Magnetorheological fluids with an additive package

Details Magnetorheological fluids with an additive package Magnetorheological fluids with an additive package

2003-06-23

2004-11-30

Koslow, C. Melissa (Department: 1755)

Compositions

Magnetic

Flaw detection or magnetic clutch

Reexamination Certificate

active

06824701

ABSTRACT:

TECHNICAL FIELD
This invention pertains to fluid materials which exhibit substantial increases in flow resistance when exposed to a suitable magnetic field. Such fluids are sometimes called magnetorheological fluids because of the dramatic effect of the magnetic field on the rheological properties of the fluid.
BACKGROUND OF THE INVENTION
Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and on the order of milliseconds under the influence of an applied magnetic field. An analogous class of fluids are the electrorheological (ER) fluids which exhibit a like ability to change their flow or rheological characteristics under the influence of an applied electric field. In both instances, these induced rheological changes are completely reversible. The utility of these materials is that suitably configured electromechanical actuators which use magnetorheological or electrorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, for controllable suspension systems, vibration dampers in controllable powertrain and engine mounts and in numerous electronically controlled force/torque transfer (clutch) devices.
MR fluids are noncolloidal suspensions of finely divided (typically one to 100 micron diameter) low coercivity, magnetizable solids such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base carrier liquid such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, e.g., about one Tesla. At the present state of development, MR fluids appear to offer significant advantages over ER fluids, particularly for automotive applications, because the MR fluids are less sensitive to common contaminants found in such environments, and they display greater differences in rheological properties in the presence of a modest applied field.
Since MR fluids contain noncolloidal solid particles which are often seven to eight times more dense than the liquid phase in which they are suspended, suitable dispersions of the particles in the fluid phase must be prepared so that the particles do not settle appreciably upon standing nor do they irreversibly coagulate to form aggregates. Examples of suitable magnetorheological fluids are illustrated, for example, in U.S. Pat. No. 4,957,644 issued Sep. 18, 1990, entitled “Magnetically Controllable Couplings Containing Ferrofluids”; No. 4,992,190 issued Feb. 12, 1991, entitled “Fluid Responsive to a Magnetic Field”; No. 5,167,850 issued Dec. 1, 1992, entitled “Fluid Responsive to a Magnetic Field”; No. 5,354,488 issued Oct. 11, 1994, entitled “Fluid Responsive to a Magnetic Field”; and No. 5,382,373 issued Jan. 17, 1995, entitled “Magnetorheological Particles Based on Alloy Particles”.
As suggested in the above patents and elsewhere, a typical MR fluid in the absence of a magnetic field has a readily measurable viscosity that is a function of its vehicle and particle composition, particle size, the particle loading, temperature and the like. However, in the presence of an applied magnetic field, the suspended particles appear to align or cluster and the fluid drastically thickens or gels. Its effective viscosity then is very high and a larger force, termed a yield stress, is required to promote flow in the fluid.
SUMMARY OF THE INVENTION
Certain aspects of prior art MR fluids such as those described in the above-identified patents will illustrate the benefits and advantages of the subject invention. A first observation in characterizing MR fluids is that for any applied magnetic field (or equivalently for any given magnetic flux density), the magnetically induced yield stress increases with the solid particle volume fraction. This is the most obvious and most widely employed compositional variable used to increase the MR effect. This is illustrated in
FIG. 1
, which is a graph recording the yield stress in pounds per square inch of suspensions of pure iron microspheres dispersed in a polyalphaolefin liquid vehicle at increasing volume fractions. The strength of the magnetic field applied is 1.0 Tesla. It is seen that the yield stress increases gradually from about 5 psi at a volume fraction of iron microspheres of 0.1 to a value of about 18 psi at a volume fraction of 0.55. In order to double the yield stress from 5 psi at a volume fraction of 0.1, it is necessary to increase the volume fraction of microspheres to about 0.45. However, as the volume fraction of solid increases in the on-state, the viscosity in the off-state increases dramatically and much more rapidly as well. This is illustrated in FIG.
2
.
FIG. 2
is a semilog plot of viscosity in centipoise versus the volume fraction of the same suspension of iron microspheres. It is seen that a small increase in the volume fraction of microspheres results in a dramatic increase in the viscosity of the fluid in the off-state. Thus, while the yield stress may be doubled by increasing the volume fraction from 0.1 to 0.45, the viscosity increases from about 15 centipoise to over 200 centipoise. This means that the turn-up ratio (shear stress “on” divided by shear stress “off”) at 1.0 Tesla actually decreases by more than a factor of 10.
In terms of basic rheological properties, the turn-up ratio is defined as the ratio of the shear stress at a given flux density to the shear stress at zero flux density. At appreciable flux densities, for example of the order of 1.0 Tesla, the shear stress “on” is given by the yield stress, while in the off state, the shear stress is essentially the viscosity times the shear rate. With reference to
FIG. 1
, for a volume fraction of 0.55, at 1.0 Tesla the yield stress is 18 psi. This fluid has a viscosity of 2000 cP, which, if subjected to a shear rate of 1000 reciprocal seconds (as in a rheometer), gives an off-state shear stress of approximately 0.3 psi (where 1 cP=1.45×10
−7
lbf s/m
2
). Thus, the turn-up ratio at 1.0 Tesla is (18/0.3), or 60. However, in a device in which the shear rate is higher, e.g., 30,000 seconds
−1
, the turn-up ratio is then only 2.0.
The observation that the on and off-states of MR fluids have been coupled in the sense that any attempt to maximize the on-state yield stress by increasing the solid volume fraction will carry a great penalty in turn-up ratio because the viscosity in the off-state will increase at the same time, as illustrated by the above example. This has been generally recognized in the prior art and has been stated explicitly in, for example, U.S. Pat. No. 5,382,373 at column 3. For a given type of magnetizable solid, experience has identified no other variable such as fluid type, solid surface treatment, anti-settling agent or the like which has anything like the effect of volume fraction on the yield stress of the MR fluid. Therefore, it is necessary to find a means of decoupling the on-state yield stress and the off-state viscosity and their mutual dependence on solid volume fraction.
In accordance with the subject invention, this decoupling is accomplished by using a solid with a “bimodal” distribution of particle sizes instead of a monomodal distribution to minimize the viscosity at a constant volume fraction. By “bimodal” is meant that the population of solid ferromagnetic particles employed in the fluid possess two distinct maxima in their size or diameter and that the maxima differ as follows.
Preferably, the particles are spherical or generally spherical such as are produced by a decomposition of iron pentacarbonyl or atomization of molten metals or precursors of molten metals that may be reduced to the metals in the form of spherical metal particles. In ac

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