Levitation melting

This article presents a general discussion of actinide metallurgy, including advanced methods such as levitation melting and chemical vapor-phase reactions. A section on purification of actinide metals by a variety of techniques is included. Finally, an element-by-element discussion is given of the most satisfactory metallurgical preparation for each individual element actinium (included for completeness even though not an actinide element), thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, and einsteinium. 

Levitation melting (Section III, A) of Pa metal in high vacuum results in a considerable increase in purity. Metallic Pa resembles Th metal in that it has a very high melting point and a low vapor pressure. 

The method of choice for kilogram-scale preparations of Np metal is direct oxide reduction by Ca metal in a molten CaCl2 solvent system as described above, followed by electrorefining. This metal can be further refined by levitation melting (Section III,A). 

Single crystals of e.g. UPds have been prepared by floating zone technique Large and perfect single crystals of e.g. UC02 or UNi5 with high purity have been drawn from a levitated melt (Fig, 10). 

Fig. 10. Pulling of a single crystal of UNi from the levitated melt by the Czochralski method...

Cervellcro. P. "Levitation-Melting Method Intrigues Investment Casters," Advanced Material Prove.sse.s. 41 (March 1991),... 

YPd2Sb was reported to be isotypic with the crystal structure of MnCu2Al with a lattice parameter a = 0.6691 (Ishikawa et al., 1982 powder diffraction). The sample was prepared by levitation melting followed by annealing at 1173 K for several days. Riani et al. (1995) confirmed the crystallographic characteristics for this compound, MnCu2Al type, a = 0.6691. 

The more frequently used forms of electrical heating, such as resistance and induction, have limited use above 3000°K. (Ml, Dl, C2, FI, F4, B3). Tungsten, carbon, and some of the carbides are the only solid materials capable of use as resistors or susceptors above that temperature. Conducting liquids, although potentially capable of reaching higher temperatures than these solids as electrical resistance elements, suffer from problems of containment and have not been used above 3000°K. except in levitation melting (01, W6). 

Another melting method is the so-called levitation melting in which the sample is freely suspended in vacuum or in an inert atmosphere by a field developed by means of induction coils, which also supply the heat. This promising method is, however, still in the experimental stage, [E. C. Ocress, D. M. Wroughton, G. Com-netz, P. H. Brace and J. C. R. Kelly, J. Appl. Phys. 23, 545 (1952) J. Electrochem. Soc. 99, 205 (1952)]. 

Uda] Uda, M., Wada, T., Solubility ofNitrogen in Arc-Melted and Levitation-Melted Iron and Iron Alloys , Trans. Nat. Res. Inst Met (Jpn.), 10(2), 79-91 (1968) (Experimental, Phase Relations, Thermodyn., 40)... 

Don, 1971Don] Levitation melting technique, chemical analysis 1525-1755°C / Cr-Fe-S up to 40 mass% Cr... 

Most compounds can be prepared by arc-melting and induction melting of the component metals, followed by an appropriate heat treatment. Induction melting should be carried out by levitation melting to avoid contamination from the crucible material. TJie choice of crucible material will vary depending upon the non-rare earth component(s) and the composition desired. Materials that have been used as crucibles include Ta, W, Mo, BN and, occasionally, refractory oxides. 

Frohberg, M.G. (1999) Thirtyyears of levitation melting calorimetry—a balance. Thermochim. Acta, 337, 7-17. 

Skinner L, Barnes AC (2006) An oscillating coil system for contactless electrical conductivity measurements of aerodynamically levitated melts. Rev Sci lustrum 77(12) 123904... 

A recent and promising modification of the transpiration method incorporates levitation heating of the specimen as problems associated with specimen + container reactions are thereby avoided. Thus Svyzahineto/. have combined levitation melting with transpiration measurements, using argon as carrier gas, in determinations of the vapour pressure of iron at temperatures around 2275 K, and Mills and Kinoshita have used... 

Levitation melting in ultra-high vacuum (10 torr) eliminates volatile impurities by evaporation. In this way residual iodides are removed from Van Arkel-produced metals and excess halides are removed from metals produced by halide reduction. 

Possibly, removal of zinc from the sample before analysis, by means of high frequency levitation melting, may provide a more elegant solution in this case, as it has done in the analysis of aluminium alloys. 

One of the most common methods of preparation is melting the parent metals together in an oxide crucible (AI2O3, ThOj or MgO) using either a resistance furnace or an inductance furnace. If a relatively high purity is required levitation melting or arc melting are to be preferred since these methods avoid the possibility of a contamination due to the reaction with the crucible material. 

Nanocomposites Pd C, Ni C, and NiPd C were produced in Institute of Metal Physics, Ural Branch of the RAS. The piece of Pd, Ni or Ni-Pd alloy was heated by induction levitation melting inside of two oppositely directed turns of inductive coil in closed system filled with hydrocarbon containing inert gas (Ar). Evaporation of strongly overheated ( 2000°C) liquid metal drop was performed in the flow of Ar, containing butane or methane. The metal vapors were taken away by the flow of argon into the colder part of reactor where nucleation and condensation of nanoparticles occurred. This... 

Thus, different types of nanocomposites can be produced by the method of levitation melting in Ar-hydrocarbon flow depending on the synthesis conditions. Surface properties of nanocomposites, including specific surface area and pore stmcture, are determined by the thickness and properties of surface caibon shell. 

Sar] investigated the micro structure of Fe-(8 to 10) mass% Al-(1.8 to 2.4) mass% C alloys by melt spinning and levitation melting. They found the melt spun alloys yielded y-i-7 (Z,l2) microstracture while levitation melting always yielded a+y+FesC microstructure. 

Very recently, [2004Euml, 2004Eum2] reported an isothermal section at 1000°C. They prepared 25 ternary alloys by levitation melting and using elements of following purity 99.999% Al, 99.95% Fe, 99.95% Mo. The alloys were annealed at 1000°C for 200 h followed by quenching in iced brine. The alloys were characterized by conventional metallography, electron probe microanalysis and X-ray diffraction. 

---