Boron-carbon coatings: structure, morphology and mechanical properties

Boron-doped carbon coatings have been produced by a method combining the deposition of a pulsed carbon plasma coating and a boron flow formed as a result of the evaporation of a boron target by pulsed YAG: Nd3+ laser irradiation. Phase, chemical composition, structure, and mechanical properties of composite boron-carbon coatings have been determined. Changes in the coatings’ roughness depending on the boron concentration have been established using atomic force microscopy. It has been shown that the grain size is on the rise with increasing boron concentration. Raman spectroscopy has revealed that at a boron concentration of 43.2 at. %. There is a sharp increase in the ID/IG ratio, which indicates the carbon component’s graphitization. Low ID/IG ratios are observed in the coating at low boron concentrations (no more than 17.4 at. %), suggesting a high content of carbon atoms with sp3 bond hybridization. The coating studies, carried out by X-ray photoelectron microscopy, showed that boron could be in a free state or in the form of carbide or oxide depending on the concentration in the coating. In this case, with an increase in boron concentration, there is a decrease in the concentration of carbon atoms in the state with sp3 bond hybridization, accompanied by an increase in the number of B-C bonds and a reduction in the boron concentration not associated with carbon and oxygen. These coating and chemical composition features determine the boron concentration’s established non-monotonic nature on their microhardness, elastic and mechanical properties

산소 작용기 제어를 통한 그래핀 옥사이드의 밴드 갭 조절

학위논문 (석사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2019. 2. 박종래.Graphene is a two-dimensional nanomaterial with carbon atoms arranged in a hexagonal lattice. Due to its excellent physical, optical and electrical properties, various applications have been proposed. Particularly, when graphene is applied to a transistor device, it becomes possible to integrate more transistors on a circuit than a silicon semiconductor. However, graphene is semimetal with band gap close to zero, which make it not suitable for realizing a logic circuit. Therefore, studies for increasing the band gap of graphene have been actively conducted in various fields. Among them, the method of utilizing the oxygen functional group of graphene oxide, which is an intermediate stage of graphene synthesis, is excellent in commercial value in that the process is simple and mass production is possible. In addition, analogous effects can be obtained without introducing hetero elements, and the hydrophilicity of the graphene is enhanced due to the oxygen functional group, which enables various applications. However, experimental data on how various oxygen functional groups affect the band structure of each graphene are insufficient, and the application of graphene oxide to the transistor material is very limited. Therefore, in order to overcome these limitations, this study investigate the specific relationship between the oxygen functional group of graphene and the electrical properties including band gap. For the purpose, different reduction reactions were applied to graphene oxide, and the composition of the functional groups were adjusted by controlling degree of reduction. The changed band gaps were measured to analyze the quantitative relationship with the functional groups. Analysis of the relationship between the material properties and electrical properties of graphene showed that single and double bonds of carbon and oxygen have significantly different effects on the band gap. Also, the band gap tuning effect of the hydroxyl group was figured out to be tenuous. Based on the analytical results, we suggested the synthesis direction for synthesizing graphene with specific performance. The results of this experiment can be expected to improve the usability of graphene oxide in electronic devices.그래핀은 탄소 원자가 육방 격자로 배열된 2차원 나노물질이며, 뛰어난 물리적, 광학적, 전기적 성질로 인해 다양한 응용이 제시되어 왔다. 특히 그래핀을 트랜지스터 소자에 적용하면 실리콘 반도체 이상의 집적화가 가능하게 된다. 그러나 그래핀은 밴드 갭이 0에 가까운 반금속으로, 논리 회로를 구현하는 데에는 적절하지 않다. 따라서 그래핀의 밴드 갭을 증가시키기 위한 연구가 다방면에서 활발하게 진행되어 왔다. 그 중에서도 흑연을 산화시켜 박리한 형태인 그래핀 옥사이드의 산소 작용기를 활용하는 방법은, 공정이 경제적이고 대량 생산이 가능하다는 점에서 상업적 가치가 뛰어나다. 더불어 이종 원소를 도입하지 않고도 유사한 효과를 이끌어낼 수 있고, 산소 작용기로 인해 그래핀의 친수성이 향상되어 다양한 응용이 가능하다. 그런데 다양한 종류의 산소 작용기들이 각각 그래핀의 밴드구조에 어떻게 영향을 미치는지에 대한 실험적인 자료가 미비하고, 그로 인해 그래핀 옥사이드의 트랜지스터 재료에의 적용이 매우 제한적이다. 따라서 본 연구에서는 이러한 한계점을 극복하기 위해서, 그래핀의 산소 작용기와 밴드 갭을 비롯한 전기적 성질과의 실험적 관계를 연구하였다. 이를 위해 그래핀 옥사이드에 서로 다른 종류의 환원 반응을 적용하였고, 환원 정도를 조절하여 작용기의 조성을 제어하였다. 그리고 그로써 변화한 밴드 갭을 측정하여 작용기와의 정량적 관계를 분석하였다. 이와 같이 그래핀의 재료적 성질과 전기적 성질의 관계를 분석한 결과, 그래핀 원자와 산소 원자의 서로 다른 결합이 밴드 갭에 미치는 영향이 상당히 다름을 보였다. 또한 수산기의 밴드 갭 조율 효과는 매우 미비한 것으로 나타났다. 이러한 분석 결과를 바탕으로 특정 성능을 갖는 그래핀을 합성하기 위한 합성 방향성을 제시할 수 있었다. 본 연구를 통해 전자 소자에서 그래핀 옥사이드의 활용성이 향상될 것을 기대할 수 있을 것이다.1. Introduction 1 1.1. Introduction to Graphene as Transistor Material 1 1.1.1. Properties of Graphene 1 1.1.2. Advantages of Graphene as Transistor Material 3 1.1.3. Issues on Graphene TransistorBand Gap Opening 5 1.1.4. State-of-the-Arts of Band Gap Opening in Graphene 6 1.1.5. Limitations of State-of-the-arts 8 1.2. The strategy and aim 10 1.2.1. Control of Degree of Reduction and Functionality 10 1.2.2. Identification of Relationship between Performance and Chemistry of Reduced Graphene Oxide 11 1.2.3. The Goal of the Study 11 2. Experimental 12 2.1. Preparation of Highly Oxidized Graphene 12 2.1.1. Materials 12 2.1.2. Synthesis of Graphene Oxide: Modified Hummers Method 12 2.2. Reduction of Graphene Oxide 13 2.2.1. Sodium Borohydride (NaBH4) Reduction 13 2.2.2. Hydroiodic Acid (HI) Reduction 14 2.2.3. Thermal Reduction 15 2.3. Characterization of Reduced Graphene Oxide 15 2.3.1. Material Properties Measurement 15 2.3.2. Electrical Properties Measurement 16 3. Results and Discussion 17 3.1. Chemical Characterization of rGO 17 3.1.1. Variation of Oxygen-containing Functionality with ReductionQualitative Analysis 17 3.1.2. Variation of Oxygen-containing Functionality with ReductionQuantitative Analysis 20 3.1.3. Defect Level Comparison through Raman Spectroscopy of Reduced Graphene Oxide 25 3.2. Electronical Characterization of rGO 27 3.2.1. Band Gap Changes of RGO with Oxygen-containing Functionality 27 3.2.2. Conductivity of Reduced Graphene Oxide Film 28 3.3. Empirical Analysis on Relation between Oxygen Functionality and Band Gap of Graphene 33 3.3.1. Simplified Model for Determining the Effect of oxygen Functionality 33 3.3.2. Band Gap Variation According to sp2 Conjugation Domain 37 3.3.3. Electronical Effects of Individual Functionalities on Band Gap 38 3.4. Increased Efficacy of Reduced Graphene oxide and Future Research 42 3.4.1. Directional Suggestion to Synthesize Graphene of Desired Specification 42 3.4.2. Future Research Direction 43 4. Conclusion 45 Bibliography 46 국문 초록 50Maste

Hydrogen Isotope Transport and Separation via Layered and Two-Dimensional Materials

The enrichment of heavy hydrogen isotopes (deuterium, tritium) is required to fulfill their increasing application demands, e.g., in isotope related tracing, cancer therapy and nuclear reaction plants. However, their exceedingly low natural abundance and the close resemblance of physiochemical properties to protium render them extremely difficult to be separated. In this thesis, we investigate hydrogen isotope transport and separation via layered and two-dimensional materials. Three different theoretical challenges are undertaken in this work: (1) identification of the transported hydrogen species (proton H+ or protium H atom) inside interstitial space of layered materials (hexagonal boron nitride, molybdenum disulfide and graphite) and elucidation of their transport mechanism; (2) separation of hydron (proton H+, deuteron D+, and triton T+) isotopes through vacancy-free graphene and hexagonal boron nitride monolayers; (3) capture of the extremely rare light helium isotope (3He) with atomically thin two-dimensional materials. In the case of hydrogen transport, the essential challenges are investigation of its mechanism as well as the identification of transported particles. Regarding the case of hydron isotope separation, the essential questions are whether or not pristine graphene is permeable to the isotopes, and how quantum tunneling and topological Stone-Wales 55-77 defects affect their permeation and separation through graphene. In the last case, to capture the light helium isotope, quantum tunneling, which favors the lighter particles, is utilized to harvest 3He using graphdiyne monolayer. Our results provide novel theoretical insights into hydrogen particle transport inside the interstitial space of van der Waals materials; they uncover the mechanism of hydron isotope separation through 2D graphene and hexagonal boron nitride monolayers; and they predict the influence of pure quantum tunneling on the enrichment of 3He through graphdiyne membrane

Universal quench dynamics of interacting quantum impurity systems

The equilibrium physics of quantum impurities frequently involves a universal crossover from weak to strong reservoir-impurity coupling, characterized by single-parameter scaling and an energy scale $T_K$ (Kondo temperature) that breaks scale invariance. For the non-interacting resonant level model, the non-equilibrium time evolution of the Loschmidt echo after a local quantum quench was recently computed explicitely [R. Vasseur, K. Trinh, S. Haas, and H. Saleur, Phys. Rev. Lett. 110, 240601 (2013)]. It shows single-parameter scaling with variable $T_K t$. Here, we scrutinize whether similar universal dynamics can be observed in various interacting quantum impurity systems. Using density matrix and functional renormalization group approaches, we analyze the time evolution resulting from abruptly coupling two non-interacting Fermi or interacting Luttinger liquid leads via a quantum dot or a direct link. We also consider the case of a single Luttinger liquid lead suddenly coupled to a quantum dot. We investigate whether the field theory predictions for the universal scaling as well as for the large time behavior successfully describe the time evolution of the Loschmidt echo and the entanglement entropy of microscopic models.Comment: 14 pages, 10 figure

Hybridization effect on mechanical properties of composite laminates

Composite materials are increasingly being used in the aeronautical industry. Although carbon fibers are the strongest and most used in the aeronautical sector, these fibers collapse quite suddenly due to their fragile nature leading to catastrophic damage. In order to minimize this effect, an alternative technique is used, which consists of combining these fibers with another less fragile type, such as Kevlar fiber, in order to obtain a material with a more ductile behavior. As the viscoelastic behavior is not much discussed in the available open literature, this work intends to study this mechanical property in several hybrid composites involving carbon, kevlar and glass fibers. For a better understanding of this phenomenon, the static behavior and tenacity of these materials were also studied. For this purpose, the effect of hybridization on flexural properties, interlaminar shear strength, creep and stress relaxation was studied in eighteen hybrid combinations combined with an epoxy matrix. It was observed that hybridization can create a more tenacious and balanced composite. The stacking sequence has a significant influence on the mechanical properties of laminates. As such, for all mechanical tests, carbon fibers are better in compression if hybridized with kevlar and better in tension if hybridized with glass. Glass fibers have always performed better under compression and kevlar fibers always perform better under tension, regardless of which other fiber they are hybridized to. With these positions in the laminate, the composites achieve greater tension and stiffness, but less deformation, greater interlaminar shear strength, less creep and less stress relaxation. As for the number of fiber layers, in the bending properties, a lower percentage of kevlar in the laminate results in higher bending stress and interlaminar shear strength. However, for the viscoelastic behavior of hybrid composites, the number of layers has no direct influence on the creep and stress relaxation values, since molecular rearrangements occur. In addition, a study of the bending properties for different strain rates in carbon fiber composites and fiberglass composites was carried out. In this way, it could be shown that there is a relationship between the strain rate and the flexural stress and stiffness of the composites. As the strain rate increases, there is an increase in bending stress and stiffness.Os materiais compósitos estão cada vez mais a ser utilizados na indústria aeronáutica. Apesar das fibras de carbono serem as mais resistentes e as mais usadas no setor aeronáutico, estas fibras colapsam de maneira bastante repentina devido a sua natureza frágil levando a danos catastróficos. Com o intuito de minimizar este efeito utiliza­se uma técnica alternativa que consiste em combinar estas fibras com outro tipo menos frágil, como por exemplo a fibra de kevlar, de modo a obterem material com um comportamento mais dúctil. Como o comportamento viscoelástico não é muito abordado na literatura aberta disponível, este trabalho pretende então estudar esta propriedade mecânica em vários compósitos híbridos envolvendo fibras de carbono, kevlar e vidro. Para o melhor entendimento deste fenómeno estudou­se igualmente o comportamento estático e tenacidade destes materiais. Para este propósito, o efeito da hibridização nas propriedades de flexão, resistência ao cisalhamento interlaminar, fluência e relaxamento de tensões foi estudado em dezoito combinações hibridas combinadas com uma matriz epoxídica. Observou­se que a hibridização pode criar um compósito mais tenaz e balanceado. A sequência de empilhamento tem uma influência significativa nas propriedades mecânicas dos laminados. Como tal, para todos os testes mecânicos, as fibras de carbono são melhores na compressão se hibridizadas com kevlar e melhores em tensão se hibridizadas com vidro. As fibras de vidro sempre apresentaram melhores resultados sob compressão e as fibras de kevlar sempre apresentam melhores resultados sob tensão, independentemente da outra fibra com a qual são hibridizadas. Com essas posições no laminado, os compósitos alcançam maior tensão e rigidez, mas menor deformação, maior resistência ao cisalhamento interlaminar, menor fluência e menor relaxamento de tensão. Quanto ao número de camadas de fibras, nas propriedades de flexão, uma menor percentagem de kevlar no laminado resulta em maior tensão de flexão e resistência ao cisalhamento interlaminar. Porém, para o comportamento viscoelástico dos compósitos híbridos, o número de camadas não tem influência direta nos valores de fluência e relaxamento de tensão, uma vez que ocorrem rearranjos moleculares. Além disso, foi feito um estudo das propriedades de flexão para diferentes taxas de deformação em compósitos de fibra de carbono e compósitos de fibra de vidro. Desta forma, pôde­se mostrar que existe uma relação entre a taxa de deformação e a tensão de flexão e rigidez dos compósitos. Com o aumento da taxa de deformação, ocorre um aumento da tensão de flexão e da rigidez

Selection of the spraying technologies for over-coating of metal-stampings with thermo-plastics for use in direct-adhesion polymer metal hybrid load-bearing components

The suitability of various polymer-powder spraying technologies for coating of metal-stampings used in polymer metal hybrid (PMH) load-bearing automotive-component applications is considered. The suitability of the spraying technologies is assessed with respect to a need for metal-stamping surface preparation/treatment, their ability to deposit the polymeric material without significant material degradation, the ability to selectively overcoat the metal-stamping, the resulting magnitude of the polymer-to-metal adhesion strength, durability of the polymer/metal bond with respect to prolonged exposure to high-temperature/high-humidity and mechanical/thermal fatigue service conditions, and compatibility with the automotive body-in-white (BIW) manufacturing process chain. The analysis revealed that while each of the spraying technologies has some limitations, the cold-gas dynamic-spray process appears to be the leading candidate technology for the indicated applications

In-situ monitoring of transport properties of graphene during plasma functionalization and it\u27s applications in energy storage.

Monolayer Graphene synthesized by chemical vapor deposition was subjected to controlled and sequential hydrogenation using RF plasma while monitoring its electrical properties in-situ. Low temperature transport properties were measured for each sample and correlated with ex-situ Raman scattering and X-ray photoemission characteristics. The dominant transport mechanism for weak hydrogenation was found to be electron diffusion and low temperature transport for strong hydrogenation is governed by variable range hopping. This investigation of transport properties of hydrogenated graphene supports to develop the universal scaling functions for metallic and insulating graphene by identifying the critical hydrogen concentration. A clear transition from Weak to Strong localization was identified by the pronounced negative magneto resistance. Variable temperature measurements done on sequentially fluorinated graphene too demonstrated a transition from metallic to semiconductor behavior. The temperature dependence of resistance supports the emergence of a bandgap in the fluorinated graphene films. Controllably fluorinated carbon nanotubes showed promise for high capacity primary and secondary battery performances

COMPUTATIONAL ANALYSIS OF FEASIBILITY AND UTILITY OF DIRECT-ADHESION POLYMER-TO-METAL HYBRID TECHNOLOGIES FOR USE IN LOAD BEARING BODY-IN-WHITE AUTOMOTIVE COMPONENTS

Traditionally, metals and plastics are fierce competitors in many automotive engineering applications. This paradigm is gradually being abolished as the polymer-metal-hybrid (PMH) technologies, developed over the last decade, are finding ways to take full advantage of the two classes of materials by combining them into a singular component/sub-assembly. By employing one of the several patented PMH technologies, automotive original equipment manufacturers (OEMs) have succeeded in engaging flexible assembly strategies, decreasing capital expenditures and reducing labor required for vehicle manufacture. The basic concept utilized in all PMH technologies is based on the fact that while an open-channel thin-wall sheet-metal component can readily buckle under compressive load, with very little lateral support, provided by a thin-wall rib-like injection-molded plastic subcomponent, the buckling resistance (and the stiffness) of the component can be greatly increased (while the accompanied weight increase is relatively small). In the present work, the potential of direct-adhesion PMH technologies for use in load-bearing structural automotive components is explored computationally. Within the direct adhesion PMH technology, load transfer between stamped sheet-metal and injection-molded rib-like plastic subcomponent is accomplished through a variety of nanometer-to-micron scale chemical and mechanical phenomena which enable direct adhesion between the two materials. Multi-disciplinary computations are carried out ranging from: (a) computational investigation of the sheet-metal stamping process including determination of the residual stresses and the extent of stamped-component warping; (b) computational fluid mechanics of the filling, packing and cooling stages of the injection-molding process including determination of flow-induced fiber orientation in the molded plastic and the extent of residual stresses and warping in the injection-molded sub-component: and (c) structural-mechanics computational investigation of the effect of injection-molded component residual stresses and warping on their ability to withstand thermal loading encountered in the paint shop and mechanical in-service loading. The results obtained revealed that a minimal level of the polymer-to-metal adhesion strength (5-10MPa) must be attained in order for the direct-adhesion PMH technologies to be a viable alternative in the load-bearing body-in-white (BIW) components. In the present work, also various PMH approaches used to promote direct (adhesive-free) adhesion between metal and injection-molded thermoplastics are reviewed and critiqued. The approaches are categorized as: (a) micro-scale polymer-to-metal mechanical interlocking; (b) in-coil or stamped-part pre-coating for enhanced adhesion; and (c) chemical modifications of the injection-molded thermoplastics for enhanced polymer-to-metal adhesion. For each of these approaches their suitability for use in load-bearing BIW components is discussed. In particular, the compatibility of these approaches with the BIW manufacturing process chain (i.e. (pre-coated) metal component stamping, BIW construction via different joining technologies, BIW pre-treated and painting operations) is presented. It has been found that while considerable amount of research has been done in the PMH direct-adhesion area, many aspects of these technologies which are critical from the standpoint of their use in the BIW structural applications have not been addressed (or addressed properly). Among the PMH technologies identified, the one based on micro-scale mechanical interlocking between the injection-molded thermoplastic polymer and stamped-metal structural component was found to be most promising. Lastly, the suitability and the potential of various polymer-powder spraying technologies for coating metal stampings and, thus, for enhancing the polymer-to-metal adhesion strength in direct-adhesion PMH load-bearing automotive-component applications is considered. The suitability of the spraying technologies is assessed with respect to a need for metal-stamping surface preparation/treatment, their ability to deposit the polymeric material without significant material degradation, the ability to selectively overcoat the metal-stamping, the resulting magnitude of the polymer-to-metal adhesion strength, durability of the polymer/metal bond with respect to prolonged exposure to high-temperature/high-humidity and mechanical/thermal fatigue service conditions, and compatibility with the automotive BIW manufacturing process chain. The analysis revealed that while each of the spraying technologies has some limitations, the cold-gas dynamic-spray process appears to be the leading candidate technology for the indicated applications