How does copper thermite differ from other thermites? What makes it special?
Broadly speaking, a thermite is simply a mixture of a metal oxide and a more-reactive metal. When ignited, the oxide gives up oxygen, which then combines with the other metal, making a different oxide.
The thermite mixture that most people are familiar with is Iron Oxide (aka "rust") and Aluminum. This mixture is extremely hard to light, but once it does get going, it produces a slag of aluminum oxide floating atop a blob of molten iron as its reaction products. Railroads used to use this thermite mixture to weld train tracks together.
Aluminum is a very reactive metal, with a high affinity for oxygen. In fact, if you saw through a block of aluminum in the open air, a thin coating of aluminum oxide will form over the newly-exposed surface within seconds.
Iron holds on to its oxygen fairly tightly, but not as strongly as aluminum does. Therefore, while it takes a fair bit of energy input to get iron oxide to let go of its oxygen, the overall reaction is "exothermic" and self-sustaining.
Copper oxide is much easier to break down than iron oxide. It only takes a little bit of energy input to get it to give up its oxygen, which then combines immediately with the aluminum.
In chemistry terms, we look at balanced equations for the reactions, and evaluate the net change in enthalpy, based on each compound's enthalpy of formation.
|Compound||Heat of Formation|
|Iron (III) Oxide, Fe2O3||-824 kJ/mol|
|Copper (II) Oxide, CuO||-157 kJ/mol|
|Aluminum Oxide, Al2O3||-1676 kJ/mol|
So, if we scale to multiples of 1 mol quantities:
1 mol (Fe2O3 -824 kJ/mol) + 2 mol Al ---> 1 mol (Al2O3 -1676 kJ/mol) + 2 mol Fe + 852 kJ excess heat
3 mol (CuO -157 kJ/mol) + 2 mol Al ---> 1 mol (Al2O3 - 1676 kJ/mol) + 3 mol Cu + 1205 kJ excess heat
It only takes 157 kJ to disassociate each mol of CuO, compared to 824 kJ to dissasociate each mol of Fe2O3. Therefore, it's much easier to light copper thermite than iron thermite. In both cases, however, a lot of excess heat is produced, making the reactions exothermic (literally "outputting heat") and self-sustaining.
To reliably light an iron thermite mixture, it takes something like a burning piece of magnesium ribbon, or a fireworks sparkler. To reliably light a copper thermite mixture, you just need the tiny flame from an electric match. Copper thermite is MUCH more practical to use for motor ignition.