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Atomistic Simulation @ Cambridge

Department of Materials Science & Metallurgy

Studying at Cambridge

 

Current projects

Ni-based Superalloys

E. Schmidt and P.D. Bristowe

Sponsors: EPSRC and Rolls Royce PLC

  • Atomistic calculations of faulted superlattice structures, precipitates and interfaces.
  • Effect of alloying additions (Ti, Ta, W).
  • Collaboration with the Rolls-Royce UTC

Research Highlight

Thermodynamic stability and electronic structure of η-Ni6Nb(Al,Ti) from first principles

N. Eurich and P. D. Bristowe, Scripta Mater. 77, 37-40 (2014)

First principles calculations in combination with special quasirandom structures are used to investigate the thermodynamic stability and electronic structure of a partially ordered hexagonal phase with chemistry Ni6Nb(Al,Ti) observed in Allvac 718Plus. The results agree with the experimental observations by confirming the structural stability of the alloy over a wide range of compositions. At finite temperature vibrational and configurational contributions to the free energy stabilise a competing orthorhombic phase which for high Ti concentrations is shown to become energetically favourable.

superalloys.jpg
First principles formation enthalpies of Ni6NbTi1-xAlx for various structures as a function of Ti concentration. 

Copper-Carbon Nanotube Composites

P. D. Bristowe, K. Koziol and K. Milowska

Sponsors: European Commission (FP7)

  • First principles modelling of the microscopic properties of copper/carbon nanotube composites
  • Electronic, mechanical and transport properties of the copper/carbon interface
  • Collaboration involving 14 partners from academia, research institutes and industry

Research Highlight:

A computational study of the quantum transport properties of a Cu-CNT composite

Mahdi Ghorbani-Asl, Paul D. Bristowe and Krzysztof Koziol, Phys. Chem. Chem. Phys. 17, 18273-18277  (2015)

The quantum transport properties of a Cu-CNT composite are studied using a non-equilibrium Green’s function approach combined with the self-consistent-charge density-functional tight-binding method. The results show that the electrical conductance of the composite depends strongly on CNT density and alignment but more weakly on chirality. Alignment with the applied bias is preferred and the conductance of the composite increases as its mass density increases. 

 

nanotube_composites.jpg
Electron difference density maps through cross-sections of the scattering region (upper) and the average electrostatic difference potential along the transport direction (lower) for the three (6,6) Cu-CNT composite orientations considered. The peak positions are indicated with red arrows to highlight the good correspondence between the upper and lower panel.

Hybrid Perovskites

A.K. Cheetham, P. D. Bristowe, F. Brivio and Z. Deng

Sponsors: Winton Programme for the Physics of Sustainability

  • DFT calculations of hybrid perovskites for solar cell applications
  • Derivatives of (CH3NH3)PbI3
  • Phase stability, bond structure and dielectric properties

Research Highlight

Role of hydrogen-bonding and its interplay with octahedral tilting in CH3NH3PbI3

J-H Lee, N. C. Bristowe, P. D. Bristowe and A. K. Cheetham, Chem. Commun. 51, 6434 (2015)

First principles calculations on the hybrid perovskite CH3NH3PbI3 predict strong hydrogen-bonding which influences the structure and dynamics of the methylammonium cation and reveal its interaction with the tilting of the PbI6 octahedra. The calculated atomic coordinates are in excellent agreement with neutron diffraction results.

perovskites.jpg
   (a) Schematic view of three methylammonium rotational modes  (b) The computed total energies as a function of the torsion angle θ for the three rotational modes and a staggered conformation (inset) with the torsion angle θ = 0° 

Glass Coatings

P. D. Bristowe, T. Wang and P. Warren

Sponsors: EPSRC in collaboration with NSG (Pilkington)

  • Classical and first principles calculations of atomic diffusion across thin-film optical coatings
  • Na diffusion into the coating and Ag diffusion into the glass
  • Development of thin-film diffusion barriers

Research Highlight

A first principles study of the properties of Al:ZnO and its adhesion to Ag in an optical coating

Z. Lin and P. D. Bristowe, J. Appl. Phys. 106, 013520 (2009)

A first principles density functional study of the atomistic properties of Al:ZnO nd its adhesion to Ag is presented. Optical coatings often contain interfaces between ZnO (0001) and Ag (111) layers whose bonding can be improved by incorporating small amounts of Al into the ZnO but the underlying strengthening mechanism remains unclear. It is assumed that Al relaxes the internal compressive stress in the film but the situation is complicated by the presence of hydrogen and/or water which can adsorb on the ZnO surface during fabrication of the coating. Hydrogen and/or water are known to weaken the Ag/ZnO interface particularly when it is O-terminated. In this paper it is shown that aluminum substitutes on Zn sites in ZnO and this does indeed reduce the internal stress in the layer under compression. However, it is also shown that Al segregates to the ZnO surface when it is O-terminated (but not Zn-terminated) and this reduces the propensity for hydrogen adsorption. Thus by eliminating some of the hydrogen from the ZnO surface which is more likely to be O-terminated than Zn-terminated under ambient conditions, the strength of the Ag/ZnO interface can be increased. The effect of aluminium incorporation into the ZnO layer is therefore two-fold: it relaxes the residual stresses in the coating and also improves the chemical bonding at the metal/oxide interface by removing the weakening effects of gaseous adsorption. The changes in interfacial bonding are explained in terms of an electron redistribution and compensation model. 

glass_coatings.jpg
The calculated work of separation (-Wsep), shown as scatter points, as a function of Ag-ZnO(AZO) inter-slab spacing for (a) the O-terminated and (b) the Zn-terminated interfaces with and without Al incorporation. The curves show the data fitted to a binding-energy equation. The data labeled adj have been adjusted to remove the strain energy associated with the applied compressive stress. The data labeled AZO-1 and AZO-2 refer to the position of the Al substituent adjacent to the Zn-terminated interface.

Carbides for Future Fission Environments (MAX Phases)

P. D. Bristowe, S. H. Shah and S. Azadi

Sponsors: EPSRC

  • First principles modelling of the phase stability of MAX phases, Zrn+1ACn, (A = S, Al, Sn, Pb; n = 1, 2)
  • Irradiation tolerance of MAX phases and their resistance to amorphization
  • Strength of grain boundaries and other interfaces in MAX-phase systems
  • Collaboration between Cambridge, Imperial College and Manchester University involving support from Westinghouse, National Nuclear Laboratory and Rolls-Royce plc.

Research Highlight

Point defect formation in M2AlC (M = Zr, Cr) MAX phases and their tendency to disorder and amorphize

S. H. Shah and P. D. Bristowe, Sci. Rep. 7, 9667 (2017)

First principles calculations are performed on Zr2AlC and Cr2AlC MAX phases to compare their ability to accommodate point defects under irradiation. Interatomic bonding is stronger in Cr2AlC than Zr2AlC but contrary to expectation Zr2AlC exhibits higher vacancy and antisite pair formation energies. However, interstitials and Frenkel defects are generally more difficult to form in Cr2AlC. The results are attributed to the mixed covalent/ionic/metallic nature of the bonding. Detailed comparison of all the energies suggests that the preferred defects in Zr2AlC and Cr2AlC are the VAl+Ali Frenkel and CrAl+AlCr antisite respectively. Thus the potential response of the two phases to irradiation is different and taking account of other competing defects it is suggested that Zr2AlC is less susceptible to amorphization. 

MAX phase image

 

Formation energies (Edefect) of different point defects in Zr2AlC and Cr2AlC. Here M is Zr/Cr. (a) vacancy defects, (b) antisite pair defects, (c) different interstitial configurations (I), (d) interstitial defects. For the vacancies and interstitials the chemical potentials of the pure constituent elements are used.


 

Structure and Dynamics of Small Metal Clusters

T. Furnival, P.D. Bristowe and P.A. Midgley

Sponsors: ERC

  • Atomistic simulation of metal clusters embedded in a crystalline or amorphous matrix
  • Collaboration with the Electron Microscopy Group

Research Highlight

Anomalous diffusion of single metal atoms on a graphene oxide support

Tom FurnivalRowan K. LearyEric C. TyoStefan VajdaQuentin M. Ramasse, John Meurig Thomas, Paul D. Bristowe and Paul A. Midgley, Chemical Physics Letters, 683, 370-374 (2017)

Recent studies of single-atom catalysts open up the prospect of designing exceptionally active and environmentally efficient chemical processes. The stability and durability of such catalysts is governed by the strength with which the atoms are bound to their support and their diffusive behaviour. Here we use aberration-corrected STEM to image the diffusion of single copper adatoms on graphene oxide. We discover that individual atoms exhibit anomalous diffusion as a result of spatial and energetic disorder inherent in the support, and interpret the origins of this behaviour to develop a physical picture for the surface diffusion of single metal atoms.

diffusion image