The B-C System

In the B-C system one binary phase, boron carbide, is established. This phase shows an extensive homogeneity range and to distinguish it from stoichio- 

Type of experiment Type of data Temp, range [K] Cone, range, Xc Literature 

Thermal analysis, metallography, Liquidus, solidus, solvus 1473-2743 0-0.2 Elliot and Van Thyne (1961) [142] 

Different types of thermodynamic data were measured for the boron carbide phase. An overview is given in Table 12 [91, 154-166]. 

---

In group III (B, Al, Ga, In, Tl), only B and A1 form carbides. Because of the ease with which C can substitute for B, the B-C system is complex at least seven carbides (B50C, BijC, BgC, B13C2, BjjCj, B4C, BCj) are proposed. However, only black B4C, with a wide composition range, exists in the pure system at high T. ... 

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. 

Mi) Hot Pressing of Boron-Rich Carbides (2273-2373 K, 20- 0 MPa, vacuum or inert atmosphere, 30-60 min). The hot pressing of these compounds is a delicate operation. Extensive carbon diffusion takes place from the mold toward the sample, and the sintering of pure boron produces only boron carbide . The diffusion diagram in the B-C system is established . Refractory metallic foils (Ta, Mo, W, etc.) can be used for protection " . Diffusion of the Mo barrier is noted (1.3-1.9 wt %) . 

Table 11 shows an overview of the experimental phase diagram data for the B-C system [142-146],... 

Seifert HJ, Kasper B, Lukas HL, Aldinger F (2001) Tliennodynamic assessment of the B-C system. In Marin-Ayral RM, Record MC (eds) Compte-Rendns, Journees d Etude des Equilibres entre Phases (XXVII JEEP). March 22 and 23,2001, Montpellier, France, p 123... 

Seifert, H.J., Kasper, B., Lukas, R.L., and Aldinger, F. (2001) Thermodynamic Assessment of the B-C System, in Joumees d Etude des Equilibres Entire Phases XXVII JEEP, March 22-23, Montpellier (eds R.M. Marin-Ayral and M.C. Record), pp. 123-126. 

Fig. 4.1. Definition of the Jacobi coordinates for A+BC. c.m. denotes the centre of mass of the B-C system.

The method of evaluation of the rate constants for this reaction scheme will depend upon the type of analytical information available. This depends in part upon the nature of the reaction, but it also depends upon the contemporary state of analytical chemistry. Up to the middle of the 20th century, titrimetry was a widely applied means of studying reaction kinetics. Titrimetric analysis is not highly sensitive, nor is it very selective, but it is accurate and has the considerable advantage of providing absolute concentrations. When used to study the A —> B — C system in which the same substance is either produced or consumed in each step (e.g., the hydrolysis of a diamide or a diester), titration results yield a quantity F = Cb + 2cc- Swain devised a technique, called the time-ratio method, to evaluate the rate... 

Several splice variants of MOP (formerly MOR-1) have been cloned (MOP-1A to MOR-1X). The B, C, andD variants differ in their amino acid sequence at the C-terminal end [4]. These receptor valiants differ in their distribution in the central nervous system and in the rate of internalization and desensitization upon... 

Since compounds with six electrons in the outer shell of an atom are usually not stable, the A—B—C system is actually one canonical form of a resonance hybrid, for which at least one other form can be drawn (see Table 15.3). 1,3-Dipolar compound can be divided into two main types ... 

The reaction of Na with B, detectable at 850°C, becomes complete at 900°C after 24 h. At 1000°C, the reaction rate is independent of the variety of boron (a- or -rh), if the particle size is the same. The B-Na system contains two borides with the... 

The B-Na system includes two phases with different thermal stabilities. Either of these two borides can be obtained by direct synthesis on adjusting the alkali-metal pressure in the vapor phase. Thus, the preparation of NaB can be carried out in isothermal reactors at < 1100°C (p a = 45 X 10 N m" ) where the equilibrium... 

The systems reported here are a single turn of B-DNA with G-C, A-T base pair sequence and the left handed Z-DNA with G-C base pair sequence. The B-DNA system is simulated for 4.0 psec and Z-DNA is simulated for 3.5 psec after equilibration. The simulation results are then analyzed for structural and dynamical properties. ... 

In a system, when 0 = 180°, the distance between spins A and C (tac) will be a maximum and the nOe between A and C a mimimum. When 9 decreases from 180°, r c decreases and the direct nOe between A and C increases. As a result of the consequently more effective A-C relaxation, the B-C relaxation process becomes relatively less important, and the indirect negative A-C contribution is correspondingly decreased. When 0 = 78°, r c = 1.26, so there will be no net nOe between A and C, even though the A and C spins are very close to each other. This is because the direct (positive) and indirect (negative) nOe effects are equal and opposite. 

In a B,C spin system, the multiplet of spin B contains the /(B, C) coupling constant among other couplings. This coupling can be determined by comparison of two spectra a coupled spectrum in which the J(B, C) coupling evolves and a decoupled reference experiment in which the J(B,C) is removed. 

Fig. 28. Typical Monte Carlo equilibrium arrangements of atoms adsorb on the center rectangular lattice illustrating the wetting transition A C->A B C. Systems of sizes L X Af = 20 X 60 are used at (a) ktTI J = A, /f/ Jj = 2.8 (b) k,r/ J, = 1.4, fl/l JjI = 2.3 and (c) k,TI J = 2.8. The solid lines show the borders of the A and C domains. Case (c) is rather close to the transition to the disordered phase.

B,c,Oj. Mixed phases of the B-C—O system have been prepared from amorphous B, B2O3, and C in various ratios at 1700°C and up to 7.5 GPa. A wide range of compositions from Be.sC and B6C0.45O0.77 was observed with intermediate stmcmres between B4C and BoO [548]. 

---