Thermite questions

[Uarbor]

So I still have this odd fascination with thermite. Part of it is trying to figure out the recipe for the exothermic welding used on copper ground rods. I want to be able to cast things of copper using thermite. It seems there is always a finely ground metal and an oxide used. My question is they must be using copper as the metal and copper oxide as the oxide otherwise the result would be something other than pure copper. The exothermic welding process really looks like pure copper. Do the metal and the oxide have to be different or can you use copper oxide and powdered copper?

[cmjlab]

For what it's worth, copper conductors are welded using Al and Copper or Copper Alloy. I've watched a few videos on how they weld industrial copper cables and conductors together - it's pretty cool.

 

Charles

[cmjlab]

My limited understanding is the Aluminum is necessary to interact with and/or prevent formation of copper oxides during the exothermic welding process (thermite welding).

 

Charles

 

**Edit** Here is a link to a product sold for that exact purpose, it looks like a rather course metal is used as well, as well as a graphite mold and lots of preheating (due to the heatsink effect of copper): https://acrobat.adobe.com/link/review?uri=urn:aaid:scds:US:63a960b7-30a0-31f9-8fac-d403a697e865

Edited December 29, 2022 by cmjlab

[Crazy Swede]

My question is they must be using copper as the metal and copper oxide as the oxide otherwise the result would be something other than pure copper. The exothermic welding process really looks like pure copper. Do the metal and the oxide have to be different or can you use copper oxide and powdered copper?

Why should copper oxide produce anything but pure copper if reduced? The oxides of the reactive metal will partly leave as smoke or stay on top like a porous crust.

 

The problem with copper thermites is that the process is so exothermic that if the ingredients are too finely powdered, the metallic copper often is spread like a cloud in an explosion, that partly will oxidise due to the atmospheric oxygen, so no copper will be cast.

 

Coarser particles help and the mixture can probably be diluted with something that does not dissolve in the copper. Maybe using non-stoichiometric formulas could also help?

[Uarbor]

Why should copper oxide produce anything but pure copper if reduced? The oxides of the reactive metal will partly leave as smoke or stay on top like a porous crust.

 

The problem with copper thermites is that the process is so exothermic that if the ingredients are too finely powdered, the metallic copper often is spread like a cloud in an explosion, that partly will oxidise due to the atmospheric oxygen, so no copper will be cast.

 

Coarser particles help and the mixture can probably be diluted with something that does not dissolve in the copper. Maybe using non-stoichiometric formulas could also help?

my copper oxide is extremely fine I have been considering granulating it for this process

[WSM]

Having worked in the industrial electrical trades for over twenty years, I can assure you the process uses course copper oxide and aluminum. The most common trade name is Cad-Weld.

 

The Cad-Weld shots come in a soft polyethylene container, sized to the mold and amount of copper desired to be cast in the graphite mold. The top 95% of the shot is course material (to control the rate of the reaction*) and the bottom portion is fine material, caked into the bottom, used for the ignition of the course material. The fine material is the same composition as the course, but with much more surface area (and MUCH more ignitable).

 

As with painting a house, PREPARATION is the biggest part of using thermitic welding. First, gather all the materials you'll need. Next prepare the mold by thoroughly heating it with a propane torch to drive off any moisture (IMPORTANT!). The molds require a thin metal cup or disk to hold the thermite above the casting part until the thermite is molten (these disks come with the Cad-Weld shots). If welding metals together, heat them too (to drive off any moisture present).

 

*Don't be deceived, even though the copper thermite is course, the entire reaction happens in about one second. After a few seconds, the mold can be removed and the weld inspected. CAUTION, even though the copper has solidified, EVERYTHING it touched is still very hot.

 

There is a lot more information needed to proceed safely. Google search Cad-Weld and see if you can learn more from them. I have practical experience using their products and can advise further, if needed.

 

WSM

Edited January 1, 2023 by WSM

[MadMat]

All thermites contain a metal oxide and a reducing agent (which in almost every case is a metal with a higher electronegativity) In the case of copper thermite, aluminum works just fine. If you want to learn more simply google electronegativity (chart). another example is; iron has a higher electronegativity than copper which means if you put a piece of iron into a solution of copper sulfate, metallic copper will coat the iron and the blue solution will begin to turn black from the formation of iron sulfate. Iron doesn't work very good for thermite though because there isn't a very high potential difference in electronegativity and that would seriously limit the amount of heat produced. I've never tried it, but it is possible that this limitation would result in a reaction that isn't self-sustaining. BTW lithium has the highest electronegativity as far as metals go, which makes it a great component of batteries.

[Uarbor]

Having worked in the industrial electrical trades for over twenty years, I can assure you the process uses course copper oxide and aluminum. The most common trade name is Cad-Weld.

 

The Cad-Weld shots come in a soft polyethylene container, sized to the mold and amount of copper desired to be cast in the graphite mold. The top 95% of the shot is course material (to control the rate of the reaction*) and the bottom portion is fine material, caked into the bottom, used for the ignition of the course material. The fine material is the same composition as the course, but with much more surface area (and MUCH more ignitable).

 

As with painting a house, PREPARATION is the biggest part of using thermitic welding. First, gather all the materials you'll need. Next prepare the mold by thoroughly heating it with a propane torch to drive off any moisture (IMPORTANT!). The molds require a thin metal cup or disk to hold the thermite above the casting part until the thermite is molten (these disks come with the Cad-Weld shots). If welding metals together, heat them too (to drive off any moisture present).

 

*Don't be deceived, even though the copper thermite is course, the entire reaction happens in about one second. After a few seconds, the mold can be removed and the weld inspected. CAUTION, even though the copper has solidified, EVERYTHING it touched is still very hot.

 

There is a lot more information needed to proceed safely. Google search Cad-Weld and see if you can learn more from them. I have practical experience using their products and can advise further, if needed.

 

WSM

thanks, I have been an electrician for over 30 years but I have not done a ton of cad welding. I do have a mold however so at least that's a start.

[Uarbor]

All thermites contain a metal oxide and a reducing agent (which in almost every case is a metal with a higher electronegativity) In the case of copper thermite, aluminum works just fine. If you want to learn more simply google electronegativity (chart). another example is; iron has a higher electronegativity than copper which means if you put a piece of iron into a solution of copper sulfate, metallic copper will coat the iron and the blue solution will begin to turn black from the formation of iron sulfate. Iron doesn't work very good for thermite though because there isn't a very high potential difference in electronegativity and that would seriously limit the amount of heat produced. I've never tried it, but it is possible that this limitation would result in a reaction that isn't self-sustaining. BTW lithium has the highest electronegativity as far as metals go, which makes it a great component of batteries.

very interesting thank you

[Arthur]

Simply as an observation, crackle and dragons egg compositions all rely on black copper oxide. They burn too fast for thermites.