Properties and applications of metastable precious metal intermetallic compounds

University of Technology, Sydney. Faculty of Science.Precious metal alloys and compounds have myriad applications in the fast-expanding horizons of the commercial and industrial worlds. They are also fascinating topics for scientific research. These materials have a long history, with gold and silver amongst the very earliest metals used by humans. Over the past millennia, the primary applications of the precious metals and their alloys have been in the ever-lucrative jewellery manufacturing industry. The traditional alloys have been perfected in over three thousand years of experience. However, in the recent past, precious metal alloys and compounds have also found themselves a crucial place of pride in the burgeoning ‘advanced materials’ sector. Gold-based and platinum-based alloys and compounds are amongst the candidates being investigated for serving in those applications. In the present project I sought to explore how gold aluminide and platinum aluminide could be developed for further innovative applications. In particular, I initially became interested in the optical properties of these materials, with a view to developing their application in the jewellery industry. The PtᵪAl alloys are, however, also useful as precursors for producing nanoporous metal sponges. The availability of such samples from the first part of the project encouraged me to consider technological applications of the aluminides in the chemical catalysis industry in the second part of the project. The two parts are linked by virtue of starting with the same materials, which are fabricated and mostly characterized the same way. In both cases the samples are fabricated as thin films by direct-current magnetron sputtering and then various techniques are used to characterize their chemical composition, structures, morphologies and specific properties. The main difference comes only at the very end of each part, with the first group of materials being evaluated on their optical properties and the second on their sponge-forming properties. My work is developed around two hypotheses. First, I hypothesized that the compounds PtAl₂ (brassy yellow) and AuAl₂ (metallic purple) can be alloyed to yield a range of intermediate colours. It is generally stated that these compounds would be immiscible but I proposed that a series of metastable solid solutions could be formed by means of magnetron sputtering. Secondly, I hypothesised that the preparation of nanoporous platinum sponges from metastable (PtᵪAl) precursors would produce a different result than producing them from well-crystallized precursors, and that this could be exploited to provide a new way to control the morphology of such sponges. The work has showed that the attractive colours of the intermetallic compounds AuAl₂ (‘purple gold’) and PtAl₂ (‘golden platinum’) can be combined or mixed to produce an interesting colour spectrum. This may be of interest to the jewellery industry. A series of metastable solid solutions could be formed by using the magnetron sputtering technique, which enables users to produce any desired stoichiometry. In addition, procedures to reliably produce pure AuAl₂ and PtAl₂ thin films have been established. These have lattice parameters of 0.599 nm and 0.594 nm respectively, which are similar to those of bulk samples produced by vacuum arc melting. Addition control may be obtained by designing multilayer stacks of these intermetallic compound films, with both bi-layer and multi-layer films being produced in the present project. It was also shown that a metastable solid solution of Au and Pt could be formed by sputtering, with a co-deposited film of 54 at.%Au- 46 at.%Pt film forming a solid solution with a lattice parameter of 0.401 nm, which lies between that of pure Au films (0.408 nm) and pure Pt films (0.394 nm). This metastable solid solution could be reacted with a pure Al film to form a metastable solid solution of (Au,Pt)Al₂ after annealing. However, thin film stacks of AuAl₂ and PtAl₂ may be a better choice to tune colours of these two compounds as they are easier to control. Next I showed that Pt-Al alloys and intermetallic compounds can be de-alloyed in alkaline solutions to produce nanoporous platinum sponges. These nanoscale sponges can be used as chemical catalysts although I did not pursue this aspect myself. Rather, in this part of the project I considered how the microstructure of the precursor alloys could control the morphology of subsequent sponges. Once again, metastable precursors could be prepared by using magnetron sputtering, and produced a different morphology of sponges compared to those produced from well-crystallized precursors. Other processing parameters have also been studied. It was found that mole fraction (χAl) of Al in the precursor and the deposition temperature are the two most important factors. Precursors with χAl 0.90 were de-alloyed. These had originally consisted of a mixture of PtAl₆ and pure Al. It was also found that precursors that had been deposited at room temperature produced very different sponge morphologies to those that had been deposited at elevated temperature: in this case the amorphous precursors with 0.67 < χAl <0.96 produced sponge morphologies ranging from pinhole to unusual isotropic foamy. This work has shown that different morphologies of nanoporous platinum sponges can be produced by controlling the processing parameters. These sponges might be considered for use in specific catalytic or sensor applications because they can be fabricated using simple and cost-effective production techniques

Classroom Façade Design for Daylighting in a Tropical Hot-humid Climate

Previous studies have confirmed that the learning performance of school students can improve when good levels of daylight are available in classrooms. In tropical climates, it might be difficult to utilise and control natural light due to its high levels. Various activities in classrooms, such as taking notes and viewing screens and whiteboard, can also become difficult. Consequently, daylight in tropical classrooms may be less welcome and utilised less frequently. In order to deal with natural light utilisation, building facade is one of significant architectural design elements influencing energy consumption and human comfort. This study aims to investigate classroom facade designs that attempt to optimise visual and thermal comfort while reducing energy consumption. Focusing on facade appearance, daylighting systems and occupants’ behaviour, there are three stages of study using occupants and classrooms in the Faculty of Architecture, Urban Design and Creative Arts, Mahasarakham University, Thailand as the case study. The research commenced the first stage with finding the actual problems of daylight utilisation in tropical classrooms in terms of room form, brightness levels, and users’ behaviours and attitudes. In order to study brightness pattern and human sensation several survey methods: illumination measurements, observations, questionnaires and interviews; were applied. In the second stage, computer simulation was undertaken in order to analyse the problems and suggest solutions using DesignBuilder package. The suggestions were verified in the last stage by surveying occupants’ satisfaction comparing the modified classroom to the existing classrooms. The measurements and surveys demonstrated availability of the daylight and positive attitude in using natural light of the occupants whereas façade and systems are not appropriated for applying natural light: provided insufficient daylight level and allowed occurrence of glare. Daylight environment appeared to proper for general visual tasks while more control was required for using projector. The simulation result showed the significance of window area, shading device and window orientation respectively. The use of two opposite fully glazed walls with shading depth of 50% of optimised device is recommended for all orientations. The suggested size of shading device usually allowed penetration of the sun into the classroom. The influence of direct sun which theoretically could be a serious problem was confirmed acceptable by occupants. It implies that direct sunlight can rather be a daylighting opportunity if correct shading design is applied. A limitation of this research is that, although the DesignBuilder package can facilitate study in both daylighting and thermal aspects, its daylighting analysis function has limit capacity for light reflected strategies. Results of this research are recommendations of facade characteristics and their operation systems which are suitable for the case study. The visually environmental improvement of one specific building which can be adapted for general classrooms and other types of low maintenance buildings. Moreover, research findings can be extended to be public guidelines for designers in their approach to sustainability

Strategies to control the spectral properties of Au-Ni thin films

Gold and nickel have quite different dielectric functions. Here we use a combination of calculation and sample manufacture to assess two strategies by which thin films of these elements can be produced with a controlled range of far-field optical properties. In the first approach, control can be achieved by manipulating the density of states of metastable solid solutions, which in turn controls the dielectric function. In the second approach the optical properties of the films are controlled by varying the geometry of stacks fabricated from the constituent elements. We show that the two approaches can produce equivalent results so both are viable options in practice. Modeling is used to reveal how the structure controls the optical properties and to map out the possible color gamut. Predictions are tested with thin film samples fabricated by magnetron sputtering. © 2013 Elsevier B.V

Thin films of PtAl2 and AuAl2 by solid-state reactive synthesis

The intermetallic compounds AuAl2 and PtAl2 are colored purple and yellow respectively. In the past they have been prepared by bulk melting techniques or by co-deposition in a magnetron sputterer. Here, however, we investigate films of AuAl2, PtAl2 and (Au,Pt)Al2 prepared by sequential physical vapor deposition of the elements, followed by in situ solid-state reaction. The microstructure, dielectric functions, optical properties and thermal stability of the resulting films are characterized and compared to those prepared by bulk melting or codeposition. The (Au,Pt)Al2 films show a color gamut that stretches from purple to brassy yellow depending on composition and microstructure. High temperature synchrotron X-ray diffraction experiments show that the (Au,Pt)Al2 phase is metastable, decomposing when heated above 420 °C. In contrast, the pure AuAl2 or PtAl2 phases are stable to about 580 °C before they oxidize or decompose. The alternative possibility of producing the purple-to-yellow color gamut by depositing optical stacks of very thin films of AuAl2 and PtAl2 is also assessed. Either scheme will provide a range of colors lying between those of the binary compound endpoints. Calculations predict that deposition of AuAl2 onto PtAl2 will produce more intense colors than vice versa, an unexpected finding that is worth further investigation.Royal Thai Scholarship. Part of this research was undertaken on the Powder Diffraction beamline at the Australian Synchrotron, Victoria, Australia.http://www.elsevier.com/locate/tsf2016-08-31hb201

Scalable, ultra-resistant structural colors based on network metamaterials

Structural colors have drawn wide attention for their potential as a future printing technology for various applications, ranging from biomimetic tissues to adaptive camouflage materials. However, an efficient approach to realize robust colors with a scalable fabrication technique is still lacking, hampering the realization of practical applications with this platform. Here, we develop a new approach based on large-scale network metamaterials that combine dealloyed subwavelength structures at the nanoscale with lossless, ultra-thin dielectric coatings. By using theory and experiments, we show how subwavelength dielectric coatings control a mechanism of resonant light coupling with epsilon-near-zero regions generated in the metallic network, generating the formation of saturated structural colors that cover a wide portion of the spectrum. Ellipsometry measurements support the efficient observation of these colors, even at angles of 70°. The network-like architecture of these nanomaterials allows for high mechanical resistance, which is quantified in a series of nano-scratch tests. With such remarkable properties, these metastructures represent a robust design technology for real-world, large-scale commercial applications

Localized surface plasmons in platinum aluminide semi-shells

© 2019 IOP Publishing Ltd. The dielectric function of the intermetallic compound PtAl2 is assessed and found to be comparable to that of titanium nitride, suggesting that nanostructures of PtAl2 may be suitable for plasmonic devices. In order to probe this further, the optical properties of experimentally produced arrays of nanoscale PtAl2 semi-shells of about 300 nm diameter were examined and compared to the results of numerical simulations. The structures showed a broad localized surface plasmon resonance centered on ∼1.3 eV (∼950 nm), which matched the simulations. Calculations showed that a ten-fold enhancement of the electric field of the incident light will be achieved around the rim of suitably oriented PtAl2 semi-shells. The phase of the oscillation induced by 1060 nm light will be retarded by π/2 relative to the incident light. This is indicative of a resonant condition. These observations suggest that it could be worthwhile to investigate possible applications for this and other intermetallic compounds in nanoscale plasmonic devices

AuAl<inf>2</inf> and PtAl<inf>2</inf> as potential plasmonic materials

The dielectric functions of PtAl2, AuAl2 and hypothetical intermediate alloys of the two in the form of Aux- Pt1-xAl 2 were calculated from first principles using density functional theory (DFT) and the random phase approximation (RPA). From these, the reflectivity, electron energy-loss spectra (EELS) and small sphere extinction spectra are predicted. The experimental reflectivity and EELS were measured for PtAl2 and showed good agreement with the theoretical spectra. The yellow color of PtAl2 is associated with a bulk plasmon at 3 eV. We predict that the optical properties of hypothetical intermediate alloys would show a smooth evolution with composition. The details of this change can be understood by examination of the underlying density of states (DOS). The predicted small sphere extinction spectra and quality factors show a strong surface plasmon resonance for these materials, with PtAl2 having the optimum performance. The results indicate that these materials are good candidates for applications in plasmonics. © 2013 Elsevier B.V. All rights reserved

The role of plasmons and interband transitions in the color of AuAl₂, AuIn₂, and AuGa₂

First principles calculations of the optical properties of the intermetallic compounds AuAl₂, AuIn₂, and AuGa₂ have been performed. Analysis of the dielectric functions showed that AuAl₂ is unique because a bulk plasmon is seen in the optical region and contributes to the purple color of this material. An experimental electron energy-loss spectrum showed excellent agreement with the theoretical prediction and confirmed the presence of the bulk plasmon