Aluminum boride structures

The process of forming boron-boron bonds is carried on further in aluminum boride, AIB, which has a very simple hexagonal structure, consisting of hexagonal layers of boron a,toms, like the layers of carbon atoms in graphite, with aluminum atoms in the spaces between the layers (Pig. 11-15). The B—B bond length is 1.73 A, corresponding to n = 0.66 that is, two valence electrons per boron atom are used in the B—B bonds, which are two-thirds bonds. 

There are several aluminum boride phases known.One of the more common, AIB12, is employed in the manufacture of lightweight armor, as a catalyst for organic reactions, and as a economical replacement for diamond abrasive. It has a complex structure composed of B12 icosahedra. Big units (twinned icosahedra), and single boron atoms in a 2 1 1 ratio. The aluminum atoms are distributed statistically over all the boron sites. AIB12 is synthesized by direct combination of the elements at 1100 °C or by reaction of aluminum with Na4B407 at 1100°C. 

An aluminum boride that is intermediate between the two extremes above, AIB4, has a structure consisting of Be octahedra that are linked by Bg units forming tunnellike cavities in which the aluminum atoms lie see Boron Inorganic Chenustr. ... 

Numerous ceramics are deposited via chemical vapor deposition. Oxide, carbide, nitride, and boride films can all be produced from gas phase precursors. This section gives details on the production-scale reactions for materials that are widely produced. In addition, a survey of the latest research including novel precursors and chemical reactions is provided. The discussion begins with the mature technologies of silicon dioxide, aluminum oxide, and silicon nitride CVD. Then the focus turns to the deposition of thin films having characteristics that are attractive for future applications in microelectronics, micromachinery, and hard coatings for tools and parts. These materials include aluminum nitride, boron nitride, titanium nitride, titanium dioxide, silicon carbide, and mixed-metal oxides such as those of the perovskite structure and those used as high To superconductors. 

Other interesting directions are hardening with intermetallics, quasi-crystals, borides, silicides, discrete fibres, creation of natural composites. Conscious regulation of structure and properties of such materials requires studying phase equlibria in multi-component systems, in particular on the basis of light metals like aluminum, magnesium, titanium. The important direction is also a creation of specially organized porous structures. To some extent these directions are presented in a number of papers of the present book. 

Binary borides are formed with transition and inner transition elements, with alkali and alkaline-earth elements, and with the p elements, aluminum, and silicon. In addition, a group of more electronegative elements (C, 0, P, S, As, Se) form compounds with boron, which display properties and structures closely related to those of some borides. This group of compounds is also briefly described in the present article as well as the polymorphs of elemental boron. 

The occurrence of the binary borides of the alkaline, alkaline earth, aluminum, and transition elements has been collected in Table 1, together with boron compounds of the right main group elements (carbides, etc.). Only relatively well-established phases have been included. Noncorroborated and/or badly characterized borides lacking precise composition and structure data are not included. The reader is referred to other sources for references. There are no binary borides among the Cu, Zn, Ga, and Ge group elements with the exception of a noncorroborated early report on diborides in the Ag-B and Au-B systems. Two silicon borides have been established, namely, SiB3 4 and SiBe. 

Moreover the presence of the bases to heterogeneous nucleation in form of hard deformable phases for example titanium borides in structure in aluminum, generate possibility of point cracks formation (Fig. 11) and in result of this delamination of sheet (foil) during rolling (Keles Dundar, 2007). 

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