MAGNETOSTRICTIVE POWDER COMPOSITE AND METHODS FOR THE MANUFACTURE THEREOF

Abstract: Magnetostrictive powder composite and method for the manufacturing of the magnetostrictive powder composite. The powder composite according to the invention is preferably used as a magnetostrictive element in sound projectors and vibration generators, transducers, actuators and in linear motors. The powder composite consists of magnetostrictive powder grains with chemical composition (RE)xT1-x, where RE represents one or a mixture of several rare earth metals, T represents Fe, Ni, Co or Mn or a mixture of these metals and x represents atomic fraction assuming a value between 0 and 1, whereby the grains are held together by a binder. The powder composite is formed in such a way that it exhibits a homogeneous magnetic flux, has a good strength and can be made with a built-in precompression. It can be equipped with coolant channels and/or with coil loops. The powder composite is produced by pressing together the magnetostrictive powder grains and the binder in order to plastically deform the grains, either completely or partially, which causes the magnetic domains in the grains to align perpendicularly to the applied compression pressure. In an alternative mode of execution according to the invention isostatic pressing is used.

Description: MAGNETOSTRICTIVE POWDER COMPOSITE AND METHODS FOR THE MANUFACTURE THEREOF. The invention relates partly to a magnetostrictive powder composite according to the preamble to claim 1, and partly to a method for the manufacturing of the magneto¬ strictive powder composite according to the preamble to claims 16 and 17. The powder composite according to the invention is preferably used as a magnetostrictive element in sound projectors and vibration generators, transducers, actuators and in various types of linear motors. To clarify the difference between permanent magnets and magnetostrictive powder composites, key properties and areas of applications for both materials are listed below: PERMANENT MAGNETS: Properties 1. Permanent magnets, usually compounds of rare earth metals and transition metals (Fe, Ni, Co) like for instance SmCo 5 and Nd 2 Fe 14 B, are passive devices used for generating a magnetic field. 2. Permanent magnets can only generate static magnetic fields. 3. Permanent magnets are magnetized initially and posses high remanence and high coercive force. Unreasonably high energy would be needed to change the magnetic field, which makes it practically impossible to use permanent magnets for purposes other than to generate static magnetic fields. 4. Permanent magnets do not need an electric current flowing in a coil or a solenoid to generate and maintain the magnetic field. Application areas A. Permanent magnets are used for generation of large static fields in situations where it is difficult to provide electric power or where the availability of electric power is limited, or where geometrical constraints such as space restrictions generate their use rather than electromagnets. B. The main applications of permanent magnets are in electric motors (in which electric energy is converted into mechanical energy) , generators (in which mechanical energy is converted into electrical energy) , loudspeakers, control devices for electron beams such as in TV sets, magnetic levitation systems, and various forms of holding magnets such as door catches. For example Nd 2 Fe 14 B magnets developed by General Motors are used in the starter motors of their cars and trucks. MAGNETOSTRICTIVE POWDER COMPOSITE: Properties 1. Magnetostrictive powder composite is an active device consisting of rare earth metals (RE) and transition metals (Fe, Ni, Co and Mn) , (RE) x Fe.,_ x , which changes its length extremely much when exposed to an external magnetic field. In contrast to traditional magneto¬ strictive materials, such as Fe and Ni which display magnetostrictive change in length of 9 ιm/m and 40 j ra/ respectively, a magnetostrictive powder composite displays length changes of more than 1000 ^ιm/m and is therefore called giant magnetostrictive material. Beacause of this, the magnetostrictive powder composite is used to generate large and fast movements of high precision and large force. In most applications this large force is used to increase change in length and to generate larger movements. 2. Magnetostrictive powder composite is usually used in high frequency applications (up to 60 kHz) , e.g. for ultrasonics. In this application the purpose of the magnetostrictive composite is that it should work as an acoustic projector i.e. to generate fast mechanical movements and ultrasound. 3. Magnetostrictive powder composite is initially a material with low ferromagnetism. Magnetic moments within the magnetic domains in the material are randomly oriented i.e. the material is not magnetized as in the case of the above mentioned permanent magnets. For a powder composite to produce a length change one has to apply mechanical stress on the material to rotate magnetic domains relative the direction of the applied stress, as well as to apply a high magnetic field by feeding current into a coil surrounding the material. Typical magnetic fields are 1 - 8 kOe. 4. The material constituting the magnetostrictive powder composite has low remanence and low coercive force. Chemical composition of the powder is chosen so that the anisotropic energy is minimized. If one omitted to do so it would be very difficult to use the material in practice. 5. Magnetic powder composite has been put forward with a purpose to increase the bandwidth of the casted giant magnetostrictive material available on the market. Magnetostrictive powder composite can manage a frequency region of 0 - 60 kHz, while casted giant magneto¬ strictive material only can manage 0 - 2 kHz. Giant magnetostrictive alloys made of terbium, dysprosium and iron are usually called Terfenol-D. Application areas Giant magnetostrictive powder composite is used in: A. acoustic underwater sound projectors for high frequencies, - acoustic projectors for ultrasound applications (20 - 60 kHz) , - vibration generators (0 - 60 kHz) , - positioners (to generate fast, high precision motion) , and B. - wide bandwidth sound projectors and vibrators in which the amplitude does not change with frequency or load, which is the case with conventional electromagnets. Magnetostrictive powder composite according to the invention presented has not been known of before. For example the patent documents US, A, 4865 660, DK, B, 157 222, FR, A, 2065 359 and EP, Al, 175 535 do certainly refer to magnetic powder composite materials, which nevertheless all are permanet magnets and which find their applications because of their capability to maintain permanent magnetization. Magnetostrictive properties are not mentioned in the above referred documents. The fact that the materials mentioned in these documents include powder grains of rare earth metals and transition metals is of no importance in this context. When using conventional magnetostrictive materials and in particular alloys of type (RE) X T.,_ X , where RE represents one or a mixture of several rare earth metals, T represents Fe, Ni, Co or Mn or a mixture of two or more of these metals and x assuming a value 0 < x ^ 1 represents atomic fraction, below mentioned rods, the following inconven- iences will be manifested: 1. The magnetostrictive materials are manufactured in the form of rods by casting. The casted rods hereby get brittle characteristics and are beacuse of this very difficult to machine with conventional techniques. 2. Scrap from crashed rods is difficult to reuse. 3. The rods are brittle and can only withstand very small tensional stress. 4. Due to a relatively low resistivity of casted magnetostrictive rods, like for instance Terfenol-D rods, in order to increase the frequency performance of the said rods, it is often neccessary to slice the rods and to glue them together again in order to decrease the electrically conductive cross section of the material and to thereby decrease eddy current losses. 5. Due to the low permeablity of a conventional casted magnetostrictive rod it is difficult to magnetize homogeneously such a rod by the use of permanent magnets applied at its ends. Usually a fairly homogeneous magnetic flux can only be achieved if the rod length is not larger than 3 times its diameter. 6. The low permeability of the casted rod also causes magnetization at the rod ends to be lower compared to the rod centre when a conventional coil is used to magnetize the rod. 7. So far it has only been possible to produce magneto¬ strictive elements in form of rods with circular cross sections. This causes a large material wastage and a costly machining if another geometrical form is required. By either crushing the scrapped magnetostrictive rods in an oxygen free atmosphere, or crushing magnetostrictive ingots or directly atomizing magnetostrictive powder or by hydrogen decrepitation producing a magnetostrictive powder, and thereafter pressing the crushed scrap or powder together with a binder, all of the above accounted inconveniences can be decreased or eliminated. To maximize the magnetostriction one can magnetically align the material before it is pressed isostatically and the binder has been cured. This is accomplished by applying a magnetizing field along the working direction of the magnetostrictive powder composite. The above mentioned disadvantages 1 - ' 6 with the existing technique are matched by the following advantages if the invention is utilized: 1. The powder composite is so tough that it can be shaped with a conventional cutting technique. 2. Scrap from crushed rods can be ground in an oxygen free atmosphere and thereafter reused for new rods. 3. If for example reinforcement fibres, preferably of aluminium oxide, silicon carbide, Kevlar, carbon, glass or titanium, are pressed into the rod and aligned longi- tudinally or perpendicularly, tensile strength and elastic modulus will be increased. 4. By coating the grain surface with an electrically insulating material or by using a binder that insulates the grains from each other, eddy current losses can be decreased. The invention utilizes such binding agents which wet said grains and bind them together and possibly also form an electrically conducting layer between the powder grains or between the grain agglomerates. These requirements are fulfilled e.g. by a number of known resins and thermoplastics. Ceramics and oxides, preferably rare earth oxides because of a high reactivity of Terfenol-D, can also be used as an insulating coating. 5. A homogeneous magnetic field generated by permanent magnets can be achieved if a powder of a permanent magnet type, preferably Nd 2 Fe 14 B, is mixed with the magnetostrictive powder, preferably along the rod axis, in order to decrease the leakage flux. This will make it possible to manufacture rods with length/diameter ratios larger than 3:1. 6. To avoid lowering of magnetization at the rod ends high permeability and high resistivity powder grains, preferably of coated iron, nickel, cobalt or amorphous iron, like for instance metglas, or alloys of these, can be pressed into the rod ends. 7. Magnetostrictive powder composite can be directly pressed to a final shape, whereby expensive material wastage is avoided. In addition, the invention provides for the following advantages: - The surface friction of the magnetostrictive powder composite can be lowered, so that it can glide easier against other objects. Also, its chemical resistance can be increased by coating the magnetostrictive powder composite, after it has been pressed, with a thin layer of non-organic material, such as aluminium oxide or an organic material, such as teflon, or if during pressing the composite surface is provided with a powder coating made of the above mentioned organic or non-organic materials. - The strength of the magnetic powder composite can be increased by coating its surfaces, which are in contact with other objects and thereby are exposed to a mechanical load, with a layer made of e.g. aluminium oxide or silicon carbide. - In addition, by the use of powder technology, additional coil loops and/or coolant channels can be integrated into the pressed form. Different embodiments of the invention are shown in the enclosed figures. Fig 1 shows a magnetostrictive composite rod 1 which, besides the magnetostrictive powder, possible coating and a binder, also has permanent magnets 2 of a conventional type at the rod ends and aligned permanent magnet powder 3, mainly along the longitudinal axis of rod 1, which makes the working magnetization in the composite rod 1 more homogeneous. Fig 2 shows a magnetostrictive composite rod 1, an excitation coil for generating magnetizing field 4 and iron powder, coated with a thin electrically insulating layer of Fe 2 0 3 or equivalent material, which has been pressed into the ends 5 of the rod 1. With this design a homogeneous magnetic flux in the composite rod 1 is achieved. Fig 3 shows a magnetostrictive composite rod 1 with longitudinal fibre reinforcement 6 which, ' besides reinforcing the rod 1 and increasing its strength against tensile stress, also makes it possible to build in a prestress into the rod 1. The magnetostrictive composite material according to the invention must exhibit low anisotropic energy and high magnetostriction in order to find practical use. It is therefore important to minimize the anisotropic energy and at the same time to optimize the room temperature magneto¬ striction of the composite material. A number of composite materials with chemical composition (RE) X T.,_ X , where RE represents one or a mixture of several rare earth metals, T represents Fe, Ni, Co or Mn or a mixture of two or more of these metals and x assuming a value 0 < x ≤ 1 represents atomic fraction, will have the mentioned properties. At evaluating different compositions of magnetostrictive composite rods 1 according to the invention the applicant has found that the following compositions A) - F) give good such properties in the composite rods: A) Tb x y 1 . x Fe 2 . H wherein x and w represent atomic fractions within 0.2 x ^ 1.0 and 0 ^ w ^ 0.5. B) Tb x Ho t _ x Fe 2 _ w wherein x and w represent atomic fractions within 0.1 ^ x ^ 1.0 and 0 < w ^ 0.2. C) Sm x Dy 1 . x Fe 2 . H wherein x and w represent atomic fractions within 0.8 x ^ 1.0 and 0 -sζ w $ 0.2. D) Sm^Ho^Fe^ wherein x and w represent atomic fractions within 0.6 $ x^ 1.0 and 0 ^ w ^ 0.2. E) Tb x Hθ y Dy z Fe 2 . H wherein x, y, z and w represent atomic fractions within 0.1.$ x ≤ 1.0, 0 •$ y ≤ 0.9, 0 ^ z ≤ 0.8, and 0 -ξ w 0.2 and x + y + z = 1. F) Sm x Hθ y Dy z Fe 2 . H wherein x, y, z, and w represent atomic fractions within 0.6 ,< x ^ 1.0, 0 < Y 0.4, 0 ^ z ^ 0.4, and 0 w < 0.2 and x + y + z = 1. Although some particularly favourable compositions of magnetostrictive composite materials are accounted for in the above it is understood that even other compositions with good properties are contained within the scope of the invention. In order to improve the magnetostrictive composite material described by the invention, to increase the derivative dλ/dH, where λ is magnetostriction and H is the magnetizing field, as well as magnetostriction at saturation one can, after pressing and after the binder has been cured, expose the magnetostrictive composite material to the following heat treatment: - composite material is heated to a temperature above its Curie temperature, which means about 400°C, - thereafter, a magnetizing field of 40 kA/m amplitude is applied, - finally the composite material is cooled down, with the magnetizing field still being applied, to a temperature below its Curie temperature. The composite material can be further improved if it is exposed to external vibrations during pressing. This will increase the density and the permeability as well as facilitate the magnetic alignement of the magnetostrictive grains. The above described method of manufacture of the magnetostrictive powder composite according to the invention often demands high pressing forces. In an alternative mode of execution according to the invention isostatic pressing is used, which usually means a lower pressing force than in the above described method. In said alternative method, the magnetostrictive powder grains and the binder are pressed together isostatically, at which the composite material is directly pressed to an arbitrary final shape. This isostatic pressing can be improved by magnetically aligning the magnetostrictive grains before the composite material has been pressed and before the binder has been cured. This is achieved by applying the magnetizing field along the working direction of the magnetostrictive powder composite.

Claims: . PATENT CLAIMS 1. Magnetostrictive powder composite consisting of magnetostrictive powder grains with chemical composition (RE) X T.,_ X , where RE represents one or a mixture of several rare earth metals, T represents Fe, Ni, Co or Mn or a mixture of two or more of these metals and x represents atomic fraction assuming a value between 0 and 1, characterized in that the magnetostrictive grains are held together by a binder, the binder is such that it wets the magnetostrictive grains and is preferably a resin or a thermoplastic, the grains are prevented from electric contact with each other by the binder and/or an electrically insulating layer, preferably ceramics or oxides, in particular rare earth oxides, that encapsulates each one of the magnetostrictive powder grains. 2. Magnetostrictive composite material according to claim 1, characterized in that it has chemical composition wherein x and w represent atomic fractions within 0.2 ^ x ^ 1.0 and 0 ^ w ^ 0.5. 3. Magnetostrictive composite material according to claim 1, characterized in that it has chemical composition Tb x H 0l . x Fe 2 . H wherein x and w represent atomic fractions within 0.1.≤ x .≤ 1.0 and 0 w < 0.2. 4. Magnetostrictive composite material according to claim 1, characterized in that it has chemical composition Sm x° i-x Fe 2-w wherein x and w represent atomic fractions within 0.8 : x ^ 1.0 and 0 < w < 0.2. 5. Magnetostrictive composite material according to claim 1, characterized in that it has chemical composition Sm x H 0l . x Fe 2 . H wherein x and w represent atomic fractions within 0.6. x ^ 1.0 and 0 ^ w ^ 0.2. 6. Magnetostrictive composite material according to claim 1, characterized in that it has chemical composition Tb x Ho y Dy z Fe 2 _ w wherein x, y, z and w represent atomic fractions within 0.1 ? x < 1.0, 0 -≤ y ^ 0.9, 0.≤ z . 0.8, and 0 w .$ 0.2 and x + y + z = 1. 7. Magnetostrictive composite material according to claim 1, characterized in that it has chemical composition Sm x Ho y Dy z Fe 2 _ w wherein x, y, z and w represent atomic fractions within 0.6 <: x ^ 1.0, 0 -≤ y ^ 0.4, 0 ^ z -≤ 0.4, and 0 -$ w ^ 0.2 and x + y + z = 1. 8. Magnetostrictive composite material according to any of the above claims characterized in that it has a rod shape. 9. Magnetostrictive composite material according to claim 8, characterized in that the rod has anarbitrary cross section. 10. Magnetostrictive composite material according to any of the above claims, characterized in that the opposite ends of the composite material contain high permeability, or high permeability and high resistivity powder grains, preferably of surface coated iron, nickel, cobalt or amorphous iron like for example metglas, or alloys of such, which make said ends into electrical insulators, even at high frequencies. 11. Magnetostrictive composite material according to any of the above claims, characterized in that the composite material also includes fibres, preferably of aluminium oxide, silicon carbide, Kevlar, carbon, glass or titanium, oriented lonqitudinally or transversally in the composite material, intended as mechanical reinforcement and/or strength improvement against tensile stresses and/or as means to produce a prestress in the composite material and/or as means to increase elastic modulus of the composite material. 12. Magnetostrictive composite material according to any of the above claimes, characterized in that the composite material also includes permanent magnet powder grains, preferably of Nd 2 Fe 14 B, in such concentration and at such locations in the composite material, preferably along the axis of the composite material, that it has a homogeneous operating magnetization. 13. Magnetostrictive composite material according to any of the above claims, characterized in that the composite material is formed with coolant channels and/or with coil loops. 14. Magnetostrictive composite material according to any of the above claims, characterized in that the composite material, with the aim to lower its surface friction so that it can glide easier against other objects and to increase its chemical resistance, is after pressing coated with a thin layer of a non-organic material, such as aluminium oxide, or an organic material, such as teflon, or that during the pressing its surface is provided with a surface layer consisting of a powder of said organic or non-organic materials. 15. Magnetostrictive composite material according to any of the above claims, characterized in that the composite material, aimed at increasing its strength, has its surfaces that are in contact against other objects and thereby are exposed to mechanical stresses, coated with a surface layer of powder, preferably aluminium oxide or silicon carbide. 16. Method for the manufacturing of the magneto¬ strictive composite material according to any of the above claims, characterized in that the magnetostrictive powder grains and the binder are pressed together at a pressure that is at least high enough to plastically deform the grains, either completely or partially, which causes the magnetic domains in the grains to align perpendicularly to the applied compression force, and that the composite material is directly pressed into a final arbitrary shape. 17. Method for the manufacturing of the magneto- . strictive composite material according to any of claims 1-15, characterized in that the magnetostrictive powder grains and the binder are isostatically pressed together, and the composite material thereby is pressed into a final arbitrary shape. 18. Method for the manufacturing of the composite material according to claim 17, characterized in that the magnetic alignement of the magnetostrictive powder grains takes place by applying a magnetizing field along the working axis of the magnetostrictive powder composite, before the composite material has been pressed and before the binder has been cured. 19. Method for the manufacturing of the magneto¬ strictive composite material according to any of claims 16-18, characterized in that high permeability, or high permeability and high resistivity, powder grains, preferably of surface coated iron, nickel, cobalt or amorphous iron like for example metglas, or alloys of these, had been pressed into the opposing ends of the composite material in such a way that said ends are made into electrical insulators, which provides a more homogeneous magnetic flux inside the composite material. 20. Method for the manufacturing of the magneto¬ strictive composite material according to any of claims 16-19, characterized in that the magnetostrictive powder grains are pressed together with fibres, preferably of aluminium oxide, silicon carbide, Kevlar, carbon, glass or titanium, which are preferably oriented longitudinally or transversally in the composite material, in such a way that said fibres act as mechanical reinforcement and/or strength improvement against tensile stresses and/or as means of producing a prestress in the composite material and/or as means of increasing elastic modulus of the composite material. 21. Method for the manufacturing of the magneto¬ strictive composite material according to any of claims 16-20, characterized in that the powder grains, preferably of Nd 2 Fe 14 B, in such concentration and at such locations in the composite material, preferably along the axis of the composite material, are added during its manufacture, that the composite material has a homogeneous operating magnetization, and that the permanent magnetic powder grains are oriented by means of an external magnetic field. 22. Method for the manufacturing of the magneto¬ strictive composite material according to any of claims 16-21, characterized in that the composite material, with the aim to decrease its surface friction so that it can glide easier against other objects and to increase its chemical resistance, after the pressing is coated with a thin layer of non-organic materials, such as aluminium oxide, or an organic material, such as teflon, or that during the pressing its surface is provided with a surface layer consisting of a powder of said organic or non- organic materials. 23. Method for the manufacturing of the magneto¬ strictive composite material according to any of claims 16-22, characterized in that the composite material, with the purpose to increase its strength, has its surfaces that are in contact against other objects and thereby are exposed to mechanical stresses coated with a surface layer of powder, preferably of aluminium oxide or silicon carbide. 24. Method for the manufacturing of the magneto¬ strictive composite material according to any of claims 16-23, characterized in that the magnetostrictive composite undergoes the following heat treatment after it has been pressed and the binder has been cured in order to increase its derivative dΛ/dH, where A is the magnetostriction and H is the magnetizing field as well as its saturation magnetostriction: - the composite material is heated to a temperature above its Curie temperature, which means about 400°C, thereafter, a magnetizing field with an amplitude of 40 kA/m is applied, - finally, the composite material is cooled down,. said magnetizing field still being applied, to a temperature below its Curie temperature. 25. Method for the manufacturing of the magneto¬ strictive composite material according to any of claims 16-24, characterized in that the composite material is exposed to external vibrations during the pressing, at which its density and its permeability are increased as well as the magnetic alignement of the magnetostrictive grains is facilitated.

Inventors: CEDELL TORD, CEDELL, TORD
Applicants: PROENGCO AB
Application year: 1992
Main classification: C22C1
IPC codes: B29C70, C04B14, C22C1, C22C32, C22C33, C22C38, C22C49, H01F1, H01L41