Boron, amorphous

Amorphous boron has not been obtained in the pure state. Crystalline boron is a black powder, extremely hard, with a metallic appearance but with very low electrical conductivity. 

Amorphous boron and the amphoteric elements, aluminium and gallium, are attacked by aqueous solutions of sodium hydroxide and... 

Boron exists naturally as 19.78% lOB isotope and 80.22% IIB isotope. High-purity crystalline boron may be prepared by the vapor phase reduction of boron trichloride or tribromide with hydrogen on electrically heated filaments. The impure or amorphous, boron, a brownish-black powder, can be obtained by heating the trioxide with magnesium powder. 

Amorphous boron is used in pyrotechnic flares to provide a distinctive green color, and in rockets as an igniter. 

Crystalline boron (99%) costs about 5/g. Amorphous boron costs about 2/g. 

Many materials have been deposited by PECVD. Typically, the use of a plasma allows equivalent-quaUty films to be deposited at temperatures several hundred degrees centigrade lower than those needed for thermal CVD techniques. Often, the plasma-enhanced techniques give amorphous films and films containing incompletely decomposed precursor species such as amorphous siUcon (i -Si H) and amorphous boron (i -B H). 

The physical properties of elemental boron are significantly affected by purity and crystal form. In addition to being an amorphous powder, boron has four crystalline forms a-rhombohedral, P-rhombohedral, a-tetragonal, and -tetragonal. The a-rhombohedral form has mp 2180°C, sublimes at approximately 3650°C, and has a density of 2.45 g/mL. Amorphous boron, by comparison, has mp 2300°C, sublimes at approximately 2550°C, and has a density of 2.35 g/mL. 

Amorphous boron (not crystalline) ignites on heating in the dry oxide. 

Semi-metal impure (amorphous) boron is a brown powder the pure element is shiny, silver grey. 

Uses. Amorphous boron is used as an addictive in pyrotechnic mixtures, solid rockets propellants, explosives, etc. Refractory metal borides are used as addic-tives to cemented carbides. High purity boron is used in electronics as a dopant to... 

Wang, P, Orimo, S., Tanabe, K., Fujii, H. 2003. Hydrogen in mechanically milled amorphous boron. J Alloys Comp 350 218-221. 

Fig. 3.18 Secondary electron micrographs showing the morphology of Mg-2B powders reac-tively mechanically alloyed, (a) Made with oxidized amorphous boron after 200 h, (b) made with oxide-free boron after 50 h and (c) made with oxide-free boron after 200 h of milling...

Fig. 3.20 DSC curves of the Mg-2B powder mixture made with oxide-free amorphous boron after milling for (a) 50 h and (b) 200 h...

The most important application of boron is to make fibers or whiskers of single crystal or ceramic crystal. The addition of boron to metals, alloys, or other solids, imparts resistance to plastic flow, and thereby produces unusual strength in the material. Amorphous boron is used in rockets as an igniter, and in pyrotechnic flares to give green color. Many boron compounds, such as borax, boron hydrides, and boron hahdes, have important commercial applications (see individual compounds). 

Amorphous boron and bismuth oxide Amorphous boron reacts vigorously with bismuth oxide to form fusible metallic bismuth and volatile boric oxide (Equation 5.10) ... 

Preparation of Amorphous Boron from Boric Acid Anhydride. (Each student performs only one of the experiments for preparing amorphous boron.) Put 2 g of boric acid anhydride into an iron crucible or onto an iron plate and roast it if the boric acid anhydride contains moisture, then when it is heated with magnesium an explosion may occur ). Triturate the boric acid anhydride as rapidly as possible (why ) in 4 mortar, weigh 1 g and mix it with 2 g of a magnesium powder. Place the mixture into a refractory test tube and fasten it in an inclined position in a clamp of a stand in a fume cupboard . First carefully heat the test tube with the mixture with the flame of a burner, and then strongly heat the bottom part of the tube. What do you observe ... 

---