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Imagine a cheap and light-weight material that has the same appearance and qualities of the traditional 18-karat gold but weighs five to ten times less.
A novel technology developed at the Swiss Federal Institute of Technology in Zurich (ETH Zurich) makes possible the fabrication of an innovative metal-organic nanocomposite material that shines like metallic gold and can be machined and polished as the traditional gold alloys.
The extraordinary lightness of the new material is achieved by embedding micron-sized gold single crystals into an organic matrix. The unique properties of the 'plastic gold' open up a myriad of possible applications from jewelry and watchmaking to the electronics and aerospace industry.
Conventional 18-karat yellow gold is made from 75% gold and 25% other metals, such as copper, silver, and platinum. Because of the gold’s rarity and high price, manufacturers are always exploring ways to reduce the amount of gold required or to use cost-effective substitutes in their manufacturing processes.
Base metals plated with gold alloys have traditionally been used as a means to reduce the amount of gold used in jewelry and electronic components. Many industrial products are constantly being redesigned to reduce the amount of gold required for their manufacture while maintaining their utility properties.
Environmentally Friendly Gold Nanocomposite
The new light-weight ‘plastic gold’ was developed by using gold single crystals embedded in a polymer matrix formed of biodegradable protein fibrils and polystyrene latex particles in a simple and environmentally friendly water-based self-assembly process.
Self-Assembled 3D Scaffold for Gold Nanocrystals
The researchers at ETH Zurich began by mixing gold nanocrystals with β-lactoglobulin fibrils (BLG, an inexpensive milk protein), polystyrene (PS) latex nanoparticles with a diameter of around 500 nm, and salt in an aqueous dispersion. The fibrils are just 4 nm in diameter and several micrometers in length. These serve as a 3D scaffold to which the gold nanocrystals attach.
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The PS nanoparticles provide the glassy matrix into which the gold crystals are embedded and determine the mechanical properties of the final product. Adding salt to the mixture modifies the electrostatic interaction between the components and leads to a well-controlled hydrogel formation.
During the next step, the water in the hydrogel is replaced with ethanol and the modified hydrogel is subjected to a process called supercritical carbon dioxide (CO2) drying. During that step, under high pressure, the ethanol in the hydrogel is replaced by CO2 in a supercritical liquid state.
Upon lowering the pressure, the CO2 transitions from a supercritical liquid state to a gas state, removing the liquid from the hydrogel in a precise and controlled way, and without disturbing the hydrogel’s initial structure.
Gossamer-Like Aerogel Matrix Significantly Reduces Weight
The resulting material is called aerogel and at this stage the polystyrene nanoparticles significantly scatter light, giving it a matt white appearance. In the final step, the aerogel is annealed (heated up) above the glass transition temperature (the temperature at which a glassy amorphous polymer becomes a viscous liquid) of PS of around 105 °C.
This allows the PS nanoparticles to fuse and form a homogeneous glassy matrix with embedded gold nanocrystals in it.
During the final step, the volume of the aerogel is reduced by a factor of four, resulting in a final density of 1.7 g cm−3 and final composition of 76% w/w gold, 20% w/w PS, and 4% w/w BLG with a porosity (the volume fraction of the air cavities in the material) of 57% v/v.
After the annealing, the ‘plastic gold’ can be polished to reveal its shining gold color and the countless tiny air cavities give its extraordinary lightness. For comparison, the density of conventional 18-karat gold is around 15 g cm−3. The 76% gold content is what makes it true 18-karat gold.
Plastic Gold with Tailored Properties
In addition to being light-weight, the ‘plastic gold’ has other advantages over traditional gold alloys. It can be molded into shape at a temperature of 190 °C, much lower than the melting point of the conventional gold alloys (1,064 °C for 24-karat gold).
The material’s mechanical properties can be easily tailored by tweaking the initial composition of the hydrogel. It is capable to withstand shocks and can be machined and shaped by standard manufacturing processes.
By changing the shape of the gold nanocrystals from flat polygonal platelets to spherical nanoparticles one can vary the color of the final product from the conventional yellow shimmer to a violet hue.
Unlike the traditional 18-karat gold substitutes (gold-plated base metals), the new material retains the long-term color and brilliance of gold without tarnishing.
Light-Weight Gold for Watches, Jewelry, and Spacecraft
The unique physical properties of the 18-karat ‘plastic gold’ are brand new to the field of gold alloys and together with its affordability and environmentally friendly processing would allow it to fill an unoccupied niche in the market of industrially relevant gold alloys.
When used for the manufacturing of watches and jewelry items, the light-weight ‘plastic gold’ can significantly enhance the wearability of the products (by reducing the wear fatigue). The new material can replace the gold alloys and coatings that are widely used for radiation and thermal shielding in spacecraft where weight saving plays a crucial role. These are just a few examples of the wide range of possible applications for the ‘plastic gold’.
References and Further Reading
L. van't Hag et al., (2020) Light Gold: A Colloidal Approach Using Latex Templates. Advanced Functional Materials, 30, 1908458. Available at: https://doi.org/10.1002/adfm.201908458
P. Rüegg (2020) An 18-carat gold nugget made of plastic, [Online] www.ethz.ch Available at: https://ethz.ch/en/news-and-events/eth-news/news/2020/01/gold-nugget-made-of-plastic.html (Accessed on 17 May 2020).