Nanoparticles and Nanocomposites

Modern nanocomposite materials are based on the production, surface modification, mixing and shaping of materials. The group focuses on plasma processing and light weight metal matrix composites.

Thermal plasma processing is a versatile technique used for nanopowder synthesis as well as for functionalization or modification of particles. Functionalized nanopowders are for instance investigated in large collaborative European projects for new batteries or as filler in polymer based composites. An inductively coupled thermal plasma facility equipped with several in-situ monitoring techniques has been developed at Empa. Another potential of the plasma process is the surface treatment of microscale powders for improving their flowability, their purity as required for instance in additive manufacturing.

Light weight metal matrix composites are also investigated using nanoparticles as reinforcement material. The simple powder metallurgy approach investigated involves high energy ball milling and compaction at high temperature. Therewith very promising mechanical performances could be achieved with various aluminium alloys.

Finally, the group investigates also laser welding processes based on the in-situ diagnostic techniques derived from the plasma activities. Therefore a unique high vacuum capability chamber with multiple viewports allowing the adaptation of the diagnostic sensors has been developed.

Plasma synthesis of nanoparticles

Inductively coupled plasma apparatus de-veloped at Empa for the thermal plasma processing of nanopowders.

Synthesis of nanopowders is carried out with an inductively coupled plasma setup. Dedicated to research activities it is equipped with numerous view ports for process observation, monitoring, and in-situ characterization. The nanoparticles plasma synthesis is based on rapid condensation, also called quenching, of a supersaturated gas phase. The quenching dynamics influence the nanoparticles properties such as size, size distribution and eventually the final chemical composition. The control and understanding of this condensation is of prime importance for guaranteeing the quality of the processed nanopowders.

The high temperatures (10000 K) and moderate velocities (100-200 ms-1) observed in radio frequency (RF) inductively coupled plasmas are particularly suitable for the evaporation of solid precursors. The starting material in our case is thus typically microscale powders that are mostly commercially available and easy to handle in a safe way. Furthermore, the process is performed under a controlled atmosphere (reducing, oxidizing or inert) allowing then the production of a large variety of nanoparticles.

In parallel to scientific and technological investigations, the Health, Safety and Environment (HSE) issues have been evaluated and best practice guidelines have been developed regarding production and manipulation of nanopowders. Particular attention has been also paid to the design of a special filtration unit.

Finally, the thermal plasma can be also used for spheroidizing microscale particles enhancing so their flowability. This is of great interest for additive manufacturing processed based a powder bed.

Nanostructured materials and nanoparticulate strengthened composites

New light and strong materials are needed to meet a wide range of energy efficient applications with increased specific stiffness, specific strength, damage tolerance, ductility, creep and reliability in extreme environments.

Adding nanoparticles into metals results in composite materials with drastically improved properties, unachievable by using conventional materials and chemicals. Besides the intrinsic properties of filler and matrix material, control of the degree of dispersion of the nanofiller in the matrix is a key issue towards functional nanocomposite materials. The research is targeted on the development of processing parameters for uniform dispersion of nanoparticulate reinforcements in various metal matrices. The degree of dispersion, size, concentration, and interfacial properties are related to the functional properties and the performance of the nanocomposite material for specific applications.
Hardness of hot pressed Al-1 vol% nano-SiC composites as a function of normalized total input milling energy for the blend preparation (J. Alloys and Compounds 489 (2010) 394-400).

Laser welding process

Prototype for the study of laser beam–material interactions at Empa

The interaction of a Laser beam with a metallic surface may lead to the formation of a plasma plume. The characterization of this plasma using spectroscopy and high speed imaging for instance can help in understanding and optimizing the welding process.

A prototype dedicated to the investigation of the interactions between a laser beam and a material has been developed for research purposes. It is made of an universal laser head allowing the rapid change of different lasers (Nd:YAG, fiber lasers…) and a high vacuum chamber equipped with many viewports and a sample moving stage. The plasma is characterized by optical spectroscopy (UV-Vis-IR) and high speed imaging, while the atmosphere is monitored by mass-spectrometry and a humidity sensor (ppm range). Therewith some correlations could be found between some properties of the plasma plume and the quality of the weld.