Solid solution phases boron

A calculated isothermal section for the Si-B-C system at T = 1400 K was presented by Goujard et al. (1994) [75, 76]. The data were derived from JANAF [140] and assessments of the phases SiBi4, SiBg, SiB3 and the solid solution phase boron carbide. The isothermal section calculated using the data of Kasper (1996) [33] (Fig. 10b) deviates significantly. Experimental investigations at these temperatures suffer from slow reaction kinetics and decisions... 

The descriptions of all ternary systems were combined in one dataset to simulate the phase equilibria in the quaternary Si-B-C-N system. The results of the thermodynamic calculations of individual systems are shown in the corresponding sections. Thermodynamic models used are the Redlich-Kister polynomial [39], extrapolations according to Muggianu et al. [40] and the compound energy formalism [41] to describe the solid solution phases )S-boron, SiBn, SiBg, SiBs and B4+ C. 

An early investigation of the La-B-Ge system reported on the existence of solid solution of boron in the binary LasGes compound which was observed by X-ray methods (Mayer and Fefrier 1974). Samples were obtained by heating of mixture of starting components up to 1870K in the He atmosphere in tantalum crucibles. The phase relations in the complete isothermal section at 970 K have been determined by Marko et al. (1978). For sample preparation, see Y-B-Ge. Phase relations are characterized by the absence of ternary compounds (fig. 12). No solubility of boron in the LasGes compound has been observed. 

Table 1 fists many metal borides and their observed melting points. Most metals form mote than one boride phase and borides often form a continuous series of solid solutions with one another at elevated temperatures thus close composition control is necessary to achieve particular properties. The relatively small size of boron atoms facilitates diffusion. 

The structure of CaB contains bonding bands typical of the boron sublattice and capable of accommodating 20 electrons per CaB formula, and separated from antibonding bands by a relatively narrow gap (from 1.5 to 4.4 eV) . The B atoms of the B(, octahedron yield only 18 electrons thus a transfer of two electrons from the metal to the boron sublattice is necessary to stabilize the crystalline framework. The semiconducting properties of M B phases (M = Ca, Sr ", Ba, Eu, Yb ) and the metallic ones of M B or M B5 phases (Y, La, Ce, Pr, Nd ", Gd , Tb , Dy and Th ) are directly explained by this model . The validity of these models may be questionable because of the existence and stability of Na,Ba, Bft solid solutions and of KB, since they prove that the CaB -type structure is still stable when the electron contribution of the inserted atom is less than two . A detailed description of physical properties of hexaborides involves not only the bonding and antibonding B bands, but also bonds originating in the atomic orbitals of the inserted metal . ... 

The solubility of rare-earth metals in /3-rh boron is unknown. Rare-earth-boron systems are cubic borides - with an composition (E = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb). The occurrence of this phase excludes extensive solid solutions in... 

The Suzuki-Miyaura tactic carried out on solid support (Scheme 28) [52] provides routes to small libraries of condensed heterocycles. Thus, Merrifield resin with the LeznofF-linked bromobenzene derivative 78 undergoes cross-coupling under normal solution-phase conditions with boron pinacolate 79 or boronic acid 80, prepared by DoM, to afford phenan-thridines 81 or, via 82 and some manipulation, dibenzopyranones 83 in good yields and with high purities. The Stille solid-support reaction has also been successfully executed [53]. 

Other reports for the boron-Mannich reaction include the synthesis of aminophenol derivatives,561 a-arylglycines,576 a growth hormone secretagogue NN703,577 a polyhydroxyindolizine alkaloid uniflorine A,578 and cyclic ct-amino acids.579 The reaction has been applied to solution-phase reactions580 and the solid-phase reaction581-585 for the synthesis of libraries of peptides, a-amino acids, and bicyclic diketopiperazines. The reactions were accelerated by the irradiation of microwave.586... 

The doped semiconductor materials can often be considered as well-characterized, diluted solid solutions. Here, the solutes are referred to as point defects, for instance, oxygen vacancies in TiC - phase, denoted as Vq, or boron atoms in silicon, substituting Si at Si sites, Bj etc. See also -> defects in solids, -+ Kroger-Vink notation of defects. The atoms present at interstitial positions are also point defects. Under stable (or metastable) thermodynamic equilibrium in a diluted state, - chemical potentials of point defects can be defined as follows ... 

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. 

Boron carbide is rhombohedral (a = 5.163 A a = 65.59°). The stoichiometric composition of boron carbide is Bi3C2(B6.5C), so that B4C is a carbon-saturated solid solution. The phase diagram for the B-C system is shown in Figure 1. The density as a function of stoichiometry is shown in Figure 2. 

Reviews are concerned with structures, properties, preparations ° and applications.The boron carbide phase is a complex solid solution." ... 

Along the EUB6-B4C section again the formation of the solid solution EuBg- Q occurs, but no ternary compound was found. The isothermal section of the tentative phase diagram of the europium-boron-carbon system at 1500°C has been deduced (see fig. 17). To give a better overview, the Eu(B, C)g and i2+xCz-x solid solution has not been included within the diagram. 

As it can be derived from the compositions proposed by Pollmarm et al. [107], there are also the solid solutions of layer aluminates from the AFm series with chromium and boron. These phases are of significant importance as the immobilization of heavy metals from the waste materials is concerned. 

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