957 research outputs found

    ‘When is a hotspot a good nanospot’:review of analytical and hotspot-dominated surface enhanced Raman spectroscopy nanoplatforms

    Get PDF
    Substrate development in surface-enhanced Raman spectroscopy (SERS) continues to attract research interest. In order to determine performance metrics, researchers in foundational SERS studies use a variety of experimental means to characterize the nature of substrates. However, often this process would appear to be performed indiscriminately without consideration for the physical scale of the enhancement phenomena. Herein, we differentiate between SERS substrates whose primary enhancing structures are on the hundreds of nanometer scale (analytical SERS nanosubstrates) and those whose main mechanism derives from nanometric-sized gaps (hot-spot dominated SERS substrates), assessing the utility of various characterization methods for each substrate class. In this context, characterization approaches in white-light spectroscopy, electron beam methods, and scanning probe spectroscopies are reviewed. Tip-enhanced Raman spectroscopy, wavelength-scanned SERS studies, and the impact of surface hydrophobicity are also discussed. Conclusions are thus drawn on the applicability of each characterization technique regarding amenability for SERS experiments that have features at different length scales. For instance, while white light spectroscopy can provide an indication of the plasmon resonances associated with 10 s–100 s nm-scale structures, it may not reveal information about finer surface texturing on the true nm-scale, critical for SERS’ sensitivity, and in need of investigation via scanning probe techniques

    Accurate quantum transport modelling and epitaxial structure design of high-speed and high-power In0.53Ga0.47As/AlAs double-barrier resonant tunnelling diodes for 300-GHz oscillator sources

    Get PDF
    Terahertz (THz) wave technology is envisioned as an appealing and conceivable solution in the context of several potential high-impact applications, including sixth generation (6G) and beyond consumer-oriented ultra-broadband multi-gigabit wireless data-links, as well as highresolution imaging, radar, and spectroscopy apparatuses employable in biomedicine, industrial processes, security/defence, and material science. Despite the technological challenges posed by the THz gap, recent scientific advancements suggest the practical viability of THz systems. However, the development of transmitters (Tx) and receivers (Rx) based on compact semiconductor devices operating at THz frequencies is urgently demanded to meet the performance requirements calling from emerging THz applications. Although several are the promising candidates, including high-speed III-V transistors and photo-diodes, resonant tunnelling diode (RTD) technology offers a compact and high performance option in many practical scenarios. However, the main weakness of the technology is currently represented by the low output power capability of RTD THz Tx, which is mainly caused by the underdeveloped and non-optimal device, as well as circuit, design implementation approaches. Indeed, indium phosphide (InP) RTD devices can nowadays deliver only up to around 1 mW of radio-frequency (RF) power at around 300 GHz. In the context of THz wireless data-links, this severely impacts the Tx performance, limiting communication distance and data transfer capabilities which, at the current time, are of the order of few tens of gigabit per second below around 1 m. However, recent research studies suggest that several milliwatt of output power are required to achieve bit-rate capabilities of several tens of gigabits per second and beyond, and to reach several metres of communication distance in common operating conditions. Currently, the shortterm target is set to 5−10 mW of output power at around 300 GHz carrier waves, which would allow bit-rates in excess of 100 Gb/s, as well as wireless communications well above 5 m distance, in first-stage short-range scenarios. In order to reach it, maximisation of the RTD highfrequency RF power capability is of utmost importance. Despite that, reliable epitaxial structure design approaches, as well as accurate physical-based numerical simulation tools, aimed at RF power maximisation in the 300 GHz-band are lacking at the current time. This work aims at proposing practical solutions to address the aforementioned issues. First, a physical-based simulation methodology was developed to accurately and reliably simulate the static current-voltage (IV ) characteristic of indium gallium arsenide/aluminium arsenide (In-GaAs/AlAs) double-barrier RTD devices. The approach relies on the non-equilibrium Green’s function (NEGF) formalism implemented in Silvaco Atlas technology computer-aided design (TCAD) simulation package, requires low computational budget, and allows to correctly model In0.53Ga0.47As/AlAs RTD devices, which are pseudomorphically-grown on lattice-matched to InP substrates, and are commonly employed in oscillators working at around 300 GHz. By selecting the appropriate physical models, and by retrieving the correct materials parameters, together with a suitable discretisation of the associated heterostructure spatial domain through finite-elements, it is shown, by comparing simulation data with experimental results, that the developed numerical approach can reliably compute several quantities of interest that characterise the DC IV curve negative differential resistance (NDR) region, including peak current, peak voltage, and voltage swing, all of which are key parameters in RTD oscillator design. The demonstrated simulation approach was then used to study the impact of epitaxial structure design parameters, including those characterising the double-barrier quantum well, as well as emitter and collector regions, on the electrical properties of the RTD device. In particular, a comprehensive simulation analysis was conducted, and the retrieved output trends discussed based on the heterostructure band diagram, transmission coefficient energy spectrum, charge distribution, and DC current-density voltage (JV) curve. General design guidelines aimed at enhancing the RTD device maximum RF power gain capability are then deduced and discussed. To validate the proposed epitaxial design approach, an In0.53Ga0.47As/AlAs double-barrier RTD epitaxial structure providing several milliwatt of RF power was designed by employing the developed simulation methodology, and experimentally-investigated through the microfabrication of RTD devices and subsequent high-frequency characterisation up to 110 GHz. The analysis, which included fabrication optimisation, reveals an expected RF power performance of up to around 5 mW and 10 mW at 300 GHz for 25 μm2 and 49 μm2-large RTD devices, respectively, which is up to five times higher compared to the current state-of-the-art. Finally, in order to prove the practical employability of the proposed RTDs in oscillator circuits realised employing low-cost photo-lithography, both coplanar waveguide and microstrip inductive stubs are designed through a full three-dimensional electromagnetic simulation analysis. In summary, this work makes and important contribution to the rapidly evolving field of THz RTD technology, and demonstrates the practical feasibility of 300-GHz high-power RTD devices realisation, which will underpin the future development of Tx systems capable of the power levels required in the forthcoming THz applications

    Beam scanning by liquid-crystal biasing in a modified SIW structure

    Get PDF
    A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

    Optimización de una metodología para el uso de herramientas de diseño estructural paramétrico

    Get PDF
    La metodología propuesta busca establecer una forma de diseño de estructuras paramétricas mediante la interacción de la geometría del modelo de diseño. Se llevará a cabo una investigación utilizando casos de estudio en el software Rhinoceros 3D con el complemento de Grasshopper/Karamba 3D para el análisis estructural. Los objetivos son analizar el uso y proceso de modelado en Rhinoceros y Grasshopper, aplicar el diseño paramétrico en los casos de estudio, optimizar una metodología basada en el análisis de casos, y promover el uso de herramientas paramétricas en el análisis y diseño estructural desde las etapas iniciales de un proyecto arquitectónico. Las estructuras paramétricas en Karamba 3D tienen aplicaciones prácticas en la arquitectura al permitir controlar el diseño de manera eficiente y responder al entorno. La integración con Grasshopper mejora la exploración del espacio de diseño y agiliza el procesamiento de datos complejos. El diseño paramétrico utiliza algoritmos y ecuaciones matemáticas para generar formas basadas en proporciones matemáticas y armoniosas. El uso de herramientas como Karamba 3D y Galápagos permite realizar análisis estructurales paramétricos detallados, considerando cargas, materiales y normas de diseño. Estas herramientas ofrecen ventajas al permitir explorar diversas configuraciones estructurales y optimizar el diseño según criterios de desempeño específicos. Se aplicaron análisis estructurales paramétricos en casos específicos, como el Museo Twist, el Restaurante de los Manantiales y la Casa Mirador, para optimizar momentos, desplazamientos, esfuerzos, forma y peso. Estos análisis mejoraron la eficiencia y seguridad de las estructuras mediante ajustes paramétricos y optimizaciones.The proposed methodology seeks to establish a design form of parametric structures through the interaction of the geometry of the design model. The research will be conducted using case studies in the Rhinoceros 3D software with the Grasshopper/Karamba 3D complement for structural analysis. The objectives are to analyze the use and process of modeling in Rhinoceros and Grasshopper, apply the parametric design in the case studies, optimize a methodology based on case analysis, and promote the use of parametric tools in the analysis and structural design from the initial stages of an architectural project. The parametric structures in Karamba 3D have practical applications in architecture by allowing them to control the design efficiently and respond to the environment. Integration with Grasshopper improves design space exploration and streamlines complex data processing. The parametric design uses mathematical algorithms and equations to generate shapes based on mathematical and harmonious proportions. The use of tools such as Karamba 3D and Galapagos allows detailed parametric structural analysis, considering loads, materials, and design standards. These tools offer advantages by allowing you to explore various structural configurations and optimize the design according to specific performance criteria. Parametric structural analyses were applied in specific cases, such as the Twist Museum, the Restaurant of the Springs, and the Casa Mirador, to optimize moments, displacements, efforts, shape, and weight. These analyses improved the efficiency and safety of the structures through parametric adjustments and optimizations.0000-0003-1841-412

    Models of polymer solutions in electrified jets and solution blowing

    Full text link
    Fluid flows hosting electrical phenomena make the subject of a fascinating and highly interdisciplinary scientific field. In recent years, the extraordinary success of electrospinning and solution blowing technologies for the generation of polymer nanofibers has motivated vibrant research aiming at rationalizing the behavior of viscoelastic jets under applied electric fields or other stretching fields including gas streams. Theoretical models unveiled many original aspects in the underpinning physics of polymer solutions in jets, and provided useful information to improve experimental platforms. This article reviews advances in the theoretical description and numerical simulation of polymer solution jets in electrospinning and solution blowing. Instability phenomena of electrical and hydrodynamic origin are highlighted, which play a crucial role in the relevant flow physics. Specifications leading to accurate and computationally viable models are formulated. Electrohydrodynamic modeling, theories for the jet bending instability, recent advances in Lagrangian approaches to describe the jet flow, including strategies for dynamic refinement of simulations, and effects of strong elongational flow on polymer networks are reviewed. Finally, the current challenges and future perspectives of the field are outlined and discussed, including the task of correlating the physics of the jet flows with the properties of realized materials, as well as the development of multiscale techniques for modelling viscoelastic jets.Comment: 135 pages, 42 figure

    Multi-scale Modelling for Materials Design in Additive Manufacturing

    Get PDF
    Additively manufactured (AMed) austenitic stainless steels (SSs) possess exceptional properties like high strength and toughness. However, it is unclear how they perform under long-term exposure to high-temperature conditions, such as those found in nuclear reactors. These properties arise due to complex microstructures that develop during additive manufacturing (AM), including nanoscale dislocation cellular structures, microscale sub-grains with a high density of low-angle grain boundaries (LAGBs), and high dislocation density. Although the quasi-static mechanical properties of AM austenitic SSs, such as 316L SS, have been systematically investigated, the creep behaviour of such alloys is still a new area of research, with some experimental studies conducted in recent years. Additionally, the mechanical properties of most AMed alloys are anisotropic due to texture formation, and creep behaviour can be significantly influenced by microstructural differences in various building directions. Furthermore, the presence of AM-characterised microstructure is a notable feature of AM materials, and the size and shape of the pores can greatly influence stress concentration during loading. Thus, it is critical to quantify the effects of AM-characterised microstructure on the mechanical properties of materials. As experimental methods have limitations for studying material properties, it is necessary to use computational modelling to extrapolate existing experimental data, especially for highly time-consuming experiments such as creep testing. This study aims to provide a modelling framework for characterizing the evolution of microstructure and high-temperature creep behaviour in AM and wrought austenitic SSs, considering the impact of the initial microstructure. For AM materials, there are two types of samples: horizontally-built samples (loading direction parallel to the building direction) and vertically-built samples (loading direction vertical to the building direction). The choice of AM materials with different built directions is for studying the effect of the relative loading direction to the building direction on material creep behaviour. The materials strengthening mechanisms, including lattice friction, solid solution strengthening, dislocation hardening, and precipitation hardening, are quantified in detail. In addition to data from literature and experiments used to evaluate each strengthening mechanism, the precipitation evolution during the creep process is simulated through the thermokinetics calculation using Thermo-Calc software. Differently fabricated materials are originally simulated under the visco-plasticity self-consistent (VPSC) framework, using the materials' own characteristics as input. The creep mechanical responses of AM and wrought materials are compared, and the dominant deformation mechanisms are revealed and quantitatively compared. Due to the limitations of the VPSC, only the primary stage and secondary stage of creep behaviour are captured. Based on this, the same physics-based model is employed under the crystal plasticity finite element method (CP-FEM) framework, which is full-field, and combined with the Gurson-Tvergaard-Needleman (GTN) damage model to capture the tertiary stage creep deformation. The original crystal plasticity model is highly microstructure-sensitive, and the detailed local structure can be analyzed through the finite element method. Therefore, the original electron backscatter diffraction (EBSD) information is pictured by MATLAB and used for materials input under the CP-FEM framework. In addition, DREAM3D software is used to extract microstructure information from raw EBSD data. The tertiary creep stages of horizontally-built and vertically-built AMed samples are simulated and compared, revealing that damage tends to accumulate on grain boundaries that are perpendicular to the loading direction. Additionally, the effects of AM-induced pores on creep deformation are evaluated by introducing them into the CP-FEM input. As selecting a specific region on the original EBSD data cannot summarize the overall AM materials characteristics, an artificial input is randomly generated through a Voronoi diagram by MATLAB with assigned grain orientation. The artificial input is characterized by AM-induced elongated grain structure to study the effects of high-angle grain boundaries (HAGBs) on materials creep behaviour, especially the damage evolution

    SuperCDMS HVeV Run 2 Low-Mass Dark Matter Search, Highly Multiplexed Phonon-Mediated Particle Detector with Kinetic Inductance Detector, and the Blackbody Radiation in Cryogenic Experiments

    Get PDF
    There is ample evidence of dark matter (DM), a phenomenon responsible for ≈ 85% of the matter content of the Universe that cannot be explained by the Standard Model (SM). One of the most compelling hypotheses is that DM consists of beyond-SM particle(s) that are nonluminous and nonbaryonic. So far, numerous efforts have been made to search for particle DM, and yet none has yielded an unambiguous observation of DM particles. We present in Chapter 2 the SuperCDMS HVeV Run 2 experiment, where we search for DM in the mass ranges of 0.5--10⁴ MeV/c² for the electron-recoil DM and 1.2--50 eV/c² for the dark photon and the Axion-like particle (ALP). SuperCDMS utilizes cryogenic crystals as detectors to search for DM interaction with the crystal atoms. The interaction is detected in the form of recoil energy mediated by phonons. In the HVeV project, we look for electron recoil, where we enhance the signal by the Neganov-Trofimov-Luke effect under high-voltage biases. The technique enabled us to detect quantized e⁻h⁺ creation at a 3% ionization energy resolution. Our work is the first DM search analysis considering charge trapping and impact ionization effects for solid-state detectors. We report our results as upper limits for the assumed particle models as functions of DM mass. Our results exclude the DM-electron scattering cross section, the dark photon kinetic mixing parameter, and the ALP axioelectric coupling above 8.4 x 10⁻³⁴ cm², 3.3 x 10⁻¹⁴, and 1.0 x 10⁻⁹, respectively. Currently every SuperCDMS detector is equipped with a few phonon sensors based on the transition-edge sensor (TES) technology. In order to improve phonon-mediated particle detectors' background rejection performance, we are developing highly multiplexed detectors utilizing kinetic inductance detectors (KIDs) as phonon sensors. This work is detailed in chapter 3 and chapter 4. We have improved our previous KID and readout line designs, which enabled us to produce our first ø3" detector with 80 phonon sensors. The detector yielded a frequency placement accuracy of 0.07%, indicating our capability of implementing hundreds of phonon sensors in a typical SuperCDMS-style detector. We detail our fabrication technique for simultaneously employing Al and Nb for the KID circuit. We explain our signal model that includes extracting the RF signal, calibrating the RF signal into pair-breaking energy, and then the pulse detection. We summarize our noise condition and develop models for different noise sources. We combine the signal and the noise models to be an energy resolution model for KID-based phonon-mediated detectors. From this model, we propose strategies to further improve future detectors' energy resolution and introduce our ongoing implementations. Blackbody (BB) radiation is one of the plausible background sources responsible for the low-energy background currently preventing low-threshold DM experiments to search for lower DM mass ranges. In Chapter 5, we present our study for such background for cryogenic experiments. We have developed physical models and, based on the models, simulation tools for BB radiation propagation as photons or waves. We have also developed a theoretical model for BB photons' interaction with semiconductor impurities, which is one of the possible channels for generating the leakage current background in SuperCDMS-style detectors. We have planned for an experiment to calibrate our simulation and leakage current generation model. For the experiment, we have developed a specialized ``mesh TES'' photon detector inspired by cosmic microwave background experiments. We present its sensitivity model, the radiation source developed for the calibration, and the general plan of the experiment.</p

    Experimentally supported computational method for the optimal design selection of 3D printed fracture healing implant geometries

    Get PDF
    The development of AM technologies has brought about very promising opportunities in the field of tissue regeneration, especially due to the design freedom they enable. However, the tools and procedures needed to enable medical designers to make use of these revolutionary technologies still need to be developed. In particular, design tools to make implants with optimal geometries for tissue regeneration and procedures to manufacture and test such implants need to be developed to enable the adoption of these technologies by medical designers and biologists designing implants. This thesis aims to address this need. In order to best use the design freedom that AM brings; it is necessary to define the optimal geometries for specific applications. A novel tool that enables the design of optimal scaffold geometries and could be easily adopted by medical designers was developed here by proposing an intuitive design selection framework that graphically allows the user to gain an understanding of how design variables affect the chosen response variables. The novel framework is flexible, enabling the incorporation of any number of necessary computational models. Triply periodic minimal surface (TPMS) equations were used to simplify the design variables needed to generate an optimal porous scaffold geometry. The potential of this framework was demonstrated by using it to find the optimal TPMS type and volume fraction for a fracture fixation scaffold. Experiments were carried out to demonstrate that TCDMDA biocompatible scaffolds of appropriate pore size could be manufactured via projection micro stereolithography. The experiments successfully demonstrate for the first time that TCDMDA scaffolds can be manufactured via PµSLA by using a suitable combination of UV intensity and layer time. It was also demonstrated for the first time that hMSCs adhere to the surface of TCDMDA samples manufactured via PµSLA. To further enhance the cell adhesion, an oxygen plasma treatment was carried out. For the second part of this study it was found that the media could not penetrate the scaffold pores sufficiently, invalidating the results. The presented results highlighting a permeability challenge with TCDMDA scaffolds manufactured via PµSLA are nevertheless expected to contribute to future studies in this area. Experiments were also carried out to demonstrate the biocompatibility of scaffolds manufactured via stereolithography using Dental LT resin (Formlabs, UK). Successful adhesion of hMSCs to the surface of these scaffolds was shown in Chapter 4. Another novel finding of this thesis was that the Dental LT scaffolds manufactured via SLA were able to successfully enable cell growth, cell differentiation and mineralization in the presence of osteogenic media and BMP-2. The final part of the thesis focused on expanding the developed design selection framework to include not only a scaffold for fracture healing, but also a matching fracture fixation plate. Fracture fixation plates have been studied for centuries, but there is little research investigating the combination of a fracture fixation plate and a scaffold. The rise of AM has inspired the development of auxetic geometries, which have been applied to fracture fixation plates before and shown to reduce stress shielding. Moreover, stiffness grading has also proved very promising in improving fracture healing. In this thesis these two promising concepts are combined for the first time demonstrating reduced stress shielding compared to a conventional fixation plate geometry. Moreover, the thesis presents a novel computational design selection framework to find optimal scaffold and fracture plate geometries which lead to an improved healing outcome. The framework may be easily adopted by medical designers

    N-type Organic Materials for Thermoelectric Applications

    Get PDF
    Harvesting waste heat as a renewable energy source could allow us to power small devices in everyday life, from medical devices to wireless networks from heat sources all around us. In particular, the use of organic materials as the active thermoelectric component opens up the possibility of flexible, printed electronics and ease of cheaper mass reproducibility. In this work, 3 topics are explored: (i) the use of graphene-based materials for thermoelectric applications, (ii) understanding how heat can move through polymer thin films with topographical features, in particular P3HT, and (iii) the effect ladderisation has on the polymer BBB and the resulting thermal and structural properties of the laddered structure, BBL. Graphene is a versatile material with intrinsically high carrier mobility. However, having impressive electronic properties is not wholly advantageous for thermoelectric energy harvesting as it usually leads to a high electronic thermal conductivity which reduces a material’s zT. In an attempt to remedy this, many researchers have successfully merged graphene with polymer. One approach is to covalently bond functional groups on to the graphene surface, then polymerise a layer of monomers on top. The functional groups interrupt the repetitive structure of graphene, inducing more phonon scattering events which reduces the lattice thermal conductivity. This process also reduces carrier mobility, hence reducing electronic thermal conductivity and carrier conductivity which is an unwanted but inevitable effect. A layer of polymer is hence grown on top to restore some electronic pathways. This was the approach deployed in this work, the polymer of choice was PEDOT, thus a sulfonate group was chosen to be the functional group. While the thermoelectric properties of pure graphene in this work yield values agreeing with literature, it was clear that the substrate choice played a noticeable part. It was found that graphene on a silicon nitride (Si3N4) terminated substrate, a higher Seebeck coefficient of ~25 μV/K was measured in comparison to ~17 μV/K for graphene on aluminium oxide (Al2O3) terminated substrates. This lead to zT reaching a maximum value of ~3x10-3. This may be explained by a possible band gap opening of 0.22 eV, observed with UPS, which was not observed for graphene on Al2O3. Raman spectroscopy showed that the D-band associated with disorder and defects within the graphene lattice was present for graphene on Al2O3, and not for graphene on Si3N4 which could also explain the lower Seebeck coefficient as this parameter is also dependent on carrier mobility. For functionalised graphene/PEDOT films, again, samples on Si3N4 performed better than films on Al2O3. Using XPS, it was found that a larger concentration of functional groups were bonded to the graphene surface for films on Al2O3 which could be due to the different fermi levels of the graphene on their respective substrate materials, and also due to the presence of more graphene edges on graphene/ Al2O3 films (as shown by the D-band in the Raman spectrum). The surface functionalisation was successful in reducing the thermal conductivity, however, the electrical conductivity was heavily damaged, in particular for films on Al2O3 where the electrical conductivity is almost an order of magnitude less and thermal conductivity was approximately half that values seen with films on Si3N4. The lower concentration of functional groups seen in the films on Si3N4 were hence beneficial to the system. Raman spectroscopy also revealed a different morphology between the two sample types where a higher degree of crystallinity due to shorter chains is seen in the films on Si3N4 and can also contribute to the higher electronic properties. Overall, it is shown that graphene works as a good base material for thermoelectric materials, and it is possible to exploit parameters such as substrate choice and functionalisation efficiency to tune the thermoelectric parameters. All in-plane thermal conductivity measurements throughout my work rely on the simple assumption that heat flux is homogeneous and one dimensional. However, for thin films, where topographical roughness is inevitable, heat flux will begin to deviate from the ideal scenario and the measured values will begin to deviate from true intrinsic values of the material. My second project focuses on understanding how measured values are affected by a simple rectangular dip/trough on the surface of a thin film and whether modelled scenarios can be used to represent realistic scenarios. Finite element modelling was used to represent a segment of a doped P3HT thin film, of thermal conductivity 0.4 W/mK, and a thickness and width of 300 nm and 1500 nm respectively, and a single surface feature. The film is modelled with a membrane layer underneath, of thickness 144 nm, representing a substrate with a thermal conductivity of 2.6 W/mK. Fourier’s law was then used to extract a thermal conductivity value that represents a real measured value. Fourier’s law states that the local heat flux is proportional to the area in which it travels through – therefore we’d only expect a measured thermal conductivity value to deviate from the material’s intrinsic value if the area of the film changes perpendicular to the heat flux. By comparing the extracted thermal conductivity to the intrinsic thermal conductivity of the material, defined in the model, it is shown that as the feature for deeper (at constant width), the extracted value deviated super linearly from the intrinsic value. However, with a full 500 nm wide crack in the film, the extracted value is only 40% lower than the intrinsic thermal conductivity which is only possible due to the presence of a more thermally conductivity membrane (membrane is 10 times more conductive than film). Colour scaled images show that heat is redirected into the membrane to allow continuous heat flow to the other side of the crack. In the case the membrane is less thermally conductive than the film, the membrane is not able to redirect the heat effectively enough and the extracted thermal conductivity drops by almost 100% (film is 20 times more conductive than the membrane). A much less significant effect is seen when keeping the depth of the feature constant whilst varying the width. This is because the change is now parallel to the heat flow. For this section, the feature depth was kept constant at 210 nm with varying widths. The peak deviation from intrinsic thermal conductivity is seen when the width of the feature is ~66% of the full simulated length, beyond this point the extracted thermal conductivity begins to converge back to the material’s intrinsic value. At this point, the maximum deviation was ~25% lower than the intrinsic thermal conductivity. When the width of the surface feature is 50% of the segment width, the original film height gets treated as peaks. Thus, the asymmetric nature of this curve tells us that heat is redirect much more efficiently into the constricted areas, as opposed to the peaks. This is, again, due to the aid of a more thermally conductive membrane. When the film is more thermally conductive than the membrane, no aid is given and the maximum deviation of the extracted thermal conductivity peaks symmetrically when the feature is at 50% of the simulated segment. In this case, the maximum deviation is much higher, at ~35%. The same is therefore seen for a suspended film. My final project explores the affect that ladderising a polymer from a single strand to a double strand has on thermal conductivity. The two polymers studied were the single stranded polymer, BBB and the double stranded ladder polymer, BBL, both of which are very similar in structure. The thermal conductivity of BBB was 0.28 W/mK which is typical of an amorphous polymer. The thermal conductivity of BBL was significantly higher at ~1 W/mK. The high thermal conductivity can be attributed to the fact that ladder polymers are much stiffer due to the double bonds between units and may also exhibit a higher order of crystallinity in comparison to an amorphous polymer. The amorphous and semi-crystalline nature of BBB and BBL respectively are confirmed by GIWAX data. This is interesting due to the significant percent increase in thermal conductivity in doped BBB, where it a small degree of crystallinity might be expected. BBB had an activation energy of 0.25 eV whereas the activation energy of BBL was lower, at 0.10eV. This suggests that the BBL structure has shallower traps associated with disorder which promotes carrier mobility and phonon propagation, agreeing with the thermal conductivity data. The structure of these polymers were analysed further using FTIR where it is clear that doping affected the two structures differently. Doped BBL is seen to have a slight peak shift associated with the neutral C=O bond which can suggest a stronger electron-phonon coupling. The spectra also suggests the dopant is affecting the C=N, C-N, C=O and C-C units within BBL, however, the dopants are residing locally near the C=O units within BBB. The more delocalised nature of polarons within doped BBL may explain the wider polaron band seen in the UV-VIS spectrum
    corecore