I. Porosity occuring in diffusion-bonding
II. Thermal Transport in cellular metallic matrices
III. Interdiffusion and segregation in materials
IV. Hollow nano-structures
V. Metal dusting
VI. Hydrogen from water and sunlight: titanium dioxide photo-anode
Diffusion-bonding is a very attractive process for the strong bonding of dissimilar engineering materials in order to form engineering devices and structures. However, because of the differing diffusion rates of the components, porosity often forms in the bonding zone during fabrication or in-service conditions. This results in a very substantial loss of strength of the bonds that is greatly limiting the uptake of this bonding process to other technologies. Based on new research into the fundamental principles of inter-diffusion and with the assistance of computer simulation we are developing a robust and versatile theory that will predict the onset and extent of porosity formed during diffusion-bonding and will guide methods for its control.
These materials are of great interest as light-weight heat sinks. This research is concerned with steady-state and transient thermal transport in cellular metallic matrices. Using a new Lattice Monte Carlo method developed in the Centre and finite element analysis we are investigating the effective thermal conductivity and transient temperature profiles in various models of cellular metallic matrices (see Figure 1) and related materials.
Figure 1 - a) Open-cell M-Pore ® aluminium foam, b) Portion of the corresponding simulated structure.
Many materials depend on interdiffusion for their formation. In service, the components of many materials segregate as a result of the high temperatures and large driving forces present. This can lead to a degradation of designed properties. In this research program we have been investigating all aspects of interdiffusion and demixing processes in technologically important materials. At present, we are examining cation demixing processes in the solid electrolytes: yttria-stabilized zirconia and magnesium and strontium doped lanthanum gallate as well as interdiffusion in a number of multicomponent alloys including austenitic stainless steel.
Hollow nanostructures are of great interest as vehicles for drug delivery, as nano-reactors for gas phase reactions and for their unique optical, magnetic and electronic properties. In this research program, making use of Monte Carlo molecular dynamics simulation we are investigating the formation of hollow nano-objects by tailored interdiffusion of coated nanospheres, see Figure 2. Since hollow nano-objects are, in principle, thermodynamically unstable, we are also making use of Monte Carlo and molecular dynamics methods to investigate the stability and shrinkage mechanisms of various hollow nano-objects by diffusion.
Figure 2 - Melting (starting from the external surface) of a hollow Pd nanosphere at 1500K
Metal dusting refers to the very rapid disintegration of iron and low alloy steels in a highly carburizing (carbon super-saturated) atmosphere at temperatures ranging from 400 to 750 °C. Degradation of structural components by metal dusting is a major problem in chemical plants involved in the production of hydrogen, the synthesis of ammonia, the reforming of methanol and the production of syngas (hydrogen/carbon monoxide mixtures). In this research program, we are undertaking a major computational and theoretical program to determine the detailed mechanisms occurring in the metal dusting of irons and low alloyed steels and, with this knowledge, to point the directions for designing new materials to withstand metal dusting.
By means of a photo-electrochemical cell hydrogen can be produced directly from sunlight and water. For this highly attractive technology to be commercially viable the key component in the cell (the photo-anode) needs to be highly corrosion resistant and have a band-gap of about 2eV. Easily the leading candidate for this task is titanium dioxide because of its extreme corrosion resistance, but it has a band gap of 3eV. In this program we are investigating many aspects of titanium dioxide including near-surface segregation, anion doping and sophisticated hollow nanostructures in order to tailor its electrical properties and retain its outstanding corrosion properties for this demanding technology.