When you go to a sports shop you are inundated with new "graphite" based materials for sports equipment: golf clubs, tennis rackets, bicycles (frames and wheel disks), ultralight airframes, and even America's Cup yachts feature these new lightweight materials. But, we are also familiar with graphite as being a very common and mundane substance. Graphite has long been a component of pencil lead, and is used as a basic lubricant. How is it that graphite is both a hi-tech and low-tech material? Could we take a bunch of pencil leads and epoxy them together into a cutting edge tennis racquet? Anyone who has used mechanical pencils knows that the leads break far too easily to provide a strong frame. It would seem as if there are two different kinds of graphite. In fact, this is true. When vendors market "graphite fiber" products they are usually selling a "carbon fiber" product. The correct name for the fibers used in all strengthening and reinforcing applications is carbon fibers. But, there is more to the story than just a general misconception over the term "graphite fibers." Surprisingly, if we look at a small section of graphite and carbon fibers on the atomic level they appear to be identical.
Figure 3.1 Headline from Chicago Tribune, May 23, 1993.
Carbon fibers are made from organic polymers such as poly(acrylonitrile). Although a full description of polymers is not really appropriate at this point, it should be noted that polymers are giant molecules comprised of repeating units. Poly(acrylonitrile) is a polymer with chains of carbons connected to one another. To make carbon fibers, the polymer is stretched into alignment parallel with what will eventually be the axis of the fiber. Then, an oxidation treatment in air between 200 and 300 °C transforms the polymer into a nonmeltable precursor fiber. This precursor fiber is then heated in a nitrogen environment. As the temperature is raised, volatile products are given off until the carbon fiber is composed of at least 92% carbon. The temperature used to treat the fibers varies between 1000 °C and 2500 °C depending on the desired properties of the carbon fiber. The process used to make carbon fibers is summarized in Figure 3.2. Each carbon fiber is very thin; the total diameter of a carbon fiber is 6-10um or about five times thinner than an average human hair (Figure 16). When carbon fibers are used in industry, they are woven into sheets, tubes, or other desired shapes (Figure 3.3). Epoxy resins or other binders are often added to the carbon fibers. The resulting composite of epoxy and carbon fibers is stronger than either component individually.
Figure 3.2 Schematic showing the process for making carbon fibers
Figure 3.3 Braiding of carbon fibers
If during the treatment process the temperature is raised above 2500 °C, graphite will be formed instead of carbon fibers! We will return to why the higher temperature leads to graphite later. Graphite can also be found in nature as flakes mixed with clay and other impurities. While graphite can be mined or formed through the carbon fiber process, most of the graphite used in industry is manufactured by heating petroleum byproducts to about 2800 °C. The petroleum byproducts are similar to the polymers used in the carbon fiber process in that both contain chains of carbon atoms.
Both graphite and carbon fibers are rigid materials that are resistant to stretching and compression. They are also chemically inert or unreactive materials. This is critical because it means that carbon fibers don't react with the outer part of a bicycle frame or a tennis racket and graphite pencil leads don't react with paper. The inertness of carbon fibers makes them suitable for medical applications such as hip and knee ligament replacements. Graphite, a black, lustrous solid, is shown in Figure 3.5. Carbon fibers, tiny black strands, are shown in Figure 3.6.
Figure 3.5 A sample of graphite
Figure 3.6 A spool of carbon fibers
The biggest difference between carbon fibers and graphite is that graphite is flaky and breaks apart very easily while carbon fibers are strong materials that do not break until much more force is applied. Figures 3.7 and 3.8 show carbon fibers after tensile failure and flexural failure respectively. Tensile failure occurs when the fibers are stretched along their axis; flexural failure occurs when the fibers are compressed along their axis. Chemists are usually interested in examining structures on an atomic level which is far smaller than the microscopic views in Figures 3.7 and 3.8. If the regions around break points have definable atomic structural features, stronger carbon fibers can be produced by excluding these characteristics. Chemists have learned that the straight lines around the fracture in Figure 3.8 are regions where the carbon fiber was the most graphite-like. But, how do chemists get such detailed information about atomic structures which are almost inconceivably small? There are a wide variety of tools available to explore substances on the atomic level. One of the most powerful methods for structural determination is called x-ray diffraction. Chemists often favor this method because the results of a successful analysis is a clear picture of the atomic arrangements - perhaps one of the closest things to an "atomic photograph." The drawback to the technique is that it can only be used on crystalline solids. The difference between crystalline and amorphous materials is an important distinction to make before discussing the details of the x-ray diffraction experiment.
Figure 3.7 Microscopic view of tensile failure in carbon fibers
Figure 3.8 Microscopic view of flexural failure in carbon fibers
Although the heart of the difference between crystalline and amorphous solids occurs on the atomic level, there are several physical characteristics which can often indicate one type or the other. Crystalline substances have regular shapes, and form flat faces when they are cleaved or broken. When they are heated, crystalline solids melt at a definite temperature (unless they decompose before melting). The regularity of crystalline solids is due to the arrangement of structural units into an orderly array or lattice. A unit cell is the smallest repeating unit of a crystal and the smallest portion from which the structure of the crystal can be understood. Stacking unit cells together forms the crystal much as stacking bricks together forms a wall. The unit cells of diamond and graphite are shown in Figure 3.9.
Figure 3.9 Unit Cells
Most solids you encounter regularly --wood, papers, and plastics--are amorphous rather than crystalline solids. When amorphous solids are broken, curved and irregular faces are formed. An amorphous solid does not have a melting point. Instead, it softens over a broad temperature range. The change from solid to liquid is slow. A summary of the properties of amorphous and crystalline solids is given in Table 3.1.
Table 3.1 Comparison of amorphous and crystalline materials
| type of solid | Characteristics | Examples |
|---|---|---|
| crystalline | consist of individual crystals, each with well-defined shape. Cleave to give well-defined faces. Melt at definite temperatures. | sodium chloride, sucrose, metals |
| amorphous | have no well-difined shape. Break to give curved or irregular faces. Soften and then melt over a temperature range. | asphalt, paraffin, window glass, obsidian, glassy form of glycerol |
There are three important subdivisions of crystalline solids.(Figure3.10) The first two are covalent and ionic networks. Network solids have an undefined size and all atoms are indirectly connected to one another. That is, you can start at any atom and trace a path to any other atom through bonds in the structure. Networks are classified by the kind of bonds that are formed between the atoms in the structure. The bonds in covalent networks are formed by the equal sharing of electrons between two atoms (called covalent bonds). When atoms of different elements form bonds, they don't share the electrons equally. Some elements have a strong pull for electrons when they form bonds resulting in an uneven distribution of electrons around the two atoms. In an extreme case, atoms can sometimes remove electrons from their neighbors. When atoms gain or lose electrons, they become charged atoms called ions. The attractions between oppositely charged ions are called ionic bonds.
The final classification of crystalline solids which will be discussed is molecular extended lattices. The repeating unit for this class are molecules which are packed next to each other. It is important to note that there are no covalent or ionic bonds between the individual molecules -- they are held together by weaker attractions called intermolecular forces.
Figure 3.10 Classification scheme for pure solids
X-ray diffraction is an analytical technique which uses a beam of x-rays to probe repeating planes of atoms. The reflection of x-rays off of repeating planes of atoms creates a series of spots called a diffraction pattern. The orientation of the x-ray and the crystal is of utmost importance. As the angle between the x-ray beam and the crystal face is varied, the diffraction pattern will change as well. By collecting data from a series of orientation angles, the three dimensional atomic structure can be calculated. You can think of this in simpler terms. Say you wanted to document a sculpture for a museum. A single photograph of the sculpture would record only a fraction of its overall form. Instead, you could take a series of photographs from several angles to flush out the entire sculpture. From your collection of photographs someone who had never seen the actual sculpture could build a replica.
The sample used in x-ray diffraction must not only be crystalline, but it must also be a single crystal. We have defined crystalline solids as being arranged into an orderly array or lattice. But, in reality, there can be small defects within the lattice, and crystals often fuse together. Both of these situations imply that repeating planes of atoms are interupted. To get good x-ray diffraction data, chemists use only single crystals with minimum defects.
The diffraction patterns of graphite and diamond are shown in Figure 3.11. Diffraction patterns from early experiments were recorded on photographic film, and the analysis of data was performed by hand calculations. Modern methods, which employ automated diffractometers and high speed computers, have made diffraction studies a very powerful technique for determining the structure of crystalline solids.
Figure 3.11 X-ray diffraction patterns of graphite and diamond.
A schematic of an x-ray diffractometer is shown in Figure 3.12. The main components are an x-ray source, a goniometer (or crystal orienter), a detection system, and a computer control system. The x-ray source is a high-vacuum tube, and the x-ray beam passes out of the tube through a thin window. A single crystal is generally mounted on the end of a glass fiber. This fiber is then attached to a metal pin which is secured to the goniometer head. The goniometer precisely orients the sample in the x-ray beam. As the x-rays pass through the crystal, the detector collects information to generate a diffraction pattern. Finally, the computer control system processes the information from the detector, and the structure of the crystal is solved.
Figure 3.12 Schematic of X-Ray Diffractometer.
To understand how a diffraction pattern occurs, consider the diffraction patterns of two waves. Two waves of the same wavelength can come together either in phase or out of phase. If the two waves come together in phase, this means that the maxima and minima on both waves are at the same points. In other words, the hills and the valleys from both waves are lined up together. The waves reinforce or cause constructive interference, and the intensity of the resultant wave is increased. Conversely, two waves coming together out of phase have each minimum from one wave combining with a maximum of the other wave. The hills from the first wave line up with the valleys from the second wave while the valleys from the first wave line up with the hills from the second wave. This is called destructive interference and destroys the wave. Figure 3.13 shows constructive and destructive interference.
Figure 3.13 Constructive and Destructive interference.
In a crystal, x-rays are reflected from the different planes of atoms that are present. If two x rays travel to two different planes, then one x-ray must travel further than the other. The x-rays may end up out of phase after they are reflected. Only at certain angles of reflection do the two rays remain in phase (Figure 3.14). In the diffraction pattern, dark areas are caused by constructive interference while lighter areas are caused by destructive interference. Thus, the diffraction pattern can be related to the structure of the crystal, and the position of each atom in a molecule as well as the type and size of a unit cell can be determined by using x-ray diffraction.
Figure 3.14 Constructive(a) and destructive(b) interference.
To see how different patterns of atoms result in different diffraction patterns, we can use a series of optical transform experiments. Optical transform experiments use the same physics of x-ray diffraction, but, the scale is much larger. Instead of a series of repeating atoms, the experiment uses a plastic slide with printed patterns. Instead of x-rays, the optical transform experiment uses visible light. You may wonder why x-rays are used for experiments on the atomic level and the visible light is used for the optical transform experiment. The diffraction phenomena depends on using a wavelengths which are of the same order as the distance between the pattern components (atoms or dots). The much shorter wavelengths of x-rays are needed for the tiny distances between atoms, while the longer wavelengths of visible lights are suitable for the printed dots in the optical transform experiments. Of course, visible light is also convenient because we can see the diffraction patterns with the naked eye! You should note that the optical transform slides are one dimensional, and the light source is always perpendicular to the slide. This is a simplification of the x-ray diffraction experiments where a three dimensional structure is probed by varying the angle between the crystal face and the x-ray beam. The schematics of the two experiments are presented below (Figure 3.15).
Figure 3.15 X-ray and optical transform experiment schematics.
Figures 3.16 Optical slide patterns with differing distances between repeating units and their resulting diffraction patterns.
Figures 3.17 Optical slide patterns with differing repeating unit shapes and their resulting diffraction patterns.
The atomic structure of graphite has been determined by x-ray diffraction and other analytical techniques and is shown in Figure 3.18. Parallel sheets of hexagonal rings are spaced 3.35 Å apart. Bonds within the chickenwire-like sheets are very strong, but interactions between the sheets are weaker and can be broken easily. Given this atomic arrangement, we can begin to explain some of the properties of graphite. When the interactions between sheets break, the planes slide over one another. It is this sliding that makes graphite such a good lubricant, and also explains why it is a soft brittle substance.
Figure 3.18 Structure of graphite
So, what does the structure of carbon fiber look like? The truth is that it is very difficult to get an accurate description of the atomic structure of carbon fibers because these materials are amorphous. Chemists have used similar techniques to x-ray crystallography and can provide a qualitative description of the structure of carbon fibers. A very tiny piece of a carbon fiber would look like graphite, but carbon fibers have less long-range ordering. Instead of the planar layers of carbon atoms which are found in graphite, carbon fibers consist of ribbons of carbon atoms aligned parallel to the axis of the fibers. Although the ribbons are essentially parallel on the surfaces of a carbon fiber, the fiber's inner layers fold in a "hairpin" fashion as seen in Figure 3.19. This is a stark contrast to graphite in which the carbon sheets remain parallel on a long range scale. The layer planes along the axis of the carbon fiber are interlinked in a complex way (Figure 3.20). It is believed that the great strength of carbon fibers is due to the interlocking and folding of ribbons (the sheets of carbon atoms can not slide past each other as they did in graphite).
Figure 3.19 Model of how sheets are arranged in carbon fiber. Inset shows hexagonal array of carbon atoms.
Figure 3.20 Interlinking nature of sheets of carbon atoms
Recall that graphite is an extremely stable form of carbon, due to the extensive resonance of single and double bonds. Carbon fibers are also stabilized by resonance, but because the structure is irregular, the effect is not as extensive. Ultimately, graphite is more stable than carbon fibers. The figure below shows a depiction of the reordering from carbon fibers to graphite as temperature is increased.
Figure 3.4 The transition from amorphous carbon to graphite
Why is temperature related to the reordering process? You may not have given much thought to what temperature physically represents. In a formal sense, temperature is a measure of the average kinetic energy in a system. So, when higher temperatures are applied to the carbon fibers, eventually enough energy is present to break the bonds in the carbon fibers, allowing them to reorganize to the more stable graphite form. We will return to a more detailed account of energetics in the next section concerning diamonds.
Although the crystalline forms of carbon (diamond, buckyball, and graphite) are often in the limelight, amorphous carbon materials cannot be overlooked. Along with carbon fibers, carbon black and activated carbon are also amorphous materials composed completely of carbon.
Carbon black is used as a pigment, in printer's ink, and as a filler for rubber goods. It is made by combustion of materials with a high carbon-content under oxygen-free conditions. Small parts of carbon black are like graphite, and the overall structure of carbon black is believed to consist of a folded version of the graphite network.
Activated carbon, prepared from the controlled pyrolysis of organic material, is useful because of its very high surface area. It is an efficient absorbent and finds applications as filters for fish tanks, drinking water, and air pollution. Chemists use activated carbon to remove impurities from reaction mixtures. Parts of the surfaces are believed to be covered with oxidation products which may account for some of the surface activity.
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