U.S. patent number 3,764,540 [Application Number 05/148,206] was granted by the patent office on 1973-10-09 for magnetofluids and their manufacture.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the. Invention is credited to Sanaa E. Khalafalla, George W. Reimers.
United States Patent |
3,764,540 |
Khalafalla , et al. |
October 9, 1973 |
MAGNETOFLUIDS AND THEIR MANUFACTURE
Abstract
Magnetofluids, comprising a stable, colloidal suspension of
magnetite and elemental iron, are produced by comminuting a
nonmagnetic or antimagnetic precursor compound to colloidal size,
dispersing the precursor in a carrier fluid and thereafter
converting the precursor to ferromagnetic forms while in stable
suspension.
Inventors: |
Khalafalla; Sanaa E.
(Minneapolis, MN), Reimers; George W. (Minneapolis, MN) |
Assignee: |
The United States of America as
represented by the Secretary of the (Washington, DC)
|
Family
ID: |
22524754 |
Appl.
No.: |
05/148,206 |
Filed: |
May 28, 1971 |
Current U.S.
Class: |
252/62.55;
252/62.52; 252/62.56; 516/33 |
Current CPC
Class: |
H01F
1/445 (20130101) |
Current International
Class: |
H01F
1/44 (20060101); H01f 001/28 () |
Field of
Search: |
;252/62.56,62.51,62.52,62.55,309,314 ;23/200 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Vertiz; Oscar R.
Assistant Examiner: Cooper; J.
Claims
We claim:
1. A process for the preparation of ferromagnetic fluids which
comprises the steps of comminuting and dispersing a solid,
nonmagnetic suboxide of iron having the formula Fe.sub.1.sub.-x O
wherein x has a value of 0.01 to 0.20 in a liquid chosen from the
group consisting of hydrocarbons, fluorocarbons, silicone oils and
esters to form a stable, colloidal suspension of the suboxide in
the liquid and thereafter heating the liquid suspension to
temperatures in the range of about 200.degree.C to about
570.degree.C but below the decomposition temperature of the liquid
for a time sufficient to cause substantial conversion of the
nonmagnetic suboxide to ferromagnetic forms.
2. The process of claim 1 wherein said dispersing step comprises
agitation of the particulate suboxide and liquid mixture together
with an aliphatic monocarboxylic acid dispersing agent having from
about eight to about 24 carbon atoms.
3. The process of claim 1 wherein the suboxide of iron is wustite
having a composition in the range of Fe.sub.0.95 O to Fe.sub.0.85
O.
4. The process of claim 3 wherein the wustite is converted to
ferromagnetic forms by heating for a time greater than about
one-half hour.
5. The process of claim 4 where the wustite is produced from finely
divided ferric oxide by reduction at temperatures within the range
of 570.degree. to 800.degree.C.
6. The process of claim 5 wherein the wustite is recovered from the
reduction step in pyrophoric form and is protected from reoxidation
by coating it with an aliphatic monocarboxylic acid dispersing
agent having from about eight to about 24 carbon atoms.
7. The process of claim 6 wherein the dispersing agent-coated
wustite is comminuted by wet grinding in the liquid.
8. The process of claim 7 wherein the liquid is a hydrocarbon and
wherein the dispersing agent is oleic acid.
9. The process of claim 8 wherein the hydrocarbon is kerosene.
Description
BACKGROUND OF THE INVENTION
Ferromagnetic liquids, commonly known as "ferrofluids" comprise a
permanent suspension of ferromagnetic particles in a liquid
carrier. Typical compositions consist of submicron-sized magnetite
particles suspended in an organic liquid, such as kerosene, with a
dispersing agent added to prevent flocculation of the particles.
Properties of the fluids are highly dependent upon particle size
and upon the concentration of ferromagnetic particles in the
liquid. A detailed discussion of ferrofluids, their properties and
their uses may be found in an article by R. E. Rosensweig published
in International Science and Technology, July 1966, pages
45-56.
Ferromagnetic liquids are conventionally produced by the long-term
grinding of magnetite in a carrier liquid. Grinding times required
in order to obtain a true colloidal suspension range to 1,000 hours
or more. Grinding ferromagnetic materials is complicated by
interparticle magnetic flocculation arising from the attraction of
one particle to another. This magnetic attraction is in addition to
the usual interparticle attraction caused by van der Waal's forces.
Consequently, the conventional preparation processes are extremely
inefficient.
SUMMARY OF THE INVENTION
We have found that the grinding or comminuting time necessary to
prepare magnetofluids may be greatly reduced by appropriate use of
a non-magnetic precursor compound. The nonmagnetic precursor
compound, preferably a suboxide of iron, is comminuted to colloidal
size and dispersed in a carrier liquid. It is then converted to
ferromagnetic forms while in suspension to form a stable
magnetofluid.
A preferred embodiment comprises preparing submicron sized wustite
particles by controlled reduction of ferric oxide and further
comminuting the wustite particles in the presence of a dispersing
agent until colloidal dimensions are obtained. The wustite
particles are then converted to ferromagnetic forms by
disproportionation to iron and magnetite.
Hence, it is an object of our invention to prepare magnetofluids
from nonmagnetic precursor compounds.
Another object of our invention is to reduce the grinding time
required in the production of stable magnetofluids.
Yet another object of our invention is to provide a magnetofluid of
novel composition.
DETAILED DESCRIPTION OF THE INVENTION
We have discovered that ferromagnetic liquids, hereafter referred
to as ferrofluids, may be produced in a simple fashion by
comminuting a non-magnetic or antimagnetic precursor compound to
produce a stable, colloidal suspension and thereafter converting
the precursor compound to ferromagnetic forms. As used herein, the
term "nonmagnetic" will be understood to include antimagnetic
compounds. This technique reduces grinding time required to produce
a stable, submicron sized particulate suspension to as little as 5
percent of that required by conventional methods.
Substitution of magnetite by a nonmagnetic precursor allows
grinding or other comminution techniques to be accomplished without
complications arising from interparticle magnetic attraction.
Precursor compounds may comprise the lower or suboxides of iron or
ferrous oxides generally. The preferred precursor is wustite which
is FeO with a defect structure. Wustite is also commonly
represented by formula Fe.sub.1.sub.-x O wherein x will typically
vary from about 0.01 to about 0.20.
Wustite has been the subject of a number of theoretical studies. It
is thermodynamically unstable below its eutectoidal temperature of
570.degree.C but the mestable phase can be prepared by rapid
quenching from higher temperatures to room temperature. Quenching
will preserve or freeze the wustite structure due to extreme
sluggishness of solid-solid transformations at ordinary
temperatures. Heating the metastable phase to temperatures below
the eutectoidal point causes wustite to disproportionate to the
thermo-dynamically stable phases of .alpha.-iron and magnetite
according to the following general formula:
4 FeO .revreaction. Fe + Fe.sub.3 O.sub.4
The kinetics of this reaction were studied by Ilschner and Mlitake,
who found that Fe.sub.1.sub.-x O decomposes in the temperature
range of 220.degree. to 450.degree.C. They found that the apparent
activation energy varied between 17 and 12 KCal per mole magnetite
formed as x changed from 0.06 to 0.11, respectively. Growth of
Fe.sub.3 O.sub.4 particle with a specific orientation relationship
with respect to the matrix was controlled by an interfacial
reaction. This work was reported in Acta Metallurgica, V. 13, pages
855-867 (1965).
It is also known that wustite has a sodium chloride structure and
that the number of metal ion vacancies in Fe.sub.1.sub.-x O is
twice as large as the iron deficiency would indicate. The extra
iron is located interstitially in tetrahedral sites while vacancies
in the crystal lattice represent extra octahedral sites. As a
result the crystal has a local configuration analogous to that of
Fe.sub.3 O.sub.4 and magnetite resulting from partial decomposition
of the wustite phase is oriented identically with and grows
epitaxially on the parent substrate.
In practicing our invention, it is highly advantageous to obtain
wustite of a very small particle size in order to substantially
reduce the amount of grinding required to reach colloidal
dimensions. Hence, we prefer to prepare wustite by the low
temperature reduction of finely divided iron oxides such as
hematite. It is known that metallic iron produced by low
temperature hydrogen reduction of iron oxides is pyrophoric. This
form of iron ignites easily, even spontaneously in air and is
reoxidized. Presumably when reduction proceeds at relatively low
temperatures, only a skeleton framework of iron atoms remain where
the iron oxide was before; thermal motion of the atoms being too
sluggish to cause crystallization into a denser and more stable
form. Pyrophoric iron has been reported at reduction temperatures
as high as 600.degree.C.
While nothing is known about pyrophoric iron oxides, it was
hypothesized that low temperature reduction of hematite might yield
wustite in the pyrophoric state. These predictions were verified by
experiments in which reagent grade iron oxide powder (hematite) was
reduced at 650.degree.C with 25% CO-75% CO.sub.2 in one case and
with 50% CO-50% CO.sub.2 in a second case to yield magnetite and
wustite respectively. Both products were found to be pyrophoric in
the sense that they reoxidized spontaneously and caught fire when
exposed to air. Hence, it was clear that wustite having a very
small particle size could be prepared but the product could not be
further processed by conventional means because of its pyrophoric
nature.
We then found that wustite in the pyrophoric state could be
protected from oxidation during further processinb by coating or
covering it with a dispersing agent. A dispersing agent is a
necessary component of any ferrofluid composition. Its purpose is
to maintain a coating around each individual ferromagnetic particle
of sufficient thickness to prevent agglomeration or flocculation
induced by the attractive van der Waals force. Essentially, the
dispersing agent acts as an elastic coating around each colloidal
particle preventing the close approach of one particle to another.
Dispersing agents generally useful in ferrofluid preparation
comprise long-chain organic molecules having a reactive end group.
Aliphatic monocarboxylic acids having from about eight to about 24
carbon atoms are typical of dispersing agents useful in ferrofluid
preparation. Oleic acid in particular is commonly used and produces
excellent results. Dispersing agents useful in ferrofluid
stabilization were found to be completely satisfactory for
protecting wustite from pyrophoric oxidation. Hence the dispersing
agent performed a dual function; first to prevent pyrophoric
oxidation and second to stabilize the ferrofluid product.
Broadly, our process comprises comminuting a non or antimagnetic
suboxide of iron to colloidal dimensions, dispersing the oxide in a
carrier liquid to form a stable non-settling suspension and
thereafter converting the nonmagnetic iron oxide to ferromagnetic
forms while maintaining the particles in stable suspension. Our
preferred lower iron oxide precursor compound is wustite.
Our most preferred embodiment comprises first subjecting a higher
oxide of iron, such as hematite, to low temperature reduction to
produce wustite of similar particle size. The wustite is then
quenched to a low temperature, ambient or somewhat above, to
recover the wustite in a metastable but pyrophoric state. Next the
pyrophoric wustite is protected against oxidation by coating it
with a dispersing agent such oelic acid. The mixture is then added
to a carrier liquid, typically a hydrocarbon such as kerosene, and
subjected to grinding for a time sufficient to reduce the wustite
particles to colloidal dimensions. After formation of a stable
suspension, the wustite is converted to ferromagnetic forms,
comprising metallic iron and magnetite, by refluxing at
temperatures above about 200.degree.C. The resulting ferrofluid
product displays a strong magnetic response and is stable and
non-settling under the influence of gravitational or magnetic
fields. It is to be noted that our ferrofluid compositions differ
from those of the prior art in that they comprise a mixture of
colloidal particles of magnetite and elemental iron. Ferrofluids of
conventional manufacture comprise a suspension of ferrite
material.
It is not necessary that the precursor compound be prepared or
obtained in a finely divided form prior to the comminution step.
Large particle, non-pyrophoric wustite could be used with
substantial advantage over prior art methods. However, preparation
of wustite in a finely divided form substantially decreases
comminution or grinding time required to obtain a colloidal
suspension. Ferric oxide having a median particle diameter
substantially less than 1 micron is commerically available and
controlled reduction of such products will given wustite particles
of approximately the same size. A stable, colloidal suspension is
formed when the maximum particle size is less than about 0.01
microns (100 A), provided of course that flocculation of the
particles is avoided. Advantages arising from production of the
wustite precursor in a finely divided form are readily
apparent.
Reduction of ferric oxides must be carried out at temperatures
above the wustite eutectoidal temperature, or 570.degree.C.
Temperatures within the general range of about 570.degree. to
800.degree.C produce satisfactory results. A preferred reduction
temperature is in the range of 600.degree. to 700.degree. C. Gases
are advantageously used as the reducing agents and a gaseous
mixture of carbon monoxide and carbon dioxide is preferred. Wustite
composition may vary from a nominal Fe.sub.0.99 O to Fe.sub.0.80 O
while a preferred composition range is from Fe.sub.0.95 O to
Fe.sub.0.85 O. Conversion of the wustite precursor to ferromagnetic
forms is most conveniently accomplished by refluxing the colloidal
suspension at temperatures above about 200.degree.C and below
570.degree.C. Care must also be taken to avoid thermal
decomposition of either the carrier liquid or the dispersing agent.
Refluxing time required for substantially complete conversion of
wustite to metallic iron and magnetite is temperature dependent but
will typically range from about 1/2 to about 10 hours at
temperatures ranging from 200 to 300.degree.C.
Carrier liquids useful in our ferrofluid compositions include
hydrocarbons, fluorocarbons, silicone oils, or esters. Choice of
carrier liquid does not affect the process except that the
dispersing agent chosen must be compatible with the carrier.
Magnetic properties of the ferrofluid may be adjusted by varying
the concentration of ferromagnetic particles suspended in the
carrier liquid. This may readily be accomplished either by simple
dilution or by evaporating a portion of the carrier. Concentrations
of ferromagnetic particles as high as 40 to 50 g per 100 ml fluid
are obtainable while retaining the liquid properties of the
composition.
Ferrofluids produced by the method of this invention are useful in
a host of different applications. For example, when a ferrofluid is
subjected to an applied magnetic field, a magnetic levitation force
is created within the ferrofluid thus giving the effect of an
apparent increase in specific gravity. Nonmagnetic bodies immersed
in the fluid may be levitated by the fluid even if they have
substantially greater densities than the fluid. By adjusting the
strength of the applied magnetic field, bodies of differing density
may be selectively levitated thus providing a separation process
based upon differing densities.
Other uses include accelerometers, attitude control devices,
oscillation damping devices, rotating shaft seals and bearings. It
has also been proposed to use hydrocarbon base or oil soluble
ferrofluids for cleaning up oil spills. A relatively concentrated
ferrofluid is sprayed on or otherwise mixed with an oil slick thus
making the entire mixture magnetic. The oil slick is then skimmed
from the water surface using a traveling magnet.
The following examples serve to illustrate specific embodiments of
our invention.
EXAMPLE 1
A 200 g sample of reagent grade ferric oxide powder was reduced at
650.degree.C using a 50 percent carbon monoxide -50 percent carbon
dioxide mixture. According to the thermodynamic phase diagram of
the iron-oxygen system, the resulting product should comprise
wustite having a composition of Fe.sub.0.91 O. A thermobalance was
used to monitor the progress of reduction which was complete in
about 1 hour. The resulting wustite, weighing 184 g, was removed
from the furnace crucible and quenched to room temperature using an
inert gas atmosphere of helium. One hundred ml of oleic acid,
enough to cover the oxide phase, was then added. The resulting
mixture was then transferred to a ball mill containing 1 liter of
kerosene and 7.3 kg of steel balls. Grinding continued for a total
of 40 hours.
At the end of that time, the fluid was separated from the balls,
poured into a graduated cylinder and allowed to stand for 48 hours.
No solid sedimentation occured after that time, indicating the
fluid to be a true colloidal suspension. The fluid had a specific
gravity of 0.806 g/ml and a viscosity of 2.87 cp compared to the
carrier fluid (kerosene containing 10 percent oleic acid) which had
a specific gravity of 0.722 g/l and a viscosity of 1.46 cp. A
negative magnetic response was exhibited by the fluid when placed
near the poles of an electromagnet.
EXAMPLE 2
The antimagnetic colloidal wustite suspension of Example 1 was
converted by disproportionation into magnetite and iron by
refluxing at 250.degree.C. Refluxing was continued for 6 hours,
resulting in a fluid which exhibited a strong magnetic response.
This resulting ferrofluid had the same specific gravity, 0.806
g/ml, as did the wustite suspension but the viscosity decreased to
2.61 cp. The fluid was stable and non-settling thus indicating that
no flocculation or agglomeration occurred during the conversion of
the wustite particles to ferromagnetic forms.
EXAMPLE 3
Samples of the ferrofluid of Example 2 were concentrated by
evaporation. Nitrogen was bubbled through the fluid at a
temperature of 200.degree.C until the volume was reduced to 72.5%
of that originally present. At this point the specific gravity
(measured at 20.degree.C) was 0.820 g/ml and the viscosity was 3.50
cp. Evaporation was then continued as before until the volume was
reduced to 50 percent of that originally present. Specific gravity
increased at this point to 0.848 g/ml and the viscosity increased
to 4.55 cp. The resulting fluid was again concentrated to a volume
25 percent of that originally present. This fluid, now concentrated
4-fold, had a specific gravity of 0.916 g/ml. Viscosity was not
measured.
All of the ferrofluid concentrates retained the properties of a
true colloidal suspension displayed by the original fluid.
Progressively increasing magnetic response was noted as the
concentration increased.
EXAMPLE 4
A comparative test was performed using the same reagent grade
hematite powder as was utilized in Example 1. In this case, 190
grams of the powder was completely converted to magnetite by
controlled reduction. Weight of the magnetite product was 184 g.
The magnetite was cooled under an inert atmosphere and transferred
under 100 ml oleic acid to the ball mill containing 1 liter of
kerosene. Small samples of the product were intermittently
withdrawn as grinding proceeded. These samples were centrifuged and
the supernatant portion was tested for magnetic response. The first
positive response was obtained after 300 hours of grinding time. At
this point, the specific gravity of the fluid was 0.827 mg/l and
its viscosity was 2.86 cp.
EXAMPLE 5
In another test ferric oxide, in the form of .gamma. -Fe.sub.2
O.sub.3 or maghemite, was used to prepare magnetite. The finely
divided ferric oxide had an average particle size of 0.02 microns
according to the manufacturer and displayed a surface area of 27
m.sup.2 /g. A particle diameter of 0.02 microns is approximately
twice the particle diameter which will form a stable ferrofluid.
The maghemite was reduced under controlled conditions and
relatively low temperature in order to obtain particulate magnetite
of approximately the same particle size. After reduction, the
magnetite product was treated in a fashion identical to that set
out in Examples 1 and 4.
After 56 hours of grinding, the resulting fluid gave a very feeble
magnetic response. At this point the fluid had a specific gravity
of 0.787 g/ml and a viscosity of 1.77 cp. Magnetite particles in
the fluid settled out after 48 hours of standing in a graduated
cylinder. Additional grinding was required in order to reach the
colloidal state.
* * * * *