JP2008072003A - Electrode substrate and method for manufacturing electrode base material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode substrate whose surface is covered with a porous oxide layer and is suitable as an anode element etc. of a capacitor, and to provide a method for manufacturing an electrode base material. <P>SOLUTION: (1) The surface of the electrode substrate is covered with a nanoporous oxide layer of a transition metal (titanium, vanadium, zirconium, niobium, molybdenum, tantalum, and tungsten), and its specific surface area is very large. If the basis of the electrode substrate is a porous sintered compact of powder of those metals, the specific surface area increases rapidly. (2) By anodizing the metal base material or the porous sintered compact of metal powder in an electrolyte of mixed acid ammonium fluoride and hydrogen peroxide, a nanoporous oxide layer is formed on the surface of the base material or the surface of the porous sintered compact. In addition, by anodizing a titanium base material or porous sintered compact of titanium in an electrolyte of mixed ammonium phosphate and ammonium fluoride, in the same way, a nanoporous oxide layer is formed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electrode substrate suitable for an anode element of a capacitor and the like, which is covered with a porous oxide layer having a very large specific surface area, and a method for producing the electrode substrate.

  In recent years, with the progress of miniaturization of electronic devices, a capacitor as an electronic element is required to be small and have a large capacity, and the development of an electrode substrate having a large specific surface area that can be used as an anode element is desired.

The capacitor basically has a structure in which metal electrodes face each other on both surfaces of a dielectric, and has a function of storing electric charges proportional to a voltage applied between the electrodes (electrode plates). The electric capacity C is generally expressed by the following formula (i). In equation (i), ε ₀ is the dielectric constant of vacuum, ε _r is the relative dielectric constant of the dielectric between the plates, A is the area of the plates, and d is the distance between the plates.

C = (ε ₀ ε _r A) / d (i)
In small capacitors used in electronic devices such as personal computers, sintered bodies such as tantalum powder are used as base materials for electrodes. For example, in an electrolytic capacitor, tantalum powder is sintered in a vacuum, and the obtained sintered body is anodized in a phosphoric acid aqueous solution to form a dielectric tantalum oxide layer (Ta ₂ O ₅ ) on its surface. And then immersed in an aqueous manganese nitrate solution, heated to form manganese dioxide that becomes a solid electrolyte on the surface of the tantalum oxide layer, which was coated with graphite and further baked with a silver paste. Here, a sintered body of tantalum powder is used as the anode element.

  In this case, since the effective area of the sintered body of the tantalum powder is the electrode plate area A of the capacitor, the capacity C increases by increasing the specific surface area of the sintered body from the equation (i).

  A capacitor using such a sintered body as an anode element is required to have a small leakage current and excellent electrical characteristics such as withstand voltage in addition to increasing the capacity.

  Therefore, many researches and developments have been made. For example, Patent Document 1 discloses an electrolytic capacitor in which a predetermined amount of tantalum powder having an average particle diameter of 10 to 500 nm is mixed with tantalum powder having an average particle diameter of 1.0 to 5.0 μm. Tantalum powder as an anode material and an electrolytic capacitor using the same are described.

  Further, in Patent Document 2, a niobium powder having a predetermined average particle diameter and a BET specific surface area, a niobium sintered body, and a capacitor having a large capacity and having good leakage current characteristics and the like are disclosed in Patent Document 3. In addition, a capacitor powder containing zirconium and containing niobium and / or tantalum as a main component, and a sintered body and a capacitor using the powder have been proposed.

  However, there is no limit to the demand for an increase in the capacity of a capacitor as an electronic element and a reduction in size and quality achieved thereby.

  Further, for example, in a dye-sensitized solar cell, titanium oxide of nanoporous particles adsorbed with a dye that absorbs sunlight is used as a base material. The larger the specific surface area of the base material, the more advantageous. Furthermore, in the catalyst carrier and the like, the reaction occurs on the surface, and the increase in the specific surface area directly contributes to the improvement of the reaction rate. Thus, there is a great expectation for an increase in the specific surface area of a dye-sensitized solar cell substrate, a catalyst carrier, or the like.

JP-A-8-97095 JP 2003-213302 A JP 2002-173371 A

  The present invention has been made in view of such a situation, and provides an electrode substrate suitable for an anode element of a capacitor whose surface is covered with a porous oxide layer, and a method for producing the electrode substrate. It is an object.

In order to solve the above-mentioned problems, the present inventors made an object of a transition metal such as tantalum, niobium, and titanium and a sintered body of the powder of these metals by anodization, and the surface thereof was porous. A method for forming an oxide layer having an extremely large surface area and the optimum conditions thereof were studied. As a result, the metals and their sintered bodies are anodized in a mixed electrolytic solution of acidic ammonium fluoride (NH ₄ F · HF) and hydrogen peroxide (H ₂ O ₂ ), thereby forming the surfaces thereof. It has been found that a nanoporous oxide layer having a very large specific surface area and a thickness of several tens of μm can be formed.

FIG. 1 shows that after niobium is anodized (constant voltage electrolysis at 20 V for 1 hour) in an aqueous solution (5 ° C.) of 3% by mass NH ₄ F · HF (hereinafter “%” means “% by mass”). It is a field emission scanning electron microscope (FE-SEM) photograph which illustrates the cross section of. It can be seen that a columnar (tube-like) nanoporous oxide layer having a thickness of about 2 μm and growing almost perpendicular to the surface is formed on the surface of niobium. The diameter of one columnar body is about 100 nm.

FIG. 2 is an FE-SEM photograph illustrating a cross section after anodizing (constant voltage electrolysis at 40 V for 2 hours) in a mixed aqueous solution (20 ° C.) of 15% NH ₄ F and 6% H ₂ O ₂ with tantalum. (A) is an overall view, and (b) to (d) are enlarged views of a portion surrounded by a square in the drawing.

  In the example shown in FIG. 2, a nanoporous oxide layer having a thickness of about 16 μm is formed on the surface of tantalum (see FIG. 2A), and the vicinity of the surface (surface layer) is a columnar nanoporous oxide layer. It turns out that there exists (FIG.2 (b)). The inner layer of the oxide layer is nanoporous, but has a layer shape as shown in FIG. 2C or a granular shape as shown in FIG.

The present invention has been made on the basis of such findings, and the gist of the present invention is the following (1) electrode substrate and (2) and (3) electrode substrate manufacturing methods.
(1) An electrode substrate whose surface is covered with a nanoporous oxide layer of a transition metal.

  The “transition metal” includes a large number of metals, and the object here is seven elements (metals) of titanium, vanadium, zirconium, niobium, molybdenum, tantalum and tungsten, and alloys thereof. An oxide layer of the metal is formed on the surface of these metals.

“Substrate” means a material used for an element, a substrate, or the like. Usually, the material is a flat plate, but is not limited thereto. The “electrode substrate” is intended for application to the anode element of a capacitor, which is the main use of the material of the present invention.
(2) The surface of the base material or the porous sintered body obtained by anodizing a metal base material or a porous sintered body of metal powder in a mixed electrolytic solution of acidic ammonium fluoride and hydrogen peroxide. The manufacturing method of the electrode base material which forms a nanoporous oxide layer in the.

  Here, the “metal substrate” is a metal substrate (material) that can form a nanoporous oxide layer on its surface by anodizing. If the metal is the seven elements (metals) of the transition metal, an electrode substrate used for the electrode substrate of (1) can be obtained.

The “porous sintered body of metal powder” is a sintered body of metal powder that can be an object of the anodization, and the “porous sintered body” It is an expression intended to be porous compared to the material.
(3) A porous sintered body of a titanium substrate or titanium powder is anodized in a mixed electrolytic solution of ammonium phosphate and ammonium fluoride, thereby nanoporously forming the surface of the substrate or the porous sintered body. The manufacturing method of the electrode base material which forms an oxide layer.

  When the metal substrate is a titanium substrate or a porous sintered body of titanium powder, the electrode substrate used for the electrode substrate (1) can be obtained by this production method.

  The “electrode substrate” defined in the present invention means a material used for obtaining the electrode substrate. In other words, there is no essential difference from the “electrode substrate”, but the “electrode substrate” is shaped to some extent so that it can be easily processed into a capacitor anode element, whereas the “electrode substrate” is an anode. It is a material with a shape and dimensions that take into account that it is easily oxidized.

In the electrode substrate of (1) above, if the specific surface area is 70 m ² / g or more, the effective area becomes very large, which is desirable as the substrate characteristics. The “specific surface area” is a value determined by the BET method.

  In the electrode substrate of (1), the nanoporous oxide layer may be a granular or layered crystalline or amorphous oxide layer.

  Similarly, in the electrode substrate of (1) above, if the thickness of the nanoporous oxide layer is 10 μm or more, the thickness is obtained by stacking a large number of layers, and the specific surface area becomes extremely large.

  Similarly, if the substrate of the electrode base is a porous sintered body of transition metal powder, the specific surface area will be greatly increased.

If the specific surface area of the electrode substrate using this porous sintered body is 700 m ² / g or more, the specific surface area becomes extremely large, which is more desirable as the substrate characteristics. The “specific surface area” is a value obtained by the BET method.

  Further, in the electrode substrate of (1) above, if the metal powder to be sintered is spherical, the fluidity at the time of charging into the sintering container is good, and a high strength and uniform porous sintered body can be obtained. So desirable.

  The electrode substrate of the present invention has a very large specific surface area because the surface of the substrate base metal or the porous sintered body of the metal powder is covered with the nanoporous oxide layer. Therefore, if this electrode substrate is used as an anode element for a capacitor, the capacity of the capacitor can be remarkably increased, and a high-capacity and thin capacitor element can be obtained.

  The electrode substrate can be used not only for the anode element but also for a dye-sensitized solar cell substrate, a catalyst carrier, and the like. Further, this electrode substrate can be manufactured by the method of the present invention in which anodization is performed in a mixed electrolytic solution of ammonium fluoride and hydrogen peroxide.

  Hereinafter, an electrode substrate of the present invention and a method for producing the electrode substrate will be described in detail with reference to the drawings. As described above, the electrode substrate of the present invention is an electrode substrate whose surface is covered with a nanoporous oxide layer of a transition metal.

  FIG. 3 is a diagram schematically showing a cross-sectional structure of the electrode substrate of the present invention, and (a) to (e) are diagrams illustrating the growth process of the nanoporous oxide layer.

As shown in FIG. 3 (a), a substrate substrate 2 that is a base material of the electrode substrate 1 is subjected to anodization in an aqueous solution of ammonium fluoride containing hydrogen peroxide, and is a nanometer-sized columnar (tubular) porous material. The oxide layer 3 (oxide film) grows. As shown in FIG. 2B, the grown oxide layer 3 is dissolved mainly in the vicinity of the tip of each columnar body by the action of fluorine ions (F ⁻ ), and further passes through the columnar body. The interface 4 between the oxide layer 3 and the substrate substrate 2 is dissolved by F ⁻ reaching the substrate substrate 2, and the oxide layer 3 is peeled off.

  Simultaneously with the peeling of the oxide layer 3, a new oxide layer 3 grows at the interface between the oxide layer 3 and the substrate substrate 2 as shown in FIG. Therefore, the peeled oxide layer 3 does not peel off from the base substrate 2. Subsequently, the newly grown oxide layer 3 is also peeled off during the growth, and at the same time, a new oxide layer is further grown. In this manner, the growth and separation of the oxide layer are repeated, and the oxide layer 3 is multilayered to form a layer shape as shown in FIG.

  FIG. 3E shows a case where the crystalline oxide 5 is generated in the layered region in the process of growing the oxide layer 3. Although the oxide 5 is granular or sponge-like, the formation of the oxide proceeds simultaneously with the growth of the oxide layer 3, whereby the growth of a thick nanoporous film can be easily realized.

  The oxide layer 3 formed in this way is crystalline or amorphous, and the oxide 5 is mainly crystalline, and the multi-layered portion is layered or granular as shown in FIG. Presents. Note that any of the columnar, layered, and granular regions is nanoporous.

  Since the surface of the electrode substrate of the present invention is covered with such a nanoporous oxide layer, the specific surface area is very large. For this reason, when used in an anode element of a capacitor, the capacity is remarkably increased. Therefore, it is possible to reduce the size of the entire capacitor by making the anode element smaller than the conventional one.

  The method for producing an electrode base material of the present invention includes the step of anodizing a metal base material or a porous sintered body of metal powder in a mixed electrolytic solution of ammonium acid fluoride and hydrogen peroxide solution. Alternatively, a nanoporous oxide layer is formed on the surface of the porous sintered body.

  By adding and mixing hydrogen peroxide, the anodic oxidation can be remarkably promoted while the chemical dissolution of the oxide layer generated by the anodic oxidation is remarkably suppressed. That is, a nanoporous oxide layer having a thickness of several tens of μm can be formed by performing electrolysis with addition of hydrogen peroxide and dielectric breakdown at a high electric field.

The thick oxide layer formed here is particulate (sponge-like). For example, the layered oxide layer produced when hydrofluoric acid is used in the electrolytic solution is easy to peel off from the substrate due to the action of fluorine (F ⁻ ), whereas the oxide layer formed above peels off. It is difficult.

  The concentration of acidic ammonium fluoride in the electrolytic solution is not particularly limited, but is preferably 1 to 20%.

  In the production method of the present invention, the concentration of hydrogen peroxide is not particularly limited. However, as described above, hydrogen peroxide is effective in suppressing the dissolution of the oxide layer and increasing the thickness of the nanoporous oxide layer at the time of anodic oxidation. The concentration is preferably 0.1 to 10%. A particularly desirable concentration is estimated to be about 0.5% from FIG.

  FIG. 4 is a current-time curve when niobium is anodized at 40 V by changing the amount of hydrogen peroxide added to acidic ammonium fluoride (concentration 3%). From FIG. 4, the current value (steady current value at which the current value becomes almost constant) is the highest when the hydrogen peroxide concentration is 0.5%, and the current value decreases as the concentration increases, and the growth of the oxide layer increases. It turns out that it is suppressed.

  There are no limitations on the voltage at the time of anodization, the electrolysis time, the temperature of the electrolytic solution, and the like. It is desirable that the voltage is 20 to 40 V, the time is 1 to 2 hours, and the temperature is within a temperature range from room temperature to about 5 ° C.

  The shape of the electrode substrate obtained by the production method of the present invention is preferably a shape that allows anodization to be performed efficiently and suitably. Usually, a flat plate is used as the shape of the electrode substrate. By appropriately adjusting the shape and dimensions of this electrode substrate, the electrode substrate of the present invention whose surface is covered with a nanoporous oxide layer can be obtained.

In the electrode substrate of the present invention, a specific surface area of 70 m ² / g or more is desirable because the effective area can be greatly increased. For example, the specific surface area of the zirconium-containing niobium sintered body described in Patent Document 3 (for example, 1.8 m ² / g described in Table 2 of the same document) is approximately 40 times larger. Even if the specific surface area (0.2-7 m ² / g) of the niobium sintered body described in claim 16 of Document 2 is compared, it becomes 10 times.

The nanoporous oxide layer on the surface of this electrode substrate has an extremely large specific surface area, and can have a specific surface area of 70 m ² / g or more even if the substrate substrate is a flat plate. This was confirmed for an electrode substrate having an oxide layer obtained by anodic oxidation of tantalum, niobium, or titanium under the desirable conditions described above.

  In the electrode substrate of the present invention, if the nanoporous oxide layer is a granular or layered crystalline or amorphous oxide layer, the oxide layer is thicker than a columnar oxide, and the specific surface area is significantly increased. It is.

  When the film is thickened, for example, if the substrate substrate is niobium, a granular crystalline oxide layer is likely to be formed, and tantalum is likely to be mixed with a slightly layered oxide layer and the crystallinity tends to be low.

  In the electrode substrate of the present invention, if the thickness of the nanoporous oxide layer is 10 μm or more, the thickness is formed by stacking a large number of layers, and the effective area increases, so that the specific surface area is extremely high. Can be bigger.

  In the electrode substrate of the present invention, if the substrate substrate is a porous sintered body of transition metal powder, the specific surface area of the electrode substrate can be dramatically increased.

  This is because if the base material is a porous sintered body, the specific surface area of the base material is larger than that of a metal base, and as shown in FIGS. This is because the nanoporous oxide layer is formed over almost the entire surface including the surface of the film.

  FIG. 5 is a diagram schematically showing a cross section in the vicinity of the surface of the porous sintered body, where (a) shows a state before anodization and (b) shows a state after anodization.

  FIG. 6 is a plan view illustrating a porous sintered body, and is shown to compare the size of particles and voids constituting the sintered body with the size of an oxide layer generated by anodization. Is. From the figure, based on the scale displayed in the lower right, it is possible to confirm the approximate size of the particles and voids constituting the sintered body and the existence of numerous voids.

  As shown in FIG. 5B, the nanoporous oxide layer 3 is formed on the surface of the sintered body by anodization. As described above, if the oxide layer is columnar (or granular), 1 Since the diameter of each columnar body is about 100 nm, the oxide layer is also formed on the surface in the voids of the sintered body, and the specific surface area dramatically increases. When the substrate substrate is a porous sintered body, an electrode substrate having a specific surface area 10 to 100 times that of a metal can be obtained. This is not limited to the case where the shape of the particles constituting the sintered body is the spherical shape shown in FIG. 6, and the same applies to crushed powder having an irregular shape obtained by crushing.

  Therefore, the particles constituting the porous sintered body may be any of irregularly crushed powder, spherical powder produced by gas atomization method, powder produced by hydrodehydrogenation, etc., and the shape is limited. Absent.

If the specific surface area of the electrode substrate using this porous sintered body is 700 m ² / g or more, the specific surface area is very large. Therefore, the capacity of the capacitor using this as an anode element is greatly increased, and the capacity is increased. A thin capacitor element can be realized.

  In addition, if the metal powder constituting the porous sintered body is spherical, a porous sintered body having good fluidity and uniform mechanical strength can be obtained. Is desirable.

  If the electrode base body using the sintered body of the spherical metal powder as a base is used as the anode element of the capacitor, the capacitor can be made thinner than in the case of using a sintered body of irregularly shaped powder. This spherical powder is excellent in fluidity, and when it is put into a sintering vessel, it is filled to a sufficient density without pressure, so that a uniform sintered body with high mechanical strength can be obtained and the anode element can be made thin. As a result, the entire capacitor element can be made thin.

  As a suitable example of this spherical powder, constituent particles of “TIPOROUS (trade name)” manufactured and sold by the applicant as a titanium porous body can be mentioned. This is a true spherical titanium powder or titanium alloy powder produced by a gas atomization method called “TILOP (trade name)”, and is a sintered body (“TIPOROUS-45 (trade name) made of the spherical powder shown in FIG. 6. ) ") Is obtained by sintering this spherical powder.

  In the method for producing an electrode base material of the present invention, when a titanium plate is used as the porous sintered body of the above titanium powder or titanium alloy powder, or a titanium plate is used as the titanium base material, a mixed electrolytic solution of ammonium phosphate and ammonium fluoride. By performing anodization therein, a nanoporous oxide layer can be formed on the surface of the porous sintered body or the substrate surface.

Specifically, a titanium porous sintered body after chemical polishing (volume ratio hydrofluoric acid: nitric acid = 1: 4) and a titanium material are mixed with 1 mol (NH ₄ ) H ₂ PO ₄ + 0.5% NH ₄ F ( For example, a nanoporous oxide layer can be formed on the surface by anodizing at 20 ° C. at various generation voltages. At this time, the optimum range of ammonium phosphate is a range from 0.1 mol to a saturated concentration, and a desirable concentration of ammonium fluoride is 0.1 to 10%.

  The obtained oxide layer can be evaluated for fine structure using a field emission scanning electron microscope (FE-SEM), and the crystallinity of the oxide film obtained by X-ray diffraction (XRD) can be evaluated. it can. The evaluation results showed that the basic anodic oxidation behavior was the same regardless of the titanium porous sintered body or titanium material, and the nanoporous oxide layer was formed so as to cover the surface.

  According to the method for producing an electrode base material of the present invention, as described above, if the electrode base is a porous sintered body of metal powder, the specific surface area can be dramatically increased, but the metal powder is substantially spherical. If the secondary particles are aggregated, the nanoporous oxide layer is also formed on the surface of the fine primary particles constituting the secondary particles. Therefore, the specific surface area can be further increased as compared with the spherical powder (“TILOP (trade name)”) constituting the porous sintered body illustrated in FIG. As a method for agglomerating the primary particles into a substantially spherical shape, a thermal aggregation method or the like can be applied.

  As described above, the electrode substrate of the present invention has been described on the assumption that the electrode substrate is applied to an anode element of a capacitor. However, the material constituting the electrode substrate should be sufficiently used as a substrate for a dye-sensitized solar cell and a catalyst carrier. Is possible.

  The electrode substrate of the present invention has a very large specific surface area. If this electrode substrate is used as an anode element for a capacitor, the capacity of the capacitor can be remarkably increased, and a high-capacity and thin capacitor element can be obtained. become. Further, this electrode substrate can be produced by the method of the present invention in which anodization is performed in a mixed electrolytic solution of ammonium fluoride and hydrogen peroxide.

  Therefore, the electrode substrate and the electrode substrate manufacturing method of the present invention can be effectively used for manufacturing a capacitor as an electronic element.

It is a FE-SEM photograph which illustrates the cross section of the electrode base material (base | substrate: niobium) obtained by the manufacturing method of this invention. It is a FE-SEM photograph which illustrates the cross section of the electrode base material (base material: tantalum) obtained by the manufacturing method of this invention. It is a figure which shows typically the cross-section of the electrode base body of this invention, and the growth process of a nanoporous oxide layer. It is a figure which shows the electric current-time curve in the case of the anodic oxidation of niobium. It is a figure which shows typically the cross section in the surface vicinity of the electrode base | substrate with which the surface of the porous sintered compact was covered with the nanoporous oxide layer. It is a top view which illustrates the shape etc. of the particle which constitutes a porous sintered compact.

Explanation of symbols

1: Electrode substrate 2: Substrate substrate 3: Oxide layer 4: Interface 5: Oxide

Claims (9)

  An electrode substrate having a surface covered with a nanoporous oxide layer of a transition metal. ^2. The electrode substrate according to claim 1, wherein the specific surface area is 70 m ² / g or more.   The electrode substrate according to claim 1, wherein the nanoporous oxide layer is a granular or layered crystalline or amorphous oxide layer.   The electrode substrate according to claim 3, wherein the nanoporous oxide layer has a thickness of 10 μm or more.   The electrode substrate according to claim 1, wherein the electrode substrate is a porous sintered body of a transition metal powder. 6. The electrode substrate according to claim 5, wherein the specific surface area is 700 m ² / g or more.   The electrode substrate according to claim 5 or 6, wherein the metal powder is spherical.   Nanoporous oxidation of the metal substrate or porous sintered body of metal powder on the surface of the substrate or porous sintered body by anodizing in a mixed electrolytic solution of ammonium acid fluoride and hydrogen peroxide solution The manufacturing method of the electrode base material characterized by forming a physical layer. A nanoporous oxide layer is formed on the surface of the base material or porous sintered body by anodizing a titanium base material or a porous sintered body of titanium powder in a mixed electrolytic solution of ammonium phosphate and ammonium fluoride. A process for producing an electrode substrate, characterized in that

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