scholarly journals Atomistic Simulation of Nanocrystalline Materials

1995 ◽  
Vol 400 ◽  
Author(s):  
D. Wolf ◽  
S. R. Phillpot ◽  
P. Keblinski
Keyword(s):  
Grain Size ◽  
Grain Boundaries ◽  
Nanocrystalline Materials ◽  
Atomistic Simulation ◽  
Nanocrystalline Silicon ◽  
High Energy ◽  
Atomistic Simulations ◽  
Silicon Based ◽  
Critical Grain Size ◽  
Energy Argument

AbstractAtomistic simulations show that high-energy grain boundaries in nanocrystalline copper and nanocrystalline silicon are highly disordered. In the case of silicon the structures of the grain boundaries are essentially indistinguishable from that of bulk amorphous silicon. Based on a free-energy argument, we suggest that below a critical grain size nanocrystalline materials should be unstable with respect to the amorphous phase.

What is Behind the Inverse Hall-Petch Behavior in Nanocrystalline Materials?

2006 ◽  
Vol 976 ◽  
Author(s):  
Christopher Carlton ◽  
P. J. Ferreira
Keyword(s):  
Grain Size ◽  
Yield Strength ◽  
Strain Rate ◽  
Grain Boundaries ◽  
Nanocrystalline Materials ◽  
Critical Value ◽  
Petch Relationship ◽  
And Diffusion ◽  
Critical Grain Size

AbstractAn inverse Hall-Petch effect has been observed for nanocrystalline materials by a large number of researchers. This result implies that nanocrystalline materials get softer as grain size is reduced below a critical value. Postulated explanations for this behavior include dislocation based mechanisms and diffusion based mechanisms. In this paper, we report an explanation for the inverse Hall-Petch effect based on the statistical absorption of dislocations by grain boundaries, showing that the yield strength is both dependent on strain rate and temperature, and that it deviates from the Hall-Petch relationship at a critical grain size.

scholarly journals Comparison of the Structure of Grain Boundaries in Silicon and Diamond by Molecular-Dynamics Simulations

1997 ◽  
Vol 472 ◽  
Author(s):  
P. Keblinski ◽  
S. R. Phillpot ◽  
D. Wolf ◽  
H. Gleiter
Keyword(s):  
Molecular Dynamics ◽  
Grain Size ◽  
Molecular Dynamics Simulations ◽  
Grain Boundaries ◽  
Amorphous Silicon ◽  
Electrical Conduction ◽  
Large Fraction ◽  
Nanocrystalline Silicon ◽  
High Energy ◽  
Dynamics Simulations

ABSTRACTMolecular-dynamics simulations were used to synthesize nanocrystalline silicon with a grain size of up to 75 Å by crystallization of randomly misoriented crystalline seeds from the melt. The structures of the highly-constrained interfaces in the nanocrystal were found to be essentially indistinguishable from those of high-energy bicrystalline grain boundaries (GBs) and similar to the structure of amorphous silicon. Despite disorder, these GBs exhibit predominantly four-coordinated (sp3-like) atoms and therefore have very few dangling bonds. By contrast, the majority of the atoms in high-energy bicrystalline GBs in diamond are three-coordinated (sp2-like). Despite the large fraction of three-coordinated GB carbon atoms, they are rather poorly connected amongst themselves, thus likely preventing any type of graphite-like electrical conduction through the GBs.

Mechanical Properties of Novel Nanocrystalline Materials

2001 ◽  
Author(s):  
J. Narayan ◽  
H. Wang ◽  
A. Kvit
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Nanocrystalline Materials ◽  
High Surface ◽  
Grain Boundary Sliding ◽  
Boundary Shear ◽  
Functionally Gradient ◽  
Boundary Sliding ◽  
First Time ◽  
Critical Grain Size

Abstract We have synthesized nanocrystalline thin films of Cu, Zn, TiN, and WC having uniform grain size in the range of 5 to 100 nm. This was accomplished by introducing a couple of manolayers of materials with high surface and have a weak interaction with the substrate. The hardness measurements of these well-characterized specimens with controlled microstructures show that hardness initially increases with decreasing grain size following the well-known Hall-Petch relationship (H∝d−½). However, there is a critical grain size below which the hardness decreases with decreasing grain size. The experimental evidence for this softening of nanocrystalline materials at very small grain sizes (referred as reverse Hall-Petch effect) is presented for the first time. Most of the plastic deformation in our model is envisioned to be due to a large number of small “sliding events” associated with grain boundary shear or grain boundary sliding. This grain-size dependence of hardness can be used to create functionally gradient materials for improved adhesion and wear among other improved properties.

scholarly journals Spatially-varying inversion near grain boundaries in MgAl2O4 spinel

RSC Advances ◽  
2020 ◽  
Vol 10 (20) ◽  
pp. 11737-11742
Author(s):  
Blas P. Uberuaga ◽  
Romain Perriot
Keyword(s):  
Grain Size ◽  
Grain Boundaries ◽  
Atomistic Simulations ◽  
Mgal2o4 Spinel ◽  
Spatially Varying ◽  
Apparent Level

Atomistic simulations reveal increased cation inversion at grain boundaries in spinel. As the grain size is reduced, the apparent level of inversion in the material will increase as the grain boundaries become an increasing fraction of the material.

Effect of RF or VHF Plasma on Nanocrystalline Silicon Thin Film Structure: Insight from OES and Langmuir Probe Measurements

2013 ◽  
Vol 1536 ◽  
pp. 161-166
Author(s):  
Lala Zhu ◽  
Ujjwal K Das ◽  
Steven S Hegedus ◽  
Robert W Birkmire
Keyword(s):  
Grain Size ◽  
Langmuir Probe ◽  
Volume Fraction ◽  
Nanocrystalline Silicon ◽  
Low Pressure ◽  
High Energy ◽  
Rf Plasma ◽  
Crystalline Volume Fraction ◽  
Langmuir Probe Measurements ◽  
Probe Measurements

ABSTRACTOptical emission spectroscopy (OES) and Langmuir Probe were used to characterize RF and VHF plasma properties under conditions leading to nanocrystalline silicon film deposition. Films deposited by RF plasma at low pressure (3 Torr), even with high crystalline volume fraction, show weak X-ray diffraction signals, suggesting small grain size, while RF films at higher pressure (8 Torr) and VHF films at both high and low pressure have larger grain sizes. The preferential growth orientation is controlled by the H2/SiH4 ratio with RF plasma, while the film deposited by VHF shows primarily (220) orientation independent of H-dilution ratio. Langmuir Probe measurements indicate that the high energy electron population is reduced by increasing pressure from 3 Torr to 8 Torr in RF plasma. Compared with RF plasma, the VHF plasma shows higher electron density and sheath potential, but lower average electron energy, which may be responsible for the larger grain size and crystal orientation. The growth rate and crystalline volume fraction of the film is correlated with OES intensity ratio of SiH* and Hα/SiH* for both RF and VHF plasmas.

Novel Nanostructured Metal and Ceramic Composites

2002 ◽  
Vol 750 ◽  
Author(s):  
J. Narayan
Keyword(s):  
Grain Size ◽  
Nanocrystalline Materials ◽  
Ceramic Composites ◽  
Interfacial Region ◽  
Uniform Size ◽  
Boundary Shear ◽  
Synthesis Procedure ◽  
And Control ◽  
Critical Grain Size ◽  
Nanostructured Metal

ABSTRACTWe have designed a unique synthesis procedure to create nonomaterials of uniform grain size and control the chemistry of interfaces between the grains. The metastability of nanocrystalline materials is a major challenge which can be addressed by controlling the chemistry of interfaces. The hardness of these films having a uniform size was measured as a function of grain size using a nanoindentation technique. It was found that hardness increased with decreasing grain size in accordance with Hall-Petch model. However, below a critical grain size we observed a decrease or softening with a further decrease in grain size. These observations in metals and ceramics are modeled in view of intragrain deformation (Hall-Petch regime) and intergrain deformation (grain boundary shear/sliding )in the softening regime. Since we can change the alloying of interfacial region, we can address the metastability as a function of temperature, which is crucial from applications viewpoint of these materials.

Diffraction studies of cubic phase stability in undoped zirconia thin films

2000 ◽  
Vol 15 (2) ◽  
pp. 369-376 ◽  
Author(s):  
S. C. Moulzolf ◽  
R. J. Lad
Keyword(s):  
Grain Size ◽  
Room Temperature ◽  
Elevated Temperatures ◽  
Electron Cyclotron Resonance ◽  
High Energy ◽  
Electron Beam Evaporation ◽  
Fiber Axis ◽  
Cubic Phase ◽  
X Ray Diffraction ◽  
Critical Grain Size

Pure stoichiometric ZrO2 films were deposited on amorphous silica substrates by electron beam evaporation of Zr in the presence of an electron cyclotron resonance oxygen plasma. Grain size, strain, and texture were analyzed by x-ray diffraction and reflection high-energy electron diffraction. Films grown at room temperature are polycrystalline and exist in the cubic phase. Growth at elevated temperatures produces coexisting cubic and monoclinic phases and shows a maximum critical grain size of ??~10 nm for stabilization of the cubic phase. Pole figure analysis indicates a preferred cubic [200] fiber axis for room-temperature growth and dual monoclinic {111} and in-plane textures for films grown at 400 °C. Postdeposition annealing experiments confirm the existence of a critical grain size and suggest mechanisms for grain growth.

Analysis of Oxygen Vacancy Trapping at [001] Symmetric Tilt Grain Boundaries in Barium Titanate by Atomistic Simulations

2010 ◽  
Vol 445 ◽  
pp. 39-42 ◽  
Author(s):  
Takashi Oyama ◽  
Nobuyuki Wada ◽  
Hiroshi Takagi
Keyword(s):  
Barium Titanate ◽  
Grain Boundaries ◽  
Atomistic Simulation ◽  
Oxygen Vacancies ◽  
Atomistic Simulations ◽  
Multilayer Ceramic Capacitors ◽  
Simulation Techniques ◽  
Electrical Degradation ◽  
Symmetric Tilt

The role of grain boundaries (GBs) in the diffusion of oxygen vacancies (VO••s) in barium titanate (BaTiO3) and its mechanism were investigated using atomistic simulation techniques. It was found that GBs trapped VO••s at specific sites in the course of the diffusion, and the excess energy reflecting structural distortion of the GB was closely related to the availability of the trapping. GBs therefore act as a resistance of the diffusion of VO••s, suggesting that electrical degradation of multilayer ceramic capacitors (MLCCs), which is derived from vacancy diffusion, enables to be additionally improved by controlling GB structures in BaTiO3-based dielectrics.

Atomistic Simulations of [001] Symmetric Tilt Boundaries in Ni3Al

1986 ◽  
Vol 81 ◽  
Author(s):  
S. P. Chen ◽  
A. F. Voter ◽  
D. J. Srolovitz
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Simulation Study ◽  
Atomistic Simulation ◽  
Atomistic Simulations ◽  
Boundary Composition ◽  
Symmetric Tilt ◽  
Tilt Boundaries ◽  
Cohesive Energies

AbstractWe report a systematic atomistic simulation study of [001] symmetric tilt grain boundaries (GB) in Ni3Al, Ni, and Al. We found that the grain boundary energies and cohesive energies of Ni3Al and pure fcc Ni are approximately thesame. Grain boundary energies aid cohesive energies in Ni3Al depends stronglyon the grain boundary composition. The Al-rich boundaries have highest grain boundary energies and lowest cohesive energies. This offers an explanation for the stoichiometric effect on the boron ductilization

Properties of Copper-Based Composite Reinforced with Al2O3 Particles of Different Size

2014 ◽  
Vol 875-877 ◽  
pp. 318-323
Author(s):  
Viseslava Rajkovic ◽  
Dusan Bozic ◽  
Jelena Stasic ◽  
Milan T. Jovanovic ◽  
Huai Wen Wang
Keyword(s):  
Grain Size ◽  
Grain Boundaries ◽  
Grain Growth ◽  
Synergetic Effect ◽  
Copper Matrix ◽  
High Energy ◽  
Inert Gas ◽  
Al2o3 Powder ◽  
Al Powder ◽  
Al2o3 Particles

Copper matrix was simultaneously reinforced with nano- and micro-sized Al2O3 particles via high-energy milling of the mixture of inert gas-atomized prealloyed Cu-1 wt.% Al powder and 0.6 wt.% commercial Al2O3 powder. At the maximum of microhardness (2400 MPa) the grain size reaches the smallest value as a result of the synergetic effect of nano- and micro-sized Al2O3 particles. The relatively low decrease in microhardness during HTE may be explained by grain growth which is retarded by Al2O3 nano-sized particles precipitated at the grain boundaries.

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