Production of Co, Ni, and Cu nanoparticles by hydrogen reduction

(Nanowerk News) In this thesis, production of Co, Ni, and Cu nanoparticles by hydrogen reduction (pdf) of metal chlorides in gas phase was studied. Nanoparticles have unique properties not found in bulk or micron-scale materials. These enable new products or reduced use of raw materials. Metal nanoparticle production has been studied widely, but especially for coated metal particles, research of coating mechanisms and economic production methods is still needed. The method used in this thesis combines a high yield, a high production rate, low production costs, high particle quality, and a good range of available particle number average diameters and other properties. These particles could be utilised in conductive inks, antenna substrates, medical imaging, or as sensors and catalysts.
The number average primary particle diameter (NAD) of Co particles increased from 20 to 84 nm as the particle mass concentration increased from 0.5 to 10 g/m3. For even higher particle mass concentrations, the NAD did not increase. The Co and Cu particles were coated in-situ with carbon, by adding ethene to the reaction flow. Copper particles were also coated with carbon and carbon nanotube-like structures by adding ethene and water to the reaction flow. When ethene concentration was increased from zero to 9.2 mol-%, the NAD decreased from 84 to 17 nm for cobalt. Particle mass concentration was 10 g/m3 or higher in these experiments. The standard deviation was 17 nm when NAD 84 nm and 7 nm when NAD was 17 nm. For copper, the NAD increased from 20 to121 nm. The particles were crystalline with an FCC structure in all cases.
The particle growth was modelled in a simplified way. It appears, that surface reaction is an important part of the particle growth process. The most likely scenario is that first, seed particles form by gas phase reaction and nucleation. Then, these particles grow partly by surface reaction and partly by condensation growth. In the gas phase, some backward reaction of metal to metal chloride occurs.
The saturation magnetic moment of the Co and Co+C particles was 141147 emu/g, which is close to bulk Co (159 emu/g). No hysteresis was observed for these particles. Cobalt particles with and without carbon coating were used to fabricate polymer composites, which could be applied in RF antennae. The relative permeability of the composite was increased from 1 to 3 as the loading of the particles increased from zero to 28 vol-%.
Copper particles coated with carbon coating and CNT-like structures were used to fabricate inkjet fluid capable of producing conductive lines after printing a single layer. The best achieved conductivity was 6.4 S/m.
Source: Technical Research Centre of Finland (VTT)
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