Materials Physics

Group Members

Head of Group: Prof. G. Neville Greaves
Staff: Prof. Ken Walters, Prof. D. Andrew Evans, Dr. Simon Cox, Dr. Rudi Winter, Dr. Dave Binding, Dr. Martin Wilding, Dr. Nigel Poolton, Dr. Edwin Flikkema, Dr. Dave Langstaff
Technicians: Steve Fearn, Matt Gunn
PhD Students: Owain Roberts, Geraint Jones, Gruffudd Williams, Twilight Barnardo, Kristin Hoydalsvik, I. Tudur Davies, and Aled Wyn

Overview

Materials research investigates the relationshiop between the microscopic structure of materials and their macroscopic properties. By understanding the fundamental nature of materials it is possible to design new materials for specific applications. 

The materials reasearch group uses a variety of measurement and modeling techniques in order to learn more about industrially relevant materials. Materials research at Aberystwyth is centred around several key areas: glasses, zeolites and ceramics, foams and complex fluids and semiconductor thin films and surfaces. In each of these areas appropriate techniques are brought to bear, measuring aspects of the materials are they are formed or processed. Parameters derived from measurements are used in sophisticated computer models, enabling further insights as to the exact nature of the processes taking place in the systems under study.

Structure of non-crystalline materials

Intense synchrotron radiation X-ray techniques, combined with neutron methods, have been exploited to determine the complex architecture of non-crystalline materials. The complementary structural probes of X-ray Absorption Fine Structure (XAFS), X-ray Diffraction (XRD) and Small Angle X-ray Scattering (SAXS) have been directed at elucidating the atomic structure of silicate glasses and the physics underlying the formation of glass ceramics. By combining these measurements with neutron scattering performed at the Spallation Neutron Source at the Rutherford Appleton Laboratory, the powerful computational techniques of Reverse Monte Carlo and Molecular Dynamics are being exploited to determine three-dimensional structures in which the movement of ions responsible for electrical conductivity, chemical resistance and thermal expansion can be realistically studied.

Zeolites and their analogues are of major importance as the substrates for catalysis - their gigantic cavernous microporous structures can be doped with metals like Ni and Co to generate the microscopic centres for catalysis. Using combined synchrotron X-ray techniques, the local geometry of the metal centre have been followed in conjunction with the crystallography of the substrate as the catalyst is formed, begins to operate and eventually fails - unique experiments allowing the phenomenon of catalysis to be physically traced from birth to death. Inelastic neutron scattering experiments reveal not only the interatomic vibrations within the open network but also the low frequency modes that relate to the microporous structure - floppy modes which can also undermine the stability of the network, leading it to collapse to an amorphised phase of conventional density.

Nanocrystalline and high-temperature ceramics

Ceramics are the industrial material of choice when it comes to severe conditions such as high temperatures and steep thermal gradients in melting furnaces, rapid thermal cycling in reactors or chemical corrosion in filter beds. While only a fairly small group of chemical compounds are sufficiently resilient to withstand such conditions, the physical make-up of the material plays an important role in providing strength. Granular ceramics also play an important role as sensors of industrial process parameters such as temperature, pressure or reactant concentration. Again, the physical microstructure, i.e. the size, shape and arrangement of the grains in the granular ceramic determine the behaviour of the material.

In the Materials Physics group, a combination of experimental techniques are applied to study the structure and its response to severe thermal and chemical conditions on all length scales from the atomic level to individual grains to macroscopic specimens. The atomic structure is determined by Nuclear Magnetic Resonance (NMR), a technique in which the electron density of the material under study is determined by subjecting the sample to radio-frequency magnetic fields. The structure of crystalline components and the strain caused in them by thermal effects are probed by X-Ray Diffraction (XRD), and the three-dimensional arrangement of both granular and continuous phases in a granular ceramic is investigated by Small-Angle X-ray Scattering (SAXS), a synchrotron-based experiment which exploits the fact that interfaces between components with different index of refraction scatter light (including x-rays) at an angle determined by the curvature of the interface.

With all three techniques, NMR, XRD and SAXS, we aim to conduct experiments under controlled in-situ conditions, i.e. with well-characterised thermal gradients and chemical conditions. This requires the design and construction of sample cells for dedicated experiments. Currently, we operate a laser-heated NMR probe, and a laser-heated diffraction cell with integrated pyro-microscope is currently being planned.

Optical Characterisation

Optical techniques are important in the characterisation of materials in addition to providing important information on the fundamental physics associated with them. A range of optical instrumentation is available in-house of which the most important are Raman spectroscopy and ellipsometry. A triple monochromator Raman system provides data on fundamental excitations in materials, which can be related to material parameters such as stress or crystallinity. Ellipsometry measures the refractive properties of materials as well as thin film thicknesses and provides complimentary information to Raman spectroscopy. The Materials Physics laboratory houses the latest state-of-the-art spectroscopic ellipsometer with variable temperature facilities, enabling optical properties to be studied from temperatures as low as 4K up to temperatures of 500K. Currently, these instruments are being used to study a range of material systems, including thin buried semiconductor layer systems, quantum dot devices, high temperature superconductors, glasses, organic semiconductors and bio-molecules.

Semiconductor thin films and surfaces

At Aberystwyth, a range of spectroscopic and imaging techniques have been developed to study the electronic and optical properties of semiconductors, in particular organic semiconductors and wide-gap semiconductors such a diamond and boron nitride. Organic semiconductors are becoming increasingly used in a range of applications such as display and photovoltaic technologies and in the field of mobile telecommunications. The Aberystwyth group has a particular interest in hybrid organic-inorganic structures: for example combining inorganic transparent oxides with organic semiconductors in new solar cell devices and combining the inorganic semiconductors such as GaAs with organic semiconductors to improve the operation of devices such as mixer diodes. Organic semiconductors currently studied include small molecules (e.g. phthalocyanines) and polymers (e.g. P3HT). In the area of wide-gap semiconductors, the focus is on the superhard materials, diamond and cubic boron nitride (cBN). These share many desirable mechanical and thermal properties, for example extreme hardness and thermal conductivity that are widely exploited, but they also have potential as optoelectronic materials, having similar electronic band gaps that lie in the UV part of the electromagnetic spectrum. Studies on diamond include the fabrication and real-time characterisation of thin film electronic device structures as well as developing new methods for correlating local chemical bonding with light emission in natural and synthetic single crystals. Studies on boron nitride involve the application of synchtrotron and optical methods to correlate local chemical bonding in multi-phase materials with light absorption and emission.

Techniques such as Real-time Electron Spectroscopy (REES), Raman Spectroscopy, Low Energy Electron Diffraction (LEED) and Scanned Probe Microscopy (SPM) are applied in the Materials Physics laboratory to study surface electronic and physical structure. Complementary in-house techniques such as IV/CV are used to study electronic devices (e.g. hybrid diodes) and synchrotron techniques such as Soft-x-ray photoelectron spectroscopy (SXPS), Grazing Incidence X-ray Reflectometry (GXR), X-ray Absorption Near-Edge Spectroscopy (XANES), Optically-detected X-ray Absorption Spectroscopy (ODXAS) and X-ray Excited Optical Luminescence (XEOL) are applied to study local bonding, structure, morphology and light emission at surfaces, interfaces and within the bulk materials.

Integrated Detectors

Often the main limitation in spectroscopic techniques is the efficient detection of radiation (both electromagnetic and particulate) and improvements in detection technology lead inevitably to significant advances in scientific knowledge. In electron spectroscopy, there is a considerable effort world-wide to develop the electron equivalent of CCD detectors, but few of the detector systems under development have been able to demonstrate the required electrical stability needed in continuous operation in commercial systems. Building on the 5mm, 192-channel ion detector originally developed at Aberystwyth, the EPSRC-funded REES (Real-time Electron Energy Spectroscopy) project has provided a 19mm, 768-channel electron detector that is currently the only one fully-integrated on a single silicon chip that efficiently detects individual electrons with a parallel multi-channel array and also has the necessary robustness. The detector is applied to real-time photoelectron spectroscopy of semiconductor surfaces and interfaces using x-ray and UV sources at Aberystwyth and has also been coupled to intense synchrotron light sources to provide the ultimate source/detector combination.

Complex Fluids and Flow Processes

The Rheology research group is mainly interested in the flow behaviour of elastico-viscous liquids, namely materials that are predominantly fluid in behaviour but have some of the elastic properties usually associated with solids. These "elastic effects" are present in a variety of liquids including foams, emulsions, polymer solutions and various materials associated with the food and pharmaceutical industries. Some of the research activity of the group is concerned with the problem of formulating equations to describe the behaviour of elastico-viscous liquids, but the main effort involves the analytical and numerical solution of flow problems in an attempt to predict the important flow characteristics which are likely to be present in the behaviour of real liquids. There is particular interest in the rheology of foods, printing inks and in high shear-rate high temperature processes such as those that occur in fuel injectors, etc.The theoretical work is supported by a corresponding experimental programme. The facilities in this connection are excellent, with ample space in the Complex Fluids Laboratory and sufficient financial support from various outside sources.

Research students in rheology and computational fluid dynamics benefit from the full resources of the University of Wales Institute of Non-Newtonian Fluid Mechanics whose Director is Professor K Walters, FRS. The Institute links research personnel at Aberystwyth, Swansea and Cardiff. The aims of the Institute are: enhanced collaboration between the three centres, the development of strong ties with industry by offering a wide spectrum of expertise, the sharing of state of the art computing facilities and the organisation of workshops and research meetings.

Structure and Dynamics of Foams

The Foams group in Aberystwyth is interested in modelling the structure and dynamics of foams and related materials. This involves solving partial differential equations, developing numerical simulations and devising related experiments. This work has industrial support and is regularly presented at conferences and workshops, and students can expect to travel widely and interact with a number of research groups around the world.

Minimal surfaces: Each soap film in a static foam is a surface of constant mean curvature. Plateau's equilibrium rules determine the local geometry of a foam but not the topology. To examine different foam structures, we take the foam energy to be its total surface area, and then seek local minima of this energy using the Surface Evolver software. In particular, we seek candidates to the global minimum, representing the optimal packings of bubbles. A recent extension to this work is in finding the global minimum of a confined foam, with possible applications to morphological development in biology.

Rheology: The flow of foams is studied both numerically, by assuming that the foam passes through a sequence of equilibrium states, and experimentally, using the facilities of the rheology laboratory. Our simulations are used to design networks for "discrete microfluidics" - the use of ordered foams and emulsions for automated testing of gas and liquid samples. We are currently extending the celebrated Stokes problem to the motion of spheres through a foam, and developing models of the dissipation that occurs in soap films as they move.

Drainage: A standing foam loses liquid from between the bubbles due to gravity, a problem that we study by deriving partial differential equations for the liquid content of the foam as a function of space and time. Recent work has extended this theory to the case when the acceleration due to gravity is reduced or negligible, such as in microgravity experiments on the International Space Station, and has included the effects of solidification, such as in the fabrication of metallic foams.