Refractory metals borides

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

The refractory-metal borides have a structure which is dominated by the boron configuration. This clearly favors the metallic properties, such as high electrical and thermal conductivities and high hardness. Chemical stability, which is related to the electronic... 

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... 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

Boron forms B—N compounds that are isoelectronic with graphite (see Boron compounds, refractory boron compounds). The small size also has a significant role in the interstitial alloy-type metal borides boron forms. Boron forms borides with metals that are less electronegative than itself including titanium, zirconium, and hafnium. 

Although the results obtained can be considered as approximate rather than accurate, they give some information about the relative strength of oxocompounds of the Group VIb elements. Nevertheless, these results were used as initial data for the development of the processes of reduction of these oxocompounds in chloride melts [150-152]. Theoretical bases and principles for monitoring the electrochemical processes of deposition of the free refractory metals and their compounds with some non-metals (carbides, borides, silicides, etc.) from molten ionic media [153] were developed. 

Steel bars, coated with tantalum by the FLINAK process, have been borided under low pressure and the properties evaluated. The process (Fig. 1) is conducted by consecutive coating procedures. To define the boride processing data for the diffusion treatment of refractory metals preliminary investigations have been carried out, such i.e. 

THE SYNTHESIS OF BORIDES, CARBIDES AND SBLICIDES OF REFRACTORY METALS IN IONIC-ELECTRONIC MELTS... 

Like the carbon black, fine amorphous boron is poorly wetted with salt melts and emerges to the surface of the salt immediately after the salt is molten, while the refractory metal powder is left on the bottom. Thus, the first stage of the S5mthesis is the formation of lithium or calcium borides. 

LiB has been so far unknown. We think they probably have a polar-covalent type of the chemical bonds, which is similar to the ionic type. In the LiCl melt, which contains a certain amount of free delocalized electrons, the said borides may dissociate to lithium cations and boron anions having the hypothetical composition B . The latter may serve as the boron carriers from pure boron or the higher lithium borides to the metal particles. As a result, thermodynamically stronger borides of refractory metals are formed through diflhision. 

A somewhat different picture is observed for the Ca-CaCl2 melt. Two strong refractory borides CaB4 and CaBe, which are insoluble in the ionic melt, are formed in the Ca-B system. The density of these borides is higher than the density of the CaCl2 melt and therefore they should subside to the tube bottom and may contact the refractory metal powder. We have been unaware of the thermodynamic data on CaB4. The standard values of the CaB6 formation are A 298 = -58.0 kJ/mole and = 76.89 J/K-mole. 

More close to the above-stated requirements are some carbides, borides, and nitrides of refractory metals and their compositions. The chemical requirements for such materials are... 

Nitrides of refractory metals interact with aluminium, giving aluminium nitride [114]. The absence of interaction of borides and carbides, which are on the left side of the schemes, was proved experimentally [115]. 

Finally, three elements of the periodic system occupy an intermediate position with regard to the ability to form refractory metal-like and non-metallic compounds. These elements, beryllium, magnesium, and aluminum, are capable of forming fairly refractory semiconductor compounds with nonmetals (beryllium, magnesium, aluminum borides, aluminum nitride, magnesium silicides), and they may also enter into the composition of intermetallic compounds of the beryllide, aluminide, etc., type. 

The materials deposited by PVD techniques include metals, semiconductors (qv), alloys, intermetaUic compounds, refractory compounds, ie, oxides, carbides, nitrides, borides, etc, and mixtures thereof. The source material must be pure and free of gases and inclusions, otherwise spitting may occur. 

Next to Cr C2, TiC is the principal component for heat and oxidation-resistant cemented carbides. TiC-based boats, containing aluminum nitride, AIN, boron nitride, BN, and titanium boride, TiB2, have been found satisfactory for the evaporation of metals (see Boron compounds, refractory boron compounds Nitrides). 

Other electropositive elements have been used (e.g. Li, Na, K, Be, Ca, Al, Fe), but the product is generally amorphous and contaminated with refractory impurities such as metal borides. Massive crystalline boron (96%) has been prepared by reacting BCI3 with zinc in a flow system at 900°C. 

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