Methods for producing porous bodies

Abstract

The present invention relates to a method for producing porous bodies, in which a starting material (3) is introduced into a sintering tool comprising a die (1) and a punch (2), and subsequently the starting material (3) is subjected to pressure by the punch (2), wherein a mechanical stop (4) for the punch (2) is provided in or on the die (1), which serves as a limit for the travel of the punch (2). Simultaneously with and / or after the application of pressure, a heat treatment of the starting material (3) in the sintering tool is carried out, so that a porous body with a porosity defined by the application of pressure is produced.

Description

[0001] The present invention relates to a method for producing a porous body.

[0002] When manufacturing porous bodies, it is desirable to precisely control the porosity, as this (co-)determines the properties of the body. It is equally desirable to minimize the use of auxiliary materials or to introduce density gradients into the body.

[0003] In powder metallurgy, the fundamental aim is to produce a compact with the highest possible density using die pressing. If the density needs to be reduced, the pressing pressure is lowered to create compacts that are still manageable. Similarly, various porous structures are generated through molding processes, although the use of additives is unavoidable in these cases. Primarily, organic compounds are used, leading to increased oxygen and carbon content in the sintered body or material. Additionally, the design of tools has traditionally focused on ensuring that the punch and die do not come into contact during the sintering process. For example, CN 107921721 B discloses a method for compacting powder in which a powder compaction die is designed to compact powder between an upper and lower punch to produce a powder compact.However, a disadvantage of this is that no further customization of the tablet's properties is possible.

[0004] The present invention therefore aims to propose a method for producing porous bodies that avoids the aforementioned disadvantage, thus enabling a simple and efficient adaptation of the properties of the pressed material and the component produced therefrom.

[0005] This problem is solved according to the invention by a method according to claim 1. Advantageous embodiments and further developments are described in the dependent claims.

[0006] In a process for producing a porous body, a starting material is placed in a sintering tool comprising a die and a punch, and subsequently the starting material is subjected to pressure by the punch. A mechanical stop for the punch is provided or arranged in or on the die, serving as a limit to the punch's travel. Alternatively or additionally, the mechanical stop can also be located on or within the punch. Simultaneously with and / or after the pressure application, the starting material in the sintering tool undergoes a heat treatment, resulting in a porous body with a porosity defined by the pressure application.

[0007] Limiting the ram stroke also limits the pressure applied, thus precisely adjusting the compression. Since this also defines the porosity of the compact formed from the raw material in the sintering tool, this porosity can be precisely controlled. The inclusion of a mechanical stop offers the possibility of achieving the desired pressure simply and efficiently with a low probability of error. The ram can be designed in two parts, consisting of an upper and a lower ram, with the upper ram typically being moved and its stroke prevented by the mechanical stop. However, it is also possible to design the ram as a single piece, meaning that usually only the ram, essentially the upper ram, is pressed onto the die, which is closed on one side and has an opening on one side to receive the ram.The piston can typically be moved hydraulically by means of a control device, typically uniaxially.

[0008] The starting material can comprise or consist of a composite material, a ceramic, and / or a metal. If a ceramic is used, it is preferably aluminum oxide, zirconium oxide, boron carbide, and / or silicon carbide, or at least comprises these materials. If a metal is used, titanium, iron, nickel, niobium, tungsten, tantalum, aluminum, molybdenum, or an alloy of these metals is preferably used, or the starting material comprises at least these metals. The addition of reinforcing phases or particles is possible, e.g., graphite and / or a metal oxide.

[0009] If the starting material includes or consists of a metal, this metal can be introduced into the sintering tool in the form of sponge powder to create preferred conditions for the formation of the compact.

[0010] It can be stipulated that the starting material is introduced into the sintering tool free of organic additives. The absence of organic additives simplifies the process, especially since the porosity can already be adjusted by limiting the punch travel.

[0011] The porous body typically exhibits a porosity between 10% and 80% by volume; that is, the stop is chosen such that, after completion of the sintering process, the porous body has the aforementioned values. Preferably, a local region with a porosity of less than 5% by volume is formed within a local region with a porosity of more than 10% by volume, or the local region with a porosity of less than 5% by volume surrounds the local region with a porosity of more than 10% by volume.

[0012] It may also be possible to remove the porous body from the sintering tool and subsequently perform an etching step to remove any remaining but undesirable substances and / or to further increase the porosity.

[0013] During the pressurization process, at least one additional element, preferably a metal structure, but particularly preferably a nonwoven fabric, a foam, a wire, and / or a (preferably additively manufactured) preform, can be inserted into the sintering tool and bonded to the porous body. This allows for flexible adaptation to the requirements of the porous body to be produced. Alternatively or additionally, it can be provided that, to form a cavity in the porous body, a mold, for example a mandrel, is inserted into the sintering tool, the starting material is arranged accordingly around the mold, the process is carried out as described, and after the porous body has been produced, the mold is removed from the cavity that has now formed in the porous body.The mold body itself can also serve as a mechanical stop and, for example, be arranged around the perimeter or off-center in the die.

[0014] The starting material is typically introduced into the sintering tool in powder form. The term "powder" here refers to a granular medium consisting of a multitude of solid particles. In particular, the particles forming the powder can have a diameter or width of less than 1 mm. Preferably, the starting material is introduced in at least two powder layers, wherein the two powder layers differ from each other with respect to the particle size and / or particle shape of the respective powders, so that the porous body is preferably formed with a porosity gradient. Thus, the mechanical properties of the porous body can be specifically influenced by layer formation.

[0015] The mechanical stop can be designed so that the introduced or filled starting material achieves a desired compression density when the punch is resting directly on the stop.

[0016] Alternatively or additionally, the die can also be open on one side and box-shaped, with the powder being introduced into an opening in the die and the box subsequently closed with a lid acting as a ram. The lid can be attached to the box with a specific force or torque (usually by screwing it on), thus exerting pressure on the raw material inside the box. The closed box can then be moved through a furnace, in particular a belt furnace, tube furnace, or muffle furnace, for sintering.

[0017] The porous body produced using the described method can be used as an electrode, as a substrate for growing an additional coating, as a heating structure, as a filter material, friction lining and / or as a capillary structure for conveying liquids.

[0018] Exemplary embodiments of the invention are shown in the drawings and are described below with reference to the Fig. 1 to 7 explained. Fig. Figure 1 shows a schematic side view of a sintering tool with a stop integrated into a die; Fig. 2 shows in a Fig. 1 corresponding view shows an embodiment with a stop integrated into a punch; Fig. Figure 3 shows an embodiment for creating a porosity gradient in a view corresponding to the previous figures; Fig. Figure 4 shows, in a view corresponding to the previous figures, a further embodiment for producing a porosity gradient; Fig. Figure 5 shows an embodiment using a molded body in a view corresponding to the previous figures; Fig. Figure 6 shows, in a view corresponding to the previous figures, the Fig. 5 shown embodiment with closed sintering tool and Fig. Figure 7 shows an embodiment with indicated external pressure in a view corresponding to the previous figures.

[0019] In Fig. 1. A powdered starting material 3 is uniformly filled into a die 1 and held there in a recess with a constant fill height and correspondingly constant bulk density. The starting material 3 should be filled into the die 1 with as homogeneous a fill density as possible to ensure homogeneous porosity in the finished state. A mechanical stop 4 made of metal is arranged within the recess. The stop 4 can be integrally formed with the die 1, i.e., integrated into it or bonded to it by a material bond, or it can be inserted into or removed from the die 1. The die 1 is typically made of hot isostatically pressed graphite, while the stop 4 is usually made of the same material as the die 1, but it can also be inserted into the die 1 as an external component and held there.A punch 2, inserted into the recess of the die 1, exerts pressure on the starting material 3, thus compacting it. The travel of the punch 2 is limited by the stop 4, mechanically preventing it from further compressing the starting material 3. The mechanical stop 4 can also be part of the punch 2, i.e., materially bonded to it, or attached to the punch 2 by friction or form-fitting.

[0020] Simultaneously with the insertion of the punch 2 and the application of pressure, the starting material 3 undergoes a heat treatment, i.e., it is heated, so that this sintering process generates a porous body with a porosity defined by the pressure. In further embodiments, it may also be possible to perform the heating only after the pressure is applied, or to perform the heat treatment both during and after the pressure is applied. In each case, the starting material 3 remains in the sintering tool during the heat treatment, i.e., pressure and heat are applied simultaneously. The sintering tool shown, consisting of the die 1 and the punch 2, generally has a control unit that controls the insertion of the punch 2, as well as a heating device, which, for the sake of clarity, is not shown.

[0021] The technical solution therefore consists of the design and use of a sintering tool (preferably for pressure-assisted sintering processes) that has a limit to the punch travel. This limit is implemented directly in the tool by the mechanical stop 4 of the punch 2 against the die 1. Knowing the fill density and the desired compression density, the necessary punch travel can be calculated directly.

[0022] The stop 4, or more precisely the height of the mechanical stop 4, is chosen so that the introduced or filled starting material 3 achieves a desired pressing density when the punch 2 rests directly on the stop 4.

[0023] The starting material 3 is typically a metal such as titanium, iron, nickel, niobium, tungsten, tantalum, or aluminum, which is usually introduced into the sintering tool as a sponge powder with internal porosity (preferably generating capillary forces). In other embodiments, alloys of the aforementioned metals can also be used, or a ceramic can be used as the starting material 3, preferably aluminum oxide, zirconium oxide, boron carbide, or silicon carbide. A preform, for example, made of foam with large pores, can also be inserted into a cavity of the die 1. These pores are subsequently filled with the starting material 3, and everything is sintered as described. The powder used then partially or completely fills the porosity in the inserted element.

[0024] The starting material 3 may also contain additives, in particular organic additives, but is generally introduced into the sintering tool without these additives. During the application of pressure, at least one additional element, such as a metal structure, a nonwoven fabric, a foam, a wire, or a preform, can be inserted into the die 1 and connected to the porous body to be produced, at least by a form-fit or force-fit connection.

[0025] In a further embodiment, the starting material 3 can also be introduced into the die 1 in powder form, with several different powder layers being introduced and sintered on top of each other. The powder layers differ in pairwise adjacent layers at least with regard to particle size and / or particle shape; however, different materials can of course be used for the layers. This allows a density gradient to be generated in the resulting porous body.

[0026] The resulting porous body is removed from the sintering tool after the sintering process is complete and exhibits a final porosity of between 10 and 80 volume percent. Alternatively or additionally, an etching step can be performed to partially or completely remove at least one component in order to increase the final porosity. Through a targeted design of the press tool or a corresponding configuration of the mechanical stop 4, porous (porosity greater than 10 volume percent) and dense (porosity less than 5 volume percent) areas can be produced simultaneously in a single sintering step. The final porosity is generally open, allowing fluids such as gases or liquids to pass through. The combination of porosity and electrical conductivity also permits its use as an electrode material.By inserting a shaped element into the die 1 before the sintering process, which is removed from the porous body after sintering, through-holes or passage volumes can be selectively created. These can serve for subsequent contacting or the insertion of, for example, pipes. This saves on the one hand the starting material 3 and on the other hand minimizes subsequent machining. The shaped element, like the stop 4, can be inserted into the die 1 as a separate component or be metallurgically bonded to it. It is also possible to position the shaped element on the punch 3 or to form it as a blind hole in the punch, typically in the upper punch.

[0027] A porous body with a defined porosity can therefore be sintered, whereby the porous body is formed by a combination of pressing, i.e., applying pressure (uniaxial and / or isostatic pressure) with limitation of the compression path. The mechanical stop 4 can mechanically block the punch 2 and can be realized by forming at least one step and / or at least one edge and / or at least one insert of one or more additional tool elements, which typically have a porosity of less than 5 volume percent. The surfaces of the porous body are directly aligned with the surface of the punch used. The use of auxiliary materials is not mandatory. The body produced in this way can be used as an electrode (anode or...It can serve as a cathode, but it can also be a substrate for growing an additional coating (for example, for water treatment), a heating structure (for example, in apparatus for evaporating liquids and for liquid separation), a filter material where a circumferential sealing surface can be created simultaneously due to the adjustable density gradient, a friction lining for tribological applications, or a capillary structure for conveying liquids (e.g., within a heat pipe or capillary pump). The electrical conductivity of the porous body also allows for heating when used as a resistance heater.

[0028] Fig. 2 shows in a Fig. Figure 1 shows a further embodiment of a sintering tool comprising the die 1 and the punch 2. Recurring features in this and the following figures are designated with identical reference numerals. The die 1 is a box open on one side, and the opening of the die 1 is filled to the edge with the starting material 3, i.e., the starting material 3 is flush with the edge. The punch 2 is provided with a recess that allows the punch 2 to be placed onto the die 1, with a central part of the punch 2 compressing the starting material 3 located in the die 1. The bearing surface of the punch 2 serves as a stop 4, so that, as in the previous embodiment, a compression density with a limited punch travel is achieved. The punch 2 can also be screwed onto the die 1 as a cover, and the die 1 can then be passed through a belt furnace.

[0029] Fig. Figure 3 shows a further embodiment in which the stop 4 is provided on both sides or around the perimeter of the die 1, but the starting material 3 is applied above the stop 4. During sintering, in addition to the central porous body with a porosity greater than 10 volume percent, a circumferential edge 5 of the porous body with a porosity less than 5 volume percent is formed. As in the previously described embodiments, a force or pressure can also act uniaxially here, i.e., from above onto the punch 2 and from below onto the die 1, as shown only schematically in Figure 3. Fig. 3 indicated by the arrows.

[0030] Fig. Figure 4 shows another variant in which the starting material 3 comprises a first starting powder 7 and a second starting powder 6, which are poured one on top of the other into the die and according to the Fig. The two procedures shown will be discussed. Alternatively, the one described in Fig. The procedures described in section 1 can be used to achieve the same result as in [reference to relevant document]. Fig. 3. To form a porosity gradient in the final porous body.

[0031] As in Fig. As shown in a further embodiment 5, a forming element 8, for example a mandrel, can also be inserted into the die 1 and subsequently the starting material is poured in, so that the starting material surrounds the mandrel 8. In this case, the punch 2 has a corresponding recess for receiving the forming element. As shown in Fig. As shown in Figure 6, the sintering tool can then be closed, and a through-hole is formed in the porous body after sintering and removal of the molded part 8. Typically, a hot press is used for this, which can of course also be used in the other embodiments.

[0032] Finally, as in Fig. As shown in section 7, in principle classic sintering furnaces such as a belt furnace, a tube furnace or a muffle furnace can also be used, by adjusting them according to the instructions in Fig. In the process shown in step 2, a closed box is produced and optionally fitted with a support weight 9. Alternatively or additionally to the support weight, a screw connection or a tension strap can also fix the punch 2 and the die 1. As before, the porous body is obtained after sintering and demolding.

[0033] However, the porous body can also be provided with further coatings on its inner and / or outer surface and thus be used as a carrier for functional layers and / or catalysts.

[0034] Features disclosed only in the exemplary embodiments can also be combined and claimed individually. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] CN 107921721 B

[0003]

Claims

[1] Method for producing a porous body wherein a starting material (3) is introduced into a sintering tool comprising a die (1) and a punch (2), and subsequently the starting material (3) is subjected to pressure by the punch (2), wherein a mechanical stop (4) for the punch (2) is provided in or on the die (1), which serves as a limit for a path of the punch (2), wherein, simultaneously with and / or after the application of pressure, a temperature treatment of the starting material (3) located in the sintering tool is carried out, so that a porous body with porosity defined by the application of pressure is produced. [2] Method according to claim 1, characterized by, that the starting material (3) comprises a composite material, preferably a composite material with graphite or a metal oxide as a reinforcing phase, a ceramic, preferably aluminium oxide, zirconium oxide, boron carbide and / or silicon carbide, and / or a metal, preferably titanium, iron, nickel, niobium, tungsten, tantalum, aluminium, molybdenum and their alloys. [3] Method according to claim 2, characterized by , that if the starting material (3) comprises a metal, the metal is introduced into the sintering tool in the form of sponge powder. [4] Method according to any one of the preceding claims, characterized by , that the starting material (3) is introduced into the sintering tool free of organic additives. [5] Method according to any one of the preceding claims, characterized bythat the porous body has a porosity between 10 volume percent and 80 volume percent, wherein preferably a local area with a porosity of less than 5 volume percent is formed within a local area with a porosity of more than 10 volume percent, or the local area with a porosity of less than 5 volume percent surrounds the local area with a porosity of more than 10 volume percent. [6] Method according to any one of the preceding claims, characterized by , that the porous body is removed from the sintering tool and an etching step is performed. [7] Method according to any one of the preceding claims, characterized by, that during the application of pressure at least one additional element, preferably a metal structure, particularly preferably a nonwoven, a foam, a wire and / or a preform, is inserted into the sintering tool and connected to the porous body and / or a shaped body is introduced into the sintering tool to form a cavity and is removed from the cavity of the porous body after the porous body has been produced. [8] Method according to any one of the preceding claims, characterized by , that the starting material (3) is introduced into the sintering tool in powder form, preferably at least two powder layers are introduced which differ from each other with respect to particle size and / or particle shape, so that the porous body is particularly preferably formed with a porosity gradient. [9] Sintering tool for carrying out a method according to any of the preceding claims, characterized by, that the mechanical stop (4) is designed such that the introduced starting material (3) achieves a desired compression density when the punch (2) rests on the stop (4). [10] Use of the porous body produced by a method according to one of claims 1-9 as an electrode, as a substrate for growing an additional coating, as a heating structure, as a filter material, as a friction lining and / or as a capillary structure for conveying liquids.

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