The Si-B System

Since the early work of Moissan and Stock (1900) [42] on the synthesis of the silicon borides SiB3 and SiBg, the Si-B system has been the subject of numerous investigations. Experimental phase diagram data available on this system are summarized in Table 3 [27, 43-61]. 

Comprehensive phase diagram investigations were published by Knarr (1959) [43], Samsonov (1963) [46], Arabei (1979) [52], Lugscheider et aL (1979) [53], Viala et al. (1980) [55] and Armas et al. (1981) [56]. Olesinski and Abbaschian (1984) [62] presented a critical assessment and calculation of the Si-B system. 

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

X-ray diffraction, metallography, DTA Solvus, liquidus eutectic equilibrium 1473-1800 0.14-1.0 Lugseheider et al. (1979) [53] 

Thermal analysis. X-ray Liquidus, solvus, SiBn phase 1700-2340 0-0.42 Armas et al. (1981) [56] 

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Reassessment of the Si-B system was based primarily on the model parameters given by Fries and Lukas [25], Modifications have been made on the thermodynamic properties of the liquid and solid diamond phases Experimental liquidus data reported by Brosset and Magunsson [26], Armas et al. [27], and Male and Salanoubat [28], solid solubility data reported by Trum-bore [18], Hesse [29], Samsonov and Sleptsov [30], and Taishi et al. [52], as well as the boron activities in liquid phase measured by Zaitsev et al. [32], Yoshikawa and Morita [33], Inoue et al. [7], and Noguchi et al. [31] were all used to determine the model parameters. Figure 13.4 shows the new assessed phase equilibria in the Si-rich Si-B system. 

Few experimental thermodynamic data are known for the Si-B system. Relative enthalpies ( heat contents ) [65], enthalpies of mixing of the liquid phase [66, 67], enthalpies of formation data [68] and activity measurements [43, 69-72] are reported. For an overview, see Table 4. 

Jansen M, Jaschke B, Jaschke X (2002) Amorphous Multinary Ceramics in the Si-B-N-C System. 101 137-192... 

Schulz S (2002) Synthesis, Structure and Reactivity of Group 13/15 Compounds Containing the Heavier Elements of Group 15, Sb and Bi 103 117-166 Seifert HJ, Aldinger F (2002) Phase Equilibria in the Si-B-C-N System. 101 1-58 Stahlke D, see Mahalakshmi L (2002) 103 85-116... 

Examination of Fig. 9.4 for the B-N2 system reveals that BN decomposes into gaseous nitrogen and liquid boron. Since these elements are in their standard states at 1 atm and the decomposition temperature, the A/fy must equal the enthalpy of formation All" of the BN at the decomposition temperature. Indeed, the A/fy (300kJ/mol) calculated by the means described agrees with the value of AH°T (300kJ/mol) given in the JANAF tables, as it should. The same condition holds for the Si-N2 system. 

Addition of Nb into the Fe-Co-Si-B system from the previous case leads to transformation of 40 vol.% into grains of bcc-Fe(Co) with dimensions 30nm (Fig. 2). Even smaller nanograins of bcc-Fe(Mo), not exceeding 8nm are obtained by crystallization of Fe-Mo-Cu-B (Fig. 3), where the stability of the clustered amorphous remains keeps the content of nanocrystalline phase lower than 25 vol.% till almost lOOOK. The reasons for this behavior can be traced to drastically enhanced nucleation rate via heterogeneous or instantaneous nucleation, which can decrease the amount of nanocrystallized volume in the first transformation stage even below 20 vol.% [5]. 

All the as-synthesized samples ofTi-MWW-PI and Ti-MWW-HM showed the XRD patterns totally consistent with those of the lamellar precursor of MWW topology, generally designated as MCM-22(P) [64, 65], Upon calcination at 803 K, all the samples were converted into the porous three-dimensional (3D) MWW structure with good quality. The amount of B incorporated into the produds was in the Si/B range 11-13, which is far lower than that in the gel with a Si/B ratio of 0.75. In contrast, there was little difference in the Si/Ti ratios between the gel and the solid product, except for the gel of Si/Ti = 100, indicating that the synthesis system is very effedive for Ti incorporation. 

Silicon has an affinity both for B (at 1500°C Si dissolves up to 17 at.% B (Massalski 1990)) and N (although Si3N4 is less stable than BN). Naidich (1981) achieved stationary contact angles of 95° and 110° for Si on cubic and hexagonal BN at 1500°C in a high vacuum in a few minutes. These values are much higher than those observed for Al, but significantly lower than the 140° or so observed for non-reactive metals. A detailed interpretation of these values is not possible in the absence of information on interfacial reactions which could occur in the Si/BN system. 

One of the most effective and commonly used procedures to modify the synthesis conditions is to dilute the initial mixture with a chemically inert compound, often the final product. The typical dependencies of combustion temperature and velocity on the amount of dilution, b, are shown in Fig. 3e. In general, an increase in dilution decreases the overall heat evolution of the system, and therefore decreases the combustion temperature and velocity. Some examples of this type are presented in the review by Rice (1991), where experimental data for group IV metals with boron and carbon, as well as Mo-Si systems are summarized. A similar result was obtained for the Hf-B system shown in Fig. 35 (Andreev et al, 1980). 

The dependence of combustion temperature and velocity for the Si-N2 system as a function of dilution with /3-Si3N4 powder is shown in Fig. 38. In this case, at constant gas pressure, remains constant and is equal to the dissociation temperature of silicon nitride, for dilutions up to 60 wt %. However, with increasing dilution, the combustion front velocity increases. Also, increasing the overall heat evolution, smaller dilution by nonmelted nitride powder promotes coalescence of liquid Si particles, leading to an increase in the average reactant particle size, as well as to formation of thin liquid Si films, which blocks nitrogen infiltration to the reaction zone (Mukasyan et al, 1986). The same effects were also observed for the AI-N2 and B-N2 systems (Mukasyan, 1994), which are characterized by dissociation of the final product in the combustion wave. 

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