JP2011077998

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DESCRIPTION JP2011077998
Abstract: To provide a carbonaceous acoustic diaphragm having sufficient rigidity while having
low density. SOLUTION: Periodical irregularities 20 are provided on the outer surface of a
carbonaceous acoustic diaphragm in two directions of longitudinal direction and lateral direction.
[Selected figure] Figure 1
Carbon quality acoustic diaphragm
[0001]
The present invention relates to a carbonaceous acoustic diaphragm.
[0002]
Speaker diaphragms used in various audio devices, video devices, mobile devices such as mobile
phones, etc. are required to have the property of being able to faithfully reproduce clear sounds
in a wide frequency band, particularly in a high frequency range.
Therefore, the material of the diaphragm is required to have seemingly contradictory properties
such as a high elastic modulus to give sufficient rigidity to the diaphragm and a low density to
reduce the weight of the diaphragm. In particular, in the diaphragm for digital speakers that has
been attracting attention in recent years, these properties are strongly demanded from the
demand for vibration response.
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[0003]
Patent Documents 1 and 2 described below disclose diaphragms made of a material in which
carbon nanofibers (gas phase grown carbon fibers) and graphite are uniformly dispersed in
amorphous carbon. However, since this material has a high density of 1.0 g / cm <3> or more, it
is necessary to blend a large amount of expensive carbon nanofibers and graphite to increase the
elastic modulus in order to obtain desired acoustic properties. Need to be thinner. Therefore,
there is a problem of breakage due to handling and the like, and a problem is left in productivity.
[0004]
In Patent Document 3, powder of resin before firing (carbonization) to form glassy carbon
(amorphous carbon) is heated and point-fused to make a porous body, and then carbonized to
form low density amorphous carbon It is described that it is a porous body. However, in this
method, it is difficult to obtain a porous body having a high porosity of 40% or more, and the
case where the density of the whole diaphragm is 1.0 g / cm <3> or less has not been obtained.
[0005]
Patent Document 4 describes an acoustic carbon diaphragm in which gas phase pyrolytic carbon
is deposited on a non-woven fabric or woven fabric of carbon fibers impregnated with a resin and
carbonized. Also in this method, it is difficult to obtain a porous body having a high porosity of
40% or more.
[0006]
Patent Document 5 describes an acoustic diaphragm in which the surface of a foamed graphite
film is etched to impregnate a plastic. The expanded graphite refers to a state in which a gas
generated inside disturbs a layered structure unique to graphite when carbonizing a polymer at a
high temperature, and it is difficult to design and control pores. Therefore, the graphite in the
foamed state is impregnated with the resin to reinforce the defects of the partially thinned
graphite to flatten the regeneration frequency, and the resin reinforces the graphite defects. That
is the main point. In addition, since the resin is impregnated by etching, the process is long and
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the management is likely to be complicated.
[0007]
JP-A 2004-32425 (Patent No. 3630669) JP-A No. 2002-171593 JP-A No. 01-185098 JP-A No.
62-163494 JP-A No. 05-22790
[0008]
Therefore, an object of the present invention is to provide a carbonaceous acoustic diaphragm
which has low density and light weight, yet has sufficient rigidity, exhibits good acoustic
characteristics, and can be manufactured industrially at low cost.
[0009]
According to the present invention, there is provided a carbonaceous acoustic diaphragm
including amorphous carbon and having irregularities formed on at least one surface thereof.
The unevenness is desirably periodic, and more desirably periodic in two directions crossing each
other.
[0010]
The above-mentioned carbonaceous acoustic diaphragm includes amorphous carbon and a
carbon powder dispersed uniformly in the amorphous carbon, and includes a low density layer
made of a porous body having a porosity of 40% or more, and amorphous carbon, It is preferable
that the high density layer comprises two high density layers sandwiching them, and the
asperities are provided on the outer surface of at least one of the high density layers.
[0011]
Here, the porosity is a percentage of the volume of the pores to the volume of the whole porous
body including the pores, and the density of the amorphous carbon serving as the skeleton is 1.5
g / cm <3>, calculated from the volume and mass of the whole porous body Defined as the
porosity to be
[0012]
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The surface irregularities as described above are, for example, PTFE (tetrafluorinated) on a glass
cloth obtained by knitting glass fiber into a sheet, for example, on one side or both sides of the
resin formed into a sheet before firing and carbonizing. It can be formed by pressing the one
impregnated with ethylene resin and fired (hereinafter referred to as PTFE impregnated cloth).
Similar surface shape molds may be used.
[0013]
By providing the unevenness on the surface, the weight can be reduced without losing the
rigidity, and good acoustic characteristics can be obtained.
[0014]
It is a notional sectional view of an acoustic diaphragm obtained in an example.
It is a graph of the measurement result of the surface roughness of a PTFE impregnation cloth.
[0015]
(Example 1) 35% by mass of vinyl chloride resin as an amorphous carbon source, 1.4% by mass
of carbon nanofibers having an average particle diameter of 0.1 μm and a length of 5 μm, and
PMMA as a drilling material for forming pores After adding diallyl phthalate monomer as a
plasticizer to the above composition and dispersing it using a Henschel mixer, kneading is
repeated sufficiently using a pressure kneader to obtain a composition, which is pelletized by a
pelletizer for molding The composition was obtained.
Pellets of this molding composition are extruded into a sheet-like molded product with a
thickness of 400 μm, and both sides are coated with furan resin, and then PTFE impregnated
cloth 970-4UL (manufactured by Nitto Denko Corporation) is pressed on both sides After curing,
the PTFE-impregnated cloth was peeled off to give a multilayer sheet.
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The multilayer sheet was treated in an air oven at 200 ° C. for 5 hours to form a precursor
(carbon precursor). Thereafter, the temperature was raised in nitrogen gas at a temperature
rising rate of 20 ° C./h, and maintained at 1000 ° C. for 3 hours. After natural cooling, it was
maintained at 1400 ° C. in vacuum for 3 hours, and then naturally cooled to complete firing. As
a result, as schematically shown in FIG. 1, the low density of the porous body having the
spherical pores 14 remaining after the powder 12 of carbon nanofibers is uniformly dispersed in
the amorphous carbon 10 and the particles of PMMA disappear. An acoustic diaphragm was
obtained having the layer 16 and the high density layer 18 made of amorphous carbon covering
both surfaces and having periodic irregularities 20 on the outer surface.
[0016]
The porosity of the low density layer 16 of the acoustic diaphragm thus obtained was 70%, and
the number average pore diameter was 60 μm. The graph of the measurement result of the
surface roughness of the PTFE-impregnated cloth used to form periodical irregularities on the
surface is shown in FIG. 2 (a). Table 1 shows the measurement results of the thickness of the
entire diaphragm, bending elasticity, density, and surface roughness according to JIS B0601:
2001. Example 2 An acoustic diaphragm was obtained by the same process as in Example 1
except that a smooth type 9700 UL was used as the PTFE impregnated cloth in place of that of
970-4 UL. The graph of the measurement result of the surface roughness of the PTFE
impregnated cloth used is shown in FIG. 2 (b). Table 1 shows the measurement results of the
thickness of the entire diaphragm, bending elasticity, density, and surface roughness according to
JIS B0601: 2001. Comparative Example An acoustic diaphragm was obtained in the same process
as in Example 1 except that a PET film was used in place of the PTFE-impregnated cloth. Table 1
shows the measurement results of the thickness of the entire diaphragm, bending elasticity,
density, and surface roughness according to JIS B0601: 2001.
[0017]
[0018]
As apparent from the results in Table 1, by providing periodic asperities on the surface, the
bending elasticity is improved and the density is reduced.
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Regarding the surface roughness parameters, it is desirable that arithmetic mean roughness Ra is
0.5 μm or more and maximum height Rz is 3.1 μm or more, and it is more desirable that Ra is 1
μm or more and Rz is 5 μm or more. I understand.
[0019]
FIG. 2 shows the measurement results of the surface roughness of the PTFE-impregnated cloth
used to form periodic irregularities on the surface. The PTFE-impregnated cloth has periodic
asperities with a period of 30 and several μm, that is, 50 μm or less, as shown in FIG. 2, in two
directions, the warp direction and the transverse direction. Ru.
[0020]
As can be seen from FIG. 2, in the PTFE-impregnated cloth, the line defining the lower recess is
sharper than the line defining the upper protrusion in the figure. That is, the curvature of the
surface defining the convex portion of the PTFE impregnated cloth is smaller than the curvature
of the surface defining the concave portion. Conversely, in the multilayer sheet to which this is
transferred, the curvature of the surface defining the concave portion is smaller than the
curvature of the surface defining the convex portion. Such a shape is considered to maintain the
structural rigidity.
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