包含少於1000 個原子的金屬納米顆粒會與此金屬的常規尺寸材料有很大差別。它們擁有一些特殊的物理和化學的性質，因而在過去的二十年中吸引了大量的研究工作。但是由於尺寸太小，對於這些納米結構的深入和全面的研究十分困難。掃描隧道顯微鏡方法是一種強大的研究此類問題的技術。它擁有的極強的空間分辨力和原子操縱的能力使得我們可以獲得一些納米顆粒的細緻信息。在這篇論文中，我將報道我的關於在半導體Si(111)-(7x7)的重構表面的貴金屬（銀/金）顆粒的研究。 / 我首先研究了在Si(111)-(7x7)上的金屬顆粒的製備方法。使用熱蒸發方法，我製備了在Si(111)-(7x7)上的金屬顆粒。在研究了這些金屬顆粒在半元胞中的的整體生長過程之後，我的研究集中在本論文的主要目標--獨立的銀納米顆粒上。通過仔細觀察記錄單個銀原子的擴散過程，我們確認了Ag3‐Ag13 顆粒中的原子數目。這些掃描隧道顯微鏡的圖像同時也指出了這些顆粒的動態性質，包括自發跳動、分解和與鄰近的單個Ag 原子的合成。為了獲得對這些Ag 顆粒的控制，我們找到了一種垂直操縱單個Ag 原子的方法。在樣品的表面，掃描隧道顯微鏡的針尖通過特定的程序被用來抓起和放下單個Ag 原子。它被用來組裝和拆裝Ag的顆粒。各種Ag 顆粒在半元胞中被組裝，同時我們發現單個半元胞中的最大的顆粒是Ag25。同時這項技術被用來組裝複雜的Ag 顆粒的圖形。 / 使用上面這些操縱的技術（熱蒸發方法和原子操縱方法），我們細緻的研究了顆粒的兩方面性質：第一，我們發現對於單個元胞中的Ag 顆粒都存在着顯著的電學的整流效應，同時我們比較了它們各自的特點。通過實驗測量和第一性原理計算，我們發現這種電學整流效應源自電子態密度的耦合效應。當這些顆粒在平面內連接起來從而增加橫向的尺寸時，這種顆粒的整流效應將會繼續存在；但當顆粒形成多層Ag 的結構即垂直方向尺寸增加時，這個整流效應將會消失。另外一個所研究的性質是所有的Ag顆粒都存在一種對周圍單個Ag原子的吸引作用，從而讓自身長大的過程。這一過程對於Ag 顆粒的生長起到了至關重要的作用。我們選擇了幾個有代表性的顆粒，通過原子操縱的方法測量了它們與一個鄰近的Ag 原子的融合過程。測量的結果說明一個顆粒的存在通常會降低一個鄰近Ag 原子向此顆粒擴散的勢壘高度，同時也會影響此單個原子在靠近此顆粒的吸附位上的吸附能。 / Metal nanoclusters with less than 1000 atoms differ very much from their bulk counterparts. They possess many unique chemical and physical properties and have attracted extensive studies during the past two decades. However, in-depth and comprehensive researches of these structures are difficult for their small size. Scanning tunneling microscopy (STM) is a versatile technique to study the nanostructures. Its extreme powerful spatial resolution as well as its atomic manipulation capability enabled us to obtain the detailed information of some nano-clusters. In this thesis, I report on my studies on the noble metal (mainly Ag) clusters on the reconstructed Si(111)-(7x7) surface. / I firstly studied the fflbrication methods of the metal clusters on the Si(111)-(7x7) surface. By thermal deposition, I fabricated Ag clusters on Si(111)-( 7x7). After studying the overall growth process of the Ag clusters in half unit cells, I focused on the main target of this thesis: individual Ag clusters. By carefully observing the diffusion process of the Ag atoms on the surface, we identified of the number of atoms inside Ag3-Ag13 clusters. The STM images also showed the dynamic behaviors of each cluster, including self-hopping, dissociation and association with nearby Ag atoms. For a better controllability, a vertical manipulation technique was developed to transfer single Ag atoms across difference locations on the Si(111)-(7x7) surface. The STM tip was used to pick up or drop off metal atoms from or to the sud"ace following specific procedures. The assembly and disassembly of Ag clusters were achieved by this method. Various Ag clusters were built and the largest occupation of a FRUC was found to be 25 Ag atoms. This technique was demonstrated to be able to manipulate Au atoms and to assemble Au clusters. / With the developed fabrication methods, by thermal deposition as well as by atomic manipulation, two properties of the clusters were studied carefully: first, a prominent electrical rectifying behavior was observed for all the Ag clusters. Their individual behaviors were compared with each other. A combination of experimental measurements and first principle calculations unveiled the mechanism of the electronic rectifying behaviors to be a wave function coupling effect. The rectifying behavior for the clusters was found to persist when clusters connected laterally, whereas to disappear when the vertical size of the cluster increased to form multiple Ag layers. The second property studied is that all the clusters are found to attract nearby single Ag atoms to grow larger. This behavior plays a critical role in the growth process of the Ag clusters. we chose several typical Ag clusters and measured the merging process with single neighboring Ag atoms by atomic manipulation. The results indicate that the presence of a Ag cluster will mostly reduce the diffusion barrier of a single Ag atom in the neighboring half unit cells to diffuse to the cluster and even affect / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Ming, Fangfei = 在Si(111)-(7x7)表面的金屬納米顆粒的製備和性質 / 明方飛. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 116-122). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese. / Ming, Fangfei = Zai Si(111)-(7x7) biao mian de jin shu na mi ke li de zhi bei he xing zhi / Ming Fangfei. / Chapter 1 --- Introduction: metal nano-clusters on semiconductor surfaces --- p.2 / Chapter 1.1 --- Background and Motivation --- p.2 / Chapter 1.1.1 --- Nano-science and nano-technology --- p.2 / Chapter 1.1.2 --- Metal nano-clusters --- p.3 / Chapter 1.2 --- Scanning tunneling microscopy (STM) --- p.6 / Chapter 1.2.1 --- Introduction to STM --- p.6 / Chapter 1.2.2 --- Basic components of STM --- p.7 / Chapter 1.2.3 --- STM working modes --- p.9 / Chapter 1.2.4 --- STM experiments in this thesis --- p.11 / Chapter 1.3 --- Substrate: the Si(111)-(7 x 7) reconstruction surface --- p.12 / Chapter 1.3.1 --- Atomic Structure --- p.12 / Chapter 1.3.2 --- Preparation in ultra high vacuum --- p.14 / Chapter 1.4 --- Growth of Ag on Si(111)-(7 x 7) at room temperature --- p.14 / Chapter 1.4.1 --- Growth method --- p.15 / Chapter 1.4.2 --- Adsorption and diffusion of single Ag atoms --- p.17 / Chapter 1.4.3 --- Forming Ag clusters --- p.20 / Chapter 1.4.4 --- Forming Ag islands --- p.23 / Chapter 1.5 --- Conclusion --- p.25 / Chapter 2 --- Individual Ag clusters: size, dynamics and stability --- p.27 / Chapter 2.1 --- Introduction --- p.27 / Chapter 2.2 --- Experiment --- p.29 / Chapter 2.3 --- Results and discussion --- p.31 / Chapter 2.3.1 --- Growth kinetics --- p.31 / Chapter 2.3.2 --- STrvr images of typical Ag structures Intra-HUC diffusion --- p.33 / Chapter 2.3.3 --- Inter-HUC diffusion --- p.37 / Chapter 2.3.4 --- Inter-HUC diffusion & identification of the number of atoms inside each cluster --- p.38 / Chapter 2.3.5 --- Stability of Ag structures. --- p.45 / Chapter 2.4 --- Conclusion --- p.49 / Chapter 3 --- Assembling and disassembling Ag/ Au clusters by atomic manipulation --- p.51 / Chapter 3.1 --- Introduction --- p.51 / Chapter 3.2 --- Experiment --- p.54 / Chapter 3.3 --- Results and disscussion --- p.55 / Chapter 3.3.1 --- Basic procedures for manipulating a single Ag atom --- p.55 / Chapter 3.3.2 --- Assembly of Ag clusters --- p.58 / Chapter 3.3.3 --- Disassembly of Ag clusters --- p.66 / Chapter 3.3.4 --- Assembly of Ag cluster patterns --- p.70 / Chapter 3.3.5 --- Comparison with the manipulation of Au atoms --- p.71 / Chapter 3.3.6 --- STrvr tip for atomic manipulation --- p.74 / Chapter 3.4 --- Conclusion --- p.75 / Chapter 4 --- Study on the electronic properties of the Ag clusters --- p.76 / Chapter 4.1 --- Introduction --- p.76 / Chapter 4.2 --- Experiment --- p.78 / Chapter 4.2.1 --- Experimental parameters. --- p.78 / Chapter 4.2.2 --- Precise tip positioning --- p.79 / Chapter 4.2.3 --- First principle calculation parameters --- p.80 / Chapter 4.3 --- Results and Discussion --- p.81 / Chapter 4.3.1 --- Grow Ag clusters --- p.81 / Chapter 4.3.2 --- I-V of the clusters. --- p.82 / Chapter 4.3.3 --- Theoretical calculation --- p.88 / Chapter 4.3.4 --- Transition from cluster to bulk in I-V. --- p.91 / Chapter 4.4 --- Conclusion --- p.94 / Chapter 5 --- Dynamics of Ag atoms near an interacting Ag cluster --- p.95 / Chapter 5.1 --- Introduction --- p.95 / Chapter 5.2 --- Results and Discussion --- p.97 / Chapter 5.2.1 --- Different Ag clusters and their orientations --- p.97 / Chapter 5.2.2 --- Association time measurement --- p.99 / Chapter 5.2.3 --- Energy barrier and diffusion prefactor --- p.101 / Chapter 5.2.4 --- Influenced intra-HUC diffusion --- p.105 / Chapter 5.2.5 --- Energy diagram --- p.107 / Chapter 5.3 --- Conclusion --- p.109 / Chapter 6 --- Conclusion and Future Work --- p.111 / Chapter 6.1 --- Conclusion --- p.111 / Chapter 6.2 --- Future work --- p.113 / Bibliography --- p.116

Thesis (M.App.Sc.) - Carleton University, 2002. / Includes bibliographical references (p. 72-87). Also available in electronic format on the Internet.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2000. / David L. McDowell. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (Ph.D.)--Mississippi State University. Department of Mechanical Engineering. / Title from title screen. Includes bibliographical references.

Orientador: Auteliano Antunes dos Santos Junior / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-28T12:03:37Z (GMT). No. of bitstreams: 1 PereiraJunior_Paulo_D.pdf: 3933527 bytes, checksum: ce6440b45089eda939114fc7869bdc4d (MD5) Previous issue date: 2015 / Resumo: As técnicas não destrutivas que utilizam ondas ultrassônicas para avaliação de tensões mecânicas se destacam pela sua simplicidade e baixo custo. A medição de tensão por ultrassom utiliza o princípio da acustoelasticidade que relaciona a variação da velocidade de ondas elásticas com a variação da tensão no material. Entretanto, não apenas a tensão influencia a velocidade de onda, mas também a microestrutura do material inspecionado. A influência da microestrutura pode levar a erros de medição de tensão. O objetivo deste trabalho é verificar a viabilidade da utilização de imagens ultrassônicas para avaliar o efeito de variações microestruturais na velocidade da onda ultrassônica longitudinal criticamente refratada (Lcr) utilizada na medição de tensões. As imagens são geradas a partir de sinais obtidos por uma sonda de transdutores estratificados (array) ultrassônicos e aplicando a técnica Total Focusing Method (TFM). Os sinais obtidos com o array contêm reflexões causadas pela microestrutura granular do material que resultam em um ruído no sinal conhecido como ruído de grão. Por esta razão, os sinais e consequentemente a imagem formada contêm informação acerca da microestrutura do material. Neste trabalho foram verificadas as relações entre tamanho e forma de grão, velocidade de onda e intensidade de ruído em imagens de TFM para amostras de aço ASTM A36. A influência da tensão e temperatura nas imagens ultrassônicas também foi analisada. Os resultados mostraram que assim como a velocidade de onda, a intensidade do ruído de grão nas imagens de TFM também sofre influência do tamanho e forma do grão do material. Entretanto, não foram verificadas variações significativas no ruído de grão em função da tensão aplicada e temperatura. Sendo a intensidade do ruído de grão nas imagens sensível somente às variações microestruturais do material, a técnica pode ser empregada para corrigir valores de velocidade de onda para medições de tensão. No trabalho também é proposto um modelo matemático para obtenção de imagens de TFM para uma dada distribuição de grãos. Os resultados obtidos com o modelo mostraram boa concordância qualitativa com os dados experimentais para a frequência de 2,5 MHz / Abstract: Non-destructive techniques using ultrasonic waves to evaluate stresses excel for exhibiting simplicity and low cost. Stress evaluation by ultrasound applies the acoustoelastic theory that relates the variation in velocity of elastic waves with the variation in stress in the material. However, not only stress has influence in wave velocity, but also the microstructure of the specimen being evaluated. The microstructure influence can lead to errors in the stress measurement. The aim of this work is to verify the feasibility of using ultrasonic images to evaluate the effect of microstructural variations in the velocity of longitudinal critically refracted waves (Lcr) used in stress measurements. The images are generated from signals obtained with a probe of ultrasonic array applying the Total Focusing Method (TFM). The signals recorded using the array contain reflections caused by the granular structure of the material that creates a noise in the signal known as grain noise. For this reason, the signals and consequently the images formed carry information about the material¿s microstructure. In this work, it was verified the relations between size and shape of grains, wave velocity and noise intensity in TFM images for samples of ASTM A36 steel. The influence of stress and temperature in the ultrasonic images was also verified. The results showed that as for the wave velocity, the grain noise intensity in TFM images are also influenced by size and shape of grains that constitute the material under inspection. Nevertheless, no significant variations were seen in the noise intensity due to the applied stress or temperature changes. Being the noise intensity of the images sensitive only to microstructural variations, the technique presented can be used to correct values of ultrasonic wave speed in the stress evaluations. In this work, it is also proposed a mathematical model to obtain TFM images for a given distribution of grains. The results showed qualitative good agreement between simulated and experimental data for the frequency of 2,5 MHz / Doutorado / Mecanica dos Sólidos e Projeto Mecanico / Doutor em Engenharia Mecânica

Understanding the relationships between microstructures and properties of materials is a key to developing new materials with more suitable qualities or employing the appropriate materials in special uses. In the present world of material research, the main focus is on microstructural control to cost-effectively enhance properties and meet performance specifications. This present work is directed towards improving the fundamental understanding of the microscale deformation mechanisms and mechanical behavior of metallic alloys, particularly focusing on face centered cubic (FCC) structured metals through a unique computational methodology called three-dimensional dislocation dynamics (3D-DD). In these simulations, the equations of motion for dislocations are mathematically solved to determine the evolution and interaction of dislocations. Microstructure details and stress-strain curves are a direct observation in the simulation and can be used to validate experimental results. The effect of initial dislocation microstructure on the yield strength has been studied. It has been shown that dislocation density based crystal plasticity formulations only work when dislocation densities/numbers are sufficiently large so that a statistically accurate description of the microstructure can be obtainable. The evolution of the flow stress for grain sizes ranging from 0.5 to 10 µm under uniaxial tension was simulated using an improvised model by integrating dislocation pile-up mechanism at grain boundaries has been performed. This study showed that for a same initial dislocation density, the Hall–Petch relationship holds well at small grain sizes (0.5–2 µm), beyond which the yield strength remains constant as the grain size increases.

Orientador: Auteliano Antunes dos Santos Júnior / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-24T07:24:11Z (GMT). No. of bitstreams: 1 Buenos_AlexandreAparecido_D.pdf: 36097019 bytes, checksum: d270fa6d123f9a141621191306944498 (MD5) Previous issue date: 2014 / Resumo: Um método alternativo para a medição das tensões em componentes em serviço emprega o ultrassom, utilizando ondas longitudinais criticamente refratadas (LCR) e a teoria acustoelástica. Este método é influenciado por diversos fatores, dentre os quais os ligados à microestrutura do material, ainda não modelados. Este trabalho avalia a influência da microestrutura do material na velocidade de propagação das ondas LCR. Foram estudados os efeitos do tamanho dos grãos e da orientação preferencial dos grãos (textura) do material. Para isso, foram ensaiadas amostras de aço carbono de média resistência ASTM A36, laminadas, utilizadas em componentes estruturais. Parte das amostras foi tratada termicamente para crescimento dos grãos em temperaturas variadas, o que permitiu obter diferentes tamanhos de grão. Outras amostras foram utilizadas como recebidas para estudo do efeito da textura do material. O trabalho mostra que ambos, o tamanho dos grãos e a textura, têm efeito sobre a propagação das ondas LCR. Os resultados mostraram um aumento na variação do tempo de percurso médio (?TOF) das ondas LCR com o aumento do diâmetro dos grãos austeníticos médios (DGAM). Como consequência, houve uma diminuição na variação da velocidade da onda LCR média (?VLCR) devido ao aumento do DGAM. Os resultados mostraram também o efeito da direção de laminação (DL) na propagação das ondas LCR, mostrando que o tempo de percurso (TOF) é maior na direção de laminação (DL) do que na direção transversal à laminação (DT). Isso é atribuído ao alongamento dos grãos devido ao processo de laminação. O efeito da textura do material foi associado ao desvio padrão (DP) encontrado e ainda representa um desafio para o emprego do método em aços com orientação cristalográfica pouco definida. Tais resultados permitem direcionar as pesquisas atuais sobre o assunto para o desenvolvimento de técnicas que minimizem os efeitos dos fatores de influência estudados / Abstract: Critically refracted longitudinal waves (LCR) and the acoustoelastic theory can be an interesting alternative for measuring stress in components in service. This method is influenced by several factors, among which the ones linked to the material microstructure, not modeled yet. This work evaluates the influence of material microstructure on the LCR waves speed. The effects of grain size and preferred grain orientation (texture) of the material were studied. We tested samples of carbon steel ASTM A36 of medium strength, made from rolled plates, used in structural components. Part of the samples was heat treated for grain growth at various temperatures, generating different grain sizes. Other samples were used as received to study the effect of the material¿s texture. The work shows that both the grain size and texture affects the propagation of LCR waves. The results showed that the increase in variation of the mean time-of-flight (?TOF) of the LCR waves can be related to the increase of the mean austenitic grain diameter (DGAM). As consequence a decrease in the variation of the mean LCR wave speed (?VLCR) is linked to the increase in DGAM. The results also reveals the effect of the rolling direction (DL) in the propagation of LCR waves, showing that the time-of-flight (TOF) is greater in the DL direction than in the transverse direction to rolling (DT). This is attributed to the grain elongation due to the rolling process. The effect of texture was associated with the standard deviation (DP) found. This effect still represents a challenge to the employment of the method in steels with poorly defined crystallographic orientation. The results allow directing the current research on the subject to develop techniques that minimize the effects of influence factors studied / Doutorado / Mecanica dos Sólidos e Projeto Mecanico / Doutor em Engenharia Mecânica

No description available.

M. Tech. (Department of Metallurgical Engineering, Faculty of Engineering Technology), Vaal University of Technology. / This work is an investigation in the development and characterisation of porous Ti-Sn-SiC and Ti-Nb-SiC composites. Pure Titanium (Ti), Tin (Sn), Niobium (Nb) and Silicon carbide (SiC) powders were used as starting materials. The Ti-Sn-SiC and Ti-Nb-SiC composites were produced by powder metallurgy (PM) press-and-sinter route. The Sn is an α-phase stabilizer while Nb is a β-phase stabilizer in Ti alloys. A systematic study of binary Ti-Sn and Ti-Nb alloys was conducted with the addition of SiC particles. The addition of Sn influences the microstructure of the titanium alloy. With increasing the percentage of Sn content, the density of the samples decreases on the Ti-Sn alloys. An increase in the Sn content from 10 to 25 wt. % content resulted in decreased hardness. The Ti-Sn binary revealed stability of the HCP phase with increasing composition of the Sn content. The porous structures of the Ti-Sn-SiC composites were evenly distributed throughout the materials. The sintered densities increase from 94.69% to 96.38%. XRD analysis detected the HCP crystal lattice structure for the Ti5.4Sn3.8SiC and Ti5.6-Sn3.8-SiC composites. XRD pattern of the Ti5.8-Sn3.8-SiC reveals both the HCP and FCC crystal structures. The HCP phase has lattice parameters a= 2.920 Å; c=4.620 Å with smaller c/a ratio of 1.589. Additionally, FCC lattice parameter a=5.620 Å Fm-3m # 225 was obtained both for Ti5.8Sn3.8SiC and Ti6.0Sn3.8SiC XRD patterns. On the other hand, Optical microscopy analysis of the Ti-Nb alloys revealed the equiaxed grains composed of the light β-phase segregating on the grain boundaries. The Ti9Nb1 has low Vickers hardness of all alloys while Ti8Nb2 and Ti7.5Nb2.5 alloys are harder due to high amount of Nb content. Generally, the densities of the Ti–Nb alloys increased with increasing Nb content. HCP and BCC phases have the lattice parameters a = 2.951 Å, c = 4.683 Å and 3.268 Å, respectively. An HCP (α′) phase was detected in the Ti8.5Nb1.5 alloy with lattice parameters a = 5.130 Å, c = 9.840 Å while a BCC phase had a = 3.287 Å. The sintered Ti8Nb2 alloy also had the α′-phase with a = 5.141 Å, c = 9.533 Å and BCC phase with a = 3.280 Å lattice parameters. On the contrary, the Ti7.5Nb2.5 alloy formed the α′-phase of a = 5.141 Å, c = 9.533 Å and BCC with a = 3.280 Å lattice parameters. For the 10 and 15 wt.% Nb alloys, very porous structures were observed. The pores appear spherical and widely distributed. As the Nb content is increased to 20 wt.% (Ti7Nb2SiC) and 25 wt.% (Ti7Nb2.5SiC), porosity was minimized. The sintered densities of the Ti-Sn alloys are decreasing from 95.90% to 92.80% with increased amount of Sn in the Ti, while the sintered densities of Ti-Sn-SiC are increasing from 94.69% to 96.38%. The high porosity, which developed in Ti7Nb1SiC and Ti7Nb2.5SiC, affected the densities of these composites. The sintered densities of Ti-Nb alloys are increasing from 92.08% to 97.65% with increased amount of Nb in the Ti. In terms of hardness Ti7Nb1SiC and Ti7Nb2.5SiC resulted in the lowest while Ti7Nb1.5SiC and Ti7Nb2SiC composites were 511.74 HV and 527.678 HV. The porosity levels were increased by the addition of SiC in the Ti-Sn-SiC and Ti-Nb-SiC composites. The XRD analysis revealed phase transformation on the Ti-Nb alloys and Ti-Nb-SiC composites.