Jump to content
Sign in to follow this  
Enthalpy

Bathyscaphe Float

Recommended Posts

Hello everybody!

Few vehicles have dived deep in the Ocean, with a crew or not.
http://en.wikipedia.org/wiki/Bathyscaphe
Very few: Piccard's ones to the deepest point of 11km around 1960, a handful much later to 5 or 7km, and recently to 11km


The float is technically difficult. A hull that resists the 114MPa water pressure is too heavy to float, unless it uses the best materials in an optimized design.
http://mseas.mit.edu/publications/Theses/Alex_Vaskov_BS_Thesis_MIT2012.pdf
What has worked up to now is:

  • Piccard used a liquid at outer pressure, lighter than water to get buoyancy and lift the heavy crew sphere. Hexane weighs 663kg/m3 without the hull and lifts little, only cryogenic gases would improve.
  • Syntactic foam. Tiny hollow glass spheres load an epoxy to make it lighter. Not very efficient.
  • Nereus uses many hollow spheres of alumina. This ceramic is light, stiff, and strong against compression. The spheres weigh 233kg/m3 hence outperform the previous methods but don't receive universal confidence.

A float with titanium or steel shell is no alternative. If breaking at 1.5 times 114MPa, it lifts nothing - it's a candidate for 6km depth only. Maybe silica gel or some zeolite covered with a metal film would resist the pressure, but not lift much. Very hollow solid molecules exist, I don't know their strength. A hull of graphite fibre composite is widely considered, but to break at 1.5 times 114MPa, it would be heavy.

----------

I propose to help a shell with a gas at high pressure. I suggested it there already
http://unmoissouslesmers.blog.lemonde.fr/2014/10/07/sous-leau-le-temps-setire/#comment-221 (sorry for ze lãnguage)
and meanwhile this idea doesn't seem already common, so here are indications if any necessary.

Carbon prepreg is wound on a 0.5mm titanium liner, like tanks for planes and rockets are made. The composite resists 1141Pa compression and 1712MPa tension (other manufacturers give rather 2400MPa), reduced to the half as winding makes it isotropic. Helium at 67MPa helps the shell at depth (114MPa) but must be contained at the surface.

According to Soave-Redlich-Kwong, helium's density is 82kg/m3 at +4°C and 67MPa
HeliumSoaveRedlichKwong.zip
and at +40°C but with the hull streched by 4.6%vol, helium's pressure reaches 71MPa.

A 31mm thick sphere for arbitrary D=1m resists 71MPa tension and 114-67MPa compression with 1.5 safety factor (more against buckling) and weighs 142kg. Helium adds 35kg, the liner 6kg. The 524dm3 float weighs 349kg/m3.

----------

Without the gas, the carbon composite float would weigh ~689kg/m3, and with full 114MPa gas pressure ~572kg/m3.

Hydrogen is too dangerous, neon leaks less but weighs a bit.

Gas pressure would help a shell of 6-2-4-6 titanium also, but the float would still weigh ~634kg/m3, and buckling demand difficult integral stiffeners. Steel is as good but makes the stiffeners more difficult.

As a child I wanted to put pressure air in boats so they carry more load. Good idea after all.
Marc Schaefer, aka Enthalpy

 

Share this post


Link to post
Share on other sites

Weighing 534kg/m3, lithium would make a nice float, with a liner to separate it from the water. This sounds a bit bizarre, but after all we have already metallic lithium in batteries all around us. If the float consists of many lined elements, a failure in water wouldn't propagate to the whole float, and would result in a fire if near the surface, and have little consequences at depth.

While being lighter than hexane and syntactic foams, lithium is also stiff - much stiffer than water. From 6000m/s sound velocity, the bulk modulus is 19GPa, so 114MPa water pressure shrinks lithium by 0.6%vol or 0.2% in each dimension.

The elements can be spherical, without any void: cold isostatic pressing, radiography. They must receive a perfectly conformal coating, for which metal sputtering or evaporation looks feasible. Over this first coating, the liner can be:

  • Malleable, maybe niobium or tantalum. This would resist a finite but big number of cycles, easily experimented on the ground.
  • Hard, with a yield strength exceeding the 0.2% dimension change. At E=200GPa, both electrodeposited nickel-cobalt and electroless phosphorus nickel have margin. Maybe evaporation or sputtering can also make the desired thickness.

I'd have a protection against mechanical aggressions, especially if the liner is malleable. Something like strong polymer fibres woven around the liner.

While the performance of a lithium float is second to ceramic balls and pressure-aided graphite tanks, it's better than graphite tanks without pressure and syntactic foam, its resistance to pressure gives more confidence, and I prefer the risk of lithium to the one of a vessel with high permanent gas pressure.

Marc Schaefer, aka Enthalpy

 

Share this post


Link to post
Share on other sites

In contrast to potassium, lithium is said (I didn't try) to react not very brutally with water, and itself and the evolved hydrogen not to ignite usually. This would represent a limited risk in open space.

Hard chromium is an other candidate as a liner: big elastic strain, thicker layer easily deposited. Though, because lithium is so soft, I'd prefer an electrodeposited nickel or nickel-cobalt layer, which accepts deformations, over the more brittle chromium and phosporus nickel.

Share this post


Link to post
Share on other sites

Here are a few paths to cover the lithium with a primer, after which a barrier against water can be deposited, possibly by aqueous chemistry or electrochemistry.

post-53915-0-06145800-1469268272.png

Many metals can be sputtered, including corrosion resistent ones, diffusion-tight, hard or malleable, and their alloys. To cover a lithium ball everywhere, I propose to lay it on three or more motorized rolls that turn it around alternating axes - remove previously any linden leaf. Vacuum O-rings on the rolls can improve the adherence if unreactive. Sets of rolls for one rotation axis can carry the ball while others sink to avoid rubbing. The screen can also carry the ball from time to time, and then one roll, or finger(s) or a shaker can rotate the ball. A magazine would process several balls after pumping the chamber once - or have an airlock rather.

Metal evaporation looks less easy than sputtering, as the machines I know need the metal source below the target.

A material molten below +180°C can solidify upon contact with colder lithium. This enables more varied shapes. Some metals and known alloys melt easily, here eutectic examples: 43Sn-57Bi at +138°C, 48Sn-52In at +117°C, Sn-Bi-In below. Maybe polyolefins and other polymers don't react with lithium and would then make a watertight shell, possibly without any added layer. Injection, in two steps and preferably under vacuum, would be better than deposition.

Some electrolytes aren't too corrosive to lithium, for instance the ethylene and propylene glycol carbonates used in lithium batteries. A metal less soluble than lithium might perhaps deposit at the lithium surface, in a displacement reaction similar to iron that covers with copper in copper sulphate.

Marc Schaefer, aka Enthalpy

Share this post


Link to post
Share on other sites

In contrast to potassium, lithium is said (I didn't try) to react not very brutally with water, and itself and the evolved hydrogen not to ignite usually. This would represent a limited risk in open space.

Reaction of Lithium with water:

 

Lithium floats on the surface of water, so it has smaller area contact with water than if it would sink.

 

Comparison of alkali metals video

https://www.youtube.com/watch?v=HvVUtpdK7xw

Edited by Sensei

Share this post


Link to post
Share on other sites

It would be interesting to coat "lithium sand" with polythene and then you could put it in a cloth bag to use as s float. The failure of a small number wouldn't matter much.

 

Interesting!

 

Up to now, I imagined balls of 1dm3 for instance, or any shape that makes a tighter pack, hold in a net or a holed reservoir. For sure, I want the bathyscaphe to float even if some elements fail, and the failure of one element have consequences of limited size.

 

Whether the elements' size should be 10L, 0.1L or 1mm3, I have no clear opinion. One remote lower limit is the diffusion of water through a too thin barrier. An upper limit is how much alkali and hydrogen are aceptable on the open deck of a large boat, with prepared passive and active safety. If one size makes coating easier, it's a strong argument.

 

Fun: one bathyscaphe had spheres (hollow) of alumina as floats. Brilliant idea, as alumina has a huge resistance to compression compared with its density, and is also extremely stiff, which is all-important because spheres use to fail by buckling under 114MPa. Alas, it wasn't quite clear whether the spheres would really survive the implosion of a neighbour. This bathyscaphe never emerged from one dive, but some little parts did, so people suppose a chain reaction ended the toy's useful life.

 

On this aspect, lithium gives confidence. So much that I feel its drawbacks would be accepted if reasonably under control.

Share this post


Link to post
Share on other sites

One more primer option is Parylene. Together with halogenated variants, it serves as a conformal coating against moisture on electronic boards.
https://en.wikipedia.org/wiki/Parylene

post-53915-0-31062800-1469872877.png

Heat splits the precursor to a diene which polymerizes when touching surfaces near room temperature, leaving no voids. Used commonly on metals, ceramics and polymers, but often after a first siloxane layer probably unsuited to lithium. Since adherence isn't vital at the float, I hope to spare the first layer. What lithium does to the monomer is unclear to me; a bit of untypical material is acceptable if the outer material is sound.

The price is a drawback: up to 1k$/kg for the precursor and one day to deposit 0.1mm, after what handling and processing is easier.

The coating chamber can process many balls. Some kind of moving support like the previous rolls must avoid shadowed locations.

----------

I like increasingly a polyolefin hull on the lithium. Low density permits several mm thickness, and then polyethylene or polypropylene resists shocks, deformations and tearing better than thin metal does.

If a polymer is injected around a lithium ball, a shell in two successive parts looks easier. A primer like parylene must ease the operations.

Other polyolefins use to need a higher injection temperature, but lithium limits it.

----------

Supposedly, lithium can be injected instead of cast, much like a polymer: heated to a creeping solid rather than a liquid, with much pressure to inject it. Material pressure fills the mould better, preferibly in combination with vacuum. Production is faster as the part is obtained solid. Solidification can make internal voids and a less accurate shape, which injection improves. The shape is more accurate.

Lithium injection could be made in the protective hull then. The hull would be nearly complete instead of two halfs, with the injection hole tapped when the lithium is cold.

Marc Schaefer, aka Enthalpy

Share this post


Link to post
Share on other sites

Coating an alkali metal with a halogenated hydrocarbon probably counts as manufacturing an explosive under most jurisdictions.

Share this post


Link to post
Share on other sites

That's why I recommend the non-halogenated Parylene.

 

Do you see other difficulties with that particular coating (or with others I suggested)? Woud lithium prevent the polymerization or induce other reactions? Other alkaline metals have a strong action on alkenes, used for instance for metathesis.

Share this post


Link to post
Share on other sites

About a million years ago when I was a kid, there was in Scientific American magazine, an advert for a company who was demonstrating their ability to "think outside the box" by suggesting that power cables could be made from sodium. They had a point; it's not a bad conductor, and it's very cheap on a dollars per cubic metre basis.

Obviously, three was a potential issue with rain- but, as they pointed out, many cables are coated with insulators.

So they actually made cables from extruded polythene (which is cheap + boringly unreactive towards sodium) filled with sodium.

 

I'm pretty sure you could do something similar with lithium. I can't be bothered to look up the melting temperatures at the moment.

The metal and plastic won't have the same melting point so, if the metal is more refractory, extrude wires of it and dip them in molten plastic. if, on the otehr hand, the metal has the lower melting point, extrude plastic tubing and then pour molten metal into it.

 

Halogenated materials are, in general, denser than hydrocarbons anyway.

Share this post


Link to post
Share on other sites

Thank you!

 

Li melts at +180°C, Na at +98°C only, polyolefins are injected at a mild temperature that makes them softer.

Share this post


Link to post
Share on other sites

Yes. I hesitated at it, and got discouraged by the alleged brittleness. Though, I haven't tested it with my own fingers. I'd prefer the balls to accept significant crushing before the lithium gets exposed, and PP is a candidate for that. I've also had an exorbitant price for PMP in the past, but I suppose it was dishonest.

 

The choice of processing temperature looks rather harmless if lithium is injected in a polyolefin sphere: I suppose it could be done at room temperature, or with very little heating.

 

On the other hand, polyethylene is commonly injected around +200°C which wouldn't fit a lithium sphere, unless thermal inertia does a trick. A lower-melting polyolefin would ease this option of shell-injected-around-lithium. Or if not, back to the liner like Parylene, but thick enough to protect the lithium a bit.

Share this post


Link to post
Share on other sites

As an alternative to parylene, the US patent 2,917,499 from 1959
https://www.google.com/patents/US2917499
describes a monomer that can be applied on a surface and polymerizes by air contact. The resulting hydrocarbon polymer layer is water-tight and resists solvents.

That would make a thick coating faster than parylene. The patent doesn't describe the possible drawbacks.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.