Magnetic fluid power generator device and method for generating power

[0001] The present invention is generally directed to generating power, and more particularly to Nano-Electro-Mechanical System (NEMS) power generators including magnetic fluids.

[0002] Micro-Electro-Mechanical Systems (MEMS) are miniaturized devices that integrate mechanical and electrical components on a common silicon substrate. Examples of MEMS mechanical devices are motors, pumps, relays, and actuators. MEMS are used in all types of miniaturized technologies from remote sensors and cellular communications to space based electronic systems and phased-array antennas. All of MEMS mechanical and electrical components require power, therefore unconventional, uninterrupted, and portable power sources for miniaturized technologies are in high demand.

[0003] Recently, there has been a strong need for NEMS, nanoscale systems of mechanical elements, such as motors, pumps, relays, and actuators and electronics on a common silicon substrate. The advent of NEMS has increased the need for miniaturized and especially nanoscale power generators.

[0004] Magnetic fluids are magnetic field responsive fluids containing magnetic particles coated with a surfactant and dispersed in a carrier liquid. Magnetic field responsive fluids provide variable stress levels created by magnetic coupling of the magnetic particles in the form of chains or bent wall-like structures upon interaction with an external magnetic field. Force absorbing devices, such as dampers, shock absorbers, seals, valves, commonly employ magnetic fluids. However, few devices that generate electric power employ magnetic fluids.

[0005] Power generators employing magnetic fluids are magnetically driven generators, while most other types of power generators are electrically driven. Magnetically driven generators have two primary advantages over electrically driven generators. First, they are not subject to any catastrophic failures analogous to electrical breakdown, and second, they have a higher energy density (about two orders of magnitude greater). Thus, it is advantageous to have a power generator for a NEMS utilizing a magnetic fluid to produce electricity.

[0006] U.S. Pat. No. 4,064,409 discloses a closed ferrofluid circuit surrounded by a magnet. Heating and cooling the ferrofluid takes advantage of the ferrofluid's magnetic properties to pump the ferrofluid through a solenoid and generate electric power. The device disclosed in this patent is thermally driven, while the present invention is directed to a magnetically driven power generator. In the present invention, alternating and traveling magnetic, fields rather than heating and cooling, take advantage of the magnetic fluid's magnetic properties to pump the magnetic fluid through external pickup coils and generate electric power.

[0007] U.S. Pat. No. 4,613,304 discloses a hydrogen gas generator system containing permanently magnetically polarized particles in a hydrogen/oxygen collection chamber. An electrical and/or mechanical pump circulates the magnetic particles in a non-magnetic, non-conductive closed loop of tubing. The circulation of the magnetic particles induces a voltage in a coil surrounding the tubing which may be utilized as an electrical power source. This electrical power source is therefore electrically or mechanically driven, while the present invention is magnetically driven.

[0008] U.S. Pat. No. 5,714,829 discloses a method for cooling a system having predictable bursts of heat dissipation using an electromagnetic heat engine. The heat engine includes a magnetic medium having susceptibility that varies with temperature and is magnetized between bursts of heat dissipation to create a magnetic field. An electrical load absorbs energy from the magnetic field by demagnetization. Again, the system disclosed in this patent is thermally driven, while the present invention is directed to a magnetically driven power generator.

[0009] U.S. Published Patent Application 2001/0033384 discloses the application of an electric or magnetic field on a thin ferrofluid sample to modulate light. The present invention is designed to generate electricity, not modulate light.

[0010] The principal object of the present invention is to provide a magnetically driven power generator device and method for generating power.

[0011] An object of the present invention is to provide a magnetically driven power generator device which has high energy density (about two orders of magnitude greater). In addition, the power generator device of the present invention offers increased reliability as it involves no catastrophic failure mechanisms analogous to electrical breakdown.

[0012] According to the invention, a pump pumps a magnetic fluid from a reservoir into a non-magnetic, non-conductive loop of tubing. Then an alternating and traveling magnetic field circulates the magnetic fluid through the non-magnetic, non-conductive loop, magnetizes the magnetic fluid, reduces the effective fluid viscosity, and induces rotations in the magnetic particles suspended in the magnetic fluid. The magnetic particle rotations induce a time varying magnetic flux and consequently an electric current through the surrounding external pickup coils. A permanent DC magnet ensures a positive magneto-motive force.

[0013] In accordance with the present invention, a device for generating power includes a fluid including magnetic particles. A source magnetizes the fluid thereby inducing rotations in the magnetic particles for creating a magnetic flux. The rotations of the magnetic particles induce an electromagnetic force in a coil associated with the fluid.

[0018] The present invention provides a power generator device PG wherein a magnetic fluid MF produces electricity. The magnetic fluid MF used in the present invention includes a fluid that has magnetic particles 10 coated with a surfactant and/or a coating 12 and dispersed in a carrier liquid 14 (FIG. 1). The surfactant includes long-chain molecules, such as polyethylene glycol, starch, dextran, or a combination thereof, or an electrostatic layer of sodium dodecyl sulfate, sodium salt of cipric acid, hexadecyltrimethyl ammonium bromide, or a combination thereof.

[0019] As illustrated in FIG. 2, the power generator device PG of the invention includes a pump 16 that pumps a magnetic fluid out of a reservoir 18 and into a non-magnetic, non-conducting loop of tubing 20. An alternating and traveling magnetic field 22 then circulates the magnetic fluid MF through the non-magnetic, non-conducting loop of tubing 20, part of which is surrounded by a permanent DC magnet 24, causing ferrohydrodynamic rotation. The alternating and traveling magnetic field 22 magnetizes the magnetic fluid MF as the magnetic susceptibility of the magnetic fluid lies between the ferromagnetic and paramagnetic materials. Fluid viscosity acting on the magnetic particles 10 suspended in the magnetic fluid MF causes the magnetization to lag behind the alternating and traveling magnetic field 22 and a torque results. The torque induces rotations in the magnetic particles 10 and decreases the effective fluid viscosity (the decrease in effective fluid viscosity is termed negative viscosity). The effective fluid viscosity is capable of reaching zero or negative values, suggesting that there is close to zero resistance to the motion of the fluid thereby reducing the input energy requirements.

[0020] External pickup coils 26 also surround a portion the non-magnetic, non-conductive loop of tubing 20 containing the magnetic fluid MF. Physical motion of the magnetic fluid MF and collective interaction among the magnetic particles 10, causes time variation in the magnetic moments and produces electromagnetic forces in the coils 26. The electromagnetic forces are generated magnetic field source and the particles do not move relative to each other, according to the classical theory of magnetohydrodynamics. The permanent DC magnet 24 assures alignment of the magnetic dipoles so that the magneto-motive forces produced by each magnetic particle add, rather than sum to zero. It is also noted that a non-magnetic solid moving in a magnetized magnetic fluid would disturb the magnetic field and also induce electromagnetic forces in the external pickup coils 26. Therefore, other particles, such as copper, silicon, alumina, aluminum or the like, may be added to the magnetic fluid to increase its power output. Preferably, these particles are non-magnetic and are thermally conducting (such as copper), electrically conducting (such as copper), or non-conducting (such as alumina), or a combination thereof. The device including the magnetic fluid MF, alternating and traveling magnetic fields 22, pump 16, reservoir 18, non-magnetic and non-conductive loop of tubing 20, permanent DC magnet 24, and external pickup coils 26, therefore, acts as an electric power generator.

[0021] The magnetic particles may be synthesized by methods commonly known in the art, such as chemical synthesis, sol-gel, chemical co-precipitation, or microwave plasma technique. The microwave plasma technique, described in U.S. Pat. No. 6,409,851 by Sethuram et al. (incorporated herein in its entirety by reference) is the preferred technique as it is unique in that it gives better control over particle size, shape and purity, and can be readily extended to produce different compositions of powders. The magnetic particles may be made of, but are not limited to, iron, iron oxide, cobalt, cobalt oxide, nickel, nickel oxide, an alloy such as steel, or a combination thereof. Preferably, the magnetic particles are made of iron, iron oxide, or a combination thereof. The composition of the particles is important as it determines the magnetic moment of the particles. For example, iron has ten times more saturation magnetization than its oxides, and at least twice as much as its nitrides.

[0022] Colloidal particles have an inherent tendency to aggregate and form clusters or agglomerate due to attractive van der Waals (vdW) forces. To stabilize the particles against these attractive forces, it is preferable to introduce a repulsive interparticle force, either by an electrostatic or by a steric means. Electrostatic stabilization utilizes the surface charge typically present on the particles, which is effective in a medium having a high dielectric constant, such as water, while in steric stabilization, a sufficiently thick layer of a polymeric or surfactant molecules is introduced around the particles. The surface layer functions as a steric barrier to prevent particle agglomeration, and thereby ensures stability of the fluid. The surface layer also prevents dissolution of the magnetic materials. This technique is preferred for the present invention. The particles are coated with a surfactant, and/or coating by adsorption of surfactant, and/or coating molecules onto the particles in the presence of ultrasonic irradiation in a high shear field. The types of surfactants that may be utilized in the present invention include, but are not limited to, polyethylene glycol, lecithin, oleic acid, or Surfynol® surfactants (available from Air Products). The types of coatings that may be utilized in the present invention include, but are not limited to, silica, gold, silver, platinum, steel, cobalt, carbon, a polymer, or a combination thereof. The polymer can be polyethylene glycol, polystyrene, dextran, or a combination thereof. Preferably, the particles are only coated with lecithin or Surfynol® surfactants (available from Air Products).

[0023] The magnetic particles coated with a surfactant are dispersed in a carrier liquid by high shear mixing followed by ultrasonification to form a homogenous fluid. The carrier liquid helps to retain the fluidity of the magnetic fluid. It is also important as it partially determines the effective fluid viscosity. Carrier liquids are water based and oil based liquids, such as glycerol/water and/or mineral oil mixtures. Preferably, the carrier liquid is water, hydraulic oil, mineral oil, silicone oil, or biodegradable oils such as cocoa oil, or a combination thereof.

[0024] The average diameter or size of the magnetic particles can be from about 1 nm to about 1000 nm. The preferred size is about 10 nm to 500 nm, while the most preferred size is about 10 nm to 100 nm. Due to their small size, the magnetic particles remain suspended and are subjected to minimal settling.

[0025] The shape of the particles is important for two reasons. First, the magnetic effect is dependent upon the particle volume fraction, which in turn is a function of the particle shape. For instance, needle-shaped particles exhibit similar magnetic effect at concentrations ten times smaller than spherical particles because of larger surface area per volume. Second, the flow characteristics of the particles in a liquid medium are dependent upon their shape. The shapes utilized in this invention include, but are not limited to, spherical, needle-like, cubic, irregular, cylindrical, diamond, oval, or a combination thereof.

[0026] The particulate volume or weight fraction is also important as the magnetic character and zero field viscosity are dependent upon the particulate volume or weight fraction. Zero field viscosity refers to the viscosity of the magnetic fluid when it is not acted upon by a magnetic field. A magnetic fluid with a larger particulate volume or weight fraction exhibits greater magnetic character and zero field viscosity than a magnetic fluid with a smaller particulate volume or weight fraction. Preferably, the particulate weight fraction is about 1% to 95%.

[0027] Ultrafine powders of iron with a particle size less than about 20 nm were produced using the proprietary microwave plasma chemical synthesis process described in U.S. Pat. No. 6,409,851 by Sethuram et al. Vapors of iron pentacarbonyl were fed into the plasmatron with argon as the plasma gas. The plasma gas flow rate was about 0.003–0.0034 m³/min and that of the carrier gas was about 0.0003–0.0004 m³/min. The plasma temperature was about 900–950° C., the powder feed rate was about 50–60 gm/hr, and the quenching water flow rate was about 2.0–2.5 liter/min at 20° C. The reactor column diameter was about 48 mm and its length was about 10″. The microwave forward power was about 4 kW, the reflected power was about 0.7 kW, and the operating frequency was about 2450 MHZ.

[0028] The iron particles were coated with polyethylene glycol in solution phase and dispersed in water by high-speed shear mixing and ultrasonification.

[0029] While this invention has been described as having preferred sequences, ranges, steps, materials, or designs, it is understood that it includes further modifications, variations, uses and/or adaptations thereof following in general the principle of the invention, and including such departures from the present disclosure as those come within the known or customary practice in the art to which the invention pertains, and as may be applied to the central features hereinbeforesefforth, and fall within the scope of the invention and of the limits of the appended claims.