NASA CASE NO. LAR152 -79_1-

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NOTICE

The invention disclosed in this document resulted from research in aeronautical

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Address informational requests for this invention to the Technology

Applications Team, NASA Langley Research Center, Code 200, Hampton, Virginia

23681-0001.

Address applications for license to the Patent Counsel Office, Mail Stop 212,

3 Langley Boulevard, Hampton, Virginia 23681-0001.

Serial No.: 08/524,855

Filed: 09/07/95 LaRC
THERMALLY STABLE, PIEZOELECTRIC AND PYROELECTRIC 

POLYMERIC SUBSTRATES AND METHOD RELATING THERETO 

NASA Case Number: LAR 15279-1 

Production of an electric voltage in response to mechanical excitation 

(piezoelectricity) or thermal excitation (pyroelectricity) requires a material to have a 

preferred dipole orientation in its structure. This preferred orientation or 

polarization occurs naturally in some crystals such as quartz and can be induced 

into some ceramic and polymeric materials by application of strong electric or 

mechanical fields. For some materials, a combination of mechanical and electrical 

orientation is necessary to completely polarize the material. The only commercially 

available piezoelectric polymer is poly(viny1idene fluoride) (PVF,). However, this 

polymer has material and process limitations which prohibit its use in numerous 

device applications where thermal stability is a requirement. 

By the present invention, thermally stable, piezoelectric and pyroelectric polymeric 

substrates were prepared from polymers having a softening temperature greater 

than 100°C. A metal electrode material is deposited onto the polymer substrate 

and several electrical leads are attached to it. The polymer substrate is heated in a 

low dielectric medium to enhance molecular mobility of the polymer chains. A 

voltage is then applied to the polymer substrate inducing polarization. The voltage 

is then maintained while the polymer substrate is cooled "freezing in" the molecular 

orientation. 

The novelty of the invention resides in the process of preparing the piezoelectric 

and pyroelectric polymeric substrate. The nonobviousness of the invention is 

found in heating the polymeric substrate in a low dielectric medium while applying 

a voltage. 

Inventors: Joycelyn 0. Simpson Terry L. St. Clair 

41 4 Woodview Lane 17 Roberts Landing 

Employer: NASA-LaRC 

Poquoson, VA 23662 

NASA LaRC 

Initial Evaluator: John W. Connell 

Serial No.: 081524,855 

Filing Date: 9/7/95 
NASA CASE NO. LAR 15279-1 -1- PATENT APPLICATION

THERMALLY STABLE, PIEZOELECTRICAND PYROELECTRIC

POLYMERIC SUBSTRATESAND METHOD RELATING THERETO

5

Origin of the Invention

The invention described herein was made by employees of the

United States Government and may be manufactured and used by or for

the Government without payment of any royalties thereon or therefor.

10 Background of the Invention

1_ Field of the Invention

15

This invention relates to thermally stable polymeric substrates

which exhibit piezoelectric and pyroelectric properties. In particular, it

relates to thermally stable, piezoelectric and pyroelectric polymeric

substrates which do not require polarization solvents or mechanical

orientation to prepare them.

20 2. Description of the Related Art

25

Production of an electric voltage in response to mechanical

excitation (piezoelectricity) or thermal excitation (pyroelectricity) requires

a material to have a preferred dipole orientation in its structure. This

preferred orientation or polarization occurs naturally in some crystals

such as quartz and can be induced into some ceramic and polymeric

materials by application of strong electric or mechanical fields. For some

materials, a combination of mechanical and electrical orientation is

necessary to completely polarize the material. Such is the case for the
NASA CASE NO. LAR 15279-1 -2- PATENT APPLICATION

only commercially available piezoelectric polymer, poly(vinylidene

fluoride) (PVF2).

Poly(vinylidene fluoride) is a highly crystalline material (40 - 60%)

possessing at least three major crystal forms: a-form, p-form, and y5 form. Of these forms, only the p-form can be polarized to exhibit

significant piezoelectric or pyroelectric responses. The I_-form of

crystallinity is induced by drawing the PVF 2 sheet at a temperature of

approximately 70°C to several times its original length. This produces a

morphology that has vinylidene fluoride groups aligned in an

10 orthorhombic, planar zigzag conformation.

When the PVF 2 is subsequently poled in a DC field of at least 100

MY/m, the molecular dipoles (negatively charged fluorine atoms and

positively charged hydrogen atoms) will orient perpendicular to the plane

of the film producing a net dipole in the field direction. This process is

15 known as poling.

After poling, the crystalline phase in the PVF 2 serves as a

structural support for the induced polarization. It is widely considered

that crystallinity is a necessary morphological feature for making a stable

polarized material.

20 However, material and process limitations prohibit the use of PVF 2

in numerous device applications, particularly where thermal stability is

required. At temperatures significantly above the glass transition

temperature of the non-crystalline regions of PVF 2, the polarization

induced in the poling process is randomized. This dipole randomization

25 occurs around 60°C and is accompanied by a tremendous loss in the

induced polarization. This destroys or significantly decreases the desired

piezoelectric and pyroelectric properties. Moreover, the need to

mechanically draw the film prior to electrical poling presents a significant

disadvantage in that drawing in the film direction causes an anisotropic
NASA CASE NO. LAR 15279-1 -3- PATENT APPLICATION

elastic modulus in the stretching or axial direction as compared to the

transverse direction.

Scheinbeim et al. (US 4,830,795 and 4,863,648) disclose a

process for making polarized material which does not require mechanical

5 drawing. In their process, polarized material is prepared by applying an

electrical field to a material in a polarization solvent. During poling the

solvent is removed from the material to the desired extent or totally to

provide a polarized material which is substantially free of mechanically

induced orientation. This process has been used to pole PVF 2 and nylon

10 materials such as Nylon 7 and Nylon 11. However, this process is only

suitable for polymers which are soluble in an appropriate polarization

solvent. This process cannot be used for polymers which are not soluble

in a polarization solvent.

Improper removal of the solvent from the polarization material also

15 adversely affects the final product. If the percentage of solvent

remaining in the poled material is too large, the enhanced mobility of the

material may actually promote randomization of the dipolar units leading

to lower desired polarization properties.

Lastly, the use of solvent dictates the conditions of the process.

20 For example, the temperature at which the solvent is removed must be

well below the degradation temperature of the polymer. In addition, the

poling temperature must be lower than the boiling point of the solvent.

An object of the present invention is to provide a polymeric

substrate which is thermally stable and exhibits piezoelectric and

25 pyroelectric properties.

Another object of the invention is to provide a process for

preparing a thermally stable polymeric substrate which exhibits

piezoelectric and pyroelectric properties without requiring mechanical

orientation.
5

NASA CASE NO. LAR 15279-1 -4- PATENT APPLICATION

Another object of the invention is to provide a process for

preparing a thermally stable polymeric substrate which exhibits

piezoelectric and pyroelectric properties without requiring the use of

solvents.

Summary of the Invention

By the present invention, thermally stable, piezoelectric and

pyroelectric polymeric substrates were prepared. These polymeric

10 substrates retain their orientation, piezoelectric and pyroelectric

properties at temperatures greater than about 100°C up to the softening

temperature of the polymer. They have an advantage over piezoelectric

inorganic materials because they are easily processable and conformable

to a variety of different shapes. As a result of these properties, these

15 polymeric substrates may be used for many applications such as: an

electromechanical transducer, a thermomechanical transducer, an

accelerometer, an acoustic sensor, an infrared sensor, a pressure sensor,

a vibration sensor, an impact sensor, an in-situ temperature sensor, an

in-situ stress/strain sensor, a micro actuator, a switch, an adjustable

20 fresnel lense, a speaker, a tunable reflector, a microphone, a tactile

sensor, a weather sensor, a micropositioner, an ultrasonic device, and a

power generator.

The thermally stable, piezoelectric and pyroelectric polymeric

substrate comprises: a polymeric substrate having a softening

25 temperature greater than about 100°C; a metal electrode material

deposited onto the polymeric substrate to form a metal electrode coated

polymeric substrate; and a polar field applied to the metal electrode

coated polymeric substrate. The polar field is applied to the metal

electrode coated polymeric substrate by attaching a plurality of electrical
5

10

NASA CASE NO. LAR 15279-1 -5- PATENT APPLICATION

leads to the metal electrode coated polymeric substrate and heating the

metal electrode coated polymeric substrate in a low dielectric medium to

about the softening temperature of the polymeric substrate. A voltage is

applied to the heated metal electrode coated polymeric substrate to

induce polarization. The voltage applied is less than the voltage wherein

dielectric breakdown occurs in the metal electrode coated polymeric

substrate. The polarized metal electrode coated polymeric substrate is

then cooled to a temperature below the softening temperature of the

polymeric substrate while maintaining a constant voltage.

Description of the Drawing

Figure 1 is a diagram of the poling apparatus.

1 5 Desc.rintion of the Preferred Embodiments

2O

25

The thermally stable, piezoelectric and pyroelectric polymeric

substrates of the present invention can be prepared from a variety of

polymers such as polyarylates, polyquinoxalines, polyphenylene ethers,

polycarbonates, polyphenylene sulfides, polysulfones, polyaryletherketones, polyimides, polyarylene ethers, polybenzimidazoles,

polyazomethines and any other thermally stable polymers known to those

skilled in the art. Of particular interest, are those polymers which have

the structural repeat unit:

--_ R--Ar --X--(Ar-- X_-Ar-)-

wherein R is selected from the group consisting of:
NASA CASE NO. LAR 15279-1 -6- PATENT APPLICATION

Ar' N_

0 0

H

I

H

and o

10 and as a preferred embodiment, R is:

--N "--CH --Ar' -CH =N--

t

_0 _Ar' _0

15

----N Ar' N_

0 0

2O

Ar' is selected from the group consisting of: any aromatic moiety and

any aromatic moiety having a polarizable group. As a preferred

embodiment, Ar' is a member selected from the group consisting of:
5

NASA CASE NO. LAR 15279-1 -7- PATENT APPLICATION

0

As a most preferred embodiment, Ar' is:

10

15

Ar is:

A

z

20 wherein the catenation is ortho, para, or meta. Z is hydrogen or Q-Y and

Q is a direct bond or any aromatic moiety. As a preferred embodiment,

Q is a direct bond. Y is a radical having a polarizable group. A

polarizable group is defined as a group having a dipole moment. As a

preferred embodiment, Y is selected from the group consisting of -CF 3,

25 -CN, -NO 2, and -SF s. As a most preferred embodiment, Y is -CN.

X is any linking group known to those skilled in the art. In

particular, X is selected from the group consisting of -0-, -CO-, -S02-,

and -CH2-. AS a most preferred embodiment, X is defined as -O-.

n is an integer ranging from about 1 to about 10. As a preferred
NASA CASE NO. LAR 15279-1 -8- PATENT APPLICATION

embodiment, n is 1.

Care must be taken in preparing polymeric substrates for high

voltage processing in order to avoid excessive arcing of the substrate. It

has been found that ultra-filtration of the precursor materials along with

5 casting and curing the substrate in a particulate free environment is

useful to minimize particulate contamination and point defects.

The thermally stable, piezoelectric and pyroelectric polymeric

substrate further comprises a metal electrode material deposited onto the

polymeric substrate to form a metal electrode coated polymeric

10 substrate. The metal electrode material may be any metal material

known to those skilled in the art such as gold or silver that is of low

resistivity and does not produce oxides at elevated temperatures. Gold is

the preferred metal electrode material for the present invention. The

metal electrode material may be applied to the film at opposing,

15 overlapping surfaces by either sputtering or evaporating as a thin sheet.

As a preferred embodiment, the metal electrode material is deposited by

evaporating as a thin sheet.

Lastly, the thermally stable, piezoelectric and pyroelectric

polymeric substrate comprises a polar field such as an electric field, a

20 magnetic field or a combination thereof. The polar field is applied to the

metal electrode coated polymeric substrate by heating the metal

electrode coated polymeric substrate in a low dielectric medium to about

the softening temperature of the polymer. Any low dielectric medium

known to those skilled in the art such as a silicone oil medium may be

25 used. The metal electrode coated polymeric substrate is heated to a

temperature in the vicinity of its softening temperature to increase the

molecular mobility of the polymer and thus facilitating poling of the

polymer. In some cases, field sweeping of ionic impurities by applying a

low static voltage for an extended period of time is necessary to minimize
NASA CASE NO. LAR 15279-1 -9- PATENT APPLICATION

conduction current during poling. A voltage is applied to the heated

metal electrode coated polymeric substrate to induce polarization wherein

the voltage is less than the voltage wherein dielectric breakdown occurs

in the metal electrode coated polymeric substrate. The voltage may be

5 either a low frequency biased AC voltage or a DC voltage applied at a

range from about 50 MV/m to about 200 MV/m. As a preferred

embodiment, a DC voltage applied at a range from about 100 MV/m to

about 200 MV/m was found to provide good results. The best results

occurred when a DC voltage of 100 MV/m was applied. This voltage or

10 field strength was found to be suitable to induce preferential alignment of

the molecular dipoles. The voltage is maintained for a period of time

which is sufficient to obtain the desired degree of polarization of the

polymeric substrate. The polarized metal electrode coated polymeric

substrate is then cooled while maintaining a constant voltage to a

15 temperature below the softening temperature of the polymer. This

allows the induced orientation to become "frozen" or preserved. As a

preferred embodiment, this temperature is 80°C.

These resulting thermally stable, piezoelectric and pyroelectric

polymeric substrates can be used to prepare a variety of articles. These

20 articles are selected from the group consisting of: electromechanical

transducers, thermomechanical transducers, accelerometers, acoustic

sensors, infrared sensors, pressure sensors, vibration sensors, impact

sensors, in-situ temperature sensors, in-situ stress/strain sensors, micro

actuators, switches, adjustable fresnel lenses, speakers, tunable

25 reflectors, microphones, hydrophones, tactile sensors, weather sensors,

micro positioner, ultrasonic devices, and power generators.

Referring now to the drawing. Figure 1 depicts the poling

apparatus 1 of the present invention. A low dielectric medium 2 is

heated to about the softening temperature of the polymeric substrate
5

10

15

NASA CASE NO. LAR 15279-1 -10- PATENT APPLICATION

using a temperature controller 3. The metal electrode material 4 is

coated onto a polymeric substrate 5 and the metal electrode coated

polymeric substrate 5 is placed in the heated low dielectric medium 2. A

high voltage source 6 is connected to the metal electrodes 4 using

electrical leads 7. The current generated by the metal electrode coated

polymeric substrate 5 in response to the applied voltage is monitored by

the current meter 8. Data from the current meter 8 and the voltage

meter 9 is relayed to the computer 10.

The following examples illustrate the process of the present

invention. These examples are merely illustrative and intended to enable

those skilled in the art to practice the invention in all of the embodiments

flowing therefrom, and do not in any way limit the scope of the invention

as defined by the claims.

EXAMPLES

2O

A 1 mil thick polyimide film having the following repeat unit:

O O

25

and a glass transition temperature of 175°C was prepared and

evaporated with a 2000 Angstrom layer of silver as an electrode

material. The electroded surface area measured approximately 144 mm 2.

The film was placed in a silicone oil medium in a poling apparatus of the
NASA CASE NO. LAR 15279-1 -11- PATENT APPLICATION

type shown in Figure 1 and connected to a high voltage DC source. The

film in the silicone oil medium was heated to 165°C and a DC voltage of

120 MV/m was applied and maintained for approximately 15 minutes.

Subsequently, the temperature was allowed to cool to ambient while the

5 voltage was held constant at 120 MV/m. The electric field was reduced

to zero when the temperature was within 10°C of ambient.

The polarized film was analyzed for a piezoelectric response by

applying a compressive force ranging from 5 to 200 Ibs and measuring

the voltage generated. The film was found to exhibit a piezoelectric

10 response where the unpolarized film prepared from the same polymer did

not exhibit a piezoelectric response.

The pyroelectric response was measured by connecting the film to

a Fluke 87 multi meter and heating the film with a heat gun. An average

maximum output voltage equal to 200 mY was achieved upon heating

15 the sample to approximately 150°C. The maximum detectable voltage

level using a Fluke 87 multi meter is 400 mY. Only minimal output

voltage was achieved upon heating an unpolarized sample prepared from

the same polymer.

20

A 1 mil thick polyimide film having the repeat unit:

25 and a glass transition temperature of 180°C was prepared and
NASA CASE NO. LAR 15279-1 -12- PATENT APPLICATION

evaporated with a 2000 Angstrom layer of silver as an electrode

material. The electroded surface area measured approximately 144 mm 2.

The film was placed in a silicone oil medium in a poling apparatus of the

type shown in Figure 1 and connected to a high voltage DC source. The

5 film in the silicone oil medium was heated to 170°C and a DC voltage of

120 MV/m was applied and maintained for approximately 15 minutes.

Subsequently, the temperature was allowed to cool to ambient while the

voltage was held constant at 120 MV/m. The electric field was reduced

to zero when the temperature was within 10°C of ambient.

10 The polarized film was analyzed for a piezoelectric response by

applying a compressive force ranging from 5 to 200 Ibs and measuring

the voltage generated. The film was found to exhibit a piezoelectric

response where the unpolarized film prepared from the same polymer did

not exhibit a piezoelectric response.

15 The pyroelectric response was measured by connecting the film to

a Fluke 87 multi meter and heating the film with a heat gun. An average

maximum output voltage in excess of 400 mY was achieved upon

heating the sample to approximately 150°C. The maximum detectable

voltage level using a Fluke 87 multi meter is 400 mY. Only minimal

20 output voltage was achieved upon heating an unpolarized sample

prepared from the same polymer.

25 A 1 mil thick polyimide film having the repeat unit:

0 O
NASA CASE NO. LAR 15279-1 -13- PATENT APPLICATION

and a glass transition temperature of 176°C was prepared and

evaporated with a 2000 Angstrom layer of silver as an electrode

material. The electroded surface area measured approximately 144 mm 2.

The film was placed in a silicone oil medium in a poling apparatus of the

5 type shown in Figure 1 and connected to a high voltage DC source. The

film in the silicone oil medium was heated to 165°C and a DC voltage of

120 MV/m was applied and maintained for approximately 15 minutes.

Subsequently, the temperature was allowed to cool to ambient while the

voltage was held constant at 120 MV/m. The electric field was reduced

10 to zero when the temperature was within 10°C of ambient.

The polarized film was analyzed for a piezoelectric response by

applying a compressive force ranging from 5 to 200 Ibs and measuring

the voltage generated. The film was found to exhibit a piezoelectric

response where the unpolarized film prepared from the same polymer did

15 not exhibit a piezoelectric response.

The pyroelectric response was measured by connecting the film to

a Fluke 87 multi meter and heating the film with a heat gun. An average

maximum output voltage in excess of 400 mV was achieved upon

heating the sample to approximately 150°C. The maximum detectable

20 voltage level using a Fluke 87 multi meter is 400 mV. Only minimal

output voltage was achieved upon heating an unpolarized sample

prepared from the same polymer.

25

A 1 mil thick polyimide film having the repeat unit:

0 O
NASA CASE NO. LAR 15279-1 -14- PATENT APPLICATION

and a glass transition temperature of 220°C was evaporated with a 2000

Angstrom layer of silver as an electrode material. The electroded surface

area measured approximately 144 mm 2. The film was placed in a

silicone oil medium in a poling apparatus of the type shown in Figure 1

5 and connected to a high voltage DC source. The film in the silicone oil

medium was heated to 210°C and a DC voltage of 120 MV/m was

applied and maintained for approximately 15 minutes. Subsequently, the

temperature was allowed to cool to ambient while the voltage was held

constant at 120 MV/m. The electric field was reduced to zero when the

10 temperature was within 10°C of ambient.

The polarized film was analyzed for a piezoelectric response by

applying a compressive force ranging from 5 to 200 Ibs and measuring

the voltage generated. The film was found to exhibit a piezoelectric

response where the unpolarized film prepared from the same polymer did

15 not exhibit a piezoelectric response.

The pyroelectric response was measured by connecting the film to

a Fluke 87 multi meter and heating the film with a heat gun. An average

maximum output voltage in excess of 400 mV was achieved upon

heating the sample to approximately 150°C. The maximum detectable

20 voltage level using a Fluke 87 multi meter is 400 mY. Only minimal

output voltage was achieved upon heating an unpolarized sample

prepared from the same polymer.

What is claimed is:
NASA CASE NO. LAR 15279-1 -32- PATENT APPLICATION

THERMALLY STABLE, PIEZOELECTRICAND PYROELECTRIC

POLYMERICSUBSTRATESAND METHOD RELATINGTHERETO

5

Abstract of the Disclosure

A thermally stable, piezoelectric and pyroelectric polymeric

substrate was prepared. This thermally stable, piezoelectric and

pyroelectric polymeric substrate may be used to prepare

electromechanical transducers, thermomechanical transducers,

10 accelerometers, acoustic sensors, infrared sensors, pressure sensors,

vibration sensors, impact sensors, in-situ temperature sensors, in-situ

stress/strain sensors, micro actuators, switches, adjustable fresnel

lenses, speakers, tactile sensors, weather sensors, micro positioners,

ultrasonic devices, power generators, tunable reflectors, microphones,

15 and hydrophones. The process for preparing these polymeric substrates

includes: providing a polymeric substrate having a softening temperature

greater than 100°C; depositing a metal electrode material onto the

polymer film; attaching a plurality of electrical leads to the metal

electrode coated polymeric substrate; heating the metal electrode coated

20 polymeric substrate in a low dielectric medium; applying a voltage to the

heated metal electrode coated polymeric substrate to induce polarization;

and cooling the polarized metal electrode coated polymeric electrode

while maintaining a constant voltage.
O0

77

m.

tO

t'- Cr',

O1

I

.,q

(,,o