double-layer thickness constant. The conformation was carried out by pouring the slips into plaster moulds with a plaster/water ratio of
AND
3.2 Floc properties From particle size analysis at each pH value, the dimensions and concentration of flocs formed in suspension were determined. Variation of the maximum diameter of flocs with the pH of the suspension can be seen in Fig. 3. These
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pH Alumina Suspensions R e l a t i o n s h i p b e t w e e n Ringharn yield s t r e s s , ~B, and p H
Fig. I. Variation of Bingham-yieldstress with pH of suspensions (90% wt% solids).
0 0
I 2
I 4
I 6
I 8
I 10
12
pH Alumina Suspensions Variation of plastic viscosity with pH Fig. 2. Variation of plastic viscosity with pH of suspensions
(90wt% solids).
123
Slip-casting of alumina bodies with differential porosities 1.0
1.8
----
pH = 4 . 0 pH: 8.5
1.4 A
E
E 1.0
t~
O E ._m a
II
0.5 0.6
e, 0.2
I 2
0
I 4
I 6
I 8
\
I 10
12
0 2.0
pH Alumina Suspensions Flow units m a x i m u m diameter
3.0
2.5 L o g r p ( ~, ) Alumina
Fig. 3. Variation o f maximum diameter of flow units with pH of suspensions (0"01 w t % solids).
casts
(a)
1.0 ----
pH= pH=
4.0 9.5
100 @
E
8o
:1 >
C
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t
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8
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12
3.0
2.5 Log rp ( A i
pH Alumina Suspensions % of f l o w units below 0.8 i~m
Alumina
casts
(b)
Fig. 4. Variation of percentage o f flow units below 0"8/lm with pH of suspensions (0'01 wt% solids). 1.0 -----
pH = 4 . 0 pH=10.5
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0.4
O
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4
5
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7
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9
10
11
12
pH Alumina Suspensions Variation of floc volume fraction with pH Fig. 5. Variation of floc volume fraction with pH of suspensions (0"01 w t % solids).
Alumina
casts
(c) Fig. 6. Pore-size distribution in pre-sintered pellets, as measured by intruded mercury volume: (a) ( - - p H = 4.0; - - pH = 8.5); (b) ( - - p H = 4.0; - - pH = 9.5); (c) ( - - p H = 4-0; - - pH = 10.5).
124
results confirm what was expected from rheological measurements, i.e. flocs with a m a x i m u m diameter are formed at pH values around 8.5-9.0, whereas, at a pH of 6.0, the flocs show a minimum diameter. The same conclusion can be drawn from plastic-viscosity measurements (Fig. 2). The variation of the number o f flocs with a minimum diameter in suspension also confirms what was said, as can be seen from Fig. 4. A greater percentage of flocs with a minimum diameter is formed at pH = 6"0, whereas, at pH values around 8-5-9-0, the number is considerably less. The effective floc-volume fraction, q~v, was obtained from the plastic viscosity with the aid of the Thomas equation. 3"v Results obtained are shown in Fig. 5. Thus, one can conclude that the measurements of floc properties confirm the results obtained from rheological determinations, since the variation of the plastic viscosity with the pH of the suspending medium may be related to variations in the size and concentration of flow units. Furthermore, the variation in flow-unit size with pH can be explained in terms of the elastic-floc model and is related to the particle-particle interactions. 3.3 Pore-size distributions After calcination and pre-sintering of pellets, they were cut into two parts (one more dense and the other more porous) and pore-size distribution was measured by mercury intrusion. In Figs 6(a), 6(b), and 6(c), one can observe the results for pellets in which the denser part was obtained from stabilized suspensions at p H = 4 . 0 and the more porous from suspensions with pH = 8.5, 9.5, and 10.5. F r o m these graphs, one can see that there is a real
A. G. Lamas, Margarida Almeida, H. M. M. Diz
difference between the pore-size distribution in the denser part and that in the porous part. If the poresize distribution for the three different porous parts are compared, one can conclude that they are very similar. In addition, for the denser part of each pellet, there is some variation in the shape of the pore-size frequency curve, reflecting a small influence by the different porous parts through the interface between the denser and the porous parts of each pellet.
4 CONCLUSIONS F r o m the present investigation, it can be concluded that the flow-unit structure in suspension influences the porosity of the cast wall: voluminous, lowdensity flocs lead to more porous walls; denser flocs lead to less porous walls.
REFERENCES 1. MORENO, R., MOYA, J. S. & REQUENA, J., Electroquimica de suspensiones ceramicas, BoL Soc. Esp. Ceram. VMr., 26 (1987) 355-65. 2. THOMAS, D. G., Transport characteristics of suspension. VIII: A note on the viscosity of Newtonian suspensions of uniform spherical particles. J. Colloid Sci., 20(1965) 267-73. 3. FIRTH, B. A. & HUNTER, R. J., Flow properties of coagulated colloidal suspensions. 1: Energy dissipation in the flow units. J. Colloid Interlace Sci., 57 (1976) 248. 4. MOYA, J. S. & AZA, S. (editors), Proc. Adv. Cer., Soc. Esp. Cer. Vid., 1986, pp. 20-36. 5. CHOU, C. C. & SENNA, M., Correlation between rheological behavior of aqueous suspensions of AI20 3 and properties of cast bodies: effects of dispersant and ultrafine powders. Amer. Ceram. Soc. Bull., 66 (1987) 1129. 6. REQUENA, J., MORENO, R. & MOYA, J. S., Alumina and alumina/zirconia multilayer composites obtained by slip casting, J. Amer. Ceram. Soe., 72 (1989) 1511. 7. KUNO, H. & SENNA, M., A practical analysis of pseudoplastic flow of suspensions. J. Colloid Interlace Sci., 89 (1982) 591 9.
Slip-Casting of Alumina Bodies w i t h Differential Porosities A. G. Lamas, Margarida Almeida & H. M. M. Diz Departamento de Engenharia Cer~mica e do Vidro, Universidade de Aveiro, 3800 Aveiro, Portugal (Received 20 January 1992; accepted 30 March 1992)
Abstract: Rheological and sedimentation studies were used to determine ftoc
structure in aqueous alumina suspensions in which the degree of flocculation was varied by modifying the pH of the suspensions. A slip-casting technique was developed to allow fabrication of alumina bodies with differential porosities by varying the degree of flocculation of the suspensions during the forming period. The porosity and density of the pre-sintered bodies were determined. The results show that the conditions of the starting suspensions influence the density of the bodies and that their variation during the slip-casting process enable samples to be fabricated with differential porosity.
1 INTRODUCTION
isoelectric point, there is no structure in suspension. At the isoelectric point, there is no repulsion between the particles, van der Waals forces bring them together, and flocculation occurs. If the particles combine to form flocs and floc networks (aggregates), a dynamic equilibrium is established between the aggregate growth and destruction at any given shear rate. Assuming that the flocs are the basic flow units, the plastic and pseudo-plastic behaviour of suspensions can be explained in terms of an elastic-floc model. 2 According to this model, the Bingham-yield value is a measure of the degree of coagulation of the suspension. It is dependent on: the volume fraction of flocs, Or; its radius; the number of bonds between particles per floc; and the energy required to separate a doublet of flow units. Plastic viscosity can be related to ~F by using empirical equations (Thomas's equation2), and, this being so, one can say that plastic viscosity is a measure of the concentration of flow units. Floc density can be estimated from another parameter:
The main objective of this investigation was the production, by slip-casting, of alumina bodies with differential porosities, so that impregnation with molten metal could be carried out. It was then necessary to study the factors that influence the porosity of cast walls, namely the degree of deflocculation of the starting suspension, the water/plaster ratio, and the casting time. The first factor studied was the degree of deflocculation of the alumina suspension. The main thought was that, whereas deflocculated suspensions lead to dense cast walls, flocculated ones give rise to porous cast walls. Oxide particles in aqueous suspension carry a charge that is dependent on the pH of the suspension and adsorption of ions. This charge results in the formation of an electrical double layer around the particles, and an isoelectric point may be defined at a certain pH value. Interaction between the particles in suspension is dependent on the nature and thickness of the electrical double layers associated with them. When the ionic strength of the suspending medium is low, particles with similar electrical double layers repel each other. Thus, at pH values away from the
OF. -- CF/~.
which means the relation between the floc-volume fraction, Jr, and the particle-volume fraction, ~p. 121
122
A. G. Lamas, Margarida Almeida, H. M. M. Diz
Firth and Hunter 3 have been able to explain the flow behaviour of several systems of oxide particles, based on this model. They estimated the distance of closest approach of colliding flow units. Calculations of the hydrodynamic force which arises from the field acting on the doublet have shown that it is adequate to separate the flocs.
1.4/1. Cylindrical pellets with differential porosities were obtained by first pouring a focculated suspension into the mould, this being followed by a deflocculated one. Moulds were filled with suspensions. Drainage of excess of suspension was carried out after formation of the wall. After mould release, the samples were dried at 40°C for two hours and then dried at 110°C to constant weight. Pre-sintering at 1450°C for three hours was carried out and was followed by cutting off the samples and analyzing them by mercury porosimetry.
2 EXPERIMENTAL
2. 1 Material The starting material was an alumina powder from Reynolds (RC 172 DBM), which had already been characterized. 4 Hydrochloric acid and sodium hydroxide solutions (10-1 m o l d m -3) were used to vary the degree of flocculation/deflocculation of the particles in suspension.
3 RESULTS
DISCUSSION
3. 1 Rheological properties Variation of the Bingham-yield value with the pH of the suspensions is shown in Fig. 1. From this graph, one can see that alumina suspensions are deflocculated at low pH values (4-4.5) and also at high pH values (9,5). These variations may be related to variations in the surface potential of the particles. The results point out to an isoelectric point around a pH of 8.0. This conclusion is in agreement with other authors. 5,6 Variation of the plastic viscosity of the suspensions with pH confirms these results, as can be seen from Fig. 2.
2.2 Equipment and procedure The theological properties of aqueous suspensions with a 90% (wt) concentration were measured with a Ferranti-Shirley cone-and-plate viscometer, reaching a maximum speed of, 1000r/min, followed by mechanical stirring, for a period of 30 min. Particle-size analysis for the determination of flow units was carried out in dilute suspensions (0.01% wt) with a sedimentograph (Lumosed-RETCH). Electrophoretic mobilities were determined in dilute suspensions (0.01% wt) by using a Rank Brothers microelectrophoresis apparatus and flat cells. All dispersions that were used in the electrophoresis experiments were prepared in 10 - 3 m o l d m - 3 KC1 solution in order to maintain the electrical double-layer thickness constant. The conformation was carried out by pouring the slips into plaster moulds with a plaster/water ratio of
AND
3.2 Floc properties From particle size analysis at each pH value, the dimensions and concentration of flocs formed in suspension were determined. Variation of the maximum diameter of flocs with the pH of the suspension can be seen in Fig. 3. These
o. m
4
300d
#.
in
o
3-
i.
•;
=oo0 0
"o m
o
"~
100
-
o
= c
0 0
I 2
I 4
2-
rj
E
m ,1=
-
I 6
I 8
I 10
¢U O.
1
12
pH Alumina Suspensions R e l a t i o n s h i p b e t w e e n Ringharn yield s t r e s s , ~B, and p H
Fig. I. Variation of Bingham-yieldstress with pH of suspensions (90% wt% solids).
0 0
I 2
I 4
I 6
I 8
I 10
12
pH Alumina Suspensions Variation of plastic viscosity with pH Fig. 2. Variation of plastic viscosity with pH of suspensions
(90wt% solids).
123
Slip-casting of alumina bodies with differential porosities 1.0
1.8
----
pH = 4 . 0 pH: 8.5
1.4 A
E
E 1.0
t~
O E ._m a
II
0.5 0.6
e, 0.2
I 2
0
I 4
I 6
I 8
\
I 10
12
0 2.0
pH Alumina Suspensions Flow units m a x i m u m diameter
3.0
2.5 L o g r p ( ~, ) Alumina
Fig. 3. Variation o f maximum diameter of flow units with pH of suspensions (0"01 w t % solids).
casts
(a)
1.0 ----
pH= pH=
4.0 9.5
100 @
E
8o
:1 >
C
60
¢D
40
@
0.5
t
o ~-
o
\\ 2o o
I
I
2
o
I
4
I
6
I
8
10
0 2.0
12
3.0
2.5 Log rp ( A i
pH Alumina Suspensions % of f l o w units below 0.8 i~m
Alumina
casts
(b)
Fig. 4. Variation of percentage o f flow units below 0"8/lm with pH of suspensions (0'01 wt% solids). 1.0 -----
pH = 4 . 0 pH=10.5
E 0.5
B O >
0.4
O
r~
0.5
I\
I
~0.3 n,
E o>
/\/',I
0.2
0 2.0
_o o.1 [[
2.5
3.0
L o g r p I ~, ) 3
4
5
6
7
8
9
10
11
12
pH Alumina Suspensions Variation of floc volume fraction with pH Fig. 5. Variation of floc volume fraction with pH of suspensions (0"01 w t % solids).
Alumina
casts
(c) Fig. 6. Pore-size distribution in pre-sintered pellets, as measured by intruded mercury volume: (a) ( - - p H = 4.0; - - pH = 8.5); (b) ( - - p H = 4.0; - - pH = 9.5); (c) ( - - p H = 4-0; - - pH = 10.5).
124
results confirm what was expected from rheological measurements, i.e. flocs with a m a x i m u m diameter are formed at pH values around 8.5-9.0, whereas, at a pH of 6.0, the flocs show a minimum diameter. The same conclusion can be drawn from plastic-viscosity measurements (Fig. 2). The variation of the number o f flocs with a minimum diameter in suspension also confirms what was said, as can be seen from Fig. 4. A greater percentage of flocs with a minimum diameter is formed at pH = 6"0, whereas, at pH values around 8-5-9-0, the number is considerably less. The effective floc-volume fraction, q~v, was obtained from the plastic viscosity with the aid of the Thomas equation. 3"v Results obtained are shown in Fig. 5. Thus, one can conclude that the measurements of floc properties confirm the results obtained from rheological determinations, since the variation of the plastic viscosity with the pH of the suspending medium may be related to variations in the size and concentration of flow units. Furthermore, the variation in flow-unit size with pH can be explained in terms of the elastic-floc model and is related to the particle-particle interactions. 3.3 Pore-size distributions After calcination and pre-sintering of pellets, they were cut into two parts (one more dense and the other more porous) and pore-size distribution was measured by mercury intrusion. In Figs 6(a), 6(b), and 6(c), one can observe the results for pellets in which the denser part was obtained from stabilized suspensions at p H = 4 . 0 and the more porous from suspensions with pH = 8.5, 9.5, and 10.5. F r o m these graphs, one can see that there is a real
A. G. Lamas, Margarida Almeida, H. M. M. Diz
difference between the pore-size distribution in the denser part and that in the porous part. If the poresize distribution for the three different porous parts are compared, one can conclude that they are very similar. In addition, for the denser part of each pellet, there is some variation in the shape of the pore-size frequency curve, reflecting a small influence by the different porous parts through the interface between the denser and the porous parts of each pellet.
4 CONCLUSIONS F r o m the present investigation, it can be concluded that the flow-unit structure in suspension influences the porosity of the cast wall: voluminous, lowdensity flocs lead to more porous walls; denser flocs lead to less porous walls.
REFERENCES 1. MORENO, R., MOYA, J. S. & REQUENA, J., Electroquimica de suspensiones ceramicas, BoL Soc. Esp. Ceram. VMr., 26 (1987) 355-65. 2. THOMAS, D. G., Transport characteristics of suspension. VIII: A note on the viscosity of Newtonian suspensions of uniform spherical particles. J. Colloid Sci., 20(1965) 267-73. 3. FIRTH, B. A. & HUNTER, R. J., Flow properties of coagulated colloidal suspensions. 1: Energy dissipation in the flow units. J. Colloid Interlace Sci., 57 (1976) 248. 4. MOYA, J. S. & AZA, S. (editors), Proc. Adv. Cer., Soc. Esp. Cer. Vid., 1986, pp. 20-36. 5. CHOU, C. C. & SENNA, M., Correlation between rheological behavior of aqueous suspensions of AI20 3 and properties of cast bodies: effects of dispersant and ultrafine powders. Amer. Ceram. Soc. Bull., 66 (1987) 1129. 6. REQUENA, J., MORENO, R. & MOYA, J. S., Alumina and alumina/zirconia multilayer composites obtained by slip casting, J. Amer. Ceram. Soe., 72 (1989) 1511. 7. KUNO, H. & SENNA, M., A practical analysis of pseudoplastic flow of suspensions. J. Colloid Interlace Sci., 89 (1982) 591 9.