Theory of Hydrogen Interactions with Amorphous Silicon

1999 ◽  
Vol 557 ◽  
Author(s):  
Chris G. Van De Walle ◽  
Blair Tuttle
Keyword(s):  
Amorphous Silicon ◽  
First Principles ◽  
Molecular Hydrogen ◽  
Hydrogen Exchange ◽  
Defect Formation ◽  
Exchange Process ◽  
Current Understanding ◽  
First Principles Calculations ◽  
Microscopic Mechanism ◽  
Enhanced Stability

AbstractWe present an overview of recent results for hydrogen interactions with amorphous silicon (a-Si), based on first- principles calculations. We review the current understanding regarding molecular hydrogen, and show that H2 molecules are far less inert than previously assumed. We then discuss results for motion of hydrogen through the material, as relating to diffusion and defect formation. We present a microscopic mechanism for hydrogen-hydrogen exchange, and examine the metastable ≠ SiH2 complex formed during the exchange process. We also discuss the enhanced stability of Si-D compared to Si-H bonds, which may provide a means of suppressing light-induced defect generation.

scholarly journals First Principles Calculations of Defects in Unstable Crystals: Austenitic Iron

2011 ◽  
Vol 1363 ◽  
Author(s):  
G.J. Ackland ◽  
T.P.C. Klaver ◽  
D.J. Hepburn
Keyword(s):  
Point Defects ◽  
First Principles ◽  
Defect Formation ◽  
Single Layer ◽  
Temperature Limit ◽  
Reference State ◽  
First Principles Calculations ◽  
General Picture ◽  
Bcc Iron ◽  
Defect Energies

ABSTRACTFirst principles calculations have given a new insight into the energies of point defects in many different materials, information which cannot be readily obtained from experiment. Most such calculations are done at zero Kelvin, with the assumption that finite temperature effects on defect energies and barriers are small. In some materials, however, the stable crystal structure of interest is mechanically unstable at 0K. In such cases, alternate approaches are needed. Here we present results of first principles calculations of austenitic iron using the VASP code. We determine an appropriate reference state for collinear magnetism to be the antiferromagnetic (001) double-layer (AFM-d) which is both stable and lower in energy than other possible models for the low temperature limit of paramagnetic fcc iron. Another plausible reference state is the antiferromagnetic (001) single layer (AFM-1). We then consider the energetics of dissolving typical alloying impurities (Ni, Cr) in the materials, and their interaction with point defects typical of the irradiated environment. We show that the calculated defect formation energies have fairly high dependence on the reference state chosen: in some cases this is due to instability of the reference state, a problem which does not seem to apply to AFM-d and AFM-1. Furthermore, there is a correlation between local free volume magnetism and energetics. Despite this, a general picture emerge that point defects in austenitic iron have geometries similar to those in simpler, non-magnetic, thermodynamically stable FCC metals. The defect energies are similar to those in BCC iron. The effect of substitutional Ni and Cr on defect properties is weak, rarely more than tenths of eV, so it is unlikely that small amounts of Ni and Cr will have a significant effect on the radiation damage in austenitic iron at high temperatures.

New Pt-based Superalloy System Designed from First Principles

2008 ◽  
Vol 1128 ◽  
Author(s):  
Vsevolod I. Razumovskiy ◽  
Eyvaz I. Isaev ◽  
Andrei V. Ruban ◽  
Pavel A. Korzhavyi
Keyword(s):  
Elastic Properties ◽  
First Principles ◽  
Defect Formation ◽  
Alloy System ◽  
First Principles Calculations ◽  
Phonon Spectra ◽  
Ultrahigh Temperature ◽  
New Generation ◽  
Formation Energies ◽  
Temperature Applications

AbstractPt-Sc alloys with the γ-γ′ microstructure are proposed as a basis for a new generation of Pt-based superalloys for ultrahigh-temperature applications. This alloy system was identified on the basis of first-principles calculations. Here we discuss the prospects of the Pt-Sc alloy system on the basis of calculated elastic properties, phonon spectra, and defect formation energies.

First Principles Study of Point Defects in Uranium Dioxide

2007 ◽  
Vol 561-565 ◽  
pp. 1971-1974 ◽  
Author(s):  
Ying Chen ◽  
Misako Iwasawa ◽  
Yasunori Kaneta ◽  
Toshiharu Ohnuma ◽  
Hua Yun Geng ◽  
...  
Keyword(s):  
Point Defects ◽  
First Principles ◽  
Defect Formation ◽  
Lattice Relaxation ◽  
Atomic Displacement ◽  
Grain Structure ◽  
First Principles Calculations ◽  
Fine Grain ◽  
Unit Cells ◽  
Formation Energies

To clarify the origin of a characteristic fine grain structure formed under the high burn-up of the nuclear fuel, the comprehensive first-principles calculations for UO2 containing various types of point defect have been performed by the PAW-GGA+U with lattice relaxation for supercells containing 1, 2 and 8 unit cells of UO2. The electronic structure, the atomic displacement and the defect formation energies of defective systems are obtained, and the effects of supercell size on these properties are discussed. Based on this relatively high precise self-consistent formation energies dataset, thermodynamic properties of various types of point defects in UO2 are further investigated in the framework of the point defects model.

Correlations between higher-order rings and microvoids in hydrogenated amorphous silicon

2015 ◽  
Vol 1757 ◽  
Author(s):  
Parthapratim Biswas ◽  
David Alan Drabold
Keyword(s):  
Amorphous Silicon ◽  
First Principles ◽  
Molecular Hydrogen ◽  
Hydrogenated Amorphous Silicon ◽  
Experimental Information ◽  
Higher Order ◽  
Hydrogenated Amorphous

ABSTRACTIn this paper we report the structure of voids in several thousand atom models of hydrogenated amorphous silicon. The models are produced by jointly employing experimental information from Smets and coworkers [1] and first principles simulations [2]. We demonstrate the existence of a useful correlation between the presence of large irreducible rings and the voids in hydrogenated amorphous silicon networks. Molecular hydrogen is observed in the models, and discussed.

A Density Functional Theory Study of the Catalytic role of Ti atoms in Reversible Hydrogen Storage in the Complex Metal Hydride, NaAlH4

2005 ◽  
Vol 884 ◽  
Author(s):  
Santanu Chaudhuri ◽  
James T Muckerman
Keyword(s):  
Density Functional Theory ◽  
First Principles ◽  
Molecular Hydrogen ◽  
Density Functional ◽  
First Principles Calculations ◽  
Density Functional Theory Study ◽  
Functional Theory ◽  
Catalytic Role ◽  
Present Part

AbstractPresence of ∼2-4 % Ti is critical for reversible hydrogenation/rehydrogenation in NaAlH4. We have investigated the probable catalytic role of Ti in this complex multi-step process. The present part of our study concentrates on the rehydrogenation reaction, i.e., the reverse reaction that forms NaAlH4 from its constituent binary hydrides. First principles calculations using density functional theory (DFT) show that a particular arrangement of Ti atoms on the surface of Al metal promotes the chemisorption of molecular hydrogen. We also present comparisons with existing experimental data (EXAFS and TEM) to support the existence of such an arrangement on the surface.

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