PLATINUM METALS REVIEW A quurterly survey of reseurch on the platinum metuls urrd of dwelopments in their applications in industry VOL. 8 JULY 1964 NO. 3 Contents Platinum Alloy Permanent Magnets 82 Alternating Current Polarisation of Noble Metal Surfaces 90 Production of Ultra-pure Hydrogen 91 Platinum Metal Salts and Complexes as Homogeneous Catalysts 92 Electrical Resistivity of Refractories 98 Platinum Metals in Electrical Contacts 99 Carbon and the Platinum Metals at High Temperatures 101 Deformation of Zone-melted Iridium Single Crystals 102 Ethylenediamine Complexes of Ruthenium 106 Abstracts 107 New Patents 115 Communications should be addressed to The Editor, Platinum Metals Review Johnson, Mutthey & Co., Limited, Hatton Garden, London, E.C.1 Platinum Alloy Permanent Magnets THE DESIGN OF MAGNETIC CIRCUITS FOR PLATINAX I1 By L. A. Ford, B.s~. Research Laboratories, Johnson Matthey & Co Limited In applications of this type, it is essential Plalinax 11, fl: cobalt-platinurn allw, is for magnetic circuits to be properly designed, one of the most powerful permanent so as to ensure that effective use is made of magnet materials knwvn. Because of its the properties of the magnetic material. This high performance, its principal uppli- is particularly important when a high per- cations are in miniatwised tinits where formance material such as Platinax I1 is to size and weight considerations are ofthe be used, and necessitates redesign of the utmost importonce, or where the geo- magnetic circuit if a material of lower per- metry of de magnet excludes the use formance is to be replaced. of odier materials. It is reudib fabri- cated and machined, mabiing magnets Operating Characteristics of complex shape to be mlariufac!ured to The performance of a permanent magnet close lolerancos, and it will operate for material is shown by its demagnetisation long periods in highly corrosive environ- curve. A typical curve for Platinax I1 is ments. maximum advani- To obtain fhe given in Fig. I illustrating, together with age from its use, it is necessary to design the derived energy-product curve, the three magnetic circuits carefully so IW to properties of residual induction (BJ, coercive exploit as fully as possible thmagnetic force (H,) and maximum energy-product characteristics of the alloy. Data are @H)rnax. given in this ortick that will assisc Although these figures are valuable in engineers make the most effeccive use to making comparisons between different mater- Phtinax IT. of ials, they give only an approximate guide to the way a magnet will function in service. Final performance depends very largely upon It is some years since investigations by a the point at which the magnet works on, or Johnson Matthey research team into alloys of adjacent to, its demagnetisation curve. This is the cobalt-platinum system led to the de- governed by factors that can often be con- velopment of Platinax 11, an extremely trolled, such as magnet shape and the type powerful permanent magnet material. of external circuit in which it operates. Since then the principal applications of this For example, the working point of a magnet alloy have been for miniaturised units, such that operates free from stray magnetic fields as hearing aids and electric watches, where lies on the demagnetisation curve provided advantage is taken of the exceptionally high that the magnet is magnetised in its circuit magnetic performance of the material, and in and is not subsequently disturbed. Its instruments where powerful magnets must be magnetic performance is determined by the subjected to highly corrosive conditions, such amount of energy that it makes available, and as meters that measure and control the flow this is proportional to the product (B xH) at of corrosive liquids. the working point. The available energy Platinum Metals Rev., 1964, 8, (3), 82-90 82 This hearing aid by Forti- phone Limited contains two Platinax 11 magnets in disc form, one .003 inch thick by -& inch and the other .005 inch thick by fc inch. By the use of Platinax II magnets tlLe combined microphone, amplifier and earphone unit has been considerably reduced in size and weight without loss of sensitivity therefore reaches a maximum value at the The working points of four cylindrical (BH),, position and in order to obtain magnets are shown in Fig. 2 on the demagneti- maximum economy of magnetic material a sation curve for Platinax 11. Each magnet was magnet should, whenever possible, operate at measured in three separate circuits, covering this point, i.e. at BdHd in Fig. I. a wide range of self-demagnetising conditions. It can be seen that even under arduous BH PRODUCT ‘open-circuit’ conditions (Fig. za) relatively I o7 5 I06 I I 7000 short Platinax I1 magnets operate satis- 8, - 6480 factorily. The addition of a cylindrical iron extension produces some improvement in 6000 economy (Fig. zb), while still further im- provement is obtained by placing the magnets 5000 in an iron yoke (Fig. zc). The effect of the improvement is clearly shown hy comparing 4000 the sizes of magnets operating at approxi- _-. VI mately (BH)ma,. For an open-circuit magnet, 3000 j a length/diameter ratio of one-half is required, v m whereas for a magnet operating in the yoke - 2000 1000 Fig. 1 The high values of coerciue,force (HJ and energy-product ( BH)for Platinax 11 are indicated by the demagnetisation and energy-product curves. r I For maximum eronomy of material a magnet - so00 -3000 -1000 should work at a point on the demagnetisation curue H (OERSTEDS) rorresponding to (BH)max,i.e. at RdHd Platinum Metals Rev., 1964, 8, (3) 83 fB a: OPEN CIRCUIT A 6,O c H 0 Fig. 3 The working point of a magnet subjected to a demagnetisingfield follows the curve from M to N. Whenthe$eld is removed, a partial recovery is made and the working point moves along a recoil line to the position P. The performance of the magnet undergoes a corresponding change the ratio is reduced to one-quarter, represent- ing a saving of 50 per cent in magnet material. The overall size of the magnetic circuit is naturally increased by such measures, but this may be justifiable if space and weight limitations are not severe. 1000 Recoil Permeability A magnet that operates in a demagnetising I' I I -5000 - 3000 - 1000 field suffers a loss in performance that is only FIELD H (OERSTEDS) partially regained when the field is removed. 7000 The extent of its recovery is determined by the recoil characteristics of the material from 6000 - which it is made. This can be appreciated v) 5000 $ by considering a magnet that operates at a -< point M on the demagnetisation curve (Fig. 3) 4000 until, on being partially demagnetised by a z field, its working point follows the curve to 3000 U the point N. In the absence of further 2000 g demagnetising forces it will remain there until the field is removed, whereupon, instead of LOO0 retracing the curve to M, it will move along a recoil line to the point P, which becomes the -5000 ' - 3000 - 1000 new working point. The performance of the FIELD H (OERSTEDS) magnet undergoes a corresponding change in magnitude. Fig. 2 A magnet's performance is governed by In order that the magnet shall have the its shape and the type of circuil in which it works. minimum reduction of performance, it is iWeaaurement of the working points of Platinax 1 I magnet systems shows that (a)high performance is necessary for the line Nl', whose slope is a obtained from short, open-circuit magnets. Greater measure of recoil permeability, to lie as economy is achieved by (b) adding a short iron close as possible to the demagnetisation extension and by (c) placing the magnet in an iron yokr curve. In the ideal case when both lines have Platinum Metals Rev., 1964, 8, (3) 84 Fig. 4 The demagnetisa- I 16000 tion curves of various per- AVERAGE RECOIL PERMEABILITIES manent magnet materials A= ALCOMAX m 14000 are shown, together with B = ALNICO (High Coercivity) 5.2 values of average recoil per- C = 357oCOBALT STEEL l2000 meability. Unlike high- D COLUMAX E = MAGNADUR 3 (B~F~~~o,~)1.05 energy materials having 10000 more “rectangrdar” curves, F = PLATINAX II the recoil lines of Platinax G=RECO 2A H =TKONAL GX aooo 2 II lie close to the main BH 34 curve for a relatively large 6000 range, indicating high SLOPE-1.16 capacity to recover from the eflects of demagnrtising 40# .fields 2000 -5000 -4000 -3000 -2000 -1000 H (OERSTEDS) the same slope, point P would coincide with points would recover almost completely on point M, and the change in energy would removal of the field. be zero. The demagnetisation curves of some per- Platinax I1 is one of the few materials whose manent magnet materials are shown in Fig. 4, recoil lines lie very close to the main BH curve. together with published values of average Many other high-energy materials have recoil permeability. Curves for the more demagnetisation curves of more rectangular common high-energy materials, such as shape, and although these exhibit good recoil Ticonal GX and Alcomax 111, are clearly properties over the upper sections of the of a more rectangular form. curve, they are not maintained where the curves become steep. As a result, a de- The Magnetic Circuit magnetising field that causes a temporary Energy produced by a permanent magnet is lowering of working point on to the steep used in two ways. Its purpose normally is to section of the curve causes, on its removal, a provide a flux across a gap, but to do this substantial reduction of available energy.