Additives boron carbides

Depending on the desired boride (e.g., diborides or hexaborides) free C, metal carbide or free B are included in addition to the boron carbide ... 

Boron carbide (B C) is a hard, black crystal that is used as an abrasive powder and as an additive to strengthen composite parts in aircraft. 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

Sodium borates are the most important industrial boron compounds. They are mainly utilized as such, but are also used as starting materials (in addition to calcium borates) for the manufacture of industrially interesting boron compounds (boric acid, di-boron trioxide, inorganic borates, refractory boron-derivatives, boron carbide, boron... 

The B mas and static NMR spectra of a series of boron carbides show a broad major resonance at about 1.3 to - 4.6 ppm, the peak position varying almost linearly with carbon content (Figure 7.10A). This resonance has been assigned to boron in the B-rich icosahedral units which are bonded together both directly and via three-atom chains (Kirkpatrick etal. 1991). A small additional shoulder on the major resonance of the static B spectra (Figure 7.10B) which increases in intensity with decreasing C content and can be simulated as a second-order quadrupolar lineshape has been assigned to the boron site in the centre of the various possible C-B-C chains (Kirkpatrick et al. 1991). 

Boron carbide (B4C). In the contact zone between the two components, tungsten boride layers can be detected after heat treatment between 1100 and 1600 °C. Depending on the reaction conditions, a-WB or W2B is formed. Additions to NaF as activator. 

Porous boron-carbide pellets useful in control rods for nuclear reactors are obtained by using organic precursors (furfuryl alcohol and maleic anhydride) , or a bimodal particle-size distribution of B C powders , or 40 wt % Al addition ... 

The addition of free graphite yields fine-grained compounds near the theoretical density (94-100 %) . Carbon is better added by the in-situ pyrolysis of a phenolfor-maldehyde resin (i 9 wt Pressure-less sintering of boron-carbide is now... 

Boron suboxides have boron oxygen mole ratios equal to or greater than one. These compounds range from molecular species to refractory solid-state materials. Monomeric vapor-phase BO and B2O2 have been studied by spectroscopic techniques. In addition to these rather unstable high-temperature species, several forms of solid noncrystalline boron suboxides have been reported. A water-soluble low-temperature form is obtained by the vacuum dehydration of tetrahydroxydiborane at 220°C (equation 5). At 500 °C, this form converts to a light brown modification that has also been obtained by reactions of boric oxide with elemental boron, boron carbide, or carbon at high temperatures (>1250 °C). 

S.C. Zhang, GE. Hilmas, et al.. Pressureless Densification of Zirconium Diboride with Boron Carbide Additions, J. zlm. Ceram. Soc., 89 5, 1544-1550(2006). 

A thermodynamic optimization of the system was performed by Domer (1982) [167]. This dataset was later refined by Lim and Lukas (1996) [36]. Due to additional crystallographic information concerning the extended homogeneity range of the boron carbide phase [152, 168] a further assessment was necessary [33, 34,169]. Data for the calculated invariant reactions are given in Table 13. Boron carbide of composition 16.4 at.% C melts congruently at 2731 K. 

Boron has been included as a neutron absorber in various materials in addition to concrete. For example, borated graphite, a mixture of elemental boron and graphite, has been used in fast-reactor shields. Boral, consisting of boron carbide (B4C) and aluminum, and epoxy resins and resin-impregnated wood laminates incorporating boron have been used for local shielding purposes. Boron has also been added to steel for shield structures to reduce secondary gamma-ray production. In special situations, where a shield has consisted of a heavy metal and water, it has been beneficial to add a soluble boron compound to the water. 

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