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Bathyscaphe Float

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Hello nice people!

Very few vehicles have dived deep in the Ocean, with a crew or not.
Piccard's ones reached the deepest point of 11km around 1960, a handful much later 5 or 7km, and recently 11km

* Kaiko (lost in a typhoon)
* Its successor Abismo
* Nereus (lost by implosion)
* The manned Deepsea Challenger
The float is difficult. A hull for 114MPa water pressure sinks unless it uses the best materials in an optimized design. (6.8MB)
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 used 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 have already failed, supposedly in a chain reaction.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 density nor resistance to isostatic pressure. Data welcome!

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. A hull of graphite fibre composite is widely considered, but to break at 1.5 times 114MPa, it would be heavy.

I've already proposed to help a graphite tank filled with helium at about half the maximum outer pressure. Figures tell the combination is lighter, but I'm not enthusiastic about m3 of gas at 60MPa near the operators.


My preferred option presently is float elements of plain lithium (534kg/m3) 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. I prefer this risk to pressure gas and to hollow floats.

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 or not, but 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. Hard chromium would resist the elastic strain too, but because lithium os so soft, I'd prefer the more resilient nickel or nickel-cobalt.
Marc Schaefer, aka Enthalpy

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.

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: 43/57 Sn/Bi at +138°C, 48/52 Sn/In 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.

Proposals, comments, suggestions, objections...?

Marc Schaefer, aka Enthalpy

One more primer option is Parylene. Together with halogenated variants, it serves as a conformal coating against moisture on electronic boards.

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

As an alternative to parylene, the US patent 2,917,499 from 1959
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.

A liner of corrosion-resisting metal, for instance nickel, tin... would protect lithium against water. Maybe a simple displacement reaction deposits the noble metal on lithium. The process would drop and turn the lithium part in a solution of the noble metal salt.

Water won't fit, but polar solvents without O-H bonds serve in lithium batteries. "Lithium metal" batteries (search words) are a fashionable research topic. Carbonates seem less favoured, but dimethylsulfoxide (DMSO) and dimethylacetamide (DMA) are considered among many more.

Both nickel chloride and lithium chloride are well soluble in DMSO, as mere examples.

The noble metal layer deposited by the displacement reaction is thin, but once the lithium part is easier to handle, other processes can deposit a thicker liner, of metal, alloy, organic materials like polymers...

Marc Schaefer, aka Enthalpy


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