Researchers at ETH Zurich developed a nugget of super lightweight, 18-carat gold, which is made using a matrix of plastic, rather than a metallic alloy.

Gold is a popular material for the use in jewellery, but also a pretty heavy material (in weight). Weighing 19.3 g/cm3, 24-carat (or pure) gold is not very practical in jewellery. That is why often, the gold is mixed with copper to make a 18-carat gold alloy (75% gold, 25% copper), weighing about 15 g/cm3, which is still relatively heavy.

The new gold/plastic mixture weighs just a tenth of that, 1.7 g/cm3, while being still 18-carat gold. Instead of a metal alloy element, the researchers used protein fibres and a polymer latex to form a matrix in which they embedded thin discs of gold nanocrystals. Additionally, the lightweight gold contains countless tiny air pockets, which are invisible to the naked eye.

First, the ingredients are added to water to create a dispersion. The dispersion is then turned to gel by adding salt, after which the water is replaced with alcohol. In a pressure chamber, the gel is turned into an aerogel. The researchers can adjust the hardness of the material by changing the composition. The latex can also be replaced by other plastics, like polypropylene.

The plastic gold glimmers like gold, but when it falls, it sounds like plastic.

While the plastic gold will be in particular demand in the manufacture of watches and jewellery, it is also suitable for chemical catalysis, electronics applications or radiation shielding. The researchers have applied for patents for both the process and the material.

Photo: ETH Zurich