A pure molybdenum grid- not

[Finn Hammer]

A survey of the refractory metals suggested that Molybdenum could be the ultimate grid material. High thermal conductivity, high melting point, easier to machine than tungsten.
I ordered a couple of rods from a German Zintermetalle dealer and initially felt good about it. It machines like very hard cast iron, so no problem there.
But it is sintered, put together from powder, it is perhaps porous on some micro or macroscopic level, I wonder what's in the inevitable voids.
I also wanted to see, if it was possible to weld it, so I ignited the TIG electrode to the end of the 6mm rod, which was easy to melt. So far so good, this material can be manufactured with comparative ease.
When I took the welders shield off, the stick was red hot about 60mm up from the end, indicating the good thermal conductivity, but it was also smoking as if it had been dipped in oil.
But it was cleaned thoroughly before heating it.
I tried to heat it 3 times more, and it keeps smoking excessively.
I do not think I am going to outgass this material with my fine vacuum station as the fume extractor, and so, this looks like the bitter end of an interesting venture.

Cheers, Finn Hammer

[Victor Gonzalez]

Try to find electron beam melted Molybdenum, it should solve all of those problems, it will be expensive, just don't know how much.

Molybdenum crucibles are used to grow pure synthetic sapphire (>2000°C). The same supplier for the crucibles should be able to sell a rod of it.

I think samaterials.com should have it.

[Frank Sanns]

Sintering can lead to porous or non porous end product depending upon the temperature and pressures used to form the item. My guess is that the molybdenum is non porous for the applications that it is normally used for.

Outgassing does not normally produce smoke. Sintering is done in an inert or vacuum atmosphere so there should be nothing that should produce smoke in there.

A metal can have a high temperature melting point but it does not mean it is inert in an oxygen atmosphere. I suspect there is reaction with surrounding air (oxidation) going on and not outgassing. Oxides of metals are typically heavy white smoke. That would be my bet.

In a Fusor, there is no air so it is a non issue.

[Richard Hull]

My first fusion grids were pure Tantalum. I just assembled them, slapped them in the fusor and they did great! I have also used Hafnium as well. No prep, no worries, fusion.... I will never use any grid but tungsten now.

If you persist with molybdenum.... Make your grid. Install it in a junk chamber take it down with a junk mechanical pump and bombard the hell out of it at high current. This will do for an outgassing.
Or make your grid and heat it in a carburizing oxy-acet flame to orange heat. Frank is right about oxide formation. Oxides are fine in most refractories in vacuums.

Richard Hull

[Victor Gonzalez]

I don't know about molybdenum powder metallurgy, if pure Mo powder can sintered. I know that tugsten carbide needs to be sintered with cobalt, but it's a compound and not a pure element.

Mo does oxidize, Periodic Videos on youtube made a nice video about it:

https://youtu.be/0cUYNaJvSbY

SA Materials does have Mo, including Mo boats to melt metals in high vacuum.

[Finn Hammer]

Guys, thank you all, you are amazing.
When I posted, it was with a faint feeling: I am giving up too early!
Mo reacts with atmosphere, I had read about it, but forgot in the heat of action.
Now I am going to push forward with the manufacture of the Mo version of the grid.

Cheers, Finn Hammer

[Dan Knapp]

Research groups in Japan have made extensive use of Mo grids, particularly Kai Masuda’s group in Kyoto. The highest reported neutron yields have been obtained with Mo grids made from rings of sheet Mo.

[Frank Sanns]

The other interesting fact about refractories is their starting base metal.

In the case of aluminum, the metal has a melting point of 660 C. Heating it in air will form the oxide which has a much higher melting point of 2,072 C. Alumina is a great refractory.

Here is a question for you (rhetorical), why then is steel (iron) cut with an oxygen torch? It is heated to red heat, then oxygen is blasted out of the central torch nozzle. The steel just melts away but only in the path of the oxygen jet. Should not the oxide be higher melting and form a skin of high melting oxide layer that protects he base metal?

Using aluminum, that indeed would be the case. You cannot cut aluminum with an oxygen torch.

It turns out that different group metals have different behaviors. The oxides of iron actually have a significantly LOWER melting point than the base metal. Blasting oxygen to the hot iron makes the oxides (and more heat) that melt at a lower temperature and they flow away in the stream.

Molybdenum is like iron in that its oxides have much lower melting and boiling points than the base metals. Molybdenum melts at 2,900 C while the oxide melts at 795 C. The boiling point is also similarly drastically reduced with the molybdenum oxide boiling at only 1,155 C.

Heating to incandescence will definitely make the oxide and even if below the actual boiling point, the vapor pressure will put much oxide into the air as dense white smoke.

I know the video provided earlier in this thread shows this but this more chemical explanation might augment what you were seeing.