Graphite clay-bonded

Carbon Electrode carbon, retort graphite, graphite (clay-bonded)... 

Natural graphites are unctuous and soft (hardness 0.5-1.5 Mohs), bum slowly, are chemically inert and have a sublimation temperature >3500°C. Graphite has excellent weathering properties, is hydrophobic, and tends to form water in oil-type emulsions. It is used in clay-bonded refractories and may be glazed to prevent oxidation. 

The shape of molecules and the types of bonding between them have significant effects on properties of materials. Examples include the wide variety of molecular shapes of silicates and the planar structures of talc, mica, clay, molybdenum disulfide, and graphite. 

The chemical composition of the German Klingenberg clay has been given for comparison inasmuch as it represents one of the best known bonding clays known, especially for the manufacture of graphite crucibles. 

The purest as well as the strongest clays are probably the so-called ball clays, the plasticity and bonding power of which is reinforced by the presence of organic matter. This type of clay seems to be preferred in the manufacture of graphite crucibles, and other special refractories. 

Brick made from a mixture of crushed coke and clay or from coke bonded by means of tar have been made many years ago. These materials are usually hand molded, dried and fired in muflBles with as complete exclusion of air as possible. The carbonization of the tar cements together the coke particles to a compact mass. The coke may also be replaced by graphite. Refractories of this type combine good heat-resisting power with excellent thermal conductivity. 

The function of the clay is that of a bonding material, which makes possible the shaping of the crucible and the cementing together of the graphite flakes. At the same time, it covers the particles, and thus protects them from oxidation. In order to be most effective in regard to this point, it is necessary that the clay contract and become dense at as low a temperature as is consistent with the required refractoriness. 

Figure 1.10, Ciriiphite swollen by iniercalatioTi of mbidlum between layers of cathnn (C Rb). Layer compounds like graphite, MoSj. or TaSi, and m.iny clays have relatively weak bonding between the atomic sheets. This is an OKample of bonding at internal surfaces of micro-porous maierials. (Courtesy of P. Guith and M. Guntheirod, Institute of Physics, Univeisity of Basel.)...

Graphite-bonded clay (crucible) ca. 1700 1700- 1800 almost impermeable to gases when glazed excel- lent 1.6 very good... 

Secondary bonding is also important in many ceramics. The most familiar properties of graphite, hexagonal-BN, and clay minerals are determined by the presence of weak secondary bonds. 

Graphite is also found in nature. This natural graphite is found in flake form and, if used in a refractory product, is usually part of a mixture with ceramic material or used for the binder. However, clay or ceramic bonded, natural graphite containing refractory is considered a ceramic product. 

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