Advanced Materials

Solid metal oxides-magnetic, superconducting and battery materials

The majority of advanced inorganic materials used in magnetic, conductivity, superconductivity, ferroelectric, catalytic, and battery applications are solid metal oxides. Numerous groups across Australia are actively studying the properties and structures of metal oxides. In general, such inorganic materials are prepared and used as polycrystalline ceramics or powders. Powder X-ray diffraction is a key characterization method and synchrotron radiation is often required

Metal oxide chemistry is dominated by classes of materials having crystal structures derived from a simper parent structure, such as perovskite or rutile. Small lattice distortions which are critical to the key electronic and physical properties of these oxides, usually lead to lower symmetries and superstructures. These distortions are characterized by subtle peak splittings and the appearance of weak superlattice reflections in the diffraction data. The detection and understanding of such distortions using powder diffraction methods usually requires high resolution that is only afforded by synchrotron radiation. Typical examples include the polar distortions in bismuth oxide ferroelectrics, Jahn-Teller distortions in manganese oxide battery materials, and valence ordering in colossal magnetoresistance (CMR) materials. Phase transitions between the distorted and structural variants influence the stability and processing of the materials. Such distortions often lead to twinning, making it impossible to synthesize readily diffraction-quality single crystals. This is critical in emerging areas. Powder diffraction does not suffer from this problem and historically as been the tool of choice to study novel oxides.

An underlying feature of many of the most interesting materials is the strongly correlated behaviour of the electrons and coupling of the electronic charge and spin degrees of freedom with those of the electron orbitals and the lattice. The greatest potential for functionality is in materials at the edges of a structural and/or structural instability, where small changes in chemical or physical conditions lead to a major change in properties. Establishing the role of these perturbations requires careful variation. The success of such parametric studies is reliant on rapid data collection (about five minutes per data set) without compromise of data quality. The importance of such studies is illustrated in the identification of an intermediate thermal phase in the unusual 4d ferromagnet SrRu)₃. This phase was only correctly identified after collecting high resolution data in 5C temperature intervals over a wide temperature range.

Australian researchers have an enviable reputation in the study of incommensurate structures. Previously such studies have been limited to samples where high quality single crystals are available. However, the presence of structural modulations can by themselves preclude the formation of suitable single crystals. The superb signal-to-noise at synchrotron powder diffractometers can reveal the extremely weak superlattice reflections associated with such modulations, and considerable effort is being directed towards the analysis of such structures from powder diffraction data. A number of key materials exhibit modulated structures, including Cu superconductors and layered bismuth oxide ferroelectrics.

Electronic and opto-electronic materials

A niche area in which Australian research has made a significant contribution is in the development of thin film materials for electronic and optoelectronic devices. Thin films of materials with particular chemical and/or physical properties such as piezoelectricity are typically deposited onto an appropriate substrate by one form of chemical vapour deposition, and during the development phase for both precursor and deposition conditions the physical and chemical properties of the film must be determined. As in most surface characterization, conventional X-ray photoelectron spectroscopy (XPS) is used for initial chemical analysis, but XPS is rarely able to reveal the orientation of film crystallites. Synchrotron-based variable angle XAS is the most effective technique for the determination of the important characteristic.

A major thrust of new knowledge and understanding of spin-dependent phenomena in atoms, clusters, ferromagnetic films and surfaces is developing from the "two=particle incidence reflection spectroscopy" technique, which leads to surface information unobtainable by any other method. The technique allows a description of spin-dependent interaction potentials and electron correlations that determine the enhanced magnetic moments of atoms, surfaces, and magnetic coupling between, for example, a magnetic and non-magnetic material or magnetic layers separated by a spacer layer. This information is the basis of spin-electronics (or magneto-electronics) in which the spin as well as the charge is a determining factor. For example, a system of alternating ferromagnetic and non-magnetic metal layers can change its electrical resistance from small (with parallel magnetization) to large (with anti-parallel magnetization) to form a "spin-valve" in the read-head of computer hard disks. It allows the "write/read head" to be made smaller and the storage density on the hard disk to be increased to more than 10¹⁰ bits/cm². Research using a synchrotron is expected to provide a basis for a further reduction in a size and increase in storage density.

Trace magnetic impurities at ultra-dilute levels have become a significant concern for silicon microchip manufacturers seeking ever-improving performance. recent research has shown that it is possible to trap these impurities on the inner surface of nanocavities in the silicon substrate. The very high intensity of a beam derived from a multipole wiggler insertion device (planned for Beamline 5 at the Australian Synchrotron) enables measurement of concentrations of these impurities at the parts per billion level using XAS and can be used to study the trapping mechanism.

Catalysts

Catalysts are vital for many industrial, biotechnological and food manufacturing processes, but how they work is often not well understood. Understanding their reaction mechanisms on an atomic scale is a key part of developing new and better catalysts. Combining microspectroscopy and imaging will provide new information on the chemistry and physical structure of a material's surface, which will give important clues to how catalysts function.

The redox state dependence of the reactions and reactivity of transition-metal complexes is a key distinguishing characteristic of the d-block elements. This characteristic is pivotal to their remarkable ability to act as catalysts for an extraordinary range of reactions and explains the vital role of transition metals in a broad range of enzymes. Both in biological and abiological contexts transition metal catalysis is essential for life as we know it. Metalloenzymes are examples of exquisitely designed low-temperature, low-pressure, low-volume catalysts. In contrasts, the catalysts typical of the chemical industry are capable of high-volume chemical transformation but only at the cost of high temperature and/or high pressure. Elucidation of the molecular details of the chemistry associated with enzyme catalysis is thus important because of the potential this offers for the discovery of energy-efficient large-volume catalysts and the light that these investigations cast on our general understanding of chemistry. Accordingly, XAS studies are in progress to improve the understanding of the influence of redox or charge state on the electronic and molecular structure of metal complexes or clusters so as to better anticipate (and ultimately control) the reactions and reactivity of transition metal catalysts. The metal clusters that lie at the active site of the nitrogenase and hydrogenase enzymes provide challenging but important target molecules for this work. Techniques including the use of infrared, ultraviolet and electron paramagnetic resonance spectroscopies have been developed to permit in situ spectroscopic examination of reactive electro-generated species to complement direct structural characterization with XAS.

XAS studies will be used to identify efficient abiological catalysts to provide a low-cost, energy-efficient production of ammonia and catalysts for H₂ fuel cells based on cheap and abundant chemicals. progress depends upon the determination of the molecular structure of reactive intermediates that, with the aid of the increasingly powerful suite of theoretical techniques can the be used to drive catalyst design. XAS studies enable time-resolved XAS measurements of dynamics systems. These advances will be essential toward the implementation of the catalyst into a process control environment.

Microporous materials such as zeolites have great potential as catalysts, sorbants, and microreactors. These materials typically have large unit cells and often contain large voids that are responsible for their key properties. Analysis of their structures is vital to understanding the way they function, but is very difficult. It is not uncommon to observe weak and complex X-ray diffraction patterns due to low symmetry of subtle distortions. The proposed small molecule diffraction end station on Beamline 2 at the Australian Synchrotron will be an important new tool for this task.

Metals and alloys

The development and production of metals and alloys are of fundamental importance for any advanced society that is dependent on sophisticated and elaborately transformed manufactures.

Many different types of metals and alloys are now available, each tuned with the required combination of physical, mechanical and chemical properties to suit a specific application. These combinations of properties are achieved by the development of highly complex microstructures through the addition of alloying elements together with thermal and mechanical treatments.

The understanding of the role of microstructures and their influence on the alloy properties has been made possible by access to a wide range of measurement and imaging techniques, especially optical and electron microscopes and microprobes, X-ray imaging and X-ray diffraction. As a result, remarkable progress has been made, but there are still many aspects of allow design that are not fully understood and plenty of opportunity for further improvement. The advanced techniques possible with the synchrotron are bringing new tools to this task.

Particular techniques that will make valuable new contributions are

• X-ray absorption spectroscopy to obtain knowledge on the short and medium range order, coordination numbers and local coordination geometry of the elements at an atomic level in complex alloys
• higher resolution X-ray diffraction on a micro scale to be able to understand subtle changes to crystal structure that occur with the addition of alloying elements and to be able to measure internal thermal and residual strain distributions in microstructures.
• phase contrast enhanced hard X-ray imaging to be able to see microstructures and defects in structures inside the material on a three dimensional basis
• in situ imaging of the liquid to solid transformation that occurs during the casting of an allow, to observe the nucleation and growth of grains and dendrites and the segregation of alloying elements
• in situ observation of the corrosion mechanisms that occur at the surfaces of alloys under a wide range of environmental conditions.

Australia is a major producer and exporter of metals and alloys, particularly for manufactured goods such as motor vehicles. While the major materials used in modern motor vehicles is steel, the move to improve fuel economy in order to reduce greenhouse gas emissions has led to the search for high strength, low density alloys. Australia is responding to this with the Light Metal Action Agenda and initiatives such as CSIRO's Light Metals Flagship. Major programs are underway to develop new, low-cost magnesium, aluminium, and titanium alloy systems. Synchrotron techniques will be used extensively by the researchers in these programs because of the ability of high energy, high brightness X-rays to penetrate deeply into these metals.

In some of the most advanced light metals the incorporation of ultra-fine refractory fibres has been considered to strengthen the material. The performance of this type of strengthening mechanism depends critically on the residual strains and the efficiency of stress transfer at the fibre/matrix interface. Synchrotron X-ray diffraction techniques are an excellent method for measuring internal stresses, particularly for monitoring in real time the changes that can occur when straining or thermally treating the material.

Engineered Components

The measurement of residual strain fields in the surface and subsurface (0.01 mm to 1.0 mm) region is important in understanding the long-term performance of mechanical engineering components. This depth range is where most of the degradation of mechanical components during service originates. It also covers the thickness range of many protective coatings (for example, thermal barrier coatings) and surface engineering treatments (for example, laser shot peeling).

The large flux of high energy X-rays coupled with the ability to scan components within the proposed configuration of Beamline 10 at the Australian Synchrotron will enable the two-dimensional mapping of strain and grain texture in practical times. It will be a major advance over the alternative techniques that are currently used-neutron methods that have insufficient spatial resolution (1 mm) and laboratory sourced moderate energy X-rays which are limited to investigating the top 0.01 mm to 0.05 mm.