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boiling off electrons

Discussion in 'Electronic Basics' started by RichD, Jan 22, 2009.

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  1. RichD

    RichD Guest

    I don't know much about vacuum tubes, but often
    I have read statements like "the cathode heats up,
    causing electrons to boil off and fly to the anode"..
    which makes me scratch my head..

    How does an electron boil? Can anyone explain this?

    Are there degrees of boiling, like a pot of water,
    or is there an on/off threshold?

    Then there's the grid mask, whatever that is... how
    come the electrons don't smash into that? How do
    they find the holes? Is this like synchronized diving,
    they're trained to hit the water at specified spots?
     
  2. Guest

    To put it simply, the cathode material is chosen to be a material
    that releases electrons when heated.

    Google thermionic emission if you want details.
    They do sometimes but they mostly don't as the grid is negative and
    serves to throttle the electron flow.

    Google control grid if you want details.
     
  3. Phil Allison

    Phil Allison Guest

    "RichD"

    ** It's analogous to a liquid boiling and releasing molecules of itself as
    vapour.


    ** Yeah - like there's " old boiler " & " pot boiler " ......


    ** Strictly temperature related, just like ordinary boiling.

    Only it needs to be in a vacuum to be effective.


    ** Cos it is 99% open space.

    Look at a pic of a grid for god's sake.


    ** Sigh......


    ** Yeah - they do clever back flips and all.

    Wanker.


    ...... Phil
     
  4. Jon Kirwan

    Jon Kirwan Guest

    + Richardson equation; work function; partial field emission; LaB6;
    tungsten; pure field emission (hot [schottky] or cold [very tiny
    tip]); quantum tunneling; Fowler/Nordheim curves; Wehnelt.

    Jon
     
  5. jmfbahciv

    jmfbahciv Guest

    Oh, man! I've only seen two of the items on your list in my
    lifetime. :)

    /BAH
     
  6. Jon Kirwan

    Jon Kirwan Guest

    I worked for a short time (a year +) with _the_ electron microscope
    company in a small technical capacity. Some things managed to rub
    off. The emitters I worked with used a combination of thermionic
    emission and field emission (partial) and the Wehnelt is pretty much
    impossible to miss, as well. LaB6 was one of those materials
    sputtered on for enhanced effect, as I recall, on some emitters (other
    materials were also used.) Work function is obvious -- can't avoid
    the concept for even a few seconds, as it permeates everything
    interesting in the world. Richardson, Fowler, and Nordheim get name
    dropped from time to time, as do others.

    Jon
     
  7. Salmon Egg

    Salmon Egg Guest

    Look at: http://en.wikipedia.org/wiki/Thermionic_emission

    In other responses red herrings like anodes and grids were mentioned.
    While important for devices, such elements have little to do with the
    fundamental process at the electron emitter.


    Bill
     
  8. Benj

    Benj Guest

    Not quite a correct description, because once they "boil off" it is
    the electric fields in the tube that cause them to be accelerated and
    "fly" to the anode.
    Pretty much like water does. In water you have a "sea" of water
    molecules bouncing around off each other. The amount of action if
    determined by the temperature of the water. And the distribution of
    water molecule velocities has a statistical distribution with some
    having high velocities and some low. When you start to have lots of
    high velocity molecules some of them start to leave the liquid and are
    vapor (or a cloud of individual molecules).

    In a metal, one pretty much has a "sea" of free electrons. These also
    have a statistical distribution of velocities that depends on
    temperature. Again, if you get the temperature high enough there are
    enough "fast" electrons that they are leaving the metal.
    There is not a distinct 'boiling point" like with water, but the
    number of electrons escaping does depend on temperature. Usually this
    temperature has to be pretty high. Edison discovered the tube by
    noting the current that flowed when he put a metal plate inside one of
    his light bulbs. However there are distinct lifetime advantages if
    you can get eletron emission at lower temperatures. 20th century tube
    "technology" was very much involved in the black magic of producing
    long-lived low temperature high current cathodes. Today, some of this
    old technology appears to be lost, like the technology of how to make
    a decent mummy.
    A grid is just a bunch of wires that is use to create an electric
    field to accelerate the electrons away from the cathode. (or repel
    them back to it). Electrons DO smash into it. But like a garden hose
    at chicken wire, most of them go through the holes. The grid is MOSTLY
    "holes"! There is no need to have them all go through the holes.
    [Well usually, if we were to discuss the theory of beam power tubes,
    we'd talk about the problems of grid heating due to the electrons that
    smash in, but this is no problem in smaller tubes.] PLUS, it depends
    on how the tube is being operated. If the grid is negative, then it is
    creating a repelling field. The field gets stronger as you get near
    any given grid wire so the electrons tend to be deflected away from
    the grid wires. OK?
     
  9. Benj

    Benj Guest

    Oh look Muffy! It's a guy who knows "everything" but is willing to
    explain nothing!
    (much of which has little to do with the subject in question except
    tangentially)

    Electron microscopes are not "tubes".
     
  10. Jon Kirwan

    Jon Kirwan Guest

    I honestly don't have the time to carefully walk someone from 0 to 60,
    right now. But there is plenty on the web to read.
    No, but that doesn't mean that the terms I provided are not relevant
    to a search, does it?

    Jon
     
  11. Rich Grise

    Rich Grise Guest

    They certainly are! They're just significantly larger than, say, a 4-400A,
    and the target (DUT) is the anode. ;-)

    Hope This Helps!
    Rich
     
  12. BradGuth

    BradGuth Guest

    Thorium gives off electrons rather nicely when heated, although radium
    (Ra226) gives off electrons when stone cold or even when cryogenic
    cold. Radium is a cold cathode electron emitter, and otherwise
    extremely nifty beyond most imaginations. Radium is also one of the
    most secretly horded elements on Earth, with a 1600+ year half life to
    boot.

    ~ BG
     
  13. Salmon Egg

    Salmon Egg Guest

    Radium is an ALPHA emitter, not an electron emitter. It is possible that
    some daughter isotopes are BETA (electron) emitters. Radium is in the
    same column of the periodic chart as is barium and strontium. These are
    sometimes used, especially as oxides, for low temperature cathodes,
    Thus, radium is also likely to be a relatively low temperature
    thermionic emitter, stupid as it may be to use it as such,

    Bill
     
  14. neon

    neon

    1,325
    0
    Oct 21, 2006
    AT absolute zero there is not much going on but as you elevate the temperature of metal electrons orbits becomes unstable. Electrons actualy do not boil off but they migrate to the plate which is guess what positive therefore electron flow.
     
  15. Guest

    ROFL Bill,

    This thread is getting more humorous as it proceeds. Let me add my bit
    to the chuckle heap.

    Radium indeed emits Alpha particles naturally. Still, when heated to
    near incandescent termperature, is capable of emitting electrons. Not
    strange, very little information is published on Radium's surface
    barrier potential for electron emission.

    Perhaps our govenment will find a way to fund such "Important"
    research. :-<

    Harry C.
     
  16. Benj

    Benj Guest

    This could explain a factoid I know. I do know that right before
    transistors put tubes mostly out of business, tube research was making
    great strides. I was assured that a cold cathode had been developed
    that produced enough current to power a vidicon tube. I was told that
    developers were quite confident the current levels could be easily
    increased to more typical tube service with some development. (As you
    know the big shortcoming of tubes was the energy wasted heating the
    cathodes). But all that technology seems to have disappeared along
    with the big tube manufacturers of the day. Today even the standard
    tube technology seems to have largely evaporated leaving former
    communist countries and third world countries to make some less than
    stellar tubes to satisfy tube-loving guitar players.

    It is in interesting story that I once applied for a job at MIT and
    based on the above information I told the interviewer that I thought
    transistors were a "passing fad". I didn't get the job! Later my
    mistake became obvious. Do you know what it was? In those days both
    tubes and transistors were constructed as 3 dimensional devices. Hence
    with no heater losses they were basically equivalent devices with the
    tubes having MUCH superior performance in the day. But it was the
    Fairchild "planar" process that made the "integrated circuit" possible
    that eventually made solid state the hands-down winner over 3-D wired
    and spot welded construction. I doubt that the MIT interviewer knew
    this at the time, but doubtless thought I was "wrong" based on the
    large power needed to run tube heaters. It's hell knowing too
    much! :)
     
  17. Salmon Egg

    Salmon Egg Guest

    I had posted that the extra elements in a vacuum had little effect on
    the basic thermionic emission processes. While basically true, some
    tubes with field emission cathodes have been produced. In particular,
    x-ray tubes requiring relatively little current at high voltage fit that
    bill. A high voltage on a nearby anode can produce significant current
    from field emission points without preheating the cathode.

    Bill
     
  18. Eeyore

    Eeyore Guest

    Some of them DO ! Look for example at limits for screen current.

    Graham
     
  19. Eeyore

    Eeyore Guest

    And thoriated filaments / heaters were found to work rather well around
    the time of WW2 luckily for the radar pioneers.

    Graham
     
  20. Salmon Egg

    Salmon Egg Guest

    With the average energy of one electron volt corresponding to 11,600?K
    per half degree of freedom, you are not going to get very large currents
    of several electron volt electrons.

    Bill
     
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