Borides thermodynamics

A combination of thermodynamic analysis and experimental data on the deposition rates, efficiencies and deposit morphologies as a function of CVD variables may be used to develop models for the deposition processes. In the case of CVD of borides such a predictive model has been created so far only for a CVD system in which TiBj is obtained by reduction of TiCl4 and BCI3 with... 

Considering other families of similar compounds, the contributions given by Guillermet and Frisk (1992), Guillermet and Grimvall (1991) (cohesive and thermodynamic properties, atomic average volumes, etc. of nitrides, borides, etc. of transition metals) are other examples of systematic descriptions of selected groups of phases and of the use of special interpolation and extrapolation procedures to predict specific properties. 

J The concept of counter-phases. When a stable compound penetrates from a binary into a ternary system, it may extend right across the system or exhibit only limited solubility for the third element. In the latter case, any characterisation also requires thermodynamic parameters to be available for the equivalent metastable compound in one of the other binaries. These are known as counter-phases. Figure 6.16 shows an isothermal section across the Fe-Mo-B system (Pan 1992) which involves such extensions for the binary borides. In the absence of any other guide-... 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

Nadal M, Grenet T, Teyssandier F (1993) Titanium borides deposited by chemical vapor deposition thermodynamic calculation and experiments. J Phys IV 2 C3-511-518... 

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. 

Intermetallics are not treated in this review, therefore, thermodynamic data regarding borides are not included in this chapter. 

The propensity of boron to form polyhedral structures is reflected also in the structures of elemental boron and boron-rich metal borides. In hydrocarbon chemistry, benzene is characterized by its extra stability the thermodynamically most stable allotrope of carbon, namely, graphite is formed by the condensation of benzene units. This beautiful relationship between compounds and allotropes exists in boron chemistry as well, where the stable allotropes of elemental boron and many of the boron-rich metal borides are made up of icosahedral subunits. 

Some of the thermodynamic properties for a few diborides, including HfB2 and ZrB2 are listed in Tables 3 and 4. The data in these Tables are from Pankratz et al, who reviewed available data in the early 80 s and included only the data they deemed to be reliable. Enthalpies of formation are strongly correlated with Gibbs free energies of formation for borides, because the entropy terms are small. This also means that the free energy is relatively insensitive to temperature. ... 

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. 

SHS is particularly suited to the synthesis of refractory ceramic powders and compacts such as carbides of Ti, Si, Cr, Ta, and B, borides of Ti, V and Cr, nitrides of B, Ti, Al, Si, sihcides of Mo, Ti and V, or even more complex compounds such as YBa2Cu307-,. The thermodynamic basis of the process, the individual types of SHS processing techniques, and the equipment and post-synthesis processing to obtain powder compacts have been reviewed by Yukhvid (1992). 

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