Process for producing a piezo- and pyro-electric film

A process for producing a piezo- and pyro-electric film, which comprises biaxially stretching a single- or multi-layered thermoplastic film containing from 3 to 80% by volume, as an average based on the entire film, of a ferroelectric substance, then subjecting the stretched film to polarization treatment by applying a high voltage at a temperature within a range of from 10.degree. C. to a temperature lower than the melting point of said thermoplastic resin, and cooling the film while continuously applying the high voltage.

The present invention relates to a process for producing a very thin 
thermoplastic film having piezo- and pyro-electric characteristics and 
high strength. 
Piezo- and pyro-electric characteristics are usually observed in 
ferroelectric substances. In the case of inorganic substances, barium 
titanate, titanic acid and a lead zirconate solid solution (i.e. so-called 
PZT), are well known. In the case of organic substances, fluorine 
compounds such as polyvinylidene fluoride, are well known. For instance, 
Japanese Examined Patent Publication No. 22271/1973, or Japanese 
Unexamined Patent Publication No. 80202/1980 discloses a process for 
producing a ferroelectric porcelain. Further, Japanese Examined Patent 
Publication No. 13830/1975, or Japanese Unexamined Patent Publication No. 
22974/1972, No. 20870/1973 or No. 39580/1977, discloses an instance 
wherein polyvinylidene fluoride is used. Furthermore, with respect to a 
piezo-electric element prepared by a combination of these inorganic and 
organic substances, Japanese Unexamined Patent Publication Nos. 
139398/1975, 139399/1975, 23698/1976, 29998/1977, 97199/1977, 157297/1979, 
96689/1980 or 155496/1979, or Japanese Examined Patent Publication No. 
47159/1977, discloses a process wherein a ferroelectric porcelain powder 
is added to a high molecular compound, particularly rubber or 
polyvinylidene fluoride, and the mixture is formed into a sheet by means 
of a pair of rolls. 
These conventional materials have fairly good piezo-electric 
characteristics. However, in the case of a material composed solely of 
ceramics, there has been a serious drawback that it is brittle and likely 
to be broken. Further, even when combined with a high polymer compound, no 
adequate strength can be obtained since the composite material is used in 
a non-stretched state, and the durability or vibration characteristics are 
inferior, since the Young's modulus is low. Further, these materials can 
only be formed into a thick plate or sheet, and thus have poor 
applicability. 
Under the circumstances, an extensive research has been conducted with an 
aim to overcome the above-mentioned drawbacks. As a result, it has now 
been found that a thermoplastic film having excellent piezo- and 
pyro-electric characteristics and high strength can readily be obtained in 
a thin film form by a specific combination of biaxial stretching treatment 
and subsequent polarization treatment. 
Namely, the present invention provides a process for producing a piezo- and 
pyro-electric film, which comprises biaxially stretching a single- or 
multi-layered thermoplastic film containing from 3 to 80% by volume, as an 
average based on the entire film, of a ferroelectric substance, then 
subjecting the stretched film to polarization treatment by applying a high 
voltage at a temperature within a range of from 10.degree. C. to a 
temperature lower than the melting point of said thermoplastic resin, and 
cooling the film while continuously applying the high voltage. 
Now, the present invention will be described in detail with reference to 
the preferred embodiments. 
The piezo- and pyro-electric film of the present invention is extremely 
strong as compared with the conventional products, since the strength is 
maintained by the biaxially stretched thermoplastic resin, and yet 
flexible. Thus, it has excellent durability and response characteristics. 
Further, it can be fabricated into a very thin film, which can readily be 
deformed by a very small stress, and which is thus capable of providing a 
quick response. During the biaxial stretching for the preparation of the 
piezo- and pyro-electric film of the present invention, particles of the 
ferroelectric substance are subjected to the shearing stress resulting 
from the molecular chain orientation of the thermoplastic resin, and 
undergo a change in their state from a randomly dispersed state in the 
non-stretched film to an oriented state as stretched in an axial direction 
or in an in-plane direction. This change in the state serves 
advantageously for a piezo-electric film. Namely, the piezo-electric film 
is frequently used for the conversion of a stress exerted in the in-plane 
or axial direction to an electric energy and vice versa, and such a 
conversion can be efficiently conducted by the film of the present 
invention. Whereas, in a case where particles of a ferroelectric substance 
are merely incorporated into a non-stretched film, the particles are 
dispersed randomly and thus serve to one another to cancel out the 
electric energy, whereby the electro-mechanical conversion efficiency is 
poor. 
As the thermoplastic resin to be used in the present invention, there may 
be mentioned polyethylene, polypropylene, poly-4-methylpentene-1, 
polyvinyl chloride, polyethylene terephthalate, polybutylene 
terephthalate, polyvinyl alcohol, polyvinylidene fluoride, a 
polyvinylidene-ethylene trifluoride copolymer, a 
polyethylene-tetrafluoroethylene copolymer and poly-.epsilon.-capramide, 
or a mixture or a copolymer thereof. However, the thermoplastic resin is 
not restricted to these specific examples, and various other types of 
thermoplastic resins may also be employed. Among them, polypropylene, 
polyethylene terephthalate, polyvinylidene fluoride, a polyvinylidene 
fluoride-ethylene trifluoride copolymer and poly-.epsilon.-capramide are 
particularly preferred in view of the properties and workability. 
As the ferroelectric substance for the present invention, a single 
substance such as a metal titanate, a metal stannate or a metal zirconate, 
or a mixture or solid solution thereof, is particularly suitable. However, 
the ferroelectric substance is not restricted to these specific examples, 
and any ferroelectric substance may be employed so long as it has a 
dieletric constant of at least about 5 and piezo- and pyro-electric 
characteristics. The effect of the addition is obtainable when it is added 
in an amount of at least 3% by volume. However, in order to obtain a 
remarkable effectiveness, the ferroelectric substance is used preferably 
in an amount of at least 7% by volume. On the other hand, if it is added 
in an excess amount, the dispersibility into the thermoplastic resin tends 
to be poor, and the stretchability tends to be impaired. Accordingly, it 
is preferred to incorporate it in an amount of not higher than 80% by 
volume. As specific examples of the matal titanate, the metal stannate, 
and the metal zirconate, there may be mentioned BaTiO.sub.3, SrTiO.sub.3, 
CaTiO.sub.3, Mg.sub.2 O.sub.4, MgTiO.sub.3, Bi.sub.2 (TiO.sub.3).sub.3, 
PbTiO.sub.3, NiTiO.sub.3, CaTiO.sub.3, ZnTiO.sub.3, Zn.sub.2 Tio.sub.4, 
BaSnO.sub.3, Bi.sub.2 (SnO.sub.3).sub.3, CaSnO.sub.3, PbSnO.sub.3, 
MgSnO.sub.3, SrSnO.sub.3, ZnSnO.sub.3, BaZrO.sub.3, CaZrO.sub.3, 
PbZrO.sub.3, MgZrO.sub.3, SrZrO.sub.3 and ZnZrO.sub.3. However, they are 
not restricted to these specific examples. These materials may be used 
alone. However, it is possible to improve the effectiveness by a 
combination thereof. No substantial improvement will be obtained by a mere 
mixture thereof. In order to obtain an improvement, it is usually 
necessary to heat and sinter the mixture to change the crystal structure. 
In such a case, the mixing ratio and the sintering temperature may be 
varied depending upon the particular purpose. For instance, when 
CaTiO.sub.3 is added to BaTiO.sub.3, the transformation temperature of the 
crystal moves to the low temperature side and becomes advantageous for use 
at a normal temperature. Further, when PbTiO.sub.3 is added to 
BaTiO.sub.3, the Curie point rises, whereby the useful temperature range 
can be widened as in the case of the addition of CaTiO.sub.3. However, if 
the amount of the addition is excessive, the electro-mechanical coupling 
coefficient tends to be small, whereby the piezo-electric characteristics 
will be impaired. Accordingly, it is preferred that the amount of addition 
is not greater than 10 mol %. Among various combinations, a combination of 
PbZrO.sub.3 and PbTiO.sub.3 is most effective to obtain excellent 
piezo-electric characteristics. At a ratio of PbZrO.sub.3 to PbTiO.sub.3 
being Zr/Ti=55/45, there is a border line of the crystal system between 
the tetragonal system phase and the trigonal system phase. With the 
composition in the vicinity of this border line, the electro-mechanical 
coupling coefficient becomes to be the greatest, and a great 
piezo-electric effect is obtainable. 
The best result may be obtained with a composition comprising from 35 to 65 
mol % of PbTiO.sub.3 and from 65 to 35 mol % of PbZrO.sub.3. 
Further, a solid solution of Pb, La, Zr and Ti makes an effective 
piezo-electric element. This is so-called PLZT. The solid solution 
preferably has a composition represented by the formula: 
EQU Pb.sub.1-x La.sub.x (Zr.sub.y Ti.sub.z).sub.1-(x/4) O.sub.3 
where x is from 1 to 0.05 and y/z is from 50/50 to 80/20. In this case, a 
transparent solid solution is obtainable. Accordingly, the composite film 
thereby obtained will have excellent transparency. To maximize the 
transparency, it is preferred to employ a composition satisfying y/z=65/35 
and x=0.8-0.1. 
As a method for dispersing the ferroelectric substance in the thermoplastic 
resin, followed by film-forming, it is common to employ a method wherein a 
ferroelectric substance presintered at a temperature of from 1000.degree. 
to 1300.degree. C., is pulverized into powder, and then mixed with a 
thermoplastic resin, and the mixture is molded into a film by 
melt-extrusion molding. The mixing method includes a method wherein the 
ferroelectric substance is added during the polymerization of the 
thermoplastic resin, or a method wherein the ferroelectric substance and 
the thermoplastic resin are directly blended by means of a single-screw or 
double-screw extruder. However, the mixing method is not restricted to 
these specific methods. For the film-forming operation, it is common to 
empoly a method wherein the thermoplastic resin containing the 
ferroelectric substance, is heated and melted by an extruder, extruded 
from a T-die or a ring die, and then cooled for solidification. As an 
alternative method, it is possible to employ a method wherein the 
ferroelectric substance is added to a solution in which the thermoplastic 
resin is dissolved or dispersed, and then the solution is spread on a 
substrate and the solvent is evaporated. Furthermore, it is possible to 
employ a method wherein at the time of the addition of the ferroelectric 
substance, a silane-type, a titanate-type or an aluminum-type coupling 
agent is coated on the surface of the ferroelectric substance to improve 
the adhesion of the interface. As specific examples of such a coupling 
agent, there may be mentioned, for example, 
.gamma.-aminopropyltriethoxysilane, 
.gamma.-glycidoxypropyltrimethoxysilane, 
.gamma.-mercaptopropyltrimethoxysilane, 
.gamma.-chloropropyltrimethoxysilane, 
.gamma.-methacryloxypropyltrimethoxysilane, vinyltriethoxysilane, 
isopropyltriisostearoyl titanate, isopropyldodecyl benzenesulfanyl 
titanate, isopropyltris(dioctylbiphosphate) titanate. However, the 
coupling agent is not restricted to these specific examples. 
The film to be formed may be a single layered or multi-layered. The film is 
required to contain at least 3% by volume, preferably at least 7% by 
volume, as an average of the entire film, of the ferroelectric substance 
to obtain distinct piezo-electric effectiveness. There is no particular 
restriction to the method for forming a multi-layered film. It is possible 
to employ an extrusion lamination method, a dry lamination method, a 
solution-coating method, a hot melt coating method or a co-extrusion 
method. Further, the types of the resins for the respective layers may be 
varied taking the stretchability into consideration. In a case where the 
content of the ferroelectric substance is great, particularly more than 
15% by volume, or in a case where the stretchability of the thermoplastic 
resin is poor, good results may be obtained by laminating a thermoplastic 
resin containing no substantial amount of the ferroelectric substance or 
other thermoplastic resin having good stretchability, followed by 
stretching. For instance, in the case of polyvinylidene fluoride 
containing at least 8% by volume of the ferroelectric substance, the 
biaxial stretching is difficult, but the stretching becomes possible by 
employing a method wherein it is laminated with poly-.epsilon.-capramide 
and then stretched. 
For the biaxial stretching, there may be employed either a so-called 
successive biaxial stretching wherein the film is first longitudinally 
stretched and then transversely stretched, or a simultaneous biaxial 
stretching in which the stretching in the longitudinal and transverse 
directions is conducted simultaneously. However, the simultaneous biaxial 
stretching is preferred, since the stretching can smoothly be operated and 
good results are obtainable even when the amount of the ferroelectric 
substance is great. 
In the case of the successive biaxial stretching, the film is first 
oriented in the longitudinal direction and then oriented in the transverse 
direction, whereby anisotropy of orientation is likely to form in the 
in-plane of the film, and the piezo-electric constant tends to be 
irregular. Whereas, in the case of the simultaneous biaxial stretching, 
uniform orientation can be maintained, whereby the piezo-electric constant 
will not be irregular. The stretching rate is at least 1.1 times in both 
longitudinal and transverse directions to obtain the effectiveness. In 
order to obtain a remarkable effect, the stretching rate is preferably at 
least 1.5 or 2 times in both longitudinal and transverse directions. 
The stretching temperature may vary depending upon the amount of the 
ferroelectric substance and the glass transition temperature and the 
melting point of the thermoplastic resin. In the case of the simultaneous 
biaxial stretching, the stretching temperature is preferably within a 
range of from (Tg-10) to (Tm-20+V).degree. C., and in the case of the 
successive biaxial stretching, it is preferred to conduct the longitudinal 
stretching at a temperature of from (Tg-10) to (Tm-30+V).degree. C. and 
then transversely stretching at a temperature of from Tg to 
(Tm-20+V).degree. C., to obtain good results, where Tg is the glass 
transition temperature (.degree.C.) of the thermoplastic resin, Tm is the 
melting point (.degree.C.) of the thermoplastic resin and V is the content 
(% by volume) of the ferroelectric substance as an average value of the 
entire film. As a method for improving the strength of the film or for the 
formation of a thin film, it is possible to employ mono-axial stretching 
instead of the biaxial stretching as in the present invention. In such a 
case, however, the differences in the elasticity and the strength between 
the stretching direction and the direction perpendicular thereto, are 
substantial and great anisotropy in the piezo-electric characteristics 
will be formed, although the strength in the stretching direction can be 
improved and a thin film may be obtained. 
The polarization treatment after the stretching is conducted at a 
temperature of at least 20.degree. C., preferably at least 40.degree. C. 
while applying a direct current voltage. If the temperature is too high, 
the fluidity or melting of the thermoplastic resin becomes excessive. 
Accordingly, the treatment has to be conducted at a temperature lower than 
the melting point or the pour point of the thermoplastic resin, preferably 
at a temperature lower by 10.degree. C. than the melting point or pour 
point. The voltage to be applied is usually within a range of from 2 
V/.mu. to 300 v/.mu., preferably 10 V/.mu. to 200 v/.mu., whereby good 
results are obtainable. The time for the polarization treatment is 
selected within a range of from 10 minutes to 4 hours depending upon the 
types of the ferroelectric substance and the thermoplastic resin used. 
If the applied voltage is removed at a temperature higher than the 
temperature for the polarization treatment, the piezo- and pyro-electric 
characteristics will diminish. Therefore, after the completion of the 
polarization treatment, it is necessary to cool the film to a temperature 
lower than the temperature for the polarization treatment while 
continuously applying the voltage. 
The piezo- and pyro-electric film of the present invention may be used for 
a headphone, a speaker, a microphone, a phonocartridge or a telephone 
apparatus in the audio field, by utilizing its piezo-electric 
characteristics. Further, as a supersonic transducer, it is applicable to 
a hydrophone, a supersonic microscope, a fish finder or a submarine sonar. 
As other applications, it may be used for a supersonic diagnostic 
appatatus, a sphygmomanometer, a phonocardiographic microphone, a pressure 
meter, an impact meter, a vibrator, a keyboard, etc. As an application of 
its pyro-electric characteristics, it may be employed for an infrared 
sensor, a laser power meter, an infrared vidicon, a radiation thermometer, 
a millimeter wave detector, a fire alarm, an invader detector, a copy 
machine, etc.

Now, the present invention will be described in further detail with 
reference to Examples. However, it should be understood that the present 
invention is by no means restricted by these specific Examples. 
EXAMPLE 1 
Powders of PbTiO.sub.3 and PbZrO.sub.3 were mixed to obtain a composition 
comprising 47 mol % of PbTiO.sub.3 and 53 mol % of PbZrO.sub.3. The 
mixture was sintered at a temperature of 1250.degree. C. for about 1 hour, 
and then pulverized to obtain a powder. This powder was added to 
polyethylene terephthalate chips in an amount of 20% by volume, and the 
mixture was heated and melted by an extruder to obtain blend chips. This 
blend chips were further heated and melted at 280.degree. C. by an 
extruder, and extruded from a T-die to obtain a film having a thickness of 
70 .mu.m. This non-stretched film was simultaneously biaxially stretched 
at 100.degree. C. at a stretching rate of 2.8.times.2.8 times (i.e. 2.8 
times in each of the longitudinal and transverse directions). Then, 
aluminum was vapor-deposited on both sides, and then a direct current 
voltage of 50 V/.mu. was applied at 120.degree. C. for 30 minutes. Then, 
the film was oC for 30 minutes. Then, the film was cooled to room 
temperature while continuously applying the voltage. The piezo-electric 
characteristic of the film thereby obtained was 18.times.10.sup.-12 C/N at 
d.sub.31. Further, the pyroelectricity at 50.degree. C. was as strong as 
1.2.times.10.sup.-8 C/cm.sup.2 .multidot.deg. Thus, the film had excellent 
piezo- and pyro-electric characteristics. 
EXAMPLE 2 
The same blend chips as used in Example 1 and non-blended polyethylene 
terephthalate chips were subjected to co-extrusion so that the blend layer 
constituted the center layer, whereby a three layered non-stretched film 
having a thickness of 10/40/10 m was prepared. This non-stretched film was 
simultaneously biaxially stretched at a stretching rate of 3.times.3 times 
at 100.degree. C. Then, aluminum was vapor-deposited on both sides, and 
then a direct current voltage of 100 V/.mu. was applied at 110.degree. C. 
for 50 minutes. The film was cooled to room temperature while continuously 
applying the voltage. The piezo-electric characteristic of the film thus 
obtained, was 14.times.10.sup.-12 C/N at d.sub.31. Further, the 
pyroelectricity at 50.degree. C. was 0.9.times.10.sup.-8 C/cm.sup.2 
.multidot.deg. Thus, the film had excellent piezo- and pyro-electric 
characteristics. 
EXAMPLE 3 
The sintered powder as used in Example 1 was added to polyvinylidene 
fluoride in an amount of 30% by volume, and the mixture was heated and 
melted in an extruder to obtain blend chips. The blend chips and 
poly-.epsilon.-capramide were subjected to co-extrusion to obtain a 
non-stretched double-layered film having a thickness of 70/30 .mu.m. This 
non-stretched film was simultaneously biaxially stretched at 130.degree. 
C. at a stretching rate of 3.times.3 times. The polyvinylidene fluoride 
layer was peeled off and aluminum was vapor-deposited on both sides, and 
then a direct current voltage of 120 V/.mu. was applied at 110.degree. C. 
for 2 hours. The film was cooled to room temperature while continuously 
applying the voltage. The piezo-electric characteristic of the film thus 
obtained, was 28.times.10.sup.-12 C/N at d.sub.31. Further, the 
pyroelectricity at 50.degree. C. was 2.3.times.10.sup.-8 C/cm.sup.2 
.multidot.deg. Thus, the film had excellent piezo- and pyro-electric 
characteristics. 
EXAMPLE 4 
PbTiO.sub.3 powder was added to polypropylene chips in an amount of 30% by 
volume. The mixture was heated and melted in an extruder to obtain blend 
chips. The blend chips were heated and melted in an extruder at 
240.degree. C., and extruded from T-die to obtain a non-stretched film 
having a thickness of 100 .mu.m. This non-stretched film was 
longitudinally stretched at 130.degree. C. at a stretching rate of 4 
times, and then transversely stretched at 150.degree. C. at a stretching 
rate of 4 times. To both sides of the biaxially stretched film, aluminum 
was vapor-deposited in a thickness of 500.ANG., and then a direct current 
voltage of 150 V/.mu. was applied at 100.degree. C. for 1 hour. The film 
was cooled to room temperature while continuously applying the voltage. 
The pyroelectricity of the film thus obtained, was as good as 
1.8.times.10.sup.-8 C/cm.sup.2 .multidot.deg. 
EXAMPLE 5 
To PbTiO.sub.3 powder, 0.3% by weight of .gamma.-aminopropyltriethoxysilane 
was added, and mixed by a Henschel mixer. The mixture was added to 
polypropylene chips in an amount of 30% by volume. Then, in the same 
manner as in Example 4, a non-stretched film having a thickness of 100 
.mu.m was prepared. This non-stretched film was longitudinally stretched 
at 130.degree. C. at a stretching rate of 4 times, and then transversely 
stretched at 150.degree. C. at a stretching rate of 4 times. The breakage 
at the time of the stretching was very little as compared with the case 
where no silane treatment was applied. The polarization treatment was 
conducted in the same manner as in Example 4. The pyroelectricity of the 
film thus obtained was as good as 1.9.times.10.sup.-8 C/cm.sup.2 
.multidot.deg. 
EXAMPLE 6 
PbO, La.sub.2 O.sub.3, ZrO.sub.2 and TiO.sub.2 were mixed for about 3 hours 
while adding acetone, to obtain a composition represented by Pb.sub.0.8 
La.sub.0.2 (Zr.sub.0.65 Ti.sub.0.35).sub.0.95 O.sub.3. The mixture was 
dried at 100.degree. C. for 24 hours. Then, the mixture was presintered at 
900.degree. C. for 2 hours, then pulverized, again presintered at 
700.degree. C. for 2 hours, and again pulverized. The powder thus obtained 
was added to polyethylene terephthalate in an amount of 20% by volume. The 
mixture was melted in an extruder to obtain blend chips. The blend chips 
were further heated and melted at 280.degree. C. in an extruder, and 
extruded from a T-die to obtain a film having a thickness of 100 .mu.m. 
The non-stretched film thus obtained was simultaneously biaxially 
stretched at 120.degree. C. at a stretching rate of 3.times.3 times. Then, 
aluminum was vapor-deposited on both sides, and then a direct current 
voltage of 80 V/.mu. was applied at 110.degree. C. for 40 minutes. The 
film was cooled to room temperature while continuously applying the 
voltage. The piezo-electricity of the film thus obtained was 
12.times.10.sup.-12 C/N at d.sub.31. 
COMATIVE EXAMPLE 1 and EXAMPLES 7 to 18 
Non-stretched films were prepared in the same manner as in Example 1 except 
that the concentration of the ferroelectric substance was varied. The 
non-stretched films were simultaneously biaxially stretched at a 
stretching rate of 3.times.3 times at various stretching temperatures, and 
then subjected to the polarization treatment in the same manner as in 
Example 1. The stretchability and the piezo-electric characteristics were 
evaluated. The results are shown in Table 1. The polyethylene 
terephthalate had a glass transition temperature of 80.degree. C. and a 
melting point of 260.degree. C. as determined by DSC measurement. 
TABLE 1 
______________________________________ 
Ferro- 
electric 
Stretching Piezoelectric 
substance 
temper- coefficient 
(% by ature Stretch- d.sub.31 
volume) 
(.degree.C.) 
ability (10.sup.-12 C/N) 
______________________________________ 
Comparative 
10 60 Broken -- 
Example 1 
Example 7 
10 80 Good 9 
Example 8 
10 100 Good 9 
Example 9 
10 120 Good 11 
Example 10 
20 85 Good 16 
Example 11 
20 110 Good 18 
Example 12 
20 130 Good 19 
Example 13 
30 90 Good 25 
Example 14 
30 120 Good 27 
Example 15 
30 140 Good 27 
Example 16 
50 100 Good 37 
Example 17 
50 130 Good 39 
Example 18 
50 150 Good 37 
______________________________________ 
COMATIVE EXAMPLES 2 and 3, and EXAMPLES 19 to 25 
Non-stretched films were prepared in the same manner as in Example 4 except 
that the PbTiO.sub.3 powder was added to polypropylene chips in various 
concentrations. The non-stretched films were successively biaxially 
stretched at a stretching rate of 4 times in each of the longitudinal and 
transverse directions at various stretching temperatures, and then 
subjected to the polarization treatment in the same manner as in Example 
4. The stretchability and the piezo-electric characteristics were 
evaluated. The results are shown in Table 2. The polypropylene had a glass 
transition temperature of -18.degree. C. and a melting point of 
167.degree. C. as determined by DSC measurement. 
TABLE 2 
__________________________________________________________________________ 
Longitudinal stretching 
Transverse stretching 
Piezoelectric 
Stretching Stretching coefficient 
PbTiO.sub.3 
temperature 
Stretch- 
temperature 
Stretch- 
d.sub.31 
(% by volume) 
(.degree.C.) 
ability 
(.degree.C.) 
ability 
(10.sup.-12 C/N) 
__________________________________________________________________________ 
Example 19 
10 120 Good 130 Good 6 
Example 20 
10 130 Good 150 Good 8 
Comparative 
10 140 Good 160 Broken 
-- 
Example 2 
Example 21 
20 130 Good 140 Good 13 
Example 22 
20 140 Good 150 Good 13 
Comparative 
20 150 Good 160 Slightly 
4 
Example 3 necked by 
stretching 
Example 23 
30 140 Good 150 Good 15 
Example 24 
30 150 Good 160 Good 19 
Example 25 
30 150 Good 165 Good 17 
__________________________________________________________________________