Graphite crucible

Fig. 3. Metal freezing-point ceU A, pure graphite crucible B, thermometer weU C, highly pure metal and D, quartz envelope.

Ipser Tiegel, a. kind of graphite crucible, irden, a. earthen. — irdenes Geschirr, earthenware. 

For the preparation of samples for X-ray fluorescence spectroscopy, lithium metaborate is the preferred flux because lithium does not give rise to interfering X-ray emissions. The fusion may be carried out in platinum crucibles or in crucibles made from specially prepared graphite these graphite crucibles can also be used for the vacuum fusion of metal samples for the analysis of occluded gases. 

Rhodium and platinum metals can be more conveniently vaporized with electron-beam vaporization source, in a stainless steel reactor, starting from metal powders deposited into a graphite crucible. 

The industrial practice for the production of tantalum consists of two steps. In the first, the carbide is made by charging a graphite crucible with an intimate, pelletized mixture of lamp black and tantalum pentoxide and heating it in a high-frequency furnace under a dynamic vacuum (10 torr). In the next step, the ground carbide and the requisite amount of tantalum pentoxide are mixed, palletized, and fed to a reduction furnace where the reduction to the metal occurs. The formation of tantalum carbide as well as the reduction to the metal occur at about 2000 °C. The product leaving the reduction furnace is in the form of pellets or roundels (small cylinders) of porous metal, usually sintered together. 

The electrolyte is made by in situ chlorination of vanadium to vanadium dichloride in a molten salt bath. Higher valent chlorides are difficult to retain in the bath and thus are not preferred. The molten bath, which is formed by sodium chloride or an equimolar mixture of potassium chloride-sodium chloride or of potassium chloride-lithium chloride or of sodium chloride-calcium chloride, is contained in a graphite crucible. The crucible also serves as an anode. Electrolysis is conducted at a temperature about 50 °C above the melting point of the salt bath, using an iron or a molybdenum cathode and a cathode current density of 25 to 75 A dnT2. The overall electrochemical deposition reaction involves the formation and the discharge of the divalent ionic species, V2+ ... 

Sulphides. MoS2 was prepared by electrolysis at 1000°C of a melt consisting of sodium tetraborate, sodium fluoride, sodium carbonate in which molybdenum (VI) oxide and sulphur were dissolved. The electrolysis was carried out at 1000°C with the melt contained in a graphite crucible also acting as anode. After electrolysis, the excess electrolyte was dissolved in water to obtain crystalline MoS2, containing however up to 2% carbon. A similar method was used for WS2 carbon was the principal impurity in the sulphides. 

In practice, a mixture of actinide dioxide and graphite powder is first pelletized and then heated to 2275 K in vacuum in a graphite crucible until a drop in the system pressure indicates the end of CO evolution. The resulting actinide carbide is then mixed with tantalum powder, and the mixture is pressed into pellets. The reduction occurs in a tantalum crucible under vacuum. At the reduction temperature, the actinide metal is vaporized and deposited on a tantalum or water-cooled copper condenser. 

MetaUic strontium is produced by electrolysis of a mixed melt of strontium chloride and potassium chloride in a graphite crucible using an iron rod as cathode. The upper cathodic space is cooled and the strontium metal coUects over the cooled cathode and forms a stick. 

Thorium can be obtained from its halides by electrolysis. A fused salt bath of NaCl—KCl—ThCh or NaCl—KCl—KF—ThF4 or similar eutectic mixtures is employed in electrolysis. The electrolysis may be carried out in a graphite crucible, and thorium is deposited as a coarse powder on the electrode, which is made of molybdenum or other suitable material. 

In the electrolytic process, a fused bath of yttrium fluoride and lithium fluoride is heated to nearly 1,700°C and electrolyzed. The electrolysis is done in a graphite crucible using molybdenum cathodes at which yttrium is produced as molten metal. 

The principle is simple. A graphite crucible is partially filled with SiC powder and a seed is attached on the lid of the crucible. The whole system is closed and heated up to temperatures above which SiC starts to sublime appreciably. A thermal gradient is applied such that the seed is slightly colder than the powder source. Material will thus transport from the source to the seed where it will condense. The principal constituents during sublimation are Si, SqC, and SiC and the ratio between them is determined by the temperature. 

Some of HTCVD s initial challenges have been solved. For example, using a coaxial injector solved the blocking of the inlet. Another issue is the purity of the graphite material, which may be overcome using properly coated graphite crucibles. 

Fig. 8. Blackbody and furnace A, furnace shell B, furnace tube C, thermal insulation D, heater winding E, outer wall of graphite crucible F, pure metal...

The anneals were interrupted at 1-, 5-, and 8-hour intervals to transfer the particles to fresh graphite crucibles (and to change cold fingers). 

This procedure made it possible to measure concomitantly the releases of 140Ba and 91Sr from the particles since at the temperatures of the anneals these nuclides are highly retained as sorbed species by the graphite crucibles (13). The release of the metallic nuclides is determined by gamma counting the empty crucibles. 

Copper Metavanadate.—Addition of copper sulphate solution to sodium metavanadate throws down a precipitate which consists mainly of copper metavanadate, which is light yellow. The precipitate may, however, be green or blue, because its composition varies considerably. Copper metavanadate can also be produced eleetrolytically. On being fused at a high temperature in a graphite crucible it forms copper and vanadium carbide.10... 

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