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Preparation of Laboratory Refractory Crucibles P. S. ROLLER, D. RITTENBERG,
AND
A. GABRIEL, U. S. Bureau of Mines, New Brunswick, N. J .
F
OR studies a t very high temperature special refractory crucibles are required. Above 1500" C. the refractory
materials usually considered are alumina, zirconia, magnesia, beryllia, and thoria and certain silicate compounds of these, such as mullite and zircon. These materials are all nonplastic, so that under ordinary circumstances they are not adapted to the operations of hand molding or casting so successfully employed with plastic clay. For the nonplastic refractories, forming the crucibles under mechanical pressure
horizontal plane, so as to break the bond between it and the walls of the crucible. The core is similarly twisted by means of a bolt that screws into it, and is then lifted out with a rotary motion. If the sleeve does not come with the core it is lifted out separately. By this procedure, even though the crucible wall be tamped in firmly, a gradient exists along the wall from the upper edge to the base. For example, for a zircon crucible, a load of 20 kg. on a 5-mm. wall 50 mm. long showed for the oven-dried product a density of 3.580 at the top third section and 3.319 a t the bottom section, a difference of nearly 10 per cent. Corresponding to this difference in density, the top section of the green crucible was obviously much stronger than the bottom section. Where it is desired to have a uniformly dense wall, the procedure may be altered by applying the final maximum pressure to the wall in three or more sections. To minimize the discontinuity thus incurred, the lower edge of the sleeve is made saw-toothed. Before applying pressure to each section of the wall, it is essential that the powder that has been tamped in be leveled by means of a rotary motion of the sawtoothed sleeve. Although the place where the sections meet is made visible by etching of the steel mold, no trouble during drying or firing has been experienced at that point. The topmost section of the wall is made with a smooth-edged sleeve.
Plug
FIGURE1. ASSEMBLY FOR FORMING CRUCIBLES
seems to be the most generally suitable. However, especially with thinwalled crucibles, considerable attention must be given to technic. A mode of operation has been described ( I ) , the essential feature of which consists in securing an equal density after compression in the base and walls. The pressures required for this purpose are generally different for the walls and base. Equal density of compression results in a unifordy strong crucible, which can be made on a dependable production schedule. The previous procedure consisted in forming the walls, leaving a very small ridge and welding the base to the walls a t the ridge. Subsequent more extended work has shown that this two-stage procedure is not always successful, because of a relatively large discontinuity at the weld. A modified procedure was therefore developed which obtains the same results of equal density of compression without the variable discontinuity that exists in the two-stage method.
2
1
3
4
FIGURE2. CRUCIBLEMOLDDISASSEMBLED 1. 3. 2.
Die Core Base and plug auxiliary core
4. Sleeve
By means of the above procedures satisfactory large crucibles 5.0 cm. in diameter have been produced with a wall as thin as 3 mm. A thinner wall could probably also be had if desired. The moistening solution consists solely of the slightly acid chloride of the refractory metal oxide. Small amounts of phosphate promote stability on firing (1). The green crucible may be placed in the drying oven within an hour after pressing. Firing is almost invariably to be done in an oxidizing atmosphere.
Modified Procedure The assembled mold is shown in Figures 1 and 2. The core is hollowed out to reduce its weight. Operation is as follows: With the steel die and base plug in place, sufficient damp powder is tamped in to provide the base. The core is then centered in osition, and powder tamped in by means of the sleeve. W i d the help of the removable auxiliary core, pressure is successively applied to the walls and base in predetermined small steps until the final maximum pressure has been reached independently for both walls and base-usually different values. The relative ressure depends on the nature of and fineness of the the amount of moistening agent. Its value is material determined by density measurements on an oven-dried crucible. After pressing, the mold is loosened and the sleeve is twisted in a
Literature Cited (1) Roller, P. S., and Rittenberg, D., IND. ENG.CHEM.,24, 436-40 (1932). R ~ C ~ I VAugust E D 6, 1936. Presented before the Division of Industrial and Engineering Chemistry at the 92nd Meeting of the American Chemical Society, Pittsburgh, Pa., September 7 to 11, 1936. Published by permission of the Director, U. S. Bureau of Mines. (Not subject to copyright.) This work W&B carried out at the Nonmetallic Minerals Experiment Station, U. 8. Bureau of Mines, Rutgers University, New Brunswick, N. J.
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