TWI477437B - Nanosize structures composed of valve metals and valve metal suboxides and process for producing them - Google Patents

Description

Nanostructure composed of valve metal and valve metal suboxide and methods of manufacturing same

SUMMARY OF THE INVENTION The present invention is directed to a novel layered structure having valve metal and valve metal suboxide having a dimension of less than 100 nm in one direction, and a method of making the same.

The microstructure of the metal and metal suboxide present in the form of a powder or a larger metal substrate surface region has various application fields, such as a catalyst, a catalyst carrier material, a film and a porous filter material, Implant materials in the medical field, and anode materials for storage materials and capacitors for secondary batteries due to their large specific surface area characteristics.

WO 00/67936 discloses reducing metal oxides in the gas phase, such as Mg, Al, Ca, Li and Ba, as a method of preparing finely divided metal powders. Due to the volume shrinkage caused by the reduction of the oxide into a metal, and the volume expansion caused by the formation of the solid oxide by the reducing metal, a highly porous valve metal powder having a high specific surface area is formed, which is particularly suitable for the manufacture of a solid electrolytic capacitor.

It has now been found that under special reducing conditions, a layered structure can be formed with a lateral nanometer scale, and the starting layer comprises alternating reducing metal oxide layers and oxidized reducing metals. Floor.

Dissolving and filtering the oxide of the reducing metal in the inorganic acid completely removes the oxide of the reducing metal of the nano-valve metal structure.

Depending on the geometry of the starting valve metal oxide, a finely divided powder having a layered structure or a strip or layered surface structure on a metal substrate having a relatively coarse/large size structure having a width of less than 100 nm can be obtained. The strips or layers of metal and/or metal suboxide, and their interstitials (interlayer distance) may be up to two times the strip width, depending on the oxidation state of the valve metal oxide and it.

Therefore, when a finely dispersed valve metal oxide powder having an average particle diameter of a basic structure of 50 to 2000 nm, preferably less than 500 nm, more preferably less than 300 nm is used, a finely dispersed metal or suboxide can be obtained. a powder having a layered structure, and a metal or suboxide strip having a width of 5 to 100 nm, preferably 8 to 50 nm, particularly preferably at most 30 nm; and a lateral dimension of 40 to 500 nm, The surface area is higher than 20 m ² /g, preferably higher than 50 m ² /g.

When a relatively large valve metal oxide substrate having a scale exceeding, for example 10 μm, is used, a metal or suboxide strip having a width of at most 100 nm, preferably 5 to 80 nm, particularly preferably 8 to 50 nm, more preferably up to 30 nm, and the gap is one to two times the width of the strip. The depth of the groove between the strips is at most 1 μm.

Larger metal structures or substrates, such as wires or foils, can be oxidized by chemical or anodic methods, and then reduced in accordance with the teachings of the present invention to obtain a strip-like structure, the strip depth being determined by The thickness of the oxide layer formed at the beginning.

Still further, the structure according to the present invention can be obtained by providing a substrate comprising, for example, another metal or ceramic having a valve metal oxide layer, such as by vapor deposition or by application. Electrodeposition of the resulting valve metal layer can be oxidized and reduced in accordance with the present invention to form a metal or suboxide.

The valve metal oxide used in accordance with the purpose of the present invention may be an oxide of a transition element of Groups 4 to 6 of the periodic table, such as Ti, Zr, V, Nb, Ta, Mo, W and Hf, and an alloy thereof (mixed) Oxide) and Al are preferably Ti, Zr, Nb and Ta, especially Nb and Ta. Nb ₂ O ₅ , NbO ₂ and Ta ₂ O ₅ are particularly preferred starting oxides. The reaction product obtained according to the present invention is preferably a metal of a starting oxide. The lower oxidation state oxide (suboxide) of the starting valve oxide can also be obtained by reducing the product. A particularly preferred reduction product is a niobium (Nb) suboxide having metallic conductivity characteristics, which has a chemical formula of NbOx, wherein 0.7 < x <1.3; in addition to metal niobium (Nb) and tantalum (Ta), it is also suitable as The anode material of the capacitor; and the material according to the invention is particularly suitable for use in low activation potential ranges of up to 10 V, particularly preferably in the range of up to 5 V, more preferably in the range of up to 3 V.

As the reducing metal, Li, Mg, Ca, B, and/or Al may be used in the present invention, and an alloy therewith. Preferred materials may be Mg, Ca and Al as long as these reducing metals are more reactive than the starting oxide, especially a co-dissolved alloy of Mg or Mg with Al.

The reduced product of the present invention is characterized in that the content of the reducing metal is in the range of more than 10 ppm, especially 50 to 500 ppm, due to the doping caused during the reduction.

The method of preparing a nanoscale structure in the present invention is substantially similar to that described in WO 00/67936 for the reduction of a metal oxide by a reducing metal in the form of a vapor. Here, the valve metal oxide to be reduced to a powder form is contacted with a reducing metal vapor in the reactor. The reducing metal is volatilized and propagated by the action of the carrier gas stream, such as generally at a high temperature of 900 to 1200 ° C, with argon passing through the valve metal oxide powder on the net or in the crucible, and generally holding the temperature Time is from 30 minutes to hours. Since the molar volume of the valve metal oxide is about two to three times that of its corresponding valve metal, there is a considerable reduction in volume during the reduction process. In the reduction procedure, a highly porous sponge-like structure in which an oxide of a reducing metal is deposited is formed. Since the molar volume of the reducing metal oxide is higher than the change between the valve metal oxide and the valve metal molar volume, when it is mixed into the pores, residual stress is generated. The oxide of the reducing metal in the structure can be removed from the structure by dissolving it, and a highly porous metal powder can be obtained. The reduction reaction mechanism, the formation and distribution of pores are described as follows: from the metal oxide particles of the reduction valve or the small reaction nuclei on the surface of the substrate, the layered structure with a nanometer scale, in the valve metal/valve metal Formed after the reaction stage in the initial reaction phase of the oxide, the layers are initially oriented perpendicular to the surface of the particle/substrate region near the surface, however, when the reaction front moves toward the oxide particle/substrate deep, the layer The direction and scale of the material are determined by the crystal orientation and scale of the major particles in the valve metal oxide and the reaction conditions. A certain number of lattice faces in the valve metal oxide grains are replaced by a lattice face of a chemically equivalent number of valve metals and oxides of the reducing metal. These nano-sized layered structures, which are actually very strongly unfavorable due to their high interfacial stress, can still be prepared and become feasible due to the reduction of the intense exothermic reaction and at least part of the excess energy not in the form of heat. Escape, but "input" to the formation of the structure, making it a quick response to the dynamics. Many flat interfaces in a layered structure act as "fast roads" for reducing metal atoms, in other words they allow rapid diffusion, and thus they cause rapid reaction kinetics and effectively reduce the total energy of the reaction system. However, the layered structure comprising the valve metal and the reducing metal oxide is formed only in the metastable state, and when the thermal energy is introduced, it is formed into a lower energy structural state. In a reduction process, the relatively long heat treatment time and the fixed reaction conditions (such as temperature, vapor pressure of reducing metal) are "normally", and the structural transformation is inevitable, in other words, the nano-sized layered structure is converted into A coarser and alternating structure comprising a region of the valve metal region and a reducing metal oxide.

It has now been found that the layered structure can be frozen if the layered structure at this temperature remains stable until the cooling of the reduced product is carefully cooled to a certain temperature before the structural transformation occurs. According to the present invention, the reducing conditions can be determined such that the reduction can be performed uniformly in a short time. In other words, if a powdered starting oxide is used, after the reduction procedure is completed, it is cooled as quickly as possible in the powder bed of the oxide and the reduction product as soon as possible.

For this reason, it is preferred to use a powder bed of a lower thickness to ensure uniform vapor permeation of the reducing metal into the powder bed. The thickness of the powder bed is preferably less than 1 cm, more preferably less than 0.5 cm.

In addition, it is ensured that the reducing metal vapor can be uniformly infiltrated into the powder bed by providing a longer free diameter length of the reducing metal vapor. According to the invention, the reduction reaction is therefore more suitable for carrying out at low pressure, more preferably under conditions of no carrier gas. The reduction reaction is particularly suitable in an oxygen-free environment where the vapor pressure of the reducing metal vapor is from 10 ^-2 to 0.4 bar, more preferably from 0.1 to 0.3 bar. An acceptable low carrier gas pressure is at most 0.2 bar, more preferably less than 0.1 bar, without any problem. Suitable carrier gas species can be (especially) blunt gases such as argon and helium, and/or hydrogen.

The rate of increase in the depth of the layered structure decreases as its depth increases because the diffusion path along the interface between the reduced metal layer and the reducing metal oxide formed between the metal layers increases. It has now been found that when the depth of material reduction is as high as 1 μm, there is substantially no transition of the layered structure.

According to the present invention, the valve metal oxide powder used preferably has a minimum cross-sectional dimension of the main structural particle size (grain size) of not more than 2 μm, more preferably not more than 1 μm, and particularly preferably no more than 0.5 μm. If the main structure of the valve metal oxide powder has a relatively small particle size, it can be used for the porous sintered agglomerate. It is beneficial to the main particles that want to be strongly sintered together, but the class structure network of the open pores exists between the main particles of the aggregation, so the pore size distribution of the open pores can directly reach the vapor of the reducing metal and restore a large part. The main particle surface.

Although the effect is significantly smaller than the pores, the grain boundaries between adjacent major particles can accelerate diffusion. Therefore, in addition to the small primary particles and the open porosity, a high proportion of grain boundaries between the main particles to be formed in the aggregated valve metal oxide particles is favored. The above can be formed by the optimization of the particle size of the main particles and the precipitation of the oxide precursor to form a hydroxide in the calcination of the hydroxide and the hydroxide to form a valve metal oxide. The temperature at which the calcination is carried out is preferably from 400 to 700 ° C, particularly preferably from 500 to 600 ° C.

In the preparation of the metal foil and the wire having the layered surface structure, it is preferable that the thickness of the oxide layer is less than 1 μm for the metal foil or the surface of the wire, and more preferably less than 0.5 μm.

After the reduction reaction under low pressure, the progress time may be from several minutes to several hours, preferably from 10 to 90 minutes, depending on the reducing metal vapor or metal vapor mixture used and its vapor pressure, and the stop of the reduction reaction may be interrupted. Supply of reducing metal vapor, and rapid cooling of the reduced valve metal to a temperature of less than 100 ° C to stabilize the nanostructure of the valve metal or valve metal suboxide layer and the reducing metal oxide, adjacent layers in different directions The structure is sintered and forms an acceptably slightly coarser size. The cooling process can be performed by introducing a protective gas (cooling gas) such as argon gas and helium gas to perform rapid pressurization. A more appropriate procedure is to cool to 300 ° C in 3 minutes, then cool to 200 ° C in another 3 minutes, and cool to 100 ° C in another 5 minutes.

According to the present invention, the reduction procedure is preferably carried out at a lower temperature, with the aim of reducing the size of the nanolayer structure. The reduction temperature of the valve metal oxide is preferably from 500 to 850 ° C, preferably less than 750 ° C, especially less than 650 ° C. Here, the actual temperature should exceed the initial temperature of the reduction reaction because the reduction reaction is an exothermic reaction.

In the present invention, in order to avoid the incompleteness or size increase of the nano-layered structure including the reaction product and the redox metal oxide which is formed at the beginning of the reduction reaction, different measurement methods may be selected or Combination method.

For example, at high reduction temperatures, short reduction times can be adequately ensured by providing an efficient and rapid collection of reducing metal vapors, such as a smaller starting metal oxide powder bed and/or a lower carrier gas pressure. Force, in other words, increases the atomic free path of its reducing metal vapor. On the other hand, at low reduction temperatures, longer reduction times can be accepted.

The starting valve metal oxide agglomerate having an advantageous open pore structure allows the layered structure of the present invention to be achieved with less stringent process conditions. After the reduction reaction is completed and the reduced valve metal oxide is cooled and gradually passed through oxygen or air to passivate, the reducing metal oxide therein can be filtered out from the formed nanostructure, such as in the form of a mineral acid. For example, it is washed with sulfuric acid or hydrochloric acid or a mixture thereof, and then neutralized with deionized water to be dried.

In the example of reducing the finely divided powder, it contains a main structure having a flat shape, and a portion thereof grows in a dendritic form to other structures.

After the reducing metal oxide is filtered out, the independent valve metal layer structure remains geometrically stable because it is sufficiently sintered with adjacent layers, usually by different ends of the individual layers. In the layered structure, the original (polycrystalline) valve metal oxide particles are thus converted into aggregated valve metal particles, the main particles of which contain layered structures in different directions, and which are sintered to each other. In summary, a stable, interpenetrating metal structure and a "flat" hole are thus formed.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram illustrating the apparatus for carrying out the process of the present invention, broadly indicated by 1 having a reduction chamber 2. Reference numeral 3 denotes a temperature controller including a heating coil and a cooling coil. The protective or filling gas or cooling gas is introduced into the reduction reaction chamber through a valve, and its introduction direction is arrow 4. The direction in which the reduction reaction chamber is evacuated or the gas is removed is arrow 5. The reduction chamber 2 is joined to the gasification chamber 6 of the reducing metal and additionally provides a separate heating source 7. The gasification chamber and the reduction chamber are effectively thermally insulated by the valve region 8. The valve metal oxide is reduced in the boat body 10 in the form of a thin powder bed. If a valve metal oxide foil or wire or a foil or wire having a valve metal oxide surface is used, it is preferred to hang vertically and parallel to the flow direction of the reducing metal vapor in the reduction chamber. The reducing metal in the boat 9 is heated to a temperature that provides the desired vapor pressure.

The oxide powder was introduced into a powder bed having a height of 5 mm in a boat body, and a boat-shaped body containing magnesium chips was placed in a gasification chamber, and the reactor was filled with argon gas. The reduction chamber was then heated to a reduction temperature and the pressure was evacuated to 0.1 bar. The gasification chamber is then heated to 800 ° C at which time the magnesium vapor pressure (static) is about 0.04 bar. After 30 minutes, the heating source of the reduction chamber and the gasification chamber was closed, and the argon gas cooled from 200 bar was introduced into the reduction chamber for another period of time, while the chamber wall of the reduction chamber was cooled by water. It.

Figures 2, 3 and 4 are transmission electron microscope images at different magnifications after preparation of the reaction product via ion beam focusing in accordance with the present invention. The dark strips in the figure are layers of tantalum, while the lighter strips are magnesium oxide layers. The layer structure in different directions corresponds to the crystal orientation of different starting ruthenium pentoxide.

1. . . reactor

2. . . Ring original chamber

3. . . Temperature Controller

6. . . Gasification chamber

7. . . Heating source

8. . . Valve area

9. . . Boat body

10. . . Boat body

Figure 1 is a diagram illustrating the apparatus for carrying out the procedure of the present invention,

2 is a transmission electron microscope image of a reduced tantalum powder prepared by ion beam focusing in accordance with the present invention.

Figure 3 is a transmission electron microscope image of a reduced tantalum powder prepared by ion beam focusing in accordance with the present invention.

Figure 4 is a transmission electron microscope image of a reduced tantalum powder prepared by ion beam focusing in accordance with the present invention.

1. . . reactor

2. . . Ring original chamber

3. . . Temperature Controller

6. . . Gasification chamber

7. . . Heating source

8. . . Valve area

9. . . Boat body

10. . . Boat body

Claims (14)

A strip or sheet of valve metal and valve metal suboxide structure having a lateral dimension of 5 to 100 nm. The valve metal and valve metal suboxide structure according to claim 1 of the patent application has a main structure in the form of a sheet or layer of powder. The valve metal and valve metal suboxide structure according to claim 1 of the scope of the patent application is in the form of a surface strip structure. A valve metal structure according to the third aspect of the patent application, which has a strip having a width of 5 to 100 nm and a gap of one to two times the strip width in the form of a foil or a wire. The valve metal and valve metal suboxide structure according to any one of claims 1 to 4, wherein the strips or sheets are arranged in parallel in the group. The valve metal and valve metal suboxide structure according to any one of claims 1 to 4, wherein the lateral dimension or the strip width is 8 to 50 nm. The valve metal structure according to any one of claims 1 to 4, which comprises Ti, Zr, V, Nb, Ta, Mo, W, Hf or Al, in particular Nb or Ta or an alloy thereof. The valve metal suboxide structure according to any one of claims 1 to 4, which has the chemical formula NbO _x , wherein 0.7 < x < 1.3. A valve metal and valve metal suboxide structure according to any one of claims 1 to 4, characterized in that at least one reduction The content of the metallic metal is from 10 to 500 ppm. A method of reducing a valve metal oxide by forming a layered nanostructure at a temperature sufficient to carry out a reduction reaction by a reducing metal vapor, characterized in that the reduced valve metal oxide is heated in a layered structure Freeze before decomposing and converting into a coarser structure. The method according to claim 10, characterized in that the reduction reaction is carried out at an inert gas pressure of less than 0.2 bar, and the reducing metal has a vapor pressure of from 10 ^-2 to 0.4 bar. The method according to claim 10 or 11, wherein the reduced product is cooled to less than 100 ° C in a few minutes immediately after the reduction procedure is completed. The method according to claim 10 or 11, characterized in that Li, Al, Mg and/or Ca, particularly Mg, is used as the reducing metal. The method according to claim 10 or 11, characterized in that Al, Hf, Ti, Zr, V, Nb, Ta, Mo and/or W and its mixed oxide, in particular Nb or Ta, are reduced Oxide.