From the Surface to the Scientific Melting Pot

 

Jim Metson doesn’t have to dig deep to achieve big changes. As a professor of chemistry at the University of Auckland and a Principal Investigator for the MacDiarmid Institute he’s involved in several research projects exploring different materials. But his focus is always on the surface.

“People have different ideas about what a surface is,” he says. “Physicists tend to view a surface as a few atoms deep, engineers take a much deeper view and chemists sit somewhere in between. For us the combination of chemical, electronic and microstructural properties of the surface is frequently important.”

Even tiny changes in the composition or microstructure of surfaces can make a big difference in how a material performs, from the efficiency or purity of the manufacturing process itself right through to the actual function of the material.

Take aluminium as an example. Its production alone consumes two percent of the world’s electricity, but despite the macro-scale of manufacture, it’s the nanoscale of the surface structures involved in the raw materials and process that can make all the difference when it comes to new developments.

Aluminium and magnesium smelting is one of the research areas at the University of Auckland’s Light Metals Research Centre, which Jim Metson helped found in 2002 to meet a global need for a concentrated research effort focusing on these materials. Together with colleagues Associate Professor Margaret Hyland and the centre’s current director, Mark Taylor, who joined the group in 2003 after working for Rio Tinto Aluminium (formerly Comalco) and managing smelters at Tiwai Point and Boyne Island, Australia, Professor Metson has been involved in the development of new dryscrubbing technology to reduce unwanted emissions and to improve the environmental performance of smelters. “We are investigating better production technologies, particularly to achieve improved energy effi ciency and a high-purity metal. Aluminium is versatile and fits high and low-tech applications in both large and small high-value markets. The raw material used in smelting, aluminium oxide, is itself a versatile commodity, used at one end of the spectrum as a highly valuable catalyst support, while at the consumer end it is a widely used abrasive.

Although tens of millions of tonnes of alumina are produced globally per annum, Professor Metson says the microstructure of this material, particularly of the surface, is still poorly understood.

In contrast to this applied research for the aluminium industry, another project at the centre focuses on a material that is difficult to produce in the first place – zinc oxide. “It’s a fickle material in terms of many desired properties, and the difficulty starts with making it with the right stoichiometry. But it’s very useful. Its band gap is in the blue to ultra violet part of the spectrum, which is why it’s already used in sunscreens. It’s potentially a superb emitter of blue light, something that the more expensive gallium nitride does in diodes and blue lasers. So if we could make zinc oxide consistently, or even just understand why it’s so hard to make, it would be much cheaper and provide a more readily available, blue light source.”

In collaboration with University of Canterbury and GNS Science, Professor Metson’s team is developing methods of growing very precise thin films of zinc oxide. “Relatively coarse films can be made rapidly but it is not so much about the deposition of the material, but about control of the microstructure, and figuring out what controls conductivity. The idea is to introduce or implant other elements, such as aluminium ions, into the surface lattice in an attempt to modify conductivity, or the nature of conductivity, and to profile what you’ve made in a structural sense.”

The Light Metals Research Centre brings together a wealth of expertise through its staff and a range of analytical and characterization techniques through its state-of-the-art equipment. High resolution electron microscopy is used to study and image the morphology of a surface, while an X-ray photoelectron spectrometer characterizes surface chemistry and depth profiles surfaces. A Time of Flight Secondary Ion Mass Spectrometer (TOF SIMS) analyses the chemical composition of the outer few atomic layers of a surface.

Professor Metson has also pioneered the use of synchrotrons in examining surfaces and structures of materials, which has led to improvements in understanding environmental emission from aluminium reduction cells and a “surprising discovery” in collaboration with MacDiarmid Institute investigators Joe Trodahl and Ben Ruck. “I worked with them on the microstructure of amorphous gallium nitride thin films and we observed that molecular nitrogen was being incorporated in the lattice. It was a major surprise to find significant levels of nitrogen are stable within the lattice and this discovery would not have been possible without the use of a synchrotron.”

So far, he has used the synchrotron facilities in Madison, Wisconsin, in Japan and in Grenoble, but says he looks forward to the opening of the synchrotron in Melbourne later this year. “It has the potential to expose a far wider spectrum of material scientists to synchrotron based techniques and a lot more people will be able to try things. One of the great things about synchrotrons is that communities of scientists are working on them, it’s a scientific melting pot and people start talking and broadening their horizons. The Melbourne synchrotron is an excellent facility with specifications comparable if not better than those of the current third generation rings internationally.”