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Item Open Access Chemical vapour deposition of freestanding sub-60 nm graphene gyroids(AIP Publishing, 2017-12-18) Cebo, Tomasz; Aria, Adrianus Indrat; Dolan, James A.; Weatherup, Robert S.; Nakanishi, Kenichi; Kidambi, Piran R.; Divitini, Giorgio; Ducati, Caterina; Steiner, Ullrich; Hofmann, StephanThe direct chemical vapour deposition of freestanding graphene gyroids with controlled sub-60 nm unit cell sizes is demonstrated. Three-dimensional (3D) nickel templates were fabricated through electrodeposition into a selectively voided triblock terpolymer. The high temperature instability of sub-micron unit cell structures was effectively addressed through the early introduction of the carbon precursor, which stabilizes the metallized gyroidal templates. The as-grown graphene gyroids are self-supporting and can be transferred onto a variety of substrates. Furthermore, they represent the smallest free standing periodic graphene 3D structures yet produced with a pore size of tens of nm, as analysed by electron microscopy and optical spectroscopy. We discuss generality of our methodology for the synthesis of other types of nanoscale, 3D graphene assemblies, and the transferability of this approach to other 2D materials.Item Open Access Complexity of combinatorial ordering genetic algorithms COFFGA and CONFGA(AIP Publishing, 2019-08-23) Hallawi, Huda; He, HongmeiThis paper analyses the complexity of two Algorithms called COFFGA (Combinatorial Ordering First Fit Genetic Algorithm) and CONFGA (Combinatorial Ordering Next Fit Genetic Algorithm). It also identifies the parameters that affect the performance of these algorithms. The complexity of the GA depends on the problem being solved by this GA, as well as the operators of the GA itself. The complexity of COFFGA and CONFGA are analysed individually. Even of these algorithms are slightly different, they may have extremely different complexities depending on the differences in their fitness function or termination condition. To provide a provable bound on a problem, there must be a bound on the evaluation function as well as a manner by which the underlying problem is tied to the representation. Given that there is no standard complexity of the GA, and the complexity of any GA depends on the problem that being solved by this GA and its operators, then CONFGA and COFFGA are analysed with different complexities; although they built upon the same algorithm and they are used to solve the same problem (Cloud resource allocation problem), but they are different in their operators their fitness function and termination condition.Item Open Access Contact cleaning of polymer film solar reflectors(AIP Publishing, 2016-01-31) Sansom, Christopher L.; Fernández-García, Aránzazu; Sutter, Florian; Almond, Heather; King, HelenThis paper describes the accelerated ageing of polymer film reflecting surfaces under the conditions to be found during contact cleaning of Concentrating Solar Power (CSP) collectors in the presence of dust and sand particles. In these situations, contact cleaning using brushes and water is required to clean the reflecting surfaces. Whilst suitable for glass reflectors, this paper discusses the effects of existing cleaning processes on the optical and visual properties of polymer film surfaces, and then describes the development of a more benign but effective contact cleaning process for cleaning polymer reflectors. The effects of a range of cleaning brushes are discussed, with and without the presence of water, in the presence of sand and dust particles from selected representative locations. Reflectance measurements and visual inspection shows that a soft cleaning brush with a small amount of water can clean polymer film reflecting surfaces without inflicting surface damage or reducing specular reflectance.Item Open Access Control of flow separation over an aerofoil by external acoustic excitation at a high Reynolds number(AIP Publishing, 2024-01-05) Coskun, Seyfettin; Rajendran, David John; Pachidis, Vassilios; Bacic, MarkoThe effectiveness of acoustic excitation as a means of flow control at high Reynolds number turbulent flows is investigated numerically by using Improved Delayed Detached Eddy Simulations. Previous studies on low Reynolds number laminar flows have shown that acoustic excitation can substantially suppress flow separation for specific effective frequency and amplitude ranges. However, the effect of acoustic excitation on higher Reynolds number turbulent flow separation has not yet been explored due to limitations on appropriate fidelity computational methods or experimental facility constraints. Therefore, this paper addresses this research gap. A NACA (National Advisory Committee for Aeronautics) 0015 aerofoil profile at 1 million Reynolds number based on the aerofoil chord length is used for the investigations. Acoustic excitation is applied to the baseline flow field in the form of transient boundary conditions at the computational domain inlet. A parametric study revealed that the effective sound frequency range shows a Gaussian distribution around the frequency of the dominant disturbances in the baseline flow. A maximum of ∼ 43% increase in lift-to-drag ratio is observed for the most effective excitation frequency F+ = 1.0 at a constant excitation amplitude of Am = 1.8%. The effect of excitation amplitude follows an asymptotic trend with a maximum effective excitation amplitude above which the gains are not significant. A fully reattached flow is observed for the highest excitation level considered (Am = 10%), that results in ∼ 120% rise in aerofoil lift-to-drag coefficient. Overall, the findings of the current work demonstrate the higher Reynolds number effectiveness of acoustic excitation on separated turbulent flows, thereby paving the way for application in realistic flow scenarios observed in aircraft and gas turbine engine flow fields.Item Open Access Cryogenic scintillation properties of n-type GaAs for the direct detection of MeV/c2 dark matter(AIP Publishing, 2018-03-20) Derenzo, S.; Bourret, Edith D.; Hanrahan, S.; Bizarri, GregoryThis paper is the first report of n-type GaAs as a cryogenic scintillation radiation detector for the detection of electron recoils from interacting dark matter (DM) particles in the poorly explored MeV/c2 mass range. Seven GaAs samples from two commercial suppliers and with different silicon and boron concentrations were studied for their low temperature optical and scintillation properties. All samples are n-type even at low temperatures and exhibit emission between silicon donors and boron acceptors that peaks at 1.33 eV (930 nm). The lowest excitation band peaks at 1.44 eV (860 nm), and the overlap between the emission and excitation bands is small. The X-ray excited luminosities range from 7 to 43 photons/keV. Thermally stimulated luminescence measurements show that n-type GaAs does not accumulate metastable radiative states that could cause afterglow. Further development and use with cryogenic photodetectors promises a remarkable combination of large target size, ultra-low backgrounds, and a sensitivity to electron recoils of a few eV that would be produced by DM particles as light as a few MeV/c2.Item Open Access Design and experiment of a bionic flapping wing mechanism with flapping–twist–swing motion based on a single rotation(AIP Publishing, 2020-06-12) Ji, Bing; Zhu, Qiaolin; Guo, Shijun; Yang, Fan; Li, Yushuai; Zhu, Zenggang; Chen, Si; Song, Rui; Li, YibinIn the present study, a bionic flapping mechanism of a spatial six-bar configuration was designed to transform a single rotation of a motor into a three degrees of freedom “flapping–twist–swing” cooperative motion of a flapping wing. The kinematics model of the flapping mechanism movement was constructed. The flapping trajectory of the wing based on the kinematics model was to mimic the motion of a pigeon wing in landing flight. To reduce the manufacturing complexity, the flapping mechanism was simplified with only two degrees of freedom (flapping and twist) retained. Finally, a prototype model with a 0.9 m wing span was built and tested. A comparison among the experimental data, theoretical calculation results, and ADAMS simulation results revealed that the difference in the flapping and the twist amplitude between experimental observations and theoretical calculation results was 12.5% and 2.3%, respectively. This was owing to the elastic deformation of the bar and the mechanism simplification. The comparison results also indicated that the maximum difference in the inertial force was 5.9% in up-stroke and 6.7% in down-stroke, respectively. The experimental results showed that the inertial force of the model with the wing patagium was approximately 2.2 N, and the maximum positive and negative lift was 2.1 N and −1.5 N, respectively. It is hoped that this study can provide guidance for the design of bionic flapping wing mechanisms of a flapping wing aircraft for short landing flight.Item Open Access Experimental and numerical investigation of the aerodynamic characteristics of high-performance vehicle configurations under yaw conditions(AIP Publishing, 2024-04-05) Rijns, Steven; Teschner, Tom-Robin; Blackburn, Kim; Ramos Proenca, Anderson; Brighton, JamesThis study investigates the impact of yaw conditions on the aerodynamic performance and flow field of three high-performance vehicle model configurations by means of wind tunnel testing and unsteady Reynolds-Averaged Navier–Stokes-based computational fluid dynamics simulations. While yaw effects on automotive vehicles have been explored, the effects on far more complex flow fields of high-performance vehicles remain insufficiently researched. This paper reveals that yaw conditions have a significant negative influence both downforce and drag performance. Spoiler and rear wing devices enhance downforce but increase the vehicle's sensitivity to yaw. Furthermore, yaw conditions significantly alter vortex structures and local flow velocities, affecting downstream flow behavior. Surface pressure measurements on the slant confirm these findings and highlight notable yaw effects and upstream effects from spoiler and rear wing devices. Wake analyses through total pressure measurements show that yaw induces a substantial deviation from straight-line wake characteristics, which become dominated by an inboard rotating vehicle body vortex. Overall, this research enhances the understanding of the effects of yaw conditions on high-performance vehicle aerodynamics and provides valuable data for future vehicle aerodynamics research in real-world operating conditions.Item Open Access Graphene-passivated nickel as an efficient hole-injecting electrode for large area organic semiconductor devices(AIP Publishing, 2020-04-20) Di Nuzzo, Daniele; Mizuta, Ryo; Nakanishi, Kenichi; Martin, Marie-Blandine; Aria, Adrianus Indrat; Weatherup, Robert; Friend, Richard H.; Hofmann, Stephan; Alexander-Webber, JackEfficient injection of charge from metal electrodes into semiconductors is of paramount importance to obtain high performance optoelectronic devices. The quality of the interface between the electrode and the semiconductor must, therefore, be carefully controlled. The case of organic semiconductors presents specific problems: ambient deposition techniques, such as solution processing, restrict the choice of electrodes to those not prone to oxidation, limiting potential applications. Additionally, damage to the semiconductor in sputter coating or high temperature thermal evaporation poses an obstacle to the use of many device-relevant metals as top electrodes in vertical metal–semiconductor–metal structures, making it preferable to use them as bottom electrodes. Here, we propose a possible solution to these problems by implementing graphene-passivated nickel as an air stable bottom electrode in vertical devices comprising organic semiconductors. We use these passivated layers as hole-injecting bottom electrodes, and we show that efficient charge injection can be achieved into standard organic semiconducting polymers, owing to an oxide free nickel/graphene/polymer interface. Crucially, we fabricate our electrodes with low roughness, which, in turn, allows us to produce large area devices (of the order of millimeter squares) without electrical shorts occurring. Our results make these graphene-passivated ferromagnetic electrodes a promising approach for large area organic optoelectronic and spintronic devices. Organic semiconductors serve as a platform for (opto)electronic devices with tunable characteristics by molecular design, enabling versatile device integration, and processing strategies.1 However, ambient processing techniques such as solution processing can facilitate oxidation of metal contacts, resulting in an uncontrolled electronic interface, which is deleterious to performance in semiconductor devices.2,3 New techniques are, therefore, required to control the interface between organic semiconductors and oxidizing metals while maintaining the possibility of solution processing. Graphene has been shown to act as an atomically thin permeation barrier.4–6 Graphene grown via chemical vapor deposition (CVD) directly on the surface of strongly interacting7 catalytic metals, such as Ni, Co, or Fe, acts as a barrier layer to prevent oxidation.8–11 These oxide-free ferromagnetic interfaces have been shown to hold significant benefits within the field of spintronics,12,13 as they enable oxidative fabrication processes, such as solution processing9,10 or atomic layer deposition,14 to be used to fabricate devices with a wider range of relevant materials. One appealing possibility would be to develop graphene-passivated ferromagnets as electrodes9,10 for organic semiconductor spintronics,15–18 where the quality of the electronic interface between the ferromagnetic electrode and the organic semiconductor is of paramount importance.19,20 Another important advantage of an ambient-stable ferromagnetic layer is that it can be used as a bottom electrode in vertical metal-organic semiconductor–metal structures, allowing one to employ techniques such as sputtering to obtain high quality and thickness-controlled metal layers or multi-layers; note that sputtering cannot be used for top-electrodes as it would destroy21 the organic semiconductor. Previous reports using graphene passivated ferromagnets as electrodes for organic semiconductor devices have studied the spin injection properties10 as well as charge injection in lateral organic semiconductor field effect transistors.9 In this work, we investigate few-layer graphene-passivated nickel (Ni/FLG) as a bottom electrode for injection of holes into organic semiconducting polymers in a vertical device structure, demonstrating efficient injection into two standard semiconducting polymers deposited from solution and in air, directly on top of Ni/FLG. Compared to previous reports on graphene-passivated ferromagnetic electrodes, where lithographic techniques had to be used in order to produce micrometer-sized features,8–10,12,14 here, we were able to produce working devices with several orders of magnitude larger active area (4.5 mm2). Our results are, thus, encouraging for the further development of organic optoelectronic and spintronic devices processed from solution under ambient conditions. Nickel was initially sputtered on thermally oxidized silicon wafers, producing films with a thickness of 150 nm. FLG domains were grown on such sputtered Ni films in a custom low-pressure Chemical Vapor Deposition (CVD) reactor (base pressure ∼1 × 10−6 mbar). All substrates were cleaned by sonicating in acetone followed by isopropyl alcohol and blow-dried with a nitrogen gun before loading. Samples were heated to approximately 450 °C using a resistive heater (temperature measurements by a K-type thermocouple) with a rapid ramp rate of 100 °C/min and annealed at ∼1 mbar of H2 for 10 min. This reduces the native oxide prior to graphene growth. After annealing, the H2 flow was stopped and the chamber was evacuated back to approximately base pressure over a period of 5 min. For graphene growth, C2H2 gas was gradually introduced into the reactor via a mass flow controller by incrementally increasing the flow rate over 5 min to achieve a partial pressure of C2H2 of 2.5 × 10−4 mbar. Subsequently, the samples were held at 450 °C in 2.5 × 10−4 mbar of C2H2 for a further 25 min, before rapid cooling (initially ∼300 °C/min) while maintaining the C2H2 flow. All gases were stopped once room temperature had been reached. Upon graphene growth, a roughening of the Ni sputtered on thermally oxidized Si was observed, with an RMS = 67 nm [Fig. 1(a)]. The roughness was found to increase with increasing growth temperature. The roughening of Ni upon graphene growth is explained by grain growth in the sputtered Ni films, occurring at high temperatures during the CVD process: under these conditions, the internal forces in the film are larger than those between the film and the substrate, and diffusion of the film material is appreciable.Item Open Access A multi-criteria decision aid methodology to design electric vehicles public charging networks(AIP Publishing, 2015-05-08) Rasposo, João; Rodrigues, Ana; Silva, Carlos; Dentinho, TomazThis article presents a new multi-criteria decision aid methodology, dynamic-PROMETHEE, here used to design electric vehicle charging networks. In applying this methodology to a Portuguese city, results suggest that it is effective in designing electric vehicle charging networks, generating time and policy based scenarios, considering offer and demand and the city’s urban structure. Dynamic-PROMETHE adds to the already known PROMETHEE’s characteristics other useful features, such as decision memory over time, versatility and adaptability. The case study, used here to present the dynamic-PROMETHEE, served as inspiration and base to create this new methodology. It can be used to model different problems and scenarios that may present similar requirement characteristics.Item Open Access Performance analyses of active aerodynamic load balancing designs on high-performance vehicles in cornering conditions(AIP Publishing, 2024-08-29) Rijns, Steven; Teschner, Tom-Robin; Blackburn, Kim; Brighton, JamesThis study presents a comprehensive investigation into the impact of active aerodynamic load balancing (AALB) on the cornering performance of high-performance vehicles. The research explores the use of active asymmetric aerodynamic devices, specifically split and tilted rear wing concepts, capable of manipulating vertical wheel loads and counteracting effects of lateral load transfer during cornering. The performance potential of AALB is assessed through quasi-steady static coupling of aerodynamic data with a detailed vehicle dynamics model. The findings show that inside bias operating states of the split rear wing and tilted rear wing concepts, which favor loads on the inside tires, can improve cornering velocities up to 0.5% and 2% compared to high symmetric operating states, respectively. Noteworthy, through effective distribution of aerodynamic loads, the inside bias operating states produce less downforce and drag, thereby reducing the propulsion power required to overcome drag by 15%–20%, depending on the cornering condition. The tilted rear wing concept demonstrates the highest AALB capability and most consistent response to its control strategy, accredited to its ability to generate vertical and horizontal aerodynamic force components. It can, therefore, achieve over 1% higher maximum cornering velocities compared to the split rear wing, while also offering efficiency benefits. Overall, the research highlights the effectiveness of AALB in improving cornering performance and efficiency, offering valuable insights for the development of advanced active aerodynamic solutions in automotive design and paving the way for future advancements in the field.Item Open Access Two-dimensional perovskite functionalized fiber-type heterostructured scintillators(AIP Publishing, 2023-02-21) Rogers, Edith; Birowosuto, Muhammad Danang; Maddalena, Francesco; Dujardin, Christophe; Pagano, Fiammetta; Kratochwil, Nicolaus; Auffray, Etiennette; Krause, Philip George; Bizarri, GregoryA fiber-type heterostructured scintillator based on bismuth germanate (Bi4Ge3O12) functionalized with the 2D-perovskite butylammonium lead bromide ((BA)2PbBr4) has been fabricated, and its scintillation performance analyzed toward its use for fast timing applications such as time-of-flight Positron Emission Tomography. The pixel shows energy sharing between the matrix and filler component, confirming that the two components are in synergy.