The control of electrons through patterning of superstructures

As concern grows over the environment, energy generation and climate change, there will be an increasing demand for new materials with improved performance to make technological applications cleaner and more efficient. One way to go about optimizing a material's performance is to start with a simple 'parent' compound and vary its chemical composition in a continuous and systematic way, for example, by substituting one of its chemical constituents by another. This strategy, known as doping, has been extremely successful. For example, in 1988, J.G. Bednorz and K.A. Müller replaced about 15% of the La ions in the insulating ceramic La2CuO4 with Ba and found that the product became a superconductor (i.e. lost all its electrical resistance) at an unprecedented temperature of 35 K, significantly higher than the previous highest known superconducting temperature. This discovery was the starting point for the development of the high temperature copper oxide superconductors, which now have operating temperatures as high as 135 K and which are increasingly being used in applications where high magnetic fields or electric currents are required.

Although the consequences of doping can be spectacular (witness the copper oxide superconductors) they can also be complex, and the link to changes in physical properties is not always well understood. One effect that can play an important role is the formation of superstructures, in which either the dopant atoms or the charges they transfer to the host organise themselves into patterns which extend over long distances and are periodically modulated on a nanometre scale. Superstructures modify the electrostatic potential in the host material, which can in turn strongly influence the physical properties of the material. This raises an interesting possibility: If one can control the formation of superstructures then it should be possible to tune the properties of a material and thereby enhance its performance.

The aims of this project are twofold, first, to understand why superstructures occur in certain materials and, secondly, to study the consequences of superstructure formation for the physical properties of those materials. To provide a testing ground for these ideas we have identified several different materials in which superstructures appear to play a prominent role. These include sodium cobaltate (a very promising p-type thermoelectric material), lithium cobaltate (the main component of the type of rechargeable batteries used in mobile phones and laptops), and two recently-discovered iron-based high temperature superconductors. As well as being good models on which to conduct experiments, these systems are chosen because they offer good prospects to underpin technological solutions for environmental and societal issues through their potential to improve the efficiency of energy harvesting and storage devices.

We will prepare single crystal samples whose composition can be varied via different doping strategies. X-ray and neutron scattering will be employed to probe deep inside the crystals to reveal the presence of superstructures and to refine the associated structural and electronic patterns, and we will correlate the results with bulk measurements of the electrical, thermal and magnetic properties of the materials. With the help of theoretical modelling, our programme will lead to a clearer understanding of the degree to which superstructures can be used control physical behaviour, and will contribute towards the development of materials with improved performance for practical applications.

WHO WILL BENEFIT FROM THIS RESEARCH?

Today's information-rich, mobile society has increasing demands for portable entertainment, computing and telecommunications equipment. Reversibility during battery cycles is a key factor, and it is affected by the formation of superstructures. Battery manufacturers would benefit from the better understanding of battery operation, and if this leads to portable batteries with better performance, the whole of society benefits from the improved operation of their iPods, laptops and mobile phones.

The Seebeck effect enables the conversion of waste heat into electrical power. This is useful when direct access to the grid is not possible. The largest market for this technology is the automotive industry, but the space industry may be important in the future. Energy scavenging will also be important in miniaturized electronic devices such as MEMS, micro-electrical systems and even "systems on a chip". All consumers will benefit from lower running costs and the consequences of the reduction in energy consumption on global warming.

In the Peltier effect electrical potential differences are converted into temperature differences, and this is exploited in solid state refrigerators. Thermoelectric coolers have a number of advantages, including long life, low maintenance, low waste and a means to cool devices on the nanoscale. The recent advent of massive data centres has shown that cooling energy costs are already comparable to, or even higher than, the cost of the computational equipment itself, primarily because the entire system is cooled down in bulk. Viable chip-scale refrigeration technology has now been demonstrated to cool "hot spots", drastically reducing energy consumption.

High-temperature superconductors are already employed widely in high-field magnets used extensively in scientific facilities, and in MRI scanners in hospitals. Other applications include low-loss power transmission cables, transformers, current-limiting devices, and microwave resonators and filters for telecommunications. Improved performance would benefit hospital patients, and the producers and users of this wide range of applications. Much more difficult to achieve, but weighted by an even greater benefit to society, would be the extension of the transition to room temperature.

HOW WILL THEY BENEFIT FROM THE RESEARCH?

The proposed single-crystal neutron scattering studies of lithium-ion batteries promises to offer new understanding of the formation of superstructures, and their effect on the battery life cycle.

The thermoelectric oxides under development here offer environmentally clean, competitive alternatives to current thermoelectrics, which often contain harmful or scarce elements.

The prospect of thermoelectric oxides exceeding current performance at elevated temperature is very promising for applications in the automotive industry.

A relatively modest enhancement of thermoelectric performance would create huge potential for cooling and energy harvesting in IT applications.

The potential benefit of work on a fundamental understanding of the iron-based superconductors is over a much larger time frame.

WHAT WILL BE DONE TO ENSURE THAT THEY BENEFIT FROM THIS RESEARCH?

Dissemination to industry will be via our industrial partner JMTC, who are expert on industrial scale processing of materials and provide the link with industrial end users. The Research and Enterprise Office at RHUL and the ISIS Innovation Ltd at Oxford will draw up an Exploitation Plan and advise on Intellectual Property. It is anticipated that progress on developing new materials will enable future bids for the translation of this research into industrial applications. Wider engagement activities with industry will be organised by our dedicated Physics outreach officers.