Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
2001-10-30
2003-09-09
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C075S010150, C075S010180
Reexamination Certificate
active
06616724
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for evaporating given components from initial multiple-substance mixtures and multiple-substance systems at subatmospheric pressure in a vacuum chamber in which individual portions of the multiple-substance mixture or multiple-substance system are arranged in ring crucibles at a plurality of levels, from which vapors of the lower-boiling component in each case are withdrawn through at least one vapor exhaust duct, the topmost ring crucible being covered except for the vapor exhaust duct.
2. Description of the Related Technology
The purification of pasty, flowing, fluid or liquefied multiple-substance mixtures or systems by heating in a vacuum and evaporating relatively lower-boiling components becomes more difficult when the boiling points are close together at a given pressure (specific vapor pressure curves of the single components). The difficulties increase further when very low residual contents of the components to be evaporated are required, as for example when trace elements and/or trace compounds interfere with the application for which the purified material is intended. These applications especially appear in the fields of electronics (semiconductor technology) and optics (lens materials), the term, “multiple-substance mixtures and systems,” being understood to mean alloys, glasses, sludges, etc., that is to say, multiple-substance mixtures containing solid-and-solid, solid-and-fluid and fluid-and-fluid phases.
Such multiple-substance mixtures and systems may also contain water, hydrocarbons, mercury, sulfur, zinc, cadmium, sodium, lithium, calcium, antimony, lead, manganese, selenium and tellurium.
To obtain short diffusion distances to the phase boundary surfaces (e.g., fluid-to-vapor) in the wholly or partially fluid multiple-substance mixtures and systems, it is desirable to divide the charge into single portions at several levels with a shallow depth of fill and a favorable ratio of surface area to volume, as is described in connection with resistance heating in DE 31 44 284 C2, which deals with a different subject, namely the embrittlement of hard metals in machining technology. In a stacked arrangement, however, temperature equalization is lost due to convection flows and/or inductive stirring between the individual portions. The consequence is considerable axial and radial temperature differences between the single portions or levels in the stack and thus different amounts of residual impurities in the individual portions, even in the radial direction.
When the entire charge is held in a single crucible, convection flows and/or inductive stirring can produce uniformity, to a certain extent, of temperature and composition of the charge axially and radially. However, convection flows presume temperature differences, and in both cases certain torus-like flow patterns develop, and individual elements of the volume of the liquid are momentarily in the vicinity of the phase boundary (evaporative surface). On average, with respect to the amount of the charge, considerably long diffusion paths are formed, and production suffers. Upon cooling, the melt gradually “freezes” and the solid-to-liquid phase boundary pushes particular impurities ahead of it into the liquid part—the so-called residual melt—and thereby causes gradients in the impurities in the block that finally solidifies. That is a consequence of the action of the effective coefficient of distribution.
keff=C
solid/
C
liquid
In this case the removal of the heat of solidification over great distances is troublesome. Moreover, the migration of the phase boundary presumes temperature differences.
Now, it is common practice to perform such processes in vacuum furnaces which have a cylindrical wall made of a material permeable to magnetic fields, such as quartz or plastic (glass fiber-reinforced) and an induction coil arranged outside of the furnace wall.
Now, it is not possible in all cases to hit upon the temperature selection for fractional distillation on the basis of isolated considerations of the specific equilibrium vapor pressure curves of the substances, since such evaporations take place under “non-equilibrium conditions.” Evaporation under such conditions represents the typical, real case with which one has to deal in practice. The evaporation process as a whole is considered to be a transporting of components out of the bulk into the vapor phase, or into the vacuum in some cases, in specific steps, and both the whole process and each individual step can be characterized by specific kinetic parameters. The first step is the transport of the particular component through the diffusion boundary layer, and this transport is proportional to the concentration gradient in this boundary layer. The second step is the free evaporation of the particular component at the surface, and this evaporation is proportional to the concentration of the component at the surface.
At the same time, however, the individual components exercise different interactions with one another, which shift the values from the equilibrium vapor pressure curves against one another. In the case of alloys, this takes place, for example, by the formation of intermetallic phases. These interactions must and can be determined experimentally.
See in this connection the book, “Metallurgische Thermochemie,” by Kubaschewski and Evans, 1959, VEB Verlag Technik, Berlin, pages 53 to 55, in which the authors deal with the concept of “activity” of the substances and components in a solution and refer to bonding and attraction forces between the molecules which have to be overcome, and to repulsive forces which can be exploited.
The individual problems and their possible solutions and effects are therefore diametrically opposed.
SUMMARY OF THE INVENTION
The invention is therefore addressed to the problem of offering a method and an apparatus by which short diffusion path lengths and great uniformities in the temperature configuration in the radial and axial directions are possible.
The solution of the stated problem is therefore accomplished according to the invention in the method described above by the fact that:
a) ring crucibles of a material which acts as a susceptor towards alternating electromagnetic fields,
b) under the lowest ring crucible and over the topmost ring crucible a heat body is disposed, made of a material which likewise acts as a susceptor towards alternating electromagnetic fields,
c) the ring crucibles and the heat bodies are contained in at least one induction coil and are heated by the inductive coupling such that the lowest ring crucible and the top ring crucible are additionally heated by the heat bodies, and
d) the ring crucibles are heated with timing such that at least one of the given components of the multiple-substance mixture or system is obtained in the greatest possible purity.
Thus the advantages are achieved that short diffusion path lengths and great uniformities in the temperature configuration in the radial and axial direction are possible, and especially that the axial and radial temperature differences between the individual portions or stack levels are extremely small, so that the residual impurity contents in the individual portions can be minimized, even in the radial direction.
The collection of the high-purity component can be accomplished in two alternative ways, namely,
1. by holding back the given component or components in the ring crucibles, or
2. by capturing the given component or components in a condenser independent of the other components. In this case a condenser is used that is not already “occupied” by other substances.
The invention also relates to an apparatus for evaporating given components from initial multiple-substance mixture and system at subatmospheric pressure with a vacuum chamber and a furnace wall in which ring crucibles are arranged at various levels, each crucible having a vapor exhaust passage, while the topmost ring crucible is closed except for the vapor exhaust passage.
The solutio
Goy Wilfried
Hugo Franz
Melber Albrecht
Raschke Manfred
Wanetzky Erwin
Ald Vacuum Technologies Aktiengesellschaft
Fulbright & Jaworski L.L.P.
Wyszomierski George
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