US20030056863A1

US20030056863A1 - Intermetallic compound and method of manufacturing the same

Info

Publication number: US20030056863A1
Application number: US10/105,451
Authority: US
Inventors: Kyosuke Yoshimi; Shuji Hanada; Akihisa Inoue
Original assignee: Tohoku University NUC
Current assignee: Tohoku University NUC
Priority date: 2001-09-26
Filing date: 2002-03-26
Publication date: 2003-03-27
Also published as: CA2379744A1; JP2003105460A; JP3536095B2

Abstract

A porous intermetallic compound (B2-type FeAl) is obtained by rapidly solidifying and cooling down a molten alloy with an extremely large number of thermal vacancies and by heating the solidified and cooled alloy at temperature within the range of 400 to 450 degrees centigrade to transform the thermal vacancies into micro pores of nanometer order in size.

Description

BACKGROUND FOR THE INVENTION

1. Field of the Invention

The present invention relates to intermetallic compounds and methods of manufacturing the same, particularly relating to intermetallic compounds having a lot of micro pores of nanometer size distributed uniformly and manufacturing methods for such porous intermetallic compounds.

2. Related Art Statements

Various kinds of intermetallics compounds having a variety of compositions, phases and crystal structures have been developed in the past However, any porous intermetallic compound, i.e. intermetallic compound having a lot of micro pores uniformly distributed therein has never been known until now. Furthermore, there is no technology in practice such that a method for transforming intermetallic compounds into a porous body having a lot of micro pores in nano-order size.

Cellular metallic materials (i.e. metal foams) are known as one of the porous metals and alloys having a number of pores formed therein. The cellular metallic materials are generally manufactured by the way that expandable resins serving as a blowing agent are dispersed in metals and alloys. Then, the dispersed resins are gasified from them to form pores. However, the known cellular metallic materials manufactured by the way explained generally above have pore sizes of 100 to 1000 μm and have relatively low specific surface areas. Thus, if the cellular metallic materials were utilized as catalysts or catalyst substrates, it would be difficult to obtain high catalytic activity. Consequently, the conventional methods for manufacturing cellular metallic materials could not produce porous intermetallic compounds with micro pores of nanometer order in size.

As another method, porous intermetallic compounds may be produced by sintering. However, sintered porous intermetallic compounds have pore sizes of 1 to 100 μm generally, so that the specific surface areas are still low. Thus, if such sintered porous intermetallic compounds are used as catalysts or catalyst substrates, it is also difficult to obtain sufficiently high catalytic activities. Therefore, the known methods for manufacturing the porous metals and alloys described above are not applicable to produce porous intermetallic compounds having micro pores of nanometer order in size.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide an intermetallic compound and a method for manufacturing the same, in which an extremely large number of micro pores (that have been never possible to form so far) are uniformly distributed in the intermetallic compound.

According to the present invention, it is characteristic that a lot of micro pores of nanometer order in size uniformly distributed are provided to an intermetallic compound in order to attain the objective mentioned above.

According to the invention, it is expected that the intermetallic compound having fine pores with a significantly enlarged specific surface area can be utilized for various applications. For instance, when the porous intermetallic compound is used for a catalyst, the catalytic activity can be improved to a large extent due to a large specific surface arm

According to the invention, the pore size is controllable within the rage of several to several hundreds nanometers by a preferable embodiment of the intermetallic compound, particularly within the range of several tens to several hundreds nanometers.

According to another aspect of the invention, an intermetallic compound having a lot of micro pores of nanometer order in size distributed uniformly is produced by:

rapidly solidifying and cooling a molten alloy consisting of one or more kinds of metal elements and/or one or more kinds of semimetal elements with large amount of thermal vacancies; and

subjecting the rapidly solidified alloy to a heat treatment within a predetermined temperature range.

In the intermetallic compound having a nano-porous structure (i.e. having an extremely large number of micro pores of nanometer order in size), due to a great number of micro pores, a specific surface area of the intermetallic compound is significantly increased. Accordingly, the intermetallic compound is expected to have sufficiently high catalytic activity and to be used for various applications. For examples, the intermetallic compound may be used as a catalyst or catalyst substrate, and the catalytic activity of the intermetallic compound can be increased to a large extent. Furthermore, the pores (i.e. voids) formed near the surfaces of the intermetallic compound produced along the present invention may be utilized as a metallic container of nanometer size (that is, a micro chemical reactor) and various chemical reactions could be controlled in nanometer order.

Also, according to the invention, the intermetallic compound does not include a blowing agent, and thereby an amount of impurities contained therein is much smaller than that of the conventional cellular metallic materials. According to the invention, it is possible to provide the intermetallic compound having inherently high purity and being additive free. Moreover, according to the invention, since the intermetallic compound consists of metal and for semimetal elements, the compound has good water and abrasion resistance.

According to the present invention, the intermetallic compound may have a composition corresponding to B2-type FeAl, B2-type NiAl, B2-type CoAl, B2-type AgMg, B2-type AuCd, B2-type CoFe, B2-type CoTi, B2-type FeTi, or B2-type CuZn, Particularly, according to the invention, B2-type FeAl is preferable among the intermetallic compounds. Since the B2-type intermetallic compounds generally have a great quantity of thermal vacancies at high temperature (i.e. an equilibrium concentration of thermal vacancies is significantly high at high temperature), and such a large number of thermal vacancies is utilized to easily form pores of nanometer sizes.

According to the present invention, a method of manufacturing an intermetallic compound having a lot of micro pores of nanometer order in size distributed uniformly comprises the steps of:

rapidly solidifying a molten alloy consisting of one or more kinds of metal elements and/or one or more kinds of semimetal elements with large amount of thermal vacancies in a molten state; and

subjecting the rapidly solidified alloy to a heat treatment within a predetermined temperature range to form and distribute uniformly a lot of micro pores of nanometer order in size in the alloy.

According to the invention, the intermetallic compound having a lot of micro pores of nanometer order in size distributed therein can be easily manufactured at a low cost in large quantities. Especially, according to the invention, the method needs no special apparatus, but can be conducted with conventional rapid solidification apparatuses (such as an apparatus for single-roll process) and conventional heating ovens.

According to the invention, in an embodiment of the method, the said heat treatment step is done in vacuum or reduced pressure or in an atmosphere of one or more inert gases. In this embodiment, oxidation on the surface and interior of the intermetallic compound can be avoided. According to the invention, it is preferred to conduct the method in an oxygen-free atmosphere (there is no chemical species which promote oxidation). Therefore, the heat treatment is carried out in one of the above mentioned states, vacuum, reduced pressure and inert gas atmosphere such as nitrogen and argon gases.

According to the invention, in another preferable embodiment of the method, the molten alloy is consisting of B2-type FeAl, and the said predetermined temperature range in the said heat treatment step is set to a temperature within the range of 400 to 450 degrees centigrade. A lower limit of the temperature range in the heat treatment step may be set to a temperature above which a transformation of the supersaturated thermal vacancies into micro pores occurs sufficiently. If the heat treatment temperature is set to a temperature lower than the lower limit, a speed of the transformation of the supersaturated thermal vacancies into pores is too late to attain a high manufacturing efficiency. An upper limit of the temperature range of the heat treatment step may be set to a temperature lower than the melting point of the alloy and/or the temperature that phase transformation does not occur. If the heat treatment temperature is set to a temperature higher than an upper limit, a speed of the transformation of supersaturated thermal vacancies into pores is too fast and frozen-in, supersaturated thermal vacancies are combined with each other into pores in a short time. Consequently, pores might grow larger, and be difficult to control the heating time to obtain a desired pore size. It is noted that the heat treatment should be set to an optimum temperature depending on material systems for a desired pore size as well as a desired pore density.

For example, when the B2-type FeAl is heat-treated at 500 degrees centigrade, the frozen-in thermal vacancies are easily clustered into pores, and it is considerably difficult to generate micro pores of nanometer order in size. In other words, the heat treatment temperature of 500 degrees centigrade is too high for this composition and the speed of the transformation of supersaturated thermal vacancies into pores is too fast. Although the micro pores are formed in the early stage of the heat treatment, they would turn into bigger within a short time or the undesired phase transformation occurs to plug up the pores. On the contrary, if the heat treatment is done at 300 degrees centigrade, it is impossible to generate micro pores in a relatively short time. Therefore, as mentioned above, it is preferable to set an optimum temperature range depending on particular materials to be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating basic steps of the method for manufacturing the intermetallic compound having micro pores of nanometer size by the invention; [0024]
FIG. 2 is a block diagram showing basic steps of the method of the present invention and equipments to be used in the method; [0025]
FIG. 3[0026] a is a photograph of rapidly solidified ribbons of Fe-46 mol % Al formed by a single-roll process along the invention, and FIG. 3b is a schematic diagram illustrating the photograph of the FIG. 3a schematically;
FIG. 4[0027] a is a photograph of a surface by scanning electron microscope of the rapidly solidified ribbon of Fe-46 mol % Al of the FIG. 3 after a heat treatment at 450 degrees centigrade for 24 hours, and FIG. 4b is a schematic diagram illustrating the photograph of the FIG. 4a schematically; and
FIG. 5[0028] a is a photograph by transmission electron microscope interior to the rapidly solidified ribbon of Fe-46 mol % Al of the FIG. 3 after the heat treatment, and FIG. 5b is a schematic diagram illustrating the photograph of the FIG. 5a schematically.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The methods and the products of the present invention will be described in detail with reference to the accompanying drawings. [0029]
FIG. 1 is a block diagram illustrating basic steps of the method for manufacturing the intermetallic compound having micro pores of nanometer size through the invention. As illustrated in FIG. 1, two or more different metal elements, e.g. Fe-46 mol % Al (i.e. Fe:AI mole ratio is 54:46) corresponding to the composition of B2-type FeAl are heated up to obtain a molten alloy. A melting point of the molten alloy is approximately 1350 to 1400 degrees centigrade. Then, the molten alloy is rapidly solidified and cooled using a rapid solidification method such as a single-roll or twin-roll melt-spinning process to produce the intermetallic compound ribbons. In some intermetallic compounds, a large quantity of thermal vacancies existing at just below the melting point are frozen at the extremely high cooling rate. [0030]
Now, the supersaturated thermal vacancies frozen in the intermetallic compound ribbons by the rapid solidification will be described in detail. Ideally, metals, alloys and intermetallic compounds have no thermal vacancy at the absolute zero degrees Kelvin. When once they are heated from the absolute zero degrees Kelvin, thermal vacancies are generated in them. As temperature arises, the concentration of the thermal vacancies becomes higher. In other words, an equilibrium concentration of thermal vacancies in metals, alloys and intermetallic compounds is a “function of temperature”. Atomic vacancies in them are generally introduced by various factors, and the thermal vacancies are characterized by the equilibrium concentration as a function of temperature. Based upon the behavior mentioned above, it is considered that the equilibrium concentration of thermal vacancies in metals, alloys and intermetallic compounds reaches the maximum at just below their melting points. It has been known that the B2-type intermetallic compounds advantageously used in the present invention have considerably high equilibrium concentrations of thermal vacancies. Thus, it is understood that the B2-type intermetallic compounds are very suitable materials for generating a large quantity of pores in this invention. It should be noted that an equilibrium concentration of thermal vacancies is a concentration of thermal vacancies just in a thermally balanced state. [0031]
According to the present invention, by means of the rapid quenching, a large quantity of thermal vacancies generated at a high temperature in B2-type intermetallic compounds can be frozen in an unbalanced state. However, since the concentration of the thermal vacancies has a tendency to approach to an equilibrium concentration all the time, if a cooling rate is low enough, thermal vacancies disappear by various factors and its concentration is reduced as compared with that generated at a high temperature. According to the invention, the thermal vacancies frozen in a supersaturated (non-equilibrium) state by the rapid cooling are clustered to form pores by a heat treatment. That is to say, when the rapidly solidified B2-type intermetallic compounds are heated to a suitable temperature at which the frozen thermal vacancies can move relatively easily, the vacancies are diffused over short distances and are combined with each other to form pores which are different from the frozen thermal vacancies. Through this process, the concentration of the thermal vacancies approaches to an apparent equilibrium state. It should be noted that even if thermal vacancies are combined with each other, pores are not always generated and other structural defects might be formed in some cases. In this context, it is very important to select the suitable heating temperature carefully. [0032]
The rapidly solidified intermetallic compound ribbons are put into a chamber kept in vacuum (at approximately 10[0033] ⁻⁴Pa) at approximately 425 degrees centigrade. And then, the ribbons are heat-treated for approximately 24 hours in the chamber. The heating temperature and time have to be determined such that the frozen thermal vacancies can be transformed into pores of nanometer order in size. By heat-treating the intermetallic compound ribbons at a suitable temperature for a suitable period, the frozen thermal vacancies in the ribbons are clustered to form micro pores of nanometer sizes. In the present embodiment, the sizes of the pores near the surface and interior to the ribbons are about several tens nanometers as will be described later. According to the invention, pore size and density are variable by controlling concentration of frozen thermal vacancies and process conditions for clustering (such as heating temperature and time). Origins of the micro pores are crystal lattice defects which are induced by a fact that atoms are dropped off at positions where atoms should inherently exist. Thus, it is considered that several to several hundreds vacancies (atom voids) are combined with each other to form spatial voids in the intermetallic compounds by clustering. This means that, conceptually, it is possible to produce pores of several nanometers. If the clustering is promoted highly enough, it would be possible to make an intercommunicated pore structure. Intermetallic compounds having such an intercommunicated pore structure (i.e. spongy metal) may be utilized for filters or catalyst substrates.
FIG. 2 is a block diagram showing basic steps of the method along the present invention and equipments to be used in the method. As shown in FIG. 2, the alloy consisting of Fe and Al, which are prepared in the composition of Fe-46 mol % Al, is melted in a container [0034] 1. The molten alloy is flow out from a nozzle of the container 1 and is fed to a rotating roll 2. Then, the alloy is rapidly solidified and cooled on the roll 2 rotating at a high speed, so that rapidly solidified ribbons are obtained. And then, the solidified ribbons are supplied into a chamber 3. The chamber 3 is equipped with a heater 4 which keeps the chamber to the predetermined temperature of 425 degrees centigrade. The ribbons are kept in the chamber 3 for approximately 24 hours. During the heat treatment, it is preferred that the chamber 3 is evacuated to make a vacuum (approximately 10⁻⁴Pa) by a vacuum pump 5. Through the heat treatment, minute pores of nanometer order in size are formed near the surfaces and interior of the ribbons.
In order to show the micro pores formed in the intermetallic compounds produced by the method along the present invention, following photographs were taken. [0035]
FIG. 3[0036] a is a photograph of rapidly solidified ribbons of Fe-46 mol % Al composition formed by a single-roll melt-spinning process in the method along the invention. FIG. 3b is a schematic diagram illustrating the photograph of FIG. 3a schematically. As shown in FIGS. 3a and 3 b, the rapidly solidified ribbons 10 before the heat treatment are metallic thin films, and micro pores are not generated. That is to say, no micro pore can be observed at this time. Just for reference, a size AA dry battery is placed to see the size and quantity of the rapidly solidified ribbons 10.
FIG. 4[0037] a is a photograph taken by a scanning electron microscope, showing a surface of the rapidly solidified ribbon of Fe-46 mol % Al shown in FIG. 3 after the heat treatment. FIG. 4b is a schematic diagram illustrating the photograph of FIG. 4a schematically. As shown in FIGS. 4a and 4 b, it can be observed that micro pores 20 of several tens nanometers are generated uniformly with a high density near the surface of the ribbon.
FIG. 5[0038] a is a photograph taken by a transmission electron microscope, showing the inside of the rapidly solidified ribbon of Fe-46 mol % Al shown in FIG. 3 after the heat treatment. FIG. 5b is a schematic diagram illustrating the photograph of FIG. 5a schematically. As shown in FIGS. 5a and 5 b, it is observed that micro pores 30 of nanometer order in size and having rectangular shape are uniformly generated inside the ribbon. The density of the micro pores is in die range of 10¹⁹to 10²¹m³. Also, it is observed in FIGS. 5a and 5 b that a small number of pores of several nanometers are produced
It is noted that the described pore sizes are no more than exemplification, and the pore size may be changed by controlling both of processing time and temperature of the heat treatment. When the heat treatment period is shortened as described in the above explained embodiment, pores of several nanometers may be produced at a very high density, and when the heating temperature is increased, pores may be grown to several hundreds nanometer. In other words, pore size should depend on the processing time and temperature of the heat treatment as well as on characteristics of starting materials. [0039]
Another factor for controlling pore size is cooling rate which is mainly determined by a type of the rapid solidification process. The concentration of frozen vacancies in solidified ribbons is changed by cooling rate, and consequently the clustering process dominating pore density and size is influenced by the cooling rate. That is to say, at a high cooling rate, the concentration of frozen thermal vacancies is high. Therefore, in order to efficiently and easily produce intermetallic compounds having micro pores uniformly distributed therein with a high density, it is important to solidify and cool down the molten alloy as rapid as possible. [0040]
According to the present invention, it should be appreciated that those skilled in the art will be able to modify or alter the intermetallic compound and the method for manufacturing the same in various ways without departing the spirit and scope of the present invention. For instance, the solidification and cooling process in the method of the invention may be easily carried out by various rapidly solidification techniques (e.g. a fiber spinning process) other than the single-roll melt-spinning process described herein. [0041]
As explained above in detail, the intermetallic compounds having pores of nanometer size and the method of manufacturing the same along the present invention are widely and effectively useful to various kinds of applications such as filters, catalysts and catalyst substrates. [0042]

Claims

What is claimed is,

1. An intermetallic compound, characterized in that a lot of micro pores of nanometer order in size are uniformly distributed.

2. The intermetallic compound according to claim 1, wherein the said intermetallic compound is produced by:

rapidly solidifying and cooling a molten alloy consisting of one or more kinds of metal elements and/or one or more kinds of semimetal elements; and

subjecting the rapidly solidified alloy to a beat treatment within a predetermined temperature range.

3. The intermetallic compound according to claim 2, wherein the said molten alloy has a composition corresponding to B2-type FeAl, B2-type NiAl, B2-type CoAl, B2-type AgMg, B2-type AuCd, B2-type CoFe., B2-type CoTi, B2-type FeTi, Or B2-type CuZn.

4. The intermetallic compound according to claim 3, the said micro pores have a pore size set to a value within the range of several to several hundreds nanometers.

5. The intermetallic compound according to claim 3, the said micro pores have a pore size set to a value within the range of several tens to several hundreds nanometers.

6. The intermetallic compound according to claim 3, wherein the said molten alloy is B2-type FeAl, and the said micro pores have a pore density therein set to a value within the range of 10¹⁹to 10²¹m³.

7. A method of manufacturing an intermetallic compound having a lot of micro pores of nanometer order in size distributed uniformly, comprising the steps of:

rapidly solidifying a molten alloy consisting of one or more kinds of metal elements and/or one or more kinds of semimetal elements; and

subjecting the rapidly solidified alloy to a beat treatment within a predetermined temperature range to form and uniformly distribute a lot of micro pores of nanometer order in size in the alloy.

8. The method according to claim 6, wherein the said heat treatment is done in vacuum or reduced pressure or in an atmosphere of one or more inert gases.

9. The method according to claim 7, wherein the said molten alloy has a composition corresponding to B2-type FeAl, B2-type NiAl, B-2-type CoAl. B2-type AgMg, B2-type AuCd, B2-type CoFe, B2-type CoTi, B2-type FeTi, or B2-type CuZn.

10. The method according to claim 7, wherein the said molten alloy is B2-type FeAl, and the said heat treatment is conducted at a temperature within the range of 400 to 450 degrees centigrade.