We have a number of fully-funded 4-year PhD studentships available to start in October 2017. These studentships are in collaboration with industry partners of the CDT and will be of particular interest to applicants from the EU who do not meet the EPSRC residency requirements for full funding as these studentships would be fully-funded for such applicants. Successful candidates will have an aptitude for theory and simulation and already have, or expect to achieve, a first class (or equivalent) Bachelor’s or Master’s degree in the physical sciences or a branch of engineering. Applicants should apply via the usual route, making it clear in their personal statement which project they are interested in. For further information on these projects please contact the CDT Senior Administrator.

1. Accurate Modelling of Sintering and Residual Stresses in Polycrystalline Diamond

Supervisors: Dr Dini and Dr Balint (Mechanical Engineering, Imperial College London)
Collaborators: Serdar Ozbayraktar (Element Six)

Polycrystalline diamonds (PCD) are systems composed of diamond granules sintered in the presence of a catalyst (e.g. cobalt) under very high pressure and temperature.  They are hard abrasive materials used in oil and gas drilling. PCD cutting discs are embedded within the drill head, and experience a complicated multi-axial loading state in contact with irregular rock, and high temperature from frictional interaction [1, 2]. Cutting disc failures lead to the removal of the drill from the rock, resulting in lost drilling time and significant associated costs; discs typically fail by a combination of intra- and trans-granular fracture through the PCD, and or spallation of the PCD from the cutting disc substrate (typically tungsten carbide). Being able to predict the microstructure and residual stress state generated by the manufacturing process affect the mechanism of fracture is a very important problem for companies such as Element 6 (E6), who manufacture the cutting discs [3-5].  A better understanding could lead to a change in process parameters to produce better microstructures and residual stress states for performance and durability, or lead to new possibilities in the sintering process (e.g. the choice of catalyst).

Understanding how the pressure versus temperature cycle that the PCD experiences during compaction, sintering and bonding to the tungsten carbide substrate of the cutting disc, affects the microstructure, residual stress state and component performance, is the overall objective of this project. The effect of the pressure-temperature cycle is felt at the microscopic and macroscopic levels; generally in the former via its effect on the PCD microstructure and locally varying residual stress state, and in the latter via its effect on layer stresses and strains in the cutting disc and macroscopic residual stress. The layers in the cutting disc have different thicknesses, coefficients of thermal expansion, elastic properties, time-independent and time-dependent (creep) plastic properties, and the PCD materials (carbon, cobalt) have differing physical parameters relevant to the sintering process. Grain size and its distribution, relative grain orientations and the precise role of the catalyst are all expected to play a role.  This project is linked to a previous project undertaken by one of the TSM CDT students in Cohort 4 and a Diamond and Science Technoloy (DST) CDT project started in 2015.

References
[1] Irifune, A., Kurio, T., Sakamoto, S., Inoue, T., Sumiya, H., Materials: Ultrahard polycrystalline diamond from graphite, Nature, 621(6923):599-600, 2003.
[2] Katzman K., Libby, W.F., Sintered diamond compacts using metallic cobalt binders, Science, 172:1132-1133, 1971.
[3] Pagget, J.W., Drake, E.F., Krawitz, A.D., Winholtz, R.A., Griffin, N.D., Residual stress and stress gradients in polycrystalline diamond compacts, International Journal of Refractory Metals and Hard Materials, 20:187–194, 2002.
[4] Kanyanta, V., Ozbayraktar, S., Maweja, K., Effect of manufacturing parameters on polycrystalline diamond compact cutting tool stress-state, International Journal of Refractory Metals and Hard Materials, 45:147–152, 2014.
[5] Kanyanta, V., Dormer, A., Murphy, N., Invankovic, A., Impact fatigue fracture of polycrystalline diamond compact (PDC) cutters and the effect of microstructure, International Journal of Refractory Metals and Hard Materials, 46: 145–151, 2014.

2. Unravelling the role of mechanochemistry in lubrication mechanisms

Supervisors: Dr Dini (Mechanical Engineering, Imperial College London) and Dr. Mostofi (Materials/Physics, Imperial College London)
Collaborator: Joseph Remias (Afton Chemical Corp.) and Hugh Spikes (Mechanical Engineering, Imperial College)

Mechanochemistry is attracting much attention within the chemistry community since it may lead to the synthesis of new compounds as well as shedding light on how chemical reactions progress.  It also impinges on stem cell research and on how the mechanical environment controls cell growth and differentiation.  In studying lubrication and polymer melts, it has been applied to describe irreversible polymer breakdown in solution at high shear rates.

It might be expected that mechanochemistry would have particular relevance to Tribology (the science of friction, lubrication and wear) since lubricants are often subjected to large mechanical forces during their operation.  It is surprising therefore that it has received little attention in research on tribofilm formation. However, the Tribology Group at Imperial College (led by Professors Dini and Spikes) has recently shown conclusively that the applied shear stress and thus mechanochemistry controls the reaction of the most widely applied antiwear additive, zinc dialkylthiphosphate (ZDDP) [1]This research greatly simplifies our understanding of how some additives work, to the extent that we can begin to relate film formation quantitatively to both the applied conditions and the additive structure. In principle, we can design molecules that respond to particular levels of applied stress; and develop quantitative models of film formation a priori.

A general and important issue is the relationship between applied shear stress and the actual mechanical forces experienced by bonds within additive molecules.  This is not at all straightforward since, based on our current understanding, it will depend on the flexibility and shape of both the solvent and the additive molecules [1]. This will be studied in the proposed project using molecular dynamics (MD) simulations, extending the work recently carried out by Professor Dini and his team [2,3]In particular, solutions of ZDDP, MoDTC and other additive molecules of interest to the industrial sponsor and experimentally explored by Professor Spikes, will be modelled with emphasis on the estimation of intramolecular forces during shear.  Classical MD using reactive potentials which allow bond-breaking as well as quantum mechanical MD, will be used to improve the description of additive systems and to determine molecular reaction pathways towards the formation of tribofilms.

References
[1]      Zhang J, Spikes H. On the Mechanism of ZDDP Antiwear Film Formation. Tribol Lett 2016;63:24. doi:10.1007/s11249-016-0706-7.
[2]      Maćkowiak S, Heyes DM, Dini D, Brańka AC. Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study. J Chem Phys 2016;145:164704. doi:10.1063/1.4965829.
[3]      Ewen JP, Gattinoni C, Morgan N, Spikes H, Dini D. Nonequilibrium Molecular Dynamics Simulations of Organic Friction Modifiers Adsorbed on Iron Oxide Surfaces. Langmuir 2016;32:4450–63. doi:10.1021/acs.langmuir.6b00586.

3. Thermodynamic Modelling of Small-Molecule Interactions with Amorphous Solids

Supervisors: Amparo Galindo and Daryl Williams (Chemical Engineering, Imperial College London)
Collaborators: George Jackson (Chemical Engineering, Imperial College London) and Steve Page (P&G Cincinnati)

The process by which small gas phase molecules are taken up by solids is commonly described as gas sorption. Gas sorption, especially of water, flavor and fragrance molecules, into amorphous polymers, foods, pharmaceutical solids and biomaterials is an important problem in many academic and industrial fields. Recent work has highlighted the complexity of these sorption processes in amorphous solids, highlighting the importance of the small molecule solubility into the amorphous phases present.

The project will use molecular based models to predict small molecule solubility in amorphous solids in order to model the sorption isotherms of small molecules in amorphous solids, using of both equilibrium and non-equilibrium approaches. The challenge is to incorporate in the SAFT approach a way to account for crystalline or amorphous polymers (usually, we take them always to be amorphous). We will calculate phase diagrams related to the “VLE” (of fluid-vapour”) or solubility boundaries of the gas and the polymers at different conditions and for different polymers/gases. There will be no simulations at this level of modelling.  We will compare some of these results with more classic polymer equations based on equilibrium lattice fluid models (e.g., Sanchez–Lacombe), non-equilibrium lattice fluid theory (NEFL) and the molecular-based statistical associating fluid theory (SAFT)1.  The NELF model can describe the solubility of gases and liquids in amorphous polymers whilst the SAFT models have been applied the solubility of CO2 and gases in polymers.  The SAFT approach is firmly based on statistical mechanics, starting from the proposition of a molecular-scale model (an intermolecular pair potential), and delivering accurate bulk properties at the macroscale.

A further challenge of bridging across scales will be addressed in the project by the development and use of coarse-grain models, at the nanoscale, which will be developed using the SAFT methodology and which can be implemented at the nanoscale via molecular dynamics simulations to study longer time-scale processes. The united atom SAFT models with be mapped into coarse grain models then linked to finite element modelling of single amorphous particles. It is interesting, in parallel to develop, SAFT-based models at the coarse-grain level, where a bead accounts for a larger number of atoms, usually 3 carbons+corresponding hydrogens. With SAFT we have quick access to characterising the bead-bead intermolecular interactions at this level, and in previous works we have characterised the intramolecular (bending, torsions) aspects of the force field. We will use MD simulations at this level to study time-dependent phenomena, and to look into structural changes of the polymers as the gases are adsorbed.

Macroscopic models based on COMSOL or similar FE environments will be used to model single amorphous solid particle properties on the 1 to 100m dimensional scale using key input data sets from the coarse grain models such as time dependent chemical, transport and mechanical descriptors. Solids of interest include polyethylene, polypropylene, MCC, collagen and hair.

Experimental data to inform and validate this modelling project will come from studentship in the ACM-CDT which will in start October 2017. Both projects are supported by P&G USA, and it is anticipated that both students will regularly visit and collaborate with researchers in the USA.

References
T. Lafitte, A. Apostolakou, C. Avendaño, A. Galindo, C.S. Adjiman, E.A. Müller, and G. Jackson, J. Chem. Phys. 139, 154504 (2013).
Dilatometry of powder compacts: Characterizing amorphous-crystalline transformations,  G.D. Wang et al, Powder Technology 236, 12–16 (2013).

4. Theory and simulation of charge injection at metal-polymer interfaces

Supervisors: Dr Mostofi (Materials and Physics, Imperial College London) and Dr Unge (Principal Scientist, ABB Corporate Research, Sweden)

Wind- or wave- power farms and solar cell parks are attractive sustainable energy sources for reducing CO2 emissions and tackling climate change. Wind- and wave power farms are preferably placed off-shore and solar cell parks in desert regions, i.e., at large distances from where the energy is used. High voltage direct current (HVDC) electrical grids are the preferred technology over HV alternating current to connect with long distance sources due to lower losses. The applied voltage should be as high as possible in order to transmit power with as small a current as possible.

One of the key scientific and technological challenges associated with insulation material for HVDC components, capacitors, bushings, cables, cable accessories, transformers, surge arresters etc, is developing a microscopic understanding of the electrical properties of the insulation material, in particular charge injection at the interface between the insulation material and the HV electrode. The chemistry of the polymer material and additives in the polymer (e.g. antioxidants) influence electrical properties such as conductivity, loss, charge injection and, in the end, electrical breakdown.

The aim of this project is to develop a better understanding of charge injection at the interface between a conductor and a polymer insulator from first‐principles, and how this is influenced by local chemistry in the polymer. Different polymer insulation systems are of interest, from amorphous polymers (eg, epoxy, silicone rubber) to semi-crystalline polymers (eg, polypropylene and polyethylene). We will start with simpler model structures in order to evaluate the level of theory needed, whether density functional theory (DFT) or beyond, e.g., many‐body perturbation theory within the GW approximation. More realistic and structurally complex phases of the insulation and their interfaces with the electrode will be generated using a combination of Monte Carlo and molecular dynamics simulations (which may be performed in collaboration with partners working on similar systems at larger length‐scales), and their electronic structure investigated with state‐of‐the‐art linear‐scaling DFT methods. In practice, both metals and conducting polymer‐composites are used as electrodes and the aim is to consider both types in this project. Effect of grain-boundaries at the metal electrode on the charge injection may also be considered. Comparison with experiments is important and will be used to validate the results of simulations wherever possible. Such experimental input may be both from literature and experiments done by ABB and partners.

The project offers the opportunity for visits to the group of Dr Unge at ABB Corporate Research in Sweden.

5. PhD in Multiscale Modelling of Corrosion Scales

Supervisors: Nicholas M Harrison & Mike Finnis
Collaborators: Ming Wei (Naperville, Illinois), Sheetal Handa (Sunbury Upon Thames), BP International Ltd.

We are all familiar with corrosion and its significant economic and environmental consequences. The current cost to industry is estimated to be over $2 trillion per annum [1]. Current theories of corrosion are in large part based on the phenomenology of average behaviour and predict more of less successfully average corrosion rates for widely used metallurgies [2,3]. This is often insufficient to allow us to generate new strategies for detecting, controlling and ultimately preventing corrosion especially in extreme environments. Recent advances in multi-scale modelling and in the in situ measurement of atomic scale processes in corrosion layers [4] suggests that it may well be possible to generate a new predictive model of corrosion scale formation that addresses behaviour at macroscopic length and time scales but is rigorously based on a new understanding of atomic scale processes.Corrosion scales

Applications are invited for a fully funded 4-year studentship in the combined quantum, atomistic and continuum modelling of the nucleation, growth and degradation of corrosion scales in the Computational Materials Science Group at Imperial College London (http://www.imperial.ac.uk/computational-materials-science).

The project involves the development of a continuum model to describe ionic and charge transport within realistic models of granular films. The model will be used to analyse the growth and degradation of, for example, oxide, sulphide and carbonate scales that form on steel surfaces in various environments. The reaction kinetics, diffusion and charge transport processes underpinning the model will be obtained from large scale quantum mechanical calculations.

This work will be conducted as part of a wider collaboration involving the Universities of Leeds, Edinburgh, Manchester and Cambridge within which state of the art in situ measurements of microscopy and spectroscopy will be used to elucidate the composition and structure of growing scales. The long term aim is to develop strategies for the prevention, mitigation and detection of corrosion.

This PhD studentship will be part of the BP International Centre for Advanced Materials (BP-ICAM: http://www.icam-online.org) community. BP-ICAM was set up by BP in autumn 2012 with a $100 million investment over 10 years.  It brings together the strengths of four world-leading universities and BP’s expertise in oil and gas to create an international centre of excellence in advanced materials research. The academic partnership between The University of Manchester (Dr Robert Lindsay), the University of Cambridge (Dr Stuart Clark), Imperial College London (Prof Mary Ryan) and the University of Illinois at Urbana-Champaign, combines game-changing capabilities in structural materials, corrosion, separations, surfaces, deposits, imaging, modelling and self-healing materials.

Applicants should submit a CV, a brief statement of research interests, and the names of two referees by e-mail to Prof. Nicholas Harrison (nicholas.harrison@imperial.ac.uk).

1. Hays G. F. Now is the Time, World Corrosion Organisation (2012)
2.  Atkinson A, Rev. Mod. Phys., 57, 437 (1985)
3. Cabrera N, Mott N, Theory of the Oxidation of Metals, Rep. Prog. Phys. 12 163 (1949)
4. M Tautschnig, NM Harrison, MW Finnis, Acta Mater., 132, 503 (2017) [https://doi.org/10.1016/j.actamat.2017.04.059]