Can electrically conductive silicone rubber block microwaves?

[bjbdts]

Hi everyone:
I'm working on a microwave chamber design(2.45GHz). The chamber needs to be pumped down to a certain vacuum level, at the mean time needs to be microwave safe. I'm wondering if anybody here has experiences about using electrically conductive silicone rubber in order to meet the requirement, or any other suggestions?

Thanks in advance.

bjbdts

 

[VEBill]

Your question is not perfectly clear. A chamber made from silicone rubber is a poor place to have a vacuum, assuming you need a non-zero volume. ;-)

Or are you proposing to use the conductive rubber to seal the gaps around the door? If so, then you should obviously study what the commercial microwave oven market does, as the requirement would be very similar.

Conductive but lossy materials exposed to high RF will get hot and be destroyed. Usually you would want a good conductor to efficiently reflect the energy, as opposed a (somewhat) conductive material that will try to absorb it. Unless the levels are low and your just doing a final "clean up" of residual leakage.

Can you clarify your question?

 

[bjbdts]

VE1BLL:
Sorry I didn't state the problem clearly. The main part of the chamber is made of metal, the electrically conductive rubber I asked is supposed to fill the gap between two metal pieces. The gap between the two metal pieces is less than 50mil. Please refer to the simplified drawing attached.
I think my situation fits your third category of "clean up" of residcual leakage.

bjbdts

 

[VEBill]

I'm not an expert in slot antennas, but I'm concerned that your one-inch long slot might allow a lot of RF to hit the seal. But I don't know.

Maybe a metal EMI seal would be better. There are a myriad styles available.

 

[Higgler]

If you can find any conducting gasket that will work, but watch out for problems in arcing if you have high power inside your oven, see below.


One real problem about the location of the gasket as shown in your drawing, if you put a short 1/4 wavelength down a slot like you have it (and you are close to 1/4 wave or 1.2 inches length), it'll make an open circuit with high voltage at the slot edge on the inner edge of your oven slot and you may accidentally create a point for arcing. You don't want an arc generator, it's noisy and can cause bad stuff, fires, etc.

Standard home use Microwave oven doors are 1/4 wave open circuits, i.e. when you close them, they don't short circuit the door to the oven, they leave a gap at the door perimeter that's specifically 1/4 wave (electrical length) so that when current reflects off that edge (it's an antenna like mismatch with air that makes the reflection), that reflected current heading back into the oven recombines with the original field inside to minimize the voltage at the gap.

A quarter wave short is exactly the wrong distance, it interrupts the current at the door gap point inside your oven and will make a large voltage at the gap and set you up for ugly arc's possibly (power dependent). You want your short at closer to 2.4 inches down the gap of your door closure, not 1". 2.4 inches would be a half wave short, which reflects as a short and minimizes the voltage at the gap on the inner part of the oven.

k

 

[biff44]

You can just jam some of this stuff in the gap:

http://www.leadertechinc.com/wire.shtml

Chomerics makes a similar all-metal emi shield.

An elastomeric absorber will not work very well at stopping leakage, will outgas, and might catch on fire if it absorbed too much microwave power.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[bjbdts]

Thanks for the responses.I tried to use a regular silicone o-ring(not electrically conductive)in the similar posion as in the drawing previously attached. The microwave leakage detected is a few mW. The power input is serveral hundred Watts. This is why in the new design I'm thinking using electrically conductive silicone ring may eliminate the few mili-watts leakage. In this case,Higgler, Is the 1.2" slot length still a problem? I just don't want the metal piece unnecessarily long.

 

[Comcokid]

There is a company that makes EMI O-rings that are spirally wrapped with metal.

http://www.spira-emi.com/index.htm

Although this may not seal the vacuum in your chamber, I know that many O-ring flanges setups will accommodate two O-rings. This would allow you to use one to seal the chamber as needed, and the second to seal the RF.

 

[Higgler]

If the power is truly tiny, then you shouldn't worry too much. The power may be small because your gap is just the right length (N x 1/4wave open length), and adding a short may actually interrupt a fair amount of current. Hard to guess at that. Darn Murphy and his law.


One common conductive o ring is a "spira-shield", it's a silicone gasket with a thin metal tape wrapped around it. The thin metal is usually around 0.1" wide or less and it's wrapped around the silicone noodle like you'd wrap tape around a hockey stick or bandage a leg or arm. This provides conductivity and flexibility. It comes in a long roll and is usually set in a machined groove like a normal O ring.

k