WO2015183293A1

WO2015183293A1 - Mixed ferrofluid and a rotary seal incorporating a mixed ferrofluid

Info

Publication number: WO2015183293A1
Application number: PCT/US2014/040131
Authority: WO
Inventors: Kuldip Raj
Original assignee: Ferrotec (Usa) Corporation
Priority date: 2014-05-30
Filing date: 2014-05-30
Publication date: 2015-12-03

Abstract

A ferrofluid mixture for use in a ferrofluid rotary seal includes either (1) a first ferrofluid having a predefined carrier liquid and a second ferrofluid mixed with the first ferrofluid where the second ferrofluid has a predefined carrier liquid that is not identical to the predefined carrier liquid of the first ferrofluid and where the first ferrofluid is at least partially miscible with and chemically compatible with the second ferrofluid, or (2) a first predefined carrier liquid, a second predefined carrier liquid that is not identical to the first carrier liquid but is at least partially miscible and chemically compatible with the second carrier liquid, at least one or more dispersants partially miscible in both carrier liquids, and a plurality of finely divided magnetic particles.

Description

MIXED FERROFLUID AND A ROTARY SEAL INCORPORATING A

MIXED FERROFLUID

BACKGROUND OF THE INVENTION

1 . Field of the Invention

[0001] The present invention relates generally to ferrofluids used in vacuum seals. More particularly, the present invention relates to ferrofluidic seals utilizing a plurality of ferrofluids.

2. Description of the Prior Art

[0002] Ferrofluidic rotary seals have been widely used in vacuum applications over the past forty-five years and are commonly used for moving of

components inside of a process chamber under high vacuum. One type of rotary seal includes magnets, a rotary shaft, magnetic pole pieces or poles, and a housing. The magnets, the poles, and the shaft form magnetic circuits with air gaps between the poles and the shaft. A ferrofluid is attracted to the air gap and forms a dynamic seal between the poles and the rotary shaft. In contrast, a seal between stationary parts, such as the seal between a pole and its housing, is usually provided by a rubber O-ring at the radial interface.

[0003] Ferrofluidic seals have been effective for a wide variety of vacuum applications, such as in semiconductor manufacturing, optical coatings, and rotary gas unions. In recent years, the performance requirements of ferrofluids have become more demanding. These requirements include, but are not limited to, ultra-high vacuum (UHV) capability, high speed rotary movement, low

starting/running torque, low heat generation, long life, chemical stability, and chemical compatibility. These requirements frequently contradict one another. For example, a ferrofluid suitable for UHV applications typically has a low volatility and a high viscosity. The same ferrofluid also tends to have a high

starting/running torque that makes it unsuitable for high-speed applications.

[0004] The properties of ferrofluids control the application for which a particular ferrofluid is suited. Ferrofluids are magnetically responsive colloidal liquids. The main constituents are nano-sized magnetic particles, one or more surfactants or dispersants, and a base carrier liquid. Each particle is a permanent magnet of substantially spherical shape with a diameter of about ten nanometers. The particles are coated with a surfactant that separates the particles from each other and prevents them from coalescing under the attractive Van der Waals and magnetic forces. The liquid medium in which the particles are suspended is often referred to as the carrier. In a seal-grade ferrofluid, the constituents typically have the following volume fractions: magnetic particles (8%), surfactant (16%), and carrier (76%).

[0005] The magnetic properties of a ferrofluid are determined by the volume fraction of the solid component. The greater the solids content (also known as the magnetization of ferrofluid), the higher the pressure holding capacity of the sealing device. By far the dominant component of the ferrofluid is the carrier. It determines the physical characteristics of a ferrofluid, such as viscosity, vapor pressure, operating temperature range, volatility, thermal conductivity,

environmental compatibility, seal service life, and power consumption of the seal. Thus, a proper selection of a carrier is crucial to the successful performance of a magnetic fluid rotary seal.

[0006] Due to the strong influence of the choice of carrier on seal operation, ferrofluids are designated by their carrier type. For example, ferrofluids based on any oil in the class of perfluoropolyethers (fluorocarbons), hydrocarbons, esters, polyphenyl ethers, and silicones are called fluorocarbon, hydrocarbon, ester, polyphenyl ether, and silicone-based ferrofluids, respectively. These ferrofluids may all have the same colloidal sized magnetic particles but each would require a different surfactant with a molecular structure compatible with that of the carrier.

[0007] Current sealing applications typically utilize one of the three families of ferrofluids, namely fluorocarbons, hydrocarbons, or esters depending upon the environments to be sealed. All stages in a traditional multi-stage seal are charged with only one type of ferrofluid. This often leads to compromising the seal performance in regard to power consumption, environmental compatibility, or life of the seal.

[0008] The ferrofluid in rotary shaft seals forms distinct O-rings with intervening air cavities. Pressure in a chamber changes as the chamber is evacuated and then back filled with process gases. For a detailed discussion, see MAGNETIC FLUIDS AND APPLICATIONS HANDBOOK (Begell House, New York), incorporated herein by reference. Like any other liquid, the volatility of a ferrofluid depends on ambient pressure. Under high vacuum conditions (a pressure of approximately 10^"8 torr or less), the evaporation rate of a typical seal grade ferrofluid is roughly twenty times greater than the evaporation rate at atmospheric pressure

(approximately 760 torr). At one torr, the volatility is about four times higher than at 760 torr. In general, ferrofluid life is reduced both by evaporation and by any chemical reaction with the gaseous medium. Ferrofluid stages on the vacuum side degrade much faster than those on the atmospheric side. This is due in part to exposure to high vacuum and/or in part to hazardous gases (when present), which is not experienced by the stages on the atmospheric side.

[0009] Rotary shaft seals utilizing magnetic fluids are typically designed with a sufficient safety margin in pressure holding capacity so that several of the atmospheric-side stages act as reserves. The atmospheric-side stages

experience only ambient air, which contains mostly nitrogen (approximately 80%) and oxygen (approximately 20%). These stages do not experience the conditions that exist on the process side of the seal. Like other liquid O-rings located inside of the seal, the first vacuum-side ferrofluid O-ring (or stage) has two free surfaces. The first free surface is exposed to the process chamber and evaporates rapidly due to prevailing high vacuum conditions. On the other hand, the second free surface of the same ferrofluid O-ring evaporates more slowly due to the higher pressure of the interstage region, which is typically between about 2 to 5 psi, depending upon the seal design.

[0010] Similarly, the second vacuum-side ferrofluid O-ring also experiences different evaporation rates at its two free surfaces due to different pressures in adjoining cavities. However, the evaporation rate of the second stage ferrofluid is much lower than that of the first stage. For this reason, the second stage is expected to last longer than the first stage. The third stage has an even longer life than the second stage because of exposure to even higher pressures than at the second stage. The stages on the atmospheric side have the longest life and lowest volatility because of exposure to atmospheric pressure (760 torr or 14.7 psi) in adjoining cavities. Nonetheless, environments surrounding a ferrofluid are an additional mechanism by which the fluid seal may fail. [0011] For instance, when a hydrocarbon-based ferrofluid is used in a seal, the atmosphere-side stage seals deteriorate faster (even when the volatility is low) than when an ester or a fluorocarbon-based ferrofluid is utilized. The presence of oxygen in air trapped in interstage regions reacts with hydrocarbons in the hydrocarbon-based ferrofluid and causes the fluid to congeal over time. On the other hand, ester-based ferrofluid exposed to high condensable humidity of the atmosphere-sideis not stable and thus hydrocarbon-based and fluorocarbon- based ferrofluids are the preferred ferrofluids. Overall, fluorocarbon-based ferrofluids are far superior to other classes of ferrofluids regarding environmental compatibility, long service life, and ultra-low vapor pressure. They are durable under radiation, humidity, reactive gases, and high temperature. Typically, fluorocarbon-based ferrofluids have a life one to two orders of magnitude longer than other fluids. However, the disadvantage of the fluorocarbon-based ferrofluids is their high viscosity. This results in greater start-up and running torque for the seal, requiring large and expensive motors to operate the device. The high viscosity also increases seal temperature, which requires liquid cooling of the device and adds to the cost of the product.

[0012] U.S. patent no. 4,407,518 to Moskowitz et al. discloses a non-bursting multiple-stage ferrofluid seal and system. The system includes an annular permanent magnet and annular first and second pole pieces. One end of the pole pieces defines a single-stage ferrofluid seal under one end of one pole piece and a multiple-stage ferrofluid seal under the one end of the other pole piece with the surface of the shaft element with which the seal is employed. The first and second pole pieces and the magnet define therebetween an interstage volume between the single-stage ferrofluid seal and the multiple-stage ferrofluid seal. A conduit extends into the stage volume and connects to a means to maintain a desired pressure in the stage volume. This device uses a single, common ferrofluid throughout the seal and uses a compensating pressure to maintain seal integrity.

[0013] U.S. patent no. 4,865,334 to Raj et al. discloses a long-life, multi-stage ferrofluid seal incorporating a ferrofluid reservoir. The reservoir is located between the seal stages and contains a quantity of ferrofluid sufficient to replace ferrofluid in the seal stages, which is lost due to evaporation or contamination. This device also uses a single type of ferrofluid, but has a reservoir system to replenish failed seals.

[0014] U.S. patent no. 4,335,885 to Hooshang Heshmat discloses a plural fluid magnetic/centrifugal seal to provide a hermetically sealed space between a rotated shaft member and a close-fitting, spaced-apart stationary housing. The housing and the shaft are shaped to provide magnetic pole-like close-clearance gap regions between their opposed surfaces. A high-viscosity ferromagnetic fluid normally is disposed in the magnetic gap region with the rotating shaft member at rest and at low rotational speeds. A magnet forms a closed magnetic circuit through the magnetic gap region with the high viscosity ferromagnetic fluid. A circumferentially-arranged centrifugal seal-forming region is radially disposed outward from the magnetic gap region and is located between the rotatable shaft and the stationary housing member. A low-viscosity centrifugal sealing fluid is disposed in the centrifugal seal-forming region and is centrifugally thrown outwardly during high speed rotation of the rotating shaft member to form a centrifugal hermetic seal between the rotating shaft member and the housing at high rotational speeds of the rotating member.

[0015] Although this device uses plural fluids, the centrifugal fluid is not a ferrofluid, but instead is comprised of water, lubricating oil, or other low viscosity fluid which does not heat up at the higher speeds of the speed range over which the seal is designed to operate. Also, the seal is designed to provide a reservoir for collecting the ferromagnetic fluid to isolate the two fluids of different viscosity from one another by keeping them in separate, but communicating spaces.

Further, the area surrounding the vane that rides in the casing where the centrifugal seal is created is nonmagnetic.

[0016] U.S. patent no. 6,899,338 to Li et al. discloses an apparatus

incorporating a multi-stage ferrofluid rotary seal adapted to provide a ferrofluid pressure-type seal about a shaft extending between a first environment and a second environment. The apparatus utilizes two of more different types of ferrofluids within the seal apparatus. The apparatus can have multiple pole pieces where each pole piece uses a different type of ferrofluid within that pole piece's radial gap. Each stage forms an independent magnetic fluid O-ring and mixing the different types of ferrofluid can be avoided by using fluid splash guards between adjacent pole pieces and/or a having a large separating space between pole pieces. A first magnetic fluid is disposed at one stage of the multi-stage rotary seal that faces the first environment. A second magnetic fluid is disposed at another stage of the multi-stage rotary seal that faces the second environment. For example, the first few stages of the seal are charged with a fluorocarbon- based ferrofluid and the remaining stages are charged with an ester or

hydrocarbon-based ferrofluid.

SUMMARY OF THE INVENTION

[0017] Traditionally, magnetic fluid seals use only one type of ferrofluid as the sealing medium. More recently, distinct ferrofluids have been used at each stage of a multi-stage ferrofluidic seal. The ferrofluid at each stage of multi-stage seals is selected from low-volatility lubricating oils, such as esters, hydrocarbons, fluorocarbons, and silicones based on seal requirements of vapor pressure, life, compatibility, and viscosity. A first fluorocarbon-based ferrofluid is selected for the first few stages of the seal based on having optimal vapor pressure, life, and environmental compatibility. The fluorocarbon-based ferrofluid, however, also has undesirable features such as high viscosity and low magnetization. Thus, a second ferrofluid such as esters, hydrocarbons, and silicones occupying other stages of the seal is therefore selected to overcome the drawbacks of the first fluorocarbon-based ferrofluid selected for the first stages.

[0018] A problem with using distinct ferrofluids above described in a multistage ferrofluidic seal is that the two ferrofluids may accidentally mix during assembly of the seal apparatus. Because the two fluids are not compatible with each other, the mixture of the two fluids is unstable and causes the seal to fail.

[0019] Another problem is that a ferrofluid that is used only in a few stages sometimes requires its own magnetic circuit. This increases the length and expense of the seal.

[0020] Therefore, what is needed is a ferrofluid mixture having two or more carrier liquids useful for ferrofluid devices, such as a vacuum rotary seal, operating simultaneously in different ambient environments.

[0021] Accordingly, it is an object of the present invention to provide a ferrofluid with a plurality of carrier liquids for use in ferrofluidic seals. [0022] It is another object of the present invention to increase ferrofluid gel time in air, in nitrogen, and/or under vacuum as compared to other ferrofluids with a single carrier liquid.

[0023] The present invention achieves this and other objectives by providing a ferrofluid for use in a magnetic fluid rotary seal that is a mixture of (1 ) miscible or partially miscible magnetic fluids or (2) miscible or partially miscible carrier liquids used to make the ferrofluid mixture. In one embodiment, ferrofluid mixture for use in a magnetic fluid rotary seal includes at least a first ferrofluid having a carrier liquid selected from the group consisting of polyphenyl ethers,

perfluoropolyethers, silicones, silahydrocarbons, hydrocarbons, halocarbons, and esters, and a second ferrofluid having a carrier liquid selected from the group consisting of polyphenyl ethers, perfluoropolyethers, silicones, silahydrocarbons, hydrocarbons, halocarbons, and esters where the second ferrofluid is not the same as the first ferrofluid and is mixed with the first ferrofluid where the first ferrofluid is at least partially miscible with and chemically compatible with the second ferrofluid.

[0024] In another embodiment, the ferrofluid mixture has a first carrier liquid with a first viscosity and a first volatility. The ferrofluid has a second carrier liquid mixed with the first carrier liquid, where the second carrier liquid has a second viscosity and a second volatility. The first carrier liquid is at least partially miscible with and chemically compatible with the second carrier liquid. Ferromagnetic particles are dispersed throughout the mixture of the first carrier liquid and the second carrier liquid and dispersant(s) provide colloid stability to the mixed system.

[0025] In still another embodiment of a ferrofluid mixture, the mixture includes a quantity of a first predefined carrier liquid, a quantity of a second predefined carrier liquid that is not identical to the first predefined carrier liquid where the first predefined carrier liquid is at least partially miscible with and chemically compatible with the second predefined carrier liquid, a predefined quantity of at least one or more dispersants that is miscible or partially miscible in both the first predefined carrier liquid and the second predefined carrier liquid where the quantity of the first predefined carrier liquid, the quantity of the second predefined carrier liquid and the predefined quantity of the at least one or more dispersants together form a carrier liquid mixture, and a plurality of finely divided magnetic or magnetizable particles disposed in the carrier liquid mixture.

[0026] In another embodiment of a ferrofluid mixture, the first carrier liquid is an ester-based oil and the second carrier liquid is a hydrocarbon-based oil.

[0027] In a further embodiment of a ferrofluid mixture, the first ferrofluid is an ester-based ferrofluid and the second ferrofluid is a hydrocarbon-based ferrofluid.

[0028] In another embodiment of the ferrofluid mixture, the predefined carrier liquid of the first ferrofluid and the predefined carrier liquid of the second ferrofluid is selected from the same chemical family of oils but the predefined carrier liquid of the first ferrofluid and the predefined carrier liquid of the second ferrofluid have different molecular structures.

[0029] In still another embodiment of the ferrofluid mixture, a ratio of the first ferrofluid to the second ferrofluid is in a range between 1 to 0 and 0 to 1 .

[0030] In yet another embodiment of the ferrofluid mixture, a ratio of the first ferrofluid to the second ferrofluid is 1 to 1 .

[0031] In another embodiment, there is disclosed a ferrofluid seal apparatus. The ferrofluid seal apparatus includes a multi-stage ferrofluid rotary seal adapted to provide a ferrofluid pressure-type seal about a shaft element extending between a first environment and a second environment and a ferrofluid mixture disposed at each stage of the multi-stage ferrofluid rotary seal. The ferrofluid mixture includes a first ferrofluid having a predefined carrier liquid and a second ferrofluid having a predefined carrier that is not identical to the predefined carrier liquid of the first ferrofluid and where the first ferrofluid is at least partially miscible with and chemically compatible with the second ferrofluid.

[0032] In a further embodiment, the predefined carrier liquid of the first ferrofluid is selected from the group consisting of polyphenyl ethers,

perfluoropolyethers, silicones, silahydrocarbons, hydrocarbons, halocarbons, and esters. The predefined carrier liquid of the second ferrofluid is selected from the group consisting of polyphenyl ethers, perfluoropolyethers, silicones,

silahydrocarbons, hydrocarbons, halocarbons, and esters.

[0033] In still another embodiment, a combination of the first ferrofluid and the second ferrofluid is selected based on a predefined viscosity and molecular weight of the ferrofluid mixture whereby the ferrofluid mixture imparts a predefined startup and running torque of the ferrofluid rotary seal.

[0034] In yet another embodiment, the first ferrofluid has an ester-based carrier liquid and the second ferrofluid has a hydrocarbon-based ferrofluid.

[0035] In another embodiment, a ratio of the first ferrofluid to the second ferrofluid is in a range between 1 to 0 and 0 to 1 .

[0036] In another embodiment, a method of providing a multistage, rotary feed- through seal with improved performance is disclosed. The method includes providing a multi-stage ferrofluid rotary seal adapted to provide a ferrofluid pressure-type seal about a shaft element extending between a first environment and a second environment, and disposing a predefined quantity of a ferrofluid mixture at each stage of the multi-stage ferrofluid rotary seal. The ferrofluid mixture includes a first ferrofluid having a predefined carrier liquid and a second ferrofluid having a predefined carrier that is not identical to the predefined carrier liquid of the first ferrofluid wherein the first ferrofluid is at least partially miscible with and chemically compatible with the second ferrofluid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIGURE 1 illustrates an exemplary experimental setup used to test mixed ferrofluid samples of the present invention.

[0038] FIGURE 2 shows one method used for visually inspecting a mixed ferrofluid sample.

[0039] FIGURE 3 is bar graph of gel time in air at 760 torr for five different mixed ferrofluid samples.

[0040] FIGURE 4 is a bar graph of gel time in nitrogen at 760 torr for five different mixed ferrofluid samples.

[0041] FIGURE 5 is a bar graph of gel time under vacuum at 1 x 10^"8 torr for five different mixed ferrofluid samples.

[0042] FIGURE 6 is a partial cutaway of a rotary feed through seal showing a mixed ferrofluid in use between a rotary shaft and a pole piece. DETAILED DESCRIPTION

[0043] Embodiments of the present invention are described with reference to Figs. 1 -6.

[0044] Ferrofluids with two or more different types of carrier liquids are useful for magnetic fluid rotary seal devices operating simultaneously in different ambient environments. Magnetic fluid rotary seal devices include, but are not limited to, a vacuum rotary seal or feed through.

[0045] A ferrofluid mixture of the present invention is based on mixing ferrofluids made with carrier oils or liquids from different chemical families, such as polyphenyl ethers, perfluoropolyethers, silicones, silahydrocarbons,

hydrocarbons, halocarbons, and esters; or a carrier oils or liquids from the same chemical family of oils but having different molecular structures. In one

embodiment, the mixture includes an ester-based oil and a hydrocarbon-based oil. Many combinations of ester- and hydrocarbon-based oils are also acceptable. In one embodiment, the mixed ferrofluid has two or more component carrier fluids. In other embodiments, the mixed ferrofluid has three or more component carrier fluids. The dispersants used for and in the present invention should be either completely or partially soluble in the carrier liquids used in mixed ferrofluids.

Selection of dispersants and synthesis of various types of ferrofluids have been discussed in test books on magnetic fluids such as Ferrohydrodynamics by R.E. Rosensweig, Cambridge University Press, New York, 1 985 and Magnetic Fluids: engineering applications by B.M. Berkovsky, V.F. Medvedev and M.S. Krakov, Oxford University Press, Inc., New York, 1993, which references are incorporated herein by reference. Those skilled in the art of organic chemistry are capable of synthesizing dispersants which may have solubility in various mixtures of carrier liquids, e.g. fluorocarbons, silicones etc. A general description of various types of oils and their physical properties can be found in the book "Synthetic Lubricants and High Performance Functional Fluids" edited by Ronald L. Shubkin, Marcel Dekker, Inc (new York) 1993. Alternately, the molecular structures of various oils may be modified to accept a given dispersant by those skilled in the art. Current ferrofluids are typically based on four chemical families of lubricating oils namely, hydrocarbons, esters, fluorocarbons and silicones. Within each family there are many oils that exist with different chemical structures, molecular weights and viscosity. Mixed ferrofluids (or ferrofluid mixtures) as used and described for the present invention means not only mixtures of ferrofluids from different chemical families but also mixtures of ferrofluids from the same chemical family having different chemical structures. An example of ferrofluids from the same chemical family include, but are not limited to, esters such as diesters, polyol esters, trimellitate esters, phosphate esters, etc.

[0046] In one embodiment, the mixed ferrofluid is a mixture of component ferrofluids that each contains ferromagnetic particles evenly distributed throughout a liquid as a colloidal dispersion. Each component ferrofluid has a value for magnetization, viscosity, volatility, pour point, thermal conductivity, and

environmental compatibility. Each of these values can be the same or different among the various component ferrofluids. Each component ferrofluid also has an average particle diameter size for the ferromagnetic particles. Values for average particle diameter size (or the distribution thereof) may be the same or different among the various component ferrofluids. For example, particle size is selected to achieve a stable colloidal dispersion. In one embodiment, magnetite particles have a particle size between about 3-20 nm.

[0047] In another embodiment, a mixed ferrofluid is prepared by mixing two or more compatible carrier fluids and a surfactant or dispersant. Ferromagnetic particles are evenly distributed throughout the mixture to form a colloidal dispersion.

[0048] The components of the mixed ferrofluids may be based on different types of ferromagnetic particles with chemical stability, such as magnetite, maghemite, mixed ferrites, magnetic alloys, and pure iron, cobalt or nickel. The dispersants of the components in mixed ferrofluids can be the same or of different chemical nature as long as they are mutually compatible.

[0049] Chemical compatibility is a measure of how stable a substance is when mixed with another substance. For this disclosure, chemical compatibility of two or more ferrofluids or carrier liquids means that the two or more ferrofluids or two or more carrier liquids, as the case may be, do not react or change when mixed and remain stable at specified storage and operating temperatures. Carrier liquids are selected by considering one or more properties of the liquids, such as molecular weights, molecular structures, viscosity, volatility, temperature limits, and environmental compatibility.

[0050] Two or more substances are considered miscible if the substances can be combined to form a homogeneous mixture (i.e., a solution of any phase). The present disclosure considers miscibility for liquid solutions. Carrier liquids may be 100% miscible, but carrier liquids that are at least partially miscible when mixed are also acceptable.

[0051] As a practical example, ferrofluid mixtures of ester-based and

hydrocarbon-based ferrofluids were prepared. Starting ferrofluids had ester and hydrocarbon oils as the respective ferrofluid carrier liquid. The starting ferrofluids had the same magnetization and viscosity values, namely 450 Gauss and 450 cP, but the volatility of the ester ferrofluid (designated E100H0) was about two times higher than that of the hydrocarbon ferrofluid (designated E0H100). The average particle size in each starting ferrofluid was the same and identical dispersants were employed to stabilize the particles.

[0052] The lifetime of a ferrofluid mixture is measured by gel time and depends on the concentration of the component ferrofluids as well as the environments. . Five ferrofluid sample mixtures were tested with each sample containing between 0 and 100% of an ester-based ferrofluid and the balance being a hydrocarbon- based ferrofluid. . . The experimental mixtures of hydrocarbon and ester fluids were based on commercial ferrofluids available as VSG 903 (hydrocarbon-based ferrofluid) and VSG01 1 (ester-based ferrofluid) from Ferrotec Corporation, Japan. Magnetite with a particle size between about 3-20 nm was dispersedn each of the carrier liquid.

Five test mixtures were prepared and identified as follows:

E100H0 =100% ester-based ferrofluid, 0% hydrocarbon-based ferrofluid E75H25 =75% ester-based ferrofluid, 25% hydrocarbon-based ferrofluid E50H50 = 50% ester-based ferrofluid, 50% hydrocarbon-based ferrofluid E25H75 = 25% ester-based ferrofluid, 75% hydrocarbon-based ferrofluid E0H100 =0% ester-based ferrofluid, 100% hydrocarbon-based ferrofluid

[0053] Figure 1 illustrates an example of an experimental setup 10 used to test ferrofluid samples 20. Experiments were performed using an air-tight chamber (not shown) with a mounting plate 12 maintained at a temperature of 150 degrees Centigrade. A ferrofluid sample 20 with a volume of about 0.3 ml was dispensed into a steel vial 16 having a surface area of 2.51 cm². Steel vials 16 containing samples 20 were placed in recesses 18 in an aluminum block sample holder 14 and the block placed on mounting plate 12.

[0054] Each ferrofluid sample 20 was separately exposed to three

environments. In one experiment, the chamber was filled with air to atmospheric pressure (760 torr). In another experiment, the chamber was filled with nitrogen to atmospheric pressure (760 torr). In a third experiment, the chamber was evacuated and maintained under high vacuum (10^"8 torr). During each experiment ferrofluid samples 20 were periodically removed from the chamber, cooled and then weighed and examined. Ferrofluid samples were visually inspected. As shown, for example, in Figure 2, vials 16 containing ferrofluid sample 20 were tipped to check if gravitational force caused sample 20 to flow. Samples 20 were also checked to see if the ferrofluid flowed when a strong magnet was placed next to vial 16. A sample was considered to have failed when it reached a gel-like condition without exhibiting any flow under gravitational forces or when a strong magnet is placed next to the vial. The time to reach this condition is referred to as the gel time. In its gel-like condition, a ferrofluid has a viscosity exceeding

100,000 cP at 27° C.

[0055] Both component ferrofluids in the experiments had about the same magnetization and viscosity values: the ester-based ferrofluid had a magnetization value of 450 Gauss and a viscosity of 450 cP; the hydrocarbon-based ferrofluid had a magnetization value of 450 Gauss and a viscosity of 450 cP. Compatible dispersants were used in synthesizing each of the sample ferrofluids. . In other ferrofluid mixtures, the respective viscosities and volatilities could be different for the carrier liquid in each ferrofluid.

[0056] Experiments indicate that any combination of ester oil and hydrocarbon oil can be selected with acceptable miscibility and chemical compatibility. For example, two carrier liquids may have the same or different viscosities and the same or different volatilities. With the addition of ferromagnetic particles, each component ferrofluid may also have the same or different values for magnetization and/or viscosity. The main requirement is that the each of the component ferrofluids be compatible and at least partially miscible with one another. The end use of such a mixed ferrofluid dictates the selection of component carrier liquids and the resulting magnetization, viscosity, and volatility of the finished mixed ferrofluild product desired for a particular application.

[0057] Table 1 lists the five experimental ferrofluid mixtures and the gel times in air at 760 torr, in nitrogen at 760 torr, and under high vacuum at 10^"8 torr.

Table 1

[0058] Referring now to Figures 3-5, the values in Table 1 are plotted to graphically illustrate the gel times of the various ferrofluid mixtures under different ambient conditions.

[0059] Figure 3 is a bar graph of gel time in air for each of the five mixed ferrofluid samples. As expressed in Table 1 and shown in Figure 3, the gel time decreases linearly with decreasing concentration of ester-based ferrofluid. Stated differently, using 100% ester-based ferrofluid resulted in the longest gel time in air compared to the mixtures with hydrocarbon-based ferrofluid. Exposed to oxygen at atmospheric pressure, the 100% ester-based ferrofluid E100H0 had the longest gel time (i.e., the longest useful life) and the 100% hydrocarbon-based ferrofluid E0H100 had the shortest gel time (i.e., the shortest useful life). This is the case even though the volatility of the ester-based ferrofluid E100H0 is about half that of that of the hydrocarbon-based ferrofluid E0H100. [0060] Figure 4 is a bar graph of gel time in nitrogen for each of the five mixed ferrofluid samples. In nitrogen at atmospheric pressure, the 50-50 mixture of ester-based and hydrocarbon-based ferrofluids had the longest gel time of the five mixtures, namely 1 ,215 hours. This gel time is more than six times as long as the gel time of the same 50-50 mixture in air.

[0061] The gel time for the 50-50 mixture in nitrogen was a significant improvement over the gel times for either 100% ester-based ferrofluid or 100% hydrocarbon-based ferrofluid.

[0062] Figure 5 is a bar graph of gel time in a vacuum for each of the five mixed ferrofluid samples. As expressed in Table 1 and shown in Figure 5, the gel time at high vacuum increases with increasing concentration of hydrocarbon- based ferrofluid. Under conditions of high vacuum (10^"8 torr), the pure ester- based ferrofluid E100H0 had the shortest gel time of the five samples. Even having 25% hydrocarbon-based ferrofluid in the mixture resulted in significantly increased gel time. Notably, no gel time datum end point is shown in Figure 3 for the 100% hydrocarbon-based ferrofluid E0H100 because no gelling of the ferrofluid was observed up to 3,980 hours.

[0063] Figure 5 suggests an exponential increase in gel time under high vacuum conditions with increasing concentration of hydrocarbon-based ferrofluid, i.e. moving from sample E100H0 to E0H100.

[0064] Referring now to Figure 6, a partial cross-sectional view of a ferrofluid rotary feed-through seal 100 illustrates an example of using a mixed ferrofluid 200. Feed-through seal 100 has a rotary shaft 120 with a plurality of teeth 122 that define interstage regions 124. Magnetically-permeable pole piece 150 carries magnetic flux from a magnet 170 positioned adjacent to pole piece 150. The magnetic flux of magnet 170 is concentrated at teeth 122 and traps mixed ferrofluid 200 at stages 124, forming a seal between rotary shaft 120 and pole piece 152.

[0065] A variety of ambient conditions exist within a ferrofluid vacuum rotary feed-through. Interstage regions 124 may be filled with air, filled with inert gases (such as nitrogen and argon), or maintained at high vacuum during operation of the device. When only one type ferrofluid is used to furbish seal 100, some regions of the seal degrade faster than others and the overall life of seal 100 is compromised. For example, an ester-based fluid E100H0 exhibits satisfactory gel times when exposed to air but not under vacuum. On the other hand,

hydrocarbon fluid E0H100 exhibits satisfactory gel times under vacuum, but not when exposed to air. The 50/50 mixed ferrofluid E50H50 provides an improved life of the seal.

[0066] In another embodiment of a mixed ferrofluid, the start-up and running torques of the seal can be controlled with mixed ferrofluids. By adjusting the concentration of ferromagnetic particles, the viscosity and density of the mixed ferrofluid can be tailored for specified torques.

[0067] Although the preferred embodiments of the present invention have been described herein, the above description is merely illustrative. Further modification of the invention herein disclosed will occur to those skilled in the respective arts and all such modifications are deemed to be within the scope of the invention as defined by the appended claims.

Claims

What is claimed is:

1 . A ferrofluid mixture for use in a magnetic fluid rotary seal, the mixture

comprising:

a first ferrofluid having a predefined carrier liquid; and

a second ferrofluid mixed with the first magnetic fluid, the second ferrofluid having a predefined carrier that is not identical to the predefined carrier liquid of the first ferrofluid;

wherein the first ferrofluid is at least partially miscible with and chemically compatible with the second ferrofluid.

2. The ferrofluid mixture of Claim 1 wherein the predefined carrier liquid of the first ferrofluid is selected from the group consisting of polyphenyl ethers, perfluoropolyethers, silicones, silahydrocarbons, hydrocarbons, halocarbons, and esters.

3. The ferrofluid mixture of Claim 1 wherein the predefined carrier liquid of the second ferrofluid is selected from the group consisting of polyphenyl ethers, perfluoropolyethers, silicones, silahydrocarbons, hydrocarbons, halocarbons, and esters.

4. The ferrofluid mixture of Claim 1 wherein the predefined carrier liquid of the first ferrofluid and the predefined carrier liquid of the second ferrofluid is selected from the same chemical family of oils but the predefined carrier liquid of the first ferrofluid and the predefined carrier liquid of the second ferrofluid have different molecular structures.

5. The ferrofluid mixture of Claim 1 wherein a combination of the first ferrofluid and the second ferrofluid is selected based on a predefined viscosity and molecular weight of the ferrofluid mixture whereby the ferrofluid mixture imparts a predefined start-up and running torque of the ferrofluid rotary seal.

6. The ferrofluid mixture of Claim 1 wherein the first ferrofluid has an ester- based carrier liquid and the second ferrofluid has a hydrocarbon-based ferrofluid.

7. The ferrofluid mixture of Claim 1 wherein a ratio of the first ferrofluid to the second ferrofluid is in a range between 1 to 0 and 0 to 1 .

8. The ferrofluid mixture of Claim 6 wherein the ratio of the first ferrofluid to the second ferrofluid is 1 to 1 .

9. The ferrofluid mixture of Claim 5 wherein the ratio of the second ferrofluid to the first ferrofluid is in a range between 1 to 0 and 0 to 1 .

10. A ferrofluid mixture for use in a multi-stage, magnetic fluid rotary seal, the mixture comprising:

a quantity of a first predefined carrier liquid;

a quantity of a second predefined carrier liquid that is not identical to the first predefined carrier liquid wherein the first predefined carrier liquid is at least partially miscible with and chemically compatible with the second predefined carrier liquid;

a predefined quantity of at least one or more dispersants that is miscible or partially miscible in both the first predefined carrier liquid and the second predefined carrier liquid, wherein the quantity of the first predefined carrier liquid, the quantity of the second predefined carrier liquid and the predefined quantity of the at least one or more dispersants together form a carrier liquid mixture; and

a plurality of finely divided magnetic or magnetizable particles disposed in the carrier liquid mixture.

1 1 . A ferrofluid seal apparatus comprising:

a multi-stage ferrofluid rotary seal adapted to provide a ferrofluid pressure- type seal about a shaft element extending between a first environment and a second environment; and a ferrofluid mixture disposed at each of stage the multi-stage ferrofluid rotary seal, the ferrofluid mixture comprising one of (1 ) a first ferrofluid having a predefined carrier liquid, and a second ferrofluid having a predefined carrier that is not identical to the predefined carrier liquid of the first ferrofluid wherein the first ferrofluid is at least partially miscible with and chemically compatible with the second ferrofluid, or (2) a quantity of a first predefined carrier liquid, a quantity of a second predefined carrier liquid that is not identical to the first predefined carrier liquid wherein the first predefined carrier liquid is at least partially miscible with and chemically compatible with the second predefined carrier liquid, a predefined quantity of at least one dispersant that is miscible or partially miscible in both the first predefined carrier liquid and the second predefined carrier liquid, wherein the quantity of the first predefined carrier liquid, the quantity of the second predefined carrier liquid and the predefined quantity of the at least one dispersant forms a carrier liquid mixture, and a plurality of finely divided magnetic or magnetizable particles disposed in the carrier liquid mixture.

12. The seal of Claim 1 1 wherein the predefined carrier liquid of the first ferrofluid is selected from the group consisting of polyphenyl ethers,

perfluoropolyethers, silicones, silahydrocarbons, hydrocarbons, halocarbons, and esters.

13. The seal of Claim 1 1 wherein the predefined carrier liquid of the second

ferrofluid is selected from the group consisting of polyphenyl ethers, perfluoropolyethers, silicones, silahydrocarbons, hydrocarbons, halocarbons, , and esters.

14. The seal of Claim 1 1 wherein a combination of the first ferrofluid and the

second ferrofluid is selected based on a predefined viscosity and density of the ferrofluid mixture whereby the ferrofluid mixture imparts a predefined startup and running torque of the ferrofluid rotary seal.

15. The seal of Claim 1 1 wherein the first ferrofluid has an ester-based carrier liquid and the second ferrofluid has a hydrocarbon-based ferrofluid.

16. The seal of Claim 1 1 wherein a ratio of the first ferrofluid to the second

ferrofluid is in a range between 1 to 0 and 0 to 1 .

17. The seal of Claim 1 1 wherein the ratio of the second ferrofluid to the first

ferrofluid is in a range between 1 to 0 and 0 to 1 .

18. The seal of of Claim 17 wherein the ratio of the first magntic fluid to the

second ferrofluid is 1 to 1 .

19. A method of providing a multistage, rotary feed-through seal with improved performance, the method comprising:

providing a multi-stage ferrofluid rotary seal adapted to provide a ferrofluid pressure-type seal about a shaft element extending between a first environment and a second environment; and

disposing a predefined quantity of a ferrofluid mixture at each stage of the multi-stage ferrofluid rotary seal wherein the ferrofluid mixture includes a first ferrofluid having a predefined carrier liquid and a second ferrofluid having a predefined carrier that is not identical to the predefined carrier liquid of the first ferrofluid wherein the first ferrofluid is at least partially miscible with and chemically compatible with the second ferrofluid.

20. The method of Claim 19 further comprising selecting the predefined carrier liquid of the first ferrofluid from the group consisting of polyphenyl ethers, perfluoropolyethers, silicones, silahydrocarbons, hydrocarbons, halocarbons, , and esters and selecting the predefined carrier liquid of the second ferrofluid from the group consisting of polyphenyl ethers, perfluoropolyethers, silicones, silahydrocarbons, hydrocarbons, halocarbons, and esters.