Hi Ana,
thanks for replying!
To answer your last questions first. :-)
I have enjoyed the IIW Assembly very much! It were five great days in Graz and I have met again some of the very greatest ones in the welding world over there! By the way, the weather was fine as well was Graz. A 100% recommendation for a visit - even a pure university's city, many students and an appealing old downtown, amazingly pulsating... All in all it was a true blessing again to having had the honor to participate this event.
To come back to our particular subject. Yes! I have met Prof. Manabu Tanaka as I have met Prof. Yoshinori Hirata as well, who is the Chairman of the IIW Study Group 212 (Physics of Welding).
In the course of the SG 212 Meeting, TANAKA has presented his paper on "Visualizations of 2D Temperature Field of Molten Metal in Arc Welding Process" (IIW Doc. 212-122-08). Hmmm, most excellent! Even TANAKA!
HIRATA has presented his paper "Magnetic Control of Arc Plasma and its Modelling" (IIW Doc. 212-1128-08). As well and as always an enjoyment to listen to...
One of the - at least for me, but surely not only for me - most impressive papers however came from John LOWKE (et TANAKA) dealing with "Electrode Heat Transfer in MIG Welding" (IIW Doc. 212-1119-08). To be honest with you. For me Prof. Lowke is one of the most outstanding arc physicists in the world and it is a true blessing for everyone who has the honor to listen devotionally to his wise words. He has presented a new and self developed theory on even the subjects we are discussing about here presently in the AWS forum. He has tried to find a reasonable explanation of the cathode sheath behavior in GMAW by checking the existing theories dealing with this subject (field emission, thermionic emission) and has found out that no existing approach can precisely explain the voltage drop with a GMA cathode. He has suggested an approach dealing with "Metastables" as he calls the particles which might arrange and govern the observable GMA regimes. Extremely interesting and presented by John Lowke himself(!). An unforgettable experience...
But to come to the point - forgive me that I am raving... :-).
Well, I have asked Prof. Tanaka as Prof. Hirata on their experiences in finding values for metal oxide work functions or the accuracy of even the known existing values. Both answered that they are getting the values they are using for their work from even the FORMENKO book, you have already found, as you said. These are the information they are using for their calculations and they have never tried to doubt on these values or have tried to question even those. I felt reminded on myself, since I have learned once the values for Al2O3 and pure Aluminum and since then I am using them for any discussion on this subject. You were the one who has "stung" me for questioning the values for the first time in my life, to be honest.
Well, to keep the long story short. There was another, extremely likeable, fellow, Michael Schnick, a great expert in numerical simulation coming from the University of Dresden and who has held a presentation on his paper: "Numerical Investigations of the Influence of Design Parameter, Gas Composition and Electric Current in Plasma Arc Welding" (IIW Doc. 212-1127-08). He has participated with our discussion and he has given me the following very interesting information. First of all, he said, the discussion on the validity of work function values is a very reasonable one, since nobody knows exactly how the values have been taken - just as we have mentioned and assumed now for several times. He said that he has visited the Max Planck Institute Department for Molecular Physics in Berlin. Here they should have a very particular apparatus making it possible to deposit metal oxide layers of 1(!) Angstrom upon substrates to subsequently measure their work functions.
To exactly determine the correct work function - e.g. Al2O3 - which should actually normally exist in this stoichiometric composition - even just a layer of that minimal thickness is necessary, the fellow said.
The "normal" layers of metal oxides whereas but in particular Aluminum Oxide (let us remain with Al2O3 to not unnecessarily complicate the issue) showing thicknesses of 50... 60 Angstrom(!). And now it comes...
When we are considering the tunnel effect as the main effect for that a valence electron may escape from the oxides surface structure, then we have to consider as well the electron's wave length in relation to the width of the potential well. Are both values - please correct me when I am wrong (you know I am nothing but a layman!!) - comparable similar, the tunneling is caused.
Now were my question, how great is the likelihood for an electron to escape when it has to tunnel a layer's thickness of 50... 60 Angstrom? Wouldn't the resonant frequency of all the atoms a/o molecules hinder an interior electron to tunnel throughout the oxide layer thus the likelihood of an electron to escape drops down to ~ 0 ?
Hmmm, questions over questions - as usual. However, as the colleague from the University of Dresden said, there are presently investigations being performed to approve the well-known values for metal oxide work functions or to determine even new values with the highest technological accuracy achievable at present, respectively.
And now coming finally to a work function value for Al2O3 I could find out with the great help of my colleagues. The value comes from the book:
"Gaseous Conductors - Theory and Engineering Applications"; James Dillon Cobine, 1941.
Cobine should state within the mentioned book another reference:
A.L. Hughes and L.A. DuBridge, "Photoelectric Phenomena", McGraw-Hill Book Company, Inc., N.Y. 1932
and
S. Dushman, Rev. Mod. Phys., 2, 381 ,1930.
for the Al2O3 work function value:
3.77 eV.
As my fellas from the numerical simulation department say, by comparing other metal oxide work functions the value appears to be quite sufficient as to be accepted for calculations.
As you can see, there is not that much left from the very first beginnings of this thread where we have used 1.77 eV for the work function of Al2O3. However, perhaps the people who have prepared the papers (data sheets) have failed by writing 1.77 eV instead of 3.77 eV. Assuming that the pure Aluminum might have a work function of a slightly > 3.77 (e.g. 3.95eV) the oxide would nonetheless provide the first electrons under the influence of an exterior force (field or thermionic). At least as long Prof. Lowke were wrong by stating that the field forces with welding are quite too low to dissolve an electron from the cathode (workpiece) and thus the existing theories might prove. :-)
To be honest, I truly trust Prof. Lowke and his calculations very much, thus I doubt currently on the existing theories. So to speak... I am appearing to be a little confused, since the more I try to learn the less I mean to know.
Perhaps you, Ana, can point all of us welders in the right direction when you have prepared your paper, which I guess will be extraordinary good!
Please let me make one short sentence to your wonderful predication:
" In welding, if I understand this correctly, you charge an electrode in a welding machine and use the substrate (material you want to weld) as ground, which makes a capacitive plasma. So, when you use AC power on your charged electrode, don't you have trouble on polarity crossings? Your arc doesn't extinguish? And why do you use AC power? DC with positive charge on the electrode would produce nice, thin and powerful arcs. Wouldn't it?"
Yes, you are absolutely right in your understanding. And yes also, we would having troubles when we were using AC for - in particular - Gas Shielded Tungsten Arc (GTAW) Aluminum Welding (destroying the oxide layer) without having power supplies who would support us in the periods the arc does extinguish in polarity crossings. GTAW DC arcs (electrode as cathode) are feasible here when using Helium as a shielding gas. This however runs smooth and quick and yields a high fusion depth at all.
My best regards and please keep us updated with your fascinating profession!
Stephan