Preparation of Transition Metal Borides

Large-scale production of metal borides occurs preferentially in electric furnaces by the following high-temperature reactions  

The carbothermic method yields carbon contaminated powders and is suitable for materials in which a C content of up to 3 mass-% can be tolerated. For instance, TiB2, ZrB2, and the technically important hexaboride CaBg are synthesized by this method. 

The boron carbide process can also start from blends of metal carbides, metal hydrides, boron oxide, boron carbide and carbon black  

This material usually contains only small amounts of residual carbon or boron carbide but no metals, and is thus the favored process for the technical synthesis of less contaminated borides. The process is carried out in tunnel furnaces under hydrogen or in a vacuum at 1600-2000°C, i.e., below the melting point of the boride. It is thus a reaction sintering procedure yielding a high-porosity product which can easily be crushed and milled. Additional refinement is obtained by multiple vacuum treatments with metallic or B4C additives to compensate nonstoichiometries. The final product is then called vacuum quality . 

The yield is usually contaminated by residual metals or oxides and thus has to be purified by subsequent leaching, or a high-temperature vacuum treatment. 

---