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Effect of powder size on the stability of concentrated aqueous suspensions of Y-TZP
Pergamon
NanoStruduredMaterials.Vol. 10. No. 6. pp. 1081-1086.1998 Elsevier Science Ltd Q 1998 Acta MetallurgicaInc. F’rintedin the USA. All rightsreserved 0965-9773’98 $19.00 + .OO
PII SO9659773(98)00127-S
EFFECT OF POWDER SIZE ON THE STABILITY OF CONCENTRATED AQUEOUS SUSPENSIONS OF Y-TZP Jing Sun, Lian Gao and Jingkun Guo State Key Lab of High Performance Ceramics & Superfine Microstructure Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R.China (Accepted September 18,1998)
AbstractThe particle size differences among four Y-lZP powders are shown to have a significant e#ect on the rheological properties of concentrated aqueous suspensions of the powders. The viscosity of slurries is sensitive to solid loading, especially at high solid content level. As a result, thefourpowders require deferent amounts of dispersant to achieve their highest solid loadings. Under the same solid content, the smaller the particles and the more absorbed dispersant needed, the higher the viscosity of the slurry. 01998 Acta Metallurgica Inc.
1. INTRODUCTION
Colloidal processing of fine particles has been suggested as a preferred way to produce certain high quality ceramic materials (l-3). In all types of suspension shape forming techniques, ranging from
i 082
J SUN,
L @O
AND
J Guo
2. EXPERIMENTS Yttria stabilized tetragonal zirconia polycrystals containing 2.8 mol% ythia were prepared by coprecipitation method using zirconium oxychloride and yttrium chloride as starting materials and ammonia solution as the precipitation medium. The pH value during coprecipitation was maintained to be greater than 9. Cl- ions were removed completely by washing with distilled water and coprecipitation was then dispersed to prevent the formation of agglomerate. The precipitated precursor was then dried at 120°C overnight and calcined at 450°C (No. l), 700°C (No. 2), 900°C (No. 3), 1050°C (No. 4) for 2 hours respectively. Table 1 summarizes the physical properties of these powders. A polyelectrolyte dispersant used in the preparation of dispersed Y-TZP slurries was the ammonium salt of poly acrylic acid (abbreviated as NH&IA) with molecular weight of about 3000. The suspension was transferred into a plastic bottle which was loaded with zirconia grinding media, then agitated by a tarbomixer for 24 hours. Theological characteristics of the Y-TZP suspensions were determined using cylindrical measuring system on rotational viscosimeter (Model Rheomat 260, Mettler Toledo AG, Switzerland). Measurements were performed with a concentric cylinder measurement geometry MSO. All measurements were performed at a temperature of 25°C. To eliminate artifacts from different treatments during the filling procedure, the samples were presheared for 3 minutes, followed by 5 minutes at rest. Several modes of test are available.
3. RESULTS AND DISCUSSION 3.1 Optimization of the Amount of Dispersant Figure 1 shows the viscosity of powder No. 2 suspensions at a constant volume fraction of solids as a function of shear rate and amount of NH&AA added. Two of these suspensions display a shear thinning behavior with a drop of at least one order of magnitude of the viscosity Erom low shear rate to high shear rate. This change in the degree of shear thinning can be related to a change TABLE 1 The Properties of Four Powders Calcined at Different Temperatures
SSA (m2/g>
1050°C
900°C
700°C
450°C
8.01
18.3
48.1
81.9
125
27.3
10.4
6.0
83
36.8
24.1
10.0
grain size/nm (BET) grain size /fun O-EM)
EFFECTOF POWDERSIZE ONTHESTA~ILIWOF GDNCENTRATED ACUJEOUS SUSPENSIONS OFY-TZP
1083
shear ascending curve
120
180
240
300
Shear Rate ( s-‘) Figure 1.The relationship between the viscosity and shear rate when the solid content is 50 wt%.
of the colloidal stability. Concentrated colloidally stable suspension display a shear thinning behavior because of a perturbation of the suspension structure by shear. At low shear rates, the suspension structure is close to the equilibrium structure at rest because thermal motion dominates over the viscous forces. At higher shear rates, the viscous forces dominate and the plateau in viscosity in this region is a measure of the resistance to flow of a suspension with a completely hydrodynamically controlled structure. With increasing shear rate, the viscous forces tend to reduce the size of the aggregates, hence facilitating flow. The degree of shear thinning evaluated from the difference between the high and low shear rate viscosity :inFigure 1 appears to be constant when 1.5 wt% NH#AA is added, but increases rapidly at additions of 2.0 wt%. The adding of dispersant forms the repulsive effect on particles, making the attractive force decrease. Other parameters such as viscosity relaxation time and elasticity modules also decreased. More adding of dispersant can occupy a substantial volume and thus raise the effective volume fractioa of suspension and subsequently lead to higher viscosity. The addition of dispersant NH4pAA can effectively improve the flowability of Y-TZP slurry, but there exists an optimum amount of dispersant. 3.2 Influence of Particle Size on the Rheological Properties of Slurries Keeping; the solid content at 49 wt%, and dispersant at 1.12 wt% (relative to the weight of the powder), which is enough to reach minimum viscosity, we get the flow curve (Figure 2) of
J SUN,
1084
13760
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l.*...
.’ mmsam.~m . . ...*
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AND
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I
l***...*..
l**....
l **.
l
.,.....**
..........m........... .............=
.=.... :* . . l....* . . 2 ...*...*.*~~*~-~............._,.*. . . . ......*........................................~*.*.~.~' 8256mm =J'.' 2' c u .+ . . f . .. 5504w.
2752
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! . .. .. . .. ““...~.~.;‘;‘:‘;;,,.~.~~~~.,.~.‘.~*.‘.,.‘~~~,~~ .,-‘L...*... . 0 3.’ -LC-’ cym%e.M-0
20
40
Shear Rate
. . . . . . .(., ~.~rc.r.r.*.*r*.b~*.“.*.b*~.~~ 4 4 + . &._.y 80
60
100
( s-l )
Figure 2a. Keeping the solid content as 49 wt%, the relationship of shear stress with shear rate ascending curve: of slurries formed by different nano-size Y-‘El? 1,2,3,4-shear l’, 2’, 3',4-shear descending curve.
Shear Rate Figure2b.Keeping
( s-l)
the solid content as 49 wt%, the relationship Of ViSCOSitY w ith shear rate of slurries formed by different nano-size Y-Tzp.
EFFECTOF POWDER SIZEONTHESTABILINOF CONCENTRATED ACUEWSSUSPENSWS OFY-TZP
1085
slurries formed by different nano-size Y-TZP. Figure 2a is the relationship of shear stress with shear rate and Figure 2b is the relationship of viscosity with shear rate. The behavior of the shear stress as a function of shear rate can be fitted by the Ostwald model.
Where z is the shear stress, r is the shear rate, k is the fluid factor and n is the flow exponent. With increasing particle size, the fluid factor drops drastically, k of powder 1 is 7381, k of powder 2 is 6139, k of powder 3 is 795, and that ofpowder 4 is 218. The four kinds of slurries show reduction in apparent viscosity with increasing shear rate. That is typical of pseudoplasticity and indicates thixotropic rheology. Keeping the shear rate as 40 s-l, the viscosity is 269 mPa - s, 204 mPa - s, 31.0,mPa - s and 8.0 mPa - s respectively. This means under the same condition, the smaller the particle, the higher viscosity of its slurry. These phenomena can be explained by the following equation:
20
40
60
80
Specific Surface Area (m*/g) Figure 3. Relationship between specific surface area and viscosity. l-the solid content is 49 wt%, 2-the solid content is 65 wt%.
1086
J
SUN,
L GAO AND
J Guo
whereV&s the effective volume fraction which determine the viscosityof slurry, Sis the thickness of the polymer layer, a is the radius of the spherical particle and V is the volume fraction of the solids. Postulating that the dispersant forms a monolayer of polymer on the particles, the smaller the particles, the bigger the effective volume. That means although the net solid content of the four kinds of slurries is the same, the smaller powder needs more water to be wet and holds more space than the coarser powder. So the smaller powder has higher viscosity. 3.3 The Relationship
of Wscosity and Specific Surface Area (SSA)
We also studied the relationship between the SSAand viscosity when the slurry contains the same solid content. From Figure 3 we can see that if the solid content is 49 wt%, the viscosity does not change greatly with SSA. The volume of the powder is much lower compared to that of the solvent. There is enough water between particles to act as moving agents; so the viscosity changes little. When the solid content is 65 wt%, the viscosity increases with SSAexponentially, and the finest particle with higher SSA needs more dispersant to adsorb to be flowable. In fact, the upper limit of flowable shmy formed by powdercalciuated at 45O’C is 65 wt%, that of 700°C is 72 wt%, the upper limit of 900°C powder is 75 wt%, and the highest loading of 105O’C powder is 80 wt%. Specimens formed by small particle size powder are easy to sinter but difficult to make into concentrated slurries. In order to get high solid loading, control of particle size distribution is au advisable method. 4. CONCLUSIONS The addition of dispersant NH@AAcan effectively improve the flowability of slurries. An optimum condition exists for dispersant at certain solid content. The solid content and the powder size affect the rheological properties of slurry greatly. With the same solid loading, the smaller the particles, the higher viscosity of its slung. In order to have similar flowability, the smaller particles which have high SSA need to adsorb more dispersant. The upper limit of solid content changes with particle size greatly. In order to get concentrated slurry, control particle size distribution is au advisable method. REFERENCES 1.
2. 3.
4. 5. 6. 7 8.
Lange, Journal of the American Ceramic Society, 1989,72(l), 3. Roosen, A. and Brown, H.K., Journal of the American Ceramic Society, 1988,71(11), 970. Calve& P.D.,Tormey, E.S. and Pober, R.L., American Ceramic Society Bulletin, 1986,65(4), 669. Hyun,M.J., ChemicalProcessing ofceramics, eds. I.L.Burtrandand 3-A. Edward,Marcel Dekker, 1994, p. 157. Kramer, T. and Lange, F.F., Journal of the American Ceramic Society, 1!994,77(4), 922. Hacldey, V.A., Journal of the American Ceramic Society, 1997,80(g), 2315. Hider, PC., Graule, T.J. and Gauckler, L.J., Journal of the European Ceramic Society, 1997, 17, 239. Bergstrom, L., Journal of Materials Science, 1996,3 1,5257.
Published witboutb the benefit of Authors’ final corrections as they were not available at press time.
NanoStruduredMaterials.Vol. 10. No. 6. pp. 1081-1086.1998 Elsevier Science Ltd Q 1998 Acta MetallurgicaInc. F’rintedin the USA. All rightsreserved 0965-9773’98 $19.00 + .OO
PII SO9659773(98)00127-S
EFFECT OF POWDER SIZE ON THE STABILITY OF CONCENTRATED AQUEOUS SUSPENSIONS OF Y-TZP Jing Sun, Lian Gao and Jingkun Guo State Key Lab of High Performance Ceramics & Superfine Microstructure Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R.China (Accepted September 18,1998)
AbstractThe particle size differences among four Y-lZP powders are shown to have a significant e#ect on the rheological properties of concentrated aqueous suspensions of the powders. The viscosity of slurries is sensitive to solid loading, especially at high solid content level. As a result, thefourpowders require deferent amounts of dispersant to achieve their highest solid loadings. Under the same solid content, the smaller the particles and the more absorbed dispersant needed, the higher the viscosity of the slurry. 01998 Acta Metallurgica Inc.
1. INTRODUCTION
Colloidal processing of fine particles has been suggested as a preferred way to produce certain high quality ceramic materials (l-3). In all types of suspension shape forming techniques, ranging from
i 082
J SUN,
L @O
AND
J Guo
2. EXPERIMENTS Yttria stabilized tetragonal zirconia polycrystals containing 2.8 mol% ythia were prepared by coprecipitation method using zirconium oxychloride and yttrium chloride as starting materials and ammonia solution as the precipitation medium. The pH value during coprecipitation was maintained to be greater than 9. Cl- ions were removed completely by washing with distilled water and coprecipitation was then dispersed to prevent the formation of agglomerate. The precipitated precursor was then dried at 120°C overnight and calcined at 450°C (No. l), 700°C (No. 2), 900°C (No. 3), 1050°C (No. 4) for 2 hours respectively. Table 1 summarizes the physical properties of these powders. A polyelectrolyte dispersant used in the preparation of dispersed Y-TZP slurries was the ammonium salt of poly acrylic acid (abbreviated as NH&IA) with molecular weight of about 3000. The suspension was transferred into a plastic bottle which was loaded with zirconia grinding media, then agitated by a tarbomixer for 24 hours. Theological characteristics of the Y-TZP suspensions were determined using cylindrical measuring system on rotational viscosimeter (Model Rheomat 260, Mettler Toledo AG, Switzerland). Measurements were performed with a concentric cylinder measurement geometry MSO. All measurements were performed at a temperature of 25°C. To eliminate artifacts from different treatments during the filling procedure, the samples were presheared for 3 minutes, followed by 5 minutes at rest. Several modes of test are available.
3. RESULTS AND DISCUSSION 3.1 Optimization of the Amount of Dispersant Figure 1 shows the viscosity of powder No. 2 suspensions at a constant volume fraction of solids as a function of shear rate and amount of NH&AA added. Two of these suspensions display a shear thinning behavior with a drop of at least one order of magnitude of the viscosity Erom low shear rate to high shear rate. This change in the degree of shear thinning can be related to a change TABLE 1 The Properties of Four Powders Calcined at Different Temperatures
SSA (m2/g>
1050°C
900°C
700°C
450°C
8.01
18.3
48.1
81.9
125
27.3
10.4
6.0
83
36.8
24.1
10.0
grain size/nm (BET) grain size /fun O-EM)
EFFECTOF POWDERSIZE ONTHESTA~ILIWOF GDNCENTRATED ACUJEOUS SUSPENSIONS OFY-TZP
1083
shear ascending curve
120
180
240
300
Shear Rate ( s-‘) Figure 1.The relationship between the viscosity and shear rate when the solid content is 50 wt%.
of the colloidal stability. Concentrated colloidally stable suspension display a shear thinning behavior because of a perturbation of the suspension structure by shear. At low shear rates, the suspension structure is close to the equilibrium structure at rest because thermal motion dominates over the viscous forces. At higher shear rates, the viscous forces dominate and the plateau in viscosity in this region is a measure of the resistance to flow of a suspension with a completely hydrodynamically controlled structure. With increasing shear rate, the viscous forces tend to reduce the size of the aggregates, hence facilitating flow. The degree of shear thinning evaluated from the difference between the high and low shear rate viscosity :inFigure 1 appears to be constant when 1.5 wt% NH#AA is added, but increases rapidly at additions of 2.0 wt%. The adding of dispersant forms the repulsive effect on particles, making the attractive force decrease. Other parameters such as viscosity relaxation time and elasticity modules also decreased. More adding of dispersant can occupy a substantial volume and thus raise the effective volume fractioa of suspension and subsequently lead to higher viscosity. The addition of dispersant NH4pAA can effectively improve the flowability of Y-TZP slurry, but there exists an optimum amount of dispersant. 3.2 Influence of Particle Size on the Rheological Properties of Slurries Keeping; the solid content at 49 wt%, and dispersant at 1.12 wt% (relative to the weight of the powder), which is enough to reach minimum viscosity, we get the flow curve (Figure 2) of
J SUN,
1084
13760
.
. . .*
l.*...
.’ mmsam.~m . . ...*
I1008 -
L &IO
**.. I’
AND
J Guo
I
l***...*..
l**....
l **.
l
.,.....**
..........m........... .............=
.=.... :* . . l....* . . 2 ...*...*.*~~*~-~............._,.*. . . . ......*........................................~*.*.~.~' 8256mm =J'.' 2' c u .+ . . f . .. 5504w.
2752
*
- .
! . .. .. . .. ““...~.~.;‘;‘:‘;;,,.~.~~~~.,.~.‘.~*.‘.,.‘~~~,~~ .,-‘L...*... . 0 3.’ -LC-’ cym%e.M-0
20
40
Shear Rate
. . . . . . .(., ~.~rc.r.r.*.*r*.b~*.“.*.b*~.~~ 4 4 + . &._.y 80
60
100
( s-l )
Figure 2a. Keeping the solid content as 49 wt%, the relationship of shear stress with shear rate ascending curve: of slurries formed by different nano-size Y-‘El? 1,2,3,4-shear l’, 2’, 3',4-shear descending curve.
Shear Rate Figure2b.Keeping
( s-l)
the solid content as 49 wt%, the relationship Of ViSCOSitY w ith shear rate of slurries formed by different nano-size Y-Tzp.
EFFECTOF POWDER SIZEONTHESTABILINOF CONCENTRATED ACUEWSSUSPENSWS OFY-TZP
1085
slurries formed by different nano-size Y-TZP. Figure 2a is the relationship of shear stress with shear rate and Figure 2b is the relationship of viscosity with shear rate. The behavior of the shear stress as a function of shear rate can be fitted by the Ostwald model.
Where z is the shear stress, r is the shear rate, k is the fluid factor and n is the flow exponent. With increasing particle size, the fluid factor drops drastically, k of powder 1 is 7381, k of powder 2 is 6139, k of powder 3 is 795, and that ofpowder 4 is 218. The four kinds of slurries show reduction in apparent viscosity with increasing shear rate. That is typical of pseudoplasticity and indicates thixotropic rheology. Keeping the shear rate as 40 s-l, the viscosity is 269 mPa - s, 204 mPa - s, 31.0,mPa - s and 8.0 mPa - s respectively. This means under the same condition, the smaller the particle, the higher viscosity of its slurry. These phenomena can be explained by the following equation:
20
40
60
80
Specific Surface Area (m*/g) Figure 3. Relationship between specific surface area and viscosity. l-the solid content is 49 wt%, 2-the solid content is 65 wt%.
1086
J
SUN,
L GAO AND
J Guo
whereV&s the effective volume fraction which determine the viscosityof slurry, Sis the thickness of the polymer layer, a is the radius of the spherical particle and V is the volume fraction of the solids. Postulating that the dispersant forms a monolayer of polymer on the particles, the smaller the particles, the bigger the effective volume. That means although the net solid content of the four kinds of slurries is the same, the smaller powder needs more water to be wet and holds more space than the coarser powder. So the smaller powder has higher viscosity. 3.3 The Relationship
of Wscosity and Specific Surface Area (SSA)
We also studied the relationship between the SSAand viscosity when the slurry contains the same solid content. From Figure 3 we can see that if the solid content is 49 wt%, the viscosity does not change greatly with SSA. The volume of the powder is much lower compared to that of the solvent. There is enough water between particles to act as moving agents; so the viscosity changes little. When the solid content is 65 wt%, the viscosity increases with SSAexponentially, and the finest particle with higher SSA needs more dispersant to adsorb to be flowable. In fact, the upper limit of flowable shmy formed by powdercalciuated at 45O’C is 65 wt%, that of 700°C is 72 wt%, the upper limit of 900°C powder is 75 wt%, and the highest loading of 105O’C powder is 80 wt%. Specimens formed by small particle size powder are easy to sinter but difficult to make into concentrated slurries. In order to get high solid loading, control of particle size distribution is au advisable method. 4. CONCLUSIONS The addition of dispersant NH@AAcan effectively improve the flowability of slurries. An optimum condition exists for dispersant at certain solid content. The solid content and the powder size affect the rheological properties of slurry greatly. With the same solid loading, the smaller the particles, the higher viscosity of its slung. In order to have similar flowability, the smaller particles which have high SSA need to adsorb more dispersant. The upper limit of solid content changes with particle size greatly. In order to get concentrated slurry, control particle size distribution is au advisable method. REFERENCES 1.
2. 3.
4. 5. 6. 7 8.
Lange, Journal of the American Ceramic Society, 1989,72(l), 3. Roosen, A. and Brown, H.K., Journal of the American Ceramic Society, 1988,71(11), 970. Calve& P.D.,Tormey, E.S. and Pober, R.L., American Ceramic Society Bulletin, 1986,65(4), 669. Hyun,M.J., ChemicalProcessing ofceramics, eds. I.L.Burtrandand 3-A. Edward,Marcel Dekker, 1994, p. 157. Kramer, T. and Lange, F.F., Journal of the American Ceramic Society, 1!994,77(4), 922. Hacldey, V.A., Journal of the American Ceramic Society, 1997,80(g), 2315. Hider, PC., Graule, T.J. and Gauckler, L.J., Journal of the European Ceramic Society, 1997, 17, 239. Bergstrom, L., Journal of Materials Science, 1996,3 1,5257.
Published witboutb the benefit of Authors’ final corrections as they were not available at press time.