Defects and Imperfections Flashcards
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Crystalline materials are inherently imperfect, as they contain defects or imperfections that disrupt the ideal atomic arrangement. These defects can significantly influence physical and chemical properties.
Classification of Defects
Defects in crystalline materials are classified based on geometry and dimensionality into three main categories.
Point Defects
Point defects are local, so-called zero-dimensional defects in a lattice. They consist of an atom and its immediate surroundings, no more than a few atomic layers away. As a reminder, the individual atoms in a covalent solid have no charge relative to each other.
Point defects in ceramics include vacancies, interstitials, impurities, Frenkel defects, and Schottky defects.
Linear Defects
One-dimensional disruptions along a line. Can weaken or strengthen a solid.
Linear defects, known as dislocations, are one-dimensional defects in the crystal structure.
Interfacial Defects
Interfacial defects are two-dimensional imperfections in a material that separate regions with different crystal structures. They are also known as planar defects.
Interfacial defects are two-dimensional defects occurring at the boundaries between different regions of a crystal.
Point: Vacancies
Occur when an atom is missing from its lattice site. They can form due to thermal vibrations or non-stoichiometric conditions.
Point: Interstitial Cations
Occupy normally unoccupied interstitial sites in the lattice, causing local distortion.
Point: Impurities
Foreign atoms introduced into the lattice that can substitute for host atoms or occupy interstitial sites.
Point: Frenkel Defects
Occur when a cation leaves its normal lattice position and occupies an interstitial site, creating a vacancy at its original position.
Point: Schottky Defects
Arise when equal numbers of cations and anions are missing from the lattice, creating vacancies for both types of ions.
Linear: Edge Dislocation
An extra half-plane of atoms is inserted into the crystal, creating a localized distortion at the edge of this plane.
Linear: Screw Dislocation
The layers of atoms are displaced in a spiral pattern around a central line.
Interfacial: Grain Boundaries
Interfaces where two grains of different orientations meet within a polycrystalline ceramic.
Interfacial: Twin Boundaries
Boundaries characterized by mirror symmetry in the arrangement of atoms.
Interfacial: Phase Boundaries
Boundaries between different phases of a ceramic material.
Porosity
Porosity refers to the void spaces within a material, expressed as a percentage of the total volume, and significantly affects mechanical, thermal, and transport properties. It is common in ceramics, metals, polymers, and composites during processes like sintering, casting, or additive manufacturing.
Open Porosity
Pores that are interconnected and accessible from the surface, influencing permeability and absorption.
Closed Porosity
Isolated pores that are not connected to the surface, affecting density and strength but not permeability.
Total Porosity
The sum of open and closed porosity, representing the total void fraction.
Methods of measuring porosity
- Archimedes’ Method: Measures material volume and the displaced fluid volume.
- Mercury Intrusion Porosimetry: Injects mercury into pores under pressure to determine pore size and total porosity.
- Gas Pycnometry: Uses gas displacement to measure solid volume and open porosity.
Effects of Porosity
Higher porosity can reduce strength and stiffness due to stress concentrators, leading to increased brittleness. It also decreases density by reducing the solid material per volume. In terms of thermal properties, porosity results in lower thermal conductivity due to trapped air or gas acting as an insulator, which is beneficial for insulation applications. Additionally, open porosity increases permeability for fluids and gasses, which is crucial for filtration and biomedical implants, while porous materials can absorb sound waves, making them useful for noise reduction.
