I was poking through my Google Drive and came across an “academic article” we were tasked to write and submit in my materials engineering course, circa 2006. It was a really cool topic to research and it gains nothing by sitting in my Drive. A lots has changed, both technology and in my writing (yes, I have since discovered paragraphs and hyphens), so take this with a grain of salt.
This is unedited and comes as it was written then, except for the bibliography. The article was written in LaTeX (as required by the professor) and the bib is formatted weirdly. See the PDF (linked below) for all sources.
If you’re a Bill Bryson fan, you’ll also note some similar writing styles included. I had recently finished reading A Short History of Nearly Everything, still one of my favorite books. It’s written in an anyone-can-understand-this way, with jokes and real-world comparisons. I don’t think I’ve veered far from that writing style since.
Feel free to poke at the complete PDF version.
Nanotubes and the nanohype: A brief biography from birth to present
Originally published November 1, 2006
If there is anything in science that has gotten a lot of press in the past decade, it is the miniscule carbon nanotube (CNT¹). CNTs have been boasted as being nothing short of the plastics of the 21st century, but as of right now, there has been a lot more in the line of promises than notable products. One can’t deny the potential, though — CNTs have recently been labeled the number one research topic in theoretical and applied physics and it seems like everyone is dumping money into research. Why? Well it boils down to four things: CNTs are extremely strong, electrically and thermally conductive, and very light — a combination that just reeks of potential. They have a theoretical (and proven) strength well beyond that of any other material known to man, conductivity that is said to be resistance free, and density one third that of steel. They are expected to be used in items as simple as clothing — for fire and rip resistivity — and as complicated as electron-beam-firing guns used to sculpt a molecule at the atomic level.
Taxonomy and Anatomy of CNTs
Nanotubes are allotropes (one specific variation) of elemental carbon (the same thing that makes up graphite, charcoal and diamond). They are members of a group called buckminsterfullerenes (or fullerenes for short), which were discovered by Sir Harold W. Kroto and his colleagues at Rice University in 1985². What they discovered at that time is a spherical make up of 60 carbon atoms which looks distinctly like a soccer ball (see Figure 1), comprised of mostly hexagons, but also pentagons (the pentagons force the ‘sheet’ of carbon to curl into a sphere). They named the molecule after the late Richard Buckmister Fuller since the shape of the molecule remarkably resembles that of Fuller’s infamous geodesic dome design. The spherical molecule is now commonly referred to as a ‘buckyball’. It turns out that fullerenes are actually the most stable allotrope of any of those known for carbon. Nanotubes are very much like buckyballs, and indeed, are found in the same places, but instead of a spherical shape, they are each made of a single-atomic layer of graphite rolled like a sheet of paper with its opposite edges fused.
Nanotubes can only be seen at the nanometer scale. A nanometer is defined as one billionth of a meter (10^-9 m), infinitely and incomprehensibly small. One photographer, an outsider to the nano-world, described the situation as, “if a big shopping mall was a blood cell, nanotechnology are the coins in the fountains.” That said, nanotubes are tiny — really tiny. The only reason we can even see them today is because of special, extraordinarily high-powered microscopes.
All CNTs share the same cylindrical shape, comprised of a rolled up sheet of pure graphite,³ which is comprised of carbon atoms laid out in a hexagonal array (see Figure 2). They are typically between 1 nm and 90 nm in diameter and have been created as long as a millimeter. One common comparison is to consider a wrapping paper roll, but one that is anywhere from 75 m (246 ft) to 38 km (23.6 mi) long. There are two types of CNTs: single walled (SWNT) and multi walled (MWNT).⁴ SWNTs are the simple version, as described by the paper roll analogy, and have diameters no larger than 2 nm.
MWNTs can be of two different variations. The first type is analogous to many rolls inside each other (see Figure 3), looking like a bulls-eye if you sliced through one, and the other is a rolled up sheet of graphite similar to a full roll of wrapping paper (the paper being the graphite this time). For both types, diameters can exceed 90 nm.
Both SWNTs and MWNTs are usually capped on either end by a half-buckyball, making it possible to trap other molecules within a CNT. The main differences between the two include ease of synthesis (creation), strength, and functionality. SWNTs are much harder to create as it is more common for a graphite sheet to roll up in itself or for layers to be added to already stable tubes. They are both extremely strong and it turns out that the SWNTs that make up MWNTs can slide essentially frictionlessly between each other.⁵
Who Gets the Credit?
It’s funny because for all their importance, the discoverer and discovery date of these molecules⁶ is kind of up in the air and seems to depend on who you ask. Carbon filaments (CNT’s much much larger cousins) have been known since the late 1880s and were even suggested for use in Edison’s lightbulb. So while filaments may have had (and probably did have) nanotubes, no one could prove it let alone identify them if they knew they existed (at that point, microscopes could only view to the micro⁷ scale).
Typically (and in some respects, erroneously), a Japanese scientist named Sumio Iijima is credited with the CNT discovery in 1991, but others claim that the real credit should go to others. For one, a patent filed in 1984 by one Dr. Howard Tennent, an employee of Hyperion Catalysis International, Inc, describes a synthesis method for what he called “carbon fibrils” whose description matches exactly that of a MWNT. What’s more interesting is the company, which still exists today,⁸ doesn’t really take part in the nano-hype. In a recent email correspondence I had with Patrick Collins, a representative of the company,⁹ I was told that the main reason for the company’s tranquility in this organized chaos that is the science of CNTs is that since the discovery took place outside of academia, few were alerted to the significance and no articles were submitted to scholarly journals, which are the main source for information sharing of scientific discovery and invention. After all, Hyperion’s process for creating MWNTs is their intellectual property, their bread and butter. He added that the company also does not sell pure CNTs to anybody; they sell composite materials that make use of their CNTs to increase strength. Collins claims that, “We [Hyperion] believe the Tennent patent represents the first deliberate synthesis of a nanotube.” Tennent still works at the company — now in his late 80s. But the apparent nonchalance regarding the ignorance of Tennent’s discovery in the history of nanotubes is still a curious notion considering the possible earth-shattering ramifications that CNTs could have on humanity — economically, socially and historically.
But the trail leads even further back to a pair of Russian scientists in 1952. At this time, the Cold War was in high gear and scientific information sharing between the United States and Russia was minimal at best.¹⁰ Indeed, the Russian duo did include photos of CNTs in their report, but did not openly identify them. They only noted that the structures were hollow (e.g. implying they were tubes) and though there was no scale, one can calculate their diameters to be roughly 50 nm. So it can be stated that the Russian pair indirectly discovered and noted the first evidence of MWNTs (SWNTs cannot have diameters as large as 50 nm). But at those times — to put it quite bluntly — nobody cared. These guys were way before their time.
It was not until 1991, when Iijima ‘revisited’ the idea that science realized the importance. Marc Monthioux, editor of the journal Carbon, says that until 1991, there wasn’t really “a fully mature scientific audience ready to surf on the ‘nano’ wave.” As for SWNTs, they were indisputably discovered¹¹ by our Japanese friend, Dr. Iijima and his colleague Dr. Toshinari Ichihashi in 1993.
Creating CNTs
CNTs are only created in small quantities as of now (e.g. orders in grams), but there are many people working on mass distribution. Presently, there are typically three synthesis methods with which to choose from. The first two processes make use of carbon sublimation, in which carbon metal will transform from a solid to a gas, skipping the liquid phase, and then eventually condense back into a solid, typically as a fullerene. This occurs through a drastic temperature change. The third process will be discussed later as it is important to another breakthrough in CNT technology.
In the first process, called arc discharge, two graphite electrodes are placed in close proximity to each other, separated by an inert gas (typically helium) and a very large potential difference is created between the two. Eventually, the electric discharge (e.g. lightning) passing from the anode (+) to the cathode ( — ) creates such high temperatures, the graphite on the cathode sublimes. The temperature then decreases just as rapidly and the carbon gas condenses into a solid on the electrodes. These condensations will include, among other things, fullerenes, including CNTs. Both Kroto and Iijima used this process in their experiments, and for more than ten years it was the more common route to take when one wanted to create CNTs.
The second sublimation technique, laser ablation, does essentially the same thing as arc discharge, but it uses a high powered, pulsed laser beam to sublime a graphite sample. The chamber with which the experiment is contained is usually cooled by water or a gas, so as the laser continues its work, the gaseous carbon finds its way to the cooler chamber surface where it condenses to form the fullerenes. Considering how much money it costs and how much research it took to produce reliable means of CNT synthesis, it is ironic — indeed comical — that fullerenes (including CNTs) occur naturally and can be found in everyday fireplace soot.¹²
While it is debatable that Iijima should be credited with the discovery of CNTs, his research is considered one of the most important steps in their short history. The next advancement in the science that can rival that of Iijima’s came from the laboratories of Ray Baughman at the University of Texas in mid 2004 when he and his colleagues were able to create CNT yarn and make exquisite knots from said yarn, without causing damage. Knot-making may not seem all too exciting, but these knots were examples of the types of knots that will degrade even the strongest of materials (including the bullet proof vest material, Kevlar) when exposed to many types of abrasion.
But an almost more important outcome of this experiment is Baughman’s successful use of the third process of synthesis for CNTs, chemical vapor deposition (CVD¹³). This process requires a chamber with a mixture of low pressure gases (which contain the element(s) needed to create the compound you want in the end) that is heated typically to the upper hundreds of degrees Celsius.¹⁴ The gases are then rapidly cooled, which, through the magic of science, causes condensation (almost a ‘raining’ effect) of the material you are after, upon a disk on the floor of the chamber. In Baughman’s case, ‘forests’¹⁵ of MWNTs were deposited on his disks. The MWNTs were all roughly the same height and tightly packed — more than 150 billion tubes for every square centimeter. Figure 4, Baughman’s amazingly clear images of the yarn spinning process, shows in great detail the CNT forests deposited through CVD. The forests in this case stand vertically and the yarn is being pulled orthogonally. Breakage doesn’t occur because atomic level forces, such as van der Waals forces, cause the recently removed CNTs to pull along their neighbors.
But Baughman and his colleagues were not done there. They continued the revolution in CNT synthesis and design, and in mid 2005 created a CNT sheet which one article compared more to “the stuff of Superman’s cape [than] industrial material.” Indeed they had invented a way to create 5 cm wide, almost infinitely long ribbons of super-strong, aligned CNTs. The sheets themselves are virtually transparent and incomprehensibly thin. They were drawn using the same process used for the CNT yarn, but this time pulled off laterally and not spun. Baughman used an aerogel ribbon¹⁶ and initiated the draw “using an adhesive strip, like that on a 3M Post-it Note,” as he put it in his article. The sheets are insanely thin, so to make them thicker and stronger, he laid them out in a crosshatch pattern and finally melted away the original aerogel ribbon, leaving behind true CNT ribbon. See Figure 5 for another great image of the drawing process.
Baughman’s ribbon, it turns out, was essentially the first synthesized CNT product to live up to all the hype we’ve been hearing about. He placed a 2.5 mm drop each of water, orange juice and grape juice on the newly created ribbon and it held. That sounds unimpressive until you hear that the mass of the drops was 50,000 times that of the CNTs they were resting on.
This brings us to a description of just how strong CNT materials are. Theoretically, MWNTs are said to be able to take stresses up to 150 GPa¹⁷ in the longitudinal direction. Just reading that may not sound all too impressive; you’ll take my word for it that it is. But when put in real terms, it means that a tether of pure, perfect CNTs with an area of one square centimeter — a little smaller than a key on a keyboard — can hold, by itself, almost four fully loaded Boeing 747s. In comparison, the strongest steels can only take 1.86 GPa, less than 1.5 percent that CNTs can take. Kevlar, the material of bullet-proof vests, makes it up to 4 GPa on a good day, which is more than twice that of the steel, but still 2.3 percent that of the CNT tether.
Obviously, theoretical numbers don’t do us much good. In reality, CNTs have still seen tensile strengths much greater than any other material, but also much less than their theoretical values. One experiment, using one MWNT, put the stress it took to fracture the specimen at 66 ± 4 GPa, less than half of the proposed 150 GPa. Baughman’s CNT yarn had an ultimate strength of 4.2 GPa. It’s important to note that point defects (errors or vacancies in the CNT walls) will adversely affect the overall strength of the entire nanotube. Every man-made material deals with this problem, as it is impossible to create a perfect material that contains no defects. In Baughman’s case, defects plus the fact that his material was a spun yarn are the reasons for ‘low’ strengths. Had Baughman had the resources to create nanotubes that were the length of the prescribed yarn, it would have lived up to the defect-only expectations.
In addition to strength, CNTs are known for exceptional electrical and thermal conductivity. Transfer of both electricity and heat are considered to be just about resistance free, assuming no defects along the nanotube. In the case of heat, CNTs within a compound could be used simply to channel heat away from the other material that could otherwise be negatively affected by it. As for its electrical aspects, nanotubes could eventually be used in circuits and computer processors, cutting down run time and increasing efficiency at the same time. SWNTs are said to be “the physical realizations of ideal quantum wires with ballistic conduction,” while noting that MWNTs are bit more complicated. The effect of electricity on CNTs is different depending on its chirality (its twist¹⁸). Depending on the chirality, the nanotube can either act as a semiconductor or a metal. When acting as a metal, it is said that the electron transfer efficiency can be 1000 times that of copper.
Practical Applications of CNTs
Now it’s time to consider some of the actually-proposed uses for CNTs. There seem to be equally as many structural applications as electrical and thermal, and as stated earlier, everyone is pouring money to research, hoping to make it big on consumer applications. I put these examples in general order of abstract to tangible,¹⁹ ending with arguably the grandest possibility CNTs can offer.
CNTs are now being considered for use as base products to create nanowires of other materials. Being that CNTs are so stable, it’s been shown that many metals have the ability to be deposited upon a CNT, using it as a template of sorts. These wires can then be used in nanoelectronic devices. It turns out that just about any metal can be made into a nanowire as long as the CNT template is first coated with a titanium buffer layer.
Baughman’s research has gone into that of artificial muscles. As of now, no material has shown resistance to the typical torsion, twist and fatigue that muscles endure over time like CNTs have. That, added to CNTs ability to conduct electricity no matter their orientation, makes them a prime candidate for research in the field.
One group at Berkeley was able to create the first ever nanoscale electromechanical actuator through use of a CNT. In engineering applications, an actuator is any object that changes one form of energy into another to send commands down a line. Consider a computer mouse, which transfers the mechanical energy of your finger click to a digital, electronic signal. In this case, the nanotube served as the primary motion-enabling part, successfully spinning the metal plate of this rotational actuator: the first tamed CNT.
Another group at the George Washington University are planning on using CNTs as “material vaporizers.” This technique can be used to etch or carve other materials at the atomic level. Using the CNT’s properties of excellent conduction, electrons will actually come out the end of the CNT in a ‘beam’ of sorts, pounding the atoms within its cross hairs with an exceptional amount of energy, eventually evaporating them. This technology also allows for two dimensional pictures to be etched onto a material.²⁰ Incredibly, the hardest parts of this process include being able to see the procedure in action, and stabilizing the microscope used.
One major consumer electronics application is the ability to create flat panel displays with much better attributes at a lower cost. Motorola, one of America’s largest consumer and portable electronics producing companies, was successful in creating a five-inch prototype display in early 2005, showing “the potential to create longer-lasting NED [Nano Emissive Display] flat panel displays with high brightness, excellent uniformity and color purity,” also placing emphasis on the cost-effectiveness of this production technique. They go on to say that this type of display shows an extremely pure image, excellent response time and is perfect for use in a high definition, flat panel television of thickness less than one inch.
The use of CNTs in lighting and antennas has come up a lot as well. Baughman mentions both in his paper regarding the CNT ribbons. It turns out that when a current is applied along one of his ribbons, it glows incandescently. Little research has been done regarding how long a light bulb filament made of CNTs could last, but once they are able to be made cheaply enough, CNTs could become common in such consumer applications. Additively, Baughman’s group also discovered that when one of his sheets is placed between two polymer layers and exposed to microwave radiation,²¹ it will actually fuse them together, showing little evidence that the CNT layer even exists (because it’s so thin and almost completely transparent). This has been proposed for use as antennas and window heaters in automobiles. Imagine the day you no longer have to wait for your windshield defroster to warm up; you can quickly melt the ice and snow using the CNT sheet between the windshield panes.
Clothing has also become a place for CNT use and testing. Nanotube-laced polymer materials used to coat fabric have the potential of being extremely fire retardant and may stop flames from igniting the fiber of the cloth. The clothing world has already seen the introduction of wrinkle-free and stain-proof fabrics made possible through nanotech applications.
The sports world is also becoming (and will become) a large benefactor of CNT advancements. Your golf game could soon go from bad to, well, less bad. New nano-engineered balls are being created to travel straighter and further and CNT-based club shafts could replace their graphite predecessors, increasing strength while lowering the center of gravity, a plus when trying to get the most out of your swing. Tennis rackets have gone from wooden to metal to graphite and soon, onto CNT-based composites. The nylon cords within the racket could soon be replaced by CNT-composite based cords, which will not stretch as much, relaying more of the power the player intended back to the ball. Carbon fiber bikes could soon evolve into nanofiber bikes, increasing strength while allowing an estimated 20 percent of the bike’s weight to be lost. Even the bowling world has seen the introduction of a nick-proof, nanocomposite ball.
The Space Elevator
And last but not least, probably the most talked about of all the potential applications is the space elevator. It has been proposed that a CNT tether could be used as a track for an elevator that has two floors: ground level (earth) and geosynchronous orbit (space). And the idea is getting some real respect these days. In 2005, NASA began their annual Space Elevator Competition, awarding $150,000 to whoever can successfully build a robot to climb a 60 m cable at a velocity of at least 1 m/s using only light from a 10,000 Watt searchlight as its power.²² The idea isn’t all that new. Arthur C. Clarke, the renowned science fiction author of 2001 A Space Odyssey, actually wrote about it in his 1979 novel The Fountains of Paradise whose plot centered around the building of a space elevator made of what he called a “hyperfilament” tether, consisting of “psuedo-one-dimensional diamond crystal.” Oh, how close he was to reality — just 25 years too early.
The details of the space elevator tend to be sketchy and don’t match up directly from plan to plan, but what does seem to stay constant is that the ground-based station would have to be along the equator and would most likely float somewhere at sea. The tether would be connected to a large counterweight in space (most likely a space station) and will stay taught because of Newtonian physics: centrifugal force. As the world spins, the counterweight will be pushed away from earth, keeping the tether firm and straight.²³ Having the ground-level base at sea means that it is movable, allowing the tether and counterweight (believe it or not) to be moved out of the way of harmful space junk, which will be monitored by radar. Many look at a spot in the Pacific Ocean roughly 1000 mi west of the Galápagos Islands. For one, it is the best place along the equator (on the water) for sunny skies. If anything could be detrimental to a CNT tether, it is a lightning strike.
The tether would have to take on a tapered design and be able to withstand at least 65 GPa of stress, plus a safety factor. Note that earlier I referenced a MWNT that withstood a max of 66 GPa. Needless to say, we have a ways to go. But a CNT tether would be optimal because of its extremely low density, implying that the centrifugal effects of its own weight will be much smaller than those of a conventional material. As CNT research goes on, it is expected that we will have the material strong enough to do the job on that scale by 2013, assuming the present climb of fifty percent strength increase per year continues. Right now, a price tag of at least $10B is proposed for the project at that date.
As for the car that will ascend the cable, plans are for it to be powered by solar cells, but not by solar power. Instead, high powered lasers will sit on earth, always pointing at the cells, giving it power to climb. The reason for this is that carrying along the extra weight of batteries or fuel cells is a waste of load. Projections have it that the space elevator could lower costs of bringing items to space by fifty to ninety-nine percent.²⁴ One source even says it could cut the price by one thousand. One company, LiftPort Group, is so serious about the elevator’s construction, it actually has a countdown on its website to the first liftoff, slated to be on October 27, 2031. Mark your calandars!
Working Through the Disadvantages
But all this advancement is not to say that CNTs are perfect. There is some evidence that CNTs could be potentially toxic, possibly comparable to the toxicity of asbestos. One toxicity review points out that CNTs have toxicity potentials mainly because of their small size and weight, allowing them to be taken into the air as additive particulate matter which can adversely affect the lungs. In their research, the group notes that CNT exposure to rodents has “resulted in inflammation, epithelioid granulomas [spots of inflamed granulation tissue], fibrosis [development of excess fibrous tissue] and biochemical/toxicological changes in the lung.” The study goes on to say that the method of synthesis is unimportant, that all forms of CNTs have the same reactions on the rodent test group.
But the possibilities really do seem endless, regardless of danger.²⁵ It’s expected that CNT fibers will be part of composite materials used in automobiles, aircraft (especially military), and any other applications that could use an increase in strength while losing a little weight at the same time (almost any if the price is right). Currently the biggest drawback to advancement is the cost. CNTs retail for as low as $25 per gram for low purity MWNTs to beyond $2000 per gram for greater than ninety percent pure SWNTs. The cost of highly pure (research grade: 90% pure) MWNTs is steady at about $110 per gram. Sound like a lot? The US Drug Enforcement Administration estimates the average gram of pure cocaine goes for $118.70 in the US.
But as stated earlier, we haven’t gotten very far. Many of our leaps and bounds have been baby steps. The government doesn’t seem to be all that interested in CNT production — at least not publicly. CNT production is its own little niche, selling mainly to academia. Indeed most progress seems to come from academia, albeit I did mention Motorola. The sporting world is beginning to see potential, and interestingly enough may be the place commercially-produced CNTs end up coming from first. I would predict next would come some electronics industries: mostly displays. Aircraft and automobile parts come next, citing the upcoming (predicted) energy crisis: lighter vehicles mean better mileage. I don’t really see the nanowires and nanoelectronics coming into being before 2030 or later, but then again the pace of progress accelerates with time. Ten years in today’s development time could be seven or even five in ten year’s time. But my prediction is leave it to the people in the sports world to make CNTs popular: Tiger Woods and his new Titleist Nano-305R driver, Roger Federer and his new Wilson NanoFederer series racket. But don’t worry, we’ll get there somehow.
Footnotes
¹ Although other elements can exist as nanotubes, for this discussion “carbon nanotubes,” “nanotubes,” “tubes,” and “CNTs” are all to be considered equivalent.
² They subsequently won the 1996 Nobel Prize in chemistry for their discovery.
³ These are referred to as graphene sheets.
⁴ Note that there is a third, lesser known nanotube structure called a nanotorus, which is effectively a carbon nanotube donut. These are mainly used for their magnetic and thermal properties. But the main emphasis here will be on SWNTs and MWNTs.
⁵ The same properties that make graphite a good solid lubricant are responsible for this.
⁶ This debate is specifically about MWNTs, as SWNTs were not discovered until afterwards and can be credited to two people working in one laboratory. We’ll discuss this later.
⁷ E.g. 1000 times larger than nano.
⁸ It actually has a trademark for the name “Fibril” nanotubes and owns the domain name fribrils.com.
⁹ 2 Nov 2006, Patrick Collins, Marketing Manager, Hyperion Catalysis International, Inc.
¹⁰ Even if sharing did take place, you have to consider the report was in Russian.
¹¹ The discovery of SWNTs will probably go down in similar light to Bell’s patent for the telephone, as Iijima and Ichihashi beat a US team at IBM in submitting their results to the journal Nature by a month — even though their findings were both published in the June 17 issue of that year. The IBM team’s article actually follows that of the Japanese. But that’s not to say it was a race. Iijima and Ichihashi were indeed searching for multiwalled nanotubes with only one wall (hence SWNTs), while the IBM team was mainly trying to create buckyballs with magnetic metal atoms trapped inside and stumbled upon SWNTs by chance.
¹² In nature, fullerenes exist in much lower quantities, making it very difficult to find them if one wanted to. So don’t go digging through your fireplace trying to make a quick buck.
¹³ This seems to be the process that Tennent used in 1983, though he doesn’t call it such. Either way, he began using it before it was even fashionable.
¹⁴ 1550°F ± 250°F.
¹⁵ Referred to as such because they resemble a very densely packed bamboo forest, with all CNTs aligned vertically
¹⁶ Aerogels are highly porous, very airy solids containing small vesicles that once held gel but are now only filled with gas. This isn’t really that important here. Just know they melt at lower temperatures than CNTs.
¹⁷ Stress is in units of force per area, meaning a constant force produces less stress when placed over an increasingly larger area.
¹⁸ Think about the difference between a straight pipe and the helix shape of DNA.
¹⁹ Or better stated: “For scientist’s eyes only” to the mass public.
²⁰ Apparently it’s not long until we’re branding things at the nano level.
²¹ It’s not said what tempted them to do it, but Baughman’s team actually decided to put the CNT sheet between two pieces of plastic and then place this sandwich in a microwave oven. That’s actually where these results came from.
²² Nobody won the 2005 contest, though one group was awarded the “Most promising for 2006" title.
²³ Just like swinging a ball on the end of a string above your head.
²⁴ But don’t forget, it will be quite the monopoly and its owners could technically charge whatever they’d like.
²⁵ After all, we still use asbestos, just not in such harmful ways.
Bibliography
See the pretty PDF version for a fully-sourced version of this document. It looks like I have full-text copies of all of the sources still, so if you want to read any, let me know.
