Compressed Powdered Molybdenum Permalloy for High Quality Inductance Coils *

By V. E. LEGG and F. J. GIVEN

M o ly b d e n u m -P e r m a llo y is n o w p r o d u ce d in th e form o f c o m ­ p r e sse d p o w d e re d c o res for in d u c ta n c e co ils. I t s h ig h p e r m e a b ility a n d lo w lo sse s m a k e p o s sib le im p r o v e d coil q u a lity , or d e c re a se d size w it h o u t sa cr ificin g c o il p er fo rm a n c e . I t s lo w h y s te r e s is lo ss red u ces m o d u la tio n e n o u g h t o p e r m it a p p lic a tio n w h ere la rg e air core c o ils w o u ld o th e r w is e b e req u ire d .

H E in tro d u c tio n of loading coils in th e telep h o n e sy stem a t a b o u t th e tu rn of th e c e n tu ry b ro u g h t special d em an d s on m ag n etic an d electrical p ro p e rtie s of core m aterials, a n d se t in m o tio n in v estig atio n s w hich h av e h a d w ide influence on th e th eo re tical an d p ra c tic a l asp ects of ferro m ag n etism . T h e first ste p in th is d ev elo p m en t led to cores of iron w ire, w hich sufficed for loading coils on c ircu its of m o d erate le n g th.1 W ith th e d ev e lo p m en t of telephone re p e a te rs an d th e ex­

ten sio n of c ircu its to tra n sc o n tin e n ta l len g th som e tw enty-five y ears ago, th e re arose need n o t o n ly for loading coils, b u t also for n etw o rk coils, w hich w ould h a v e high s ta b ility w ith tim e, te m p e ra tu re a n d accid en tal m ag n e tiz atio n . M ag n etic s ta b ility w as a t first secured 2 by em ploying iron w ire cores p ro v id e d w ith several air gaps. L a te r, com m ercial a n d technical co nsiderations led to a core stru c tu re m ad e from com pressed in su lated pow dered m a te ria l, first electro ly tic iron 3 a n d la te r perm allo y p o w d er.4 T h is ty p e of core is m echanically s ta b le ; it in tio d u c e s in a n evenly d is trib u te d fashion th e requisite air-gaps, w hile av o id in g u ndesirable leakage field s; a n d it sub-divides th e m ag n e tic m a te ria l so as to reduce e d d y -c u rre n t losses. A lthough o th e r m ean s h av e been su g g este d,5,6 no w ay h a s y e t been devised w hich provides th ese fea tu res so well and a t so low a co st as th e com pressed pow dered ty p e of core.

L oading coil cores m ade from electro ly tic iron pow der generally satisfied th e s ta b ility req u irem en ts for long lines, b u t on ac c o u n t of th e ir low m ag n etic p erm eab ility th e y were large a n d costly. T h e search for m ateria ls w ith higher p erm eab ility a n d lower h y steresis loss

* Presented at W inter C onvention of A .I.E .E ., N ew York, N . Y ., January 22-26, 1940.

In t r o d u c t io n

385

led to perm allo y 7 w hich, b y 1925, h a d been p ro d u ced in pow dered

M O L Y B D E N U M P E R M A L L O Y FOR I N D U C T A N C E COI LS 387 alloy is m a n u fa c tu re d com m ercially by th e W estern E lectric C o m p an y for use in loading coils a n d filter coils.

Ph y s i c a l a n d Ma g n e t i c Ch a r a c t e r i s t i c s

T h e raw m a te ria ls an d necessary e m b rittlin g a g e n ts 11 are m e lte d to g e th e r a n d c a st in to in g o ts w hich are rolled to develop th e desired grain s tru c tu re . T h e d e n sity of th is alloy is 8.65 g m /cm 3. T h e b r ittle m aterial is pulverized to th e desired fineness an d finally an n ealed to soften th e alloy p a rtic le s before in su latio n an d pressing in to core form .

T h e d is trib u tio n b y w eight of th e p a rticle sizes of a sam ple of 120- m esh pow der is given in Fig. 1, show ing a ro o t m ean sq u are size of 50

Fig. 1— Distribution of particle size of 120-mesh powder, by weight.

m icro n s.12 Since th e effective resistan ce of a coil d u e to e d d y -c u rre n t losses in its core is p ro p o rtio n al to th e m ean sq u are p article d ia m e te r,13 it can be decreased w hen desired b y th e use of m ore finely pulverized m aterial.

T h e problem of in su latin g 2-81 m olybd en u m -p erm allo y pow der is to co a t th e p articles w ith a m in im u m thick n ess of a m a te ria l w hich will n o t b re a k aw ay d u rin g th e pressing o p e ratio n , w hich will n o t fuse and flux th e m a g n etic p articles to g e th e r d u rin g th e core h e a t tre a tm e n t, w hich will p re v e n t th e flow of e d d y c u rre n ts betw een m etallic p articles, a n d w hich will be chem ically in e rt th ro u g h o u t th e lifetim e of th e m ag n etic core. T h e difficulty of th e problem will be a p p re c ia ted from th e fa c t t h a t th e se p a ra tio n betw een a d ja c e n t p article s of a core of 125 p e rm e a b ility is a p p ro x im ate ly equal to th e w ave-length of visible

lig h t (0.5 m icro n ). T h is th ic k n e ss of in su la tin g film m a y be show n to

M O L Y B D E N U M P E R M A L L O Y FOR I N D U C T A N C E COI LS 389

less p e rm a n e n t sh ift of p e rm e a b ility d u e to a c c id e n tal stro n g m a g n e ti­

z atio n . S uch v a ria tio n s h a v e re c e n tly been overcom e to a degree in co n tin u o u s cores m ad e of h a rd rolled nickel-iron alloy s h e e t,6 b u t th e y h av e been found to be r a th e r large im m ed iately a fte r stro n g m a g n e tiz a ­ tio n , decreasing slow ly to to le ra b le lim its o n ly a fte r tw o o r th re e d a y s.

W ith com pressed p o w dered m o ly b d e n u m -p erm allo y cores, p e rm e a ­ b ility sh ift d u e to stro n g m a g n e tiz a tio n is re m a rk a b ly sm all even w ith in a fra c tio n of a m in u te a fte r th e m ag n e tiz a tio n is released, an d a n y

Fig. 3— Effect of core forming pressure on density and tensile strength.

fu rth e r d rift of p erm e a b ility w ith tim e is negligible. In ty p ical cores of th e new m a te ria l, th e sh ift in p e rm e a b ility a fte r stro n g m a g n e tiz a ­ tion is less th a n 0.2 per c e n t for cores of p e rm e a b ility 125, a n d less th a n 0.05 per c e n t for cores of p erm e a b ility 14. F ig u re 4 show s th e residual effect of th e ap p lic a tio n a n d rem oval of v a rio u s m a g n e tizin g forces on cores of b o th th ese p erm eab ilities.

W h en a d ire c t c u rre n t is su p erp o sed on an a lte rn a tin g c u rre n t in th e w indings of a coil, th e in d u c ta n c e is a lte re d because th e m a g n e tic field se t u p b y th e d ire c t c u rre n t m odifies th e core p e rm e a b ility . F ig u re 5 show s th e effect of su p erp o sed d-c. fields on th e p e rm e a b ility of 2-81 m o ly b d e n u m -p erm allo y pow der cores o f v a rio u s p erm eab ilities.

A fu rth e r im p o rta n t core p ro p e rty is th e c o n s ta n c y of p e rm e a b ility w ith resp ect to flux d e n sity B . T h is is of p a rtic u la r im p o rta n c e in

M O L Y B D E N U M P E R M A L L O Y FOR I N D U C T A N C E COILS 391

DC MAGNETIZING FORCE IN OERSTEDS

Fig. 4— Residual effect of d-c. magnetization on initial permeability—-measured three m inutes after release of direct current.

precision filters, to insure t h a t changes in transm ission level do n o t produce serious a lte ra tio n s in th e frequency d iscrim in atio n c h a ra c te r­

istics. F igure 6 show s th e su p erio rity of th e new m aterial over th e earlier perm alloy.

A new req u irem en t for cores has been in tro d u ced b y q u a rtz cry stal filters used in w ide-band carrier system s. In o rd er to secure th e neces­

sary precision in th is ty p e of filter, m easures h av e to be ta k e n to p re ­ v e n t d e p a rtu re s from th e initial frequency a d ju s tm e n t due to changes in core perm eab ility o rd in arily occurring w ith room te m p e ra tu re changes. E x trem ely sm all te m p e ra tu re coefficients of p erm eab ility have now been achieved b y ad d in g to th e new 2-81 m o lybdenum - perm alloy pow der a v e ry sm all p ercen ta g e of special perm allo y pow der having a m olybdenum c o n te n t of a b o u t 12 per cent. Such an alloy has a non-m agnetic or C urie p o in t close to room te m p e ra tu re, an d for a

INITIAL PERMEABILITY

Fig. 5— Effect of superposed magnetization on permeability.

sm all te m p e ra tu re ran g e ju s t below its C urie p o in t, it h a s a n e g a tiv e te m p e ra tu re coefficient several h u n d re d tim es as large as th e p o sitiv e coefficient of 2-81 m o ly b d en u m -p erm allo y . B y choosing su ita b le c o m p o sitio n s a n d p e rcen tag es of such co m p e n sa tin g alloys, th e n e t te m p e ra tu re coefficient of p e rm e a b ility of a core can be a d ju s te d to a n y reaso n ab le value, p o sitiv e o r n e g a tiv e, o v er a desired te m p e ra tu re range. F ig u re 7 show s a p e rm e a b ility vs. te m p e ra tu re c u rv e for a

FLUX DENSITY IN GAUSSES Fig. 6— Perm eability-induction characteristics.

core stab ilized to give a sm all n eg a tiv e coefficient, c o m p a re d to a sim ilar cu rv e for a core n o t stabilized.

Co r e Lo s s e s

T h e d e sira b ility of core m a te ria ls increases in g eneral as th e ir loss c h a ra c te ristic s decrease. Low to ta l e d d y -c u rre n t a n d h y ste re sis losses give low c o n trib u tio n s to a tte n u a tio n . H y ste re sis loss is fre q u e n tly of especial im p o rta n c e because it a p p e a rs fu n d a m e n ta lly as a re sistan ce w hich v aries w ith coil c u rre n t, an d because it in c id e n ta lly g en erates h arm o n ic voltages. A low v alu e of h y steresis loss th u s

M O L Y B D E N U M P E R M A L L O Y FOR I N D U C T A N C E C O ILS 393

2-81 m o ly b d e n u m -p erm allo y in su la te d to several perm eab ilities.

M O L Y B D E N U M P E R M A L L O Y FOR I N D U C T A N C E COILS 395 in m o st resp ects an d hav e been m ade a p p ro x im a te ly 50 p er c e n t sm aller in volum e b y using a m olyb d en u m -p erm allo y core w ith a n om inal p e rm eab ility of 125. T a b le I I sum m arizes d a ta co m p arin g

T A B L E II

Co m p a r a t iv e Si z e a n d We i g h t Da t a o f Ty p i c a l Ne w a n d Su p e r s e d e d Co i l s

Type of Coil Type of Compressed

Powdered Core Inductance

(Henrys)

Coil Volume (Cu. In.)

WeightCoil (Lbs.)

Sm all E xchange Area Perm alloy 0.088 2.5 0.4

«< M olybdenum Permalloy 0.088 1.5 0.2

Program Circuit Permalloy 0.022 11.8 1.6

i 4 M olybdenum Perm alloy 0.022 4.4 0.6

Toll-Side'C ircuit Permalloy 0.088 13.5 1.7

It M olybdenum Permalloy 0.088 5.1 0.7

th e electrical c h aracteristics, sizes, an d w eights of coils com m only used on exchange a n d toll cables. F ig u re 8 show s th e im p ro v ed

O 0 .5 1.0 1.5 2 .0 2 .5 3 .0 3 .5 4 .0 4 .5 5 .0 5 .5 6 .0 6 .5 7.0 7.5 8 .0 FREQ U EN CY IN KILOCYCLES PER SECOND

, EXCHANGE-AREA

' COI LS

0 .0 8 8 HENRY 0 .0 0 1 A M PER E

TOLL COILS 0 . 0 8 8 HENRY 0 .0 0 2 AMPERE

PROGRAM COILS 0 . 0 2 2 HENRY 0 .0 0 2 AM PERE

Fig. 8— Effective resistance-frequency characteristics of typical loading coils.

re sistan c e-fre q u en cy c h a ra c te ristic s for ty p ical coils. F ig u re 9 show s th e im p ro v e d te le g ra p h flu tte r 18 c h a ra c te ristic s of a co m m o n ly used toll ty p e coil.

T E LEG R A PH C U R R EN T IN M ILLIA M PER ES

Fig. 9— Flutter characteristics of typical toll loading c o il; P— with permalloy core, M P — w ith m olybdenum -perm alloy core.

F igure 10 p ic tu res th e re d u ctio n in size of cores a n d coils w hich are com m only used in toll an d exchange a re a c ircu its. In th e p re p a ra tio n for com m ercial m a n u fa c tu re of th e sm allest of th e new coils, a difficult problem in th e d e v e lo p m en t of w in d in g m a c h in e ry w as in v o lv ed because of th e sm all dim ensions of th e hole in th e finished coil. T h is problem has been successfully solved b y th e W e ste rn E le c tric C o m p a n y ,

M O L Y B D E N U M P E R M A L L O Y FOR I N D U C T A N C E COILS 397

A side from m a n u fa c tu rin g econom ies, th e re d u ctio n in coil size is of

M O L Y B D E N U M P E R M A L L O Y FOR I N D U C T A N C E COILS 399

o b ta in p ro p e r se p a ra tio n of th e frequency b a n d s of th e v ario u s c h a n ­ nels or to insure su itab le transm ission p ro p erties of th e individual channels. W hile th ese n etw o rk s involve coils, condensers a n d cry stals, it is fre q u en tly th e case t h a t th e ir size, co st a n d perfo rm an ce are d e ­ term in ed chiefly b y th e q u a lity fa c to r Q of th e in d u c ta n c e coils. T h is follows from th e fa c t t h a t Q values of coils are u su ally co nsiderably

Fig. 11— N ew and superseded cases containing 200 exchange area coils.

lower th a n th o se o b ta in ab le read ily in condensers a n d cry stals. Ac­

cordingly, it is v e ry desirable to hav e as high a v alu e of Q as possible econom ically. In a d d itio n , such coils m u st h av e low hysteresis resistance to lim it m o d u latio n , a n d a low te m p e ra tu re coefficient of in d u ctan ce to secure s ta b ility of a tte n u a tio n or im pedance ch a ra c ­ teristics of th e filters an d netw orks.

D ue to th e im p ro v em e n ts in th ese respects, m o lybdenum -perm alloy core coils can be used q u ite extensively in new ty p es of carrier

tele-ph o n e system s. In such sy stem s for existing lines a n d cables, as well a s for p ro je c te d new ty p e s of cables, th o se filters are of k ey im ­ p o rta n c e w hich s e p a ra te in d iv id u al m essage ch an n els in th e freq u en cy ran g e from 3 to 108 kc. B y using coils of p ow dered m o ly b d en u m - p erm allo y in su la te d to perm eab ilities of 14 or 26, v a lu a b le econom ies in space a n d c o st of filters a re realize d.21’22 F ig u re 14 show s a ty p ic a l coil em ploying a 14 p erm e a b ility core designed for use in one of th ese ch an n el filters h a v in g its tra n s m itte d b a n d in th e v ic in ity of 108 kc,

Fig. 12— N ew and superseded cases containing six program loading coils.

to g e th e r w ith a shielded solenoidal air core coil w hich m ig h t be e m ­ ployed for th e sam e p urpose. T h e m o ly b d en u m -p e rm allo y coil h as a Q a t 100 kc a b o u t tw ice t h a t of th e air core coil, y e t it occupies a p ­ p ro x im a te ly 1/10 as m u ch space. T h e th ird o rd er m o d u la tio n p ro d ­ u c ts are a p p ro x im a te ly 80 d b below th e level of th e n o rm al ch an n el c u rre n ts. T h is is considered to be to lera b le from th e s ta n d p o in t of in te rc h a n n e l c ro ssta lk on circ u its used for o ne-w ay tra n sm issio n . A n in d u c ta n c e -te m p e ra tu re coefficient of a b o u t —20 X 10-6 p e r d e ­ gree F. h a s been chosen to c o m p en sate for th e p o sitiv e c a p a c ity - te m p e ra tu re coefficient of asso cia ted condensers.

In F ig. 15 d a ta a re p resen ted illu stra tin g th e (/-frequency c h a ra c ­ teristics t h a t can be o b ta in e d on ty p ical coil designs using th e new m a te ria l in th e frequency ran g e from 300 cycles to 200 kc. T h e ch a ra c teristic s show n ap p ly to coils w ound on th ree sizes of cores t h a t

M O L Y B D E N U M P E R M A L L O Y FOR I N D U C T A N C E C O ILS 401

Fig. 13— Comparative size of equivalent 1939 and 1922 cases.

are su itab le for use in th is range. F o r com parison, sim ilar c h a ra c ­ te ristics are also included for coils using cores of equal size b u t m ad e of perm alloy a n d e lectro ly tic iron pow der. T hese d a ta include all effects on Q resu ltin g from w inding capacities an d losses, w hich have

been m ad e to le ra b ly sm all b y s u ita b le choice of in su la tin g m a te ria ls, s tra n d in g of c o n d u c to r a n d co n fig u ra tio n of w inding.

Co n c l u s i o n

C om pressed pow dered cores of 2-81 m o ly b d e n u m -p erm allo y h a v e p ro p e rtie s w hich are su p erio r to th o se of earlier p ow dered cores in re sp e c t to p e rm e a b ility range, h y steresis loss a n d e d d y c u rre n t loss.

B ecause of th e low er losses a n d g re a te r p e rm e a b ility ran g e, in d u c ta n c e coils are now possible w hich h a v e g re a tly increased Q v alu es for a given v olum e. B ecause of th e low h y ste resis losses an d a tte n d a n t low ering

Fig. 14— Com parative size of non-m agnetic core and m olybdenum permalloy core filter coils.

of m o d u latio n effects to to lerab le levels, m ag n etic core in d u c ta n c e coils can now be em ployed w here n o n -m ag n e tic core coils h a v e p re ­ v io u sly been necessary, w ith a v e ry g re a t increase in Q v a lu es for a given volum e. T e m p e ra tu re coefficients c an now be o b ta in e d w hich are equal to th e te m p e ra tu re coefficients of o th e r high g ra d e electrical e lem en ts such as m ica condensers a n d q u a rtz c ry sta ls. M oreover, th e a b ility to m ak e th e te m p e ra tu re coefficient of coils n e g a tiv e or p o sitiv e a t will p e rm its th e a tta in m e n t of re m a rk a b le s ta b ility in re s o n a n t co m b in atio n s of coils a n d condensers. T w o im p o r ta n t a p p lic a tio n s h a v e been m ade in th e field of c o m m u n ic a tio n a p p a ra tu s . F o r voice- freq u en cy circu its, new loading coils of im p ro v ed q u a lity a n d red u ced

M O L Y B D E N U M P E R M A L L O Y FOR I N D U C T A N C E C OI LS 403

a3

US

"c5

o>

a£ Uo

mbf) E

QUALITY FACTOR, Q

size h av e been s ta n d a rd iz e d . F o r b ro a d b a n d c a rrie r sy stem s, th e

M O L Y B D E N U M P E R M A L L O Y FOR I N D U C T A N C E COILS 405

W hen th e core p e rm eab ility p is red u ced b y d ilu tio n of a given m a te ria l, th e hysteresis a n d residual loss coefficients a a n d c v a r y so as to m ak e th e p ro d u c ts pc = k\ a n d pa = k2 ap p ro x im a te ly c o n sta n t, a s m a y be seen b y reference to T a b le I. T h e core loss resistan ce in o hm s a t freq u en cy / c y c l e s p e r second m a y th erefo re be expressed w ith reasonable ac cu racy as

(

)

T h e coil quality7 fa c to r is th u s

( 7 ) Q = W - Ï t !

R< + R m 19pf X 109

spd-Case I: If th e v alu e of mis fixed a n d da n d H can be freely chosen, it is desirable to know th e v alu e of p. w hich will yield th e h ig h est possible v a lu e of Q. B y s u b s titu tin g in (7) th e v alu e of d2o b ta in ed from (5) a n d s e ttin g th e d e riv a tiv e w ith re sp e c t to p eq u al to zero, th e following is o b ta in e d for th e o p tim u m p e rm e a b ility :

, , 52.5 X 1016Pc3w4 _

( ) ^ ' e?s^pkx*I*Ls

T h e corresponding valu es of da n d R c can be o b ta in e d from eq u atio n s (5) a n d (2). T h e corresp o n d in g v alu e of Q, w hich is th e g re a te st o b ta in a b le u n d e r th e se co n d itio n s is

(9) =

5rmr

,¿

, ,

— + 2 + 5 “ ef

If a sm aller v alu e of Q th a n t h a t o b ta in e d from (9) is accep tab le, e q u atio n s (7) a n d (5) c an be solved sim u lta n eo u sly for d a n d p. A sm aller value of p th a n t h a t o b ta in e d from (8) an d a correspondingly sm aller v alu e of d will re su lt.

Case I I : If m o d u la tio n is u n im p o rta n t a n d th e h y steresis loss re ­ sistan ce is negligible in com parison w ith o th e r c o m p o n en t losses, th e n d a n d p can be selected w ith o u t reg ard to m o d u la tio n . E q u a tio n (7) can be d iffere n tia ted directly7 a n d solved for th e p e rm e a b ility re q u ired to yield th e m ax im u m v alu e of Q. T h is o p tim u m p e rm e a b ility is

(1 0) 19pe X 109

> ^-

sei‘r

T h e co rresp o n d in g v a lu e of Q is

4. “ Compressed Powdered Perm alloy M anufacture and M agnetic Properties,”

W. J. Shackelton and I. G. Barber, T rans. A .I .E .E ., v. 47, 1928, p. 429.

18. “ H ysteresis Effects with V arying Superposed M agnetizing Forces,” W . Fondiller and W. H . M artin, T rans. A .I . E .E ., v. 40, 1921, p. 553.