Missile?
Tell me more please, how will small bit of gold coated grid wire go balistic?
Ok not organic enough for a MRI type of machine. Pick your spectrometer to count the bits of barium that migrate into the matrix of the gold.
Nickel is ferromagnetic, NMR spectrometers have a very big coil in order to obtain high magnetic field, then the valve could be threw away like a missile, a beautiful glass bottled missile.
Not my spectrometer, but a friend of mine.
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So it is said and no doubt believed in some texts, but does it stand scrutiny and is it true ?
Three problems cause me to doubt. 1) BaO prefentially evaporates from the cathode, so seems more likely that is what is deposited on the grid rather than Ba metal, no reason for dissociation AFAIK 2) Ba does not 'readily dissolve' in gold in the solid phase, even the self-diffusion coefficient of gold is lower than most common metals used for grids. Metal diffusion is a slow process especially at likely temperatures. 3) Only a thin film of gold is required to be effective, seemingly insufficient to conserve a concentration gradient (required for diffusion) for long.
Sorry, I did not read correctly, the grid will be vaporized because is a short circuited loop.
The grid has usually two nickel side rods, then in addition of a vaporized grid you will have two missiles. 😀
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Is it energetically possible that Ba3Au2 bonding forms as a compound (alloy?) on the gold surface, liberating O2 ?
Something like 6BaO + 2Au2 > 2Ba3Au2 + 302
Hermann/Wagener Vol 1 p100 states that most evaporation happens in the process of activation or early in use, and elsewhere that grid current typically stabilises within 25s of manufacturing initial switch on. Very little evaporation in use, it says. I wondered whether any such burst of evaporation landing on the grid might get dealt with as per the above dissociation ? I did say I wasn't a chemist ..............
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Something like 6BaO + 2Au2 > 2Ba3Au2 + 302
Hermann/Wagener Vol 1 p100 states that most evaporation happens in the process of activation or early in use, and elsewhere that grid current typically stabilises within 25s of manufacturing initial switch on. Very little evaporation in use, it says. I wondered whether any such burst of evaporation landing on the grid might get dealt with as per the above dissociation ? I did say I wasn't a chemist ..............
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^ Given the bulk and heavy nature, EDX would work, I was thinking TOF-SIMS. XRD/XRR might be nice to see any subdomain characteristics.
*I have access to these as well, but I'm not authorized on the machines. Sigh.
*I have access to these as well, but I'm not authorized on the machines. Sigh.
You're good then. We were doing metal oxides in my old lab and teh EDX wasn't good enough to tell %oxygen for us to know the specific composition of our materials.
Who sez?
The vapour pressure of Ba at 1000K is 10^9 times that of BaO. See CRC manual for Ba constants. See Blewett J P, Liebhafsky H A, Henneley E F, The Vapor Pressure and Evaporation Rate of Barium Oxide, J Chem Phys 7 478 1939.
All compounds dissociate at any non-zero kelvin temperature - but the rate goes up very sharply with temperature, many substances increasing their dissociation rate of the order nx10^6 for each 100K rise in temperature at ordinary temperatures.
The very slow dissociation of BaO is driven by the cathode operating temperature, 1050 K.
Who sez? References?
It only has to be a very slow process, taking 1000's of hours to absorb picogrammes.
You have ansolutely no basis for saying that, as you have no numbers to put on it.
Assumptions and old musty books
Assumptions and old musty books.
I love the old musty books. We can not sit down with the Grey beards and talk tube teck.
The serious Comercial study and development of tube teck perty much stopped with the dominance of semiconductor teck.
Today we have 2015 tools to test and calculate down to the PPM level. I have teck on my bench to test noise and distortion in the frequency domain. Wish I had tools to grind up a 1953 Buick and test for gold to the PPM level, I do not.
Thasks all for sharing your thoughts in this thread.
DT
Assumptions and old musty books.
I love the old musty books. We can not sit down with the Grey beards and talk tube teck.
The serious Comercial study and development of tube teck perty much stopped with the dominance of semiconductor teck.
Today we have 2015 tools to test and calculate down to the PPM level. I have teck on my bench to test noise and distortion in the frequency domain. Wish I had tools to grind up a 1953 Buick and test for gold to the PPM level, I do not.
Thasks all for sharing your thoughts in this thread.
DT
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There's a certain analogy at work, yes. Electrons in equilibrium (zero applied voltage) don't go anywhere, so they just mill around due to heat, most being resorbed in the cathode, some being absorbed in the plate. More of a thermal diffusion condition (the same phenomenon occurs in semiconductors, if you apply a temperature gradient to a junction), plus a barrier (the surfaces separated by vacuum, too wide to tunnel through, requiring thermal activation).
Down in this region, current is exponential with voltage, due to the statistical tail of energy levels.
As you turn up the bias, more and more electrons go through. At some point, this begins to deplete the space charge and increase emission (at least from the local cathode area, though presumably the total exchange of electrons with the surface, at the exact surface itself, and within crevices near and inside the bulk, remains constant).
Eventually, the space charge is depleted or stripped away, and full emission is had. This region is less like Le Chatelier's principle and more like kinetically limited reaction rates: it "wants" to proceed (there is a large energy gain to be had, so that the reaction proceeds substantially forward only), but there is a limited supply of reactive species (conduction electrons) to proceed. Increasing the temperature would speed up the process (in both cases), but have undesirable side effects (side reactions, cathode failure..).
Tim
The jury is still out as to whether there is any notable 'carrier emission surplus' at all under conditions of a retarding field which arises in typical grid controlled valve where conduction is carrier limited........ IMO anyways.
There is indeed a surplus.
Under normal operation of a vacuum tube (ie grid negative), there is a substantial space charge (electron cloud) between the cathode and the grid.
This electron cloud acts as a virtual cathode much closer to the grid than the real cathode is. In consequence, the mu of a triode, for example, is substantially greater than the relative grid-cathode and anode-cathode spacing would suggest.
The build up of electrons before the grid can be viewed as somewhat like cutomers at the check-out of a grocery store. So long as the check-out chick processes purchases just faster than customers arive, there is virtually no queue. But if she is just a bit slower, the queue builds up until there is so many customers waiting, the sight of them induces others to leave without buying.
So it is with vacuum tubes. So long as the grid is zero or positive, all electrons go through, and the space charge between cathode and grid is negligible. But with a negative grid, more electrons are emitted from the cathode than get through the grid. An electron cloud builds up to an equlibirium where the number turned back to the cathode by the space charge is equal to the surplus of emitted over those let through the grid.
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So it is said, but whether it stands scrutiny and is always true is another matter........and that's another (old) thread topic. A matter of whether the external field is sufficient to (mostly) contain charge carriers within the physical boundary of the cathode surface, or whether they are penned up beyond the boundary by it. Sometimes, not always or necessarily was my opinion IIRC.
But in terms of primary emission from the grid, at issue here, it's a moot point, because the external field is accelerating, and grid emission current is saturated there is no equilibrium...........
Nine order of magnitude just can't be right ! Perhaps the units or conditions are confused between the two sources ?
Below is a marked up plot of evaporation rate of BaO and Ba versus temperature, obtained from the same source (Hermann/Wagener The Oxide Cathode 1951 Vol1 pp100, Vol2 sec 31), clearly illustrating that BaO is preferentially sublimed/evaporated over Ba by perhaps an order of magnitude or two.
What's more, there's a weight of literature discussing the evaporation of BaO from oxide cathodes as an issue, mostly associated with the process of activation when the cathode is briefly at elevated temperatures.
As to diffusion constant of Ba in Au, for the range of temperatures likely to be found in a grid, I can't find one. So I base my opinion on self-diffusion rate for Au, which is published, and describes intrinsic lattice migration - it is smaller for Au than any other likely grid metal. If you know a value for Ba-Au diffusion constant for a compatible range of temperatures, bring it on.
And no, if Ba is on the surface of BaO even in small quantities, the surface will be active and emit - so there's not '1000s of hours' to wait around for metallic diffusion.
In fairness, apparently not much Ba is necessary to activate BaO emission, so seems reasonable that sublimation of an activated BaO/Ba cathode and deposit of a mix of BaO/Ba on the grid would be a viable primary emission source - hence the issue. The challenge is to find a satisfying answer that stands scrutiny as to why gold, silver and a few other materials significantly supress this........
Anyways, here's the plot showing relative evaporation/sublimation rates for BaO and Ba:
Attachments
Boundries
Boundries
My concern with these triode strapped pentodes is between the boundries of diode operation and cutoff, in the sweet space of good gm controlled hf noise and excess flicker noise at the bottom end. About the point where the tube just begins to break a sweat.
DT
Boundries
My concern with these triode strapped pentodes is between the boundries of diode operation and cutoff, in the sweet space of good gm controlled hf noise and excess flicker noise at the bottom end. About the point where the tube just begins to break a sweat.
DT
If there's any interest in this, if someone can send me some grid wire (don't handle with bare hands!) taken from an unused gold-grid tube, one of the same type with a few minutes of use, and one of the same type with longer term use), I can run them. Code them so I don't know which is which.
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