FIELDOFTHEINVENTION The purpose of the method developed is to form a porous surface layer on top of a heat transfer surface, and to make it attach itself firmly to the surface below it at a temperature and time suitable for industrial production. The heat transfer surface is copper or a copper alloy, preferably oxygen-free copper or deoxidised high phosphorous copper. The powder forming the porous surface is fine-grained copper powder or copper alloy powder. In the method according to the invention, brazing solder is brought to the heat transfer surface to bind the copper powder to the substrate. The brazing solder preferably contains nickel, tin and phosphorus alloyed with copper. The invention also relates to the heat transfer surface onto which the porous layer is formed by means of the copper or copper alloy powder and said brazing solder.

BACKGROUND OF THE INVENTION The idea in the development of heat exchangers has always been to get the largest possible heat transfer capacity for the heat transfer surface. A smooth surface can be considered the first stage of development when thinking of a tube. The second generation of development is surfaces that are grooved and ridged in different ways, where the pattern may be both on the inner and outer surface. In recent years a third generation of heat transfer surfaces has been developed, namely porous surfaces. A porous surface is formed by attaining a fine-grained powder on the heat transfer surface, fixed to the heat exchange surface in various ways. The powder forms a porous layer on the surface of the tube or other heat exchanger, which allows an increase in heat transfer capacity.

The increase in heat transfer capacity is based on the fact that with a porous surface, boiling begins at a lower temperature than normal. When nuclear boiling starts at temperatures lower than normal, the temperature difference between the heat transfer surface and the liquid remains smaller. For example, when using water as the liquid the temperature must not reach a hundred degrees, because in that case it is no longer a question of the intended nuclear boiling in the porous surface, but the whole liquid boils instead.

Heat transfer surfaces that may use a porous surface are for instance heat exchanger tubes, of which a porous layer may be formed on both the inner and outer surface. In addition, other devices used for heat transfer include heat sink, heat spreader, heat pipe and vapour chamber devices, boiling surfaces for cooling electronic components as well as solar panels, cooling elements, car radiators and other coolers such as casting moulds and casting coolers.

US patent publications 3,821 ,018 and 4,064,914 describe the formation of a porous metallic layer on a copper surface. A metallic layer is formed from copper powder, steel powder or copper alloy powder, bonding metal alloy powder and an inert liquid binder. The bonding metal alloy powder comprises either a powder with 90.5-93 wt % copper and 7-9.5 wt % phosphorous, a powder with 25-95 wt % antimony and the rest copper, or a powder with 56% silver, 22% copper, 17% zinc and 5% tin. The grain size of both the powder forming the porous layer and the bonding metal alloy powder is between 32- 500 μm and the amount of bonding metal alloy powder is 10-30% of the total amount of powder. The surface onto which the porous layer is formed is coated first with a binder. After that a combined layer of copper powder and bonding metal alloy powder is spread on top of the binder. The piece is heated in non-oxidising conditions first at a temperature below 5380C to vaporise the binder. The temperature is raised at a rate of approximately 20O0CVh. In the second heating stage, the temperature is increased quickly to a range between 732-8430C. At the temperature in question the bonding metal alloy powder melts and brazes the entire powder mass to its base material.

JP patent application 61228294 presents a method for the formation of a porous layer on the inner surface of a heating pipe. First the binder is spread onto the pipe. After this, the porous layer is formed of metal particles with a grain size of the magnitude of 100 - 300 μm. As fluxing agent tin chloride may be used for example, which is sprayed on top of the powder layer and dried, so that the binder is removed. If several layers are desired, the procedures are repeated several times. Finally the powder is fixed tightly to the surface of the pipe by means of a braze. The braze is tin or a tin-lead alloy. The soldering temperature is 300-3500C.

JP patent application 2175881 describes the formation of a layer of powder- like substance on the inner surface of a heat transfer tube. The tube is copper or aluminium. By means of a suitable binder or fluxing agent an integral layer of a mixture of two powders is formed on the inner surface of the tube. One of the powders is a metal with a lower melting point such as tin, and the other has a higher melting point such as copper. The particle size of the powders is 0.01 - 3 mm. In addition, a spiral groove is formed on the inner surface of the tube. The tube is heated to the melting point of the powder with the lower melting point, whereby the powder with the higher melting point is also fixed to the surface of the tube. Simultaneously, a stable porous layer is formed on the surface of the tube.

CN patent application 1449880 presents a low-temperature sintering process for forming a porous layer on the surface of a pipe. According to this patent, glue is brushed onto the surface of the pipe, which is then sprayed with a copper-tin powder alloy and the component is then transferred to a furnace, where it is treated in a shielding gas. In the first stage the pipe is kept at a temperature of 400-500 0C for 5-30 minutes, after which the temperature is raised quickly to 670-700 0C, at which temperature the pipe is kept for 60-90 min. The tin content of the powder alloy is 9 -13 wt%.

In the above-mentioned US patent publications 3,821 ,018 and 4,064,914, a method is presented, in which fine-grained powder is fixed to a heat transfer surface using a binder and bonding metal alloy powder. The binder is removed slowly by heating, after which the temperature is raised to a minimum of 7320C, so that the bonding metal alloy powder melts and brazes the powder to the heat transfer surface. This refers to brazing, where the heating temperature required is high and the heating time is long for implementation on industrial scale.

The use of silver-containing braze filler is restricted by its higher melting and brazing temperature as well as the price. In other methods of the prior art, tin or a tin alloy is used, which help fix the powder to the heat transfer surface by soft soldering. It is well-known that the strength of a soft-soldered join is considerably lower than that of a brazed join. The liquid flowing on the heat transfer surface may loosen powder particles from the heat transfer surface with prolonged use if the bond between the powder particles is weak. As a result the heat transfer capacity of the component is weakened. Indeed in some of the methods described above, separate fluxing agents are used, which is why a separate stage is required to remove the fluxing waste in order to prevent corrosion.

PURPOSE OF THE INVENTION The purpose of the method now developed is to form on top of a heat transfer surface a porous layer, which can be fixed strongly to the surface below it at a temperature and in a time applicable for industrial production.

SUMMARY OF THE INVENTION The invention relates to a method for forming a strongly adhesive porous surface layer on a heat transfer surface. The heat transfer surface is copper or copper alloy, preferably oxygen-free or deoxidised high phosphorous copper. The powder forming the porous surface is fine-grained copper powder or copper alloy powder. In the method according to the invention, brazing solder is brought to the heat transfer surface, and preferably contains nickel, tin and phosphorous alloyed with copper. In order to perform the brazing, the heat transfer surface is taken for heat treatment, where the temperature is a maximum of 7250C.

The brazing layer may be brought to the heat transfer surface in many different ways, and afterwards or simultaneously the powder forming the porous layer itself is brought. The powder particles forming the porous surface are brazed to each other and to the heat transfer surface acting as base material by annealing.

The method also relates a heat transfer surface of copper or copper alloy, onto which a porous surface has been formed of copper powder or copper alloy powder using Ni-Sn-P-Cu-containing brazing solder.

The essential features of the invention will be made apparent in the appended claims.

The heat transfer surface onto which the porous layer is fixed is preferably of oxygen-free copper or deoxidised high phosphorous copper, with a phosphorous content of the order of 150-400 ppm, i.e. the heat transfer capacity of the material is already naturally very high. It is described in the prior art how heat exchanger pipes and many other devices are considered to be heat transfer surfaces. The method according to our invention for manufacturing a permanent porous surface as well as the heat transfer surface according to the invention may be used in the manufacture of these devices. The brazing solder to be used in the method and product according to the invention is a metal alloy, which in addition to copper, contains nickel, tin and phosphorous. The metal content of the solder alloy is preferably in the following range: 0.8-5.2 weight % Ni, 0-27.4 weight % Sn, 2.2-10.9 weight % P with the remainder copper. One solder composition that has proved advantageous is as follows: 3.9-4.5 weight % Ni, 14.6-16.6 weight % Sn, 5.0-5.5 weight % P with the remainder copper. The amount of solder to be used is 1-50 weight % of the total amount of powder fed to the heat transfer surface. The melting point of the brazing solder is preferably in the range of 590 - 6050C.

In order to obtain a porous surface fine-grained copper powder or copper alloy powder with a fairly narrow particle size distribution and preferably with a round or rounded particle shape, is brought to the heat transfer surface. Due to the narrow particle size distribution, plenty of pores remain on the coating, in which the heat transfer fluid starts to boil at low temperatures.

The particle size distribution may be for instance a narrow range of between 35 - 500 μm, preferably between 35-300 μm. One preferred particle size range is 37-90 μm. If the particle size distribution is large, the coating may form too densely and the benefits of a porous surface are lost.

The heat transfer surface may be treated with a binder or a binder may be mixed into the metal powder to be used in preparing a coating, as described in the prior art, but this is not necessary. If a binder is used, its removal takes place by annealing according to known techniques.

The brazing powder may be brought to the heat transfer surface in many different ways. According to one method of the invention, the brazing powder is mixed into copper powder. This method is possible particularly if it is desired to use a separate binder. For the sake of simplicity we use the term copper powder in this text for the powder used to form a porous surface, although it may also be a copper alloy powder. In another method, the brazing layer is made on the heat transfer surface before the copper powder is put on it. The brazing may be placed on the heat transfer surface for example on top of a binder before the copper powder is put on the surface. In a third method, the heat transfer surface may be first immersed in molten brazing and then the copper powder put on the surface. The brazing powder may also be brought to the heat transfer surface by means of thermal spraying or by brushing or spraying the brazing powder mixed into a binder using gas pressure.

The copper powder that forms the actual porous surface may also be fed to the heat transfer surface in several different ways. One way is to mix a binder, brazing powder and copper powder together and spray the mixture onto the heat transfer surface. According to one embodiment the brazing is brought to the surface of the material to be treated separately and the copper powder is sprayed on top of the brazing layer. The thickness of the powder layer is preferably in the range of 35 - 500 μm and advantageously 35 - 300 μm.

Heat treatment of the heat transfer surface is preferably performed in shielding gas to prevent the oxidation of copper. Generally used gases or gas mixtures such as argon, nitrogen, hydrogen, nitrogen-hydrogen mixture, carbon monoxide or cracked ammonia may be used as the shielding gas. Treatment at a temperature of around 300 - 4000C is sufficient to dry/evaporate the binder. A strong joint is obtained between the powder particle and the heat transfer surface when the component to be treated is briefly, for 1 -10 minutes, at a maximum temperature of 7250C, preferably in the range of 650-7000C. The brazing material may then be molten or mushy. In this case the furnace used may be for example a batch furnace or a strand annealing furnace, through which the heat transfer component to be treated is routed. When the component is at the temperature in question only momentarily, it means a clear energy saving in comparison to the known technique. At the same time, momentary heating in practice means that the furnace to be used may be relatively short, reducing investment costs.

In the method according to the invention, the brazing solder has proved to form a strong joint between the heat transfer surface and the copper powder particles as well as binding the particles to each other. Since using a solder according to the invention means that a separate fluxing agent is not required, the flux waste removal stage is also omitted from the treatment. The solder does not include any toxic ingredient such as lead, so that a product removed from service that contains a heat transfer surface may safely be recycled and used in the manufacture of a similar new product.

The invention also relates to a heat transfer surface of copper or copper alloy, onto which a porous surface of copper powder or copper alloy powder is formed by means of Cu-Ni-Sn-P-containing brazing solder. In addition to heat exchanger tubes, the heat transfer surface may be formed on other devices used for heat transfer, including heat sink, heat spreader, heat pipe and vapour chamber devices, and boiling surfaces for cooling electronic components as well as solar panels, cooling elements, car radiators and other coolers such as various casting moulds and casting coolers.

LIST OF DRAWINGS Figure 1 shows an enlargement of a porous heat transfer surface manufactured in accordance with example 1 , Figure 2 shows an enlargement of a cross-section of a porous surface copper pipe manufactured in accordance with example 1 , and Figure 3 shows the heat transfer capacity of a porous surface copper pipe manufactured in accordance with example 1 as a function of the temperature difference between the surface and the liquid with comparisons between a smooth and ridged surface. EXAMPLES Example 1 Deoxidised high phosphorous copper strip (Cu-DHP) was used as the heat transfer surface. The copper powder was water-atomised powder, with a copper content of 99.86 % and a particle size in the range of 37-90 μm. The brazing powder used was a powder with the following composition: 3.9 -4.5 weight % Ni, 14.6 -16.6 weight % Sn, 5.0-5.5 weight % P with the remainder copper. The particle shape of the brazing powder was spherical and the particle size somewhat smaller than that of the copper powder. The melting range of the brazing powder was 593-6040C. Both powders were mixed with a commercial organic binder, whereby a powder paste was formed. The composition of the paste in percentage by weight was 77% copper powder, 18% binder and 5% brazing powder.

The paste was sprayed onto the surface of the copper strip. The thickness of the sprayed coating layer was approximately 100 μm. The strip was conveyed through a resistance furnaces acting as a drying and brazing furnace at a rate of 10 cm/min. The temperature of the binder drying and evaporation furnace was approximately 3000C and that of the brazing furnace about 7000C. Nitrogen atmosphere was used as shielding gas, which included some hydrogen to prevent the oxidation of the component.

After brazing, the strip was taken for inspection, where it was found that the powder particles had adhered tightly to the surface of the strip and to each other. The strip could also be bent without dislodging any powder from the surface. The porosity of the surface and the surface area were large and numerous channels extending from the surface of the strip to the surface of the powder layer had formed, as can be seen in Figs. 1 and 2. Figure 1 is a SEM picture (SEM = Scanning Electron Microscopy) of a manufactured porous heat transfer surface and Figure 2 a microscopy picture of a cross- section of the porous surface of a copper pipe. Figure 3 shows the heat transfer capacity of a porous heat transfer surface manufactured in accordance with example 1 as a function of the temperature difference (Ts - TIq) between the surface (q) and the liquid (pentane) with comparisons between a smooth surface and a ridged surface.

After the formation of the porous surface, the strip was welded into a tube so that the porous surface formed the inner surface and/or the outer surface of the tube. The welding was very successful despite the porous surface. The porosity of the finished inner surface coating of the heat transfer tube was around 40 volume %.