486 research outputs found

    Strategies for Red-Light Photoswitching

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    Vuorovaikutteiset, muotoutuvat ja jopa älykkäät molekyylirakenteet ovat avain uuden sukupolven lääkeaineisiin ja toiminnallisiin materiaaleihin. Valokytkimet eli yhdisteet, jotka isomeroituvat reversiibelisti valon vaikutuksesta johtaen makroskooppisten ominaisuuksien muutoksiin, ovat erottamaton osa tätä tulevaisuutta. Mahdolliset sovelluskohteet ulottuvat lääketieteestä elektroniikkaan ja robotiikkaan. Valitettavasti useimmat valokytkinrakenteet, esimerkiksi laajalti käytetyt atsobentseenit, absorboivat ultraviolettivaloa, joka on vahingollista monille materiaaleille ja erityisesti eläville soluille. Jotta valokytkinten koko potentiaali voidaan hyödyntää, tarvitaan harmittomalla näkyvällä valolla toimivia yhdisteitä. Puna- tai infrapunavalo olisi ihanteellinen ärsyke biologian alalla käytettäville kytkimille. Sama pätee myös molekyylimoottoreihin eli yhdisteisiin, jotka pyörivät valon vaikutuksesta yksisuuntaisesti. Lisäksi sekä kytkinten että moottorien tulisi isomerisoitua valon vaikutuksesta tehokkaasti ja nopeasti, termisten isomerisaatioreaktioiden tulisi olla sovelluskohteesta riippuen hitaita tai nopeita ja yhdisteiden tulisi toimia hyvin erilaisissa ympäristöissä. Näiden ominaisuuksien hallitsemiseksi on tärkeää ymmärtää niiden taustalla olevat mekanismit. Tässä väitöskirjassa tutkimme kolmea keinoa toteuttaa valokytkentä punaisella valolla: (i) atsobentseenien absorptiospektrin siirtäminen rakennetta muokkaamalla, (ii) uusien, valmiiksi punaista valoa absorboivien rakenteiden hyödyntäminen ja (iii) epäsuora valokytkentä punavalolla aktivoitavia katalyyttejä hyödyntäen. Tarkastelemme strategioita teoreettiselta kannalta ja osoitamme, että niistä jokainen mahdollistaa valokytkennän punaista valoa käyttäen. Kullakin strategialla on etunsa ja haasteensa tehokkaan, nopean ja kestävän valokytkennän toteuttamiseksi. Tästä johtuen yksi ihanteellinen valokytkinmalli ei voi saavuttaa kaikkia eri sovelluksille asetettuja tavoitteita, vaan tulevaisuuden haaste on löytää kuhunkin käyttöön paras ratkaisu. Samoja periaatteita voidaan soveltaa myös molekyylimoottoreihin, jolloin molekulaarisen tason yksisuuntainen kiertoliike voidaan saada aikaan näkyvällä valolla. Lisäksi punaisella valolla toimivien valokytkinten rakenteita hyödyntämällä moottorien rotaatiota saadaan tehostettua.Responsive, adaptive and even intelligent molecular systems have been identified as the key to next-generation pharmaceuticals and functional materials. Photoswitches, compounds that isomerise reversibly between two distinct ground-state species upon excitation with light and consequently give rise to a macroscopic effect, are an integral part of this future. Their potential application areas range from photopharmacology to optoelectronics and soft robotics. However, most conventional photoswitch structures such as azobenzenes absorb ultraviolet light, high-energy photons that are detrimental to many artificial materials and especially to living systems. To harness their full potential, photoswitches should function efficiently with visible light that is benign to the environment. Red or near-infrared light would be the ideal stimulus for switches utilised in biological context, as these wavelengths are least absorbed by living tissue. The same applies to light-driven molecular motors, compounds that exhibit unidirectional rotation upon photoexcitation. In addition to absorption in the red part of the visible spectrum, both switches and motors should exhibit efficient and fast photoisomerisation, favourable thermal isomerisation kinetics and tolerance towards different environments in order to be useful in real-life applications. In this light, it is crucial to understand the underlying fundamental mechanisms that govern these attributes. In this thesis, we explore three different approaches to realise photoswitching with red light: (i) synthetic modifications of azobenzenes, (ii) utilisation of new photoswitch cores that inherently absorb low-energy photons, and (iii) indirect isomerisation with red-light photocatalysts. We study each strategy from a theoretical viewpoint and demonstrate that they all provide means to induce isomerisation with red light, each with unique advantages and challenges in terms of promoting efficient, fast and robust switching. As a result, a single optimal photoswitch system cannot be designed; instead, the challenge lies in identifying the best design for each application. The same principles can also be applied to molecular motors, giving rise to visible-light-powered unidirectional rotary motion on a molecular level. We show that drawing inspiration from red-light-absorbing photoswitches has repercussions not only on the visible-light absorption but also on enhanced rotation dynamics

    A holistic review on fatigue properties of additively manufactured metals

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    Additive manufacturing (AM) technology is undergoing rapid development and emerging as an advanced technique that can fabricate complex near-net shaped and light-weight metallic parts with acceptable strength and fatigue performance. A number of studies have indicated that the strength or other mechanical properties of AM metals are comparable or even superior to that of conventionally manufactured metals, but the fatigue performance is still a thorny problem that may hinder the replacement of currently used metallic components by AM counterparts when the cyclic loading and thus fatigue failure dominates. This article reviews the state-of-art published data on the fatigue properties of AM metals, principally including SS--NN data and fatigue crack growth data. The AM techniques utilized to generate samples in this review include powder bed fusion (e.g., EBM, SLM, DMLS) and directed energy deposition (e.g., LENS, WAAM). Further, the fatigue properties of AM metallic materials that involve titanium alloys, aluminum alloys, stainless steel, nickel-based alloys, magnesium alloys, and high entropy alloys, are systematically overviewed. In addition, summary figures or tables for the published data on fatigue properties are presented for the above metals, the AM techniques, and the influencing factors (manufacturing parameters, e.g., built orientation, processing parameter, and post-processing). The effects of build direction, particle, geometry, manufacturing parameters, post-processing, and heat-treatment on fatigue properties, when available, are provided and discussed. The fatigue performance and main factors affecting the fatigue behavior of AM metals are finally compared and critically analyzed, thus potentially providing valuable guidance for improving the fatigue performance of AM metals.Comment: 201 pages, 154 figure

    Laser Micromachining: An Enabling Technology for Functional Surfaces and Materials

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    L'abstract è presente nell'allegato / the abstract is in the attachmen

    Quantum computation of stopping power for inertial fusion target design

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    Stopping power is the rate at which a material absorbs the kinetic energy of a charged particle passing through it -- one of many properties needed over a wide range of thermodynamic conditions in modeling inertial fusion implosions. First-principles stopping calculations are classically challenging because they involve the dynamics of large electronic systems far from equilibrium, with accuracies that are particularly difficult to constrain and assess in the warm-dense conditions preceding ignition. Here, we describe a protocol for using a fault-tolerant quantum computer to calculate stopping power from a first-quantized representation of the electrons and projectile. Our approach builds upon the electronic structure block encodings of Su et al. [PRX Quantum 2, 040332 2021], adapting and optimizing those algorithms to estimate observables of interest from the non-Born-Oppenheimer dynamics of multiple particle species at finite temperature. Ultimately, we report logical qubit requirements and leading-order Toffoli costs for computing the stopping power of various projectile/target combinations relevant to interpreting and designing inertial fusion experiments. We estimate that scientifically interesting and classically intractable stopping power calculations can be quantum simulated with roughly the same number of logical qubits and about one hundred times more Toffoli gates than is required for state-of-the-art quantum simulations of industrially relevant molecules such as FeMoCo or P450

    First-principles study of the enhancement of electrochemical performance of a SnS2 monolayer for lithium/sodium-ion batteries via vacancy defects

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    Dissertation (MSc (Physics))--University of Pretoria, 2023.Various transition metal dichalcogenides materials have been investigated from bulk to monolayer phases for different advanced technological applications. Tin disulfide monolayer offers advantages as an anode material for Li/Na-ion batteries, although it cannot be considered an ideal for direct exploitation. We systematically performed a comparative study of the adsorption and diffusion behaviour of Li/Na on a pristine SnS2 monolayer and on a SnS2 monolayer with a S-vacancy for enhancement of electrochemical performance, using the density functional theory approach. Although all the adsorption sites are exothermic, it was established that Li/Na adatoms mostly prefer to bind strongly on a SnS2 monolayer with a S-vacancy but avoiding the S-vacancy site. It was established that avoiding the S-vacancy site along the path, an excellent diffusion barriers of 0.19 eV for Li and 0.13 eV for Na were achieved, suggesting possible ultrafast charge/discharge rate. Due to reduced molar mass, the SnS2 monolayer with a S-vacancy has a slightly higher storage capacity than its pristine counterparts for both Li and Na adatoms. The obtained open circuit voltage values are within the range of 0.25–3.00 V assuring that the formation of dendrites can surely be averted for the envisaged battery operation. Understanding the effects of an S-vacancy on the electrochemical properties of Li/Na on the SnS2 monolayer allows us to consider possible improvements to energy storage devices that can be applied as a result of improved anode material.PhysicsMSc (Physics)Unrestricte

    Cosmic Microwave Background Anomalies meet Primordial Non-Gaussianity

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    openDespite the Standard Cosmological Model being well established, several unexpected statistical features (anomalies) are present in the data of the Cosmic Microwave Background (CMB) temperature anisotropies. If these anomalous features were to have a primordial origin, they could indicate an apparent violation of statistical isotropy of the Universe at the largest cosmological scales, thus providing an exciting opportunity to probe the exact physical mechanism operating at the very first epochs of the Universe. This Thesis will investigate the CMB anomalies from an observational point of view, providing an up-to-date review of the main models and explanations proposed in literature, focusing in particular on the possibility that such anomalies are related to non-Gaussian features of the primordial cosmological perturbations arising from inflationary models in the Early Universe

    On the glass transition of bulk and confined polyamorphic liquids: A molecular dynamics simulations study

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    Supercooled liquids and the glass transition are not satisfactorily understood to date. The temperature dependence of dynamical properties eludes theoretical prediction. No model can be successfully applied to all liquids. One liquid is particularly complex in its supercooled regime - water. This seemingly simple liquid exhibits the most anomalies of any neat liquid, and most of these are thought to be related to the existence of two distinguishable liquid phases with different density in the supercooled regime, i.e., water exhibits polyamorphism. However, most of the relevant temperature range lies in the so-called no-man's land, a region of the phase diagram in which bulk water rapidly crystallizes and which is therefore experimentally inaccessible to the bulk liquid. Therefore, experimental studies often exploit the fact that crystallization of water is suppressed in nanoscopic confinements or water mixtures. The present work deals with both areas of research, water's polyamorphism and dynamics of supercooled liquids, confined and mixed, with the use of molecular dynamics simulations. They allow for detailed analysis and systematic variation of the liquid and enable easy supercooling. Partial charges of the TIP4P/2005 and SPC/E water models were scaled which led to strong shifts of dynamics in temperature. These were reconciled by using the high-temperature activation energy as the relevant energy scale as long as structural properties were the same. For the TIP4P/2005 model and a set of reduced charges, isochore crossing in the phase diagram confirmed the existence of a liquid-liquid critical point (LLCP) in the supercooled regime at positive or negative pressures, depending on the molecular polarity. The two-structure equation of state (TSEOS) formalism was used to describe the data and determine the location of the LLCP. In addition, reduction of the partial charges accelerated dynamics at the LLCP and simulations with elongated boxes in the double metastable regime allowed for the coexistence of high-density (HDL) and low-density (LDL) liquid phases and the determination of their dynamics as a function of temperature. The results are in agreement with observations from isochoric and isobaric simulations and translational motion was observed for all state points. It was found that the temperature dependence of the dynamics at a constant fraction of the low-density state (LDS) is Arrhenius-like. Thus, the presumed fragile-to-strong transition (FST) of water is not caused by a transition from fragile HDL to strong LDL but by the fast transition between these liquid states when the system is cooled through the Widom line at constant pressure. This is consistent with experimental observations slightly above water's glass transition temperature Tg and reinforces the question of whether HDL or LDL on their own exhibit an FST. Models for the temperature dependence of reactive mixtures were tested but were unable to describe simulation results at the lowest studied temperatures. A family of functional forms for the temperature dependence of dynamical properties of supercooled liquids was derived. These functions allow their description over the entire temperature range from the boiling point to the glass transition and with or without an FST. The second-order functions predict a high and low-temperature Arrhenius regime connected by an intermediate fragile regime. Knowledge of the path in the phase diagram of charge-scaled water-like systems, whether they cross the Widom line at increased charges or not, allowed for more rigorous testing of these functional forms. They are sensitive to deviations from Vogel-Fulcher-Tammann (VFT) behavior and apply well to data from charge-scaled water and silica simulations, which have a pronounced FST, as well as to real liquids. The possibility that supercooled liquids in general have a low-temperature Arrhenius regime and the characteristics of such FSTs were discussed. Simulations of charge-scaled water models in chemically neutral pores were performed and static and dynamic length scales associated with changes of water's structure and dynamics near the pore wall were extracted. These correlation lengths were used to test theories of the glass transition and discussed in the context of water's two phases. Signs of crossing the Widom line could not be found in the temperature dependence of the correlation lengths within the moderately supercooled temperature range. The slowdown at the pore wall relative to the pore center was characterized using two empirical functions for additional activation energies caused by the liquid-confinement interface. Furthermore, the potential energy landscape (PEL) imprinted on the liquid was quantified using a novel approach based on Boltzmann statistics and predicted and measured mobility gradients are in agreement. Lastly, the origin of slow solvent processes observed in dielectric spectroscopy studies of dynamically asymmetric binary mixtures was determined in simulations. For mixtures of picoline and poly-methylmethacrylate and of water and polylysine, fractions of slow solvent molecules were not found. Instead, the PEL imprinted by the slow polymer molecules causes preferred locations and orientations for the solvent molecules. A mechanism was proposed in which the solvent molecules exchange fast compared to the relaxation of the polymer molecules but have correlated orientations. This causes long-lived cross correlations that can be misinterpreted as slow solvent contributions in coherent measurements. Other sources of cross correlations were quantified and the dependency on measured molecular property and correlation function were discussed. The dynamical heterogeneity of solvent dynamics was traced back to the variation of the local solvent concentration and it is broad but unimodal. The same observations, slowly decaying cross correlations and absence of self correlation on these time scales, were made for other binary mixtures, suggesting that these effects are relevant to a wide range of systems

    Charting new physics territories with cosmological observations

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    In this Thesis we exploit the latest cosmological observations to constrain fundamental physics scenarios that go beyond the standard models of cosmology and particle physics. We start by considering Macroscopic Dark Matter (MDM) candidates, which represent an appealing alternative to particle dark matter. We focus on the process of proton capture by MDM and constrain the parameter space of Macros using three cosmological probes: (i) the change in the baryon density between the epochs of the Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB) decoupling; (ii) the production of spectral distortions in the CMB spectrum; (iii) the kinetic coupling between charged MDM and baryons at the time of CMB decoupling. We also show how future spectral distortions experiments, like PIXIE and SuperPIXIE, will allow us to improve these bounds. Then, we focus on thermal axion-like particles (hereafter axions), which are produced in the early Universe from scatterings between particles belonging to the primordial thermal bath. Depending on their mass, thermal axions can behave as a hot, warm or cold dark matter component. Using the latest observations of CMB anisotropies by Planck and of Baryon Acoustic Oscillations from galaxy surveys, we constrain the couplings of axions to photons and gluons. We compare these bounds with the constraints derived from laboratory and astrophysical probes. In the second part of the Thesis, we focus on the polarization of the CMB as a probe to test possible violations of fundamental symmetries. First, we consider models that extend Maxwell’s electrodynamics by introducing renormalizable operators which break Lorentz invariance. These consist in two terms, one of which violates also CPT symmetry. Using the most recent observations of CMB polarization, we derive strong bounds on the Lorentz-violating coefficients. In particular, for the CPT-odd coefficients we obtain the strongest constraints to date, even considering non-CMB probes. Finally, we analyze the effects of chiral scalar-tensor theories of gravity during inflation, focusing on the non-Gaussianity of primordial gravitational waves. We compute the theoretical prediction for the primordial bispectrum and we discuss the prospects for detecting such parity-violating signatures with future CMB experiments.In questa Tesi sfruttiamo le più recenti osservazioni cosmologiche per vincolare scenari di fisica fondamentale che vanno oltre i modelli standard della cosmologia e della fisica delle particelle. Per cominciare consideriamo candidati di materia oscura macroscopica (MDM), che rappresentano una interessante alternativa ai modelli di materia oscura particellare. Ci concentriamo sul processo di cattura di protoni da parte della MDM e vincoliamo lo spazio dei parametri delle Macro sfruttando tre osservabili cosmologiche: (i) la variazione nella densità di barioni tra le epoche della Nucleosintesi Primordiale (BBN) e della Radiazione Cosmica di Fondo (CMB); (ii) la produzione di distorsioni spettrali nello spettro della CMB; (iii) l’accoppiamento tra la MDM carica e i barioni all’epoca del disaccoppiamento della CMB. Inoltre, mostriamo come esperimenti futuri per distorsioni spettrali, come PIXIE e SuperPIXIE, potranno migliorare i vincoli attuali. Successivamente ci concentriamo su particelle axion-like (d’ora in avanti assioni) termiche, che sono prodotte nell’Universo primordiale tramite processi di scattering tra particelle presenti nel plasma primordiale. A seconda della loro massa, gli assioni termici possono comportarsi come una componente di materia oscura calda, tiepida o fredda. Sfruttando le più recenti osservazioni delle anisotropie della CMB dal satellite Planck e delle Oscillazioni Acustiche Barioniche dalle survey di galassie, vincoliamo gli accoppiamenti degli assioni ai fotoni e ai gluoni. Questi risultati vengono poi confrontati con i vincoli ottenuti da esperimenti in laboratorio e tramite sorgenti astrofisiche. Nella seconda parte della Tesi ci concentriamo sulla polarizzazione della CMB per testare possibili violazioni di simmetrie fondamentali. Come prima cosa, consideriamo modelli che estendono l’elettrodinamica di Maxwell introducendo operatori rinormalizzabili che violano la simmetria di Lorentz. Questi consistono di due termini, uno dei quali viola anche la simmetria CPT. Sfruttando le più recenti osservazioni della polarizzazione della CMB, deriviamo vincoli molto stringenti sui coefficienti che parametrizzano la violazione di Lorentz. In particolare, per i coefficienti che violano CPT otteniamo i vincoli più forti ad oggi, anche considerando osservabili non cosmologiche. Per concludere, analizziamo gli effetti di teorie della gravità scalari-tensoriali che violano la parità durante l’inflazione, concentrandoci sulla non-Gaussianità delle onde gravitazionali primordiali. Calcoliamo la predizione teorica per il bispettro primordiale e discutiamo le prospettive per un’osservazione di questi effetti di violazione di parità tramite esperimenti di CMB futuri
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