I need to find a substance with the following physical properties:
December 11, 2013 1:23 PM   Subscribe

I need to find a substance that is : a) transparent over the visible or infrared, b) melts between 10 and 100 degrees C c) has modest freeze/melt expansion e)is at least a bit volatile in the liquid phase.

I need to find a material that is :

a) transparent over the visible and infrared, or at least infrared, both when solid and when liquid.

b) melts between 10 and 100 degrees C. The lower bound matters and is pretty firm, the upper bound is more flexible. Really it's just a matter of convenience and the desire to avoid complications caused by thermal expansion caused at higher temperatures.

c) Does not have a particularly large expansion or contraction upon melting or freezing, 0.7% or less is good, some paraffin waxes are roughly this (they are not transparent, though).

d)Is not terribly toxic, although I can manage some toxicity.

e)is volatile enough in the liquid phase that we can remove it by vacuum pumping to reduce the ambient pressure, then boiling it away.

What I want to do with it is to selectively melt out a void from a block of material using a cone of light to selectively heat areas inside the block under computer control, carefully remove the remaining liquid, and then fill the resulting void with plaster, allow the plaster to set, then remove the remaining transparent material, so you have a custom cast.

I will read comments on the viablility of this process and am of course interested in talking about it some other time, but right now I am really only looking for the material, as the details look promising in other regards, upon close inspection.

Water would be ok, except for the large freeze melt volume change, darn it. We can add calcium chloride to the plaster so it can set at a little bit below zero degrees... but it's better not to, because it affects the accuracy of the process, most likely.

There's gotta be some material or another that has this. Benzene might be ok. Camphor, oxalic acid. There's gotta be something better, though. Something like water that is relatively easy to handle and stuff.

I'm doing this out of the local hacker space here in Ottawa, it is a sort of art project, but if the accuracy of the process can be pushed, it could be used a bit like a 3d printer.
posted by Nish ton to Science & Nature (9 answers total) 1 user marked this as a favorite
Material-wise, you want a wax, I think. A lot depends on the accuracy you need, and the size of the object. Could you be more specific about those parameters? What's your desired end-product?

In terms of the optics, what you are trying to do is very hard with linear optics. To my knowledge, people in industry do such "light sculpting" using extremely expensive nonlinear optical methods like multi-photon excitation. I can send you material on that if you're interested.
posted by serif at 1:49 PM on December 11, 2013

Crystal-bond is a thermoadhesive used for mounting ceramic wafers for analysis. It's clear, has a flow point well below 100°C and has very little melt-freeze hysteresis. It would cost a fortune in the volume you need but maybe you can prepare your own (Phthalic Anhydride 66-85% Ethylene Glycol remainder according to MSDS) or find something similar in bigger bricks. I'm not certain that it would be as volatile as you want though since at least the solid is fairly vacuum-compatible

Otherwise, some thermoplastic polymer. Depends how hot you can get and what you mean by infrared. Delrin? 180°C?

BTW I don't understand why the melt-freeze volume change matters as long as you start by melting a drain channel from the outside in.
posted by Pre-Taped Call In Show at 1:49 PM on December 11, 2013

I think n-pentanol has the melting point and optical properties you're looking for. Consider other long chain alcohols too.
posted by u2604ab at 2:17 PM on December 11, 2013

Transparent in the infrared is really hard---thats what chemical bonds resonate to. Are there particluar frequency windows that might work for you? That's acheivable. Otherwise, the best you can do is something with an uncomplicated IR spectrum, like a paraffin or a wax. A refined mineral oil is typically used in IR spectroscopy because most of the spectrum is clear and the stretches that it does have are relatively easy to subtract from a result.
posted by bonehead at 2:23 PM on December 11, 2013

Also, this:

melts between 10 and 100 degrees C

and this:

is volatile enough in the liquid phase that we can remove it by vacuum pumping

...are going to be tough to achieve in a single material. Most things that melt at that high a temperature aren't very volative (benzene is 5.5 C, with a vapour pressure of 10 kPa). I don't think the alcohols will work: 1-pentanol is -78C (mp) and has a vapor pressure of only 200 Pa (water is about 10 times this).

Also, both have simple IR spectra, but neither is truly transparent in the IR.
posted by bonehead at 2:37 PM on December 11, 2013

Here's a decent resource for IR transparency. As you can see, those materials which have high transparencies have no covalent bonds, or relatively exotic ones. This also means that most of them are solids, unfortunately.

Is there some other way to do this for you? This is a difficult criterion to get around.
posted by bonehead at 2:40 PM on December 11, 2013

I'm sorry, this strikes me as a completely bass-ackwards way of making a cast.

First you sculpt the object that you're trying to reproduce. Then you use it to make a cast. You're trying to make a cast without a physical copy of the object, and you're finding out why no one does it that way.
posted by valkyryn at 4:16 PM on December 11, 2013 [1 favorite]

At the risk of stating the obvious I am also going to point out that if a material is "transparent" at the excitation wavelength, it then can't absorb the energy to induce a phase change. Hence the need for two-photon processes.
posted by Pre-Taped Call In Show at 4:54 PM on December 11, 2013 [2 favorites]

Are you thinking of only melting the point of the cone of light? I so, you don't really need a very transparent material. Also, are you thinking of melting a pocket, and then pulling vacuum and having it move through the solid? If not, just do it upside down, and let gravity drain the liquid as you are sculpting.
posted by 445supermag at 9:12 PM on December 11, 2013

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