Crucibles, refractory materials

Metals which have been used (generally in inert or reducing atmosphere) as container materials are W (melting point 3422°C), Mo (2623°C), Pt (1769°C), Fe (1538°C), Ni (1455°C), Cu (1085°C), Au (1064°C), Ag (962°C). W and Mo do not react with many elements they must be protected however from air oxidation. Pt and Au cannot be used, owing to their reactivity, for melting metallic materials they are useful for other types of synthesis. Fe, of very high purity and with very low carbon content, could possibly be used for melting alkaline and alkaline earth metals and a number of their alloys. 

Materials based on amorphous silica fibres are of special interest these are manufactured in a variety of textile forms (cloth, tape, rope, etc.) which can be used for several applications (thermal, electric insulation) for service temperatures generally up to about 1000°C. Typically they contain 93-96% Si02 about 4% Al203 and small quantities of Ca or alkali oxides. 

Thorium oxide, has the highest melting point of the usual ceramic materials (3390°C). It is used to form ceramics, Th02, as the so-called meta-Th02, freshly prepared by low temperature decomposition of thorium oxalate it is fairly soluble in acids and tends (especially in the presence of nitrate ions) to form colloidal solutions which can be dried to form stable gels that can be sintered to give high-density ceramic bodies. 

On this subject notice that, possibly combined with various heating methods, several physical effects may be considered which allow free flotation of solid and even liquid matter. Materials may be levitated for instance by a jet of gas, by intense sound waves or by beams of laser light. Conductors levitate in strong radiofrequency fields, charged particles in alternating electric fields, magnets above superconductors or vice versa. A review on levitation in physics with the description of several techniques and their principles and applications was made by Brandt (1989). 

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Melting Crucible (Tundish) Material Ceramics/Refractory... 

Alternatively, refractory wools can be wrapped about the crucibles, however, in some cases pyrolysis on the large surface area can prevent reaction or cause free radical polymerizations of alkenes.(15) In larger than milligram quantities, Ti, V and the more refractory materials are best evaporated by electron beam. (13,14) In any case, for metals it is usual to degas the... 

Vitreous silica optical fibers for modern telecommunication systems must be made from ultrapure materials. Glasses produced from melts are incapable of reaching the quality levels required by these fibers. The glasses used in current fibers are produced in situ as the preform is formed by vapor deposition processes. Since the raw materials are liquids, purification by distillation can radically reduce impurity contents. Furthermore, since the glass never contacts crucible or refractory materials, the purity of the glass is maintained throughout the process. 

Table 9.6 lists some of the important crucible materials. Crucibles and other furnace equipment such as boats and setter plates must meet the same requirements as refractory materials used for furnace insulation, i.e., they must be able to withstand high temperatures and also contact with any corrosive liquids or gases used. Items such as crucibles and boats should also possess good thermal... 

CeS and MgS are very stable. Why can CeS be used as a refractory material for crucibles while MgS is unsuitable ... 

Plugging Compound. See filler. Plumbago. A refractory material composed of a mixture of fireclay and graphite some silicon carbide may also be included. It finds considerable use as a crucible material for foundries. In USA refractories of this type are termed... 

Several methods are employed, some involving the use of crucibles, some not. One of the situations where use of a crucible is unavoidable is in the crystal growth from the melt of highly volatile compounds involving one or more high vapour pressure constituents. In these circumstances a sealed refractory material crucible is necessary unlike the instance of volatile semi-conductor melts, encapsulated single crystal growth is not feasible due to the absence of a suitable encapsulant. 

With all these refractory material crucibles, considerable problems with wetting are encountered, leading to difficulty in extracting the boule subsequent to solidification. An additional problem resulting from wetting is the strain induced... 

Being a hot crucible technique, a suitable choice of refractory material has to be made. Mo, Ta and W are frequently used and with induction heating, direct... 

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