Properties and applications of metastable precious metal intermetallic compounds
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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 < 0.60 did not form sponges after either deposition at elevated or room temperature. 'Mud-cracked' mesoporous sponges could be formed by preparing precursors with χAl =0.67 and deposited at elevated temperature. The Pt₈Al₂₁ and meta-stable phase (ɛ-phase) were formed in precursors with 0.67< χAl < 0.90 that had been deposited at elevated temperature. In this case de-alloying produced classic isotropic fibrous sponges. Disordered and fragile masses were obtained when 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.
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