The structure and composition of catalytic silver nanoparticles (Ag-NPs) fabricated through a novel gas condensation process has been characterized by Scanning Electron Microscopy (SEM) and Atom Probe Tomography (APT). SEM was used to confirm the number density and spatial distribution of Ag-NPs deposited directly onto standard silicon microposts used for APT experiments. Depositing nanoparticles (NPs) directly by this method eliminates the requirement for focussed ion beam (FIB) liftout, significantly decreasing APT specimen preparation time and enabling far more NPs to be examined. Furthermore, by encapsulating deposited particles before final FIB sharpening, the APT reconstruction methodologies have been improved over prior attempts, as demonstrated by comparison to the SEM data. Progress in these areas is vital to enable large-scale catalyst research efforts using APT, a technique, which offers significant potential to examine the detailed atomic-scale chemistry in a wide variety of catalytic NPs.
Oxidational wear continues to present an economic challenge for the replacement of components subject to high temperature fretting and sliding contacts in applications such as gas turbine engines. At elevated temperatures, low friction oxide 'glaze' layers can form and act as an interface between the contact and the substrate material. Whilst desirable, the glaze is formed from wear debris and often consumes the underlying substrate material. In order to induce rapid formation of low friction oxide layers without a severe 'running-in' period, nano particles of Fe in the range 5-10nm were deposited on ground flat ended pin and plate 080M40 substrates using a terminated gas condensation PVD process, to a thickness of 600nm.Coatings were tested in a reciprocating geometry at a fixed stroke length of 0.4mm, frequency of 31Hz and 40N normal load (1MPa contact stress) and at ambient, 300°C and 540°C. At ambient temperature the coated surfaces exhibited higher friction but lower wear compared to the uncoated substrates, whereas at elevated temperatures, the coated surfaces exhibited slightly lower steady state dynamic friction coefficients, and minimal changes in wear depth after a short incubation period. SEM of the worn surfaces indicated that hard oxide plateaus were responsible for the load bearing contact area at elevated temperatures. Cross sectional FIB, TEM and SIMS confirmed that at elevated temperatures, the nano-particle coating induced rapid formation of a nano-crystalline porous surface oxide film of mixed composition which protected the substrate from severe wear during the running-in period.
Silver nanoparticles have been prepared using a “terminated gas condensation” technique. An unprecedented control of 5-6 nm-size nanoparticles with well defined shape and variable surface densities has been achieved. The technology is shown to permit independent control of both, plasmon resonance intensity and frequency position. On the basis of optical measurements, a smart tuning of plasmon resonance intensity with particle density is indeed demonstrated. Moreover, the embedding of NPs in different surrounding medium enables to control the resonance wavelength as experimentally demonstrated and theoretically confirmed.
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