Feature

Metallic Super-Sponges

Sponge-like metals make great catalysts.

Bernard Carpinter

When inert gas atoms are fired at a million kilometres per hour into metal surfaces, materials with a "honeycomb" or "sponge-like" structure are formed. These "metal sponges", which have now been studied at Victoria University for several years, could find important applications in industry, for example as catalysts.

The research team leader, Associate Professor Peter Johnson of the Physics Department, explains that the sponges are produced by bombarding thin foils of metals with inert gases, usually helium, in the charged particle accelerator. A close look at the implanted metals, using the Victoria University's Electron Microscope Facility, reveals bubbles of helium at high concentration.

For low-dose implantations at room temperature, a striking feature is that the bubbles are ordered on a three-dimensional array. Remarkably, the bubbles are arranged in a "superlattice" in exactly the same pattern as the atoms in the host metal crystal. These bubbles are on such a fine scale that their spacing is just 20 times greater than the spacing of the metal atoms.

The phenomenon of bubble ordering is of interest internationally because advances in clarifying the mechanisms involved could make a valuable contribution to understanding the behaviour of materials at the atomic level. Two recent examples of contributions by group members are the PhD research of Kevin Stevens, which clarifies important aspects of the imaging of bubble structures in the electron microscope, and the MSc research of Fenella Lawson establishing the existence of a lower temperature threshold for bubble ordering.

When higher doses of helium were shot into the metals, early results gave evidence of a random bubble structure with larger cavities. This structure had some unique characteristics that suggested the material could have significant potential for use in industrial applications. But there was a problem, as Johnson explains.

"At these high doses the metal in the implanted layer contains high levels of gas, damage and strain. This makes both the preparation of specimens and the imaging of fine-scale cavities particularly demanding. We realised that extra resources were needed."

The group was strengthened by the participation in the work of Dr Peter Gilberd of the Physics Department and Dr Ian Vickridge of the Institute of Geological and Nuclear Sciences (IGNS), with Yvonne Morrison as a research officer.

For high-dose implantations into vanadium -- the first metal studied in detail in the FRST programme -- regular cavities were found on the scale of a nanometre (billionth of a metre).

"These cavities exhibit a high degree of interconnection and an exceptionally high internal surface area," Johnson says. "The metal walls separating neighbouring cavities are so thin that their width corresponds to a few atom spacings only."

Gilberd continues: "The cavity concentration is so high that the degree of swelling in the implanted layer could be greater than 100% -- we are coming to think of the modified layer as a metal foam or sponge."

Such structures have great potential as the basis for sophisticated catalysts for producing high value chemicals, such as pharmaceuticals, and for possible magnetic or optical applications.

The advantage of using such "nanoporous" forms of these materials is the greatly increased surface area -- chemical reactions are promoted at the surface of catalysts, so the greater the surface area, the more effective is the catalyst.

In discussing the involvement of GNS, Vickridge points out the long-standing association between the accelerator groups at Victoria and GNS.

"We see our expertise, centred on materials science and ion beam analysis techniques, and Victoria expertise which is centred on electron microscopy and ion implantation, as mutually complementary. Our emphasis is on the more applied aspects -- the development of materials and processes for eventual use by industry."

Vanadium pentoxide (V₂O₅) is a widely-used catalyst, and current work at GNS concentrates on using ion beam analysis to determine the nature and thickness of the oxides formed when the nanoporous vanadium layers are exposed to flowing oxygen. The oxides are also studied at Victoria using electron microscopy and, with help from Professor Joe Trodahl's group, Raman spectroscopy. Experiments are showing that the sponge-like form of the metal is enhancing the formation of the pentoxide on its surfaces.

Platinum in a finely divided state is an extremely effective catalyst, so platinum has been included in the metals studied at Victoria. Morrison summarises the results.

"In platinum, the group has successfully identified the conditions required to produce a range of different cavity structures with sizes from two nanometres to hundreds of nanometres. The formation of porous structures with cavity sizes in excess of 10 nanometres is of particular interest."

Recently, under similar conditions, helium implantation into titanium has been found to produce structures which are more regular in shape, uniform in size, and smaller in scale. Titanium dioxide is also a catalyst. One of the team's technical successes has been to cut very thin cross-sections of the implanted metals, using a technique developed in conjunction with the Electron Microscope Facility.

The success of the research in vanadium, platinum and titanium has laid the foundation for future work in other metals and FRST funding has been extended through to June 2000 for this purpose. As this research develops, there will be an emphasis on strengthening links with industry. Already at IGNS, a start is being made on investigating alternative ion implantation techniques, such as plasma immersion, that could be more suited to industrial-scale production.

Bernard Carpinter is a journalist at Victoria University.