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:
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.htmAnyone know enough about material science to answer this question?
Thanks!
