Chemistry in its element: carbon
You're listening to Chemistry in Your Element presented byworld of chemistry, Journal of the Royal Society of Chemistry.
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Hello this week to the element that unites marriage, war, conflict and cremation and to explain how, here's Katherine Holt.
Any chemist could talk about carbon for days. After all, it's a common, everyday element, found in almost everything and ubiquitous to us carbon-based life forms. A whole branch of chemistry is devoted to their reactions.
In its elementary form, it presents some surprises in the contrasting and intriguing forms of its allotropes. It seems like every few years a new form of carbon comes into vogue: a few years ago, carbon nanotubes were the new black (or should I say “the new bucky ball”), but graphene is now!
But today I'm going to talk about the most glamorous form carbon can take: diamond. For millennia, the diamond has been associated with wealth and prosperity because it can be cut to form gemstones of great clarity, brilliance, and durability. Diamonds are truly eternal! Unfortunately, the diamond also has a dark side: the greed it provokes leads to the trade in so-called "conflict diamonds", which support and finance civil wars.
Man's desire for diamonds has led alchemists and chemists for many centuries to attempt to synthesize the material. After many initial fraudulent claims, diamond was finally artificially synthesized in the 1950s. Scientists took inspiration from nature by looking at the conditions under which diamonds naturally form deep in the earth's crust. Therefore, they used high temperatures (more than 3000ÖC) and high pressures (>130 atm) to convert graphite into carbon. This was an impressive feat, but the extreme conditions required made it prohibitively expensive as a commercial process. Since then the process has been refined and the use of metal catalysts means lower temperatures and pressures are required. Crystals a few microns in diameter can form in minutes, but a 2-carat gem-quality crystal can take several weeks.
These techniques now make it possible to artificially synthesize gem-quality diamonds that are indistinguishable from natural diamonds without the help of specialized equipment. Needless to say, this can be a headache for companies that sell natural diamonds! It is possible to turn any carbon-based material into a diamond, including hair and even cremation human remains! Yes, you can turn your beloved pet into a diamond forever if you want! Man-made diamonds are chemically and physically identical to natural stones and come without any ethical ballast. Psychologically, however, it remains a barrier - ifRIGHTloves you, would buy youRIGHTDiamond - right?
From the perspective of a chemist, materials scientist or engineer, we quickly run out of superlatives when describing diamond's amazing physical, electronic and chemical properties. It is the hardest material known to man and more or less inert, able to withstand the strongest and most corrosive acids. It has the highest thermal conductivity of any material, making it excellent at dissipating heat. This is why diamonds always feel cold. With a wide bandgap, it is the textbook example of an insulating material, and for the same reason it has amazing transparency and optical properties over the broadest wavelength range of any solid material.
So you can see why diamonds are exciting to scientists. Its hardness and inertness suggest applications as protective coatings against abrasion, chemical corrosion and radiation damage. Its high thermal conductivity and electrical insulation require it to be used in high-performance electronics. Its optical properties are ideal for windows and lenses and its biocompatibility can be used in implant coatings.
These properties have been known for centuries, so why hasn't diamonds been used more widely? This is because natural diamonds and diamonds formed by high-pressure, high-temperature synthesis are limited in size, usually a few millimeters at most, and can only be cut and shaped along certain crystal faces. This avoids the use of diamonds in most of the proposed applications.
However, about 20 years ago, scientists discovered a new way to synthesize diamonds, this time underunderPressure, high temperature conditions, using chemical vapor deposition. If the thermodynamic stability of carbon were taken into account, one would find that at room temperature and pressure the most stable form of carbon is actually graphite, not diamond. From a purely energetic or thermodynamic point of view, diamond should spontaneously transform into graphite under ambient conditions! This is clearly not the case as the energy required to break the strong bonds in diamond and rearrange them to form graphite requires a large input of energy and hence the whole process is so slow that the reaction is scaled up of millennia does not take place. appear.
It is this metastability of diamond that is exploited in chemical vapor deposition. A gas mixture of 99% hydrogen and 1% methane is used and an activation source such as e.g. B. a hot filament, is used to generate highly reactive methyl and hydrogen radicals. The carbon-based molecules then settle on a surface to form a diamond coating or thin film. In fact, both graphite and diamond initially form, but under these highly reactive conditions, graphite deposits are removed from the surface, leaving only diamond. The films are polycrystalline, composed of micron-sized crystallites, thereby lacking the diamond-like clarity and brilliance of gemstones. While not as pretty, these diamond films can be deposited on a variety of surfaces of different sizes and shapes, greatly expanding diamond's potential applications. Challenges remain in understanding the complex chemistry of intergranular boundaries and surface chemistry of films and learning how to better utilize them. This material will keep chemists, materials scientists, physicists and engineers busy for many years to come. However, today we all agree that the diamond is more than just a pretty face.
Katherine Holt praises the merits of the jewel in the carbon crown. Next week we head to the front of group one to hear the story of the metal that revolutionized the treatment of manic depression.
Its calming effects on the brain were first noted in 1949 by an Australian doctor, John Cade, of the Victorian Department of Mental Hygiene. He had injected the guinea pigs with a 0.5 percent lithium carbonate solution, and to his surprise these normally overexcited animals became compliant. Cade then gave his most mentally disturbed patient an injection of the same solution. The man responded so well that within days he was transferred to a regular infirmary and soon back to work.
And it's still used today, although despite 50 years of medical advances, we still don't know how it works. That was Matt Wilkinson, who will be in his element here next week with the lithium story in chemistry. I hope you can join us. I'm Chris Smith, thanks for listening and goodbye.
Chemistry in Its Element is presented by the Royal Society of Chemistry and produced bythenakedscientists.com. More information and more episodes of Chemistry in Your Element on our website atChemicalworld.org/elementos.
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