Note: Descriptions are shown in the official language in which they were submitted.
~4~34
.
FIELD OF THE INVENTION
This invention relates to films of polyvinyli-
dene fluoride, abbreviated herein as PVF2, dielectric
uses thereof, for example, capacitors~ and methods o
making such films.
BACKGROUND OF THE INVENT~ON
Many electronic circuits, ~uch as photofla~h
circuits of cameras, require ~ capacitor to store ~nd
deliver large amounts of charge, for example, to fire
the flash. Conventionally, such capacitors sre electro-
lytic in nature, thflt is, they use an electrolyte $n
sddition to a dielectric oxide fllm on a layer of metal,
the oxide film being disposed between the layers of the
met&l. Although they are advantageously sm~ll and
inexpensive, such cspacitors suff~r from a number of
disadvantages: a) They have to be reformed after
sitting idle on the shelf--that is, the oxide film
becomes deformed, resulting in decreased capacitsnce.
Reforming acts to restore the oxide film. The reforming
voltage can be a significant and undesirable draw on the
power source when that power source is one or more
batteries, as in a cam~ra. b) Electrolytic c~pacitors
do not maintain uniform capacitance over time. Vari-
ations of from -20% to +15070 of the nominal value are
common. This is due in part to thP fact that capaci-
tance in such capscitors is sub~ect to vari~nce with
changes in temperature. c) The electrolyte causes h$gh
leakage currents, high equivalent series reslstance ~nd
high dielectric loss tangents or dissipation factors.
d) Electrolytic capacitors are polarized so as to be
mountable in a cireult in only one direction. e)
Finally, they must be used within the voltage range for
which they were designed.
Attempt~ to replace such capac~tor~ w~th dry
polymeric film capacltor~ lacking the electrolyte have
-2- ~ 2 ~
suffered from the fact that the dielectric films have
had low dielectric constants and relatively large thick-
nesses. Since capacitance C of a capacitor is de~er-
mined by the well-known equation
l) C 5 KA X 8.85 X 10-l2/ t farads
where K is the relatlve dielectric constant~ A i6 the
capacitance area in square meter~ and t i6 the thlck-
ness of the fllm in meters, the resulting capacit~nces
have been less than desired. The capacLtance den~ity
C/V of such a capacitor is al80 undesirably reduced,
where C/V is determined by the equation
2) C/V = K X 8.85 X 10-l2/t2 farads/m 3
and K and t are as defined above. As a result of K
being lower and t being too large in capacitors using
such films, a larger volume capacitor ls required to
obtain the same capacitance as in an electrolytic
capacitor. Since the trend in csmeras is to make the
cameras smaller rather than larger, there is no room for
enlarged capacitors in a camera fla~h circuit.
Improvements in both the value of K and of t,
that is, a larger K and a smaller t, h~ve been obtained
heretofore in PVF2 films. However, prior to this
invention only one of three polycrystalline ph~es of
such film was thought to produce significant value6 of
K. That was ~he beta phase that is produced by first
quenching the film as it is extruded from the melt, and
thereafter stretching it at temperstures signiflcantly
below its melt point (about 185~C), e.g., between 60
and 100C. ~See, for example, Murayama, J. of Pol2m.
Sci., Polym. Physics Ed., Vol. 13, pp. 929-945 (1975),
and especially Films A and E or F of Table I, p. 930.)
The alpha phase that occurs when the film i6 quenched
and not stretched, or stretched while xtill molten9 has
not been con~idered particularly useful as a cap~citor
dielectric,as its K values have been no better than
about lO.
Nevertheless~ ~uch beta phase PVF2 has not
been completely successful in capacitors~ During charg-
ing of the capacitor, the d~electric film un~void~bly
becomes poled. One reason for the laclc of Buccess i8
bel~eved to be the tendency of the beta phase to hAve
high piezoelectric properties after it has been ~o
poled. That is, unlike alpha ph~se PVF2, bet~ ph~se
PVF2 film has high piezoelectric constant~, for
example, at leas~ 1 X 10-1l meter~/volt after pol~ng
with a field of 1 megavolt/cm (MV/cm) for 1 hour at room
temperature. Such piezoelectric properties are detri-
mental to capacitors because they cau~e substantial
dimensional changes when high electric fields are
applied. This property is reported, for example, as
1'ferroelectricity", in Edwards, "Radiation Response and
Electrical Proper-ties of Polymer Energy Storage
Capacitors: PVF23 Polysulfone, and Mylar") Nasa
Conference Publication No. 2186, p. 1 (1981) and
especially p. 2. Such dimensional changes in turn tend
to stress the capacitor to the point ~t which fractures
and other mechanical fa~lures occur. Still another
disadvantage of the beta phase PVF2 is that production
of it requires &dditional equipment and 6teps in the
manufacturing process. That is, after the ~xtruded film
is quenched to form the alpha phase, additional equip-
ment is required to stretch the film to convert the film
to the beta phase. Even if the alpha phase were known
heretofore to produce high dielectric constants, whlch
is believed not to be the case, the thicknesses l't'l, in
equations 1) and 2) above, obtainable for alpha phase
film by merely quenching the f~lm without sub6equen~
~tretching, have been too large to produce 8~8nificant
values of C or C/V.
Therefore, there has been a great need to pro-
vide PVF2 film with the increased dielectric eonstant~
that are characteristic of beta phase PVF2, but
~ 2
--4--
withou~ the piezoelectric constantg thAt h~ve been
characteristic of poled bet~ phase film.
SUMMARY OF THE INVENTION
I have discovered that PVF2 film c~n be manu-
factured with the dielectric constants that heretoforehave been characteristic of only bet~ ph~se PVF2 film,
but without the piezoelectric beha~ior of poled beta
phase film. Dry fllm capacitors compri6ing such PVF2
film have high energy denslty values coupled w$th
dimensionally sound construction. Such capacitors in
turn provide improved camera flash ~pparatus having
reduced volume, enabling ~maller cameras to be con~
s~ructed.
More specifically, in accord with one aspect of
the invention there is provided a ~heet of film com-
prising poly(vinylidene fluoride) characterized by a
dielectric constant of at least about 12, and a piezo
electric constant no greater than about 4 X 10-12
meters/volt when poled at 1 MV/cm for 1 hour at room
temperature. Such piezoelectric constants are less than
hslf the constants that occur when using beta phase. In
8 preferred embodiment, ~uch a fllm has a crystalline
structure that i~ predom~nantly alpha phese.
In accord with another aspect of the invention,
there i~ provided a method for forming such ~ film. In
the method which comprises the ~teps of B) extruding
molten poly(vinylidene fluoride) generelly ~n the shape
of a f~lm, ~nd b) stretching said film while 8~ill
molten, the improvement resides ~n stretching the film
during step b) by an amount effective to reduce the film
thickness to a v~lue no gre~ter than about l/50th the
originsl thickness.
In ~till another aspect of the inventlon, there
is provided a wound capacitor comprising a core and a
pair of interleaved ln~ulative sheets each bearing an
electrically conductive layer 9 the ~heet~ a) having an
insul~tive thickne~s no greater than about 5 microns,
~L~3~
and b) being wrapped around the core with the conductive
layer of each member of the pair being ~oined to a sep-
arate respective one of a pair of electrodes. The
capacitor is improved in that the in~ulative sheets are
each a film comprising poly(vinylidene fluoride) char-
acterized by dielectric and piezoelectric constant~ ns
noted in the previous paragraph. The preferred ~orm~ of
such capacitors feature PVF2 sheets that are pre-
dominantly alpha phase in cry~talline itructure, wlth
film thicknesses and dielectric constants that produce
C/V charge densities of at least about 5 farad~/m3.
In accord with yet another ~spect of the lnven-
tion, there is provided a flash spparatus for use w~thin
a circuit of a camera, comprising an electronic fl~sh
tube, and means for firing the flash. The app~ratu6 i~
improved in that the flash firing means includes the
capacitor described above.
Thus, this invention advantageously fea~ures a
PVF~ film of high dielectric constant, without the
piezoelectric drawbacks of beta phase PVF2.
It is another advantageous feature of the
invention that such film6 have h~gh voltage breakdown
strengths.
Still another advantageous feature ~8 that such
PVF2 film has been found to produce superior, reduced
dissipation factors, which in turn lnRures that the
equivalent series resistance is reduced for a given
capacitance.
Still another advantageous feature is that such
films are readily made by the 6impler procedure of uni-
axial stretching only.
It is a related advantageous feature ~hat dry
polymeric f~lm capacitors made from ~uch PVF~ film
hsve high energy dencities without the piezoelectric
stre6ses here~ofore characteristic of PVF2 film pro~
viding such energy densitie6.
-6- ~3~8~
It is another related advantageous feature of
the ~nvention th~t such a capacitor can be made from
such film w~thout the c~pacitor cracking or failing dur-
ing use with high electric field6.
Yet another advantageous feature i6 that Quch
capaci~ors are manufacturable in sizes, for at least
certain voltage ratings, that are smaller than compar-
ably-rated electrolytic capacitor~, producing photoflash
circuits with reduced volumes.
Other advantageous features will become sppar~
ent upon reference to the follow1ng "Descript~on of the
Preferred Embodiments", when rePd in light of the
attached drawings.
BRIEF DESCRIPTION OF THE DRAWI~GS
Fig. 1 is a plot of an infrared absorbance
spectrograph of a PVF2 film made in accordance with
the invention;
Fig. 2 is a schematic illustrstion, partially
in section, of apparatus used to manufacture the PVF2
film;
Fig. 3 is a graph illus~rating the relation-
ship, in one example, of the surface speed of the chill
wheel to the dielectric constant of the PVF2 fllm;
F~g. 4 is a fragmentary sectional view of one-
half of a capacitor manufactured in accordance with theinvention, taken along a radius of the capacitor; and
Fig. 5 is a schematic view of a camera utiliz-
lng the flash circu~t o the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
~0 I have discovered that PVF2 film can be pre
pared with the dielectric const~nts of bets pha~e
PVF2, while retaining the electr~cal properties, and
their attendan~ advantages, of alpha phase PVF2.
Capacitors made from such dielectric film have 6uperior
properties, including stable dimensional properties.
Such capacitors are useful in a wide range of circui~s.
~ ~ 3
--7--
More specific~lly, stretching PVF2 ~llm 8uf-
fic~ently while it is still in the molten Rt~te 60 ~6 to
prevent formation of the beta phase, causes the film to
h~ve dielectric constants ranging from 12 to about 16
and higher. Proper stretching causes the film to have
final thicknesses no greater than 5 microns, and as thin
as 1 micron, and still be free of pinhole6 and ~oids.
When the cryst~lline structure o~ the finished ~ilm iB
examined, it is found to be predominantly alpha phase.
As used herein, "predominantly alpha phase" means at
least 75% ~by weight) of the crystall~ne portion of the
fi~m is alpha phase in crystalline structure.
In the embodiments which follow, the film of
the invention is described as useful to form a dry
capacitor wherein no dielectric liquid i~ included to
remove ~ir pockets. In addition, useful forms of the
capacitor of this invention include those wherein the
~ilm of thls invention is combined with such a dielec-
tric liquid to form a wet capacitor. Furthermore, the
film is useful beyond its use in a capacitor, for exam-
ple as tubing insulation, diaphragms for instruments ~r
pumps, and protective surfaceæ for materials exposed to
weather or corrosion.
To determine the percent of alpha phase present
in the film's crystalline structure, infrared ab60rption
spec~roscopy i~ used, as is described in U.S. Patent No.
4,298,719, col. 5, lines 23-42. Specifically, the
absorption 6pectroscopy curve is exsmined for the curve
values at 510 and 530 cm~l, where 510 is character-
istic of beta phase and 530 of alpha phase. The amountof absorbsncy D is measured as the area under the curve
for the peak in question. Thus, ~he proportion of the
alpha phase crystal~ present, by crystalline ~eigh~,
compared to the total crystalline weight (alpha phase
plus beta phase), is determined by the equat~on
-8- 12~4~B4
% = 530 X loo
D510 + D530
(The third crystalline phase, gamma, is co ~mall in
quantity that it can be ignored 3 Referring to the
curve of Fig- 1, Ds30 and D510 are readily measured
from the point~ of the curve identified, and the per-
centage determined. For the particular curve ahown a~
Fig. 1, the percent that is alpha pha6e i8 greater t~lan
90% .
The apparatu~ and method apparen~ from F~g. 2
are particularly useful in manufacturing ~heets of
PVF2 of this invention. A conventional extruder 10 is
supplied with particulate PVF2 polymer Yia a hopper 20
so that the screw 30 of the hopper i8 driven at B
desired RPM by motor 40. Any weight average molecular
weight (Mrw) of the PVF2 is useful, as long as it
insures the PVF2 is in particulate form. For example,
a Mw of 105 is useful.
Heaters, not shown, preferably ~upply suxiliary
heat to extruder 10. Molten polymer i8 delivered from
the extruder to a conventional die 50 having a rectang-
ular opening 60 with a fixed length and a variable width
"w." The hot polymer melt M flows out of die 50 acro6s
a distance "Y" to a conventional, rapidly rotst~ng 6ur-
fsce such as chill wheel or roller 70 operated a~ RPM'sand temperatures hereinsfter described. A~ter ~ol~di-
fying on the chill wheel 70, the film i8 carried off to
edge slitters 80 and tske-up roller 90 ehst operate~ at
RPM's sufficient to maintain tension on the film and
avoid wr~nkling. Optionally, an air ~et 100 or a vacuum
holddown (not shown) i6 added to temporarily "pin" the
polymer film to chill wheel 70. Preferably9 temperature
control means, such a~ a coolant, are added to wheel 70,
to ma~ntain the temperature of the 6urface of the wheel
below the melt temperature (160-185~C) of the PVF~.
~234l~
g
Most preferably, ~uch surf~ce temperature i6 m~ntained
at a value between about 30C and about 120~C.
The features of the app~ratus of Fig. 2 that
are consldered important to the invention are the
relative speeds of the extruded mel~ M and the surfsce
speed of wheel 70, flow distance Y, and die openlng
width w. As to the relative speeds, the desired
properties of the final film, including thickness ~nd
dielectric constant, are achieved only if the surface
speed of wheel 70 is selected to be a high multiple of
the lineal speed of extruded melt M. The necessary RPM
for the chill wheel depends upon the particular
apparatus selected, and is readily determined for a
given apparatus by experimentation. Fig. 3 is a plot of
the RPM's needed for a 20 cm diameter wheel 70 to
produce dielectric constant~ K of at least 12 when ehe
melt M is ex~ruded at a lineal speed of about 34.5
cm/min from a die opening width w ~ 254 ~. This
particular apparstus should be operated with wheel 70
rotated at a minimum of about 57 RPM, a value which,
when converted to 43 cm/sec peripheral speed, is at
lesst sbou~ 105 times thst of the lineal speed of melt M.
In another form of the apparatus (Example~ 1-4
hereinafter), it was found that the ratio of ~urface
speed of the chill wheel and the lineal speed of extru-
ded melt should be at least about 45 for best results.
Faster relative speeds of rotation of wheel 70
will provide even larger dielectric con~tants and thin-
ner f~lms, and greater uniaxial orieDtation within the
film.
Although an exact understanding of the mechan-
i~m has not been achieved and is not needed to practice
this invention, it i8 believed that the high diele~ric
constantR, and the relative lsck of piezoelectric
activity when poled, of the film of the inventi~n are
achieved by ~tretching the film, or equivalently,
-10- ~ ~ 3 4
reducing its thickness, by a particul~r amount.
Specifically, the film is stretched or reduced in thick-
ness by a stretch ratio of at least about 50, during or
before the chilling of the film below lts molten point.
For example, if the film as extruded from the die h~s an
initial thickness of 254 micron6, it should have a f~nal
thickness after stretching that i~ no gre~ter ~han about
5 microns (1/51 reduction) to insure that the high
dielectric constants and low piezoelectric constants sre
~chieved. A final thickne~ greater than 5 micron~ al80
provid~s such constants, if the initial extruded thick-
ness is also larger than the final thickness by a factor
of 50. E.g., an initial thickness of 500 microns, when
reduced to 10 microns by the procedure of this inven-
tion, can be expected to hsve a dielec~ric constant ofat least 12 and a piezoelectric constant no greater than
about 4 X 10-l2 meters/volt when poled as described
above.
A variety of flow dist~nces Y is useful within
the invention. The most critical aspect of distance Y
is that it not be so large as to allow the melt M to
601idify before reaching wheel 70, or fiQ as to prevent
adherence of the film to wheel 70. Useful value~ of Y
range from about 0.1 to about 5 cm. Mo6t preferably,
dlstance Y does not exceed about 2.5 cm.
Preferably, die opening width w i~ selected to
minimize the thickness of melt M that is extruded,
thereby reducing the final thickness of the film that ls
achieved. U6eful value6 of width w range from 25 to
about 1000 ~, with 250 ~ being preferred. Thus,
although final film thicknesses greater than 5 ~ are
also useful, if the film is to be u6ed in a photofla~h
capacitor ~s in the preferred embodiment, the fin~l
thickne~s should be <5 ~, U6~ ng a 6tretch ratlo
>50.
~3~
--11--
The indi~idusl components ~f the afore-
described appar~tus sre conventional. U~eful example~
include the Brabender Model 2523 Deluxe Vented Extruder,
and chrome-pl~ted ~tainless steel chill wheel~ operated
at from about 30 to about 80 revolution~ per minute,
depending upon the diameter of the wheel.
Alternatlvely, the film i8 formed with the
aforesaid propertles by extruding melt M onto a plastic
6upport, such as poly(ethylene tereph~halate), not
shown. This support with the PVF2 still molten there-
on is partially wrapped around wheel 70 so that both the
suppor~ and the PVF2 are stretched by ~he rapld rots-
tion of the wheel.
Yet another alternative manufacturing technique
comprises the coextrusion of such a plastic ~upport
along with the P~F2, so that both are driven (not
shown~ by wheel 70, while still molten, and thereby
; stretched.
It is readily app~rent from the preceding
description that the manufacturing process is improved
in thst only uniaxial stretch~ng is required. Thus, the
additional equipment that would be needed to obtain
biaxial stretching is not necessary.
The PVF2 film described above has the follow-
ing superior properties, in addition to the afore-
- mentioned high dielectric constant and low piezoelectric
constant: reduced dissipation factors and high voltage
breakdown strengths. Therefore, the film is us~ful in a
variety of applications, partlcularly those requiring
high dielectric constants. One such u e is as the d~-
electric for a capacitor. A capacitor 200, Fig. 4, ~8
prepared from sheet~ of the afore-described PVF2 film,
by applying conductive, metallic electrode layerx 212
and 212' on two such PVF2 sheets 214 and 214' so that
the edges 216 and 216', respect~vely, of the two sheet~
are left uncoated with metal. Any conventional pro~
-12- ~4~
cedure can be used to apply ~he met~llic l~yers. The
insulative thickness of the sheet~, that is, the
thickness measured without includlng the metallic
layers, is preferably no greater than about 5 m~crons.
The met~llic layers have any suitable resistivity, for
exsmple, 1 to 4 ohms/square, with thickne6se6 preferably
from 500 to 2000A. The thus-coated sheet6 (identlfied
as composites A and B) are then wrapped ~n interleaved
relation around a core 220 of any de~ired shape~ one
composite stacked on ~he other9 60 that edges 216 and
216' are at opposite ends of the core. Soft conductive
metal pieces 221, 221', such as flame sprayed metal, are
applied at the edges 222 and 224 of the wrappings so as
to separately electrically interconnect hll of the
layers 212 at one end, and all the layers 212' at the
other. The metal pieces 221 and 221' are wired to the
capacitor's lead wires, not shown, and encasing plastic
ends 230 and a cover 240 are applied.
The capacitor constructed ~s described above is
useful in any electrical circuit. Its shape is that of
the core 220. It is particularly useful in flash
apparatus for cameras. The increased C~V v~lues permit
the capacitor to have reduced dimensionsg a property
especially needed in new lines of pocket cameras being
introduced by camera makers. As depicted in Fig. 5,
such a camera 300 comprises flash appara~us that
includes an electronic flash tube 318 which is wired to
a high voltage power supply 326 via a control c~rcuit
324. Power supply 326 also supplies power to the lens
motor drive circuit 330 that is controlled by an
optional automatic focus detector 328. The drive
circuit in turn operates the positioning of lens 342 V~3
motor 332 so that the image t'I" is properly focused on
film 344. All these components are generally described
in U.S. Patent No. 4,291,958, issued September 29 3 1981,
by Lee Frank et al.
-13- ~23~
The firing means for the fl~sh appar~tufi
includes the flash control circuit 324 and of course
power supply 326. Control circuit 324 in turn includes
two c~pacitors -one which is a triggering capacitor (not
shown) controlled by the circuit 324, and the other of
which is the firing capacltor that supplie~ the energy
to actu~lly fire tube 318. The c~pacitor of this inven-
tion is particularly useful ~s the firing capacitor.
The cap~citor is fired ~nd the tube flashed when ~he
camera shutter release button (not shown) i~ actu~ted,
if the camera needs additionsl light for the exposure in
question.
Examples
The following examples further illustrate the
invention.
Examplés 1-8
Two different forms of the app&ratus shown in
Fig. 2 and described above, were used to prepare PVF2
film. The following features of the appsratus were
selected:
-14- ~ 2 3 ~
Table I - Ap~ar~tus P~ra_ eters
Examp_es 1-4 Examples 5-8
Screw Diameter 1.9 cm 2.54 cm
Length/Diameter Ratio 25/1 24/1
Compression Ratio 3/1 3/1
No. of Feed Flights 15 12
No. of Taper Flights 5 6
No. of Metering Flights 5 6
10 No. of Heating Zones in Barrel 3 3
No. of Heating Zone~ in Die
Temperature of Hea~ing ~See T~ble II) (See T~ble
III)
Width of Die 10 cm 15 cm
Die Opening 254 ~ 305 ~
Chill Wheel ~iameter 7.6 cm 20.3 cm
Chill Wheel RPM (See Table II) (See Table
III3
Extruder Motor Horse-
power 3.0 3~0
Extruder RPM 10 15
Wind-up Drive No Yes
Air pinning No Yes
Die opening geome~ry horizontal vertical
Chill wheel
temperature 50~C 40UC-70C
~3~
_1
.,. tn
~ ~ 8 ~ ~ ~o ~ ~ o~
Cll 0 ~ c~
a~ 0 ~E
o~
. . . . ,~ a
_I C ~ o~
~ ~ ~ 0~ ,~ ~ ~ ~ O
v u~ ~ _Iv :~ IY ~
_, _, q,
~1 a~
~ ~ ~ ~ o o C~ ~
I _J 0
~ .u~ ~ u~
Q~ ~ U O CO ~ 1 "~_ _
P~ J :~ a~ ~ E O
C 0 X ~ ~ ~ ~ ~ ~
~0 ~ ~1 J-i ~ _ - _ ~ 1~ r ~ ~ :1 ~ N X 1
~q ~ ~ ~ . ~ a~ ~ v o ~
O ~ O ~0 ~ c~
C~
o ~ O o o o ~ a~ c~
o~ a c~
~ ~ ~ C~ ,Q ~ ~ C~
~ 1~ t`~ O o O oh ~) e~ ~ 0C ~
_1 V 4~1 ~ ~ ~ ~ 41 t'r) ~ ~) `;t
,~:) a~ o c~ o c~
E~ ~ ~ ~
~ ~ O O O O ~ O~ O~ ~ O
P...... ~ ~ c~ J cr~ a~ O
C~
Q~
~1 O O O O ~ ~ t~
C~ ,~ ~ r_ oO
IE~ c~ ,~
a~
_I
~ ~ u~
~16~
After each film was ~tre~ched and rolled onto
roller 90, it w~s measured for its cry6talline phase.
All examples were found by infrared ~bæorption spectro-
scopy to have predominantly slph~ ph~se crystalline
structure. This was confirmed by X-ray ~nalysi6 wherein
the crystalline structure was ~ound to be at lea~t 95
by weight alpha phase.
Each example was also measured for thickness,
birefringence (~n), dielectric constQnt ~t 1000 Hz,
10 charge density C/V~ and dissipation factors, reported in
Table IV hereinafter~ Thicknesæ measurement6 were made
by three different techniques as follows:
In the first method, the films were placed
between a flat gauge block and the head of ~ miniature
linear variable differential ~ransformer (Daytronic
Model DC20A LVDT~. The LVDT developed an output voltage
proportional to distance from a reference position. The
transducer output was amplified with a Daytronic Model
300D transducer amplifier indicator followed by a C3140
20 operational amplifier with a gain of 20. Voltage
through the LVDT was measured with the film 6ampleæ both
in place and out of position. The difference between
readings yielded ~ voltage proportion~l to the s~mple
thickness. A cslibration curve was made using conven-
25 tional 6, 9 and 16 ~ thick biaxially stretched PVF2film obtained from Kureha Chem$cal ~ndustry Co., Ltd.,
Japan, and 12.5 ~ and 25.4 ~ polyethylene tere-
phthalate shim stock. All measurements were made at
least 4 times and the average value determined.
In the second method of thickness determ~nation
an IR interference technique was uæed. Constructive
interference between the direct ray and the ray w~ch is
internally reflec~ed once off each f{lm Rurfsce occurs
in trsnsmission when
1) m~ ~ 2nt
-17- ~23~
where t is the film thickness, n i6 the index o~
refraction of the film, ~ i6 the incident wavelength
and m i6 an integer. Two different wavelength6 were
selected to obtain constructive interference. The num-
ber of fringes between interference maxima (~m) i8given by
2) ~m - 2nt (1/~ 2), or
l~m
3) t ' 2n~ (
Fourier Transform Infrared Spectra (FTIR) were obtained
on each example. The film thicknesses were determined
from the interference fr~nges observed in the 4000-1800
cm~l range.
The third method of film thickness ~easurement
utilized a precision micrometer gauge (Federal Gauge
Model E3BS-2) with 2.54 ~ scale divisionsO The exam-
ple films were folded 4, 8 and 16 times with special
care to avoid wrinkllng. A 6tatic neutral~zer gun
obtained from Quantum Instruments was used to eliminate
static charges during folding.
The average thicknesses measured by each of
these techniques were then further aver~ged to obtaiD
the re~ults set forth in Table IV.
Birefringence was determined using a polarizing
microscope equipped with a Bere~ compensator. The bire-
frlngence i~ given by the equation:
4) ~n ~ R/t
where R is the retardation of the film measured by rota-
ting a calcite crystal to the two positions of maximum
extinction, and ~ is the thickness already determlned.
Dielectric constants were calculated from the
equation
Ct
5) K -
A X 8.85 X 10-l2
-lB-
where C (capacitance) W8S measured on a dielectric
bridge ~t 1000 Hz after a measured ~res A of the film
was electroded.
Voltage breakdown strengths were determined by
ramping a high voltage power ~upply through the ex~mples
deposited wlth 800A thick aluminum electrodes, while
monitoring ~he current flow. Breakdown was defined to
be the voltage at which the current surged from le88
than 1 ~ ~mp to greater than 10 ~ amps. The values
lis~ed in Table IV are average values for 10 sample6.
Charge density C/V was of course c~lculated
from the equat~on
6) C/~ ~ K X 8.85 X 10-l2/ t2.
The dissipation factors were obtained as pha~e
measurements made directly on the dielectric bridge
noted above for the dielectric constants, as i8 conven-
tional.
~23~
--19-
O ~
~ ~ C~
g ..
O O o u~ u~ o O O O U~ ~D
cq J ~ O O O ~ O O O~ ::
,~ O ~
O O O O O O O C~ , r!
r~ CO
~q
~, . . . . . . . . ~;
o ~I r~ ~ ~ I~
:'1 ~
--I o
l; t~
3 r
~ O ~ JJ 0
H ~ ~ O O 1~ C`~ 00 1~ 1/'l 0~ _~
~ ~ 3 ~ ~ ~
E~ o o r~ U~ o U~ v
+ I + I ~1 +l $1 + l + l + I .~ ~
o .
o ~ ~ ~ ~ n o
o ~ ~ o ao
O O O O O O O O ' C
O O O C:~ O O O O O ~rl
c +~ jl +1+1~1 +1 +1 ~ ~
a x U~ o s~
o ~ N N ~) J~ C
O O O O O O O O t,)
. . . . - -
u~
~q ~ O O O
a~ Q) C ~ C~l _I ~ --
~4 ~ O ' ' O ' O O '
~ ~ ~ $1 $", +1 $, ~1 +1 $, ~ ~
~j ~ E ~ ~ ~
a~ :, cd
~ Cl ~
-20-
In addition to th~ propert~es listed ~n Table
IV, piezoelectric const~nts were also measured for
Examples 1, 3, 5, 6 and 8 by stressing the film6 along
their length and measuring the induced charge, after
polin~ a~ 1 megavolt/cm at room temper~tur~ ~25UC) for 1
hour. Example 1 wa~ found to h~ve a piezoelectrlc con-
stant of 1.2 X 10-12 meter~/volt, Ex. 3 was 1.9 X
10-l2, Ex. 5 was 2.1 X 10-l2, Ex. 6 W88 2.6 X
lo-12 and Ex- 8 was 3 X 10-12 meters/volt- Examples
2, 4 and 7 not tested are presumed to have a value less
than that of Ex. 8, inasmuch as Ex. 8 ha6 the highe~t
birefringence and dielectrlc constant, which, as is well
known, produce the highest piezoelectric constant when
poled.
Thus a the examples of the invention produced a
piezoelectric constant which in many cases is an order
of magnitude lower than that occurring in beta phase
PVF2 having comparable dielectric const~nts.
As Comparative Examples, Example 1 and Example
5 were e&ch repeated, except that the RPM of the chill
wheel was reduced to only 17 and 21.3, respectively.
This produced an avera~e final thickness of the PVF2
film that was 8 8 microns and 7.3 microns, respectively~
a reduction ln thickness of only l/2g.4 and 1/41.8,
respectively. This was found to produce dielectric con-
stants of only 10.3 and 9.4, respectively, de20nstrating
that the stretch r~tio needs to be a~ least about 50 to
obtain Applicant's results.
The invention has been described in detail with
particular reference to preferred embodiment6 thereof~
but it will be understood that vari~tions and modifica
tions can be effected within the spirit ~nd ~cope of the
invent~on.
