Patent Description:
Metal foams can be applied to various fields including lightweight structures, transportation machines, building materials or energy absorbing devices, and the like by having various and useful properties such as lightweight properties, energy absorbing properties, heat insulating properties, refractoriness or environment-friendliness. In addition, metal foams not only have a high specific surface area, but also can further improve the flow of fluids, such as liquids and gases, or electrons, and thus can also be usefully used by being applied in a substrate for a heat exchanger, a catalyst, a sensor, an actuator, a secondary battery, a gas diffusion layer (GDL) or a microfluidic flow controller, and the like.

<CIT> discloses a fine nickel powder is applied on the skeleton of a urethane resin foamed body and sintered to produce a battery plaque. In this case, the fine nickel powder at the time of application is treated with an alternating magnetic field and magnetized to form a dense and uniform layer of fine Ni powder around the skeleton and simultaneously to form a stably shaped sheet in which the fine Ni powders are magnetically combined, and the sheet is dried, dewaxed and sintered in a short time by using a highfrequency coil.

<CIT> discloses raw materials which contain metal or ceramic powder, an aqueous polymer solution having viscosity, and a foaming agent are mixed under pressure in a pressure vessel. The resultant pressurized slurry is jetted into the atmosphere to undergo foam formation while forming a fine cell structure. Immediately after the foam formation, the resultant foamed slurry is rapidly cooled and freezed to undergo the stabilization of the shape of foam and is then freeze-dried into a bulky foamed body, followed by sintering.

<CIT> discloses a porous metallic material having an overall porosity of <NUM> to <NUM>%, and a skeleton in a three dimensional network structure which is entirely composed of a sintered metal powder having a porosity of <NUM> to <NUM>%.

It is an object of the present invention to provide a method capable of manufacturing a metal foam comprising pores uniformly formed and having excellent mechanical strength as well as a desired porosity.

The above problems are solved in accordance with claim <NUM>. Further embodiments result from the sub-claims and the following written description.

In the present application, the term metal foam or metal skeleton means a porous structure comprising two or more metals as a main component. Here, the metal as a main component means that the proportion of the metal is <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, or <NUM> wt% or more based on the total weight of the metal foam or the metal skeleton. The upper limit of the proportion of the metal contained as the main component is not particularly limited and may be, for example, <NUM> wt%.

The term porous property may mean a case where porosity is <NUM>% or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, or <NUM>% or more. The upper limit of the porosity is not particularly limited, and may be, for example, less than about <NUM>%, about <NUM>% or less, or about <NUM>% or less or so. Here, the porosity can be calculated in a known manner by calculating the density of the metal foam or the like.

The method for manufacturing a metal foam of the present application comprises a step of sintering a green structure comprising a metal component having metals. In the present application, the term green structure means a structure before the process performed to form the metal foam, such as the sintering process, that is, a structure before the metal foam is formed. In addition, even when the green structure is referred to as a porous green structure, the structure is not necessarily porous per se, and may be referred to as a porous green structure for convenience, if it can finally form a metal foam, which is a porous metal structure.

In the present application, the green structure may be formed using a slurry containing at least a metal component, a dispersant, and a binder.

In one example, the metal component may comprise at least a metal having appropriate relative magnetic permeability and conductivity. According to one example of the present application, the application of such a metal can ensure that when an induction heating method to be described below is applied as the sintering, the sintering according to the relevant method is smoothly carried out.

As the metal, a metal having a relative magnetic permeability of <NUM> or more is used. Here, the relative magnetic permeability (µr) is a ratio (µ/µ<NUM>) of the magnetic permeability (µ) of the relevant material to the magnetic permeability (µ<NUM>) in the vacuum. The metal used in the present application may have a relative magnetic permeability of <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, <NUM> or more, or <NUM> or more. The upper limit of the relative magnetic permeability is not particularly limited because the higher the value is, the higher the heat is generated when the electromagnetic field for induction heating as described below is applied. In one example, the upper limit of the relative magnetic permeability may be, for example, about <NUM>,<NUM> or less.

The metal may be a conductive metal. In the present application, the term conductive metal may mean a metal having a conductivity at <NUM> of about <NUM>/m or more, <NUM>/m or more, <NUM>/m or more, <NUM>/m or more, <NUM>/m or more, <NUM>/m or more, or <NUM>/m, or an alloy thereof. The upper limit of the conductivity is not particularly limited, and for example, may be about <NUM>/m or less, <NUM>/m or less, or <NUM>/m or less.

In the present application, the metal having the relative magnetic permeability and conductivity as above may also be simply referred to as a conductive magnetic metal.

By applying the conductive magnetic metal, sintering can be more effectively performed when the induction heating process to be described below proceeds. Such a metal can be exemplified by nickel, iron or cobalt, and the like, but is not limited thereto.

The metal component may comprise, if necessary, a second metal different from the conductive magnetic metal together with the metal. In this case, the metal foam may be formed of a metal alloy. As the second metal, a metal having the relative magnetic permeability and/or conductivity in the same range as the above-mentioned conductive magnetic metal may also be used, and a metal having the relative magnetic permeability and/or conductivity outside the range may be used. In addition, the second metal may also comprise one or two or more metals. The kind of the second metal is not particularly limited as long as it is different from the applied conductive magnetic metal, and for example, one or more metals, different from the conductive magnetic metal, of copper, phosphorus, molybdenum, zinc, manganese, chromium, indium, tin, silver, platinum, gold, aluminum or magnesium, and the like may be applied, without being limited thereto.

The ratio of the conductive magnetic metal in the metal component is not particularly limited. For example, the ratio may be adjusted so that the ratio may generate an appropriate Joule heat upon application of the induction heating method to be described below. For example, the metal component may comprise <NUM> wt% or more of the conductive magnetic metal based on the weight of the total metal component. In another example, the ratio of the conductive magnetic metal in the metal component may be about <NUM> wt% or more, about <NUM> wt% or more, about <NUM> wt% or more, about <NUM> wt% or more, about <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, or <NUM> wt% or more. The upper limit of the conductive magnetic metal ratio is not particularly limited, and may be, for example, less than about <NUM> wt%, or <NUM> wt% or less. However, the above ratios are exemplary ratios. For example, since the heat generated by induction heating due to application of an electromagnetic field can be adjusted according to the strength of the electromagnetic field applied, the electrical conductivity and resistance of the metal, and the like, the ratio can be changed depending on specific conditions.

The metal component forming the green structure may be in the form of powder. For example, the metals in the metal component may have an average particle diameter in a range of about <NUM> to about <NUM>. In another example, the average particle diameter may be about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, or about <NUM> or more. In another example, the average particle diameter may be about <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less. As the metal in the metal component, one having different average particle diameters may also be applied. The average particle diameter can be selected from an appropriate range in consideration of the shape of the desired metal foam, for example, the thickness or porosity of the metal foam, and the like, which is not particularly limited.

The green structure may be formed using a slurry comprising a dispersant and a binder together with the metal component comprising the metal.

The ratio of the metal component in the slurry as above is not particularly limited, which may be selected in consideration of the desired viscosity and process efficiency. In one example, the ratio of the metal component in the slurry may be from about <NUM> to <NUM> wt%, but is not limited thereto.

Here, as the dispersant, a monohydric alcohol having <NUM> to <NUM> carbon atoms such as methanol, ethanol, propanol, pentanol, octanol, ethylene glycol, propylene glycol, pentanol, <NUM>-methoxyethanol, <NUM>-ethoxyethanol, <NUM>-butoxyethanol, glycerol, texanol, or terpineol, or a dihydric alcohol having <NUM> to <NUM> carbon atoms such as ethylene glycol, propylene glycol, hexane diol, octane diol or pentane diol, or a polyhydric alcohol, etc., is used.

The slurry may further comprise a binder. The kind of the binder is not particularly limited, and may be appropriately selected depending on the kind of the metal component or the dispersant, and the like applied at the time of producing the slurry. For example, the binder may be exemplified by alkyl cellulose having an alkyl group having <NUM> to <NUM> carbon atoms such as methyl cellulose or ethyl cellulose, polyalkylene carbonate having an alkylene unit having <NUM> to <NUM> carbon atoms such as polypropylene carbonate or polyethylene carbonate, or a polyvinyl alcohol-based binder such as polyvinyl alcohol or polyvinyl acetate, and the like, but is not limited thereto.

The ratio of each component in the slurry as above is not particularly limited. This ratio can be adjusted in consideration of process efficiency such as coating property and moldability upon a process of using the slurry.

For example, in the slurry, the binder may be included in a ratio of about <NUM> to <NUM> parts by weight relative to <NUM> parts by weight of the above-described metal component. In another example, the ratio may be about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, or about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, or about <NUM> parts by weight or more, and may be about <NUM> parts by weight or less, about <NUM> parts by weight or less, or about <NUM> parts by weight or less.

Also, in the slurry, the dispersant may be contained at a ratio of about <NUM> to <NUM>,<NUM> parts by weight relative to <NUM> parts by weight of the binder. In another example, the ratio may be about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, about <NUM> parts by weight or more, or about <NUM> parts by weight, and may be about <NUM>,<NUM> parts by weight or less, about <NUM>,<NUM> parts by weight or less, about <NUM>,<NUM> parts by weight or less, about <NUM>,<NUM> parts by weight or less, or about <NUM>,<NUM> parts by weight or less.

In this specification, the unit part by weight means a weight ratio between the respective components, unless otherwise specified.

The slurry may further comprise a solvent, if necessary. As the solvent, an appropriate solvent may be used in consideration of solubility of the slurry component, for example, the metal component or the binder, and the like. For example, as the solvent, those having a dielectric constant within a range of about <NUM> to <NUM> can be used. In another example, the dielectric constant may be about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, or about <NUM> or more, or may be about <NUM> or less, about <NUM> or less, or about <NUM> or less. Such a solvent may be exemplified by water, an alcohol having <NUM> to <NUM> carbon atoms such as ethanol, butanol or methanol, DMSO (dimethyl sulfoxide), DMF (dimethyl formamide) or NMP (N-methylpyrrolidinone), and the like, but is not limited thereto.

When a solvent is applied, it may be present in the slurry at a ratio of about <NUM> to <NUM> parts by weight relative to <NUM> parts by weight of the binder, but is not limited thereto.

The slurry may also comprise, in addition to the above-mentioned components, known additives which are additionally required.

The method of forming the green structure using the slurry as above is not particularly limited. In the field of manufacturing metal foams, various methods for forming the green structure are known, and in the present application all of these methods can be applied. For example, the green structure may be formed by holding the slurry in an appropriate template, or by coating the slurry in an appropriate manner.

The shape of such a green structure is not particularly limited as it is determined depending on the desired metal foam. In one example, the green structure may be in the form of a film or sheet. For example, when the structure is in the form of a film or sheet, the thickness may be <NUM>,<NUM> or less, <NUM>,<NUM> or less, <NUM>,<NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, about <NUM> or less, about <NUM> or less, about <NUM> or less, about <NUM> or less, about <NUM> or less, or about <NUM> or less. Metal foams have generally brittle characteristics due to their porous structural features, so that there are problems that they are difficult to be manufactured in the form of films or sheets, particularly thin films or sheets, and are easily broken even when they are made. However, according to the method of the present application, it is possible to form a metal foam having pores uniformly formed inside and excellent mechanical properties as well as a thin thickness.

The lower limit of the structure thickness is not particularly limited. For example, the film or sheet shaped structure may have a thickness of about <NUM> or more, <NUM> or more, or about <NUM> or more.

The metal foam can be manufactured by sintering the green structure formed in the above manner. In this case, a method of performing the sintering for producing the metal foam is not particularly limited, and a known sintering method can be applied. That is, the sintering can proceed by a method of applying an appropriate amount of heat to the green structure in an appropriate manner.

As a method different from the existing known method, in the present application, the sintering can be performed by an induction heating method. That is, as described above, the metal component comprises the conductive magnetic metal having the predetermined magnetic permeability and conductivity, and thus the induction heating method can be applied. By such a method, it is possible to smoothly manufacture metal foams having excellent mechanical properties and whose porosity is controlled to the desired level as well as comprising uniformly formed pores.

Here, the induction heating is a phenomenon in which heat is generated from a specific metal when an electromagnetic field is applied. For example, if an electromagnetic field is applied to a metal having a proper conductivity and magnetic permeability, eddy currents are generated in the metal, and Joule heating occurs due to the resistance of the metal. In the present application, a sintering process through such a phenomenon can be performed. In the present application, the sintering of the metal foam can be performed in a short time by applying such a method, thereby ensuring the processability, and at the same time, the metal foam having excellent mechanical strength as well as being in the form of a thin film having a high porosity can be produced.

Thus, the sintering process may comprise a step of applying an electromagnetic field to the green structure. By the application of the electromagnetic field, Joule heat is generated by the induction heating phenomenon in the conductive magnetic metal of the metal component, whereby the structure can be sintered. At this time, the conditions for applying the electromagnetic field are not particularly limited as they are determined depending on the kind and ratio of the conductive magnetic metal in the green structure, and the like. For example, the induction heating can be performed using an induction heater formed in the form of a coil or the like. In addition, the induction heating can be performed, for example, by applying a current of <NUM> A to <NUM>,<NUM> A or so. In another example, the applied current may have a magnitude of <NUM> A or less, <NUM> A or less, <NUM> A or less, <NUM> A or less, <NUM> A or less, or <NUM> A or less. In another example, the current may have a magnitude of about <NUM> A or more, about <NUM> A or more, or about <NUM> A or more.

The induction heating can be performed, for example, at a frequency of about <NUM> to <NUM>,<NUM>. In another example, the frequency may be <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less. In another example, the frequency may be about <NUM> or more, about <NUM> or more, or about <NUM> or more.

The application of the electromagnetic field for the induction heating is performed within a range of <NUM> minutes or more, about <NUM> minutes or more, or about <NUM> minutes or more and <NUM> hours or less. In another example, the application time may be about <NUM> hours or less, about <NUM> hours or less, about <NUM> hours or less, about <NUM> hours or less, about <NUM> hours or less, about <NUM> hours or less, about <NUM> hours or less, about <NUM> hours or less, about <NUM> hour or less, or about <NUM> minutes or less.

The above-mentioned induction heating conditions, for example, the applied current, the frequency and the application time, and the like may be changed in consideration of the kind and the ratio of the conductive magnetic metal, as described above.

The sintering of the green structure may be carried out only by the above-mentioned induction heating, or may also be carried out by applying an appropriate heat, together with the induction heating, that is, the application of the electromagnetic field, if necessary.

For example, the sintering may also be performed by applying an external heat source to the green structure together with the application of the electromagnetic field or alone.

In this case, the heat source may have a temperature in a range of <NUM> to <NUM>.

The present application also relates to a metal foam. The metal foam may be one manufactured by the above-mentioned method. Such a metal foam may comprise, for example, at least the above-described conductive magnetic metal. The metal foam may comprise, on the basis of weight, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, or <NUM> wt% or more of the conductive magnetic metal. In another example, the ratio of the conductive magnetic metal in the metal foam may be about <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, or <NUM> wt% or more. The upper limit of the ratio of the conductive magnetic metal is not particularly limited, and may be, for example, less than about <NUM> wt% or <NUM> wt% or less.

The metal foam may have a porosity in a range of about <NUM>% to <NUM>%. As mentioned above, according to the method of the present application, porosity and mechanical strength can be controlled, while comprising uniformly formed pores. The porosity may be <NUM>% or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, or <NUM>% or more, or may be <NUM>% or less, or <NUM>% or less.

The metal foam may also be present in the form of thin films or sheets. In one example, the metal foam may be in the form of a film or sheet. The metal foam of such a film or sheet form may have a thickness of <NUM>,<NUM> or less, <NUM>,<NUM> or less, <NUM>,<NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, about <NUM> or less, about <NUM> or less, about <NUM> or less, about <NUM> or less, about <NUM> or less, or about <NUM> or less. For example, the film or sheet shaped metal foam may have a thickness of about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, about <NUM> or more, or about <NUM> or more.

The metal foam may have excellent mechanical strength, and for example, may have a tensile strength of <NUM> MPa or more, <NUM> MPa or more, <NUM> MPa or more, <NUM> MPa or more, <NUM> MPa or more, or <NUM> MPa or more. Also, the tensile strength may be about <NUM> MPa or more, about <NUM> MPa or more, about <NUM> MPa or more, about <NUM> MPa or more, or about <NUM> MPa or less. Such a tensile strength can be measured, for example, by KS B <NUM> at room temperature.

Such metal foams can be utilized in various applications where a porous metal structure is required. In particular, according to the method of the present application, it is possible to manufacture a thin film or sheet shaped metal foam having excellent mechanical strength as well as the desired level of porosity, as described above, thus expanding applications of the metal foam as compared to the conventional metal foam.

The present application can provide a method for manufacturing a metal foam, which is capable of forming a metal foam comprising uniformly formed pores and having excellent mechanical properties as well as the desired porosity, and a metal foam having the above characteristics. In addition, the present application can provide a method capable of forming a metal foam in which the above-mentioned physical properties are ensured, while being in the form of a thin film or sheet, and such a metal foam.

<FIG> are SEM photographs of metal foams formed in Examples.

Hereinafter, the present application will be described in detail by way of examples and comparative examples, but the scope of the present application is not limited to the following examples.

Nickel (Ni) having a conductivity of about <NUM>/m at <NUM>, a relative magnetic permeability of about <NUM> and an average particle diameter of about <NUM> to <NUM> was used as a metal component. The nickel was mixed with a mixture in which ethylene glycol (EG) as a dispersant, ethyl cellulose (EC) as a binder and methylene chloride (MC) as a solvent were mixed in a weight ratio (EG: EC: MC) of <NUM>:<NUM>:<NUM>, so that the weight ratio (Ni: EC) of the binder and the nickel was about <NUM>:<NUM>, thereby preparing a slurry. The slurry was coated in the form of a film to form a green structure. Subsequently, the green structure was dried at a temperature of about <NUM> for about <NUM> minutes. An electromagnetic field was then applied to the green structure with a coil-type induction heater while purging with hydrogen/argon gas to form a reducing atmosphere. The electromagnetic field was formed by applying a current of about <NUM> A at a frequency of about <NUM>, and the electromagnetic field was applied for about <NUM> minutes. After the application of the electromagnetic field, the sintered green structure was cleaned to produce a sheet having a thickness of about <NUM> in the form of a film. The produced sheet had a porosity of about <NUM>% and a tensile strength of about <NUM> MPa. <FIG> is an SEM photograph of the sheet produced in Example <NUM>.

A sheet having a thickness of about <NUM> was produced in the same manner as in Example <NUM>, except that hexanol was used instead of ethylene glycol as the dispersant. The produced sheet had a porosity of about <NUM>% and a tensile strength of about <NUM> MPa.

A sheet having a thickness of about <NUM> was produced in the same manner as in Example <NUM>, except that <NUM>,<NUM>-hexanediol was used instead of ethylene glycol as the dispersant. The produced sheet had a porosity of about <NUM>% and a tensile strength of about <NUM> MPa.

A sheet having a thickness of about <NUM> was produced in the same manner as in Example <NUM>, except that texanol was used instead of ethylene glycol as the dispersant. The produced sheet had a porosity of about <NUM>% and a tensile strength of about <NUM> MPa.

A sheet having a thickness of about <NUM> was produced in the same manner as in Example <NUM>, except that texanol was used instead of ethylene glycol as the dispersing agent, no solvent was used, and a slurry was used, which was prepared by mixing the nickel with a mixture in which the texanol and ethyl cellulose (EC) as the binder were mixed in a weight ratio (texanol: EC) of about <NUM>:<NUM>, so that the weight ratio (Ni: EC) of the binder and the nickel was about <NUM>:<NUM>. The produced sheet had a porosity of about <NUM>% and a tensile strength of about <NUM> MPa. <FIG> is an SEM photograph of the sheet produced in Example <NUM>.

A sheet having a thickness of about <NUM> was produced in the same manner as in Example <NUM>, except that propylene glycol was used instead of ethylene glycol as the dispersant.

Claim 1:
A method for manufacturing a metal foam comprising steps of: forming a green structure using a slurry consisting of a metal component having a conductive metal with relative magnetic permeability of <NUM> or more or an alloy containing the conductive metal, a dispersant and a binder or consisting of a metal component having a conductive metal with relative magnetic permeability of <NUM> or more or an alloy containing the conductive metal, a dispersant, a binder and a solvent; and sintering the green structure,
wherein the green structure is formed only by using the slurry,
wherein the sintering of the green structure is performed by applying an electromagnetic field to the structure,
wherein the electromagnetic field is applied for a time in a range of <NUM> minutes to <NUM> hours and
wherein the dispersant is a monohydric alcohol having <NUM> to <NUM> carbon atoms, dihydric alcohol having <NUM> to <NUM> carbon atoms or polyhydric alcohol.