Sintered metal filter medium, method for producing the same, filter cartridge, and method for filtering supercritical carbon dioxide

Abstract

The present application relates to a kind of sintered metal filter medium and its preparation method, filter core and supercritical carbon dioxide filtering method, at least including thin layer, the sintered metal filter medium is hollow tubular structure, the thin layer is porous structure, the porous structure of the thin layer has pinning node, the particle size of the pinning node is not less than 0.4 μm, the average particle size of the pinning node is 0.5-2.6 μm, the surface distribution density of the pinning node is 1-10 per 10 square microns, the area ratio of pinning node is not more than 85%;The IPA initial foaming point of the sintered metal filter medium is 50-500kPa.The sintered metal filter medium of the present application can reduce the cracking probability in post-processing process, especially cutting process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of filter materials, in particular to a sintered metal filter medium, a preparation method thereof, a filter cartridge and a supercritical carbon dioxide filtering method. BACKGROUND

[0002] The sintered metal filter medium is made of metal powder as raw material, is formed by pressing / spraying / deposition filtering and the like, and is further sintered at high temperature, and can be used to separate impurities in a fluid and achieve precision filtration. For example, it can be used to filter high-purity fluid required by the semiconductor and microelectronic manufacturing industry, and can be used to filter high-purity fluid required by other surface manufacturing steps such as etching and cleaning.

[0003] The fluid can be in the form of a gas or a liquid, or in the form of a supercritical fluid such as supercritical carbon dioxide. Filtering supercritical carbon dioxide needs to be carried out under relatively harsh supercritical conditions, for example, a temperature higher than 30℃ and a pressure higher than 25MPa, 30MPa, 35MPa or even 40MPa, which puts a very high requirement on the pressure resistance of the filter element. The sintered metal filter medium can be used for filtering such high-pressure fluid due to its high mechanical strength.

[0004] Prior art with publication number USRE36249E1 discloses a high porosity metal membrane filter device for semiconductor industry gas purification, wherein the preparation method of the high porosity metal membrane element includes: depositing a bed of substantially uniform low-density sinterable dendritic material into a mold suitable for applying a compressive force thereto, then compressing the low-density bed of sinterable dendritic material to form a green body, and finally sintering the green body at a temperature lower than the melting point of the metal material. The porous metal membrane element obtained by sintering in this way is in the form of a sheet. When the production scale needs to be increased and the filtration area of the sheet-shaped porous metal membrane element needs to be increased, the overall volume of the filter device will become very large, which is not suitable for the installation and production line environment of high integration of semiconductor processes.

[0005] In order to solve the defect that the existing sheet-shaped porous metal filter medium element is not convenient to expand the filtration area, a hollow tubular sintered metal filter medium has been developed.

[0006] However, the sintered metal filter medium needs to go through post-processing procedures such as cutting and welding to be made into a filter cartridge product. For example, the excess part at both ends of the sintered metal filter medium is cut off by a cutting knife to obtain the required size, and the sintered metal filter medium with the determined size is welded and assembled with a plug or a shell assembly and the like to obtain the final filter cartridge product. In these post-processing procedures such as cutting operation, the sintered metal filter medium often cracks unpredictably, which greatly restricts the yield of the filter cartridge.

[0007] Therefore, there is a need to provide a sintered metal filter medium with low cracking possibility in a post-processing procedure. SUMMARY

[0008] To reduce the cracking rate of the sintered metal filter medium in a post-processing procedure, especially in a cutting procedure, the present application provides a sintered metal filter medium and a preparation method thereof. A filter cartridge comprising the sintered metal filter medium and a supercritical carbon dioxide filtration method are also provided.

[0009] A sintered metal filter medium comprising at least a fine layer, the sintered metal filter medium being in a hollow tubular structure; wherein the fine layer is a porous structure, and the porous structure of the fine layer has pinning nodes with a particle size not less than 0.4 μm, an average particle size of the pinning nodes being 0.5-2.6 μm, a surface distribution density of the pinning nodes being 1-10 per 10 square microns, and an area ratio of the pinning nodes being not more than 85%; and the sintered metal filter medium has an IPA initial foaming point of 50-500 kPa.

[0010] It is generally believed that the cracking of the sintered metal filter medium is caused by poor mechanical strength or uneven stress, and thus the probability of cracking can be reduced by improving the mechanical strength or the stress uniformity. By controlling the sintering process to control the sintering degree of the metal filter medium, for example, increasing the sintering degree of the metal filter medium to form thicker sintering necks between the metal particles, the mechanical strength can be improved; and using raw materials with narrower particle size distribution and more uniform particle size for sintering can improve the stress distribution uniformity of the metal filter medium. Therefore, reducing the distribution width of the particle size of the metal particles and appropriately increasing the sintering degree of the metal filter medium, it is generally believed that a sintered metal filter medium with lower cracking probability can be obtained.

[0011] Correspondingly, based on the above cognition, fine layer metal particles with smaller particle size and narrower distribution width of particle size should be used in combination with an appropriately increased sintering degree to improve the strength and stress uniformity of the fine layer of the sintered metal filter medium, so that the sintered metal filter medium has higher natural strength and is less likely to have local weak points due to stress concentration, and is not easy to form cracks in the post-processing process, even if local micro-cracks are generated, they are not easy to further spread through some stress concentration points, to solve the problem of cracking of the sintered metal filter medium in the cutting process.

[0012] However, the inventors of the present application found that the technical solutions expected to solve the cracking problem of the metal filter medium in the post-processing process cannot solve the cracking problem; and due to the slightly higher sintering degree and the small particle size of the fine layer metal particles, the final metal filter medium has a smaller pore size, although the interception capacity is good, but the flux has a more obvious decline. On the contrary, the inventors unexpectedly found that when the final sintered metal filter medium includes a plurality of pinning nodes with a larger size, a particle size of not less than 0.4 μm, and an average particle size of 0.5-2.6 μm in the fine layer porous structure, and a surface distribution density of 1-10 per 10 square microns, and the area ratio of the pinning nodes is not more than 85%, the sintered metal filter medium has a significantly reduced cracking phenomenon in the post-processing process.

[0013] The surprise of this result is that the larger size pinning nodes will inevitably lead to an increase in the sintering driving force required by the fine layer metal particles in the surrounding area due to their small specific surface area and poor diffusion speed, which on the one hand will lead to a decrease in the uniformity of sintering in each region of the fine layer, which often means a further increase in stress inhomogeneity, and on the other hand means a decrease in the sintering degree in the surrounding area of the pinning nodes, which will inevitably lead to a decrease in the overall mechanical strength of the fine layer. Whether it is the increase in stress inhomogeneity or the decrease in mechanical strength, the expected effect should be an increase in the cracking probability of the sintered metal filter medium, rather than a result of obtaining a lower cracking probability.

[0014] The reason why the special solution of the present application can significantly reduce the cracking rate may be due to the fact that:

[0015] The sintered metal filter medium provided by the present application has an IPA initial bubble point of 50-500 kPa, which means that the fine layer in the sintered metal filter medium can effectively intercept particles with a particle size of 100 nm or less, and has pores of nanometer order. Considering that the pores of the fine layer porous structure are obtained by sintering the fine layer metal particles as boundaries, the pore size is highly related to the particle size of the fine layer raw metal particles, so the nanometer order of the pores generally also means that the particle size of the fine layer metal particles is also of nanometer order.

[0016] As aforementioned, the existence of pinning nodes in the nanoscale fine layer inevitably leads to a wider distribution of the particle size of the fine layer metal particles, which results in the difference in sintering degree of different regions. Specifically, the existence of pinning nodes with larger particle size means that during the sintering process of the sintered metal filter medium, the process of mutual fusion of the metal particles around the pinning nodes and the formation of larger grains through grain boundary migration is delayed, preventing the rapid growth of grains due to the rapid migration of grain boundaries, so that the fine layer around the pinning nodes has a moderately refined grain size and a lower sintering degree; while the fine layer in the region without pinning nodes has a larger grain size and a higher sintering degree due to lower required sintering driving force, larger specific surface area of metal particles and faster diffusion speed. In short, the existence of pinning nodes leads to the existence of high-sintering regions with higher sintering degree and low-sintering regions with lower sintering degree in the fine layer, which indeed inevitably affects the stress uniformity and the overall mechanical strength of the fine layer.

[0017] However, for the fine layer sintered from nanoscale metal powder, the refined grains can improve the toughness of the material, and the smaller grain size increases the number of grain boundaries, which can hinder the propagation of cracks and thus improve the fracture toughness of the material. In addition, the high-density dislocations and cracking points in the fine-grained material can better improve the toughness and deformation resistance. That is, the moderately refined grains, i.e. the reduction of the grain size, are beneficial to improve the toughness of the sintered body. Therefore, the high-sintering regions in the fine layer have higher hardness and become actual hard regions, while the low-sintering regions near the pinning nodes in the fine layer have higher toughness and become actual soft regions, and the hard regions and soft regions are arranged alternately to form the porous structure of the fine layer.

[0018] In the post-processing process of the metal filter medium, such as the cutting process, whether the cutting tool is rapidly rotated or the metal filter medium is rapidly rotated, the tangential relative motion will occur at the contact position between the outer surface of the fine layer and the cutting tool, which makes the position of the fine layer contacting the cutting tool subjected to a greater tangential force. Under the action of the tangential force, the fine layer has a tendency to separate the circumferentially adjacent regions.

[0019] Under this tendency, if fine layer metal particles with more uniform particle size and the sintering degree of the metal filter medium are appropriately increased, the porous structure of the fine layer has a high sintering degree, and the metal particles form a whole with higher hardness through a large number of sintering necks. When the tangential force acts on a region, it does not directly cause the region to separate from the axially adjacent region. The porous structure with higher integrity inevitably transmits the tangential force to the adjacent position in the axial direction, so that the fine layer produces circumferential separation between the blocky porous structures, which is manifested as the axial extension cracking phenomenon.

[0020] The fine layer of the present application is a porous structure with special hard zones and soft zones distributed in intervals. Therefore, although the tangential force is inevitably transmitted axially in the hard zone, it is also inevitably transmitted to the soft zone with good toughness and certain plastic deformation capacity. The soft zone dissipates the tangential force through certain plastic deformation, making it difficult to further transmit axially and even more difficult to tear the next hard zone circumferentially. The pinning effect of the pinning node itself, on the other hand, hinders the axial transmission of the tangential force, making the tangential force have to bypass the pinning node to reach the next hard zone, thereby prolonging the cracking path and naturally increasing the energy required for cracking. Under the action of multiple factors, the crack propagation is inhibited at the micro level and does not produce macro cracking.

[0021] In short, the existence of the pinning node makes the porous structure of the fine layer a special structure with hard zones and soft zones distributed in intervals. On the one hand, if the tangential force directly acts on the soft zone, the plastic deformation of the soft zone reduces the probability of micro-crack formation to a certain extent. On the other hand, if the tangential force directly acts on the hard zone, a micro-crack is formed. When the micro-crack extends a certain length to reach the soft zone, the plastic deformation of the soft zone makes the micro-crack unable to continue to extend, and even more unable to tear the next hard zone circumferentially. In addition, the pinning node hinders the further extension of the micro-crack, which prolongs the length of the cracking path and increases the energy required for cracking. The combination of multiple factors makes the sintered metal filter medium provided by the present application have a significantly reduced probability of cracking during cutting operation. Since the sintered metal filter medium has a high temperature at the welding point during welding, the temperature on both sides of the circumference is low. The uneven temperature leads to different shrinkage and expansion of each region, thereby forming an uneven stress region in the circumference and having a tendency to separate the circumferentially adjacent regions. Therefore, the special structure of the present application can also reduce the cracking probability of the sintered metal filter medium during the welding process.

[0022] It should be noted that, as mentioned above, local micro-cracks still exist, but their size is controlled at the micro level. Moreover, after the sintered metal filter medium is cut, these local micro-cracks will eventually be fused and sealed during the subsequent welding process, and will not affect the integrity of the filter core.

[0023] More specifically, in the present application, the surface distribution density of the pinning nodes not less than 1 per 10 square microns means that the number of soft zones is sufficient, and the particle size not less than 0.4 μm and the average particle size not less than 0.5 μm means that it can affect more surrounding metal particles, so that the volume of a single soft zone is large enough, and the number of soft zones is sufficient, and the volume of the soft zones is sufficient. The large number of soft zones not only greatly reduces the probability of micro-crack formation, but also effectively prevents the continuous extension of micro-cracks, and the pinning effect of the pinning nodes itself also helps to prevent the spread of cracks. Of course, since the metal filter medium in the present application needs to be used in high-pressure supercritical fluid filtration and other application scenarios, its mechanical strength still needs to ensure that it does not break during use, therefore, the average particle size of the pinning nodes should not be greater than 2.6 μm, so as to avoid the formation of a single soft zone with too large a volume, and the surface distribution density of the pinning nodes should not be more than 10 per 10 square microns, and the area ratio of the pinning nodes should not be more than 85%, so as to avoid too many soft zones, and too many soft zones will inevitably be continuously extended rather than being distributed at intervals. Although this indeed can further reduce the possibility of cutting and cracking, it is difficult to ensure that the fine layer has a high enough hardness and strength to avoid being crushed under high-pressure filtration conditions. That is, a large number of soft zones need to be distributed at intervals to avoid axial cracking when subjected to tangential force during the post-processing stage, but a large number of hard zones also need to be distributed at intervals to avoid insufficient self-supporting ability of the fine layer porous structure under positive pressure and being crushed.

[0024] The particle size and average particle size of the pinning nodes are obtained by the following method: in the SEM image of the outer surface of the fine layer of the sintered metal filter medium, randomly select a pinning node, draw four intersecting line segments on the surface of each pinning node, the end points of each line segment are located on the boundary of the pinning node, the included angle between adjacent two line segments is close to 45°, and the length of each line segment is measured using measurement software such as NanoMeasure, and the average value of the lengths of the four line segments is taken as the particle size of the pinning node. Further, the average particle size of the pinning nodes is obtained by averaging the particle sizes of more than 20 pinning nodes.

[0025] The surface distribution density of the pinning nodes is obtained by the following method: count the number of pinning nodes in the entire field of view of the SEM image of the outer surface of the fine layer of the sintered metal filter medium, and then divide the counted number by the area of the corresponding field of view to obtain the surface distribution density of the pinning nodes.

[0026] The area ratio of pinning nodes is calculated by the following method: the sum of the areas of all pinning nodes in the SEM image of the outer surface of the fine layer of the sintered metal filter medium, for example, 20 μm*20 μm, 30 μm*30 μm or the entire field of view, is calculated, and then the sum of the areas of all pinning nodes is divided by the area of the corresponding field of view, i.e. the area ratio of pinning nodes is obtained, wherein the individual pinning nodes are approximately regarded as circles, the area of each pinning node is approximately calculated, and the sum of the areas of all pinning nodes is obtained by adding the areas of all pinning nodes.

[0027] Preferably, the average particle size of the pinning nodes is 0.6-2 μm, and the surface distribution density of the pinning nodes is 2-7 per 10 square microns.

[0028] Further preferably, the average particle size of the pinning nodes is 0.7-2 μm.

[0029] When the average particle size of the pinning nodes decreases, the pinning nodes themselves weaken the resistance to the axial propagation of microcracks, the corresponding soft zone has a smaller volume, and the soft zone also weakens the resistance to the axial propagation of microcracks, but is beneficial to improving the uniformity of stress distribution of the fine layer; conversely, when the average particle size of the pinning nodes increases, the pinning nodes themselves strengthen the resistance to the axial propagation of microcracks, the corresponding soft zone has a larger volume, and the soft zone also strengthens the resistance to the axial propagation of microcracks, but intensifies the non-uniformity of stress distribution of the fine layer. When the surface distribution density of the pinning nodes decreases, the pinning nodes are more sparse in the porous structure of the fine layer, the number of soft zones is smaller, the probability of microcrack formation is relatively higher, and the probability of the resistance to the axial propagation of microcracks is lower; conversely, when the surface distribution density of the pinning nodes increases, the pinning nodes are more dense in the porous structure of the fine layer, the number of soft zones is larger, the probability of microcrack formation is relatively lower, and the probability of the resistance to the axial propagation of microcracks is higher.

[0030] Therefore, when the pinning nodes with a relatively smaller average particle size are used, the surface distribution density thereof can be simultaneously increased, i.e. when the particle size of the pinning nodes and the volume of the soft zone are smaller, the resistance and interception of the individual pinning nodes and the individual soft zone to the axial extension of microcracks are relatively weaker, and appropriately increasing the number of pinning nodes and soft zones is beneficial to reducing the probability of microcrack formation in the cutting process and increasing the probability of the resistance to the axial propagation of microcracks, so that the sintered metal filter medium has a lower probability of cutting and cracking.

[0031] When the average particle size of the pinning nodes is relatively larger, the surface distribution density of the pinning nodes can be reduced simultaneously, that is, when the particle size of the pinning nodes and the volume of the soft regions are larger, the single pinning node and the single soft region have stronger hindering and intercepting effects on the axial extension of the micro cracks, and the uniformity of the stress distribution of the sintered metal filter medium can be improved, the hardness and the strength of the sintered metal filter medium can be improved, and the mechanical stability of the sintered metal filter medium under the high-pressure filtration working condition can be improved by appropriately reducing the number of the pinning nodes.

[0032] Specifically, when the average particle size of the pinning nodes is not less than 0.6 μm, the minimum value of the average particle size of the pinning nodes is increased, the stress non-uniformity of the fine layer can be aggravated by the pinning nodes with the relatively larger minimum value of the average particle size, and the maximum value of the surface distribution density of the pinning nodes is appropriately reduced, and the maximum value of the number of the soft regions is also reduced accordingly, that is, the pinning nodes with the relatively larger minimum value of the average particle size are matched with the slightly lower maximum surface distribution density, the uniformity of the stress distribution of the sintered metal filter medium can be improved, the strength and the hardness of the sintered metal filter medium can be improved, and the mechanical stability of the sintered metal filter medium under the high-pressure filtration working condition can be improved; at the same time, the surface distribution density of the pinning nodes is increased to not less than 2 per 10 square microns, the minimum value of the surface distribution density of the pinning nodes is appropriately increased, and the minimum value of the number of the soft regions is also increased accordingly, that is, the pinning nodes with the relatively larger minimum value of the average particle size are matched with the slightly larger minimum surface distribution density, the probability of the micro cracks formed in the cutting process can be reduced, the probability of the micro cracks hindered when the micro cracks axially spread can be improved, and the cracking probability of the sintered metal filter medium in the cutting process is lower.

[0033] When the average particle size of the pinning nodes is not more than 2 μm, the maximum value of the average particle size of the pinning nodes is reduced, the stress uniformity of the fine layer can be improved by the pinning nodes with the relatively smaller maximum value of the average particle size, and the minimum value of the surface distribution density of the pinning nodes is appropriately increased by increasing the surface distribution density of the pinning nodes to not less than 2 per 10 square microns, and the minimum value of the number of the soft regions is also increased accordingly, that is, the pinning nodes with the relatively smaller maximum value of the average particle size are matched with the larger minimum surface distribution density, the probability of the micro cracks formed in the cutting process can be reduced, the probability of the micro cracks hindered when the micro cracks axially spread can be improved, and the cracking probability of the sintered metal filter medium in the cutting process is lower; at the same time, the surface distribution density of the pinning nodes is increased to not more than 7 per 10 square microns, the maximum value of the surface distribution density of the pinning nodes is appropriately reduced, and the maximum value of the number of the soft regions is also reduced accordingly, that is, the pinning nodes with the relatively smaller maximum value of the average particle size are matched with the smaller maximum surface distribution density, the stress distribution uniformity of the fine layer can be improved, the strength and the hardness of the sintered metal filter medium can be improved, and the mechanical stability of the sintered metal filter medium under the high-pressure filtration working condition can be improved.

[0034] Preferably, the pinning nodes include at least first pinning nodes with a particle size of 0.5-1.5 μm and second pinning nodes with a particle size greater than 1.5 μm, and the number of the first pinning nodes accounts for 30-90%.

[0035] The first pinning nodes have a smaller particle size and correspondingly a smaller volume of soft zone, which is beneficial to improve the uniformity of stress distribution of the fine layer, but the first pinning nodes and the corresponding soft zone have a weaker hindering effect on the axial propagation of micro-cracks; while the second pinning nodes have a larger particle size and correspondingly a larger volume of soft zone, and the second pinning nodes and the corresponding soft zone have a stronger hindering effect on the axial propagation of micro-cracks, and meanwhile exacerbate the non-uniformity of stress distribution of the fine layer.

[0036] If the number of the first pinning nodes is too high and the particle size of the first pinning nodes is relatively small, i.e. the number of small pinning nodes is too large, and correspondingly the number of soft zones with small volume is also too large, the hindering effect on the axial propagation of micro-cracks is reduced. If the number of the second pinning nodes is too high and the particle size of the second pinning nodes is relatively large, in order to ensure that the fine layer has a high enough hardness and strength to ensure its mechanical stability in the high-pressure filtration working condition, the overall surface distribution density of the pinning nodes in the porous structure of the fine layer must be reduced, i.e. the pinning nodes are more sparse in the porous structure of the fine layer, i.e. the distance between any adjacent pinning nodes is larger, and correspondingly the probability of micro-crack formation in the cutting process of the sintered metal filter medium is increased, and the probability of the hindering effect on the axial propagation of micro-cracks is reduced.

[0037] In the present application, the number of the first pinning nodes with a particle size of 0.5-1.5 μm is set to 30-90%, and correspondingly the number of the second pinning nodes with a particle size greater than 1.5 μm is set to 10-70%, and the number of the second pinning nodes is not less than 10%. The particle size and number of the first pinning nodes and the second pinning nodes are both appropriate, i.e. the particle size is not too large or too small, and the number is not too large or too small. The first pinning nodes and the second pinning nodes cooperate to ensure that a certain number of the second pinning nodes and the corresponding soft zones have a high enough hindering effect on the axial propagation of micro-cracks, and at the same time, a large enough number of the first pinning nodes and the corresponding soft zones are used to improve the uniformity of stress distribution of the fine layer, reduce the probability of micro-crack formation, and increase the probability of the hindering effect on the axial propagation of micro-cracks, so as to further reduce the cutting and cracking probability of the sintered metal filter medium.

[0038] More preferably, the second pinning nodes include crack-resisting nodes with a particle size not less than 3 μm, and the number of the crack-resisting nodes is not more than 10% of the number of all pinning nodes and not less than 3% of the number of all pinning nodes.

[0039] The particle size of the crack blocking node is not less than 3 μm, that is, the particle size is relatively large, the volume of the corresponding soft zone is also relatively large, the strength of the hindering effect of the crack blocking node and the corresponding soft zone on the axial propagation of the micro crack is higher, and the axial size of the micro crack can be controlled to be smaller. Therefore, the existence of a small amount of crack blocking nodes is beneficial to reduce the probability of cutting cracking of the sintered metal filter medium. However, due to the relatively large particle size of the crack blocking node, the stress inhomogeneity of the corresponding soft zone is more serious. If the content is too much, it may affect the mechanical stability of the sintered metal filter medium under high pressure filtration conditions. Therefore, the amount of the crack blocking node should not be too much.

[0040] Specifically, in the present application, when the number of the crack blocking nodes is controlled to be not less than 3%, the probability of cutting cracking of the sintered metal filter medium is reduced by the appropriate amount of crack blocking nodes and the corresponding soft zone, and when the number of the crack blocking nodes is controlled to be not more than 10%, the mechanical stability of the sintered metal filter medium under high pressure filtration conditions is ensured.

[0041] Preferably, the average shape factor of the pinning node is not more than 1.15.

[0042] The average shape factor is obtained by the following method: selecting a pinning node on the SEM image of the outer surface of the sintered metal filter medium, drawing four intersecting line segments in the pinning node, the intersection point of the line segments is the approximate geometric center of the pinning node, the starting point and the ending point of the line segments are the intersection points of the line segments and the edge of the pinning node, the included angle between any two adjacent line segments is about 45°, the length of each line segment is measured by measurement software such as NanoMeasure, and the ratio of the difference between the maximum value and the minimum value of the lengths of the four line segments to the average value is taken as the shape factor of the pinning node. At least 10 pinning nodes are selected in the same way, the shape factor of each pinning node is calculated respectively, and then the average value is obtained to get the average shape factor.

[0043] When the average shape factor of the pinning node is not more than 1.15, for example, the length and the short diameter of the pinning node are 3 and 1 respectively, that is, the size of the pinning node in each direction does not differ too much, the shape of the pinning node is similar to ellipsoidal or spherical, the specific surface area is relatively small, the heat absorption is slow, the sintering degree of the pinning node and the fine particles near the pinning node is reduced during sintering, the grain growth rate of the fine particles in the fine layer is slowed down, the grains are refined, and the toughness of the corresponding soft zone is improved. Therefore, when the tangential force directly acts on the soft zone, the better plastic deformation capacity of the soft zone can further reduce the probability of micro crack formation; when the tangential force acts on the hard zone and the micro crack extends to the soft zone, the better plastic deformation capacity of the soft zone can better inhibit the axial propagation of the micro crack, that is, the hindering effect on the axial propagation of the micro crack is stronger.

[0044] More preferably, the pinning nodes include a plurality of quasi-spherical nodes with a shape factor of not more than 0.7, and the number ratio of the quasi-spherical nodes is not less than 60%.

[0045] The smaller the shape factor, the higher the uniformity of the pinning nodes in each direction, and the closer the shape of the pinning nodes to a spherical shape. The pinning nodes with a shape factor of not more than 0.7 are defined as quasi-spherical nodes, the toughness of the soft zone corresponding to the quasi-spherical nodes is higher, the quasi-spherical nodes and the corresponding soft zone can reduce the probability of micro-crack formation and the strength of the hindering effect on the axial propagation of micro-cracks is higher, but the quasi-spherical nodes will significantly reduce the sintering degree of the corresponding soft zone due to the slow heat absorption during the sintering process, thereby reducing the sintering integrity of the fine layer.

[0046] In the present application, the number ratio of the quasi-spherical nodes with a shape factor of not more than 0.7 is not less than 60%, which ensures that the number of soft zones with high toughness is sufficient, can greatly reduce the probability of micro-crack formation, and reduce the probability of further axial propagation of micro-cracks, thereby reducing the probability of cutting and cracking of the sintered metal filter medium.

[0047] Preferably, the average distance between the two adjacent pinning nodes is 0.2-2 μm.

[0048] The two adjacent pinning nodes refer to two pinning nodes without other pinning nodes between them, and the minimum distance between the edges of the two adjacent pinning nodes is taken as the distance between them. In the SEM image of the outer surface of the sintered metal filter medium, at least 10 groups of adjacent two pinning nodes are selected, and the distance between the 10 groups of adjacent two pinning nodes is measured by using measurement software such as NanoMeasure, and the average value of the 10 distances is taken as the average distance between the two adjacent pinning nodes.

[0049] The average distance between the two adjacent pinning nodes is not less than 0.2 μm, and the surface distribution density of the pinning nodes in the porous structure of the fine layer is not more than 10 per 10 square microns, which can avoid the soft zone to be connected in one body in the fine layer, improve the uniformity of stress distribution, and ensure that the fine layer has high enough strength and hardness to ensure the mechanical stability of the sintered metal filter medium under high pressure filtration working conditions.

[0050] On the premise that the surface distribution density of the pinning nodes in the porous structure of the fine layer is not less than 1 per 10 square microns, the average distance between the two adjacent pinning nodes is further controlled to be not more than 2 μm, which can reduce the probability of micro-crack formation and the possibility of micro-cracks passing between the two pinning nodes when propagating in the axial direction, thereby achieving the purpose of reducing the probability of cracking of the sintered metal filter medium during cutting.

[0051] More preferably, the sintered metal filter medium further comprises a coarse layer located radially inside the fine layer, the coarse layer being a porous structure, the coarse layer and the fine layer being sintered as a whole; or, the sintered metal filter medium further comprises a coarse layer located radially inside the fine layer and at least one functional layer located radially inside the coarse layer, the coarse layer and the functional layer being porous structures, the functional layer, the coarse layer and the fine layer being sintered as a whole, the functional layer being at least one of a pre-filter layer, a reinforcing layer, and a separation layer.

[0052] The porous metal film element provided by the prior art with the disclosure number USRE36249E1 has relatively uniform pore size and porosity as a whole, but the process gas to be treated may carry both large-particle-size particulate pollutants and small-particle-size particulate pollutants, and in order to ensure the trapping ability for small-particle-size pollutants, the pore size of the porous metal film element should not be too large, which makes the particulate pollutants of various particle sizes be relatively concentrated in a certain area, so that the porous metal film element often has the problem of local pores being concentrated and rapidly clogged by particulate pollutants of different particle sizes during use, resulting in short service life of the filter device.

[0053] The sintered metal filter medium of the present application can be a two-layer porous structure or a three-layer or more porous structure. When the sintered metal filter medium is a two-layer porous structure, the surface of the coarse layer away from the fine layer is the inner surface of the sintered metal filter medium, the coarse layer is formed by sintering of metal particles with larger particle size, and has a larger pore size, so that the coarse layer acts as a pre-filter layer to trap large-particle-size particulate impurities, while the fine layer with smaller pore size is used to trap small-particle-size particulate impurities, and the coarse layer and the fine layer work together to achieve high-efficiency regional trapping of particulate impurities of various size ranges, and the non-concentrated trapping structure also improves the pollutant carrying capacity of the sintered metal filter medium as a whole, avoiding the possibility of concentrated and rapid clogging. In addition, the sintered neck formed by the metal particles of the coarse layer has a larger size and higher bonding strength between particles, so that the coarse layer also plays a role in supporting and reinforcing the fine layer to improve the mechanical strength of the sintered metal filter medium as a whole.

[0054] When the sintered metal filter medium is a three-layer or more porous structure, at least one functional layer is integrally sintered to the surface of the coarse layer away from the fine layer, and the functional layer is at least one of a pre-filter layer, a reinforcing layer, and a separation layer, and can also have the functions of pre-filtering and supporting, etc.

[0055] Preferably, when the surface of the coarse layer away from the fine layer is the inner surface of the sintered metal filter medium, the pore area ratio of the surface of the coarse layer away from the fine layer is 10-40%.

[0056] The pore area ratio can be measured by the following method: the SEM image of the inner surface of the sintered metal filter medium is imported into an image processing software such as ImageJ, the sum of the areas of all the pores on the inner surface and the total area of the inner surface are calculated by the software, and the ratio of the sum of the areas of all the pores to the total area of the inner surface is the pore area ratio.

[0057] The pore area ratio of the inner surface of the coarse layer away from the fine layer is controlled to be not more than 40%, and the sintering degree of the whole coarse layer is ensured to be high enough to ensure the mechanical strength of the coarse layer itself, so that the coarse layer can effectively support the fine layer, improve the mechanical strength of the whole sintered metal filter medium, and make the coarse layer have high enough interception efficiency and ensure a certain flux to play a full pre-filtering effect. At the same time, the pore area ratio of the inner surface of the coarse layer away from the fine layer is controlled to be not less than 10%, and the coarse layer itself is ensured to have enough pores to reduce the flow resistance, so that the coarse layer and the whole sintered metal filter medium have high enough flux.

[0058] Preferably, the average width of the sintering neck of the inner surface of the coarse layer away from the fine layer is 0.2-6 μm, the average maximum Fretter diameter of the pores is 0.5-7 μm, and the ratio of the maximum Fretter diameter to the minimum Fretter diameter of the pores is 1.2-5.

[0059] The sintering neck of the coarse layer refers to the part of the coarse particles that is formed by welding during sintering, and the pore is the area surrounded by the boundary of the coarse particles. The average width of the sintering neck and the maximum Fretter diameter and the minimum Fretter diameter of the pores can be obtained by the following method:

[0060] The SEM image of the inner surface of the sintered metal filter medium is imported into a measurement software such as NanoMeasure, the width of at least 10 sintering necks is measured, and the average value of the widths of the 10 sintering necks is taken as the average width of the sintering neck. The SEM images of the inner surfaces of at least 10 sintered metal filter medium products are imported into an image processing software such as ImageJ, the maximum Fretter diameter and the minimum Fretter diameter of the pores in the SEM images of each product are measured, the average value of the maximum Fretter diameters of the 10 products is calculated to obtain the average maximum Fretter diameter of the pores, and the ratio of the maximum Fretter diameter to the minimum Fretter diameter of the pores is further calculated.

[0061] The average width of the sintering neck of the coarse layer is controlled to be not less than 0.2 μm, the average maximum Fretter diameter of the pores is controlled to be not more than 7 μm, and the ratio of the maximum Fretter diameter to the minimum Fretter diameter of the pores is controlled to be not more than 5, which ensures that the coarse particles of the coarse layer are sintered sufficiently, the mechanical strength of the coarse layer itself is high enough to better support the fine layer, the pore size of the coarse layer is not too large, and the size difference in each direction of the pore is not too large, i.e. the shape of the pore is relatively regular, so that the coarse layer can provide a better pre-filtering effect.

[0062] Further control the average width of the sintering neck of the coarse layer to be no more than 6 μm, and the average maximum Fretter diameter of the hole is no less than 0.5 μm, the ratio of the maximum Fretter diameter to the minimum Fretter diameter of the hole is no less than 1.2, to avoid excessive sintering of coarse particles, leading to the occlusion of the pores of the coarse layer, so that the coarse layer has enough and large pores, and the uniformity of the size of the pores in each direction is better, to ensure that the coarse layer and the sintered metal filter medium can provide a higher flux.

[0063] Preferably, the thickness of the sintered metal filter medium is preferably 120-3000 μm, and the thickness of the fine layer is 1 / 6-2 / 3 of the thickness of the sintered metal filter medium.

[0064] Firstly, the thickness of the sintered metal filter medium as a whole is not less than 120 μm, and the thickness of the fine layer is not less than 1 / 6 of the thickness of the sintered metal filter medium as a whole, i.e. the thickness of the fine layer is not less than 20 μm, to ensure that the sintered metal filter medium itself is thick enough, and thus has a high enough mechanical strength, so as to be able to withstand the high pressure difference under the filtering working condition, and has a large enough pollution capacity and a relatively longer service life, at the same time, the fine layer is also thick enough, so that the pinning nodes can be uniformly dispersed in the fine layer and fully welded with the fine particles, to ensure the sintering integrity of the fine layer and the sintered metal filter medium as a whole, and also to ensure the interception efficiency and the pollution capacity of the fine layer; secondly, the thickness of the sintered metal filter medium as a whole is not more than 3000 μm, and the thickness of the fine layer is not more than 2 / 3 of the thickness of the sintered metal filter medium as a whole, i.e. the thickness of the fine layer is not more than 2000 μm, to avoid that the sintered metal filter medium as a whole and the fine layer are too thick, resulting in too large flow resistance and too low filtering flux.

[0065] Preferably, the porosity of the sintered metal filter medium is 20-40%.

[0066] The porosity of the sintered metal filter medium is measured by the drainage method, and the specific operation steps are as follows:

[0067] Firstly, the apparent volume V0 of the sintered metal filter medium is calculated from the inner and outer diameters and the axial height of the hollow tubular sintered metal filter medium;

[0068] Then, the hollow tubular sintered metal filter medium is completely immersed in water with a volume of V1, vacuumed for 30 min, to ensure that the water completely fills the pores of the hollow tubular sintered metal filter medium, and the total volume V2 of the water in which the hollow tubular sintered metal filter medium is immersed is read out, then the porosity of the sintered metal filter medium is .

[0069] The porosity of the sintered metal filter medium is not less than 20%, so that a greater flux can be provided and a better interception effect can be provided, and the porosity of the sintered metal filter medium is not greater than 40%, so that the sintering is sufficient to ensure that the sintered metal filter medium has higher strength and hardness.

[0070] Preferably, the fine layer is formed by sintering a mixture, the mixture comprising at least fine particles and doping particles, the doping particles forming pinning points of the porous structure of the fine layer, the doping particles having a particle size greater than the particle size of the fine particles.

[0071] The second aspect of the present application further provides a preparation method of the sintered metal filter medium, comprising the following steps:

[0072] S1: providing a mixture, the mixture comprising fine particles and doping particles uniformly dispersed in the fine particles, compressing the mixture to obtain a hollow tubular green body, the mixture being prepared by the following steps:

[0073] A1: mixing a dispersing agent having a surface tension of not greater than 30 mN / m with fine particles and doping particles to obtain a mixed precursor, wherein the fine particles have a particle size of 50-300 nm, the doping particles have a particle size of not less than 0.4 μm and an average particle size of 0.5-2.6 μm, and the mass of the doping particles is 15-85% of the total mass of the fine particles and the doping particles;

[0074] A2: standing the mixed precursor, and then performing ultrasonic operation on the mixed precursor to obtain a dispersed precursor;

[0075] A3: stirring and heating the dispersed precursor, and drying to obtain a mixture;

[0076] S2: sintering the green body obtained in S1 to obtain a sintered metal filter medium preform;

[0077] S3: cutting at least one end portion of the sintered metal filter medium preform obtained in S2 to obtain a sintered metal filter medium.

[0078] In step A1, by controlling the particle size of the doping particles to be not less than 0.4 μm and the average particle size to be 0.5-2.6 μm, and the mass of the doping particles to be 15-85% of the total mass of the fine particles and the doping particles, the particle size of the pinning nodes in the fine layer obtained by sintering and the surface distribution density of the pinning nodes in the porous structure of the fine layer can be controlled within the range of the present application. The fine particles and the doping particles are mixed with a dispersant having a surface tension of not more than 30 mN / m to obtain a mixed precursor. The dispersant has a low surface tension and good wetting properties, which is beneficial to the uniform mixing of the doping particles and the fine particles, better wets the surface of all the particles, effectively avoids the formation of agglomerates of the fine particles in the mixing process, and preliminarily disperses the agglomerates of the fine particles, thereby reducing the difficulty and energy consumption of step A2. More preferably, the mass of the doping particles is 30-85% of the total mass of the fine particles and the doping particles.

[0079] In step A2, when the mixed precursor is left to stand, the agglomerates of the fine particles and the doping particles will deposit to the bottom of the mixed precursor. By setting the mass of the doping particles to be 15-85% of the total mass of the fine particles and the doping particles, the content of the doping particles is high enough to cause the agglomerates of the fine particles and the doping particles to be distributed at intervals in the bottom of the mixed precursor after the mixed precursor is left to stand. The probability of the doping particles being dispersed around the agglomerates of the fine particles is relatively high. Therefore, when the mixed precursor left to stand is subjected to ultrasonic treatment, the agglomerates of the fine particles are not only impacted by the ultrasonic waves, but also impacted by the doping particles. During the impact or collision process of the agglomerates of the fine particles, once a small gap is formed between the fine particles, the dispersant having a surface tension of less than 30 mN / m can quickly penetrate into the gap, the surface of the fine particles in the agglomerates that have not been completely dispersed is gradually wetted, the speed of the complete dispersion of the agglomerates is accelerated, and the surface of the fine particles dispersed is fully wetted by the dispersant, so that the agglomerates are not formed again, and finally the dispersed precursor in which the agglomerates of the fine particles are completely dispersed is obtained.

[0080] It should be noted that it is generally believed that the standing operation cannot improve the dispersion effect, but can cause the separation of particles with different particle sizes. However, the present application uses the standing operation in combination with the ultrasonic operation to achieve the dispersion of the agglomerates of the nano-scale particles, i.e., the standing operation generally used for the separation of materials is used for the dispersion of the agglomerates of the fine powder, thereby widening the application range of the standing operation.

[0081] Since the doped particles in the dispersion precursor obtained in step A2 are basically at the bottom of the dispersion precursor and have not been uniformly dispersed in the fine particles, the dispersion precursor obtained in step A2 is further stirred and heated in step A3. The stirring is to uniformly disperse the doped particles in the fine particles, and the heating is to evaporate the dispersant at the same time, thereby solidifying the dispersion of the doped particles in the fine particles, avoiding the doped particles uniformly dispersed in the fine particles from depositing again at the bottom of the dispersion precursor, and obtaining a dry mixture of uniformly dispersed doped particles in fine particles.

[0082] The mixture obtained in step A3 is further compressed to obtain a hollow tubular green body, i.e., the green body obtained in step S1 is completed, and the green body is sintered in step S2 to obtain a sintered metal filter medium pre-product. Finally, at least one end of the sintered metal filter medium pre-product obtained in step S2 is cut in step S3 to obtain a sintered metal filter medium.

[0083] It should be noted that the sintered metal filter medium of the present application can be a two-layer porous structure or a three-layer or more porous structure, so step S1 can provide a porous structure raw material according to the overall structure of the sintered metal filter medium. For example, when the sintered metal filter medium further includes a coarse layer located radially inside the fine layer, step S1 can also provide coarse particles with a particle size of 1-10 μm. When the mixture is located radially outside the coarse particles during the pressing of the green body, the coarse particles and the mixture are compressed to obtain a hollow tubular green body, and then steps S2 and S3 are sequentially performed to obtain a sintered metal filter medium.

[0084] Steps A1, A2 and A3 cooperate with each other to obtain the above-mentioned dry mixture in which the doping particles are uniformly dispersed in the fine particles, and due to the fact that the mixture and the coarse particles are sufficiently compressed to obtain the green body before sintering, the particles form certain contact and extrusion, and further solidify the dispersion result of the doping particles in the fine particles. Therefore, during the sintering process, the dispersion state of the doping particles in the fine particles is basically unchanged, and the uniform dispersion state of the doping particles in the mixture before compression is basically maintained, and correspondingly, after the sintering is completed, the coarse particles are sintered to form the porous structure of the coarse layer, the mixture forms the porous structure of the fine layer, and the doping particles form the pinning points in the porous structure of the fine layer, and correspondingly, the dispersion state of the pinning points in the porous structure of the fine layer is similar to the dispersion state of the doping particles in the mixture; and due to the fact that the particle size of the doping particles is relatively large, the heat absorption is slow, the deformation is small and can be ignored, and the particle size is basically unchanged during the sintering process, so that the particle size of the pinning points in the sintered metal filter medium is still not less than 0.4 μm, and the average particle size is 0.5-2.6 μm, the surface distribution density of the pinning points in the porous structure of the fine layer is 1-10 per square micrometer, and the area ratio of the pinning points is not more than 85%. In addition, due to the fact that the particle size of the fine particles is 50-300 nm, the pore size of the fine layer is also in the nanometer level, so that the IPA initial foaming point of the sintered metal filter medium is 50-500 kPa.

[0085] It should be noted that in the preparation process of the sintered metal filter medium provided in the present application, before sintering, the doping particles with a corresponding particle size are uniformly dispersed in the fine particles in a certain mass ratio to form a mixture, the mixture is compressed to obtain a hollow tubular green body, and the green body is further sintered to obtain the porous structure of the fine layer, and the pinning points are dispersed in the porous structure of the fine layer with a corresponding surface density. Among them, the uniform dispersion of the doping particles in the fine particles is a key step, which directly determines the dispersion state of the pinning points in the porous structure of the fine layer after sintering. Therefore, other processes such as spraying, filtering and retaining are not suitable for the preparation of the sintered metal filter medium, because the large doping particles and the fine particles are often layered, which cannot guarantee the uniform dispersion of the doping particles in the fine particles, and correspondingly, the pinning points in the porous structure of the fine layer are not uniformly dispersed.

[0086] Preferably, step A2 is repeated 2-4 times; and / or, in step A2, the standing time is 30 min-90 min, the ultrasonic power is 100-300 W, and the ultrasonic time is 3-15 min.

[0087] The mixed precursor is subjected to at least two times of standing and ultrasonic operation, which ensures that the agglomerates of fine particles in the mixed precursor are thoroughly dispersed, facilitates the full sintering of the fine particles, and avoids the presence of unsintered fine particles; and the mixed precursor is subjected to at most four times of standing and ultrasonic operation, which avoids the breakage and damage of the doped particles, resulting in irregular shapes of the doped particles, which is not conducive to the formation of the soft zone or causes the toughness of the soft zone to decrease.

[0088] The standing time of the mixed precursor is at least 30 min, and accordingly, more agglomerates of fine particles and doped particles are separated from the fine particles and deposited at the bottom of the mixed precursor after the completion of one standing operation, so that a large number of agglomerates of fine particles are dispersed in the subsequent ultrasonic operation, thereby improving the dispersion efficiency of a single ultrasonic operation, and further reducing the number of standing and ultrasonic operations. When the standing time is less than 30 min, the number of standing and ultrasonic operations needs to be increased to sufficiently disperse the agglomerates of fine particles due to the relatively short standing time. When the standing time of the mixed precursor reaches 90 min, the amount of agglomerates of fine particles and doped particles separated from the fine particles has reached a maximum value, and the amount of agglomerates of fine particles and doped particles deposited at the bottom of the mixed precursor does not significantly increase even if the standing time is further prolonged, so that there is no need to set the standing time to be more than 90 min, and it is also not conducive to improving the dispersion efficiency of the mixed precursor.

[0089] Therefore, the standing time is controlled to be 30 min to 90 min, which improves the dispersion efficiency under the premise of ensuring that the agglomerates of fine particles are uniformly and sufficiently dispersed.

[0090] In order to ensure that the agglomerates of fine particles can be quickly and sufficiently dispersed, the ultrasonic power in the ultrasonic operation should not be less than 100 W, and accordingly, the ultrasonic time should not be shorter than 3 min, and at the same time, in order to avoid damage to the doped particles caused by the impact of ultrasonic waves with high power for a long time, the ultrasonic power should not be greater than 300 W and the ultrasonic time should not be longer than 15 min.

[0091] The mixed precursor is subjected to at least two times of standing and ultrasonic operation, and the standing time of each time is not less than 30 min, and the ultrasonic power of each time is not less than 100 W and the ultrasonic time of each time is not shorter than 3 min, so as to achieve the full and efficient dispersion of the agglomerates of fine particles; and the mixed precursor is subjected to at most four times of standing and ultrasonic operation, and the standing time of each time is not more than 90 min, and the ultrasonic power of each time is not higher than 300 W and the ultrasonic time of each time is not longer than 15 min, which improves the dispersion efficiency under the premise of achieving the full dispersion of the agglomerates of fine particles, and avoids the damage to the doped particles, so as to ensure that the doped particles play an effective pinning role in the sintering process, thereby improving the toughness of the soft zone.

[0092] Preferably, the material of the fine particles and the doping particles is stainless steel, and the material of the coarse particles is nickel or carbonyl nickel. In step S2, the sintering process includes a heating stage and a subsequent holding stage. In the heating stage, the temperature is uniformly raised from ambient temperature to 780-820°C at a rate of 2-8°C / min. In the holding stage, the temperature is held at 780-820°C for 20-40 min.

[0093] The heating rate in the heating stage should not be too fast. If the temperature is raised too fast from ambient temperature to the holding temperature, the temperature difference between the coarse particles and the mixture will be too large, which will result in a large difference in sintering shrinkage between the coarse particles and the mixture in the holding stage, and finally cause interlayer cracking between the porous structure of the coarse layer and the porous structure of the fine layer, i.e., the porous structure of the coarse layer and the porous structure of the fine layer fail to form effective fusion at the interface. Of course, the heating rate in the heating stage should not be too slow either. If the heating rate is too slow, the sintering efficiency will be low.

[0094] The holding time in the holding stage will affect the sintering effect. If the holding time is too short, the coarse particles will absorb too little heat and fail to melt sufficiently, the coarse particles will not form sufficient fusion between them, and the fine particles will also fail to form sufficient fusion between them and between them and the doping particles. As a result, the strength and hardness of the sintered metal filter medium will be poor, and it will not be able to withstand high pressure difference filtration conditions. If the holding time is too long, the coarse particles will be severely over-sintered, and the fine particles will also be severely over-sintered, which will cause the pores of the coarse layer and the pores of the fine layer to be blocked. Accordingly, the sintered metal filter medium will have a very low porosity and a very small pore size, and the flow resistance will be large. During filtration, the flux will be low.

[0095] Specifically, in the present application, the material of the fine particles and the doping particles in the mixture is stainless steel, the material of the coarse particles is nickel or carbonyl nickel, the particle size of the coarse particles is 1-10 μm, the particle size of the fine particles is 50-300 nm, the particle size of the doping particles is not less than 0.4 μm and the average particle size is 0.5-2.6 μm, the mass of the doping particles is 15-85% of the total mass of the fine particles and the doping particles, the green compact is uniformly heated at a heating rate of not less than 2 ℃ / min from room temperature (e.g. 25 ℃) to a holding temperature of not less than 780 ℃, and is held at the holding temperature for at least 20 min; or the green compact is uniformly heated at a heating rate of not less than 2 ℃ / min from room temperature (e.g. 25 ℃) to a holding temperature of not more than 820 ℃, and is held at the holding temperature for not more than 40 min; or the green compact is uniformly heated at a heating rate of not more than 8 ℃ / min from room temperature (e.g. 25 ℃) to a holding temperature of not less than 780 ℃, and is held at the holding temperature for at least 20 min; or the green compact is uniformly heated at a heating rate of not more than 8 ℃ / min from room temperature (e.g. 25 ℃) to a holding temperature of not more than 820 ℃, and is held at the holding temperature for not more than 40 min; in this way, when the green compact reaches the holding temperature through the heating stage, the temperature of the coarse particles and the mixture is substantially uniform, and the holding time in the holding stage is reasonable, not too long or too short, effective fusion is formed between the coarse particles, between the fine particles, between the fine particles and the doping particles, and effective fusion is also formed at the interface between the coarse layer and the fine layer, and problems such as clogging of the pores of the coarse layer and the fine layer do not occur, thereby obtaining a sintered metal filter medium pre-product which is fully sintered and has a uniform pore structure, and the sintering efficiency is also relatively high.

[0096] Preferably, in step A1, the dispersant is selected from one or more of methanol, ethanol, n-propanol, isopropanol or n-butanol, and the volume ratio of liquid to solid in the mixed precursor is 2:1-8:1.

[0097] Methanol, ethanol, n-propanol, isopropanol or n-butanol have relatively low surface tension, and have good preliminary dispersion effect on the agglomerates of the fine particles in the mixed precursor, and these solvents are common and inexpensive.

[0098] One or more of methanol, ethanol, n-propanol, isopropanol or n-butanol is used as the dispersant, and the volume ratio of liquid to solid in the mixed precursor is not less than 2:1, i.e. the volume ratio of the dispersant to the total volume of the fine particles and the doping particles is not less than 2, and the amount of the dispersant is sufficient to facilitate mixing of the fine particles and the doping particles, to sufficiently wet the surfaces of all the particles, and to ensure that the dispersant has good preliminary dispersion effect on the agglomerates of the fine particles, while the volume ratio of liquid to solid in the mixed precursor is not more than 8:1, so as to minimize the amount of the dispersant used on the premise of ensuring the preliminary dispersion effect on the agglomerates of the fine particles, and to facilitate subsequent rapid removal of the dispersant.

[0099] Preferably, before heating and stirring in step A3, the supernatant of the dispersant is removed first, the stirring rate is 300-600r / min, the vacuum degree during stirring is 0.02-0.06MPa, and the temperature is 60-80℃.

[0100] Before heating and stirring, the supernatant of the dispersant is removed first, which can quickly remove more dispersant in the shortest time and greatly shorten the removal time of the dispersant. After the supernatant of the dispersant is removed, the stirring rate is further controlled to be not less than 300r / min, the vacuum degree during stirring is not less than 0.02MPa, and the temperature is not less than 60℃; or, the stirring rate is controlled to be not more than 600r / min, the vacuum degree during stirring is not more than 0.06MPa, and the temperature is not more than 80℃, that is, by matching the stirring rate with the evaporation rate of the dispersant, when the doped particles are uniformly dispersed in the fine particles, the dispersant is just completely evaporated, which better solidifies the dispersion result of the doped particles in the fine particles, further improves the dispersion uniformity of the doped particles in the fine particles, and further improves the dispersion uniformity of the pinning nodes in the porous structure of the fine layer.

[0101] The third aspect of the present application also provides a filter element, which comprises the sintered metal filter medium.

[0102] The fourth aspect of the present application also provides a supercritical carbon dioxide filtration method, in which the supercritical carbon dioxide passes through the sintered metal filter medium. BRIEF DESCRIPTION OF DRAWINGS

[0103] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0104] Figure 1 SEM diagram of the outer surface of the sintered metal filter medium product of the present application;

[0105] Figure 2 SEM diagram of the inner surface of the sintered metal filter medium product of the present application;

[0106] Figure 3 Structure diagram of the filter element of the present application;

[0107] Figure 4 Structure diagram of the filter of the present application.

[0108] Figures 3-4 In the figure, 100 - filter core, 10 - sintered metal filter medium, 20 - end plate, 30 - joint member, 31 - hollow channel, 200 - housing, 210 - joint. DETAILED DESCRIPTION

[0109] Hereinafter, the present application will be further described by specific preparation examples, examples and comparative examples.

[0110] Preparation example of mixture

[0111] Preparation example 1

[0112] Step A1: fine particles with a particle size of 50-150 nm and an average particle size of 100 nm and doped particles with an average particle size of 1.95 μm were mixed to obtain mixed metal particles, and the mixed metal particles were mixed with a dispersant-isopropyl alcohol to obtain a mixed precursor, and the volume ratio of liquid to solid in the mixed precursor was 2:1.

[0113] In the figure, the particle size distribution of the doped particles is as follows: the first doped particles have a particle size of 0.5-1.5 μm and an average particle size of 0.62 μm, and the mass ratio is 0.66%; the second doped particles have a particle size greater than 1.5 μm and an average particle size of 2.64 μm, and the mass ratio is 99.34%, and the number ratio of the blocking particles with a particle size greater than 3 μm in the second doped particles is 14.4%; the average shape factor of the doped particles is 0.81, and the number ratio of the doped particles with a shape factor not greater than 0.7 is 86.51%; the materials of the fine particles and the doped particles are both 316 stainless steel, and the mass of the doped particles is 83.85% of the total mass of the fine particles and the doped particles. The blocking particles with a particle size greater than 3 μm after sintering form blocking nodes in the fine layer of the sintered filter medium, and the doped particles with a shape factor not greater than 0.7 after sintering form spherical nodes in the fine layer of the sintered filter medium.

[0114] Step A2: the mixed precursor was simply mixed and then stood for 30 min, and then the mixed precursor was ultrasonically treated under the condition of a power of 100 W for 6 min, and the standing and ultrasonic treatment were repeated twice to obtain a dispersed precursor, and the dispersed precursor was removed from the top of the dispersed precursor.

[0115] Step A3: the dispersed precursor was stirred and heated to a constant weight to obtain a dried mixture, wherein the stirring rate was 600 r / min, the vacuum degree during stirring was 0.06 MPa, and the temperature was 80°C. The specific raw materials and process parameters are shown in Tables 1 and 2.

[0116] Preparation example 2

[0117] The main difference between Preparation Example 2 and Preparation Example 1 is that in step A1, the mass of the doping particles is 83.18% of the total mass of the fine particles and the doping particles, wherein the average particle size of the first doping particles is larger and the mass ratio is higher, the average particle size of the second doping particles is smaller and the mass ratio is lower, and the total average particle size of the doping particles is 1.46 μm. The average shape factor of the doping particles is 0.64, and the number ratio of the doping particles with a shape factor not greater than 0.7 is 95.32%. In addition, the volume ratio of liquid to solid in the mixed precursor is 5:1.

[0118] Due to the small difference in the mass content of the doping particles and the smaller average particle size, in step A2, the number of standing and ultrasonic operation can be appropriately increased to 3 times, the ultrasonic power is correspondingly increased to 200 W, and the ultrasonic time is 3 min; in step A3, the stirring rate is increased to 450 r / min, the stirring vacuum degree is 0.04 MPa, and the heating temperature is 70°C. The specific raw materials and process parameters are shown in Tables 1 and 2.

[0119] Preparation Example 3

[0120] The main difference between Preparation Example 3 and Preparation Example 1 is that in step A1, the total amount of the doping particles is smaller, specifically, the mass of the doping particles is 50.63% of the total mass of the fine particles and the doping particles, wherein the mass ratio of the first doping particles is higher, the average particle size of the second doping particles is smaller and the mass ratio is lower, and the total average particle size of the doping particles is 0.77 μm. The average shape factor of the doping particles is 1.12, and the number ratio of the doping particles with a shape factor not greater than 0.7 is 75.78%. In addition, the volume ratio of liquid to solid in the mixed precursor is 8:1.

[0121] Due to the lower mass content of the doping particles and the smaller average particle size than the total average particle size of the doping particles in Preparation Example 1, in step A2, the number of standing and ultrasonic operation can be appropriately increased to 4 times, the standing time is prolonged to 60 min, and the ultrasonic power is correspondingly increased to 300 W, and the ultrasonic time is 12 min; in step A3, the stirring rate is reduced to 300 r / min, the stirring vacuum degree is 0.02 MPa, and the heating temperature is 60°C. The specific raw materials and process parameters are shown in Tables 1 and 2.

[0122] Preparation Example 4

[0123] The main difference between Preparation Example 4 and Preparation Example 3 is that in step A1, the total amount of the doping particles is less, specifically, the mass of the doping particles is 32.08% of the total mass of the fine particles and the doping particles, wherein the mass of the first doping particles accounts for 100%, that is, no second doping particles with a larger particle size are added, and the average particle size of the total doping particles is 0.53 pm. The average shape factor of the doping particles is 1.04, and the number percentage of the doping particles with a shape factor not greater than 0.7 is 76.37%.

[0124] Since the mass content of the doping particles is lower and the average particle size is smaller, in step A2, the appropriate standing time can be extended to 90 min, and the ultrasonic time is correspondingly extended to 15 min. The specific raw materials and process parameters are shown in Tables 1 and 2.

[0125] Preparation Example 5

[0126] The main difference between Preparation Example 5 and Preparation Example 1 is that in step A1, the total amount of the doping particles is slightly more, specifically, the mass of the doping particles is 84.92% of the total mass of the fine particles and the doping particles, and in the doping particles, the mass of the second doping particles accounts for 100%, that is, no first doping particles with a smaller particle size are added, and the average particle size of the doping particles is 2.58 pm. The average shape factor of the doping particles is 0.78, and the number percentage of the doping particles with a shape factor not greater than 0.7 is 89.59%. The specific raw materials and process parameters are shown in Tables 1 and 2.

[0127] Preparation Example 6

[0128] The main difference between Preparation Example 6 and Preparation Example 1 is that in step A1, the total amount of the doping particles is slightly more, specifically, the mass of the doping particles is 85.05% of the total mass of the fine particles and the doping particles, wherein the mass of the first doping particles is lower, the mass of the second doping particles is higher, and the average particle size of the total doping particles is 2.21 pm. The average shape factor of the doping particles is 0.61, and the number percentage of the doping particles with a shape factor not greater than 0.7 is 96.72%.

[0129] Since the amount of the doping particles added is not much different from that of Preparation Example 1, but the average particle size is larger, in step A2, the ultrasonic time can be shortened to 4 min. The specific raw materials and process parameters are shown in Tables 1 and 2.

[0130] Preparation Example 7

[0131] Preparation Example 7 mainly differs from Preparation Example 2 in that in step A1, the particle size of the fine particles is 150-300 nm, the average particle size is 250 nm, the total amount of the doped particles is less, specifically, the mass of the doped particles is 72.41% of the total mass of the fine particles and the doped particles, the amount of the first doped particles is more, the amount of the second doped particles is less, and the average particle size of the total doped particles is 1.14 μm. The average shape factor of the doped particles is 0.67, and the number of doped particles with a shape factor of not more than 0.7 accounts for 94.85%.

[0132] Since the amount of the doped particles is less and the average particle size is smaller than that of Preparation Example 2, in step A2, the number of standing and ultrasonic operation can be appropriately increased to 4 times, the standing time is prolonged to 60 min, and the ultrasonic power is correspondingly increased to 300 W; in step A3, the stirring rate is reduced to 300 r / min. The specific raw materials and process parameters are shown in Tables 1 and 2.

[0133] Preparation Example 8

[0134] Preparation Example 8 mainly differs from Preparation Example 3 in that in step A1, the average shape factor of the doped particles is 1.20, and the number of doped particles with a shape factor of not more than 0.7 accounts for 62.17%.

[0135] Since the sphericity of the doped particles is relatively lower, in step A2, the standing time is prolonged to 90 min, and the ultrasonic time is prolonged to 15 min. The specific raw materials and process parameters are shown in Tables 1 and 2.

[0136] Preparation Example 9

[0137] Preparation Example 9 mainly differs from Preparation Example 2 in that in step A1, the average shape factor of the doped particles is 1.26, and the number of doped particles with a shape factor of not more than 0.7 accounts for 54.21%.

[0138] Since the sphericity of the doped particles is relatively lower, in step A2, the number of standing and ultrasonic operation can be appropriately increased to 4 times, the standing time is prolonged to 60 min, and the ultrasonic power is increased to 300 W. The specific raw materials and process parameters are shown in Tables 1 and 2.

[0139] Preparation Example 10

[0140] Preparation Example 10 mainly differs from Preparation Example 2 in that in step A1, the particle size of the fine particles is 150-300 nm, the average particle size is 250 nm, the average shape factor of the doped particles is 0.58, and the number of doped particles with a shape factor of not more than 0.7 accounts for 98.34%.

[0141] Due to the relatively higher sphericity of the doped particles, the number of times of standing and ultrasonic operation in step A2 can be appropriately reduced to 2. The specific raw materials and process parameters are shown in Table 1 and Table 2.

[0142] Preparation Example 11

[0143] Preparation Example 11 and Preparation Example 4 mainly differ in that in step A1, the particle size of the doped particles is 0.40-0.55 μm, the average particle size is 0.45 μm, and the mass of the doped particles is 1.8% of the total mass of the fine particles and the doped particles. The specific raw materials and process parameters are shown in Table 1 and Table 2.

[0144] Preparation Example 12

[0145] Preparation Example 12 and Preparation Example 5 mainly differ in that in step A1, the particle size of the doped particles is 2-4.5 μm, the average particle size is 3.05 μm, and the mass of the doped particles is 95.03% of the total mass of the fine particles and the doped particles. In step A2, the ultrasonic time is 3 min. The specific raw materials and process parameters are shown in Table 1 and Table 2.

[0146] The particle size distribution, shape parameters, and mass ratio of the doped particles of the above Preparation Examples 1-12 are shown in Table 1 below; the process parameters of Preparation Examples 1-12 are shown in Table 2 below.

[0147] Table 1 Raw material parameter table of Preparation Examples 1-12

[0148]

[0149] Table 2 Process parameter table of Preparation Examples 1-12

[0150]

[0151] Examples and comparative examples of sintered metal filter medium Example 1

[0152] Step S1: Provide coarse particles and a mixture (obtained from Preparation Example 1) and a compression mold, the compression mold includes a compression mandrel and an annular rubber sleeve located on the outer periphery thereof and two plug heads located at the axial ends of the two, an annular cavity is formed between the compression mandrel, the annular rubber sleeve and the two plug heads, the coarse particles and the mixture are filled in the annular cavity, the coarse particles are filled on the outer periphery of the compression mandrel, and the mixture is filled on the radial outer side of the coarse particles, the filling thickness of the coarse particles is 499.6 μm, and the filling thickness of the mixture is 423.6 μm, and the coarse particles and the mixture are compressed by isostatic pressing to obtain a hollow tubular green body; wherein the compression pressure P1 is 120 MPa, the compression time T1 is 100 s, the coarse particles are nickel carbonyl powder with a particle size of 1-3 μm and an average particle size of 2.3 μm.

[0153] Step S2: sintering the green compact obtained in step S1 in a vacuum furnace, wherein the absolute pressure P2 in the vacuum furnace is 0.008 Pa, the temperature in the vacuum furnace is uniformly raised from room temperature to a holding temperature of 820 ℃ at a raising rate of 2 ℃ / min, then the temperature is kept at 820 ℃ for 20 min, and then the temperature is cooled to room temperature, thereby obtaining a sintered metal filter pre-product.

[0154] Step S3: cutting the two ends of the sintered metal filter pre-product obtained in step S2, thereby obtaining a sintered metal filter. The specific raw material selection and process parameters are shown in Tables 3 and 4. Example 2

[0155] The main difference between Example 2 and Example 1 is that in step S1, the filling thickness of the coarse particles is 559.4 μm, and the filling thickness of the mixture (obtained from Preparation Example 2) is 421.7 μm; wherein the compression pressure P1 is 90 MPa, the compression time T1 is 90 s, the coarse particles are nickel carbonyl powder with a particle size of 3-6 μm and an average particle size of 4.6 μm; in step S2, the absolute pressure P2 in the vacuum furnace is 0.004 Pa, the temperature in the vacuum furnace is raised from room temperature to a holding temperature at a raising rate of 5 ℃ / min, the holding temperature is 800 ℃, and the holding time is 30 min. The specific raw material selection and process parameters are shown in Tables 3 and 4. Example 3

[0156] The main difference between Example 3 and Example 1 is that in step S1, the filling thickness of the coarse particles is 536.1 μm, and the filling thickness of the mixture (obtained from Preparation Example 3) is 505.0 μm; wherein the compression pressure P1 is 70 MPa, the compression time T1 is 60 s, the coarse particles are nickel carbonyl powder with a particle size of 6-10 μm and an average particle size of 8.2 μm; in step S2, the absolute pressure P2 in the vacuum furnace is 0.002 Pa, the temperature in the vacuum furnace is raised from room temperature to a holding temperature at a raising rate of 8 ℃ / min, the holding temperature is 780 ℃, and the holding time is 40 min. The specific raw material selection and process parameters are shown in Tables 3 and 4. Example 4

[0157] The main difference between Example 4 and Example 3 is that in step S1, the filling thickness of the coarse particles is 539.1 μm, and the filling thickness of the mixture (obtained from Preparation Example 4) is 501.7 μm; in step S2, the holding time is shortened to 30 min. The specific raw material selection and process parameters are shown in Tables 3 and 4. Example 5

[0158] The main difference between Example 5 and Example 1 is that in step S1, the filling thickness of the coarse particles is 515.7 μm, the filling thickness of the mixture (prepared in Preparation Example 5) is 439.9 μm, and the compression time T1 is 120 s; in step S2, the holding time is 40 min. The specific raw material selection and process parameters are shown in Table 3 and Table 4. Example 6

[0159] The main difference between Example 6 and Example 1 is that in step S1, the filling thickness of the coarse particles is 506.0 μm, the filling thickness of the mixture (prepared in Preparation Example 6) is 437.1 μm, and the compression time T1 is 120 s; in step S2, the holding time is 30 min. The specific raw material selection and process parameters are shown in Table 3 and Table 4. Example 7

[0160] The main difference between Example 7 and Example 2 is that in step S1, the filling thickness of the coarse particles is 139.0 μm, the filling thickness of the mixture (prepared in Preparation Example 7) is 27.2 μm, and the compression pressure P1 is 80 MPa. The specific raw material selection and process parameters are shown in Table 3 and Table 4. Example 8

[0161] The main difference between Example 8 and Example 3 is that in step S1, the filling thickness of the coarse particles is 1311.9 μm, the filling thickness of the mixture (prepared in Preparation Example 8) is 2463.5 μm; in step S2, the holding time is shortened to 30 min. The specific raw material selection and process parameters are shown in Table 3 and Table 4. Example 9

[0162] The main difference between Example 9 and Example 2 is that in step S1, the filling thickness of the coarse particles is 521.4 μm, the filling thickness of the mixture (prepared in Preparation Example 9) is 430.2 μm, and the compression pressure P1 is 80 MPa; in step S2, the holding time is 20 min. The specific raw material selection and process parameters are shown in Table 3 and Table 4. Example 10

[0163] The main difference between Example 10 and Example 2 is that in step S1, the filling thickness of the coarse particles is 1140.9 μm, the filling thickness of the mixture (prepared in Preparation Example 10) is 912.3 μm, and the compression time is extended to 100 s; in step S2, the holding time is extended to 40 min. The specific raw material selection and process parameters are shown in Table 3 and Table 4. Example 11

[0164] The main difference between Example 11 and Example 4 is that there is no coarse particle, in step S1, the mixture obtained from Preparation Example 4 is directly compressed at 8 MPa for 50 s to obtain a hollow tubular green body; in step S2, the holding time is shortened to 25 min. The specific raw material selection, process parameters are shown in Table 3 and Table 4. Example 12

[0165] The main difference between Example 12 and Example 5 is that there is no coarse particle, in step S1, the mixture obtained from Preparation Example 5 is directly compressed at 30 MPa for 80 s to obtain a hollow tubular green body; in step S2, the holding time is shortened to 30 min. The specific raw material selection, process parameters are shown in Table 3 and Table 4. Comparative Example 1

[0166] The main difference between Comparative Example 1 and Example 11 is that in step S1, only fine particles with a particle size of 50-150 nm and an average particle size of 100 nm are compressed at a compression pressure of 8 MPa for 50 s to obtain a hollow tubular green body, wherein the fine particles are not mixed with the doped particles to form a mixture; in step S2, the holding time is shortened to 20 min. The specific raw material selection, process parameters are shown in Table 3 and Table 4. Comparative Example 2

[0167] The main difference between Comparative Example 2 and Example 11 is that in step S1, the mixture obtained from Preparation Example 11 is compressed at a compression pressure of 10 MPa for 50 s to obtain a hollow tubular green body, wherein the average particle size of the doped particles in the mixture is 0.45 μm and the mass ratio is only 1.8%; in step S2, the holding time is shortened to 22 min. The specific raw material selection, process parameters are shown in Table 3 and Table 4. Comparative Example 3

[0168] The main difference between Comparative Example 3 and Example 12 is that in step S1, the fine particles obtained from Preparation Example 12 are compressed at a compression pressure of 30 MPa for 100 s to obtain a hollow tubular green body, wherein the average particle size of the doped particles in the mixture is 3.05 μm and the mass ratio is 95.03%; in step S2, the holding time is 40 min. The specific raw material selection, process parameters are shown in Table 3 and Table 4.

[0169] Table 3 Summary of related parameters of coarse particles of Examples 1-10

[0170]

[0171] Table 4 Summary of process parameters of Examples 1-12 and Comparative Examples 1-3

[0172]

[0173] Performance test method and test results

[0174] I. Morphology parameters of sintered metal filter medium products

[0175] The average particle size, surface distribution density, area ratio, shape coefficient and other parameters of pinning nodes of the sintered metal filter medium products prepared from Examples 1-12 and Comparative Examples 1-3 can be measured by combining the SEM images of the outer surface and the inner surface of the products with manual or NanoMeasure, imageJ measurement software. The specific test method is described in the summary section.

[0176] In addition, the total thickness and the thickness of the fine layer of the sintered metal filter medium products can be directly measured from the cross-sectional SEM images of the products or directly measured using a thickness gauge or other methods.

[0177] II. Cutting and cracking performance of sintered metal filter medium products

[0178] The test method is as follows: 10 hollow tubular sintered metal filter medium products are provided for each of Examples 1-12 and Comparative Examples 1-3, each sintered metal filter medium product is cut 10 times (10 axial positions), that is, a total of 100 times (200 cuts) are cut for the products prepared for each example and each comparative example (both sides of the cut are cuts, a total of 200 cuts), whether there is obvious cracking at the cut is observed by naked eye or under about 10-100 times magnification equipment, the number of axial cracking from the cut position is counted, and the corresponding cutting and cracking probability can be calculated.

[0179] III. Service performance of sintered metal filter medium products

[0180] 3.1 Pressure holding performance of sintered metal filter medium products

[0181] The test method is as follows: first, the filter cartridges made of the sintered metal filter medium prepared from Examples 1-12 and Comparative Examples 1-3 are provided, the membrane holes of the filter medium are impregnated with molten paraffin and cooled to seal the membrane holes; then, high-pressure water is injected into the hollow interior of the filter cartridge, the pressure of the high-pressure water is gradually increased from 1 MPa, each time by 1 MPa, each pressure is held for 1 hour, until the pressure of the high-pressure water suddenly drops (the filter cartridge is broken), that is, the highest pressure resistance value of the corresponding filter cartridge is measured. For example, when the pressure of the high-pressure water is 5 MPa, the pressure holding passes, but when the pressure of the high-pressure water is increased to 6 MPa, the pressure of the high-pressure water suddenly drops, it can be preliminarily determined that the pressure holding pressure of the filter cartridge is not less than 5 MPa and less than 6 MPa; further, the pressure of the high-pressure water is increased by 0.1 MPa as the gradient, until the accurate pressure holding pressure is measured. When measuring the pressure holding pressure of the filter cartridge made of the sintered metal filter medium prepared from Comparative Example 3, it is found that the filter cartridge is broken when the pressure of the high-pressure water is 1 MPa, therefore, the pressure of the high-pressure water is decreased by 0.1 MPa as the gradient, until the pressure holding pressure thereof is measured.

[0182] It is further noted that during the pressure retention test, the pressure on the downstream side of the filter element is atmospheric pressure, and the pressure on the upstream side is the pressure of the high-pressure water. At this time, the pressure difference between the upstream and downstream sides of the filter element is almost equal to the pressure of the high-pressure water. In fact, under the working condition of filtering supercritical carbon dioxide, the pressure difference between the upstream and downstream sides of the filter element generally does not exceed 2 MPa. Therefore, when the pressure retention pressure of the filter element exceeds 10 MPa, it is almost impossible to be damaged due to the pressure during use (even if the pressure suddenly increases due to pressure fluctuations). Therefore, the test for a larger pressure retention pressure is no longer performed.

[0183] 3.2 Vickers hardness of the fine layer

[0184] Test procedure: The Vickers hardness can be measured by Innovatest Falcon 507 microhardness tester. The test method is to select a suitable load (0.1 kg in this application) and a holding time (usually 10-15 s) according to the characteristics of the material to be tested. After testing, the size of the indentation is measured by software to display the Vickers hardness value. Of course, in addition to the aforementioned type of equipment, the results measured by other types of Vickers hardness testing equipment that meet the relevant standards are also acceptable. It should be noted that when measuring the Vickers hardness of the fine layer, the indentation is located on the outer surface of the fine layer.

[0185] 3.3 IPA initial bubble point of the sintered metal filter medium

[0186] Test procedure: First, completely immerse the sintered metal filter medium in IPA; then, slowly introduce gas (such as nitrogen), gradually increase the pressure, and when the first continuous bubble appears at the outlet end, the pressure value is the IPA initial bubble point. Other operation details can refer to ASTM F316-70 or ISO 2942:2018.

[0187] 3.4 Porosity of the sintered metal filter medium

[0188] The specific test method is described in the summary section.

[0189] The above test results are recorded in Tables 5 to 7.

[0190] Table 5 Characterization results of the fine layer

[0191]

[0192] Table 6 Characterization results of the coarse layer

[0193]

[0194] Table 7 Performance test results of the sintered metal filter medium

[0195] Total thickness / pm Fine layer thickness / pm Porosity of the filter medium / % IPA initial foaming point / kPa Probability of cut cracking / % Hardness of the fine layer / HV0.1 Dwell pressure / MPa Example 1 732.1 346.1 34.6 323.4 2 108.3 >10 Example 2 776.7 344.5 31.9 334.5 2 114.2 >10 Example 3 826.8 412.6 25.2 363.2 3 125.8 >10 Example 4 814.5 401.8 22.1 369.6 8 143.2 >10 Example 5 759.7 361.3 39.8 327.8 3 104.4 8.8 Example 6 751.9 361.0 37.5 330.1 4 106.8 9.1 Example 7 129.6 22.2 27.0 51.8 4 95.1 4.8 Example 8 3026.1 2012.6 24.7 497.5 9 157.3 >10 Example 9 743.7 344.5 32.8 330.1 8 118.9 >10 Example 10 1626.7 745.3 33.5 313.2 3 110.3 9.5 Example 11 411.2 411.2 21.3 332.6 9 141.7 9.2 Example 12 353.7 353.7 38.6 295.0 6 102.1 7.1 Comparative Example 1 348.3 348.3 11.2 354.0 78 217.3 / Comparative Example 2 350.9 350.9 14.5 339.3 62 165.4 / Comparative Example 3 357.6 357.6 60.7 280.3 6 67.3 0.4

[0196] Data analysis and conclusion

[0197] The performance test results of Comparative Examples 1-3 and Comparative Example 1 show that when the fine layer has no pinning nodes in the porous structure, the probability of cutting and cracking of the corresponding sintered metal filter medium product is as high as 78%. The performance test results of Comparative Examples 1-3 and Comparative Example 2 show that even if the fine layer has pinning nodes in the porous structure, but the particle size is too small and the surface distribution density is too low, that is, the pinning nodes are small and few, although the probability of cutting and cracking of the corresponding sintered metal filter medium product is reduced, it is still as high as 62%. In summary, when the sintered metal filter medium product has no pinning nodes, or although it contains pinning nodes, the average particle size of the pinning nodes is less than 0.5 μm and the surface distribution density is less than 1 per 10 square microns, correspondingly, no soft zone is formed in the sintered metal filter medium product, or the volume of the soft zone in the sintered metal filter medium product is small and the number is few. When it is cut, on the one hand, micro-cracks are easily formed, and on the other hand, the micro-cracks are not blocked in time and effectively, but quickly spread or extend axially. The final result is that the probability of cutting and cracking of the filter medium product is very high.

[0198] The performance test results of Comparative Examples 1-3 and Comparative Example 3 show that even if the fine layer has pinning nodes in the porous structure, but the particle size is too large and the surface distribution density is too high, that is, the pinning nodes are large and many, although the probability of cutting and cracking of the corresponding sintered metal filter medium product is also less than 10%, but its mechanical properties drop sharply, and the pressure holding pressure of the filter core prepared is only 0.4 MPa, which is less than the pressure difference under the supercritical filtration working condition, and cannot meet the pressure resistance requirement under the supercritical filtration working condition. When the average particle size of the pinning nodes in the sintered metal filter medium product is greater than 2.6 μm and the surface distribution density is more than 10 per 10 square microns, the volume of the soft zone in the sintered metal filter medium product is large and the number is many, which leads to that the hardness of the whole filter medium is very low and the pressure resistance performance is very poor, and cannot meet the requirement of the pressure difference under the supercritical fluid filtration working condition.

[0199] The performance test results of Comparative Examples 1-3 and Example 4 show that when the average particle size of the pinning nodes in the porous structure of the fine layer is less than 0.6 μm, the volume of the soft zone in the porous structure of the fine layer is relatively small, the blocking effect on the extension of micro-cracks is relatively weak, and the probability of cutting cracking rises to close to 10%. The performance test results of Comparative Examples 1-3 and Example 5 show that when the average particle size of the pinning nodes in the porous structure of the fine layer is greater than 2.0 μm, the probability of cutting cracking changes little, but because the volume of the soft zone is relatively large, the pressure resistance of the product is significantly deteriorated, and the holding pressure is reduced from more than 10 MPa to 8.8 MPa. The probability of cutting cracking of the sintered metal filter medium products prepared in Examples 1-3 is all below 3%, and the holding pressure is all more than 10 MPa.

[0200] The performance test results of Comparative Example 4 and Example 11 and the performance test results of Comparative Example 5 and Example 12 show that when the sintered metal filter medium product has no coarse layer and the porous structures of the fine layers are basically the same (mainly referring to the particle size distribution and the surface distribution density of the pinning nodes being basically the same or similar), the probability of cutting cracking of the sintered metal filter medium product is not much different, but the pressure resistance of the sintered metal filter medium product without the coarse layer is worse, which is manifested in that the holding pressure value is lower.

[0201] The performance test results show that when the porous structure of the fine layer of the sintered metal filter medium has pinning nodes, the particle size of the pinning nodes is not less than 0.4 μm, the average particle size is 0.5-2.6 μm, the surface distribution density is 1-10 per 10 square microns, and the area ratio is not more than 85%, the probability of cutting cracking of the sintered metal filter medium product can be stably controlled below 10%, and the mechanical stability of the filter element prepared from the sintered metal filter medium product under high-pressure filtration working conditions is also high enough, and the holding pressure is all more than 4.8 MPa. Moreover, the sintered metal filter medium product prepared in Example 8 has very low flux, very high bubble point close to 500 kPa, and very small pore size, which is suitable for intercepting small-size (for example, 10 nm) particulate pollutants, and the sintered metal filter medium product prepared in Example 7 has very low bubble point, about 50 kPa, and relatively large pore size, which is suitable for treating fluid containing large-size (for example, 100 nm) particulate pollutants and can provide higher flux.

[0202] The approximate steps of preparing the filter element 100 from the sintered metal filter medium prepared in Examples 1-12 are as follows: welding the closed end plate 20 and the joint member 30 having the hollow channel 31 to the two ends of the hollow tubular sintered metal filter medium 10 respectively, the hollow channel 31 of the joint member 30 communicates with the surface of the coarse layer of the sintered metal filter medium 10 away from the fine layer, and the structural schematic diagram of the filter element 100 is shown in FIG. 1. Figure 3The filter core 100 and the shell 200 are assembled together to form a filter, wherein the shell 200 has a joint 210, as shown in Figure 4 During filtration, the supercritical carbon dioxide to be filtered enters the hollow interior of the filter core 100 through the hollow channel 31 of the joint member 30, and passes through the coarse layer and the fine layer in sequence, and the large-particle and small-particle contaminants are trapped in sequence, and the clean supercritical carbon dioxide flows out from the surface of the fine layer away from the coarse layer (the arrow indicates the flow direction of the fluid), and finally is discharged from the joint 210 of the shell 200.

[0203] The technical features of the above-described embodiments can be combined in any manner. In order to make the description concise, all possible combinations of the technical features in the above-described embodiments are not described, but as long as the combinations of the technical features do not contradict, they should be considered within the scope of the present disclosure.

[0204] The above-described embodiments only express several implementation manners of the present application, and should not be construed as limiting the scope of the patent. It should be pointed out that, without departing from the concept of the present application, several modifications and improvements can be made, which are within the protection scope of the present application.

Claims

1. A sintered metal filter medium comprising at least a fine layer, the sintered metal filter medium being in a hollow tubular structure; characterized in that, the fine layer is a porous structure, the porous structure of the fine layer having pinning nodes; the pinning nodes have a particle size of no less than 0.4 μm, an average particle size of 0.5-2.6 μm, a surface distribution density of 1-10 per 10 square microns, and an area ratio of no more than 85%; the sintered metal filter medium has an IPA initial foaming point of 50-500 kPa.

2. The sintered metal filter medium of claim 1, wherein, the pinning nodes have an average particle size of 0.6-2 μm, and a surface distribution density of 2-7 per 10 square microns.

3. The sintered metal filter medium of claim 1, wherein, the pinning nodes comprise at least first pinning nodes having a particle size of 0.5-1.5 μm and second pinning nodes having a particle size greater than 1.5 μm, and the first pinning nodes have a quantity ratio of 30-90%.

4. The sintered metal filter medium of claim 3, wherein, the second pinning nodes comprise blocking nodes having a particle size of no less than 3 μm, and the number of the blocking nodes is no more than 10% of the number of all pinning nodes and no less than 3% of the number of all pinning nodes.

5. The sintered metal filter medium of claim 1, wherein, the pinning nodes have an average shape factor of no more than 1.

15.

6. The sintered metal filter medium of claim 5, wherein, the pinning nodes comprise spherical-like nodes having a shape factor of no more than 0.7, and the spherical-like nodes have a quantity ratio of no less than 60%.

7. The sintered metal filter medium of claim 1, wherein, the average distance between adjacent two pinning nodes is 0.2-2 μm.

8. The sintered metal filter medium of claim 1, wherein, the sintered metal filter medium further comprises a coarse layer located radially inside the fine layer, the coarse layer being a porous structure, and the coarse layer and the fine layer are sintered into one body; or, the sintered metal filter medium further comprises a coarse layer located radially inside the fine layer and at least one functional layer located radially inside the coarse layer, the coarse layer and the functional layer are both porous structures, the functional layer, the coarse layer and the fine layer are sintered into one body, and the functional layer is at least one of a pre-filtering layer, a reinforcing layer and a separation layer.

9. The sintered metal filter medium of claim 8, wherein, a surface of the coarse layer away from the fine layer is an inner surface of the sintered metal filter medium, and the coarse layer has a pore area ratio of 10-40% on the surface away from the fine layer.

10. The sintered metal filter medium of claim 9, wherein, the average width of a sintering neck on the surface of the coarse layer away from the fine layer is 0.2-6 μm, the average maximum Fretter diameter of a pore is 0.5-7 μm, and the ratio of the maximum Fretter diameter to the minimum Fretter diameter of the pore is 1.2-5.

11. The sintered metal filter medium of claim 9, wherein, the sintered metal filter medium has a thickness of 120-3000 μm, and the thickness of the fine layer is 1 / 6-2 / 3 of the thickness of the sintered metal filter medium.

12. The sintered metal filter medium of claim 8, wherein, the sintered metal filter medium has a porosity of 20-40%.

13. The sintered metal filter medium of claim 1, wherein, the fine layer is formed by sintering a mixture, the mixture comprising at least fine particles and doping particles, the doping particles forming the pinning nodes of the porous structure of the fine layer, and the doping particles have a particle size greater than that of the fine particles.

14. A method of producing a sintered metal filter medium as claimed in any one of claims 1-13, characterized in that, comprising the following steps: S1: providing a mixture composed of fine particles and doping particles uniformly dispersed therein, compressing the mixture to obtain a hollow tubular green body, and preparing the mixture by the following steps: A1-mixing a dispersant with surface tension not more than 30 mN / m with fine particles and doping particles to obtain a mixed precursor, A2-resting the mixed precursor, and then performing ultrasonic operation on the mixed precursor to obtain a dispersed precursor, A3-stirring and heating the dispersed precursor to dry to obtain a mixture; wherein the fine particles have a particle size of 50-300 nm, the doping particles have a particle size not less than 0.4 μm and an average particle size of 0.5-2.6 μm, and the mass of the doping particles is 15-85% of the total mass of the fine particles and the doping particles; S2-sintering the green compact obtained in S1 to obtain a sintered metal filter medium pre-product; S3-cutting at least one end of the sintered metal filter medium pre-product obtained in S2 to obtain a sintered metal filter medium.

15. The method of making a sintered metal filter medium according to claim 14, wherein, Step A2 is repeated 2-4 times; and / or, in step A2, the resting time is 30-90 min, and the ultrasonic power is 100-300 W and the ultrasonic time is 3-15 min.

16. The method of making a sintered metal filter medium according to claim 14, wherein, In step S1, coarse particles with a particle size of 1-10 μm are further provided, wherein the mixture is located radially outside the coarse particles, and the coarse particles and the mixture are compressed to obtain a hollow tubular green compact, the fine particles and the doping particles are made of stainless steel, and the coarse particles are made of nickel or carbonyl nickel; in step S2, the sintering process comprises a heating stage and a subsequent holding stage, the heating stage is uniformly raised from the ambient temperature to 780-820 ℃ at a rate of 2-8 ℃ / min, and the holding stage is held at 780-820 ℃ for 20-40 min.

17. The method of making a sintered metal filter medium according to claim 14, wherein, The dispersant is selected from one or more of methanol, ethanol, n-propanol, isopropanol or n-butanol, and in step A1, the liquid to solid volume ratio is 2:1-8:1; in step A3, before heating and stirring, the dispersant supernatant above the dispersed precursor is removed first, the stirring rate is 300-600 r / min, the vacuum degree during stirring is 0.02-0.06 MPa, and the temperature is 60-80 ℃.

18. A filter cartridge, characterized by The filter element comprises the sintered metal filter medium according to any one of claims 1-13.

19. A method of filtering supercritical carbon dioxide, characterized by, During filtration, supercritical carbon dioxide is passed through the sintered metal filter medium according to claims 1-13.

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