Field of the invention

The invention relates to a heat conduction constructional element, as well briefly referred to as "heat conductor" and a method for manufacturing the heat conductor.

Description of the related art

Thermal management is an essential issue in almost all fields of technology and often requires heat transport or heat transfer from a first point or area to second point or area. Heat transport can be accomplished by thermal radiation, thermal conductivity and convection. Thermal conductivity of solids is due to the thermal conductivity of the solid's conduction electrons and the thermal conductivity of the underlying lattice. The thermal conductivity κ of metals was found empirically to follow K = σ LT (Wiedemann & Franz law), where σ is the electrical conduc- tivity, L a constant for a wide range of metals (Lorentz constant) and T the absolute Temperature. A theoretical understanding of the Wiedemann & Franz law was obtained by Drude's theory assuming that the bulk of the thermal current through a metal is transported by the conduction electrons. Of course the lattice vibrations as well contribute to the thermal conductivity, however their contribu- tion is in most cases poor, which becomes evident considering that the thermal conductivity of metals is far higher than the thermal conductivity of insulators. A theoretical understanding can be obtained by departing from the assumption that a local thermal equilibrium is maintained by an energy exchange due to phonon collisions (Solid State Physics, Ashcroft Mermin, Saunders Coll. Publ., 1976, Chap. 25). Q.U reports in the US-patent application US 6,132,823 a hollow rod like heat conduction constructional element having an extremely high thermal conductivity. The interior of the rod is coated with three basic layers, the first layer being a combination of sodium, beryllium, a metal such as manganese or aluminum, cal- cium, boron and dichromate radical; the second layer formed over the first layer and being a combination of cobalt, manganese, beryllium, strontium, rhodium, copper, 13-titanium, potassium, boron, calcium, a metal such as manganese or aluminum and the dichromate radical; and the third layer formed over the second layer and being a combination of rhodium oxide, potassium dichromate, radium oxide, sodium dichromate, silver dichromate, monocrystalline silicon, beryllium oxide, strontium chromate, boron oxide, 13-titanium and a metal dichromate, such as manganese dichromate or aluminum dichromate. A theoretical understanding for the extremely well heat conductivity of the heat conduction constructional element was not obtained yet. Summary of the invention

The problem to be solved by the invention is to provide heat conduction constructional element, or in other words to provide a thermal conductor having good thermal conductivity and which is simple to manufacture.

Solutions of the problem are described in the independent claims. The depend- ent claims relate to further improvements of the invention.

The invention is based on the observation that the coating according to Qu's method imposes difficulties as there is an incompatibility between the typical heat pipe material which is to be coated and the coating layers. In addition the coating solution appeared to be unstable. Example for preparing an aqueous solution as coating liquid

Provide the following ingredients:

1. 100ml of deionized water

II. further ingredients (List 1):

Dissolve in the given order and one after the other the ingredients of list 1 in the given amount in deionized water. Use for example an Erienmeyer conical flask as container. Ensure that the container is thoroughly cleaned for example using ultra pure water and dryed afterwards. Before adding the next ingredient to the water, stir the water for at least lOmin.

After adding all ingredients of list 1, keep stirring for at least 40min.

Store the obtained solution, subsequently referred to as coating solution, in an airtight sealed container under ambient temperature (temperature 5-30°C). Avoid contact of the liquid to air and in particular to oxygen. During this process one should maintain clean room conditions satisfying ISO 14644- 1/ISO 6 standard. The relative humidity should be below 40% and the temperature be about 20°C (±2°C). The background and acoustic vibrations should be minimized, as well as the presence of magnetic fields. The container should be cleaned using ultra pure water and oven-dried prior to use.

Example for preparing a heat conductor:

1. Prepare an inner surface of a hollow profile which can be made e.g. of a metal like aluminum, stainless steel (e.g. material number 1.4301/ composition: X5CrNil8-10 or material number 1.4404 /composition

X2CrNiMol7-12-2) or carbon steel, such that is cleaned from dust, oxides oil and the like.

2. Close the ends of the profile, e.g., by welding or bonding, to thereby obtain a hollow rod like intermediate product having an inner space.

3. Prepare at least one injection opening having for example a diameter of about 3 to 8mm. The diameter can vary (e.g. 1mm to 5cm) dependent on the volume of the inner space being enclosed by the inner surface and the size of the inner surface. The injection opening is preferably prepared before cleaning the inner surface and before closing the profile, to avoid that drilling chips reside inside the intermediate product.

4. An injection pipe may be connected to the opening by welding or bonding.

5. Evacuate the inner space to a pressure of about lPa even better to about 10 ^_1 Pa or lower. To this end a vacuum pump can be connected to the pipe. Now the pump and the inner space are in fluid communication. After the evacuation of the space, the pump can be disconnected from the pipe. 6. Inject the coating liquid prepared as explained above into the inner space, for example via the pipe. The volume of the injected liquid should be between 1000ml and 500ml per m ² of the surface of the inner space. For example using a tubular profile having a diameter of 44mm and length of 2m, one should add about 150ml of the coating liquid. This is about 5% (3 to 8%) of the volume of the inner space. To this end one may use an injector being connected to the pipe. The pipe may preferably be connected to a valve. The injector may be coupled to a third branch of the valve. Thus using the valve the injector can brought in fluid communication with the inner space and thereby, the coating liquid is drawn into the inner space due to the low pressure inside the inner space.

7. After inserting the coating liquid, one may optionally evacuate the inner space again to at least about lPa even better to about 10 ^_1 Pa.

8. Seal the injection opening preferably permanently, for example by clamp- ing the root of the pipe using mechanical tools which may be actuated hydraulically or pneumatically.

When first heating the such prepared thermal conductor, the liquid inside the inner space coats the surface of the inner space. An example of how to use the heat conductor is explained with respect to the figures. Description of Drawings

In the following the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment with reference to the drawings.

Figure 1 is a flow diagram illustrating the method for manufacturing the heat conductor. Figure 2 is a sketch of a test set up.

Figure 3 is table showing experimental data obtained by the test set up of Fig. 2

The procedure of manufacturing a heat conductor using the coating liquid as prepared according to the above example is explained with respect to Fig. 1.

The procedure starts with cleaning the inner surface of a hollow cylindrical profile 10 made of for example stainless steel, as well referred to as hollow cylindrical rod 10. Subsequently the inner space 15 of the rod is closed by inserting discs 11, 12 of stainless steel into both ends of the inner space of the rod 10. The discs 11, 12 can be connected by welding or bonding, to ensure an airtight connection to the rod 10. The inner space is thus a compartment 15 inside the profile 10. The disk 12 has an opening 14 to which an injection pipel6 is connected by welding or bonding. The other end of the pipe 16 is connected via a valve to vacuum pump via tube 19 and the inner space 15 of the rod 10 is evacuated to at least about lPa even better to about 10 ^_1 Pa. Now, the fluid communication of the tube 19 and the pipe 16 is closed by the valve. Instead the inner space is connected to a container 20 with the coating liquid 30 until a predefined amount of the coating liquid 30 is sucked into the inner space 15. For an inner space having a diameter of 44mm and length of 2m one should add about 150ml (±20%) of the coating liquid. The connection between the container 20 and the pipe 16 is disconnected using the valve and the inner space is again evacuated to at least about lPa even better to about 10 ^_1 Pa by connecting the tube 19 with the pipe 16 using the valve. Subsequently the inner space of the rod 10 is sealed for example by clamping or welding the pipe, if possible close to the disc 12. In Figure 2 a test step up is briefly sketched. A heat conductor was prepared starting from a circular hollow body or profile 10. The profile 10 has a length of 2000mm and an outer diameter of 225mm. The inner space 15 as well referred to compartment 15 has a circular cross section with an inner diameter of 42mm. The heat conductor differs from the heat conductor in Fig. 2 in that end cap 12 supports an electrical heating unit 66 having the form of an electrical heating cartridge being positioned inside the compartment 15 and has an outer diameter of about 32mm. The electrical heating unit 66 was inserted into the compartment together with end cap 12. On the outer surface of the profile 10 are 5 spots Tl to T5 along the length of the profile 10 for measuring the surface temperature using a thermocouple. The profile 10 is oriented horizontally.

The table in Fig. 4 shows that while heating the heat conductor the temperature as measured at the spots Tl to T5 on the outer surface is almost homogenously distributed.

List of reference numerals

10 profile, hollow rod having inner space

11 disc for closing the inner space

12 disc for closing the inner space

14 opening

16 pipe

18 tube to container 30 with coating liquid 20

17 valve

19 tube to vacuum pump

20 coating liquid

66 electrical heater