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Rheomagnetic properties of mixed magnetic particle suspensions
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 80 (1993) 39-46 0927-7757/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved.
Rheomagnetic suspensions
39
properties of mixed magnetic particle
T.M. Kwod’, M.S. Jhon”, H.J. Choib,‘, T.E. Kariz+* ‘Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, bDepartment of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA “IBM Research Division, Almaden Research Center, San Jose, CA 95120-6099, USA (Received
18 November
1992; accepted
USA
8 April 1993)
Abstract Single-domain magnetic particles are the essential ingredient of magnetic tapes, particulate recording disks and magnetic stripes. The particles are single-domain y-Fe,Oj, CrO, or barium ferrite, and non-magnetic a-Fe,O, mixture. Each of these particles has intrinsic coercivity, which should be matched with the magnetic field strength of the writing element of a particular device. In this study a magnetic inductance measurement with low field strength was employed to obtain the magnetic permeability of suspensions containing two of the particle types mixed together as a function of composition and volume fraction of particles. The bulk magnetic property B is a linear combination of the contributions from each particle type such that the “excess” inductance is L - L, = L$J$ where 4i is the volume fraction and B, is the magnetic property of particle type i. For the non-magnetic a-Fe,03, Bi=O. This allows the formulation of mixed particle suspensions to obtain a desired property for custom-designed magnetic particle coatings. However, mixing magnetic particle types will broaden or produce a bimodal switching field distribution. This may affect the squareness of the magnetic hysteresis loop. These properties should be taken into account for the design of a practical magnetic coating with mixed particle suspension. Another requirement of the magnetic particle suspensions is that they remain well dispersed, even though strong magnetic forces between the particles promote flocculation. An extension of the inductance measurement technique is employed to study the flocculation of a suspension containing magnetic y-Fe,O, and non-magnetic a-Fe,O,. The presence of the a-Fe,O, decreases the flocculation state of the suspension. Thus the suspension stability is enhanced by incorporating a small amount of non-magnetic particles in addition to surfactant. Key words: Magnetic
particles;
Rheomagnetic
measurement;
Suspensions
Introduction Conventional particulate media are manufactured in a coating process similar to a painting or printing process (Bhushan Cl]). The magnetic particle suspension is prepared by dispersing dried particles in a suspending fluid (mixture of binder resins and solvent). The suspension is filtered to remove large floes and agglomerates. The filtered *Corresponding author. ‘Permanent address: Department of Polymer Engineering, Inha University, Inchon, Korea.
Science
and
suspension is coated on a polymer substrate. While the particles are well dispersed, the magnetic particles in the film are oriented by a magnetic field. After the orientation step, the coated film is cured in an oven, leaving a cross-linked magnetic film. The magnetic particles are single-domain Codoped y-Fe,03, CrO,, or barium ferrite, several tenths of a micrometer in size (Bate [2], Sharrock [3]). Since the magnetic particles retain a permanent magnetic moment, the magnetic particles in suspension are subject to aggregation and flocculation. Flocculation adversely affects the coating
T.M. Kwon
40
and the magnetic
orientation
poor media performance roughness
and
media
process,
through noise.
resulting
increased The
final
et al./Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 39-46
in
surface product
quality depends on the “quality of dispersion” and the concentration of magnetic particles in the coating (Bate [4]). Control of the magnetic particle suspension is a prime concern in manufacturing. The recently developed rheomagnetic measurement (RM) technique (Karis and Jhon [5,6]), which measures hydrodynamic orientation of magnetic particles using a magnetic sensing field, is particularly well suited to process control because it can perform on-line measurement of both the concentration and the dispersion quality of the magnetic particle suspensions. Previous studies focused on the development of an efficient flow orientation measurement and detection scheme (Karis and Jhon [6,7]). Experimental observations have demonstrated that the RM technique employed to measure the hydrodynamic orientation provides a measure of the flocculation state for both rod- and plate-like particles in suspension (Karis and Jhon [7], Kwon et al. [S-IO]). A theory was also derived to relate the observations to the effective particle (or floe) diameter and the aspect ratio of suspended particles. This study considers the effects of mixtures of two different types of particle on the suspension microstructure. The effects are related to the inductance measurement of the RM apparatus. A previous study (Kwon et al. [lo]) focused on suspensions containing only one type of magnetic particle. The current study is the first one to report RM measurements on mixtures. Mixtures are of industrial interest because formation of pinholes in barium ferrite particle coatings for ultrahigh density recording media are reduced by the addition of a small amount of a different type of particle to the suspension (Takahashi et al. [ 111). Very little is known about this effect. Our purpose in studying mixtures of particles is to investigate the effects of interparticle forces exerted between magnetic particles on suspension properties. The effects of magnetic attraction can be systematically studied through adjusting the interparticle forces by the following two methods. One approach is to
alter the magnetic interaction by replacing some of the magnetic particles in suspension with nonmagnetic
particles.
Another
approach
is to elimi-
nate the interaction by raising the temperature above the Curie temperature of the magnetic particles. Above the Curie temperature, the magnetic particles lose their magnetic domain and become non-magnetic (Liu [12]). In this paper, we consider only mixtures. The heating method requires modification of the apparatus and will be described in a later publication. The initial magnetic susceptibility of the coating is dependent on that of the constituent particles. This is controlled by adjusting the chemical composition of the particles. For example, the magnetic properties of y-Fe,O, are adjusted by doping cobalt in the particle surface layers. Alternatively, the magnetic property of the coating could, in principle, be adjusted by controlling the relative proportion of two or more types of particle each having different magnetic properties. The mechanical properties of the composite magnetic film can then be maintained by incorporating the same total amount of magnetic particles. Tests are carried out using inductance measurement by placing sample suspensions in a high frequency (l-10 MHz) sensing coil. The increase in inductance is proportional to the volume fraction of magnetic particles placed in the coil and the initial magnetic susceptibility of the suspension. Binary mixed particle suspensions over a range of compositions are prepared and measured using the coil inductance method. A mixture of magnetic with non-magnetic particles is also explored to address the effect of nonmagnetic particles on the flocculation stability of the mixed particle suspension. The flocculation process is an equilibrium between the primary particles and clusters of particles bound together by magnetic attractive forces. For primary particles to form a floe, two particles must approach within the range of the forces and stick together. The probability of close contact between particles is usually reduced by adding a surfactant. The surfactant can sterically or electrostatically stabilize the
TM.
Kwon et al./Colloids
Surfaces A: Physicochem.
Eng. Aspects 80 (1993) 39-46
41
suspension by providing a physical or electrostatic repulsive barrier between the particles. In this
the particles’ major axis. Plate-like magnetic particles’ magnetic moment vector is perpendicular to
study no surfactant is added. Instead, in the case of a mixed magnetic and non-magnetic particle
the face. Consequently, the inductance provides a measure of the particles’ orientation by the flow
suspension, the suspension stability can also be improved. When a magnetic and non-magnetic particle approach one another, attractive magnetic
field. Since the orientation
forces are absent, and even a small amount of electrostatic stabilization will prevent flocculation. Effectively, the probability of contact between two magnetic particles is decreased by the presence of a non-magnetic species. The coil magnetic inductance measurement is employed on a flowing suspension to study and non-magnetic
the effect of a mixed particle suspension.
magnetic
Experimental The main element in the test apparatus is an inductor coil and oscillator circuit. Stable oscillation is obtained in the range 1- 10 MHz by selection of the appropriate capacitor. The coil is made by wrapping IO-50 turns of silver or copper wire around a glass tube. The oscillator method was selected for its low cost and accuracy (Karis and Jhon [7], Kwon et al. [lo]). The oscillator is designed to provide a 5 V peak-to-peak square wave output with a 50% duty cycle. Frequency is measured to within six significant figures using a frequency counter. The inverse square frequency is proportional to the coil inductance. For quiescent measurements, the suspension is placed in the coil and the oscillator frequency shift gives the change of inductance relative to that with pure suspending fluid in the coil. Further information regarding the state of suspension flocculation is obtained if the coil is wrapped closely downstream from a circular converging flow of the suspension. A circular converging flow (7.5 cm to 3 mm in diameter) was used in this study. The contribution of the magnetic particles to the inductance is related to the projection of the particle magnetic moment vector onto the magnetic field vector. For rod-like magnetic particles, the magnetic moment vector is parallel to
depends
on the effective
particle shape and flow rate relative to rotary Brownian diffusion, the inductanceeflow rate curve provides an indirect measure of dispersion quality. For rod-like magnetic particles, the decreases with increasing orientation,
inductance so that a
larger decrease in inductance with flow rate indicates improved dispersion quality. The detailed description of the apparatus and the experimental procedure are given by Karis and Jhon [7] and Kwon et al. [lo]. The mixed magnetic particle suspension characteristics are emphasized here. The magnetic particles used in this study were single-domain particles as used for commercial media production. Rod-like y-Fe,O, particles were obtained from Magnox. In order to study the effects of intrinsic magnetic properties, nonmagnetic cr-Fe20a particles were also obtained from Magnox. According to the vendor’s note, the a-Fe,O, particles were thermally treated to manufacture y-Fe20, particles and therefore the size and the shape of these two particle types were nearly the same. Rod-like Cr02 particles from Du Pont and plate-like barium ferrite particles from Toda (Japan) were also obtained for testing. The size and shape of the particles were characterized using a transmission electron microscope (TEM). A vibrating sample magnetometer (VSM), EG&G PARC Model 155, was employed to characterize the particle magnetic properties for randomly oriented dry powders. The particle density was calculated using the volume increase when a known mass of particles was immersed into the fluid. The size and shape from the TEM micrographs and the density and magnetic properties from VSM measurements for each particle type are listed in Table 1. Actual suspensions for media production contain not only particles and solvent (usually cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone,
T.M.
42 Table 1 Physical properties
of the magnetic
Property
M, (emu g-t)
Eng. Aspects 80 (1993) 39-46
particles Plate-like
Rod like” ;‘-Fe,O,
H, (Oe) Density pp (g cm 3, Particle sizeb (pm) Aspect ratio p = L/a Curie temp. T, (“C)
Kwon et al.,‘Colloids Surfaces A: Physicochem.
71.8 321 4.7 L=O.5 7.0 590
ac-Fe,O,
CrOz
Barium
4.7 L=O.5 7.0 _
81.49 585 4.8 L = 0.46 13 1155126
34.97 635 5.2 a=0.13 0.1 320
ferrite
“According to the vendor’s note the r-Fe,O, _ _.particles were thermally and shape of these two types of particle were identical. bL, length of the particle; a, diameter of the particle. ‘Cited from Ref. 4.
treated
or a mixture of these) but also approximately 5 wt.% of binder polymer. In this study, ethylene glycol was adopted as a suspending fluid to simulate the actual coating formulation. Ethylene glycol was obtained from Aldrich. Particle suspensions were prepared by a consistent procedure to obtain reproducible initial suspension characteristics. First, 400 g of the particle powder was combined with 600 ml of suspending fluid in a paint shaker (Red Devil, Model 5410-00) for 10 min. Then the mixture was passed through a media mill (Eiger Minimotor Mill Exp., 0.6 mm ceramic ball media) at 1000 rev min ‘. An additional amount of suspending fluid was stirred into this millbase, and the diluted suspension was once again passed through the mill. No dispersant or surfactant was included. Mixed particle suspensions were made by combining two different types of magnetic particle suspensions prepared according to the above procedure.
suspensions, we studied rod-like + rod-like and rod-like + plate-like magnetic particle mixtures, and magnetic-t non-magnetic particle mixed suspensions. Figures l(a) and l(b) show the measured inductance behavior for a mixture of plate-like barium ferrite and rod-like y-Fe,03 and a mixture of rod-like y-Fe,O, and CrO, particle suspensions
Results RM characterization includes the concentration measurement for a quiescent suspension and the orientation measurement for the suspension subjected to flow. Thus we performed the RM characterizations focusing on these two measurements for mixed suspensions. In the concentration measurement for quiescent
to manufacture
y-Fe,O,
particles
and therefore
the size
respectively. As shown in Figs. l(a) and l(b), the inductance of a quiescent mixed suspension is a linear combination of all particle species in the mixture. A previous study (Kwon et al. [lo]) revealed that the measured inductance of a quiescent suspension is proportional to the volume concentration of the particles in the suspension and that the proportionality constant is related to the initial magnetic susceptibility of the particles. The observation of a linear combination of measured inductance for a quiescent mixed suspension implies that the intrinsic magnetic property, i.e. the initial susceptibility of the particle mixture, is the volume average of the constituent particles’ magnetic properties. To study the dependence of the measured inductance on the content of non-magnetic particles in the suspension of magnetic particles, we prepared mixed suspensions of y-Fe,O, and a-Fe,03 particles because the z-Fe,O, particles are not only non-magnetic but also similar to y-Fe,O, particles in shape and size. Figure l(c) shows the measured inductance dependence on the level of z-Fe,O, in
T.M. Kwon et al./Colloids
Surfaces
A: Physicochem.
Eng. Aspects 80 (1993) 39-46
I2950
43
12950 (a)
TO,.ll P.i”xlr
12890
1 0
Fraction
Fr.,‘l,“n
20
(c)
10
of Chromium
hrl
Dioxide,
’ X0
@caz/@
1 100
(%)
Fig. 1. (a) Measured inductance dependence on barium ferrite particle content in rod-like ;I-Fe,O, and plate-like Ba-ferrite mixed suspensions. (b) Measured inductance dependence on rod-like CrO, particle content in rod-like y-Fe,O, and CrO, mixed suspensions. (c) Measured inductance dependence on non-magnetic a-Fe,O, particle content in magnetic y-Fe,O, and non-magnetic r-Fe,O, mixed suspensions. The inductance of the mixed suspension is a linear combination of the inductance contributions from the two constituent particle species. The filled and unfilled symbols indicate suspensions from different batches.
the particle suspension. The inductance of the pure cc-Fe,O, particle suspension was the same as that found with pure suspending fluid. The initial susceptibility of a-Fe,O, is negligibly small under the test conditions owing to the non-magnetic nature of a-Fe,O, (Chikazumi and Charap [13]). The inductance for the y-Fe,O, and a-Fe,O, mixed suspensions was also linearly dependent on the sum of inductance contributions by each particle type. From the linear dependence of the inductance on the particle concentration observed in the mixture studies, we conclude that the measured inductance of a quiescent suspension is determined solely by the particle concentration and the intrinsic
magnetic properties of the particles constituting the suspension. Microstructural changes in the mixed suspensions may be reflected in the inductance of the suspensions subjected to flow, because the orientational characteristics depend on particle shape. To study the effect, we performed flow orientation measurements on mixed suspensions of y-Fe203 and cc-Fe,O, particles in the converging flow cell. The results are shown in Fig. 2. In this figure, the vertical axis is scaled with respect to the volume fraction of magnetic y-Fe,O, particles because non-magnetic a-Fe,O, does not contribute to the inductance. In these tests, the overall change in L from zero flow rate to the maximum flow rate
T.M.
44
Kwon et al./Colloids
Surfaces A: Physicochem.
Eng. Aspects 80 (1993) 39-46
2400
2300
2100[ 0
500
1000
I500
0
r-Fe,O,
content
on flow orientation of magnetic y-Fe,O, sions: (a) qh=0.030; (b) C#J =0.023.
increases with increasing orientation. For suspensions of total particle volume concentration 4= 0.030, shown in Fig. 2(a), the orientation of the yFe,O, particle suspension is slightly increased by a low level of a-Fe,O, substitution. Further increasing the z-Fe,O, fraction beyond 0.6 did not appreciably increase the flow orientation. This was considered to occur because the suspension is in the semidilute regime of concentration [lo]. Interparticle interaction is not further reduced by the replacement with non-magnetic r-Fe,O,. The same test was done on a suspension of lower total particle concentration (4 = 0.023). The results are presented in Fig. 2(b). Compared to the flow orientation of higher concentrations shown in Fig. 2(a), the suspension showed more overall orientation change with a-Fe,O, content. From the above results, a low level of substitution of non-magnetic particles reduces interparticle attraction, increasing the presence of orientable microstructures. An extreme of the microstructure would be primary particles. Therefore the content of non-magnetic particles reduces the level of flocculation. Summary and discussion The effects of microstructural changes produced by mixing two different types of particle suspension
’ IO00
1500
Flow Rate, Q (ml/min)
Flow Rate, Q (ml/min) Fig. 2. Effect of non-magnetic
._
500
and non-magnetic
r-Fe,O,
mixed suspen-
were characterized using the RM technique. The measured permeability, or initial susceptibility of mixed suspensions (rod-like y-Fe,O, + rod-like CrO,, rod-like y-Fe,03 + plate-like barium ferrite, and rod-like magnetic y-Fe,O, + rod-like nonmagnetic cc-FezO,), is a linear combination of the constituent particles’ magnetic properties. The correlation coefficient for the linear regression was greater than 0.99. Microstructural changes produced by substituting non-magnetic sr-Fe,O, particles for magnetic y-Fe,O, had little effect on the inductance of a quiescent ;J-Fe,03 particle suspension. The permeability of the quiescent suspension depends only on the concentration and intrinsic magnetic properties of the constituent particles in mixed suspension. This is consistent with the independence of the initial susceptibility from volume fraction reported by Bottoni et al. [14]. In flow orientation measurements on mixed suspensions of non-magnetic a-Fe,O, and magnetic y-Fe,03 particles, reduced interparticle interaction by nonmagnetic a-Fe,O, particle substitution increased the flow orientation of magnetic y-Fe,O, particles owing to the formation of microstructures that could be more easily oriented by the imposed flow. The observations from this mixed suspension study are expressed in terms of the previously developed theory. For quiescent suspension of a
T.M. Kwon et al./Colloids
Surfaces
A: Physicochem.
single particle type, the inductance in Kwon et al. [lo])
L is (see Eq. (10)
45
1500
TotalPanicle Fracrmn
L-LL,=4B where 4 is the volume
Eng. Aspects 80 119931 39-46
(1) fraction
of suspended
mag-
netic particles, L, is the measured inductance for the suspending medium (C#J =O), and B is a constant which is related to the intrinsic initial magnetic susceptibility of the particles. For mixed suspensions, one can expect B to be related to the effective magnetic property of the mixture. From the observed linear combination shown in Figs. l(a)-l(c), Eq. (1) for a quiescent mixed suspension is written as L-L,=$IB,+m,B,=d(~~+~R2)
(2)
where Bi (i = 1, 2) are intrinsic quantities and $i is the volume concentration of particle type i in the mixed suspension. Equation (2) can also be expressed as Y=$(B,-B,)+B,
0
20
40
$1/$
60
80
100
WI)
Fig. 3. (L-L,)/+ vs. +I/4 plot for magnetic y-Fe,O, and nonmagnetic a-Fe,O, mixed suspensions in Fig. l(c). The data fall on the master curve given by Eq. (3). Here the intercept is zero owing to non-magnetic r-Fe,O,(E,=O).
the equation for the inductance of a suspension subjected to hydrodynamic orientation (see Eqs. (1) and (9) in Kwon et al. [lo]), the inductance of a mixed suspension is
L_
$Bl(I
- S,) + $B,(1
- S,)
1
Equation (3) implies that plots of (L - L,)/cj~ vs. c$~/c$for quiescent mixtures should yield a master curve of slope B, - B, and intercept B,, regardless of the total particle concentration 4. If a suspension shows a deviation from the master curve, it implies that there is a significant magnetic property change due to structural changes in the mixture. Thus, using Eq. (3), we can study structural changes in a
L-L ---S=B,(l 41
mixture. For the non-magnetic cl-Fe,O, and magnetic y-Fe203 mixtures shown in Fig. l(c), we present the (L - L,)/$J vs. cjl/~ plot in Fig. 3. The mixed suspensions of different concentrations fall on the master curve of Eq. (3); here the intercept is zero owing to the presence of non-magnetic aFe,03 (BZ =O). We thus consider that structural changes in the non-magnetic/magnetic particle mixture have little effect on the intrinsic magnetic property. Flow orientation of mixed particle suspensions is also expressed as a linear combination. From
[lo]) that S1 depends on the rotary P&let number Pe, i.e. Si = S,(Pe). In this apparatus, the rotary P&let number is given as Pe = yQ/d3D, (where y is a hydrodynamic constant, Q is the flow rate, d is the diameter of the small tube, and D, is the rotary diffusion coefficient) (Karis and Jhon [6,7], Kwon et al. [lo]). S, depends on particle size and shape through D,. Therefore changes in the inductance flow curve are attributed to microstructural changes in the suspension. In flow orientation of mixed suspensions, as shown in Figs. 2(a) and 2(b),
4
where Si (i = 1, 2) are the order particle type i. For a non-magnetic particle mixture, Eq. (4) becomes
(4)
parameters for and magnetic
-S,)
It was shown
(Karis
and Jhon
[6], Kwon
et al.
46
T.M.
Kwon ef al.lColloids
the changes in the inductance flow curve with nonmagnetic particle content imply that changes in the flocculation state of the magnetic particles occur with non-magnetic particle content, i.e. S, = f[II,(42)]. The flocculation state changes through changes in magnetic interaction owing to nonmagnetic particles. Further tests are in progress to characterize the mixed-suspension inductance flow
I 2 3 4 5 6 7 8
solid films suspensions.
9
from
the
mixed
particle
Acknowledgment
10 II 12
Thanks are due to Dr. J.R. Lyerla, who arranged a joint study agreement with IBM, and provided the opportunity and encouragement to work on this project.
Eng. Aspects 80 (1993) 39-46
References
curves using barium ferrite with magnetic and nonmagnetic rod-like particles. Tests should also be done to measure the magnetic hysteresis curve for prepared
Surfaces A: Physicochem.
13 14
B. Bhushan, Tribology and Mechanics of Magnetic Storage Device, Springer-Verlag, New York, 1990. G. Bate, in E.P. Wohlfarth (Ed.), Ferromagnetic Materials, Elsevier, Amsterdam, 1986, pp. 381-507. M.P. Sharrock, IEEE Trans. Magn., 25 (1989) 4374. G. Bate, IEEE, 74 (1986) 1513. T.E. Karis and MS. Jhon, Proc. Natl. Acad. Sci. USA, 83 (1986) 4973. T.E. Karis and M.S. Jhon, J. Appl. Phys., 64 (1988) 5843. T.E. Karis and M.S. Jhon, Colloids Surfaces, 53 (1991) 393. T.M. Kwon, M.S. Jhon and T.E. Karis, IEEE Trans. Instrum. Meas., 41 (1992) 10. T.M. Kwon, M.S. Jhon and T.E. Karis, Adv. Inf. Storage Syst., 4 (1992) 87. T.M. Kwon, M.S. Jhon and T.E. Karis, J. Appl. Phys., 72 (1992) 3770. J. Takahashi, K. Itoh and S. Ogawa, IEEE Trans. Magn., 22 (1986) 713. P.C. Liu, Ph.D. Thesis, Carnegie Mellon University, PA, 1990. S. Chikazumi and S.H. Charap, Physics of Magnetism, Krieger, Huntington, New York, 1978. G. Bottoni, D. Candolfo, A. Cecchetti and F. MasoIi, IEEE Trans. Magn., 8 (1972) 770.
Rheomagnetic suspensions
39
properties of mixed magnetic particle
T.M. Kwod’, M.S. Jhon”, H.J. Choib,‘, T.E. Kariz+* ‘Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, bDepartment of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA “IBM Research Division, Almaden Research Center, San Jose, CA 95120-6099, USA (Received
18 November
1992; accepted
USA
8 April 1993)
Abstract Single-domain magnetic particles are the essential ingredient of magnetic tapes, particulate recording disks and magnetic stripes. The particles are single-domain y-Fe,Oj, CrO, or barium ferrite, and non-magnetic a-Fe,O, mixture. Each of these particles has intrinsic coercivity, which should be matched with the magnetic field strength of the writing element of a particular device. In this study a magnetic inductance measurement with low field strength was employed to obtain the magnetic permeability of suspensions containing two of the particle types mixed together as a function of composition and volume fraction of particles. The bulk magnetic property B is a linear combination of the contributions from each particle type such that the “excess” inductance is L - L, = L$J$ where 4i is the volume fraction and B, is the magnetic property of particle type i. For the non-magnetic a-Fe,03, Bi=O. This allows the formulation of mixed particle suspensions to obtain a desired property for custom-designed magnetic particle coatings. However, mixing magnetic particle types will broaden or produce a bimodal switching field distribution. This may affect the squareness of the magnetic hysteresis loop. These properties should be taken into account for the design of a practical magnetic coating with mixed particle suspension. Another requirement of the magnetic particle suspensions is that they remain well dispersed, even though strong magnetic forces between the particles promote flocculation. An extension of the inductance measurement technique is employed to study the flocculation of a suspension containing magnetic y-Fe,O, and non-magnetic a-Fe,O,. The presence of the a-Fe,O, decreases the flocculation state of the suspension. Thus the suspension stability is enhanced by incorporating a small amount of non-magnetic particles in addition to surfactant. Key words: Magnetic
particles;
Rheomagnetic
measurement;
Suspensions
Introduction Conventional particulate media are manufactured in a coating process similar to a painting or printing process (Bhushan Cl]). The magnetic particle suspension is prepared by dispersing dried particles in a suspending fluid (mixture of binder resins and solvent). The suspension is filtered to remove large floes and agglomerates. The filtered *Corresponding author. ‘Permanent address: Department of Polymer Engineering, Inha University, Inchon, Korea.
Science
and
suspension is coated on a polymer substrate. While the particles are well dispersed, the magnetic particles in the film are oriented by a magnetic field. After the orientation step, the coated film is cured in an oven, leaving a cross-linked magnetic film. The magnetic particles are single-domain Codoped y-Fe,03, CrO,, or barium ferrite, several tenths of a micrometer in size (Bate [2], Sharrock [3]). Since the magnetic particles retain a permanent magnetic moment, the magnetic particles in suspension are subject to aggregation and flocculation. Flocculation adversely affects the coating
T.M. Kwon
40
and the magnetic
orientation
poor media performance roughness
and
media
process,
through noise.
resulting
increased The
final
et al./Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 39-46
in
surface product
quality depends on the “quality of dispersion” and the concentration of magnetic particles in the coating (Bate [4]). Control of the magnetic particle suspension is a prime concern in manufacturing. The recently developed rheomagnetic measurement (RM) technique (Karis and Jhon [5,6]), which measures hydrodynamic orientation of magnetic particles using a magnetic sensing field, is particularly well suited to process control because it can perform on-line measurement of both the concentration and the dispersion quality of the magnetic particle suspensions. Previous studies focused on the development of an efficient flow orientation measurement and detection scheme (Karis and Jhon [6,7]). Experimental observations have demonstrated that the RM technique employed to measure the hydrodynamic orientation provides a measure of the flocculation state for both rod- and plate-like particles in suspension (Karis and Jhon [7], Kwon et al. [S-IO]). A theory was also derived to relate the observations to the effective particle (or floe) diameter and the aspect ratio of suspended particles. This study considers the effects of mixtures of two different types of particle on the suspension microstructure. The effects are related to the inductance measurement of the RM apparatus. A previous study (Kwon et al. [lo]) focused on suspensions containing only one type of magnetic particle. The current study is the first one to report RM measurements on mixtures. Mixtures are of industrial interest because formation of pinholes in barium ferrite particle coatings for ultrahigh density recording media are reduced by the addition of a small amount of a different type of particle to the suspension (Takahashi et al. [ 111). Very little is known about this effect. Our purpose in studying mixtures of particles is to investigate the effects of interparticle forces exerted between magnetic particles on suspension properties. The effects of magnetic attraction can be systematically studied through adjusting the interparticle forces by the following two methods. One approach is to
alter the magnetic interaction by replacing some of the magnetic particles in suspension with nonmagnetic
particles.
Another
approach
is to elimi-
nate the interaction by raising the temperature above the Curie temperature of the magnetic particles. Above the Curie temperature, the magnetic particles lose their magnetic domain and become non-magnetic (Liu [12]). In this paper, we consider only mixtures. The heating method requires modification of the apparatus and will be described in a later publication. The initial magnetic susceptibility of the coating is dependent on that of the constituent particles. This is controlled by adjusting the chemical composition of the particles. For example, the magnetic properties of y-Fe,O, are adjusted by doping cobalt in the particle surface layers. Alternatively, the magnetic property of the coating could, in principle, be adjusted by controlling the relative proportion of two or more types of particle each having different magnetic properties. The mechanical properties of the composite magnetic film can then be maintained by incorporating the same total amount of magnetic particles. Tests are carried out using inductance measurement by placing sample suspensions in a high frequency (l-10 MHz) sensing coil. The increase in inductance is proportional to the volume fraction of magnetic particles placed in the coil and the initial magnetic susceptibility of the suspension. Binary mixed particle suspensions over a range of compositions are prepared and measured using the coil inductance method. A mixture of magnetic with non-magnetic particles is also explored to address the effect of nonmagnetic particles on the flocculation stability of the mixed particle suspension. The flocculation process is an equilibrium between the primary particles and clusters of particles bound together by magnetic attractive forces. For primary particles to form a floe, two particles must approach within the range of the forces and stick together. The probability of close contact between particles is usually reduced by adding a surfactant. The surfactant can sterically or electrostatically stabilize the
TM.
Kwon et al./Colloids
Surfaces A: Physicochem.
Eng. Aspects 80 (1993) 39-46
41
suspension by providing a physical or electrostatic repulsive barrier between the particles. In this
the particles’ major axis. Plate-like magnetic particles’ magnetic moment vector is perpendicular to
study no surfactant is added. Instead, in the case of a mixed magnetic and non-magnetic particle
the face. Consequently, the inductance provides a measure of the particles’ orientation by the flow
suspension, the suspension stability can also be improved. When a magnetic and non-magnetic particle approach one another, attractive magnetic
field. Since the orientation
forces are absent, and even a small amount of electrostatic stabilization will prevent flocculation. Effectively, the probability of contact between two magnetic particles is decreased by the presence of a non-magnetic species. The coil magnetic inductance measurement is employed on a flowing suspension to study and non-magnetic
the effect of a mixed particle suspension.
magnetic
Experimental The main element in the test apparatus is an inductor coil and oscillator circuit. Stable oscillation is obtained in the range 1- 10 MHz by selection of the appropriate capacitor. The coil is made by wrapping IO-50 turns of silver or copper wire around a glass tube. The oscillator method was selected for its low cost and accuracy (Karis and Jhon [7], Kwon et al. [lo]). The oscillator is designed to provide a 5 V peak-to-peak square wave output with a 50% duty cycle. Frequency is measured to within six significant figures using a frequency counter. The inverse square frequency is proportional to the coil inductance. For quiescent measurements, the suspension is placed in the coil and the oscillator frequency shift gives the change of inductance relative to that with pure suspending fluid in the coil. Further information regarding the state of suspension flocculation is obtained if the coil is wrapped closely downstream from a circular converging flow of the suspension. A circular converging flow (7.5 cm to 3 mm in diameter) was used in this study. The contribution of the magnetic particles to the inductance is related to the projection of the particle magnetic moment vector onto the magnetic field vector. For rod-like magnetic particles, the magnetic moment vector is parallel to
depends
on the effective
particle shape and flow rate relative to rotary Brownian diffusion, the inductanceeflow rate curve provides an indirect measure of dispersion quality. For rod-like magnetic particles, the decreases with increasing orientation,
inductance so that a
larger decrease in inductance with flow rate indicates improved dispersion quality. The detailed description of the apparatus and the experimental procedure are given by Karis and Jhon [7] and Kwon et al. [lo]. The mixed magnetic particle suspension characteristics are emphasized here. The magnetic particles used in this study were single-domain particles as used for commercial media production. Rod-like y-Fe,O, particles were obtained from Magnox. In order to study the effects of intrinsic magnetic properties, nonmagnetic cr-Fe20a particles were also obtained from Magnox. According to the vendor’s note, the a-Fe,O, particles were thermally treated to manufacture y-Fe20, particles and therefore the size and the shape of these two particle types were nearly the same. Rod-like Cr02 particles from Du Pont and plate-like barium ferrite particles from Toda (Japan) were also obtained for testing. The size and shape of the particles were characterized using a transmission electron microscope (TEM). A vibrating sample magnetometer (VSM), EG&G PARC Model 155, was employed to characterize the particle magnetic properties for randomly oriented dry powders. The particle density was calculated using the volume increase when a known mass of particles was immersed into the fluid. The size and shape from the TEM micrographs and the density and magnetic properties from VSM measurements for each particle type are listed in Table 1. Actual suspensions for media production contain not only particles and solvent (usually cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone,
T.M.
42 Table 1 Physical properties
of the magnetic
Property
M, (emu g-t)
Eng. Aspects 80 (1993) 39-46
particles Plate-like
Rod like” ;‘-Fe,O,
H, (Oe) Density pp (g cm 3, Particle sizeb (pm) Aspect ratio p = L/a Curie temp. T, (“C)
Kwon et al.,‘Colloids Surfaces A: Physicochem.
71.8 321 4.7 L=O.5 7.0 590
ac-Fe,O,
CrOz
Barium
4.7 L=O.5 7.0 _
81.49 585 4.8 L = 0.46 13 1155126
34.97 635 5.2 a=0.13 0.1 320
ferrite
“According to the vendor’s note the r-Fe,O, _ _.particles were thermally and shape of these two types of particle were identical. bL, length of the particle; a, diameter of the particle. ‘Cited from Ref. 4.
treated
or a mixture of these) but also approximately 5 wt.% of binder polymer. In this study, ethylene glycol was adopted as a suspending fluid to simulate the actual coating formulation. Ethylene glycol was obtained from Aldrich. Particle suspensions were prepared by a consistent procedure to obtain reproducible initial suspension characteristics. First, 400 g of the particle powder was combined with 600 ml of suspending fluid in a paint shaker (Red Devil, Model 5410-00) for 10 min. Then the mixture was passed through a media mill (Eiger Minimotor Mill Exp., 0.6 mm ceramic ball media) at 1000 rev min ‘. An additional amount of suspending fluid was stirred into this millbase, and the diluted suspension was once again passed through the mill. No dispersant or surfactant was included. Mixed particle suspensions were made by combining two different types of magnetic particle suspensions prepared according to the above procedure.
suspensions, we studied rod-like + rod-like and rod-like + plate-like magnetic particle mixtures, and magnetic-t non-magnetic particle mixed suspensions. Figures l(a) and l(b) show the measured inductance behavior for a mixture of plate-like barium ferrite and rod-like y-Fe,03 and a mixture of rod-like y-Fe,O, and CrO, particle suspensions
Results RM characterization includes the concentration measurement for a quiescent suspension and the orientation measurement for the suspension subjected to flow. Thus we performed the RM characterizations focusing on these two measurements for mixed suspensions. In the concentration measurement for quiescent
to manufacture
y-Fe,O,
particles
and therefore
the size
respectively. As shown in Figs. l(a) and l(b), the inductance of a quiescent mixed suspension is a linear combination of all particle species in the mixture. A previous study (Kwon et al. [lo]) revealed that the measured inductance of a quiescent suspension is proportional to the volume concentration of the particles in the suspension and that the proportionality constant is related to the initial magnetic susceptibility of the particles. The observation of a linear combination of measured inductance for a quiescent mixed suspension implies that the intrinsic magnetic property, i.e. the initial susceptibility of the particle mixture, is the volume average of the constituent particles’ magnetic properties. To study the dependence of the measured inductance on the content of non-magnetic particles in the suspension of magnetic particles, we prepared mixed suspensions of y-Fe,O, and a-Fe,03 particles because the z-Fe,O, particles are not only non-magnetic but also similar to y-Fe,O, particles in shape and size. Figure l(c) shows the measured inductance dependence on the level of z-Fe,O, in
T.M. Kwon et al./Colloids
Surfaces
A: Physicochem.
Eng. Aspects 80 (1993) 39-46
I2950
43
12950 (a)
TO,.ll P.i”xlr
12890
1 0
Fraction
Fr.,‘l,“n
20
(c)
10
of Chromium
hrl
Dioxide,
’ X0
@caz/@
1 100
(%)
Fig. 1. (a) Measured inductance dependence on barium ferrite particle content in rod-like ;I-Fe,O, and plate-like Ba-ferrite mixed suspensions. (b) Measured inductance dependence on rod-like CrO, particle content in rod-like y-Fe,O, and CrO, mixed suspensions. (c) Measured inductance dependence on non-magnetic a-Fe,O, particle content in magnetic y-Fe,O, and non-magnetic r-Fe,O, mixed suspensions. The inductance of the mixed suspension is a linear combination of the inductance contributions from the two constituent particle species. The filled and unfilled symbols indicate suspensions from different batches.
the particle suspension. The inductance of the pure cc-Fe,O, particle suspension was the same as that found with pure suspending fluid. The initial susceptibility of a-Fe,O, is negligibly small under the test conditions owing to the non-magnetic nature of a-Fe,O, (Chikazumi and Charap [13]). The inductance for the y-Fe,O, and a-Fe,O, mixed suspensions was also linearly dependent on the sum of inductance contributions by each particle type. From the linear dependence of the inductance on the particle concentration observed in the mixture studies, we conclude that the measured inductance of a quiescent suspension is determined solely by the particle concentration and the intrinsic
magnetic properties of the particles constituting the suspension. Microstructural changes in the mixed suspensions may be reflected in the inductance of the suspensions subjected to flow, because the orientational characteristics depend on particle shape. To study the effect, we performed flow orientation measurements on mixed suspensions of y-Fe203 and cc-Fe,O, particles in the converging flow cell. The results are shown in Fig. 2. In this figure, the vertical axis is scaled with respect to the volume fraction of magnetic y-Fe,O, particles because non-magnetic a-Fe,O, does not contribute to the inductance. In these tests, the overall change in L from zero flow rate to the maximum flow rate
T.M.
44
Kwon et al./Colloids
Surfaces A: Physicochem.
Eng. Aspects 80 (1993) 39-46
2400
2300
2100[ 0
500
1000
I500
0
r-Fe,O,
content
on flow orientation of magnetic y-Fe,O, sions: (a) qh=0.030; (b) C#J =0.023.
increases with increasing orientation. For suspensions of total particle volume concentration 4= 0.030, shown in Fig. 2(a), the orientation of the yFe,O, particle suspension is slightly increased by a low level of a-Fe,O, substitution. Further increasing the z-Fe,O, fraction beyond 0.6 did not appreciably increase the flow orientation. This was considered to occur because the suspension is in the semidilute regime of concentration [lo]. Interparticle interaction is not further reduced by the replacement with non-magnetic r-Fe,O,. The same test was done on a suspension of lower total particle concentration (4 = 0.023). The results are presented in Fig. 2(b). Compared to the flow orientation of higher concentrations shown in Fig. 2(a), the suspension showed more overall orientation change with a-Fe,O, content. From the above results, a low level of substitution of non-magnetic particles reduces interparticle attraction, increasing the presence of orientable microstructures. An extreme of the microstructure would be primary particles. Therefore the content of non-magnetic particles reduces the level of flocculation. Summary and discussion The effects of microstructural changes produced by mixing two different types of particle suspension
’ IO00
1500
Flow Rate, Q (ml/min)
Flow Rate, Q (ml/min) Fig. 2. Effect of non-magnetic
._
500
and non-magnetic
r-Fe,O,
mixed suspen-
were characterized using the RM technique. The measured permeability, or initial susceptibility of mixed suspensions (rod-like y-Fe,O, + rod-like CrO,, rod-like y-Fe,03 + plate-like barium ferrite, and rod-like magnetic y-Fe,O, + rod-like nonmagnetic cc-FezO,), is a linear combination of the constituent particles’ magnetic properties. The correlation coefficient for the linear regression was greater than 0.99. Microstructural changes produced by substituting non-magnetic sr-Fe,O, particles for magnetic y-Fe,O, had little effect on the inductance of a quiescent ;J-Fe,03 particle suspension. The permeability of the quiescent suspension depends only on the concentration and intrinsic magnetic properties of the constituent particles in mixed suspension. This is consistent with the independence of the initial susceptibility from volume fraction reported by Bottoni et al. [14]. In flow orientation measurements on mixed suspensions of non-magnetic a-Fe,O, and magnetic y-Fe,03 particles, reduced interparticle interaction by nonmagnetic a-Fe,O, particle substitution increased the flow orientation of magnetic y-Fe,O, particles owing to the formation of microstructures that could be more easily oriented by the imposed flow. The observations from this mixed suspension study are expressed in terms of the previously developed theory. For quiescent suspension of a
T.M. Kwon et al./Colloids
Surfaces
A: Physicochem.
single particle type, the inductance in Kwon et al. [lo])
L is (see Eq. (10)
45
1500
TotalPanicle Fracrmn
L-LL,=4B where 4 is the volume
Eng. Aspects 80 119931 39-46
(1) fraction
of suspended
mag-
netic particles, L, is the measured inductance for the suspending medium (C#J =O), and B is a constant which is related to the intrinsic initial magnetic susceptibility of the particles. For mixed suspensions, one can expect B to be related to the effective magnetic property of the mixture. From the observed linear combination shown in Figs. l(a)-l(c), Eq. (1) for a quiescent mixed suspension is written as L-L,=$IB,+m,B,=d(~~+~R2)
(2)
where Bi (i = 1, 2) are intrinsic quantities and $i is the volume concentration of particle type i in the mixed suspension. Equation (2) can also be expressed as Y=$(B,-B,)+B,
0
20
40
$1/$
60
80
100
WI)
Fig. 3. (L-L,)/+ vs. +I/4 plot for magnetic y-Fe,O, and nonmagnetic a-Fe,O, mixed suspensions in Fig. l(c). The data fall on the master curve given by Eq. (3). Here the intercept is zero owing to non-magnetic r-Fe,O,(E,=O).
the equation for the inductance of a suspension subjected to hydrodynamic orientation (see Eqs. (1) and (9) in Kwon et al. [lo]), the inductance of a mixed suspension is
L_
$Bl(I
- S,) + $B,(1
- S,)
1
Equation (3) implies that plots of (L - L,)/cj~ vs. c$~/c$for quiescent mixtures should yield a master curve of slope B, - B, and intercept B,, regardless of the total particle concentration 4. If a suspension shows a deviation from the master curve, it implies that there is a significant magnetic property change due to structural changes in the mixture. Thus, using Eq. (3), we can study structural changes in a
L-L ---S=B,(l 41
mixture. For the non-magnetic cl-Fe,O, and magnetic y-Fe203 mixtures shown in Fig. l(c), we present the (L - L,)/$J vs. cjl/~ plot in Fig. 3. The mixed suspensions of different concentrations fall on the master curve of Eq. (3); here the intercept is zero owing to the presence of non-magnetic aFe,03 (BZ =O). We thus consider that structural changes in the non-magnetic/magnetic particle mixture have little effect on the intrinsic magnetic property. Flow orientation of mixed particle suspensions is also expressed as a linear combination. From
[lo]) that S1 depends on the rotary P&let number Pe, i.e. Si = S,(Pe). In this apparatus, the rotary P&let number is given as Pe = yQ/d3D, (where y is a hydrodynamic constant, Q is the flow rate, d is the diameter of the small tube, and D, is the rotary diffusion coefficient) (Karis and Jhon [6,7], Kwon et al. [lo]). S, depends on particle size and shape through D,. Therefore changes in the inductance flow curve are attributed to microstructural changes in the suspension. In flow orientation of mixed suspensions, as shown in Figs. 2(a) and 2(b),
4
where Si (i = 1, 2) are the order particle type i. For a non-magnetic particle mixture, Eq. (4) becomes
(4)
parameters for and magnetic
-S,)
It was shown
(Karis
and Jhon
[6], Kwon
et al.
46
T.M.
Kwon ef al.lColloids
the changes in the inductance flow curve with nonmagnetic particle content imply that changes in the flocculation state of the magnetic particles occur with non-magnetic particle content, i.e. S, = f[II,(42)]. The flocculation state changes through changes in magnetic interaction owing to nonmagnetic particles. Further tests are in progress to characterize the mixed-suspension inductance flow
I 2 3 4 5 6 7 8
solid films suspensions.
9
from
the
mixed
particle
Acknowledgment
10 II 12
Thanks are due to Dr. J.R. Lyerla, who arranged a joint study agreement with IBM, and provided the opportunity and encouragement to work on this project.
Eng. Aspects 80 (1993) 39-46
References
curves using barium ferrite with magnetic and nonmagnetic rod-like particles. Tests should also be done to measure the magnetic hysteresis curve for prepared
Surfaces A: Physicochem.
13 14
B. Bhushan, Tribology and Mechanics of Magnetic Storage Device, Springer-Verlag, New York, 1990. G. Bate, in E.P. Wohlfarth (Ed.), Ferromagnetic Materials, Elsevier, Amsterdam, 1986, pp. 381-507. M.P. Sharrock, IEEE Trans. Magn., 25 (1989) 4374. G. Bate, IEEE, 74 (1986) 1513. T.E. Karis and MS. Jhon, Proc. Natl. Acad. Sci. USA, 83 (1986) 4973. T.E. Karis and M.S. Jhon, J. Appl. Phys., 64 (1988) 5843. T.E. Karis and M.S. Jhon, Colloids Surfaces, 53 (1991) 393. T.M. Kwon, M.S. Jhon and T.E. Karis, IEEE Trans. Instrum. Meas., 41 (1992) 10. T.M. Kwon, M.S. Jhon and T.E. Karis, Adv. Inf. Storage Syst., 4 (1992) 87. T.M. Kwon, M.S. Jhon and T.E. Karis, J. Appl. Phys., 72 (1992) 3770. J. Takahashi, K. Itoh and S. Ogawa, IEEE Trans. Magn., 22 (1986) 713. P.C. Liu, Ph.D. Thesis, Carnegie Mellon University, PA, 1990. S. Chikazumi and S.H. Charap, Physics of Magnetism, Krieger, Huntington, New York, 1978. G. Bottoni, D. Candolfo, A. Cecchetti and F. MasoIi, IEEE Trans. Magn., 8 (1972) 770.