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3.0 ATOMIC STRUCTURE
3.1 Introduction
The structure of atoms affects the types of bonds that exist in different types of materials. These different types of bonds directly affect suitability of materials for real world engineering applications.
Both the composition and the structure of a material have a profound influence on its properties and behavior. Engineers and scientists who study and develop materials must understand their atomic structure. The properties of materials are controllable and can be tailored to the needs of a given application by controlling their structure and composition.
We can examine and describe the structure of materials at five different levels: (a) Macrostructure; (b) Microstructure; (c) Nanostructure; (d) Short- and long-range atomic arrangements; (e) Atomic structure. Engineers and scientists concerned with development and practical applications of advanced materials need to understand the microstructure and macrostructure of various materials and the ways of controlling them. Microstructure is the structure of material at a length-scale of ~10 to 1000 nm. Length-scale is a characteristic length or range of dimensions over which we are describing the properties of a material or the phenomena occurring in materials. Microstructure typically includes such features as average grain size, grain size distribution, grain shape, grain orientation, and other features related to defects in materials. A grain is a small crystal of the material within which the arrangement of atoms and repeats in a particular fashion in all three dimensions. Macrostructure is the structure of a material at a macroscopic level where the length scale is~>100,000 nm. Features that constitute macrostructure include porosity, surface coatings, and such features as internal or external micro-cracks.
It is also important to understand atomic structure and how the atomic bonds lead to different atomic or ionic arrangements in materials. The atomic structure includes all atoms and their arrangements, which constitute the building blocks of matter. It is from these building blocks that all the nano, micro, and macro-levels of structures emerge. The insights gained by
understanding atomic structure and bonding configurations of atoms and molecules are essential for the proper selection of engineering materials, as well as for developing new, advanced materials.
A close examination of atomic arrangement allows us to distinguish between materials that are amorphous or crystalline (those that exhibit periodic arrangements of atoms or ions). Amorphous materials have only short-range atomic arrangements while crystalline materials have short and long-range arrangements. In short-range atomic arrangements, the atoms or ions show a particular order only over relatively short distances.
For crystalline materials, the long-range atomic order is in the form of atoms or ions arranged in a three-dimensional pattern that repeats over much larger distances (from ~>100 nm to up to few cm).
3.2 The structure of materials: technological relevance In today’s world, information technology (IT), biotechnology, energy technology, environmental technology, and many other areas require smaller, lighter, faster, portable, more efficient, reliable, durable, and inexpensive devices. We want batteries that are smaller, lighter, and longer lasting. We need cars that are affordable, lightweight, safe, highly fuel efficient, and ‘‘loaded’’ with many advanced features, ranging from global positioning systems (GPS) to sophisticated sensors for airbag deployment.
Some of these needs have generated considerable interest in nanotechnology and micro-electro- mechanical systems (MEMS). A real-world example of the MEMS technology, Figure 3.1 shows a small accelerometer sensor obtained by the micromachining of silicon (Si). This sensor is used to measure acceleration in automobiles. The information is processed by a central computer and then used for controlling airbag deployment.
TABLE 3.1 Levels of structures
Level of Structure Example of Technologies
Atomic Structure Diamond: Diamond is based on carbon-carbon (C-C) covalent bonds. Materials with this type of bonding are expected to be relatively hard. Thin films of diamonds are used for providing a wear-resistant edge in cutting tools.
Atomic Arrangements:
Long-Range Order
(LRO)
Lead-zirconium-titanate [Pb(Zrx Ti1-x)O 3 ] or PZT: When ions in this material are arranged such that they exhibit tetragonal and/or rhombohedral crystal structures, the material is piezoelectric (i.e., it develops a voltage when subjected to pressure or stress). PZT ceramics are used widely for many applications including gas igniters, ultrasound generation, and vibration control.
Atomic Arrangements:
Short-Range Order
(SRO)
Ions in silica-based (SiO 2 ) glasses exhibit only a short-range order in which Si+^4 and O-^2 ions are arranged in a particular way (each Si+1^ is bonded with 4 O-^2 ions in a tetrahedral coordination). This order, however, is not maintained over long distances, thus making silica-based glasses amorphous. Amorphous glasses based on silica and certain other oxides form the basis for the entire fiber optical communications industry.
Nanostructure Nano-sized particles (~^5 – 10 nm) of iron oxide are used in ferro fluids or^ liquid magnets. These nano-sized iron oxide particles are dispersed in liquids and commercially used as ferrofluids. An application of these liquid magnets is as a cooling (heat transfer) medium for loudspeakers.
Microstructure The mechanical strength of many metals and alloys depends very strongly on the grain size. The grains and grain boundaries in this accompanying micrograph of steel are part of the microstructural features of this crystalline material. In general, at room temperature a finer grain size leads to higher strength. Many important properties of materials are sensitive to the microstructure.
Macrostructure Relatively thick coatings, such as paints on automobiles and other applications, are used not only for aesthetics, but also to provide corrosion resistance.
Figure 3.2 Diamond-coated cutting tools. [~Up to 10-^10 m (1 A˚]
Figure 3. 3 Piezoelectric PZT-based gas igniters. When the piezoelectric material is stressed (by applying a pressure) a voltage develops and a spark is created between the electrodes. [~ 10 -^10 to 10 -^9 m (1 to 10 A˚), ordering can exist up to a few cm in larger crystals]
Figure 3.6 A number of organic and inorganic coatings protect the steel in the car from corrosion and provide a pleasing appearance. [~>10-^4 m (100,000 nm)]
