Sintering device and sintering method

Abstract

The invention relates to a sintering device and a sintering method. The sintering device (10) comprises an electrically conductive first component (11), an electrically conductive second component (12) and at least one electrically conductive surface element (20, 20') for heating a green body (14) to be sintered. The first component (11) and the second component (12) are movable relative to each other and / or to the surface element (20, 20') such that an electric circuit with the first component (11), the surface element (20, 20') and the second component (12) can be closed by the relative movement. In this way, rapid sintering processes are possible on an industrial scale. The device can be integrated in a particularly simple manner into existing sintering plants, for example FAST / SPS sintering plants.

Description

[Technical field]

[0001] The present invention relates to a sintering apparatus and a sintering method.

[0002] Sintering is a method for processing materials. In this case, a compact of the material is heated and optionally pressurized, whereby the compact is compacted by a diffusion process. Sintering is carried out at an elevated temperature, but below the melting temperature of the main components, so that the shape of the workpiece is retained during sintering. Shrinkage of the workpiece occurs when the starting material is compressed. The total surface area and interfacial energy are reduced. As the sintering time increases, shrinkage is superimposed by grain growth. Again, the reduction in interfacial energy is the main driving force. Sintering results in a solid workpiece, and properties such as hardness, strength, and thermal conductivity can be influenced by appropriate process parameters.

[0003] Ceramic and metal components can be manufactured by sintering, which have multiple applications with regard to their properties such as hardness, strength, wear resistance, heat resistance, thermal conductivity, and electrical conductivity. The sintering process is often preceded by molding, in which a green body is manufactured from the starting material, which is usually in powder form. The green body is a compact of ceramic and / or metal powders in the unsintered state. During sintering, the starting material is usually exposed to temperatures above 700° C. for metal powders and above 1000° C. for ceramic powders. Due to the high temperatures involved, the method is technically complex and energy intensive. With long hold times at the sintering temperature, excessive grain growth may result in undesirable properties of the manufactured components. In addition, the long duration of the method, which usually ranges from one hour to several days, is a major drawback.

[0004] To reduce the duration, electric field assisted sintering was developed, where the heating is done by electric current. In this method, also known as field activated sintering, "field assisted sintering technology" (FAST) or "spark plasma sintering" (SPS), an electric current (constant direct current, pulsed direct current or alternating current) is passed through a conductive tool (= a press mold consisting of an upper punch, a lower punch and a die, all parts made of a conductive material such as graphite). The tool is heated by the Joule effect (= resistive heating). In FAST / SPS, a 10% reduction in the powder is usual and is higher compared to conventional sintering. 2 Heating rates in the °C / min range result from direct contact between the tool and the powder by thermal conduction. If electrically conductive powders are sintered, additional direct heating of the powder occurs by the flow of electric current through the powder, further increasing the heating rate and the uniformity of the temperature distribution. In addition, uniaxial pressure is applied to the powder via the hydraulically movable punch system of the FAST / SPS system, and sintering is usually carried out under inert gas or vacuum.

[0005] A method has been developed that allows sintering to be performed on a laboratory scale with a significantly reduced duration. This is called "Ultrafast High-Temperature Sintering" (UHS) and is described in the publications "A general method to synthesize and sinter bulk ceramics in seconds" by Wang et al., Science 368, 521-526 (2020) and "Tailoring grain growth and densification toward a high-performance solid-state electrolyte membrane" by Hong et al., Materials Today, Volume 42, Jan-Feb 2021, pp 41-48. The UHS method uses resistively induced heating of a carbon mat to generate heat and transfer it particularly quickly to the green body to be sintered. However, the method has only been used on a laboratory scale so far. So far, only samples with an area size of 6 mm have been sintered.

[0006] The present invention is based on the problem of providing an apparatus, a sintering plant and a method which allows rapid sintering on a technical scale. In particular, quasi-continuous or continuous sintering according to the UHS principle should be made possible.

[0007] This problem is solved by an apparatus according to claim 1 and by a sintering plant and a method according to the additional claims. Advantageous embodiments are given in the dependent claims.

[0008] To solve this problem, a sintering device is used. The device comprises an electrically conductive first component, an electrically conductive second component and at least one electrically conductive surface element for heating the green body to be sintered. The first component and the second component are movable relative to each other and / or relative to the surface element, so that an electric circuit can be closed by the relative movement. The electric circuit comprises the first component, the surface element and the second component.

[0009] The current heats the conductive surface elements to high temperatures very quickly, so that heat is available to the green body very quickly. In this way, sintering is possible in a very short time, typically less than a minute.

[0010] The relative movement allows the green bodies to be sintered to be brought quickly, precisely and reproducibly to the sintering position. The sintering position is the position where the green bodies are located during sintering. In this way, a (large-scale) technical implementation of the UHS process is possible. A relative movement can be performed to position the green bodies between the components. The closing of the circuit for heating the surface elements is thus performed with particularly little technical effort.

[0011] In other words, the two components transport the green body together with at least one surface element. In this case, the circuit is closed as soon as the surface element contacts the components. The surface element is heated as soon as it is conductively placed between the two components.

[0012] The first and second components are electrically conductive. In particular, the first and / or second components are made of graphite or metal. The components are sometimes called electrodes. In particular, the first and / or second components have contact surfaces for contacting surface elements. In particular, the first and / or second components have an electrical connection area to which an electrical conductor can be connected. The electrical connection area is not particularly located on the contact surface.

[0013] The electrically conductive surface element heats up very quickly due to its resistance. The generated heat is transferred rapidly, in particular by thermal radiation, to the green body in order to sinter the green body. Preferably, the surface element is in direct contact with the green body. In this case, the surface of the green body is in contact with the surface of the surface element. In this way, heat can also be transferred particularly quickly and with low losses by direct thermal conduction. However, it is also possible that a small distance remains between the surface element and the green body.

[0014] A surface element is an element whose average height is small compared to its length and width. In particular, the average height is less than 1% of the length and / or width, preferably less than 0.1% of the length and / or width. The thickness of the surface element may be between 0.1 mm and 25 mm, in particular between 1 mm and 10 mm, preferably between 2 mm and 6 mm. These thicknesses have been found to be particularly useful, since if the thickness is too low, high temperatures cannot be kept stable and very high currents are required for heating due to the small volume of the surface element. On the other hand, if the thickness is too high, the heating and cooling rates are reduced. The surface element may have a length resistivity of between 0.5 Ω·mm and 20 Ω·mm, for example between 1 Ω·mm and 12 Ω·mm, in particular between 2 Ω·mm and 5 Ω·mm, preferably between 3 Ω·mm and 4 Ω·mm.

[0015] In particular, the surface element is disposed or positionable between the first and second components such that when a current is applied to the first and second components, current flows through the surface element, heating it. Typically, at least one surface element is arranged to be resistively heated when a circuit is closed.

[0016] In one embodiment, the surface elements comprise or consist of carbon. The surface elements typically comprise more than 50% carbon, preferably more than 95% carbon. In one configuration, the surface elements comprise or consist of carbon fibers. In one configuration, the surface elements comprise or consist of carbon nanotubes.

[0017] Alternative materials for the surface elements can include electrically conductive high temperature ceramics, in which case the surface elements can include or consist of silicon carbide, silicon nitride, tungsten carbide, titanium carbide, or mixtures thereof.

[0018] In one configuration, the surface element is mechanically flexible. This means that the surface element can be flexibly bent to be placed at an angle of 90° over a length of up to 5 mm. This improves the heat input into irregular or differently shaped green bodies. In an alternative configuration, the surface element is rigid. In this case, the surface element is specifically adapted to the geometry of the sample. For example, the surface element can have a cavity or a contact side for contacting the green body, the contour of which is adapted to the contour of the green body. This can ensure a particularly reproducible process, especially in the case of large volumes.

[0019] In one configuration, the conductive surface element is a felt, paper, nonwoven fabric, or fabric. It can be, for example, a carbon felt, i.e., a felt that contains or consists of carbon, a carbon paper, i.e., a paper that contains or consists of carbon, a nonwoven fabric of carbon fibers, or a woven fabric of carbon fibers. Like a nonwoven fabric, a felt is a woven structure made of irregular fibrous material. In contrast to a nonwoven fabric, the fibers of a felt are mechanically reinforced and therefore typically cannot be separated from one another or can only be separated with difficulty. In one configuration, the fibers are partially or entirely carbon fibers.

[0020] Carbon is suitable for rapid heating and is particularly heat resistant. This requires that the oxygen partial pressure be low enough so that the carbon does not oxidize or burn. For example, the device can be constructed so that the surface elements can be heated in an inert gas atmosphere and / or in a vacuum.

[0021] The electric circuit can be closed by the relative movement. This means that the relative movement brings about a conductive contact such that an electric current can then flow through the first component, the at least one surface element and the second component. In particular, the electric circuit can be selectively opened or closed by the relative movement. It is not necessary to close the circuit and allow the current to flow automatically. Of course, the circuit may comprise and be closed by an additional switch, for example as part of a suitable control of the device. In particular, the circuit further comprises a suitable contact structure and a power source or a connector for connecting a power source.

[0022] In one configuration, the relative movement occurs between the first component and the second component. For example, the relative movement may be from an open position of the device to a closed position of the device. In this case, the two components move towards each other. This is done especially after positioning the green body at the sintering position and / or the surface element. The relative movement can be achieved by moving the first component, the second component, or both components.

[0023] In another configuration, the relative movement takes place between the two components on the one hand and the surface element on the other hand, where the first component and the second component can move in particular uniformly together.

[0024] In one embodiment, the apparatus is configured for integration into a FAST / SPS sintering plant and / or a continuous furnace. In particular, the first component and the second component are configured such that they can be mechanically and electrically connected to an existing punch.

[0025] In one embodiment, a first region of the surface element is conductively connectable or connected to a first component and / or a second region of the surface element is conductively connectable or connected to a second component.

[0026] The first region is different from the second region, in particular the two regions are located at different length and / or width positions of the surface element and are not located on opposite sides of the same length and width position of the surface element.

[0027] In particular, a third region is located between the first and second regions, which typically heats up most quickly and / or the most when the circuit is closed. In particular, the third region is mechanically attached only through the first and second regions. The third region is typically used for heat generation and heat transfer to the green body.

[0028] The first region of the surface element may be connected or connectable, directly or indirectly, to a first component, and the second region of the surface element may be connected or connectable, directly or indirectly, to a second component.

[0029] In particular, the first region is electrically insulated from the second component. This can be achieved, for example, by a sufficient distance between the first region and the second component and / or by an electrical insulator located between the first region and the second component. In particular, the second region is electrically insulated from the first component. This can be achieved, for example, by a sufficient distance between the second region and the first component and / or by an electrical insulator located between the second region and the first component. In this way, the current is forced to flow through the surface element. This prevents short circuits bypassing the surface element.

[0030] In one configuration, there are two surface elements arranged one above the other, and the green body is placed between the surface elements. In the areas adjacent to the green body where the surface elements overlap, they are in conductive contact with each other. Thus, the first area of ​​the first surface element is conductively connected to the first area of ​​the second surface element, and the second area of ​​the first surface element is conductively connected to the second area of ​​the second surface element. Here, the first area of ​​the first surface element may contact the first component. Indirectly, the first area of ​​the second surface element is also conductively connected to the first component. Thus, the second area of ​​the second surface element may contact the second component. Indirectly, the second area of ​​the first surface element is also conductively connected to the second component. As above, the first area of ​​the second surface element is electrically insulated from the second component, and the second area of ​​the first surface element is electrically insulated from the first component.

[0031] In one embodiment, one side of the sintering location of the green body is defined by a surface element and / or the other side of the sintering location of the green body is bounded by a counter element.

[0032] In particular, the sintering position is or can be located between the first component and the second component. The sintering position may also be referred to as a receiving space for receiving the green body for sintering. The green body is located in the sintering position during sintering.

[0033] In particular, in this embodiment, the device comprises only one electrically conductive surface element. In this embodiment, the green body is heated from only one side. In particular, this can be used to sinter a coating applied to a workpiece. Here, the coating is a green body in the sense of the present invention. In this case, the heat input from one side may be sufficient and / or even necessary to protect the substrate on which the coating is located from excessive thermal effects.

[0034] In particular, the two sides are opposite sides. For example, the sintering position is bounded from below by the surface element and from above by the counter element, or vice versa. The counter element serves to limit the sintering position. For example, the counter element can hold the green body in a fixed position during sintering, serve to transfer the green body to the sintering position and / or serve to transfer the sintered body from the sintering position. The counter element may have the same dimensions and / or the same shape as the surface element, or may be different. The counter element may be stationary, for example attached to one of the components, or may be movably arranged.

[0035] In particular, the counter element is heat-resistant up to more than 1000° C., preferably up to more than 1600° C. Heat resistance can refer to an inert gas or vacuum atmosphere. In particular, the counter element is inert so as not to undergo any chemical reaction with the green body, other objects and / or the atmosphere at said temperatures. In particular, the counter element is non-conductive. The counter element may be manufactured from a ceramic material. The counter element is typically also a flat element. For example, the counter element may be a non-woven fabric, a felt, a paper, a woven fabric and / or a tape. The counter element may be circular, quasi-endless or limited in area. The counter element may be flexible or flexibly arranged to compensate for the shrinkage of the green body during sintering and / or to reduce mechanical stress on the green body. The counter element may be of the same material as the surface element.

[0036] In one embodiment, the device comprises two electrically conductive surface elements, each surface element defining one side of the sintering position of the green body. In other words, the at least one electrically conductive surface element comprises two surface elements. In particular, the two sides are opposite sides in this embodiment as well. For example, the sintering position is defined from above and below by the respective surface elements. The surface elements may be configured similarly.

[0037] In particular, the two surface elements are arranged parallel to one another and / or one above the other. Thus, the two first regions of the surface elements may overlap one another conductively and the two second regions of the surface elements may overlap one another conductively. The sintering position may be formed between the two third regions of the surface elements. In this embodiment, the heating is carried out from both sides. In this way, the green body can be heated particularly quickly and uniformly.

[0038] In one embodiment, the device is designed to allow relative movement of the first component and the second component along a common axis. In particular, the sintering position of the green body is accessible in the open position. In particular, in the closed position, electrical contact is established between the first component, the surface element, and the second component.

[0039] In other words, the two components can move towards and away from each other. If the sintering location between the first and second components is accessible, the sintered component can be removed therefrom and / or the green body to be sintered can be placed therein. In this case, the circuit is not closed. If the circuit is closed, heating and therefore sintering takes place. The sintering location is then inaccessible.

[0040] The electrical contact between the first component, the surface element, and the second component may be such that, on the one hand, electrical contact exists between the first component and the surface element, and, on the other hand, electrical contact exists between the surface element and the second component. In particular, the axis is a linear axis, in which case the movement is a linear movement.

[0041] In one embodiment, a quasi-continuous process can be performed in which the supply and removal of the green body and / or body to be sintered is continuous, with the sintering itself taking place during interruptions in the supply and removal. Such a cycle can be repeated many times and includes, for example, opening, removing the sintered body, placing the green body, and closing for sintering.

[0042] In one configuration, the surface element is attached to the first component or the second component. Thus, when the respective components move, the surface element also moves. The sintering location may be adjacent to the surface element. The sintering location may be located between two surface elements. The second surface element may be attached to the respective other component. Thus, in the open position, the sintering location located between the surface elements is accessible.

[0043] In one configuration, the apparatus includes a drive for the first component and / or the second component, whereby relative movement can be effected. In particular, the drive is configured to apply a compressive force along an axis between the first component and the second component.

[0044] In one embodiment, the apparatus comprises a first transport device for moving the surface element or the counter element in a substantially linear manner, so that the green body and / or the sintered body arranged on the surface element or the counter element can be moved relative to the first component and the second component.

[0045] In particular, the first conveying device serves to convey the green bodies to the sintering position and / or to convey the sintered bodies away from the sintering position. In particular, the first conveying device is configured such that the surface elements and / or the counter elements move in a substantially horizontal direction. This allows the respective bodies to be easily placed. In one configuration, the surface elements are designed as an endless belt which is moved by the first conveying device. In the area between the outer rollers, this belt performs a partially linear movement.

[0046] This embodiment allows for a technically uncomplicated quasi-continuous sintering process in which the green and sintered bodies can be transported by a transport device. If the relative movement is a movement along an axis, the movement of the surface element and / or the counter element may in particular be aligned perpendicular to the direction of the relative movement.

[0047] In a further embodiment, the surface element is provided in wound-up form on a roll. The first transport device is configured to hold and transport the partially unwound surface element.

[0048] In other words, the surface element is provided in the form of a quasi-endless roll-up on a roll so that it is used only once. It has been found that, depending on the green body to be sintered and the selected process parameters, wear or damage to the surface element can occur during sintering. In order to prevent quality losses in the subsequent sintering process, the surface element is used only once in this embodiment. This allows a particularly high and reproducible quality of the sintering process.

[0049] In particular, the first conveying device comprises a rotatable roller with the rolled up surface element. The first conveying device in particular further comprises a second roller spaced apart from the first conveying device, which holds the surface element on the other side. The first conveying device in particular further comprises a drive for the movement of the surface element. The drive can drive the roller with the rolled up surface element and / or the second roller and / or engage the surface element at another position. The sintering position is typically located between the roller with the rolled up surface element and the second roller. In particular, the device further comprises means for discharging and / or stacking, for example rolling up, the used surface element. The used surface element can be collected in this way.

[0050] In one embodiment, the surface element and / or the counter element are designed as an endless belt. The first transport device can comprise two transport rollers for holding and moving the endless belt.

[0051] In other words, the surface elements are designed as conveyor belts, which allows a particularly simple transport of the bodies or sintered bodies to be sintered, which allows a particularly simple quasi-continuous process.

[0052] Conveyor rollers are rotatably mounted rollers configured to hold and move the endless belt. Conveyor rollers may be driven or undriven.

[0053] In a further embodiment, the apparatus comprises a second conveying device for moving the further surface or counter element substantially linearly, so that the green body and / or the sintered body arranged on the surface or counter element can be covered with the further surface or counter element when moved relative to the first and second components.

[0054] In this case, the counter element is a flat element. In this embodiment, the sintering position is therefore defined from below and above by a flat element. Among the flat elements there is at least one surface element and possibly one counter element.

[0055] In one configuration, a first conveying device and a second conveying device are provided. At least one of the conveying devices serves to move the surface element. The other conveying device may also serve to move the surface element or to move the counter element. In particular, the movement by the first conveying device and the movement by the second conveying device are at least partially performed in parallel. The first component and the second component are typically arranged in parallel parts. This embodiment allows for a particularly operationally reliable quasi-continuous method, in which heating can be performed on one side or on both sides.

[0056] The further surface element may also be provided in wound-up form on a roll. The second transport device may be configured to hold and transport the partially unwound further surface element.

[0057] In a further embodiment, the first and second components are configured as rotatably mounted roller-shaped electrodes, which are in particular configured to bring the surface elements into conductive contact with their circumferential surfaces when the surface elements are moved relative to the first and second components, the movement may be essentially linear. This embodiment allows, in a technically simple manner, a continuous process in which the feeding of the green body, the sintering and the removal of the sintered body are carried out without interruption.

[0058] In one embodiment, the device comprises a first counter component facing the first component and / or a second counter component facing the second component. The first component and the first counter component may be arranged to apply a compressive force from opposite sides to a first region of the surface element. The second component and the second counter component may be arranged to apply a compressive force from opposite sides to a second region of the surface element.

[0059] The first and second components may be arranged on the same side of the surface element. This allows a particularly space-saving arrangement. In particular, the first and / or second counter components are non-conductive and / or heat-resistant. The application of force allows for improved support during sintering and / or more accurate transfer of the green and sintered bodies. In addition, the application of force can improve the sintering process. For example, sintering may be carried out under pressure.

[0060] In one configuration, the device is configured to allow relative movement between the first component and the first counter component along a first axis. In one configuration, the device is configured to allow relative movement between the second component and the second counter component along a second axis. The first and second axes may be different axes or may be parallel. In one configuration, the first and second components are movable together relative to the surface element, the first and / or second counter component. In one configuration, the first and second counter components are movable together relative to the surface element, the first and / or second component. For example, the two components may be mechanically connected to each other.

[0061] In one configuration, the first component and the first counter component are configured as rollers and are arranged to rotate relative to each other. In particular, they are arranged so that together they allow the transfer and / or application of a force to a first area of ​​a surface element arranged between them. In one configuration, the second component and the second counter component are configured as rollers and are arranged to rotate relative to each other. In particular, they are arranged so that together they allow the transfer and / or application of a force to a second area of ​​a surface element arranged between them. In particular, the first component and the second component are arranged to rotate around the same axis of rotation. In particular, the first counter component and the second counter component are arranged to rotate around the same axis of rotation. In particular, the axis of rotation of the components is parallel to the axis of rotation of the counter component.

[0062] In a further embodiment, the apparatus comprises a manipulator for at least partially moving the green body to be sintered to the sintering position and / or for at least partially moving the sintered body away from the sintering position.

[0063] The manipulator is configured to move the green body along at least a portion of a path to the sintering position and / or to move the sintered body along at least a portion of a path away from the sintering position. In particular, the manipulator is configured to place the green body on the surface element and / or the counter element and / or to remove the sintered body from the surface element and / or the counter element.

[0064] In one configuration, the manipulator is configured to directly access the sintering location, which allows the green body to be positioned at the sintering location and / or the sintered body to be removed, particularly when the relative movement between the first and second components is along an axis.

[0065] In another configuration, the manipulator is configured to access another location of the surface element and / or the counter element that is not between the first and second components. In particular, the first transport device can function to transport the green body from another location to the sintering location and / or to transport the sintered body from the sintering location to another location.

[0066] This embodiment allows for a fully automated process. In this way, a high throughput can be achieved without compromising quality. Control based on detected process parameters and flexible reactions to deviations are also possible in this way.

[0067] In a further embodiment, the apparatus comprises a pyrometer for determining the temperature of the surface element, in particular a continuous cavity is disposed in the first component or the second component, and an observation axis extends between the pyrometer and the surface element through the first component or the second component.

[0068] Here, the temperature of the surface element and thus the central parameter of the sintering process can be continuously detected. This allows for a precise control of the sintering process. The reproducibility of the quality and properties of the sintered body is further improved. In addition, the automatic control of the process is facilitated.

[0069] A further aspect of the invention is a sintering plant, in particular a continuous furnace or FAST / SPS sintering plant. The sintering plant comprises a sintering device according to the invention. In particular, the sintering plant comprises a power source for applying an electric current to an electric circuit. The electric circuit comprises a first component, a surface element and a second component. All the features, configurations and advantages of the device described above also apply to the sintering plant and vice versa.

[0070] A FAST / SPS sintering plant is an apparatus suitable for carrying out the FAST / SPS method ("electric field assisted sintering technology" or "spark plasma sintering"). In the case of a FAST / SPS sintering plant, the apparatus in particular has an upper punch and a lower punch designed as electrical contacts. These punches are usually additionally configured for uniaxial pressing of the green body, where in the conventional manner, two geometrically simple components (cylinders or rectangular parallelepipeds) enclosing the powder and a die are placed between the punches. Instead, in the apparatus according to the invention, a die is placed between the punches here. The punches are also designed here as electrical contacts. For example, the first and second components can be positioned and, if necessary, fixed between the punches so that the upper punch is in electrical contact with the first component and the lower punch is in electrical contact with the second component and vice versa. A power supply is then connected or connectable to the two punches. The movement of the punches is then performed together with the movement of the corresponding components. The punches of the FAST / SPS sintering plant in particular consist of a heat-resistant metal alloy and / or graphite. The punches are usually water-cooled. In particular, the sintering plant has a control for controlling and / or regulating the flow of electric current. In particular, at least one of the punches is arranged to be hydraulically movable.

[0071] A further aspect of the invention is a method for sintering a green body. It comprises providing an apparatus having an electrically conductive first component, an electrically conductive second component and at least one electrically conductive surface element. In particular, the apparatus is an apparatus according to the invention. The method may further comprise placing the green body to be sintered on the surface element. The method may further comprise carrying out a relative movement between the first component and the second component, or between the first component and the second component on the one hand and the surface element on the other hand. The method may further comprise applying an electric current to the first component, the at least one surface element and the second component. The electric current may heat the surface element so that the green body is sintered.

[0072] All features, configurations and advantages of the apparatus described in the first place also apply to the method, and vice versa. The placement of the green body and the implementation of the relative movement may be performed continuously or simultaneously in at least a time section. The implementation of the relative movement and the application of the electric current may be performed continuously or simultaneously in at least a time section. The green body is in particular placed so that it is in contact with the surface element. For example, the green body is placed on the surface element. The green body may be placed between two surface elements or two parts of a surface element. The green body may be placed between a surface element and a counter element. The surface element may be circular, quasi-endless or a part with a limited area.

[0073] In one embodiment, the current is applied for a period of less than 5 minutes, preferably less than 2 minutes, and in one embodiment less than 1 minute or less than 30 seconds. In one embodiment, the method is carried out such that complete compaction is achieved in a period of less than 5 minutes, preferably less than 2 minutes, and in one embodiment less than 1 minute or less than 30 seconds. In one embodiment, the surface elements, the surfaces of the green body facing the surface elements, and / or the entire green body are compacted in a period of less than 10 minutes, preferably less than 2 minutes, and in one embodiment less than 1 minute or less than 30 seconds. 3 ~10 4 With a temperature ramp rate of °C / min and / or to a temperature of more than 1500°C, in particular more than 2000°C, in one configuration more than 2500°C. The maximum temperature may be 3000°C.

[0074] In one configuration, near-net-shape bodies, particularly with complex geometric shapes, are produced as sintered bodies. In one configuration, components for electrochemical cells (e.g., batteries, fuel cells, and / or electrolysis cells) are produced as sintered bodies. In one configuration, the sintered bodies produced are porous bodies, such as porous electrodes. In one configuration, the sintered bodies produced are bodies that are impermeable to gases and / or liquids. In one configuration, the sintered bodies produced are multiphase composites and / or ion-conducting electrolytes, such as single-phase membrane layers. In one configuration, the green bodies are produced using generative manufacturing processes, such as 3D printing, or screen printing, dip coating, spin coating, inkjet printing, electrophoresis, or thermal spraying. In particular, the green bodies are pre-compressed green bodies, such as pre-formed powder compacts, and / or essentially stable green bodies. Alternatively, the green bodies may be loose powders placed in a combustible mold, for example made from cardboard, so that the mold burns during sintering. Alternatively, the green bodies may be loose powders placed in a mold to be sintered. The mold is subsequently separated (lost-mold principle). In one embodiment, the lost mold consists of graphite. In one embodiment, the green body is a coating arranged on a substrate. In this case, heating is carried out in particular only from the side of the coating. In one embodiment, the green body is, for example, a stack of film cast layers, part of a stack, a multi-layer material composite or part of a multi-layer material composite. Due to the short sintering times and the targeted heat input, undesired disturbances due, for example, to interface reactions or material decomposition are minimized or avoided completely.

[0075] In one embodiment of the method, the surface element and / or the counter element are moved in a substantially linear manner. The green body may be placed on the surface element or the counter element and may be moved together with the first component and the second component. After the completion of the substantially linear movement, the relative movement may be performed. In this case, the first component and / or the second component may, for example, be moved in a substantially linear manner. Thus, a circuit through the first component, the surface element and the second component may be closed. In particular, the circuit passes through the first component, a first area of ​​the surface element contacting the first component, a third area of ​​the surface element, a second area of ​​the surface element contacting the second component and the second component in this order or in the reverse order. The surface element or the counter element may be moved in a substantially linear manner after the sintering is completed. The sintered body may be placed on the surface element or the counter element and may be moved together with the surface element or the counter element with respect to the first component and the second component.

[0076] In other words, movement to the sintering position occurs first and stops when the green body reaches the sintering position. The circuit is closed and sintering takes place. The sintered body is then moved away from the sintering position. Sintering can take place in up to 30 seconds, up to 20 seconds, or within about 10 seconds. Immediately after, linear movement of the surface element and / or counter element can continue.

[0077] The essentially linear (rectilinear) movement of the surface element or counter element, which moves the sintered body away from the first and second components, can simultaneously move the subsequent green body relative to the first and second components and bring it into the sintering position. In this way, a quasi-continuous process is performed, which allows a high throughput at the same time as a precise setting of reproducible sintering parameters. The quality of the sintered body is therefore particularly high and constant. In particular, the surface element and / or counter element in this embodiment is present as a circular belt or quasi-endless.

[0078] In another embodiment, the first and second components are each designed as a rotatably mounted roller-shaped electrode. The relative movement is performed as a substantially linear movement of the surface element. The first and second components are conductively contacted with the surface element during the relative movement with their respective circumferential surfaces. In this way, a continuous process is provided that allows a high throughput with low technical effort.

[0079] In the following, exemplary embodiments of the invention are also described in more detail with reference to the figures. The features of the exemplary embodiments can be combined individually or in multiples with the claimed subject matter, unless otherwise specified. The claimed area of ​​protection is not limited to the exemplary embodiments. [Brief description of the drawings]

[0080] The diagram is as follows: [Figure 1] FIG. 1 shows a first exemplary embodiment of a sintering apparatus. [Diagram 2] FIG. 2 shows a second exemplary embodiment of a sintering apparatus. [Diagram 3] FIG. 13 shows a third exemplary embodiment of a sintering apparatus. [Figure 4] FIG. 4 illustrates a fourth exemplary embodiment of a sintering apparatus. [Diagram 5] FIG. 5 illustrates a fifth exemplary embodiment of a sintering apparatus. [Figure 6] FIG. 13 illustrates a sixth exemplary embodiment of a sintering apparatus. [Figure 7] FIG. 13 illustrates a seventh exemplary embodiment of a sintering apparatus.

[0081] Figures 1 and 2 each show a cross section of a sintering device 10. Both devices 10 comprise a first component 11 shown at the top and a second component 12 shown at the bottom. The components 11, 12 are rectangular shaped and made of graphite. In the middle region, both the first component 11 and the second component 12 have a cavity inside. In other words, the components 11, 12 are U-shaped.

[0082] In the exemplary embodiment shown in FIG. 1, a surface element 20 made of carbon felt is attached to the first component 11. There is also a surface element 20' made of carbon felt attached to the second component 12. The surface elements 20, 20' are each partially guided around a leg of a U-shape and fastened laterally with a screw. In the middle, they do not rest on the material of the respective component 11, 12. The two surface elements 20, 20' have the same shape and design. A closed position is shown in which the surface elements 20, 20' lie flat against each other and form a sintering position 16 between them, in which the green body 14 to be sintered is received. A first area 21 of each surface element 20, 20' is clamped on the side shown on the left between the mutually facing legs of the U of the two components 11, 12. A second area 22 of each surface element 20, 20' is clamped on the side shown on the right between the opposite legs of the U of the two components 11, 12. Between them is a third region 23 of each surface element which is in direct mechanical contact with the green body 14 .

[0083] Relative movement of the two components 11, 12 apart along a vertical axis common to the two components 11, 12 brings about an open position (not shown) in which the sintering station 16 is freely accessible so that the sintered body can be removed and / or the green body 14 can be introduced.

[0084] In the closed position 19 shown, electrical contact is established between the first component 11, the two surface elements 20, 20' and the second component 12. In the illustrated exemplary embodiment, the first component serves as a positive electrode and the second component 12 serves as a negative electrode. In particular, a direct or pulsed direct current is applied. However, this does not exclude the use of an alternating current. According to the technical direction of the current, the current therefore flows from the first component 11 to the second component 12. On the right side, an electrical insulator 62 is arranged between the first component 11 and the surface element 20. A conductor 63 is arranged between the surface element 20' and the second component 12. Similarly, a conductor 63 is arranged on the left side between the first component 11 and the surface element 20. In contrast, the electrical insulator 62 is arranged between the surface element 20' and the second component 12. The insulator 62 and / or the conductor 63 are in particular cap-shaped and rest on the respective legs of the U. Conductor 63 may be made from the same material as components 11, 12, e.g., graphite, and serves not only to make electrical contact but also to protect components 11, 12 from wear. The cap-shaped design allows for easy replacement if worn. In one configuration, conductor 63 may be omitted.

[0085] Thus, the current flows from the first component 11 on the left side through the conductor 63 to the first region 21 of the top element 20. On the left side, the insulator 62 prevents the current from flowing to the top element 20. In this region, the current also flows to the first region 21 of the bottom element 20', since the two surface elements 20, 20' are adjacent to each other. However, the insulator 62 prevents the current from flowing directly to the second component 12. Thus, the current flows from left to right in both surface elements 20, 20' through the third region 23 of each surface element 20, 20' and into the respective second region 22. Heating is greatest in the third region 23, since here there is no contact with any of the components 11, 12 and therefore there is no significant heat flow from the surface elements 20, 20' or the green body 14. The current flows to the right from the second region 22 through the conductor 63 to the second component 12, thus ensuring that the current flows completely through the surface element 20 or surface element 20' to generate thermal energy for sintering.

[0086] The fastening of the surface elements 20, 20' in the region of the electrical insulator 62 is realized by means of a non-conductive screw 61, in particular made of boron nitride. The screw 61 extends through the respective insulator 62 into the material of the respective component 11, 12. The screw 61 can also serve to fasten the respective cap to the respective component. Alternatively, it is also possible to fasten the surface elements only to the insulator 62. In this case, conventional screws can be used. The surface elements 20, 20' are fastened in the region of the electrical conductor 63 by means of a conductive screw 60, in particular made of graphite. The screw 60 extends through the respective conductor 63 into the material of the respective component 11, 12. The screw 60 can also be used to fasten the respective cap to the respective component.

[0087] A vertical linear cavity 46 is arranged in the first component 11, above which a pyrometer 45 is located. This is used for continuous non-contact temperature measurement of the third region 23 of the surface element 20 and thus for monitoring the temperature during sintering. The cavity 46 provides an observation axis between the pyrometer 45 and the surface element 20. The cavity 46 can have a channel-like configuration, for example when implemented as a bore with a circular cross-section.

[0088] The main advantage of the apparatus 10 shown here is that it is suitable for a simple integration into existing systems for "field-assisted sintering technology" (FAST) or "spark plasma sintering" (SPS). In particular, for this purpose, the first component 11 and the second component 12 are positioned between two punches of the system and fixed in position as required. An electric current can be applied to the two components 11, 12 via the two punches. The punches can carry out a vertical relative movement of the two components 11, 12. In this way, the operating conditions required for sintering according to the invention can be easily realized by means of existing equipment. The established control and possible operating parameters, such as the power supply, of this equipment are optimally suited for carrying out the method according to the invention. For example, vacuum or an inert gas atmosphere, such as Ar, Ar / H 2 , or N 2 Existing process controls and controls can be used, including the setting of the pressure and gas composition. Gas pressure and gas composition can be detected and controlled. Such a system allows sintering of workpieces up to 500 mm in diameter. The system is established, commercially available, and meets safety requirements. Optionally, existing heating, debinding, and cooling zones can also be used. Manipulators available for these plants allow for an automated method.

[0089] In contrast to FIG. 1, in the exemplary embodiment shown in FIG. 2, there is only one surface element 20. It is arranged at the top and attached to the first component 11. The underside of the sintering position 16 is defined by a counter element 25, which in the example shown carries the green body 14. The counter element 25 is designed as a flexible felt element that compensates for the shrinkage of the component during sintering. An inert support element 26 is located below the counter element 25. In contrast to the example shown, the felt element can also be omitted. In this case, the support element 26 serves as a counter element for limiting the sintering position and / or for holding the green body 14.

[0090] In this case, the green body 14 to be sintered is a coating arranged on a substrate 17. The substrate 17 rests on a counter element 25. The coating is in contact with a surface element 20. The current thus flows here from the first component 11 to a first region 21 located on the left side of the surface element 20, through a third region 23 to the right, into a second region 22 of the surface element 20 and from there directly to the second component 12. Apart from the differences described, the example of FIG. 2 is constructed similarly to the example of FIG. 1 described above. In this respect, reference is made to the above.

[0091] A further exemplary embodiment is shown in Fig. 3. The apparatus 10 comprises a first conveying device 31 with two conveying rollers 38', 39', which move the surface element 20, designed as a circular belt 37, in the manner of a conveyor belt. This is indicated by an arrow in the conveying rollers 38'. The green bodies 14 can be placed on the surface element 20, for example by a manipulator (not shown), in a loading zone 65, which is shown diagrammatically on the left. Thus, between the conveying rollers 38', 39', there is a linear (rectilinear) movement of the surface element 20 and the green bodies located thereon relative to the first component 11 and the second component 12. The exemplary embodiment shown here also has the advantage that the apparatus 10 can be integrated in a simple manner into existing systems, for example into a continuous oven, in order to use the equipment that is present as described above. The first conveying device 31 can be part of a continuous oven.

[0092] The two components 11, 12 are here designed as rod-shaped electrodes with a cylindrical cross section in order to minimize the mechanical load on the surface element. The electrodes extend in particular over the entire width of the surface element. The electrodes are movable individually or together along their respective axes vertically along the arrow 59 in order to allow a relative movement with respect to the surface element 20 that can close a circuit through the first component 11, the surface element 20 and the second component 12. A power supply connected to both components 11, 12 is shown. Again, as in FIG. 1, the current flows from the first component 11 to the first region 21, then through the third region 23 to the second region 22 and then to the second component 12. In particular, the third region 23 is strongly heated.

[0093] The device 10 further comprises a second conveying device 32 with two conveying rollers 38, 39 for moving the circulating surface element 20' configured as a circulating belt 37 in the manner of a conveyor belt. The second conveying device 32 is a mirror image of the first conveying device 31 about a horizontal axis and is otherwise configured similarly to the first conveying device 31. Therefore, reference is made to the above. In one embodiment, the upper conveying device may have a shorter length than the lower conveying device, so that the lower conveying device overhangs both sides of the upper conveying device. In this way, it is easier to place the green body 14 on the lower conveying device and to remove the sintered body 15.

[0094] The device comprises a first counter element 51 facing the first element 11 and a second counter element 52 facing the second element 12. The counter elements 51, 52 are movable individually or with each other along their respective axes in the vertical direction along the arrow 58, in order to allow a relative movement with respect to the surface element 20' and to apply a compressive force to the first and second areas 21 and 22 of the surface element 22. In the illustrated configuration, the elements 11, 12 and the counter elements 51, 52 are in the closed position. It can thus be seen that the surface elements 20, 20' deform and enclose the green body 14 for sintering. In this position, both surface elements 20, 20' are immobile and both transport devices 31, 32 are stopped. When the circuit is closed, for example by a switch and / or a control, both surface elements 20, 20' are heated by resistance, which results in heating of both sides of the green body 14. Sintering can take place, for example, within 10 seconds. After sintering, the open position is reached by an upward movement of the counter elements 51, 52 and a downward movement of the components 11, 12, the surface elements 20, 20' are again freely movable and the movement of the transport devices 31, 32 is resumed. In this way a quasi-continuous process is performed. The sintered body can be removed from the surface element 20, for example by a manipulator (not shown), in an unloading zone 66, shown diagrammatically on the right.

[0095] Alternatively to the configuration shown here, the two upper elements may also be designed as a first component 11 and a second component 12, and the two lower elements may be designed as a first counter component 51 and a second counter component 52.

[0096] The exemplary embodiment shown in Fig. 4 has a similar construction, so that only the differences are described here and reference is otherwise made to the above. In contrast to Fig. 3, here instead of the underside element a non-conductive counter-element 25 is arranged in the form of a circular belt 37, which moves together with a first conveying device 31, on which the green body 14 is arranged. Meanwhile, the above-mentioned second conveying device 32 moves the surface element 20, in the vicinity of which the first component 11 and the second component 12 are arranged. In this way, the green body 14 is heated from above on one side, for example for sintering of the coating.

[0097] The exemplary embodiment shown in FIG. 5 is also similar in construction to FIG. 3, so that only the differences are described here and reference is otherwise made to the above. Deviating from FIG. 3, both surface elements 20, 20' are now arranged quasi-endless on respective rolls 34. Both the first conveying device 31 and the second conveying device 32 are configured to hold and transport the partially unwound surface elements 20, 20'. For this purpose, they each have a second roller 35 which, together with the roller 34, clamps and moves the partially unwound surface elements 20, 20'. In this way, sintering with damaged surface elements is avoided and a particularly high and consistent quality of the sintered body is ensured.

[0098] Other possibilities are also conceivable: for example, it is also possible to combine a lower conveying device with a counter element according to FIG.

[0099] Another exemplary embodiment is shown in FIG. 6. Here, the first conveying device 31 comprises a number of parallel transport rollers 33. These are configured to move the respective parts of the surface elements 20, 20' with limited area, clamping the green or sintered body, in a substantially linear manner relative to the first component 11 and the second component 12. The first conveying device 31 may be configured in one part (integral) so that the clamping body can be moved before and after sintering. The first conveying device 31 may also be configured in two parts, as shown, so that the first part transfers the clamped green body 14 to the components 11, 12 and the second part moves the clamped sintered body 15 away from the components 11, 12. This device 10 can also be easily integrated into existing continuous furnaces in order to use existing equipment as described above. The first conveying device 31 can be part of the continuous furnace.

[0100] In the example shown, the components 11, 12 are each designed as a rectangular block with a contact area on the underside for contacting the surface element and an electrical connection area arranged on the side. A corresponding counter component 51, 52 is arranged below each component 11, 12.

[0101] The components 11, 12 are shown in the open position 18. Thus, by means of the first transport device 31, the sintered body 15 can be transported and the green body 14 can be conveyed between the components 11, 12 and the counter component 51, 52. For closing, the components 11, 12 can be moved individually or together vertically downwards along the arrow 54, such that the first component 11 contacts the first area 21 of the surface element 20' and the second component 12 contacts the second area 22 of the surface element 20'. Now, the circuit can be closed so that the third area 23 with the green body 14 located at the sintering position 16 is heated for sintering. Here too, a quasi-continuous method is possible. The clamped green body can be placed on and / or removed from the transport roller 31, for example by a manipulator (not shown).

[0102] Here too, the surface elements 20, 20' are used only once for the reasons explained. It is also possible to deviate from what is shown and perform heating on one side by replacing one of the surface elements 20, 20' with a corresponding counter element.

[0103] A similar device 10 is shown in FIG. 7, so that here too only the differences are described. Deviating from FIG. 6, each of the components 11, 12 is now designed as a roller-shaped electrode 40 rotatable about a horizontal axis. The electrode 40 has a circumferential surface 41 for conductively contacting the surface element 20' and an electrical connection area 42. In particular the size and proportions of the electrodes are not to scale. The relative movement here is between the electrode 40 and the surface element 20'. The clamped green body is moved along the direction 56 by the transport roller 33 until it reaches the electrode 40. Through the contact between the electrode 40 and the surface element 20', both surface elements 20, 20' are heated and sintering takes place. Now the two surface elements 20, 20' are clamped between the components 11, 12 and the counter-components 51, 52. In this configuration it is not necessary to stop the movement of the first transport device for sintering. A completely continuous method is thus provided in which the transport and sintering take place without interruption. List of reference signs

[0104] 10 equipment 11 First Component 12 Second component 14 Unsintered body 15 Sintered body 16 Sintering position 17 Substrate 18 open position 19 Closed position 20,20' Surface element 21 The First Area 22 The Second Region 23 The Third Region 25 Counter elements 26 Supporting Elements 31 First conveying device 32 Second conveying device 33 Transfer roller 34 Lola 35 The Second Roller 37 Belt 38,38',39,39' Conveyor roller 40 electrodes 41 Circumferential surface 42 Connection area 45 Pyrometer 46 Cavity 51 First Counter Component 52 Second Counter Component 54,56,58,59 Arrows 60 Conductive Screw 61 Non-conductive screws 62 Insulators 63 Conductor 65 Load Zone 66 Unloading Zone

Claims

1. A sintering apparatus (10) comprising an electrically conductive first component (11), an electrically conductive second component (12) and at least one electrically conductive surface element (20, 20') for heating a green body (14) to be sintered, wherein said first component (11) and said second component (12) are movable relative to each other and / or relative to said surface element (20, 20'), whereby an electric circuit comprising said first component (11), said surface element (20, 20') and said second component (12) can be closed by said relative movement.

2. 2. The device (10) according to claim 1, characterized in that said surface elements (20, 20') comprise or consist of carbon.

3. The device (10) according to any one of claims 1 to 2, characterized in that a first region (21) of the surface element (20, 20') is conductively connectable or connected to the first component (11) and a second region (22) of the surface element (20, 20') is conductively connectable or connected to the second component (12).

4. The apparatus (10) according to any one of claims 1 to 3, characterized in that one side of the sintering position (16) of the green body (14) is defined by a surface element (20) and the other side of the sintering position (16) of the green body (14) is defined by a counter element (25).

5. The apparatus (10) according to any one of claims 1 to 3, characterized in that the apparatus (10) comprises two electrically conductive surface elements (20, 20'), each surface element (20, 20') defining one side of a sintering location (16) of the green body (14).

6. The device (10) according to any one of claims 1 to 5, characterized in that the device (10) is configured to allow relative movement of the first component (11) and the second component (12) along a common axis, characterized in that in an open position (18), a sintering location (16) of the green body (14) is accessible and in a closed position (19), electrical contact is established between the first component (11), the surface elements (20, 20') and the second component (12).

7. The device (10) according to any one of claims 1 to 6, characterized in that it comprises a first conveying device (31) for moving the surface element (20) or the counter-element (25) in a substantially linear manner so that the green body (14) and / or the sintered body (15) arranged on the surface element (20) or the counter-element (25) can be moved relative to the first component (11) and the second component (12).

8. 8. The device (10) according to claim 7, characterized in that the surface element (20, 20') is provided in wound-up form on a roll (34) and the first conveying device (31) is configured to hold and transport the partially unwound surface element (20, 20').

9. 8. The device (10) according to claim 7, characterized in that the surface element (20, 20') and / or the counter element (25) are configured as an endless belt (37) and the first conveying device (31) comprises two conveying rollers (38, 38', 39, 39') for holding and moving the endless belt (37).

10. Apparatus (10) according to any one of the three preceding claims, characterized in that the apparatus (10) comprises a second conveying device (32) for moving the further surface element (20') or counter-element (25) substantially along a straight line such that the green body (14) and / or the sintered body (15) arranged on the surface element (20, 20') or counter-element (25) can be covered with the further surface element (20') or counter-element (25) when moved relative to the first component (11) and the second component (12).

11. 10. The device (10) according to any one of the four preceding claims, characterized in that the first component (11) and the second component (12) are configured as a rotatably mounted roller-shaped electrode (40) configured to bring the surface elements (20, 20') into conductive contact with their circumferential surfaces (41) when the surface elements (20, 20') are moved substantially linearly relative to the first component (11) and the second component (12).

12. The device (10) comprises a first counter-component (51) facing the first component (11) and a second counter-component (52) facing the second component (12), the first component (11) and the first counter-component (51) being arranged to exert a compressive force from opposite sides on a first region (21) of the surface element (20, 20'), and the second component (12) and the second counter-component (52) being arranged to exert a compressive force from opposite sides on a second region (22) of the surface element (20, 20'), characterized in that the device (10) according to any one of claims 1 to 11.

13. The device (10) according to any one of claims 1 to 12, characterized in that the device (10) comprises a manipulator for at least partially moving the green body (14) to be sintered to a sintering position (16) and / or for at least partially moving the sintered body (15) away from the sintering position (16).

14. The device (10) comprises a pyrometer (45) for determining the temperature of the surface element (20, 20'), In particular, a continuous cavity (46) is arranged in the first component (11) or in the second component (12) through which an observation axis passes between the pyrometer (45) and the surface element (20, 20'). A device (10) according to any one of the preceding claims, characterized in that

15. A sintering plant, in particular a FAST / SPS sintering plant or a continuous furnace, comprising a sintering device (10) according to any one of claims 1 to 14, in particular said sintering plant comprising a power source for applying an electric current to said electric circuit comprising said first component (11), said surface elements (20, 20') and said second component (12).

16. A method for sintering a green body (14), comprising the steps of: Providing a device (10) having an electrically conductive first component (11), an electrically conductive second component (12) and at least one electrically conductive surface element (20, 20'), said device (10) being in particular a device (10) according to any one of claims 1 to 14, placing a green body (14) to be sintered on said surface elements (20, 20'); performing a relative movement between said first component (11) and said second component (12) or between said first component (11) and said second component (12) on the one hand and said surface element (20, 20') on the other hand; applying an electric current to the first component (11), the at least one surface element (20, 20'), and the second component (12), the electric current heating the surface element (20, 20') such that the green body (14) is sintered; A method comprising:

17. the surface element (20, 20') or the counter element (25) is moved substantially linearly, the green body (14) is placed on the surface element (20, 20') or the counter element (25) and is moved together with the first component (11) and the second component (12), after completion of said substantially linear movement, said relative movement is performed in which said first component (11) and / or said second component (12) are moved such that a circuit passing through said first component (11), said surface elements (20, 20') and said second component (12) is closed, After the sintering is completed, the surface element (20, 20') or the counter element (25) is moved substantially linearly and the sintered body (15) is placed on the surface element (20, 20') or the counter element (25) and is moved together with the first component (11) and the second component (12).

17. The method according to claim 16.

18. 17. The method according to claim 16, characterized in that the first component (11) and the second component (12) are each configured as a rotatably mounted roller-shaped electrode (40), the relative movement being performed as a movement of the surface element (20, 20') in a substantially straight line, and the first component (11) and the second component (12) bring the surface element (20, 20') into conductive contact with a respective circumferential surface (41) during the relative movement.

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