Academic literature on the topic 'Mangetic and optical propertie'

A simple method was applied to fabricate phase-pure hollow CuS microspheres. The obtained product was characterized by X-ray diffraction, scanning electron microscopy, photoluminescence spectra and UV-Vis absorption spectroscopy. Further, the catalytic activity of CuS spheres was evaluated by the decolorization of Rhodamine B in the presence of hydrogen peroxide solution at room temperature. The results indicated that the product showed a good optical propertie, and the hollow sphere CuS could be an effective catalytic material.

During the last few years, liquid crystalline elastomers (LCEs) have been systematically produced by cross-linking liquid crystalline side-chain polymers. In these networks, a liquid crystalline molecule is fixed at each monomeric unit. LCEs exhibit a novel combination of properties. Due to liquid crystalline groups, they show anisotropic liquid crystalline properties similar to conventional liquid crystals (LCs); but due to the three-dimensional network-structure of the polymer chains, they show typical elastomer properties, such as rubber elasticity or shape stability. One exceptional property of this combination is demonstrated when a mechanical deformation to the LCE causes macroscopically uniform orientation of the long molecular axis of the LC units (the so-called “director”).This response of the LC-phase structure to an applied mechanical field is similar to the effect of electric or magnetic fields on low molecular weight liquid crystals (LMLC), as illustrated in Figure 1. Figure la shows an undeformed LCE. Because of the non-uniform orientation of the director, the sample scatters light strongly so the elastomer is translucent like frosted glass. On the other hand, applying a mechanical field the director becomes uniformly aligned and the sample is transparent (Figure 1b). Such a macroscopically ordered rubber exhibits optical properties very similar to single crystals. These propertie s of LCEs offer new prospects for technical application, e.g., in nonlinear and integrated optics.

Abstract The MnBi2Te4 family compounds have drawn enormous attention in recent years due to their potential to realize the high-temperature quantum anomalous Hall effect (QAHE). As one of the most direct techniques to probe electronic structure, angle-resolved photoemission spectroscopy (ARPES) has been widely applied to investigate the interplay between the magnetism and non-trivial topological band structure of MnBi2Te4 family materials. Here, we briefly review some of latest progress of ARPES on MnBi2Te4·(Bi2Te3) (n=0, 1, 2, 3) and analogous MnSb2Te4·(Sb2Te3) (n=1,2) series of materials. Aside from the direct observation of the non-trivial topological band structure, ARPES results reveal that their surface band gap can be well modulated by both temperature and Bi2Te3 interpolation. In addition, although ARPES results of the topological surface states are still under hot debate, the extensive experimental results have provided an opportunity to shed light on the complex interactions in MnBi2Te4 family materials.

Water is the major chemical constituent of our planet's surface and it is essential for living organism survival; many biochemical and industrial processes occur in aqueous solution and the role of the solvent in the reactions is crucial. The comprehension of the chemical and physical nature of water has been a long-standing goal of science, and liquid water continues to attract intense interest and motivate a large number of experimental and theoretical works. Recently, however, the theoretical studies of water have mostly focused on its structure and ground state properties whereas less effort has been dedicated to its electronic structure and optical absorption spectrum. As a consequence, experimental data about excited states are not yet completely understood. Simultaneously, in the last years, great attention has been devoted to the study of water confined in different nanoporous systems, or in proximity of macromolecules and surfaces, because of its biological and technological importance (water in biology is always confined). Up to now, however, there is no general theory predicting the behavior of confined liquids or the relative importance of surface interaction versus confinement. The present work focuses on two complementary aspects of water: its excited state properties, very important in many chemical reactions and therefore fundamental to advance in many research fields, and the proton microscopic dynamics in confined water, which is interesting for many biological processes such as catalysis, protein folding or ionic transport in membranes. These topics are faced with different investigative approaches, both theoretical and experimental. The electronic and optical properties of liquid water are studied with ab-initio theoretical calculations, taking into account both self-energy and excitonic effects in the framework of many-body perturbation theory. The study of the proton microscopic dynamics of confined water has been instead made with deep inelastic neutron scattering experiments, performed at the ISIS spallation neutron source.

Le eccitazioni elettroniche assumono un ruolo di fondamentale importanza in diverse proprietà fisiche dei materiali, ad esempio nel trasporto quantistico, trasporto termico, conducibilità e proprietà ottiche. A seconda della carica elettrica dello stato finale, possiamo distinguere tra eccitazioni cariche e neutre. Nelle eccitazioni cariche, consideriamo elettroni che vengono aggiunti o sottratti al sistema. Questi processi possono essere studiati tramite esperimenti di fotoemissione diretta o inversa. Invece, nelle eccitazioni neutre, la carica totale del sistema viene conservata durante il processo di eccitazione. Questo tipo di eccitazioni possono essere rilevate attraverso misure di assorbimento ottico, sia in regime lineare che non lineare. La teoria del funzionale della densità (DFT) e la sua estensione dipendente dal tempo (TDDFT) sono spesso il primo quadro teorico di riferimento nella descrizione a primi principi dei processi di eccitazione. Tuttavia, è noto che la DFT e la TDDFT danno luogo ad errori e limiti di applicabilità a causa delle approssimazioni funzionali che devono essere necessariamente utilizzate in pratica. Pertanto, lo sviluppo di approssimazioni più accurate ed altre estensioni teoriche è un campo di ricerca interessante e attivo. In questo lavoro, mi occupo dello sviluppo di nuovi metodi per il calcolo delle eccitazioni cariche e neutre. Nella prima parte, mostro come il gap fondamentale dei punti quantici bidimensionali può essere accuratamente stimato al costo computazionale di un calcolo di stato fondamentale, seguito da uno step non autoconsistente dal costo computazionale trascurabile, il tutto eseguito all'interno dell'approssimazione della densità locale. Nonostante ciò, la procedura include formalmente la discontinuità del potenziale di scambio espressa attraverso il metodo del potenziale effettivo orbitale. Nella seconda parte, ricavo un'approssimazione del potenziale che include esplicitamente un gap di scambio diverso da zero per sistemi bidimensionali sia finiti che periodici. Sebbene la forma funzionale utilizzata coinvolga direttamente gli orbitali a singola particella, il costo computazionale è paragonabile ai calcoli DFT standard. L'approssimazione del potenziale è applicata al grafene artificiale, in cui una distorsione di Kekulé utilizzata per aprire un gap nei punti di Dirac. Nella terza parte di questo lavoro mi occupo di studiare le eccitazioni neutre non lineari. In particolare, derivo la sezione d'urto ottica per un sistema a più elettroni soggetto a un campo elettrico impulsivo nel regime non perturbativo, cioè per valori arbitrari dell'intensità del campo incidente, partendo dallo stato fondamentale. Successivamente mostro che la sezione d'urto include assorbimenti da stati eccitati all'aumentare dell'intensità del campo elettrico. Queste transizioni ottiche non sono possibili all'interno del regime lineare. Come esempio di applicazione, considero il caso di un sistema modello unidimensionale composto da due elettroni interagenti. L'analisi rivela che gli stati eccitati di tipo gerade, che non partecipano alle proprietà ottiche nel regime lineare, sono popolati nel regime non lineare a causa dell'assorbimento da stato eccitato. Questa analisi aiuta a interpretare le simulazioni di TDDFT in tempo reale che impiegano campi elettrici impulsivi oltre il regime lineare, come per lo studio dei processi coinvolti nei fenomeni di limitazione ottica. I risultati ottenuti in questa tesi di dottorato contribuiscono allo sviluppo di metodi accurati e praticabili per studiare le eccitazioni elettroniche nei sistemi quantistici e, in particolare, allo sviluppo teorico nel campo degli approcci a principi primi basati sui funzionali della densità.
Electronic excitations play a prominent role in a large variety of physical properties of materials, e.g., quantum transport, heat transport, conductivity, and optical properties. Depending on the electric charge of the final state, we may distinguish between charged and neutral excitations. In charged excitations, we consider electrons that are added or subtracted to the system. Direct or inverse photoemission experiments are a primary tool for the experimental observation of such processes. Instead, in neutral excitations, the total charge of the system is conserved during the excitation process. These can be probed through optical absorption measurements, both in the linear and nonlinear regimes. Density functional theory (DFT) and its time-dependent extension (TDDFT) are often the theoretical framework of first choice in the first-principles description of excitation processes. However, it is well known that DFT and TDDFT show failures and limitations due to the functional approximations which are necessary in practice. Thus, the development of more accurate approximations and theoretical extensions is an interesting and intense field of research. In this work, I develop new advances in the calculation of charged and neutral excitations. In the first part, it is shown that the fundamental gap of two-dimensional quantum dots can be accurately estimated at the effort of a standard ground-state calculation supplemented with a non-self-consistent step of negligible cost, all performed at the level of the local-density approximation. Yet, the procedure formally exploits the exchange discontinuity as expressed through the orbital-effective-potential method. In the second part, I derive an approximate potential that can capture non-vanishing exchange gaps both infinite and periodic two-dimensional systems. Although the procedure involves single-particle orbitals directly, the computational cost is comparable to standard DFT calculations. The potential approximation is applied to the artificial graphene, Kekulé distorted to open a gap at the Dirac points. In the third part of this work, nonlinear neutral excitations are investigated. In particular, I derive the optical cross section of a many-electron system subject to an impulsive electric field in the nonperturbative regime, i.e. for arbitrary values of the field strength, starting from the ground state. I show that the cross section includes absorptions from excited states for increasing intensities of the electric field - which are optical transitions that cannot be captured within the linear regime. As an example, I consider the case of a 1D two-electron model system. The analysis reveals that gerade excited states, which are dark in the linear regime, are populated in the nonlinear regime due to excited-state absorption. This analysis helps to interpret real-time TDDFT simulations which employ impulsive electric fields beyond the linear regime, as for studying processes in optical limiting phenomena. The results obtained in this Ph.D. thesis contribute to the development of accurate and feasible methods to investigate electronic excitations in quantum systems, and, more generally, to the theory development of first-principles density-functional approaches.

Tissue healing is in general a complex process, which involves both local and systemic responses, and bone regeneration in particular is much slower than repair in any other human tissue. Thus, it exhibits a great challenge in clinical practice and in the field of research. Bone regeneration is comprised of a series of biological events, involving a number of cell types and intracellular and extracellular molecular- signaling pathways, with a definable temporal and spatial sequence, in an effort to optimize the skeletal repair and restore its functionality. Photobiomodulation (PBM) therapy has been shown to be effective in modulating both local and systemic responses, by enhancing cellular activities resulting in an increase in function, especially in injured tissues, leading to optimization of tissue repair and regeneration. In bone tissue, application of the photonic energy leads to bone healing by the activation of osteoblasts, leading to proliferation and differentiation, as well as osteoclast inhibition and, consequently, neoformation of bone matrix. The process of the in vitro pre-osteoblasts maturation, mimicking their in vivo behavior, passes through three distinct stages of development: proliferation, early differentiation (maturation) and late differentiation (mineralization). Despite the extensive research on the effects of photobiomodulation (PBM) light on bone regeneration, the current outcomes ranging from positive to negative effect remain controversial. These contradictory data are thought to be due to; incomplete knowledge and understanding of the mechanistic effects of laser light on cells, lack of standardized laser dosimetry, inefficient laser beam profile, improper study design and varied methods of investigation. The literature is hindered by a considerable heterogeneity of the irradiation parameters of PBM, as well as, the methods utilized to evaluate the results and the type of osteoblast-like cells irradiated. This has led to a need of standardization. Moreover, heterogeneity of the current studies and their limitations could be due to study designs and inefficient beam profile, resulting in undesirable effects and accounting for negative and inconclusive outcomes. Ultimately, lack of intimate knowledge and understanding of the PBM light behavior impinging on the target tissue, as well as the optical tissue properties, can compromise optimization of the therapeutic outcomes. Thus, an evidence-based decision for definite therapeutic application of PBM in bone regeneration is required. In this thesis, we addressed the above issues and challenges via two elements, the electromagnetic (EM) modeling experiments and the molecular and cellular impact of PBM on bone regeneration (In vitro studies). The Electromagnetic models In my PhD proposal, I intended to both create an EM model, for the first time, and examine the mechanism of interaction of the electromagnetic fields (EMFs) with cells/tissues and establish the link that can be utilized in my cellular experiments. As the project evolved, it became clear this work was breaking new grounds and was significantly more complex than initially envisaged. As it is a small part of a much larger exciting project undertaken by University of Genoa, it has meant that I need to coordinate my work with the overall timetable of this larger project. As a result, I decided to defer, the interaction of the EMF with cells of interest part, to my Post doctorate study. We developed, for the first time, a set of simple models to examine the behavior of the local electromagnetic field (EMF), determining the PBM effects on mitochondria. This set of models was tested and crosschecked for its validity by evaluating various variables in terms of, polarization, absorption and scattering coefficient, dissipated energy density and irradiance, as well as the refractive index. Ultimately, our model and preliminary data are the first stepping-stone for further experiments, in order to understand the mechanism of interaction between electromagnetic fields and cells or tissues. Our conclusions showed that when these set of models are utilized, for the phenomenon of interest, the incident field polarization had small effects on the electromagnetic field and negligible consequences on the average energy, as well as, on the dissipated power densities. The same was shown to hold true for different orientations that the mitochondria can assume. The analogous conclusions were obtained by taking into account the possible changes in the dimensions or of the real part of the refractive index of the considered organelles. The variations of the absorption coefficient were shown to have significant effects on the average dissipated power density in the mitochondria but these effects can be predicted in a surprisingly simple way. It was proved that the numerical analysis, of the problems of interest, could be computed by using three-dimensional models, involving only a few mitochondria in the plane, which was transverse to the direction of propagation of the illuminating light that generated a uniform distribution of the energy over 1cm2 area. The one- dimensional models provided significant information on the EMF, utilized to stimulate the mitochondria. Mitochondria behaved like weak scatterers. Therefore, it was not necessary to analyze large extension of such organelles to understand what happen inside one of them. The molecular and cellular impact of 980nm PBM on osteoblast maturation: in vitro studies Our pilot study data, on the bone marrow stromal cells (BMSCs), strongly suggested that the high fluence concept (over 60J/cm2 in continuous emission mode (CW)) delivered by flattop beam profile device (FT) can promote BMSCs differentiation towards osteogenesis. Moreover, the results showed an increase in cytokines synthesis with potent anti-inflammatory properties and a decrease in the release of proinflammatory mediators. This provided me with a platform, demonstrating the validity of high fluence in facilitating osteoblasts differentiation through BMSCs. Based on this; I formulated three PBM protocols for 980nm to be tested on pre- osteoblast cell line in my definitive in vitro studies. The first phase of in vitro studies aimed to evaluate the 980nm bio-stimulatory effects on osteoblasts maturation, optimise the PBM effects on bone healing with various beam profiles delivery devices, and establish protocol/protocols of 980nm PBM in bone regeneration. The primary objective was to determine the optimal 980nm dosimetry, which exerts bio- stimulatory effects to accelerate and enhance the bone regenerative process. The secondary objective was to evaluate the intra-cellular pathways of the photon-cell interaction across the metabolic proliferative and differentiation changes, which ultimately lead to bone healing and repair. The results of this study validated the contribution of PBM in bone regeneration and elucidated the biochemical effects at a cellular level. Moreover, the role of different dosages of 980nm PBM irradiation delivered by FT; in comparison to the Gaussian beam profiles (Standard (ST)) on bone regeneration were highlighted. The setup of the power outputs on the laser device was 1.1Watt (W) for the ST and 1W for the FT. However, the real (the threshold) power output reaching the target, measured by power meter, was as ∼0.9 W, (Irradiance ∼ 0.9W/cm2, Exposure time 60 seconds, energy ∼55 J (Joule), fluence ∼55 J/cm2) delivered with the FT beam profile in CW in comparison to the ST, on MC3T3-E1 pre-osteoblast maturation. The protocol was based on 60 seconds exposure time for two consecutive weeks, which employed for all the groups. The laser grouping and their associated irriadtied energies were as follows: Group 1- Irradiation once per week (Total enrgy 110J). Group 2- Irradiation three times per week (Alternate day) (Total energy 330J). Group 3 - irradiation five- times per week (Total energy 550 J). The control cultures were processed in identical conditions except that the laser device was kept off all the time. The total energy was 0J.
The metabolic activity and the osteoblasts maturation were analyzed by alkaline phosphatase assay, alizarin red S histological staining, immunoblot and/or double immunolabeling analysis for Bcl2, Bax, Runx-2, Osx, Dlx5, osteocalcin, and collagen Type 1. Our data, for the first time, prove that laser irradiation of 980 nm wavelength with flattop beam profile delivery system, compared to standard-Gaussian profile, has improved photobiomodulatory efficacy on pre-osteoblastic cells differentiation. Mechanistically, the irradiation enhances the pre-osteoblast differentiation through activation of Wnt signaling as well as the Smads 2/3-βcatenin pathway. Our results indicated and valued the intra-cellular pathways of the photon-cell interaction across the metabolic, proliferative and differentiation changes in the cells. Additionally, our data showed that the cells irradiated THREE times a week (Total energy of 330 J) and ONCE a week (Total energy of 110 J) for two consecutive weeks protocols have increased the proliferation and differentiation of the osteoblasts in both ST & FT hand-pieces but the data showed increasingly statistical significant in the FT group. The only Runx2 was detected when the cells were irradiated with the ST hand-piece. Therefore, total energy of 110 J when either of the hand-pieces utilized, has influenced early differentiation markers. Interestingly, when the process was carried out, until the mineralization and maturation (Late osteogenesis), the ST hand-piece irradiation failed to induce an effective process, and did not lead to matrix deposition, while the FT profile showed a significant effect. In conclusion, our data, for the first time, prove that laser irradiation of 980 nm wavelength with the FT beam profile delivery system in comparison to the ST profile has a great photobiomodulatory efficacy on pre-osteoblastic cells differentiation, which would assist in accelerating bone regeneration, due to its homogeneous energy distribution at each point of its cross-section. Moreover, the irradiation protocols of three times a week and once a week for two consecutive weeks were able to increase the pre-osteoblasts and osteoblasts transcription factors, which were strongly and statistically significantly increased when the FT hand-piece was utilized. Therefore, the 980 nm laser irradiation protocol was able to promote the MC3T3-E1 cell differentiation. Researchers have demonstrated that the major barrier for an effective biological healing is insufficient laser photonic energy delivered to the injured site. PBM can modify the cell metabolism by increasing the mitochondria's ATP production. Currently, the challenge is to understand the target tissues optical properties and its cellular pathway when irradiated with laser phonic energy. In this way, modification of various energy exposure values can influence clinical outcomes predictability. Therefore, in the second phase of my in vitro study, we evaluated the effect of 980nm irradiation delivered with ST and FT beam profile hand-pieces on monolayer cell, at various power outputs; 0.8W, 0.5W and 0.25W. However, the exact power output values reaching the target, measured by power meter, were as follows: 0.75W, 0.45W, and 0.20W respectively. The MC3T3-E1 cells irradiated for two consecutive weeks, according to the following protocols: once a week (Total energy 90, 54, 24 J), respectively); three times a week (total energy 270, 162, 72J, respectively); five times a week (total energy 450, 270, 120 J, respectively). Metabolic activity of viable cells evaluated as follows: Hoechst staining; Western blotting for Runx-2, Bcl2, Bax, Osx, Dlx5, β-catenin, Smads 2/3, TGFβ, p.PI3K, PI3K, p.AKt, AKt, and p.ERK. Our data, for the first time, prove that the 980 nm irradiation at power output setting at 0.75W (0.75W/cm2) for 60 seconds in CW stimulated the MC3T3-E1 pre- osteoblasts viability, by affecting the critical pre-survival markers such as p.PI3K, p.Akt, Bcl2 and Bclxl. Moreover, we concluded that 980nm PBM delivered with FT at 0.75W power output was comparable to results with the ST. However, 0.45W and 0.20W did not modulate the cell metabolic features. Additionally, none of the laser protocols delivered with FT or ST had any influence on the cell differentiation process. In summary, our in vitro studies data, for the first time, have demonstrated the potential of utilizing the FT beam profile with our established protocols in bone regeneration, as a therapeutic tool for future pre-clinical and clinical applications. Moreover, these studies have shown the mechanistic effects of the PBM light on intracellular pathway across the metabolic and differentiation of the osteoblasts towards bone regeneration.

Il mercato del solare termodinamico offre varie soluzioni tecnologiche e impiantistiche in funzione dei livelli di temperatura che si vogliono ottenere. Le esigenze energetiche nei vari settori industriale, residenziale e commerciale, però, spingono il mercato verso i collettori solari capaci di operare con rendimenti maggiori del 50% a temperature superiori a 100 °C, fino anche a 250 °C (nel range cosiddetto “a media temperatura”'). In questo ambito, la tecnologia che dimostra di essere più matura per la penetrazione del mercato risulta essere quella dei collettori parabolici lineari (PTC), e in particolar modo quelli di piccola taglia. L'assorbitore solare riveste un ruolo di estrema importanza per il buon funzionamento dell'intero PTC. In particolare la scelta del coating superficiale per il tubo rappresenta un punto focale per lo sviluppo e l'ottimizzazione del sistema in termini tecnici ed economici. Per il raggiungimento degli obiettivi è necessario orientarsi verso soluzioni tecnologiche che abbiano proprietà chimiche, fisiche e ottiche tali da garantire elevate prestazioni in termini di efficienza energetica e stabilità nel tempo alle temperature di esercizio desiderate. I coatings a base di cromo nero presentano ottime caratteristiche ottiche (α≈0.90-0.92; εT≈0.10-0.15) e risultano essere stabili anche fino a 300 °C. Il più grande problema legato alla realizzazione di rivestimenti cromati è legato all'inquinamento conseguente all'utilizzo nel bagno elettrolitico di ossidi di cromo esavalente. Agli inizi del nuovo secolo, con l'avvento di nuove soluzioni chimiche meno inquinanti per la produzione di oggetti cromati e con il crescente interesse verso i collettori solari piani, le tecniche galvaniche hanno trovato largo uso nella produzione di impianti solari termodinamici. L'assorbitore che è stato studiato è un assorbitore a base di cromo nero, e questo rientra nella categoria dei “tandem-absorber”: lo studio è cominciato quindi dal substrato. Lo studio del substrato ha portato alla comprensione delle caratteristiche che questo deve possedere e quali sono le condizioni operative per ottenerle. Il substrato per il cromo nero deve possedere appropriate caratteristiche ottiche, ovvero bassa emittanza, ma deve anche favorire la deposizione e l'adesione dell'assorbitore. I materiali candidati a questo scopo sono stati il nichel, ottenuto con due diversi processi di deposizione e il rame. Poiché questi tre materiali favoriscono egualmente deposizione ed adesione del cromo nero, è da preferire il materiale che garantisce la minore emittanza, ovvero il nichel ottenuto con il processo di deposizione di Watts (ε300 °C≈ 0.4). Si è mostrato inoltre, facendo chiarezza rispetto a quanto riportato in letteratura, come gli spessori dei substrati non influenzino le caratteristiche ottiche. Quindi al fine di contenere i costi di produzione è da preferire il minore spessore che garantisca una buona adesione del substrato e questo è stato individuato in uno spessore di 2 µm. Si sono poi trattate le caratteristiche ottiche dell'assorbitore, ponendo particolare attenzione al contesto in cui questo verrà utilizzato: infatti il parametro di selettività, comunemente utilizzato in letteratura per il confronto degli assorbitori solari selettivi, non fornisce indicazioni valide sul comportamento dell'assorbitore nell'impianto solare. Si è quindi introdotto il parametro di efficienza η che tiene conto delle condizioni in cui verrà impiegato l'assorbitore. Ipotizzando un plausibile caso di lavoro con temperatura di esercizio 300 °C e rapporto di concentrazione di 40, si è mostrato come sia necessario cercare di massimizzare l'assorbanza del materiale assorbitore al fine di ottimizzare l'efficienza, piuttosto che limitarne l'emittanza. L'analisi dei parametri di deposizione che ha portato a determinare l'insieme di condizioni da cui deriva la migliore efficienza ha mostrato la fondamentale importanza della composizione chimica del bagno galvanico: infatti, oltre alla presenza del costituente principale, ovvero il Cr+3, si è verificato il contributo determinante apportato dai ``catalizzatori''. Questi facilitano il trasporto dello ione principale in soluzione e la sua deposizione al catodo, limitando al contempo le reazioni collaterali. In questo modo si riesce ad ottenere il cromo nero con un miglioramento di η del 5-8 % ed a densità di corrente molto inferiori rispetto al caso in cui i catalizzatori non sono presenti. Densità di corrente e temperatura del bagno galvanico sono i parametri principali su cui operare. I migliori risultati sono stati ottenuti a 20 °C con una densità di corrente di 60 A dm-2. Il tempo della deposizione è molto importante: infatti, dagli studi condotti, il tempo ottimale di deposizione è 1 minuto, poiché sia per tempi minori che maggiori si ha un peggioramento dell'efficienza. Dall'analisi della superficie si è visto che il cromo nero è uno strato di materia soffice, non compatta, con aspetto estremamente frastagliato e composta da globuli di piccole dimensioni costituiti da un nucleo di cromo metallico circondato da uno strato di ossidi e idrossidi di cromo. L'aspetto della superficie influenza le caratteristiche ottiche del materiale: infatti esiste una correlazione tra la rugosità superficiale e α/εT secondo cui all'aumentare di Rz si ha una perdita di selettività. Numerose e importanti informazioni si sono ottenute dalla valutazione degli effetti provocati dai trattamenti termici: infatti per un assorbitore solare è di fondamentale importanza conoscere le caratteristiche ottiche alla temperatura di funzionamento. Il cromo nero analizzato è caratterizzato da un miglioramento della selettività dopo essere stato esposto alle alte temperature (300 °C e 400 °C) soprattutto nei casi in cui il substrato sia Ni Watts. Si è potuto valutare anche che il rame non è un buon substrato per applicazioni che possano trovarsi a temperature superiori ai 300 °C a causa della sua facile interdiffusione con altri metalli. L'esposizione dell'assorbitore alla temperatura di esercizio si comporta inoltre come una sorta di livellante nei confronti di η il cui valore medio si attesta a circa 0.8. Infatti il trattamento termico a 300 °C provoca un miglioramento delle efficienze degli assorbitori che inizialmente possedevano delle η abbastanza basse e un cambiamento esattamente opposto per gli assorbitori che appena deposti presentavano le migliori efficienze. La presenza dello ione fluoruro nella composizione del bagno galvanico comporta invece una minore resistenza dello strato assorbitore nei confronti della temperatura. Alla luce di queste considerazioni la composizione ottimale del bagno galvanico individuata è costituita da CrCl3·6H2O 266 g l-1, H2SiF6 10 g l-1, NaH2PO4 4 g l-1 e CoCl2 ·6H2O 15 g l-1 . Infine, nonostante in precedenza si sia individuato il miglior substrato in base alle sue caratteristiche ottiche, si è visto sperimentalmente come questo parametro non si rifletta in maniera determinante sull'efficienza finale. Infatti, come già detto, è importante massimizzare l'assorbimento piuttosto che limitare l'emissione dell'assorbitore. Per questo motivo e considerate le prove effettuate, si può affermare che i substrati considerati sono tra loro equivalenti. Alla luce di ciò il substrato più adatto è il nichel ottenuto con il metodo di Wood, poiché è quello che necessita di minor lavorazione e non presenta le limitazioni riguardo alle temperature di utilizzo viste per il rame. Il miglior campione ottenuto, che rispetta le condizioni appena elencate, presenta una efficienza di conversione energetica η=0.88: questo valore non è molto lontano, e talvolta migliore, delle efficienze dei ben più costosi CERMET (η=0.85-0.93), oltre ad essere migliore delle efficienze dei campioni ottenuti da cromo esavalente. ***************** The use of a low-intensity source like sunlight, for energy generation requires an efficient system to concentrate and capture radiation and to transfer the energy to the exchange fluid. Sunlight is abundant, renewable and free of charge. Therefore the development and diffusion of solar energy exploitation is a key issue for the future. However, at present solar energy technologies are generally affected by a not high enough efficiency and a high cost, making them not fully competitive yet over conventional fossil fuels. Thus, it is clear that both increasing the efficiency and reducing the cost is mandatory to promote solar energy exploitation. Systems operating at mid-temperatures (i.e. using fluids at about 200-300 °C) and in particular parabolic trough collectors (PTCs) offer several advantages in comparison with conventional flat plates thanks to their higher efficiency and reduced receiver surface. In these systems the incident solar radiation is converted into heat either by sunlight absorption by blackened or specially developed absorbing surfaces that collect the solar energy and transfer it to the fluid. Required characteristics for the absorber surface are chemical and physical stability at the operating temperatures, as well as good performances in terms of energy efficiency. Moreover a production process characterized by a low cost and a high repeatability should promote a large scale diffusion. Several direct industrial applications, like Direct Steam Generation (DSC) and Solar Heating and Cooling (SHC), could exploit mid-temperature solar energy as energy source. This interest drives the research of novel technologies focused on this market sector where the technologies developed for systems operating at higher temperatures (e.g. CSP plants) cannot be used. Electrodeposition techniques are a promising route to obtain surfaces with tailored optical characteristics. Black chrome coatings have excellent optical properties, as they are strongly absorbing in the sunlight spectral region, with a high absorbance α ≈ 0.90-0.92 and a low thermal emittance ε ≈ 0.10-0.15. Moreover they remain stable up to 300 °C. However, a relevant drawback correlated to chrome electrodeposition is represented by pollution derived from Cr6+ ions. Because of that, the technological development of these processes underwent a sharp slowdown since '90. Only with the advent of new studies about Cr3+ baths, since the beginning of 2000's, the electrodeposition processes have found new interest in mass production of components for thermal solar plants. To obtain a good coating by black chrome, a preliminary deposition of a nickel layer on the substrate is needed to ensure a better chrome adherence to the surface and an improved wear and corrosion resistance. Moreover this creates an ``absorber/reflector tandem'' having both the high solar absorbance of the black exterior deposit and the low thermal emittance of the metallic inner coating. The first step of this study was the investigation of structural features and optical properties of the nickel and copper surfaces, correlating them to coating thickness and deposition process, in the perspective to assess optimal conditions for solar absorber applications. The second step of this study was the investigation of structural features and optical properties of the black chrome absorber taking into account several bath's operational parameters. This black chrome was obtained by a solution of Cr+3. In order to compare the performance obtained by the materials in a working configuration has been paid attention to a parameter that can provide some information: this parameter is the efficiency η that take into account the working temperature and the concentration ratio. Moreover has been done several thermal aging cycle on the materials in order to predict the effect of the aging on the optical properties. The optimal set-up that has been found is: for a galvanic bath composition CrCl3·6H2O 266 g l-1, H2SiF6 10 g l-1, NaH2PO4 4 g l-1 and CoCl2 ·6H2O 15 g l-1; for the operational parameters 20 °C and current density of 60 A~dm-2. With this set-up the best result is a sample with η=0.88: this value is rather similar to the efficiency of the more expensive CERMET (η=0.85-0.93).