Wednesday, 3 September 2014

Nanotech wedding ring

New Zealand is known for the Lord of the Rings trilogy and so I took the opportunity recently when I got married to make the precious. But I couldn't resist including some nanotechnology in the design.

Shane Hartdegen a great teacher and designer of jewellery made both my wife and my wedding rings and I wanted to explain the design as we had a lot of fun with the materials. There are four concentric rings of different metals.

From the outside in there is electrochemically recycled gold, followed by white gold which was made from forged gold and palladium, then a copper layer giving a nice orange colour and inside is titanium which would normally be grey in appearance but by heating it in the flame a very thin layer of titanium oxide is formed that allows light to interfere and produce this blue-purple colour.

I was intrigued about how a grey metal could produce so many amazing colours. It all comes down to a small layer of oxide that is formed when you heat treat the metal. This nanocoating is on the order of the wavelength of light.

Figure of a 50 nm in (a) and (b), 550 nm in (c) and (d), 750 nm in (e) and (f), layer of Titanium dioxide grown on a layer of indium tin oxide a conductive film. The layer was grown with atomic layer deposition and is as close as I could find to oxide growth on Titanium metal however you would probably not get such a porous structure. 

What optical effect causes some colours to be reflected (such as blue and purple in this case) but not other colours such as yellow. The phenomenon responsible is called interference and occurs as the light bouncing off the top of the oxide layer and off the oxide metal interface interfere. Only certain wavelengths will constructively interfere and be reflected.

To work out what wavelengths will be reflected you can use similar formulas to Bragg's law which is used for measuring the thickness of layer of crystals, however that require much shorter wavelength x-rays.

This effects allows us to determine the thickness of the layer by determining the colour of the film (as has been done by harry sherwood on his blog)

A more common way to deposit the oxide layer is by using electricity through a process called electrolysis where titanium ions in the solution are neutralised by the electrons given from the anode allowing for the depositing of the metal. This method allows for a wide range of colours to be created.

Electrolysis was the method used to recycle the gold used in the ring. Most gold is made through dissolving rocks in cyanide solutions. These are harmful for the environment. We decided to recycle gold from old jewelery. First the jewelery was dissolved in an acid solution followed by electrolysis onto the anode and finally smelting it to form the metals needed.