1,721,007 research outputs found
Ab initio characterization of Hafnium oxides HfO2 and related derivatives as emerging nanomaterials for electronics applications
No full textQuesta tesi di dottorato mira a studiare, attraverso simulazioni atomistiche, materiali che possono fungere da efficienti raccoglitori di energia. In un'epoca in cui la domanda di energia e la necessità di fonti di energia sostenibili sono sempre più fondamentali, le tecniche di raccolta di energia possono offrire un modo efficiente per utilizzare forme di energia già disponibili nell'ambiente che altrimenti andrebbero perse. In letteratura sono descritte molte soluzioni per recuperare energia da radiazione solare ed elettromagnetica, vibrazioni meccaniche o differenziali termici. Tra i materiali che possono essere utilizzati a questo scopo, l'ossido di afnio (HfO2) ha ricevuto notevole attenzione per la sua combinazione unica di costante dielettrica elevata, stabilità termica e compatibilità con la tecnologia metallo-ossido-semiconduttore complementare (CMOS). In questo contesto, vengono esplorati alcuni aspetti relativi a questo materiale, avvalendosi di simulazioni teoriche ab initio; in particolare, HfO2 esiste in natura come diversi polimorfi associati a peculiari stabilità e proprietà. Inoltre, drogando l'HfO2 con diversi elementi, come Zr e Y, e con varie quantità, è possibile comprendere come le proprietà del materiale siano influenzate in termini di celle unitarie, energia di formazione, proprietà ottiche e funzioni dielettriche. Quindi, è stata condotta un'esplorazione teorica di diodi metallo-isolante-metallo (MIM) basati su HfO2 per studiare gli effetti dei polimorfi sulle proprietà del dispositivo e per stimare le curve IV (corrente-tensione). Analogamente, sono stati esplorati altri diodi MIM basati su HfO2 con un approccio di modellazione atomistica seguita da convalida sperimentale. Un dispositivo MIM è stato fabbricato interponendo uno strato dielettrico di HfO2, con uno spessore compreso tra 3 e 4 nm, tra due metalli diversi, Platino e Titanio. La curva IV ottenuta dalle simulazioni ab initio è stata sovrapposta alla curva ottenuta dalla misura sperimentale del diodo fabbricato e i risultati hanno mostrato l'eccellente capacità della metodologia proposta di descrivere accuratamente il comportamento del dispositivo MIM da un punto di vista atomistico. Inoltre, è stato valutato il comportamento piroelettrico in condensatori ferroelettrici basati su HfZrO2. Questa tesi descrive in dettaglio le simulazioni di dinamica molecolare ab initio utilizzate per analizzare le deformazioni della densità elettronica in funzione della temperatura, che determinano una modifica della permittività dielettrica dell'HfZrO2. Questa tesi esplora il potenziale delle simulazioni ab initio per prevedere, investigare ed esaminare le proprietà di una nuova classe di materiali, nonché dispositivi governati da fenomeni chimico-fisici in cui sono coinvolti i materiali stessi.This PhD thesis aims to investigate, through atomistic simulations, materials that can act as efficient energy
harvesters. In an era of increased energy demands and the need for sustainable power sources, energy
harvesting techniques can offer an efficient way to collect energy already available in the environment that
would otherwise be lost. In this context, many solutions have been presented in the literature to recover energy
from solar and electromagnetic radiation, mechanical vibrations, or thermal differentials. Among materials
that can be used for this purpose, hafnium oxide (HfO2) has received significant attention for its unique
combination of high dielectric constant, thermal stability, and compatibility with complementary metal-oxide-
semiconductor (CMOS) technology. In this context, a few aspects related to this material are explored, making
use of ab initio theoretical simulations; in particular, HfO2 exists in nature as different polymorphs associated
with peculiar stability and properties. Moreover, by doping the HfO2 with different elements, such as Zr and
Y, and with various amounts, it is possible to understand how the properties of the materials are affected in
terms of unit cells, formation energy, optical properties, and dielectric functions. Then, a first-principle
exploration of HfO2-based metal-insulator-metal (MIM) diodes was carried out to investigate the effects of
polymorphs on the device’s properties and to estimate the IV (Current-Voltage) curves. Similarly, other HfO2-
based MIM diodes have been explored with an atomistic-to-continuum modeling approach followed by
experimental validation. A MIM device has been fabricated by interposing a dielectric layer of HfO2, with a
thickness between 3 and 4 nm, between two different metals, platinum (Pt) and titanium (Ti), which are the
source and drain electrodes, respectively. The IV curve obtained by ab initio simulations has been traced back
to experimental measurements conducted on the fabricated diode, and the results showed the excellent
capability of the proposed methodology to accurately describe the behavior of a MIM device from an atomistic
point of view. Additionally, the pyroelectricity behavior in ferroelectric capacitors based on HfZrO2 has been
evaluated to test the harvesting capability of this material. This thesis details the ab initio molecular dynamics
simulations used to analyze the deformations of the electron density as a function of temperature, which results
in a change in the dielectric permittivity of the HfZrO2.
This thesis explores the potential of ab initio simulations to predict, investigate, and examine the properties of
a novel class of materials, as well as devices governed by chemical-physical phenomena in which the same
materials are involved
Structural study on methacrylamide-grafted Tussah silk fibroin fibres
No full textTussah silk fibroin fibres were modified by grafting with methacrylamide (MAA), with weight gains ranging between 2.6% and 71.4%. Raman and IR spectroscopic analyses showed that upon grafting the fibres underwent slight conformational changes towards a more unordered state, due to the covalent and hydrogen bonds interactions occurring between the polymer (polyMAA) and the amorphous domains of silk fibres. To test the stability towards alkaline hydrolysis, the untreated and MAA-grafted silk fibres (weight gain of 71.4%) were immersed in NaOH 5% at 50 °C for different times; the IR and Raman spectroscopic techniques were utilized to elucidate the degradation mechanism as well as the rearrangements of the fibres induced by the treatment. Upon hydrolysis, both the untreated and grafted fibres underwent an enrichment in β-sheet conformation, due to the preferential removal of the unordered domains. As a result of the covalent interactions with silk fibroin, the polymer increased its stability towards alkaline hydrolysis, since its complete solubilization was avoided and the transformation of its CONH2 groups into COO- and COOH was delayed. Vibrational spectroscopy proved to be a valid technique to investigate the mechanism and the effects of the hydrolytic attack, which are both fundamental to design new-generation silk-based materials
Raman and photoemission spectroscopic analyses of explanted Biolox® delta femoral heads showing metal transfer
Full text linkBiolox® delta has been widely used in joint replacements thanks to its high strength and wear resistance. In this study, eleven Biolox® delta femoral head retrievals affected by metal transfer (MT) were analysed by Raman spectroscopy to estimate the tetragonal to monoclinic zirconia phase transformation, whose occurrence may compromise ceramic chemical stability and mechanical strength. The residual stress state was evaluated by both Raman and photoemission spectroscopy. Vmmonoclinic zirconia contents were higher near the centre of the articulating surface and in the MT area than in the border control area of the retrievals. In only one retrieval, stress related to MT appeared a more severe condition, able to induce zirconia phase transformation; for all the others, stresses related to loading in the central region and related to MT, were conducive to a zirconia phase transformation of nearly the same extent. Vmdepth profiling analyses showed that the transformation involved different thicknesses in different samples. Raman data allowed for the investigation of the mechanism of zirconia phase transformation and confirmed that the growth stage was absent and the nucleation stage was not occurring as freely as it would in unconstrained zirconia
Transfer of metallic debris after in vitro ceramic-on-metal simulation: wear and degradation in Biolox® Delta composite femoral heads
No full textThis study was aimed at investigating the effects of the transfer of metallic debris during an in vitro wear test on ceramic femoral heads articulating against metallic acetabular cups. In particular, Biolox® Delta ceramic femoral heads of two different diameters (32 and 36 mm, three samples of each set) were run for five million cycles onto a hip wear simulator using bovine calf serum as lubricant. Wear and degradation of Biolox® Delta Composite femoral heads were evaluated by a gravimetric method and by micro-Raman spectroscopy, which was used to investigate possible phase changes upon the particles deposit, by assessing the monoclinic zirconia content and quantifying the tetragonal → monoclinic zirconia transformation.
Our results showed that after five million cycles, the 32 mm-acetabular cups lost a higher mass than the 36 mm-ones. Metal transfer was observed on all the Biolox® Delta ceramic femoral heads and determined a worsening of all roughness parameters. The micro-Raman analyses of the in vitro tested femoral heads confirmed the results previously obtained on retrievals, i.e. the stress related to metal transfer appeared a particularly severe condition able to induce the tetragonal→monoclinic zirconia phase transformation. The extent of the transformation appeared higher than that observed in vivo for Biolox® Delta-on-Biolox® Delta couplings. On average, the 36 mm-femoral heads seemed more detrimentally affected by metal transfer than the 32 mm-ones; one femoral head belonging to the first set of samples underwent the highest extent of phase transformation and the highest depth involved, which in the worn centre area appeared meanly higher for the 36 mm-components than for the 32 mm-ones
Stability toward alkaline hydrolysis of B. mori silk fibroin grafted with methacrylamide
No full textBombyx mori silk fibroin fibers were grafted with methacrylamide (MAA) and characterized by Raman and infrared (IR) vibrational spectroscopy before and after hydrolysis in NaOH 5% to elucidate the possible interactions between the two components and the stability of the fibers toward alkaline hydrolysis. Upon grafting, the fibers underwent conformational rearrangements toward a more unordered state and lost orientation at weight gains higher than 60%. Vibrational spectroscopy disclosed the occurrence of intermolecular interactions (mainly hydrogen bonds) between B. mori silk fibroin and polyMAA in the grafted fibers, and the formation of covalent bonds has been explored. These strong interactions made the grafted fibers as a whole more stable toward alkaline hydrolysis because they prevented the solubilization of the polymer upon hydrolysis and made slower the transformation of its CONH2 groups into COOH and COO− groups. Upon hydrolysis, silk fibroin underwent an enrichment in the β-sheet crystalline domains, because of the preferential removal of the unordered domains, which were more prone to the OH− attack. IR and Raman spectroscopy proved valid techniques to investigate the degradation mechanism and kinetics of grafted silk fibroin fibers and so for designing high-performing silk-based materials. The A731/A1004 Raman intensity ratio was proposed to spectroscopically evaluate the composition of the grafted samples; its value was found to linearly increase with weight gain (R2 = 0.998), envisaging the possibility of using Raman spectroscopy as a routine analytical technique for qualitative and quantitative characterization of grafted industrial samples. Copyright © 2016 John Wiley & Sons, Ltd
Wear performance of neat and vitamin E blended highly cross-linked PE under severe conditions: The combined effect of accelerated ageing and third body particles during wear test
Full text linkThe objective of this study is to evaluate the effects of third-body particles on the in vitro wear behaviour of three different sets of polyethylene acetabular cups after prolonged testing in a hip simulator and accelerated ageing. Vitamin E-blended, cross-linked polyethylene (XLPE_VE), cross-linked polyethylene (XLPE) and conventional polyethylene (STD_PE) acetabular cups were simulator tested for two million cycles under severe conditions (i.e. by adding third-body particles to the bovine calf serum lubricant). Micro-Fourier Transform Infrared and micro-Raman spectroscopic analyses, differential scanning calorimetry, and crosslink density measurements were used to characterize the samples at a molecular level. The STD_PE cups had twice mass loss than the XLPE_VE components and four times than the XLPE samples; statistically significant differences were found between the mass losses of the three sets of cups. The observed wear trend was justified on the basis of the differences in cross-link density among the samples (XLPE>XLPE_VE>STD_PE). FTIR crystallinity profiles, bulk DSC crystallinity and surface micro-Raman crystallinity seemed to have a similar behaviour upon testing: all of them (as well as the all-trans and ortho-trans contents) revealed the most significant changes in XLPE and XLPE_VE samples. The more severe third-body wear testing conditions determined more noticeable changes in all spectroscopic markers with respect to previous tests. Unexpectedly, traces of bulk oxidation were found in both STD_PE (unirradiated) and XLPE (remelting-stabilized), which were expected to be stable to oxidation; on the contrary, XLPE_VE demonstrated a high oxidative stability in the present, highly demanding conditions
Does cyclic stress and accelerated ageing influence the wear behavior of highly crosslinked polyethylene?
Full text linkFirst-generation (irradiated and remelted or annealed) and second-generation (irradiated and vitamin E blended or doped) highly crosslinked polyethylenes were introduced in the last decade to solve the problems of wear and osteolysis. In this study, the influence of the Vitamin-E addition on crosslinked polyethylene (XLPE_VE) was evaluated by comparing the in vitro wear behavior of crosslinked polyethylene (XLPE) versus Vitamin-E blended polyethylene XLPE and conventional ultra-high molecular weight polyethylene (STD_PE) acetabular cups, after accelerated ageing according to ASTM F2003-02 (70.0±0.1 °C, pure oxygen at 5 bar for 14 days). The test was performed using a hip joint simulator run for two millions cycles, under bovine calf serum as lubricant. Mass loss was found to decrease along the series XLPE_VE>STD_PE>XLPE, although no statistically significant differences were found between the mass losses of the three sets of cups. Micro-Raman spectroscopy was used to investigate at a molecular level the morphology changes induced by wear. The spectroscopic analyses showed that the accelerated ageing determined different wear mechanisms and molecular rearrangements during testing with regards to the changes in both the chain orientation and the distribution of the all-trans sequences within the orthorhombic, amorphous and third phases. The results of the present study showed that the addition of vitamin E was not effective to improve the gravimetric wear of PE after accelerated ageing. However, from a molecular point of view, the XLPE_VE acetabular cups tested after accelerated ageing appeared definitely less damaged than the STD_PE ones and comparable to XLPE samples
Insights into the interfaces of VO2(M) and VO2(B) polymorphs with different substrates
Full text linkThe phenomena arising at the interface between oxide materials and substrates can fundamentally and practically change the physical and chemical properties of the materials themselves. In this study, we employed density functional theory (DFT) calculations to elucidate the stability and optical properties of VO2(M) and VO2(B) interactions with substrates of sapphire(c-cut), sapphire(r-cut), SrTiO3(001), SrTiO3(111), SrNbO3(001), SrNbO3(111), LaAlO3(c-cut), and LaAlO3(r-cut). The surface calculations showed that the fully relaxed SrTiO3(111) and SrNbO3(001) possess the highest energies of 31.8 eV nm−2 and 21.15 eV nm−2 relative to other slabs, before optimizing with VO2(M) and VO2(B) polymorphs, respectively. The calculated adsorption energy indicated that the interface of VO2(M) and VO2(B) structures showed more stability on SrTiO3(001) and SrTiO3(111), with adsorption energy values of 2.83 eV and 0.91 eV, respectively. Furthermore, the optical absorption properties of the highest and lowest stable interfaces have been evaluated. The outcomes predicted that VO2(M)@SrTiO3(001) and VO2(B)@SrTiO3(111) have their optical adsorption in the visible light range, while VO2(M)@sapphire(c-cut) and VO2(B)@sapphire(c-cut) showed the main adsorption peak in the infrared region
Towards graphene-based asymmetric diodes: a density functional tight-binding study
No full textSelf-consistent charge density functional tight-binding (DFTB) calculations have been performed to investigate the electrical properties and transport behavior of asymmetric graphene devices (AGDs). Three different nanodevices constructed of different necks of 8 nm, 6 nm and 4 nm, named Graphene-N8, Graphene-N6 and Graphene-N4, respectively, have been proposed. All devices have been tested under two conditions of zero gate voltage and an applied gate voltage of +20 V using a dielectric medium of 3.9 epsilon interposed between the graphene and the metallic gate. As expected, the results of AGD diodes exhibited strong asymmetric I(V) characteristic curves in good agreement with the available experimental data. Our predictions implied that Graphene-N4 would achieve great asymmetry (A) of 1.40 at |VDS| = 0.2 V with maximum transmittance (T) of 6.72 in the energy range 1.30 eV. More importantly, while the A of Graphene-N4 was slightly changed by applying the gate voltage, Graphene-N6/Graphene-N8 showed a significant effect with their A increased from 1.20/1.03 under no gate voltage (NGV) to 1.30/1.16 under gate voltage (WGV) conditions. Our results open up unprecedented numerical prospects for designing tailored geometric diodes
Strong enhancement of graphene plasmonic emission by quantum Čerenkov effect in confined structures
Full text linkOne notable issue in low terahertz (THz) applications is to achieve sources with higher output power than the state of the art. One possible solution to the foregoing problem is to amplify the electromagnetic field emitted by already accessible THz generators. Here, we study the quantum Cerenkov effect as a possible explanation for low-THz amplification, which has been found experimentally elsewhere. Specifically, the emission of surface plasmons from traveling electrons in mono-dimensional graphene, mediated by charge–field interaction, is shown to provide in-plane electromagnetic radiation down to THz and mm-wave frequencies. We focus on a structure consisting of a graphene layer between metal electrodes, which enhance the field confinement and lead to a linearization of the plasmon dispersion in the frequency domain. When compared to a non-confined plasmonic radiation, the above-mentioned configuration shows emission rates ten times larger, which make it promising for THz amplification
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