Item Published version Open AccessMaterials design for artificial pinning centres in Superconductor PLD Coated Conductors(IoP, 2017-11-01) Feighan, JPF; driscoll; kursumovic; Feighan, John [0000-0002-5222-7034]; Driscoll, Judith [0000-0003-4987-6620]To date the strategy for inducing pinning in REBa2Cu3O7-x (REBCO where RE = rare-earth) coated conductors has largely been empirical. Hence, we are not yet at a point where we can dial-in the compositions and process parameters to optimise pinning for particular applications having specific temperature, field, and field angle requirements. In this review, we cover the critical materials science aspects which enable the understanding, design and engineering of desired pinning centre morphologies. Our main emphasis is on in in-situ epitaxial growth of REBCO films by vapour deposition. We review the optimal pinning centre morphologies which have been determined to be effective for different operation regimes. We also highlight how the interplay of thermodynamics (including epitaxial effects), film-particle interfacial strain, and kinetics determine pinning morphologies. Finally, we also briefly cover pinning in rapid ex-situ, liquid assisted growth which is likely to be a necessary universal approach for applications where low cost is critical. Item Published version Open AccessThe effect of crystal structure on the electromechanical properties of piezoelectric Nylon-11 nanowires.(Royal Society of Chemistry (RSC), 2018-06-19) Choi, Yeon Sik; Kim, Sung Kyun; Williams, Findlay; Calahorra, Yonatan; Elliott, James A; Kar-Narayan, Sohini; Choi, Yeonsik [0000-0003-3813-3442]; Kim, Sungkyun [0000-0002-4675-2617]; Calahorra, Yonatan [0000-0001-9530-1006]; Elliott, James [0000-0002-4887-6250]; Kar-Narayan, Sohini [0000-0002-8151-1616]Crystal structure is crucial in determining the properties of piezoelectric polymers, particularly at the nanoscale where precise control of the crystalline phase is possible. Here, we investigate the electromechanical properties of three distinct crystalline phases of Nylon-11 nanowires using advanced scanning probe microscopy techniques. Stiff α-phase nanowires exhibited a low piezoelectric response, while relatively soft δ'-phase nanowires displayed an enhanced piezoelectric response. Item Accepted version Open AccessDesign of a Vertical Composite Thin Film System with Ultralow Leakage To Yield Large Converse Magnetoelectric Effect(American Chemical Society (ACS), 2018-05-30) Wu, Rui; Kursumovic, Ahmed; Gao, Xingyao; Yun, Chao; Vickers, Mary E; Wang, Haiyan; Cho, Seungho; MacManus-Driscoll, Judith L; Wu, Rui [0000-0003-2010-5961]; Yun, Chao [0000-0002-4027-0510]Electric field control of magnetism is a critical future technology for low-power, ultrahigh density memory. However, despite intensive research efforts, no practical material systems have emerged. Interface-coupled, composite systems containing ferroelectric and ferri-/ferromagnetic elements have been widely explored, but they have a range of problems, for example, substrate clamping, large leakage, and inability to miniaturize. In this work, through careful material selection, design, and nanoengineering, a high-performance room-temperature magnetoelectric system is demonstrated. The clamping problem is overcome by using a vertically aligned nanocomposite structure in which the strain coupling is independent of the substrate. To overcome the leakage problem, three key novel advances are introduced: a low leakage ferroelectric, Na0.5Bi0.5TiO3; ferroelectric–ferrimagnetic vertical interfaces which are not conducting; and current blockage via a rectifying interface between the film and the Nb-doped SrTiO3 substrate. The new multiferroic nanocomposite (Na0.5Bi0.5TiO3–CoFe2O4) thin-film system enables, for the first time, large-scale in situ electric field control of magnetic anisotropy at room temperature in a system applicable for magnetoelectric random access memory, with a magnetoelectric coefficient of 1.25 × 10–9 s m–1. Item Accepted version Open AccessPredicting microstructure and strength of maraging steels: Elemental optimisation(Elsevier BV, 2016) Galindo-Nava, EI; Rainforth, WM; Rivera-Díaz-del-Castillo, PEJ; Galindo-Nava, EI [0000-0001-5401-0480]A physics–based modelling framework to describe microstructure and mechanical properties in maraging steels is presented. It is based on prescribing the hierarchical structure of the martensitic matrix, including dislocation density, and lath and high–angle grain boundary spacing. The evolution of lath–shaped reverted austenite is described using grain–boundary diffusion laws within a lath unit. The dislocation density provides the preferential nucleation sites for precipitation, whereas descriptions for particle nucleation, growth and coarsening evolution are identified for Ni 3 Ti, NiAl and its variants, and BCC–Cu clusters. These results are combined to describe the hardness at different ageing temperatures in several [Formula presented], [Formula presented] and [Formula presented] steels. A critical assessment on individual contributions of typical alloying elements is performed. Ni and Mn control the kinetics of austenite formation, where the latter shows stronger influence on the growth kinetics. Ti additions induce higher hardness by precipitating stronger Ni 3 Ti, whereas Cu clusters induce low strength. A relationship between the reverted austenite and the total elongation in overaging conditions is also found. This result allows to identify optimal process and alloy design scenarios to improve the ductility whilst preserving high hardness in commercial maraging steels. Item Accepted version Open AccessUnderstanding martensite and twin formation in austenitic steels: A model describing TRIP and TWIP effects(Elsevier BV, 2017) Galindo-Nava, EI; Rivera-Díaz-del-Castillo, PEJ; Galindo-Nava, EI [0000-0001-5401-0480]A unified description for the evolution of ε– and α ′ – martensite, and twinning in austenitic steels is presented. The formation of micron—scale ε and twin bands is obtained by considering the evolution of hierarchically arranged nano–sized ε and twins (embryos). The critical size and applied stress when these structures form is obtained by minimising their free energy of formation. The difference between forming an ε plate or a twin lies in the number of overlapping stacking faults in their structure. A nucleation rate criterion is proposed in terms of the critical embryo size, resolved shear stress and embryo number density. Based on Olson and Cohen's classical α ′ –martensite transformation model, the nucleation rate of α ′ is considered proportional to that for ε. These results, combined with dislocation–based approximations, are employed to prescribe the microstructure and flow stress response in steels where transformation–induced–plasticity (TRIP) and/or twinning–induced–plasticity (TWIP) effects operate; these include austenitic stainless and high–Mn steels. Maps showing the operation range of ε, α ′ and twinning in terms of the stacking fault energy at different strain levels are defined. Effects of chemical composition in the microstructure and mechanical response in stainless steels are also explored. These results allow identifying potential compositional scenarios when the TRIP and/or TWIP effects are promoted in austenitic steels. Item Published version Open AccessBottom-up Formation of Carbon-Based Structures with Multilevel Hierarchy from MOF-Guest Polyhedra.(American Chemical Society (ACS), 2018-05-16) Wang, Tiesheng; Kim, Hyun-Kyung; Liu, Yingjun; Li, Weiwei; Griffiths, James T; Wu, Yue; Laha, Sourav; Fong, Kara D; Podjaski, Filip; Yun, Chao; Kumar, R Vasant; Lotsch, Bettina V; Cheetham, Anthony K; Smoukov, Stoyan K; Wang, Tiesheng [0000-0001-7587-7681]; Kim, Hyun-Kyung [0000-0002-7897-5065]; Griffiths, James T [0000-0002-1198-1372]; Wu, Yue [0000-0003-2874-8267]; Fong, Kara D [0000-0002-0711-097X]; Lotsch, Bettina V [0000-0002-3094-303X]; Cheetham, Anthony K [0000-0003-1518-4845]; Smoukov, Stoyan K [0000-0003-1738-818X]Three-dimensional carbon-based structures have proven useful for tailoring material properties in structural mechanical and energy storage applications. One approach to obtain them has been by carbonization of selected metal-organic frameworks (MOFs) with catalytic metals, but this is not applicable to most common MOF structures. Here, we present a strategy to transform common MOFs, by guest inclusions and high-temperature MOF-guest interactions, into complex carbon-based, diatom-like, hierarchical structures (named for the morphological similarities with the naturally existing diatomaceous species). As an example, we introduce metal salt guests into HKUST-1-type MOFs to generate a family of carbon-based nano-diatoms with two to four levels of structural hierarchy. We report control of the morphology by simple changes in the chemistry of the MOF and guest, with implications for the formation mechanisms. We demonstrate that one of these structures has unique advantages as a fast-charging lithium-ion battery anode. The tunability of composition should enable further studies of reaction mechanisms and result in the growth of a myriad of unprecedented carbon-based structures from the enormous variety of currently available MOF-guest candidates. Item Accepted version Open AccessPredicting whether a material is ductile or brittle(Elsevier BV, 2018) Thompson, RP; Clegg, WJ; Thompson, Rob [0000-0001-9459-5014]In this paper we discuss the various models that have been used to predict whether a material will tend to be ductile or brittle. The most widely used is the Pugh ratio, G/K, but we also examine the Cauchy pressure as defined by Pettifor, a combined criterion proposed by Niu, the Rice and Thomson model, the Rice model, and the Zhou-Carlsson-Thomson model. We argue that no simple model that works on the basis of simple relations of bulk polycrystalline properties can represent the failure mode of different materials, particularly where geometric effects occur, such as small sample sizes. Instead the processes of flow and fracture must be considered in detail for each material structure, in particular the effects of crystal structure on these processes. Item Published version Open AccessAlloy fluctuations at dislocations in III-Nitrides: identification and impact on optical properties(Society of Photo-optical Instrumentation Engineers (SPIE)) Massabuau, FCP; Chen, P; Rhode, SL; Horton, MK; O'Hanlon, TJ; Kovacs, A; Zielinski, MS; Kappers, M; Dunin-Borkowski, RE; Humphreys, C; Oliver, R; Massabuau, Fabien [0000-0003-1008-1652]; Oliver, Rachel [0000-0003-0029-3993]We investigated alloy fluctuations at dislocations in III-Nitride alloys (InGaN and AlGaN). We found that in both alloys, atom segregation (In segregation in InGaN and Ga segregation in AlGaN) occurs in the tensile part of dislocations with an edge component. In InGaN, In atom segregation leads to an enhanced formation of In-N chains and atomic condensates which act as carrier localization centers. This feature results in a bright spot at the position of the dislocation in the CL images, suggesting that non-radiative recombination at dislocations is impaired. On the other hand, Ga atom segregation at dislocations in AlGaN does not seem to noticeably affect the intensity recorded by CL at the dislocation. This study sheds light on why InGaN-based devices are more resilient to dislocations than AlGaN-based devices. An interesting approach to hinder non-radiative recombination at dislocations may therefore be to dope AlGaN with In. Item Published version Open AccessA review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals(Informa UK Limited, 2019) Clyne, TW; Troughton, SC; Clyne, TW [0000-0003-2163-1840]; Troughton, SC [0000-0003-2413-5455]The review describes recent progress on understanding and quantification of the various phenomena that take place during plasma electrolytic oxidation, which is in increasing industrial use for production of protective coatings and other surface treatment purposes. A general overview of the process, and some information about usage of these coatings, are provided in the first part of the review. The focus is then on the dielectric breakdown that repeatedly occurs over the surface of the work-piece. These discharges are central to the process, since it is largely via the associated plasmas that oxidation of the substrate takes place and the coating is created. The details are complex, since the discharge characteristics are affected by a number of processing variables. The inter-relationships between electrical conditions, electrolyte composition, coating microstructure and rates of growth, which are linked via the characteristics of the discharges, have become clearer over recent years and these improvements in understanding are summarized here. There is considerable scope for more effective process control, with specific objectives in terms of coating performance and energy efficiency, and an attempt is made to identify key points that are likely to assist this. Item Accepted version Open AccessVoltage control of magnetic single domains in Ni discs on ferroelectric BaTiO3(IOP Publishing, 2018) Ghidini, M; Zhu, B; Mansell, R; Pellicelli, R; Lesaine, A; Moya, X; Crossley, S; Nair, B; Maccherozzi, F; Barnes, CHW; Cowburn, RP; Dhesi, SS; Mathur, ND; Ghidini, M [0000-0002-1905-2455]; Zhu, B [0000-0001-5601-6130]; Mansell, R [0000-0002-6026-0731]; Moya, X [0000-0003-0276-1981]; Mathur, ND [0000-0001-9676-6227]For 1 μm diameter Ni discs on a BaTiO3 substrate, the local magnetization direction is determined by ferroelectric domain orientation as a consequence of growth strain, such that single domain discs lie on single ferroelectric domains. On applying a voltage across the substrate, ferroelectric domain switching yields non volatile magnetization rotations of 90°, while piezoelectric effects that are small and continuous yield non volatile magnetization reversals that are non-deterministic. This demonstration of magnetization reversal without ferroelectric domain switching implies reduced fatigue, and therefore represents a step towards applications. Item Accepted version Open AccessRecent progress in the high-cycle fatigue behaviour of γ-TiAl alloys(Informa UK Limited, 2018) Edwards, TEJ; Edwards, TEJ [0000-0002-3089-0062]The high cycle fatigue (HCF) properties of γ-TiAl alloys are reviewed, particularly with regards to the deformation mechanisms active in the near-threshold cyclic loading regime. By examining the influence of lamellar orientation and thickness on the HCF threshold, in addition to more conventional microstructural considerations such as the grain size or the volume fraction of lamellar colonies, factors to improve the γ-TiAl microstructure for HCF are assessed. Finally, experimental methods and loading strategies are surveyed to identify techniques for improving HCF testing of γ-TiAl alloys. In this, we consider both the conservativism of differing approaches, and the possibility to measure with suitable resolution the local mechanical behaviour under HCF of the lamellar γ-TiAl microstructure. Item Accepted version Open AccessResonant photoluminescence studies of carrier localisation in c-plane InGaN/GaN quantum well structures.(IOP Publishing, 2018-05-02) Blenkhorn, WE; Schulz, S; Tanner, DSP; Oliver, RA; Kappers, MJ; Humphreys, CJ; Dawson, P; Oliver, Rachel [0000-0003-0029-3993]; Humphreys, Colin [0000-0001-5053-3380]In this paper we report on changes in the form of the low temperature (12 K) photoluminescence spectra of an InGaN/GaN quantum well structure as a function of excitation photon energy. As the photon energy is progressively reduced we observe at a critical energy a change in the form of the spectra from one which is determined by the occupation of the complete distribution of hole localisation centres to one which is determined by the resonant excitation of specific localisation sites. This change is governed by an effective mobility edge whereby the photo-excited holes remain localised at their initial energy and are prevented from scattering to other localisation sites. This assignment is confirmed by the results of atomistic tight binding calculations which show that the wave function overlap of the lowest lying localised holes with other hole states is low compared with the overlap of higher lying hole states with other higher lying hole states. Item Open AccessMeasurements of crossed-field demagnetisation rate of trapped field magnets at high frequencies and below 77 K(IOP Publishing, 2018) Baskys, A; Patel, A; Glowacki, BA; Baskys, A [0000-0002-1875-8106]; Patel, A [0000-0002-3979-3517]; Glowacki, BA [0000-0003-2165-6378]Design requirements of next generation electric aircraft place stringent requirements on the power density required from electric motors. Future prototype planned in the scope of European project “Advanced Superconducting Motor Experimental Demonstrator” ASuMED considers a permanent magnet synchronous motor, where the conventional ferromagnets are replaced with superconducting trapped field magnets, which promise higher flux densities and thus higher output power without adding weight. Previous work has indicated that stacks of tape show lower cross-field demagnetisation rate to bulk (RE)BCO whilst retaining similar performance for their size, however the crossed-field demagnetisation rate has not been studied in the temperature, magnetic field and frequency range that is relevant for the operational prototype motor. This work investigates crossed-field demagnetisation in 2G HTS stacks at temperatures below 77 K and frequency range above 10 Hz. This information is crucial in developing designs and determining operational time before re-magnetisation could be required. Item Accepted version Open AccessDedoping of Lead Halide Perovskites Incorporating Monovalent Cations.(American Chemical Society (ACS), 2018-07-24) Abdi-Jalebi, Mojtaba; Pazoki, Meysam; Philippe, Bertrand; Dar, M Ibrahim; Alsari, Mejd; Sadhanala, Aditya; Divitini, Giorgio; Imani, Roghayeh; Lilliu, Samuele; Kullgren, Jolla; Rensmo, Håkan; Grätzel, Michael; Friend, Richard H; Abdi-Jalebi, Mojtaba [0000-0002-9430-6371]; Pazoki, Meysam [0000-0001-6776-5460]; Philippe, Bertrand [0000-0003-2412-8503]; Dar, M Ibrahim [0000-0001-9489-8365]; Sadhanala, Aditya [0000-0003-2832-4894]; Lilliu, Samuele [0000-0002-8449-2211]; Kullgren, Jolla [0000-0003-3570-0050]; Grätzel, Michael [0000-0002-0068-0195]; Friend, Richard H [0000-0001-6565-6308]We report significant improvements in the optoelectronic properties of lead halide perovskites with the addition of monovalent ions with ionic radii close to Pb2+. We investigate the chemical distribution and electronic structure of solution processed CH3NH3PbI3 perovskite structures containing Na+, Cu+, and Ag+, which are lower valence metal ions than Pb2+ but have similar ionic radii. Synchrotron X-ray diffraction reveals a pronounced shift in the main perovskite peaks for the monovalent cation-based films, suggesting incorporation of these cations into the perovskite lattice as well as a preferential crystal growth in Ag+ containing perovskite structures. Furthermore, the synchrotron X-ray photoelectron measurements show a significant change in the valence band position for Cu- and Ag-doped films, although the perovskite bandgap remains the same, indicating a shift in the Fermi level position toward the middle of the bandgap. Such a shift infers that incorporation of these monovalent cations dedope the n-type perovskite films when formed without added cations. This dedoping effect leads to cleaner bandgaps as reflected by the lower energetic disorder in the monovalent cation-doped perovskite thin films as compared to pristine films. We also find that in contrast to Ag+ and Cu+, Na+ locates mainly at the grain boundaries and surfaces. Our theoretical calculations confirm the observed shifts in X-ray diffraction peaks and Fermi level as well as absence of intrabandgap states upon energetically favorable doping of perovskite lattice by the monovalent cations. We also model a significant change in the local structure, chemical bonding of metal-halide, and the electronic structure in the doped perovskites. In summary, our work highlights the local chemistry and influence of monovalent cation dopants on crystallization and the electronic structure in the doped perovskite thin films. Item Accepted version Open AccessLighting up two-dimensional lanthanide phosphonates: Tunable structure-property relationships toward visible and near-infrared emitters(American Chemical Society (ACS), 2014) Patterson, AR; Schmitt, W; Evans, RC; Evans, Rachel Claire [0000-0003-2956-4857] Item Accepted version Open AccessCharge-modulated self-assembly and growth of conjugated polyelectrolyte-polyoxometalate hybrid networks.(Royal Society of Chemistry (RSC), 2014-05-25) Houston, Judith E; Patterson, Adam R; Jayasundera, Anil C; Schmitt, Wolfgang; Evans, Rachel C; Evans, Rachel Claire [0000-0003-2956-4857]Self-assembly of an anionic polyoxometalate with cationic conjugated polyelectrolytes leads to hybrid supramolecular networks whose dimensionality is controlled by the chain length and steric charge distribution. Item Accepted version Open AccessSelf-assembled conjugated polyelectrolyte-surfactant complexes as efficient cathode interlayer materials for bulk heterojunction organic solar cells(Royal Society of Chemistry (RSC), 2015) Chevrier, M; Houston, JE; Kesters, J; Van Den Brande, N; Terry, AE; Richeter, S; Mehdi, A; Coulembier, O; Dubois, P; Lazzaroni, R; Van Mele, B; Maes, W; Evans, RC; Clément, S; Evans, Rachel Claire [0000-0003-2956-4857]
Conjugated polyelectrolyte–surfactant cathodic interface layers lead to improved power conversion efficiencies in organic solar cells.
Item Accepted version Open AccessAll-conjugated cationic copolythiophene "rod-rod" block copolyelectrolytes: Synthesis, optical properties and solvent-dependent assembly(Royal Society of Chemistry (RSC), 2014) Thomas, A; Houston, JE; Van Den Brande, N; De Winter, J; Chevrier, M; Heenan, RK; Terry, AE; Richeter, S; Mehdi, A; Van Mele, B; Dubois, P; Lazzaroni, R; Gerbaux, P; Evans, RC; Clément, S; Evans, Rachel Claire [0000-0003-2956-4857]
The optical and thermal properties and solvent-dependent assembly of all-conjugated cationic copolythiophene block copolyelectrolytes are investigated.
Item Accepted version Open AccessExpanding the light absorption of poly(3-hexylthiophene) by end-functionalization with π-extended porphyrins.(Royal Society of Chemistry (RSC), 2016-01-04) Chevrier, Michèle; Richeter, Sébastien; Coulembier, Olivier; Surin, Mathieu; Mehdi, Ahmad; Lazzaroni, Roberto; Evans, Rachel C; Dubois, Philippe; Clément, Sébastien; Evans, Rachel Claire [0000-0003-2956-4857]Poly(3-hexylthiophene)s end-functionalized with π-extended porphyrins have been synthesized in a one-pot procedure. The polymers show a broad absorption profile extending to 700 nm and a fibrillar microstructure, which can be tuned through judicious selection of the porphyrin molar ratio. Item Published version Open AccessSynergistic photoluminescence enhancement in conjugated polymer-di-ureasil organic-inorganic composites.(Royal Society of Chemistry (RSC), 2015-12-01) Willis-Fox, Niamh; Marques, Ana-Teresa; Arlt, Jochen; Scherf, Ullrich; Carlos, Luís D; Burrows, Hugh D; Evans, Rachel C; Arlt, Jochen [0000-0002-4152-5896]; Carlos, Luís D [0000-0003-4747-6535]; Burrows, Hugh D [0000-0003-3127-2298]Poly(fluorene) conjugated polyelectrolyte (CPE)-di-ureasil organic-inorganic composites have been prepared using a versatile sol-gel processing method, which enables selective localisation of the CPE within the di-ureasil matrix. Introduction of the CPE during the sol-gel reaction leads to a homogeneous distribution of the CPE throughout the di-ureasil, whereas a post-synthesis solvent permeation route leads to the formation of a confined layer of the CPE at the di-ureasil surface. The CPE and the di-ureasil both function as photoactive components, contributing directly to, and enhancing the optical properties of their composite material. The bright blue photoluminescence exhibited by CPE-di-ureasils is reminiscent of the parent CPE; however the distinct contribution of the di-ureasil to the steady-state emission profile is also apparent. This is accompanied by a dramatic increase in the photoluminescence quantum yield to >50%, which is a direct consequence of the synergy between the two components. Picosecond time-correlated single photon counting measurements reveal that the di-ureasil effectively isolates the CPE chains, leading to emissive trap sites which have a high radiative probability. Moreover, intimate mixing of the CPE and the di-ureasil, coupled with their strong spectral overlap, results in efficient excitation energy transfer from the di-ureasil to these emissive traps. Given the simple, solution-based fabrication method and the structural tunability of the two components, this approach presents an efficient route to highly desirable CPE-hybrid materials whose optoelectronic properties may be enhanced and tailored for a targeted application.