FLUIDES AIMANTES A AIMANTATION A SATURATION ELEVEE

MAGNETOFLUID WITH HIGH SATURATION MAGNETISATION

Abrégé français

L'invention vise à créer un fluide magnétique présentant une aimantation à saturation élevée, dont la concentration de saturation est supérieure à 100 mT afin de permettre le transfert de forces supérieures. A cet effet, on forme des dispersions stables de particules nanométriques et de cobalt modifiées par des couches d'adsorption monomoléculaires, dans des fluides de support, la concentration volumique des composants ferromagnétiques étant comprise entre 5 et 35 %, de préférence entre 15 et 30 %, et la concentration volumique des particules modifiées (particules nues avec enrobage d'adsorption) atteignant jusqu'à 50 % en volume.

Abrégé anglais

It has been the task of the invention to form a magnetofluid with a high
saturation magnetisation which has a saturation concentration that lies
above 100 mT in order to be able to transmit higher forces.
According to the invention, this is attained by stably dispersing cobalt and
nanometer particles modified with mono-molecular adsorption layers in
carrier liquids, whereby the volume concentration of the ferromagnetic
component comes to 5 to 35 per cent, preferably to 15 to 30 %, and the
volume concentration of the modified particles (naked particles with
adsorption shell) comes up to 50 per cent by volume.

Revendications

CLAIMS
1. A magnetofluid with an extremely high saturation polarization, the
magnetofluid having a
ferromagnetic component, a surface-active agent, and a carrier liquid, in
which the particle
size of the ferromagnetic component is between 5 and 20 nm, the ferromagnetic
component
containing finely dispersed particles selected from the following group:
cobalt and iron
- nanometer particles, the particles being modified with mono-molecular
adsorption layers in
carrier liquids having a low viscosity between 0.7 and 10 mPas, whereby the
volume
concentration of the ferromagnetic component is between 5 and 35 per cent by
volume, the
volume concentration of the modified particles is up to 50 per cent by volume,
and the
concentration of the surface-active agent is maintained above its micelle
concentration to
ensure stability and mono-dispersity of the modified particles in the carrier
liquid.
2. A magnetofluid as defined in Claim 1 in which the modified particles are
manufactured by
thermolysis of one-molecular metal carbonyl solutions in organic carrier
liquids.
3. A magnetofluid as defined in any one of claims l and 2 in which the volume
concentration
of the ferromagnetic component is between 20 and 30 per cent by volume.
4. A magnetofluid as defined in any one of claims 1 to 3 in which the
viscosity of the carrier
liquid at room temperature is between 0.8 and 2 mPas.
5. A magnetofluid as defined in any one of claims 1 to 4 in which the surface-
active agents
have corrosion-inhibiting characteristics.
6. A magnetofluid as defined in any one of claims 1 to 5 in which unmodified
nanometer
particles have a particle size between 5 and 20 nm.
7. A magnetofluid as defined in any one of claims 1 to 6 in which said mono-
molecular
adsorption layers have a thickness of between 1.2 and 2 nm.
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8. A magnetofluid as defined in any one of claim 1 to 7 in which the ratio of
the modified
particle radius to the unmodified particle radius is between 1.10 and 1.30.
9. A magnetofluid as defined in claim 8 in which the ratio of the modified
particle radius to the
unmodified particle radius is between 1.15 and 1.20.

Description

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Magnetofluid with a high saturation magnetisation
Magnetofluids are so-called "smart" materials that are locally fixed in in-
teraction with strong external magnetic fields or magnetic field gradients
as well as brought in positions that are previously determined - even
against the force of gravity. Thereby, their colloidal stability is
maintained,
and they behave like a continuous liquid phase.
The term of a magnetofluid has usually been introduced for them.
A decisive factor for their quality is the value of the saturation
polarization
because this value proportionally becomes part of the specific interaction
force of the magnetofluid with the external magnetic field.
The magnetofluids are used in blood pumps, for the manufacture of mag-
netofluid sensors with a high sensitivity or for the production of magneto-
fluid gaskets with a higher sealing power.
In general, such magnetofluids are stable dispersions with super-
paramagnetic properties. They are composed of ferri- or ferromagnetic
nanometer particles, surface-active agents and a carrier liquid. The sur-
face-active materials effect a dispersing and colloidal stabilisation of the
particles in the carrier liquids.
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Very different carrier liquids, such as water, hydrocarbons, perfluorocar-
bon ether, diester, and silicone oils are used. Magnetite which is modified
with
special surface-active materials, such as medium-chained organic acids,
is used as a magnetic component.
Magnetofluids with saturation magnetisations of 10 to 40 mT (Ferrofluidics
Corporation) and up to 90 mT are commercially offered
The manufacture of magnetic liquids which contain metallic nanometer
particles instead of magnetite particles is also known. Typical ferromag-
netic metals are iron, cobalt and nickel. Their particles are made by ther-
molysis of their carbonyls or by aerosol formation.
Polymers, such as polybutadiene derivatives with a molecular weight of at
least 1,000 or polybutenyl succinpolyamine and surface-active agents,
such as oil-soluble alkyl sulfonates or sarcosyl-0 and duomeen-TDO, are
described as stabilising agents. However, the value of the saturation
magnetisation lies far below 30 mT, mostly only at 5 mT. In this way, low
forces can also be transmitted regularly.
Accordingly, the surface-active agent concentrations in solvents are so
low that the critical micelle concentration for the inverse micelles is not
reached.
Therefore the invention is based on the task to create a magnetofluid
having a saturation concentration that lies above 100 mT in order to be
able to transmit higher forces.
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According to the invention, this is done by stably dispersing modified co-
balt and nanometer particles in carrier liquids with mono-molecular ad-
sorbed layers, whereby the volume concentration of the ferromagnetic
component comes to 5 to 35 per cent, mainly to 15 to 30 %, and the vol-
ume concentration of the modified particles (naked particles with adsorp-
tion shell) up to 50 per cent by volume.
Due to the high volume concentration of the ferromagnetic component and
the value which is by the factor 3 to 4 times higher than magnetite for the
saturation polarization of the naked particles which have a size of 5 to
20 nm, the value resulting for the saturation polarization lies between
100 and 400 mT, however, preferably between 150 and 300 mT.
The modified particles are manufactured on the basis of the known
chemical procedure through nucleation/crystalline growth processes,
namely the thermolysis of their carbonyls directly conducted in an organic
solvent in the presence of polymers and surface-active agents. According
to the invention, polymers or surface-active agents with reactive nitrogen
compounds are clearly used above the critical inverse micelle concentra-
tion in order to produce iron-nanometer particles. In the same way, poly-
mers or surface-active agents with reactive sulfo- or nitrogen compounds
are also used above their micelle concentration for the stabilisation of the
cobalt particles.
The reactive groups are adsorbed at the surface of the metallic particles
while forming mono-molecular layers.
The strong adsorptive capacity and the firm bonding of the surface-active
agents result in the fact that these agents have an oxidation inhibiting ef-
feet on the ferromagnetic particles.
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The following prerequisites must be met in order to produce a magneto-
fluid with a high saturation polarization by means of ferromagnetic par-
ticles: use of ferromagnetic particles, such as iron and cobalt which have
a 3 to 4-fold higher saturation polarization than magnetite particles: use
of iron and cobalt particles in particle sizes of up to 20 nm under consid-
eration of ensuring a sufficient, colloidal particle stability; use of
polymers
and/or surface-active agents which adsorb mono-molecularly with layer
thicknesses of 1.2 to 2 nm under consideration of ensuring a sufficient
colloidal particle stability.
Surprisingly, it has been found out that ferromagnetic iron or cobalt parti-
cles which have mono-molecular adsorption layers as a shell can still be
dispersed stably in colloidal respect in carrier liquids if the proportion of
the radii of the modified to the naked particles lies between 1.10 and 1.30,
preferably between 1.15 and 1.20. In this way, a solids content of the fer-
romagnetic component from 5 to 35 per cent by volume is attained. This
increases the saturation magnetisation again up to 400 mT.
Even at a high particle concentration, the magnetofluid shall remain in
such a state that it is still flowable. This is attained by the viscosity of
the
carrier liquid at room temperature lying between 0.7 and 10 mPas, mainly
between 1 and 5 mPas on the one hand, and by dissolving only the re-
quired amount of polymers and/or surface-active agents in the carrier liq-
uid on the other.
The cobalt and iron nanometer particles are manufactured by modification
of the chemical methods described according to the state of the art by
thermolysis of their carbonyls in organic carrier liquids. One-molecular
metal carbonyl solutions are used for the manufacture of highly concen-
trated particles. Thereby, the thermolysis causes that a much higher num-
ber of metallic nuclei is created by thermolysis - compared with known
methods.
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From the very beginning, the concentration of the surface-active
agents/polymers in the solution has been clearly kept above its micelle
concentration in order to ensure the stability and mono-dispersity of the
particles. Therefore, it can be based on the idea that the nucleation of the
metallic particles takes place within the interstice of the inverse micelle
and thus in the direct nearness of the reactive groups of the surface-
active agents/polymers enabled for the naked particle surface. It stands to
reason that this reaction runs fully under the exclusion of air and water.
The particle size of the naked metal core can be changed in a controllable
way by the reaction control (temperature), by the chemical composition
and concentration of the reaction partners, in particular the chemical
composition of the solvent.
Cobalt and particularly iron particles tend to oxidation to non-magnetic
particles in contact with air if the surface is not protected (retarded). A
protection can also be reached by the chemical adsorption of surface-
active agentslpolymers, in particular of surface-active agents with internal
loads, such as the betaine structure. The reaction half-time of the oxida-
tion can be decreased in orders of magnitudes by the use of such bi-
functional surface-active agents which effect the colloidal as well as the
chemical stabilisation of the ferro-magnetic particles.
In the following, the invention is described in detail by means of embodi-
ments.
Embodiment 1:
40 g dicobalt octacarbonyl are mixed with 100 ml anhydrous 1,3,5 triiso-
propylbenzene (boiling point 235°) in which 8 g dehydrated sodium bis
(ethyl, hexyl) sulfosuccinate had been dissolved.

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The mixture is fed into a three-neck bottle with stirrer, cooler and gas inlet
pipe and heated at 120°C under superpure argon until the carbon mon-
oxide formation is completed. The formed Co-magnetofluid has an Ms-
value of 30 mT. Its magnetisation curve has no hysteresis; the viscosity
comes to 4 mPas only. The half-life period of the oxidation comes to sev-
eral months. The Ms-value has been increased up to 250 mT by evapora-
tion of the solvent.
Embodiment 2
In the same way as in Embodiment 1, 50 g iron pentacarbonyl, together
with a mixture consisting of 10 g oleylsarcoside and 10 g polyisobutylene
succinimide in 100 ml anhydrous decane are dropped into a surface-
active solution at a temperature of 180 °C. Afterwards, the solution is
kept
at this temperature until the formation of carbon monoxide has been com-
pleted.
The Ms-value of the formed iron-magnetofluid of 150 mT has been in-
creased by partially evaporating the decane to 250 mT. The product has
no hysteresis in the magnetisation curve and its viscosity lies at 2 Pas.
The half-life period of the oxidation in contact with air comes to several
weeks.
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