The Nonlinear Library LW - cold aluminum for medicine by bhauth
Dec 17, 2023
06:47
Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: cold aluminum for medicine, published by bhauth on December 17, 2023 on LessWrong.
cold aluminum
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
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.
some blog
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...
cold aluminum
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
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.
some blog
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...