GB2232165A - Magnetic compositions - Google Patents

Abstract

A magnetic composition is provided which comprises a combination of two or more magnetic phases having different magnetic properties which interact synergistically. The consequence of this synergism is that a completely new set of properties can be produced by combining two or more materials. The composition may have a significantly better temperature stability than that of neodymium-iron-boron alloys and may also be less susceptible to failure when subjected to a reverse field such as often occurs under load conditions.

Description

MAGNETIC COMPOSITIONS The present invention relates to magnetic compositions and, in particular, to magnetic compositions in which two or more magnetic phases are combined.

Magnetic materials and permanent magnets are important materials for use in electrical and electronic applications. In view of the increasing demands for smaller devices and for higher efficiency, there has been an increasing demand for improved magnetic materials and permanent magnets.

Sintered or bonded samarium-cobalt (Sm-Co) magnets have been successfully used in applications where high magnetic remanence and coercivity are needed in a shaped permanent magnet. However, such Sm-Co magnets are very expensive.

More recently high performance rare earth-transition metal magnets have been developed which have properties similar to those of samariumcobalt magnets. These magnets are based on the light rare earth metal elements neodymium and praseodymium, in combination with iron and boron.

Accordingly, European Patent No 0101552 (Sumitomo Special Metals Co. Ltd.) describes permanent magnetic materials comprising 8-30 atomic percent of Nd and/or Pr, 2 to 28 atomic percent of boron and the balance iron. The permanent magnetic material preferably has a mean crystal grain size of from 1 to 80 micrometres and is prepared by sintering the Nd-Fe-B alloy powder and subjecting the sintered alloy powder to a heat treatment.

European Patent No. 0108474 (General Motors Corporation) discloses magnetically hard alloy compositions comprising at least 10 atomic percent of one or more rare earth elements, preferably neodymium and/or praseodymium; 0.5 to 10 atomic percent boron; and iron; the alloy containing a major proportion of a magnetically hard crystalline phase containing crystallites having an average diameter of less than 400 nanometres. These compositions are prepared by melt spinning the alloy mixture.

Metal magnets are often fashioned either as fully dense sintered compacts using conventional powder metallurgy processing, or by bonding metal magnet powders together using organic polymer binders. This latter process has found wide acceptance for the preparation of the Sm-Co and Nd-Fe-B magnets described above.

The neodymium-iron-boron alloys suffer from two major problems. First their performance can be severely degraded due to corrosion at ambient temperatures, although the bonding technique disclosed above help to ameliorate this problem.

Secondly, these alloys have a low Curie temperature.

As the Curie temperature for these alloys is just above 3000C, there can be a noticeable and irreversible decrease in magnetic performance at any temperature above about 1000C. This severely limits the operating temperature range of all currently available magnets using these alloys.

Some success has been achieved in increasing the Curie temperature of these neodymium-iron-boron alloys by the addition of other elements, such as gallium and silicon. This modification, together with other alloying additions extends the use of these materials. However, this improvement is only achieved with an increase in cost.

We have now developed a method whereby the properties of a magnetic material may be enhanced or altered.

Accordingly, the present invention provides a magnetic composition which comprises a combination of two or more magnetic phases having different magnetic properties which interact synergistically.

When two magnetic phases are combined, the resultant magnet properties may not be the arithmetic mean of the individual sets of properties, i.e. the materials interact synergistically. The consequence of this synergism is that a completely new set of properties can be produced simply by combining the two or more materials. Some of the effects which can be produced by combinations of different magnet phases are described below.

Because of the poor temperature stability of neodymium-iron-boron alloys, the combination of these alloys with a magnetic material having a greater temperature stability, such as Alnico or samarium-cobalt alloys, can provide a product which has a significantly better temperature stability from that of the neodymium-iron-boron alloy. The additive material therefore acts in a supportive role reducing the degradation of the neodymium-iron-boron properties and extending the working range of the composite product. The retained remanence of the composite product can be greater than the arithmetic mean of the remanences of the two components when considered in isolation.A neodymium-iron-boron material, which has become totally demagnetised by heating to a temperature greater than the Curie temperature, can also be partially re-energised by combining the material with a second magnetised phase and re-heating. In this case the recovered remanence will be no greater than that of the second phase at the temperature to which the combination is heated.

Another mode of failure of magnets is by imposing a reverse field. This would normally occur under load conditions and may lead to total demagnetisation of the material. This can again be prevented by utilising as a secondary phase in a composite magnetic material a magnetic phase with a higher resistance to demagnetisation. When the reverse field is removed, the material which has been magnetised, or polarised in the reverse direction, will recover some or all of its original magnetisation, providing that the coercivity of the secondary phase has not been exceeded, and that it is present in a sufficient quantity to create a re-polarising field. This behaviour is of significant importance as a fail-safe mechanism or in applications where a two stage reaction to an imposed field is required.

The magnetic phases which may be combined to provide the magnetic compositions of the present invention are materials which have different magnetic properties and which, when combined, interact synergistically. The magnetic phases may be chosen from rare earth-iron-boron alloys, such as neodymium-iron-boron or praseodymium-iron-boron, samarium-cobalt alloys, such as the 2:17 type, aluminium-nickel-cobalt alloys generally referred to as Alnico alloys, or ferrite magnetic materials such as barium ferrite. The neodymium-iron-boron alloys have a high remanence and high coercivity, both of which properties can be utilised for different applications of combination magnets. The fernite magnetic materials are not considered to be suitable for counteracting the losses associated with the poor temperature stability of neodymium-iron-boron alloys.

The present invention also includes within its scope a method of producing a magnetic composition with novel properties which method comprises combining two or more magnetic phases having different magnetic properties.

Two or more magnetic phases may be combined in one of a number of ways depending on the application. Powders of the magnetic materials may be sintered together using conventional powder metallurgy processing techniques, or may be bonded at room temperature or elevated temperatures using thermoplastic or thermosetting polymers, resins or glasses. Alternatively, the composite may be constructed by assembling a number of pieces of the materials to give the required final shape. As magnetic properties are sensitive to the geometry of the final article, this must be considered when such items are designed. The final composite will then exhibit magnetic characteristics dissimilar to any of the parent materials and may be tailored so as to give advantageous magnetic properties.

The present invention further includes a method of extending the working temperature range of a magnetic material having a high temperature coefficient of remanence, which comprises combining this material with a second magnetic material having a low temperature coefficient of remanence.

The present invention still further includes a method of extending the working range of magnetic materials exhibiting low coercivity, which method comprises combining the magnetic material exhibiting low coercivity with a second magnetic material having a higher coercivity.

The present invention will be further described with reference to the following examples which are intended to illustrate but not to limit the scope of the present invention.

EXAMPLE 1 A neodymium-iron-boron melt spun alloy (of composition 26.6%Nd, 72%Fe, 1.4%B, by weight) was ground to pass a 150 pm sieve and mixed with a simiarly sized powder of Alnico alloy (of composition 8%Al, 13%Ni, 23%Co, 3%Cu, 53%Fe, by weight) in the proportions 55wt% neodymium-iron-boron alloy with 45wt% Alnico alloy. This mixture was then injection moulded in a nylon carrier to produce samples 10mm in diameter and 20mm long. The total amount of magnet material in each of the finished samples was 60% by volume. After magnetisation, the sample was heated to a temperature of 2000C in air, and cooled.

The sample retained over 85% of its initial magnetisation as measured before heating. By comparison, a similar sample of neodymium-iron-boron alloy containing no Alnico alloy powder, retained only 30% of its initial magnetisation after being subjected to the same heating schedule.

The thermal range of neodymium-iron-boron alloys can thus be increased by the incorporation of a material such as an Alnico alloy which has a much greater temperature stability.

EXAMPLE 2 Samples containing an intimate mixture of barium ferrite and a neodymium-iron-boron (NFB) alloy (of the composition given in Example 1) were compression bonded using a proprietary epoxy resin system, in the following proportions: NFB alloy 38wt% Ferrite 54wt% Epoxy resin 8wt% When the epoxy resin had set the sample was magnetised and subjected to a reverse field which would normally be sufficient to demagnetise the ferrite component. On removal of the field the original magnetic properties were restored, i.e. the total demagnetisation of the ferrite component was prevented by the incorporation of the neodymium-iron-boron material which has a higher coercivity.

Claims (9)

CLAIMS:

1. A magnetic composition which comprises a combination of two or more magnetic phases having different magnetic properties which interact synergistically.

2. A magnetic composition as claimed in claim 1 wherein one of the magnetic phases is a rare earth-iron-boron alloy, preferably a neodymium-ironboron alloy.

3. A magnetic composition as claimed in claim 2 wherein the other of the magnetic phases is a samarium-cobalt alloy, aluminium-nickel-cobalt alloy or a ferrite material.

4. A method of producing a magnetic composition with novel properties which method comprises combining two or more magnetic phases having different magnetic properties.

5. A method as claimed in claim 4 wherein the two or more magnetic phases are combined by sintering powders of the magnetic materials together.

6. A method as claimed in claim 4 wherein the two or more magnetic phases are combined by bonding using thermoplastic or thermosetting polymers, resins or glasses.

7. A method as claimed in claim 4 wherein the final composite item is constructed by assembling a number of pieces of the two more magnetic materials to give the required final shape.

8. A method of extending the working temperature range of a magnetic material having a high temperature coefficient of remanence, which comprises combining this material with a second magnetic material having a low temperature coefficient of remanence.

9. A method of extending the working range of magnetic materials exhibiting low coercivity, which method comprises combining the magnetic material exhibiting low coercivity with a second magnetic material having a higher coercivity.