Metal borides, preparation methods

Borides of Al, Ti, Zr and Hf are prepared according to reaction (a), although even with careful measuring of reactants it is not always possible possible to form the metal boride in stoichiometric ratio, and free metal B may be deposited . This method is not suitable when the free metal deposits at T below those required for boride formation, as in the cases of Nb, Ta, Mo and W. 

In this method " - the melt eontains boric oxide and the metal oxide in a suitable electrolyte, usually an alkali or alkaline-earth halide or fluoroborate. The cell is operated at 700-1000 C depending on electrolyte composition. To limit corrosion, the container serving as cathode is made of mild steel or of the metal whose boride is sought. The anode is graphite or Fe. Numerous borides are prepared in this way, e.g., alkaline-earth and rare-earth hexaborides " and transition-metal borides, e.g, TiBj NijB, NiB and TaB... 

The reduction of a metal oxide or other metal compound by C, B or boron carbide requires higher T than in the other methods available for boride preparation. 

This method is the simplest and cheapest for making borides. It is used to prepare transition-metal borides and alkaline-earth metal hexaborides. 

The crystal growth of metal borides by gas-phase methods permits preparation of products at moderate T (1000-1500°C). This is an important advantage since most borides melt at high T (ca. 3000°C), which makes their crystal growth from melts difficult. In addition, the gas-phase methods lead to the formation of single crystals and solid films of incongruently melting borides. 

More than one boride phase can be formed with most metals, and in many cases a continuous series of solid solutions may be formed. Several methods have been used for the relatively large-scale preparation of metal borides. One that is commonly used is carbon reduction of boric oxide and the appropriate metal oxide at temperatures up to 2000 °C. Fused salt electrolysis of borax or boric oxide and a metal oxide at 700 1000 °C have also been used. Small-scale methods available include direct reaction of the elements at temperatures above 1000 °C and the reaction of elemental boron with metal oxides at temperatures approaching 2000 °C. One commercial use of borides is in titanium boride-aluminum nitride crucibles or boats for evaporation of aluminum by resistance heating in the aluminizing process, and for rare earth hexaborides as electronic cathodes. Borides have also been used in sliding electrical contacts and as cathodes in HaU cells for aluminum processing. 

In the mid-1970s S. Friberg and the late F. Gault proposed an original method using microemulsions to prepare monodisperse nanosized particles. These ideas were followed by a rapid increase in original research works related to the preparation of metal and metal boride nanoparticles [1-5]. 

Large quantities of boron chloride are best prepared by the chlorination of a mixture of boron oxide (or borax) and carbon. Smaller amounts may be obtained (1) by the chlorination of amorphous boron or metallic borides or (2) by the reaction between boron oxide and phosphorus (V) chloride in a sealed tube. A more convenient laboratory method for the preparation of boron chloride makes use of the reaction between boron fluoride and aluminum chloride. For the preparation of boron bromide, the reaction of boron fluoride and aluminum bromide is recommended as much less troublesome than other methods. Potassium tetrafluoborate may be used in place of the boron fluoride in each of these syntheses. 

Metal borides are generally prepared by the direct reaction of the elanents at high temperatures or by the reduction of metal oxides or halides. Thus, reduction of mixtures of BjOj and metal oxides by carbothermic reaction yields metal borides. Reaction of metal oxides with boron or with a mixture of carbon and boron carbide is another route. Some metal borides are prepared by fused salt electrolysis (e.g. TaBj). Borides of IVA-VIIA elements as well as ternary borides have been reviewed by Nowomy [1], The method employed to prepare TiB starting with TiCl is interesting [2], TiCl and BCI3 react with sodium in a nonpolar solvent (e.g. heptane) to produce an amorphous precursor powder along with NaCl. NaCl is distilled off and the precursor crystallized at relatively low temperatures (-970 K). 

Methods of preparation for silicides and aluminides are very similar to those used for carbides, nitrides and borides (1) synthesis by fusion or sintering, (2) reduction of the metal oxide by silicon or aluminum, (3) reaction of the metal oxide with SiOj and carbon, (4) reaction of the metal with silicon halide or (5) fused salt electrolysis. The simplest preparation method consists of... 

New processes for the production of powders have been developed. Molecular precursor methods for preparing nitrides are currently being explored. A precursor processible to metal boride is obtained by dispersing a metal source in a boron carbide polymeric precursor. Electro-chemically prepared polymeric precursors have been used for the formation of pure nitride and carbide powders. Direct formation of carbides and diborides by mechanical alloying of metal powder and carbon or boron powder is also an effective method for synthesizing nanocrystalline powders. 

Preparation. The simplest method of preparation is a combination of the elements at a suitable temperature, usually ia the range of 1100—2000°C. On a commercial scale, borides are prepared by the reduction of mixtures of metallic and boron oxides usiag aluminum, magnesium, carbon, boron, or boron carbide, followed by purification. Borides can also be synthesized by vapor-phase reaction or electrolysis. 

This method is used extensively in the laboratory because it is particularly suitable for preparing borides of rare or expensive metals, e.g., the transition-metal-rich borides CrB, Cr3B4, CrB2 (except Cr3B2 and Cr4B), the diborides ScB2, TiB2 the rare-earth hexaborides, dodecaborides and MB -type borides. 

The reduction of a metal oxide by a mixture of B and C is easier than the reduction by the borothermic process described above. The rate of reduction depends on the removal of CO, so operation under vacuum increases the rate and allows the reaction to proceed at a lower T than the borothermic process. The metal oxide may be volatile and the borides can be contaminated by C. Accordingly, this method is not suitable for preparing pure alkaline-earth and rare-earth hexaborides because in all cases borocarbides of formula MBg C, (e.g., M = Sr, Eu, Yb) are formed . 

Solid-state metathesis reactions. For a number of compounds, solid-state metathesis (exchange) reactions have the advantages of a rapid high-yield method that starts from room-temperature solids and needs little equipment. The principle behind these reactions is to use the exothermicity of formation of a salt to rapidly produce a compound. We may say that for instance a metal halide is combined with an alkali (or alkaline earth) compound of a /7-block element to produce the wanted product together with a salt which is then washed away with water or alcohol. Metathesis reactions have been used successfully in the preparation of several crystalline refractory materials such as borides, chalcogenides, nitrides. 

Borides of Nb and Ta have been reviewed67 and a method for their preparation from boron and metal oxide has been reported.68... 

An attractive method to produce single crystals (in the dimension range from pm to several nun) is the high-temperature solution (flux) method, becanse of its simphcity and the low temperature required. The elements are dissolved in the solvent metal (often Al) and subseqnently the solntion is slowly cooled to room temperature. A prerequisite is, of course, that the solubility of the used solvent in the desired boride is insignificant. The solubility of Al in most boron-rich binary borides has been found to be extremely small. Crystals prepared in this manner are suitable for measurement of physical properties, for instance, microhardness, electrical resistivity, and so on. 

There are few useful reactions in which new B—H bonds are formed. Although the formation of boranes from the protolysis of borides or the reduction of boron compounds with Hj, either in electrical discharges or in the presence of active metals, have historical importance, these methods have no importance or utility today. Indeed, the preparation of boranes is so dominated by the single common starting material, the tetrahydroborate ion, that the only important reactions in which B—H bonds are formed are those in which hydride ion either reduces species with B—O or B-halogen bonds to form boranes or adds to trifunctional boron compounds to form hydroborates. 

Industrially, borides are prepared in various ways, including reduction of metal oxides by mixtures of carbon and boron carbide, electrolysis in fused salts, and direct combination of the elements. For research purposes, the last method is usually used. Cobalt boride, made in aqueous solution by reduction of Co2+ salts with NaBH4 is an active catalyst for reduction of various substrates.6... 

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