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Up-scaling cluster beam flux of magnetron-sputtering sources – Towards industrial applications

In the last decades multi-atomic systems and clusters of atoms have increasingly attracted more and more interest among researchers, for their unique properties deeply influenced by their particle sizes just sitting in the transitional range from single atoms to the bulk solid-state. These nanoparticles/clusters have found wide applications, for example in catalysis or sensing due to their high surface to volume ratio; in novel optical and electronical devices due to the electron confinement effect. Although many laboratories have demonstrated exciting results on the application of clusters, new technologies and approaches for the cluster production are needed to bring these laboratory results closer to early-stage industrial exploitation.

The methods employed to synthetize clusters mainly belong to two categories: chemical methods and physical methods. The former is more widely used in practice, as they typically are characterized by higher cluster production rates, which make them more commercially viable. However, solvents used in chemical methods make the production process less clean, and the ligands used to avoid aggregation can block the active sites on the cluster surface. On the other hand, physical methods rely on much cleaner vacuum preparation processes, and no ligands are involved to keep clusters apart. In addition, the clusters produced in this way are characterized by a much narrower size distribution. Among the physical methods, cluster preparation from the gas-phase using a magnetron sputtering source has several advantages, such as easy tunability of cluster stoichiometry, size and flux by changing the key parameters including the gas pressure and temperature, power applied to the magnetron or the aggregation length. However, although magnetron cluster sources have a high cluster throughput compared with the other sources, the typical cluster deposition rate is still far below the requirements for industrial production of cluster-based devices or catalysts. The scale-up of the cluster-throughput in magnetron sputtering cluster sources would help this technology to meet the industrial demand and to produce commercially available cluster-based materials.

In this thesis, the gas dynamics inside the gas aggregation zone is studied, in order to increase the cluster flux produced in a magnetron sputtering cluster source. In particular, the gas inlet and chamber shape are varied and optimized to achieve a higher cluster throughput. It has been found that when the gas inlet is placed close to the target, the gas drag force would not only decrease the amount of sputtered atoms which deposit back to the target surface, but also help increase their density in the gas phase, which would increase the cluster nucleation rate. Furthermore, results have shown that a conical shape and a smaller cross-sectional diameter of the chamber would increase the drag force clusters experience from the carrier gas, especially close to the chamber walls, decreasing the probability of cluster attachment to the walls and so increasing the final cluster flux.

Then, two application examples are given where a high-flux magnetron sputtering cluster source is employed: Ag-cluster doped amorphous carbon antimicrobial coatings for aerospace application and Pd/Ni bi-metallic clusters on alumina powder as catalyst for the oxidation of carbon monoxide (CO). In the first example, the combination of the cluster source with standard magnetron sputtering is used to prepare amorphous carbon (a-C) nano-composite coatings with silver (Ag) nanoparticles embedded, and the aging as well as antimicrobial properties are studied. Due to the diffusion of Ag inside the a-C film, it slowly migrates to the coating surface, where it eventually agglomerates to form whiskers. If the Ag mobility is too high, the coating may degrade too quickly, not only decreasing its lifetime but also increasing the Ag leaching to the level where toxicity may become a concern. It has been found that with the combination of the cluster source and standard magnetron technology, it is possible to not only limit the Ag diffusion to the surface, but also increase the antimicrobial performance of the coatings despite lowering the total amount of Ag content.

In the second application example, palladium-nickel (Pd-Ni) bi-metallic clusters have been deposited directly onto alumina powder. Clusters have been prepared using the combination of a hollow cathode with a hollow cylindric Ni target and a magnetron provided with a planar circular Pd target. Samples have been characterized with STEM, EDX and ICP to analyse the cluster composition and structure. The CO oxidation catalytic performance has been evaluated using a plug flow reactor, and then compared with analogous samples prepared with monometallic Pd clusters. Results show that the temperature of 100% of CO conversion decreases by 50 °C, and that the turnover frequency increases by 5 times.

Date:7 Nov 2016 →  28 Nov 2022
Keywords:Alloy Clusters, Cluster-beam System, Nanoparticles
Disciplines:Condensed matter physics and nanophysics
Project type:PhD project