The present invention relates to a high performing aluminium component such as busbars and other electrical conductors and heat sinks with a surface coating suitable for thermal radiation, a method for preparation and application of the coating on the components.

The current rating of a busbar is limited by the maximum ambient temperature and the maximum permitted working temperature. In use, heat is generated in the bar due to the electrical resistance, and the heat is evacuated or tapped from the bar by convection and radiation. Hence, an alloy with reduced electrical resistivity in combination with a high emissivity coating will improve the performance of electrical conductors. In addition performance may be increased by providing ribs, corrugations or other surface modifications which enhances convection and thereby increase heat transfer. The result should be the smallest possible bar size. As energy costs rise, it is worth considering the lifetime cost of a busbar system, including capital cost and the cost of waste energy.

Aluminium can substitute copper in many new application for electrical conductors. The increased copper prize increase the market pull for aluminium conductors and busbars.

With high thermal radiation from a conductor the temperature of the conductor is reduced and the conductivity is improved at the lower temperature. It is commonly known that black matt surfaces are better at radiating heat than bright and shiny metal surfaces, and that is why coating is sometimes suggested. The efficiency of radiation is the emissivity of the surface and the total surface area per volume. Theoretically, for a perfect radiator (matt black), the emissivity is 1 and for a perfect reflector, emissivity is zero. Thus, the thermal radiation is governed by the emissivity of the surface of the conductor.

Aluminium metal has a very low emissivity, typical ε = 0.06 for untreated aluminium. But the emissivity changes very rapidly by surface treatment of aluminium (see table below). High emissivity will give high radiation heat loss.

As to busbars, thermal conductivity is very important due to the maximum limit on operational temperature. General known requirements as regards he maximum temperature limits for bus bars are in the range of 140 and 105°C.

With the present invention is provided a high performing aluminium component suitable for thermal radiation applications such as aluminium busbars or heat sinks with high emissivity. Further is provided an aluminium component with improved electrical and thermal conductivity based on the combination of selected alloy quality and improved thermal radiation by surface treatment including the inventive coating and serrated profile design.

The invention will be further described in the following by way of example and with reference to the attached figures where: Fig. 1 shows a longitudinal view of part of a busbar according to the invention provided with a coating according to the invention,

Fig. 2 shows a cross sections a) with a tape and b) with a copper deposition of the same busbar as is shown in Fig. 1 .

Fig. 3 shows an alternative bus bar design with serrated surface in the form of longitudinal ribs.

Standard black powder coating typically has an emissivity from 0.84 to 0.88. The emissivity is influenced by the binder system, pigmentation and the surface roughness. As is indicated above, black matt coatings have higher emissivity then black and glossy surfaces.

The pigmentation of a black powder coating for interior applications can apply all types of pigments and binder systems since it is not affected by solar radiation or corrosive environment.

By selecting and applying carbon pigments with high emissivity it has, according to the invention, proved possible to develop powder coatings with emissivity larger than the current 0.90.

The coating may be a dry powder coating type or wet type coating containing black pigments. In turn if the coating is dry, it may be a thermosetting or thermoplastic polymer such as polyester or polyurethane, polyester epoxy, straight epoxy or acrylics. On the other hand, if wet, the, coating may be any polymer based type such as an acrylic polymer coating, polyester resin type or polyurethane type.

Figs. 1 and 2 shows respectively in perspective view and cross section, a busbar 1 of aluminium with a coating 2 according to the invention. The aluminium busbar is preferably made by extrusion of an age hardening 6000 aluminium alloy with high electrical conductivity and yield strength in the range from 100 to 180MPa. For electrical conductors with lower requirements on mechanical strength it is an alternative to use a high purity 1000 aluminium alloy. (EC 6000 and EC 1000 each containing by weight %:

Type: EC 6000 EC 1000

Si 0,3 - 0,6 0,15 max

Fe 0,1 - 0,3 0,30 max

Cu 0,01 max 0,01 max

Mn 0,005 max 0,010 max

Mg 0,3 - 0,6 0,02 max

Cr 0,005 max 0,005 max

Zn 0,02 max 0,02 max

Ti 0,01 max 0003 max

V 0,005 max 0,010max

B 0,010 max

Others each 0,02 max 0,02 max

Others total 0,10 max 0,010max

It has been documented that the electrical conductivity can be increased by reducing the content of elements like Fe, Cu, Mn, Cr, Zn, Ti and V in a standard 6101 aluminium alloy and the above selected alloy lies within the desired electrical conductivity level.

As can be seen in the example shown in Figs. 1 and 2, part of the bus bar is un-coated in the longitudinal direction to obtain good connectivity with electrical components and connections.

This is done as follows:

After extrusion, the extruded profile 1 is preferably subjected to mild degreasing before being provided with a masking tape 3 (see Fig 2 a) in its longitudinal direction. The tape may be applied without degreasing, but mild degreasing is preferred to obtain improved adhesion. The purpose of the tape, which should be chemical and temperature resistant, is to keep the masked part of the busbar surface free from further treatment and coating, thereby obtaining good connectivity with electrical components and connections.

After masking the un-masked part of the busbar is subjected to further degreasing and surface treatment. Such process may include the following process steps:

1. Degreasing , preferred alkaline degreasing

2. Rinsing.

3. Etching / neutralisation / desmutting, an acid process step if the degreasing is alkaline

4. Rinsing.

5. Passivation preferred in no-chromate processes for improved adhesion and corrosion protection.

6. Rinsing.

7. Drying with hot air typical in a small furnace.

After surface treatment, the un-masked surface of the busbar is provided with the high emissivity coating according to the invention. The coating may be applied to the busbar surface by spray, dip or rolling with a wet lacquer or by electrostatic or tribiostatic application using powder coating, as stated above.

Depending on type of coating, the busbar will be held or stored at required temperature for some time to cure (dry or harden etc.) the coating. The tape will then finally removed after the coating has been cured / dried. .

Masking of the busbar with tape before coating represents a cheap alternative to the costly machining process used today.

A temperature rise test has documented that the total effect of the selected alloy and use of the high emissivity black powder according to the invention was -10°K, or 7% performance improvement.

Fig. 3 shows an alternative bus bar 5 where performance is increased by providing ribs 6, which enhances convection and thereby increase heat transfer. Other designs may also be provided such as corrugations, dents or other surface modifications. The bus bar may as well, as in the former example shown in Figs. 1 and 2 be provided with an uncoated part 7 in the longitudinal or transversal direction.