scholarly journals Cooperative Chemical Rebonding In The Segregation Of Impurities In Silicon Grain Boundaries

1996 ◽  
Vol 442 ◽  
Author(s):  
A. Maiti ◽  
M. F. Chisholm ◽  
S. J. Pennycook ◽  
S. T. Pantelides
Keyword(s):  
Grain Boundary ◽  
Ab Initio ◽  
Grain Boundaries ◽  
Structural Transformation ◽  
Atomic Resolution ◽  
Segregation Energy ◽  
Site Variation ◽  
Segregation Of Impurities

AbstractWith ab initio calculations we show that the experimentally observed large segregation energies of As at Si grain boundaries can be explained by the formation of isolated dimers or ordered chains of dimers of threefold-coordinated As along the cores of grain boundary dislocations. We also find the intriguing possibility that As segregation may drive structural transformation of certain grain boundaries. Recently we have obtained the first atomic-resolution STEM images of As in a Si grain boundary, consistent with the formation of As dimers. Segregation energy of As dimers was found to be significantly higher in isolated dislocation cores, where larger site-variation in strain than in grain boundaries lead to further lowering of the electronic levels of As deep into the bandgap.

scholarly journals Segregation of P and S Impurities to A Σ9 Grain Boundary in Cu

Metals ◽  
2020 ◽  
Vol 10 (10) ◽  
pp. 1362
Author(s):  
Cláudio M. Lousada ◽  
Pavel A. Korzhavyi
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Coordination Number ◽  
High Performance ◽  
Chemical Properties ◽  
Atomic Scale ◽  
Atomic Site ◽  
Segregation Energy ◽  
Physical Chemical ◽  
Symmetric Tilt

The segregation of P and S to grain boundaries (GBs) in fcc Cu has implications in diverse physical-chemical properties of the material and this can be of particular high relevance when the material is employed in high performance applications. Here, we studied the segregation of P and S to the symmetric tilt Σ9 (22¯1¯) [110], 38.9° GB of fcc Cu. This GB is characterized by a variety of segregation sites within and near the GB plane, with considerable differences in both atomic site volume and coordination number and geometry. We found that the segregation energies of P and S vary considerably both with distance from the GB plane and sites within the GB plane. The segregation energy is significantly large at the GB plane but drops to almost zero at a distance of only ≈3.5 Å from this. Additionally, for each impurity there are considerable variations in energy (up to 0.6 eV) between segregation sites in the GB plane. These variations have origins both in differences in coordination number and atomic site volume with the effect of coordination number dominating. For sites with the same coordination number, up to a certain atomic site volume, a larger atomic site volume leads to a stronger segregation. After that limit in volume has been reached, a larger volume leads to weaker segregation. The fact that the segregation energy varies with such magnitude within the Σ9 GB plane may have implications in the accumulation of these impurities at these GBs in the material. Because of this, atomic-scale variations of concentration of P and S are expected to occur at the Σ9 GB center and in other GBs with similar features.

The “Solidification” of Grain Boundaries with Increasing Temperature

1994 ◽  
Vol 357 ◽  
Author(s):  
Witold Lojkowski ◽  
Bogdan Palosz
Keyword(s):  
Solid State ◽  
Melting Point ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
High Temperatures ◽  
Low Temperatures ◽  
Melting Behavior ◽  
Wetting Transitions ◽  
Increasing Temperature ◽  
Segregation Of Impurities

AbstractThe aim of the paper is to explain the recently observed de-wetting grain boundary transition with increasing temperature. On the example of a bicrystal from the Fe-6at.%Si alloy, it was found recently that as temperature is increased, the following GB transitions take place: “solid” (or regular) GB-→“premelted” GB →“solid” GB. At the same time the wetting/de-wetting transitions have taken place. Another example of such GB behavior was discovered during sintering of alumina. The inverse melting behavior is explained as follows: low melting point impurities cause GB premelting at low temperatures, However de-segregation of impurities at high temperatures causes return of the GB structure to its regular “solid” state.

Modeling Boron Diffusion in Polycrystalline HfO2 Films

2002 ◽  
Vol 716 ◽  
Author(s):  
Chun-Li Liu
Keyword(s):  
Grain Boundary ◽  
Ab Initio ◽  
Grain Boundaries ◽  
Boundary Diffusion ◽  
Bulk Diffusion ◽  
Concentration Profiles ◽  
Boron Diffusion ◽  
Gate Stacks ◽  
Modeling Prediction ◽  
Sims Analysis

AbstractWe present ab-initio modeling results including formation, migration, and activation energies for B diffusion through bulk and grain boundaries in polycrystalline HfO2 films. Modeling results clearly indicate that B can penetrate through a 40 Å HfO2 film via grain boundary diffusion, but not by bulk diffusion. SIMS analysis of B concentration profiles for polysilicon/HfO2/Si gate stacks after different anneals showed double B peaks at the interfaces and thus confirmed the modeling prediction.

scholarly journals Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO3

1999 ◽  
Vol 586 ◽  
Author(s):  
Miyoung Kim ◽  
Nigel D. Browning ◽  
Stephen J. Pennycook ◽  
Karl Sohlberg ◽  
Sokrates T. Pantelides
Keyword(s):  
Grain Boundary ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
Grain Boundaries ◽  
Theoretical Calculations ◽  
Intrinsic Property ◽  
Dislocation Core ◽  
Positive Temperature Coefficient ◽  
Positive Temperature ◽  
Z Contrast

ABSTRACTThe understanding of electrical properties of grain boundaries in perovskites is essential for their application to capacitors, varistors and positive-temperature coefficient resistors. The origin of the electrical activity is generally attributed to the existence of charged defects in grain boundaries, usually assumed to be impurities, which set up a double Schottky barrier as they are screened by dopants in the adjacent bulk crystal. Microscopic understanding of the origin of the grain boundary charge, however, has not been achieved. It is not known yet if the charged grain boundary states are an intrinsic property of a stoichiometric grain boundary, arise from nonstoichiometry, or are caused by impurities. Here, the relation between atomic structure and electronic properties is studied by combining experiment with ab-initio calculations. The starting structures for theoretical calculations were obtained from Z-contrast images combined with electron energy loss spectroscopy to resolve the dislocation core structures comprising the boundary. Dislocation core reconstructions are typical of all grain boundaries so far observed in this material. They avoid like-ion repulsion, and provide alternative sites for cation occupation in the grain boundaries. Optimized atomic positions are found by total energy calculations. Calculated differences in vacancy formation energies between the grain boundaries and the bulk suggest that vacancy segregation can account for the postulated grain boundary charge.

Effects of Impurities And Alloying Elements on Iron Grain Boundary Cohesion

1997 ◽  
Vol 475 ◽  
Author(s):  
D.E. Ellis ◽  
X. Chen ◽  
G.B. Olson
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Physical And Mechanical Properties ◽  
Alloying Elements ◽  
Metallic Materials ◽  
Crucial Importance ◽  
Intergranular Embrittlement ◽  
Long Time ◽  
Impurity Doped ◽  
Segregation Of Impurities

In metallic materials, where grain boundaries(GB) are of crucial importance, impurities and alloying elements play an important role in determining their physical and mechanical properties because the behavior of a grain boundary may change drastically with the presence of impurities and alloying elements. For example, in iron and its alloys, including industrially important steels, the intergranular embrittlement is usually associated with segregation of impurities, like P and S, toward the GBs. On the other hand, alloying elements, like Mo and Pd, are helpful for intergranular cohesion in iron, due to either direct cohesion effect or effect upon embrittling potency of other impurities. Understanding the mechanisms of impurity-promoted embrittlement and the consequent cohesion(decohesion) effects is becoming more and more important and remains as a challenge for materials scientists. There have been intensive investigations on these mechanisms for a long time and with the progress in computing techniques in recent years, calculations on more realistic representations of impurity-doped grain boundaries have become possible[1–4].

STEM Investigation of Segregation and Local Structure at Grain Boundary in Tetragonal Zirconia

1997 ◽  
Vol 3 (S2) ◽  
pp. 549-550
Author(s):  
H. Gu ◽  
F. Wakai
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Solid Electrolytes ◽  
Tetragonal Zirconia ◽  
Boundary Structure ◽  
Contrast Imaging ◽  
Amorphous Films ◽  
Grain Boundary Structure ◽  
Z Contrast ◽  
Segregation Of Impurities

Y or Ca stabilized tetragonal ZrO2 (TZP) exhibits superplasticity at high temperature, and can also be used as solid electrolytes. Those properties are dictated by structure and chemistry of grain boundaries, which can be controlled by segregation of impurities or additives. The grain boundaries were found either covered by amorphous films or free of the film. Co-segragation of additives and stabilizers has also been observed. To fully understand the correlation between segregation and grain boundary structure, a dedicated STEM (VG HB601) capable of EDX/EELS analysis and phase/Z-contrast imaging is employed to study 3Y-TZP doped with 0.3 and 0.9 mol% SiO2.Although Y-L lines arc dominated by overlapping Zr-L lines in EDX, Y excess at grain boundaries can still be measured by “spatial difference” which removes Zr signal with a spectrum from the bulk. The co-segregation of Si and Y is also observed (Fig. 1) at many boundaries. Their average excesses arc 5±2 nm−2and 25±10 run−2 respectively, close to 1 monolayer each of SiO2 and Y2O3.

Investigations of the Bonding Changes Associated with Grain Boundary Embrittlement

1996 ◽  
Vol 458 ◽  
Author(s):  
V. J. Keast ◽  
J. Bruley ◽  
D. B. Williams
Keyword(s):  
Electronic Structure ◽  
Grain Boundary ◽  
Energy Loss ◽  
Grain Boundaries ◽  
Edge Structure ◽  
Grain Boundary Embrittlement ◽  
Impurity Elements ◽  
Electron Energy Loss ◽  
Segregation Of Impurities ◽  
Boundary Embrittlement

ABSTRACTThe embrittlement of materials through the segregation of impurities to the grain boundaries is a common and industrially important problem. Despite considerable investigation, the mechanism by which the impurity elements cause embrittlement is not well understood. A change in the electron energy loss near edge structure (ELNES) has been observed at Cu grain boundaries containing Bi. This result provides experimental evidence that a change in the electronic structure at the grain boundary is responsible for embritdement.

scholarly journals First-Principles Simulations and Z-Contrast Imaging of Impurities at Tilt Grain Boundaries in Mgo

1997 ◽  
Vol 492 ◽  
Author(s):  
Y. Yan ◽  
M. F. Chisholm ◽  
G. Duscher ◽  
S. J. Pennycook ◽  
A. Maiti ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
First Principles ◽  
Density Functional Calculations ◽  
Density Functional ◽  
Equilibrium Structure ◽  
Atomic Resolution ◽  
Contrast Imaging ◽  
Metastable Structures ◽  
Z Contrast

ABSTRACTFirst-principles density-functional calculations were used to study the effects of Ca impurities on the Σ=5 (310) <001> tilt grain boundaries in MgO. An equilibrium structure and two metastable structures of the grain boundaries in pure MgO have been established. The calculations further demonstrated that Ca impurities segregate at particular sites in the metastable grain boundary and induce a structural transformation. This result is consistent with atomic resolution Z-contrast imaging. The calculations also found that the impurities at the grain boundaries do not induce states in the band gap. The mechanism of the transformation is also discussed.

Atomic Resolution Studies of Tilt Grain Boundaries in NiO

1985 ◽  
Vol 60 ◽  
Author(s):  
K. L. Merkle ◽  
J. F. Reddy ◽  
C. L. Wiley ◽  
David J. Smith ◽  
G. J. Wood
Keyword(s):  
Electron Microscopy ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Mirror Symmetry ◽  
High Resolution Electron Microscopy ◽  
Atomic Resolution ◽  
High Angle ◽  
Structural Units ◽  
Atomic Structures ◽  
Resolution Electron

AbstractThe atomic structures of a number of <001> high-angle tilt grain boundaries in NiO have been studied by high-resolution electron microscopy (HREM). Crystal 1inity is always maintained right up to the grain boundary (GB). Grain boundary planes bounded by a (100)-plane are preferred, however symmetrical facets are also found at each misorientation. A tendency to match atomic planes across the GB is not only observed in symmetrical, but also in asymmetrical GBs. Structural units can be clearly recognized in symmetrical GBs. Contrast differences suggest that a multiplicity of structural units exists for some GB configurations. Frequently symmetric GBs also show deviations from mirror symmetry. Multislice simultations indicate that the image contrast associated with HREM GB images is not particularly sensitive to GB relaxation.

Ab-Initio Determination of the Atomic Structure of Symmetrical Tilt Grain Boundaries in BCC Transition Metals

1997 ◽  
Vol 492 ◽  
Author(s):  
C. Elsässer ◽  
O. Beck ◽  
T. Ochs ◽  
B. Meyer
Keyword(s):  
Transition Metals ◽  
Grain Boundary ◽  
Ab Initio ◽  
Grain Boundaries ◽  
Density Functional ◽  
Local Density ◽  
High Symmetry ◽  
Many Body ◽  
Local Density Functional Theory ◽  
Symmetrical Tilt

ABSTRACTAtomistic simulations of grain-boundary structures in body-centered cubic transition metals have revealed that angle-dependent contributions to interatomic interactions are essential. Unfortunately, the results of presently available empirical many-body potentials are not yet always sufficiently reliable for quantitative theoretical predictions of grain-boundary structures, which are consistent with experimental observations, e.g. by high-resolution transmission electron microscopy.Ab-initio electronic-structure calculations based on the local-density-functional theory offer the possibility to determine accurately the microscopic structures of special, high-symmetry grain boundaries, which can be used as data bases for the improvement of empirical many-body potentials. Such ab-initio calculations, with a mixed-basis pseudopotential method and grain-boundary supercells, are presented for Σ5 (310) [001] 36.87° symmetrical tilt grain boundaries in Niobium and Molybdenum.

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