=materials =industrial design =medicine
Very pure aluminum at the boiling
point of hydrogen is a very cool material. At 20K, 99.999% pure aluminum has
1000x the electrical conductivity it has at 0 C. At 4K, it has maybe 4000x.
At such low temperatures, electron free paths in aluminum become
macroscopic, which is why even small amounts of impurities greatly increase
resistance. Even wire diameter has a noticeable effect. Magnetic fields can
also increase resistance, but this is also a purity-dependent effect: 99.999%
aluminum might have 3x the resistance at 15T, but even purer aluminum is
much less affected.
Yes, aluminum purification costs some money, but
it's not particularly expensive. It might cost 3x as much as standard
aluminum, but it's far cheaper than superconductors. Another point to note
is that superconductors only have 0 resistance for constant current.
Cryogenic aluminum isn't affected by current changes like superconductors
are.
It seems like that sort of interesting effect that massively
increases a figure of merit should have some sort of application, don't you
think? Yet, while there are superconducting electric motors for ships, I'm
not aware of cryogenic aluminum conductors being used for any commercial
applications. What could it be used for?
power lines
An obvious application
for low-resistance conductors is long-distance power transmission. My
estimations indicated that using cryogenic aluminum for that is somewhat too
expensive, because the (cryocooler cost)*(insulation cost) product is too
high for reasonable line currents. Connecting it to ambient-temperature
lines is also an issue, because cold pure aluminum also has high thermal
conductivity.
As temperatures decrease, resistance
decreases, but cryocoolers become more expensive and less efficient. In
general, liquid hydrogen seems better than liquid helium or liquid nitrogen
for cryogenic aluminum conductors.
At such temperatures, it's worth
using multilayer vacuum
insulation. That's
far more effective than typical insulation like fiberglass or polyester, but
it still doesn't seem good enough to make insulation + cryocoolers
sufficiently cheap for large underground power lines.
While the
economics don't work out, it is possible to use cryogenic aluminum for
high-power electricity transmission. It's merely expensive, not unfeasible.
Feel free to use that for flavor in hard SF stories.
What are some
attributes of applications that make cryogenic aluminum more suitable?
- Large
currents per surface area.
- Superconductors would be used but resistance
from changing current is a problem.
- Low weight is important.
-
Cooling at low temperatures is easily available.
One application that's been proposed is electric motors with cryogenic aluminum conductors in aircraft fueled by liquid hydrogen, which would provide free cooling for the aluminum. Obviously, such aircraft don't currently exist, and I don't think they're very practical, but that's beyond the scope of this post.
MRI machines
So, the only good application for cryogenic aluminum that comes to mind is MRI machines. Yes, it would be hard for a new company or new technology to enter that market at this point, but there are some theoretical advantages that cryogenic aluminum could have over superconductors.
You've probably heard that MRI
scans are expensive because the machines are expensive, but they're ~5x more
expensive in the USA than in Mexico. You might then think they're expensive
because of labor requirements, but the Netherlands has among the lowest
prices for MRI scans.
In any case, yes, the machines are somewhat
expensive.
Here
are some approximate machine prices. Supposing a $400k machine is used for
10 people a day with 5 year amortization, that's $22/use. Considering
typical price multipliers for US healthcare, you can see how that could
become expensive...?
Some of that cost is for superconducting magnetic
coils. Is there some way to potentially reduce that expense by using cold
aluminum instead, or perhaps improve MRI performance somehow?
A typical approach in MRI
machines is to have 2 superconducting coils (with constant current) to make
an approximately homogeneous field, and use copper coils to create a (much
smaller) magnetic field gradient that changes at perhaps 2 kHz. NMR
precession frequency depends on field strength, so this field gradient can
be used to localize emissions to a particular slice.
With aluminum,
the same coils could theoretically be used for both purposes. How expensive
would replacing superconductors with aluminum be? I'm estimating that would
involve:
- multiple tons of pure aluminum
-
>1 kW of
resistive loss
-
cryocooler energy usage comparable to current MRI energy use
- (cryocooler + aluminum) cost comparable to total cost of comparable
current MRI machines
So, cold aluminum is usable for MRI machines,
but not strictly better than superconductors. How about high-temperature
superconductors? People have of course considered that, but they're really
not as good a
choice as many
people seem to think.
Cold aluminum could also be used together with
superconductors for gradients. Is that better than using copper?
A
larger field gradient allows better localization between slices. This can
improve resolution (with higher field strength) or scan speed (with higher
repetition rate). Both those things also obviously add to other costs. The
slew rate (field change / time) is limited by induced current triggering
longer nerves, but only the most expensive MRI machines are close to that
limit.
The cost of cryocoolers and pure aluminum for making the
highest gradients seen in MRI machines today actually seems very reasonable.
Using cold aluminum instead of copper could potentially reduce their size,
weight, and even cost. It could allow larger maximum field gradients in a
given volume; alternatively, it could allow having some open space between
coils, which could make scans less unpleasant.
There's a reason I
think very high field gradients could become more important for MRI machines
in the future: simultaneous multi-slice imaging. In the past, higher
repetition frequency was favored over that because of the higher
computational requirements for MRI reconstruction, but today that's not a
problem; that calculation is less demanding for a GPU than running Starfield
or Cities Skylines 2. High-gradient multi-slice imaging could potentially
make scans several times faster at the same quality. That not only increases
machine throughput and reduces labor costs, it also improves patient
experiences and makes real-time performance better.
Using cryogenic
high-purity aluminum to make large field gradients in MRI machines using
simultaneous multi-slice imaging not only seems to give better
cost-performance — it's also, finally, a real application for cryogenic
high-purity aluminum.