The specific distinct properties that separate one material from the other lie inside the internal atomic structure of materials. By examining the atomic structure of the material, an understanding of how different structure affects the properties, behavior, and resulting applications of engineering materials can be obtained.
Scientists have invented various instruments in order to study the material structure at various levels of observation. The finer details we can find in each level varies from one to the other. The degree of magnification and resolution applied to study material by various methods is termed a measure of the level of observation. While considering the structures at various levels it is useful to have some concept of relative sizes. The most popular “logarithmic scale of the universe (Fig. 1)” serves as one method of size comparison.
Type of Material Structures
Basic knowledge of the material structure will help science and engineering students to study the properties of the material as each type of material possess a specific composition for a specific property. Depending on the increased magnification level, the structure of materials can be classified into the following groups:
- Crystal structure
- Electronic structure
- Nuclear structure.
Macrostructure of a material:
- is examined with the naked eye or under low magnification.
- deals with the shape, size, and atomic arrangement in crystalline materials such as quartz.
- can be observed directly on a fracture surface or on a forging specimen
- exposes defects, segregations, cracks, etc.
Microstructure refers to the structure of the material that is
- observed under a low magnification (about 1500 times linear) optical microscope.
- Cracks, porosity, and non-metallic inclusions within materials can be exposed by examining them under an optical microscope.
Note that, Optical microscopes can
- magnify a structure about 1500 to 3000 times linear, without having any loss of resolution of details of the material structure.
- resolve two lines separately when their difference of separation is 10–7 m (= 0.1 µm).
When it is required to examine or study finer details of metal structure like crystal imperfections, displacement (dislocations) in a structure, etc. a special microscope possessing with much higher magnification and resolution than the optical microscope is used. This structure is termed as Sub Structure.
- For this purpose, an Electron microscope with magnifications of 106 times linear can be used.
- Other important modern-day microscopes are scanning electron microscope, electron probe micro analyzer, field ion microscope, etc. Field Ion Microscope has the ability to produce images of individual atoms as well as defects in atomic arrangements.
Crystal Structure reveals the atomic arrangement within a crystal.
- X-ray diffraction techniques and electron diffraction methods are generally used for examining the crystal structure of materials.
- It provides sufficient information regarding the arrangement of atoms within a unit cell.
- Crystals are composed of a large number of unit cells that form a pattern that repeats regularly in space.
In the center of an atom, the nucleus is present and on the outside of the nucleus, electrons are present. These electrons occupy energy levels in atoms. Different energy levels (known as electron shells) hold maximum numbers of electrons. The lowest available energy level occupies first in an atom and this is nearest to the nucleus. Energy levels increase gradually based on the levels up to the outermost level. Electronic Structure refers to the electrons in the outermost shells of individual atoms that form the solid. Spectroscopic techniques are commonly used to determine the electronic structure.
In the case of Nuclear Structure, every atom consists of a small dense nucleus at the center. And it carries more than 99.97% of the atomic mass. Protons and neutrons are tightly bounded with strong force at the nucleus. Nuclear Structure is studied by nuclear spectroscopic techniques, e.g. Mossbauer studies and nuclear magnetic resonance (NMR).
Reference and Further Studies
- Materials Science and Engineering by V. Raghavan