Alternative to Fused Quartz as Tube Material in Diffusion Furnace

Hello
I'm designing a three zone oxidation- / diffusion furnace for my own, small semiconductor laboratory. The furnace itself is more or less
finished, but the diffusion tube has still to be choosen / bought. A perfect material for the tube would be fused quartz (like HSQ300/400 from Haeraus or 214 LD / LDH from GE). But once the tube has been run in, the temperature mustn't go below about 300 C, otherwise the tube will crack as an effect of devitrification. Now I have the following problem: As I use this furnace only a few times a month, it's a) too expensive for me, to keep the furnace always running at about 300-400C standby, as a lot of current and nitrogen is used. b) a potential fire hazard if something should happen (although there are overtemperature sensors) while I'm not in the lab for a longer time. c) too expensive to buy a new quartz diffusion tubing for every run.
So I looked for alternative materials. Finally I found two interesting materials: -Alsint 99.7% (recrystallized Al2O3 min. 99.7%, nonporous) -Halsic-I (SiSiC, Silicon carbide matrix filled with Silicon, nonporous)
Now my questions are:
-Do these materials withstand repeated ramping up from room temperature to the max. process temperature of 1300 C and after processing back to room temperature again without any cracking? -Are these materials usable as a process tube in a diffusion-/oxidation furnace for semiconductor processing? Or is there too much outgassing of alkali- and other ions which will contaminate the silicon wafers?
My process / furnace parameters are:
Outer atmosphere: air at atmospheric pressure Inner atmosphere(s): nitrogen, argon, oxygen, oxygen with H20 vapor, all at atmospheric pressure with a slow laminar flow along the process tube. Temperature: max. 1300 C, usually 1000 - 1100 C Tube outer diameter: max. 150 mm Zones: 3 Power: max. 7,5 kW
I hope someone can help me along with this problem, as it's really difficult to get informations about the suitability of these materials for semiconductor processing.
Many thanks in advance and kind regards,
Alex
Student of Electrical- and Electronics Engineering
University of Applied Sciences Bern, Switzerland
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Alex wrote:

Mullite (2[SiO2] - 3[Al2O3]) would be a better choice for thermal shock resistance and repeated thermal cycling. It is usable to 1700+ C in air and up to 1600C under vacuum and tends to be significantly cheaper than high alumina. I have used mullite tubes up to 1500C under vacuum, oxidizing, reducing, and inert in the past with good success. Of course high alumina is fine as well, but you can run into thermal shock/cycling problems. Vesuvius is a great place to get alumina, zirconia, and mullite tubes. They slip cast their tubes and can custom make anything you need for a very reasonable price.
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