Chemical transition metal borides

Pierson, H., A Survey of the Chemical Vapor Deposition of Refractory Transition Metal Borides, in Chemical Vapor Deposited Coatings, pp. 27-45, Am. Ceram. Soc. (1981)... 

Selected Chemical Properties of Transition-Metal Borides... 

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

In 1963 Dr. Danbk joined the Institute of Inorganic Chemistry of the Slovak Academy of Sciences in Bratislava, of which he was the director in the period 1991-1995. His main field of interest was the physical chemistry of molten salts systems in particular the study of the relations between the composition, properties, and structure of inorganic melts. He developed a method to measure the electrical conductivity of molten fluorides. He proposed the thermodynamic model of silicate melts and applied it to a number of two- and three-component silicate systems. He also developed the dissociation model of molten salts mixtures and applied it to different types of inorganic systems. More recently his work was in the field of chemical synthesis of double oxides from fused salts and the investigation of the physicochemical properties of molten systems of interest as electrolytes for the electrochemical deposition of metals from natural minerals, molybdenum, the synthesis of transition metal borides, and for aluminium production. 

A pronounced bloating of the samples is observed due to the release of gaseous compoimds. Eq. (38) resembles the so-called boron carbide route for the production of the particular transition metal borides. Another limiting factor is the chemical, geometrical and mechanical destabilization of tetragonal zirconia if combined... 

The high enthalpy of formation of both Zr02 and transition metal borides can be used to enhance densification by a chemical driving force starting from, e.g., Ti02 and ZrB2 [303, 304] ... 

The metal-rich transition metal phosphides (<60% P) are dark coloured and insoluble in water, they have high chemical and thermal stability, they are dense, hard and brittle and have high thermal and electrical conductivities. These properties they have in common with the transition metal borides and silicides (and in some cases carbides and nitrides) to which they are often structurally related. With few exceptions, the transition metal phosphides, borides and silicides are not attacked by dilute acids and bases and may remain unaffected by hot concentrated mineral acids. 

Boron carbide is chemically inert, although it reacts with oxygen at elevated temperatures and with white hot or molten metals of the iron group, and certain transition metals. B4C reacts with halogens to form boron halides—precursors for the manufacture of most nonoxide boron chemicals. B4C also is used in some reaction schemes to produce transition metal borides. Boronizing packings containing B4C are used to form hard boride surface layers on metal parts. 

INTERSTITIAL. < 11 Descripiive of a nnnstoichiuiiiclric compound ol a metal and a nometal whose structure conforms to. 1 simple chemical formula, bill exists over a limited range of chemical composition. Imersiinal compounds are represented by borides, nitrides, and carbides ol the transition metals. 21 Descriptive n an atom of an impurity lhai causes a del cel... 

With purely ionic compounds, appropriate ionic radii must evidently be compared. Complications arise, however, with compounds formed by a metal with a non-metallic element, having partly covalent bonds. Though the values of covalent radii are available as well,152 153 the precise nature of the chemical bond in any particular chemical compound is usually not known. It is yet unclear whether the tabular values can be used to predict the mobility of the components, for example, in the crystal lattices of transition metal carbides, borides or silicides. 

Such a dependence is frequently observed experimentally in the oxida-tion of transition metals forming volatile oxides, and also of their alloys, carbides, nitrides, borides, silicides and other chemical compounds.350 365 An example taken from the work of E.A. Gulbransen and K.F. Andrew389 is presented in Fig. 5.22. 

Chromium nitrides have been prepared by several routes heating of chromium metal in N2, reaction of chromium borides with NH3, and heating of CrCly in gaseous NH3. The two stable nitrides have the composition Cr2N and CrN see Nitrides Transition Metal Solid-state Chemistry). At very high temperatures, both decompose into the constituent elements (CrN, > 1425 °C CryN, >700 °C). CrN is very stable chemically, while CryN dissolves in dilute acid with liberation OfH2. 

Recently, there has also been an increase in the importance of melts in their use as a reaction medium for chemical and electrochemical synthesis of compounds for functional and construction ceramics, e.g. double oxides with spinellitic and perowskite structure and binary compounds with prevailing covalent bond character, mainly borides and carbides of transition metals. 

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