Isn't vacuum the best insulator?

[enroger0]

Hi, I always thought vacuum is the best insulator, until I find figures that state breakdown field for vacuum is about 2*10^6V/cm, while some dielectrics breakdown in field higher than that(Al2O3 is 4~5*10^6).

So I'm wondering why? I mean electrons in vacuum has the highest potential energy compared to electrons confined in some lattices right? Vacuum is supposed to be the highest possible potential barrier?

 

[ZapperZ]

These are phenomenological numbers, i.e. they were obtained out of experiment. You need to figure out the circumstances that these were arrived at.

Zz.

 

[Dadface]

And it is impossible to create a perfect vacuum.

 

[Creator]

Hi, I always thought vacuum is the best insulator, until I find figures that state breakdown field for vacuum is about 2*10^6V/cm, while some dielectrics breakdown in field higher than that(Al2O3 is 4~5*10^6).

So I'm wondering why? I mean electrons in vacuum has the highest potential energy compared to electrons confined in some lattices right? Vacuum is supposed to be the highest possible potential barrier?

Probably has to do with the 'branching' characteristics of the breakdown streamers in that particular material. Usually it starts non linear, but as the branching progresses, it can get much more linear since the streamers have a self-avoidance character.
see here ... http://math.nist.gov/mcsd/savg/vis/dielectric/index.html

especially figure 3 , for example...
"
"The front of the growth, facing the counterelectrode, has become rounded and brush-like; this rounding counteracts the field enhancement from the diminished gap distance, so that forward growth proceeds at a nearly constant rate".

Creator

 

[Minich]

Electron in vacuum is a FREE electron and vacuum is not a barrier at all. You may consider a vacuum as a very highconducting medium (due to rare collisions).
When you switch electric field on, vacuum does not resist to electron motion.
For solids ionization electric field is about (1-10 ev)*(dielectric constant)/((atom size)*10).
10 is for reliability )
Vacuum does not screen electric field (dielectric constant=1). Many dielectrics have dielectric constant >> 1.

 

[enroger0]

Thanks for the replies guys. I realize I didn't state the condition of these numbers, they measure those number by applying voltage between two metal separated by dielectric/vacuum and see at what field strength there will be a significant increase in current.

I think maybe the workfunction of those metal conductor they use will have a big effect, smaller workfunction => less field needed to bring a electron to vacuum?

So I think bringing a electron from whatever metal into a dielectric(lattice)'s conduction bandedge should require less energy to bring that same electron into vacuum?

So my question really is: given two fixed metal plate, you either separate them with vacuum or dielectric materials like Al2O3/SiO2...etc. So which one have a higher breakdown voltage?

some random paper I found over the internet:

vacuum breakdown field: wire plane config
http://www.springerlink.com/content/rq05360171217x27/

aluminum oxide breakdown in metal insulator silicon config
http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=00817577

silicon dioxide breakdown
http://adsabs.harvard.edu/abs/1996SPIE.2874..114C

 

[ZapperZ]

You need to be a bit more careful here. "Contact potential" need not be the same as the work function. In fact, in most cases, it isn't.

The mechanism for a breakdown, especially vacuum breakdown, is highly complicated and still being debated. The "spark" that one sees is the ionization of low pressure gas that exists surrounding a high field-enhancement point. How the gas gets there is still being debated. One could argue that the field current generates a localized heating and causes an outgassing. That's just one mechanism. There are many.

So it isn't a surprise that a vacuum breakdown limit for a material can some time be less than a material interface breakdown. The mechanism can be very different for different interface.

Zz.