JP2013144847A - Method for producing metallic porous sintered body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a metallic porous sintered body which can be used as a filter etc. having a high trapping ratio of fine objects to be removed, such as soot.SOLUTION: It is found that performance of trapping soot etc. can be improved significantly by employing structure of the metallic porous sintered body, wherein the structure does not have three-dimensional network structure of a fine skeleton but has dispersed hollow spaces communicated, and pores having a relatively large diameter are formed on the walls of the hollows. The metallic porous sintered body has dispersed hollow spaces inside, a part of or all of which are communicated and which are obtained by eliminating resin particles. On the wall surfaces constituting the hollows, pores obtained by sintering metal powders are formed. The metallic porous sintered body has a BET surface area of at least 700 cm/cm, and an average diameter of the pores on the wall surfaces measured by a method of mercury penetration is at least 1 μm. In the method for producing the metallic porous sintered body, a formed body obtained by mixing the metal powders having an average particle diameter of 200 μm or less with resin particles having an average particle diameter of 0.1-10 mm and a binder is subjected to heating degreasing, and then sintering.

Description

  The present invention is a filter material for removing soot in exhaust gas of a diesel engine, that is, a filter for a dust collector such as a diesel particulate filter (hereinafter abbreviated as DPF), a combustion gas of an incinerator and a thermal power plant, or the like. The present invention relates to a method for producing a porous metal sintered body that can be used.

Conventionally, a cordierite (ceramics) honeycomb having heat resistance has been used as a filter for DPF. However, ceramics are easily damaged by vibration and thermal shock, and local heating (heat spots) is generated when soot containing carbon as a main component trapped in the filter for low heat conduction burns, causing cracks. And melting damage is a problem. Accordingly, a metal DPF filter having a strength higher than that of ceramics and a high thermal conductivity has been proposed.
For example, it has been proposed to employ a metal porous body having a three-dimensional network structure as a filter (see Patent Document 1). This proposal is excellent in terms of resistance to cracking and melting damage, and can be simplified in terms of structure compared to a honeycomb.

JP-A-5-312017

  The metal porous body having the above-described three-dimensional network structure is advantageous in terms of resistance to cracking and melting damage due to thermal shock, but has a problem of low scavenging performance. This is because a conventional metal porous body having a three-dimensional network structure has a small surface area because it has a thin skeleton. For this reason, the probability of collision of the soot particles with the skeleton is low, and the surface of the skeleton is smooth, so the attached soot is unstable. It is considered low. As a result, the rate at which soot generated in the engine is released into the atmosphere as it is without being captured by the filter increases. Further, in order to increase the capture rate, it is necessary to make the filter thick and large, and the DPF itself becomes large, which causes a problem for in-vehicle use.

  The objective of this invention is providing the manufacturing method of a metal porous sintered compact which can be used for the filter etc. with a high capture | acquisition rate of fine to-be-removed objects, such as soot.

  The present inventor is not a three-dimensional network structure having a thin skeleton, but a metal porous sintered body structure in which communicating hollow spaces are dispersed and relatively large-diameter pores are formed on the walls of the cavities. As a result, it has been found that the trapping performance and the like of the soot can be greatly improved by adopting.

That is, according to the present invention, a hollow space formed by removing resin particles is dispersed in a part or all of the inside, and a fine powder formed by sintering metal powder on the wall surface constituting the space. A method for producing a porous metal sintered body having pores, having a BET surface area of 700 cm ² / cm ³ or more and an average wall pore diameter of 1 μm or more measured by mercury porosimetry, having an average particle diameter of 200 μm This is a method for producing a porous metal sintered body characterized by heating and degreasing a sintered body obtained by mixing the following metal powder with resin particles having an average particle diameter of 0.1 mm to 10 mm and a binder. Preferably, the method is a method for producing a porous metal sintered body in which resin particles are removed from the above-mentioned molded body with a solvent, heated and degreased, and sintered.

  The metal powder is preferably stainless steel containing Cr: 16 wt% or more, or heat resistant steel containing Al: 1-10 wt% and Cr: 5-30 wt%. Alternatively, the porous metal sintered body preferably has a porosity of 85% to 95%. The metal porous sintered body is suitable for use as a filter, a catalyst carrier, a liquid absorber, a liquid holder, and a member for separating water vapor in a gas.

  According to the present invention, it is possible to provide a porous metal sintered body that can be used for a filter having a high capture rate of fine objects to be removed such as soot. In addition, the porous metal sintered body of the present invention having relatively large pores and a large specific surface area is used as a catalyst carrier because of its large specific surface area and air permeability, in addition to its use as a filter. be able to. Furthermore, the pores of the cavities and wall surfaces are characterized by the ability to absorb and retain liquids by capillary action and capillaries to condense vapors such as water vapor. It can also be suitable for use as a member.

It is the scanning electron micrograph (SEM photograph) which shows the cross-sectional form of the example 1 of this invention, and an optical micrograph. It is the scanning electron micrograph (SEM photograph) which shows the cross-sectional form of the example 3 of this invention, and an optical micrograph. It is the scanning electron micrograph (SEM photograph) which shows the cross-sectional form of a comparative example, and an optical micrograph. It is a schematic diagram which shows the evaluation method (state which set the test piece) of the filter characteristic. It is a figure which shows the relationship between the injection amount of carbon microparticles | fine-particles, and pressure loss, and a capture rate. It is a scanning electron micrograph (SEM photograph) which shows the adhesion condition of the carbon microparticles | fine-particles to the test piece after the capture rate evaluation test (invention example 1). (Invention Example 1) A scanning electron micrograph (SEM photograph) showing the state of adhesion of carbon fine particles to the wall surface of a test piece. It is an optical microscope photograph which shows the adhesion state of the carbon microparticles | fine-particles to the test piece after the capture rate evaluation test (comparative example).

As described above, an important feature of the present invention is that a metal porous sintered body structure in which hollow spaces are dispersed and relatively large-diameter pores are formed on the walls of the cavities is employed. When using a porous metal sintered body as a filter, in order to increase the probability that fine objects to be removed, such as soot, will be adsorbed to the filter, the object to be removed in the gas passes through the inside of the sintered body. It is effective to increase the area of the sintered part that can collide. In the present invention, the specific surface area is improved by using a wall surface formed by dispersing a hollow space instead of a conventional mesh structure having a thin skeleton, where the object to be removed collides and is adsorbed. In addition, by forming pores on the wall surface constituting the space, the surface roughness of the wall surface is increased, and by achieving a higher specific surface area, the probability of adsorption is further increased. When the above characteristics are specifically evaluated as, for example, the specific surface area by the BET method, a high specific surface area of 700 cm ² / cm ³ or more, which is difficult in the present invention with a metal porous body having a conventional three-dimensional network structure, is assumed. Can do. This increases the probability that the object to be removed is adsorbed, and improves the capture performance of the object to be removed. The specific surface area according to the BET method is preferably 900 cm ² / cm ³ or more.

  In addition, the pores formed on the wall surface increase the specific surface area and become uneven on the wall surface, making it easier to attach the object to be removed. Add the function. At this time, the average pore diameter (diameter) of the pores formed on the wall surface by the mercury intrusion method needs to be 1 μm or more. This is because the trapping rate tends to decrease when it is smaller than 1 μm. This is because if the pores on the wall surface are small, the gas will not easily permeate the wall surface, and the gas will flow along the wall surface. This is thought to be due to an increase in the rate of outflow. Preferably it is 10 micrometers or more, More preferably, it is 20 micrometers or more.

The hollow space formed inside the porous metal sintered body of the present invention described above needs to be partially or entirely connected.
When using a metal porous sintered body as a filter, a gas containing an object to be removed passes through the inside of the sintered body. If each cavity is isolated, gas etc. must permeate | transmit the wall surface which comprises a hollow space. In the present invention, since there are pores on the wall surface, there is air permeability, but if the gas path is only pores, the pressure loss of the gas when passing through the filter may be too high. In particular, when used as a filter such as a DPF, a sudden increase in pressure loss due to clogging of pores occurs with an increase in the amount of trapped objects such as soot. Accordingly, it is necessary that a part or all of the hollow space communicates. The pressure loss decreases as the opening size of the communicating space increases and the frequency of communication increases.
By satisfying the above conditions, each hollow space of the porous body functions as a trap for the object to be removed.

  In the present invention, in order to prevent damage caused by vibration or thermal shock when using a sintered body, cracks due to heat spots at the time of burning soot when used as a filter for DPF, and melting damage, compared with ceramics, Uses a sintered metal that is resistant to impacts and has high heat conductivity, so it is difficult for heat to accumulate. At this time, the filter is heated by the exhaust gas, and depending on the DPF method, the filter may be heated to a high temperature of 600 ° C. or more, which is the ignition temperature of the soot, because corrosion resistance at high temperature is required. Therefore, it is necessary to select a material that meets the usage conditions.

In the porous metal sintered body of the present invention, the porosity is preferably 85% or more and 95% or less.
In the metal porous sintered body of the present invention constituted by a hollow space and its wall surface, when the porosity is lower than 85%, the communication between the cavities is insufficient, so that when used as a filter, there is no pressure loss. Get higher. On the other hand, if it is higher than 95%, the wall surface is reduced, the strength of the porous metal sintered body is insufficient, and the capture performance of the object to be removed when used as a filter is lowered, and the characteristics of the present material are lost.

Moreover, in the metal porous sintered body of the present invention having a thickness of 10 mm, the pressure loss when flowing air at a flow rate of 5 m / s at 23 ° C. is preferably 1 kPa or more and 10 kPa or less.
The degree of communication of the cavity of the metal porous sintered body can be evaluated as the above-mentioned pressure loss on a macro scale. If the pressure is less than 1 kPa, the degree of communication is too high, and when used as a filter, the frequency of collision between the object to be removed and the wall surface is reduced, and the capture rate is reduced. On the other hand, when the pressure is higher than 10 kPa, since the degree of communication of the cavity is low, when used as a filter, clogging due to an object to be removed occurs from an early stage, and the rise in pressure loss is high. For example, as a DPF filter If used, it may cause a reduction in engine output.

The manufacturing method of the metal porous sintered body applied to the present invention described above is as follows.
First, metal powder is prepared. When using a porous metal sintered body as a DPF filter, stainless steel containing Cr: 16 wt% or more or alumina at a high temperature is used in order to make it compatible with filter applications where the filter is heated and regenerated at a high temperature of 600 ° C or higher. Metal powders such as heat-resisting steel containing Al: 1 to 10 wt% and Cr: 5 to 30 wt% for forming a film are effective. The average particle size is 200 μm or less. Resin particles and a binder are mixed with the metal powder. The resin particles are mixed for the purpose of forming a cavity in the sintered body, and the average particle size is 0.1 mm to 10 mm. The resin particles are vaporized at the time of sintering, or after forming a metal powder into a molded body, the resin particles are dissolved and removed using a solvent before sintering. When the deformation or collapse of the sintered body accompanying the melting or vaporization of the resin particles during sintering becomes a problem, the latter removal by dissolution is preferred.

In addition, the latter method of dissolving and removing the resin particles is an effective production method because a stable production of a porous sintered body having a thickness of 10 mm or more is possible and the degree of freedom with respect to the thickness is increased. Also, in terms of process, depending on the combination of the solvent and the resin, there is an advantage that both the resin and the solvent can be recycled by distillation.
Although a resin can be used as the binder, in the case of applying a method of removing resin particles with a solvent, it is effective to use, for example, a binder mainly composed of methylcellulose and water that does not dissolve in the solvent.

  Subsequently, a molded object is produced. At the time of molding, it is preferable to increase the contact area between the resin particles by applying a pressure that does not pulverize the resin particles. Thereby, the opening dimension of the communication part of the space in the completed porous body becomes large, and the frequency of communication also increases. Thereafter, the molded body is heated and degreased and sintered. When water is put into the binder, it is preferable to put a drying step after molding, and when the resin particles are removed with a solvent, it is preferable to give a solvent extraction and drying step before heat degreasing.

  The method for producing a porous metal sintered body of the present invention having relatively large pores on the wall surface of the cavity described above and having a large specific surface area has a large specific surface area and is breathable. In addition to the above applications, it can be used as a catalyst carrier. In addition, the pores of the cavities and wall surfaces are characterized by absorbing and holding liquids by capillarity and condensing steam such as water vapor into capillaries. It is also suitable for applications such as an absorber for water vapor and a separation member for water vapor in gas.

(Preparation of test piece)
SUS316L water atomized powder with an average particle size of 60 μm, commercially available methylcellulose, and paraffin wax particles with an irregular average particle size of 2.5 mm are mixed as resin particles, and water and a plasticizer are added and mixed and kneaded to produce a kneaded body. did. The mixing amount of the resin particles was set so that the volume ratio of the resin particles was 90% when the total volume of the metal powder and the resin particles was 100%.

  Then, the kneaded body was press-molded with a press at a pressure of 0.7 MPa to produce a disk, and dried at 50 ° C. Next, paraffin wax particles in the molded body were extracted from the molded body with a solvent and dried at 90 ° C. Subsequently, after heating and degreasing in nitrogen at 600 ° C., sintering was performed in a vacuum of 1200 ° C., thereby producing two kinds of disk-shaped test pieces having thicknesses of 7 mm and 10 mm, respectively, and a diameter of φ144 mm. Hereinafter, a test piece having a thickness of 7 mm is referred to as Invention Example 1, and a test piece having a thickness of 10 mm is referred to as Invention Example 2.

The kneaded body produced under the same conditions as in the case of Invention Examples 1 and 2 was formed by extending with a roller, and the formed body was processed again in the same process as in Invention Examples 1 and 2, and a disk having a thickness of 10 mm and a diameter of 144 mm A test piece was prepared. Hereinafter, this test piece is referred to as Invention Example 3.
Further, as a comparative example, a test piece of a metallic porous body having a three-dimensional network structure of Ni—Cr alloy was produced by plating on a urethane foam subjected to conductive treatment. This was processed into a disk shape having a thickness of 10 mm and a diameter of 144 mm.

(Comparison of cross-sectional forms)
Scanning electron micrographs (SEM photographs) and optical micrographs, which are examples of cross-sectional forms of Examples 1 and 3 of the present invention and comparative examples, are shown in FIGS. The SEM photograph was obtained by observing a fractured surface obtained by dividing the test piece in two, and the cross-sectional structure photograph (optical micrograph) was obtained by observing the cross section of the test piece by polishing the test piece embedded in the resin. Is.
In Example 1 of this invention, it turns out that the communicating hole has opened in the adjacent cavity. The wall surface has a structure in which metal powder is sintered, and irregularities and pores are seen. In addition, the same form could be confirmed also in Invention Example 2.
It can be seen that Example 3 also has a communication hole in an adjacent cavity, although the degree of communication of the cavity is small compared to Example 1 of the invention. The wall surface has a structure in which metal powder is sintered in the same manner as Example 1 of the present invention, and irregularities and pores are observed.
In contrast to these inventive examples, the comparative examples have a thin skeleton, a hollow structure, a smooth surface, and no pores.

(Comparison of surface area etc.)
The surface area per unit volume (BET surface area), the pore diameter (average diameter), the porosity, and the pressure loss when permeating the atmosphere in the inventive examples 1 to 3 and the comparative example were measured. The surface area per unit volume was measured by the BET method, the average diameter of the pore diameter was measured by the mercury intrusion method, and the pressure loss was measured by a differential pressure when passing through the atmosphere at 23 ° C. and a flow rate of 5 m / s.
In the comparative example having a smooth surface, the pore diameter was not measured. Regarding the BET specific surface area, the specific surface area per 1 g of the sintered body portion constituting the wall surface in the present invention example and the skeleton portion in the comparative example are measured, and the specific surface area per 1 cm ^{3 of} the porous body is measured using the porosity. It is converted to. Further, the comparative example shows a value corrected by subtracting the surface area of the hollow portion that does not contribute to the filter performance using the image processing data of the cross-sectional structure because the skeleton portion is hollow. The results are shown in Table 1.

It can be seen that the inventive example has the same porosity as the comparative example, but the specific surface area of the present invention is nearly twice as large. Moreover, from the cross-sectional form shown above and the result of Table 1, in the example of the present invention, it can be seen that the pressure loss decreases as the degree of communication of the hollow portion increases and the thickness of the porous body decreases.
Moreover, it turns out that the pressure loss of a comparative example is very low compared with the example of this invention. This is presumably because the resistance of the porous body to the gas flow permeated through the porous body is smaller than that of the above-described structure of the present invention because the structure of the comparative example is a network structure with a smooth and thin skeleton.

(Evaluation of filter characteristics)
The present invention examples 1 to 3 and the comparative example were used as filters, and the relationship between the amount of carbon fine particles charged and the pressure loss and the carbon fine particle capture rate were evaluated.

-Evaluation method-
First, the test piece is previously dried at 130 ° C. for 2 hours and then weighed. Next, as shown in FIG. 4, the test piece 2 is set in the holder 1 through the sealing material 3, sucked from one side so that the flow rate is constant at 5 m / s, and the average particle size is 0.042 μm from the other side. Of carbon fine particles were added at a rate of 0.1 g / min, and the change in pressure loss of the test piece with respect to the input amount was investigated. The ventilation portion of the test piece at this time was φ139.5 at the center of the test piece. The pressure loss was determined from the differential pressure before and after the test piece. The test was conducted in a room where the temperature was maintained at 23 ° C.

  The test ends when the flow rate cannot be controlled at a constant 5 m / s, and the test piece is taken out of the holder. Then, after drying at 130 ° C. for 2 hours, weighing is performed, the amount of carbon fine particles captured is determined from the increase in the number of test pieces, and the value obtained by dividing the amount captured by the total input amount of carbon fine particles under test is the capture rate of the test pieces. It was. Further, the adhesion state of the carbon fine particles to the test piece was also investigated.

-Evaluation results-
FIG. 5 shows the relationship between the amount of carbon fine particles introduced and the pressure loss, and Table 2 shows the capture rate measured after the test of each test piece.

When comparing specimens of (Invention Example 2) and (Invention Example 3) having the same thickness of 10 mm, the pressure loss is lower when the degree of penetration of the cavity is higher (Invention Example 2), and the capture rate is lower. It can be seen that the degree of penetration of the cavity is lower (Invention Example 3).
In addition, when the test pieces of 7 mm thickness (Invention Example 1) and 10 mm (Invention Example 2) are compared, the pressure loss is lower in the thinner thickness (Invention Example 1), and the capture rate is thicker. It can be seen that the thicker one (Invention Example 2) is higher.
Further, when three sheets of (Invention Example 1) having a thickness of 7 mm are stacked with a 3 mm gap as shown in FIG. 4, the pressure loss is lower than that of (Invention Example 3), but the capture rate is 95. It can be seen that the value is as high as 2% (invention example 3).

  From the above, it can be seen that the pressure loss and the capture rate can be controlled by the degree of communication of the cavities, the thickness of the metal porous sintered body, and the number of stacked layers.

  On the other hand, (Comparative Example) has a lower pressure loss than (Invention Example 1) to (Invention Example 3), and no increase in pressure loss is observed with respect to the amount of carbon fine particles added, but the capture rate is 2 It can be seen that it is very low at 7%. In order to increase the capture rate to a level equivalent to (Example of the present invention), it is presumed that a considerable thickness is required. Therefore, when used as a filter that requires a high capture rate, the volume becomes very large.

FIG. 6 shows SEM photographs of the surface (front surface) of the fracture surface of the 7 mm-thick (invention example 1) after the test, near the surface (front surface) of the carbon particulate input side, and near the back surface. The SEM photograph which expanded the fracture | rupture part (cross section of a wall surface) of the wall surface which shows is shown in FIG. Moreover, the optical micrograph which observed the adhesion state of the carbon microparticles | fine-particles to the internal frame | skeleton from the surface and back surface after a test (comparative example) is shown in FIG.
(Invention Example 1), it was confirmed from FIG. 6 that carbon fine particles were accumulated on the wall surface of each cavity facing the flow of the carbon fine particles from the carbon fine particle charging side to the opposite side. Further, the fine particles that appear white in FIG. 7 are carbon fine particles. From now on (Example 1 of the present invention), the fine particles are deposited on the wall surfaces forming the hollow space where the carbon fine particles communicate with each other, and at the same time in the pores. It was confirmed that it was deposited.
On the other hand, it can be seen from FIG. 8 that (Comparative Example) is attached to the surface of the carbon fine particle input side of an elongated and smooth skeleton. It can also be seen that there is a portion that is not attached even on the input side surface.

From the above, in the present invention, each cavity inside the porous metal sintered body functions as a trap, and further, the wall surface having the pores of the cavity contributes greatly to capturing the object to be removed as a filter. I understand. In addition, it can be seen that the object to be removed after capture has a wall surface having a sufficient area and has large unevenness due to the sintered structure, so that it adheres more stably than the comparative example.
On the other hand, in the comparative example, it is considered that the amount of adhesion is quickly saturated because the surface on which graphite fine particles collide and adhere is thin and narrow. Further, since the skeleton surface is smooth, it may be peeled off if attached to some extent. For this reason, it is considered that the pressure loss is constant at a low value regardless of the input amount of the carbon fine particles, and the capture rate is low.

1. Holder 2. Test piece 3. Sealing material

Claims (5)

Inside, a part or all of the hollow spaces formed by removing resin particles are dispersed, and pores formed by sintering metal powder are formed on the wall surfaces constituting the spaces. , A method for producing a porous metal sintered body having a BET surface area of 700 cm ² / cm ³ or more and an average wall pore diameter of 1 μm or more measured by mercury porosimetry, wherein the metal powder has an average particle diameter of 200 μm or less. A method for producing a porous metal sintered body, comprising heating and degreasing a molded body in which resin particles having an average particle diameter of 0.1 mm to 10 mm and a binder are mixed and sintering. The resin particles are removed with a solvent from a molded product obtained by mixing resin particles having an average particle size of 0.1 mm to 10 mm with a metal powder having an average particle size of 200 μm or less and a binder, heated and degreased, and sintered. The method for producing a porous metal sintered body according to claim 1. The metal powder is stainless steel containing Cr: 16 wt% or more, or heat-resistant steel containing Al: 1-10 wt% and Cr: 5-30 wt%. A method for producing a metal porous sintered body. The method for producing a porous metal sintered body according to any one of claims 1 to 3, wherein the porous metal sintered body has a porosity of 85% to 95%. The metal porous sintered body is used for any one of a filter, a catalyst carrier, a liquid absorber, a liquid holding body, and a member for separating water vapor in a gas. A method for producing a porous sintered body.