Liquid boron carbides

When nucleation takes place throughout a reacting solid particle and it is of the same speed as the growth of these grains, a different kinetic mechanism must be used. The mechanism called nucleation kinetics has been developed to combine these two steps into a single step. An example of this mechanism is the carboreduction of boron oxide to boron carbide. This overall reaction is written to imply that it follows a liquid-solid reaction pathway ... 

In this case, a liquid metal, molten zirconium, can be reacted (oxidized) by a bed of solid oxidant (i.e., B4C) to form products that are different than the bed (Fig 4). Just as described earlier, there is a directed oxidation reaction of a liquid metal and the product may contain some residual metal. In this case, the boron carbide bed is consumed according to the reaction ... 

The two-phase boride/carbide layer described in item 1 extends downward by reaction as the boron-rich liquid below it is drawn into the porous boron carbide by capillary action. Boron and carbon rapidly diffuse through this thin B-rich liquid layer, causing the two-phase layer to thicken as the directed reaction proceeds. 

In orientation to the crystallographic results [152, 168], Kasper et al. (1996) [33, 34] and Seifert et al. [169] used for the model description of the homogeneity range of boron carbide the sublattice description (hi2> BiiC)(CBC, CBB, BVaB). The sublattice model (B)93(B,C)i2 was used to describe the carbon solubility in j8-boron. The Redlich-Kister parameters for the liquid phase and graphite (ss) and general formula descriptions for the solution phases were accepted from [36]. The calculated optimized phase... 

The Si-B-C system was mainly investigated with view to the understanding of the sintering mechanisms of SiC with boron in combination with carbon and the sintering of boron carbide with silicon [4]. The silicon solubility of about 2.5 at.% in B4C at 2323 K and the comparatively low temperatures of liquid phase formation in the ternary system enhance the sintering of boron carbide. 

One maximum where boron carbide is formed from gas and liquid phases was found by calculation. Two transition reactions and two degenerated reactions were found. The calculated temperature of the reaction B4C -I- 2 N2 = 4 BN -I- C (U2 in Fig. 23) of 2597 K is experimentally not well established. In situ investigations of the reaction of BN and graphite are not reported and therefore only reaction temperatures indirectly derived from X-ray analysis of quenched materials are documented. It has to be assumed... 

Corrosion by Liquid Media Pure boron carbide is insoluble in HCl, H2SO4, and HNO3, even when boiling. Resistance of hot-pressed boron carbide to sulfuric acid solution has been investigated [97]. The material was produced from a powder containing 77.8-79% boron, 21.6-20.4% carbon, 0.25-0.7%... 

The oxidation of boron nitride as well as boron carbide is distinguished by an oxide layer remaining in a liquid state and vaporizing considerably over the whole temperature range. However, the study of the mechanisms of oxidation of boron nitride materials is complicated by the existence of several BN modifications with considerable differences in their structure and properties. The oxidation of a stable hexagonal a-modification of BN with a graphite lattice has been studied most thoroughly [104]. 

Thermodynamic calculations by Dorner [78], Lukas [93] and Lim and Lukas [52], however, clearly demonstrated the existence of a binary phase equilibrium of boron carbide and a Si- and B-containing melt above 1560°C. The theoretical results were confirmed by hot pressing, liquid phase sintering and infiltration experiments by Lange and Holleck [75], Telle [83], Telle and Petzow [94], and Telle [54], which also yielded more details on the extension of the homogeneity field of boron carbide towards the Si-rich corner of the system B-C-Si. 

Removal of materials using an axially oscillating tool at ultrasonic frequency (about 20,000 Hz) with a fine abrasive of silicon carbide, aluminum oxide, or boron carbide carried by a liquid between tool and workpiece. 

The densification of boron carbide using silicon or aluminum is difficult to obtain due to the high vapor pressures of liquid silicon or aluminum at reasonable sintering temperatures, and which may hinder densification due to the formation of degassing channels and entrapment of residual gas in the closed pores. Telle and Petzow [404, 405] have demonstrated the existence of a binary equilibrium between a Bi2(B,C,Si)3 solid solution and boron-rich liquid silicon above 1560 ° C, which might be beneficial... 

It is necessary to load 31 fuel assemblies with guide tubes for control rods (GT assemblies) to gain a satisfactory shutdown margin under cold zero power shutdown conditions. The Reactor Protection System comprises two independent fast acting shutdown systems. Shutdown System-1 (SDS-1) is based on mechanical shutdown rods with boron carbide based absorbers in 31 fuel assemblies it provides sufficient negative reactivity with all rods inserted, with one maximum worth rod not available, in the cold shut down condition. Shutdown system-2 (SDS-2) is based on liquid poison injection into the moderator. 

The CHTR incorporates a passive power regulation system (PPRS) [XXIX-5]. This system includes a gas header filled with helium gas at moderate pressure. The header is attached to a niobium driver tube, which contains lead-bismuth eutectic alloy as driven liquid. The driver tube is housed within a control tube that contains an annular control rod made of boron carbide with niobium cladding. The annular space between the driver and control tube contains lead-bismuth eutectic, on which the control rod floats. The space above the liquid... 

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