Silicon borides

Quartz batting sandwiched between AFRSI quartz and fibrous glass fabric Nylon felt with a silicone rubber FRSI coating Silica tiles, borosilicate glass coating insulation HRSI with silicon boride added... 

Table 3 summarizes the properties of the so-called nonmetallic hard materials, including diamond and the diamondlike carbides B4C, SiC, and Be2C. Also included in this category are comndum, A1203, cubic boron nitride, BN, aluminum nitride, AIN, silicon nitride, S N and silicon boride, SiB6 (12). 

Borides Carbon boride, CB6. and silicon borides SiB3 and SiB6 are hard, crystalline solids, produced in ihe electric furnace magnesium boride, Mgi B2, brown solid, by reaction of boron oxide and magnesium powder ignited, forms boron hydrides with HC1 calcium boride, Ca3 B2, forms boron hydrides and hydrogen gas with IIC1. 

Although few applications have so far been found for ceramic matrix composites, they have shown considerable promise for certain military applications, especially in the manufacture of armor for personnel protection and military vehicles. Historically, monolithic ("pure") ceramics such as aluminum oxide (Al203), boron carbide (B4C), silicon carbide (SiC), tungsten carbide (WC), and titanium diboride (TiB2) have been used as basic components of armor systems. Research has now shown that embedding some type of reinforcement, such as silicon boride (SiBg) or silicon carbide (SiC), can improve the mechanical properties of any of these ceramics. 

In nonoxide ceramics, nitrogen (N) or carbon (C) takes the place of oxygen in combination with silicon or boron. Specific substances are boron nitride (BN), boron carbide (B4C), the silicon borides (SiB4 and SiBg), silicon nitride (SisN4), and silicon carbide (SiC). All of these compounds possess strong, short covalent bonds. They are hard and strong, but brittle. Table 22.5 lists the enthalpies of the chemical bonds in these compounds. 

The authors have reported the synthesis and thermoelectric properties of arc-melted silicon borides [14]. As-melted silicon borides in the boron content range of 80 to 94mol% consisted... 

Figure 1. X-ray diffraction pattern of an arc-melted silicon boride (B=90mol%).

Figure 2. Relationship between free silicon content in arc-melted silicon borides and boron content in raw materials.

Figure 3. X-ray diffraction patterns of arc-melted silicon boride (B=90mol%) after heat-treatment. The heat-treatment conditions are (a) 1538K, 5hr (b) 1538K, 40hr (c) 1673K, 0.5hr (d) 1673K, 2hr.

Figure 4. Microstractures of arc-melted silicon boride (B=90mol%). (a) as-melteH (b) after heat-treatment at 1673K for 0.5hr. The A, B and C parts are SiB frpp-s - and SiB4, respectively. ...

Figure 5. Temperature dependence of electrical conductivity of arc-melted silicon borides before and after heat-treatment at 1673K for 0.5hr.

The occurrence of the binary borides of the alkaline, alkaline earth, aluminum, and transition elements has been collected in Table 1, together with boron compounds of the right main group elements (carbides, etc.). Only relatively well-established phases have been included. Noncorroborated and/or badly characterized borides lacking precise composition and structure data are not included. The reader is referred to other sources for references. There are no binary borides among the Cu, Zn, Ga, and Ge group elements with the exception of a noncorroborated early report on diborides in the Ag-B and Au-B systems. Two silicon borides have been established, namely, SiB3 4 and SiBe. 

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]. 

Besides the homogeneity ranges of the silicon borides mainly phase diagram data concerning the liquidus and the silicon solvus were published. The eutectic temperature between SiBg and silicon was calculated to 1658 K [36] (1657 K in the Scheil schemes) in good agreement with measurements with experimental data [52, 56, 61]. 

Bolmgren H (1994) On the crystal structure and homogeneity range of rhombohedral silicon boride. Report UUIC-B18-266. Institute of Chemistry, Uppsala University, Sweden... 

Figure 9. (a) The binary B-Si system according to experimental results with powders of technical purity. Homogeneity fields of the silicon borides according to Ettmayer et al. [76], Lugscheider et al. [77], Armas et al. 1981 [82], (b) The binary B-Si system according to recent calculations by Lim and Lukas [52],... 

Different processing routes have been applied to synthesize this class of composite material. Besides the classical method of powder mixing using submicron SiC, B4C and carbon or carbon-containing precursors, followed by forming and subsequent heat treatment ]313], the infiltration of boron carbide preforms with organic precursors such as PCS, followed by heat treatment [400, 401] or the pressure-assisted reaction sintering of silicon borides with elemental carbon [423], have been used. 

The microstructure of the composites fabricated by powder mixing consists of a fine-grained boron carbide matrix with isolated SiC grains, whereas the materials starting with organic precursors [425] or with silicon borides [423] exhibit a continuous silicon carbide phase located in the grain boundaries of the matrix phase. 

Boron Silicides. See silicon borides Borosilicate Glass. A silicate glass containing at least 5% B2O3 (ASTM definition) a characteristic property of borosilicate glasses is heat resistance. 

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