Isn't vacuum the best insulator?

In summary, the breakdown field for vacuum is relatively low, and some materials breakdown in field higher than that. Probably has to do with the 'branching' characteristics of the breakdown streamers in that particular material.
  • #1
enroger0
21
0
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?
 
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  • #2
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.
 
  • #3
And it is impossible to create a perfect vacuum.
 
  • #4
enroger0 said:
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
 
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  • #5
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.
 
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  • #6
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
 
  • #7
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.
 

Related to Isn't vacuum the best insulator?

1. What is vacuum and how does it insulate?

Vacuum is a space or container that is completely devoid of matter, including air molecules. It insulates by preventing heat transfer through conduction and convection, as there are no particles present to transfer energy.

2. Is vacuum the best insulator compared to other materials?

Yes, vacuum is considered the best insulator as it completely eliminates conduction and convection, which are the main modes of heat transfer. Other materials, such as foam and fiberglass, still have some air particles present and can still conduct heat to some extent.

3. How does vacuum insulation work?

Vacuum insulation works by creating a barrier between two environments with different temperatures. The vacuum prevents heat transfer through conduction and convection, while the reflective lining of the vacuum container prevents radiant heat transfer.

4. Can vacuum insulation be used for all types of insulation?

No, vacuum insulation is not suitable for all types of insulation. It is most commonly used for thermal insulation, but it may not be effective for sound or electrical insulation.

5. Are there any drawbacks to using vacuum insulation?

One drawback of vacuum insulation is that it can be expensive to create and maintain a vacuum-sealed environment. It also requires strong and durable materials to maintain the vacuum, which can add to the cost. Additionally, vacuum insulation may not be suitable for all applications, as it may not provide enough insulation in extreme temperatures.

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