The structure of a material may be divided into four levels: atomic structure, atomic arrangement, microstructure, and macrostructure. Although the main thrust of [the materials engineer] is to understand and control the microstructure and macro-structure of various materials, [she] must first understand the atomic and crystal structures.
Atomic structure influences how the atoms are bonded together, which in turn helps one to categorize materials as metals, ceramics, and polymers and permits us to draw some general conclusions concerning the mechanical properties and physical behavior of these three classes of materials.
Taken from The Science and Engineering of Materials, Donald R. Askeland, Alibris.
This first image shows the coordination polyhedra for a superconductor material as shown on the Ceramics web page. It represents the basic repeat unit that, when aggregated with about 10^20 similar units, will create a monolith of the superconductor somewhat less in size than one cubic centimeter. The crystalline unit cell is one aspect of structure that the materials engineer must understand to produce functional superconductor devices. However, there are other aspects of a material's structure that too must be considered. The purpose of this web page is to introduce the reader to the concept of structure.
Let us begin our discussion of structure by first considering the crystal structure of perovskites. Perovskites are a large family of crystalline ceramics that derive their name from a specific mineral known as perovskite. They are the most abundant minerals on earth and have been of continuing interest to geologists for the clues they hold to the planet's history. The parent material, perovskite, was first described in the 1830's by the geoologist Gustav Rose, who named it after the famous Russian mineralogist Count Lev Aleksevich von Perovski. Currently, the most intensely studied perovskites are those that superconduct at liquid nitrogen temperatures. Superconducting perovskites were first discovered by IBM researchers Bednorz and Mueller who were examining the electrical properties of a family of materials in the Ba-La-Cu-O system. The crystal model above is a perovskite.
The coordination polyhedra is only one way to represent a crystalline unit cell. Another way is to use a ball and stick model, with the balls representing atoms and the sticks, bonds between the atoms. Two representations of this are illustrated below.
First, let us consider a basic unit cell, a cubic crystal system, as seen in three dimensions. Those of us who lack 3-dimensional depth perception can sometimes gain 3D information by moving our heads slightly from left to right while looking at an object. Similarly, all of us can project a 3-dimensional cube onto a 2-dimensional screen and then rotate it to provide information on its 3D nature. In other words, we can use a 2D perspective projection extrapolated to a 3D impression. To the left, you see a GIF animation of a unit cell of a three-dimensional (3D) crystal. The reference source URL is University of Missouri at St. Louis
Other Internet sources showing the "ball and stick" unit cell structures of some of the more common materials are as follows: crystallography web pages from this University of Oxford site. Another interesting site explaining simple solid crystal structures is this Non-Destructive Testing site. You are encouraged to visit and explore these links.
If you link to the following pages, then you may get a better idea as to why this crystal-structure-stuff is so important. The image on the left is from a press release by the Tokyo Institute of Technology, Japan. The article is a little complex, but just consider the topic: "Development of a Superionic Conductor Exhibiting the Highest Known Lithium-Ion Conductivity. This pertains to the development of "Practical Application of All-Solid-State Lithium Batteries with High Safety", a very 'NOW' topic. The mobility of lithium ions down-the-chute in this particular crystal structure is kind of evident from the image: wouldn't you say? Crystal structure is key to the movement of these ions when an electrical field is applied. Now let's get back to basics!
If you link to the following pages, then you can download the software to do crystal projections and find other interesting things on your own. For example, there is a DOS program which allows you to take ASCII files describing the three-dimensional position of atoms in a molecule (or for that matter, furniture in a room), and view it in interactive rotation, in perspective projection, in textured red-green stereo, or you can choose to just let it 'wobble' in place! An example of such a crystal structure is illustrated on the left.
For more useful freeware concerning crystal structure and such, check out this University of Missouri at St. Louis link ! For general information on crystal structure and crystal structure types, check out the following links: (a) Chemistry at Michigan State , and (b) structural molecular biology at Duke .
So, the unit cell is the basic repeat-unit for describing a crystal. What is a crystal? Well most of us have seen mineral crystals. For example, consider amethyst. Amethyst is the purple variety of quartz and is a popular gemstone. If it were not for its widespread availability, amethyst would be very expensive. The name "amethyst" comes from the Greek and means "not drunken." This was maybe due to a belief that amethyst would ward off the effects of alcohol, but most likely the Greeks were referring to the almost wine-like color of some stones that they may have encountered. Its color is unparalleled, and even other, more expensive purple gemstones are often compared to its color and beauty. The reference URL for this image is a mineral supplier. The image has Copyright 1995,1996 by Amethyst Galleries, Inc..
The amethyst crystals, above-left, are large and well defined. Recall: there are billions and billions and billions of unit-cells that make up these individual crystals. Let us now take a look at fluorite crystals which are smaller, more regular crystals, aggregated as a group. Fluorite is a mineral with a veritable bouquet of brilliant colors. Fluorite is well known and prized for its glassy luster and rich variety of colors. Again, this image has Copyright 1995,1996 by Amethyst Galleries, Inc..
Now, let us move from aggregate fluorite crystals to aggregate galena crystals. Galena is PbS, or lead-sulfide. This fine specimen of the mineral Galena consists of hundreds of intergrown crystals. Most of these are tiny, not measuring more than 0.1" (3 mm) in diameter, but at least 20 of them exceed 0.3" (8 mm) in all dimensions. The crystals shown are of octahedral form with their tips often truncated by small cube-oriented faces. They have the standard dark-gray color, dull metallic luster, and opacity of Galena, and are dusted with a thin layer of superfine pyrite (Fe-S) or chalcopyrite (Fe/Cu-S), giving some of the crystals a dull golden appearance.
With the introduction above, the reader may appreciate that there are must necessarily be defects associated with crystals. Defects too define structure. For example, consider the boundaries between individual crystals (or grains). Since these crystalline aggregates grow together with 'random' orientation, grain-boundaries necessarily exist: and they are defects as the atomic order along them is disrupted from that within individual grains. These planar structures certainly must have something to do with, for example, how the aggregate will break apart if struck by a hammer blow. Note too the reference to a dull golden appearance of the galena specimen. The source of this discoloration is impurity particles. Iron sulfide and iron/copper sulfide grow on (and then into) the lead-sulfide crystals. These sulfides have different color properties than the lead-sulfide. It is indeed impurities and imperfections in the crystal structure of the amethyst and fluorite crystals, introduced above, that give those crystals color. Note in the case of the amethyst the non-uniformity of color, and thus the non-uniformity of chemical content! All of these concepts relate to the structure and associated defects of the materials being discussed.
Now I share with you a few micro-structural images. The first will be a look "inside" a perovskite material: lanthanum aluminate. Let's not concern ourselves with the magnification. You can assume dimensions to be in the order of tens of microns. What you will see is a non-perfect, but beautiful state of matter. In one of the superconducting perovskites, the degree of defect (such as that shown on the left) would determine how well the crystal would work as a superconductor. The structure determines properties. Enjoy the beauty and many natural wonders in the reference source, The Molecular Expressions Photo Gallery at Florida State University.
For a second look at defect structure, consider the image on the right from the NASA Science Academy web pages [no longer available]. Here, we are looking at rather high magnification at a Group II-VI semiconductor compound, possibly ZnS. The color electron photomicrograph shows such common structural defects as a grain boundary (A), twin boundaries (B), and triangular-shaped dislocation etch pits (C). These defects were revealed by chemical etching of a wafer cut from a crystal of a II-VI semiconducting alloy, which was produced by directional solidification. Dislocations are another type of defect (line-defect) common to crystalline solids, and very, very important to their properties. For a brief overview of dislocations and associated etch-pits, visit the following Carnegie Mellon website.
Reflect again on the poly-crystalline structure of the galena aggregate previously introduced. This is essentially a three dimensional view of how metals and alloys are structured. The sole difference is the scale of the grain-array. Commercial alloys are fine-grained, with grains (ie, crystals) typically 0.075 mm or so, in diameter. Perhaps comparison of the galena aggregate to the image below will convince you that grain-boundaries play a role in the behavior of metals and alloys. Shown is the fracture surface of a high-strength alloy which failed by hydrogen embrittlement. This mode of failure is highly dependent on the size, orientation and chemical make-up of the grain boundaries. Please note the similarity of the galena specimen and this failure specimen, which was subject to inter-granular (ie, along-the-grain-boundary) fracture. The individual polyhedra facets define the grains. The source URL is the 'Analyze This' web pages.
Metallography is a means to evaluate the grain-structure of materials. Shown on the right is a color photo-micrograph (a two-dimensional section through a poly-crystalline array) of a common alloy or metal (brass or nickel, for example). To the trained metallurgist or materials engineer, the structure represents a face-centered-cubic material that has been worked and then "recrystallized" during an annealing treatment. The metal or alloy is in a soft, ductile state. I know you may not know what all of these terms mean. I am trying to illustrate the link between structure, properties and processing. I am trying to illustrate the perspective of the materials engineer and the importance of the structure concept. This image is the work of George Vander Voort of the International Metallographic Society. For more photomicrographs, visit the web pages of Materials Evaluation and Engineering.
To reinforce the importance of grain structure to properties, please consider the photo-micrograph below. Again, failure along grain boundaries of an engineering alloy is featured. The alloy is stainless steel (why is it called "stainless" steel.... do you know?). The failure mode is caustic stress corrosion cracking. Here, in a micrograph of the stainless steel, one can see how failure is proceding along the grain-boundaries from the free-surface of the component (top edge). Besides grain boundaries, what other defects do you see in this photomicrograph? The reference URL is the Houston Failure Group.
Let's tie the concept of structure, and one's ability to "model" the atomic structure to a practical problem involving cracks. Consider, for example, a crack in a turbine blade in a jet engine..... or a flaw in the wall of a high-pressure gas cylinder. Can you see the value of determining the forces necessary to propagate an existing crack to catastrophic fracture in these instances? Computer modeling of atomic structures and determination of the resulting properties is a field called computational materials science. A view of the propagating crack tip in a molecular dynamic simulation of a fracture in an amorphous covalent solid is shown in the image. The solid is being pulled apart at the red atoms; the yellow arrows show the net force on each atom and the black lines show bonds. Such simulations promise atomic scale insight into the origins of cracks and the underlying mechanisms of fracture. The source URL for this image was the San Diego Supercomputer Center web page called "Image of the Week". This image and other computational images were collected by the The San Diego Supercomputer Center (SDSC) at the University of California, San Diego.
I hope this has given you some idea of the structure-concept of materials engineering. If you are interested in learning more about structure and defects in materials, please visit the VIMS multi-media web pages. For more about the use of the optical and scanning electronmicrosope to document the structure of 'stuff', you are encouraged to visit the Dennis Kunkel's Incredible microscope gallery.
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