碳(Carbon)為IV A族，每顆碳原子擁有四顆能夠鍵結的電子，碳的同素異形體有數種，最常見為石墨、鑽石、富勒烯(C_60)以及石墨烯(Graphene)，每一種同素異形體所表現的物理性質也不同。其中我以石墨烯這種二維材料進行計算，石墨烯(Graphene)是一個非常良好的導體。但在其碳原子上下交互接上氫原子(Hydrogen)可形成石墨烷扶手椅型(Graphane chair)是一個絕緣體。另一類似結構的材料為氮化硼(Boron Nitride，簡稱BN)，利用3A的硼原子(Boron)與5A的氮原子(Nitride)取代石墨烯中的碳原子形成六角形氮化硼。BN的能隙差很大，故也為不導電之絕緣體。 　　其中二維的石墨烯(Graphene)又可依特定方式裁切成一維鋸齒狀石墨烯奈米帶(1D zigzag Graphene nanoribbon)。另外若將石墨烷扶手椅型(Graphane chair)上面的氫原子(Hydrogen)拔除，考慮拔除氫原子的鏈狀(chain)的數目其導電性質與磁性將會發生變化。並將一維氮化硼(BN)連接一維鋸齒狀石墨烷奈米帶(1D zigzag Graphane nanoribbon)，於石墨烷奈米帶邊界接上兩顆氫考慮其結合能，再拔去線狀(line)與鏈狀(chain)氫原子探討其能量大小與能帶性質。 　　最後將一維鋸齒狀石墨烷奈米帶(1D zigzag Graphane nanoribbon)被拔除氫鍊(chain)處加入第一類過度金屬(1st Transition metal)，由於過度金屬擁有3d軌域之角動量，故進一步分析其磁性影響與能帶性質。以上計算皆使用Vienna Ab initio Simulation Package (VASP)計算。

La spectroscopie de perte d’énergie des électrons (EELS) et la cathodoluminescence (CL) dans un microscope électronique en transmission à balayage (STEM) sont des techniques puissantes pour l’étude des nanostructures isolées. Cependant, des électrons rapides peuvent endommager fortement des échantillons minces et fragiles, ce qui limite la résolution spatiale et l’intensité des signaux spectroscopiques. Pendant cette thèse, nous avons dépassé cette restriction par le développement de protocoles d’acquisition spécifiques pour l’étude de certains archétypes de nanosystèmes fragiles. Dans la première partie de cette thèse, nous avons caractérisé des flocons minces de graphène oxydé (GO) et GO réduit (RGO) par EELS dans le STEM. Grâce aux spécificités techniques de notre microscope et à la définition des conditions d’illumination optimales, nous avons dérivé des cartes du contenu d’oxygène dans le (R)GO à une résolution spatiale inédite. Aussi, par l’analyse des pics EELS de structure fine, nous avons révisé les modèles atomiques proposés dans la littérature. Des molécules isolées constituent une autre classe de nanomatériaux fortement sensibles à l’irradiation et aussi à l’environnement chimique et physique. Nous avons conduit des expériences de CL sur des molécules individuelles, grâce à un choix avisé du substrat. Le nitrure de bore hexagonal (h-BN) est un matériaux bidimensionnel chimiquement inerte, qui participe activement au processus de CL en absorbant l’énergie incidente. Le transfert de l’excitation aux molécules et l’utilisation d’une routine innovante d’acquisition par balayage aléatoire ont permis de réduire les effets d’illumination. Ensuite, l’intérêt porté aux propriétés optiques du h-BN ont inspiré l’étude de sa phase cubique (c-BN), qui a été peu caractérisé auparavant à cause d’impuretés dans les cristaux. Nous avons analysé des cristaux de c-BN de haute pureté par EELS, en identifiant une bande interdite d’énergie plus grande que précédemment rapportée et plus proche des calculs les plus récents. Dans des cristaux moins purs, nous avons identifié et analysé plusieurs émissions associées à des défauts, en termes d’énergie caractéristique, distribution spatiale et temps de vie, par CL et interférométrie en intensité de Hanbury-Brown et Twiss. / Electron energy loss spectroscopy (EELS) and cathodoluminescence(CL) in a scanning transmission electron microscope (STEM) are extremely powerful techniques for the study of individual nanostructures. Nevertheless, fast electrons damage extremely sensitive thin specimens, imposing strong limitations on the spatial resolution and the intensity of spectroscopic measurements. During this thesis we have overcome this restriction by developing material-specific acquisition protocols for the study of some archetypical fragile nanosystems. In the first part of this thesis we have characterized graphene oxide (GO) and reduced graphene oxide (RGO) thin flakes by EELS spectroscopy in the STEM. Thanks to the particular setup of our microscope and by experimentally defining the optimal illumination conditions, we have derived oxygen quantification maps of (R)GO at an unprecedented spatial resolution. On the basis also of EELS fine structures analysis, we have revised the existing proposed atomic models for these materials. Another class of exceedingly sensitive nanometric systems is represented by individual molecules, which are strongly affected by both illumination and chemical/physical environment. We have performed the first CL-STEM investigation on the luminescence of isolated molecules, thanks to a watchful choice of the substrate. Hexagonal boron nitride (h-BN) is a flat, chemically inert 2D material, that actively takes part in the CL process by absorbing the incident energy. Excitation transfer from h-BN to molecules and the use of an innovative random scan acquisition routine in the STEM have allowed to considerably lower illumination effects and improve CL intensity. Afterwards, the attractive optical properties of h-BN have led to the study of its cubic phase (c-BN), which has been up to now hindered by the poor quality of the crystals. By EELS in the STEM we have analysed c-BN crystals of the highest available purity, identifying a wider optical band-gap with respect to previous experimental studies and in better agreement with recent calculations. In commercial crystals, several defect-related emissions have been identified and analysed in terms of characteristic energy, spatial distribution and lifetime using CL and Hanbury-Brown and Twiss intensity interferometry.

Diploma thesis on Machining hardened steels is focused on the analysis of longitudinal turning of hardened steel 14 109 by using a tool from PKNB in terms of measuring the force load using a dynamometer and then evaluating the surface quality. The theoretical part deals with the issue of turning technology, cutting materials and heat treatment of steel. In the practical part, the influence of cutting conditions on the resulting values was assessed during the experiment. Twelve samples with different combinations of cutting conditions were tested, then was selected a sample that met the criterion of combining minimum cutting forces values and surface quality. The experiment shows that force load values can be achieved twice less by combining cutting conditions with an appropriate combination.

Strength and strain analysis of high pressure compound vessel, which is used to produce superhard materials (for example synthetic diamond). This work was elaborated to compare the stresses and strains calculated by Prof. Jan Vrbka making use of the FEM program ‘Prokop’17 years ago with those gained with the contemporary FEM Ansys program. The vessel is loaded by internal pressure of size 6 GPa. The elastic-plastic material be-haviour is taken into account. Real value of friction between rings and non-uniform temperature field is included into the calculation. The process of assembling the compound vessel is simulated step by step.

Diploma thesis is focused on the superhard cutting materials (polycrystalline diamond, polycrystalline boron nitride) and presents their physico-mechanical properties, production, efficient use, new trends. It assesses the range of cutting tool materials and individual front world producers in terms of optimum cutting conditions for turning operations and type of material to be machined. Further, these cutting materials are compared between different manufacturers to bring out thein differences in cutting conditions.

During neural activity, action potentials travel down axons, generating effective charge current pulses, which are central in neuron-to-neuron communication. Consequently, said current pulses generate associated magnetic fields with amplitudes on the order of picotesla (pT) and femtotesla (fT) and durations of 10’s of ms. Magnetoencephalography (MEG) is a technique used to measure the cortical magnetic fields associated with neural activity. MEG limitations include the inability to detect signals from deeper regions of the brain, the need to house the equipment in special magnetically shielded rooms to cancel out environmental noise, and the use of superconducting magnets, requiring cryogenic temperatures, bringing opportunities for new magnetic sensors to overcome these limitations and to further advance neuroscience. An extraordinary magnetoresistance (EMR) tunable graphene magnetometer could potentially achieve this goal. Its advantages are linear response at room temperature (RT), sensitivity enhancement owing to combination of geometric and Hall effects, microscale size to place the sensor closer to the source or macroscale size for large source area, and noise and sensitivity tailoring. The magnetic sensitivity of EMR sensors is, among others, strongly dependent on the charge mobility of the sensing graphene layer. Mechanisms affecting the carrier mobility in graphene monolayers include interactions between the substrate and graphene, such as electron-phonon scattering, charge impurities, and surface roughness. The present work reviews and proposes a material set for increasing graphene mobility, thus providing a pathway towards pT and fT detection. The successful fabrication of large-size magnetic sensors employing CVD graphene is described, as well as the fabrication of trilayer magnetic sensors employing mechanical exfoliation of h-BN and graphene. The magneto-transport response of CVD graphene Hall bar and EMR magnetic sensors is compared to that obtained in equivalent trilayer devices. The sensor response characteristics are reported, and a determination is provided for key performance parameters such as current and voltage sensitivity and magnetic resolution. These parameters crucially depend on the material's intrinsic properties. The Hall cross magnetic sensor here reported has a magnetic sensitivity of ~ 600 nanotesla (nT). We find that the attained sensitivity of the devices here reported is limited by contaminants on the graphene surface, which negatively impact carrier mobility and carrier density, and by high contact resistance of ~2.7 kΩ µm at the metallic contacts. Reducing the contact resistance to < 150 Ω µm and eliminating surface contamination, as discussed in this work, paves the way towards pT and ultimately fT sensitivity using these novel magnetic sensors. Finite element modeling (FEM) is used to simulate the sensor response, which agrees with experimental data with an error of less than 3%. This enables the prediction and optimization of the magnetic sensor performance as a function of material parameters and fabrication changes. Predictive studies indicate that an EMR magnetic sensor could attain a sensitivity of 1.9 nT/√Hz employing graphene with carrier mobilities of 180,000 cm²/Vs, carrier densities of 1.3×10¹¹ cm^-2 and a device contact resistance of 150 Ω µm. This sensitivity increments to 443 pT/√Hz if the mobility is 245,000 cm²/Vs, carrier density is 1.6×10¹⁰ cm^-2, and a lower contact resistance of 30 Ω µm. Such devices could readily be deployed in wearable devices to detect biomagnetic signals originating from the human heart and skeletal muscles and for developing advanced human-machine interfaces.

Aerogels with extraordinary mechanical properties attract a lot of interest for their wide spread applications. However, the required flexibility is yet to be satisfied, especially under extreme conditions. Herein, a boron nitride nanoribbon aerogel with excellent temperature‐invariant super‐flexibility is developed by high temperature amination of a melamine diborate precursor formed by hydrogen bonding assembly. The unique structure of the aerogel provides it with outstanding compressing/bending/twisting elasticity, cutting resistance, and recoverable properties. Furthermore, the excellent mechanical super‐flexibility is maintained over a wide temperature range, from liquid nitrogen temperature (−196 °C) to higher than 1000 °C, which extends their possible applications to harsh environments.

No description available.

Research efforts have been focused in the development of hard and wear resistant coatings over the last few decades. These protective coatings find applications in the industry such as cutting tools, automobile and machine part etc. Various ceramic thin films like TiN, TiAlN, TiC, SiC and diamond-like carbon (DLC) are examples of the films used in above applications. However, increasing technological and industrial demands request thin films with more complicated and advanced properties. For this purpose, B-C-N ternary system which is based on carbon, boron and nitrogen which exhibit exceptional properties and attract much attention from mechanical, optical and electronic perspectives. Also, boron carbonitride (BCN) thin films contains interesting phases such as diamond, cubic BN (c-BN), hexagonal boron nitride (h-BN), B4C, β-C3N4. Attempts have been made to form a material with semiconducting properties between the semi metallic graphite and the insulating h-BN, or to combine the cubic phases of diamond and c-BN (BC2N heterodiamond) in order to merge the higher hardness of the diamond with the advantages of c-BN, in particular with its better chemical resistance to iron and oxygen at elevated temperatures. New microprocessor CMOS technologies require interlayer dielectric materials with lower dielectric constant than those used in current technologies to meet RC delay goals and to minimize cross-talk. Silicon oxide or fluorinated silicon oxide (SiOF) materials having dielectric constant in the range of 3.6 to 4 have been used for many technology nodes. In order to meet the aggressive RC delay goals, new technologies require dielectric materials with K

The development of metals that can overcome the strength-ductility-weight trade-off has been an ongoing challenge in engineering for many decades. A promising option for making such materials are Metal matrix composites (MMCs). MMCs contain dispersions of reinforcement in the form of fibers, particles, or platelets that significantly improve their thermal, electrical, or mechanical performance. This dissertation focuses on reinforcement with two-dimensional (2D) materials due to their unprecedented mechanical properties. For instance, compared to steel, the most well-studied 2D material, graphene, is nearly forty times stronger (130 GPa) and five times stiffer (1 TPa). Examples of reinforcement by graphene have achieved increases in strength of 60% due to load transfer at the metal/graphene interface and dislocation blocking by the graphene. However, the superior mechanical properties of graphene are not fully transferred to the matrix in conventional MMCs, a phenomenon known as the "valley of death." In an effort to develop key insight into how the relationships between composite design, processing, structure, properties, and mechanics can be used to more effectively transfer the intrinsic mechanical properties of reinforcements to bulk composite materials, nanolayered composite systems made of Ni, Cu, and NiTi reinforced with graphene or 2D hexagonal boron nitride h-BN is studied using experimental techniques and molecular dynamics (MD) simulations. / Doctor of Philosophy / The design of new metals with concurrently improved strength and ductility has been an enduring goal in engineering for many decades. The utilization of components made with these new materials would reduce the weight of structures without sacrificing their performance. Such materials have the potential to revolutionize many industries, from electronics to aerospace. Traditional methods of improving the properties of metals by thermomechanical processing have approached a point where only minor performance improvements can be achieved. The development of Metal matrix composites (MMCs) is among the best approaches to achieving the strength-ductility goal. Metal matrix composites are a class of materials containing reinforcements of dissimilar materials that significantly improve their thermal conductivity, electrical conductivity, or mechanical performance. Reinforcements are typically in the form of dispersed fibers, particles, or platelets. The ideal reinforcement materials have superior mechanical properties compared to the metal matrix, a high surface area, and a strong interfacial bond with the matrix. Two-dimensional (2D) materials (materials made up of a single to a few layers of ordered atoms) are attractive for reinforcement in composite materials because they possess unprecedented intrinsic properties. The most well-studied 2D material, graphene, is made of a single layer of carbon atoms arranged in a hexagonal honeycomb pattern. It is nearly forty times stronger (130 GPa) and five times stiffer (1 TPa) than steel. Examples of graphene reinforcing have shown increases in strength of 60% due to load transfer at the metal/graphene interface and dislocation blocking by the graphene. Despite their exceptional mechanical properties, the superior mechanical properties of graphene are not fully transferred to the matrix when incorporated into conventional metal matrix composites. This phenomenon, known as the "valley of death," refers to the loss of mechanical performance at different length scales. One cause of this phenomenon is the difficulty of evenly dispersing the reinforcements in the matrix using traditional fabrication techniques. Another is the presence of dislocations in the metal matrix, which cause very large local lattice strains in the graphene. This atomistic-scale deformation at the interface between the metal and the graphene can significantly weaken it, leading to failure at low strains before reaching its intrinsic failure stress and strain. This dissertation aims to provide insight into how the relationships between composites' design, processing, structure, properties, and mechanics can be used to transfer intrinsic mechanical properties of reinforcements to bulk composite materials more effectively. For this, nanolayered composite systems of Ni and Cu reinforced with graphene or 2D h-BN were studied using experimental techniques and molecular dynamics (MD) simulations to elucidate the underlying mechanisms behind the composites' material structure and mechanical behavior. Additionally, we explore the incorporation of graphene in a metallic matrix that does not deform through dislocations (or shear bands), such as the shape memory alloy nickel-titanium ( Nitinol or NiTi), to avoid low strain failure of the metal/graphene interface. This theoretical strengthening mechanism is investigated by designing and fabricating NiTi/graphene composites.