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...