Producing synthetic corundum (Ruby/Sapphire) with CO2 lasers

[CurtisOliver]

One of the benefits of working in a school design technology department is that I have access to a range of materials and a co2 laser cutter. Also it helps being on good terms with the science department too. So I decided to run some experiments making crystals under laser conditions. The first crystal I was successful in producing was halite. So this set the course for something more ambitious.

I went to the science department and asked for some aluminium oxide (Al2O3) powder as well as trying my luck for some powdered dopants (Chromium, Titanium and Iron).
Out of the dopants however they only had iron powder. This did present a problem though. Iron powder of that purity is highly oxidising and presented a risk of a metal fire. And it also occurred to me that they are the ingredients used in thermite. So I went away empty handed at first.

After some thinking, I realised I had plenty of steel lying around. Steel as people know is a iron based alloy. I have lasered steel plenty of times before without issues. I was still nervous of lasering it in powdered form, but figured the contaminants would lessen the risk of drastic oxidisation. So I got myself some paper and a steel block, and began filing away creating steel dust.

The first step was to cut some 30-40mm aluminium squares and beat them to create a parabolic like dish.





The next step was to put a small heap of aluminium oxide into the aluminium dish.
The idea was the parabolic shape would help focus some of the laser into a point within the medium. Aluminium is typically a reflector at IR wavelengths.
And then I sprinkled some steel dust on top.




I tried static firing with the laser defocused to a focus height of 120mm first. Setting the laser to fire automatically runs at 100% power, which is 60W in this case. The focal length of the laser lens was 25.4mm and the beam diameter of a CO2 laser is typically around 3mm so I was able to estimate the spot size and therefore the intensity at this height.

This was the result of the first spot firings.






Bingo, corundum crystals!
What surprised me also was the production of ruby. Then I figured the steel I used must have been an old piece of stainless steel which contains chromium. Using a small 395nm torch I was able to fluoresce the ruby crystals further confirming them.






Now it was time to improve on things. The previous firing took a while to heat the aluminium oxide to exceed 2072°C that it needs to become molten. I decided to decrease the focal height to 90mm increasing the laser intensity.
This was the result:

The result produced a single black sapphire and surrounding microcrystal of ruby.

After more runs I found that if I moved the laser head manually by a couple of mm whilst the aluminium oxide was still molten, I was then able quickly heat the surrounding powder and add to the previous crystal through coalescence. The coalescence was random and sometimes it contributed to the original crystal or it began forming a new one.

Once the aluminium oxide was hot enough I found it took very little to melt the surrounding oxide to the point I could now have a slow travelling beam.

So I adopted this 10mm square cut route. I set the laser to travel at 1mm/s at line intervals of 0.125mm.






After a full pass I was able to gather a nice collection of rubies and sapphires.

I didn’t have time to get myself a sieve so the container contains some aluminium oxide powder as well.







And now the fluorescence shots!



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Using steel dust as a dopant produced some interesting results and potentially rare raw crystals. I have a couple of samples that have coalesced together containing ruby, clear sapphire and black sapphire. There also appears to be pale yellow variety present. I am yet to produce the classic blue sapphire.

To improve things further I am going to try a couple more things.

1. Adopt a circular spiral cut route. This is to hopefully allow a singular central crystal to grow outwards.

1. Mix a higher concentration of steel dust thoroughly into the oxide powder. Hopefully stronger doping may occur. I saw some evidence in microcrystals that produced very red rubies.
2. Potentially invest in pure chromium and titanium powders to try out creating higher quality and purer Cr:Al2O3 (Ruby) and Ti:Al2O3 (Titanium Sapphire) crystals.
3. Produce crystals on a layer basis. I’ve proved I can remelt and coalesce the oxide horizontally so there should be no reason I can’t build on them vertically.
4. Risk using small quantities of iron powder to see if I can make the elusive blue sapphire.

So I hope to be able to share further progress with you all.

Edit:

Some closeups of some of the crystals.

And under closer inspection I have discovered at least two microscopic blue sapphires.

Note: The image has had to be altered to show the blue tint. It was very hard to capture it on my phone.

 

[kecked]

Nice. If you grow any clear let me know I facet gems for a hobby. I’d need at least 5mm to work with it.
I buy synthetic rods for like 30.00 that weight in the hundreds of grams. Blue seems difficult as what I get is mainly clear with a few mm thick blue rind.

corrundum can be literally any color

 

[CurtisOliver]

So my largest crystals yet are around 5mm. But they are 90% at least black sapphire.
I haven't achieved large clear sapphires on their own yet. Black sapphire seems to be the easiest to form.
Btw, I'm about to edit my post. Under closer inspection I have managed to produce at least two micro samples of blue sapphire.
It was incredibly difficult to photograph though.
I'll will also share closeups of some of the most impressive crystals.

 

[kecked]

Wonder if it might work better under inert like argon? I am thinking it’s the cooling that gets you and it needs more time. Maybe preheat the mass and use the laser to drive it over but then bring the mass down slowly to room temp? The black is likely the iron. You need chromium or vanadium I think but I do remember iron was a dope as well. The amounts are really timely however.

 

[CurtisOliver]

Wonder if it might work better under inert like argon? I am thinking it’s the cooling that gets you and it needs more time. Maybe preheat the mass and use the laser to drive it over but then bring the mass down slowly to room temp? The black is likely the iron. You need chromium or vanadium I think but I do remember iron was a dope as well. The amounts are really timely however.

You are correct that iron is responsible. The steel dust doesn't exactly lend itself to being a pure dopant by any means. However I'm surprised it worked at all. So what I have worked out is that I am generating iron oxide myself during the process by melting the steel dust and then the superhot temperatures are crystallising it to become haematite inclusions within the molten aluminium oxide. It's actually a similar process to nature as black sapphires are the most common. Under 365nm I found a higher proportion of rubies than previously thought. The steel I was filing had to have been stainless steel. The chromium content contributed nicely to fluorescence is many samples.

Iron oxide is 100% a dopant in sapphires. In fact ruby Ruby and a variety of purple sapphire is one of the few corundum's that doesn't have any at all.

So after a some research I have decided to place an order for some of my own aluminium oxide powder, as well as Iron (II,III) Oxide, Titanium (IV) Oxide and Chromium (III) Oxide dopants. You need Vanadium (III) Oxide, but could only find Vanadium (V) Oxide available online.

I have also designed myself a new laser path to hopefully improve the results. I also have to take into account that impurities in the steel dust could have prevented some coalescence.

Stainless steels are steels containing at least 10.5% chromium, less than 1.2% carbon and other alloying elements. Stainless steel’s corrosion resistance and mechanical properties can be further enhanced by adding other elements, such as nickel, molybdenum, titanium, niobium, manganese, etc.

So the high chromium content is responsible for the rubies. And a trace amount of titanium could be the reason I have two tiny blue sapphires. However nickel, molybdenum, niobium and manganese aren't particularly useful and could have served as contaminants. And the worst one would have been carbon. Carbon requires a further 1500C to melt so would have remained present as small solid contaminants and could have interfered with the process.

Whilst I am waiting for my chemicals to arrive, I am going to run a new test with just pure aluminium oxide and see if I get any promising improvements in creating clear sapphire. If there are no contaminants then I expect the oxide to have an easier time heating more consistently and coalescing.

 

[CurtisOliver]

Update 18/11/22:

So I have been playing around more with making better sized/more usable crystals.

I first experimented with better techniques for producing white sapphire as at the time the aluminium oxide was the purest substance I had. And white sapphire is just pure Al2O3 with no dopants. And I started to get better results in producing larger crystals. However I received the chromium (III) oxide powder, so I am going to skip ahead and show you the progress in making SLM (Selective Laser Melting) rubies.

I decided to play around with making the crystals with various different focal heights and the last one is a different mould which I’ll get to explaining in a bit.





By varying the focal heights you can see the intensity within the mould changes. I wanted to see if increasing the intensity distribution would have any impact in keeping more of the crystal molten at any given time. In conclusion I found the laser being focused at the top of the mould was the most effective and easiest to setup. The crystal had better intensity distribution when focused half way into the mould. But it presented problems of the laser head being covered in hot aluminium oxide vapours. Increasing the focal length to 50.8mm may be the best alternative to this, however the spot intensity will be weaker under a longer focal length lens. But the Rayleigh length would also increase possibly making the distribution greater. I don’t currently have a co2 50.8 lens to try. So for now I have chosen to adopt a focus at top of the mould approach.





This was the cut route I have adopted. I decided to limit the file to the 1.5mm radius contour. As the cut duration increases significantly with each 0.5mm in diameter I add. As the diameter of the path grows, the laser head must be slowed down and rerun more times to not only achieve an equivalent amount of exposure but to exceed it. This then causes more time to reheat the already formed crystal and allow more molten oxide to coalesce with it. The multiple passes act as a way of cleaning up the join by repeatedly remelting and rejoining with the temperature rising in between exposures.

I found that I needed a small solid mould to allow me to build up the crystal vertically. Aluminium oxides powders bulk density is considerably lower than the density itself. After compacting the powder, I have found that I will need to repack the mould and then I can grow the crystal further. My aluminium dish was mainly focused on the horizontal packing, where I now needed something more compact and vertical and more thermally controlled.

I found myself a 10mm socket and placed it in a vice. I then packed it out with a Al2O3+Cr2O3 mix and began lasing.

This is what the result looks from a focus +8mm run.





The result was my largest ruby crystal yet. The only issue is, it hadn’t coalesced too well, and it featured a nasty crack. The crystal then split into two sections.

I then tried the -4.15mm and +/- 0mm run. And this is where I started to notice positives and negatives.
The positives were I noticed better coalescence and the results yielded a decent size stable crystal.

The negatives however were the vapours produced on the -4.15mm run, and the presence of impurities. Black sapphire managed to reoccur in a pure Al2O3+Cr2O3 mix. The bottom of the crystal had formed a nice black base. And then it occurred to me the intensity was high enough to etch the steel mould. The steel mould was causing impurities under those high intensities. Haematite was again allowed to form and therefore the base of the crystals became black sapphire instead.

So I then went off quickly to search for an alternative. I chose to search for aluminium, as it wouldn’t cause any impurities to creep in as its a aluminium oxide in the first place I am melting. I had melted some aluminium into a ladle previously. So I decided to use that and just drill a 7.5mm hole into it.

The results were much improved. The rubies were at least 99% pure after switching to a aluminium mould.





And now a collection of the latest progress samples.







And again another bonus, this time a strange one.

After observing one of my white sapphires under UV, I noticed a glowing yellow speck.

Turned out I had a small ruby inclusion. But this one doesn’t fluoresce red!









The fact that the white sapphire glows full stop shows that it has chromium present. However it glows a strange yellow line. Very difficult to photograph well. But the crystal speck has a similar red/pink with added yellow hue. And the speck definitely glows alternative to the typical deep red. I have also found another one with a weaker and smaller spot. So it was replicated. Another strange one is a very ordinary looking sapphire has a weak cloudy pale blue/white fluorescence, which I can only describe as being comparable to how amber fluoresces.