Here we go, with another foray into the world of chemical elements. As I’ve said, the plan is to make sure at least one element is included from each of the eighteen Groups of the Periodic Table (a.k.a. the Periodic Table of the Elements).
So far, we’ve had a canter through the stories of elements from eight of these:
- Group 1: Hydrogen, and Sodium
- Group 2: Calcium
- Group 8: Iron
- Group 10: Platinum
- Group 11: Copper
- Group 12: Mercury
- Group 14: Carbon
- Group 16: Oxygen, and Sulphur
- Group 17: Bromine
In this episode, we’re heading across the Periodic Table to Group 13, to an element which is quite unlike the metals which make up the other elements in this group. Zany, quirky, eccentric, and downright peculiar, it is an element of some considerable mystery still. Ah well, as complex, contrary, and contradictory as it can be, here we go, with the anything but boring story of boron.
Atoms of boron are formed, not by nuclear fusion within stars (as many, indeed most, naturally occurring elements are), but by nuclear fission. This happens during a process known as ‘cosmic ray spallation’. This process took place after the Big Bang (indeed it’s still happening), when highly energetic ‘cosmic rays’ hit other ‘matter’, rather inconveniently knocking bits off it (e.g. protons, neutrons, and alpha particles). Some of these fragments become boron. The process is also known as cosmogenic nucleosynthesis. With this atypical pedigree, it’s no wonder our element is a bit odd.
The chemistry of boron has been described as ‘unique’. It is said that, after carbon, it might be the most intriguing and complex of any element. In fact, it has been described by one eminent Russian chemist, Professor Artem R. Oganov, as a “…truly schizophrenic element. It’s an element of complete frustration. It doesn’t know what it wants to do. The outcome is something horribly complicated.”
Boron is present in the some of the oldest rocks on Earth, dating back 3.8 billion years. That said, boron is relatively rare, making up only around 0.001% of the Earth’s crust, and 0.0004% of seawater.
Our intriguing element is never found in nature as a native substance (in elemental form) which is, perhaps, one reason that it isn’t as well-known as it might be. Boron typically occurs in the form of borate minerals, in effect salts, where the ‘borate’ contains boron and oxygen. There are over one hundred different types of borate mineral, many of which are water-soluble. Boron compounds are usually found in sediments or sedimentary rock, but are also found in hot springs, salt beds, saltmarshes, desert lakebeds, and, of course, in seawater (where it exists primarily as boric acid).
The leading mineral sources of boron are:
- rasorite (or kernite), Na2B4O6(OH)23H2O
- colemanite, Ca2B6O115H2O or CaB3O4(OH)3·H2O
- borax (a.k.a. ‘tincal’), Na2B4O5(OH)410H2O
- ulexite, NaCaB5O6(OH)65H2O
We’ll discuss the last two of these in a bit more detail later. Boron also occurs as boric acid in some volcanic spring waters. The largest known deposits of our element (in mineral form, primarily as oxides but often including other elements) can be found in Turkey, and in the USA, chiefly in California and Nevada.
Boron has been given the symbol B, which should not, I feel, come as any great surprise. It is the fifth element on the Periodic Table; therefore, a boron atom has five protons and five electrons. This makes it, physically, a very small (though not the smallest) atom.
As a metalloid, or semi-metal, boron is the element which forms the top of the dividing line between metallic and non-metallic elements. As you’ll see from the Periodic Table above (the metalloids are the elements shown in that odd yellowy-grey), this divide isn’t a neatly straight line, but zigzags down the table from boron (B) to astatine (At).
The elements which fall into this category are ones which can’t really make up their minds what they want to be when they grow up, so hedge their bets. They may have properties which fall somewhere between those of a metal and a non-metal, or they’ll exhibit some ‘characteristic’ properties of both.
As I mentioned, boron as an element isn’t encountered in nature. It has to be obtained from the compounds which do exist in nature. The pure element is extremely difficult to prepare from the compounds in which it is usually found.
When prepared in crystalline form, it does look something like a metal, being lustrous, and darkish grey to black in appearance. It is a hard element (in fact, only second to the carbon-based diamond in hardness). But, unlike most true metals, it is also quite brittle (unless heated, when it becomes soft and malleable), so cannot be used in structural applications.
However, elemental boron can also be prepared in amorphous (non-crystalline) form and is actually most often seen in this form when produced. In this state, boron simply looks like a dull, instant coffee-like brown powder.
Despite appearances, in whichever of the forms it may take, elemental boron behaves rather more like a metal in terms of its physical properties, but its chemical properties lie closer to those of non-metals.
In in its crystalline form boron is typically unreactive (almost inert chemically at room temperature), but amorphous boron is reactive. Boron doesn’t react with air at room temperature but, at higher temperatures, it does react to form boron trioxide, B2O3, also called boric oxide or boracic acid.
Boron has a very high melting point, at 2,349 Kelvin (2,076°C), and a relatively high boiling point, at 4,200 Kelvin (3,927°C). Like its fellow metalloid elements, boron acts as an electrical insulator at room temperature and below. In contrast, it acts as a good conductor when it is heated or when it is combined with other elements. That said, the metalloids generally exhibit lower electrical and thermal conductivity than true metals.
Boron’s companion elements in Group 13 are usually referred to as the boron group. However, since they are characterised by having three electrons in their outermost orbitals (electron shells) they are sometimes known as the ‘triels’. They have been called by another name too. This relates to the icosahedral (20-faced) structures formed by these elements, thus the other name for Group 13 is the ‘icosagens’.
Chemical bonding to form boron compounds is anything but clear-cut. When it forms compounds, boron usually bonds covalently (if you recall from earlier articles, a single covalent bond is formed by two electrons which are shared between two atoms). Indeed, it does this to form boron trifluoride, BF3.
However, even when boron shares its three valence electrons with three fluorine atoms, this still leaves the boron atom with only six out of eight possible electrons in its outermost shell, an electron deficiency. To fill this shell, in order to stabilise it, would require two more electrons to be donated by another atom. As it is able to accept a pair of electrons in this manner, it is defined as a Lewis acid (an electron acceptor).
Our oddball element is quite happy to make things yet more complex. Boron neither loses nor gains electrons particularly easily, but it is capable of forming ionic bonds, as well as covalent bonds.
With there being an odd number, three, valence electrons (electrons in its outermost shell), a boron atom has one unpaired electron. It can, in theory, donate this single electron to become a B1+ cation, for example in compounds called borylenes.
It can also, though this is quite a challenge, donate all three electrons from its outermost shell to become a boron cation, B3+. This it does when the electrons are donated to three hydrogen atoms, to make borane, BH3.
But boron can also exist as an anion, where electrons have been acquired (donated by other atoms). Anions B-5 (found in aluminium boron-carbide, Al3BC), and B-1 (found in in magnesium diboride, MgB2) are known.
In total, oxidation states of -5, -1, +1, +2, +3 and +4 are theoretically possible. In fact, pure boron can exist as a mixture of boron anions (+ve ions) and cations (-ve ions).
In addition, it can exist with a zero charge, for example in compounds called diborynes, which have a boron–boron triple bond. To sum up boron bonding… it’s complicated. I will make no attempt to elucidate—quite frankly, this stuff is well beyond my ken.
Having considered bonding, now we move on to the different physical structures which boron can exhibit, the allotropes. Once again, our element is nowhere near dreary, and you’ll hardly be shocked to hear that this is also quite a complex area. Indeed, the true picture of how many allotropes there are, both 3D and 2D, and what they look like, is only now beginning to unfold.
Boron can be produced (remember, it doesn’t naturally appear in elemental form on Earth) in two different amorphous allotropic forms, lacking a definitive physical shape. These are as a brown powder, or as an opaque black glass-like solid.
But it can also take on a variety of crystalline structures. The most common of these are:
- α-rhombohedral (α-R) which has clear red crystals
- α-tetragonal (α-T)* which is black, metallic-looking, and lustrous
- β-rhombohedral (β-R) the most stable form, which is dark or silvery grey
- β-tetragonal (β-T) which can be black or red
- γ-orthorhombic (γ)* which is dark grey
The two structural forms marked with an asterisk* require particular conditions in which to be synthesised. The diagram HERE gives an idea of the complexity of the structures which make up just some of boron’s crystalline allotropes. At least fourteen more possible allotropes have been described, but not all of these have yet been proven experimentally.
I mentioned that some of the allotropes are 2D rather than 3D. The borophenes (a.k.a. boron sheet) are 2D allotropes. They are quite similar in some ways to carbon’s graphene, in that they are networks of interconnected boron atoms, just a single atom thick. Simple, eh? No, this is boron we’re discussing, so even the borophenes have three possible structures, β12 borophene, χ3 borophene, and striped borophene.
Complicating the picture further still (and boron really does like to do this), boron’s 2D allotropes can also form not only sheets, but roll up into boron nanotubes, and even wrap themselves into a spherical form, as boron fullerenes. The prime example of this spherical structure is borospherene (analogous to the carbon-based buckminsterfullerene), which comprises a slightly uneven sphere of forty linked boron atoms.
So now we come to boron’s isotopes, and there are fifteen of these in total. Two, 10B and 11B, are stable. Of these, 11B is the most abundant, making up some 80% of the boron on Earth. The other isotopes, ranging from 7B to 21B, are all radioactive, and all have short, or very short half-lives.
The nuclei (the protons and neutrons) of both stable isotopes of boron, 10B and 11B, exhibit a property called ‘nuclear spin’ when exposed to a magnetic field. This relates to the rotation of an atomic nucleus when placed into a magnetic field—nuclei can either align with or against the field.
The nuclear spin value of the boron two isotopes is different: 10B having a value of 3, and 11B with a value of 3/2. Because of this spin, both isotopes are of use in nuclear magnetic resonance spectroscopy (NMR), an analytical technique which can help researchers identify proteins, carbohydrates, nucleic acids, and other complex biomolecules. There are pros and cons to using each, but 11B is typically more abundant in samples, more sensitive, and provides sharper signals, so is more commonly used than 10B.
Boron, a trace element, is naturally present in many foods. Whilst it isn’t, like other elements we’ve looked at, one of the elements currently seen as ‘essential’ to life, it does play an important role in biological processes in animals, including ourselves. Because of this, both of the stable isotopes, 10B and 11B, are used in studies of boron metabolism.
In addition, 10B finds a use in medicine, helping treat certain cancers, including brain tumours, by selectively killing cancerous cells using a clever technique known as boron neutron capture therapy (BNCT).
This is a two-stage treatment. Firstly, a drug which incorporates boron (usually either BPA, boronophenylalanine, or BSH, mercaptoundecahydrodecaborane,) is given to the patient), and this is absorbed by the malignant cells. Then, in the second stage, the patient is subjected to a beam of low energy neutrons, which are directed at the tumour.
These neutrons, when they hit the boron within the cells, produce both alpha particles (in the form of 4He nuclei) and high-energy but stable lithium-7 isotopes (7Li nuclei). This radiation, produced within the cancerous cells, effectively destroys them from the inside out.
Additionally, as the radiation produced has a very restricted range, it cannot penetrate into or through neighbouring tissues, so cells nearby remain largely unaffected. This is particularly important where tumours have developed in sensitive areas of the body, such as the brain, where collateral damage is to be avoided at all costs.
Boron isotopes are also used in geochemistry, where they can offer valuable insight into past climates (palaeoclimates), including the pH of ancient oceans and levels of CO2 in the atmosphere.
Boron’s stable isotopes, 10B and 11B, are also used extensively in the nuclear industry. Indeed, nuclear plants typically maintain substantial amounts of boron (in some form or another) close to the reactors.
In particular, 10B is used for its radiation-absorbing effect (it is added to the steel used in reactors to help absorb neutron radiation). In addition, either borosilicate glass or boron carbide, an extremely hard ceramic, is often used to produce the control rods at the core of a nuclear reactor. The coolant waters used in reactors to help remove heat from the reactor core are also ‘borated’, with borate compounds, typically boric acid, added to the water.
In the event of an out-of-control reaction, boron compounds (such as boric acid) which contain 10B that absorbs neutrons, can be used to quench the reaction. Indeed, some forty tons of boron carbide, together with large quantities of lead, clay, and sand, were poured on Chernobyl Reactor No. 4 in an unfortunately unsuccessful attempt to control the meltdown that occurred on the 26th of April 1986.
It’s partner isotope, 11B, is also used in nuclear plants, but as a neutron reflector, preventing neutrons from passing through the reactor core, thus helping to control the fission reaction in the uranium fuel.
OK, so what do we know about boron’s discovery? Well, because our element doesn’t appear in nature as a native element, it was only ‘discovered’ in the early 1800s. Here again, boron shows its eccentricity as not one, but three people are credited with its discovery, not only in the same year, but within days of each other in June 1808. Still more befuddling, they used different means to accomplish this feat.
The first were two French chemists, Joseph-Louis Gay-Lussac and Louis-Jacques Thénard, who worked together at the Ecole Polytechnique in Paris. To acquire a sample of boron, they managed to decompose boracic acid (chemically, this is boron trioxide, B2O3). They achieved this by reacting the compound with the alkali metal potassium, which has a great affinity with oxygen.
Boron trioxide reacts with potassium to produce a solid olive-grey substance, the boron, and potassium oxide, with the reaction:
B2O3 + 6K → 2B + 3K2O
A mere nine days later, the English chemist, physicist, and inventor, Sir Humphry Davy, independently isolated the element. Davy had already isolated several elements using electricity (remember the Voltaic Pile?). Indeed, he had discovered both potassium and sodium just the year before, in 1807.
He tried using this same process to see whether it might be possible to isolate boron from its compounds. He observed that an electric current passed through a solution of borates led to the formation of a brown precipitate on one of the electrodes. Yes, this was boron. However, an exacting and pedantic man, Davy wasn’t entirely happy with his results.
Undeterred, he tried again, using a different technique which, unbeknownst to him, was very similar to that used by the Frenchmen. For this experiment, he had reacted boric acid (yes, coincidentally he chose the same compound used by his French rivals) with the potassium metal he himself had discovered. With this method, he experienced an improved result, producing rather more of the brownish powdery substance.
The main difference between Davy’s technique and that used by Gay-Lussac and Thénard is that Davy performed his reaction in a hydrogen atmosphere. He initially coined the name ‘boracium’ for his elemental discovery, after the compound he’d used to prepare it. By 1884 though he proposed that it be renamed boron.
Ultimately, however, the name derives from the Arabic word ‘buraq’ or ‘baurach’ (in Persian ‘burah’) used for the naturally occurring mineral ‘borax’ (see below). Chemically, this is sodium borate, Na2B4O5(OH)4·10H2O.
The Arabic word ‘buraq’ also means ‘white’, ‘sparkling’, or ‘lightning’ (looking at borax, above, you get a good idea why this word might have been chosen). Interestingly, the name ‘Burāq’ is used to refer to the mythological white winged steed (half-mule, half-donkey with the face of a woman and the tail of a peacock) upon which the Islamic prophet Muhammad is said to have travelled from Mecca to Jerusalem and back.
But boron’s tale gets kookier still. Whilst they are to this day credited with discovering our element, neither the Frenchmen nor Davy, had actually succeeded in isolating a sample of boron as a pure element. Although their discoveries had allowed them to produce some elemental boron, the samples they were able to prepare were in no way unsullied.
This is because, awkwardly, boron will cheerfully react with numerous substances at the elevated temperatures required for reduction of its compounds. If there’s anything else in the vicinity it will probably react with that too, thus other boron compounds end up as part of the prepared product. Only c. 60% of the material prepared in 1808 was pure boron.
It took nearly a century to reach a purity of c. 90%. This was accomplished in 1895, by the French chemist Ferdinand-Frédéric-Henri Moissan (best known for the isolation of fluorine and the development of the electric arc furnace, EAF).
Moissan used a slightly modified technique, and reduced borax with magnesium metal to generate boron. As he indicates in his book ‘Le Four électrique’ (‘The Electric Furnace’), his experiments with the element he had produced also showed that boron “passes from the solid to the gaseous state without becoming liquid”.
It wasn’t until 1909 that the American chemist, Ezekiel Weintraub, was able to produce a sample of boron which was c. 99% pure, by reducing boron halides with hydrogen. Even today, it is not usually produced at such a high purity.
Nowadays, there are four main techniques available for the preparation of elemental boron. These are the electrolysis of molten boron salts, reduction of boron compounds by metals (as we’ve seen above), hydrogen reduction, and the decomposition of boron compounds.
By and large, boron is prepared by heating borax with carbon, typically giving around 95% pure boron, but other techniques are used if boron of a particularly high purity is demanded. Using modern methods, usually involving multi-step processes, samples of elemental boron of 99.9999% purity have been prepared. Not bad, eh?
But boron has been used by mankind (albeit in compound not elemental form) for far longer than the relatively recent dates associated with its ‘discovery’. It is thought that the ancient Mesopotamians were the first to make use of borax, possibly in Babylon sometime between 1,894 and 539 BC or potentially earlier, back to the early Sumerian metalworkers c. 2,500 BC.
Borax is well known as flux in metallurgy, often valuable when crafting objects of silver, gold, electrum, bronze, and other metals. In contrast, the use of borax fluxes by goldsmiths in Europe does not commence until about AD 1,200.
However, the use of borax as a metallurgical flux was certainly known to the ancient Egyptians, who also used it medicinally (it acts as a mild antiseptic), and in the complex process of mummification. It is also likely that it was been used in the production of the precious, and permanent blue pigment they developed for ceramics and wall paintings, known as Egyptian blue.
By the first century AD, there is reason to believe from some texts, that the Roman emperors Caligula and Nero may have used borax in amphitheatres. Along with fresh sand and rich perfumes, the borax would help to clean and deodorise the gladiatorial combat zone, doubtless also minimising the influx of pests irresistibly attracted by the blood and gore.
Borax (originally called ‘tincal’ from the Sanskrit word for the mineral) was known to the civilisations in the Far East too. During the Chinese Liao dynasty (AD 916–1125), the glazes craftsmen used on their multicoloured ‘sancai’ ceramics have been analysed and found to contain c. 13% boric oxide.
Since the tradition of ‘sancai’ (characteristically made with lead glazes) began several hundred years before, during the Tang dynasty, it is possible, though as yet unconfirmed, that borax could have been incorporated into earlier glazes. Oddly, the secret of these borax glazes was lost for centuries, only re-emerging in the tenth century.
Outcrops of native borax were exploited from high altitude lake beds in Tibet (once the principal source) and were exported from the eighth century AD, at that point to the Arabian Peninsula and, eventually, beyond. The amounts traded were small-scale, and both the method of production and its source was undisclosed.
This valuable substance had quite a journey. It was protected for transport with a layer of ‘fat’. It first travelled to Hindustan in bags tied to sheep, then onwards to Europe along the ancient trade routes taken by Marco Polo’s caravans, which we now call the Silk Road (or Silk Routes). When it finally reached its destination, purification was required to separate the useful substance from the protective fatty matter. The most effective purification process had been developed by the Venetians and was a fiercely guarded secret.
But, as most secrets do, it eventually seeped out. By the end of the seventeenth century, the Dutch had ‘acquired’ both access to the source and a good understanding of the Venetian’s purification process. Though the centre for borax production may have shifted, the trade in this valuable substance from Tibet continued for centuries.
The first borax refinery to be opened in Britain was at Plaistow marshes, in Essex. This could be found at the pharmaceutical and chemical works of Allen and Howard, and dates to 1798. As a little aside, one of the firm’s founders, Luke Howard, is a fascinating man. Born on what is now the A10, a mere hop, skip and a jump away from Tottenham Hotspur’s White Hart Lane stadium, he was not only a pharmacist, but a keen and talented amateur meteorologist. Howard is the man who devised a nomenclature system for clouds and is, therefore, known as the ‘Godfather of Clouds’. The Essex borax refinery was not unique though, and similar refinery sites were to be found across Europe.
Soon afterwards, by the time we reach the early 1800s, a new source of borax (and other boron minerals, such as sassolite, H3BO3) was discovered. A conveniently situated European source dispensed with many of the costs and vagaries of the long-distance Silk Road trade.
This source was in Italy, in an area rich in geothermal activity including geysers, hot springs, pools of boiling mud, and steam vents, some producing steam as hot as 220⁰C. The ‘new’ borate source came from the ‘soffioni’, volcanic vents in the Maremma region of Tuscany. This might, perhaps, have been foreseeable, as the hot waters in this region had long been used for medicinal purposes. Indeed, even before the coming of the Romans, the Etruscans had made use of the many springs and pools to bathe in the mineral-rich, naturally hot waters. They called this region ‘Il Bagno’, building sacred thermal complexes dedicated to divinities of health, healing, and well-being.
In 1827, a Frenchman, François Jacques de Larderel, developed a method to extract boric acid from the mineral resources of this area, using steam heated cauldrons. After first exhausting supplies of wood from local trees to produce the steam (oops, this is how we learn!), the abundant natural boiling water and steam that rises at these sites was eventually exploited. This powered the production of boric acid and borax at source.
An ingenious idea, this made it one of the first places in the world where geothermal energy was harnessed to support a local industry. In fact, in 1911, the first geothermal power plant in the world, the Larderello plant, in southern Tuscany between Pisa and Siena, was built in the Valle del Diavolo (‘Devil’s Valley’). The plant was named for Monsieur Larderel, a rather fitting tribute, I feel.
A European source for precious borax and boron compounds, plus the additional cost savings achieved by this geothermal step effectively put an end to the lucrative Tibetan ‘tincal’ trade. Italy’s success was relatively short-lived though, as in 1873 massive deposits of borax and other soluble salts were discovered in San Bernardino County, California. This was soon followed by further discoveries in the South American Andes, on Turkey’s Anatolia Plain, and at further US sites in the Californian and Nevada deserts. Many of these are still in production to this day.
By now, although we’ve covered a few applications for our element, you might be wondering what else boron, borax, borates, and other boron compounds can be used for. The answer is one helluva lot!
Boron is a requisite element in a relatively recent powerful type of permanent magnet. These magnets, which were developed in the mid-1980s, are neodymium magnets (also known as Neos, NdFeB, or NIB magnets). They are produced from an alloy of neodymium (Nd), iron (Fe), and boron (B).
They vary in potency, but in terms of strength per given mass of magnet are considerably stronger that the old-fashioned horseshoe type magnets we all probably played around with as kids. A tiny neo is capable of magnetically ‘sticking to’ and lifting hundreds, if not thousands, of times its own mass. Thus, neos have all but replaced the old-style ferrite magnets in many modern applications.
Being so strong for their size and weight, they find uses in a whole range of smaller or portable equipment, such as computer hard drives, mobile phones, toys, jewellery clasps, door locks, cordless tools, microphones, loudspeakers, headphones, and ear buds, amongst other things.
Larger neos can be found in MagLev (magnetic levitation) transport systems, medical imaging systems (such as MRI scanners), generators, and electric motors of all sorts (from EVs and hybrid vehicles to servos and compressor motors). Oh yes, neos have also opened up a rather niche new, if mucky, ‘sport’ which, a few years back, Mr S and I were fascinated to watch in progress at Nottingham’s canal system, magnet fishing.
Somewhere in the region of half of all boron use is in the manufacture of vitreous products (glass, pottery glazes, vitreous or ‘porcelain’ enamels). Boron (generally as borax or boron oxide) is added to the mix as a fluxing agent to lower the melting temperature and viscosity of the melt and improve the workability of the glass. It is also the foundation for a particular type of glass that many of you will be very familiar with, though you may not know it.
This is borosilicate glass, made primarily from silica and boron trioxide. It light, strong, has excellent chemical and thermal resistance, and good optical clarity. You’ll find it in the heat-resistant glass front of your oven, and the Pyrex mixing bowls, ovenware and tableware you may have in your kitchen. It makes up almost all laboratory glass (all those beakers, test tubes, pipettes, and other essentials), also the vials and ampoules in which medicines are stored.
It can even be found in some implantable medical devices such as such as prosthetic (artificial) eyes, and hip joints. It lines vacuum flasks, coats solar panels, is used in headlight bulbs, lamp covers, halogen bulbs and fluorescent tubes. It’s used for thin-film-transistor liquid-crystal display (TFT LCD) screens, and for a vast range of specialist glass applications, for example in art glass, optical glass (prisms and lenses, and in the light-sensitive sunglasses which protect our eyes), laser glass, and telescope mirror blanks. Oh, and the encapsulation of nuclear waste wouldn’t be possible without the extreme chemical durability of borosilicate glass.
Besides the glassy applications we’ve considered, in the previous article when we explored platinum, we took a look at the production of fibreglass. Our element, boron, is another material essential to this process. The E-glass mentioned then, the most commonly used type, is alumino-borosilicate glass. In fact, since fibreglass is the foremost insulating material used in construction, fibreglass manufacturing is the single largest consumer of boron minerals worldwide. It gets more interesting though as a new composite material, boronated fibreglass, is making great strides in the automotive and the aerospace industries.
Speaking of fibres, when it comes to elemental boron, one main industrial use is in the preparation of boron fibre (a.k.a. boron filament). Boron fibre comprises linked boron atoms in a ‘ribbon’ form to produce a monofilament fibre. This yields a lightweight yet high-strength material which finds a number of applications, notably in the aerospace industry.
Boron fibres can be woven into fabrics, used in composite materials to reinforce them, and included to provide heat resistance in other materials. They have found a number of applications in many aircraft, and also for a variety of purposes in space.
Fibres of boron were first noted way back in 1911 by the American chemist, Ezekiel Weintraub, who’d produced that first sample of very pure (c. 99% pure) elemental boron. It took until 1959 before Claude Talley and his colleagues at Texaco Experiment demonstrated that it was possible to produce continuous, high-strength, high-stiffness, and low-density boron filaments (fibres) using a technique called chemical vapor deposition (CVD).
That stiffness was critical, as this allowed the fibres to be used in primary load-carrying structures in aircraft (at this point in time carbon fibre, which we doubtless know better, wasn’t available in unbroken strands). The promise shown by this new material meant that the U.S. Air Force took a very keen interest and funded further research.
To minimise potentially catastrophic ‘flutter’ (escalating vibration or oscillation which was, incidentally, the cause of the collapse of the Tacoma Narrows Bridge), boron fibre composite became the chosen material for the horizontal tail skins for the Grumman F-14 Tomcat fighter, and for the empennage (both the horizontal and vertical skins, and the rudder) on the McDonnell Douglas F-15 Eagle.
The potential and value placed on boron fibres can be summed up by a remark made in 1964 by General Bernard Schriever (who was instrumental in the USAF’s space and ballistic missile programs). He described the development of boron fibre as:
“the greatest single advance in materials in the last 3000 years”.
Boron fibres (as part of composite materials, often incorporating epoxy resin, such as in high tensile strength tapes) are still used in the repair of many commercial aircraft today. Boron offered other advantages for the aerospace and defence industries too. Boron can withstand extremely high temperatures without either melting or embrittling. It is also an excellent conductor of electricity. These qualities give boron a performance edge over most metals in the design of aircraft parts. Oh, and boron is also used in solid rocket fuels and as an additive to jet fuels as it augments their energy output. Boron is valuable because of its low molecular mass (remember, it’s a small, light atom) and its high combustion energy.
A relatively high-cost material, boron fibres have largely been superseded by carbon fibre in many composites. However, they have found a number of more prosaic applications. Their strength and rigidity means that they have been used in the production of high-quality sports equipment, for example fishing rods, bicycle frames, golf clubs, and tennis, squash, and badminton rackets, etc.
We’ve already mentioned some of the medicinal uses for boron and its compounds, but there are many more. From the 18th century boric acid was used as an eye wash, and as a mild broad-spectrum antiseptic. To bring us more up to date, two boron-containing topical medications have recently been developed and seem to be proving effective.
Tavaborole, 5-Fluoro-2,1-benzoxaborol-1(3H)-ol, is an antifungal, used to treat the persistent and irritating fungus which can infect our toenails, and Crisaborole, 4-[(1-Hydroxy-1,3-dihydro-2,1-benzoxaborol-5-yl)oxy]benzonitrile, is used to treat eczema (atopic dermatitis).
In the 1870s, borax and boric acid were used to help preserve foodstuffs such as meat, fish, cream, and butter. Indeed, one Pacific Coast Borax Co. advertising campaign referred to the “millions of pounds of borax … used annually by the British government to preserve mild cured hams”.
This use continued right through the two World Wars, until some bright spark decided it was ‘harmful’ and banned it in the 1950s. Was this a good call? Probably not.
Generally, there has been a dearth of research in this area, and specific biochemical functions for boron are still not completely understood. However, in the last twenty years or so, there have been considerable advances in boron organic chemistry. It’s looking quite likely that boron will be added to the list of ‘essential’ elements.
For example, it has been known since the 1980s that boron plays an essential part in the growth and maintenance of bone, improving the absorption and deposition of magnesium in bone. It is looking increasingly likely that boron supplementation may benefit individuals with osteoporosis or osteoarthritis.
More recently, it was discovered that boron also helps support the immune system, so contributes greatly to wound healing. Furthermore, it can help reduce the levels of inflammatory biomarkers (such as C-reactive protein, CRP)—a breakthrough discovery as elevated hs-CRP is associated with an increased risk of many disorders.
Boron also seems to enhance the body’s ability to use and regulate certain hormones (e.g. oestrogen and testosterone) and can help to prevent Vitamin D deficiency. In addition, it appears to provide some protection against oxidative stress and heavy metal toxicity and has been shown to help prevent and/or ameliorate the effects of a number of cancers. It may also help to lessen the adverse effects of traditional chemotherapeutic medication.
Interestingly, new studies (albeit limited in scope) have suggested that boron may play a role in brain function, supporting and increasing the electrical activity in the brain, and improving both short-term memory and cognitive function, possibly benefitting individuals with conditions such as dementia, but also depression.
But it isn’t only a useful element for animals, including us humans. About a hundred years ago, in the early 1920s, it was convincingly demonstrated that boron is an essential micronutrient for plants. Indeed, they cannot grow without it so in deficient soils it is applied as a fertiliser.
Boron is fundamental to ensuring and maintaining the health of all of the plants that both we, and the animals we eat as meat, rely upon.
Although only required in small amounts, it is integral to the lifecycle of plants. It plays a key role in a range of plant functions, perhaps most importantly in cell wall formation, both above and below ground. This helps the plant to maintain its structural strength. It also helps stabilise cell membranes, maintaining their function.
Boron also assists with the transport of both water and nutrients through the plant, ensuring that these reach the growing portions of the plant. It helps control flowering, the production of pollen, germination, and the development of both seeds and fruit. It is also needed for effective nitrogen fixation in legume crops.
Recent research suggests that boron could even have played an important part in the very beginnings of life on Earth. Boron is probably a prebiotic element, that is, it was present on Earth prior to living organisms arising. Certainly, boron has been identified in the oldest rocks on Earth.
Many scientists now believe that life as we know it began in the so-called ‘RNA World’. This commenced with self-assembling single-stranded molecules, RNA (ribonucleic acid), prior to the emergence of more complex DNA (deoxyribonucleic acid) and proteins. A fundamental component of RNA, its ‘backbone’ if you like, is a simple sugar, ribose. Boron (III), present on Earth in compound form, stabilises ribose. It is still early days for this hypothesis so, for now at least, we’ll just have to watch this space.
But back to our plants, and another important factor in their health and proliferation is boron’s ability to protect plants from a variety of diseases. As such, it can be applied as a pesticide and fungicide. Interestingly, for similar reasons, that is, due to its antifungal, antiseptic, and antiviral properties, boric acid is often used during the water treatment of swimming pools.
Boric acid (H3BO3) also has remarkable flame-retardant properties, so is used to reduce the flammability of a variety of materials in the textile industry, in the manufacture of curtains, carpets, clothing, mattresses, upholstery, and more.
Boron nitride (BN) is another compound of interest, and one with some remarkable properties. Because it exists in a number of different forms (differing in the structural arrangement of boron and nitrogen atoms) the properties this compound can exhibit vary. The forms, roughly speaking, include amorphous (a-BN), hexagonal (h-BN or alpha-BN), cubic (c-BN), and wurtzite (w-BN), though a variety of other forms exist.
Layers of amorphous boron nitride (a-BN) are used in some semiconductor devices (semiconductors are components which lie between conductors and insulators). This includes MOSFETs (metal-oxide-semiconductor field-effect transistors), by far the most common transistor in digital circuitry. In fact, other boron compounds, borates, are used in the manufacture of capacitors, transistors, semiconductors, and other microelectronics, including batteries, that make up the computers and mobile devices you are probably using to read this article.
Boron nitride is extremely hard, with the c-BN form coming in at only slightly less hard than diamond’s 10 on the Mohs scale, and the w-BN form even harder than diamond. This makes it extremely effective for applications like machining steel. The c-BN form is also extensively used as an abrasive (as is boron carbide), and for bits in some cutting tools.
It also has lubricating properties (especially the h-BN form), similar to graphite. This and the fact that it is stable at high temperatures make it a valuable addition to engine lubricants. Fascinatingly, it’s slipperiness also finds applications in cosmetics, like eye shadow, blusher, eyeliner pencils, and lipsticks.
It conducts heat as readily as a metal, yet it is very thermally stable. This makes it valuable for transferring excess heat from a hot source to a cooler heat sink or heat exchanger. It is also chemically stable, being insoluble in most acids. This makes boron nitride ceramics ideal for use in high-temperature equipment and metal casting. However, it is possible to etch boron nitride as is soluble in alkaline molten salts. It behaves as an electrical insulator. Interestingly here, boron nitride can form nanotubes and nanomeshes, and these, unlike carbon nanomaterials, have the advantage of being electrical insulators.
We mentioned that borates are used in the creation of microelectronics components. Well, if you are old-school, and prefer paper, they are also used to boost the quality of recycled paper, improving its brightness.
Industry also benefits from borates, as they are used in the manufacture of industrial fluids—antifreeze, brake fluids, lubricants, fuel additives, and power steering fluid for cars, lorries, and aircraft. Also, in hydraulic fluids, water treatment chemicals (including as biocides in aviation fuel), closed system heat exchanger fluid. Oh, and in metalworking, where they provide corrosion inhibition for a variety of alloys and are used in metal cutting and grinding as coolants, and to reduce friction.
Boron carbide (B4C) was also mentioned for its abrasive properties. However, it is an extremely hard boron–carbon ceramic, so is also used to produce the control rods for the core of nuclear reactors. So tough is this stuff, one of the hardest known materials, just a little behind cubic boron nitride and diamond, that it is used in the composite ‘Chobham’ armour, a multi-layered system to protect tanks and other vehicles.
It was first trialled in the American M1 Abrams tank but is also employed in the Challenger tanks used by the British Army to this day. Boron carbide is also used in similar systems to protect military aircraft, and helicopters. Sometimes referred to as ‘Black Diamond’, it is also found in body armour (ceramic body armour was first used during the Vietnam war) and bulletproof vests. Its major advantage is that it provides high-performance ballistic protection, but for the lightest possible weight.
Similarly, boron steel, an alloy of steel with a small amount (typically ≤ 1%) of boron has been used since the late 1990s in car production, greatly strengthening structural components such as the chassis and roll bars.
When we looked at the elements sulphur and copper, I mentioned a few rather resplendently coloured minerals, long used as semi-precious gemstones and, in some cases, pigments. Well, there is, perhaps not too surprisingly, another one to discuss.
This is tourmaline, a lovely translucent to opaque gemstone which comes in a wide variety of hues, from colourless (rarely) to brown, red, orange, yellow, green, blue, violet, or pink, encompassing most shades in between.
There is an ancient Egyptian myth about the abundance of colours in tourmaline. The Egyptians believed that the stones acquired their beautiful shades when they passed through a rainbow on their way to Earth. I must admit that I rather like tourmaline as it is, along with another multi-coloured stone, opal, the birthstone for those of us with October birthdays.
It is quite unusual as it can be bi-chromatic, displaying different colours within a single stone. It is not uncommon to see a stone which is delicate pink in one part and a grass green in another, or even one exhibiting several different colours, as can be seen above.
These variations in hue relate to elements (other than boron) which appear in the stone. All tourmalines are crystalline silicate minerals, so contain silicon dioxide, SiO2, but then, chemically, things get extremely complicated. Boron is always present in tourmaline, but several other elements such as aluminium, chromium, iron, lithium, manganese, magnesium, potassium, sodium, titanium, vanadium, or zinc may be included in the mineral compounds.
Tourmaline is primarily mined in Brazil nowadays but is also found in Africa, Asia, and parts of the USA, including Maine and California (in fact, it was so prized that certain colours of Californian tourmaline were given by Native Americans as funerary gifts).
It has an interesting history, as glorious green stones found in Brazil by the Spanish conquistadors in the 1500s were believed to be emeralds. This belief stood for the next few hundred years, and it wasn’t until 1703 that a package of multi-coloured gemstones sent from Ceylon (now Sri Lanka) to the Netherlands were identified by a Dutch gemmologist as a distinct gem type, previously undescribed. Tourmaline’s name comes from the term ‘turmali’, meaning ‘stones with mixed colours’ in Sinhalese.
In 1876, the American mineralogist George Frederick Kunz started a craze for tourmaline gemstones when he introduced the beautifully vibrant green tourmaline from Maine to his employer, Tiffany & Co., who still use tourmalines in their fabulous jewellery to this day.
A man passionate about his field, Kunz wrote a really rather wonderful book about precious stones covering such topics as the ‘Sentiments and Folk Lore, Superstitions, Symbolism, Mysticism, Use in Medicine, Protection, Prevention, Religion, and Divination, Crystal Gazing, Birthstones, Lucky Stones and Talismans, Astral, Zodiacal, and Planetary’.
In the late 1800s, the biggest market for tourmaline was China, as the late Qing dynasty Empress Dowager Tzu Hsi (a.k.a. Cixi, or Xiaoqinxian) was enchanted by pink tourmaline. She bought substantial amounts from the source in San Diego County, to have carvings, watch chain bars or jacket buttons produced. This trade was so important that shortly after she died in 1908, the US tourmaline trade collapsed.
Optically, boron and its minerals are a bit of an oddity (are you, like me beginning to wonder if there’s much about this element which isn’t ‘different’?). Amorphous boron is completely opaque to visible light even in very thin sections. However, although it is opaque to visible light, boron can transmit portions of infrared light.
Tetragonal boron, in thin sections (< 50 µ thick), while fundamentally opaque will, at room temperature, transmit a small observable amount of infrared light. This transmission gradually increases with increasing wavelength above 8 μm, the mid-infrared region.
Furthermore, one of the borate minerals, ulexite (chemically, repeating units of hydrated sodium calcium borate, NaCaB5O6(OH)6·5H2O), also exhibits some unusual optical properties.
Ulexite is found in the deserts and mountains of Nevada and California (there are huge veins of the compact fibres in Kramer District, Kern Co., California), though there are also sources in Argentina, Canada, Chile, Kazakhstan, Peru, Russia, and Turkey.
Ulexite can transmit in the visible part of the spectrum. It acts as one of nature’s own ‘fibre optic’ cables. Because of its optical quirks, ulexite is sometimes called ‘television stone’.
Ulexite is found as bundles of parallel silky white fibres, compacted together into ‘columns’. If you look at the fibres from the side, the rock is not transparent at all. But each of the fibres can transmit light along the long axes of its crystals by internal reflection, for the whole of the fibre’s length.
As you will see above, a stone which is cut with flat faces at right angles to the orientation of the fibres can demonstrate this phenomenon. If the stone (with nicely polished flat surfaces) is placed on some printed text, the letters appear to be on the upper surface of the stone. This shift occurs without perceptibly distorting the letters.
Before we finish, let’s just head back to borax for a moment, as it enables another oddity which bears a mention. Mixing borax solution with liquid PVA glue creates a bizarre substance called ‘oobleck’. This takes the form of a non-Newtonian fluid, or shear thickening fluid, which is one whose viscosity depends upon the forces applied to it (most liquids, well standard Newtonian ones, maintain a constant viscosity).
This mixture is a suspension of small particles in a liquid. It’s a little reminiscent of quicksand. It’s liquid-like when gently handled, say slowly poured or stirred, but instantly changes to a solid-like state in response to an externally applied stressor, for example when it is placed under pressure by squeezing or hitting it.
Oobleck (a name which comes from a fictional green goo in a Dr Suess children’s book) can also be made with a mixture of cornstarch and water. Rather fetchingly, it ‘dances’ if subjected to the vibrations produced by a loudspeaker cone. Don’t believe me? Check out this short video. Interestingly, this unusual property of a boron-related substance is thought to show some potential for use in more flexible, therefore comfortable, body armour.
Borax has many more uses than simply ‘science fair’ experiments though. It is a commonly found household substance and can be used domestically in a variety of different ways.
Borax is slightly alkaline and forms a basic solution when added to water. It is inexpensive and, despite the bad press it has received over the years, it’s non-toxic although it can, if used carelessly (as with most cleaning agents), irritate the skin and eyes.
Borax is excellent in the laundry and is found in many clothes detergent formulas. It helps to remove stains, neutralises body odours, and softens hard water. It whitens your whites and can brighten up your coloureds. It’s great for budging bothersome stains on your carpets too. It’ll clean mould and mildew off walls and window frames (maybe it should be promoted in Rochdale), remove soap scum from shower doors, and is good for cleaning windows to a smudge-free finish. It clears and deodorises drains, and while we’re thinking stink, can also detox those smelly trainers! It even helps prevent pesky pests, such as mice, or ants.
Don’t just take my word for it though. In 1966 it got the presidential seal of approval.
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