Last time we finished our discussion of diamond, and now we move to what is pretty incorrectly called amorphous carbon. Truly amorphous materials. like glass, have no true crystal structure (although there may be some local microstructures) that repeats regularly.
When used in the sense of carbon, only recently produced thin films of carbon are truly amorphous. These are of research interest for the most part, although I would be quite surprised if practical uses are not found for them before long.
We shall discuss forms of carbon traditionally called amorphous even though they are not truly amorphous. These include some of the most commonly encountered forms of carbon, and almost everyone has seen and touched at least a few examples.
One of the forms that almost everyone has handled is charcoal. This is made by heating wood in the absence of air until most of the volatiles have been driven off of what remains. There are always some minerals left, so charcoal is not pure carbon, but it is pretty close. If a very pure carbon is required, sugar can be used as the carbon source because it is already available in a high degree of purity.
It is hard to imagine the various uses of charcoal, and different types are produced for different purposes. Cooking, particularly in third world countries, consumes tremendous amounts of charcoal. This is because it burns hot, produces little smoke, and is much more convenient to use than wood itself. We use charcoal for recreational cooking, but many people use it as their primary fuel. Charcoal for cooking comes in several types, the two most common in the US being lump charcoal (simply whole chunks of wood that have been charred) and briquettes. Briquettes are usually made of sawdust of wood chips and contain a binder to hold them together after charring. I prefer lump charcoal for several reasons.
First, it has more fuel value per unit mass than briquettes because some of the binders are inert. Second, it can be put out by cutting off the air supply without falling apart like briquettes often do. Third, lump charcoal rarely has off flavors and some briquettes leave an evil taste on the food. Finally, lump charcoal produces far less ash than briquettes because it contains no binder.
Before coke was developed, charcoal was used to smelt iron from iron ore. It is actually better than coke, but is expensive and consumes lots of trees. Coke is made from coal (more on them later) and is cheap relative to charcoal. Illegally produced iron in the third world is a leading consumer of charcoal produced by rainforest deforestation.
A special type of charcoal made from maple is used to filter Jack Daniels Tennessee Whiskey. This hints at one of the larger uses for charcoal in the developed world, removing relatively small amounts of undesired materials from much larger quantities of a product being purified. For most of these applications the charcoal is "activated" to increase its power.
Activated charcoal is made from charcoal that is treated either physically or chemically to increase the interior surface area to a huge value, up to and exceeding 1,500,000 square meters per kilogram. For comparison, and American football field is 5353 square meters, so that kilogram of activated charcoal can have the surface area of 280 football fields! In addition, depending upon the specific process used to activate it and the starting woody material, it can be customized for particular uses.
Activated carbon (another name for activated charcoal) is used for all kinds of things. For those of you with an aquarium, it is used to remove toxic materials from the water. It does this by the process of adsorption (NOT aBsorption), where the material to be removed preferentially wets or coats the surface area of the charcoal. That is why a high surface area is important, since the films tend to be only one molecule thick, although thicker films are known.
Here are just a few of many examples for the uses of activated charcoal. In medicine, it is often used to give to victims of oral poisons because these are often adsorbed. This removes the poison from the GI system by making it unavailable to be absorbed into the bloodstream. It is also used in industrial respirators and military protective masks to adsorb airborne toxins. These charcoals are often chemically treated to enhance their ability to remove certain classes of compounds.
Activated charcoal has just about replaced boneblack in the sugar industry. Boneblack works a bit better, but it is so expensive that it is cheaper to use more activated charcoal. In this application, the activated charcoal adsorbs colored materials from the sugar, allowing the production of a very white material of high purity. Without the use of it or boneblack, all sugar would look like raw sugar. Once again, a special preparation of the charcoal is done to enhance its properties for this use.
There are many other variations on this theme, but the central point is that activated charcoal is an extremely useful material for removing unwanted minor components from bulk materials. If you have a home water filter, it almost certainly contains activated carbon.
Activated carbon can sometimes be reactivated after it is filled to capacity by thermal or chemical means. It all depends on the specific situation. Such reactivation is usually not something that can be done indefinitely, as the active sites eventually become irreversibly bound unless the adsorbed species are very volatile.
An extremely important (and controversial) form of relatively pure carbon is coal. I am not going to get into the relative merits and demerits of burning coal, or the politics involved, but regardless of which side of the argument one takes, everyone can agree that there is a lot of coal burnt, for good or ill or somewhere in betwixt them.
Coal is the result of anaerobic decomposition of terrestrial plants from many millions ago. The plant matter died, fell, and accumulated. In time, sediment was deposited on that plant material and eventually most of the volatiles were driven off by heat and pressure. The result is coal. Now, there are many different grades of coal, ranging from only partially carbonized lignite to almost completely carbonized anthracite. In general, the more highly carbonized the coal is, the higher the fuel value.
Lignite is lightly carbonized, ranging only from 25 to 35 per cent carbon. On the other end of the spectrum, anthracite is highly carbonized, with a carbon content of up to 98 per cent. Most of us have never seen actual coal except perhaps in a mineral collection. On the other hand, I have seen, handled, and burnt coal to heat my grandmum's house. For those of you who read my My Little Town series that appears here on Wednesday evenings at 9:00 you already know that I am originally from Hackett, Arkansas. It turns out that the region around Hackett has some pretty extensive coal deposits, and before the Great Depression lots of that coal went by rail to the steel plants around the Great Lakes. Here is a mini version of one of my stories.
Remember, I mentioned coke a while ago. Coke is to coal as charcoal is to wood, merely pyrolyzed coal. But not just any coal can be converted into coke. Lignite is too low in carbon content, and anthracite is too high. What is needed is the Goldilocks solution, a coal that is "just right". Enter bituminous coal, Goldilocks incarnate.
Bituminous coal has a carbon content around 70 per cent, give or take a few points. For it to make good coke for steelmaking, it has to be low in both phosphorous and sulfur. It also has to have a physical structure that results in a highly porous and hard chunk after pyrolysis. Hackett coal was perfect for that, and was highly sought after.
Coke is essentially pure carbon (with some mineral impurities). It burns with a smokeless flame, very hot, and is an excellent reducing agent that will take iron oxides to elemental iron. That is why it is essential for steelmaking. Hardness of the chunks is important because the loads in a blast furnace are huge, and softer materials would just clog up the furnace.
Now back to my personal experience. When I was a lad, the mines around Hackett were still being worked. Individuals still owned some of them, and they would bring coal by the pickup truck load and unload it for a small fee. I remember many a morning at my grandmum's house going out and getting kindling and a couple of loads of coal for her Warm Morning coal burning stove. For years she heated her house with coal. Sometimes I would find some neat fossils in the coal, but they were for the most part too brittle to preserve.
Whether or not one approves of burning coal for energy production, it is still essential to produce coke for steelmaking. And until the reality changes and better, greener energy alternatives become economical, coal will remain one of the primary fuels for power plants. Well, I tipped my hand just then. I do not think that we should burn coal to any significant extent for energy, but the reality is that we do. We have got to do better.
Carbon black is a form of carbon made by incomplete combustion of heavy petroleum materials, coal tar, and sometimes vegetable oil. It is available in a huge range of particle sizes and other properties, depending on the manufacturing method and intended use. Of the over 8 million metric tons produced, about 90% goes in rubber products to increase their abrasion resistance and tensile strength. Tires consume almost three quarters of the carbon black produced, and the difference in properties in rubber with and without carbon black is tremendous. Pure SBR rubber (styrene butadiene rubber) has abrasion resistance comparable to that of a gum rubber eraser and a tensile strength of only about 2.5 megapascals. Add the finest particle size carbon black and the abrasion resistance becomes like that of a tire, and the tensile strength increases by an order of magnitude.
The rest of the carbon black produced is mainly for pigments. Almost all permanent black ink and toner contain carbon black as the primary coloring agent. The pigment properties of carbon black are excellent. It is not bleached by light (or anything else) and has a high affinity for paper. Pigment grade carbon black is generally oxidized a little to make it bond better. Such inks can not be removed from paper or fabric except by some specialized processes (used mainly for recycling old newsprint), so a permanent black marker mark is almost impossible to remove from clothing or other fabrics.
Related to carbon black is soot, a sort of generic term for carbon that is produced by incomplete combustion of carbon containing materials under uncontrolled conditions. Years ago a form of soot called lampblack was used for pigments, but this is not the case any more. Lampblack is produced by allowing a flame of something, say kerosine, burning in a low oxygen atmosphere to be in contact with a cold, usually metallic, surface and then scraping the soot off of it.
Soot is a nasty bugger. Whilst carbon black contains relatively little polycyclic aromatic hydrocarbons (PAHs) with low bioavailability, soot if filled with them and they are readily absorbed. PAHs are known and potent human carcinogens. There are scores of sources for soot, and as many ways for it to be produced. Just about anything that has carbon in it and burns can produce soot. In general we think of soot as being airborne, and then it can be deposited on surfaces. The airborne kind has greater potential for toxicity since the particles are so small that they readily get into the lungs.
Large sources include things like power plants that burn coal, oil, or even natural gas. Natural gas produces less soot than most fuels, but still can if there is not enough air present. Most soot produced by these sort of large sources contain scrubbers that remove the bulk of it before the exaust is vented to the atmosphere, and least in the US. Eastern Kentudky University is nearby, and the first winter that I was here you easily could see the central steam plant boiler location by the plume of black soot coming from the stack. Evidently they have installed a scrubber, because the last couple of years I have not seen any soot.
EPA has been toughening its regulations for fixed point soot sources, and is also tightening up on some mobile sources like Diesel engines on trucks and ships. However, some soot production is not controllable. House and other building fires are huge soot producers because the synthetic materials used in flooring, piping, fabric, electrical insulation, and a plethora of other items. Forest fires produce horrific amounts of soot. Soot can also be produced inside, from oil heaters, lamps, and even candles. Even processes not involving combustion can produce soot, like dusty high temperature light bulbs, dusty electric baseboard heaters, and oil furnaces. The soot in oil furnaces is vented outside, but if the heat exchanger is defective, soot can enter the house. However, carbon monoxide is the greater threat in these cases. If you have ever felt the greasy feel of soot deposited on a surface, that is because the PAHs are relatively soft and give it that feel.
Even though we call these forms of carbon amorphous, they really are not. All of them have some microcrystalline structure depending on the manner of production. The microcrystalline structures generally fall into the diamond or the graphite configuration, and most "amorphous" carbon contains both types. As a practical matter, it really does not matter because these different forms of carbon are manufactured to specific end uses, and none of them are used either for abrasives where extreme hardness is required nor used as lubricants where slipperyness is necessary.
In all of these carbon forms the graphite or diamond domains are extremely microscopic and contribute little to the bulk properties of the material. It is possible to tell the relative amounts of these crystalline phases by certain spectroscopic methods. Remember that graphite is all sp2 hybridized whilst diamond is all sp3? These two different hybridizations absorb energy differently (actually, scatter is differently since Raman spectroscopy is often used for this). Using this technique it is possible to distinguish the different forms. As I said, there is not much practical use knowing what the major crystalline phase is present.
Well, that about does it for tonight. Do not forget that Curiosity is scheduled to make its very complicated landing on Mars tonight at around 1:00 AM Eastern. DarkSyde indicated that he will be liveblogging the event, so I shall probably slide over there later. Next week we shall cover the exotic allotropes of carbon, such as the fullerenes, glassy carbon, nanotubes, and such.
Well, you have done it again! You have wasted many more einsteins of perfectly good photons reading this sooty piece. And even though Mike Huckabee realizes that he has been eating way too much fried chicken lately when he reads me say it, I always learn much more than I could possibly hope to teach in writing this series. Thus, please keep those comments, questions, corrections, and other feedback coming! Tips and recs are also always very much appreciated.
I shall remain here tonight as long as comments warrant and return tomorrow around 9:00 PM Eastern for Review Time. Remember, no science or technology issue is off topic here. Please keep in your thoughts the deployment of Curiosity early tomorrow morning (for me, earlier for those in more western US time zones).
Doc, aka Dr. David W. Smith