Molding boron carbides

In general, the purified boron carbide is ultimately obtained as a granular soHd that subsequendy may be molded or bonded into usehil shapes. To achieve high density and strength, it is hot pressed at 1800—2400°C in graphite molds. 

Boron carbide is used in the shielding and control of nuclear reactors (qv) because of its neutron absorptivity, chemical inertness, and radiation stabihty. For this appHcation it may be molded, bonded, or the granular material may be packed by vibration. 

It is technically easier to manufacture a molded component from SiC-powder by slip casting or dry pressing, working it mechanically and then sintering pressureless at 1950 to 2000°C. Due to the low sintering activity of silicon carbides, such processes have only been recently successfully carried out with the advent of fine particulate SiC-powders (specific surface area > 5 m /g) with low oxygen-contents (< 0.2%). Boron or aluminum and free carbon or boron carbide are added as sintering aids. 

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 %) . 

Injection molding of boron carbide with 2-5 mass.-% carbon black was developed by Schwetz et al. [210]. Like in conventional processes known for oxide and nitride ceramics, the spray-dried powder blend was mixed with 18 mass-% organic binder and molded at 120°C and 45 MPa. Dewaxing was accomplished by heating in an atmosphere-controlled furnace at 100 mbar. The binder components decomposed thermally by cracking and evaporated within four days and temperatures up to... 

Silicon carbide and boron carbide to a lesser degree are important industrial materials which are produced on a large scale in the form of powders, molded shapes, and thin films. 

Boron is an important material for nuclear applications due to its high neutron absorption cross section (760 bam at neutron velocity of 2200 m/ sec). The cross section of the isotope is considerably higher (3840 bam).l l In addition, boron does not have decay products with long half-life and high-energy secondary radioactive materials. However, pure boron is extremely brittle and difficult to produce in shapes such as control rods. Boron carbide is usually the material of choice since it provides a high concentration of boron atoms in a strong and refractory form and is relatively easy to mold (see Ch. 16). 

By using a boron-nitride powder barrier (hot pressed or deposited onto graphite mold and punches), it is possible to prepare dense and pure boron and boron-rich phases, especially carbides (between B,o.4C and B4C) by hot pressing in graphite dies " . 

The increasing use of plastics with abrasive fillers and reinforcements created a demand for an even more abrasion resistant barrel than the standard iron/boron type. The use of glass fiber reinforced compounds for injection molding has been the single most important factor since a fabricator would be lucky if they could reach 6 months of continuous operation. This need has been successfully answered by the development of liner materials containing metallic carbides such as tungsten carbide and titanium carbide extending their life. 

The different types of boron nitride composites cited can be reinforced with fibrous materials such as titanium alloy fibers [287], Si/Zr oxynitride fibers [288], SiOg/TiOg/ZrOg fibers [289], and carbon fibers [290 to 292, 313] (see also Section 4.1.1.10.1, p. 58). BN-containing oxide and carbide ceramics are used to protect graphite from being attacked in metallurgical processes [293 to 295]. Porous ceramics and ceramic foams which can be infiltrated either with metals or lubricants may contain a-BN or are produced in boron nitride ceramic molds [296 to 299]. 

In some of these applications, it is necessary to coat the surface of the molded graphite with a more inert coating such as pyrolytic graphite, boron nitride (BN) or silicon carbide (SiC), to prevent contamination and reaction with the graphite at high temperature. The coating is usually done by CVD as reviewed in Ch. 7. 

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