Materials processing for future technology
The Science of Electrodeposition
You can learn more about specific aspects of supercritical fluid electrodeposition on our other science pages.
There is more than one way to coat a surface with a material, a process that scientists call 'deposition'. One of the most common ways is the process of 'electrodeposition', sometimes called 'electroplating', that uses electricity to transfer the material we want to coat with onto the surface. There are two ways of doing this. This animation shows the simplest way which transfers atoms from one conducting surface (e.g. a metal) to a different one. This works because electricity wants to flow the whole way around the circuit so metal atoms come off one surface to carry it to the other side. Because electricity flows only one way, the atoms (gold) that have crossed the gap stay there and gradually the other surface (silver) is coated with a layer of gold.
This is not the only way of doing electrodeposition. It is also possible to have the atoms we want to coat with dissolved in the bath just like you would dissolve sugar in a cup of tea. If we use the right liquid and atoms it is easier for one surface to give its electricity to these dissolved atoms than to travel across the gap themselves. Once they pick up the electricity the dissolved atoms carry it across to the other side and just like before stay there to coat it.
As an example, let's imagine that we want to coat something in copper. There are several steps to this:
- First we must mix up a bath (solution) by finding a fluid to dissolve our useful molecules in (the 'solvent') and a suitable 'reagent' (the useful molecules). In this case the reagent will be a molecule that has copper in it because this is what we want to deposit. The reagent needs to have an electrical charge (i.e. positive or negative). For our example the molecules are positively charged.
- Next we dip our conducting surface (the 'electrode') in the bath along with an extra substrate that we don't want to coat. These must not be touching.
- We then need to apply an electrical potential between the substrates. 'Potential', otherwise called 'voltage', is the driving force for electricity and is used to describe how powerful sources of electricity. A way we could apply the potential is to connect the electrode to one end of a battery and the counter-electrode to the other end.
- When a potential is applied the electrode becomes charged. For example, if we connect the electrode to the negative end of the battery, it will get a negative charge. Opposite electrical charges attract each other, so this will pull the dissolved reagents which are positively charged toward it.
- When the reagents reach the electrode the electrical attraction is so strong that is pulls the charged copper out of the reagent molecules and bonds it to the electrode surface (i.e. deposition).
For Scientists... Electrodeposition has a number of advantages over rival deposition techniques, all of which can be exploited for SCFED: (a) by masking the conducting substrate, electrodeposition allows the spatial location of deposition to be controlled, (b) as deposition can only occur at the conducting surface, deposition is directional away from a surface, (c) as electrodeposition draws the reagents out of a fluid, it can be used to deposit conformally on a curved or interior surface and can fill volumes without shrinkage, and (d) the high material efficiency not only allows significant control over the deposition process but improves the financial viability.