Coating processes – Spray coating utilizing flame or plasma heat – Nonuniform or patterned coating
Reexamination Certificate
1998-09-28
2001-10-16
Bareford, Katherine A. (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
Nonuniform or patterned coating
C427S446000, C427S453000, C427S454000, C427S456000, C427S455000, C427S233000, C427S236000, C165S104260, C029S890032, C164S046000, C264S309000
Reexamination Certificate
active
06303191
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a heat pipe for transporting heat from an evaporation area to a condensation area, comprising a housing with housing walls, a capillary structure arranged in the housing and thermally coupled with the respective, corresponding housing wall in the evaporation area as well as in the condensation area, a vapor channel arranged in the housing and leading from the evaporation area to the condensation area as well as a heat transport medium.
Heat pipes of this type are known from the state of the art; with these, a structure produced from metallic netting, felts or woven wire mesh is customarily used as capillary structure, wherein the production is complicated and costly since a secure and close contact between the capillary structure and the walls of the heat pipe must be provided by a plurality of manual spot weldings.
Furthermore, the problem exists with these solutions that during long-term usage internal corrosion can occur as a result of the residual oxygen which is difficult to avoid or as a result of diffusion processes, primarily in the region of the points of contact altered in their structure due to the spot welding.
SUMMARY OF THE INVENTION
The object underlying the invention is therefore to provide a heat pipe with a capillary structure which is as simple to produce as possible and durable in use as well as to make available a process for the production of such a heat pipe.
This object is accomplished in accordance with the invention, in a heat pipe of the type described at the outset, in that the capillary structure is an open-pored capillary layer produced by way of thermal plasma spraying of powder particles.
The advantage of the inventive solution is to be seen in the fact that the thermal plasma spraying represents a simple possibility of producing open-pored capillary layers from powder particles quickly and with high power, wherein the porosity of the capillary layer may be set in a defined manner during the plasma spraying by way of suitable operational parameters.
The capillary layer can thereby be produced from the most varied of materials. One advantageous embodiment provides, for example, for the capillary layer to be produced from powder particles consisting of a metallic starting material, wherein not only pure metals but also any type of alloy may be used in this case. For example, refractory metal or nickel or nickel-based alloys can be used for high-temperature applications, preferably of more than 1000° Celsius, whereas brass, bronze or aluminum can be used, for example, in the room temperature range.
Alternatively thereto, it is preferably provided for the capillary layer to be produced from powder particles consisting of a ceramic starting material, wherein any type of ceramic material can be used.
An important limiting condition for all the materials for the production of the capillary layer is that these are inert in relation to the respective heat carrier medium.
A particularly advantageous structure of the capillary layer is present when this has powder particles bonded to one another as a result of surface melting and the molten layer thereby forming and extending over adjacent powder particles. This means that the powder particles are bonded to one another to form a rigid layer merely in that they are melted on their surface and bear a molten layer which extends at least over part of their surface and, on the other hand, sees to it that a type of partial “coating” results for adjacent powder particles with the molten layer of adjacent powder particles and this “coating” then holds the powder particles together in the capillary layer itself.
A particularly favorable concept provides for the powder particles in the capillary layer to each have beneath the molten layer a crystal structure which is unchanged in relation to the state prior to the plasma spraying. This solution has the great advantage that the crystal structure in the powder particles does not experience any alteration, with the exception of the molten layer, and thus the formation of undesired structures or bondings also does not occur and so capillary layers of this type have a long service life with, at the same time, a high mechanical stability.
Such a bond consisting of powder particles melted on their surface may be realized with powder particles having a homogeneous composition, wherein during the plasma spraying the extent or degree of melting of the particles can be defined by adjustment of the parameters.
It is, however, even more advantageous when the powder particles are built up as particles having a melting point varying over a diameter from the inside towards the outside, wherein the melting point preferably decreases from the inside towards the outside. In the simplest case, the particles are built up of a core and a shell or also designed as particles with several shells, for example, particles with two shells, wherein core and shell or the several shells are built up from materials with different melting points, preferably such that the melting point of an outer shell is lower than that of one of the inner shells or the core, wherein the melting points preferably decrease in steps from the inside towards the outside.
The possibility thus exists during the plasma spraying, for example, of melting only the outermost shell, the material of which is then available to ensure a stable bond between the individual particles whereas the core area remains unmelted and thus ensures the formation of the porous layer with the desired pore size.
Within the scope of the embodiments described thus far, the size of the powder particles has not been defined in greater detail. One particularly advantageous embodiment, for example, provides for powder particles to have a particle size of approximately 30 &mgr;m to approximately 300 &mgr;m. It is even more advantageous when the powder particles have a particle size of approximately 50 &mgr;m to approximately 200 &mgr;m.
Furthermore, the pore size has also not been defined in greater detail in conjunction with the preceding explanations concerning the individual embodiments. One advantageous embodiment, for example, provides for the capillary layer to have pores with an adjusted average size in the range of between approximately 10 &mgr;m and approximately 1000 &mgr;m. Even more advantageous is a formation of a capillary layer which has pores with an average size in the range of between approximately 50 &mgr;m to approximately 300 &mgr;m.
The pore size could, when an average pore size is maintained, be subject to considerable variations only for this average pore size.
It is, however, particularly advantageous, especially in order to obtain a definable effect of the capillary layer, when in a volume range the smallest value and the largest value of the pore size differ at the most by a factor of approximately two, i.e., for example, the smallest value is at the most approximately half the largest value.
In principle, it would be conceivable to apply the capillary layer directly to a substrate provided for this, for example a housing wall. For reasons of mechanical stability and good heat contact, a particularly expedient solution provides for an adhesive layer to be applied between the capillary layer and a substrate for this by way of plasma spraying.
Such an adhesive layer then offers particularly great advantages when this is produced from the same powder material as the capillary layer.
The adhesive layer itself does not need to be of a porous design. The adhesive layer is preferably designed as a continuous layer which has, in particular, a lower porosity than the capillary layer or even no porosity at all.
One advantageous solution provides for the adhesive layer to have a thickness of more than approximately 10 &mgr;m.
Powder particles with an average size in the range of between approximately 5 &mgr;m and approximately 50 &mgr;m are preferably used for the production of the adhesive layer by way of plasma spraying.
In order to improve the desired effect, in particular the tra
Henne Rudolf
Laing Doerte
Thaler Heiko
Bareford Katherine A.
Deutsches Zentrum fuer Luft -und Raumfahrt e.V.
Hoppin Ralph F.
Lipsitz Barry R.
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