NL1010267C2 - Modified porous metal surfaces. - Google Patents

Description

Title: Modified Porous Metal Surfaces

The invention relates to porous metal structures, such as pipes or plates, the diameter of the pores in the surface layer of which can be adjusted within relatively narrow limits, and to porous metal substrates 5 whose surface has been modified in such a way that it is suitable for application of either ceramic membranes or porous or non-porous metal membranes.

In the prior art, in many cases, porous metal structures, plates or tubes are used in processes in which finely divided solids are separated from liquids. Porous metal structures are also often used for the separation of solid particles from gas flows. In general, particles with dimensions up to about 3 µm can be separated well from gases or liquids. Although there is a great need to separate smaller solid particles, suitable mechanical strong and relatively inexpensive porous materials are not available for this. For separating particles from liquids, it is further attractive to have fine pore filter materials that are resistant to alkaline liquids. In that case, it is preferable to use membrane layers which are built entirely of metals or alloys. However, filter elements made from oxidic materials, such as titanium dioxide or zirconium dioxide, can also be used.

Broadly speaking, porous metal structures are technically manufactured in two different ways. The first procedure is based on metals that have a relatively low melting point and therefore sinter at relatively low temperatures. A mold of the desired shape and dimensions is filled with preferably spheres of the metal from which the porous structure must consist. In general, good results are obtained with graphite molds, although in principle other materials can also be used. By heating the filled mold at a temperature at which the metal particles sinter, the desired porous structure is obtained. The weight of the metal particles is sufficient to contact the elemental metal particles such that the particles sinter together. The atmosphere in which the sintering must take place depends strongly on the metal to be sintered. Metals and alloys such as copper, silver, nickel and bronze, which can be completely reduced at low temperatures, for example 600 ° C, will be heated in a reducing gas flow. The oxide layer on the surface of the metal or alloy particles will be removed by reduction, thereby creating a strong metallic bond between the particles. With a layer that is difficult to reduce, but low-melting metal such as aluminum, the layer of oxide present on the surface cannot reduce with hydrogen. For the sintering of aluminum to porous structures, it is therefore necessary to heat in a very good vacuum. In addition to an extremely low residual pressure of oxygen, a very low residual pressure of water vapor is also essential for sintering very base metals such as aluminum. Due to the weight of the aluminum spheres, the thin oxide layer present on the surface of the aluminum is elevated at an elevated temperature at which the metallic aluminum continues, and the metallic aluminum can come into contact with particles touching each other. The metallic alumihiSÉt sintered smoothly. One can also work in an inert gas Opt exotically low levels of oxygen, carbon dioxide and water vapor.

In the case of low melting metals or alloys that are difficult to reduce, one has to proceed in a different way. In that case, use a much higher pressure to push out the oxide layer of metal or alloy on the contact surfaces 35 between contacting particles. Small payment or alloy particles must therefore be pressed into a mold of the desired shape and dimensions. Larger shapes can advantageously use, for example, a rubber mold and hydrostatic pressing. After pressing, the 'green' molding is heated for some time in a reducing gas stream at a strongly elevated temperature. The neck-shaped contact sites grow between the elementary metal particles.

According to the current state of the art, it is not quite possible to produce porous metals with only pores smaller than 2 μπι. Such porous metal filter elements are now also less attractive from a technical point of view, due to the high transport resistance. Namely, the thickness of the porous element cannot be made very small without detracting from the mechanical strength of the part. With a fine-porous material with a thickness of, for example, 3 mm, the transport resistance is high. Technically it is therefore much more favorable to apply a thin layer with much narrower pores on a coarsely porous substrate.

The object of the present invention is therefore to provide a coarsely porous metal with a thin surface layer that contains pores with dimensions of about 2 to 0.01 µm and no wider pores at all. In this context thin is understood to mean a layer thickness of 1 to less than 30 μπι. The object of the invention also includes a coarse-porous metal bottom layer in which a thin porous layer with pores of 2 to 0.01 µm is applied in or on the pore mouths.

Depending on the application of the porous metals with a fine-porous surface layer, one will opt for completely metallic layers or for a ceramic layer on or in the pore mouths of the coarsely porous metal bottom layer. A special application of porous layers is to increase the selectivity of chemical reactions. It is desirable in the art to be able to use chemical reactions between two compounds which, in reaction with themselves, give rise to undesired products. An example is the aldol condensation of two different aldehydes. Since the aldehydes can also react with each other, three different compounds are obtained here, only one of which is desirable. If one now applies a catalyst for the desired reaction to the surface of a narrow-pore membrane, one reactant is passed through the membrane and the other reactant is passed along that side of the membrane where the catalyst is present, then the chance of 10 reaction between the different aldehydes.

According to the invention, particles with dimensions of 2 to 0.01 µm of metals or alloys, of compounds to be reduced to metals or alloys or of ceramic materials are introduced into the pore mouths of a coarsely porous metal substrate. Generally, for the application of the small particles in the pore mouths of the coarsely porous metal, a suspension of the small particles in a suitable liquid will be started from. The task is to allow the suspension to flow over the bottom layer 20 with a fairly uniform thickness, while preventing the suspension from penetrating locally deep into the coarse pores of the bottom layer, while other parts of the surface of the coarsely porous bottom layer do not suspension come into contact. According to the present invention, there are two procedures to avoid this drawback.

According to the first of the above two methods according to the invention, in order to obtain a uniform distribution of the suspension over the coarsely porous substrate, it is possible to start from a suspension of the particles to be applied, the viscosity of which is otherwise added according to the means known in the art, such as hydroxymethyl cellulose or polyvinyl alcohol. The viscosity of the suspension is adjusted so that the suspension does not quickly penetrate into the coarse pores of the bottom layer 35. According to the invention, such a dip coating of the coarsely porous metal bottom layer with a suspension of the fine particles to be applied leads to the filling of the pore nozzles with the small particles.

According to the second of the above-mentioned processes according to the invention, the pores of the coarsely porous metal can be filled almost completely with a liquid or with a waxy material with a low melting point. A suspension of the small particles to be applied can then be allowed to flow over the filled metal substrate. According to a preferred form of the method according to the invention, the pores are filled with a liquid which is immiscible with the liquid of the suspension of the particles to be applied and which has a higher density than that of the suspension. After drying, the liquid or waxy material is removed. Tubes preferably use a waxy material and rotate the horizontally disposed tube about its axis. After the slurry flows out, the rotation is continued until the liquid of the applied layer has evaporated. The invention also includes "spin coating" a porous flat metal body filled with a waxy material. The porous metal body to be treated is thereby rotated and the suspension of the particles to be applied is placed centrally on the surface of the porous body. Due to the centrifugal force, the suspension is spread as a thin layer over the surface of the porous body.

In order to obtain pores of 2 to 0.01 µm, particles with a diameter of approximately the same dimensions are required. The manufacture of metal or alloy particles of the above dimensions is technically known for a number of metals or alloys. For example, by decomposing metal carbonyls in a gas stream and collecting the particles in oil, it is possible to prepare small iron, nickel or cobalt particles. These particles can be separated from the oil by magnetic means. A wide range of metal and alloy particles can also be prepared by depositing on a support such as finely divided silica or alumina a suitable precursor of the metal or alloy and finely distributing this precursor at an elevated temperature reduce appropriate gas flow. The dimensions of the metal or alloy particles 5 can be controlled by adjusting the loading of the carrier. Sintering of the metal or alloy is prevented by the support. After reduction, the carrier can be dissolved in caustic solution, leaving a suspension of extremely small metal or alloy particles, which can be easily separated from the liquid by magnetic means in the case of ferromagnetic metals or alloys.

For non-ferromagnetic metals, the same procedure can be applied with an inert thermostable support. Since the separation of the metal particles from the liquid 15 after dissolution of the carrier cannot now be separated magnetically, the small particles will now preferably be flocculated. The particles can then be separated and the impurities removed by washing. After resuspending the washed material, 20 clusters of small particles are obtained, the dimensions of which in the suspension to be applied to the coarsely porous metal can be adapted to the dimensions of the pores of the coarsely porous according to the prior art. metal. According to the prior art, it is possible to use ultrasonic treatment, a liquid flow with a high velocity gradient or a liquid flow in which cavitation is generated. One can also work with a colloid mill or an ultraturrax. Finally, according to the present invention, one can coat small ferromagnetic particles with a layer of another metal, such as copper or silver.

According to a preferred form of the method according to the invention, complex cyanides with two alloying components are applied to a solution soluble in lye to prepare small alloy particles. For example, for iron-nickel alloys, Ni2Fe (CN) 6 can be applied to, for example, aluminum oxide. In this way, an atomic mixing of the alloy components in the precursor is accomplished.

Another possibility to prepare small metal particles is to start from a finely divided precursor 5 without support and to react the precursor to the metal. Preferably, a precursor is used here, which yields the metal at a relatively low temperature, because the small metal particles 10 sinter together at a strongly elevated temperature or during a prolonged thermal treatment. According to the invention, oxalates of copper, silver or nickel are preferably started from and decomposed in an inert or reducing gas stream. Other salts of organic acids can also yield small metal particles in this way. In this way, 15 small metal particles are obtained which are not strongly related and which can be dispersed excellently, for example by ultrasonic vibration.

It is very important to prevent the presence of larger clusters of small metal particles in the suspension. To this end, according to the present invention, the suspension is filtered through a fine filter before being applied to the surface of the coarse porous metal.

An advantage of starting out from metal particles is that there is no strong shrinkage, which is the case when starting out from particles to be reduced to metals or alloys. Nevertheless, it can be advantageous to introduce small reducible particles into the mouths of the pores of a coarsely porous metal structure. After heating in an inert or oxidizing gas, the particles applied and fixed in the pore mouths can then be reduced to the corresponding metals. This is excellent with the oxides of silver, copper and nickel, which are relatively easy to reduce. For silver oxide, heating to the air is even sufficient to effect reaction to the metal. According to the invention, 1010267 8 particles to be reduced to the desired metal are then placed in the pore mouths with dimensions of 2 to 0.01 µm, after which the material is reduced if necessary after a thermal pretreatment in a non-reducing gas flow.

According to the invention it is also possible to place particles which cannot be reduced to metal in the pore mouths of the coarsely porous metal, in general the dimensions of the pores obtained with particles which cannot be reduced to metals can be better controlled. Therefore, if the dimensions of the 10 pores need to be accurately set, it is preferable to use non-reducible particles or to reduce the particles to the corresponding metals. For working in alkaline liquids, oxides such as titanium dioxide or zirconium dioxide will be used which are resistant to alkaline liquids. Non-reducible oxides, such as, for example, silicon dioxide, aluminum oxide, titanium dioxide or zirconium dioxide, are very attractive if one wants to carry out separations at higher temperatures in reducing gas flows. Small metal particles sinter under such conditions, either creating large pores or completely clogging the pore mouths.

In most cases, small metal particles can be adhered well to the walls of the pores of the coarsely porous metal. An in-termetallic bond can form under reducing conditions. When preheated at high temperatures under non-reducing conditions, oxidic materials with the oxidized metal surface can also form tight bonds. However, there are a number of technically important cases where good adhesion cannot be achieved.

It is therefore often difficult to strongly bind solid particles on the surface of metals. Of course, metal particles or particles of metal-reducible oxides can be introduced into the pore mouths of the porous metal and then heat-treated in a reducing gas stream. If the temperature and the gas atmosphere 1010267 9 can be adjusted to reduce the surface area of the porous metal and the applied small metal particles, a metallic bond can be realized between the surface area of the porous metal and the metal particles. This also applies in the case that reducible oxides are introduced into the pore mouths. However, it has been found that the bond between the metal particles and the wall of the pores of the metal is so strong that large cracks often arise in the metal layer. Moreover, it is very difficult to prevent strong sintering of the metal particles when reducing. Strong sintering leads to a rough surface with relatively wide pores. High temperatures are required to reduce the surface area of many metals, whereby strong sintering of the metal particles 15 is unavoidable. The surface of stainless steel, a very attractive material for many applications, is very difficult to reduce. Very high temperatures are required for this. On the other hand, strong sintering with metals that are easy to reduce, such as copper, is not easy to avoid.

Surprisingly, it has now been found that covering the surface of the walls of the pore mouths of the porous metal structure with a porous or non-porous layer of an oxide which adheres strongly to the pore wall and to the metal or oxide particles to be applied anchor the small particles in the pore mouths. Figure 1 gives a schematic view of the metal structure. According to the invention, therefore, a coarsely porous metal structure is used, the walls of the pore mouths of which are covered with a thin or non-porous layer of an oxide which is applied to the pore wall as well as to the metal to be applied, the alloy or the oxide strongly adheres. Thin in this context means a thickness of less than 3 μπι. In general, enamel-forming oxidic compounds are very suitable as an adhesive layer. Oxides such as silicon dioxide or titanium dioxide have been found to anchor both metal particles and oxidic particles very tightly to the surface of the pore walls. According to a preferred form of the invention, therefore, porous metal structures are used in which a thin layer of silicon dioxide or titanium dioxide or another enamel-forming compound is applied to the wall of the pore mouths.

According to a preferred form of the method according to the invention, a thin layer of silicon dioxide is applied to the walls of the pore nozzles by applying a layer of silicone rubber 10 to the wall of the pore nozzles. Pyrolysis of the dried layer of silicone rubber results in a silicon dioxide layer, which is non-porous after heating at a high temperature and is porous at a lower temperature (for example 600 ° C). Analogously, according to a preferred form of the method according to the invention, titanium dioxide can be strongly adhered to the walls of the pore mouths by pyrolysing a layer of the titanium equivalent of silicone rubber. A layer of water glass applied to the wall of the pore mouths also leads to a strong interaction with metal, alloy and oxide particles.

According to the invention, the surface of porous metal structures is therefore modified by applying a thin adhesive oxide layer to the wall of the pores, after which the pores in the surface are filled via 'dip coating' with a thin, porous layer of cohesive solid particles of which the diameter is smaller than the diameter of the pores in the surface of the metal.

The solid oxide particles with dimensions of about 2 to 0.01 µm which are applied in this manner by dip coating in the mouth 30 of the pores of the metal can be obtained according to the known art. Very suitable is a preparation of small particles by the decomposition in flowing air or in an oxidizing gas stream of metal oxalates, such as nickel oxalate or zinc oxalate, or mixed oxalates of two or more metal ions, such as nickel-magnesium oxalate. The corresponding (mixed) oxides are obtained by decomposition of oxalates in air. Oxide particles can also be prepared by flame hydrolysis of, for example, chlorides, which are excellent for manufacturing metal surfaces modified according to the present method. The reaction is for the preparation of silicon dioxide by flame hydrolysis

SiCl, + 2H 2 O = SiO 2 + 4HCl

In an analogous way, many other metal oxides are produced on an industrial scale, such as aluminum oxide and titanium dioxide.

According to the invention, the "dip coating" is carried out using conventional techniques. A suspension of the oxide particles to be applied is prepared in, for example, a water or a water / ethanol mixture to which, for example, methyl cellulose has been added to adjust the viscosity. The presence of larger conglomerates in the suspension with which the 'dip coating' is carried out should be avoided. Larger conglomerations lead to a rougher surface. According to the invention, the suspension is pre-filtered through a filter with pores of 20 µm or smaller pores.

When the "dip coating" is carried out with particles of titanium dioxide or zirconium dioxide with an average size of 0.05 μπι, a membrane is obtained which is excellent for microfiltration, using pores of 0.05 to 10 μπι .. Both the metal and the oxide particles are resistant to an alkaline environment. According to a preferred form of the method according to the invention, the "dip coating" is therefore carried out with a suspension of titanium dioxide or zirconium dioxide.

Membranes with much narrower pores than those found in porous metals or alloys and much narrower than the aforementioned pores of 2 to 0.05 µm can be used to great advantage in the art. In this case it concerns pores from 10 to 0.5 nm. Dissolved compounds can be separated from liquids by means of such membranes and gas molecules of different molecular weight or dimensions can be separated. The advantage of such porous materials has long been recognized and various materials that can perform such separations have been proposed technically.

In the prior art, membranes consisting of polymers are generally used for the separation of various gas molecules. A drawback of polymer membranes is the fact that such membranes cannot be easily cleaned by heating the membranes at high temperatures. It is also generally not easy to apply a large pressure difference over polymer membranes. Finally, polymer membranes cannot be used well at elevated temperatures. In particular, if one wants to shift the thermodynamic equilibrium by selectively removing a certain component, one generally has to work at elevated temperatures. If one wishes to use membranes at higher temperatures and greater pressure differences, the membranes must be manufactured from metal or ceramic materials.

For technical applications, the mechanical properties of metals are generally much more attractive than the properties of ceramic materials. However, it is extremely difficult to produce coarsely porous metal plates or tubes having a thin layer on the surface with pores of dimensions from 10 nm to less than 1 nm. In the art, therefore, membrane modules are usually used, which are made entirely of ceramic material. According to the prior art, a thin ceramic layer with very fine pores is applied to a ceramic layer with coarser pores, often one or more ceramic intermediate layers are applied, the size of the pores going to the top layer with the narrows pores in steps. A drawback of such fully ceramic membranes is that clamping in technical equipment is difficult, especially if clamping takes place at a low temperature and the membrane must later be used at an elevated temperature.

As noted above, the art frequently uses coarsely porous metal pipes or plates. The size of the pores in such materials can be varied to a minimum size of about 5 µm. It has therefore previously been proposed in the patent applications International Patent Application WO 91/12879 and WO 92/13637 to apply a thin fine-porous ceramic layer to the surface of coarse-porous metals. The thin porous ceramic layer seals the coarse pores of the metal. The difference in the expansion coefficient of the metal and the ceramic layer is important. The expansion coefficient of ceramics, such as aluminum oxide, is about 7x106 per ° C, while that of metals, such as iron and copper, is about 15x106 per ° C. Due to this difference in thermal expansion coefficient, only thin ceramic layers can be used. In connection with the permeability of the membrane layer, an attempt is made to create a leak-free membrane with the thinnest possible layer of ceramic material. The use of a thin ceramic layer is therefore no problem.

In the above-mentioned patent application WO 91/12879 it has been proposed to fill the pores of the porous metal with a liquid of a higher density than water. It is also possible to fill these pores with a material with a relatively low melting point, such as a wax or candle grease. Then, a sol of a suitable material, such as silicon dioxide, alumina or titanium dioxide, is allowed to flow out over the surface of the porous metal, and the sol is allowed to dry after the sol has been allowed to gel. To avoid cracks in the dried layer, drying should be done very carefully. After the sol or gel has dried, the liquid or waxy material is removed and the fine-porous ceramic layer is stabilized by heating to a temperature of, for example, 450 to 650 ° C.

1010267 14

While this procedure yields interesting membrane composites, drying the gelled sol or not is a very critical operation. In WO 92/13637 it is therefore proposed to dissolve a solution of a material such as silicone rubber or the titanium equivalent of silicone rubber over a porous metal whose pores are filled with a heavy liquid or a waxy material and the solvent to be evaporated by evaporation. remove. This results in a very thin layer of silicone rubber, which adheres very strongly to the metal surface, then the heavy liquid or the waxy material is removed, after which the organic components of the silicone rubber are removed by heating in the air or in an oxygen-containing gas stream. The result is a very thin layer of high porosity silica, which is very strongly adhered to the metal. The silicon dioxide contains pores of 0.8 and 1.2 nm. It is important that the fine-porous ceramic membrane layer is formed at high temperatures. On cooling, the layer is placed under compressive stress 20; ceramic materials can absorb a compressive stress much better than a tensile stress.

Although the last method discussed provides very good membranes, the quality of the porous metal substrates is a very big problem. In general, the surface of porous metal pipes and plates commercially produced in the manner discussed above is relatively rough, while there are also (to a small extent) pores of 15 to 20 µm in the surface. Research with a scanning electron microscope has shown that the surface of 30 commercial porous stainless steel structures contains a number of large pores with diameters up to 20 μπι. In qualitative good structures few, relatively large pores occur in the surface. In order to bridge pores of such dimensions, relatively thick ceramic membrane layers are required. As a result, the presence of a relatively small fraction of large pores leads to a considerably lower permeability of the membrane layer. It is therefore technically of great importance to provide porous metal substrates that do not contain pores with dimensions larger than about 10 µm.

The prior art has attempted to smooth the surface of commercial porous metals and eliminate larger sized pores by applying a network of very thin threads (for example, 5 to 10 microns in diameter). In practice, however, this appears to be an expensive procedure, while the result often leaves something to be desired. Examination of a large number of commercial structures in the scanning electron microscope showed that in this case too the large pores cannot be sufficiently removed. Attempts have also been made to improve the quality of the surface by spraying with small metal particles. This method has been found to eliminate the coarse pores, but this is at the expense of a greater surface roughness. This makes it difficult to cover the surface with a sealing thin layer of a fine-porous ceramic material. Also thin metal wire fabrics do not provide a surface suitable for applying thin porous ceramic layers.

A second main object of the present invention is therefore to provide a coarsely porous metal with a surface such that a thin porous layer of a ceramic material is sufficient to close all coarse pores. Such a modified porous metal surface 30 preferably has pores from 2 to 0.1 µm.

A further object of the present invention is to provide porous metals with a surface whose pores are wholly or substantially closed with thin layers of metals or alloys that selectively allow passage of certain gas molecules.

On a thus modified porous metal surface, a porous layer of silicon dioxide with pores of 0.8 and 1.2 nm can be applied relatively quickly, starting from silicone rubber. Figure 2 gives a schematic view of the metal structure. In this way a membrane with such small pores is obtained that one can separate a number of gas molecules by making use of a difference in transport 10 at Knudsen diffusion. In general, to obtain a completely dense layer, more than one layer of silicon dioxide will be applied via silicone rubber. It is also particularly attractive to apply one or more layers of silicon dioxide via sol-gel methods according to the known prior art. The invention also includes a first layer applied by pyrolysis of a layer of silicone rubber followed by one or more layers by a sol-gel method. It is known that by using sol-gel techniques one can obtain layers with pores that only allow hydrogen to pass through.

In a number of cases, for the separation of gases, advantage can be taken of the fact that certain metals selectively dissolve certain gas molecules. An example is silver in which selectively dissolves oxygen, while hydrogen selectively dissolves in palladium or palladium alloys. To avoid recrystallization, a palladium-silver alloy is preferably used. According to the present invention, a dense layer of a metal (the membrane metal) is deposited in the pores of a solid particle-modified porous metal by "dip coating" with a colloidal suspension of the desired metal. It is also possible to fill the pores of the modified porous metal with a heavy liquid and to spread a sol of the desired metal over the modified. After heat treatment in a reducing gas stream, if necessary, the * 1010267 17 small metal particles sinter together to form a tight plug that completely seals the pore mouth. Due to the strong interaction with the thin adhesive layer applied to the wall of the pore mouth, the membrane metal has no tendency to detach from the surface of the pore wall.

In a preferred form of the method of the invention, a silver sol is used prepared according to a method published by Carey Lea (Carey Lea, Am. J. Sci.

(3) 37 (1889) 476). This method of preparation leads to a highly concentrated silver sol. Surprisingly, it has been found that the silver particles preferably penetrate into the pores of the modified coarsely porous metal.

The modified porous metal surfaces of the present invention are also ideally suited for the application of oxygen ion conductors. If only transport of oxygen is intended, an electronic conductor is ideally suited. An opposite electron flow then neutralizes the flow of oxygen ions through the layer. If, on the other hand, one strives for application in a 'solid oxide fuel cell', one must use an electronic insulator. The oxidic layer will now preferably be applied to the pore mouths, with a metal conductor again applied to the oxidic layer, which does not completely cover the surface.

25 Example

Manufacture of a silicon dioxide membrane with very narrow pores

In this example, stainless steel filters manufactured by Krebsöge, Radevormwald, Germany are used. The 30 filters consisted of sintered AISI 316L steel (Cr 16.5%, Ni 13.5%, Mo 2.5%, Si 0.7%, C <0.02%, Mn <0.2% and iron) . The membrane layers were applied to discs with a diameter of exactly 24.0 mm and a thickness of 2 mm. The porosity of the filter was about 40%. In order to be able to clamp the discs 35 leak-free for measurement of the transport f 1010267 18, the discs were placed in a non-porous stainless steel disc with a diameter of 48 mm and a thickness of 5 mm in which a cylindrical hole of exactly 24.0 mm was fitted. The porous discs were placed in the 5 cylindrical hole of the heated non-porous disc. After cooling, the porous disc was found to have shrunk leak-free in the non-porous ring.

The stainless steel structure thus obtained was thoroughly cleaned by ultrasonic vibration in acetone and then by dipping in a boiling solution of 1.5 molar ammonia with 0.1 wt.% Triton-X in water for at least one hour. The discs were rinsed twice with boiling demineralized water and stored under ethanol.

In order to obtain a good adhesion between the nickel oxide particles to be applied below and the wall of the pores of the stainless steel, a silicon dioxide layer was applied to the wall of the pores. This was done by 30 to 60 μΐ of a solution of RTV silicone rubber (Bison,

Perfecta Int. Netherlands) 4.8% by weight in ethyl acetate (Merck) or diethyl ether (Merck) on the porous metal discs. The layer of silicone rubber resulting after drying was pyrolyzed to silica in air at 723 K.

Nickel oxalate dihydrate was prepared by mixing solutions of oxalic acid and nickel nitrate. A solution of nickel nitrate hexahydrate was quickly added to a solution of oxalic acid dihydrate. The volumes of both solutions were equal; the final concentration of the reactants was 1.0 molar. The precipitation vessel was kept at a fixed temperature of 293 K. After 30 min, the precipitate was filtered off and washed with demineralized water and ethanol. The filter cake was dried at 353 K in stationary air for 24 hours. The resulting material was ball milled with ethanol for 4 hours, after which the alcohol was evaporated at 393 K for at least 16 hours.

1010267 19

Nickel oxide particles were obtained by heating the nickel oxalate thus prepared in a rotary oven at a rate of 1 K / min to 723 K in a 100 ml / min 02 / A (20/80) stream. After the sample was kept at 723 K 5 for 5 hours, it was cooled to room temperature. The particle size of the nickel oxide particles was determined by scanning electron microscopy and with an optical particle counter, Accusizer 770 Optical Partial Sizer with a detection limit of 1 μπι. The last instrument indicated a mean cluster size of 1.5 μπι with a small fraction greater than 2 μπι. The BET surface varied from 10 to 17 m2 per gram. This corresponds to elemental nickel oxide particles from 0.08 to 0.03 μπι.

Scanning electron microscopy indicated that the size of the 15 individual nickel oxide particles was significantly smaller than 1 μπι.

The nickel oxide powder was suspended in alcohol milled in a ball mill for at least 4 hours. The resulting paste was diluted with ethanol and filtered through a 17 μl filter (Haver & Boecker). The solids content of the filtered suspension was adjusted to 10 to 12% by weight. After this, the suspension was placed in a stirred vessel with baffles and methyl cellulose (Genfarma) was added to an amount of 2 wt. %. The suspension was heated to 328 K with stirring just above the gelation point. An equal volume of water was then added with vigorous stirring, which led to a dramatic increase in viscosity. The suspension was stirred for 10 min and then filtered through a 25 µg mesh under 2 to 4 bar pressure. The suspension was finally cooled to room temperature.

The carefully cleaned stainless steel discs were mounted vertically in a dip-coating device. A 1010267 speed of 1 and of 3 mm / min was used. After dip coating, the disk was heated in an air flow of 100 ml / min at 723 K for 5 hours, after heating at a rate of 1 K / min in a special quartz oven. After cooling, the nickel oxide was found to be very strongly bound in the pore mouths of the stainless steel. The mean pore diameter was 2 μπι. In general, in order to obtain a total coverage of all the coarse pores of the stainless steel, a layer of nickel oxide with dip coating had to be applied three times. The final thickness of the nickel oxide layer was then 15 to 20 µm.

The pores of the stainless steel were temporarily filled with 1,1,1-trichloroethane (Aldrich), after which a solution of silicone rubber (Bison, Perfecta Int.

The Netherlands) flowed from 8 to 10% by weight in diethyl ether (Jansen) over the surface of the stainless steel disc.

After the solvent had evaporated, the silicone rubber film was air dried for at least one hour. After removal of the 1,1,1-trichloroethane

20 the covered stainless steel disc at 723 K for 5 hours

kept in a stream of air and then cooled. Heating and cooling were done at 1 K / min. After the application of 1 to 5 silicon dioxide layers, the permeance of the resulting membrane systems was 7 to 12 x 25 10-5 mol / (m2secPa). The variation of the permeance with the differential pressure across the membrane system showed that some contribution from laminar flow was still present. With a larger silicon dioxide layer, a contribution of laminar flow was virtually absent. In order to check the reproducibility of the manufacture of membrane systems according to the invention, four different membrane systems were prepared in the same manner. If there was no noticeable contribution from laminar flow, the permeance varied from 1.2 to 5.4 x 10 6 6 mol / (m2secPa). This shows that membrane systems with such a high permeance can be reproducibly manufactured.

1010267

Claims (24)

1. Coarsely porous metal structures characterized in that a thin surface layer containing only pores with dimensions of about 2 to 0.01 μπι is applied to the metal structure. 52. Method for manufacturing coarsely porous metal workpieces according to claim 1, characterized in that particles with dimensions of 2 to 0.05 µm of metals or alloys, of compounds to be reduced to metals or alloys or of ceramic materials are introduced into the 10 pore mouths of the coarsely porous metal substrate. 3. Process according to claim 2, characterized in that the particles are applied by dip coating with a suspension of particles of 2 to 0.05 µm. 4. Process according to claim 2, characterized in that the particles are applied by flowing a suspension of particles of 2 to 0.05 µm over the coarsely porous metal, the pores of the metal being largely filled with a liquid. 5. A method according to claims 3 and 4, characterized in that the suspension is filtered before application to the coarsely porous metal through a filter with openings of maximum 17 µm. 6. Method according to claim 4, characterized in that the pores of the metal structure are filled to a large extent with a liquid immiscible with the liquid of the suspension of small particles and of a greater density than the liquid of the suspension. 7. Method according to claim 4, characterized in that the pores of the metal structure are largely filled with a waxy material with a melting point below about 120 ° C. i 1010267 8. Process according to claim 6, characterized in that 'spin coating' is used for applying the suspension of small particles. 9. A method in which small metal or alloy particles are prepared by applying a suitable precursor of the metal or alloy finely divided to a thermostable, caustic-soluble carrier, then reducing this precursor at an elevated temperature in a suitable gas stream and subsequently after cooling to room temperature, the carrier is dissolved in lye, after which the small metal particles, if necessary after flocculation, are separated from the liquid and the impurities are removed by washing. 10. Process according to claim 9, wherein insoluble complex cyanides are applied as precursor 15 to alloy particles on a suitable support material. 11. Coarse-porous metal structure, characterized in that the walls of the pore mouths are covered with a porous or non-porous layer of an oxide, which covers both the metal of the coarse-porous metal structure and that of the small to be applied metal particles adhere well, after which according to one or more of claims 2 to 8, metallic or non-metallic particles with dimensions of 2 to 0.05 µm are applied in the pore mouths. Coarse-porous metal structure according to claim 11, characterized in that a thin layer of an enamel-forming oxide is applied to the wall of the pore mouths. Coarse-porous metal structure according to claim 11, characterized in that a thin layer of silicon dioxide or titanium dioxide is applied to the wall of the pore nozzles. Method for applying a thin oxide layer according to claim 13, characterized in that a thin layer of silicone rubber or the titanium equivalent thereof is applied to the wall of the pore mouths of the metal structure and after drying of the layer the organic components by oxidation at a temperature of about 400 to 700 ° C. 1010267 Coarse-porous metal structure according to claim 11, characterized in that irreducible oxides, such as silicon dioxide, titanium dioxide, or zirconium dioxide, are provided in the pore mouths. Coarsely porous metal structure according to claims 1, 9 or 10, characterized in that a thin layer with pores with diameters from 5 to 0.5 nm is applied in or on the modified pore mouths. Coarse-porous metal structure according to claim 11, characterized in that the thin porous layer with pores of 5 to 0.5 nm consists of silicon dioxide or titanium dioxide. 18. Method for applying a fine-porous layer with pores of 5 to 0.5 nm on a porous metal surface modified according to claims 1, 9 and 10 by applying a thin layer of silicone rubber to the surface and subsequently applying this layer pyrolysis. 19. Method for applying a fine-porous layer with pores of 5 to 0.5 nm on a porous metal surface modified according to claims 1, 9 and 10 by applying a thin layer of the titanium equivalent of silicone rubber to the surface and then pyrolyze this layer. Coarse-porous metal structure according to claims 1, 9 or 10, characterized in that a thin non-porous layer of a metal which selectively allows certain gas molecules to pass is disposed in or on the modified pore mouths. Coarse-porous metal structure according to claim 20, characterized in that a thin non-porous layer of silver or an oxygen-permeable silver alloy is applied in or on the modified pore mouths. Coarse-porous metal structure according to claim 20, characterized in that a thin non-porous layer of palladium or a hydrogen-permeable palladium alloy is applied in or on the modified pore mouths. Coarse-porous metal structure according to claim 20, characterized in that a 1010267 thin non-porous layer of an electronically conductive oxide or mixed oxide which selectively transmits oxygen is provided in or on the modified pore mouths. Coarse-porous metal structure according to claim 20, characterized in that a thin non-porous layer of a non-electronically conductive oxide or mixed oxide which selectively permits oxygen is applied to the modified pore mouths. 1010267