Thermites are a double-edged sword. Packing a tremendous energy density, and eager to produce tremendous heat when ignited, thermite is great for welding train tracks. But sometimes you might be looking for a little more finesse. A new approach to 3D printing thermites might just be able to tame the beast.
Most of us do our soldering while sitting safely indoors in a comfortable climate. The biggest dangers we’re likely to face are burnt fingertips, forgetting the heat shrink, or accidentally releasing the smoke monster. But outside of our homes and workshops, there’s a lot of extreme joining of metals going on. No matter where it’s done, welding and brazing in the field requires a lot of equipment, some of which is unwieldy and even more difficult to move around in harsh conditions.
The utility of brazing is limited by all the complex scaffolding of hardware required to support it. This limiting factor and the discovery of thermite led to exothermic welding, which uses an energetic material to provide enough heat to melt a filler metal and join the pieces. Energetic materials can store a lot of chemical energy and forcefully release it in a short period of time.
Thermites are made of metal oxide and metal powder, often iron oxide and aluminium. When ignited by a source of high heat, thermite compounds undergo an exothermic reduction-oxidation (redox) reaction as the aluminium reduces the number of electrons in the iron oxide atoms. More heat makes the reaction run faster, generating more heat, and so on. The result is molten iron and aluminium oxide slag.
Imagine you’re out behind enemy lines in WW2, setting up demolition charges that may save the lives of your fellow soldiers. How do we make a solid connection between wires that will last? One of the solutions that were used by the OSS and SOE, the predecessors to the CIA and British Secret Service, were self soldering sleeves that could be lit like a match. [ElementalMaker] managed to get his hands on a box of these sleeves, and found that they work incredibly well, even after more than half a century.
The sleeves consist of a copper tube with solder and flux inside, and wax-covered pyrotechnic compound around the outside. A small blob of striker compound similar to a match head is used to set the soldering process in motion, using the striker surface on the outside of the oversize matchbox that the sleeves are packed in. The pack that the [ElementalMaker] got was made in 1964, but is supposedly no different from those used in WW2.
When lit, the pyrotechnic compound does not create any flame, it only smolders, probably to make it safer to use, and avoid detection at night. As the solder inside the sleeve melts, the operator is supposed to push the wires further into the tube to make them overlap. Although [ElementalMaker] didn’t cut open the sleeves, it definitely looks like a good joint, with solder oozing from the ends. Check out the video after the break! If you want to get your hands on a pack of these sleeves, it looks like a military surplus store in the UK managed to source some.
Often it feels as if soldering is deemed to be more of an art form than something that’s underpinned by the cold, hard reality of physics and chemistry. From organic chemistry with rosin, to the material properties of fragile gold bond wires and silicon dies inside IC packages and the effects of thermal stress on the different parts of an IC package, it’s a complicated topic that deserves a lot more attention than it usually gets.
A casual inquiry around one’s friends, acquaintances, colleagues and perfect strangers on the internet usually reveals the same pattern: people have picked up a soldering iron at some point, and either figured out what seemed to work through trial and error, or learned from someone else who has learned what seemed to work through trial and error. Can we say something scientific about soldering?