Process of making an acoustic carbon diaphragm

A process for producing a diaphragm for an acoustic device of carbonaceous materials which has the steps of mixing and kneading an organic mixture composition containing one or more or mixtures of a monomer, prepolymer and low polymer of relatively polymerizable thermosetting resin of a substance exhibinting high carbon residual yield after calcining with carbon powders as a binder, preliminarily molding the same in a film or sheet shape, calcining a diaphragm molding molded in a desired diaphragm shape and a voice coil bobbin molding molded in a desired voice coil bobbin shape from the film or sheet-like molding and then forming an integral structure of the diaphragm and the voice coil bobbin by calcining a composite material integrated with the diaphragm molding and the voice coil bobbin molding by an organic liquid composition exhibiting high carbon residual yield in an inert gas atmosphere. Thus, the process can transmit a driving force generated in a voice coil to the diaphragm without loss and without ageing fatigue, such as a creep of the materials irrespective of external environments.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for producing a diaphragm for an 
acoustic device of fully carbonaceous materials. More particularly, the 
invention relates to a process for producing an acoustic carbon diaphragm 
having a high hardness, a high strength and a high elasticity as compared 
to a diaphragm manufactured from a conventional diaphragm material. The 
diaphragm may be used as a speaker and a microphone. It exhibits less 
deformation by an external force due to excellent rigidity, as well as low 
sound distortion, a wide sound reproduction range, and distinct sound 
quality. In addition, the diaphragm imparts a high rigidity to an entire 
vibration system owing to an integrated structure of the diaphragm and a 
voice coil bobbin, which eliminates energy loss during transmission of a 
driving force generated in a voice coil to the diaphragm, thereby 
providing an excellent responsiveness to an input signal. The diaphragm is 
suitable for digital audio applications. 
A diaphragm intended for a speaker and a voice coil bobbin should possess 
the following properties: 
(1) low density, 
(2) high Young's modulus, 
(3) high sound propagating velocity, 
(4) adequately large internal loss of vibration, 
(5) stability against variation in the atmospheric conditions, no 
deformation nor change of properties, and 
(6) suitability for a simple and inexpensive manufacturing process. 
More specifically, the material for the diaphragm is required to have a 
wide sound reproduction range in high-fidelity over a broad frequency 
band. To efficiently and distinctly produce sound quality, the material 
should have high rigidity, with no distortion such as creep against 
external stress. To further increase the sound velocity from the equation 
of 
EQU V=(E/p).sup.1/2 
where V: sound velocity, E: Young's modulus, p: density, the material is 
required to have small density and high Young's modulus. 
In addition to the above-mentioned conditions, in the case of a voice coil 
bobbin, the material should be resistant to the Joule heat generated by a 
voice current flowing in a voice coil. 
The conventional materials for the diaphragm and voice coil bobbin include 
paper (pulp), plastic, aluminum, titanium, magnesium, beryllium, boron as 
basic materials, and further contain glass fiber, carbon fiber compositely 
mixed with the basic material, or processed to metal alloy, metal nitride, 
metal carbide, or metal boride. However, the paper, plastic and their 
composite materials have a small Young's modulus and small density. Thus, 
the sound velocities of these materials are low. Vibration division occurs 
in a specific mode and the frequency characteristics in the high frequency 
band of the materials are particularly low, resulting in difficulty in 
producing distinct sound quality. In addition, these materials are 
feasibly affected by their external environment such as temperature and 
moisture, causing deterioration in the quality and ageing fatigue, thereby 
disadvantageously decreasing the characteristics. 
Plates of aluminum, magnesium, or titanium have also been employed. The 
sound velocities of these materials are high, but the materials have sharp 
resonance phenomenon in high frequency band with small internal loss of 
vibration, or ageing fatigue such as creep occurs in the materials, 
thereby disadvantageously deteriorating their characteristics. 
Boron, beryllium, and their nitrides, carbides and borides provide 
excellent physical properties. Tweeters which use these materials in their 
diaphragms possess sound reproduction limits in the audible frequency 
bands or higher, thereby correctly producing natural sound quality without 
transient phenomenon by the signals in the audible band. However, these 
materials are very expensive, and are difficult to machine. In particular, 
the conventional process for producing a diaphragm by rolling and press 
molding is not practical and a depositing method such as C.V.D. or P.V.D. 
should be employed. These processes are expensive and it is difficult to 
produce speakers of large size. 
Because the conventional material for the voice coil bobbin is typically 
paper (pulp), such as kraft paper, the rigidity of the entire vibration 
system decreases even if materials having excellent physical properties 
are used for a diaphragm. The rigidity of the entire vibration system also 
decreases due to the presence of a bonding layer for bonding the diaphragm 
to the voice coil bobbin, and an energy loss occurs at the bonding layer 
when transmitting the driving force generated in the voice coil to the 
diaphragm. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a process for 
producing a diaphragm for an acoustic device of completely carbonaceous 
materials, which can eliminate the above-described disadvantages 
associated with conventional materials for diaphragms and voice coil 
bobbins. Another object is to provide a process for manufacturing a 
diaphragm which can receive a driving force generated in a voice coil 
without energy loss and without ageing fatigue, such as creep, 
irrespective of external environments, such as temperature and moisture. 
Yet another object is to provide a process which can provide a diaphragm 
with excellent heat resistance, which has an integral structure of the 
diaphragm and the voice coil bobbin, can faithfully reproduce a wide 
frequency range from a low sound range to a high sound range, and which 
can generate a distinct quality of tone. 
The inventor has discovered a process for producing a diaphragm for an 
acoustic device of completely carbonaceous materials comprising the steps 
of mixing and kneading an organic mixture composition containing one or 
more or mixtures of a relatively polymerizable thermosetting resin 
monomer, prepolymer and low polymer, which exhibits a high carbon residual 
yield after calcining, together with one or more types of carbon powders 
such as natural graphite, artificial graphite, kish graphite, carbon 
black, and coke powders, preliminarily molding the same in a film or sheet 
shape, calcining a diaphragm molding molded into a desired diaphragm shape 
and a voice coil bobbin molding molded into a desired voice coil bobbin 
shape from the film or sheet-like molding and then forming an integral 
structure of the diaphragm and the voice coil bobbin by calcining a 
composite material integrated with the diaphragm molding and the voice 
coil bobbin molding by an organic liquid composition exhibiting high 
carbon residual yield in an inert gas atmosphere. 
The word "carbon" in this specification comprises both carbonaceous and 
graphite properties.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A monomer, prepolymer or low polymer of relatively easily polymerizable 
thermosetting resin of a substance exhibiting high carbon residual yield 
after calcining is mixed with one or more types of carbon powders in a 
high speed agitator, such as a Henschel mixer. The mixture is then kneaded 
into a paste-like composition by a kneader capable of imparting a high 
shearing force, such as ball mills, three rolls or two rolls. Then, a 
predetermined amount of hardener is added to the paste-like composition, 
and the mixture is again kneaded to disperse the hardener. Air bubbles may 
be removed through a reduced pressure defoaming machine, if necessary. The 
obtained raw liquid is then preliminarily molded into a film or sheet of 
the desired thickness via coater or a calender rolls by using a back sheet 
having a separable film. 
Then, the back sheet of the film or sheet is removed when the raw liquid 
has solidified into a B-stage resin having plasticity (not hardened) which 
is molded into the shape of a desired diaphragm by a press molding 
machine, a vacuum molding machine or blow-molding machine. In this case, 
the plasticity of the film or sheet may be suitably increased by 
adequately heating or it may be hardened by reaction. After the material 
is sufficiently hardened, the film or sheet is removed from the mold, and 
the molded film or sheet is removed. 
The molded film or sheet obtained by the above-mentioned operation is cut 
into a rectangular shape of desired shape, and the back sheet is then 
removed. The film or sheet is wound on a round rod or a pipe having a 
desired diameter, dimensions, and a smooth surface as a supporting base, 
and fixedly secured at both ends thereof. The voice coil bobbin molding 
obtained by the above-mentioned operation is heated at 50.degree. to 
300.degree. C., sufficiently cured, and then is removed from the mold. 
The diaphragm molding and the voice coil bobbin molding obtained by the 
above-mentioned operations are further insolubilized and infusibilized in 
a heated air oven at an ambient temperature or heated, and then bonded 
together by an organic liquid composition. 
The organic liquid compositions useful in the present invention include 
thermoplastic resins, such as polyvinyl chloride and chlorinated vinlyl 
chloride resin; thermosetting resins, such as phenyl resin, furan resin 
and polyimide; natural high molecular weight substances, such as 
tragacanth gum; asphalt pitches, such as petroleum asphalt and coal tar 
pitch; and one or more types of compositions of dry distilled pitches 
obtained by dry distilling organic high molecules. Carbon powders, such as 
natural graphite and artificial graphite, carbon black, coke powder, and 
wooden carbon, etc. may be added to 5 to 50 wt. % so as to strengthen the 
bond of the diaphragm and the voice coil bobbin during carbonization. 
The organic high molecular substances or pitches are not in the liquid 
state at ambient temperatures. Among these a solution of the initial 
condensate of the materials, or thermally melted materials may be 
preferably used. The organic liquid composition may be coated between the 
diaphragm molding and the voice coil bobbin molding, and bonds by heating 
and by removing its solvent to solidify it. 
The carbonaceous powders used in the present invention include one or more 
types of natural graphite, artificial graphite, kish graphite, superhigh 
elastic modulus graphite fiber, carbon black, wooden carbon powder, etc., 
in such a manner that the grain size of the carbonaceous powder is 
preferably 0.1 to 200 microns of mean grain size. The superhigh elastic 
modulus graphite fiber is preferably 3 mm or less of fiber length. From 10 
to 90 wt. %, more preferably 20 to 80 wt. %, of carbonaceous powder is 
added to the whole quantity of the mixture. In order to develop the 
superhigh elastic modulus function, it is preferable to employ highly 
crystallized natural graphite and superhigh elastic modulus graphite 
fiber. 
The monomer, prepolymer and low polymer of the thermosetting resins include 
furan resins, phenol resins, xylene resins, epoxy resins, and bismaleimide 
triazine resins. Among those, furan resins such as furfuryl 
alcohol/furfurals, furfural/phenols, furfural/ureas, phenol resins such as 
resoles, novolacs, and their mixture resins are preferred due to the 
easiness of operation and molding workability. From 10 to 90 wt. %, 
preferably 20 to 80 wt. %, of the whole mixture is added, or more 
preferably 30 to 80 wt. % is added to avoid problems in workability and 
shape retentivity after calcining. 
The binder capable of being mixed with the thermosetting resin preferably 
includes thermosetting resins, such as polyvinyl chloride, 
polyacrylonitrile, polyvinyl alcohol, polyvinyl chloride vinyl acetate 
copolymer, etc., natural polymer substances, such as lignin and cellulose, 
etc., asphalt pitches, such as petroleum asphalt, coal tar pitch, and 
naphtha decomposed pitch, vinyl chloride pitch suitable to be decomposed 
with an organic solvent, such as MEK, THK, etc., selected in response to 
the objects and as required. 
The diaphragm, the voice coil bobbin and the organic liquid composition 
preferably use the same composition to avoid non-uniform shrinkage at 
calcining time. 
The integral structure obtained by the above-described operation is 
contained in a calcining sheath, and thermally calcined to be carbonized 
at 1000.degree. to 1500.degree. C. in an inert gas phase of nitrogen or 
argon. In the calcining and carbonizing steps, it is important to 
gradually heat the structure at a temperature rising velocity of 
50.degree. C./hr. or lower, preferably 20.degree. C./hr. or lower, up to 
500.degree. C. so as to prevent it from being deformed and cracked. In a 
range or 500.degree. C. or higher, the structure is heated at a 
temperature rising velocity of 20.degree. to 200.degree. C./hr. more 
preferably 50.degree. to 100.degree. C./hr. for economic reasons, and then 
maintained at the highest temperature for 1 to 5 hours so as to obtain the 
homogeneous property of carbonization, and finally allowed to cool 
naturally. 
EXAMPLES 
The following examples are provided to illustrate the process for producing 
a diaphragm for an acoustic device of fully carbonaceous materials, but 
the present invention is not limited to these particular examples. 
EXAMPLE 1 
75 wt. % of initial condensate of furfuryl alcohol/furfural resin (VF-302 
produced by Hitachi Chemical Co., Ltd., Japan) and 25 wt. % of natural 
flaky graphite (having 1 micron of mean particle size) were mixed in a 
Warner mixer to be uniformly dispersed, and further highly dispersed by 
three rolls cooled with water for ink kneading, to produce a material 
paste composition. 4 wt. % of p-toluenesulfonic acid-50%-methanol solution 
was added as a hardener to 100 wt. % of the material paste composition, 
and the mixture was then defoamed through a reduced presure defoaming 
machine while sufficiently agitating keep room temperatures by a high 
velocity homogeneous mixer. 
The raw solution so prepared was coated on a back sheet having an 
exfoliating membrane by a coater having a doctor blade to produce a 90 
micron thick layer, which was preliminarily hardened. A preliminarily 
molded sheet having a sufficiently soft plasticity (B-stage state) was 
obtained. 
Then, the back sheet was removed, and the composition was molded into a 
dome shape by a vacuum molding machine which used a domed molding die 
having a bore of 27 mm in diameter. The molding was thermally hardened by 
80.degree. C. hot air and removed from the mold to produce a diaphragm 
molding. 
A preliminarily molded sheet having a thickness of 70 microns obtained by 
procedures similar to the above operations was cut into a 85.times.6 mm 
rectangle. The back sheet was removed and the sheet was wound on a ceramic 
pipe having a 27 mm outer diameter and a smooth surface, and fixed at both 
ends thereof. Then, the wound sheet on the pipe was held at 100.degree. C. 
for 10 hours and further 180.degree. C. for 24 hours in an air oven to be 
insolubilized and infusibilized. The cured molding was removed from the 
ceramic pipe to produce a voice coil bobbin molding. 
The bottom of the dome of the diaphragm molding was bonded to the voice 
coil bobbin molding using an organic liquid composition. The organic 
liquid composition was formulated by adding 2 wt. % of A-3 hardener 
(produced by Hitachi Chemical Co., Ltd., Japan) to the furan initial 
condenstate, and agitating the mixture. The bonded assembly was allowed to 
stand at ambient temperature for 3 hours to solidify the organic liquid 
composition, further heated to 180.degree. C. to be insolubilized and 
infusibilized, then contained in a calcining sheath, heated at a 
temperature rising velocity of 15.degree. C./hr. up to 500.degree. C. in a 
nitrogen gas atmosphere furnace, and then heated at a temperature rising 
velocity of 50.degree. C./hr. from 500.degree. C. to 1000.degree. C. 
Subsequently, the bonded assembly was held at 1000.degree. C. for 3 hours, 
then allowed to naturally cool, thereby obtaining an integral structure in 
which the completely carbonaceous diaphragm was bonded to the completely 
carbonaceous voice coil bobbin by means of carbon. 
The completely carbonaceous dome-shaped diaphragm (a tweeter for 
reproducing a high frequency sound range) obtained in this matter had a 
diameter of 23 mm and a thickness of 50 microns diaphragm. The voice coil 
bobbin had an outer diameter of 23 mm, a height of 5 mm, and a thickness 
of 50 microns, an elastic modulus of 175 GPa, a sonic velocity of 11.0 
km/sec., an internal loss of tan .delta.9.0.times.10.sup.-3, and a density 
of 1.45 g/cm.sup.3. 
EXAMPLE 2 
70 wt. % of resole phenol resin (PL-2818 produced by Gunei Chemical Co., 
Ltd., Japan) and 30 wt. % of carbon black (MA-8 produced by Mitsubishi 
Chemical Industries, Ltd., Japan) were mixed to produce a material paste 
composition using procedures similar to those of Example 1. Then, a 
preliminarily molded sheet having 1.1 mm of thickness was obtained by a 
similar operation. This sheet was dried, a back sheet was then removed, 
and the sheet molded in a press molding machine mounted with a metal mold 
set at 150.degree. C. into a cone shape having 32.0 cm in diameter of 
bore, hardened, and removed from the molds, thereby obtaining a diaphragm 
molding. 
The above-mentioned material paste composition was also used to obtain a 
preliminarily molded sheet having 0.6 mm of thickness obtained by 
operations similar to those of Example 1. The sheet was cut into a size of 
220.times.35 mm, and the back sheet was then removed. The cut sheet was 
wound on a cylindrical metal mold having 7.0 cm in outer diameter and 
smooth surfaces, fixed at both ends thereof, thermally cured in a press 
molding machine held at 170.degree. C. at the metal mold for 15 minutes, 
and removed from the mold, thereby obtaining a voice coil bobbin molding. 
Then the diaphragm molding and the voice coil bobbin molding were bonded 
together using an organic liquid composition similar to that in Example 1. 
The liquid composition was solidified at 100.degree. C. in a heating oven, 
and further heated to 180.degree. C. Then, similar to Example 1, it was 
calcined to 1300.degree. C. to bond the completely carbonaceous diaphragm 
to the completely carbonaceous voice coil bobbin by means of carbon into 
an integral structure. 
The completely carbonaceous cone-type diaphragm (a woofer for reproducing 
low frequency sound range) thus obtained had a size of 27.5 cm in diameter 
of bore, and 0.8 mm of thickness. The voice coil bobbin had the following 
physical properties: an outer diameter of 6.0 cm, a height of 3.0 cm, a 
thickness of 0.5 mm, an elastic modulus of 126 GpA, a sonic velocity of 
9.5 km/sec., an internal loss of tan .delta.15.times.10.sup.-3, and a 
density of 1.40 g/cm.sup.3. 
EXAMPLE 3 
30 wt. % of initial condensate of furfuryl alcohol/furfural resin (VF-302 
produced by Hitachi Chemical Co., Ltd. Japan) and 20 wt. % of polyvinyl 
chloride resin (having 800 of mean polymerization produced by Nippon Zeon 
Co., Ltd., Japan) were dissolved in tetrahydrofuran, 20 wt. % of 
dibutylphthalate was added, the resultant mixture was then used as the raw 
material of a carbonization binder, 50 wt. % of natural flaky graphite 
(having a mean grain size of 1 micron) was mixed, and the resultant 
mixture was treated in a manner similar to that of Example 1. The solvent 
was then volatilized to be removed, and a preliminarily molded sheet 
having a thickness of 120 microns was produced. 
Then, the back sheet was removed, and the composition was then molded into 
a dome shape by a press molding machine which used a domed molding die 
having a bore of 65 mm in diameter. The domed molding was thermally 
preliminarily hardened by hot air at 180.degree. C., and removed from the 
mold to produce a diaphragm molding. 
A voice coil bobbin was produced from a preliminarily molded sheet having a 
thickness of 90 microns which was obtained in a manner similar to the 
above operation. The sheet was cut in a fashion similar to Example 1 and 
heated to 180.degree. C. to produce a voice coil bobbin molding having a 
65 mm outer diameter, 8 mm height and a thickness of 90 microns. 
The organic liquid composition was formulated by adding 1 wt. % of A-3 
hardener (produced by Hitachi Chemical Co., Ltd., Japan) to the mixture, 
which was then sufficiently agitated, and mixed. The diaphragm and voice 
coil bobbin were bonded together to form an assembly using the organic 
liquid composition in a manner similar to that of Example 1. The solvent 
was volatilized at 100.degree. C. in a heating oven to solidify the liquid 
material, and further treated in an air oven heated to 240.degree. C. for 
8 hours to completely remove the plasticizer. The furan resin was 
completely cured with HCl gas generated by the decomposition of the 
polyvinyl chloride resin. In a manner similar to Example 1, the assembly 
was then calcined to 1200.degree. C. to produce an integral structure of a 
carbonaceous diaphragm and a carbonaceous voice coil bobbin bonded 
together by carbon. 
The completely carbonaceous dome-shaped diaphragm (a squawker for 
reproducing an intermediate frequency sound range) obtained in this manner 
had a diameter of 60 mm, and a thickness of 80 microns. The voice coil 
bobbin had an outer diameter of 60 mm, a height of 7 mm, and a thickness 
of 80 microns, an elastic modulus of 106 GPa, a sonic velocity of 8.0 
km/sec, an internal loss of tan .delta.20.0.times.10.sup.-3, and a density 
of 1.65 g/cm.sup.3. 
Table 1 compares the properties of the diaphragm obtained by Example 1-3 to 
diaphragms prepared from conventional materials. The conventional 
diaphragms are similar to the tweeter of Example 1, in which the diaphragm 
and the voice coil bobbin are separately molded and calcined independently 
under the same conditions, but bonded with an ordinary adhesive instead of 
an intermediate carbon bonding layer. 
TABLE 1 
______________________________________ 
Properties 
Sound Elastic 
Diaphragm velocity modulus tan.delta. 
Density 
Materials (km/sec.) 
(GPa) (.times. 10.sup.-3) 
(g/cm.sup.3) 
______________________________________ 
paper (pulp) 
1.0.about.2.4 
0.2.about.4.0 
20.about.60 
0.2.about.0.7 
polypropylene 
1.3 1.5 60 0.9 
aluminium 5.1 70.0 2.7 
titanium 4.9 110.0 4.5 
magnesium 5.1 44.0 2.about.3 
1.7 
beryllium 12.2 270.0 1.8 
Example 1 11.0 175.0 9.0 1.45 
Example 2 9.5 126.0 15.0 1.40 
Example 3 8.0 106.0 20.0 1.65 
______________________________________ 
As understood from the above table, the diaphragm of Example 1 exhibits 
excellent properties equivalent to those of the beryllium diaphragm. 
Moreover, the diaphragms of Examples 1-3 have a sonic velocity 
approximately twice as large as the conventional metal material. 
Though not shown in the table, the expansion coefficients of the diaphragms 
of Examples 1 to 3 were 2.0 to 3.0.times.10.sup.-6 /.degree. C., with an 
oxidation starting temperature of 400.degree. C. or higher. Thus, the 
material can sufficiently endure against Joule heat generated by a voice 
current flowing in the voice coil. 
As shown in the drawing, tweeter (1) of Example 1 has a higher frequency 
band limiting frequency than a conventional tweeter (2) bonded with an 
ordinary adhesive. 
The high performance diaphragm of the present invention can be 
inexpensively produced by an industrially simple process. Thus, the 
diaphragm and the voice coil bobbin can be preformed in sufficient 
quantity that makes it attractive for use in digital audio equipment such 
as a compact disk player.