Patent Grant
9589732
The present invention relates to porous sintered aluminum (porous aluminum sintered body) and the production method thereof, and the porous sintered aluminum is particularly suitable as a current collector for a lithium-ion secondary battery and an electrical double layer capacitor, and is also suitable as a buffer for joining aluminum alloys having different thermal expansion coefficients.
The present application claims priority on Japanese Patent Application No. 2009-082498 filed on Mar. 30, 2009, and Japanese Patent Application No. 2009-186616 filed on Aug. 11, 2009, the contents of which are incorporated herein by reference.
Recently, aluminum foil has generally been used as a current collector for positive electrodes of a lithium-ion battery and an electrical double layer capacitor. In addition, such a battery and a capacitor have been used in electrical vehicles and the like in recent years, and the electrode with the current collector in the battery and the capacitor has been required to have a higher output and a higher energy density with the broadening of the usage purposes. As described in Patent Documents 1 and 2, porous aluminum body which includes open pores having a three-dimensional network structure has been known as a current collector for an electrode.
As a method for producing this porous aluminum body, a foam-melting method has been known as disclosed in Patent Document 3. In this foam-melting method, a thickener is added to a melted aluminum so as to increase the viscosity, and then titanium hydride as a foaming agent is added thereto. While foaming the melted aluminum by utilizing hydrogen gas generated in a thermal decomposition reaction of the titanium hydride, the melted aluminum is solidified. However, foamed aluminum obtained by this method includes large closed pores having sizes of several millimeters.
There are other methods, and the following method is exemplified as a second method. Aluminum is pressed into a casting mold having a core of sponge urethane, and a hollow cavity formed after burning off the urethane is filled with the aluminum. Thereby, foamed aluminum having a sponge skeleton is obtained. According to this method, foamed aluminum is obtained which includes open pores having pore diameters that fulfill 40 PPI or smaller, that is, 40 cells or less per inch (pore diameters of about 600 μm or larger).
The following method is exemplified as a third method. As disclosed in Patent Document 4, aluminum alloy is infiltrated into a reinforcing material made of hollow ceramics by the pressure infiltration method; and thereby, foamed aluminum is obtained which includes closed pores having pore diameters of 500 μm or smaller in accordance with the dimension of the reinforcing material.
The following method is exemplified as a fourth method. As disclosed in Patent Document 5, a mixed powder of AlSi alloy powder and TiH2 powder is sandwiched between aluminum plate materials, and the mixed powder is heated and rolled in such a state. Thereby, aluminum is foamed due to the decomposition of the TiH2 powder. The foamed aluminum obtained by this method includes pores having large pore diameters of several millimeters.
The following method is exemplified as a fifth method. As disclosed in Patent Document 6, aluminum is mixed with metal of which eutectic temperature with aluminum is lower than the melting point of aluminum, and the mixture is heated at a temperature which is higher than the eutectic temperature and lower than the melting point of aluminum. Foamed aluminum obtained by this method has a porosity of about 40% which is low, although the pore diameters can be reduced by this method. Therefore, in the case where the foamed aluminum is used as a current collector, an amount of cathode active material or anode active material infiltrated into the pores of the foamed aluminum is small, and the desired high output and high energy density cannot be achieved.
Accordingly, among the aforementioned foam-melting method and the second to fifth methods, the second method in which aluminum is pressed into a casting mold having a core of sponge urethane, is employed as a method of producing foamed aluminum including fine open pores which can attain the high output and high energy density.
However, even in the second method, it is necessary to use sponge urethane having a fine microporous structure in order to further reduce the pore diameters of the open pores. In the case where such a sponge urethane is used, the flow of aluminum worsens; and thereby, aluminum cannot be press-filled into the hollow, or the casting pressure becomes excessively high. Therefore, it is difficult to manufacture foamed aluminum which includes pores having pore diameters that fulfill smaller than 40 PPI.
A slurry foaming method is disclosed in Patent Document 7 as a method for producing foamed metal which has a high porosity and includes open pores having small diameters and uniform dimensions, in which a plurality of fine open pores are uniformly arranged. In the slurry foaming method, foamable slurry containing metal powder and a foaming agent is foamed, and dried. Thereafter, the foamed and dried slurry is sintered. According to this method, if a raw material powder which can be sintered is prepared, it is possible to easily manufacture foamed metal which has a high porosity and includes open pores having uniform dimensions and arbitrary pore diameters that fulfill about 10 PPI to about 500 PPI, that is, within a pore diameter range of 2.5 mm to 50 μm. Here, the slurry foaming method means a method for producing foamed metal in which foaming is conducted by containing the foaming agent as described above or foaming is conducted by injecting gas and stirring, and the foamable slurry as described above is sintered in the foamed state.
However, conventionally, it is difficult to manufacture foamed aluminum by the slurry foaming method.
The reason is as follows. In the slurry foaming method, metal powder is sintered by free sintering which is performed without applying stress such as a compression stress or the like; and thereby, foamed metal is obtained. However, the surface of aluminum powder is covered with dense aluminum oxide film having a thickness of several nanometers to several tens of nanometers, and this aluminum oxide film inhibits the sintering regardless of being solid phase sintering or a liquid phase sintering. Therefore, it is difficult to proceed sintering by the free sintering; and as a result, uniform foamed aluminum cannot be obtained by the slurry foaming method.
Therefore, a method can be exemplified which employs a combination of the slurry foaming method and the aforementioned fifth method, as a method for sintering the aluminum powder by the free sintering. According to this method, copper powder is prepared as a metal whose eutectic temperature with aluminum is lower than the melting point of aluminum, and the copper powder and a foaming agent are mixed with aluminum. Then the mixture is heated and sintered at a temperature which is higher than the eutectic temperature and lower than the melting point of aluminum. Thereby, foamed aluminum is obtained. However, liquid droplets of aluminum ooze out of the surface, and the liquid droplets are solidified; and as a result, a plurality of aluminum lumps having semispherical shapes are formed. In particular, in the case where the foamed aluminum has a thin plate shape, the formation of the aluminum lumps occurs remarkably as shown in
On the other hand, a joining method such as brazing method or the like is generally employed when aluminum is joined with aluminum alloy or aluminum alloys having different compositions are joined with each other, such as a case in which an aluminum heat-sink is joined with a power module substrate made of aluminum nitride. However, there are problems in that reliability of joining is lowered and joining strength is degraded due to a thermal stress generated at the time of joining, which is caused due to a difference in thermal expansion coefficient since the thermal expansion coefficient of aluminum differs from that of the aluminum alloy.
As an approach (method) for solving such problems, it is known that in the case where aluminums and aluminum alloys having different compositions are joined, it is effective to join the aluminum and the aluminum alloys by brazing and the like in a state where porous aluminum alloy having an overall porosity of about 10 to 70% is interposed as a buffer therebetween.
Conventionally, a method is widely employed in which a porous aluminum alloy having a eutectic composition such as Al—Cu series or Al—Si series is used as the buffer.
However, even in the case where such a porous aluminum alloy is used, the melting point is lowered due to the generation of a liquid phase. As a result, the thermal resistance at the time of joining is degraded. Therefore, there is a case where the porous aluminum alloy cannot be used in practice depending on the usage conditions (in particular, temperature conditions in which the porous aluminum alloy is used).
[Patent Document 1] Japanese Patent No. 3591055
[Patent Document 2] Japanese Unexamined Patent Application, Publication No. 2009-43536
[Patent Document 3] Japanese Unexamined Patent Application, Publication No. H08-209265
[Patent Document 4] Japanese Unexamined Patent Application, Publication No. 2007-238971
[Patent Document 5] Published Japanese Translation No. 2003-520292 of the PCT International Publication
[Patent Document 6] Japanese Examined Patent Application, Publication No. S61-48566
[Patent Document 7] Japanese Patent No. 3535282
The present invention is made in view of the above circumstances, and the present invention aims to provide a method for producing porous sintered aluminum, and in accordance with this method, it is possible to obtain uniform porous aluminum which has a high porosity and includes open pores having pore diameters that fulfill 40 PPI or greater, that is, pore diameters of 600 μm or smaller, and uniform dimensions. In addition, the present invention also aims to provide porous sintered aluminum, and the porous sintered aluminum can be suitably used as a current collector for a positive electrode of a battery or a capacitor which requires high output and high energy density, and the porous sintered aluminum can also be suitably used as a buffer for joining aluminum alloys having different thermal expansion coefficients.
The present inventors found that there were conditions under which it was possible to perform sintering even by free sintering without generating lumps of liquid droplets if an aluminum powder was mixed with a sintering aid powder containing titanium and the mixture was heated and sintered at a temperature within a predetermined range, and the present inventors completed the present invention.
That is, the method for producing porous sintered aluminum of the present invention includes: mixing aluminum powder with a sintering aid powder containing a sintering aid element to obtain a raw aluminum mixed powder; forming the raw aluminum mixed powder into a formed object prior to sintering having pores; and heating the formed object prior to sintering in a non-oxidizing atmosphere to produce porous sintered aluminum. The sintering aid element is titanium, and when a temperature at which the raw aluminum mixed powder starts to melt is expressed as Tm (° C.), then a temperature T (° C.) of the heating fulfills Tm-10 (° C.)≦T≦685 (° C.).
Here, the non-oxidizing atmosphere means an atmosphere in which raw aluminum mixed powder is not oxidized, and examples thereof include an inert atmosphere, and a reducing atmosphere. In addition, the aforementioned heating temperature is not the temperature of the raw aluminum mixed powder, that is, the heating temperature means not the measured reaction temperature of the raw aluminum mixed powder but the holding temperature of the surrounding around the raw aluminum mixed powder.
In the method for producing porous sintered aluminum of the present invention, an average particle diameter of the aluminum powder may be in a range of 2 to 200 μm.
When an average particle diameter of the sintering aid powder is expressed as r (μm), and a mixing ratio of the sintering aid powder is expressed as W (% by mass), r and W may fulfill 1 (μm)≦r≦30 (μm), 1 (% by mass)≦W≦20 (% by mass), and 0.1≦W/r≦2.
The sintering aid powder may be either one or both of titanium and titanium hydride.
The porous sintered aluminum of the present invention includes aluminum sintered matrix in which Al—Ti compounds are dispersed and distributed. The aluminum sintered matrix is a sintered body produced from aluminum particles, and the Al—Ti compound is Al—Ti compound grains produced by a sintering synthesis reaction between aluminum particles and titanium particles. The Al—Ti compound grains are sintered with aluminum grains of the aluminum sintered matrix, and voids are formed between the aluminum grains of the aluminum sintered matrix, and thereby, an overall porosity is in a range of 10 to 70%.
In accordance with the method for producing porous sintered aluminum of the present invention, the aluminum powder is mixed with the sintering aid powder containing titanium to obtain the raw aluminum mixed powder, and then the raw aluminum mixed powder is heated at the temperature T which fulfills Tm-10 (° C.)≦T≦685 (° C.). Thereby, it is possible to obtain porous sintered aluminum which includes two or more open pores per linear length of 100 μm.
Here, the reason that the heating temperature is limited to not less than Tm-10 (° C.) is because the temperature at which the aluminum powder contained in the raw aluminum mixed powder starts to react with the sintering aid powder containing titanium is Tm-10 (° C.). The melting point of aluminum is described as Tm because industrially used aluminum contains impurities such as iron and silicon and the melting point thereof becomes lower than 660° C., which is the melting point of pure aluminum. On the other hand, the reason that the heating temperature is limited to 685° C. or lower is because aluminum lumps having liquid droplet shapes are generated in the sintered body in the case where the mixture is heated and maintained at a temperature which is higher than 685° C.
The aluminum powder is provided such that the viscous composition has a viscosity at a level at which the composition can be formed into a desired shape and the formed object prior to sintering obtained by drying the viscous composition in a state where air bubbles are mixed therein has a desired handling strength. That is, in the case where the average particle diameter is small, it is necessary to increase the quantity (mass) of water-soluble resin coupler with respect to the quantity (mass) of the aluminum powder so as to secure the viscosity and the handling strength. However, in the case where the quantity (mass) of the water-soluble resin coupler becomes large, the amount of carbon remaining in the aluminum is increased during the formed object prior to sintering is heated, and the remained carbon inhibits the sintering reaction. On the other hand, in the case where the particle diameter of the aluminum powder is excessively large, the strength of the porous sintered body is lowered. Therefore, it is preferable that the average particle diameter of the aluminum powder be in a range of 2 μm or greater; and thereby, the inhibition of the sintering reaction due to the large quantity (mass) of water-soluble resin coupler is prevented. Moreover, it is preferable that the average particle diameter of the aluminum powder be 200 μm or smaller; and thereby, the strength of the porous sintered body is secured. More preferably, the average particle diameter of the aluminum powder is set to be in a range of 7 μm to 40 μm.
With regard to the sintering aid powder, it is preferable that the average particle diameter r (μm) and the mixing ratio W (% by mass) fulfill 1 (μm)≦r≦30 (μm), 0.1 (% by mass)≦W≦20 (% by mass), and 0.1≦W/r≦2.
The reason is as follows. In the case where the mixing ratio W of the sintering aid powder exceeds 20% by mass, sintering aid particles have contacts with each other in the raw aluminum mixed powder; and thereby, the reaction heat between aluminum and titanium cannot be controlled, and a desired porous sintered body cannot be obtained. Therefore, it is preferable to fulfill 0.1 (% by mass)≦W≦20 (% by mass). In addition, it is more preferable to fulfill 1 (% by mass)≦W≦20 (% by mass).
Even in the case where the mixing ratio fulfills 0.1 (% by mass)≦W≦20 (% by mass), the reaction heat between aluminum and titanium became excessively high in some cases depending on the particle diameter of the sintering aid powder. In these cases, the temperature of melted aluminum due to the reaction heat further rose; and thereby, the viscosity thereof was lowered. As a result, liquid droplets were generated in some cases.
In view of these, test pieces were manufactured under various conditions, and the test pieces were observed by an electron microscope. As a result of the observation, it was found that only a surface layer portion having a substantially constant thickness from the exposed surface side of the titanium particle reacted with aluminum in the case where the amount of heat generation was controlled to be in a range controllable by the mixing ratio of titanium and the particle diameter of titanium. From the experimental results, it was found that the conditions of 1 (μm)≦r≦30 (μm) and 0.1 (% by mass)≦W/r≦2 (% by mass) are preferable in order to prevent the occurrence of liquid droplets.
Hereinafter, the meaning of 0.1≦W/r≦2 in the case of using titanium as the sintering aid powder will be described. When the average particle diameter of titanium is expressed as r, the number of titanium particles is expressed as N, the additive quantity (mass) of titanium is expressed as w, the specific weight of titanium is expressed as D, and the reduction amount in the titanium particle diameter due to the reaction with aluminum is expressed as d, the reaction heat amount Q fulfills Q∝4πr2dN since the reaction heat amount Q is proportional to the volume of reacted titanium. Moreover, since the additive amount of the titanium particles is calculated as a product of the average volume of one titanium particle and the number of titanium particles, w=4/3πr3DN is obtained. Accordingly, if the latter equation is substituted into the former equation, Q∝3wd/rD is obtained. Here, Q∝w/r is further obtained based on the fact that 3/D is a constant and the observation result that d is substantially constant regardless of the sintering conditions. Therefore, the range of W/r in which the liquid droplets are not generated is experimentally determined and the range is limited as described above. Thereby, the generation of liquid droplets due to the excessively high reaction heat between aluminum and titanium is prevented.
In addition, the titanium hydride as the sintering aid powder contains titanium at a content of 95% by mass or greater, and dehydrogenation of the titanium hydride occurs at a temperature of 470 to 530° C. to convert into titanium. Therefore, the titanium hydride is thermally decomposed into titanium by the aforementioned heating. Accordingly, it is possible to enhance the reaction efficiency with the aluminum powder by using titanium and/or titanium hydride as the sintering aid powder.
Moreover, it is possible to obtain uniform foamed porous sintered aluminum which has a high porosity and includes fine open pores having pore diameters of smaller than 600 μm and uniform dimensions by a combined method of the method for producing porous sintered aluminum according to the present invention and the known slurry foaming method.
That is, according to this method, at first, the raw aluminum mixed powder is mixed with a water-soluble resin coupler, water, and a plasticizer containing at least one selected from polyalcohols, ethers, and esters to obtain a viscous composition, and the viscous composition is dried in a state where air bubbles are mixed thereinto to obtain a formed object prior to sintering. Then, this formed object prior to sintering is heated under the aforementioned conditions to manufacture porous sintered aluminum. Thereby, it is possible to obtain a porous body which includes two different air holes (pores) including air holes which are surrounded by sponge skeletons formed by the slurry foaming method and air holes formed in the sponge skeleton itself by the sintering method of the present invention.
In this method, the viscous composition can be foamed by the addition of non-water-soluble hydrocarbon system organic solvent containing 5 to 8 carbons; and thereby, air bubbles can be mixed into the viscous composition.
In the case where the contained amount of the water-soluble coupler exceeds 7% of the quantity (mass) of the raw aluminum mixed powder, the amount of carbon remaining in the formed object prior to sintering is increased during heating, and the remained carbon inhibits the sintering reaction. On the other hand, in the case where the contained amount of the water-soluble coupler is less than 0.5%, it is not possible to secure the handling strength of the formed object prior to sintering. Therefore, it is preferable that the water-soluble coupler be contained at a content in a range of 0.5% to 7% of the quantity (mass) of the raw aluminum mixed powder.
In addition, it is possible to effectively generate air bubbles by adding a surfactant to the raw aluminum mixed powder. In the case where the added amount of this surfactant is set to be in a range of 0.02% or greater of the quantity (mass) of the raw aluminum mixed powder, it is possible to achieve an effect due to the addition of the surfactant. In the case where the added amount of the surfactant is set to be in a range of 3% or smaller of the quantity (mass) of the raw aluminum mixed powder, it is also possible to prevent the inhibition of the sintering reaction due to the increased amount of carbon remaining in the formed object prior to sintering.
In the case where the viscous composition is extended to have a thickness of 0.05 mm to 5 mm to form a plate-shaped formed object prior to sintering and this plate-shaped formed object is sintered, it is possible to obtain porous sintered aluminum having an overall porosity of 70 to 90% which is suitable as a current collector for a lithium-ion secondary battery or an electrical double layer capacitor.
On the other hand, in the case where the method for producing porous sintered aluminum of the present invention is not combined with the known slurry foaming method, and a mixture of the aluminum powder and the sintering aid powder containing the sintering aid element is heated at the heating temperature T (° C.) in a non-oxidizing atmosphere, it is possible to obtain porous sintered aluminum having an overall porosity of 10 to 70% as in the present invention because air bubbles are mixed thereinto when the aluminum powder is mixed with the sintering aid powder containing the sintering aid element.
In the obtained porous sintered aluminum, since titanium particles are used as the sintering aid agent, the titanium particles exhibit a sintering promotion effect in the sintering reaction with the aluminum particles. In addition, the titanium particles are sintered with aluminum grains of the aluminum sintered matrix; and thereby, Al—Ti compound is formed as a reactant.
The melting point of the Al—Ti compound grains is high (about 1350° C.), and the overall porosity of the porous sintered aluminum is in a range of 10 to 70%. Therefore, in the case where the aforementioned aluminums or aluminum alloys having different compositions are joined by brazing or the like, the porous sintered aluminum can be suitably used as a buffer therebetween.
Hereinafter, the method for producing porous sintered aluminum according to the present invention will be described.
The method for producing aluminum of the present embodiment includes the following steps:
a process of providing raw aluminum mixed powder in which aluminum power is mixed with titanium and/or titanium hydride to obtain raw aluminum mixed powder;
a process of providing a viscous composition in which the raw aluminum mixed powder is mixed with water-soluble resin coupler, water, and the like to provide a slurry viscous composition containing air bubbles;
a process prior to sintering in which the viscous composition is dried to obtain a formed object prior to sintering; and
a sintering process in which the formed object prior to sintering is heated at a temperature that fulfills Tm-10 (° C.)≦heating temperature T≦685 (° C.) in a non-oxidizing atmosphere.
Here, Tm (° C.) represents the temperature at which the raw aluminum mixed powder starts to melt.
In the process of providing the raw aluminum mixed powder, an aluminum powder having an average particle diameter of 2 to 200 μm is used. The reason is as follows. In the case where the average particle diameter is small, it is necessary to add a large amount of water-soluble resin coupler to the aluminum powder in order that the viscous composition has a viscosity at which the viscous composition can be formed in to a desired shape and the formed object prior to sintering has a handling strength. However, in the case where a large amount of water-soluble resin coupler is added, an amount of carbon remaining in the aluminum is increased when the formed object prior to sintering is heated, and the remained carbon inhibits the sintering reaction. On the other hand, in the case where the particle diameter of the aluminum powder is excessively large, the strength of the porous sintered aluminum is lowered. Accordingly, as described above, the aluminum powder having the average particle diameter in a range of 2 to 200 μm is used, and the average particle diameter is more preferably in a range of 7 to 40 μm.
The aluminum powder is mixed with titanium and/or titanium hydride. The reason is as follows. In the case where the aluminum powder is mixed with titanium and the formed object prior to sintering is heated at the heating temperature T which fulfills Tm-10 (° C.)≦heating temperature T≦685 (° C.), it is possible to perform free sintering of aluminum without generating lumps of liquid droplets. In addition, titanium hydride (TiH2) contains titanium at a content of 47.88 (molecular weight of titanium)/(47.88+1 (molecular weight of hydrogen)×2), which is 95% by mass or greater, and dehydrogenation of the titanium hydride occurs at a temperature of 470 to 530° C. to convert into titanium. Therefore, the titanium hydride is thermally decomposed into titanium by the aforementioned heating. Accordingly, it is possible to perform free sintering of aluminum without generating lumps of liquid droplets even in the case where the titanium hydride is mixed thereinto.
Here, when the average particle diameter of titanium or titanium hydride is expressed as r (μm), and the mixing ratio of titanium or titanium hydride is expressed as W (% by mass), the following equations are fulfilled: 1 (μm)≦r≦30 (μm), 0.1 (% by mass)≦W≦20 (% by mass), and 0.1≦W/r≦2. For example, in the case where the titanium hydride powder has an average particle diameter of 4 μm, 0.1≦W/4≦2 is to be fulfilled; and therefore, the mixing ratio W becomes in a range of 0.4 to 8% by mass. In the case where the titanium powder has an average particle diameter of 20 μm, 0.1≦W/20≦2 is to be fulfilled; and therefore, the mixing ratio W becomes in a range of 2 to 40% by mass. However, since 0.1 (% by mass)≦W≦20 (% by mass) is to be fulfilled, the mixing ratio becomes in a range of 2 to 20% by mass.
The average diameter of titanium hydride is set to fulfill 0.1 (μm)≦r≦30 (μm), the average diameter is preferably set to fulfill 1 (μm)≦r≦30 (μm) and is more preferably set to fulfill 4 (μm)≦r≦20 (μm). The reason is as follows. In the case where the average diameter is 1 μm or smaller, there is a concern of spontaneous combustion. After sintering, the titanium hydride becomes titanium grains covered with a compound of aluminum and titanium. In the case where the average particle diameter exceeds 30 μm, the compound phase of aluminum and titanium is easily peeled off from the titanium grains; and thereby, a desired strength of the sintered body cannot be obtained.
The reason that 0.1 (% by mass)≦W≦20 (% by mass) is set is as follows. In the case where the mixing ratio W of the sintering aid powder exceeds 20% by mass, the sintering aid particles contact with each other in the raw aluminum mixed powder; and thereby, the reaction heat between aluminum and titanium cannot be controlled, and a desired porous sintered body cannot be obtained.
Even in the case where the mixing ratio fulfills 0.1 (% by mass)≦W≦20 (% by mass), the reaction heat between aluminum and titanium became excessively high in some cases depending on the particle diameter of the sintering aid powder. In these cases, the temperature of melted aluminum due to the reaction heat further rose; and thereby, the viscosity thereof was lowered. As a result, liquid droplets were generated in some cases.
In view of these, test pieces were manufactured under various conditions, and the test pieces were observed by an electron microscope. As a result of the observation, it was found that only a surface layer portion having a substantially constant thickness from the exposed surface side of the titanium particle reacted with aluminum in the case where the amount of heat generation was controlled to be in a range controllable by the mixing ratio of titanium and the particle diameter of titanium. From the experimental results, it was found that the conditions of 1 (μm)≦r≦30 (μm) and 0.1 (% by mass)≦W/r≦2 (% by mass) are preferable in order to prevent the occurrence of liquid droplets.
Hereinafter, the meaning of 0.1≦W/r≦2 in the case of using titanium as the sintering aid powder will be described. When the average particle diameter of titanium is expressed as r, the number of titanium particles is expressed as N, the additive quantity (mass) of titanium is expressed as w, the specific weight of titanium is expressed as D, and the reduction amount in the titanium particle diameter due to the reaction with aluminum is expressed as d, the reaction heat amount Q fulfills Q∝4πr2dN since the reaction heat amount Q is proportional to the volume of reacted titanium. Moreover, since the additive amount of the titanium particles is calculated as a product of the average volume of one titanium particle and the number of titanium particles, w=4/3πr3DN is obtained. Accordingly, if the latter equation is substituted into the former equation, Q∝3wd/rD is obtained. Here, Q∝w/r is further obtained based on the fact that 3/D is a constant and the observation result that d is substantially constant regardless of the sintering conditions. Therefore, the range of W/r in which the liquid droplets are not generated is experimentally determined and the range is limited as described above. Thereby, the generation of liquid droplets due to the excessively high reaction heat between aluminum and titanium is prevented.
In order to produce foamed aluminum having an overall porosity of 70 to 90% which is suitably used as a current collector of a lithium-ion secondary battery or an electrical double layer capacitor, the following components are added to the raw aluminum mixed powder in the process of providing the viscous composition: at least one kind selected from polyvinyl alcohol, methylcellulose, and ethylcellulose as a water-soluble resin coupler; at least one kind selected from polyethyleneglycol, glycerin, and di-N-buthyl phthalate as a plasticizer; distillated water; and alkylbetaine as a surfactant.
In the case where at least one kind selected from polyvinyl alcohol, methylcellulose, and ethylcellulose is used as the water-soluble resin coupler, a relatively small additive amount is sufficient. Therefore, the additive amount (ratio) thereof is set to be in a range of 0.5% to 7% of the quantity (mass) of the raw aluminum mixed powder. In the case where the additive amount of the water-soluble resin coupler exceeds 7% of the quantity (mass) of the raw aluminum mixed powder, the amount of carbon remaining in the formed object prior to sintering is increased during heating, and the remained carbon inhibits the sintering reaction. In the case where the additive amount of the water-soluble resin coupler is less than 0.5%, the handling strength of the formed object prior to sintering cannot be secured.
Alkylbetain is added at an amount (ratio) of 0.02% to 3% of the quantity (mass) of the raw aluminum mixed powder. In the case where the amount (containing ratio) is set to be 0.02% or higher of the quantity (mass) of the raw aluminum mixed powder, air bubbles are effectively generated during mixing a non-water-soluble hydrocarbon system organic solvent which will be described later. By setting the amount (containing ratio) to be 3% or lower, the inhibition of the sintering reaction due to the increased amount of carbon remaining in the formed object prior to sintering can be prevented.
After kneading the mixture, foaming is performed by further mixing the non-water-soluble hydrocarbon system organic solvent containing 5 to 8 carbons; and thereby, a viscous composition including air bubbles mixed thereinto is prepared. As the non-water-soluble hydrocarbon system organic solvent containing 5 to 8 carbons, at least one kind selected from pentane, hexane, heptane, and octane can be used.
On the other hand, in order to produce porous sintered aluminum having an overall porosity of 10 to 70% which is suitably used as a buffer for joining aluminums or aluminum alloys having different compositions, at least the water-soluble resin coupler and the distillated water are added to the raw aluminum mixed powder to obtain slurry viscous composition in the process of providing viscous composition. At this time, only air bubbles which are mixed thereinto during mixing are contained in the slurry viscous composition, and the mixture is not positively foamed for containing air bubbles in this process.
Next, in both of the aforementioned cases, in the step prior to sintering, a strip-shaped polyethylene sheet is prepared of which the surface is coated with separating compound, and the viscous composition is extended to have a thickness of 0.05 mm to 5 mm by coating the viscous composition on the surface of the strip-shaped polyethylene sheet. Then, the temperature and moisture of the surrounding (circumferential atmosphere) are controlled for a specific time period so as to make the dimensions of air bubbles uniform. Thereafter, the resulting object is dried at a temperature of 70° C. by an air dryer. Here, the viscous composition is coated by a doctor blade method, a slurry extrusion method, or a screen printing method.
The dried viscous composition is peeled off from the polyethylene sheet, and then, if necessary, the dried viscous composition is cut out into a predetermined shape such as a circle having a diameter of 100 mm. Thereby, the formed object prior to sintering is obtained.
Next, in the sintering process, zirconia spinkle powder is spread on an alumina setter, and the formed object prior to sintering is placed on the alumina setter. Then, pre-sintering is performed by holding the formed object prior to sintering at 520° C. for one hour in an argon atmosphere whose dew point is −20° C. or lower. Thereby, the water-soluble resin coupler, and if added, a binder solution of the plasticizer component, the distillated water, and alkylbetaine are evaporated (removal of binder). In addition, dehydrogenation proceeds in the case where titanium hydride is used as the sintering aid powder.
Thereafter, the formed object prior to sintering which is pre-sintered is heated at a heating temperature T which fulfills Tm-10 (° C.) heating temperature T≦685 (° C.) to obtain foamed aluminum.
This is based on the following reason. It is considered that the reaction between aluminum and titanium starts by heating the formed object prior to sintering up to the melting temperature Tm (° C.). However, aluminum contains a very small amount of eutectic alloy elements such as Fe, Si, and the like as impurities in practice; and thereby, the melting point thereof is lowered. Therefore, it is considered that the reaction between aluminum and titanium starts by heating up to Tm-10 (° C.) and foamed aluminum is formed. In practice, the melting point of aluminum is 660° C.; however, the melting start temperature of an atomized powder having a purity of about 98% to 99.7%, which is marketed as a pure aluminum powder, is about 650° C.
On the other hand, in the case where the temperature reaches 665° C. which is the peritectic temperature of aluminum and titanium, and the melting latent heat is further input, the sintered aluminum (aluminum sintered body) is melted. Therefore, it is necessary to keep the temperature of a furnace atmosphere in a range of 685° C. or lower.
In addition, it is necessary to perform the heating of the sintering process in a non-oxidizing atmosphere in order to suppress the growth of oxide layers on the aluminum particle surface and the titanium particle surface. However, the oxide layers on the aluminum particle surface and the titanium particle surface do not remarkably grow even in the case of heating in the air under the conditions where the heating temperature is 400° C. or lower and the holding time is about 30 minutes. Therefore, the formed object prior to sintering may be heated and held at a temperature in a range of 300° C. to 400° C. for about 10 minutes in the air (removal of binder), and then the formed object prior to sintering may be heated at a predetermined temperature in an argon atmosphere.
The thus obtained foamed aluminum includes metal skeletons having a three-dimensional network structure of perforated sintered metal (perforated metal sintered body), and pores are included between the metal skeletons. In addition, Al—Ti compound is dispersed in the perforated sintered metal, 20 or more pores are formed per linear length of 1 cm, and the foamed aluminum has an overall porosity of 70 to 90% and is suitably used as a current collector of a lithium-ion secondary battery or an electrical double layer capacitor.
In the case where the formed object prior to sintering is sintered in a state where only air bubbles mixed during the mixing are included without the addition of foaming agent, the aluminum grains of the aluminum sintered matrix are sintered with the Al—Ti compound grains in the porous sintered aluminum. Moreover, air gap (voids) are formed between aluminum grains of the aluminum sintered matrix; and thereby, the overall porosity becomes in a range of 10 to 70%. Therefore, the porous sintered aluminum is suitably used as a buffer which is interposed between aluminums or aluminum alloys having different compositions for joining them.
The present invention is not limited to the aforementioned embodiments, and a sintering aid powder other than titanium and titanium hydride may be used as long as a sintering aid powder which contains titanium as a sintering aid element is used.
Al powders having average particle diameters of 2.1 μm, 9.4 μm, 24 μm, 87 μm, and 175 μm, Ti powders having average particle diameters of 9.8 μm, 24 μm, and 42 μm, and TiH2 powders having average particle diameters of 4.2 μm, 9.1 μm, and 21 μm were prepared. Then, in accordance with the aforementioned embodiment, the Al powder was mixed with the Ti powder and/or the TiH2 powder at the ratios shown in Table 1 to prepare raw aluminum mixed powders 1 to 10, and binder solutions 1 to 5 having the compounding compositions shown in Table 2 were prepared. They were kneaded with a non-water-soluble hydrocarbon system organic solvent at the ratios shown in Table 3 to manufacture viscous compositions of Examples 1 to 16.
220*1
25*2
Next, polyethylene sheets were prepared of which the surfaces were coated with separating compound, and the viscous compositions of Examples 1 to 16 were coated and extended on the surface of the polyethylene sheet by the doctor blade method, and the temperature and the moisture were controlled to be predetermined values for a specific time period so as to adjust the dimensions of air bubbles uniformly. Then, the viscous compositions were dried at 70° C. in an air dryer. The coating thicknesses of the viscous compositions, temperatures, moistures, and holding times at that time are shown in Table 3. Thereafter, the dried viscous compositions were peeled off from the polyethylene sheets and cut out into circular shapes having diameters of 100 mm to obtain formed objects prior to sintering in Examples 1 to 16.
Then, zirconia spinkle powder was spread on an alumina setter, and the formed objects prior to sintering in Examples 1 to 16 were placed on the alumina setter. The formed objects prior to sintering in Examples 1 to 16 were subjected to debinding in an atmosphere where argon flowed or in air. Thereafter, the formed objects prior to sintering in Examples 1 to 16 were heated to obtain foamed aluminums. The heating temperatures and heating holding times are also shown in Table 3.
Next, the contraction percentages and porosities of the obtained foamed aluminums in Example 1 to 16 were calculated. In addition, the number of three-dimensional pores was measured in a stereoscopic microscope photograph, and the number of pores in the skeletons was measured in a scanning electron microscope (SEM) photograph. The obtained SEM photograph was observed to confirm whether solidification of liquid droplets occurred. Moreover, surface analyses were conducted by an electron probe microanalyzer (EPMA) to confirm whether Al—Ti compound existed on the surface of the skeletons of the foamed aluminums. The results are shown in Table 5, the SEM photograph of the foamed aluminums in Example 1 is shown in
Next, rolling extension tests were performed on the foamed aluminums in Examples 1 to 16 at a rolling reduction rate of 20%, and whether cracking occurred was visually confirmed. Thereafter, rectangular samples having dimensions of 20 mm×50 mm were cut out from the foamed aluminums, and the electrical resistances between opposed corners were measured. Then, the rectangular samples of the foamed aluminums were wound around an outer circumference of a cylindrical object having a diameter of 5 mm, and whether cracking occurred was visually confirmed. The results are shown in Table 5.
Comparative raw aluminum mixed powders 31 to 35 were prepared by using the same Al powder, Ti powder, and TiH2 powder as those in Examples. Either one of the comparative raw aluminum mixed powders 31 to 35 and the raw aluminum mixed powder 1 of the present invention was mixed and kneaded with either one of the binder solutions 1 to 5 shown in Table 2 and the non-water-soluble hydrocarbon system organic solvent at the mixing ratios shown in Table 4. Other conditions were same as those in Examples. Thereby, foamed aluminums in Comparative Examples 1 to 9 were produced. The foamed aluminums in Comparative Examples 1 to 9 were evaluated by the same methods as those for Examples, The evaluation results are shown in Table 5, and an SEM photograph of the foamed aluminum in Comparative Example 1 is shown in
As can be understood from Table 5, with regard to the foamed aluminums in Examples 1 to 16, the numbers of pores per skeleton length of 100 μm of the perforated sintered metals were in a range of 2 to 4, and the numbers of three-dimensional pores per one inch were in a range of 52 or more, that is, the numbers of the three-dimensional pores per one centimeter in the metal skeletons were in a range of 20 or more. In addition, no lumps of liquid droplets were generated in the foamed aluminums, the electrical resistances were low, and no cracking due to the winding test were observed. Accordingly, the foamed aluminums in Examples 1 to 16 are suitable as a current collector for a positive electrode of a battery or a capacitor which requires high output and high energy density.
Next, a lithium cobalt oxide (LiCoO2) powder as an active material, polyvinylidene fluoride (PVdE) as a coupler, artificial graphite powder as a conductive material were mixed at a ratio by weight of 86:6:8 to prepare a cathode material. N-methyl-2 pyrrolidone as a solvent was mixed with the cathode material to prepare a cathode active material slurry.
Then, the foamed aluminums in Examples 1 to 16 and foamed aluminum in Conventional Example 1 were immersed into this cathode active material slurry for 10 minutes. The foamed aluminums were taken therefrom, and dried. Thereafter, the foamed aluminums were rolled to produce cathodes of lithium-ion batteries in Examples 1 to 16 having thicknesses of 0.5 mm.
Here, as the foamed aluminum in Conventional Example 1, foamed aluminum of 30 PPI was used. The foamed aluminum was produced by a method of pressing aluminum into a casting mold having a core of sponge urethane which is mentioned as the second method in the related art. In addition, the filling densities of the cathode active materials of the foamed aluminum in Examples 1 to 16 and the foamed aluminum in Conventional Example 1 are shown in Table 5.
Then, cylindrical objects having diameters of 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, and 5 mm were respectively prepared. The cathodes of lithium-ion batteries in Examples 1 to 16 and Conventional Example 1 were wound. Whether or not the active materials were peeled off was visually observed, and the minimum diameters with which peeling were not observed are shown in Table 5.
As can be understood from the results in Table 5, with regard to the cathodes of the lithium-ion batteries in Examples 1 to 16, the active materials were not peeled off even in the case where the cathodes were wound around the cylindrical objects having diameters of 1.5 mm to 2.5 mm. On the other hand, with regard to the cathode in Conventional Example 1, the active material was peeled off when the cathode was wound around the cylindrical object having a diameter of 3 mm. In addition, the active material filling density of the cathode of the lithium-ion batteries in Examples 1 to 16 were in a range of 4.1 g/cm3 or greater. In contrast, the active material filling density of the cathode in Conventional Example 1 was 3.841 g/cm3, which was small.
Next, kneading was performed without adding the non-water-soluble hydrocarbon system organic solvent shown in Table 3 to prepare a composition, and the composition was coated and extended on a surface of a polyethylene sheet by the doctor blade method. Here, the surface of the polyethylene sheet was coated with separating compound in advance. Then, the coated composition was dried. Thereafter, the composition was peeled off from the polyethylene sheet to obtain a formed object prior to sintering. Zirconia spinkle powder was spread on an alumina setter, and the formed objects prior to sintering was placed on the alumina setter. The formed object prior to sintering was subjected to debinding in an atmosphere where argon flowed or in air. Thereafter, the formed object prior to sintering was heated to obtain porous sintered aluminum.
Then, a thermal analysis (TG/DTA) was performed in order to measure the melting point of the porous sintered aluminum. In addition, the same thermal analysis (TG/DTA) was also performed on Al—Si series porous sintered aluminum and Al—Cu series porous sintered aluminum as comparative examples.
In this example, porous sintered aluminums which were produced by adding Ti, Si, or Cu at the same amount of 1.0% by weight to Al were used.
As can be seen in
Accordingly, it can be understood that Al—Ti porous sintered body according to the present invention has an excellent heat resistance. Therefore, it is possible to remarkably enhance the reliability of joining between aluminums or aluminum alloys having different compositions by using the Al—Ti porous sintered body as a buffer therebetween.
The present invention can be applied as a method for producing a current collector of a lithium-ion secondary battery or an electrical double layer capacitor, or a buffer for joining aluminum alloys having different thermal expansion coefficients as well as a method for producing foamed aluminum.
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