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JP2004015261

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DESCRIPTION JP2004015261
An object of the present invention is to use a carbon nanotube having a high elastic modulus and
a high thermal conductivity different in structure from a high elastic modulus carbon fiber as a
reinforcing material for a light metal or its alloy, thereby having high rigidity and high thermal
conductivity and complexity. It is an object of the present invention to provide a diaphragm for
an electroacoustic transducer which can be easily manufactured and which has good
processability. According to the present invention, a diaphragm is formed of a composite material
in which a light metal such as aluminum or magnesium or an alloy thereof is reinforced with a
carbon nanotube. This composite material containing carbon nanotubes has high rigidity, good
processability, and excellent thermal conductivity. [Selected figure] Figure 1
Electroacoustic transducer diaphragm
The present invention relates to a diaphragm for an electroacoustic transducer made of a metal
composite material in which a light metal such as aluminum, magnesium or the like is reinforced
with carbon nanotubes, or a metal composite material thereof. Etc., and a diaphragm for an
electroacoustic transducer used for a microphone or the like. Conventionally, aluminum or
titanium is used as a diaphragm having a low density and a high modulus of elasticity for a metal
diaphragm having a high modulus of elasticity. As a diaphragm having a specific elastic modulus
larger than that of aluminum or titanium, a magnesium-lithium alloy has been proposed as
shown in Japanese Patent Publication No. 59-26160. However, magnesium and magnesiumlithium alloys have a problem that rust occurs in the atmosphere. Therefore, as disclosed in
Japanese Patent Publication No. 62-20757, there has been proposed a diaphragm for an
electroacoustic transducer in which a surface of a diaphragm substrate is treated with a chemical
conversion film and a synthetic resin film is formed. Further, in Japanese Patent Application LaidOpen No. 60-125099, a speaker diaphragm in which a high elastic modulus material such as
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titanium carbide or nitride is formed on the surface by a ion plating method on a speaker
diaphragm substrate made of a light metal is disclosed. Proposed. Furthermore, as disclosed in
JP-A-62-91098, by providing a ceramic film such as silicon oxide or aluminum oxide on a light
metal diaphragm substrate such as titanium, aluminum or magnesium, it is possible to have high
rigidity and good corrosion resistance. Electroacoustic transducer diaphragms have also been
proposed and used. However, the diaphragm as described above is not preferable in terms of
acoustic characteristics because the specific elastic modulus is lowered due to the anti-corrosion
treatment. Further, in the method of forming the ceramic layer on the light metal, since the
ceramic layer is thin, the acoustic characteristics of the diaphragm have not been improved.
Therefore, in order to improve the physical properties of the diaphragm, the following diaphragm
is proposed and used. JP-A-59-131294 and JP-A-60-57799 produce green sheets of ceramic
crystal particles, then heat and pressure form them into a diaphragm shape with a metal mold,
and at 1400-1600 ° C. in air. Ceramic sintered diaphragms such as polycrystalline alumina
sintered for 3 hours have been proposed and used. On the other hand, a fully crystalline diamond
diaphragm has also been proposed and used in the CVD method.
However, in the method of obtaining a diaphragm by mixing the above-mentioned ceramic and
resin to produce a green sheet and then forming it into a ceramic, it has been difficult to produce
a diaphragm having a complicated shape. In addition, an expensive thin film manufacturing
apparatus is required to manufacture a diaphragm by the CVD method, and a large number of
diaphragms can not be processed at one time, resulting in a problem of an increase in
manufacturing cost. In addition, as a method of strengthening a metal diaphragm, a fiber
reinforced diaphragm has been proposed, and it has been attempted to use a composite material
in which aluminum is reinforced with a high modulus carbon fiber. In the non-oxidative
atmosphere of the composite material, no decrease in strength is observed if the temperature is
500 ° C. or lower, but at 550 ° C. or higher, a reaction occurs at the interface between the
carbon fiber and the matrix to form Al 4 C 3 and the strength There is a problem that it falls.
Therefore, in the composite material of carbon fiber reinforced aluminum, it has been tried to
coat the surface of the carbon fiber with metal plating or ceramics, but there is a problem that
the process becomes complicated. In order to solve this problem, Japanese Patent Publication No.
56-6754 proposes a speaker diaphragm in which a silicon carbide fiber that is stable even at
high temperature and an inorganic material such as aluminum are combined. However, since this
silicon carbide fiber has a density of 2550 kg / m <3>, an elastic modulus of 176 GPa and an
acoustic velocity of 8300 m / s, there is a problem that a sufficient combined effect can not be
obtained. The present invention has been proposed in view of the above, and the object of the
present invention is to use carbon nanotubes having high elastic modulus and high thermal
conductivity different in structure from high elastic modulus carbon fibers as light metals or
metals thereof An object of the present invention is to provide a diaphragm for an electroacoustic
transducer having high rigidity, high thermal conductivity, and capable of easily producing a
complicated shape and having excellent machinability by using it as an alloy reinforcement.
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SUMMARY OF THE INVENTION The present invention achieves the above object by forming a
light metal such as aluminum or magnesium or an alloy thereof with a carbon nanotube
reinforced composite material. Further, in this case, it is characterized in that 5 to 40% by volume
of the addition amount of carbon nanotubes as a reinforcing material is added. In addition, it is
characterized in that the thermal conductivity of the diaphragm is larger than that of the matrix
metal. In addition, the surface of the diaphragm is subjected to an antirust treatment by an
alumite treatment, painting, or the like.
BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, specific embodiments of the
diaphragm for electroacoustic transducer according to the present invention will be described.
The light metal used in the present invention is aluminum or an alloy thereof, magnesium or an
alloy thereof or the like. As shown in Table 1, as a reinforcing material of matrix metal, a carbon
nanotube which is lighter than high modulus carbon fiber, has high modulus and high thermal
conductivity, and has a size of nm unit is used. The carbon nanotubes can be uniformly dispersed
in a light metal or an alloy thereof by a solid phase method or a liquid phase method. Carbon
nanotubes can be produced by, for example, an arc discharge method, a hydrocarbon catalyst
decomposition method, etc., form a seamless coaxial cylinder of a sheet of hexagonal mesh faces
of carbon atoms, and mechanically with a nanometer size. The strength is excellent. And although
it is regarded as a kind of whisker crystal from its unique structure, when it is subjected to strong
processing, it causes a unique mode of plastic deformation accompanied by buckling, which
enables complex forming. Further, the thermal conductivity is good, and even when heat
treatment is performed, no reaction phase with the matrix metal is generated, which is different
from conventional high modulus carbon fibers. Physical properties of each material are shown in
Table 1. As a carbon nanotube to be used, the diameter of one carbon nanotube is about 5 to 60
nm, and the length is 0.5. The thing of about -5 micrometers is preferable, and even if it becomes
a multi-bundle, it can utilize. The carbon nanotube is characterized in that the carbon nanotube is
compounded in the range of 5 to 40% by volume to the matrix, but the reason for limiting the
range of the reinforcing material is as follows . At 5% or less, the influence of the interaction
between the carbon nanotube and the matrix interface is small because there are few carbon
nanotubes contributing to strengthening, and the composite effect is dominated by the nature of
the matrix. When the content is 40% or more, aggregation of the reinforcing material occurs, and
a portion which does not form a matrix interface with the carbon nanotube is generated, and the
decrease and the dispersion of the physical property value become large. Since the thermal
conductivity of the carbon nanotube is 10 times or more as large as that of the aluminum alloy or
the magnesium alloy, the heat generated from the voice coil or the like is easily released to the
air, so the electrical input to the speaker is improved. Can.
Further, since magnesium, aluminum and the like are easily rusted even in the atmosphere, it is
necessary to apply a rustproofing treatment corresponding to the application, but since the
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diaphragm substrate is reinforced with carbon nanotubes, it is prevented The decrease in
physical properties due to rust treatment is less than that of the unreinforced metal vibrator. In
the antirust treatment, the surface of the diaphragm may be subjected to an alumite treatment,
painting, or the like. Even if rustproofing treatment is performed, the specific elastic modulus is
less likely to be reduced, and the acoustic characteristics are not degraded. EXAMPLES The
carbon nanotubes used in this example were produced by the arc discharge method, and the
carbon nanotubes used contained granular graphite and amorphous carbon as impurities. An
aluminum alloy (2024) reinforced with carbon nanotubes can be produced by a PM method, a
squeeze casting method or the like, but in the present example, the case where it is produced by
a PM method will be described. Test pieces for measuring physical properties were produced in
the same manner as in the case of producing the diaphragm for the electroacoustic transducer.
The test piece is a mixture of aluminum alloy powder (material name 2024, particle diameter 0.1
μm) and carbon nanotube powder (diameter about 5 to 60 nm, length about 0.5 to 5 μm)
changed by 5 to 50% by volume The powder was compacted and hot pressed at a temperature
above the solidus temperature and then rolled at about 500 ° C. FIG. 1 shows a block diagram
of the above process. The diaphragm shape can be formed in this step. Alternatively, after rolling,
it can be used as a diaphragm substrate, and the diaphragm can be manufactured by a known
metal foil forming process. It should be noted that plastic working such as compression extrusion
was performed at the time of production of the composite material, but carbon nanotubes did not
break easily because they plastically deform, so various plastic working was confirmed to be
easy. The [0044] This is different from the conventional high-elasticity fibers (carbon fibers,
alumina fibers, silicon carbide fibers, whiskers thereof and the like) which are conventional fiber
reinforcements. On the other hand, when the addition amount of carbon nanotubes is 40% by
volume or more, the dispersion of carbon nanotubes becomes uneven, and the variation in
physical property value becomes large. The Young's modulus and the velocity of sound measured
by the vibrational lead method when the carbon nanotube is added to the aluminum alloy are
shown in Table 2. As is apparent from Table 2 above, the speed of sound of the aluminum alloy is
5080 m / s. The sound velocity of the 40% by volume added composite material is about 2.1
times to 10,710 m / s, and the ceramic-based vibration plate conventionally proposed (10,420 m
/ s for Al2O3, 11,057 m / s for SiC) It became comparable.
The thermal conductivity is 399 w / m · k when 40 vol% of carbon nanotubes is added, and this
value is equivalent to the thermal conductivity of copper (395 to 403 w / m · k). In the case of a
magnesium alloy, the limit of the amount of carbon nanotubes added was 40% by volume as in
the case of the aluminum alloy. The density at 40 volume% is 1,930 kg / m <3> and the speed of
sound is 9,870 m / s. This value is the density 3600 kg / m <3> of rutile type titanium oxide (TiO
2), speed of sound It was lightweight compared to 8,660 m / s, and the speed of sound was fast.
The thermal conductivity was obtained by adding 40 vol% of carbon nanotubes to 321 w / m · k,
which was equivalent to the thermal conductivity of gold (313 to 324 w / m · k). As described
above, the diaphragm for the electroacoustic transducer according to the present invention has a
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processability as compared with a fiber reinforced metal diaphragm such as silicon carbide fiber
conventionally used, a ceramic diaphragm and the like. Good, lightweight, high rigidity, high
thermal conductivity, high sound speed physical properties can be obtained. The reason is that
carbon nanotubes are based on a network of graphite with cage material (cage structure).
However, carbon nanotubes are different from graphite, and there are one longitudinal wave, two
transverse waves, and one torsional acoustic mode, and the velocity of sound of these four
acoustic modes reflects the coupling of sp <2>. The speed of sound is faster and the thermal
conductivity is higher than in The present invention is not limited to the above embodiment, and
various modifications can be made without departing from the spirit of the present invention.
That is, the carbon nanotubes may be produced by a method other than the arc discharge
method. Moreover, you may produce a diaphragm base material other than PM method.
Moreover, as a diaphragm shape, the thing of arbitrary shapes, such as cone shape, flat shape,
dome shape, can be produced. As apparent from the above description, according to the present
invention, since the carbon nanotube has good plastic deformability and excellent processability,
the vibration for the electroacoustic transducer having a complicated shape is obtained. The plate
can be easily made. In addition, since the density is small and the elastic modulus and the
thermal conductivity are large, a high-performance diaphragm can be manufactured. For
example, if this diaphragm is used as a speaker, the reproduction characteristics up to the high
frequency range and the heat generated from the voice coil can be released by the diaphragm, so
that a high input speaker can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a block diagram of the production process
of the substrate of the present invention. [Explanation of code] 1 Vibration plate 2 Carbon
nanotube powder 3 Light metal
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