Determination of boron and nitrogen in boron nitride

Improved techniques are described for the determination of boron and nitrogen in pure boron nitride. Controlled fusion of boron nitride with sodium carbonate in a muffle furnace is followed by a potentiometric titration of the boric acid. A special quartz vessel is described for the determination of nitrogen. The boron nitride is fused with sodium hydroxide and the resulting ammonia is swept into a receiver and titrated with standard hydrochloric acid. Boron and nitrogen values with their standard deviation are given for a typical pure boron nitride.     

Structure and stability of small boron and boron oxide clusters

To rationally design and explore a potential energy source based on the highly exothermic oxidation of boron, density functional theory (DFT) was used to characterize small boron clusters with 0-3 oxygen atoms and a total of up to ten atoms. The structures, vibrational frequencies, and stabilities were calculated for each of these clusters. A quantum molecular dynamics procedure was used to locate the global minimum for each species, which proved to be crucial given the unintuitive structure of many of the most stable isomers. Additionally, due to the plane-wave, periodic DFT code used in this study a straightforward comparison of these clusters to the bulk boron and B2O3 structures was possible despite the great structural and energetic differences between the two forms. Through evaluation of previous computational and experimental work, the relevant low-energy structures of all but one of the pure boron clusters can be assigned with great certainty. Nearly all of the boron oxide clusters are described here for the first time, but there are strong indications that the DFT procedure chosen is particularly well suited for the task. Insight into the trends in boron and boron oxide cluster stabilities, as well as the ultimate limits of growth for each, are also provided. The work reported herein provides crucial information towards understanding the oxidation of boron at a molecular level.     

Rapid mass-spectrometric determination of boron isotopic distribution in boron carbide

Boron isotopic ratios are measured in boron carbide by thermionic ionization mass spectrometry with no prior chemical separation. A powder blend of boron carbide and sodium hydroxide is prepared, a small portion is transferred to a tantalum filament, the filament is heated to produce sodium borate, and the filament is transferred to the mass spectrometer where the(11)B/(10)B ratio is measured, using the Na(2)BO(2)(+) ion. Variables investigated for their effect on preferential volatilization of (10)B include the sodium hydroxide-boron carbide ratio and the temperature and duration of filament heating. A series of boron carbide pellets containing natural boron, of the type proposed for the control rods of the Fast Flux Test Facility reactor, were analysed with an apparently unbiased result of 4.0560 for the (11)B/(10)B ratio (standard deviation 0.0087). The pellets contained over 3% metal impurities typically found in this material. Time of analysis is 45 min per sample, with one analyst.     

Crystalline boron nanowires

Ideal nanowire interconnects for nanoelectronics will be refractory, covalently bonded, and highly conductive, irrespective of crystallographic orientation. Theoretical studies suggest that boron nanotubes should be stable and exhibit higher electrical conductivities than those of carbon nanotubes. We describe CVD growth of elemental boron nanowires, which are found to be dense nanowhiskers rather than nanotubes. Conductivity measurements establish that they are semiconducting, with electrical properties consistent with those of elemental boron. High conductivities should be achievable through doping.     

Thermoelectric properties of photo- and thermal CVD boron and boron phosphide films

The electrical properties of n-BP films newly prepared by thermal CVD in the B₂H₆-PH₃-H₂ system were improved by a deuterium lamp excitation. High-temperature electrical conductivity and thermoelectric power of amorphous boron and polycrystalline boron phosphide films grown on silica glass were measured to evaluate the thermoelectric figure-of-merit (Z). In particular, the Z-value for photo-thermal BP films was higher (10⁻⁴/K) than that of boron films, indicating that they are promising for high-temperature thermoelectric materials.     

Polyhedral boron nitride molecules

The geometrical and electronic structures of two isomers (1 and2) of the polyhedral boron nitride molecule, B₁₂N₁₂, have been calculated using the MNDO method. Structure1 having the form of a truncated octahedron is more energetically preferable (ΔH _f ⁰=−128 kcal mol⁻¹) than isomer2, which hasC _6v symmetry. The equilibrium geometries of the N₆B₆(CH₂)₆ isomers (3 and4), which simulate fragments of structure2, have been calculated. The stabilization mechanism of the N₆ nitrogen cluster (hexaazabenzene) in polyhedral structures is discussed. The parameters calculated for molecules1 and2 have been correlated with the corresponding characteristics of their carbon analogs. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1712–1714, October, 1993.     

Elongation of planar boron clusters by hydrogenation: boron analogues of polyenes

Dihydrogenated boron clusters, H(2)B(n)(-) (n = 7-12), were produced and characterized using photoelectron spectroscopy and computational chemistry to have ladderlike structures terminated by a hydrogen atom on each end. The two rows of boron atoms in the dihydrides are bonded by delocalized three-, four-, or five-center σ and π bonds. The π bonding patterns in these boron nanoladders bear similarities to those in conjugated alkenes: H(2)B(7)(-), H(2)B(8), and H(2)B(9)(-), each with two π bonds, are similar to butadiene, while H(2)B(10)(2-), H(2)B(11)(-), and H(2)B(12), each with three π bonds, are analogous to 1,3,5-hexatriene. The boron cluster dihydrides can thus be considered as polyene analogues, or "polyboroenes". Long polyboroenes with conjugated π bonds (analogous to polyacetylenes), which may form a new class of molecular wires, should exist.     

Reactivity of boron fullerenes

The properties of carbon and boron fullerenes are compared, and similarities and distinctions are pointed out. The ionization potentials and electron affinities of boron nanoparticles are estimated, and the binding energy for these systems is calculated.     

Novel architectures of boron

Structural Chemistry - The review presents results of the recent studies of non-typical forms of boron derivatives, including flat hexagonal boron, boron fullerenes, supertetrahedral boron, and...     

Pseudohalides of boron subphthalocyanine

The synthesis and study of a series of pseudohalides of boron subphthalocyanine (BsubPc) are reported. Each pseudohalide has been compared to the more common chloride and bromide of BsubPc, and we have found that most react slower under standard phenoxylation and hydrolysis conditions. Three pseudohalides (TsO-BsubPc, MsO-BsubPc, and BsO-BsubPc) do not hydrolyze at all even after prolonged periods of time in the presence of water. Single crystals of TsO-, MsO-, and ClsO-BsubPc were obtained, and their structures were unambiguously determined.     

Rapid accurate isotopic measurements on boron in boric acid and boron carbide

A procedure is described whereby rapid and accurate isotopic measurements can be performed on boron in boric acid and boron carbide after fusion of these compounds with calcium carbonate. It allows the determination of the isotopic composition of boron in boric acid and boron carbide and the direct assay of boron or the (10)B isotope in boron carbide by isotope-dilution mass spectrometry.     

Amorphous feather-like boron nanowires

Large-scale arrays of feather-like boron nanowires have been successfully prepared using magnetron sputtering with a target of highly pure boron and boron oxide mixture. The morphology, microstructure and composition of the feather-like boron nanowires are characterized in detail using scanning electron microscopy, transmission electron microscopy, and electron energy-loss spectroscopy. Elemental mapping has been used to investigate the distribution of boron and oxygen in the boron nanowires. It is revealed that the feather-like boron nanowires possess the microstructure of outer oxidized coating layer and inner pure boron. The thickness of the outer oxidized layer is about 1–2 nm. Our results may provide opportunities to understand the fundamentals of boron chemistry and to fabricate new nanodevices.     

Photoluminescence of boron carbide

The excitation of B_4.3C with an Ar-laser () yields a photoluminescence spectrum between about 1.56 and 1.58 eV with its main maximum at 1.563 eV and a weaker maximum at 1.572 eV. It is attributed to the indirect-allowed recombination of free excitons.     

Investigations on boron compounds

Summary A method is proposed for the preparation of triallylborine by reaction between boron trifluoride and allyl-magnesium bromide; the desired product is obtained in 80% yield.     

Some physical properties of compacted specimens of highly dispersed boron carbide and boron suboxide

Structure, shear modulus and internal friction (IF) of compacted specimens of boron carbide and boron suboxide have been investigated. Microtwins and stacking faults were observed along the {100} plane systems of polycrystalline specimens of boron carbide. Electrical conductivity of the specimens was that of p-type. Concentration of holes varied from 10¹⁷ to 10¹⁹ cm⁻³. The IF was measured in the temperature range 80-300 K. It was shown that the IF of boron carbide and that of boron suboxide were characterized with a set of similar relaxation processes. Mechanisms of the relaxation processes in boron carbide and boron suboxide are discussed in terms of the Hasiguti model of interaction between dislocations and point defects.