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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Mikrostrukturen i valset kobber

Christoffersen, Henrik 01 January 1997 (has links)
No description available.
2

Characterization of AlGaN/GaN heterostructures grown by molecular beam epitaxy

Hsu, Yu-Chi 26 July 2007 (has links)
In this paper we will discuss the characteristic of AlGaN/GaN heterostructure grown on sapphire by plasma-assisted molecular-beam epitaxy. In this series of samples, we try to change the ratio of buffer layer N/Al, the ratio from sample A to sample D is 43¡B26¡B23¡B12. We used the Hall measurement¡BAFM and X-ray to analyze the series of samples. From the Hall measurement and AFM, we found that the dislocation scattering is the mainly reason which cause the mobility increasing or decreasing. From X-ray, we can get that the mainly dislocation type is edge dislocation. The density of edge dislocation decreased due to vary the growth conditions. In our samples, the mobility in the room temperature increase from 387 cm2/Vs to 1224 cm2/Vs and in the liquid nitrogen temperature the mobility achieve 3705 cm2/Vs.
3

The Study of the Relationship Between Fatigue Crack Propagation and Dislocation Structure ¡V the Dislocation Structure Variation of Crack Closure In copper

Tzeng, Ting-Hung 12 June 2000 (has links)
µL
4

Electronic characteristics of defects of GaN films grown on Si(111) substrate

Chen, Bo-Chih 28 July 2009 (has links)
The electronic properties of the defects of the GaN/Si(111) system has been successfully measured by STM in the work. Different types of the dislocations in GaN films, such as edge dislocations and screw dislocations, have been observed. Defects induce the change of the band gap from 3.4 eV to 2.2 eV. The characteristic scattering length of the edge dislocation is around 25 nm.
5

The compound-like nanosegregation at dislocations and grain boundaries in metallic materials, relevance to physics of the diffusion anomalies

Nechaev, Yury S. 17 September 2018 (has links)
No description available.
6

Theory and modeling of the mechanical behavior of nanoscale and finescale multilayer thin films

Li, Qizhen 12 October 2004 (has links)
No description available.
7

Mechanical Properties and Radiation Tolerance of Metallic Multilayers

Li, Nan 2010 May 1900 (has links)
High energy neutron and proton radiation can induce serious damage in structural metals, including void swelling and embrittlement. Hence the design of advanced metallic materials with significantly enhanced radiation tolerance is critical for the application of advanced nuclear energy systems. The goals of this dissertation are to examine the fundamental physical mechanisms that determine the responses of certain metallic multilayers, with ultra-high density interface structures, to plastic deformation and high fluence He ion irradiation conditions. This dissertation focuses on the investigation of mechanical and radiation responses of Al/Nb and Fe/W multilayers. Radiation induced microstructural evolution in Cu and Cu/Mo multilayer films are briefly investigated for comparisons. Al/Nb multilayer films were synthesized by magnetron sputtering at room temperature. The interface is of Kurdjumov-Sachs orientation relationship. In situ nanoindentation inside a transmission electron microscope (TEM) reveal that interfaces act as strong barriers for dislocation transmission and dislocations climb along the Al/Nb interfaces at a much higher velocity than in bulk. The evolution of microstructure and mechanical properties of Al/Nb multilayers has been investigated after helium ion irradiations: 100 keV He+ ions with a dose of 6x10^16/cm2. When layer thickness, h, is greater than 25 nm, hardness barely changes, whereas radiation hardening is more significant at smaller h. This study shows that miscible fcc/bcc interface with large positive heat of mixing is not stable during ion irradiation. In parallel we investigate sputtered Fe/W multilayers. Film hardness increases with decreasing h, and approaches a maximum of 12.5 GPa when h = 1 nm. After radiation, radiation hardening is observed in specimens when h >/= 5 nm, however, hardness barely changes in irradiated Fe/W 1 nm specimens due to intermixing. In comparison, Cu/Mo 5 nm multilayers with immiscible interface has also been investigated after helium ion irradiations. Interfaces exhibit significantly higher helium solubility than bulk. He/vacancy ratio affects the formation and distribution of He bubbles. The greater diameter of He bubbles in Cu than Mo originates from the ease of bubble growth in Cu via punching of interstitial loops. Finally, helium bubble migration and growth mechanisms were investigated in irradiated Cu (100) single crystal films via in situ heating inside a TEM. The activation energy for bubble growth is ~ 0.02 eV at low temperature. At higher temperatures, the activation energy for bubble coalescence is ~ 0.22 eV inside crystal, and 0.34 eV close to surface. The migration mechanisms of helium bubbles involve continuous as well as Brownian movement.
8

A microstructure analysis of pressureless sintered LiMn2O4 spinel

Wang, Chun-Chieh 22 July 2004 (has links)
The spinel structure of LiMn2O4 powders react with the 1 mole Li2CO3 and 4 mole MnO2 powders by solid-state reaction at 800 oC, and then sintered at 1300 oC to become ceramics specimen. There are accompany phase transformation and non-stoichiometric composition during the cooling process. In X-ray diffraction analysis, the sintered specimen contains principal Li-deficiency Li1-XMn2O4 composition and minor of second phases LiMnO2 and Mn3O4. Lattice parameters also distorted by John-Teller effect. In electron microscopy observation, there are lamellae grains and defects in the specimen, such as twins, dislocations and stacking faults. In TEM analysis, tetragonal-LiMn2O4 structure has lamellae domains, and reflection twinning. However, this study for cubic-LiMn2O4 structure found that edge dislocations with Burger vector of 1/2<110> slip on {110} plane, and mixed(45o) dislocation with Burger vector of 1/2<100> slip on {100} plane.
9

The Dislocation Evolutions in Polycrystalline Copper under high-low strain controlled fatigue

Zhuang, Yue-feng 29 August 2006 (has links)
The dislocation structure evolution of polycrystalline copper at constant strain amplitude during low cycle fatigue develops loop patches, vein structure, persistent slip bands, dislocation walls, dislocation cells, and cells with misorientation dislocation step-by-step by increasing fatigue cycles. However, the dislocation structure evolution will change in low cycle fatigue under reduced loading amplitude. The polycrystalline copper of 99.99 at% purity and 60&#x00B5;m in grain size was used in the low cycle fatigue test. First, the test is controlled at £G£`/2= ¡Ó0.4%, ¡Ó0.2%, and ¡Ó0.1% strain amplitude until the specimens crack. And control the fatigue test after 2500 cycles at ¡Ó0.4% strain amplitude. Then we can observe the dislocation structure of above specimens by electron microscope. After 2500 cycles at ¡Ó0.4% strain amplitude, change the strain amplitude from ¡Ó0.4% to ¡Ó0.2%. We chose the steps of low cycle fatigue test under reduced loading amplitude at 1000, 10000, and 30000 cycles. By the same token, change the strain amplitude from ¡Ó0.4% to ¡Ó0.2%. We chose the steps of low cycle fatigue test under reduced loading amplitude at 1000, and 50000 cycles. Then observe the dislocation structure of above specimens by electron microscope, and we can know the dislocation morphology of evolution process under reduced loading amplitude. After 2500 cycles at ¡Ó0.4% strain amplitude, change the strain amplitude from ¡Ó0.4% to ¡Ó0.2% and from ¡Ó0.4% to ¡Ó0.1%. After 1000 cycles, the dislocation wall can be observed at grain boundary. After 10000 cycles under changed loading amplitude from ¡Ó0.4% to ¡Ó0.2%, we can observe that the dislocation cells are broken and evolve loop patches. And after 50000 cycles under changed loading amplitude from ¡Ó0.4% to ¡Ó0.1%, large area of dislocation walls and some loop patches can be observed.
10

The dislocation reverse evolution in polycrystalline copper during low-cycle fatigue

Chang, Chi-Whei 02 July 2003 (has links)
Abstract The dislocation structure evolution of polycrystalline copper at constant strain amplitude during low cycle fatigue has been studied sufficiently. The dislocation structure develops loop patches, vein structure, persistent slip bands (PSBs), dislocation walls, dislocation cells, and misorientation dislocation cells step-by-step by increasing fatigue cycles. However, the dislocation structure evolution will change as the strain amplitude decreasing from high to low. In order to realize that the dislocation structure of polycrystalline copper how to evolve with reducing strain amplitude during low cycle fatigue, I use the copper of 99.99% purity in this experiment. The test is controlled 4¡Ñ103 cycles at 0.2% strain amplitude, and the strain amplitude is decreased from 0.2% to 0.1%. It keeps the 0.1% strain amplitude after 4¡Ñ103 cycles. I chose the four steps of the low cycle fatigue at 5¡Ñ103 , 9¡Ñ103, 15¡Ñ103 cycles, and the specimen cracking then observe the dislocation structure. Then we can know the dislocation morphology under evolution process after decreasing the strain amplitude. From the fatigue tests data by dropping the strain amplitude we can see the dislocation cells fast creaking to loop patches at 5¡Ñ103 cycles; The dislocation cells scatter and become vein structure with loop patches like a band at 9¡Ñ103 cycles. To observe clearly that the scattering loop patches normal develop dislocation walls near the grain boundary at 15¡Ñ103 cycles; At last, all of the dislocations form dislocation cells again and progress misorientation when becoming equiaxis cell. The morphology is between 0.2% strain amplitude and 0.1% strain amplitude. So we can understand the process of dislocation reverse evolution is the dislocation cells diffusing to bands. Then the bands creak to loop patches.

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