Alan Lawley, the Editor-in-Chief of the International Journal of Powder Metallurgy introduces this special issue on precious metals, which discusses the history, production and application of platinum group metal (pgm) powders, as well as gold and silver powders. Powder metallurgy involves the production of a metal powder to certain specifications and the subsequent treatment of this powder to bind and shape it into a desired form. As Lawley points out, the powder metallurgy of the base metals, especially iron, may receive more coverage, but precious metal powders have many unique properties that make them valuable for a range of applications. Five papers cover the topic from different perspectives and they are discussed separately in this review.

‘Precious Metals: A Valuable Powder Metallurgy Player’

The first paper, by Pierre W. Taubenblat, FAPMI (Promet Associates, USA), is a very good overview of the subject, which grabs attention from the start with the historical reference to early examples of decorative metal powder found in the tomb of Tutankhamun, and moves on to describe the current uses for precious metals in the field of powder metallurgy. The use of platinum powder as a catalyst, where its high specific surface area is an advantage, and the effect of using platinum and palladium powders as sintering aids are both touched on. The paper does not cover in any great detail the historical applications for silver, which would have been at least as extensively used as gold; however silver is discussed with respect to current markets, specifically its use in electronics.

‘Applications for Precious Metal Powders’

An interesting and very readable paper by Joseph Tunick Strauss (HJE Company, Inc, USA) covers a wide range of applications for precious metals from silver through to platinum. The focus is on gold and silver, which is not surprising as these are much more widely used than the higher-value pgms, although some reference is made to palladium and platinum. The predominant use for powder metallurgical processing of precious metals in general appears to be in cosmetic applications such as jewellery and dentistry; although silver, particularly, has bulk usage in solder/braze pastes and contacts. Little detail on this is provided, but this is probably appropriate, as these bulk applications use simple and established manufacturing processes. There is also little emphasis on the use of ultra-fine powders in sensors, which is the ‘high-tech’ end of the solder type applications, although this is covered in detail by the next paper reviewed below.

The clarity of this paper could have been improved by including an introduction identifying the major areas of application and the wide range of processes used to manufacture precious metal powders; a table or glossary would also have aided clarity, although the many acronyms are identified in full at the first usage. An important point is raised under the sub-heading of ‘Net-Shape Parts’, namely that there is no real small-scale method of manufacturing powders, which makes development of processes that might use pgm powders difficult. Such technology would greatly promote the development of powder metallurgical precious metal products in the medium and long term.

The introduction of rapid manufacturing (RM) to the portfolio of processes and manufacturing techniques was the most interesting part of this paper, as it appears to offer opportunities for pgms in terms of scale and product complexity that no other process can match.

‘The Manufacture of Platinum, Gold, and Palladium Powders’

This paper by Howard D. Glicksman (DuPont Electronic Technologies, USA) provides an excellent and detailed review of fine and ultra-fine precious metal powder manufacture and gives considerable detail without becoming too immersed in technical jargon. The paper concentrates on the higher-end precious metals (platinum, palladium and gold) and their application in the modern and rapidly growing market of electronics, including sensor technology, and therefore is very well focused. The detail in the text is almost sufficient to allow the reader to attempt their own manufacture of these or similar powders (Figure 1 and 2).

Of particular interest was the apparent ability to use simple additives to control shape and minimise agglomeration. It could be assumed that control of these characteristics would involve significant complication and cost but this may not necessarily be the case. Another revelation was that it is possible to manufacture alloy powders directly from solution, a fact which may not be widely known.

‘Precious Metal Powder Precipitation and Processing’

This paper, by Sean Frink and Phil Connor (Ferro Electronic Material Systems, USA) is similar in many ways to the previous article, except that it is specific to chemical processing of powders, i.e. precipitation from solution. Of particular interest was the breakdown of the difficulties of characterising fine non-spherical powders: the available technologies and their limits are well described and would be a useful reference for anyone intending to work with these products. Some additional information on the manufacturing of novel morphologies would have been interesting, although I suspect that this subject could warrant a paper in itself.

‘Additive Manufacturing of Precious Metal Dental Restorations’

To a degree, a confused introduction spoils an otherwise interesting paper by Anita L. Hancox and Jeffrey A. McDaniel (imagen, LLC, USA). The subjects are introduced out of sequence and the ‘goal’ of a non-silver alloy is mentioned when no silver content had been indicated to that point! However, the main body of the paper covers the subject of manufacturing metal foundations for permanent dental crowns and bridges well, although it does not adequately emphasise the criticality of powder size and distribution that was made clear in the earlier papers. The process of three-dimensional (3D) printing is well described and the importance of binder droplet size is highlighted, with an excellent illustration of the benefit of reducing droplet size. It was good to see some metallurgical aspects (such as grain structure and physical strength) of gold-palladium alloys being explored, as this information is often sparse and has been absent elsewhere.

Concluding Remarks

As a group of papers the Focus Issue covers the current use of precious metal powders very well, although much more could have been said about the potential new technologies deriving from powder metallurgy. Metal injection moulding and rapid manufacturing technologies such as additive laser manufacturing are covered in general by Strauss's paper but are worthy of a separate paper in themselves.