A DFT-Study of Structure and Properties of Amorphous SiCN

2002 ◽  
Vol 731 ◽  
Author(s):  
Peter Kroll
Keyword(s):  
Bulk Modulus ◽  
Density Functional ◽  
Elevated Temperatures ◽  
Amorphous Structure ◽  
Silicon Carbonitride ◽  
Random Networks ◽  
Structure And Properties ◽  
Functional Methods ◽  
Structures And Properties ◽  
Amorphous Si

AbstractStructures and properties of amorphous silicon carbonitride (a-SiCN) materials are studied using density functional methods. Topologicaly different models of a-SiCN with 112-156 atoms each were generated from continuous alternating random networks. The networks have distinct topologies which result in a different chemical bonding in the investigated structural models. A first model consists of Si-N and Si-C bonds only. For such materials with “anionic” carbon we found the highest bulk modulus. However, the network strain is largely due to the high SiC content, resulting in bond ruputre and redistribution in the amorphous structure. A second model consists of Si-N and C-N bonds only. We found that a tetrahedral coordination of “cationic” carbon in such a compound is unstable at elevated temperatures, as indicated by Car-Parrinello molecular dynamic (CPMD) simulations. A third model consists of amorphous Si3N4 as host structure and a segregation of graphitic C inside a pore. Such a model, although low in density, has an enlarged bulk modulus, comparable to the “anionic” model.

First-Principles Simulation of Hydrogen Interaction in Amorphous Silicon Nitride

2002 ◽  
Vol 719 ◽  
Author(s):  
Peter Kroll
Keyword(s):  
First Principles ◽  
Density Functional ◽  
Elevated Temperatures ◽  
Random Network ◽  
Internal Stresses ◽  
Structure And Properties ◽  
Dynamic Simulations ◽  
Functional Methods ◽  
Hydrogen Interaction ◽  
Bond Stretching

AbstractStructure and properties of amorphous hydrogented silon nitride (a- SiNx:H) are studied using density functional methods. Models of a-SiNx:H were generated using a random network algorithm. In addition we investigarted the N-H terminated surfaces of β-Si3N4.A comparison between the vibrational spectra of the surface and of the bulk models shows that within the bulk the frequency of N-H bond stretching (≈ 3150 cm-1) is lower than at the surface (≈3300 cm-1). Both spectra show an asymmetric form of the principal peak tailing towards space of hydrogen with nearby atoms. Car-Parrinello moleculat dynamic simulations at elevated temperatures show hopping of bulk hydrogen between different two bonding sites. Moreover, hydrogen acts as network transformer releasing internal stresses.

The Electronic Structure and other Properties of Amorphous Silicon Nitride Investigated with Density Functional Theory

2002 ◽  
Vol 715 ◽  
Author(s):  
Peter Kroll
Keyword(s):  
Density Functional ◽  
Elevated Temperatures ◽  
Topological Defects ◽  
Network Models ◽  
Functional Theory ◽  
Chemical Order ◽  
Amorphous Silicon Nitride ◽  
Density Functional Methods ◽  
Functional Methods ◽  
Ab Inito

AbstractStructural models of amorphous silcon nitride, a-Si3N4. consisting of 112-448 atoms were studied using density functional methods. We used continuous random alterating networks with well-defined topology for the respersentation of chemical order in the material as theoretical precursors. The models were optimized within the DFT framework and compared them to one “ab inito derived” model obtained from quenching a hypothetical melt. The strong chemical order is maintained in the network models even after Car-Parrinello molecular dynamic (CPMD) simulations at elevated temperatures for several pico-seconds, In contrast, the “ab initio derived” model exhibits n-N bonds.The optimized strutures of Si3N4 have between 2.6 and 3.2 g/vm3 and comprise few topological defects only. The dominant defects are ever over-coordinated Si and N atoms and the 2-connected is averaged over a dozen modles is, averaged over a dozen models, about 1%. Some models are even free of three-connected Si. The calculated bulk moduli decrease with decreasing density of the a-Si3N4 model. We furthermore investigated the properties of the material ater alloying elements such as H and O, espically their capacity to reduces interal strain.

Spherulite Structures in a Melt Spun Fe-Ni Austenitic Steel

1985 ◽  
Vol 43 ◽  
pp. 52-53
Author(s):  
G. M. Michal ◽  
T. K. Glasgow ◽  
T. J. Moore
Keyword(s):  
Austenitic Steel ◽  
Room Temperature ◽  
Elevated Temperatures ◽  
Large Range ◽  
Amorphous Structure ◽  
Nucleation And Growth ◽  
Ni Alloys ◽  
Melt Spun ◽  
Crystal Fibers ◽  
Rapidly Solidified

Large additions of B to Fe-Ni alloys can lead to the formation of an amorphous structure, if the alloy is rapidly cooled from the liquid state to room temperature. Isothermal aging of such structures at elevated temperatures causes crystallization to occur. Commonly such crystallization pro ceeds by the nucleation and growth of spherulites which are spherical crystalline bodies of radiating crystal fibers. Spherulite features were found in the present study in a rapidly solidified alloy that was fully crysstalline as-cast. This alloy was part of a program to develop an austenitic steel for elevated temperature applications by strengthening it with TiB2. The alloy contained a relatively large percentage of B, not to induce an amorphous structure, but only as a consequence of trying to obtain a large volume fracture of TiB2 in the completely processed alloy. The observation of spherulitic features in this alloy is described herein. Utilization of the large range of useful magnifications obtainable in a modern TEM, when a suitably thinned foil is available, was a key element in this analysis.

scholarly journals Dual role of CO in the stability of subnano Pt clusters at the Fe3O4(001) surface

2016 ◽  
Vol 113 (32) ◽  
pp. 8921-8926 ◽  
Author(s):  
Roland Bliem ◽  
Jessi E. S. van der Hoeven ◽  
Jan Hulva ◽  
Jiri Pavelec ◽  
Oscar Gamba ◽  
...  
Keyword(s):  
Density Functional ◽  
Elevated Temperatures ◽  
Time Lapse ◽  
Dual Role ◽  
Oxidation Catalyst ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Active Metal ◽  
Catalytically Active ◽  
The Stability

Interactions between catalytically active metal particles and reactant gases depend strongly on the particle size, particularly in the subnanometer regime where the addition of just one atom can induce substantial changes in stability, morphology, and reactivity. Here, time-lapse scanning tunneling microscopy (STM) and density functional theory (DFT)-based calculations are used to study how CO exposure affects the stability of Pt adatoms and subnano clusters at the Fe3O4(001) surface, a model CO oxidation catalyst. The results reveal that CO plays a dual role: first, it induces mobility among otherwise stable Pt adatoms through the formation of Pt carbonyls (Pt1–CO), leading to agglomeration into subnano clusters. Second, the presence of the CO stabilizes the smallest clusters against decay at room temperature, significantly modifying the growth kinetics. At elevated temperatures, CO desorption results in a partial redispersion and recovery of the Pt adatom phase.

Sign in / Sign up

Export Citation Format

Share Document