Towards novel boron nanostructural materials

Ihsan Boustani

DOI: 10.1039/9781849732789-00001

Nanostructures are linked to many areas of science and technology. A cornerstone of our current research is the practical development of boron cluster-based, nanostructured Materials-by-Design. It is now possible to imagine and predictably model properties, synthesis procedures, manufacturing processes, and to functionally integrate engineering principles. Super-properties such as hardness, wear, coating, as a form of resistance to many attacks (e.g. corrosion), conductivity, spin and energy conversion/ storage, neutron protection and absorption, radiation shielding, armor, hardening, enforce, nanoenergetic, propulsion and glue materials, not to mention the boron neutron capture therapy in medicine, will benefit from the functionalization and industrialization of boron clusters and predicted nanostructures. The experimentally observed physical manifestations of a number of these clusters and nanostructures and related properties are consistent with our theoretical models. Early experiments have demonstrated the near-term viability of controlled synthesis for specific engineering properties.

1 Introduction

Boron contra carbon: Advances in novel experimental techniques for fabrication and measurements together with development of new theoretical methods have resulted, not only knowing the atomistic details of a given structural configuration, but fabrication of artificial structures that required placement of atoms at specified locations for tailor-made properties not exhibited by naturally occurring materials. The arrangements of atoms at nanoscale can now be routinely achieved. Both theoretical and experimental methods have observed a dramatic variation in the physical and chemical properties of a given material with the size at such length scale. For example, carbon exhibits novel properties in the form of clusters, fullerenes, graphene sheet and carbon nanotubes (CNTs). These carbon nanostructures are now proposed as candidate materials for a wide variety of applications such as structural (clothes, fire protection and sport equipment), electronic (displays and transistors), chemical (sensors, water filters and hydrogen storage), and mechanical (oscillators, nanotube membranes and thermal radiation). However, despite their excellent materials and electrical characteristics, CNTs suffer from serious limitations due to a lack of high-purity materials. Therefore, in the recent years attention has also been focused on counterpart or alternatives to CNTs, which can not only exhibit similar properties as the CNTs, but also has some novel characteristics, that are unique to them. To this end, several different nanotubes and novel nanostructures of a wide range of elements and compounds were predicted, synthesized, and characterized.

Why boron! In this respect, boron, being the nearest neighbor to carbon in the periodic table, holds a unique promise of substituting the CNTs and also exhibiting its own novel features. The potential candidacy of boron arises from its electron deficient character and the demonstration of its diverse structural features at cluster level. Boron is the smallest and lightest atom that can form high-strength solids with covalent bonds. In fact, β-boron and γ-boron are the hardest elementary crystals after diamonds. In addition, boron solids have a series of impressive properties. All forms of boron (allotropes) have very high melting points, from 2200 to 23001 C. They are not reactive and thus have high resistance to chemical attacks. One property of special importance is boron's ability to absorb neutrons. This makes boron useful as the control rods of nuclear reactors for controlling the fission reaction, and important for a medical treatment called boron neutron capture therapy (BNCT).

2 History

Origin of boron: Boron compounds may have been known for thousands of years, starting with the Babylonians. They have been credited with importing borax from the Far East over 4000 years ago for use as a flux for working gold. Mummifying, medicinal and metallurgic applications of boron are sometimes attributed to the ancient Egyptians. Also the Chinese, Tibetans and Arabians are reported to have used such materials. The word boron originates from the Arabic word al-buraq or the Persian word burah which are names for the mineral borax. The name boron arises from the combination of borax and carbon. Borax was known from the deserts of western Tibet, where it received the name of tincal, derived from the Sanskrit. Borax glazes (Na2[B4O5(OH)4]·8H2O) were used in China from AD300, and some tincal even reached the West, where the Arabic alchemist Jabir ibn Hayyan seems to mention it in 700. Marco Polo brought some glazes back to Italy in the 13th century. Agricola, around 1600, reports the use of borax as a flux in metallurgy. In 1777, boric acid was recognized in the hot springs (soffioni) near Florence, Italy, and became known as sal sedativum, with mainly medical uses.

Discovery of boron: Boron as an element was not known until 1808 discovered by Sir Humphry Davy and independently by Joseph Louis Gay-Lussac and Louis Jacques Thénard isolated through the reaction of boric acid and potassium. The purity of their products was about 50%. Davy called the element boracium, Jöns Jakob Berzelius identified boron as an element in 1824. Fifty years later impure boron products resembling both diamond and graphite were produced. The diamond-hard material was found to be largely aluminum boride (AlB12), while the graphite variety was a complex boron-aluminum-carbon. Much later, a higher purity boron was made by reduction of boron trioxide with magnesium. Purities of about 90% can be achieved by this procedure. In 1909, the first pure boron was arguably produced by E. Weintraub. Ordinary boron is a brown-black amorphous powder. Pure boron can be made as extremely hard yellow monoclinic crystals that are a semiconductor resembling silicon. The band gap is 1.50 or 1.56 eV. Crystalline boron is an insulator at low temperatures, but becomes a conductor at elevated temperatures, as would be expected as carriers are thermally excited into the conduction band.

Boron steel: Coevally boron was found that it formed many unusual and complex compounds. It was also considered as a potential alloying element in steels. During the next 20 years various investigators studied, and even patented, steels with boron concentrations up to 2% in some cases. It was not recognized until 1921 that such large amounts of boron were responsible for making steel extremely hard and brittle and that even such a minute amount as 0.001 wt% of boron could produce significant effects on steel properties. Only in the thirties and early fourties the effect of different elements — including boron — on the hardenability of steels was recognized and boron was found to be by far the most potent. However, it has been during the last forty years that the right physical and chemical properties have been determined. High purity boron was produced by electrolysis of molten potassium fluroborate and potassium chloride (KCl) and by vapor deposition methods which have made such deter- minations possible.

Boron as absorber and propellent: The renaissance of the old boron business, although with completely different products, emerged after World War II. Elementary boron and not borax was at the top of the list of the highly ambitious industry. On the one hand, there were the operators of nuclear power plants that required boron to absorb the neutrons in the reactors. On the other was the entire pyrotechnical industry, which used elementary boron in powder form as an additive in solid rocket propellants to achieve a higher thrust force. Back in 1950, there were only very few companies in the West capable of producing boron, and none of them was located in Europe. In other words, the company that would be capable for supplying this element as early as possible would probably encounter a rapidly growing market. In the mid 1970s there was an interesting development in the field of rocket engines toward the solid-propellant ducted rocket. The central feature of this development was a new propellant system, which was referred to as the ram jet engine. A control valve was placed between the gas generator and the combustion chamber to regulate propulsion. Since the gas generator of the system was under high pressure, the opening of the regulating valve had to be appropriately narrow. As a result, reliable methods for characterizing the grain size distribution of the solid boron 95/97 additive had to be calculated because it was feared that coarse boron grains would interfere with the ability to regulate the engine. As a result, customers demanded an amorphous boron with a two-percent maximum of grains larger than 2 µm and no grains larger than 5 µm.

3 Chemistry of boron

Atomic boron: Boron, being the nearest neighbour to carbon in the periodic table, is hard, brittle, lustrous black semimetal. It is known with its short covalent radius, multicenter and directed covalent bonds, holds a unique property because of its electron deficient character and thermic properties. The electron configuration of boron is 1s22s22p1. It has only three valence electrons, so the ion is unpolarizable, and does not hydrate. Therefore, boron is not eager to donate electrons in an electrovalent bond, and can also not accept them easily. Therefore, most of its bonds are covalent, and even forms half-bonds in which only one electron is shared covalently, not the usual two. This gives boron an apparent valence of +6 that we shall see in some interesting compounds. The first ionization potential is 8.29 eV, which is usually high. Boron occurs naturally as 80% B11 and 20% B10. The latter isotope has a high cross section for thermal neutron absorption. Boron is at room temperature nonreactive with oxygen, water, alkalis or acids, and is unaffected by air. The density of crystalline boron is 2.34 g/cc, of amorphous boron, 2.37. It melts at 23001 C and boils at 2550° C (some sources say 2040° C and 4100° C), so it is a very refractory substance. Boron fibers have been used in composite materials because of their great strength.

Boron minerals: At higher temperatures, boron does burn and reacts with oxygen forming boron oxide (B2O3), and with nitrogen forming boron nitride (BN). With most metals, it forms refractory borides. Boron is sufficiently reactive to preclude its occurrence in the free state. The sources of combined boron are sassolite (H3BO3), found in Italy; colemanite (Ca2B6O11·5H2O); ulexite (CaNaB5O6(OH)6·5H2O); and kernite (rasorite, Na2B4O7·4H2O), in the United States. Ulexite is also found in Bolivia, Chile and Peru. Boracite (Mg3B7O13Cl), is found in Germany. The world's major source of boron is kernite from the Mojave Desert in California, USA. Turkey has supplied colemanite for many years to boric acid producers in Europe. Sodium borates were discovered at Kirka in 1960 and other deposits have since been found and developed in Anatolia. As a result, today Turkey is the largest producer of borate products in the world, exporting mineral concentrates of tincal, colemanite, and ulexite, plus refined borax decahydrate, borax pentahydrate (Na2B4O7·5H2O), anhydrous borax (Na2B4O7), and sassolite or boric acid (B[OH]3).

Borax and boric acid: Boron in its combined form of borax, written also as (Na2B4O7·10H2O) has been used since early times. Early uses were as a mild antiseptic and cleaner because of its detergent and water-softening properties. Later it was used as a soldering flux and ceramic flux because of its ability to dissolve metal oxides. Borax is used to produce a heat-resistant borosilicate glass for the home and laboratory, familiar to many by the trade mark "Pyrex", and is the starting material for the preparation of other boron compounds. Boric acid, is mildly antiseptic, is used widely as an eye washer and as a neutron absorber in the swimming-pool type nuclear reactors and in electroplating baths, such as those used for nickel deposition. Its anhydride is used as a source of boron in the fused salt electrolysis method for the preparation of elemental boron. The boron trifluoride is a gas produced in large quantities for gas tube neutron radiation detectors for monitoring radiation levels in the earth's atmosphere and in space. Some organizations use these devices to ascertain the best underground level at which to blast to produce oil wells of high yield. Boron triflouride is an important industrial catalyst for many organic reactions, such as polymerization reactions. It has also a major role in the electroplating of nickel, lead and tin. Amorphous boron is used in pyrotechnic flares to provide a distinctive green colour and in rockets as an igniter.

Production of boron: However, the production process for boron via potassium fluoroborate and repurification was not a solution because it was too expensive. There were three other methods used to produce boron in the 1950s: (i) reducing boron trichloride with hydrogen, (ii) thermal conversion of diboran and (iii) reducing boron trioxide with magnesium. At the beginning of the 1950s, scientists in the United States had already started to recover amorphous boron with a 90–92% boron content directly, by treating boron oxide with magnesium and following this with an acid treatment. By the early 1960s adding amorphous boron (90–92% boron) to solid propellant rocket motors in order to improve propulsion performance had gained wide acceptance. Thus a specification covering the composition of the boron and also the analytical methods to be used were introduced in 1962. The boron 90/92 that was produced in the United States largely fulfilled the requirements.

Oxidation of boron: Other production process consisted of magnesium reduction of boron oxide, acid leaching of the reaction cake, washing with water, drying and deag-glomeration of the dried cake. A new area of application for amorphous boron emerged at the end of the 1980s. Spurred by the aim of airbag producers to equip the gas generators of these lifesaving automobile safety systems with ignition mixtures that were both absolutely reliable and also nontoxic, the demand for boron 90/92 in the United States and boron 95/97 in Germany skyrocketed. A densified powder mixture of sodium nitrate and amorphous boron is triggered by an electric impulse in such airbag igniters. With the energy created by this primary ignition, sodium azide, which is distributed around the igniter in the form of pellets, decomposes in the gas generator in a fraction of a second to nitrogen and sodium. While the sodium is removed with the help of silicates, the released nitrogen inflates the airbag.

Previous applications: Aluminum boride (AlB12), has been used as a substitute for diamond dust for grinding and polishing. Boron carbide is also used with this purpose and it has found extensive use as a polishing agent, for sandblast nozzles, etc. Elemental boron is hard and brittle like glass, having similar uses. Boron can be added to pure metals, alloys or to other solids to improve its strength. Boron in the elemental form is not toxic. The finely divided powder is hard and abrasive, and may cause skin problems indirectly if the skin is rubbed after contact. Trace amounts of boron seem necessary for good growth of plant lie, but large amounts are toxic. Boron accumulated in the body through absorption, ingestion and inhalation of its compounds affects the central nervous system. The symptoms are depression of circulation, vomiting and diarrhea, followed by shock, coma and a body rash.

4 Icosahedral-based crystalline boron

First crystallines: However, as mentioned above, the first samples of highly pure crystalline boron were prepared in 1909 by Weintraub, by reduction of BCl3 with hydrogen in an electric arc. The first crystals of the a-tetragonal boron were obtained in 1943 by Laubengayer et al. They have undertaken the production of single crystals of boron, the establishment of the high purity of the samples, a comprehensive study of the properties of the element and, in particular, X-ray examination of the material in an effort to ascertain the X-ray powder diffraction pattern characteristic of boron and to obtain data adequate for the determination of its crystal structure. They used the filament method which consists essentially in the vapor phase reduction of a volatile compound of boron in the vicinity of a glowing filament, the free boron depositing upon the filament. After considerable preliminary experimentation the reduction of boron tribromide vapor by hydrogen was found to be particularly suitable for the production of pure boron, hydrogen bromide being the other product formed. They observed single crystals of boron of high purity which have been grown in appreciable size.