DE3524799A1 - METHOD FOR PRODUCING A TURNED SURFACE LAYER AND MOLECULAR SCREEN PRODUCED BY THIS METHOD - Google Patents

Abstract

Process for treating a surface layer (101, 121) of a body (1, 21) made of, in particular, porous materials such as sintered material, in which the surface in question is brought into contact with a short-lived plasma, in particular generated by wire explosion (2, 21). Based on this process, molecular-sieve membranes (30) can be produced by preparing a surface layer (121) treated according to the process, generating in this very thin layer narrow radiation-damaged channels by means of particle irradiation (34) and then clearing said channels by etching. <IMAGE>

Description

The present invention relates to a method and according to the preamble of claim 1 molecular sieve produced using this method membrane.

A wide variety of methods are known from the prior art known with which surfaces of solid materials be compensated. As a rule, the respective procedure depends ren of the material or at least on the type of material from which the surface to be coated consists. For Ceramic material and other similar sintered materials is coming primarily grinding and mechanical or chemical Polishing in question. Metals, but also e.g. Ceramics, receive surface coatings by preferably Ver nickel or chrome plating. Other allowances are art fabric coatings. It is also known to go through metals Ion bombardment with ions of the same or different ones to make elements resistant. It is also knows, especially for optical glasses that Surface of glass with a flame retardant ver improve. The latter method is very thin layered melting, with the previous one Grinding and polishing produced scratch marks and the like. to be eliminated (again).

Are surface-coating external cover layers from applications  unsuitable for reasons of use, so there are porous Materials such as for sintered ceramics, basic Problems with conventional methods on or at these Materials closed, tempered surfaces put.

The method of the present invention is thereupon aligns the surface of a material with particular porous structure, e.g. a sintered material like Ceramics, sintered metal and the like. To be tempered so that a relatively thin tempered surface layer on a be meets the body from the material of this body and the properties of this material for the restli Chen body at least essentially not changed will.

The inventive method is characterized by the features of Claim 1 given and further refinements and Developments of the invention go from the Unteran sayings. These include claims 10 to 13 to highlight, which is a particularly outstanding Application of the method according to the invention, namely for Production of a molecular sieve membrane, relates and Features of this development of the invention indicates. It is noted that on a molecular sieve membrane of claims 12 and 13 carried out for their manufacture performed, characteristic process steps retrospectively are verifiable.

The present invention is related to an older invention (DE-OS 34 35 320) incidentally made observation that a plasma elec tread coating, and in itself less favorable Selection of the parameters, a surface change on  is kicking. It was also recognized that this Surface change specifically for a desired surface area remuneration is to be used.

The inventive method provides that the ver quality surface, e.g. also the inner surface of a (narrow) tube made of, for example, piezoceramic (like it used for inkjet printers) or one flat surface in contact with such a plasma bring. According to the invention, this plasma is the energy converted in the plasma and the life of the Dimension plasma selected within narrow limits. There is also a confinement of the plasma, the surface to be coated at least a part of this limitation (e.g. the Pipe inner wall forms the entire boundary).

The already mentioned measurement of plasma energy and plasma duration is strongly dependent on the material of the surface to be coated and in particular on the thermal conductivity coefficient of the density with the specific heat of this material. The invention and thus the plasma energy to be selected and the lifetime of the plasma are based on the objective of causing only a 10 to 50, in particular up to 20 nm thick, surface layer of the material in question to melt so briefly that the material of this layer flows away. (the underlying volume of the material remains unchanged). In the context of the present invention, the thermal properties already mentioned are to be observed. If it is, for example, ceramic, ie if heat conduction (for example comparatively to metals) is relatively low, time values adapted to the invention for the life of the plasma are between 0.5 and 5 μs. With a suitable plasma energy of 1 to 5 J./cm ² (based on the supplied, converted electrical energy), there is a thin-layer melting of the ceramic surface as indicated above, the melting front penetrating into the material from the surface slowly becoming so moved into the interior of the material so that the melting zone delimited by the material surface on one side and by the melting front on the other side remains essentially intact until the plasma state of the discharge has ended. This method according to the invention applied to, for example, piezoceramic ensures that the piezoceramic material of the now surface-coated body continues to have its piezoelectric property (after polarization has taken place). A further example of (polycrystalline) silicon material is discussed for the choice of the duration of the plasma pulse according to the invention. Because of the much higher thermal conductivity, the duration of the plasma exposure may be in the range of only 100 ns. Again, an approximately equal plasma energy of 2 to 5 J./cm ^{2 is to be used} . In the short period of about 100 ns, the front of the melting process in silicon is again not significantly further than the indicated 10 to 100 nm, preferably 20 nm, penetrated. It has been shown that the plasma pulse lasts considerably longer, for example with substantially more than 100 ns for silicon, predominantly results only in disadvantages, even if the extended plasma duration should increase the plasma energy accordingly. This then results in disadvantageous overheating effects in the material of the region of the surface layer, combined with strong evaporation. Such evaporation, apart from the special dimensioning discussed further below, leads to the formation of uncontrollably large holes and / or undesirable surface roughness through localized, explosive evaporation.

The above statements give the expert necessary hints for the many possible in their properties, especially with regard to the Thermal conductivity and specific heat, relative various, according to the invention with surface treatment to determine the materials to be provided. The determination the relevant parameters can be determined by simple Series tests as well as by calculating the occurring Heating and heat conduction processes take place.

This was found to produce for the invention Plasma must be spatially limited, namely with regard to the layer thickness to values in the millimeter range. It is the plasma to be provided over the surface to be coated layer meant. The one available for the plasma Layer thickness above the surface to be coated is like this to comply with the fact that the electrical and / or optically generated In any case, plasma is already up to this surface expands during a fraction of the pulse duration. The Plasma generation is too localized for the invention ren that such a thickness is maintained. In particular it is recommended, e.g. for a flat plate, one opposite plate to provide and the in Distance enclosed between these two plates Limit layer to this thickness discussed here.

Due to the measures and dimensions according to the invention thus causes only a thin surface layer of the Material heated briefly with melting the same will not affect the remaining volume of the material in this way. The dimension is so tight  chooses that preferably no substantial evaporation of the Material of this surface layer occurs. Only in the Surface takes place within the duration of the pulse of this Melting, because of the dimensioning of the plasma energy. It can be achieved that tearing open by evaporation from the inside of the surface Material is omitted. However, it should be pointed out pointed out that a further development of the invention boils down to a certain degree of evaporation from the Interior of the material by choosing accordingly higher Energy is set, creating deliberate holes in ge targets selected diameter range in the tempered Layer are generated.

Preferably, the energy supply for it takes place generating plasma directly from electrical energy, preferably wise from a capacitor discharge. It is from before part, (by appropriate choice of the inductance of the Circuit) to provide aperiodic capacitor discharge. The discharge itself is carried out over one or more thin ones connected in parallel and arranged side by side Initiated metal wires. The Ent generating the plasma Charge is so powerful that the thin, preferably only 10 to 50 µm thick metal wire explosively ver steams. Because of this type of evaporation to be set of the metal wire is also the one chosen for this wire Material less critical. This can e.g. Copper, Silver, but also tungsten and the like.

In this context it should be mentioned that by or at the selected dimensioning for plasma energy and plasma duration the emergence of an electrically conductive metal coating tion on the surface to be coated is not applied. This Effect was observed even in an inert gas atmosphere. In  this effect is even more pronounced in the presence of Guaranteed oxygen.

It is also important to the invention to maintain a gas pressure value in the space provided for the plasma generation, specifically with regard to the pressure value before the start of the plasma generation. Among other things, this can be seen in terms of good and even spreading of the plasma discharge across the surface to be coated. For example, pressure values in the range from 10 ² to 10 ⁴ Pa must be observed, such pressure values also leading to the fact that no pressure value occurs during the plasma discharge that would lead to the destruction of the piece of material.

The distance provided according to the invention leaves one advantageous effect for the invention, namely that particles evaporated from the surface to be coated by reflection on the plasma cloud to a considerable degree get back to the material surface.

Further explanations of the invention follow from the following description of two exemplary embodiments of the Surface finishing process and an example of Production of a molecular sieve.

Figs. 1 and 2 each show an example of fiction, modern surface treatment, namely, the inner surface of a tube and the flat surface of a plate.

Fig. 3 shows the cross section of an internally coated tube and

Fig. 4 shows a device for producing a molecular sieve from a molten plate according to FIG. 2 and a molecular sieve thus produced.

Fig. 1 is a remuneration is paid inside tube 1 with, for example shows 2 mm inside diameter. Through this tube 1 , a thin metal wire 2 is stretched, which is used to generate the plasma in the interior of the tube 1 . This wire 2 consists of, for example, copper, silver or also of a high-melting metal, such as, for example, tungsten. Numerical values of the diameter of the wire 2 are between 10 and 100 μm, preferably 20-50 μm. There is an elec trical circuit 3 , which consists essentially of a switch 4, a discharge capacitor 5 , a charging resistor 6 , the connections 7 for the high voltage supply and the connections 31 and 32 for the wire he already mentioned. With 8 the reactor vessel is indicated, which includes the tube 1 and the connections 31 and 32 and thus the wire 2 . This reactor vessel 8 has, for example, a common gas inlet and gas outlet. A gas atmosphere required for the generation of the plasma is maintained within the reactor vessel 8 , for example consisting of an inert gas such as nitrogen or also of noble gas or corresponding mixtures. A (starting) pressure of 10 ² -10 ⁴ Pa is preferably provided in the interior of the reactor vessel 8 .

The surface to be coated is the inner surface of a tube 1 made of, for example, sintered ceramic such as piezo ceramic or aluminum oxide. With an inner diameter of, for example, 2 mm, a wire 2 stretched out for electrical generation of the plasma has a corresponding distance of 1 mm from the inner tube wall. A copper wire with a diameter of 50 μm, for example , is suitable for carrying out the process of tempering the inner surface 101 a . The circuit 3 has a capacitor with 0.25 μF, a charging resistor with 1 MOhm and 5 KV voltage at the connections 7 . With the closing of the switch 4 , a capacitor discharge charge impulse through the wire 2 passes through, which brings the wire 2 to the momentary evaporation. This evaporation is such that practically no surface metallization of the inner surface 101 a of the tube 1 occurs. However, the surface coating according to the invention is carried out, namely in a layer depth of 10 to 20 nm. Preferably, the initial gas pressure in the reactor vessel 8 is set to such a low pressure value that the resulting plasma is at least the distance between the wire 2 and the Inner surface of the tube 1 expands. 100 denotes an irradiation, for example of laser radiation, which is provided alternatively or additionally for electrical plasma generation.

Fig. 2 shows the application of the invention Ver proceedings for one-sided surface finishing of two plates 21 and 21 a. These two plates 21 , 21 a are arranged opposite one another at a distance of, for example, 1 to 2 mm. With 22 a number parallel to each other in the space between the plates 21 and 21 a spanned wires are referred to, which correspond to the wire 2 from the embodiment of FIG. 1. The dimension of the space between the opposing surfaces 121 a of the plates 21 , 21 a is dimensioned, for example, to 1 to 2 mm. The distance of the wires 22 from one another has approximately the same dimension. Here too, optically effective plasma generation can be seen as a support or alternatively.

For the implementation of the method, the wires 22 are preferably connected electrically in parallel, which is indicated by dashed lines. With 31 one connection of this parallel connection is designated. The associated second connection corresponding to connection 32 of FIG. 1 is not shown. At the terminal 31 and the other terminal, not shown, a circuit 3 is connected accordingly to that of FIG. 1. Accordingly, the number of wires 22 , for example, the capacitance of the capacitor is multiplied, so that in principle for each individual wire 22, the conditions are approximately the same as in Fig. 1. The same is provided for the selection of the gas atmosphere and its pressure in the reactor vessel 8 , which is only indicated.

The explosive evaporation of the wires 22 at the moment the switch 4 of the connected circuit 3 is switched on again leads to the generation of plasma in the space between the mutually opposite surfaces 121 a of the surface 21 and possibly 21 a of the surface according to the invention. The material of the respective plate 21 and / or 21 a to be tempered consists of a material as stated above. The two mutually opposite plates 21 and 21 a form by their distance from one another the plasma discharge in the vicinity of the wires 22 enclosing space. In the example of FIG. 1, this inevitably causes the tube 1 . This inclusion of the plasma is important for the implementation of the method, so that actually the surface melting takes place. If in individual cases only one plate 21 is to be remunerated, a second plate 21 a of plate 21 opposite to be assigned. The material of this second, in this case only the inclusion of the plasma serving second plate 21 a can then be made of any material that is sufficiently resistant to the plasma that occurs and does not provide such evaporation, which is the surface finish of the plate to be coated according to the invention 21 would run counter. As a rule, you will always choose such materials for such a second plate 21 a , which are also considered surface treatment for an upper surface according to the invention anyway.

FIG. 3 shows the cross section of a surface-coated tube 1 according to FIG. 1 . With 101 , the thin, he inventively surface-coated layer on the inside of the tube 1 is designated. The cross-sectional area 102 within the tube 1 is the cross-section of the plasma occurring during the implementation of the method according to the invention.

The method according to the invention is particularly suitable especially asymmetric molecular sieve membranes of higher Quality and especially with mechanically excellent Manufacture robustness. You go for manufacturing of such a molecular sieve membrane by itself coarse-pored body, e.g. a sintered body from preference wise ceramics. According to the invention Process becomes a surface layer of this coarse pore Body surface tempered. This leads to this Surface layer is largely dense. More advantageous wise this has tempered surfaces according to the invention layer only a small thickness of 10 to preferred only 20 nm. With such a thin surface layer it is relatively easy to make this layer with holes to provide the cross-sectional dimensions in values of Have molecular sizes, and that through this layer through to the coarse-pored material of the base body pass. The basic body ensures high mechanical Stability. Its large porosity ensures that this Body cannot be clogged by such material that the fine holes of the actual molecular sieve membrane forming tempered surface zone Has.  

Such holes can be technologically relatively simple Way, in a wide range of cross-sectional dimensions gene, be prepared by first the already tempered surface layer of the body with highly energetic table particles is irradiated. For radiation ions or neutral particles are suitable. Such a Particle crossing the surface zone produces im material traversed this path along its path surrounding channel of high radiation damage density. With ent speaking of low surface density of the particle irradiation there are spatially separated, side by side lying radiation damage channels. By following chemical etching becomes the radiation-damaged material of these channels detached. The length of time applied Etching makes it possible to choose large, in relatively narrow Dimensional open cross sections as a sieve holes in the molecular sieve membrane.

It has already been pointed out above that Advantage can be something larger or at the upper limit of the area to be selected to choose plasma energy. It is achieved that a certain, but still very limited, and also internal evaporation of material from the upper surface zone takes place. Such fine metered evaporation leads to fine holes in the surface layer, and this in contrast to explosive tearing open large holes with a large overdose of plasma energy.

The invention results in molecular sieve membranes selectable large holes down to hole cross sections from 10 nm.

Used for the body of such a molecular  sieve membrane piezoelectric material, so can Another significant advantage can be achieved. By To place alternating electrical fields on the electrodes of the polarized piezoceramic body that acts as an upper surface zone comprises such a molecular sieve membrane, such a body can be excited to resonate vibrations will. With this swinging of the body one can such molecular sieve membranes of the invention Clean holes that clog holes. Given if this becomes an auxiliary mud flow applied in the reverse direction through the membrane layer.

Fig. 4 shows a schematic diagram for the production of a molecular sieve membrane, starting from a plate 21 which already has a coated surface 121 produced according to FIG. 2. The plate 21 made of, for example, aluminum oxide, zirconium oxide or other highly resistant but porous material to be sintered is, apart from the thin surface layer 121 , relatively coarse in volume in its volume, for example with pore sizes down to the μm range. In a reactor vessel 28, only indicated, of a device for bombardment with corpuscular particles, for example ions. With 33 such a corpuscular part source is indicated and within the vessel 28 the further such a corpuscular particle bombardment means are arranged, which are omitted for the sake of clarity.

In carrying out the part of the method according to the invention which is used to produce a molecular sieve, the surface, ie the thin layer 121 , of the plate 21 is irradiated with the corpuscular particles, as indicated by 34 . For example, the full-area irradiation takes place in an area indicated by 35 . In this area 35 , the impinging particle particles of the rays 34 in the layer 121 create thin channels with radiation damage in the material 36 . Because of the very small thickness of the surface-coated layer 121 (10 to 20 nm) that can be achieved with the invention, it can be easily achieved that the corpuscular particles penetrate this thin layer 121 completely, ie that the channels formed pass through the layer 121 . The material is then etched out of these channels and the holes 36 forming the actual molecular sieve are formed.

With 30 4 is in Fig. Radiation to an after loading and pointed molecular sieve ready-prepared etching which can perform with the holes 36 this function as a sieve in the region 35. The part volume of the plate 21 , which is determined by the area 35 and the thickness of the layer 121 , is the actual molecular sieve. The remaining coarse-pored body of the plate 21 is used for mechanical strength. Outside the region 35 (which can also cover the entire surface of the plate 21 ), this molecular sieve 30 is sealed. The portion of the plate 21 (in the figure) lying below the surface layer 21 is functionally non-existent for the sieve function, namely because this portion of the body is permeable in comparison to the molecular sieve region due to its coarse porosity.

Claims (13)

1. A method for producing a coated surface layer on a body, in particular on one with a material on the surface which has a porous structure, characterized in that

• - that the surface to be coated ( 101 a , 121 a) is brought into contact with a plasma that is measured in microseconds to milliseconds,
• - That the layer thickness of the plasma covering this surface ( 101 a , 121 a) is kept limited to a range of approximately 0.3 to approximately 10 mm, the surface to be coated ( 101 , 121 ) being at least a part of this limitation, and
• - That the process is carried out in an inert gas atmosphere and the gas pressure before the plasma generation is dimensioned to such an initial value that the plasma generated expands at least over this limited layer thickness.

2. The method according to claim 1, characterized in that a tubular body ( 1 ) is provided, the inner surface ( 101 ) is to be tempered and the plasma from this surface ( 101 ) is limited. 3. The method according to claim 1, characterized in that the body is a plate ( 21 ), one surface ( 121 ) of which is to be tempered and that this plate opposite another plate ( 21 a) is arranged such that between these two Plates an interspace limiting the layer thickness of the plasma is seen before. 4. The method according to claim 1, 2 or 3, characterized in that the plasma is generated by means of an explosion generated by the flow of electrical current of one or more wires through which this current flows ( 2 ). 5. The method according to claim 4, characterized by that the wire material is copper. 6. The method according to claim 4, characterized by that the wire material is silver. 7. The method according to any one of claims 1 to 6, characterized by that the plasma using ionizing radiation is produced. 8. The method according to any one of claims 1 to 7, characterized by that it is applied to a body made of sintered material becomes. 9. The method according to claim 8, characterized net in that as a sintered material Ceramic is selected. 10. A method for producing a molecular sieve membrane, characterized in that

• - that starting from a coarse-pored body by applying a method according to one of claims 1 to 9, a tempered surface layer ( 121 ) with ver comparatively to the coarse-pored structure of the body ( 21 ) is at most still very fine-pored structure,
• - whereby this surface layer ( 121 ) is produced in a thickness between 5 and 50 nm and
• - That this surface layer ( 121 ) with ver into the coarse-pored structure of the body ( 21 ) reaching holes will be seen, whose cross-sectional dimensions correspond to the pre-given sieve size of the molecular sieve membrane.

11. The method according to claim 10, characterized in that the introduction of the holes in the surface layer ( 121 ) by means of irradiation with high-energy particles ( Fig. 4) and subsequent chemical etching, wherein the radiation density ( 34 ) is chosen so that the the radiation generated and corresponding radiation damage, in the surface layer ( 121 ) then existing channels are still laterally separated from each other. 12. molecular sieve membrane ( 30 ), from a coarse-pored body ( 21 ) with an at least in a surface area ( 35 ) tempered according to a method of claims 1 to 9, at least largely dense surface layer ( 121 ), in which according to a method of claims 10 or 11 holes ( 36 ) with cross sections in the range of molecular dimensions forming the molecular sieve and extending through this surface layer ( 121 ) are produced. 13. Molecular sieve membrane according to claim 12, characterized in that the body ( 21 ) is piezoceramic and is provided with electrodes for connecting an electrical excitation voltage.