Evaluation method for porous metal bodies

Abstract

To provide a method for evaluating a metallic porous body with which a thickness of a skeleton of a three-dimensional network structure constituting the metallic porous body can be more mechanically precisely found to facilitate management of the thickness of the skeleton.SOLUTION: A method for evaluating a metallic porous body including a skeleton of a three-dimensional network structure includes: obtaining an image of a cross section along a thickness direction of the metallic porous body; and calculating, as a thickness of the skeleton, a ratio of a cross-sectional area of the skeleton to a boundary length of the skeleton by obtaining each of the cross-sectional area and the boundary length on the basis of the image.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating a porous metal body, and more particularly to a method for evaluating a porous metal body having a skeleton with a three-dimensional network structure. [Background technology]

[0002] Porous metal bodies having a skeleton with a three-dimensional network structure are used in applications such as filters, battery electrode plates, catalyst carriers, and metal composites. For example, a positive electrode plate in which an active material is filled into the pores of a porous metal body that functions as a current collector is suitably used as an electrode plate for an alkaline storage battery. Known examples of porous metal bodies include nickel porous bodies containing nickel as the main component and aluminum porous bodies containing aluminum as the main component.

[0003] Patent Document 1 discloses a method for producing a porous metal body, in which a coating of a metal that forms a eutectic alloy below the melting point of Al is formed on the skeleton of a foamed resin having a three-dimensional network structure by a gas phase method such as plating, vapor deposition, sputtering, or CVD, and then the foamed resin on which the coating has been formed is impregnated and coated with a paste whose main components are Al powder, a binder, and an organic solvent, and then heat-treated at a temperature of 550°C to 750°C in a non-oxidizing atmosphere.

[0004] Patent Document 2 discloses a nickel-chrome porous body having a skeleton with a three-dimensional network structure, the skeleton being hollow and having a main metal layer and surface oxide layers formed on both sides of the main metal layer, the surface oxide layer having a thickness of 0.5 μm or more and containing chromium oxide as a main component, the main metal layer being nickel-chrome with a chromium content of 10 mass% or more as a whole, and the chromium content in a range of at least 3 μm from the interface in contact with the surface oxide layer being 20 mass% or more, and the surface oxide layer and the main metal layer being in close contact with each other with no gaps. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 8-170126 [Patent Document 2] International Publication No. 2022 / 059494 Summary of the Invention [Problem to be solved by the invention]

[0006] In a battery using such a porous metal body as a current collector for an electrode plate, if there is variation in the thickness of the skeleton (skeleton thickness) between the surface side and the inside in the thickness direction of the porous metal body, the output characteristics of the battery may be reduced. However, Patent Documents 1 and 2 do not describe a specific method for controlling such variation in skeletal thickness. Therefore, there is a problem in that it is difficult to control the skeletal thickness, and as a result, the variation in skeletal thickness may result in insufficient performance of the porous metal body.

[0007] The present invention has been made to solve these problems, and aims to provide a method for evaluating a metal porous body that can more accurately and mechanically determine the thickness of the skeleton of the three-dimensional network structure that constitutes the metal porous body, and that can easily control the thickness of the skeleton. [Means for solving the problem]

[0008] A method for evaluating a metal porous body according to one embodiment is a method for evaluating a metal porous body having a skeleton with a three-dimensional mesh structure, and includes the steps of acquiring an image of a cross section along the thickness direction of the metal porous body, acquiring the cross-sectional area and perimeter of the skeleton based on the image, and calculating the ratio of the cross-sectional area to the perimeter as the thickness of the skeleton. [Effects of the Invention]

[0009] The present invention provides a method for evaluating a metal porous body that can determine the thickness of the skeleton of the three-dimensional network structure that constitutes the metal porous body more mechanically and accurately, and can easily control the thickness of the skeleton. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing an example of a porous metal body. [Figure 2] FIG. 2 is a diagram showing an example of an image of a cross section of a porous metal body. [Figure 3] FIG. 1 is a first diagram illustrating the skeletal thickness. [Figure 4] FIG. 2 is a second diagram illustrating the skeletal thickness. [Figure 5] 1 is a flowchart illustrating a method for evaluating a porous metal body according to the first embodiment. [Figure 6] FIG. 2 is a diagram showing an example of an image of a cross section of a porous metal body. [Figure 7] FIG. 10 is a schematic diagram illustrating a method for calculating a skeletal thickness. [Figure 8] 7 is a plot diagram and a histogram obtained by digitizing the image shown in FIG. 6. [Figure 9] 10 shows plots and histograms after filtering. [Figure 10] 1 is a table showing the coefficient of variation obtained from the calculation results of the skeletal thickness ratio for each of Samples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0011] Embodiment 1 Hereinafter, an embodiment of the present invention will be described with reference to the drawings. For clarity, the following description and drawings have been simplified as appropriate. In the following description, the same or equivalent elements are designated by the same reference numerals, and redundant description will be omitted.

[0012] First, an overview of a porous metal body 1 to which a porous metal body evaluation method according to this embodiment (hereinafter sometimes simply referred to as "evaluation method") is applied will be described with reference to Fig. 1. Fig. 1 is a diagram showing an example of a porous metal body. The area within the dashed line in Fig. 1 is a partially enlarged view that schematically shows the interior of the porous metal body 1.

[0013] As shown in Fig. 1, the porous metal body 1 has a predetermined thickness TP in the Y-axis direction, which is its thickness direction. The porous metal body 1 has, for example, a flat plate-like appearance overall, and has one surface 2 and the other surface 3 that face each other in the thickness direction. The porous metal body 1 also has a skeleton 10 with a three-dimensional network structure, and pores 20 are formed inside the skeleton 10 and are three-dimensionally connected by the skeleton 10.

[0014] The skeleton 10 of the metal porous body 1 is formed of a metal layer mainly containing a metal or its alloy. The skeleton 10 may contain elements other than metal. The material (type of metal or its alloy), porosity, average pore size, thickness, and other structural features of the metal porous body 1 are appropriately selected depending on the application.

[0015] For example, when the metal porous body 1 is a nickel porous body made of nickel or its alloy, it can be suitably used as a current collector for the positive electrode plate of a nickel-metal hydride battery. For example, when the metal porous body 1 is an aluminum porous body made of aluminum or its alloy, it can be suitably used as a current collector for the positive electrode plate of a lithium-ion secondary battery. When the metal porous body 1 is used as a current collector, an active material can be supported in the pores 20. This enables uniform current collection even in a thick electrode, thereby achieving high battery capacity and improved cycle characteristics.

[0016] Methods for manufacturing such a metal porous body 1 include various methods, such as a plating method in which the surface of a substrate such as a foamed resin is made conductive and then metal is plated onto the surface of the substrate, and a sintering method in which powdered metal is made into a slurry and attached to a substrate such as a foamed resin, and then sintered.

[0017] Here, when the metal porous body 1 is subjected to image processing, the skeleton 10 may not be recognized at the desired measurement location in the image due to image processing. As a result, it may not be possible to properly measure the state of the skeleton 10, such as its thickness. Here, FIG. 2 is a diagram showing an example of an image of a cross section of the metal porous body. FIG. 2 shows an example of a cross section along the thickness direction of the metal porous body 1.

[0018] 2, the skeleton 10 present inside the metal porous body 1 can be recognized, but the skeleton 10 on the surfaces 2 and 3 sides may not be recognized. When evaluating the thickness of the skeleton 10 of such a metal porous body 1, the measurement target for measuring the thickness is biased toward the skeleton 10 present inside the metal porous body 1.

[0019] Furthermore, for example, a porous metal body 1 manufactured using a plating method may have variations in thickness among the skeletons 10. Therefore, the thicknesses of the skeletons 10 present on the surfaces 2 and 3 of the porous metal body 1 are different from each other.

[0020] 3 and 4, the individual skeletons 10 in the cross section of the porous metal body 1 will be considered. Fig. 3 is a first diagram illustrating the skeleton thickness. Fig. 4 is a second diagram illustrating the skeleton thickness.

[0021] As shown in Fig. 3, thicknesses TS1, TS2, and TS3 of each side (three sides in Fig. 3) constituting one skeleton 10 are not uniform but are different. Also, as shown in Fig. 4, when considering one of the sides constituting skeleton 10, the greater the unevenness of the surface of skeleton 10, the greater the difference is likely to be between thicknesses TS4 and TS5 at different positions within that side.

[0022] Therefore, if the measurer arbitrarily selects the measurement object and measurement position to determine the thickness of the skeleton 10, not only will the measurement accuracy decrease, but there is also the problem that the relative variation in the measurement values ​​is likely to increase because the measurement values ​​can be adjusted arbitrarily.

[0023] In contrast to these, in the evaluation method according to the present embodiment, the cross-sectional area S and perimeter L of the skeleton 10 are obtained based on an image 32 of a cross section along the thickness direction of the metal porous body 1, and the ratio of the cross-sectional area S to the perimeter L is calculated as the thickness (skeleton thickness) of the skeleton 10. According to this method, the skeletal thickness of the skeleton 10 of the three-dimensional network structure that constitutes the metal porous body 1 can be determined mechanically with higher precision, and the skeletal thickness can be easily controlled.

[0024] Furthermore, the metal porous body 1 produced by plating is prone to variations in skeletal thickness between the inside and the surfaces 2 and 3. If the metal porous body 1 having such variations in skeletal thickness is used as a current collector for an electrode plate, there is a risk that the output characteristics of a battery including this electrode plate will be reduced.

[0025] Therefore, in the evaluation method according to this embodiment, plot points P are obtained by plotting the skeletal thickness against the position coordinates of the skeleton 10 indicating the thickness direction, and an approximation curve CL2 is obtained based on the plot points P obtained for each of the multiple skeletons 10 present in the image 32. Furthermore, when the region located on the surfaces 2 and 3 sides in the thickness direction of the metal porous body 1 is defined as the surface side portion, and the region inside the surface side portion of the metal porous body 1 in the thickness direction is defined as the inner portion, the ratio (skeleton thickness ratio) of the average skeletal thickness of the skeletons included in the inner portion to the average skeletal thickness of the skeletons 10 included in the surface side portion calculated based on the approximation curve CL2 is calculated.

[0026] This method makes it possible to compare the skeletal thickness of the skeleton 10 contained on the surfaces 2 and 3 and the inside of the metal porous body 1. Therefore, by controlling the skeletal thickness using the evaluation method according to this embodiment, it is possible to obtain a metal porous body 1 that contributes to improving battery characteristics.

[0027] An overview of the evaluation method according to this embodiment will be described with reference to Fig. 5. Fig. 5 is a flowchart illustrating the evaluation method for the porous metal body according to embodiment 1. As shown in Fig. 5, the evaluation method for the porous metal body 1 includes steps S1 to S4. Each step will be described in detail below.

[0028] First, the skeletal thickness can be measured by steps S1 and S2. Steps S1 to S2 will be described in detail with reference to Fig. 6 and Fig. 7. Fig. 6 is a diagram showing an example of an image of a cross section of a porous metal body. Fig. 7 is a schematic diagram illustrating a method for calculating the skeletal thickness. Fig. 7 shows a cross section of a skeleton 10, which is schematically represented as a hollow, approximately triangular shape.

[0029] First, in step S1, an image 32 of a cross section along the thickness direction of the porous metal body 1 is acquired. The image 32 acquired here is a two-dimensional image expressed in a plane coordinate system having an X-axis and a Y-axis. The X-coordinate represents the position in the X-axis direction, which is perpendicular to the thickness direction of the porous metal body 1. The X-axis direction is the direction along the width direction or length direction of the porous metal body 1. The Y-coordinate represents the position in the Y-axis direction, which is along the thickness direction of the porous metal body 1. The image 32 includes the position coordinates (X-coordinate, Y-coordinate) of each skeleton 10.

[0030] As shown in Fig. 6, a cross section of the porous metal body 1 has a plurality of skeletons 10 with hollow portions 11 formed therein. An image 32 of the cross section of the porous metal body 1 can be obtained using, for example, a scanning electron microscope (SEM). When obtaining the image 32 using an SEM, for example, the porous metal body 1 embedded in an appropriate resin may be cut in its thickness direction, the obtained cut surface may be polished, and then the cut surface may be subjected to the SEM to obtain an SEM image 31 of the cut surface.

[0031] Image 32 can be obtained by performing binarization processing on the SEM image 31 obtained in this manner using appropriate image analysis software. The upper side of Fig. 6 shows SEM image 31. The lower side of Fig. 6 shows image 32, which has been binarized by performing binarization processing using an intermediate value between the maximum brightness and the minimum brightness in SEM image 31. Note that image 32 may be obtained using another device, such as a metallurgical microscope, instead of an SEM.

[0032] Next, in step S2, the cross-sectional area S and perimeter L of the skeleton 10 are obtained based on the image 32 obtained in step S1. In this step, for each of the multiple skeletons 10 present in the image 32, the perimeter L of the skeleton 10 shown by the dashed line in Fig. 7 and the cross-sectional area S of the skeleton 10 shown by the hatching in Fig. 7 are obtained. For each skeleton 10, the ratio (S / L) of the cross-sectional area S to the perimeter L can be calculated as the skeleton thickness.

[0033] Here, the perimeter L of each skeleton 10 is the total length of the sides that make up the skeleton 10. In other words, the perimeter L of a skeleton 10 with a substantially triangular cross section is the total length of the three outer peripheral sides that make up the skeleton 10. In this manner, the skeleton thickness of each skeleton 10 can be determined. In order to determine the skeleton thickness ratio, which will be described later, it is preferable to calculate the skeleton thickness for all skeletons 10 present in the image 32 in steps S1 and S2.

[0034] Next, steps S3 and S4 will be described in detail with reference to Fig. 8 and Fig. 9. The skeletal thickness ratio can be estimated by steps S3 and S4. Fig. 8 is a plot diagram and a histogram obtained by digitizing the image shown in Fig. 6. Fig. 9 is a plot diagram and a histogram after filtering processing. In Fig. 8 and Fig. 9, the width of the Y coordinate of plots 41 and 51 is assumed to coincide with the thickness TP of the porous metal body 1 in image 32.

[0035] In step S3, plot points P are obtained by plotting the skeleton thickness against the Y coordinate (position coordinate) of the skeleton 10 indicating the thickness direction, and an approximate curve CL2 is obtained based on the plot points P obtained for each of the multiple skeletons 10 present in the image 32.

[0036] In this step, as shown in FIG. 8, a plot diagram 41 is obtained, which includes a plurality of plot points P in which the skeleton thickness of each skeleton 10 is plotted for each Y coordinate of the skeleton 10. Specifically, by plotting the skeleton thickness on the vertical axis and the Y coordinate on the horizontal axis, the plot diagram 41 shown in the upper part of FIG. 8 is obtained. The lower part of FIG. 8 is a histogram 42 in which the frequency is on the vertical axis and the cross-sectional area S of the skeleton 10 is on the horizontal axis, which corresponds to the plot diagram 41. The Y coordinate of the skeleton 10 may be, for example, the Y coordinate of the area centroid of the skeleton 10 in the image 32.

[0037] Then, as shown in Fig. 9, a filtering process is performed on a plurality of plot points P included in the plot diagram 41 of Fig. 8. The conditions for the filtering process can be set arbitrarily. As an example, it is advisable to set tolerances for each of the cross-sectional area S, the absolute maximum length, and the hollow area ratio of the skeleton 10, and perform the filtering process under the condition that plot points P that deviate from at least one of the tolerances are discarded.

[0038] For example, by filtering using the cross-sectional area S, it is possible to exclude plot points P corresponding to skeletons 10 in which a cross-sectional shape of a substantially triangular skeleton 10 is combined with multiple other skeletons 10 to form a substantially polygonal cross-sectional shape. Furthermore, by filtering using the absolute maximum length, it is possible to exclude plot points P corresponding to skeletons 10 in which the skeleton 10 is not cut in its thickness direction and has an irregular shape. Here, the absolute maximum length is the maximum distance between any two points on the periphery of the skeleton 10 in the image 32. Furthermore, by filtering using the hollow area ratio, it is possible to exclude plot points P corresponding to skeletons 10 in which the metal layer is interrupted. Here, the hollow area ratio is the ratio of the cross-sectional area of ​​the hollow portions 11 to the total cross-sectional area in the image 32, which is the sum of the cross-sectional area S of the skeleton 10 and the cross-sectional area of ​​the hollow portions 11 formed inside the skeleton 10.

[0039] In this way, it is preferable to use filtering processing to extract plot points P corresponding to skeletons 10 whose cross-sectional shape is a hollow, approximately triangular shape from all skeletons 10 present in image 32. By performing filtering processing, plot points P that are unnecessary for calculating the skeleton thickness ratio are discarded, and multiple plot points P necessary for use in calculating the skeleton thickness ratio are extracted. This results in a plot diagram 51 such as that shown in the upper part of FIG. 9. The histogram shown in the lower part of FIG. 9 is a histogram 52 in which the vertical axis represents frequency and the horizontal axis represents cross-sectional area S, and corresponds to plot diagram 51.

[0040] Furthermore, an approximate curve CL2 is created based on the plurality of plot points P extracted by the filtering process. Fig. 9 shows the approximate curve CL2 created by performing quadratic function approximation using the least squares method or the like based on the plurality of plot points P included in the plot diagram 51 (all of the plot points P included in the plot diagram 51). Note that, although the approximate curve CL2 is expressed by a quadratic function in this embodiment, the approximate curve CL2 is not limited to this, and may be expressed by a higher-order function of cubic or higher order.

[0041] In the example shown in Figure 9, the skeleton thickness gradually decreases as the Y-axis coordinate increases until it reaches the minimum value, with the minimum value being near the midpoint between the maximum and minimum coordinate values ​​of the multiple plot points P, and then gradually increases from the minimum value as the Y-axis coordinate increases.

[0042] Next, in step S4, when the area located on the surface 2, 3 side of the metal porous body 1 in the thickness direction is defined as the surface side, and the area inside the surface side of the metal porous body 1 in the thickness direction is defined as the inner part, a skeletal thickness ratio is calculated, which indicates the ratio of the average skeletal thickness of the skeleton 10 included in the inner part to the average skeletal thickness of the skeleton 10 included in the surface side, each calculated based on the approximation curve CL2.

[0043] In this step, the skeletal thickness ratio is calculated using the approximate equation of the approximate curve CL2 created in step S3. Specifically, first, the minimum value TSa on the approximate curve CL2 is extracted. Then, using the approximate equation, the Y coordinate value Ya of point A having the value TSa can be calculated. The value Ya indicates a position near the center in the thickness direction of the metal porous body 1.

[0044] Since the metal porous body 1 has a predetermined thickness TP, a value Yb can be set that indicates a position at an arbitrary distance in the thickness direction from the position indicated by the value Ya. Then, the skeletal thickness value TSb of point B having the value Yb can be calculated using an approximation formula.

[0045] Therefore, the ratio of the value TSa to the value TSb (TSa / TSb) can be calculated as the skeletal thickness ratio. In this way, it is possible to estimate the skeletal thickness ratio between the surface side portion and the inner portion. This suggests that the closer the skeletal thickness ratio is to 1, the smaller the variation in skeletal thickness between the surface 2, 3 side and the inner side in the thickness direction of the metal porous body 1 is.

[0046] If the filtering process is omitted in step S3, the skeletal thickness ratio may be calculated based on an approximate curve CL1 created based on a plurality of plot points P included in the plot diagram 41 shown in FIG. 8. However, from the viewpoint of accuracy, it is preferable to perform the filtering process. The approximate curve CL1 may be a quadratic function, or may be an approximate curve CL1 expressed by a higher-order function of cubic or higher order.

[0047] Next, the effects of the present invention will be described based on examples. Fig. 10 is a table showing the coefficient of variation obtained from the calculation results of the skeleton thickness ratio for each of samples 1 to 3.

[0048] (Example) The examples are as follows: Three types of porous metal bodies 1, Samples 1 to 3, having different structures were prepared, and the skeletal thickness ratio was calculated for each of Samples 1 to 3 by the evaluation method according to the flow chart of FIG.

[0049] First, multiple cross-sectional SEM images 31 were acquired for each of Samples 1 to 3. Then, the acquired multiple SEM images 31 were binarized to obtain the cross-sectional area S and perimeter L of all skeletons 10 present in each image 32, and then the skeleton thickness was calculated for each of all skeletons 10 from the obtained cross-sectional area S and perimeter L.

[0050] Furthermore, for each image 32, a plot diagram 41 including a plurality of plot points P in which the calculated skeleton thickness was plotted for each Y coordinate of the skeleton 10 was obtained, and then a filtering process was performed on all of the plot points P included in the plot diagram 41 to obtain a filtered plot diagram 51. The filtering conditions were such that tolerances were set for the cross-sectional area S of the skeleton 10, the absolute maximum length, and the hollow area ratio, and plot points P that fell outside at least one of the tolerances were discarded.

[0051] Based on all plot points P included in each plot diagram 51, an approximate curve CL2 was created by quadratic approximation using the least squares method. Points A and B were determined based on the created approximate curve CL2, and then the skeletal thickness ratio was calculated for each image 32 using the skeletal thickness value TSa of point A and the skeletal thickness value TSb of point B. When setting the Y coordinate value Ya of point A to the Y coordinate value Yb of point B, the distance from value Ya to value Yb was standardized for samples 1 to 3.

[0052] Finally, the coefficient of variation (CV value) was calculated from the calculation results of the skeleton thickness ratio, indicating the variance in the calculation results for each of Samples 1 to 3. The coefficient of variation shown in Figure 10 is expressed as a percentage (%) of the standard deviation of the skeleton thickness ratio divided by the average value of the skeleton thickness ratio for each of Samples 1 to 3. It can be said that the smaller the coefficient of variation, the more accurately the skeleton thickness of the surface side and the skeleton thickness of the inner part can be compared.

[0053] (Comparative Example) The comparative examples are as follows. In the evaluation method for the comparative examples, images 32 obtained for each of Samples 1 to 3 in the same manner as in the examples were observed, and skeletons 10 having a substantially triangular cross-sectional shape were randomly selected from the surface side and inner parts. Then, the thickness of each selected skeleton 10 was measured at arbitrary measurement positions (center positions) on three sides, and the average value was taken as the skeleton thickness. Furthermore, using the skeleton thickness of each selected skeleton, the average skeleton thickness of the skeletons 10 included in the surface side and the average skeleton thickness of the skeletons 10 included in the inner part were calculated, and the skeleton thickness ratio was determined for each image 32. Furthermore, as in the examples, a coefficient of variation indicating the variation in the calculation results for each of Samples 1 to 3 was determined from the calculation results of the skeleton thickness ratio.

[0054] 10, in the Examples, the coefficients of variation were all 15% or less, and the coefficients of variation were all lower than those of the Comparative Examples in the calculation results for Samples 1 to 3. In particular, when Sample 1 was evaluated, the coefficient of variation in the Examples was significantly lower than that of the Comparative Examples, and the variation in the calculation results of the skeletal thickness ratio was significantly smaller.

[0055] In this way, it was confirmed that by using the evaluation method for the metal porous body 1 according to this embodiment, not only can the measurement accuracy of the skeletal thickness be improved, but also arbitrary adjustment of the measurement value when measuring the skeletal thickness can be avoided. [Explanation of symbols]

[0056] 1. Porous metal 2, 3 surface 10 Skeleton 11 Hollow part 20 vacancies 31 SEM images 32 images 41, 51 Plot diagram 42, 52 Histogram A, B points CL1, CL2 approximate curve P plot points

Claims

1. A method for evaluating a flat metal porous body having a skeleton with a three-dimensional network structure, comprising: acquiring an image of a cross section along a thickness direction of the porous metal body; acquiring a cross-sectional area and a perimeter of the skeleton based on the image, and calculating a ratio of the cross-sectional area to the perimeter as a thickness of the skeleton; A method for evaluating a porous metal body having the above structure.

2. a step of acquiring plot points obtained by plotting the thickness of the skeleton against the position coordinates of the skeleton indicating the thickness direction, and acquiring an approximation curve based on the plot points acquired for each of a plurality of skeletons present in the image; a step of calculating a ratio of the average thickness of the skeleton included in the inner portion to the average thickness of the skeleton included in the surface side portion, calculated based on the approximation curve, when a region located on the surface side in the thickness direction of the metal porous body is defined as a surface side portion and a region of the metal porous body located inside the surface side portion in the thickness direction is defined as an inner portion; The method for evaluating a porous metal body according to claim 1, further comprising:

3. The method for evaluating a porous metal body according to claim 1 , wherein the porous metal body has the skeleton formed by plating.

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