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Dispersion of BaTiO3 powders (Part II)
CERAMICS
INTERNATIONAL.
83
Vol. 10. n. 3.1984
Dispersion of BaTi S. MIZUTA*,
Powders (Part II)
M. PARISH and H.K. BOWEN
Massachusetts Institute of Technology Ceramics Processing Research Laboratory, Cambridge, Massachusetts ?? Now at National Chemical Laboratory for Industry, lsukuba Research Center, Ibaraki, Japan
Highly concentrated BaTi suspensions in benzaldehyde were investigated for adaptation toward tape casting. Effects of particle concentration, total particle mass, gravitational forces, and particle size distribution on sediment density were studied.
1 - INTRODUCTION In Part I ’ the fundamental properties of BaTiO, dispersions were presented for a variety of representative pure organic liquids, based on sediment volume and contact angle measurements. Benzaldehyde was found to be the best dispersing medium. Part II was undertaken to examine both the state of a highly concentrated BaTiOs suspension in benzaldehyde and the packing of settled particles. The sediment volume measurement used in Part I was a convenient method to simply evaluate the state of a dispersion for a number of systems. However, sediment volumes do not represent absolute values of dispersibility, but a relative degree of dispersibility. Also, the previous experiments were made for fairly dilute particle concentrations using as-received, wide particle size distribution powders. In order to achieve uniform packina,of the particles, the following three conditions are required 1. Particle size distribution should be narrow, preferably monosized. 2. The particles in suspension must be nonagglomerated (monodispersed), even in highly concentrated dispersions. 3. From a handling point of view, excessive thixotropy, dilatancy and high yield point should be avoided for a rheologically favorable suspension. 3*4 A BaTiO, (TAM-COF) suspension prepared and evaluated as follows:
in benzaldehyde
was
Classification of the particles was performed in order to obtain a narrow particle size distribution. Effects of particle concentration, particle size distribution, the total mass of powder in the supension and centrifugal settling on sediment volume were examined. The state of dispersion in a liquid and the particle packing in a sediment were examined by SEM observation.
2 - EXPERIMENTAL 2.1 - Preparation
of Narrow Particle Size Distribution
lsopropanol was chosen as the classification (sedimentation) liquid medium because of its low viscosity and high volatility and because it was found to be a good dispersing medium for BaTi03 (Part I). Use of benzaldehyde was avoided here because of the solid residue formed during drying.
The BaTi powder and isopropanol were mixed in a glass container and were subjected to ultrasonification to help break up agglomerates. The larger particles in the dispersion were allowed to gravitationally settle. Separation of the particles into incremental size ranges of 0.9-1.0 pm, 0.4-0.6 pm and 0.1-0.2 cm was made by centrifugal sedimentation at 1000 rpm (254 g). Settling times were estimated by using Stokes law. Settled cakes of classified particles were dried in air. Classification of powders into narrow size increments is accomplished through the sedimentation of a dispersion. The rate of sedimentation is estimated by Stokes’ law, a hydrodynamic relationship based on a balance of the drag and gravitational forces acting on a spherical particle: v = 2 R2(Pz-P1)g
9
ill
7
where v is the steady state sedimentation velocity, R is the particle radius, pl and p2 are the liquid and particle densities, respectively, g is the effective gravity, and 7 is the absolute viscosity of the liquid. Since the terminale velocity of a particle is proportional to the liquid. Since the terminal velocity of particles is proportional to the square of the particle size, a large particle or floe falls at a considerably greater rate than a smaller, individual particle. Increments of particle sizes may be extracted by allowing a desired particle size to settle out of the dispersion of a known height in a calculated time. For example, a 0.4-0.6 pm powder may be obtained by allowing all particles larger than 0.6 pm to fall out of the dispersion. The remaining dispersion, which contains only particles less than 0.6 pm, is removed and the particles greater than 0.4 pm are allowed to settle. This sediment now consists of particles in the 0.4-0.6 pm size range. An SEM photomicrograph of a sample from a 0.4-0.6 pm classified powder is shown in Fig. 1 5. Note that the particle size is close to that predicted by Stokes’ Law. A closer look at Fig. 1 shows a significant percentage of particles less than 0.4 pm. This is unavoidable since the sedimentation time for the second cut is determined for the removal of 0.4 pm particles from the dispersion. During this time, smaller particles settle from lower heights into the sediment as the larger size particles (0.4cm) settle from the top to the bottom of the dispsersion. 2.2 - Sediment centrated
Volume Measurement Suspensions
for Highly Con-
The state of a BaTi03 dispersion must remain nonagglomerated, even at high particle concentrations, if it is to be used for tape casting. The object of this series of experiments is to investigate the effect of particle concentration on the sediment density.
2.2.1 - Effect of Particle Concentration
on Sediment
Volume
Preliminary experiments suggested that the total mass of particles present may affect the sediment volume through
84
S. MIZUTA,
2.2.3 - Effect of Centrifugation
M. PARISH and H.K. BOWEN
on Sediment
Volume
These experiments were carried out to observe the effect of increased gravity on the density of sediment volume for 10, 20, and 30 ~01% particle concentrations. The total suspension volume was fixed at 15 cc. The amounts of particles and liquid used are listed in Table I. A centrifuge tube was used for this test rather than a graduated cylinder. Centrifugal sedimentation was made at 12000 rpm (17000 g) for 10 min.* 2.2.4 - Effect of Particle Size Distribution
on Sediment Volume
The experiment described in Section 2.2.1 was repeated using narrow size distribution particles (0.9 - 1 .O pm at a concentration of 30 vol%, and a total particle mass of 7.5 g as listed in Table I. 2.3 - SEM Observation
of Particles
in a Supernatant
BaTi03 dispersions in benzaldehyde were evaluated using a scanning electron microscope (SEM). The BaTiOj powder (39) and benzaldehyde (15~~) were mixed using ultrasonication for 3 min. After standing 72 hrs, a glass slide was dipped into the suspension and dried in air. The partiCleS on the surface of the glass were observed with the SEM.
2.4 - SEM Observation Sediment FIGURE 1 - SEY pho~omicraaranh TAM-COF powder.
nf the
#I AJI &.m
gravitational effects. Therefore, these experiments were all performed with a constant mass of particles, i.e., higher particle concentrations were achieved by using smaller amounts of liquid while maintaining the total mass of particles constant. A 25 cc graduated cylinder was used for mixing the liquid and the powder. The quantities used are listed in Table 1. The cylinder was shaken vigorously by hand until the powder and liquid were thoroughly mixed, and was then placed in an ultrasonic bath for 30 minutes. The suspension was allowed to stand for 2 weeks. After this time almost all of the particles had settled out and the sediment volume was measured directly. 2.2.2 - Effect of Mass on Sediment
Volume
In this set of experiments, the particle concentration was held constant at 30 vol%, while the total amount of suspension was varied. The mixing and measuring procedures followed were the same as in section 2.2.1. The amounts of liquid and particles used are listed in Table I. TABLE I Exp. No.
Particle Concentration VOI% 5
2-2-l
10 20 30
2-2-2 (Mass Effect)
: 30
2-2-3 (Centrifugal) 2-2-4 (Narrow-Sized) ?? ??
Packing
of a
Casting Experiment
A highly concentrated suspension with 52 ~01% of particles (130 g of 0.4 - 0.6 pm narrow-sized BaTiOs in 20 cc benzaldehyde) was prepared in the following way. All the BaTiOj was added by alternating steps of mixing approximately 5 g of powder into the suspension and then stirring with a glass rod and subjecting it to ultrasonification (microtip probe)* *. The suspension was found to be fairly viscous immediately after powder addition; however, it became much more fluid after standing for a few minutes. The suspension was driprEdMdirectly onto the SEM stub, dried in air, and observed by .n
Weight* of Particles (9) 7.5 7.5 7.5 7.5 7.5 15.0 22.5
15.0 15.0 15.0 4.17
14.25 14.25 12.0 2.92
4.5 9.0 16.0 7.5’
6.0 g/cc was considered as density of BaTiO,
* narrow-sized particles of 0.9 - 1.l pm
2.5 - Preliminary
Volume of Liquid (cc) 23.75 11.25 5.0 2.92 2.92 5.63 6.75
30
Particle
The particle packing of a sediment which settled out of a benzaldehyde dispersion was evaluated using an SEM. A narrow size distribution BaTi powder (29) in the range of 0.1 pm and benzaldehyde (15~~) were mixed in a centrifuge tube, then put in an ultrasonic bath for 30 min. After centrifugally settling at 12000 rpm (17000 g) for 10 min, the supernatant was removed. The sediment was then dried at 90% under a vacuum of 0.1 mm Hg for one day. Top and fractured surfaces were observed with the SEM.
Volume of Suspension (cc) 25.0 12.5 6.25 4.17 4.17 6.33 12.5
5 ::
of the
??
IEC HT Centrifughe, Damon I IEC Division, Needham Heights, MA. * SonicatorTM Cell Disruptor, Model W-220 F, Heat Systems - Ultrasonics, Inc., Plainview, NJ. ??
??
DISPERSION
OF SaTiO,
3 - RESULTS
POWDERS
85
(PART I)
AND DISCUSSION
of 3.1 - Effects Distribution
Concentration,
Mass,
Gravity
and
Results are shown in Fig. 2 Sediment densities which were approximately 40% of single crystal density (SCD) were not significantly changed by particle concentrations in the range of 5 - 30 vol%, when the total mass of the particles was kept constant at 7.5 g. As the total mass of particles increased from 7.5 to 22.5 g, the sediment density increased from 40% to 53% of SCD at 30 vol% particle concentration. The sediment of 7.5 gms (30 VOW) of narrow size distribution particles (0.5 - 0.6 pm cut) was denser than that of the original wide size distribution particles. In the case of centrifugal settling, much higher densities in the range of 48 70% of SCD were found. These values are fairly close to the value of ideally close packed spheres as shown in Fig. 2. FIGURE 3 - SEM photomlcrograph of particles which remain in the supernatant as described In section 2-3.
( A A
0
I
0 0
O
w 0 A
0 0
0
0
-
1
EFFECT
GRAVITY
CONCENTRATION
CENTRIFUGE
MASS NARROW SIZE PARTICLES CONCENTRATION
10 20 30 SOLIDS CONCENTRATION (vol %)
a
40
FIGURE 2 - The effect of concentration, mass, applied force, and particle size distribution on the sediment density of TAMCOFlbenzaldehyde systems.
Based on these results, the following three observations can be made: Sediment densities changed only slightly with concentration from 5 to 30 vol%; i.e., no flocculation was found with an increase in particle concentration. Thus, the state of the BaTiOs dispersion in benzaldehyde was independent of particle concentration in this range. A wide particle size distribution was considered to be undesirable for dense and uniform packing of particles. A poly-dispersed system is favorable only when finer particles uniformly fill the voids of close packed larger particles. However, such an ideal configuration was not found in this experiment. A wide size distribution was found to give rise to non-uniform packing, resulting in lower sediment density. Sediment density was found to be a function of both the total mass of particles and gravity, i.e., forces applied onto the particles. If each of the particles was deposited individually out of an extremely dilute supension onto the bottom of a cylinder with a very slow settling rate, no effect of mass or gravity would be observed. But in practice, as particles
b FIGURE 4 - SEM photomicrographs of the a) top, and b) fractured surfaces of a centrifugally casted 0.1-0.2 pm particles.
simultaneously start to settle down with various velocities corresponding to their respective sizes, the suspension gradually becomes highly concentrated in the lower regions of a vessel with a fairly dilute supernatant above. This concentration increase requires the displacement of the liquid which occurs more readily when the applied force is higher. Therefore, an increase in the total mass of the particles and/or gravity is considered to enhance the compaction of the highly concentrated layer of the suspension.
S.MIZUTA,
86
M. PARISH
and H.K
BOWEN
shown in Figs. 4a and 4b. Extremely dense and uniform packing of particles is observed on the top surfaces. No voids larger than the particle size are present and several areas show ideal close packing of particles. This uniform, dense packing is also seen on the fractured surfaces.
3.4 - An Evaluation
of Particle Packing of a Cast
SEM photographs of the top and fractured surfaces are shown in Fig. 5a and 5b. The packing is observed to be fairly good, even though a rather wide distribution of particle sizes (in the range of 0.2 - 1 .O pm) and several submicron voids are evident. If a more narrow size distribution is used, much better packing, similar to the centrifugal cast as shown in Fig. 4a would probably result. The results shown are considered sufficient verification that the 54 ~01% BaTiO, suspension in benzaldehyde can be used in industrial casting processes.
4 - CONCLUSIONS When BaTiOJ powder was dispersed in bezaldehyde, flocculation was not affected by particle concentrations up to 30 vol %. A narrow size distribution of the powder was shown favorable to form dense, uniform green microstructures. Applied forces, such as centrifugal forces, helped compact highly concentrated suspensions, resulting in very high particle densitites in the cast pieces.
REFERENCES
b FIGURE 5 - SEM photomicrographs of a) top, and b) fractured surfaces of a cast made from a highly concentrated TAMCOFlbenzaldehyde suspension.
3.2 - An Evaluation
1. S. MIZUTA, et al., *Dispersion of BaTiO> Powders (Part I)*, Ceramics International 10 (1984) 2. H. KENT BOWEN, *Basic Research Needs on High Temperature Ceramics for Energy Applications., Mat. Sci. Eng. 44 (1980) 1 3. R.E. COWAN, Treatise on Materials Science and Technology, Vol. 9, Ed., F.F.Y. Wand, academic Press. N.Y., 1970, p. 156. 4. Y. SHIRAKI, N. OTSUKA and H. Ninomiya, &lip Casting of Barium Titanate**, Yogyo Kyokai Shi 62 (1974) 470 5. M. PARISH, S.M. Thesis, M.I.T. (1982)
of Particles in a Supernatant
As shown in the SEM photomicrogrph in Fig. 3, the particles seem to be well dispersed. Particle sizes in the range of 0.1-0.2 v observed in this micrograph are in good agreement with the value 0.16 am calculated by Stokes law for the experimental conditions.
ACKNOWLEDGEMENTS The authors would like to thank Mark Green and Dr. R.L. Pober for their contributions, and, Mrs. LuAnne van Ultert and Tricia Normile for their work in preparation of this paper. This work was funded by the Multilayer Ceramic Chip Capacitor Consortium; their support is gratefully acknowle$ged.
3.3 - An Evaluation of the Particle Packing of a Sediment SEM pictures of the top and fractured surface of a centrifugally settled sediment with 0.1 - 0.2 pm particles are
Received August
18, 1983; accepted October 3, 1983
INTERNATIONAL.
83
Vol. 10. n. 3.1984
Dispersion of BaTi S. MIZUTA*,
Powders (Part II)
M. PARISH and H.K. BOWEN
Massachusetts Institute of Technology Ceramics Processing Research Laboratory, Cambridge, Massachusetts ?? Now at National Chemical Laboratory for Industry, lsukuba Research Center, Ibaraki, Japan
Highly concentrated BaTi suspensions in benzaldehyde were investigated for adaptation toward tape casting. Effects of particle concentration, total particle mass, gravitational forces, and particle size distribution on sediment density were studied.
1 - INTRODUCTION In Part I ’ the fundamental properties of BaTiO, dispersions were presented for a variety of representative pure organic liquids, based on sediment volume and contact angle measurements. Benzaldehyde was found to be the best dispersing medium. Part II was undertaken to examine both the state of a highly concentrated BaTiOs suspension in benzaldehyde and the packing of settled particles. The sediment volume measurement used in Part I was a convenient method to simply evaluate the state of a dispersion for a number of systems. However, sediment volumes do not represent absolute values of dispersibility, but a relative degree of dispersibility. Also, the previous experiments were made for fairly dilute particle concentrations using as-received, wide particle size distribution powders. In order to achieve uniform packina,of the particles, the following three conditions are required 1. Particle size distribution should be narrow, preferably monosized. 2. The particles in suspension must be nonagglomerated (monodispersed), even in highly concentrated dispersions. 3. From a handling point of view, excessive thixotropy, dilatancy and high yield point should be avoided for a rheologically favorable suspension. 3*4 A BaTiO, (TAM-COF) suspension prepared and evaluated as follows:
in benzaldehyde
was
Classification of the particles was performed in order to obtain a narrow particle size distribution. Effects of particle concentration, particle size distribution, the total mass of powder in the supension and centrifugal settling on sediment volume were examined. The state of dispersion in a liquid and the particle packing in a sediment were examined by SEM observation.
2 - EXPERIMENTAL 2.1 - Preparation
of Narrow Particle Size Distribution
lsopropanol was chosen as the classification (sedimentation) liquid medium because of its low viscosity and high volatility and because it was found to be a good dispersing medium for BaTi03 (Part I). Use of benzaldehyde was avoided here because of the solid residue formed during drying.
The BaTi powder and isopropanol were mixed in a glass container and were subjected to ultrasonification to help break up agglomerates. The larger particles in the dispersion were allowed to gravitationally settle. Separation of the particles into incremental size ranges of 0.9-1.0 pm, 0.4-0.6 pm and 0.1-0.2 cm was made by centrifugal sedimentation at 1000 rpm (254 g). Settling times were estimated by using Stokes law. Settled cakes of classified particles were dried in air. Classification of powders into narrow size increments is accomplished through the sedimentation of a dispersion. The rate of sedimentation is estimated by Stokes’ law, a hydrodynamic relationship based on a balance of the drag and gravitational forces acting on a spherical particle: v = 2 R2(Pz-P1)g
9
ill
7
where v is the steady state sedimentation velocity, R is the particle radius, pl and p2 are the liquid and particle densities, respectively, g is the effective gravity, and 7 is the absolute viscosity of the liquid. Since the terminale velocity of a particle is proportional to the liquid. Since the terminal velocity of particles is proportional to the square of the particle size, a large particle or floe falls at a considerably greater rate than a smaller, individual particle. Increments of particle sizes may be extracted by allowing a desired particle size to settle out of the dispersion of a known height in a calculated time. For example, a 0.4-0.6 pm powder may be obtained by allowing all particles larger than 0.6 pm to fall out of the dispersion. The remaining dispersion, which contains only particles less than 0.6 pm, is removed and the particles greater than 0.4 pm are allowed to settle. This sediment now consists of particles in the 0.4-0.6 pm size range. An SEM photomicrograph of a sample from a 0.4-0.6 pm classified powder is shown in Fig. 1 5. Note that the particle size is close to that predicted by Stokes’ Law. A closer look at Fig. 1 shows a significant percentage of particles less than 0.4 pm. This is unavoidable since the sedimentation time for the second cut is determined for the removal of 0.4 pm particles from the dispersion. During this time, smaller particles settle from lower heights into the sediment as the larger size particles (0.4cm) settle from the top to the bottom of the dispsersion. 2.2 - Sediment centrated
Volume Measurement Suspensions
for Highly Con-
The state of a BaTi03 dispersion must remain nonagglomerated, even at high particle concentrations, if it is to be used for tape casting. The object of this series of experiments is to investigate the effect of particle concentration on the sediment density.
2.2.1 - Effect of Particle Concentration
on Sediment
Volume
Preliminary experiments suggested that the total mass of particles present may affect the sediment volume through
84
S. MIZUTA,
2.2.3 - Effect of Centrifugation
M. PARISH and H.K. BOWEN
on Sediment
Volume
These experiments were carried out to observe the effect of increased gravity on the density of sediment volume for 10, 20, and 30 ~01% particle concentrations. The total suspension volume was fixed at 15 cc. The amounts of particles and liquid used are listed in Table I. A centrifuge tube was used for this test rather than a graduated cylinder. Centrifugal sedimentation was made at 12000 rpm (17000 g) for 10 min.* 2.2.4 - Effect of Particle Size Distribution
on Sediment Volume
The experiment described in Section 2.2.1 was repeated using narrow size distribution particles (0.9 - 1 .O pm at a concentration of 30 vol%, and a total particle mass of 7.5 g as listed in Table I. 2.3 - SEM Observation
of Particles
in a Supernatant
BaTi03 dispersions in benzaldehyde were evaluated using a scanning electron microscope (SEM). The BaTiOj powder (39) and benzaldehyde (15~~) were mixed using ultrasonication for 3 min. After standing 72 hrs, a glass slide was dipped into the suspension and dried in air. The partiCleS on the surface of the glass were observed with the SEM.
2.4 - SEM Observation Sediment FIGURE 1 - SEY pho~omicraaranh TAM-COF powder.
nf the
#I AJI &.m
gravitational effects. Therefore, these experiments were all performed with a constant mass of particles, i.e., higher particle concentrations were achieved by using smaller amounts of liquid while maintaining the total mass of particles constant. A 25 cc graduated cylinder was used for mixing the liquid and the powder. The quantities used are listed in Table 1. The cylinder was shaken vigorously by hand until the powder and liquid were thoroughly mixed, and was then placed in an ultrasonic bath for 30 minutes. The suspension was allowed to stand for 2 weeks. After this time almost all of the particles had settled out and the sediment volume was measured directly. 2.2.2 - Effect of Mass on Sediment
Volume
In this set of experiments, the particle concentration was held constant at 30 vol%, while the total amount of suspension was varied. The mixing and measuring procedures followed were the same as in section 2.2.1. The amounts of liquid and particles used are listed in Table I. TABLE I Exp. No.
Particle Concentration VOI% 5
2-2-l
10 20 30
2-2-2 (Mass Effect)
: 30
2-2-3 (Centrifugal) 2-2-4 (Narrow-Sized) ?? ??
Packing
of a
Casting Experiment
A highly concentrated suspension with 52 ~01% of particles (130 g of 0.4 - 0.6 pm narrow-sized BaTiOs in 20 cc benzaldehyde) was prepared in the following way. All the BaTiOj was added by alternating steps of mixing approximately 5 g of powder into the suspension and then stirring with a glass rod and subjecting it to ultrasonification (microtip probe)* *. The suspension was found to be fairly viscous immediately after powder addition; however, it became much more fluid after standing for a few minutes. The suspension was driprEdMdirectly onto the SEM stub, dried in air, and observed by .n
Weight* of Particles (9) 7.5 7.5 7.5 7.5 7.5 15.0 22.5
15.0 15.0 15.0 4.17
14.25 14.25 12.0 2.92
4.5 9.0 16.0 7.5’
6.0 g/cc was considered as density of BaTiO,
* narrow-sized particles of 0.9 - 1.l pm
2.5 - Preliminary
Volume of Liquid (cc) 23.75 11.25 5.0 2.92 2.92 5.63 6.75
30
Particle
The particle packing of a sediment which settled out of a benzaldehyde dispersion was evaluated using an SEM. A narrow size distribution BaTi powder (29) in the range of 0.1 pm and benzaldehyde (15~~) were mixed in a centrifuge tube, then put in an ultrasonic bath for 30 min. After centrifugally settling at 12000 rpm (17000 g) for 10 min, the supernatant was removed. The sediment was then dried at 90% under a vacuum of 0.1 mm Hg for one day. Top and fractured surfaces were observed with the SEM.
Volume of Suspension (cc) 25.0 12.5 6.25 4.17 4.17 6.33 12.5
5 ::
of the
??
IEC HT Centrifughe, Damon I IEC Division, Needham Heights, MA. * SonicatorTM Cell Disruptor, Model W-220 F, Heat Systems - Ultrasonics, Inc., Plainview, NJ. ??
??
DISPERSION
OF SaTiO,
3 - RESULTS
POWDERS
85
(PART I)
AND DISCUSSION
of 3.1 - Effects Distribution
Concentration,
Mass,
Gravity
and
Results are shown in Fig. 2 Sediment densities which were approximately 40% of single crystal density (SCD) were not significantly changed by particle concentrations in the range of 5 - 30 vol%, when the total mass of the particles was kept constant at 7.5 g. As the total mass of particles increased from 7.5 to 22.5 g, the sediment density increased from 40% to 53% of SCD at 30 vol% particle concentration. The sediment of 7.5 gms (30 VOW) of narrow size distribution particles (0.5 - 0.6 pm cut) was denser than that of the original wide size distribution particles. In the case of centrifugal settling, much higher densities in the range of 48 70% of SCD were found. These values are fairly close to the value of ideally close packed spheres as shown in Fig. 2. FIGURE 3 - SEM photomlcrograph of particles which remain in the supernatant as described In section 2-3.
( A A
0
I
0 0
O
w 0 A
0 0
0
0
-
1
EFFECT
GRAVITY
CONCENTRATION
CENTRIFUGE
MASS NARROW SIZE PARTICLES CONCENTRATION
10 20 30 SOLIDS CONCENTRATION (vol %)
a
40
FIGURE 2 - The effect of concentration, mass, applied force, and particle size distribution on the sediment density of TAMCOFlbenzaldehyde systems.
Based on these results, the following three observations can be made: Sediment densities changed only slightly with concentration from 5 to 30 vol%; i.e., no flocculation was found with an increase in particle concentration. Thus, the state of the BaTiOs dispersion in benzaldehyde was independent of particle concentration in this range. A wide particle size distribution was considered to be undesirable for dense and uniform packing of particles. A poly-dispersed system is favorable only when finer particles uniformly fill the voids of close packed larger particles. However, such an ideal configuration was not found in this experiment. A wide size distribution was found to give rise to non-uniform packing, resulting in lower sediment density. Sediment density was found to be a function of both the total mass of particles and gravity, i.e., forces applied onto the particles. If each of the particles was deposited individually out of an extremely dilute supension onto the bottom of a cylinder with a very slow settling rate, no effect of mass or gravity would be observed. But in practice, as particles
b FIGURE 4 - SEM photomicrographs of the a) top, and b) fractured surfaces of a centrifugally casted 0.1-0.2 pm particles.
simultaneously start to settle down with various velocities corresponding to their respective sizes, the suspension gradually becomes highly concentrated in the lower regions of a vessel with a fairly dilute supernatant above. This concentration increase requires the displacement of the liquid which occurs more readily when the applied force is higher. Therefore, an increase in the total mass of the particles and/or gravity is considered to enhance the compaction of the highly concentrated layer of the suspension.
S.MIZUTA,
86
M. PARISH
and H.K
BOWEN
shown in Figs. 4a and 4b. Extremely dense and uniform packing of particles is observed on the top surfaces. No voids larger than the particle size are present and several areas show ideal close packing of particles. This uniform, dense packing is also seen on the fractured surfaces.
3.4 - An Evaluation
of Particle Packing of a Cast
SEM photographs of the top and fractured surfaces are shown in Fig. 5a and 5b. The packing is observed to be fairly good, even though a rather wide distribution of particle sizes (in the range of 0.2 - 1 .O pm) and several submicron voids are evident. If a more narrow size distribution is used, much better packing, similar to the centrifugal cast as shown in Fig. 4a would probably result. The results shown are considered sufficient verification that the 54 ~01% BaTiO, suspension in benzaldehyde can be used in industrial casting processes.
4 - CONCLUSIONS When BaTiOJ powder was dispersed in bezaldehyde, flocculation was not affected by particle concentrations up to 30 vol %. A narrow size distribution of the powder was shown favorable to form dense, uniform green microstructures. Applied forces, such as centrifugal forces, helped compact highly concentrated suspensions, resulting in very high particle densitites in the cast pieces.
REFERENCES
b FIGURE 5 - SEM photomicrographs of a) top, and b) fractured surfaces of a cast made from a highly concentrated TAMCOFlbenzaldehyde suspension.
3.2 - An Evaluation
1. S. MIZUTA, et al., *Dispersion of BaTiO> Powders (Part I)*, Ceramics International 10 (1984) 2. H. KENT BOWEN, *Basic Research Needs on High Temperature Ceramics for Energy Applications., Mat. Sci. Eng. 44 (1980) 1 3. R.E. COWAN, Treatise on Materials Science and Technology, Vol. 9, Ed., F.F.Y. Wand, academic Press. N.Y., 1970, p. 156. 4. Y. SHIRAKI, N. OTSUKA and H. Ninomiya, &lip Casting of Barium Titanate**, Yogyo Kyokai Shi 62 (1974) 470 5. M. PARISH, S.M. Thesis, M.I.T. (1982)
of Particles in a Supernatant
As shown in the SEM photomicrogrph in Fig. 3, the particles seem to be well dispersed. Particle sizes in the range of 0.1-0.2 v observed in this micrograph are in good agreement with the value 0.16 am calculated by Stokes law for the experimental conditions.
ACKNOWLEDGEMENTS The authors would like to thank Mark Green and Dr. R.L. Pober for their contributions, and, Mrs. LuAnne van Ultert and Tricia Normile for their work in preparation of this paper. This work was funded by the Multilayer Ceramic Chip Capacitor Consortium; their support is gratefully acknowle$ged.
3.3 - An Evaluation of the Particle Packing of a Sediment SEM pictures of the top and fractured surface of a centrifugally settled sediment with 0.1 - 0.2 pm particles are
Received August
18, 1983; accepted October 3, 1983