Surface Structures Determined by Kinetic Processes: Adsorption and Diffusion of Oxygen on Pd(100)

Clean and As covered zinc-blende and wurtzite GaN surfaces have been investigated employing density-functional theory calculations. For clean GaN surfaces our calculations indicate the stability of several novel surface structures that are very different from those found on traditional III-V semiconductors. Adding impurities commonly present in significant concentrations during growth strongly modifies surface reconstructions and energies. In particular, we find that arsenic has a low solubility and significantly stabilizes the cubic GaN (001) surface making it interesting as a potential surfactant. Finally, we have studied the diffusion of Ga and N adatoms on both the equilibrium and non-equilibrium surfaces. Our calculations reveal a very different diffusivity for Ga and N adatoms: While Ga adatoms are very mobile at typical growth temperatures, the diffusion of N adatoms is slower by several orders of magnitude. These results give insight into the fundamental growth mechanisms and allow conclusions concerning optimum growth conditions.

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Nitruration par échange d'ions de l'acier rapide pour outils AISI M2

Matériaux & Techniques ◽

10.1051/mattech/199684060013s ◽

1996 ◽

Vol 84 ◽

pp. 13-16

Author(s):

Levon Capan ◽

Omer Cimen ◽

Hasan Cep ◽

Zerrin Oktem

Keyword(s):

High Speed ◽

Steel Surface ◽

Surface Structures ◽

Ion Nitriding ◽

Nitriding Time ◽

X Ray ◽

Diffusion Depth ◽

Depth Increase ◽

Nitride Layers ◽

And Diffusion

In this study, AISI M2 high speed tool steel is austenized at 1200°C, cooled in air and then double tempered at 540°C. Hardened samples are hold at the ion nitriding furnace between 500-540°C for different times. Hardened and nitrided samples were investigated for the surface structures using X-ray, microstructure of nitride layers and the hardness measurements were done. The hardness of steel surface and diffusion depth increase due to the increasing of nitriding time.

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Meridional Circulation, Turbulence, and Diffusion Processes in Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100080477 ◽

1976 ◽

Vol 32 ◽

pp. 109-116 ◽

Cited By ~ 1

Author(s):

S. Vauclair

Keyword(s):

Convection Zone ◽

Diffusion Processes ◽

Meridional Circulation ◽

Diffusion Time ◽

Work In Progress ◽

First Results ◽

Stellar Envelopes ◽

And Diffusion ◽

Turbulence And Diffusion ◽

Hr Diagram

This paper gives the first results of a work in progress, in collaboration with G. Michaud and G. Vauclair. It is a first attempt to compute the effects of meridional circulation and turbulence on diffusion processes in stellar envelopes. Computations have been made for a 2 Mʘstar, which lies in the Am - δ Scuti region of the HR diagram.Let us recall that in Am stars diffusion cannot occur between the two outer convection zones, contrary to what was assumed by Watson (1970, 1971) and Smith (1971), since they are linked by overshooting (Latour, 1972; Toomre et al., 1975). But diffusion may occur at the bottom of the second convection zone. According to Vauclair et al. (1974), the second convection zone, due to He II ionization, disappears after a time equal to the helium diffusion time, and then diffusion may happen at the bottom of the first convection zone, so that the arguments by Watson and Smith are preserved.

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Scanning and Transmission Microscopy of Nematocyst Batteries in Epitheliomuscular Cells of Hydra

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100066097 ◽

1971 ◽

Vol 29 ◽

pp. 410-411 ◽

Cited By ~ 1

Author(s):

Jane A. Westfall ◽

S. Yamataka ◽

Paul D. Enos

Keyword(s):

Electron Microscopy ◽

Scanning Electron Microscopy ◽

Osmium Tetroxide ◽

External Surface ◽

Three Dimensional ◽

Surface Structures ◽

Transmission Microscopy ◽

Transmission Electron ◽

Freeze Dried ◽

Scanning Electron

Scanning electron microscopy (SEM) provides three dimensional details of external surface structures and supplements ultrastructural information provided by transmission electron microscopy (TEM). Animals composed of watery jellylike tissues such as hydras and other coelenterates have not been considered suitable for SEM studies because of the difficulty in preserving such organisms in a normal state. This study demonstrates 1) the successful use of SEM on such tissue, and 2) the unique arrangement of batteries of nematocysts within large epitheliomuscular cells on tentacles of Hydra littoralis.Whole specimens of Hydra were prepared for SEM (Figs. 1 and 2) by the fix, freeze-dry, coat technique of Small and Màrszalek. The specimens were fixed in osmium tetroxide and mercuric chloride, freeze-dried in vacuo on a prechilled 1 Kg brass block, and coated with gold-palladium. Tissues for TEM (Figs. 3 and 4) were fixed in glutaraldehyde followed by osmium tetroxide. Scanning micrographs were taken on a Cambridge Stereoscan Mark II A microscope at 10 KV and transmission micrographs were taken on an RCA EMU 3G microscope (Fig. 3) or on a Hitachi HU 11B microscope (Fig. 4).

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Electron Microscopic Observation of Biological Specimens in Their Native State by Employing Cryogenic Techniques

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100095315 ◽

1972 ◽

Vol 30 ◽

pp. 410-411 ◽

Cited By ~ 1

Author(s):

Tokio Nei ◽

Haruo Yotsumoto ◽

Yoichi Hasegawa ◽

Yuji Nagasawa

Keyword(s):

Electron Microscopic Observation ◽

Surface Structures ◽

Specimen Preparation ◽

Native State ◽

Electron Microscopic ◽

Biological Specimen ◽

Specimen Holder ◽

Cold Stage ◽

Preparation Techniques ◽

Scanning Electron

In order to observe biological specimens in their native state, that is, still containing their water content, various methods of specimen preparation have been used, the principal two of which are the chamber method and the freeze method.Using its recently developed cold stage for installation in the pre-evacuation chamber of a scanning electron microscope, we have succeeded in directly observing a biological specimen in its frozen state without the need for such conventional specimen preparation techniques as drying and metallic vacuum evaporation. (Echlin, too, has reported on the observation of surface structures using the same freeze method.)In the experiment referred to herein, a small sliced specimen was place in the specimen holder. After it was rapidly frozen by freon cooled with liquid nitrogen, it was inserted into the cold stage of the specimen chamber.

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