Crucible Choice

The selection of the proper crucible must take into account its opening size and the material is made of. A wide opening is recommended for a rapid unfold of the reactions, as a narrow opening will create a different working atmosphere within the reaction chamber. The initial sealing of crucibles may be necessary to avoid contact with air, but the removal of the seal must be performed in the sample chamber immediately before starting the analysis. 

The crucible material should not induce further reactions or disrupt the reactions that occur during the analysis. The DTA analyses are usually performed on high temperature ranges, gold and platinum being the most commonly used materials for the manufacturing crucibles. When hard tissues are analyzed by DSC at temperatures below 500 °C, the aluminum crucibles are often used [21], The TGA analyses are made in alumina crucibles, which have the advantage that they can be heated up to 1600 °C [23], 

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Elements dissolved in boron influence its crystal structure. Dissolved impurities also influenee the physical and chemical properties of boron, especially the electrical properties, because boron is a semiconductor. Preparation of solid solutions in jS-rh boron requires a careful choice of crucible material. To avoid contamination, boron nitride or a cold, coinage-metal crucible should be used or the levitation or floating-zone melting techniques applied. 

Heat a crucible to full red heat by placing it on a stand and mounting a propane torch underneath. A number of items may be used as crucibles, such as a short length of steel pipe with cap, an old oil filter housing, or a small cast iron skillet. The latter is my personal choice. 

As many physical properties of the actinide metals depend significantly on the sample purity, refining of the metals is mandatory. The choice of the refining methods is determined by the chemical reactivity of the actinide metal in the presence of the constituents of air, by high temperature reactions with crucible materials, by the specific radioactivity and the availability of the actinide elements. 

Dumas remarks that the choice of the crucibles is very important, and that the Hessian ones are better than those of porcelain. The crucibles sometimes color the metal yellow or brown, when particles of iron are detached from the internal surface. TMb inconvenience is not to be apprehended with crucibles of hard porcelain, but these often break or run out, and are too permeable, A pottery or porcelain furnace is used to melt the material, and the crucibles remain in the fire about twenty-four hours. The more tranquil and prolonged the fusion, the greater hardness and beauty does the strass acquire. If the crucibles are of excellent quality, a porcelain furnace maybe used but when the loss occurring is too great, one must be contented with a common potter s furnace. 

In the present study, gold and tin were usually evaporated simultaneously from two separate crucibles, and the atoms cocondensed simultaneously with a high excess of the vapor of the solvent of choice. This resulted in the almost complete matrix isolation of the atoms at -196°C (the ratio of Au Sn could be determined by weighing the used crucibles later). Upon warming the atom clustering took place, mainly in the cold liquid solvent as it melted. 

Figure 3.10 Effect of choice of x-axis temperature. Top x-axis corresponding to reference temperature. Middle idealized case for 2-axis corresponding to sample temperature (immersed thermocouple junction). Bottom x-axis corresponding to sample temperature (thermocouple junction in contact with underside of sample crucible).

Alloys of Pb and alkali metals can be prepared by fusing the metallic components. The high reactivity of the alkali metals requires exclusion of air and moisture moreover, special procedures are necessary to handle the heavier alkali metals. Choice of T is important in producing alloys of defined composition or intermetalllcs, as are crucible material and the technique used to add the components. 

This reaction uses cobalt oxide and alumina as starting materials. Choice of an alumina crucible would therefore by unsuitable. This is because raising the temperature sufficiently to get the alumina starting material to react would also allow the crucible to react. Any other reaction vessel with a melting point higher than 1200 °C would be suitable (Table 4.1). 

In 4.2.6. (see above), we illustrate the t5T)ical setup for the crucible and melt in the Czochralski apparatus. We place the Ir crucible containing the mixture of oxides on a circular Zr02 platform. This acts as a thermal barrier for the bottom of the apparatus. We may then place a larger Zr02 cylinder around the crucible for further insulation (Note that the choice of a thermal insulator is not critical except that it must be able to withstand the anticipated temperatures to be used). An R.F. coil is placed around the outside of the insulation in a position where it can electrically couple with the metal crucible. Finally, an outside wall of insulation is put into place and atop cover plate is put into position. At 2000 °C., the outer wall thickness of Zr02 needs to be at least 2.5-3.0 cm. The whole is then... 

This method is most common. The evaporation material is placed in a container made of Mo, , W or C which can be in the form of a boat, crucible, coil or strip. In some cases ceramic crucibles or inserts are also employed, made of AI2O3 and BeO, BN or BN/TiB2. The container is heated by current flow and the material is evaporated or sublimed from this. Various types of resistance-heated evaporation sources are shown in Fig. 53. Undesired but possible chemical reactions can cause film contamination [274]. The formation and evaporation of vapourizable compounds of the boat material upon contact of the hot boat wall, with reactive gases, as well as with some reactive and/or decomposable film material, is not always low [275], but can, however, often be avoided by appropriate choice of the evaporation source material and by special pre-treatment. A further evaporation method uses radiation heating. The radiationheated vapour source consists generally of a resistance-heated spiral radiator of tungsten wire which is mounted above the evaporant surface in an open crucible. It can be used for the evaporation of easy volatilised materials. 

The rate of dehydration of hydroxides and hydrated oxides may be varied within limits by choice of suitable experimental conditions. Aside from the obvious effect of temperature, the following dehydration procedures are open to the experimenter and give better products than does simple heating in open dishes or crucibles ... 

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