Journal of the Royal Naval Scientific Service. Volume 27, Number 2 ...

Materials Development: Conde and Godfrey i:
L
FIG. 4. 3250 HP 200 RPM superconducting motor—
1969 with 50 HP "Model" motor—1966 and
super-conductor/copper strip winding.
(by courtesy IRD Co. Limited!
TABLE 8.
Comparative Costs of Cryogenic Materials.
Material
Approximate
order o/ Cost
£/ton
3}% nickel steel 110 to 150
9% 240 to 300
Aluminium and Al alloys 410 to 570
Stainless steels 430 to 500
36% nickel iron 800 to 900
Copper and copper alloys 900 to 1400
Nickel base alloy 718 ~ 4,500
Titanium or Ti alloy ~ 10,000
thick are also under consideration. The low
expansion characteristics of 36% nickel-iron
(Invar) make it attractive for membrane liners.
The high cost (c £800/£900/ton) of Invar also
favours efficient usage.
Satisfactory solders and brazing alloys are
available for joining cryogenic materials. Soft
solders should contain at least 80% lead for
temperatures down to — 196°C and 90% lead
for lower temperatures. Brazing fillers and
silver based solders retain excellent ductility at
low temperatures but the lead and phosphorous
content of the former should be the minimum
if impact resistance is to be unimpaired. In
general cast materials and weld metal show
similar behaviour to comparable wrought
materiails in Table 4 but the coarser grain size
of the cast structure tends to give reduced
impact values. Most of the common welding
processes are satisfactory but filler materials
of different composition to the parent material
or of greater alloy content may be required if
adequate toughness combined with 100 v joint
efficiency is to be achieved. For example the
9% nickel steel and Invar are usually welded
with nickel-chromium, nickel-chromium-iron,
or complex austenitic stainless steel filler. Cast
or welded niobium stabilised austenitic stainless
steels are not favoured for cryogenic applications
since even with carbon limited to 0 5
and niobium to 05% impact properties are
inferior to those of the unstabilised types.
304L and 316L.
Very pure metals show a dramatic reduction
in electrical resistivity at temperatures
approaching 0°K. The ratio of the resistivity
of HC copper at 293°K to that at 42°K may
be 300 to 1 and in commercial high purity
copper about 1000 to 1. Ultra pure copper may
show a ratio as high as 50,000 to 1. Aluminium
shows similar behaviour and these two
materials offer potential as highly efficient
cryo-conductors. Other materials are super
conducting below some critical temperature,
T,., which is in the range of 0-01° to 915° K
for super conducting pure metals and up to
20 7° K for superconducting alloys and intermetallic
compounds. The theory and behaviour
of superconductors are beyond the scope of
this article and it is sufficient to note' 2 " that
such materials are being considered for highly
efficient transmission lines and have already
been used in powerful magnets and superconducting
electrical motors or generators. The
IRD Co. Limited 3250 h.p. disc type homopolar
motor is the most advanced at present.
This machine employs niobium-titanium super-