COMPOSITE OF ALUMINUM AND BORON NITRIDE NANOTUBES AND METHOD FOR MANUFACTURING SAME
There is provided a composite of a metallic matrix and boron nitride nanotubes, the metallic matrix including aluminum or an aluminum alloy. Also, there is provided a method for manufacturing the composite. The method includes: a powder mixing step of mixing a powder of boron nitride nanotubes and a powder of an element soluble in a molten metal of the metallic matrix to prepare a powder mixture of boron nitride nanotubes and a metallic matrix-soluble element; an alloy melt mixing step of mixing the powder mixture and the molten metal of the metallic matrix to prepare a metallic matrix melt mixed with boron nitride nanotubes; and a casting step of solidifying the metallic matrix melt mixed with boron nitride nanotubes to obtain the composite.
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The present application claims priority from Japanese patent application serial no. 2018-078482 filed on Apr. 16, 2018, the content of which is hereby incorporated by reference into this application.
FIELD OF THE INVENTIONThe present invention relates to technology of composites of metals and fine fibrous substances, and in particular, to a composite of a matrix of aluminum or an aluminum alloy (hereinafter referred to simply as aluminum) and boron nitride nanotubes dispersed in the matrix (hereinafter referred to as aluminum and boron nitride nanotube composite) and a method for manufacturing the composite.
DESCRIPTION OF RELATED ARTIn order to improve mechanical properties of metallic materials, research and development has been carried out on technology of adding a fine fibrous substance to or mixing it into a metallic matrix. For example, JP 2010-196098 A discloses a metal matrix composite obtained by impregnating a preform including a plurality of fibers of a fibrous material entangled in a three-dimensional space with a molten metal and solidifying the metal and preform. In space defined by the fibers are trapped metal powder particles with a carbon nanomaterial (carbon nanotubes or carbon nanofibers) attached on the surface of the particles or incorporated in the particles. The metal powder and the molten metal are an aluminum alloy or a magnesium alloy.
On the other hand, in recent years, attention has been directed toward boron nitride nanotubes as a fine fibrous substance. Boron nitride nanotubes (hereinafter referred to as BNNTs as well) are nanotubes (NTs), which are cylinders formed of a sheet of alternately bonded nitrogen (N) atoms and boron (B) atoms. BNNTs are considered to have mechanical properties comparable to those of carbon nanotubes (CNTs), which are cylinders formed of a sheet of carbon (C) atoms bonded with each other, and have high thermal stability.
For example, Yanming Xue et al. (Materials and Design 88 (2015) 451-460) discloses a study in which an aluminum and boron nitride nanotube composite (hereinafter referred to as Al/BNNT composite) was fabricated by a high-pressure torsion technique. In the study, a mixture of an aluminum (Al) powder and boron nitride nanotubes (BNNTs) was subjected to a torsion process under a high pressure of 5 GPa.
According to JP 2010-196098 A, there can be provided a metal matrix composite with excellent lubricity, cohesion resistance, wear resistance, and thermal conductivity. At present, however, new problems are arising, such as poor interfacial bondability between the aluminum Matrix and the carbon nanomaterial, and insufficient homogeneous dispersion and insufficient chemical stability of the carbon nanomaterial in the aluminum matrix.
According to Yanming Xue et al, the obtained Al/BNNT composite has an amorphous ultra-thin Al—(BNO) layer (2-5 nm in thickness) at the interface region between the Al and BNNTs and exhibits an improved tensile strength of up to 420 MPa at room temperature, which is more than double the tensile strength of pure Al materials. This suggests the possibility of improvement of mechanical properties by combining Al and BNNTs to form a composite, which is enticing. However, since the technique disclosed in Yanming Xue et al. employs a special manufacturing method, a high-pressure torsion technique, it is disadvantageous in that the composite has poor shape flexibility and shape controllability, and the cost of forming the composite into a desired shape is prone to be high. For new materials such as Al/BNNT composites to find practical applications (in particular, to replace existing materials), they have to be made available at low cost, as a matter of first priority. If Al/BNNT composites can be manufactured by a casting process, their disadvantage in shape flexibility and shape controllability can be overcome and their manufacturing costs can be significantly reduced. Meanwhile, casting is considered as unsuitable for manufacturing Al/CNT composites because CNTs react chemically with an Al melt to produce compounds such as carbides.
SUMMARY OF THE INVENTIONIn view of foregoing, it is an objective of the present invention to provide an Al/BNNT composite which has excellent shape flexibility and shape controllability and is capable of cost reduction. Also, another objective of the invention is to provide a method for manufacturing the Al/BNNT composite.
According to one aspect of the invention, there is provided a composite of a metallic matrix and boron nitride nanotubes, the metallic matrix including aluminum or an aluminum alloy. The boron nitride nanotubes are dispersed in the metallic matrix, and the metallic matrix is prepared by a melt-solidification process (has a melt-solidified structure).
In the above aspect of a composite (I) of the invention, the following modifications and changes can be made.
(i) The aluminum alloy may include aluminum as a main component and at least one of silicon, copper, magnesium, and nickel.
According to another aspect of the invention, there is provided a method for manufacturing a composite of a metallic matrix and boron nitride nanotubes, the metallic matrix including aluminum or an aluminum alloy. The method includes:
a powder mixing step of mixing a powder of boron nitride nanotubes and a powder of an element soluble in a molten metal of the metallic matrix to prepare a powder mixture of boron nitride nanotubes and a metallic matrix-soluble element;
an alloy melt mixing step of mixing the powder mixture and the molten metal of the metallic matrix to prepare a metallic matrix melt mixed with boron nitride nanotubes; and
a casting step of solidifying the metallic matrix melt mixed with boron nitride nanotubes to obtain the composite.
Meanwhile, in the present invention, the molten metal of the metallic matrix in the alloy melt mixing step may be a pure aluminum melt or an aluminum alloy melt.
In the above aspect of a method for manufacturing a composite of a metallic matrix and boron nitride nanotubes (II) of the invention, the following modifications and changes can be made.
(ii) The powder of an element soluble in a molten metal of the metallic matrix may be a powder of silicon.
(iii) A ratio between the specific surface area of the powder of boron nitride nanotubes and the specific surface area of the powder of an element soluble in a molten metal of the metallic matrix may be less than 10.
(iv) A mass ratio between the powder of boron nitride nanotubes and the powder of an element soluble in a molten metal of the metallic matrix may be equal to or more than 1:2 and equal to or less than 2:1.
(v) The aluminum alloy may include aluminum as a main component and at least one of silicon, copper, magnesium, and nickel.
(vi) The powder mixing step may include:
a boron nitride nanotube suspension preparation substep of mixing the powder of boron nitride nanotubes and an organic solvent to prepare a boron nitride nanotube suspension;
a metallic matrix-soluble element suspension preparation substep of mixing the powder of an element soluble in a molten metal of the metallic matrix and an organic solvent to prepare a metallic matrix-soluble element suspension;
a boron nitride nanotube/metallic matrix-soluble element suspension preparation substep of mixing the boron nitride nanotube suspension and the metallic matrix-soluble element suspension to prepare a boron nitride nanotube/metallic matrix-soluble element suspension; and
an organic solvent elimination substep of eliminating the organic solvent from the boron nitride nanotube/metallic matrix-soluble element suspension to prepare the powder mixture of boron nitride nanotubes and a metallic matrix-soluble element.
ADVANTAGES OF THE INVENTIONAccording to the invention, there can be provided an Al/BNNT composite which has excellent shape flexibility and shape controllability and is capable of cost reduction. In addition, there can be provided a method for manufacturing the Al/BNNT composite.
From the viewpoints of shape flexibility and shape controllability of Al/BNNT composites, the present inventors carried out earnest research on methods for manufacturing Al/BNNT composites by casting. In doing so, the inventors simply mixed an Al melt and BNNTs and subsequently solidify the resultant mixture to fabricate casts and examined them. As a result, it was found that the Al melt had poor wettability with the BNNTs and the solidified Al matrix and the BNNTs easily separated from each other (details will be described later).
In the invention, “to wet” means to form a contact angle of 90° or smaller between the liquid phase (or the solidified liquid phase, i.e. originally liquid phase) and the solid phase (i.e. solid phase all along). Also, “to be wet” means the state in which the contact interface between the BNNTs and the Al matrix can be observed by electron microscopy (e.g. scanning electron microscopy (SEM), or transmission electron microscopy (TEM)) and desirably, no unwanted inclusions (e.g. reaction compounds of BNNTs and Al, voids, etc.) are present on the interface.
The composite of the invention will be hereinafter described with a cast as an example.
With an aim to improve the wettability between an Al melt and BNNTs, the inventors formulated a hypothesis as follows. In order to improve the wettability between an Al melt and BNNTs, it would be desired that a component with a high affinity for both or at least the Al melt is added.
Specifically, the inventors have deemed that to an Al melt, a component that is readily soluble in the Al melt should added. They have deemed that presence of an element that is readily soluble in the Al melt (hereinafter referred to as metallic matrix-soluble element, such as Si, Cu, Mg, and Ni) near BNNTs would increase the likelihood of direct contact between the Al melt and the BNNTs with the progress of dissolution of particles of the element, which could improve the dispersibility of the BNNTs into the Al melt.
As far boron nitride (BN), they have deemed that since BN is a group III-V compound and has chemical properties similar to those of carbon (C), which is a group IV element, a group IV element would have a high affinity for BN (i.e. a group IV element melt would have good wettability with BN). Based on the above, they have deemed a group IV element, Si in particular, would be preferable as a component readily soluble in the Al melt, and addition of Si would improve the wettability between the Al melt and the BNNTs and the dispersibility of the BNNTs.
In order to confirm this hypothesis, a powder mixture of a BNNT powder and a Si powder was prepared and mixed into an Al melt. The resultant mixture was solidified to fabricate a composite, and this composite was examined. As a result, it was confirmed that the wettability between the Al matrix and BNNTs had been improved. The present invention was made based on this finding.
Preferred embodiments of the present invention will be hereinafter described step by step of the manufacturing process with reference to the accompanying drawings. However, the invention is not limited to the specific embodiments described below, and various combinations with known art and modifications based on known art are possible without departing from the spirit and the scope of the invention.
[Method for Manufacturing Al/BNNT Composite]
The method may also include a remelting and casting step (S4), not shown in
Each step of the method for manufacturing an Al/BNNT composite according to the embodiment of the invention will be hereinafter described more specifically.
(Powder Mixing Step S1)
As described above, this step is a step of mixing a BNNT powder and a Si powder to prepare a BNNT/Si powder mixture. There is no particular limitation on the BNNTs used in the invention, and any commercially available BNNT powder may be used. For example, BNNTs with an average diameter of 10 nm or smaller and an average length on the order of μm may be used. Also, the BNNTs are not limited to those with a single-layer structure, and nanotubes with a multi-layer structure (e.g. 2-10 layers) may be used.
In general, in order to mix two powders homogeneously, it is preferable that they should be similar in average particle size. The powder mixture of the invention is of a BNNT powder and a Si powder, and as described above, the particles of the BNNT powder have a fibrous shape with an aspect ratio (length/diameter) of as large as around 102 to 103.
The inventors conducted various studies and have found that in the case where a BNNT powder and a Si powder are mixed, it is preferable that the specific surface area (unit: m2/g) should be adopted instead of the average particle size as the size selection criterion for the two powders and that the specific surface area ratio between the BNNT powder and the Si powder should be controlled to be less than 10. The specific surface area of each powder can be measured by gas adsorption (the BET theory, the BET method), for example.
For the Si powder to be used in the invention, nanoparticles are effective. The specific surface area of the Si powder is preferably over one tenth to less than ten times of the specific surface area of the BNNT powder to be mixed into it, and more preferably one third to three times. Also, a Si powder of amorphous particles (indeterminate shape particles) or scale-like particles (e.g. particles with a thickness of around 10 to 30 nm and a diameter of around 50 to 500 nm) is preferable because when it is mixed with a BNNT powder, its presence among BNNTs is expected to reduce entanglement among BNNTs (see
Moreover, as for a mixing ratio between the BNNT powder and the Si powder, the ratio between the total surface area of the BNNT powder and the total surface area of the Si powder is preferably 2 or smaller, and more preferably 1.5 or smaller. For example, in the case where a BNNT powder with the average particle size of 4 nm, the average hollow diameter of 0.84 nm, and the specific surface area of 400 m2/g is mixed with a Si powder with the average particle size of 10 nm and a specific surface area of 200 m2/g, it is preferable that the Si powder should have a mass twice that of the BNNT powder so that the total surface area of the BNNT powder and the total surface area of the Si powder are equal. By setting the mixing ratio in such a manner, aggregation of the BNNT powder can be prevented more effectively.
For mixing the BNNT powder and the Si powder, various mixing methods may be applied. For example, the powder mixing step S1 may be divided into the following substeps: a BNNT suspension preparation substep (S1a), an Si suspension preparation substep (S1b), a BNNT/Si suspension preparation substep (S1c), and an organic solvent elimination substep (S1d). The BNNT suspension preparation substep S1a is a step of mixing the BNNT powder with an organic solvent to prepare a BNNT suspension, which facilitates disentanglement of the BNNTs. There is no particular limitation on the organic solvent to be used in the BNNT suspension preparation substep S1a, and alcohols (e.g. methanol, ethanol, 1-propanol, 2-propanol, etc.) and ketones (e.g. acetone, methyl ethyl ketone, and methyl isobutyl ketone, etc.) may be used.
Similarly, the Si suspension preparation substep S1b is a step of mixing the Si powder with an organic solvent to prepare a Si suspension, which facilitates disaggregation of the Si powder. There is no particular limitation on the organic solvent to be used in the Si suspension preparation substep S1b, and alcohols (e.g. methanol, ethanol, 1-propanol, 2-propanol, etc.) and ketones (e.g. acetone, methyl ethyl ketone, and methyl isobutyl ketone, etc.) may be used.
The BNNT/Si suspension preparation substep S1c is a step of preparing a suspension mixture of the BNNT suspension and the Si suspension (referred to as BNNT/Si suspension). From the viewpoint of homogeneity in the final mixture, the mass ratio of the BNNT powder and the Si powder in the mixture is preferably within a range from 1:2 to 2:1, and more preferably within a range from 1:1.5 to 1.5:1.
The organic solvent elimination substep S1d is a step of eliminating the organic solvent from the BNNT/Si suspension to prepare a mixture of the BNNT powder and the Si powder (referred to as BNNT/Si powder mixture). There is no particular limitation on the method for eliminating the organic solvent. However, in the case where the amount of the organic solvent in the BNNT/Si suspension is relatively large, a method for filtering to roughly separate the liquid phase from the solid phase and subsequently drying the solid phase of the BNNT/Si powder mixture may be preferably used, for example.
Although the above description has been made with a BNNT/Si powder mixture as an example, a similar effect is expected to be obtained with a powder mixture of a BNNT powder and a powder of other elements readily soluble in an Al melt other than Si (e.g. Cu, Mg, and Ni). This is because metal powders of these elements have the effect of allowing the Al melt to penetrate into the vicinity of the BNNTs as they dissolve in the Al melt.
(Alloy Melt Mixing Step S2)
This step is a step of mixing the BNNT/Si powder mixture with an Al melt to prepare an Al alloy melt mixed with BNNTs. There is no particular limitation on the method for mixing the BNNT/Si powder mixture and the Al melt. However, in order to prevent scattering of the powder mixture and to make sure that the BNNTs are completely buried in the Al melt, it is preferable that the BNNT/Si powder mixture should be packed in Al foil or an Al container before it is put into the Al melt.
In the invents on, the Al melt may be a pure Al melt or an Al alloy melt. Herein, pure Al is defined as aluminum with a purity of 99.0% or higher.
In the case of an Al alloy melt, it preferably has a chemical composition that is capable of forming a eutectic structure. Preferred examples include aluminum alloys for casting specified by JIS H 5202 (e.g. AC1A: Al—Cu alloys, AC1B: Al—Cu—Mg alloys, AC2A and AC2B: Al—Cu—Si alloys, AC3A: Al—Si alloys, AC4A, AC4C and AC4CH: Al—Si—Mg alloys, AC4B: Al—Si—Cu alloys, AC4B and AC8C: Al—Si—Cu—Mg alloys, AC5A: Al—Cu—Ni—Mg alloys, AC7A: Al—Mg alloys, AC8A and AC8B: Al—Si—Cu—Ni—Mg alloys, AC9A and AC9B: Al—Si—Cu—Mg—Ni alloys). In other words, the Al alloy melt used in this step includes Al as its main component and at least one of Cu, Mg, Si, and Ni.
As described in JIS H 5202, aluminum alloys for casting may further include, as trace components, at least one of zinc (Zn), iron (Fe), manganese (Mn), titanium (Ti), lead (Pb), tin (Sn), and chromium (Cr) in addition to Cu, Mg, Si and/or Ni.
(Casting Step S3)
This step is a step of solidifying the Al alloy melt mixed with BNNTs to obtain an Al/BNNT composite. There is no particular limitation on the casting method for solidification, and any conventional method may be used.
By performing the steps above, there can be obtained an Al/BNNT composite according to an embodiment of the invention.
EXAMPLESPreferred embodiments of the invention will be hereinafter described in more detail with examples.
[Experimental 1]
Fabrication of Example 1According to the manufacturing method described above, a cast article as an Al/BNNT composite (hereinafter referred to as Al/BNNT composite cast article) of Example 1 was fabricated. First, 1 g of a BNNT powder (with the average particle diameter of 5 nm and the specific surface area more than 100 m2/g) was put into 100 mL of ethanol and subjected to ultrasonic agitation for one hour to prepare a BNNT suspension (BNNT suspension preparation step S1a). The specific surface area was measured with a vapor adsorption amount measuring instrument (BELSORP-maxII, a product of MicrotracBEL Corp.).
Similarly, 1 g of a Si powder (of scale-like particles with the average particle thickness of 30 nm, the average particle size of 400 nm, and the specific surface area more than 100 m2/g) was put into 100 mL of ethanol and subjected to ultrasonic agitation for one hour to prepare a Si suspension (Si suspension preparation step S1b).
Next, the whole of the BNNT suspension was mixed with whole of the Si suspension and subjected to further ultrasonic agitation for one hour to prepare a BNNT/Si suspension (BNNT powder of 1 g and Si powder of 1 g, total 200 mL) (BNNT/Si suspension preparation step S1c).
Next, the BNNT/Si suspension was filtered and the solid phase was dried to prepare a BNNT/Si powder mixture (organic solvent elimination step S1d).
Next, the BNNT/Si powder mixture thus prepared was packed Al foil (commercially available) to prepare an Al package. Subsequently, the Al package was put into an Al alloy melt (AC4CH: Al-7 mass % Si-0.3 mass % Mg alloy, 1 kg, 700° C.) prepared in a graphite crucible and subjected to agitation and mixing for one hour. Then, the molten metal was taken out from the furnace together with the crucible to allow the Al alloy melt mixed with BNNTs and Si to solidify by natural cooling to obtain an Al/BNNT composite cast article of Example 1.
Fabrication of Example 2An Al/BNNT composite cast article of Example 2 was fabricated in the same manner as Example 1 except that a pure Al melt (A1100, 1 kg, 70.0° C.) was used.
Fabrication f Comparative Example 1An Al/BNNT composite cast article of Comparative Example 1 was fabricated in the same manner as Example 1 except that the BNNT powder was not mixed with an Si powder.
[Experimental 2]
(Observation of Microstructure of Al/BNNT Composite)
For each of the Al/BNNT composite cast articles of Example 1, Example 2 and Comparative Example 1, the microstructure of a portion near the surface of the cast article was observed.
As shown in
In both Example 1 and Comparative Example 1, AC4CH (Al—Si—Mg alloy), containing Si, was used as the Al alloy melt to which the BNNT powder was added. Also, in Example 2, the Al melt before the addition of the BNNT/Si powder mixture did not contain Si Considering these, the above-mentioned clear difference in wettability between the molten metal and the BNNTs cannot be attributed only to presence or absence of Si in the Al/BNNT composite cast article.
Unfortunately, any detailed mechanism has not been clarified at the present stage, but if nothing else, it can be said that by mixing the powder mixture of the BNNT powder and the Si powder (BNNT/Si powder mixture) into the Al melt, the likelihood of direct contact between the Al melt and the BNNTs was increased with the progress of dissolution of the Si powder, which led to an improvement of wettability and dispersibility. As has been described above, the present invention shows an extremely interesting phenomenon.
The above embodiments and experiments are given for the purpose of detailed explanation only, and the invention is not intended to include all configurations of the specific examples described above. Also, a part of an embodiment may be replaced by known art, or added with known art. That is, a part of an embodiment of the invention may be combined with known art and modified based on known art without departing from the technical idea of the invention where appropriate.
Claims
1. A composite of a metallic matrix and boron nitride nanotubes, the metallic matrix comprising aluminum or an aluminum alloy, wherein
- the boron nitride nanotubes are dispersed in the metallic matrix, and the metallic matrix is melt-solidified.
2. The composite according to claim 1, wherein
- the aluminum alloy comprises aluminum as a main component and at least one of silicon, copper, magnesium, and nickel.
3. A method for manufacturing a composite of a metallic matrix and boron nitride nanotubes, the metallic matrix comprising aluminum or an aluminum alloy, the method comprising:
- a powder mixing step of mixing a powder of boron nitride nanotubes and a powder of an element soluble in a molten metal of the metallic matrix to prepare a powder mixture of boron nitride nanotubes and a metallic matrix-soluble element;
- an alloy melt nixing step of mixing the powder mixture and the molten metal of the metallic matrix to prepare a metallic matrix melt mixed with boron nitride nanotubes; and
- a casting step of solidifying the metallic matrix melt mixed with boron nitride nanotubes to obtain the composite.
4. The method for manufacturing a composite of a metallic matrix and boron nitride nanotubes according to claim 3, wherein
- the powder of an element soluble in a molten metal of the metallic matrix is a powder of silicon.
5. The method for manufacturing a composite of a metallic matrix and boron nitride nanotubes according to claim 3, wherein
- a ratio between the specific surface area of the powder of boron nitride nanotubes and the specific surface area of the powder of an element soluble in a molten metal of the metallic matrix is less than 10.
6. The method for manufacturing a composite of a metallic matrix and boron nitride nanotubes according to claim 3, wherein
- a mass ratio between the powder of boron nitride nanotubes and the powder of an element soluble in a molten metal of the metallic matrix is equal to or more than 1:2 and equal to or less than 2:1.
7. The method for manufacturing a composite of a metallic matrix and boron nitride nanotubes according to claim 3, wherein
- the aluminum alloy comprises aluminum as a main component and at least one of silicon, copper, magnesium, and nickel.
8. The method for manufacturing a composite of a metallic matrix and boron nitride nanotubes according to claim 3, wherein the powder: mixing step comprises:
- a boron nitride nanotube suspension preparation substep of mixing the powder of boron nitride nanotubes and an organic solvent to prepare a boron nitride nanotube suspension;
- a metallic matrix-soluble element suspension preparation substep of mixing the powder of an element soluble in a molten metal of the metallic matrix and an organic solvent to prepare a metallic matrix-soluble element suspension;
- a boron nitride nanotube/metallic matrix-soluble element suspension preparation substep of mixing the boron nitride nanotube suspension and the metallic matrix-soluble element suspension to prepare a boron nitride nanotube/metallic matrix-soluble element suspension; and
- an organic solvent elimination substep of eliminating the organic solvent from the boron nitride nanotube/metallic matrix-soluble element suspension to prepare the powder mixture of boron nitride nanotubes and a metallic matrix-soluble element.
Type: Application
Filed: Apr 15, 2019
Publication Date: Oct 17, 2019
Applicant: HITACHI METALS, LTD. (Tokyo)
Inventors: Makoto OKAI (Tokyo), Hideki YAMAURA (Tokyo), Hideya YAMANE (Tokyo)
Application Number: 16/384,483
