Hello, I am trying to anneal graphene flake on a silica substrate at 250 celcıus degree under vacuum using a standard mechanical pump and a quartz tube. The whole process will take 4 or 5 hours for temperature ramp, dwell and cooling. So during that time, I plan to purge H2 and Ar with 200 and 400 sccm respectively together from the beginning. What will be the max limit of hydrogen gas flow rate (sccm) that ı should use in order not to have a dangerous situation?
Hello! We use for the synthesis of graphene a plasma with a temperature of 16000-20000 K. We all turns into steam. Therefore, do not create a dangerous situation
My question is very clear and about graphene annealing not cvd or such methods. To clean Thé graphene surface, H2 and Ar gases are used during corresponding temperature. Due to hydrogen gas is very flammable there should be a max.limited sccm value not to have a risky case because hydrogen and argon Will Be pumped and released into Thé room. So if the hydrogen rate in air exceeds the limit, any Spark or oxygen may ignite the explosion
Hi, did you consider using forming gas (explosion-proof mixtures of N2 + H2) ?
In order to get rid of photoresist residue from the surface of graphene this mixture (H2 and Ar) generally used in literature. So I also want to use it but I dont know exactly whether it will be a problem or not because as ı mentioned in question up to desired temperature and cooling it will take approximately 3 hours so during that time ı must purge gases until cools down may be I can cut H2 off before cooling
Guy Vereecke is on the right track. Consider using 3-6% H2 in Ar. These combinations are available as a standard premixed blend from most commercial suppliers. (Although there may be a slight delay in delivery because they are not as commonly used as their nitrogen cousins.) Even in a worst case situation with a major release you can't make a flammable mixture in air.
Hidrogen is highly explosive in the mixture with air if the concentration of hidrogen is from 4,65% to 93,9%.
First of all thanks to all;
I searched and found that lower explosive limit of hydrogen is about 4% in air as Mr.Zaytsev. So I want to get your opinions on my calculation. I plan to purge 200 cc (sccm) H2 and 400 cc (sccm) Argon gases through a quartz tube while making it warms up 250 celcıus degree. If I purge gases for 5 hours (becasuse of warming and cooling process) it means 300 minutes. Meanwhile a mechanical pump will vacuum the tube and exhale into the clean room. The volume of clean room is approximately 150 cubic meter and total H2 that we get into the room by pump can be calculated as well
200 cc equals 0,0002 cubıc meter if we consider 300 min. it will be 0,06 cubıc meter in the room So the rate of total H2 will be 0,06/150 =0,0004% . İt is very small compared to LEL valu of hydrogen. As e result of that Can I say it is safe to do experiment?
Looks good.
And probably you could make an exhaust system to the outside spase?
Just in case...
If there is no Oxigen no explosion can occour...
This means that, since the mixture is in your reactor with no leak you can use any mixture. If you want the mixture not to explode when mixed with air that's another story.
In that case you need your H2 concentration in the air to be lower than 5%. If you want your Ar/H2 mixture to be iherently safe, you need it to be lower thant 3% in H2 (or so... I don't remember the exact number). In the case of N2 this limit is 5%.
If you want your mixture in the furnace to be higher in concetration (and you are sure there is no leek) but you need a safe waste, you could diliute with a propper amount of N2 your gas after you furnace in order to make the concetration as low as 5%... but I think is not very practical using 200sccm of H2
I agree with Alberto.
If you worry about the leakage of oxygen, you can firstly fill argon to a little bigger than air pressure and then heat, mix the gas without vacuum.
Alberto is in error. It makes no difference whether you use Ar or N2 as the diluent.
What matters is the H2:O2 ratio when the combined mixture is vented into air. Forming gas (either N2 or Ar based) is formulated so that it is not possible to get a flammable mixture when you mix it with air in any proportion. If a higher hydrogen concentration is needed then you need to consider how to safely dispose of your vent stream. Either venting a slow flow (small volume) into a fast moving air stream (very large volume) or pre diluting with an inert gas (e.g., N2, Ar, CO2) will work, but have someone experienced with hydrogen handling go over your plans and set-up with you before you actually start to work in any case.
Dear Alan, according to regulations it makes difference. Not sure of why, but according to European regulations, the maximum H2/N2 mixture that is considered "non flamable" is 5%, the maximum H2/Ar mixture is 3%. Above this limit it is considered flamable.
I follow U.S. rules, and we have the same limits for both, set in volume fraction, which, under most conditions is a good approximation of mole fraction. The only way different limits would make sense is if they were in mass fractions, which is an odd way to measure gas.
thanks for your contributions
I will inform you after process If I am still alive :)
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