Surface-treated titanium materials for fuel cell separators and their manufacturing methods

Abstract

A surface-treated titanium material for fuel cell separators is provided, exhibiting good toughness even under pressure molding conditions, excellent adhesion between the surface layer and the substrate, and maintaining conductivity over a long period. The surface-treated titanium material for fuel cell separators comprises a titanium substrate having a passivation film on its surface and a surface layer formed on the titanium substrate. The surface layer includes a titanium oxide layer and carbon particles, with the carbon particles dispersed within the titanium oxide layer. The combined thickness of the titanium oxide layer and the passivation film is 25 nm or more. The ratio of the peak height representing Ti₂O₃ to the peak height representing TiO₂ (Ti₂O₃ / TiO₂ ratio), when the Raman spectrum of the surface layer and the passivation film combined is separated, is 0.08 to 1.45 or less. Furthermore, the porosity at the interface between the titanium oxide layer and the titanium substrate is 0.30 or less.

Description

Technical Field

[0001] This invention relates to surface-treated titanium materials for fuel cell separators and methods for manufacturing the same, and particularly to surface-treated titanium materials for fuel cell separators that can maintain high conductivity over a long period of time and methods for manufacturing the same. Background Technology

[0002] The separators that separate the individual cells of a fuel cell form the flow path for the fuel gas and also allow the current generated from one cell to flow to the adjacent cells. Therefore, the materials used for fuel cell separators are required to have high conductivity and maintain their conductivity over a long period of time in the corrosive atmosphere within the fuel cell cells.

[0003] As a type of separator material that meets such requirements, Patent Document 1 discloses a separator material for fuel cells in which a mixed layer of rutile-based titanium oxide and carbon black is formed on the surface of a titanium substrate.

[0004] According to the separator material for fuel cells described in the aforementioned Patent Document 1, since the mixed layer contains carbon black which is relatively stable to oxidation, it can achieve both high conductivity and conductive corrosion resistance.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2016-122642 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] However, in order to improve the power generation efficiency of fuel cells, there are cases where the width of the fuel gas flow channel in the separator is made narrower.

[0010] However, when manufacturing such grooved separators using pressure molding, the mixed layer of titanium oxide containing the aforementioned rutile matrix may peel off from the titanium substrate surface. Furthermore, due to the insufficient toughness of the mixed layer, numerous cracks are introduced during pressure molding, allowing corrosive acidic liquids from the fuel cell to easily reach the interface between the mixed layer and the titanium substrate. As a result, interfacial corrosion progresses, leading to a decrease in conductivity.

[0011] Therefore, it is desirable to develop an insulating material with a titanium substrate and a surface layer that can improve the adhesion between the surface layer and the substrate, enhance the toughness of the surface layer, and maintain conductivity over a long period even when pressure molding is used to form grooves.

[0012] The present invention was made in view of such a problem, and the object is to provide a surface-treated titanium material for fuel cell separators that has good toughness, excellent adhesion between the surface layer and the substrate even under pressure molding under harsh conditions, and can maintain conductivity for a long time, as well as a method for manufacturing the same.

[0013] Methods used to solve problems

[0014] The above-mentioned objective of the present invention is achieved by means of the following [1] structure related to the surface-treated titanium material for fuel cell separators.

[0015] [1] A surface-treated titanium material for a fuel cell separator comprises a titanium substrate having a passive film on its surface and a surface layer formed on the titanium substrate; the surface layer comprises a titanium oxide layer and carbon particles; characterized in that the carbon particles are dispersed inside the titanium oxide layer; the combined thickness of the titanium oxide layer and the passive film is 25 nm or more; when performing peak separation on the Raman spectrum of the combined titanium oxide layer and the passive film, the peaks at a Raman shift of 235–252 cm⁻¹ are obtained. -1 The peak height obtained within the range of 260–276 cm is set as I1, and the peak height will be within the Raman shift range of 260–276 cm. -1 The peak height obtained within the range of 292–303 cm⁻¹ is set as I², and the peak will be in the Raman shift range of 292–303 cm⁻¹. -1 When the height of the peak obtained within the range is set to I3, (I2+I3) / I1 is 0.08 or more and 1.45 or less; and when observing any cross section with a field emission scanning electron microscope, when the length of the interface between the aforementioned titanium oxide layer and the aforementioned titanium substrate in the observation area is set to L1, and the length of the void existing at the aforementioned interface is set to L2, L2 / L1 is 0.30 or less.

[0016] The above-mentioned objective of the present invention is achieved by means of the following [2] or [3] structure related to the manufacturing method of titanium material for surface treatment of separators for fuel cells.

[0017] [2] A method for manufacturing a surface-treated titanium material for a fuel cell separator, as described in [1], is characterized by comprising: a step of annealing a titanium substrate; a step of coating carbon particles onto the surface of the annealed titanium substrate; an oxidation treatment step of heat-treating the titanium substrate coated with the aforementioned carbon particles in an oxidizing atmosphere to form the aforementioned titanium oxide layer containing the aforementioned carbon particles; a cleaning step of removing carbon particles that are not in close contact with the aforementioned titanium oxide layer from the surface of the aforementioned titanium oxide layer; and a reduction treatment step of heat-treating the titanium substrate on which the aforementioned titanium oxide layer is formed in a vacuum or an inactive gas atmosphere to form the aforementioned surface layer.

[0018] [3] A method for manufacturing a surface-treated titanium material for a fuel cell separator, as described in [1], is characterized by comprising: a step of annealing a titanium substrate; a step of coating carbon particles onto the surface of the annealed titanium substrate; an oxidation treatment step of heat-treating the titanium substrate coated with the aforementioned carbon particles in an oxidizing atmosphere to form the aforementioned titanium oxide layer containing the aforementioned carbon particles; a reduction treatment step of heat-treating the titanium substrate with the aforementioned titanium oxide layer formed in a vacuum or an inactive gas atmosphere to form the aforementioned surface layer; and a cleaning step of removing carbon particles that are not in close contact with the aforementioned titanium oxide layer from the surface of the aforementioned titanium oxide layer.

[0019] Invention Effects

[0020] According to the present invention, a surface-treated titanium material for fuel cell separators, and a method thereof, are provided that exhibits good toughness, excellent adhesion between the surface layer and the substrate even under pressure molding conditions, and maintains conductivity over a long period of time. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the structure of the surface-treated titanium material used for the fuel cell separator according to this embodiment.

[0022] Figure 2 The accompanying photograph is a substitute for the attached image showing a cross-section of a surface-treated titanium material as captured by a field emission scanning electron microscope.

[0023] Figure 3 This is a graph showing the relationship between (I2+I3) / I1 and contact resistance when the horizontal axis is set to (I2+I3) / I1 and the vertical axis is set to contact resistance.

[0024] Figure 4 This is a graph showing the measurement method for the carbon particle content in the titanium oxide layer.

[0025] Figure 5 This is a schematic diagram showing the vacuum chamber used in the oxidation process.

[0026] Figure 6 This is a graph showing the relationship between heating temperature and titanium foil temperature in an oxidation process, where the horizontal axis is set to heating time and the vertical axis is set to titanium foil temperature.

[0027] Figure 7 This is a schematic diagram showing the continuous annealing furnace used in the embodiment.

[0028] Figure 8 This is a graph showing the relationship between heating temperature and titanium foil temperature in a reduction process, where the horizontal axis is set to heating time and the vertical axis is set to titanium foil temperature.

[0029] Figure 9 This is a schematic diagram representing a contact resistance measuring instrument.

[0030] Figure 10 This is a diagram showing an example of a sample after Raman spectroscopy and peak separation.

[0031] Figure 11 This is a chart showing the standard spectrum of Ti2O3.

[0032] Figure 12 This is a chart showing the standard spectrum of TiO2 (rutile).

[0033] Figure 13 This is a graph showing the relationship between heater set temperature and film thickness when the horizontal axis is set to the heater set temperature during oxidation treatment and the vertical axis is set to film thickness.

[0034] Figure 14 This is a graph showing the relationship between heater set temperature and void ratio when the horizontal axis is set to the heater set temperature during oxidation treatment and the vertical axis is set to the void ratio.

[0035] Figure 15 These are photographs that represent cross-sections of Comparative Examples No. 7, Example No. 15, and Example No. 16 taken by SEM imaging, serving as alternative drawings. Detailed Implementation

[0036] Hereinafter, a method for implementing the present invention (hereinafter referred to as "this embodiment") will be described in detail. Furthermore, the present invention is not limited to the embodiment described below, and can be implemented in any way without departing from the spirit of the invention.

[0037] Furthermore, in this specification, the term "~" indicating a numerical range is used to mean that the numerical values ​​listed before and after it are the lower and upper limits.

[0038] Surface-treated titanium materials for fuel cell separators

[0039] The inventors of this invention have conducted focused research to obtain a surface-treated titanium material that suppresses interfacial delamination, possesses high toughness (and is less prone to cracking), and maintains high conductivity. They discovered that appropriately controlling the ratio of Ti₂O₃ to TiO₂ in the titanium oxide layer and reducing the void area is effective. The surface-treated titanium material for fuel cell separators according to this embodiment will be described in more detail below. Furthermore, in this specification, the surface-treated titanium material for fuel cell separators may be simply referred to as surface-treated titanium material.

[0040] <Construction of Surface-Treated Titanium Material for Fuel Cell Separators>

[0041] Figure 1 This is a schematic diagram illustrating the structure of the surface-treated titanium material used for the fuel cell separator according to this embodiment.

[0042] The surface-treated titanium material 1 comprises a titanium substrate 2 having a passive film 3 on at least a portion of its surface and a surface layer 4 formed on the titanium substrate 2.

[0043] Surface layer 4 comprises titanium oxide layer 5 and carbon particles 6. Furthermore, carbon particles 6a are dispersed within titanium oxide layer 5, but some carbon particles 6b are in close contact with the surface of titanium oxide layer 5. In addition, voids are inevitably formed at the interface F2 between titanium substrate 2 and surface layer 4.

[0044] Furthermore, a passive film (amorphous titanium oxide layer) with a thickness of 3 nm to 10 nm originally exists on the surface of the titanium substrate 2. This passive film 3 also remains on the surface-treated titanium material 1 obtained using the manufacturing method described in this embodiment. There are cases where the thickness of the remaining passive film 3 differs from the thickness of the passive film originally present on the surface of the titanium substrate 2. However, the term "passive film 3" in the surface-treated titanium material 1 according to this embodiment refers to the passive film remaining after manufacturing. Hereinafter, the passive film remaining after manufacturing will be simply referred to as the passive film 3, and the portion of the titanium substrate 2 excluding the passive film 3 will be referred to as the metallic titanium layer 2a.

[0045] (The total film thickness T of the titanium oxide layer and the passive film is ≥25nm)

[0046] Carbon particles 6 are stable to oxidation and serve as a conductive path for the current generated from a single cell to flow to an adjacent single cell.

[0047] Although it also depends on the size of the carbon particles 6, if the total film thickness T of the titanium oxide layer 5 and the passive film 3 is less than 25 nm, the carbon particles 6 cannot be sufficiently incorporated into the titanium oxide layer 5, and the desired conductivity cannot be obtained. Therefore, the total film thickness T of the titanium oxide layer 5 and the passive film 3 is set to 25 nm or more, preferably 30 nm or more.

[0048] On the other hand, if the total film thickness T of the titanium oxide layer 5 and the passive film 3 is less than 50 nm, it is possible to suppress the excessive formation of voids 7 along the interface F2 between the titanium oxide layer 5 and the passive film 3 in the titanium substrate 2. As a result, when using the fuel cell separator obtained by press-forming the surface-treated titanium material 1, even if corrosive acidic liquid reaches the interface F2 along the pores and cracks of the surface layer 4, the voids 7 into which the acidic liquid penetrates are less, thus preventing the acidic liquid from spreading at the interface F2. As a result, corrosion can be suppressed, and the conductivity of the fuel cell separator can be maintained. Therefore, the total film thickness T of the titanium oxide layer 5 and the passive film 3 is preferably set to be less than 50 nm.

[0049] Furthermore, the thickness of the titanium oxide layer 5 varies depending on the orientation of the crystal grains in the titanium substrate 2. Since the titanium oxide layer 5 changes color according to its thickness, ranging from light to dark yellow, orange, brown, purple, cyan, and light blue, each crystal grain will have a different color when the surface is observed using an optical microscope, indicating that the thickness of the titanium oxide layer 5 is uneven. In this specification, the total film thickness T of the titanium oxide layer 5 and the passive film 3 refers to the total film thickness of the titanium oxide layer 5 and the passive film 3 when viewed under an optical microscope, considering the cross-section of the portion showing the most prevalent color. Hereinafter, the total film thickness T of the titanium oxide layer 5 and the passive film 3 will be simply referred to as film thickness T.

[0050] Figure 2 This is a photograph, alternative to the accompanying drawing, showing an example of a cross-section of a surface-treated titanium material as captured by a field emission scanning electron microscope. Figure 2 This paper details the calculation method for the total film thickness T of the titanium oxide layer and the passive film.

[0051] First, the surface of the surface-treated titanium material 1 is observed using an optical microscope, and the part with the most proportion of color is cross-sectioned using a cross-section polisher (CP: Crosssection polisher (registered trademark)).

[0052] Next, the cross-section was observed at 300,000x magnification using a field emission scanning electron microscope (FE-SEM).

[0053] Then, within the field of view, four areas where no carbon particles 6 are found on the outermost surface F3 of the titanium oxide layer 5 are located. The film thickness (distance from F1, the interface between the metallic titanium layer 2a and the passive film 3 in the titanium substrate 2, to the outermost surface F3 of the titanium oxide layer 5) in these areas is measured, and the average value of the four locations is set as the film thickness T.

[0054] In addition, in this embodiment, a protective layer 10 is formed to protect the carbon particles 6 and the titanium oxide layer 5 before the cross-section is processed by CP. The protective layer 10 can be obtained, for example, by forming an osmium film 8 on the surface of the surface layer 4 by vapor deposition and then forming a carbon film 9.

[0055] exist Figure 2 In the diagram, the four locations where carbon particles 6 are absent on the outermost surface are designated as P1, P2, P3, and P4, respectively. The distances from interface F1 to P1, P2, P3, and P4 are designated as H1, H2, H3, and H4, respectively. Figure 2 In the diagram, H1: 45nm, H2: 55nm, H3: 46nm, and H4: 52nm. Therefore, Figure 2 The film thickness T of the surface-treated titanium material 1 shown can be calculated using (H1+H2+H3+H4) / 4, for example, it is calculated to be 50nm.

[0056] <(I2+I3) / I1: Above 0.08 and below 1.45>

[0057] In this embodiment, the titanium oxide layer 5 comprises Ti2O3 and TiO2, with Ti2O3 having a metallic bond component (Table 2.3, p40, No. 3, of the Journal of the China Association for the Study of Corrosion Prevention, ed., Metallic Materials, 3rd Printing). Therefore, if the proportion of Ti2O3 in the titanium oxide layer 5 increases, the titanium oxide layer 5 becomes more easily plastically deformed under pressure molding conditions, and interfacial peeling can be suppressed.

[0058] Therefore, in this embodiment, the content of Ti2O3 relative to the content of TiO2 is calculated by using the peak height of TiO2 representing the rutile structure and the peak height of Ti2O3 during Raman spectroscopy analysis.

[0059] After peak separation of the Raman spectrum combining the surface layer and the passive film, the Raman shift was found to be between 235 and 252 cm⁻¹. -1 The peaks obtained within the range represent TiO2, and in this embodiment, their height is set to I1. Similarly, the peaks in the range of 260–276 cm⁻¹ represent TiO₂. -1 The peaks obtained in the range of 292–303 cm⁻¹ and in the range of 292–303 cm⁻¹ -1 The peaks obtained within the range all represent Ti2O3, and the heights of these peaks are set as I2 and I3, respectively.

[0060] That is, the content of Ti2O3 relative to the content of TiO2 is represented by (I2+I3) / I1.

[0061] If (I2+I3) / I1 is less than 0.08, the content of brittle TiO2 in the titanium oxide layer 5 increases, which may lead to the peeling of the titanium oxide layer 5 under pressure molding. Furthermore, even without peeling, more cracks form in the titanium oxide layer 5. As a result, the corrosive acidic liquid generated inside the fuel cell penetrates along the peeling interface and cracks to the interface F2 between the surface layer 4 and the titanium substrate 2, causing corrosion of the passive film 3 and / or the adjacent titanium layer 2a, thus reducing the conductivity of the separator.

[0062] Therefore, (I2+I3) / I1 is set to 0.08 or higher, preferably 0.2 or higher.

[0063] Such titanium oxide mixed with Ti2O3 cannot be formed simply by oxidizing a titanium substrate coated with carbon particles such as carbon black under a low-pressure oxygen atmosphere. It is obtained by heating (reduction treatment) in a vacuum or inert gas atmosphere after oxidation, allowing the oxygen in the rutile and anatase structured titanium oxide (TiO2) formed during the oxidation process to diffuse and absorb into the titanium substrate. In other words, because some of the oxygen in TiO2 is drawn away by the titanium, TiO2 transforms into Ti2O3.

[0064] However, since the volume of titanium per mole is approximately 16% smaller than that of rutile TiO2, the more the conversion from TiO2 to Ti2O3 progresses, the greater the volume shrinkage of the titanium oxide layer 5. Therefore, it is conceivable that the titanium oxide layer 5 will become porous, or that excessive tensile stress will be generated in it. Consequently, although there are metallic bonds in Ti2O3, if the Ti2O3 content is excessive, cracks can easily be introduced into the titanium oxide layer 5 during extension by pressure molding.

[0065] If corrosive acidic liquid generated within the fuel cell penetrates into the interior of the titanium oxide layer 5 along the formed pores and cracks, it reaches the interface F2 between the titanium substrate 2 and the surface layer 4, causing corrosion of the passive film 3 and / or the adjacent metallic titanium layer 2a. As a result, an amorphous titanium oxide layer with lower conductivity is formed between the conductive carbon particles 6 and the titanium substrate 2, degrading the conductivity of the separator. Therefore, it is also necessary to specify an upper limit for the Ti2O3 content in the titanium oxide layer 5.

[0066] If (I2+I3) / I1 exceeds 1.45, the formation of Ti2O3 in the titanium oxide layer 5 becomes excessive, thus reducing the conductivity of the insulating element. Therefore, (I2+I3) / I1 is set to 1.45 or less, preferably 1.1 or less.

[0067] In addition, when indicating the content of Ti₂O₃, the reason for taking the sum of the heights of two peaks (I₂+I₃) is that the peak intensity is small if only one of the peaks is used, and when peak separation is performed by curve fitting based on Raman spectroscopy, detection may be difficult if only one of the peaks is used.

[0068] <L2 / L1:0.30以下>

[0069] Voids 7 are inevitably formed at the interface F2 between the titanium base material 2 and the surface layer 4. If a large number of regions have such voids 7, cracks and the like occur when the surface-treated titanium material 1 is compression molded, and corrosive acid liquid intrudes into the interior of the voids 7, thereby causing corrosion at the interface F2 and deteriorating the electrical conductivity of the separator.

[0070] In the present embodiment, an arbitrary cross-section is observed at a magnification of 300,000 times using an FE-SEM, and the proportion of regions where voids 7 are formed is appropriately defined. When an arbitrary cross-section is observed with an FE-SEM, if the length of the interface F2 between the surface layer 4 and the titanium base material 2 in the observation region is defined as L1, and the length of the voids 7 existing at the interface is defined as L2, the proportion of regions where voids 7 are formed can be expressed by L2 / L1.

[0071] If L2 / L1 exceeds 0.30, cracks are formed in the titanium oxide layer 5 when compression molding is performed. Then, when the obtained fuel cell separator is used, corrosive acid liquid intrudes into the voids 7 through the cracks and spreads at the interface F2, which promotes the corrosion of the passive film 3 and / or the metallic titanium layer 2a adjacent to the passive film 3.

[0072] Therefore, L2 / L1 is preferably 0.30 or less. In addition, in an arbitrary cross-section observed with an FE-SEM, if no void 7 is observed, L2 is set to 0, so L2 / L1 is 0.

[0073] Use Figure 2 The method for calculating the proportion of void regions will be described in detail. In addition, as described above, the film thickness T is calculated using a cross-sectional photograph of the region where the proportion of the thickness of the titanium oxide layer 5 is the largest; and the proportion of void regions is also calculated using a cross-sectional photograph of the region where the proportion of color in surface observation is the largest, in the same manner as the measurement method for the film thickness T. Therefore, the following uses Figure 2 to describe the method for calculating the proportion of void regions.

[0074] The void 7 has, for example, an elliptical shape extending in the horizontal direction, and the interface between the void 7 and the metallic titanium layer 2a forms a linear shape that is darker in contrast (the interface F2 between the passive film 3 and the titanium oxide layer 5). Although the interface undulates rather than being a straight line, it is regarded as a straight line and the void length is measured from the image.

[0075] exist Figure 2 In the process, the lengths of five gaps (B1, B2, B3, B4, B5) are measured, and L2 (B1+B2+B3+B4+B5) is calculated by summing them. By setting the width of the image to the interface length L1, L2 / L1 can be calculated.

[0076] Figure 3 This is a graph showing the relationship between (I2+I3) / I1 and contact resistance when the horizontal axis is set to (I2+I3) / I1 and the vertical axis is set to contact resistance. Additionally, the graph shows a contact resistance of, for example, 15 mΩ·cm. 2 In the following cases, it can be judged that the conductivity is excellent.

[0077] like Figure 3 As shown, in the range between the two thicker solid lines parallel to the vertical axis, where (I2+I3) / I1 is between 0.08 and 1.45, the contact resistance is lower compared to other ranges. In particular, in the range between the two dashed lines parallel to the vertical axis, where (I2+I3) / I1 is between 0.1 and 1.1, the contact resistance is even lower.

[0078] However, even when the value of (I2+I3) / I1 is within the range of this invention, the contact resistance still exceeds 15 mΩ·cm. 2 These are cases where the combined film thickness T of the titanium oxide layer and the aforementioned passive film is less than 25 nm, or the L2 / L1 ratio is 0.30 or higher, leading to an increase in contact resistance.

[0079] (Types of carbon particles)

[0080] In this embodiment, carbon particle 6 can be any carbon-based particle or a carbon-based particle doped with B, N, etc. Examples of carbon particles 6 include carbon black, graphite, B-doped diamond particles, and N-doped diamond particles. One type of carbon particle can be used alone, or two or more can be used together. Carbon black is a carbon particle with a chain-like structure composed of amorphous carbon. Carbon black is classified according to its manufacturing method as furnace black, acetylene black, or thermal black, and any of these can be used. Examples of graphite include synthetic graphite and natural graphite.

[0081] (Average particle size of carbon particles)

[0082] The average particle size of the carbon particles 6 used, based on the median diameter of the aggregated carbon particles (the particle diameter that constitutes 50% of the cumulative distribution: hereinafter referred to as D50), is preferably 120 nm or less. If the average particle size of the carbon particles 6 is 120 nm or less, the proportion of smaller carbon particles 6, which is less than the thickness of the titanium oxide layer 5, increases, thus increasing the amount incorporated into the titanium oxide layer. As a result, the conductive path increases, and higher conductivity can be obtained. The particle size (D50) of the carbon particles 6 is more preferably 70 nm or less.

[0083] (The carbon particle content in the titanium oxide layer)

[0084] By controlling the carbon particle content in the titanium oxide layer 5, a fuel cell separator with excellent electrical conductivity can be obtained. Excellent electrical conductivity is achieved when the carbon particle content in the titanium oxide layer 5 is 20% or more. Therefore, the preferred carbon particle content in the titanium oxide layer 5 is 20% or more, and more preferably 25% or more.

[0085] (Method for measuring the carbon particle content in the titanium oxide layer)

[0086] Figure 4 This is a graph showing the measurement method for the carbon particle content in the titanium oxide layer.

[0087] In this embodiment, the carbon particle content is calculated based on the reflected electron image of the surface layer cross-section taken using FE-SEM at an accelerating voltage of 2.0 keV and a magnification of 300,000x. Specifically, the area occupied by the carbon particles 6 inserted into the titanium oxide layer 5 is denoted as Sc, and the total cross-sectional area of ​​the titanium oxide layer 5 and the passive film 3 is denoted as St. The ratio (Sc / (St+Sc)) is calculated to obtain the content.

[0088] Reference Figure 4 The method for measuring the carbon particle content is explained in more detail.

[0089] First, such as Figure 4 As shown in (1), a cross-section of the surface-treated titanium material 1, which is the object of measurement, is processed at any position using CP, and the cross-section is photographed using FE-SEM. Then, the interface F1 between the passive film 3 and the metallic titanium layer 2a is selected to be as straight as possible.

[0090] Next, as Figure 4 As shown in (2), draw a straight line A1 on the above interface, and draw a straight line A2 parallel to the interface at a position above the film thickness T obtained by the above method from the interface.

[0091] Then, as Figure 4As shown in (3), the image of the region between lines A1 and A2 is binarized, carbon particles 6 are set to black, and the titanium oxide layer 5 is assigned a white color. At this time, the carbon particles protruding from the surface of the titanium oxide layer 5 are not included in Sc.

[0092] Then, the number of black points, which is proportional to Sc, and the number of points in the region between lines A1 and A2, which is proportional to St+Sc, are measured, and Sc / (St+Sc) is calculated. Additionally, during binarization, gaps are also colored black, so the gaps are treated as white points, and Sc is calculated accordingly.

[0093] [Manufacturing Method of Surface-Treated Titanium Material for Fuel Cell Separators]

[0094] The method for manufacturing a surface-treated titanium material for a fuel cell separator according to this embodiment includes: a step of annealing a titanium substrate; a step of coating the surface of the annealed titanium substrate with carbon particles; an oxidation treatment step of heat-treating the titanium substrate coated with carbon particles in an oxidizing atmosphere to form a titanium oxide layer containing carbon particles; a cleaning step of removing carbon particles that are not in close contact with the titanium oxide layer from the surface of the titanium oxide layer; and a reduction treatment step of heat-treating the titanium substrate with the titanium oxide layer formed in a vacuum or an inactive gas atmosphere to form a surface layer.

[0095] The manufacturing method described above will be explained in detail below.

[0096] <Substrate>

[0097] Rolled titanium foil can be used as the titanium substrate 2.

[0098] (Average carbon concentration on the substrate surface)

[0099] The average carbon concentration from the outermost surface of the titanium substrate 2 to a depth of 50 nm is preferably 10 atomic percent or less, more preferably 7 atomic percent or less. Regarding the surface carbon, the titanium wear powder generated during rolling reacts with the lubricating oil to form Ti(C,O), which has an approximately equal composition of oxygen and carbon, and is embedded in the titanium surface. This Ti(C,O) thermally decomposes in the subsequent annealing process, and C diffuses into the titanium through solid solution. However, if there is a large amount of Ti(C,O) after rolling, a large amount of Ti(C,O) that cannot be completely decomposed will remain.

[0100] In the oxidation process described later, almost no titanium oxide layer forms on Ti(C,O). Furthermore, although Ti(C,O) itself is conductive, it is easily corroded into titanium oxide in the acidic environment of the fuel cell. Therefore, if a large amount of incompletely decomposed Ti(C,O) remains, it becomes a cause of increased contact resistance. Even after annealing, the amount of Ti(C,O) remaining after annealing increases with the average carbon concentration.

[0101] If the average carbon concentration from the outermost surface of the titanium substrate 2 to a depth of 50 nm is 10 atomic% or less, the amount of incompletely decomposed Ti(C, O) is reduced, which can suppress the increase in contact resistance. Therefore, the average carbon concentration is preferably 10 atomic% or less, and more preferably 7 atomic% or less.

[0102] (Method for measuring the average carbon concentration on the substrate surface)

[0103] The following describes the method for measuring the average carbon concentration from the outermost surface of the titanium substrate 2 to a depth of 50 nm.

[0104] The carbon concentration from the outermost surface of the titanium substrate 2 to a depth of 50 nm can be measured, for example, by depth-direction compositional analysis using an X-ray photoelectron spectrophotometer (XPS). For instance, in the XPS apparatus, Ar ions are colliding with the surface of the titanium substrate at an accelerating voltage of 1 kV, and the surface is etched in depth units of 0.5 to several nm. The carbon concentration at these depths is then measured. This process of etching the titanium substrate surface in 0.5 to several nm depth units and measuring the carbon concentration is repeated until the etching depth reaches 50 nm. These operations allow the carbon concentration at various depths from the surface to be obtained. Then, the analysis depth is plotted on the horizontal axis, and the carbon concentration on the vertical axis. The measurement points are plotted, and the area bounded by the broken line formed by connecting these points sequentially with straight lines and the horizontal axis up to 50 nm is calculated. Dividing this area by 50 nm yields the average carbon concentration up to 50 nm.

[0105] Furthermore, adsorbed carbon, caused by organic matter or the like present in the atmosphere, can usually be detected on the surface of the titanium substrate 2. In this specification, in carbon concentration measurement, the portion other than the surface portion (contamination layer) of the titanium substrate 2 where organic matter or the like is adsorbed is considered the "outermost surface." Therefore, the aforementioned 50 nm represents the value from the outermost surface to a depth of 50 nm as defined above. Moreover, the area to be analyzed is preferably the area of ​​a circle with a diameter of 100 μm or more. Because Ti(C,O) ranges in size from sub-μm to several μm, to obtain the average carbon concentration on the surface of the titanium substrate, it is necessary to measure an area much larger than Ti(C,O).

[0106] <Antennary process for substrate>

[0107] The manufacturing method described in this embodiment includes a step of annealing a titanium substrate. Furthermore, in the following steps, the surface of the titanium substrate undergoes various changes, such as forming a coating layer obtained by applying carbon particles or a titanium oxide layer. However, for convenience, in the following description, the layer formed on the surface is sometimes referred to as titanium foil.

[0108] To enable pressure forming of the fine grooves, the titanium substrate 2, which has hardened due to rolling, needs to be annealed to soften it, i.e., reduce its 0.2% yield strength. If the 0.2% yield strength of the titanium substrate 2 is high, the springback after pressure forming will be greater, resulting in warping of the separators or an undesirable groove shape, making it difficult to assemble fuel cell cells. Due to the anisotropy of the titanium substrate, the 0.2% yield strength differs in the length direction (referred to as the L direction) and the width direction (referred to as the T direction) of the coil. For pressure forming of the fine grooves, the preferred 0.2% yield strength in the L direction is 110–150 MPa, and the preferred 0.2% yield strength in the T direction is 180–215 MPa.

[0109] Furthermore, if the yield strength of 0.2% is too low, the titanium foil may crack due to pressure forming. Therefore, for example, the rolling temperature of the titanium foil is increased to 700–850°C, and annealing is performed for about 20–40 seconds. Although it also depends on the Fe concentration of titanium impurities that lowers the precipitation temperature of the β phase, annealing below 850°C can suppress the precipitation of the β phase at the grain boundaries, thus improving the pressure formability. That is, it is preferable to select a temperature and time at which the β phase does not precipitate.

[0110] On the other hand, although annealing can be performed at temperatures below 700°C, it is preferable to extend the processing time if the temperature is low. Furthermore, when the surface carbon concentration of the rolled titanium foil is high, increasing the annealing temperature and extending the processing time causes Ti(C,O) to decompose and C to diffuse into the titanium substrate, thus reducing the amount of residual Ti(C,O). In this way, the annealing conditions can be appropriately adjusted according to the surface carbon concentration, provided they soften the titanium foil without affecting the press forming process.

[0111] <Carbon particle coating process>

[0112] The manufacturing method described in this embodiment includes a process of coating carbon particles onto the surface of an annealed titanium substrate 2.

[0113] Carbon particles 6 can be applied to titanium substrate 2 in the form of an aqueous or oil-based dispersion (also known as a dispersion coating). Alternatively, carbon particles 6 can also be applied directly to titanium substrate 2.

[0114] Dispersed coatings containing carbon particles 6 may also contain binder resins and / or surfactants. However, since binder resins and surfactants tend to decrease conductivity, their content is preferably as low as possible. Furthermore, the dispersed coating may contain other additives as needed.

[0115] In adhesive resins, resins that decompose without residue through heating in an oxidation treatment process are preferred. Examples of such adhesive resins include acrylic resins, polyethylene resins, polypropylene resins, polystyrene resins, and polyvinyl alcohol resins. Among these, acrylic resins are preferred from the viewpoint that lower decomposition temperatures have less impact on surface layer formation. One type of adhesive resin can be used alone, or two or more can be used in combination.

[0116] The mixing ratio of carbon particles 6 to binder resin in the dispersion coating, in terms of solid content mass ratio (binding resin solid content / carbon particle solid content), is preferably 0.3 to 2.5. The smaller this mass ratio, the greater the amount of carbon particles 6, resulting in improved conductivity. Therefore, from the viewpoint of conductivity, this mass ratio is preferably 2.5 or less, and more preferably 2.3 or less. On the other hand, the larger this mass ratio, the greater the amount of binder resin. Therefore, when this mass ratio is relatively large, the adhesion between the titanium substrate 2 and the coating film increases. Therefore, from the viewpoint of adhesion, this mass ratio is preferably 0.3 or more, and more preferably 0.4 or more.

[0117] As an aqueous medium, water or ethanol can be used, for example. As an oily medium, toluene or cyclohexanone can be used, for example.

[0118] As described above, the average particle size of carbon particles 6 is preferably 120 nm or less. Since carbon particles 6 tend to easily form aggregates in coatings, coatings carefully designed to prevent aggregate formation are preferred. For example, carbon black, which chemically bonds functional groups such as carboxyl groups to the surface and improves dispersibility by enhancing interparticle repulsion, is preferred as the carbon particles 6.

[0119] From the viewpoint of electrical conductivity, the preferred coating amount of carbon particles 6, excluding the binder resin component, on the surface of the titanium substrate 2 is 10 μg / cm³. 2 The above, more preferably, is 30 μg / cm 2 That's all. Additionally, the preferred coating amount of carbon particles 6 is 60 μg / cm³. 2 The following applies. Even if the amount of carbon particles 6 applied is greater than that applied, the effect of improving conductivity tends to saturate.

[0120] Methods for applying a dispersion containing carbon particles 6 to a titanium substrate 2 include, but are not limited to, brush coating, blade coating, roller coating, gravure coating, die coating, dip coating, and spray coating. Furthermore, methods for applying the dispersion in powder form include, for example, using a toner made from carbon particles 6 and electrostatically coating the toner onto the titanium substrate 2.

[0121] <Oxidation Treatment Process>

[0122] The manufacturing method involved in this embodiment includes an oxidation treatment step of heat-treating a titanium substrate 2 coated with carbon particles in an oxidizing atmosphere to form a titanium oxide layer 5 containing carbon particles 6.

[0123] Figure 5 This is a schematic diagram showing the vacuum chamber used in the oxidation process. (Example) Figure 5 As shown, the vacuum chamber 20 is composed of the sample chamber 11, the heating chamber 12 and the transport passage 17 that connects them.

[0124] In the sample chamber 11, a tray 14 is provided to hold the titanium foil 13 (a titanium substrate 2 coated with carbon particles 6). In addition, in the heating chamber 12, two carbon heaters 16, each 20×40cm in size, are arranged parallel to each other at a distance of 7cm.

[0125] In the sample chamber 11 of the vacuum chamber 20 configured in this way, titanium foil 13 is placed on a tray 14 and mounted, and the vacuum chamber 20 is evacuated. Then, carbon heater 16 is heated to a specified temperature, and oxygen is introduced into the vacuum chamber 20 to bring the pressure inside the vacuum chamber 20 to a specified level.

[0126] Then, the tray 14 is transported from the sample chamber 11 to the heating chamber 12, heated for a specified time, and then returned to the sample chamber 11 to be placed and cooled. Thus, the titanium oxide layer 5 is formed.

[0127] The oxygen partial pressure during oxidation is preferably set to 10–100 Pa. By setting the oxygen partial pressure to 10 Pa or higher, the diffusion rate of titanium ions can be made appropriate when they diffuse from the titanium substrate 2 to the titanium surface, combine with oxygen in the atmosphere, and form a titanium oxide layer. As a result, a surface layer 4 of sufficient thickness to absorb carbon particles that will become conductive pathways can be formed.

[0128] Furthermore, by setting the oxygen partial pressure to below 100 Pa, significant consumption caused by the accelerated combustion of carbon particles can be suppressed. This, in turn, suppresses the increase in the rate of oxygen diffusion into the titanium substrate 2, preventing the formation of a titanium oxide layer 5 on the titanium substrate 2 side from its original surface, thus achieving good electrical conductivity.

[0129] Furthermore, when the carbon concentration on the surface of the titanium substrate 2 is high before oxidation treatment, pickling is sometimes performed after annealing to dissolve and remove the surface layer with high carbon concentration in order to improve corrosion resistance. With such pickling, a surface layer of sufficient thickness is formed as long as the oxygen pressure is above 1 Pa. The reason for the variation in the lower limit of oxygen pressure for forming a surface layer 4 of sufficient thickness via outward diffusion of titanium ions is not yet clear. It is considered that the difference in structure between the passive film on the surface of the titanium substrate 2 formed by the annealing process and the passive film formed after pickling affects the diffusion rate of titanium ions from the titanium substrate 2 through the passive film to the surface.

[0130] Figure 6 This is a graph showing the relationship between heating temperature and titanium foil temperature during the oxidation process, with the horizontal axis set to heating time and the vertical axis set to titanium foil temperature. Additionally, Figure 6 The oxidation treatment conditions shown are as follows: the set temperature of the carbon heater 16 is 615°C, and the coating amount of carbon particles is 60 μg / cm². 2 The partial pressure of oxygen is 20 Pa.

[0131] like Figure 6 As shown, if the titanium foil 13 is fed between the two heated carbon heaters 16, the titanium foil 13 is heated and its temperature rises to a certain temperature. Furthermore, Figure 6 The measurement was taken by inserting a titanium foil 13 with a thermocouple 15 spot-welded to it between the heated carbon heater 16.

[0132] To form the desired titanium oxide layer 5, it is preferable to set the heating rate and the temperature reached of the titanium foil 13, as well as the processing time from the titanium foil 13 to its entry into and exit from the heating chamber 12. The heating rate can be determined by the set temperature of the heater, the emissivity of the heater and the titanium foil coated with carbon particles, and the temperature reached can be determined by the set temperature of the heater and the processing time.

[0133] If the amount of carbon particles 6 coated on the surface of the titanium substrate 2 is varied, the emissivity of the titanium foil 13 changes, and the heating rate changes. Furthermore, if the type of carbon particles 6 is varied, the emissivity of the carbon particles 6 themselves changes, thus changing the heating rate. Moreover, if the material of the heater is varied, the emissivity of the heater, i.e., the proportion of heat released from the heater, changes, thus changing the heating rate. Therefore, the heating rate varies due to various factors and cannot be generalized. However, for example, in the case of using a carbon heater, if the coating amount of carbon particles 6 is 30 μg / cm³... 2 In the above, carbon particles 6 are carbon black, so it is preferable to set the processing time to 7 to 20 seconds and the arrival temperature of the titanium substrate 2 to about 560 to 640°C.

[0134] <Cleaning Process>

[0135] The manufacturing method involved in this embodiment includes a cleaning process to remove carbon particles 6 that are not in close contact with the titanium oxide layer 5 from the surface of the titanium oxide layer 5.

[0136] In the coating layer obtained by coating carbon particles 6 onto the surface of the titanium substrate 2, only carbon particles 6 with a particle size of at least 100 nm are taken from the interface between the titanium substrate 2 and the coating layer and introduced into the titanium oxide layer 5 or closely adhered to the surface of the titanium oxide layer 5. Therefore, it is necessary to remove the carbon particles 6 that are not closely adhered to the titanium oxide layer 5 from the surface of the titanium oxide layer 5. If the non-adhered carbon particles 6 are not removed by cleaning, they may detach during pressure molding and adhere to the mold, causing pressure damage to the titanium foil 13 (the titanium substrate 2 on which the titanium oxide layer 5 is formed) and causing the titanium foil 13 to break. As a cleaning method, methods such as washing with water using a brush or ultrasonic cleaning to remove them can be used, as long as the remaining carbon particles 6 are removed, any method can be used.

[0137] Furthermore, as long as the carbon particles 6 that are not in close contact with the surface of the titanium oxide layer 5 are removed during this cleaning process, it will not have an adverse effect on subsequent processes. Therefore, the carbon particles 6 that are in close contact with the surface of the titanium oxide layer 5 can be completely removed, or some can remain in close contact with the surface.

[0138] <Reduction Process>

[0139] The manufacturing method described in this embodiment includes a reduction treatment step of heating a titanium substrate 2 on which a titanium oxide layer 5 is formed in a vacuum or an inactive gas atmosphere to form a surface layer 4.

[0140] The reduction process can use the vacuum chamber 20 used in the oxidation process.

[0141] First, titanium foil 13 (cleaned titanium substrate 2) is placed on tray 14 and installed in sample chamber 11. After evacuation, carbon heater 16 is heated to a specified temperature. During evacuation, oxygen-supplying molecules such as H2O, O2, CO, and CO2 adsorbed on the walls of vacuum chamber 20 are released. If these oxygen-supplying molecules exist in a certain quantity in the atmosphere of vacuum chamber 20, the reduction process will no longer proceed. Therefore, the vacuum level in vacuum chamber 20 is preferably less than 0.1 Pa, and more preferably less than 0.05 Pa.

[0142] Alternatively, inactive gases such as Ar can be introduced into the vacuum chamber 20 to increase the pressure inside the vacuum chamber 20 to above 0.1 Pa for reduction treatment. However, in this case, it is preferable to evacuate the vacuum chamber 20 until the pressure is less than 0.1 Pa before introducing Ar into the vacuum chamber 20.

[0143] Then, within the heating chamber 12, the titanium foil 13 is conveyed between two carbon heaters 16 that have been heated to a specified temperature. After a specified heating time, it is returned to the sample chamber 11. Thus, a reduction treatment is performed to form the desired surface layer 4.

[0144] As described above, by performing a reduction treatment step after the oxidation treatment step, oxygen can be diffused and absorbed into the titanium substrate by the titanium oxide (TiO2) with rutile and anatase structure formed in the oxidation treatment step to generate Ti2O3. As a result, a titanium oxide layer 5 containing Ti2O3 and TiO2 in a desired ratio can be formed, so a surface layer 4 that is easily plastically deformable can be obtained.

[0145] The preferred conditions for the reduction treatment are to appropriately set the heating rate and the temperature reached of the titanium foil 13, as well as the processing time from the titanium foil 13 to its entry into and exit from the heating chamber 12, based on the oxidation treatment conditions, i.e., the amount of titanium oxide formed by the oxidation treatment.

[0146] In the oxidation process, if the titanium oxide layer 5 grows to a thicker thickness, without the aid of reduction treatment to increase the amount of oxygen diffused from the titanium oxide layer 5 and the passive film 3 to the metallic titanium layer 2a, the amount of Ti2O3 formed will be less, and the adhesion and toughness between the surface layer 4 and the titanium substrate 2 cannot be obtained.

[0147] On the other hand, when the titanium oxide layer 5 is thin, if the amount of oxygen diffused from the titanium oxide layer 5 and the passive film 3 to the metallic titanium layer 2a is not reduced, too much Ti2O3 will be generated, which will easily lead to interfacial corrosion, i.e., an increase in resistance.

[0148] Furthermore, when the heating rate is slow, i.e. the emissivity of the heater is low, it is preferable to extend the processing time since it takes time for the temperature of the titanium foil 13 to rise.

[0149] On the other hand, when the heating rate is relatively fast, it is preferable to shorten the processing time. However, since the heating time is kept constant as long as the material of the heater is fixed, the reduction processing conditions only need to be set to the reached temperature and the processing time.

[0150] Furthermore, even if the thickness of the titanium oxide layer is constant, if the reduction treatment is carried out under conditions of long reduction treatment time and / or high temperature, corrosion of the interface due to excessive reduction will occur, and the resistance of the fuel cell separator will increase.

[0151] On the other hand, even if the thickness of the titanium oxide layer is constant, if the reduction treatment is carried out under conditions of short reduction treatment time and / or low temperature, the surface layer will have insufficient adhesion or toughness due to insufficient reduction.

[0152] Since the reduction treatment conditions vary due to various factors, they cannot be generalized. However, since reduction is difficult under strong oxidation conditions (high temperature or long time treatment), it is preferable to set the reduction treatment conditions to be stronger to match the oxidation conditions.

[0153] On the other hand, since reduction is easier when oxidation is weak, it is preferable to set the reduction treatment conditions to be weak as well.

[0154] For example, in the oxidation process, when the arrival temperature of the titanium foil 13 is set in a high-temperature range of 620–635°C, it is preferable to set the arrival temperature of the titanium foil 13 in the reduction process to a temperature range of 525–610°C. Furthermore, in the oxidation process, when the arrival temperature of the titanium foil 13 is set in a low-temperature range of 555–less than 595°C, it is preferable to set the arrival temperature of the titanium foil 13 in the reduction process to a temperature range of 510–600°C. Moreover, in the oxidation process, when the arrival temperature of the titanium foil 13 is set in a medium-temperature range of 595–less than 620°C, it is preferable to set the arrival temperature of the titanium foil 13 in the reduction process to a temperature range of 515–610°C. By setting these temperatures, a desired resistance value can be obtained.

[0155] Furthermore, in both the oxidation and reduction processes, any heater other than the carbon heater 16, such as a sheath heater, that can heat the titanium foil 13 without oxidizing and consuming it in an oxidizing atmosphere, can be used. However, since the emissivity varies depending on the material of the heater, it is preferable to optimize the processing time.

[0156] [Other manufacturing methods for surface-treated titanium materials for fuel cell separators]

[0157] In the manufacturing method described in the above embodiments, the manufacturing process is described in the order of annealing process → carbon particle coating process → oxidation treatment process → cleaning process → reduction treatment process. However, the cleaning process can also be set as the last one, and the process can be set as annealing process → carbon particle coating process → oxidation treatment process → reduction treatment process → cleaning process.

[0158] That is, another manufacturing method of the surface-treated titanium material for the fuel cell separator according to this embodiment includes: a step of annealing a titanium substrate; a step of coating carbon particles onto the surface of the annealed titanium substrate; an oxidation treatment step of heat-treating the titanium substrate coated with carbon particles in an oxidizing atmosphere to form a titanium oxide layer containing carbon particles; a reduction treatment step of heat-treating the titanium substrate with the titanium oxide layer formed in a vacuum or an inactive gas atmosphere to form a surface layer; and a cleaning step of removing carbon particles that are not in close contact with the titanium oxide layer from the surface of the titanium oxide layer.

[0159] However, in this case, the surface emissivity increases during the reduction process because residual carbon particles 6 adhere to the surface of the titanium foil 13. As a result, if the reduction process is performed under the same heating conditions as when the reduction process is performed after the cleaning process, the temperature of the titanium foil 13 rises more rapidly. Therefore, when the cleaning process is performed after the reduction process, it is preferable to set the heater temperature for the reduction process to be lower.

[0160] Example

[0161] Hereinafter, examples and comparative examples of surface-treated titanium materials for fuel cell separators manufactured by the manufacturing method of this embodiment will be described.

[0162] Surface-treated titanium materials were manufactured using the various manufacturing methods shown below, and the effects of manufacturing conditions on the properties of the resulting surface-treated titanium materials were investigated.

[0163] [Experiment 1. Effects of Ti₂O₃ / TiO₂ ratio and porosity ratio on performance]

[0164] By varying the Ti2O3 / TiO2 ratio ((I2+I3) / I1) and the porosity ratio (L2 / L1), surface-treated titanium materials are manufactured. Detailed manufacturing and measurement conditions are as follows.

[0165] Manufacturing of Surface-Treated Titanium Materials

[0166] (Preparation and carbon analysis of titanium substrate)

[0167] A titanium foil roll 26 with a width of 48 cm was prepared and rolled to a thickness of 0.1 mm. The surface composition of the titanium foil 26a was analyzed in the depth direction using XPS. The average carbon concentration up to a depth of approximately 50 nm (excluding the outermost surface) of the titanium foil 26a was 3.9 atomic%.

[0168] (Annealing process)

[0169] In this embodiment, using Figure 7 The continuous annealing furnace 21 shown anneals the titanium substrate. For example... Figure 7 As shown, the continuous annealing furnace 21 consists of a first chamber 22, a second chamber 23, a third chamber 24, and a fourth chamber 25. The configuration is such that the coil is unwound in the first chamber 22, heated by a carbon heater 28 in the second chamber 23, cooled in the third chamber 24, and wound up in the fourth chamber 25.

[0170] Using the continuous annealing furnace 21 configured as described above, the titanium foil roll 26 is first placed in the first chamber 22, and after evacuation, the carbon heater 28 is heated to a temperature of 810°C. Next, Ar gas is introduced into the continuous annealing furnace 21, bringing the pressure inside the furnace to approximately 2 Pa. Then, the production line speed is set so that the heating time using the carbon heater 28 is 35 seconds, and the titanium foil 26a, unwound from the titanium foil roll 26, is conveyed to the second chamber 23 for heating. The titanium foil 26a is then conveyed to the third chamber 24, cooled inside the furnace by means of radiation cooling or the like, and then wound up again in the fourth chamber 25 to form a titanium foil roll 27.

[0171] (Carbon particle coating process)

[0172] Titanium foil of approximately 10×20cm size was cut from the annealed titanium foil roll 27. A coating containing carbon black was applied to the surface of the titanium foil using a doctor blade coater, followed by drying to form a carbon black coating film on the surface of the titanium foil. A coating film was also formed on the back side. Carbon black with a particle size (D50) of 62nm was used. Furthermore, to improve the adhesion of the coating, an acrylic resin was added at a weight ratio of 0.8 relative to the carbon black.

[0173] In addition, to measure the coating weight, the weight of a titanium foil of the same size as described above, after being coated on one side and dried, was measured. The coating was then wiped off with a cloth containing ethanol, and the weight was measured again. The weight difference was calculated and divided by the coating area. The coating weight was approximately 60 μg / cm². 2 Therefore, the coating weight of carbon black was calculated to be 60 ÷ 1.8 ≈ 33 μg / cm². 2 .

[0174] (Oxidation treatment process)

[0175] Oxidation treatment Figure 5 The process is carried out in the vacuum chamber 20 shown. First, a titanium foil 13 coated with a carbon black coating on both sides is placed on a tray 14 and positioned in the sample chamber 11. After evacuating the vacuum chamber 20, the carbon heater 16 is heated to a specified temperature. Then, oxygen is introduced into the vacuum chamber 20, and the pressure inside the vacuum chamber 20 is adjusted to 20 Pa. The titanium foil 13 is then transferred to the heating chamber 12 and heated for 17 seconds. The tray 14 is then returned to the sample chamber 11, and the titanium foil 13 is allowed to cool for oxidation treatment.

[0176] Furthermore, a thermocouple 15 is welded to the center of the titanium foil 13, which has a carbon black coating on both sides. By heating it in the same manner as the oxidation process described above, the temperature change of the thermocouple when the titanium foil 13 is heated can be determined. For example, as... Figure 6As shown, the temperature of the carbon heater 16 is set to 615°C. The titanium foil 13 is fed into the heated chamber 12 after being heated, and after 17 seconds, the temperature of the titanium foil 13 reaches approximately 630°C. Therefore, it is considered that the temperature reached by the titanium foil 13 after 17 seconds is approximately 15°C higher than the set temperature of the carbon heater 16. The reason why the thermocouple temperature is higher than the set temperature of the carbon heater 16 is considered to be because, since the thermocouple measuring the temperature of the carbon heater 16 itself is located at the end of the carbon heater 16, the temperature near the center of the carbon heater 16 that actually heats the titanium foil 13 becomes higher than the set value.

[0177] (Cleaning process)

[0178] The titanium foil 13 is removed from the vacuum chamber 20, and its surface is wiped with a cloth soaked in ethanol. Then, water is added while wiping it again with a sponge and drying it. This removes the remaining carbon black that is not in close contact with the surface of the titanium foil 13.

[0179] (Reduction Process)

[0180] use Figure 5 The reduction process is performed in the vacuum chamber 20 shown. First, the cleaned titanium foil 13 is placed back on the tray 14 of the vacuum chamber 20 and placed in the sample chamber 11. After evacuating the vacuum chamber 20, the carbon heater 16 is heated to a specified temperature. Furthermore, the pressure inside the vacuum chamber 20 is set to less than 0.1 Pa to prevent the influence of oxygen, water vapor, etc., so that the reduction of titanium oxide formed during the oxidation process can proceed. Then, Ar gas is introduced into the vacuum chamber 20 to set the furnace pressure to 40 Pa. The titanium foil 13 is then transferred to the heating chamber 12 and heated for 12 seconds. The tray 14 is then returned to the sample chamber 11, and the titanium foil 13 is placed to cool, thus performing the reduction process.

[0181] Figure 8 This is a graph showing the relationship between heating temperature and titanium foil temperature during the reduction process, with the horizontal axis set to heating time and the vertical axis set to titanium foil temperature. Figure 8 In the diagram shown, the set temperature of carbon heater 16 is 615℃, and the partial pressure of Ar gas is 40 Pa. For example... Figure 8 As shown, similar to the oxidation process, the cleaned titanium foil 13, with thermocouples 15 welded to it, is fed into a heating chamber 12 of the carbon heater 16, where the temperature is set to 615°C. After 12 seconds of heating, the titanium foil 13 reaches a temperature of approximately 585°C. This is approximately 30°C lower than the set temperature of the carbon heater 16. Therefore, it is considered that the temperature reached by the titanium foil 13 after 12 seconds is approximately 30°C lower than the set temperature.

[0182] (Fabrication of pressure-molded simulation material)

[0183] The treated titanium foil (surface-treated titanium material) is cut with a length of 65 mm on one side parallel to the rolling direction of the coil and a width of 20 mm on the other side perpendicular to the rolling direction. Then, markings are drawn parallel to the center line on both sides, 12.5 mm away from the center line in the length direction, using a universal pen. The two ends in the length direction are grasped with a tensile testing machine and stretched so that the length between the markings is 32.5 mm, thereby producing a pressure-formed simulation material under harsh conditions with an elongation of 30%.

[0184] Measurement and Analysis

[0185] (Measurement of initial contact resistance)

[0186] Figure 9 This is a schematic diagram of a contact resistance measuring device. The contact resistance measuring device 30 is configured such that the test material 37 is clamped between a pair of cylindrical copper electrodes 31a and 31b, and a four-terminal resistance measuring device (Tsuruga Electric Machinery: Low Resistance Meter 356E) 33 is connected between one copper electrode 31a and the other copper electrode 31b via current terminals 32a and 32b, respectively. Furthermore, a load-applying device (not shown) is installed on the copper electrodes 31a and 31b to apply a load to the test piece in the direction indicated by the arrow.

[0187] Using such a contact resistance measuring device 30, the initial contact resistance (contact resistance before impregnation) of each simulated material is measured.

[0188] The experimental material 37 was constructed as follows: a 0.05 mm thick resin sheet 35 with a 16 mm diameter hole was stacked on top of the pressure-molded simulation material 34 near its central portion; then, a carbon paper 36 with a width of approximately 20 mm was stacked on top of the resin sheet 35. Both the pressure-molded simulation material 34 and the carbon paper 36 were stacked in such a way that they covered the hole in the resin sheet 35. Then, a 14 mm diameter carbon paper with a front end area of ​​1.54 cm² was placed on top of the resin sheet 35. 2 Between copper electrodes 31a and 31b, ensuring the resin sheet is not sandwiched between the electrodes, the three overlapping test materials 37 are inserted. Then, a load of 15.4 kg is applied, pressurizing the test material 37 using copper electrodes 31a and 31b. Next, one current terminal 32 of the four-terminal resistance meter 33 is connected to copper electrode 31a, and the other current terminal 32 is connected to copper electrode 31b. One resistance measurement terminal 38a is connected to the pressurized molded simulation material 34, and the other resistance measurement terminal 38b is connected to carbon paper 36 to measure the resistance. The measured resistance value is then multiplied by 1.54 cm², which is the contact area. 2 The initial contact resistance before impregnation was calculated.

[0189] (Measurement of contact resistance after impregnation)

[0190] When considering the use of fuel cell separators, the contact resistance of the simulated material 34 was measured after immersion in a corrosive acidic liquid. The corrosive acidic liquid was prepared by adding NaF and NaCl to ion-exchanged water adjusted to pH 3 by adding sulfuric acid, resulting in an acidic solution with a fluoride ion concentration of 30 ppm and a chloride ion concentration of 10 ppm.

[0191] First, the pressure-molded simulation material 34 was placed in a polyethylene container and immersed in the acidic solution. The container was then covered and placed in a constant temperature bath at 80°C for 4 days (96 hours). Next, the container was removed from the constant temperature bath, and the titanium foil was removed from the container, washed with ion-exchanged water, and dried. The contact resistance after immersion was measured using the same method as for the initial contact resistance measurement.

[0192] In this embodiment, the contact resistance is 15 mΩ·cm 2 The following are considered qualified.

[0193] (Measurement of Ti2O3 / TiO2 ratio)

[0194] Raman spectroscopy analysis was used to analyze samples ranging from 150 to 2000 cm⁻¹. -1 The wavenumber range, for the area of ​​the resistance measurement part of the titanium foil before the fabrication of the pressure-molded simulation material 34, is 1 cm². 2 Measurements were taken in the specified area. In the Raman spectroscopic analysis, a laser Raman microscope (RAMANtouch, Nanophoton Co., Ltd.) was used. Furthermore, the measurement conditions were set as follows: laser wavelength: 532 nm, laser power: approximately 500 W / cm². 2 Diffraction grating: 600gr / mm, slit width: 50μm.

[0195] Next, the peaks originating from titanium oxide, from 150 to 800 cm⁻¹, were observed. -1 The spectrum is extracted, and peak searching and peak separation are performed.

[0196] Figure 10 This is a diagram showing an example of a sample after Raman spectroscopy and peak separation. Figure 11 This is a chart showing the standard spectrum of Ti2O3. Figure 12 This is a chart representing the standard spectrum of TiO2 (rutile). For example... Figure 10 As shown, the detected peaks are derived from rutile TiO2, anatase TiO2, and Ti2O3.

[0197] The titanium dioxide formed during the oxidation process is almost entirely rutile, but through reduction treatment, the TiO2 content of the rutile decreases, resulting in a small amount of anatase-structured TiO2 and Ti2O3. For example... Figure 10 and Figure 12 As shown, the peak of the rutile after reduction treatment remains at 441 cm. -1 Nearby and 235-252cm -1 However, both peaks overlap with Ti₂O₃. Peak separation based on curve fitting (fitting a mixture function (Gaussian function + Lorentz function)) is needed to separate rutile and Ti₂O₃. However, since the former peak is difficult to separate, the 235–252 cm⁻¹ peak of the latter is used as the rutile peak. -1 The peak (peak height 11).

[0198] On the other hand, such as Figure 10 and Figure 11 As shown, the peaks of Ti2O3 were separated using peak separation based on curve fitting, from 260 to 276 cm⁻¹. -1 (Peak height I2) and 292–303 cm -1 The total peak height (I3) is used as an indicator of the degree of reduction, with 260–276 cm⁻¹ representing the structure of Ti₂O₃. -1 292~303cm -1 The total peak height (I2+I3) is relative to 235–252 cm, which represents the rutile structure. -1 The ratio of the peak height (I1) (hereinafter referred to as "(I2+I3) / I1" or "Ti2O3 / TiO2 ratio") was investigated in relation to the contact resistance after impregnation.

[0199] (Measurement of porosity and film thickness T)

[0200] In a portion of surface-treated titanium materials obtained by varying the temperature during oxidation, the surface was observed using an optical microscope, and the area with the highest color ratio was cross-sectionally processed using a cross-sectional polisher (CP). Then, using an FE-SEM (Hitachi High-Tech S-5000, Ltd.), with an accelerating voltage of 2.0 kV and a magnification of 300,000x, the reflected electron image of the cross-section was photographed, and the ratio of the length occupied by the voids to the length of the interface (void ratio: L2 / L1) and the film thickness T were measured.

[0201] Figure 13 This is a graph showing the relationship between heater set temperature and film thickness when the horizontal axis is set to the heater set temperature during oxidation and the vertical axis is set to film thickness. Furthermore, Figure 14This is a graph showing the relationship between heater set temperature and void ratio when the horizontal axis is set to the heater set temperature during oxidation treatment and the vertical axis is set to the void ratio.

[0202] like Figure 13 and Figure 14 As shown, if the amount of carbon particles coated and the oxidation treatment time are set to a constant, the film thickness T and the porosity ratio, although varying, are roughly determined by the oxidation treatment temperature. This is because the process of forming the titanium oxide layer is only the oxidation treatment step. Therefore, if the oxidation heater temperature, coating conditions, and treatment time are the same, although there are some differences, films with approximately the same thickness will be formed.

[0203] Furthermore, voids are also formed during the oxidation process to form the titanium oxide layer. The mechanism of titanium oxide formation during oxidation is that titanium atoms in the titanium substrate diffuse from the passive film on the surface of the titanium substrate and emerge to the surface of the passive film, where they react with oxygen in the furnace, thereby forming titanium oxide with a rutile-based structure. Then, titanium atoms diffusing from the titanium substrate diffuse into the passive film and the rutile-based titanium oxide formed on its surface, reaching the surface and continuously reacting with oxygen in the furnace, thus growing the titanium oxide layer. At this time, titanium ions diffusing in the titanium oxide layer (in the titanium oxide layer, titanium takes the ionic form) appear on the surface of the titanium oxide layer by exchanging positions with voids in the titanium lattice present in the titanium oxide layer. In addition, voids diffuse to the interface between the titanium oxide and the passive film.

[0204] Some of the pores that diffuse to the interface diffuse within the passive film, but some accumulate at the interface between the titanium dioxide and the passive film, growing into voids. Therefore, at higher oxidation temperatures or for longer oxidation times, the diffusion rate of pores increases, resulting in a greater accumulation of pores at the titanium dioxide-passive film interface. In other words, although there is a difference, the formation of voids is almost entirely determined by the oxidation conditions.

[0205] Therefore, similar to film thickness, the amount of void formation can be considered approximately the same, although there may be some differences, as long as the oxidation treatment temperature and time are the same. Thus, for the void ratio (L2 / L1) and film thickness T, for each oxidation heater set temperature, one or two samples are selected and measured. All test pieces treated under the same oxidation conditions are considered to have approximately the same void ratio and film thickness, and no measurement is performed.

[0206] The measurement results of the oxidation and reduction temperatures, initial and 4-day immersion contact resistance, Ti2O3 / TiO2 ratio, porosity, and film thickness T for each test piece are shown in Table 1 below. Note that "-" in Table 1 indicates no measurement was taken. Furthermore, in the column for oxidation temperature, the estimated temperature of the titanium foil is set to the heater setting temperature for oxidation +15°C. Similarly, in the column for reduction temperature, the estimated temperature of the titanium foil is set to the heater setting temperature for reduction -30°C.

[0207] [Table 1]

[0208]

[0209] As shown in Table 1, in Examples No. 1 to 12, the Ti₂O₃ / TiO₂ ratios observed by Raman spectroscopy were all in the range of 0.08 to 1.45, and the porosity ratios were all below 0.3. Therefore, the contact resistance after 4 days of immersion in the test solution was 15 mΩ·cm. 2 Therefore, Examples No. 1 to 12 demonstrate the ability to maintain excellent conductivity over a long period of time.

[0210] In addition, the above Figure 10 This is a graph showing the Raman spectrum and peak separation for Example No. 12. Figure 10 As shown, in Example No. 12, I1 is 434.5, I2 is 475.9, and I3 is 76.0. Therefore, the Ti2O3 / TiO2 ratio is 1.27 according to (475.9+76.0) / 434.5.

[0211] In contrast, Comparative Example No. 1, with a Ti₂O₃ / TiO₂ ratio of 1.66, exceeded the upper limit of 1.45 for the scope of this invention, resulting in a contact resistance exceeding 15 mΩ·cm after 4 days of immersion in the test solution. 2 This is because, although the oxidation treatment temperature is low, the reduction treatment temperature is high, resulting in an excess of Ti2O3 formation.

[0212] In Comparative Example No. 2, the Ti₂O₃ / TiO₂ ratio was 0.00, which is less than 0.08, the lower limit of the scope of this invention. Therefore, the contact resistance after 4 days of immersion in the test solution exceeded 15 mΩ·cm. 2 This is because, although the oxidation treatment temperature is high, the reduction treatment temperature is low, so reduction is ineffective and almost no Ti2O3 is formed. Therefore, during the fabrication of the pressure-molded simulant, partial peeling due to stretching, the formation of many cracks, and corrosion progression occur.

[0213] Although the Ti₂O₃ / TiO₂ ratios of Comparative Examples No. 3 and No. 4 are within the range of this invention, their void ratios are 0.326 and 0.348, respectively, exceeding the upper limit of 0.3 for the scope of this invention. Therefore, the contact resistance after 4 days of immersion in the test solution exceeds 15 mΩ·cm. 2 This is likely because the high temperature of the oxidation process resulted in excessive void formation.

[0214] The above Figure 2 This is a cross-sectional view of Comparative Example No. 5. Comparative Example No. 5 has a Ti₂O₃ / TiO₂ ratio of 0.00, which is less than 0.08, the lower limit of the scope of this invention. Furthermore, as... Figure 2 As shown, numerous voids are formed, with a porosity of 0.56, exceeding the upper limit of 0.3 for the scope of this invention. Therefore, the contact resistance after 4 days of immersion in the test solution exceeds 15 mΩ·cm. 2 The degradation was significant. This is likely because, although the oxidation treatment temperature was high, the reduction treatment temperature was low, resulting in insufficient Ti2O3 formation. Furthermore, due to the high oxidation temperature, interfacial porosity formation became excessive.

[0215] [Experiment 2. Effects of carbon particle size (D50) and reduction treatment conditions on performance]

[0216] By varying the particle size (D50) of the carbon particles and the conditions of the reduction treatment, surface-treated titanium materials can be manufactured. Detailed manufacturing and measurement conditions are as follows.

[0217] Manufacturing of Surface-Treated Titanium Materials

[0218] (Preparation and annealing of titanium substrate)

[0219] Annealing of a titanium foil roll with a width of 48 cm and a thickness of 0.1 mm was performed using the same method as in Experiment 1.

[0220] (Carbon particle coating process)

[0221] Titanium foil, unwound from annealed titanium foil rolls, is placed on a die-coating production line. A die-coating machine is used to coat both sides of the titanium foil. The same coating is used to achieve a coating weight of 50–70 μg / cm³ after drying. 2 (If converted to carbon black, it would be 28–39 μg / cm³) 2 In this embodiment, two types of carbon black with particle diameters (D50) of 62 nm and 109 nm are used.

[0222] (Oxidation treatment process)

[0223] use Figure 7 The oxidation process is performed in the continuous annealing furnace 21 shown. First, a titanium foil roll 26 coated with a carbon black coating on both sides is placed in the first chamber 22 of the continuous annealing furnace 21. After vacuuming, since the temperature of the titanium foil 26a cannot be measured, the sheath heater is heated and adjusted to make the estimated temperature of the titanium foil 26a 615-620°C. Next, oxygen and Ar gas are introduced so that the oxygen partial pressure in the continuous annealing furnace 21 is calculated to be 20 Pa. Then, the production line speed is set so that the time for it to pass through the second chamber 23, which is the heating zone, is 17 seconds, and the titanium foil 26a is conveyed to the third chamber 24. Then, after cooling the titanium foil 26a in the third chamber 24, it is wound up in the fourth chamber 25 and formed into a titanium foil roll 27 again.

[0224] (Cleaning process)

[0225] The oxidized titanium foil roll 27 is taken out from the continuous annealing furnace 21. In order to remove the remaining carbon black that is not in close contact with the surface of the titanium foil 13, the titanium foil roll 27 is placed in a continuous water washing equipment with a brush cleaning tank and an ultrasonic cleaning tank for production line cleaning.

[0226] (Reduction Process)

[0227] For titanium foil rolls using carbon black with a particle size (D50) of 62 nm and titanium foil rolls using carbon black with a particle size (D50) of 109 nm, a carbon heater is installed. Figure 7 The continuous annealing furnace 21 shown performs the reduction process. First, the titanium foil roll is placed in the first chamber 22 and evacuated. Since the temperature of the titanium foil 26a cannot be measured, the sheath heater is heated and adjusted to bring the estimated temperature of the titanium foil 26a to 605°C. Next, the production line speed is set so that the time to pass through the second chamber 23, which serves as the heating zone, is 12 seconds, and the titanium foil 26a is conveyed to the third chamber 24. Then, after cooling the titanium foil 26a in the third chamber 24, it is wound up in the fourth chamber 25 to form a titanium foil roll again.

[0228] (Fabrication of pressure-molded simulation material)

[0229] Similar to Experiment 1, a pressure-molded simulation material was prepared.

[0230] Measurement and Analysis

[0231] (Measurement of initial contact resistance and contact resistance after impregnation)

[0232] Similar to Experiment 1, for the pressure-molded simulant, the initial contact resistance was measured, and the contact resistance was measured after 4 days of impregnation.

[0233] (Measurement of contact)

[0234] On the surface between the markings of a pressure-molded simulant stretched to 30% elongation, approximately 2cm of tape (Cellotape (registered trademark): product number 405 (manufactured by Nichban Co., Ltd.)) with a width of 12mm and a length of approximately 5cm was pasted from one end. Then, the pasted portion was wiped with a soft cloth and stretched to a non-peeling degree, so that the portion of the tape not adhering to the adhesive surface from the other end to the boundary of the pasted portion forms a 90° angle with the adhesive surface. Then, while maintaining approximately the 90° angle, force was applied to peel the tape off from the pressure-molded simulant in approximately 1-2 seconds. This was performed as a tape peel test, and the adhesion of the surface layer was measured by visually observing the peeled surface and the removed tape.

[0235] (Measurement of Ti2O3 / TiO2 ratio)

[0236] Raman spectroscopy analysis was used to analyze samples ranging from 150 to 2000 cm⁻¹. -1 The wavenumber range was used to measure the resistance of a 0.2 mm × 0.2 mm region in the titanium foil before the fabrication of the pressure-molded simulation material 34. Similar to Experiment 1, the Ti₂O₃ / TiO₂ ratio was determined. In Raman spectroscopy analysis, a LabRAM HR-800 microlaser Raman spectrometer (manufactured by Jobin Yvon) was used. The measurement conditions were set as follows: laser wavelength: 514.6 nm, laser power: 20 mW, diffraction grating: 300 gr / mm, confocal aperture: 100 μm, exposure time × number of exposures: 120 seconds × 8 times.

[0237] (Measurement of the weight of carbon black (Sc / (St+Sc)))

[0238] The surface-treated titanium foil was cut into 2cm x 5cm pieces, weighed, and then the surface layer on one side was removed by polishing. Carbon black was then burned using a combustion method. A carbon / analysis apparatus CS844 (manufactured by LECO Japan Co., Ltd.) was used during combustion.

[0239] Then, by quantifying the amount of CO2 produced by combustion, the weight of carbon black taken into the surface layer of one side is calculated. In addition, by dividing the weight of the titanium foil in advance by the thickness of the titanium foil (0.1 mm) and the density of titanium, the surface area of ​​one side is calculated, and by dividing the calculated weight of carbon black by the surface area of ​​one side, the weight of carbon black in the surface layer per unit area is calculated.

[0240] The particle size (D50) of carbon particles, the estimated temperatures of the titanium foils after oxidation and reduction treatments, the contact resistance after initial and 4-day impregnation, the Ti2O3 / TiO2 ratio, and the weight of carbon black for each test piece are shown in Table 2 below. Additionally, the estimated temperatures of the titanium foils after oxidation treatment for Examples No. 13, No. 14, and Comparative Example No. 6, as shown in Table 2, are also presented. Figure 13 and Figure 14 It could be determined that the film thickness T was 25 nm or more and the porosity was 0.30 or less, so the film thickness T and porosity were not measured. Furthermore, Comparative Example No. 6 was a form in which no carbon black was coated, and after oxidation treatment under the same conditions as Examples No. 13 and 14, a reduction treatment was performed without cleaning. Therefore, the carbon black weight of Comparative Example No. 6 was obviously 0, so no weight measurement using the combustion method was performed.

[0241] [Table 2]

[0242]

[0243] In Examples No. 13 and No. 14, since the Ti₂O₃ / TiO₂ ratio is in the range of 0.08 to 1.45, the contact resistance after 4 days of immersion in the test solution is 15 mΩ·cm. 2 Furthermore, no visible peeling was observed in the tape peeling test, and the adhesion also showed good results. Additionally, when comparing Example No. 13 and Example No. 14, although the Ti2O3 / TiO2 ratio was almost the same, the contact resistance of Example No. 13, with a particle size D50 of 62 nm, was lower. This is likely because the contact resistance per 1 cm of the surface layer... 2 The carbon black weight, i.e., the proportion of carbon black taken into the surface layer, is greater in Example No. 13, where the particle size D50 is smaller, than in Example No. 14, where the particle size D50 is larger, thus increasing the conductive path. Therefore, the carbon particles with smaller particle size D50 exhibit greater conductivity.

[0244] In contrast, Comparative Example No. 6, which does not contain carbon particles in its surface layer, exhibits good initial contact resistance, but its contact resistance after 4 days of immersion significantly exceeds 15 mΩ·cm. 2 This result indicates that carbon particles (carbon black) are a stable conductive pathway, and without carbon black, the long-term stability of conductivity cannot be maintained.

[0245] [Experiment 3. Effect of film thickness T on performance]

[0246] By varying the film thickness T, surface-treated titanium materials were manufactured. In Experiment 3, to confirm the effect of film thickness T, test pieces were fabricated by setting the heater temperature during the oxidation treatment to three different temperatures: 615°C, 595°C, and 535°C. The manufacturing and measurement conditions for the test pieces are described below.

[0247] Manufacturing of Surface-Treated Titanium Materials

[0248] Similar to Experiment 1, surface-treated titanium materials were manufactured.

[0249] Measurement and Analysis

[0250] (Measurement of initial contact resistance and contact resistance after impregnation)

[0251] Similar to Experiment 1, for the pressure-molded simulant, the initial contact resistance was measured, and the contact resistance was measured after 4 days of impregnation.

[0252] (Measurement of Ti2O3 / TiO2 ratio)

[0253] Similar to Experiment 1, the Ti2O3 / TiO2 ratio was measured.

[0254] (Measurement of porosity and film thickness T)

[0255] For surface-treated titanium materials obtained by varying the temperature during oxidation, the surface was observed using an optical microscope, and the areas with the highest color ratio were cross-sectionally processed using a cross-sectional polisher (CP). Then, using an FE-SEM (Hitachi High-Tech S-5000, Ltd.), with an accelerating voltage of 2.0 kV and a magnification of 300,000x, the reflected electron image of the cross-section was photographed, and the film thickness T was measured. Furthermore, the ratio of the length occupied by voids to the length of the interface (void ratio: L2 / L1) was measured.

[0256] (Measurement of the carbon particle (carbon black) content in the titanium oxide layer)

[0257] The surface of the titanium material with the highest color ratio was cross-sectionally processed using CP. The cross-section was then photographed using FE-SEM. The image was binarized in the region extending from the interface between the passive film and the metallic titanium layer (where it is as straight as possible) to a position slightly above the film thickness T. Carbon black was then designated as black, and the titanium oxide layer as white. Protrusions from the surface of the titanium oxide layer were excluded from the carbon black representation. The area occupied by the carbon black was designated as Sc, and the combined cross-sectional area of ​​the titanium oxide layer and the passive film was designated as St.

[0258] Then, the number of black dots, which is a value proportional to Sc, and the number of black and white dots, which is a value proportional to St+Sc, are measured and set as {Sc / (St+Sc)}×100, and calculated as a percentage (%).

[0259] The measurement results of the oxidation and reduction treatment temperatures, initial and 4-day immersion contact resistance, Ti2O3 / TiO2 ratio, porosity, film thickness T, and carbon black content of each test piece are shown in Table 3 below.

[0260] [Table 3]

[0261]

[0262] As shown in Table 3 above, in Examples No. 15 and No. 16, since the film thickness T exceeds 25 nm and the Ti₂O₃ / TiO₂ ratio is within the range of 0.08 to 1.45, the porosity is also within the range of this invention. Therefore, the contact resistance after 4 days of immersion in the test solution is 15 mΩ·cm. 2 the following.

[0263] In contrast, although Comparative Example No. 7 has a Ti₂O₃ / TiO₂ ratio and porosity within the scope of this invention, its film thickness of 21 nm is less than the lower limit of the scope of this invention. Therefore, its contact resistance after immersion in the test solution for 4 days exceeds 15 mΩ·cm. 2 The result.

[0264] Figure 15 These are photographs, alternative figures, showing cross-sections of Comparative Examples No. 7, No. 15, and No. 16 taken by SEM imaging. Comparative Example No. 7 shows a morphology where carbon particles (carbon black) 6 are only loaded on the surface of the titanium oxide layer 5, and the carbon particles 6 are not taken into the interior of the titanium oxide layer 5.

[0265] On the other hand, Examples No. 15 and No. 16 both show that carbon particles 6 are taken into the titanium oxide layer 5 and dispersed.

[0266] In addition, the above Figure 4 This is a photograph showing a cross-section of Comparative Example No. 7, where the carbon black content is lower than that of the example, resulting in higher contact resistance. This is likely because, due to the thinner film thickness T, the carbon black cannot be adequately incorporated into the titanium oxide layer 5.

[0267] [Experiment 4. The effect of annealing on performance]

[0268] The effect of the presence or absence of annealing on the titanium substrate on the 0.2% yield strength of the obtained surface-treated titanium material was confirmed.

[0269] use Figure 7 The continuous annealing furnace 21 shown is configured with the temperature of the carbon heater 28 and the production line speed set so that the titanium foil 26a reaches a temperature of 780°C and the elapsed time in the second chamber 23, which serves as the heating zone, is 40 seconds. Then, the 0.1 mm thick titanium rolled foil is annealed and surface-treated using the same method as in Experiment 1.

[0270] On the other hand, for the unannealed titanium rolled foil, the surface treatment was performed using the same method as in Experiment 1, and the 0.2% yield strength in the length direction (L direction) and width direction (T direction) of the coil was measured. Additionally, annealing was performed on 6 coils, and annealing was not performed on 4 coils, and the 0.2% yield strength was measured.

[0271] The measurement results of 0.2% yield strength in the L and T directions are shown in Table 4 below.

[0272] [Table 4]

[0273] Table 4

[0274]

[0275] As shown in Table 4 above, Examples No. 17 to Example No. 22 are surface-treated titanium materials made using annealed titanium substrates, with a 0.2% yield strength in the L direction of 119 to 143 MPa and a 0.2% yield strength in the T direction of 185 to 208 MPa.

[0276] Furthermore, the preferred range for 0.2% yield strength is 110–150 MPa in the L direction and 180–215 MPa in the T direction. Therefore, although the surface-treated titanium materials of the embodiments exhibit variations, all fall within the preferred range of 0.2% yield strength.

[0277] On the other hand, Comparative Examples No. 8 to No. 11, which used unannealed titanium substrates to produce surface-treated titanium materials, exhibited yield strengths of 207–239 MPa in the L direction and 230–257 MPa in the T direction, values ​​higher than the preferred range. These results indicate that an annealing process for the titanium substrate is necessary; without this process, the 0.2% yield strength becomes excessively high, preventing the attainment of the desired shape during fine groove pressure molding.

[0278] The above is with reference to the appendix. Figure 1Various embodiments have been described, but the present invention is certainly not limited to such examples. Those skilled in the art will readily conceive of various modifications and alterations within the scope of the claims, and it should be understood that such modifications also fall within the scope of the present invention. Furthermore, the constituent elements of the above embodiments can be arbitrarily combined without departing from the spirit of the invention.

[0279] Furthermore, the contents of Japanese Patent Application No. 2020-197464, filed on November 27, 2020, are incorporated herein by reference.

[0280] Explanation of reference numerals in the attached figures

[0281] 1. Surface treatment of titanium materials

[0282] 2 Titanium substrate

[0283] 3 passive film

[0284] 4 Surface layer

[0285] 5. Titanium oxide layer

[0286] 6 carbon particles

[0287] 7 gaps

[0288] 13 Titanium foil

Claims

1. A surface-treated titanium material for a fuel cell separator, The titanium substrate having a passive film on its surface and a surface layer formed on the aforementioned titanium substrate; The aforementioned surface layer comprises a titanium oxide layer and carbon particles, wherein the aforementioned titanium oxide layer comprises TiO2 and Ti2O3; Its features are, The aforementioned carbon particles are dispersed within the aforementioned titanium oxide layer; The combined thickness of the aforementioned titanium oxide layer and the aforementioned passive film is 25 nm or more; When performing curve fitting-based peak separation on the Raman spectrum of the combined titanium dioxide layer and the passive film, the peaks at the Raman shift of 235–252 cm⁻¹ will be... －1 The height of the TiO2 peak obtained in the range is set as I1, and the peaks with Raman shifts of 260–276 cm⁻¹ are used. －1 The height of the peak representing Ti2O3 obtained in the range is set as I2, and the peaks with Raman shifts of 292–303 cm⁻¹ are used. －1 When the peak height of Ti2O3 obtained within the range is set to I3, (I2+I3) / I1 is 0.08 or higher and 1.45 or lower; and, When observing any cross section using a field emission scanning electron microscope, if the length of the interface between the aforementioned titanium oxide layer and the aforementioned titanium substrate in the observation area is set as L1, and the length of the void existing at the aforementioned interface is set as L2, then L2 / L1 is 0.30 or less.

2. A method for manufacturing a surface-treated titanium material for a fuel cell separator, comprising the method for manufacturing a surface-treated titanium material for a fuel cell separator as described in claim 1, characterized in that, have: The process of annealing the titanium substrate; The process of coating carbon particles onto the surface of the aforementioned titanium substrate after annealing; The oxidation process involves heat-treating the titanium substrate coated with the aforementioned carbon particles in an oxidizing atmosphere to form the aforementioned titanium oxide layer containing the aforementioned carbon particles. The cleaning process removes carbon particles that are not in close contact with the aforementioned titanium oxide layer from the surface of the titanium oxide layer. as well as The reduction process involves heating the titanium substrate with the aforementioned titanium oxide layer in a vacuum or in an inactive gas atmosphere to form the aforementioned surface layer.

3. A method for manufacturing a surface-treated titanium material for a fuel cell separator, comprising the method for manufacturing a surface-treated titanium material for a fuel cell separator as described in claim 1, characterized in that, have: The process of annealing the titanium substrate; The process of coating carbon particles onto the surface of the aforementioned titanium substrate after annealing; The oxidation process involves heat-treating the titanium substrate coated with the aforementioned carbon particles in an oxidizing atmosphere to form the aforementioned titanium oxide layer containing the aforementioned carbon particles. The reduction treatment process involves heating the titanium substrate with the aforementioned titanium oxide layer formed thereon in a vacuum or in a non-reactive gas atmosphere to form the aforementioned surface layer; and The cleaning process removes carbon particles that are not in close contact with the aforementioned titanium oxide layer from the surface of the titanium oxide layer.

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