JPH0565990B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing an anode for a molten carbonate fuel cell, and more particularly to a method for manufacturing an anode made of a nickel-chromium alloy. [Prior Art] Molten carbonate fuel cells have traditionally been known to convert the energy of chemical reactions into electrical energy. The structure is such that oxidant gas is supplied to the anode side and fuel gas is supplied to the anode side. In order for the reaction between gas and carbonate to occur efficiently in the electrode portion, the electrode needs to have a porous structure, and up to now, sintered bodies of nickel powder have been mainly used. In this case, since an oxide layer is formed on the surface of the cathode on the oxidizing gas side by the oxidizing atmosphere during battery operation, sinking of the nickel particles does not proceed. However, in the anode on the fuel gas side, the surface of the nickel particles maintains a metallic state due to the reducing atmosphere containing hydrogen during cell operation. For this reason,
At battery operating temperatures of 600 to 700°C, the electrodes are likely to sinter, and as sintering progresses, the initial porosity decreases. Furthermore, clamping pressure is always applied to the electrode when it is assembled into a battery, and this causes cleave deformation of the anode, reducing the initial porosity. Such a decrease in the porosity of the anode results in deterioration of battery performance. As a way to prevent these, the anode is
A method using a material in which a second element is added to nickel is widely used, and the addition of chromium is known to be particularly effective. When manufacturing this type of anode, the first method is to prepare an alloy powder of nickel and a second element, and then mold and sinter this powder. However, since these alloy powders are generally manufactured by an atomization method, the particles become spherical, and a product with high porosity cannot be obtained when sintered. In addition, the commonly used nickel powder for electrode materials is manufactured by the carbonyl method.
While particles with an average particle diameter of several micrometers or less can be easily obtained, in the atomization method, coarse particles with an average diameter of several tens of micrometers or more account for the majority, and the yield is poor to obtain powder with an average particle diameter of several micrometers or less. The second method is to prepare a porous body such as nickel foam or sintered nickel powder in advance, impregnate it with a second element in the form of a metal salt, and then chemically heat-treat it for reduction. The manufacturing process is complicated, and when a large amount of the second element is added, multiple impregnation treatments are required, resulting in poor industrial efficiency. The third method is to mix the base nickel powder and the second element powder, mold and sinter the mixture, and is the simplest and easiest method compared to the above two methods. However, in this method, segregation of the second element component is unavoidable, and there is a possibility that some parts do not contain the second element. The shrinkage caused by the reaction became uneven, sometimes causing cracks and deformation. Furthermore, even if no structural defects occur, long-term heat treatment at high temperatures is required to reduce the segregation of the second element, and in this case, the porosity of the sintered body may decrease. I don't like it. [Problems to be Solved by the Invention] As mentioned above, there are still various problems in the conventional methods and no satisfactory method has been found. The present invention aims to solve the problems of the conventional method and to obtain a nickel-chromium alloy anode by a simple method. [Means for Solving the Problems] In order to achieve the above-mentioned object, the present inventors have made extensive studies and found that in the production of a nickel-chromium alloy anode, chromium, which is a second element, is used in the form of a pure metal. Furthermore, the present invention was completed by discovering that by using chromium in the form of ultrafine particles produced by an evaporation method, it is possible to produce an anode that is uniformly dispersed without segregation. That is, the present invention is characterized in that, in a method for manufacturing a nickel-chromium alloy anode for a molten carbonate fuel cell, nickel powder and ultrafine chromium particles manufactured by an evaporation method are mixed, molded, and sintered. This invention relates to a method for manufacturing a nickel-chromium alloy anode. Next, the present invention will be explained in more detail. In the present invention, the second element, chromium, is added in the form of a pure metal, and its feature lies in the use of ultrafine chromium particles produced by an evaporation method as the second element. By the way, the evaporation method heats the solid material to vaporize it, cools it in a refrigerant, and condenses it into ultrafine particles.In the case of chromium, the average particle size is about 500Å.
You can get the following. The ultrafine metal particles obtained here have a very small particle size, so they are not suitable as a sintering material alone, but they are effective as a material for adding a second element. That is, since the obtained ultrafine chromium particles have a particle size two orders of magnitude smaller than the base nickel powder, they can easily be uniformly dispersed between the nickel powders and do not segregate to a large extent. Further, the ultrafine particles easily diffuse into nickel during the sintering heat treatment, and most of the added amount is alloyed with nickel, so it is only necessary to add an amount corresponding to the desired alloy composition. Furthermore, in the case of the present invention, although a pure metal element is used as the second element additive, the dispersion effect is equivalent to that of a metal salt, and moreover, the desired second element additive can be obtained by a single sintering process. An alloy sintered body containing elemental components can be obtained. In order to further improve diffusion, it is necessary to weigh and mix a predetermined amount of base nickel powder and chromium ultrafine particles at the raw material stage, and then preliminarily heat-treat the mixture at a temperature lower than the sintering temperature during electrode manufacturing. According to
It is possible to better diffuse chromium into nickel. If a sintered body is produced using the powder after this preliminary heat treatment as a raw material, there is an advantage that the distribution of chromium in the sintered body can be made more uniform. [Operation] The main driving force for the sintering reaction is the surface energy of the powder particles. Mass transfer occurs to reduce this energy, and sintering proceeds such that the surface area of the powder particles is reduced. In the present invention, there are two materials to be sintered.
It is a mixture of different types of metal powders, and alloying of the two types of metals progresses with the above-mentioned mass transfer. That is, the base nickel powder forms a porous body structure that determines the physical properties of the porous body by mutual fusion, and at that time, it is alloyed with the ultrafine chromium particles dispersed in various parts. In this case, a similar reaction occurs even if micron-order chromium powder is used instead of ultrafine chromium particles, but ultrafine particles have a higher particle surface energy and are less dispersible between nickel powders. The sintering reaction progresses easily. The method of the present invention is particularly effective as a method for producing porous alloy bodies such as electrodes for fuel cells. In other words, when simply producing a dense alloy sintered body, nickel and chromium can be easily separated by increasing the sintering temperature or by applying pressure during molding or sintering to bring the particles into close contact with each other. However, when producing a porous sintered body, high sintering temperature and application of pressure are not preferable because they reduce the porosity.
However, according to the present invention, nickel and chromium can be alloyed without reducing the porosity. [Examples] The present invention will be explained below based on Examples.
The present invention is not limited to these examples. Example 1 90g of nickel powder (Type 255 manufactured by INCO) with an average particle size of 2.2 to 2.8 μm was used as the base material, and the average particle size was prepared by an in-gas evaporation method as a second element additive.
10g of 500Å ultrafine chromium particles (made by vacuum metallurgy),
Polyvinyl butyral as an organic binder
8 g of hydroxypurmethylcellulose as a plasticizer, and 80 g of a mixed solution of ethanol and trichlene at a weight ratio of 1:1 as a solvent were weighed, respectively, and kneaded in a ball mill for 100 hours to prepare a slurry. This slurry is applied to a reinforcing material using nickel wire mesh (wire diameter 0.2 mm, 20 meshes) using a doctor blade device.
The above coating was applied and formed into a thin plate. The dried molded body is heated and held at 950°C for 1 hour in a vacuum to sinter it.
A 90 mm square anode with a thickness of 0.8 mm was obtained. Approximately 1 g of a test piece was taken from this electrode and its porosity was measured with the reinforcing material included, and it showed a value of 64%. Example 2 The same nickel powder and ultrafine chromium particles used in Example 1 were weighed so that they accounted for 90% by weight and 10% by weight of the total weight, respectively. Ethanol corresponding to 50 parts by weight based on 100 parts by weight of the total powder weight and the above two types of metal powders were mixed in a ball mill for 100 hours.
After drying the slurry after mixing, it was placed in an aluminium crucible and heated and held at 650°C in vacuum for 4 hours as a preliminary heat treatment to diffuse chromium into the nickel.
After the preheated powder was ground in a mortar, coarse particles were removed using a 35-mesh sieve. Powder prepared in this way after preheat treatment
To 100g, the same kind and amount of organic binder as in Example 1,
A plasticizer and a solvent were added, and the mixture was mixed and molded under the same conditions as in Example 1. The dried molded body was heated and held at 1020° C. for 1 hour in a vacuum to sinter it to obtain an anode having the same shape as in Example 1. Further, when the porosity was measured in the same manner as in Example 1, the porosity was 65%. Comparative Example As a base material, 90 g of the same nickel powder as used in the example, and as a second element additive, 10 g of carom powder (manufactured by Fuchigawa Metal Office) with an average particle size of 2 μm prepared by a pulverization method, the same type as in the example. , the same amount of organic binder, plasticizer, and solvent were mixed under the same conditions as in the example.
Molded. The dried molded body was heated and held at 1080° C. for 1 hour in a vacuum to sinter it to obtain an anode having the same shape as in the example. Further, when the porosity was measured in the same manner as in the example, the porosity was 59%. [Comparison of Examples 1 and 2 and Comparative Example] Figure 1-A, Figure 2-A, and Figure 3-A show the particle structure of the artificially fractured surface of the electrode for Examples 1 and 2 and Comparative Example, respectively. Figures 1-B and C, 2-B and C, and 3-B and C are scanning electron micrographs showing the nickel and A characteristic X-ray photograph of chromium is shown. As can be seen from these drawings, a chromium segregation area of about 20 μm is seen in Comparative Example 1, whereas in Examples 1 and 2, although some segregation is seen, chromium is generally uniformly distributed. . Further, the segregation of chromium is smaller in Example 2 than in Example 1. Of the artificially fractured surfaces of each electrode shown in the drawings, a quantitative analysis of chromium was performed using EPMA on a portion where chromium was thought to be uniformly distributed without segregation. The results are shown in Table 1. In addition, the above-mentioned sintering temperature and porosity are also listed in the table for reference.

〔Effect of the invention〕

According to the method for manufacturing a nickel-chromium alloy anode of the present invention, ultrafine chromium particles are uniformly dispersed in nickel powder without segregation, and the manufacturing process is also different from sintering nickel powder alone. A nickel-chromium alloy porous body of any composition can be easily manufactured using a similar process.

[Brief explanation of the drawing]

Figure 1-A, Figure 2-A, and Figure 3-A are electron micrographs showing the particle structures on the artificially fractured surfaces of the electrodes of Examples 1 and 2 and Comparative Example, respectively. Figures 2-B and 3-B are
These are characteristic X-ray photographs of Ni in each part shown in A.
Furthermore, Fig. 1-C, Fig. 2-C, and Fig. 3-C are characteristic X-ray photographs of Cr in the part shown in Fig. A, respectively.

Claims (1)

[Claims] 1. A method for producing a nickel-chromium alloy anode for a molten carbonate fuel cell, which includes mixing nickel powder and ultrafine chromium particles produced by an evaporation method, and then molding and sintering the mixture. Characteristic method for manufacturing nickel-chromium alloy anodes. 2. The method for manufacturing a nickel-chromium alloy anode according to claim 1, characterized in that the mixture of the nickel powder and ultrafine chromium particles is heat-treated at a temperature lower than the sintering temperature of the electrode, and then molded and sintered. .