November, 1948
Electronic Engineering
351
Magnetic Materials
By G. FitzGerald-Lee, F.R.S.A.
NTIL the year 1900 the only improved on by one containing 0.6 malloy is effected in vacuo in an
carbon, 35 cobalt, 8 tungsten and induction furnace and the alloys are
2.5 chromium. Later still a further poured in helium or nitrogen, at
improvement was made by reducing atmospheric pressure.
The physical reasons for the
the cobalt content to 25 per cent.
4,500, and is still used for pole and adding 20 nickel and 15 success of Supermalloy are not yet
pieces and relays. All other mag- titanium. This last steel is very definitely known. It is believed that
netic materials, with initial per- similar to the Japanese Honda steel, the presence of certain impurities
meabilities ranging up to 100,000, which has 7 per cent. less nickel. or combination of impurities, such
have been discovered and developed Honda was a development of two as are usually found in commercial
during the last 47 years.
other Japanese magnet steels, alloys, prevents the attainment of
Low -carbon steel, with 250 i.p. Mishima A and B, both with about high permeability; and that a
and 2,500 m.p., is used for the fields 26 per cent, nickel and 12 alumi- definite cooling rate must be used,
and frames of D.C. and synchronous nium, but the B having 8 cobalt as below the temperature at which
atomic ordering begins, or the specimachines; annealed cast steel (i.p.: well.
175; m.p. :1,500) for frames and
Nickel containing 21.5 per cent. men should be held for a definite
solid poles; and annealed cast iron iron is unusually susceptible to time at a temperature of about
(i.p. :125; m.p. :500) for frames. magnetism of low intensity, and 450° C.
It appears that when
Early in the century Sir Robert when it was embodied in the trans- a critical amount of ordering is
Hadfield evolved silicon -iron, which atlantic cable the time of trans- present, the magnetostriction and
is an, iron or low -carbon steel to mission was five times quicker than the magnetic crystal anisotropy
both tend to disappeir at the same
which up to 4 per cent. silicon has it had been previously.
been added; it has low magnetic
The most
highly
magnetic time in the alloy of proper composihysteresis, and one type, with 4 per material known for many years, tion, and that high permeability
cent. silicon, 0.05 carbon, 0.8 man- eventually superseded by Nifal, was then occurs in the polycrystalline
ki useful magnetic material known
was pure iron, which has an initial
permeability (i.p.) of 275 and a
maximum permeability (m.p.) of
ganese and 0.02 each of phosphorus a steel containing 0.9 per cent.
and sulphur, is commonly used in carbon, 35 cobalt, 5 tungsten and 2
sheets for transformer cores.
chromium. "Permalloy," originally
For stampings in transformers and a trade name, now appears to be
high -efficiency rotating machines a generic term for high -nickel iron
material.
The latest materials for permanent
magnets are sintered Alnico, Alcomax and Hycomax.* Cast Alnico
magnets, which have already been
alloys having high magnetic per- mentioned, have to be ground to
evolved, such as Lohys and Stalloy. meability and low hysteresis loss, size after heat treatment, as the
Fig. 1 shows the B -H curve of Lohys and which contain other elements alloy is too hard to be rolled or
low -loss
silicon
steels have been
(B = lines per square centimetre; H
= 1.256 X ampere turns per centimetre) and several other metals
mentioned here. Hipernik, a 50 per
cent. nickel steel (i.p. :6,000; m.p.:
90,000), is used in audio -frequency
and instrument transformers.
The principal requirements of
steels for permanent magnets is that
they shall have high remanence
(retentivity) and coercive force;
they often contain up to 35 per cent.
cobalt or 10 tungsten.
Nifal
(ni-f(e)-al) and Alnico (al-ni-co) are
examples of modern permanent
magnet alloys. Nifal has enabled
great improvements to be made in
aircraft instruments and reduction
as
copper, molybdenum, machined by normal methods; and,
chromium, cobalt and manganese. further, it is often impossible to pro-
such
It usually has about 78 per cent. vide for small holes and accurate
nickel, and is widely used in tele- bores in such castings. These diffi-
culties have now been overcome by
producing the magnets by the
Mu -metal, of the permalloy type, powder metallurgy process of sintering, whereby the constituents of the
contains copper and manganese.
A new magnetic material is known alloy are first reduced to powder
phone equipment (i.p. :9,000; m.p. :
100,000).
and then mixed and moulded together under heat and pressure to
the exact shape and size required.
The resultant alloy is chemically
and sulphur are much lower than in and physically identical with the
Most commercial alloys. The initial cast product except that it has very
permeability of Supermalloy, which much greater mechanical strength.
was discovered in 1943 and fully The interesting differences between
as "Supermalloy "; this has about
79 per cent. of nickel, 15 iron, 5
molybdenum and 0.5 manganese,
impurities such as carbon, silicon
developed this year, is over 100,000. the magnetic properties of cast and
in the size of magnetos and cycle In the form of 0.001 in. insulated sintered Alnico, Alcoraax and Hycodynamos. Cast Alnico, with 54 per tape the i.p. is about 90,000 as com- max are shown in Table 3, and
cent. of iron, 18 nickel, 12 cobalt, pared with about 15,000 for molyb- Fig. 3 shows sintered Alnico com-
denum-permalloy. The use of Superin magnetic chucks for turning and malloy in communication transgrinding; it gives 30 Der cent. more formers has been found to allow a
energy per unit volume than cobalt threefold increase in the range of
steel. One of the earliest magnet frequencies transmitted, and a pulse
steels contained simply 0.9 per cent. duration three times that previously
carbon with 8 cobalt. This was obtainable. The melting of Super10 aluminium and 6 copper,,. is used
pared with various magnetic steels,
each mass of alloy having the same
magnetic strength as the others.
A recent British Patent Specification (No. 583,411) relates to a pro-
7KT.& last two materials. which are anisotropie,
are manufactured on a commercial scale only in
Britain by Murex Lfd., Itainham, Essex.
352
Fig. I.
November, 1948
Electronic Engineering
B -H Curves for various magnetic
20
materials
(a) From H=0-1.2,
B=0-250
(b) From H=1.2-160, B=250-9,000
(c) From H=I60-1,000, B=9,000-20,000
18
Reference
a. Permalloy
b. Hipernik
c. Pure Iron
e. Cast steel, annealed
iron, annealed
g. Lohys.
Carbon annealed steel
d. Low
1. Cast
16
dg
C
ab
14
6
12
Rx 03
600
4
b
2
800
1000
cess of making an anisotropic permanent magnet in which an iron base alloy containing about 9 per
cent. aluminium, 25 nickel and 21
cobalt is subjected to the action
of a magnetic field while cooling
from
25
50
75
100
125
150
f
10 per cent. The cooling in the magnetic field is effected at an average
200
rate of 0.5-15° C. per second, the
maximum cooling rate of the guar ternary alloy of iron, aluminium,
6
L
150
solution temperature of
may contain up to 5 per cent. of
copper, titanium or silicon, or up to
1 per cent. zirconium, or any two
or more of these elements un to
250
rl
a
1,240° C. down to 650° C., and a
precipitation or ageing heat -treatment is applied to the alloy before
it is finally magnetised. The alloy
nickel
and cobalt being 10° per
second with 21 per cent. nickel and
15° with 30 nickel. If silicon is
present the cooling rate is less than
10° per second, and is slower still
as the silicon content is increased;
with zirconium present the cooling
2
Qc,
rate should not exceed 1° per second.
100
Table 1 shows the coefficients of
magnetic permeability and susceptibility of certain materials, those in
a
the first column being established
by dividing' the magnetic induction
produced by the magnetising force;
and in the second column by dividing the magnetic intensity by the
5
magnetising force.
41
2
(.4rr
4
-npere-turn
6
)er
CM
8
1.0
1.2
Table 2 gives the hysteretic contants, or Steinmetz coefficients, for
various materials.
November, 1948
TABLE I.-COEFFICIENTS OF MAGNETIC
PROPERTIES
Material
very
Annealed
Perm.
.
3,080
2,590
280
245
200
beneficial effect of purification of the
material with respect to non-metallic
substances such as carbon and oxygen was used to considerable advantage. It will be interesting during
25
10
439
...
manganese steel
280
18
170
1.4
11
Fig. 2 graphically recapitulates
the development of magnetic
materials from iron to Supermalloy;
in Neumann's " 1040 " alloy the
37
Glass hard piano wire
Annealed Norway iron
100
Susc.
soft iron
wire
...
Annealed soft iron wire ...
Moderately soft iron wire
Annealed steel wire
Hard -drawn steel wire
Cobalt
Hadfield
353
Electronic Engineering
the next few years to see whether
this progress can be maintained.
TABLE 11.-HYSTERETIC CONSTANTS
Material
Hys. Con.
Very soft iron wire
Very thin soft sheet iron ...
Thin good sheet iron
Thick sheet iron ...
Ordinary sheet iron
Transformer cores ...
Soft annealed cast steel
Soft machine steel
...
Cast steel
Cast iron
Hardened cast steel
0.002
0.0024
0.003
0.0033
0.004
0.005
0.038
0.0094
0.012
0.016
0.025
...
TABLE III.-COMPARATIVE MAGNETIC PROPERTIES
103
Br. in
Coercivity
Hc, in
gauss
oersteds
product
BH max.
C
7100-7900
580-480
I.4 -1.8x 106
S
6400-7700
550-450
6300-7200
660-550
1.4-1.8x 108
Remanence
Alloy
90
BO
Standard
"Alnico "
70
High
Coercive
60
Fig. 2.
Sixty years'
development in
50
magnetic
permeability.
C
-
Energy
.
1.4-1.66 x 106
" Alnico "
S
5800-6400
640-590
1.4-1.66 x 106
High
C
8000-8800
420-320
1.3-1.7 x 106
S
7300-8000
450-350
1.25-1.45 x 106
C
12700
570
4.3 x 106
S
11200
560
3.3 x 106
C
8500
790
2.7 x 106
7600-8200
820-760
2.4-2.8 x 106
Remanence
40
" Alnico "
30
" Alcomax 11 "
20
k'
" Hycomax "
10
/9/4
IRON
--
1900
1690
/9
7
1952
/934
S
SI. STEEL
1910
1920
1930
1940
1950
C = Cast
S = Sintered
Power Valve Protective C ircuit
is common practice to protect
ITthe
grid or anode circuits of
respect to A. A resistor R3 is current but not the grid current,
arranged in the cathode circuit of the current flowing in such a direr.
power valves with relays, whereby the valve so that it carries the anode tion that the potential of D is below
that of the point C. The values of
a change in current in either of
R3 and 112 are in the same ratio as
these circuits causes the anode
the maximum permissible ratio
supply to be cut off. This method,
between the grid and anode curhowever, has the disadvantage that
rents. Thus, during normal operathe relay may operate during the
tion the potential of B is not above
adjustment of the valve load or
that of D, but if the grid current
when transients are caused by
falls or if the anode current rises
switching in an associated part of
CIRCUIT
then B becomes less negative than
BREAKER
the circuit.
D, then the diode V1 conducts,
The accompanying diagram shows
allowing relay RI, to open the key
a simple and efficient safety circuit
K. The circuit breaker comes into
that does not suffer from the usual MAINS
operation and removes the anode
disadvantage outlined above, as it
supply from the valve, thus preventoperates only when the ratio of grid
ing damage.
current to anode current falls below a
RL
When the load is being adjusted,
predetermined safe operating value.
or when transients occur, the grid
A triode power valve obtains its bias
and anode currents rise together,
by means of the grid current flowand the circuit continues to operate
ing in the grid resistor 111, and a
normally.
resistor, R2, is placed in series with
-Communicated from the E.M.I.
it so that the grid current places
Laboratories.
point B at a negative potential with