Metal fiber network and method for assembling a fiber network

Abstract

The present invention relates to a method for assembling a fiber network comprising a plurality of metal fibers, the method comprising the steps of: providing a loose network from a plurality of metal fibers at an assembly site; heating the plurality of fibers to a bonding temperature selected in the range of 50-98% of their melting point temperature at a heating rate of more than 50 K / min, in particular more than 100 K / min, in particular more than 200 K / min, preferably more than 1000 K / min, thereby forming contact points between the individual metal fibers; and bonding the plurality of metal fibers to one another by cooling the plurality of fibers at a cooling rate of more than 50 K / min, preferably more than 100 K / min. The present invention further relates to a metal fiber network comprising a plurality of metal fibers attached to each other at contact points, wherein the metal fibers have a non-circular cross section, in particular a rectangular, square, partial circle, or an elliptical cross section having a major axis and a minor axis, or wherein said metal fibers have a circular cross section, the fibers having a generally constant width along the length of the fiber, the variation of said width of said fibers along their length being less than 40%, preferably less than 30%, in particular less than 20%.

Description

[Technical field]

[0001] The present invention relates to a method for assembling a fiber network comprising a plurality of metal fibers, and to a network of metal fibers. [Background technology]

[0002] Today, fiber networks are used in a wide variety of applications ranging from filters to batteries, for example.

[0003] Traditionally, filtration of gases such as air or liquids has been based on metal fiber meshes or foams, which are now part of a wide variety of devices ranging from oil filters in automotive applications to cleaning systems for fluids or gases such as air.

[0004] Previously known filters are usually based on metal fibers with a circular cross section (e.g. oil filters) or on carbon-based foams (e.g. HEPA filters). Filters made of metal fibers with a circular cross section are characterized in that such fibers have a high mechanical stability but a low surface-to-volume ratio. However, such filters usually have a fairly high weight since a large amount of fibers is required. On the other hand, filters made of carbon foam are mostly very fragile, but are light and have a very large internal surface area. Furthermore, it is noted that the filtering capacity of such previously known filters is not ideal.

[0005] In another field of application, networks of metal fibers can also improve the performance of secondary batteries when used as auxiliary electrodes, such networks of metal fibers can also contribute to the performance of catalytic materials in electrochemical applications such as fuel cells and hydrolysis, or as components in electromagnetic shielding materials, as filters, or as tissue materials and tissue hybrid materials that can also contain additives such as cotton, silk or wool.

[0006] Due to the wide variety of applications mentioned above, there is an increasing need to be able to produce fiber networks with different defined properties depending on the application.

[0007] In a conventional process for producing fiber networks, a number of fibers are fed into a hot press and exposed to high pressure. The fibers are then placed in a furnace and slowly heated to a temperature close to the melting temperature of the fibers while still being exposed to the pressure. The high temperature is maintained until the fibers are connected to each other. The produced network is then slowly cooled.

[0008] The above process is also known as "sintering". Such a process usually takes more than an hour depending on the capacity of the oven used. However, it is recognized that in conventional sintering, the fibers relax before reaching a temperature high enough to connect them to each other. These relaxation processes release stored energy from the fibers. As an example, fibers obtained by quenching techniques, such as melt spinning, can have a significant amount of stored energy.

[0009] The driving force of the above process is the reduction of the fiber surface and the associated reduction of their free energy ΔG. The free energy ΔG is the surface component ΔG S , volume component ΔG V , and grain boundary component ΔG B This relationship is shown in equation (1). During sintering of the fibers, the volume fraction remains approximately constant (ΔG V =0), while the grain boundary portion increases due to dislocations, i.e., the surface decreases (ΔG B >0), the volume fraction decreases (ΔG V <0). Volume part ΔG V is clearly the grain boundary part ΔG Bis more important than the decrease in the total free energy of the system (ΔG<0), which leads to a negative change in the total free energy of the system and the process occurs spontaneously as soon as a certain energy threshold (the activation energy) is exceeded. During conventional sintering processes, the decrease in ΔG is also associated with the rounding of the fibers, i.e., the change in the fiber diameter towards a circular shape, also referred to herein as rounding. ΔG T = ΔG V +ΔG B +ΔG S

[0010] Here, the energy threshold that must be exceeded is the activation energy of diffusion, E A (Equation (2)). Here, D0 is the temperature-dependent diffusion coefficient, k is the Boltzmann constant, T is the absolute temperature, and D is the temperature-dependent diffusion coefficient. The temperature-dependent diffusion coefficient D (unit: m 2 s -1 ), the faster the fiber becomes circular. Here, the temperature is the activation energy E A It is not only involved in the realization of the goal but also in determining its speed.

number

[0011] Thus, such known sintering processes occur through atomic rearrangement processes (diffusion) and not through processes involving the regeneration of molten fibers. The thermodynamic goal is to achieve the maximum possible volume with the smallest possible surface. The ideal ratio is achieved with a perfect sphere.

[0012] Previously known methods for producing fiber networks do not allow for practical control of this effect, for example to produce fiber networks with fibers of limited cross section. Summary of the Invention [Problem to be solved by the invention]

[0013] It is therefore an object of the present invention to provide a method for assembling a fibre network with a better controlled fibre cross-section and a corresponding fibre network. This object is solved by the subject matter of the independent claims. [Means for solving the problem]

[0014] In particular, the present invention provides a method for assembling a fiber network comprising a plurality of metal fibers, the method comprising: providing a loose network of a plurality of metal fibers at an assembly site; Bonding a plurality of metal fibers to one another by forming contact points between individual metal fibers, comprising: heating the plurality of fibers to a bonding temperature selected within the range of 50-98% of their melting point temperature at a heating rate of more than 50 K / min, in particular more than 100 K / min, in particular more than 200 K / min, preferably more than 1000 K / min; - cooling the plurality of fibers at a cooling rate of more than 20 K / min, preferably more than 50 K / min, preferably more than 100 K / min, in particular to a temperature of 60% of their melting point; and The present invention provides a method comprising:

[0015] As mentioned above, fibers exposed to heat tend to rearrange at the atomic level so that the maximum achievable volume with the smallest achievable surface is realized. The "near perfect" state, as seen in the context of fibers, is a perfect sphere. Thus, with common manufacturing methods, due to the heating step, the fibers start to rearrange at their atomic level and reach a more favorable energy level, for example by crystallization or by reducing defects in the crystal lattice of the fibers. As a result, the fibers can further change their shape by transforming their cross-section from a flat or elliptical cross-section to a circular cross-section, i.e., a rounding of the cross-sectional shape of the fibers occurs. During the transition to the thermodynamically most favorable spherical shape, not only can the rounding effect of the cross-sectional shape of the fibers be observed, but also a change in the diameter of the fibers. Before reaching a spherical shape, the fibers show areas of reduced diameter, referred to herein as constrictions. These constrictions proceed further until the fibers break apart. Finally, the fibers turn into multiple droplets, i.e., they reach a spherical shape.

[0016] The method of the invention takes advantage of the kinetics of the rearrangement process. Rearrangement only occurs if the fibers have enough time for it. By increasing the heating and cooling rates and preferably maintaining the fixing temperature for not more than 30 minutes, the formation of contact points where the metal fibers come into contact with each other can be ensured. Nevertheless, the rearrangement process is greatly reduced and in particular shape-changing effects, i.e. rounding, as well as the formation of constrictions and interruptions, can be avoided. Thus, in the method of the invention, the heating and cooling rates are maintained well above 20 K / min, preferably above 50 K / min, preferably above 100 K / min. In this connection, it is noted that it may be preferable to cool the assembled network at the cooling rates to a temperature below 60% of the melting temperature of the fibers. After cooling the fibers to the temperature rates, the cooling rate is no longer important and can therefore be reduced if necessary. Commonly known sintering processes are carried out with heating / cooling rates of about 10-20 K / min, which results in much longer heating / cooling times for the fibers (up to several hours). If the heating rate is too slow, before the adhesion temperature is reached, relaxation processes take place in the loose network of metal fibers, causing the surface component of the free energy ΔG S and grain boundary component ΔG B As a result, if a slow heating rate is used in the step of heating the metal fibers, also referred to as the first method step, an additional fixing step may be necessary in which the fixing temperature is maintained for more than 30 minutes. By keeping the heating rate slow, a fixing time of 30 minutes or less in said additional fixing step may not be sufficient to bond the fibers to each other, because the fixing does not extend the surface component ΔG to the same extent as if a faster heating rate was used in the heating step. S and grain boundary component ΔG BThe advantage of the above-mentioned advantages cannot be obtained from the fact that the fiber is heated to a temperature of about 100° C. The longer time required to reach the fixing temperature and / or the optional longer time of maintaining the fiber at the fixing temperature can change the fiber to a more thermodynamically favorable state, i.e. the fiber cross section can change towards a circular cross section. As already mentioned above, when the fiber tries to change to a more thermodynamically stable state, not only can the cross section change, but the fiber width can become non-uniform and / or necking of the fiber width can occur. As shown in the enclosed drawings and discussed in more detail below, these necking can cause further breakage of the fiber, thereby reducing the length of the fiber.

[0017] By using the method of the invention, a network of bonded fibers can be assembled from a loose network of fibers with minimal (undesirable) effects on their atomic level. As a result, the cross-sectional shape and length of the fibers can be maintained. As explained above, this was not possible with previously known methods, because the stored energy is already released from the fibers during heating before the respective bonding temperature is reached. As a result, in the case of typical sintering processes, the fibers are in a more thermodynamically stable state when the bonding temperature is reached. Higher bonding temperatures and longer bonding times are then required, resulting in changes in the cross-section, diameter and / or length of the fibers.

[0018] In the method according to the invention, the temperature used, i.e. the bonding temperature, is determined by the material of the metal fibers. In order to prevent the amorphous metal fibers from crystallizing during the welding process, it is preferable to keep the temperature used below the crystallization temperature of these fibers. The crystallization temperature can be determined, for example, by differential scanning calorimetry (DSC) measurements of the metal fibers in question. DSC measurements can be carried out using the following conditions: starting temperature 30°C, 10Kmin -1 up to 1200℃ at a heating rate of 10Kmin -1 The DSC measurement was performed at a cooling rate of 100 ml min -1The reaction can be carried out under an argon atmosphere using a constant argon flow of 1000 s and a completely oxygen-free zirconium-oxygen trap system (STA 449 F3 Jupiter, Netzsch Bj.2017).

[0019] In the context of the present description, "% of melting temperature" means the melting temperature in °C as determined, for example, by differential scanning calorimetry (DSC) measurements. Thus, if the melting temperature is 1000°C, in the context of the present description, 20% of the melting temperature is 200°C, 50% of the melting temperature is 500°C, and 95% of the melting temperature is 950°C.

[0020] Moreover, as an additional advantage, with the method according to the invention, the network can be assembled in such a way that it is flexible and can be repeatedly deformed without causing degradation of the network, i.e. without the separation of one metal fiber from the metal fiber network due to deformation. The metal fibers are fixed to each other, so that they are in contact with each other, i.e. the contact points cannot move relative to the metal fibers, as is the case for example in non-woven agglomerations of entangled metal fibers, such as metal felts. As a result, the metal fiber network according to the invention is mechanically stable and also flexible. Mechanical stability in this context means that the metal fiber network is not a loose agglomeration of metal fibers, i.e. the network does not break apart into separate metal fibers as soon as a small force acts on the network. Such a metal fiber network can therefore be flexibly deformed without being destroyed. The metal fiber network can recover its shape after deformation. However, if the metal fiber network is folded, it can also be permanently reshaped.

[0021] With the method according to the invention, it is further possible for the contact points to be distributed throughout the assembled network, whereby there are contact points throughout the three-dimensional structure of the metal fiber network. Thus, the contact points are not only provided in certain areas of the metal fiber network, such as the center or periphery of the network. The contact points can be distributed uniformly throughout the network. Furthermore, the density of the contact points can have a gradient throughout the network, i.e. the network can have areas with a high density of contact points and areas with a low density of contact points. It is also possible to have a regular or irregular spatial distribution of the contact points.

[0022] In this regard, it is further noted that each metal fiber can have at least two contact points with another metal fiber, more preferably at least three contact points, and even more preferably at least four contact points.

[0023] According to one embodiment of the present invention, the method may further comprise a step of maintaining the fixing temperature for a fixing time selected in the range of 0 seconds to 30 minutes, in particular in the range of 0 seconds to 15 minutes, preferably in the range of 0 seconds to 5 minutes, the step of maintaining the fixing temperature being carried out before the step of cooling the plurality of fibers. Thus, as already mentioned above, the method may comprise an additional step of maintaining the fixing temperature for a predetermined time. However, it has also been shown that by maintaining a high heating rate, the fixing time can be reduced to a minimum. That is to say, in some cases, it is even possible to start the cooling process as soon as the fixing temperature is reached, thereby maintaining said temperature for essentially 0 seconds. In this connection, it should obviously be noted that in practice, if the cooling process starts immediately after the fixing temperature is reached, the fixing time is not strictly 0 seconds, or to some extent 0.1 seconds or less. In the context of the present application, a fixing time of 0 seconds therefore relates to the case in which the cooling step starts immediately after the fixing temperature is reached during the heating step. For some embodiments, the fix time may be 1 second or more, 2 seconds or more, 3 seconds or more, 10 seconds or more, or even 30 seconds or more.

[0024] According to another embodiment, the method may include a cleaning step before the step of bonding the fibers together, the cleaning step comprising performing cleaning of the fibers by heating the fibers to a cleaning temperature selected in the range of 20% to 60% of the melting temperature of the fibers. The lower limit may even be selected from room temperature, in particular slightly higher than room temperature. It has been shown that metal fibers often contain different kinds of impurities and / or additives on their surface, which are usually byproducts of their respective manufacturing methods. By heating the fibers to the cleaning temperature, the impurities and / or additives decompose, i.e. evaporate or burn, whereby the remaining fibers contain a clean surface. Such clean surfaces are then easier and better able to sinter with each other.

[0025] In this connection, it should be noted that in the ideal case, the cleaning temperature should be well below the temperature at which the fibers start to lose their stored energy, i.e. below the temperature at which the above-mentioned rearrangement processes tend to occur. In this way, the fibers can be cleaned from the above-mentioned additives / impurities without already starting to sinter one another.

[0026] In undesirable cases where the cleaning temperature is selected at a temperature at which the fibers already start to reorganize at their atomic level, for example due to the properties of additives present on the surface of the fibers, special care must be taken by maintaining the fibers at said cleaning temperature for as short a time as possible, i.e. only until the fibers are clean. In such cases, it may be necessary to later adapt the fixing temperature of a subsequent heating step to a higher temperature in order to allow the cleaned fibers to be effectively sintered together. The minimum time for cleaning the fibers can be easily determined by trial and error.

[0027] The cleaning temperature can therefore be selected depending on the material of the fibres and the additives / impurities from which the fibres are to be cleaned.

[0028] The step of cleaning the fibers can be carried out in the same assembly location, i.e. in the same oven, as the step of heating the fibers to the fixing temperature. Such an arrangement is usually called a batch process. In another embodiment, there can be several different ovens, or one oven with several different heating zones, through which the fibers pass for each method step. On the other hand, such a process is called a continuous process, which is particularly advantageous when large amounts of fibers are envisaged to be processed.

[0029] The step of cleaning the fibers may further include using a gas flow at the assembly site, which may facilitate removal of evaporated / decomposed additives from around the fibers, so that the additives cannot re-collect on the fibers after the fibers are cooled again.

[0030] Said gas flow can be obtained, for example, by using suction at the assembly site to suck the burnt / vaporized additives away from the assembly site. Another possibility can be to supply a stream of gas to the assembly site, for example a reactive gas such as oxygen or air, or an inert gas such as nitrogen or argon, configured to blow away the vaporized / burned additives away from the assembly site. The type of method of supplying said stream of gas can be selected according to the other conditions present at said assembly site, for example whether the assembly site is provided in vacuum, air or protective gas. For example, if the fibers are made of a material that tends to react with oxygen, the stream of gas supplied to clean the environment from the additives can be selected to be oxygen-free, for example by supplying an inert or protective gas.

[0031] Additionally, the step of cleaning the plurality of fibers can include reducing the air pressure at the assembly site. This can be useful, for example, for additives with high vapor pressure. The air pressure can be reduced to less than 80 kPa, less than 50 kPa, or less than 10 kPa. In some embodiments, the pressure can be further reduced to less than 1 kPa, less than 0.1 kPa, or even 0.0001 kPa, i.e., a vacuum can be used. As used herein, air pressure corresponds to a pressure of 101 kPa.

[0032] The vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase (solid or liquid) at a particular temperature in a closed system. Equilibrium vapor pressure is one of the measures of the evaporation rate of a liquid. It is related to the tendency of particles to be released from the liquid (or solid). Substances that have a high vapor pressure at standard temperature are often called volatile. The pressure exhibited by a vapor present above the liquid surface is known as vapor pressure. As the temperature of a liquid increases, so does the kinetic energy of its molecules. When the kinetic energy of the molecules increases, the number of molecules that transform into vapor increases, thereby increasing the vapor pressure.

[0033] In a further aspect of the invention in this context, the cleaning of the plurality of fibers also comprises determining the compound to be removed, i.e. the additive. Furthermore, in this aspect of the invention, the step of cleaning the plurality of fibers further comprises a reduction in pressure and / or an increase in temperature based on the vapor pressure curve of the compound to be removed. The reduction in pressure and / or the increase in temperature is in particular carried out in such a way that the fibers are finally exposed to conditions in which the compound to be removed is in the gas phase according to its vapor pressure curve. This change in pressure and / or temperature can be carried out in a stepwise manner, i.e. by gradually changing the pressure and / or temperature followed by keeping these parameters substantially constant for a certain time, or the pressure and / or temperature can be changed in a continuous manner. The required pressure and / or temperature can vary widely for the different compounds to be removed. However, the available vapor pressure curves provide a suitable guide to the skilled person as to the appropriate cleaning conditions without the fibers being overheated to more than 60% of their melting point.

[0034] Thus, for some materials, the cleaning effect can be enhanced by reducing the air pressure at the assembly site, which can induce evaporation of said materials, in conjunction with heating the fibers to the cleaning temperature.

[0035] According to one embodiment, before the metal fibers are bonded to each other, the method further comprises the step of applying a predetermined pressure to the metal fibers, in particular less than 1 MPa, in particular less than 500 kPa. With the method according to the invention, a relatively low pressure can be applied to ensure the formation of contact points where the fibers are connected to each other. In the previously known methods, a much higher pressure must be applied to form the contact points.

[0036] It is further preferred to supply the assembly site with a protective gas, for example argon, nitrogen, Ar-W5 (5% H2 by volume in Ar), Ar-W2 (2% H2 by volume in Ar), forming gas (5% H2 by volume in N2) or another noble gas, in order to prevent oxidation of the fibers during the assembly process. This step may be appropriate both for the step of heating the fibers to the fixing temperature and for the step of maintaining the fibers at said fixing temperature (if any). In general, the method according to the invention may also be carried out in a vacuum. The exact conditions at the assembly site may therefore be selected depending, for example, on the material used for the fibers. Some materials, such as, for example, iron and / or some steels, cannot be used with nitrogen, because they are prone to nitridation. For such materials, therefore, another protective gas may be used.

[0037] According to another embodiment of the invention, the step of heating the fibers is carried out by a suitable heating device. Preferred examples of such heating devices are induction furnaces, infrared furnaces, high-temperature ceramic heating elements and / or zone furnaces, such as conveyor furnaces. Such heating devices can ensure rapid heating, i.e. fast heating rate, as well as rapid cooling, i.e. fast cooling rate, so that the fibers can be connected to each other before reaching the fixing temperature without the fibers losing too much energy due to rearrangement and relaxation processes or anything similar. The heating device may suitably be a continuous or batch furnace.

[0038] In some embodiments, said suitable heating device for carrying out the heating step may be a continuous oven, which is often the preferred choice for applications with high production rates, i.e. where the processing of large amounts of fibers is envisaged (industrial applications).

[0039] The fixing temperature is preferably determined in situ by electron microscopy. This can be done, for example, by placing the fibers in a heating stage of an in situ SEM (scanning electron microscope). Since heat transfer is almost non-existent in high vacuum, the fibers need to have a good thermal connection with the heating stage. For this, a heat-stable graphite paper can be used. Thus, one sheet can be used as a support between the fibers and the heating stage, and another sheet with a hole in the middle can be used to see the fibers. These fiber sandwich structures can then be moved to the heating stage and pressed down. The heating stage can then be heated to a temperature close to the melting temperature of the fibers, but still lower than the melting temperature. Meanwhile, the fiber cross-section can be observed by SEM until the fibers start to connect with each other. In this connection, the fixing temperature can be determined. In a second experiment, the fiber sandwich structure can be heated to the above determined fixing temperature with the aforementioned heating rate. Then, said fixing temperature can be optionally maintained for a fixing time until the desired degree of connection and therefore the desired connection strength is reached. In other words, in a second experiment, the method steps according to the invention can therefore be carried out to check whether the determined fixing temperature is correct.

[0040] The fixing temperature and time are determined by the fiber material, the fiber dimensions, i.e. width and thickness, and the amount of stored energy in the fiber. For example, for thin fibers of a material, with small thickness and width, the rounding process tends to proceed faster than for less thin fibers of the same material. The determination of the fixing temperature and time for a particular type of fiber can be done either in situ by the electron microscopy method described above, or by using trial and error testing. Trial and error testing can be done using actual equipment for manufacturing metal fiber networks, i.e. under actual manufacturing conditions.

[0041] It is noted in this connection that it may be preferable for the fixing temperature to be selected within the range of 80-98%, in particular within the range of 90-98%, of the melting temperature of the metal fibers. Fixing temperatures within the above range have been found to be adequate for most materials. It is noted in this connection that the exact fixing temperature may also vary depending on the fixing time, i.e. the higher the fixing temperature, the shorter the fixing time may be and vice versa.

[0042] The cooling rate in the cooling step is preferably maintained for a time sufficient to cool the metal fibers to 60% of their melting temperature.

[0043] The method steps according to the invention, namely the heating step and the cooling step, and optionally the step of maintaining the fibers at the fixing temperature, are preferably carried out for a total time of less than 30 minutes, preferably less than 15 minutes, in particular less than 5 minutes, in particular less than 1 minute. It has been shown that the faster the method is carried out, the less negative side effects there are, such as, for example, the release of energy, the formation of necking and / or breaks in the fibers, and changes in the fiber cross-sectional shape. It should also be noted that in this situation, the cooling step does not necessarily have to be cooled to room temperature. The steps can be terminated after cooling to a temperature of 60% of the melting temperature of the metal fibers.

[0044] It should be noted in this connection that it may be possible to equally distribute the predetermined time between the above steps. In another embodiment, if a step of maintaining the fixation temperature is performed, this step may take much longer than the steps of heating and cooling the fibers. In an ideal experiment, for example, the steps of heating and cooling the fibers may take 1 minute, while the step of maintaining fixation may take 30 seconds or less. However, in another experiment, the heating step may take place within the range of 1-5 minutes, the step of maintaining the fixation temperature may take place within the range of 0.5-1 minute, and the cooling step may take place within 10 minutes until the assembled network has cooled to a temperature of about 60% of the melting temperature of the fibers used. Further cooling of the assembled network to room temperature may take, for example, another 1-2 hours.

[0045] It may be desirable for the metal fibers to have a length of 1.0 mm or more, and / or a width of 100 μm or less, and / or a thickness of 50 μm or less. With metal fibers having such dimensions, a network with metal fibers that adhere to one another can be produced without the need to heat the metal fibers to temperatures close to their melting point for more than 30 minutes. Conventional sintering techniques require temperatures close to or even slightly higher than the melting temperature of the metal to be maintained for relatively long periods of time. As a result of this, the material of the metal fibers can be melted or at least softened to some extent, whereby the metal fibers form a metal foil rather than a network, especially if relatively high pressures are applied during sintering. Since the network of metal fibers is not a metal foil, i.e. the structure of the metal material used to produce the network of metal fibers can still be discerned in the network of metal fibers. Thus, in a cross-sectional view of the network of metal fibers, there are voids that are not part of the metal fibers but are between the metal fibers of the network fiber.

[0046] According to the invention, the metal fibers, before being fixed together, preferably exhibit an exothermic event when heated in a DSC measurement, where this exothermic event releases energy in an amount of 0.1 kJ / g or more, more preferably in an amount of 0.5 kJ / g or more, even more preferably in an amount of 1.0 kJ / g or more, and most preferably in an amount of 1.5 kJ / g or more. This absolute amount depends very much on the metal or metal alloy used. The extent of the exothermic event can be determined by comparing the DSC measurements of the metal fibers before and after thermal equilibrium. In other words, metal fibers exhibiting such an exothermic event are not in their thermodynamic equilibrium at ambient temperature. During heating in a DSC measurement, the metal fibers can go from a metastable state to a thermodynamically more stable state, for example by crystallization, recrystallization, or another relaxation process that reduces defects in the lattice of metal atoms. Exothermic events observed in metal fibers when heated, e.g., during DSC measurements, indicate that the metal fibers are not in their thermodynamic equilibrium, e.g., the metal fibers may be in an amorphous or nanocrystalline state with defect energy and / or crystallization energy being released during heating of the metal fibers for crystallization or recrystallization to occur. Such events can be recognized, e.g., using DSC measurements. It has been found that networks of metal fibers exhibiting such exothermic events exhibit improved strength after the metal fibers are bonded together.

[0047] According to another embodiment, the metal fibers have a non-circular cross section, in particular a rectangular, square, partial circle, or elliptical cross section with a major and a minor axis, such cross sections result in fibers that are not usually in their thermal equilibrium, i.e. in a metastable state, which may be beneficial in some applications.

[0048] In this connection, it should be noted that, obviously, the value of the minor axis must be smaller than the value of the major axis. If the minor axis has a larger value than the major axis, i.e. is longer than the major axis, then the definitions of "short" and "long" simply need to be swapped.

[0049] It may be preferable that the ratio of the minor axis to the major axis is in the range of 1 to 0.05, preferably in the range of 0.7 to 0.1, in particular in the range of 0.5 to 0.1. As is generally known, the greater the ratio between the lengths of the minor and major axes of an ellipse, the more similar the ellipse is to a circle, with a ratio of 1. The smaller the value of this ratio, the more oblate the ellipse. The ratio of the minor axis to the major axis is therefore in particular less than 1.

[0050] Alternatively, the metal fibers may include circular cross sections, where the ratio of the "long" axis to the "short" axis is clearly exactly 1. Circular cross sections include cross sections with energetically more favorable states where the aspect ratio is less than 1. Thus, fibers with circular cross sections are energetically closer to their equilibrium state than fibers with cross sections of another shape.

[0051] According to another embodiment of the present invention, the metal fibers are melted by 10 2 Kmin -1 Such metal fibers can be obtained, in particular by vertical or horizontal melt spinning, by subjecting them to a cooling rate of 10 or more. Such metal fibers produced by melt spinning may contain regions spatially trapped in a high energy state (i.e., metastable state) due to the rapid cooling effected during the melt spinning process. Rapid cooling in this context can be achieved at temperatures of 10 2 K.min -1 More than 10, preferably 4 K.min -1 A cooling rate of 10 or more is preferable. 5 K.min -1 This means a cooling rate of at least this.

[0052] Also, fibers obtained by melt spinning often include rectangular or semi-elliptical cross sections, which are preferred for certain applications due to their departure from their equilibrium state. Examples of melt spinning machines capable of producing such fibers are known, for example, from the yet unpublished international application PCT / EP2020 / 063026, and published applications WO 2016 / 020493 A1 and WO 2017 / 042155 A1, which are incorporated herein by reference.

[0053] According to another example, at least some of the metal fibers of the plurality of metal fibers are amorphous or at least some of the metal fibers of the plurality of metal fibers are nanocrystalline. The nanocrystalline metal fibers include crystalline domains. Upon heating to a temperature of about 20-60% of the melting temperature of the nanocrystalline metal fibers, recrystallization of these domains occurs, resulting in an increase in the average size of the crystalline domains compared to the average size of the initial crystalline domains in the nanocrystalline metal fibers prior to heating. It is also possible to mix non-equilibrium fibers (e.g. nanocrystalline or amorphous fibers) with equilibrium (e.g. annealed) fibers.

[0054] In some applications it is preferred that the metal fibers are in electrical contact with each other. This may be desirable, for example, when the assembled network is to be used in electrochemical applications such as batteries, fuel cells, etc.

[0055] According to one embodiment, the metal fibers are in direct electrical contact with each other so that the electrical conductivity can be maximized. In this regard, it is particularly preferred that all metal fibers are sintered to other metal fibers, most preferably directly to other metal fibers, without the need for additional binders, such as polymer binders. Therefore, it is even more preferred that the metal fibers are bonded to each other without the use of polymer binders, since such polymer binders often have insufficient electrical conductivity and high temperature performance.

[0056] The metal fibers may preferably comprise at least one of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, chromium, vanadium, titanium, aluminum, silicon, lithium, manganese, boron, combinations thereof, and alloys comprising one or more of the above, such as CuSn8, CuSi4, AlSi1, Ni, stainless steel, Cu, Al, or Vitrovac alloys. Vitrovac alloys are Fe-based and Co-based amorphous alloys. It may be particularly preferred that the metal fibers are made of copper, or aluminum, or stainless steel alloys. Different types of metal fibers can be combined with each other, whereby the filter may comprise, for example, metal fibers made of copper, one or more stainless steel alloys, and / or aluminum. Particularly preferred is a network made of metal fibers, in which the metal fibers are copper, aluminum, cobalt, stainless steel alloys, the stainless steel alloys containing copper, aluminum, silicon, and / or cobalt.

[0057] According to another aspect of the invention, there is provided a network of metal fibers, said network comprising a plurality of metal fibers attached to each other at contact points, wherein the metal fibers either have a non-circular cross section, such as a rectangular, square, partial circle, or an elliptical cross section having a major axis and a minor axis, or the metal fibers have a circular cross section, said fibers further having a width that is generally constant along the length of the fiber, the variation in the width of the fiber along its length being preferably less than 40%, more preferably less than 30%, or even more preferably less than 20%.

[0058] Preferably, the width of the fibers along their length varies by less than 20%, more preferably less than 10%, even more preferably less than 5%, or most preferably less than 1%. As used herein, the change in fiber width refers to a comparison of the width of the fibers before and after they are sintered together.

[0059] In previously known networks, the fibers usually have an irregular shape, so that it cannot be guaranteed that the width of a fiber does not vary too much along its length. For example, if the width of the fiber varies greatly, said fiber may split at the narrower part, i.e. the fiber has a constriction and then a break occurs there. On the other hand, fibers with a (near) constant width have the advantage that the individual fibers can connect to each other at any position along their length, without the risk of breaks occurring during this process.

[0060] In this connection, it is noted that the metal fibers are preferably substantially constant in width, i.e. the variation in the width of the fibers along their length is preferably less than 40%, more preferably less than 30% or even more preferably less than 20%. As mentioned above, when metal fibers are heated at a slow heating rate, a rearrangement process at the atomic level occurs in order to reach an energy level closer to their equilibrium state. This possibly leads to further changes in the shape of the fibers, as perfect spheres are the most preferred state. When such shape changes start, the fibers may begin to break apart by forming constrictions that may cause breaks in the fibers. Eventually, if heating is too long, the fibers will turn into metal droplets. In the method according to the invention, fast heating and cooling rates are used, and, if performed, short fixing times are also used. In the resulting fixed metal fiber network, the fibers are substantially free of such breaks, thereby maintaining the fiber length. Furthermore, due to the fast heating and cooling rates, shape changes in the fiber cross section can be avoided, i.e. there is dynamic control over the fiber shape. As a result, the method of the invention provides a high degree of control over the fiber shape.

[0061] It may be preferred that the fibers of the plurality of fibers are sintered to each other, more preferably directly sintered to each other, which eliminates the need for additional frames or the like to hold the fibers together. Furthermore, by sintering the metal fibers directly to each other, the connection points become conductive, which results in a relatively small internal resistance of the metal fiber network.

[0062] It may be preferred that the ratio of the minor axis to the major axis is in the range of 1 to 0.05, preferably in the range of 0.7 to 0.1, in particular in the range of 0.5 to 0.1. As already mentioned above, fibers with a flatter cross section are energetically further away from their equilibrium state and they store more energy than fibers with, for example, a circular cross section.

[0063] In fact, by using fibers with a larger or smaller ratio between the length of the minor and major axes as mentioned above, it may be possible to select the properties of the network depending on the application of the network. Thus, by using fibers with a smaller ratio, the mechanical stability of the network is reduced and the weight is reduced, while by using fibers with a larger ratio, the mechanical stability of the network is improved and the weight is increased. This can be selected depending on the application where the properties are more important. Due to the dynamic control obtained by the present invention, the shape of the fibers is substantially maintained, i.e. the aspect ratio of the fibers is substantially maintained. As a result, the properties of the network can be easily adjusted by starting with fibers with the desired final shape.

[0064] According to one embodiment, the network is a regular or irregular network. Such irregular networks have, for example, good electrical conductivity in all directions and anisotropic fluid properties. Furthermore, irregular metal fiber networks are easier to manufacture than regular fiber networks. Nevertheless, in some applications, it may be preferred that the fibers in the network are aligned in different directions to obtain individual fiber orientation. Thus, it may be preferred that in the network, some or all of the fibers have an orientation, i.e. the fiber lengths are not randomly oriented, but have a dominant orientation in one or more spatial directions. The dominant orientation of the metal fibers allows the filter to have isotropic fluid properties.

[0065] According to another embodiment of the invention, the network has open pores between the metal fibers of the plurality of metal fibers. The porosity of the network is preferably up to 95% by volume. It is also preferred that the porosity of the network is greater than 80% by volume. It is even more preferred if the porosity is in the range of 80% to 95% by volume. An active material, such as an active electrode material or an active catalyst material, can be mixed into the open pores. It is even more preferred that at least some of the metal fibers of the plurality of metal fibers in the network according to the invention are at least partially coated. This coating can be an active material, such as, for example, an electrode active material that interacts with Li ions in the battery, or a catalytically active material that converts CO to CO2, or a catalytically active material that is active in hydrolysis. A coating can also be applied on the metal fibers that improves the adhesion of the metal fibers to each other, thereby increasing the mechanical strength of the network. The porosity can be determined by reconstructing the network structure using microcomputer tomography and then evaluating the porosity using the bubble point method described below.

[0066] By way of example, the active electrode materials of such batteries are for the anode: graphite, silicon, silicon carbide (SiC), and tin oxide (SnO), tin dioxide (SnO2), and lithium-titanoxide (LTO); and for the cathode: lithium nickel manganese cobalt oxide (NMC), lithium nickel cobalt aluminum oxide (NCA), lithium cobalt oxide (LiCoO2), and lithium iron phosphate (LFP).

[0067] The network can have an average pore size selected in the range of 0.1 to 100 μm, preferably in the range of 0.5 to 50 μm, in particular in the range of 1 to 10 μm. The average pore size can be determined by reproducing the fiber structure using a microcomputer tomograph and then evaluating the average pore diameter using the bubble point method. With the bubble point method, the maximum ball diameter that can fit between two fibers is determined, which is considered as the pore size. More specifically, a point is placed in the center between the two fibers and the radius of the bubble having said point as its center is increased until it touches the surface of both fibers. The diameter of the bubble corresponds to the pore size. If, at any given parameter, the bubble diameter touches only one fiber, the center point is moved in the direction of the fiber that the bubble did not touch.

[0068] It is particularly preferred if in the network of metal fibers according to the invention the metal fibers are attached to one another at contact points which are randomly distributed over the network of metal fibers, in particular directly attached. According to another aspect of the invention, it is preferred that the contact points are not randomly distributed, but are for example provided in the peripheral area of ​​the network of metal fibers, or that the metal fibers are regular, so that the contact points are also regular. It is even more preferred that the contact points at which the metal fibers are attached to one another are localized in certain areas and are not provided uniformly over the entire network of metal fibers. If the contact points at which the metal fibers are attached to one another are only present in separate areas, the fibers lying between these areas can have a high flexibility and at the same time ensure mechanical stability and good electrical conductivity.

[0069] The thickness of the network of the present invention is not particularly limited. However, it may be preferable if the network has a thickness of 0.01 mm or more. More preferably, the thickness of the network is 0.03 mm or more, even more preferably 0.05 mm or more, even more preferably 0.07 mm or more, and most preferably 0.1 mm or more. If the thickness of the network is less than 0.01 mm, there is a risk that the mechanical stability of the network will be insufficient. The upper limit of the network thickness is not particularly limited. However, depending on the application, the upper limit may be 3.0 mm or less, or 2.5 mm or less. For battery applications, the preferred thickness of the network is in the range of 0.1 mm to 0.5 mm. A network having a thickness within this range is advantageous for stacking and winding the network coated with the active material to manufacture a battery. Another preferred thickness range is in the range of more than 0.5 mm to 5 mm, more preferably in the range of 1 to 3 mm.

[0070] According to another aspect of the invention, a network of metal fibers is provided, said network may for example be a network according to the invention obtainable by the method according to the invention.

[0071] The invention will now be described in more detail, by way of example only with reference to the accompanying drawings and photographs, by way of various examples of networks and methods of the invention. [Brief description of the drawings]

[0072] [Figure 1] FIG. 1 is a representative diagram showing a typical process that may occur during sintering. [Diagram 2] 13 is a frame from a video showing sintering and relaxation of CuSi4. [Diagram 3] 13 is a frame from a video showing sintering and relaxation of AlSi1. [Figure 4] 1 is a micrograph of a conventional sintered fiber network. [Diagram 5] 1 is a micrograph of sintered CuSi4 fibers having a flat shape. [Figure 6]1 is a micrograph of a sintered AlSi1 fiber having a flat shape. [Figure 7] 1 is a micrograph of cleaned and sintered AlSi1 fibers having a flattened shape. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0073] Figure 1 shows typical diffusion processes that cause relaxation during the sintering process. The various arrows indicate surface diffusion 1, lattice diffusion (from the surface) 2, evaporation and condensation 3, grain boundary diffusion 4, lattice diffusion (from the boundary regions) 5, and volume diffusion 6. Whereas in processes 1-3 no shrinkage occurs, but the fibers are only connected to each other, in processes 4-6 material is removed from the boundary regions and deposited on the sintered necks. This results in a reduction in the surface of the fibers and associated reduction in their free energy ΔG, as already mentioned above.

[0074] By the method of the present invention, only the boundary regions of the metal fibers are thermally activated, whereby the fibers 10 sinter together, but in order to maintain the shape and dimensions of the fibers, some of the fibers 10 are rounded, which may be associated with an enlarged surface area that provides beneficial properties for many applications, e.g. electrochemical or filtration applications.

[0075] In the method according to the invention, a loose network of fibers 10 is formed at an assembly site 12. The fibers 10 are then attached to each other by forming contact points 14 between the individual fibers 10. To form the contact points 14, the method according to the invention provides three steps. A. The plurality of fibers 10 are heated to a bonding temperature selected within the range of 50-98% of their melting point temperature at a heating rate of more than 50 K / min, in particular more than 100 K / min, in particular more than 200 K / min, preferably more than 1000 K / min. B. Optionally, the above-mentioned fixing temperature can then be maintained for a fixing time selected within the range of 0 seconds to 30 minutes, in particular within the range of 0 seconds to 15 minutes, preferably within the range of 0 seconds to 5 minutes. C. Finally, said plurality of fibers 10 are cooled at a cooling rate of more than 50 K / min, preferably more than 100 K / min, in particular to a temperature below 60% of their melting point.

[0076] Furthermore, before carrying out the three steps A, B and C, pressure can be applied to the fibers 10 so that the single fibers 10 are in contact with each other. The pressure can be relatively low, i.e. in the range of 0.05 to 1 GPa, and serves to form contacts between the unconnected metal fibers. It is not necessary to maintain pressure when carrying out steps A, B and C, i.e. it is sufficient to compress the fibers for a short time by applying the pressure only before carrying out steps A, B and C, and not during. It is preferable not to apply external pressure during steps A, B and C. By avoiding the force of external pressure, the risk of the metal fibers being deformed into metal foils can be avoided, especially when operating at a fixing temperature close to the melting temperature.

[0077] Furthermore, before carrying out steps A, B, and C above, a further cleaning step can be carried out, which includes heating the plurality of fibers 10 to a cleaning temperature, such that decomposition, i.e., evaporation or combustion, of additives and / or impurities that may be present on the surface of the fibers 10 occurs, thereby resulting in a clean surface of the fibers 10. This step of cleaning the fibers 10 is further described below in relation to FIG.

[0078] Compared to previously known methods, steps A, B and C are performed more quickly, as follows: the maximum time to perform all three steps can be defined as less than 45 minutes, or even in the range of about 15 minutes. It has already been possible to show that times of less than 5 minutes, or even less than 1 minute, are possible with the method according to the invention, which are well below the typical times used for sintering, which previously required several hours.

[0079] To make such short times feasible, step A, and optionally steps B and C, are performed using a furnace or other heating device configured for fast heating and cooling rates, such as an induction furnace, an infrared furnace, high temperature ceramic heating elements, and / or a zone furnace, e.g., a conveyor furnace (not shown in the drawings).

[0080] Of particular interest is that the fibres 10 which are intended to be connected to one another are heated to a precise fixing temperature lying in the range of 50-98%, in particular in the range of 80-98%, in particular in the range of 90-98% of the melting temperature of said fibres 10. This precise fixing temperature is determined by the material used for the fibres 10 (see also Tables 1 and 2 below). By choosing the right fixing temperature, the fibres 10 can be connected to one another and do not start to change shape, i.e. circularise due to the relaxation process described above, or melt.

[0081] To allow the determination of said fixing temperature and time, trial and error experiments and / or electron microscopy experiments can be carried out on samples of real metal fibers. In the case of electron microscopy experiments, the fiber is placed in the heating stage of an in-situ SEM (scanning electron microscope). For this, the fiber needs to have a good thermal connection with the heating stage, since in high vacuum there is almost no heat transfer. Thus, a heat-stable graphite paper can be used, for example one sheet can be used as a support between the fiber and the heating stage, and another sheet with a hole in the middle can be used to view the fiber. Such a fiber sandwich structure is then moved into the heating stage and pressed down. The heating stage is then heated to a temperature close to the melting temperature. The fiber cross-section is then observed by SEM until the fibers start to connect with each other. In this way, the fixing temperature is determined. In a second experiment, at least the above steps A and C are carried out until the desired degree of connection and therefore the desired connection strength is obtained. For such trial and error experiments, a certain amount of fiber is placed in a high-speed heating furnace. To achieve contact points between the fibers, the networks can be pressed against each other or placed on a plate with space holders and a cover plate. After removing the air / oxygen in the furnace and setting the test atmosphere, the furnace is heated to a possible, i.e. determined, fixing temperature and maintained for a certain time, which can be the fixing time. Depending on the outcome of the fibers, for example whether the fibers are connected to each other and / or whether the shape of the fibers changes, the parameters have to be adjusted. In this connection, it is noted that three possible outcomes can be expected: 1) the fibers do not sinter, 2) the fibers sinter but become rounded, or 3) the fibers do not sinter but become rounded. In the case of the first outcome, the fixing temperature should be increased and / or the fixing time should be increased. In the case of the second outcome, the fixing temperature should be decreased and / or the fixing time should be decreased, and in the case of the third outcome, the heating rate should be increased, the fixing temperature should be decreased and / or the fixing time should be decreased.

[0082] In the case of some materials, it is also beneficial to supply a protective gas, for example argon, nitrogen Ar-W5 (5% by volume H2 in Ar), Ar-W2 (2% by volume H2 in Ar), forming gas (5% by volume H2 in N2) or another noble gas, to the assembly site 12 in order to prevent oxidation of the metal fibers 10. Whether or not such a supply of protective gas is necessary can be selected depending on the material of the fibers 10.

[0083] The contact points 14 of the assembled network may be regularly or irregularly distributed throughout the network, depending on the application of the assembled network, thereby allowing the fibers to bond together. The amount of contact points 14 may also be selected depending on the application of the network by applying higher or lower pressure to the fibers 10, at least prior to performing steps A and C, so that more or fewer contact points 14 are formed. Fiber density, i.e., amount of fibers per volume, and / or fineness of the fibers may also be used to adjust the number of contact points 14.

[0084] The contact points 14 also allow electrical conductivity throughout the assembled network. Thus, a high number of contact points 14 may be beneficial in applications where high electrical conductivity of the network is required. On the other hand, for filters, the number of contact points 14 formed throughout the network may not be as critical, provided that all of the fibers 10 are still held together.

[0085] The fibers 10 used in the assembly of the network according to the invention have a length of 1.0 mm or more, and / or a width of 100 μm or less, and / or a thickness of 50 μm or less (see Figures 2-6). Such fibers 10 can be produced, for example, by the so-called vertical or horizontal melt spinning process described in the literature in International Application PCT / EP2020 / 063026 (not yet published), WO 2016 / 020493 A1, and WO 2017 / 042155 A1. These fibers 10 often have a cross-section and an overall elliptical, rectangular, or flat shape. Moreover, fibers 10 produced by melt spinning often store a large amount of energy.

[0086] In order to more fully understand the method according to the present invention, several experiments were carried out, which are described below in conjunction with FIGS.

[0087] Fibers of copper alloys (CuSi4 (4 wt.% Si and 96 wt.% Cu) and AlSi1 (1 wt.% Si and 99 wt.% Al)) were sintered together while maintaining the flat ribbon-like structure of the fibers. To perform a systematic examination of the process, the fibers were heated in an electron microscope at a heating rate of 10 K / min and videos were recorded. Figure 2 shows one frame from the video of CuSi4 at special times such as the start of sintering (left to see the blurring of the sharp transitions 14 between the fibers 10) and when the fibers 10 start to round out, forming necks 15 and breaks 16, and the fibers 10 break apart (right). The corresponding temperatures are very close to each other, which is why utmost temperature precision and control is required to obtain the best results.

[0088] FIG. 3 shows frames of a video taken with AlSi1 fibers under the same conditions as the video of FIG. 2. FIG. 3 shows the same characteristic time points described in FIG. 2, namely the beginning of the sintering and the beginning of the rounding process. It could be observed that the fibers sintered together at about 602°C and on the other hand at 624°C. Furthermore, between 602°C and 624°C, the fibers transformed from flat ribbon-like fibers towards fibers with a circular cross section. This can be recognized by the fact that the fibers become thinner at 624°C than at 602°C. It should be noted that the videos whose frames are shown in FIGS. 2 and 3 were recorded under slow heating conditions (10 K / min) in high vacuum conditions. These conditions are different from those of the present invention. This is the reason why the values ​​shown in FIGS. 2 and 3 are different from those shown in the table below. Nevertheless, FIGS. 2 and 3 show the difficulty of sintering the fibers in the conventional method using slower heating rates.

[0089] In conventional thermal sintering using furnaces such as resistance heating furnaces, the fibers 10 are heated at a rate of 10-20 K / min, i.e. relatively slowly. During this time, the fibers 10 undergo a so-called relaxation process, during which the energy stored in these fibers, for example by their production by a melt spinning process, is slowly released and is no longer available for forming connection points between the metal fibers. The release of the stored energy during the slow heating not only affects the mechanical properties of the fibers 10, but also increases the energy required during the actual sintering, since the fibers 10 are no longer in thermodynamic disequilibrium as they were after production. For this reason, untreated fibers 10 obtained from melt spinning, and fibers 10 tempered for 1 h at 300 ° C for comparison, were brought to the sintering temperature within 1 min in a fast heating furnace (here an infrared furnace). This temperature was maintained for 1 min and then cooled as quickly as possible (from the sintering temperature to less than 600 ° C in less than 30 s). In addition to infrared heaters, other possible heating devices are for example ceramic heaters or induction heaters. Very short process times, only 1 minute or less, are sufficient to sinter the fibers 10 together at the contact points 14, but the energy and time are insufficient to transform the fibers 10 into a thermodynamically favorable circular shape. This is not possible with conventional heating and cooling rates, which require a long time to reach the target temperature (several hours from the sintering temperature to less than 600° C.). It is still possible to sinter the fibers 10 together using conventional heating and cooling rates. However, the sintered fibers conform to an ideal circular shape and are damaged, for example by possible necking 15 or even breakage 16 at twisted points. Due to the very long diffusion paths when the fibers become circular, either high temperatures and / or long times are necessary to transform them into a thermodynamically favorable circular shape. This can be avoided by using fibers with stored energy, which is obtained, for example, in the production by melt spinning. The stored energy can be measured, for example, by DSC measurements, where it can be observed in the form of exothermic events.

[0090] Further tests were carried out on the length of time that the fibres 10 made of AlSi1 and CuSi4 respectively needed to be heated at a certain temperature to reach their ideal round shape. The fibres of AlSi1 had a ribbon-like structure with an average length of 30 mm, an average width of 75 μm and an average thickness of 15 μm. The fibres of CuSi4 had a ribbon-like structure with an average length of 20 mm, an average width of 35 μm and an average thickness of 7 μm. For these tests, the fibres were heated within 1 minute to the fixing temperature determined as shown in the table below. After maintaining the fixing temperature for a certain time, they were rapidly cooled naturally to about 500° C. within 30 seconds for CuSi4 and to room temperature in about 20 minutes, and to about 330° C. within 30 seconds and to room temperature in 15 minutes for AlSi1. After cooling, it was checked whether the fibres had a rounded cross section. The experiments were repeated at each fixing temperature with increasing fixing times. The results of these tests are shown in the table below.

[0091] [Table 1]

[0092] It is clear that the higher the bonding temperature is chosen, the shorter the time that must be maintained to sinter the fibers 10 together without changing their outer diameter. It is also clear that said temperature depends on the material from which the fibers 10 are made. Furthermore, the size of the fibers, especially their thickness and width, has an effect on the rate at which the cross-sectional shape of the fibers changes from flat to circular. The above experiments show how easily a person skilled in the art can determine suitable conditions by simple trial and error for each fiber material.

[0093] Although different metal fibers with respect to material and / or dimensions may require different conditions to bond together, the above experimental studies have demonstrated that if the time between the fibers 10 sintering together is minimized, the fibers 10 can be connected into a network without changes in length, shape, and / or diameter of the fibers 10.

[0094] FIG. 4 shows a conventional sintered network of previously flat fibers 10. The formation of necks 15 and breaks 16 in the fibers 10 can be clearly seen. Furthermore, the cross-section of the fibers has changed from flat to circular. The formation of sintered necks, or necks, corresponds to the current theory explained in connection with FIG.

[0095] Table 1 shows the sintering temperatures (1 min holding time in each case) of thermally untreated (obtained from melt spinning) and tempered (1 h at 300 °C under argon atmosphere) CuSi4 fibers 10 with ribbon-like structure with an average length of 20 mm, an average width of 35 μm and an average thickness of 7 μm. Similarly, table 2 shows the same for AlSi1 fibers 10 with an average length of 30 mm, an average width of 75 μm and an average thickness of 15 μm. A comparison between untreated and tempered fibers was made using a tube furnace under a protective gas atmosphere (argon) with a heating rate of 10 K / min. It was found that in the case of tempered CuSi4 fibers 10, a temperature of at least 950 °C and a holding time of at least 1 h are necessary to sinter the fibers 10 together. After sintering, the previously tempered fibers 10 are almost perfectly circular and in some cases are strongly limited in length by neckings 15 and breaks 16. In contrast, sintering of thermally untreated CuSi4 fibers begins at much lower temperatures (between 890 and 910 °C) than tempered fibers (which begin sintering above 950 °C) and is completed within 0.5–5 min for CuSi4 fibers and 0.5–5 min for AlSi1 fibers, depending on the consolidation temperature, with lower consolidation temperatures requiring longer consolidation times.

[0096] [Table 2]

[0097] [Table 3]

[0098] Comparative experiments with relaxed fibers 10 (heat treatment at 300° C. under protective gas for 1 h, no change in shape, only degradation of defects, release of stored energy) show that the sintering described here is not possible or is only possible at higher temperatures than for untreated fibers 10. For relaxed fibers, the temperature window between the onset of sintering and the change in fiber shape is very narrow. However, when using fibers with stored energy, for example fibers that show an exothermic signal during DSC measurements, the temperature window for sintering the fibers together without rounding is much wider. With slow heating and cooling rates of the known sintering process, the fibers 10 undergo a relaxation process before the sintering temperature can be reached, and the stored energy is released very quickly, which is therefore not available to drive the sintering process. If slow heating rates are used, the fibers undergo tempering before the sintering temperature is reached. As a result, they behave similarly to the annealed fibers reported in Tables 1 and 2. The more energy is stored in the fibers 10 during the manufacturing process, the lower the sintering temperature and the shorter the time that can be required.

[0099] The maximum possible energy can be introduced by fast quenching rates, for example by the well-known melt spinning process. Due to the fundamental mechanism of the process, the method according to the invention can be transposed to almost all metal, metal-inorganic and equivalent alloys and materials, provided that sufficient energy is stored.

[0100] Figure 5 shows CuSi4 fibres 10 sintered by the method according to the invention so as to still maintain their original flat shape. Figure 6 shows the same for AlSi1 fibres 10. It can be clearly seen that the fibres 10 in figures 5 and 6 have an almost constant width along their length, i.e. no necking 15 or even breaks 16 that could cause a complete breakage of the fibres 10 are visible. That is to say, the variation in width of the fibres 10 in the network according to the invention is less than 40%, preferably even less than 30%.

[0101] Finally, FIG. 7 shows a network of sintered AlSi1 fibers 10 with a flattened cross section and whose surfaces were cleaned in the aforementioned cleaning step before heating the fibers 10 to the bonding temperature. As can be seen in FIG. 7, the surface of the fibers 10 appears smoother than the AlSi1 fibers shown in FIG. 6. It can therefore be concluded that the surface of the fibers 10 has been cleaned of previously present impurities. Such impurities often arise as a byproduct of the manufacturing process of the fibers 10, which cannot be avoided. Therefore, in order to obtain a good sintering result, an additional step of cleaning the fibers 10 by heating them to a temperature selected within the range of 40% to 60% of their melting point can be a preferred option. EXAMPLES

[0102] To demonstrate the effectiveness of the additional step of cleaning the fiber 10, three experiments were performed to demonstrate the benefits of the cleaning step.

[0103] Experiment 1: Sintering of (melt-spun) metal fibers in a ceramic heating element oven In experiment 1, a self-assembled oven containing a ceramic heating element was used. A number of fibres 10 made of an aluminium-silicon alloy (1 wt% Si in Al) and produced by conventional melt spinning (as explained above) were placed on the heating surface of the oven. The heating surface then heated the fibres 10 to a temperature of 640° C. within 4 minutes, which corresponds to an average heating rate of about 155 K / min. It is further noted in this context that commonly known ovens tend to start off heating at a faster heating rate and then quickly slow down as the higher temperatures are reached.

[0104] After the fibers 10 were heated above the bonding temperature of 640° C., the temperature was maintained for 10, 20, 30, and 60 seconds, respectively, such that all of the fibers 10 were connected to each other, i.e., sintered, regardless of the time the fibers 10 were maintained at the bonding temperature.

[0105] The subsequent cooling step was carried out naturally, i.e., the fiber 10 was cooled without external intervention. After about 1 minute the temperature of the fiber 10 was already below 500° C., followed by below 300° C. after about 5 minutes, which amounts to an average cooling rate of about 68 K / min for cooling the fiber from 640° C. to 300° C.

[0106] As a result, it was found that the fibers 10 were sintered together during the process without any change in cross section.

[0107] The above experiment 1 was further carried out under protective gas (ie, Ar) to prevent oxidation of the fibers 10 .

[0108] Experiment 2: Sintering of metal (extraction wheel) fibers in a ceramic heating element oven This experiment 2 was carried out in the same manner as experiment 1. However, the fibers 10 used were made of the same alloy, namely AlSi1, but they were produced using the so-called extraction wheel method, which is a commonly known (metal) fiber production method (see, for example, Cramer, A., et al., Tailored magnetic fields in the melt extraction of metallic filaments. Metallurgical and Materials Transactions B, 2009. 40(3): p. 337-344; or Park, MH, YS Song, and JH Won. A Study on the Fabrication of Metal Fiber by Fine Melt Extraction Process. in Advanced Materials Research. 2007. Trans Tech Publ.). Due to the use of the extraction wheel method, the fibers 10 produced thereby were coarser, i.e. less fine, than those produced by melt spinning. Therefore, an additional cover plate was placed on top of the fibers 10 to allow better contact between the multiple fibers 10.

[0109] After undergoing the heating and cooling steps under the same conditions as described above, these sintered fibers 10 were also found to have their cross-section unchanged after the sintering process, still having their half-moon cross-section (formed by the manufacturing method) even after being connected to each other.

[0110] Experiment 3: Sintering of metal (melt spun) fibers in a ceramic heating element oven with additional cleaning process This experiment 3 was carried out in the same way as experiment 1. However, to demonstrate that impurities and / or additives may remain on the surface of the fibers 10 during manufacture, the fibers 10 were additionally heated to a temperature of about 400° C. before being sintered together. As an example, thin paraffin oil and PVA (polyvinyl alcohol) are used. In both cases, the fibers 10 were kept at the aforementioned cleaning temperature for about 5 minutes, which decomposed the additives present on the surface, which could be removed using a gas stream at the assembly site 12 of the fibers 10.

[0111] After removing the decomposed, i.e. evaporated / decomposed / burned additives from the assembly site 12, the fibers were heated from 400° C. to the fixing temperature of 640° C. in about 2 minutes, i.e. with an average heating rate of about 70 K / min. As in experiment 1, the fixing temperature was maintained for a few seconds and then cooled again with an average cooling rate of about 68 K / min.

[0112] Again, it could be shown that the fibers still had the same cross-section as before, and furthermore no residue remained on the surface. This can be seen in Figure 7, where sintered fibers 10 are shown that still have a flat cross-section and no paraffin oil and / or PVA is found remaining on their surface.

[0113] Thus, comparing the above three experiments 1-3, it can be seen that overall, an additional cleaning step is not necessary to make the fibers sinterable to each other without changing their cross-section, but it can help improve the sintering quality, as the resulting clean surfaces are better able to connect to each other than contaminated surfaces.

Claims

1. A method for assembling a fiber network comprising a plurality of metal fibers (10), comprising: providing a loose network of said plurality of metal fibers (10) at an assembly site (12); bonding the plurality of metal fibers (10) together by forming contact points (14) between the individual metal fibers (10); heating said plurality of fibers (10) to a bonding temperature selected within the range of 50-98% of their melting point temperature at a heating rate of more than 50 K / min, in particular more than 100 K / min, in particular more than 200 K / min, preferably more than 1000 K / min; cooling the plurality of fibers (10) at a cooling rate of more than 50 K / min, preferably more than 100 K / min; and An assembly method comprising:

2. 2. The method for assembling a fiber network according to claim 1, further comprising the step of maintaining the fixing temperature for a fixing time selected in the range of 0 seconds to 30 minutes, in particular in the range of 0 seconds to 15 minutes, preferably in the range of 0 seconds to 5 minutes, wherein the step of maintaining the fixing temperature is performed before cooling of the plurality of fibers.

3. 3. A method for assembling a fiber network according to claim 1 or 2, comprising a further step carried out before the step of fixing the plurality of fibers to one another, said further step comprising cleaning the plurality of fibers by heating the plurality of fibers to a cleaning temperature selected within the range of 20% of the melting temperature of the fibers, in particular from room temperature to 60% of the melting temperature of the fibers.

4. The method of assembling a fiber network according to claim 3 , wherein the step of cleaning the plurality of fibers comprises using a gas flow at the assembly location.

5. 4. The method for assembling a fiber network according to claim 3, wherein the step of cleaning the plurality of fibers comprises reducing the air pressure at the assembly site, preferably to a pressure of less than 80 kPa, more preferably to less than 50 kPa, even more preferably to less than 10 kPa, in particular to less than 1 kPa, to less than 0.1 kPa, or even to 0.0001 kPa or less.

6. 4. The method for assembling a fiber network according to claim 3, wherein the further step of cleaning comprises determining a compound to be removed and selecting a pressure reduction and / or temperature increase based on a vapor pressure curve of the compound to be removed, in particular further comprising reducing the pressure and / or increasing the temperature based on the vapor pressure curve in a stepwise or continuous manner.

7. 3. The method for assembling a fiber network according to claim 1 or 2, wherein, prior to bonding the plurality of metal fibers (10) to one another, the method further comprises the step of exposing the plurality of metal fibers (10) to a predetermined pressure, in particular less than 1 GPa.

8. 3. The method for assembling a fiber network according to claim 1 or 2, wherein a protective gas such as argon, Ar-W5 (5 vol.% H2 in Ar), Ar-W2 (2 vol.% H2 in Ar), forming gas (5 vol.% H2 in N2) or another noble gas is supplied to the assembly site (12).

9. 3. The method for assembling a fiber network according to claim 1 or 2, wherein the step of heating the fibers (10) is performed by an induction furnace, an infrared furnace, a high temperature ceramic heating element, and / or a zone furnace, e.g. a conveyor furnace.

10. The method for assembling a fiber network according to claim 1 or 2, wherein the step of heating is performed by a continuous furnace.

11. 3. The method for assembling a fiber network according to claim 1 or 2, wherein the bonding temperature is determined in situ by electron microscopy.

12. The method for assembling a fibre network according to claim 1 or 2, wherein the fixing temperature is selected within the range of 80-98%, in particular within the range of 90-98%, of the melting point temperature of the metal fibres.

13. 3. The method for assembling a fiber network according to claim 1 or 2, wherein the steps of heating the plurality of fibers, the optional step of maintaining a bonding temperature, and the step of cooling the plurality of fibers are performed for a predetermined time period of less than 30 minutes, preferably less than 15 minutes, in particular less than 5 minutes, in particular less than 1 minute.

14. 3. The method of claim 1 or 2, wherein the predetermined time is divided equally between the steps of heating the plurality of fibers and cooling the plurality of fibers.

15. 3. The method for assembling a fiber network according to claim 1 or 2, wherein in the step of cooling the fibers, the cooling rate is maintained above 50 K / min, preferably above 100 K / min, until the fibers are cooled to a temperature below 60% of the melting point temperature of the metal fibers.

16. 3. The method for assembling a fiber network according to claim 1 or 2, wherein the metal fibers (10) have a length of 1.0 mm or more and / or a width of 100 μm or less and / or a thickness of 50 μm or less.

17. 3. The method for assembling a fiber network according to claim 1 or 2, wherein prior to the step of heating the plurality of fibers, the width of the fibers along their length changes by less than 20%, more preferably by less than 10%, even more preferably by less than 5%, or most preferably by less than 1% compared to the initial width of the fibers.

18. 3. The method for assembling a fiber network according to claim 1 or 2, wherein the metal fibers (10), before and / or after being fixed together, exhibit an exothermic event when heated during a DSC measurement, said exothermic event releasing energy preferably in an amount of 0.1 kJ / g or more, more preferably in an amount of 0.5 kJ / g or more, even more preferably in an amount of 1.0 kJ / g or more, and most preferably in an amount of 1.5 kJ / g or more.

19. 3. The method for assembling a fiber network according to claim 1 or 2, wherein the metal fibers (10) comprise a non-circular cross section, in particular a rectangular, square, part-circular or elliptical cross section having a major axis and a minor axis.

20. 19. A method for assembling a fibre network according to claim 18, wherein the ratio of the minor axis to the major axis is in the range of 1 to 0.05, preferably in the range of 0.7 to 0.1, in particular in the range of 0.5 to 0.

1.

21. 3. The method for assembling a fiber network according to any of the preceding claims, wherein the metal fibers (10) comprise a circular cross section.

22. In particular, by vertical or horizontal melt spinning, the molten material of the metal fibers is ２ Kmin －１ 3. The method for fabricating a fiber network according to claim 1 or 2, wherein the metal fibers (10) can be obtained by using cooling rates equal to or greater than the above.

23. 3. The method for assembling a fiber network according to claim 1 or 2, wherein at least a portion of the metal fibers (10) of the plurality of metal fibers are amorphous or at least a portion of the metal fibers (10) of the plurality of metal fibers are nanocrystalline.

24. 3. The method for assembling a fiber network according to claim 1 or 2, wherein the metal fibers (10) are in electrical contact with each other.

25. 3. A method for assembling a fiber network according to claim 1 or 2, wherein the metal fibers (10) are in direct electrical contact with each other.

26. 3. The method for assembling a fiber network according to claim 1 or 2, wherein the metal fibers (10) comprise at least one of copper, silver, gold, nickel, palladium, platinum, cobalt, iron, chromium, vanadium, titanium, aluminum, silicon, lithium, manganese, boron, combinations of the above, and alloys comprising one or more of the above, such as CuSn8, CuSi4, AlSi1, Ni, stainless steel, Cu, Al, or vitrovac alloys.

27. A network of metal fibers, comprising a plurality of metal fibers secured to one another at contact points (14); The metal fibers (10) have a non-circular cross section, in particular a rectangular, square, semicircular or elliptical cross section having a major and a minor axis, or The metal fiber (10) comprises a circular cross section; 1. A network of metal fibers, wherein the fibers (10) have a generally constant width along the length of the fibers, the variation in the width of the fibers along their length being less than 40%, preferably less than 30%, in particular less than 20%.

28. 28. The network of claim 27, wherein the metal fibers (10) of the plurality of metal fibers do not include constrictions (16).

29. 30. The network of metal fibers of claim 27 or 28, wherein individual fibers of the plurality of fibers are sintered to one another.

30. A network of metal fibres according to claim 27 or 28 of the preceding claims, wherein the ratio of the minor axis to the major axis is in the range of 1 to 0.05, preferably in the range of 0.7 to 0.1, in particular in the range of 0.5 to 0.

1.

31. 29. The network of metal fibers according to any of the preceding claims 27 or 28, wherein the network is a regular or irregular network.

32. 30. The network of metal fibers of claim 27 or 28, wherein the network has open pores between the metal fibers of the plurality of metal fibers.

33. 29. A network of metal fibers according to any of the preceding claims 27 or 28, wherein the contact points (12) between the metal fibers are randomly or regularly distributed throughout the three-dimensional structure of the network.

34. A network of metal fibres obtainable by the method according to the preceding claim 1, in particular a network according to claim 27 or 28.

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