Sciencemadness Discussion Board - (Neutron detector) Melting Lithium fluoride into bigger, clean lumps-

(Neutron detector) Melting Lithium fluoride into bigger, clean lumps-

Naaaan - 13-2-2018 at 10:28

Hi there,
since i have been more of an metal-organic (catalyst-design) chemist, i don't know much about 'simple' salts like Alkali-halogenides and especially their use in 'non-chemical' settings (meaning optical devices).

Now i have a question about Lithium fluoride; since it's not soluble enough to make a - let's say 1cm² wide and 200µm thick film (doesn't have to be a single crystal) on a metal surface by solving & evaporating, i'm looking for a method to get a 'big lump of LiF'.

I know well about crystal growing (from phase diffusion methods for structural analytiks to Ar-gas protected reaction in gas-phase to form SiC from raw Si and C to be cut into wafers). But as i said, for my LiF it is not neccessary to be a single crystal, just a 'thick' film. ('thick' because my main work was with ALD and some CVD, so 200µm is relatively thick...)

I have bulk LiF grains, they melt easily (~830°C), but without a platinum i don't think i can generate a molten 'lump' of LiF (since the fluoride-ions will react with most containers... i haven't even tried melting LiF in a testtube, but i am sure it won't be clean afterwards.

So my question (after researching patents, scifinder, etc and only finding methods for single-crystal preparation) is:
Does anyone have experience or an idea how i can get my 200µm film of LiF on my metal-surface (which is part of a heat sensitive detector)?

My next step would be to get a platinum crucible, heat it, and then trying to cut the molten LiF, in hope it hadn't reacted with air, since i can't use protecting gas at the moment (and i'm pretty sure Li⁺ will react with N₂ from the air, not even thinking of what 800°C F^- will do with components of the air)

[Edited on 13-2-2018 by Naaaan]

[Edited on 14-2-2018 by Bert]

DraconicAcid - 13-2-2018 at 10:40

Lithium *ions* will not react with nitrogen, and fluoride *ions* are very stable. You can't melt LiF in test tube because glass won't stand up to 800 oC.

You say it's not soluble enough to make a film by evaporating a solution of it- have you tried different solvents? Could you repeat the addition and evaporation several times to make a thick enough film?

Does it have to be LiF? The eutectic of sodium/potassium/lithium fluorides (FLiNaK) has a much lower melting point, and can be melted in glass test tubes for short periods of time (over long periods, the lithium salts start exchanging with the ions in the glass, weakening it).

Naaaan - 13-2-2018 at 10:59

In certain, thick enough test tubes, you can melt LiF (tried it also with NaCl, which has a slightly lower boiling point). But maybe the LiF wasn't pure and some lower melting mixture had formed.

The reaction between the ions with air-components was an assumption; that F^- is very stable is true, due to the electron configuration. But i suspected the temperature to lead to elevated reactivity.

I researched the solubilities of LiF in different solvents, but none of those i found solves enough LiF to evaporate the amount i can put on my target. At least not enough, so i would have to repeat the process a couple of hundred times or with excruiating evaporation times.

Since this is a side-project, i didn't put much practical work in it yet, but i found someone claiming molten LiF in a thick test tube (from "quartz glass" or "fused silica" - i don't know the english term, but those can stand a bit over 800°C) and getting a colored molten liquid.

- i don't intend to go after melting LiF in glas (either one), since i don't trust them not to react with molten LiF or don't melt themselves.
thus, my idea of a platinum crucible.

edit: yes, it should be clean LiF, since it's for physical detectors (neutrons), the Fluoride is rather unimportant, but it is used because of its low protonnumber to increase sensitivity (as far as i found out); the Lithium is the more important part.

edit2: i will deliver more details tomorrow, but the optical properties of LiF seem to be usefull in many kinds of radiation detectors.
[Edited on 13-2-2018 by Naaaan]

[Edited on 13-2-2018 by Naaaan]

Bert - 13-2-2018 at 11:06

A long time ago, at a university physical sciences lab far, far away, we used equipment with UV windows. We bought them, they were not cost effective to make.

Here is some old tech. Yes, quartz tube furnaces and Platinum equipment.

I begin to understand why we didn't make them ourselves.

Here is a wet chemistry crystal growing scheme. The liquid is hot, concentrated hydrofluoric acid, something I wouldn't care to work with.

http://onlinelibrary.wiley.com/doi/10.1002/crat.19690040109/...

Are you certain you must use the fluoride? The chloride and especially the iodide looks more process friendly. Found a lot of references to (sparing) solubility in organic solvents and others on organic compounds with Li being used in N detectors.

[Edited on 13-2-2018 by Bert]

[Edited on 13-2-2018 by Bert]

Naaaan - 14-2-2018 at 03:39

Ok i got more information now, talking to the ppl i am working with:

They want the Li⁺ above a sensor that shall detect neutrons; from other groups they know there is no single-crystal needed (no optical detection or something like that).

But what they need is a film, that contains Li⁺ and doesnt absorb much alpha-rays. Suspending LiF in any kind of glue seems ok, but it absorbs too much alphas.
I told them to just buy a fitting LiF-crystal, but since the detector they are wanting to modify with that is f*ing expensinve and not really available (about 100 pieces in the world).
Another, lower melting, Li-Salt would be a solution, but i didn't really get to talk to them. Also i don't know wether they want to use LiF because it's used in detection of different rays or if they really out thought into that...
I'll try to figure it out, amorphous LiF would be fine, but since the Lithium is the main figure here, i smell some fishy side-project, i got dragged into from some (absolutely no chemically-trained) physicists.

Tsjerk - 14-2-2018 at 05:36

What is the material you are going to make the LiF film on? Glass? Quartz?

Edit: Ah, found it, metal... What metal is it?

[Edited on 14-2-2018 by Tsjerk]

Bert - 14-2-2018 at 05:39

Moveing this to the radiochemistey sub forum-

Ozone - 14-2-2018 at 06:55

Very thin windows of optically clear material can be created by pressing the powdered salt in a die with a hydraulic press--or, more simply, in a big nut between two screws with polished ends (tightened with wrenches).

This is how I used to make samples for IR spectroscopy--mix compound with the IR-transparent salt (typically with NaCl or KBr), homogenize with a mortar/pestle, or better, Wig-L-bug, and press into thin windows. If done correctly, the aiming laser can pass right through without scattering too much.

O3

Tsjerk - 14-2-2018 at 07:23

I believe this sensor is not an optical sensor.

Bert - 14-2-2018 at 07:30

Naaaan's associates seem to be trying for a combined neutron and alpha detector?

Quote:

They want the Li+ above a sensor that shall detect neutrons; from other groups they know there is no single-crystal needed (no optical detection or something like that).

But what they need is a film, that contains Li+ and doesnt absorb much alpha-rays.

Sulaiman - 14-2-2018 at 08:30

This sounds like a professional project,
if your career is not tied to the development of this window
then I suggest that you save an enormous ammount of R&D time
by outsourcing the film fabrication.

There is always someone... look carefully

Sales/marketing people contacted directly by 'phone are more likely to offer help than an internet query.
Sometimes I have simply cold call asked for the engineering or department manager and gone from there.

Naaaan - 14-2-2018 at 09:48

no it'm actually not tied to it at all, i was just asked to help.

the idea with the nuts and bolts (kinda like making a IR-KBr-Pellet) came to me today too.

but since i learned that this whole thing was some kind of... not very well thought through idea between some physicists without any knowledge of chemistry or technical aspects, i don't think i'll go after it anymore.