Additive manufacture with copper and silver nanoparticles

Pervan, David (2020) Additive manufacture with copper and silver nanoparticles. PhD thesis, University of Nottingham.

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Additive Manufacturing (AM) of electrically conductive copper (Cu) parts is of significant research interested. Cu’s high thermal and electrical conductivity combined with its relative abundance and low price makes it an ideal material for thermal and electrical devices. Unlike conventional subtractive manufacturing processes, AM is capable of producing highly geometrically complex parts which is beneficial for many thermal and electrical devices applications. To date there are only capital and energy intensive AM processes which can produce macro-scale Cu parts exist. There is no low-energy process capable of additively manufacturing electrically conductive copper (Cu) microparts. Additively manufactured Cu microparts have potential applications such as microchips, electronic microdevices, medical devices, microfluidic devices, and micro-optical systems. This thesis aims to lay the groundwork for a novel low-power AM process, capable of producing electrically conductive Cu microparts from nanoparticle feedstock in an air environment.

Sequential ink deposition through bar coating and inkjet printing followed by low-power laser sintering in an air environment was used to manufacture multi-layered Cu microparts. The microparts were mechanically and electrically characterised. The use of nanoparticles (NPs) reduced the energy demand for inducing particle sintering. Because of the low laser power of merely 2.5 W and fast heating and cooling rates characteristic for laser irradiation the Cu microparts were unoxidised. The Cu microparts were of low density and highly porous. This had a significant negative effect on the mechanical part properties which were far off from bulk Cu properties. An additional heat treatment to densify the manufactured microparts was suggested in order to improve these mechanical properties. The electrical resistivities of the Cu microparts were close to bulk Cu resistivity.

For two-dimensional (2D) printed electronics applications, silver (Ag) has been the most common conductive material used. Despite its similar electrical conductivity and lower price, copper (Cu) is being used significantly less, mainly because of Cu’s tendency to rapidly oxidise which is detrimental to its electrical conductivity. Ultra-fast photonic sintering is capable of “outrunning” the Cu oxidation, yet, the overall processing cost of Cu remains higher than for Ag. In the first comprehensive study the impact of additions of small amounts of Ag to Cu NPs on ultra-fast and slow sintering processes is investigated. With both ultra-fast sintering techniques, namely IPL and laser sintering, the addition of Ag to Cu NPs does not significantly impact electrical conductivity because the Cu NPs do not oxidise during the ultra-fast sintering. For slow thermal particle sintering the addition of Ag was found to be beneficial. In an air environment the Cu oxidised and hence became dielectric but at a Ag content of 3, 8 and 25 wt%, there was a significant reduction in electrical resistance, which was hypothesised to be due to the Ag content migrating forming a connected grid capable of conducting electricity. During thermal sintering in an inert environment, the Ag content accumulates primarily around the necks of adjacent Cu particles. This was reasoned to be the results of surface tension differences between the Ag-gas and the Ag-Cu interface acting on the mobile Ag. This phenomenon was further investigated in a theoretical simulation, calculating the theoretical reduction in Cu-Cu inter-particle resistance due to Ag accumulation in the necking region.

The most widely reported application of AM with metal NPs is printed electronics. The ability to print and sinter electrically conductive tracks onto heat-sensitive flexible materials enables the manufacture of flexible electronics, such as wearable sensors. To assess whether Cu NPs can be considered a viable replacement for Ag NPs for flexible electronics, the impact of cyclic bending on the electrical resistance and microstructure was investigated. Cu and Ag tracks were inkjet printed and laser sintered at various layers and widths and exposed to cycling bending. Ag NP tracks were significantly more resilient to cyclic bending than Cu NP tracks. Crack growth was the major driver for the observed increase in resistance of the Cu NP tracks which was not found to be the case with the Ag NP tracks. It was hypothesised that the greater width of the Cu particle size distribution compared to Ag likely contributed to the Cu tracks being less resilient to cyclic bending.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Wildman, Ricky
Lester, Ed
Tuck, Christopher
Hague, Richard
Keywords: Additive manufacturing, 3D Printing, Metal nanoparticles, Copper nanoparticles, Silver nanoparticles
Subjects: T Technology > TS Manufactures
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Chemical and Environmental Engineering
Item ID: 61566
Depositing User: Pervan, David
Date Deposited: 31 Dec 2020 04:40
Last Modified: 31 Dec 2020 04:40

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