GB2128849A

GB2128849A - Transducer element

Info

Publication number: GB2128849A
Application number: GB08326781A
Authority: GB
Inventors: Rodney David Greenough
Original assignee: BESTOBELL
Current assignee: BESTOBELL
Priority date: 1982-10-14
Filing date: 1983-10-06
Publication date: 1984-05-02
Also published as: GB8326781D0

Abstract

A transducer element comprising particles 3 of a magnetostrictive material, preferably of the iron/rare earth element kind, dispersed within and bound together by a matrix 4 of a plastics material e.g. epoxy resin. The magnetostrictive particles are oriented by applying a magnetic field before the plastics material sets. <IMAGE>

Description

SPECIFICATION Transducer element The most common form of electromechanical transducer for converting electrical energy to mechanical energy, and vice versa, particularly at ultrasonic frequencies, uses a piezoelectric crystal. Such crystals require comparatively high electrical voltages and this can involve danger if, for example, the piezoelectrical crystal transducer forms part of equipment for sensing the level of an inflammable liquid. An alternative form of transducer, which for the same energy consumption requires a higher current, and therefore a lower voltage, than a piezoelectric crystal, utilizes a magnetostrictive material. Such a material converts a high frequency variable magnetic field into mechanical movement, or vice versa.

One conventional magnetostrictive material is a laminated alloy of nickel, chromium and cobalt but this material has a comparatively low magnetostrictive constant. More recently, alloys of iron and rare earth elements, such as a binary terbium/iron (Tb Fe2); and a ternary dysprosium/terbium/iron alloy (Dy Tb Fe) have been developed and have a magnetostrictive constant an order of magnitude or more greater than that of the nickel/chromium/cobalt alloy. However these iron/rare earth element alloy materials are brittle and can only be cut by expensive spark machining, from the large lumps in which they are produced, down to the size and shape of the required elements. This leads to wastage of the expensive material and does not alter the brittleness which makes the machined element still liable to fracture if dropped or knocked.

In accordance with the present invention, a transducer element comprises particles of a magnetostrictive material dispersed within and bound together by a matrix of a plastics material.

An element of this composite construction can be prepared by grinding up the magnetostrictive material, particularly of the iron/rare earth element kind, and mixing it in the plastics material prior to setting of the plastics material.

If the plastics material is sufficiently fluid immediately after the mixing, particle orientation can be achieved by applying a magnetic field (e.g. of 30 kilogauss) before the plastics material sets. The particles are then fixed with a particular crystal axis along a chosen direction in, for example a rod element. This offers the possibility of using parent material with high magnetocrystalline anisotropy.

The shape of the element can be determined by causing the mixture to set in a corresponding mould shape, thereby eliminating, or reducing to a minimum, any subsequent machining and wastage of material.

Alternatively, an element may be produced from a larger block of the composite material, in which case the material can be machined more easily and with simpler and cheaper techniques than parent iron/rare earth element alloy material.

The resulting element is invulnerable to fracture owing to the strength provided by the plastics matrix.

Although the magnetostrictive constant of the composite material is not of course as great as that of the parent material from which the particles are formed, surprisingly, it is still an order of magnitude greater than that of conventional nickel/chromium/cobalt material. The reduction in magnetostrictive constant, as compared to the parent material, is more than compensated by the advantages of the composite material. In fact the magnetostrictive constant of the composite material can be maximised by utilising a hard plastics matrix material, such as an epoxy resin material, and by providing as large a content of magnetostrictive material as possible in the composite. Preferably, the ratio of parent magnetostrictive material to plastics matrix material in the composite is at least two to one by weight.

Any current loss in conventional nickel/chromium/cobalt material limits its use to about 20 kHz or less, unless the laminations are reduced to less than 0.003 inch. The bulk DC resistivity of the new composite material is very high, in excess of 5000 k. ohms for a cylindrical rod transducer element having a length of 2 cms and diameter of 0.6 cms. A uniform particle size of up to 50 micron, e. g. 20 to 30 micron, together with the high bulk resistivity offers the possibility of working with the new transducer element at frequencies of greater than 20 kHz. Experiments have indicated that the output from a rod element of the composite material at about 40 kHz is up to 12 dB higher than that from a rod of equal dimensions fabricated from laminated nickel/chromium/cobalt material.

In one example a rod transducer element was formed from a composite material comprising seventy parts by weight of a Dy/Tb/Fe alloy known as Araldite. The Terfenol consisted of seventy five parts by weight of Dy, twenty five parts by weight of Tb, and two hundred parts by weight of Fe.

A comparison of the magnetostrictive constant of this composite material, as compared with that of the parent Terfenol, and laminated conventional nickel/chromium/cobalt material, is given in the following table: Material Magnetostrictive constant (strain for a field of 3 kilogauss) Ni/Cr/Co 26 x 10-6 Terfenol (Dy Tb Fe) 1260 x 10-6 Terfenol composite 400 x 10-6 The exemplifed rod element is illustrated diagramatically in the accompanying drawings in which Figure 1 is a perspective view and Figure 2 is an axial section though the rod.

As shown in Figure 1, the rod element is in the shape of a right circular cylinder having an axial length greater than its diameter. As shown in Figure 2, the element is composed of particles of 3 of Terfenol disbursed within a matrix 4 of Araldite.

Claims

1. A transducer element comprising particles of a magnetostrictive material dispersed within and bound together by a matrix of a plastics material.

2. An element according to claim 1, in which the magnetostrictive material is of the iron/rare earth element kind.

3. An element according to claim 1 or claim 2, in which the plastics material is an epoxy resin material.

4. An element according to any one of the preceding claims, in which the ratio of magnetostrictive material to plastics matrix material is at least two to one by weight.

5. A transducer element substantially as described with reference to the accompanying drawing.

6. A method of making a transducer element according to any one of the preceding claims, in which the magnetostrictive material is ground and dispersed within a fluent plastics material which subsequently sets solid.

7. A method according to claim 6, in which the dispersion is cast in a mould prior to setting of the plastics material.

8. A method according to claim 6 or claim 7, in which the particles of ground magnetostrictive material are oriented by applying a magnetic field before the plastics material sets.