Pyrolytic technology for assembling refractory structures made of carbon-bearing materials

Abstract

The erection of refractory structures made of carbon-bearing materials involves assembling the elements-of the structure so as to ensure their durability under significant mechanical and thermal loads and allow the formation of supports and cbznr~ls for conductors within the structure. The method of assembly must also not unduly complicate the design and maintenance of the structure for service at temperatures above the melting points of most metals and alloys.The main methods that meet the stringent requirements for assembling carbon-bearing structural elements are bonding with high-temperature adhesives and mechanical fastening. In contrast to the adhesives used for metals and plastics, these adhesives --as the carbonized binders of the structural materials themselves --must have a high coke number, adequate viscosity, and wettability sufficient for penetrating the pore space of the material, filling surface cavities, and forming carbonized structures that "bond" with the substrate as a result of adhesion. Thus, a carbonaceous composite is created in the adhesive joint, this composite making it possible to transmit mechanical loads from one part to another at temperatures within the range 1000-1500"C.The ,lamzte tensile strength of adhesively joined carbon surfaces is very low --3-4 MPa. The strength of the joint is increased by reinforcing it with carbon powders, fine carbon fibers, or stronger powders of carbide-forming metals (silicon, ti~, zirconium) or their oxides. Disperse inclusions of carbides of these metals in the adhesive joint (boron carbide also being added in certain cases) increase the adhesive strength of the interface to 7-9 MPa, but their main importance is that they significantly reduce the deformation of the joint. Carbides of the indicated metals have an elastic modulus of 3500-4600 GPa, which is 10-15 times greater than the volumes of carbon-bearing materials joined together in the composite. As a result, on the average the stiffness of the adhesive joint as a whole rams out to be higher than the stiffnesses of the carbon-bearing parts being joined. There is a proportionate reduction in strain in the joint in the transmission of the external load. As regards choosing the optimum composition of composite joint and the optimum size of carbide particles, adequately sealing the cad~on-bearing structural elements within the joint realizes conditions under which the cohesive strength of the joined elements sometimes turns out to be lower than the adhesive strength of the joint itself.Additions of heat-resistant and high-strength carbides not only change the character of deformation of the components within the adhesive joint --reducing shear strain at the interfaces of the carbon surfaces to safe levels --but also improve the durability of the joint when subjected to oxidation at elevated temperatures. The same phenomenon is seen in high-temperature carbon-carbide composites. The actual strength of the joint is determined to a significant extent by the internal stresses in its vicinity, which i...

Refractory structures made of low-weight carbon-bonded carbon-fiber-reinforced composites

“…3 was made from 500´500 mm blocks, which were glued with a heat-resistant glue of the SVK type filled with metallic silicon. Figure 4 shows the temperature dependence of the tensile strength of glued carbon samples [13]. This parameter also characterizes the strength of a surface refractory ceramic coating.…”