Defects and Imperfections (Mat Sci 3)
Defects and Imperfections
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.
Defect classification. Callister, W. D., & Rethwisch, D. G. (2018). Materials Science and Engineering: An Introduction.
Classification of Defects
Defects in crystalline materials are classified based on geometry and dimensionality into three main categories:
Point Defects: Localized disruptions associated with one or two atomic positions.
Linear Defects: One-dimensional disruptions along a line.
Interfacial Defects: Two-dimensional boundaries.
Point Defects
Point defects in ceramics include vacancies, interstitials, impurities, Frenkel defects, and Schottky defects.
Vacancies: Occur when an atom is missing from its lattice site. They can form due to thermal vibrations or non-stoichiometric conditions.
Interstitial Cations: Occupy normally unoccupied interstitial sites in the lattice, causing local distortion.
Impurities: Foreign atoms introduced into the lattice that can substitute for host atoms or occupy interstitial sites.
Frenkel Defects: Occur when a cation leaves its normal lattice position and occupies an interstitial site, creating a vacancy at its original position.
Schottky Defects: Arise when equal numbers of cations and anions are missing from the lattice, creating vacancies for both types of ions.
A visualization of vacancy and interstitial. Callister, W. D., & Rethwisch, D. G. (2018). Materials Science and Engineering: An Introduction.
Linear Defects
Linear defects, known as dislocations, are one-dimensional defects in the crystal structure.
Edge Dislocation: An extra half-plane of atoms is inserted into the crystal, creating a localized distortion at the edge of this plane.
Screw Dislocation: The layers of atoms are displaced in a spiral pattern around a central line.
Interfacial Defects
Interfacial defects are two-dimensional defects occurring at the boundaries between different regions of a crystal.
Grain Boundaries: Interfaces where two grains of different orientations meet within a polycrystalline ceramic.
Twin Boundaries: Boundaries characterized by mirror symmetry in the arrangement of atoms.
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. The types of porosity include:
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.
Porosity can be measured using several techniques:
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.
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.