Patent Application
20100282429
The present invention relates to a composite metal material produced using a carbon nanomaterial as a reinforcing material.
The use of special carbon fibers referred to as carbon nanomaterials as reinforcing materials has received a great deal of attention in recent years, and various activation methods have been offered. Carbon nanofiber (CNF) which is a typical example of a carbon nanomaterial is a material in which a sheet of carbon atoms arranged in a hexahedral lattice is wound in the form of a tube. The material is referred to as a carbon nanofiber (or carbon nanotube) because the diameter is 1.0 to 150 nm (nanometers). The length is from a few micrometers to 100 μm.
When matrix metal stock is reinforced with carbon nanomaterial, it is necessary for the carbon nanomaterial to be dispersed uniformly in the matrix metal stock. This type of dispersion technique is known, for example, in Japanese Patent Application Laid-Open Publication No. 2006-265686 (JP 2006-265686 A).
A method for producing a nickel (Ni)/carbon nanotube (CNT) composite sintered body using the dispersion technology disclosed in JP 2006-265686 A referred to above is described in reference to the flow chart shown in
In step (“ST” below) 101, the carbon nanotubes (CNTs), dispersant (sodium dodecyl sulfate), and solvent (purified water) are prepared, and the materials are combined and stirred/mixed for 1 h using ultrasound (ST102). In addition, in ST103, nickel (Ni) powder, dispersant (ammonium polyacrylate), binder (polyvinyl alcohol), and solvent (purified water) are prepared, the materials are combined, and stirring/mixing is carried out for 1 h using ultrasound (ST104).
The CNT suspension obtained in ST102 and the Ni slurry obtained in ST104 are combined, stirred/mixed by ultrasonication (ST105), and then heated to 80° C. and aggregated (ST106) to obtain an Ni/CT mixed slurry (ST107).
Next, the Ni/CNT mixed slurry is dried in two stages and compressed (ST108) to obtain a green molded body (ST109).
The resulting green molded body is defatted for 30 h (ST110) and then a sintering treatment is carried out under compression in a vacuum (ST111). An Ni/CNT composite sintered body is thereby obtained (ST112). With this technology, a favorable CNT dispersion will be produced based on inspection of the resulting Ni/CNT composite sintered body using a microscope.
The following conclusions regarding the above conventional technique were confirmed by the inventors of the present invention.
Firstly, production costs are high due to the necessity of a defatting treatment step (ST110) that lasts as long as 30 h.
Secondly, although dispersibility is favorable, the increase in strength is not as great as expected.
Specifically, the conventional technology has room for improvement in regard to production costs and strength increase.
An object of the present invention is to offer a production technology whereby strength can be improved while production costs can be reduced.
The inventors of the present invention conjectured that a lack of adhesion between the CNF and matrix (Ni) is the reason that strength is not improved as much as expected, in spite of the favorable dispersion. When adhesion is insufficient, slipping between the CNF and matrix occurs when the composite deforms under external force, resulting in a loss of the CNF strengthening action.
Thus, investigations were carried out in light of the idea that increasing adhesion between CNF and the matrix is effective in addition to CNF dispersion. Sufficient results were thus obtained.
According to one aspect of the present invention, there is provided a method for producing a carbon nanocomposite metal material, which comprises the steps of: preparing a matrix metal stock and a microparticulate-coated carbon nanomaterial, obtained by bonding microparticles having an element that reacts with carbon to generate a compound, to the entire surface of a carbon nanomaterial; mixing the microparticulate-coated carbon nanomaterial and the matrix metal stock; pre-molding by packing the resulting mixture; heating the resulting preform to a temperature that is at or above the melting point of the matrix metal stock in a vacuum, inert gas, or non-oxidative gas atmosphere, and maintaining the heating temperature for a set time period; compacting the resulting heat-treated body by performing cooling to a temperature that allows hot working of the matrix metal stock and performing compression for a prescribed time period at this temperature; and cooling the resulting compacted body.
In this manner, according to the production method of the present invention, a microparticulate-coated carbon nanomaterial in which microparticles containing an element that reacts with carbon to generate a compound are bonded to the entire surface of the carbon nanomaterial was selected as the starting material.
For example, when a carbon nanomaterial is mixed directly with the matrix metal stock, the carbon nanomaterial coagulates and dispersion suffers. In order to solve this problem, a dispersant has conventionally been added.
With the microparticulate-coated carbon nanomaterial used in the present invention, the surface microparticles exhibit a separating action, and thus a dispersant is not necessary. Because dispersant is not necessary, the defatting treatment step becomes unnecessary, allowing production costs to decrease.
When a preform produced by mixing and packing microparticulate-coated carbon nanomaterial and matrix metal stock is heated to a temperature that is at or above the melting point of the matrix metal stock and then left for a set time period, the melted microparticulate-coated carbon nanomaterial infuses into the matrix metal stock.
When, in this state, the temperature is reduced to a temperature at which hot-working is possible, and a compacting treatment is carried out, the carbon nanomaterial and matrix metal are tightly bonded via the microparticles, so the strength of the composite metal material can be greatly increased.
The temperature at which hot-working is possible is made as high as possible. When this is done, compacting can be carried out with low compressive force, and there is no concern regarding constraints such as the mold.
At temperatures that are lower than the temperature at which hot-working is possible, undesirable effects such as poor workability and cracking occur, and thus the compaction treatment is difficult. At high temperatures that are above the temperature at which hot-working is possible, a liquid phase state is produced, and leakage of the liquid phase occurs due to compression, so the compression force becomes less effective, making compaction difficult.
In the cooling step described above, it is preferred that the compacted body described above be cooled during compression. At the time of cooling, stresses will arise in the carbon nanocomposite metal material due to differences in the rate of cooling. In the present invention, the generation of stress is suppressed by compression. As a result, it is possible to obtain a well-formed carbon nanocomposite metal material.
In the cooling step described above, an extrusion molding step is carried out whereby the carbon nanocomposite metal material is extruded and molded. Because the carbon nanocomposite metal material is extrusion molded, the orientation of the carbon nanomaterial is increased, and a carbon nanocomposite metal material can be obtained that has superior mechanical strength, such as tensile strength.
It is preferred that the prepared microparticulate-coated carbon nanomaterial be produced by a mixed body formation step in which carbon nanomaterial and carbide-forming microparticles are mixed to obtain a mixed body; and a vacuum vapor deposition step in which the aforementioned carbide-forming microparticles are evaporated under high temperature and vacuum and are made to deposit on the surface of the aforementioned carbon nanomaterial. Consequently, the carbide-forming microparticles are uniformly bonded to the surface of the carbon nanomaterial, because the carbide-forming microparticles are evaporated under high temperature and vacuum and are bonded to the surface of the carbon nanomaterial.
In the mixed body formation step, it is preferred that the organic solvent, the carbide-forming microparticles, and the carbon nanomaterial be introduced into the mixing container, stirred, and dried. Consequently, coagulation of the carbon nanomaterial can be prevented by the organic solvent. The dispersed carbon nanomaterial can thus be coated with carbide-forming microparticles.
It is preferred that carbide-forming microparticles of Si or Ti be used. Both Si and Ti are metals that have a melting point that allows vacuum vapor deposition, and their wettability with respect to molten matrix metal is also favorable. Si and Ti are both readily procured, and Si is particularly inexpensive and thus desirable from the standpoint of proliferation of the method of the present invention.
The matrix metal stock referred to above is preferably Mg or an Mg alloy. In the production method of the present invention, treatment is carried out in a vacuum, and Mg and Mg alloy, which are susceptible to oxygen, can both be treated. Mg and Mg alloy are light metals, and because inclusion of carbon nanomaterial in these metals increases mechanical strength, a structural material that is light and strong can be provided which also has superior thermal transfer properties and abrasion resistance.
According to another aspect of the present invention, there is provided a method for producing a carbon nanocomposite molded article, which comprises the steps of: preparing a matrix metal stock and a microparticulate-coated carbon nanomaterial, obtained by bonding microparticles having an element that reacts with carbon to generate a compound, to the entire surface of a carbon nanomaterial; mixing the microparticulate-coated carbon nanomaterial and the matrix metal stock; pre-molding by packing the resulting mixture; heating the resulting preform to a temperature that is at or above the melting point of the matrix metal stock in a vacuum, inert gas, or non-oxidative gas atmosphere, and maintaining the heating temperature for a set time period; compacting the resulting heat-treated body by performing cooling to a temperature that allows hot working of the matrix metal stock and performing compression for a prescribed time period at this temperature; cooling the resulting compacted body; and die-casting the carbon nanocomposite metal material obtained after the cooling step.
In the carbon nanocomposite metal material produced by the production method for carbon nanocomposite metal materials, carbon nanomaterial is uniformly dispersed. By providing a material having this type of uniformly mixed condition and carrying out die cast molding, molding of molded articles having complex shapes can be readily carried out, and a composite metal molded article with high mechanical strength can be produced.
Certain preferred embodiments of the present invention will be described in detail below, by way of example only with reference to the accompanying drawings, in which:
a) to 1(d) are schematic views showing the mixed body formation step and vacuum vapor deposition step of the present invention;
a) to 4(c) are schematic views showing the preparation step, the mixing step, and the preform step of the present invention;
a) to 7(c) are schematic views showing extrusion molding;
As shown in
As shown in
As shown in
In the vacuum furnace 20, heating is carried out for 20 h at 1200° C. in a vacuum. By heating in a vacuum, the Si powder in the mixed body 15 is evaporated. The evaporated Si contacts the surface of the carbon nanomaterial forming compounds, and the material is bonded as Si microparticles.
The structure of the resulting microparticulate-coated carbon nanomaterial is described in reference to
With the microparticulate-coated carbon nanomaterial 30, the entire surface of the carbon nanomaterial 13 is coated with a layer of carbide-forming microparticles 31 (microparticles containing an element that reacts with carbon to form a compound over the entire surface).
Because carbide-forming microparticles are bonded to the surface of the carbon nanomaterial 13, an SiC reaction layer, for example, is formed at the interface, and the carbide-forming microparticle layer 31 is tightly bonded to the carbon nanomaterial 13. Consequently, there is no concern regarding release of the carbide-forming microparticle layer 31 from the carbon nanomaterial 13. In addition, the carbide-forming microparticle layer 31 has additionally improved wettability with respect to the matrix metal relative to the carbon nanomaterial 13.
a), 4(b) and 4(c) show the preparation step, mixing step, and preforming step.
In the preparation step of
In the mixing step of
In the preforming step of
The treatment apparatus 50 is composed of a lower punch 51 that supports the preform 41, an upper punch 52 that is situated opposite the lower punch 51 and constricts the preform 41 or compresses (pressurizes) it at a compressive force P1, a heater 53 that surrounds the preform 41, a chamber 54 that entirely surrounds the heater 53, preform 41, and the like, an evacuation device 55 that is connected to this chamber 54 and places the interior of the chamber 54 in an evacuated state, and an inert gas suction device 56 that suctions argon inert gas into the chamber 54. This treatment apparatus 50 is controlled in accordance with the control graph shown in
In the heating step, the interior of the chamber is placed in an evacuated condition, and an inert gas such as argon or a non-oxidative gas such as nitrogen is introduced while maintaining this evacuation or subsequent thereto. Next, the preform is heated to 700° C. at a prescribed rate of heating (temperature elevation), and, upon reaching 700° C., the material is maintained for 10 min to obtain a heat-treated body 57 shown in
Because the melting point of Mg is 650° C., the matrix metal stock melts when heated to 700° C. and infuses into the microparticulate-bonded carbon material. Thorough infusion is allowed to occur by retention for 10 min.
By decreasing the setting temperature of the heater 53 shown in
Upon reaching 580° C., the top punch 52 is lowered, and a pressure of 40 MPa is applied to the heat-treated body 57. The material is maintained for 10 min at 580° C. under compression. During this retention, the upper punch 52 is gradually lowered. This lowering is continued for 5 to 7 min, and subsequently lowering is stopped. When the upper punch 52 is moving downwards, slight gaps are present in the structure, and the gaps are compacted. When the lowering of the upper punch 52 stops, it can be concluded that sufficient density has been attained. The resulting compacted body 58 is thus well compacted.
This compaction can be carried out at a temperature that allows hot working of the matrix metal stock, but the required compressive force for compaction depends on the temperature. Compaction can be carried out with a smaller compressive force when the temperature is high, and compaction can be readily carried out even with carbon molds that are not very strong. It is thus preferable to carry out compaction in a temperature range that is as high as possible.
Workability is poor at low temperatures that are below the hot working temperature, and with Mg or Mg alloy matrix metal stock in particular, cracking, fissures, and the like readily occur, making compaction difficult.
At high temperatures in excess of the hot working temperature, a liquid-phase condition is produced, and leaking of liquid phase will occur under compression, so the compressive force becomes less effective, making compaction difficult.
The resulting compacted body 58 can yield a carbon nanocomposite metal material 59 when cooled to normal temperatures while being constrained by the upper punch 52. In the compacted body 58, the surface temperature decreases first, and the temperature of the inner sections decreases slowly. Thus, there are cases where stress referred to as cooling stress is generated due to the temperature differential. By continued constraint applied using the upper punch 52, it is possible to suppress the generation of cooling stress. However, when there is no concern regarding cooling stress, cooling may be carried out without a compressive force (without the compacted body 58 being constrained by the upper punch 52).
An example of extrusion molding of the unextruded carbon nanocomposite metal material 59 will be described below.
a), 7(b), and 7(c) are explanatory diagrams for the extrusion step of this embodiment.
In
In
c) shows the exterior of the extruded carbon nanocomposite metal material 65, where carbon nanomaterial 13 oriented in the direction of extrusion can be observed on a surface 66.
A sufficient amount of carbon nanomaterial 13 is contained on the surface, thereby improving abrasion resistance.
Although not shown in the drawing, when a cross section of the carbon nanocomposite metal material 65 is observed, the carbon nanomaterial 13 oriented in the extrusion direction can be observed in the cross section. The carbon nanomaterial 13 is thus uniformly dispersed, increasing the mechanical strength.
The carbon nanocomposite metal material 65 shown in
When heating is stopped at the partial melting temperature, the matrix melt is a mixture of solid phase and liquid phase, and movement of the carbon nanomaterial is restricted. As a result, dispersion of the carbon nanomaterial is maintained.
The carbon nanocomposite metal material 65 produced by the method for producing carbon nanocomposite metal materials has a uniformly dispersed carbon nanomaterial. Because die-cast molding is carried out by supplying material in this uniformly mixed condition, it is possible to readily carry out molding, even with molded articles having complicated shapes. In addition, a carbon nanocomposite metal molded article 86 can be produced that has high thermal conductivity, mechanical strength, and abrasion resistance.
Experimental examples pertaining to the present invention are described below, but the present invention is not restricted to these examples.
Mixed body formation step and vacuum vapor deposition step: As shown in
Preparation step: As shown in
The composition of the Mg alloy as defined in ASTM AZ91D (magnesium alloy die-cast JIS H 5303; product analogous to MDC1D) had an Al content of about 9 wt %, with the remainder being trace elements, inevitable impurities, and Mg.
Mixing step: As shown in
Preforming step: As shown in
Heat treatment step: As shown in
Compacting step: As shown in
Cooling step: As shown in
First evaluation: Sample strips were cut from the carbon nanocomposite metal material prior to extrusion, and the compressive force was measured. The measured values are presented in Table 1 below.
Experiments 1 to 4 employed pure Mg as matrix metal, and Experiments 5 to 9 employed AZ91D as matrix metal. With Experiments 1 and 5, a structure was produced that contained no microparticulate-coated carbon nanomaterial for purposes of comparison. Experiment 4 gave a value of 145, taking Experiment 1 as 100, and the compression strength increased by 45% due to the 20 mass % content of microparticulate-coated carbon nanomaterial.
In Experiment 1 to Experiment 4, it was confirmed that compression strength increased in proportion to the added amount of microparticulate-coated carbon nanomaterial. In Experiments 5 to 9 as well, it was confirmed that compression strength increased in proportion to the added amount of microparticulate-coated carbon nanomaterial.
Next, an experiment was carried out in which an extrusion molding process was carried out on the carbon nanocomposite metal material prior to the extrusion treatment.
Extrusion molding step: Extrusion molding was carried out in reference to
Second evaluation: A test strip (7 mm in diameter, 7 mm in height) was cut from the extruded material (extruded carbon nanocomposite metal material), and the compression strength was measured. The measured values are presented in Table 2 below.
For purposes of convenience, the test numbers were produced by adding 10 to the number of Experiments 1 to 9, yielding Experiments 11 to 19. Specifically, extrusion was added to Experiment 1 in Experiment 11, and extrusion was added to Experiments 2 to 9 in Experiments 12 to 19.
Experiments 11 to 14 employed pure Mg as matrix metal, and Experiments 15 to 19 employed AZ91D as matrix metal. With Experiments 11 and 15, a structure was produced that contained no microparticulate-coated carbon nanomaterial for purposes of comparison. Experiment 14 gave a value of 121, taking Experiment 11 as 100, and the compression strength thus increased by 21% due to the 20 mass % content of microparticulate-coated carbon nanomaterial.
Although the details are not presented, a similar increase in mechanical strength was obtained when Ti was used instead of Si as the carbide-forming metal (element that reacts with metallic carbon to form compound). In addition to Si and Ti, zirconium (Zr) or vanadium (V) may be used as the carbide-forming metal.
In addition to Mg or Mg alloy having a melting point of about 650° C., Al or Al alloy having a melting point of about 660° C., Sn or Sn alloy having a melting point of about 232° C., or Pb or Pb alloy having a melting point of about 327° C. may be used as the matrix metal stock.
Obviously, various minor changes and modifications of the present invention are possible in light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2006-312089 | Nov 2006 | JP | national |
