Real Trials rather than Virtual Experiments
Ultralight yet extremely strong – porous nanometals offer highly promising applications for future aircraft, energy-efficient cars and safer industrial plants. Experts still do not know how nanoparticles behave in detail. Most of their findings derive from computer simulations where experts assemble dozens of atoms into virtual metal particles to conduct experiments on supercomputers. These computer experiments yield exciting and plausible results, but experimental verification is often missing because it is extremely difficult to examine the mechanical properties of these nanoparticles. Conventional material research testing machines are unsuitable for this.
Researchers at the HZG and TUHH have succeeded in developing a mechanical test procedure for nanoparticles. They combine billions of gold nanowires into a porous, sponge-like network consisting of one quarter metal and three quarters air. Millimetre-sized test cylinders can be produced from this nanoporous gold, making them large enough for a conventional testing machine. A punch presses on the cylinder from above and all the nanowires are simultaneously deformed in the process so that the experts can subsequently deduce the behaviour of the individual particle. The sample is also immersed in an acid solution acting as an electrolyte, and an electrical voltage can be applied. The researchers can thereby specifically manipulate the surface phenomena that are so critical at the nanoscale, and in some cases repeatedly switch them on and off.
The mystery of their material strength has been solved
In an article published in "Nano Letters", the scientists were able to prove that, as suspected, the processes on the surface of the nanoparticle contribute decisively to its enormous strength. In a large, macroscopic body, the vast majority of atoms are located inside the crystal, while only a fraction of the atoms is on the surface. A nanoparticle, in contrast, has a relative large fraction of the atoms sitting on its surface. Surface effects, therefore, determine its mechanical properties. In a further publication in "Nature Communications", the experts were able to shed more light on the nature of these surface effects and determine the influence of two different phenomena. Their results disproved the interpretation of computer experiments. Accordingly, it is not, as assumed, the force acting between the surface atoms that determines the mechanical properties of the nanoparticles. Instead, it is likely to be the energy in their surface that is responsible for their strength.
In future, such findings could help to develop innovative materials based on nanotechnology – extremely interesting, perhaps, not only for lightweight construction, but also for materials with built-in sensor properties. "Due to the fact that we better understand the fundamental properties of these nanowires," emphasises project leader Prof.Jörg Weißmüller, "we are better able to target the development of future materials."