At the heart of it, NanoFoil
® is simply the aluminum and nickel chemical reaction just waiting to happen. A lot of energy and a lot of heat strapped into thousands of alternating layers of atoms. Each atomic layer of aluminum is waiting for just the right energy to move into the nickel layer and combine - to release up to 1250 Joules of energy per gram of material and as much as 1500ºC (2730ºF).But, why don’t nickel and aluminum just react in real life? And more importantly, how do we make the NanoFoil react to release heat precisely where we want it?
The former question is answered by going back to basic chemistry and a concept called activation energy. Activation energy is defined as that energy that must be overcome in order for a chemical reaction to take place. In regular use, when aluminum and nickel come into contact with one another they do not react, and this is a good thing. Imagine if your nickel-coated nickel reacted with your aluminum money clip in your pocket…hot! The activation energy of the reaction is too high to promote this reaction naturally. ‘
There are a few ways to reduce this activation energy, the most common being via a catalyst, which is a substance that modifies the transition state which, in turn, lowers the activation energy of the reaction. In the case of the NanoFoil, instead of a chemical modifier we have taken advantage of a physical modifier, surface area. By layering the aluminum and nickel atoms very thinly and in a very precise method, we rely on the increase in surface area to decrease the activation energy necessary to start the reaction…in most demos we use only a 9 volt battery!
The second question is a frequent one fielded by Indium engineers, and worth a deeper look!
How is the NanoFoil Activated/Ignitied?
The reason I will use the term "activation" over "ignition" is that ignition implies the beginning of a sustained burn, where the NanoFoil is a reaction that lasts for less than a millisecond, and only requires activation.
The reaction will start with 250ºC of localized heat, or a very localized form of energy. The trick is getting a very concentrated form of energy to come into contact with the NanoFoil. Touching the NanoFoil with the point of a resistance soldering iron that is at 250ºC is much more likely to activate the NanoFoil than throwing the NanoFoil on a hot plate that has been heated to 250ºC. In general, there are three types of energy you can put into foil to activate it.
- Mechanical Energy
- Thermal Energy
- Electrical Energy
Mechanical Energy – In the case of mechanical energy, dropping the NanoFoil on a concrete or hard surface could activate it IF it lands on its edge and all of the impact energy is concentrated on the corner. Generally, the NanoFoil does not go off with contact, but friction between the NanoFoil and itself, in the form of a small shard, has produced enough energy to activate the NanoFoil.
Thermal Energy – In the case of thermal energy, as discussed above, a concentrated amount of 250C heat will activate the NanoFoil. In the case of ohmic heating, which is what we do in demos, by shorting the leads of a battery, the current must be 100-120Amps for a 15um contact diameter, and 250-300 Amps for a 300µm contact diameter. A hot filament or flame, such as a lighter, will also activate the NanoFoil.
Electrical Energy – In this case a spark will activate the NanoFoil, but it is about concentration of power, or power density. With a momentary point contact from an electrical probe, 10 Amps and 5 Volts is sufficient as long as it is POINT contact. The foil can be activated remotely through the use of a dedicated trace on a board, and this requires testing to determine the amount of energy that will travel the distance of the trace.
In my next blog post I will talk about Laser Ignition, ESD sensitivity, and some of the tools that Indium has developed to control the activation.