One method was devised for making transparent arc tubes out of aluminum oxide, and it was the process of growing a tubular single-crystal sapphire arc tube (Corstar) with the EFG process. Obviously Corstar did it's job semi-sufficiently, it was used for a little bit in HPS arc tubes. But I have a question:

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Being a single crystal, the Corstar tube had a different CTE length-wise than width-wise (problematic for sealing). Well did they have any other choice? I think they may have, but I could be wrong.

On Lamptech, it is said that YAG (a ceramic material) can be sintered with a larger grain size to yield something similar in translucency to the typical PCA used for ceramic arc tubes, or could be sintered with very very small grain size to yield something with very high transparency (while presumably remaining more or less amorphous). I also know that YAG can be grown in a single-crystal format with the EFG process, but this appears to be done only for scientific or laser applications unrelated to lighting.

So to summarize, YAG can be made translucent with larger grain sintering (done regularly with alumina to make PCA), YAG can be made transparent with EFG (done before with alumina to make Corstar), but YAG can also be made transparent through smaller grain sintering (not done with alumina as far as I am aware).

What prevents them from sintering small (~1 micron) alumina particles to make an amorphous transparent alumina product with no weird CTE problems? Just like they did with YAG? This would have all of the benefits of PCA (uniform CTE) and all of the benefits of Corstar (transparency) combined into one product, while probably being a lot cheaper than YAG and more resistant to halide salts than YAG.

Here is the transparent sintered YAG lamp on lamptech that I speak of:
https://lamptech.co.uk/Spec%20Sheets/D%20MHC%20Toto%20YAG70.htm

Anyone know enough about material science to answer this question?

Thanks!

I am for sure not competent in Al2O3 material science, but can speculate the reason is production costs, as always. Real efficiency benefits of having a transparent arctube are too small to bother.

@RRK
Oh absolutely, all transparent arc tube technologies for CMH or HPS have been discontinued, and for good reason. I was just thinking that if they tried it with the much more expensive YAG ceramic, I would assume that they would try it with alumina as well.

It was tried also with Alumina, ending up with submicron particles.  But as RRK stated, less cost effective.  Also, the submicron alumina arc tubes are less transparent than the YAG or YbAG.  At work I am pretty sure I do still have a tray of empty arc tubes of near-transparent submicron alumina tubes, along with YAG.  When I find these I can make a photo to compare the appearance.  They were made by  Dr. Yanagitani’s process at Konoshima in Japan and he offered some prototypes when visiting my lab about 20yrs ago.  His patents and technical papers explain the technical challenges, along with similar from NGK Ceramics and Toto Ceramics.

@James
Ah-ha! So they did try it! Very cool, I would love to see a comparison. Unfortunate that they didn't see commercial use, but there is of course a good reason for that. Thanks for explaining!

I think some of the Japanese lampakers used it.  For sure the Toshiba 20W ceramic metal halide of around 1997 used submicron alumina.  Also the very first briefly made SLI CMI-T lamps of 1997.  But I think no other companies! 

This is news to me! I am gonna look around here for an example for sure, I would love to see that.