Nanomaterials: Properties, Defects, and Impact on Material Behavior, Lecture notes of Environmental science

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Nano Science Properties References: Text book of Nanoscience and Nanotechnology by B S Murthy

Contents

Microstructure and defects in nanomaterials

 Dislocations, twins, stacking faults

 Voids and Grain boundaries

Effects of nanodimensions on material behavior

 Mechanical properties

 Melting point and diffusivity

Assignment Questions

Nano Materials Point defects Vacancies Interstitial Substitutional Shottky Frankle Line defects Screw Dislocations Edge Dislocations Volume defects Voids Inclusions Microcracks

Defects

Surface

Defects

Grain

boundaries

Twins,

Stacking

faults

Free

surfaces

Point Defect: 0D defect: Localized interruption in the regularity of crystal lattice Line Defect: 1D defect: eg. Missing row of atoms Surface defects: 2D interface which separates two regions Volume Defect: 3D defects

Point Defects Vacancies: Absence of an atom Produced due to thermal vibrations of atoms at high temperature Shottky: Missing ionic pair Produced to maintain charge neutrality Interstitial: small size atom in a void Substitutional: Foreign atom replacing the host (due to Impurities) Frankel : Vacancy-interstitial pair (cation into interstitial) Vacancies, Interstitial and substitutional defects are observed in metals/alloys Shottky and Frankel defects are observed in ionic solids

Surface Defects Twin Multiple Twin: five fold (decahedral structure) Perfect crystal Stacking fault External Surfaces: External surface itself is a defect Grain boundaries: (In a Separate slide) Twins: A special type of grain boundary Atoms on either side are in mirror image positions. Due to deformation under high strain rate or low temperature during growth Stacking faults: A crystal is formed by the stacking of layers. Eg: ABC, ABC, ABC… in FCC If one such layers is missing, ABC, ABC, BCA,BCA is a stacking fault. Produced during crystal growth. Twins and stacking faults usually neglected in bulk. But nanomaterials show multiple twinning resulting in various structures

Grain boundary

• (^) Important planar defect which separates two crystalline orientations (grains) in poly crystalline structure.
• (^) Produced due to tilt or twist in the atomic arrangement during solidification of crystal growth depending on rate of cooling.
• (^) Atoms are less bounded, less regular and there is a wide distribution in lattice parameter.
• (^) High energy regions, which depend on the degree of misalignment atoms.
• (^) Large density of grin boundaries in nanomaterials

Melting Point

• (^) The melting point is the temperature, at which the phase transition from solid to

liquid state begins.

• (^) The decrease in boundary energy due to enhanced surface and grain boundary area

in in nanocrystalline materials can reduce melting point.

• (^) At any higher temperature, the solid possesses energy due to the vibrations of he

atoms about the mean position. The amplitude of vibrations increase with the

increase in temperature. When the vibration amplitude exceeds a certain percentage

of bond length, melting begins at surface and propagates through the solid.

• (^) Atoms at the surface and grain boundary are less constrained to vibrate compared to

atom inside the crystal lattice. As the grain size decreases, the percentage of atoms

residing at surfaces and grain boundaries increases significantly.

• (^) Hence free standing nanoparticles may show a lower melting pint compared to bulk.
• (^) In contrast, nanoparticles with in a matrix may, in fact experience an enhancement in

melting temperature

• (^) Melting temperature does not continuously decrease with decrease in grain size but

bellow a certain critical limit of the cluster size.

Diffusivity Diffusion: Is the net movement of molecules from a region of higher concentration to region of lower concentration. Diffusivity is rate of change of diffusion.

• (^) Diffusion kinetics increase with increasing defect content of the materials, like vacancy concentration. Grain boundaries and dislocation cores provide easy diffusion paths compared to bulk lattice, because the structure is more defective/open. Defects provide a high density of short circuit diffusion paths.
• (^) Thus, nano crystalline materials are expected to exhibit enhanced self diffusivity compared to bulk.
• (^) The measured diffusivity in crystalline Cu are about 14-20 orders of magnitude higher.
• (^) where Q is activation energy [ The energy that is required to move neighboring atoms classically, where +f Where is grain boundary size; l, gb, d and s stands for lattice, grain boundary, dislocation surface respectively
• (^) [ activation energy corresponds to lattice, dislocation, grain bounadary respectively]
• (^) for bulk but for nanocrystalline materials, diffusion across grain boundary dominates (nano) by almost 20 orders of magnitude

Peculiar Properties at

nanoscale

• (^) Elastic moduli were found 30 to 50% lower
• (^) Hardness and strength were found very high
• (^) For metals increased 2-7 times
• (^) Nanoscale multi layers show ultra high hardness

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