Polyvinylfluoride (PVF) plastic film was found to be a good pyroelectric material. Radiometers using PVF were developed that exhibit high sensitivity and frequency response. Normalized detectivities of greater than 10 to the 8th power cm/Hz/w and responsivities on the order of 100,000 V/W were measured (500 C BB source, 0.1 Hz chopping frequency and 1 Hz bandwidth.

Russell, L. D.

English

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N7 3 25 47 2
NASA TECHNICAL NASATM x-62,271
MEMORANDUM
I CASE FILE
< COPY
fVF PYROELECTRIC RADIOMETER
Larry D. Russell
Ames Research Center
Moffett Field, Calif. 94035
May 1973
https://ntrs.nasa.gov/search.jsp?R=19730016745 2020-03-11T19:42:22+00:00Z
SUMMARY
Polyvinyl fluoride (PVF) plastic film has been found to be a good
pyroelectric material. Radiometers using PVF have been developed that
exhibit high sensitivity and frequency response. Normalized detectivities
of greater than 108 cm Hz1/2 W-1 and responsivities on the order of 105
V/W have been measured (500°C BB source, 0.1 Hz chopping frequency and
1 Hz bandwidth.)
Introduction
Polyvinyl fluoride plastic film (e.g., DuPont "Tedlar") has pronounced
pyroelectric characteristics. When charged at a high voltage and temperature,
and then cooled while maintaining the charging field, it acquires a permanent
polarization which is very sensitive to temperature. This pyroelectric
characteristic of polyvinylfluoride (PVF) was observed and used as a
radiometer subsequent to investigations into the persistent polarization
characteristics of polymers for radiometers1. Other descriptions of PVF
pyroelectric radiometers have been recently made by Phelan, Mahler and
Cook2, and Cohen, Edelman and Vezzetti3. Pyroelectric effects in a similar
fluorocarbon film, polyvinylidene fluoride, have also been recently reported1*,5,
Description
Tedlar PVF is a highly crystalline and ordered plastic film. It is
also quite polar for a plastic dielectric, having a dielectric constant of 9.
The pyroelectric characteristic of PVF appears to be related primarily to
volume polarization, involving orientation of crystallites of h$gh dipole
moment. When PVF is charged at a high voltage (400 Kv/cm) and elevated
temperature (80-100°C) and then cooled while imposing the external electric
field, a persistent polarization is created due primarily to dipole orien-
tation. Space charge polarization due to injected charge and ionic impurities
may also be involved, but its effects are apparently secondary to dipole
orientation. For example, the author has found that charging by the methods
of corona discharge or electron beam irradiation (which produce space charge
in the polymer at room temperature) produced negligible pyroelectric effects.
- 2 -
The persistent dipole orientation polarization formed by the charging
process described above is quite long-lived, as evidenced by the fact that
the PVF radiometers described below have not decayed in sensitivity in
nearly a year. This permanency further indicates the presence of a pyro-
electric mechanism involving a temperature-sensitive, metastable polari-
zation state as opposed to a temperature-sensitive, time-dependent depolari-
zation effect such as the gradual decay of absorption current found in many
di electri cs .
A pyroelectric radiometer is electrically a capacitor. When the PVF
dielectric is charged so as to produce persistent polarization, it acquires
the ferroelectric characteristic of a spontaneous or persistent polarization
which changes with temperature. Under constant temperature conditions, the
persistent polarization charge is not apparent because it is exactly com^-
pensated by induced charge on the electrodes. However, by removing one
electrode, the persistent polarization cah be measured.
The removed electrode is vibrated a short distance away and within the
static electric field of the exposed surface charge of the persistent polari-
zation. This induces an AC voltage which can be amplified and nulled out by
applying an opposing DC voltage. The required DC bias is a measurement of
the permanent effective surface charge of the dielectric5. For PVF charged
at 400 Kv/cm at 100°C, the charge is typically 2xlQ-7 coul./cm2. This level
of permanent charge is relatively high, being comparable to that of other
polymers used specially for their high electret charge7.
A slight increase in temperature causes a reversible reduction of the
persistent polarization which produces free charge on the electrodes. The
amount of charge, Q, per area, A, produced per unit temperature change, T,
is the pyroelectric coefficient,
S = Q/AT
- 3 -
For PVF, the coefficient is on the order of 2 x 10"8 coulomb/°C cm2 as
calculated from the radiometer response.
Response
The signal charge produced can be detected with either a voltage or
current amplifier. In the voltage mode, a high input impedance amplifier
is utilized to produce maximum sensitivity or responsivity, but it causes
the responsivity to drop off with frequency. In the current mode a low
input impedance amplifier is used. The current mode responsivity is lower
in proportion to the lower input impedance, but it remains constant with
frequency, and thus the current mode is more suitable for high frequency
response applications. These signal characteristics can be described in
terms of the detector's equivalent circuit.
The equivalent circuit (Fig. 1) of the PVF pyroelectric radiometer can
be considered to be a current source shunted by the detector resistance R
and capacitance CF and the amplifier input resistance R, and capacitance C,.5 d a
From the definition of the pyroelectric coefficient S the amplitude
of the time varying component of current produced by incident radiation that
is chopped sinusoidally at a circular frequency to is
I = toSAT
(0 0)
The thermal equation of the detector is
Pw si nut = H ^  + GT
where P is the amplitude of absorbed incident power, H is the detector element
to
heat capacity and G is the thermal conductance away from the detector element.
Solving this equation gives the amplitude of the time varying temperature rise
of the detector,
Tw = (Pu/G)(l + «Ve,»T2)
- 4 -
where l/u>y = TT = H/G, the thermal time constant. For the PVF radiometer
a)T is on the order of 0.5 Hz.
The impedance amplitude of the equivalent circuit is
|Z| = R (I +HL)
where 1/uv = tr = RC is the electrical time constant (R = R R /(R + R ) >t. t s a s a
and C = C + C,).s a
The signal voltage amplitude is then
Vu = IJZ| = (uSARP /G)(l + u>2/wT2)~1/2(l + u2/u)E
and the responsivity is
R = V /P = (o>SAR/G)(l + uVto-r2)-1/2!! + u>2/u>r2)"CD 0) ' ^ T h
In the voltage mode operation a high amplifier impedance of 10" n puts
u>rr on the order of 0.5 Hz for a small PVF radiometer, and for chopping
frequencies greater than 0.5 Hz the responsivity becomes proportional to l/u>
R,, = .^
V bd)
In the current mode where the amplifier input resistance is much less
than the film resistance, the voltage signal is effectively proportional
to the current produced. Thus, for R. « R_, ov = K-T » <o and |Z| = Ra.
a S C. "^ a" **
At frequencies above the thermal frequency u-p = .05Hz, the responsivity
becomes
n _ SARa U,
G
which is independent of frequency.
For the case of a pulse of radiation, the response can be described
as the difference between two exponentials, one decaying with the thermal
time constant TT, the other decaying with the electrical time constant
- 5 -
T^8. In the voltage mode pulses shorter than tr and TT are integrated.
Construction and Performance
The PVF radiometers were made by depositing thin (lOOnm) aluminum
electrodes on both, sides of 1.25vim DuPont "Tedlar" PVF film and blackening
one electrode with either deposited gold-black (approximately 200nm thick-
ness) or high absorptance paint (approximately lOym thickness). A schematic
diagram of the detector construction and high input impedance (10" fi) source-
follower FET preamplifier used for the voltage mode configuration is shown
in Figure 2. The detector area ranged from 0.5 cm2 for small detectors,
conveniently constructed by mounting the film on the end of a BNC connector,
to 5 cm2 for large area detectors. Two such detectors are pictured in
Figure 3.
The voltage mode responsivity of a PVF radiometer having a 1 cm2 cross-
sectional area and a FET source follower preamplifier was measured to be
R(500°K, 0..1 Hz) = 105 V/W,
where the source was a 500°K blackbody chopped at 0.1 Hz.
Detector noise is due primarily to thermal fluctuations in the detector
film, Johnson noise in the equivalent circuit resistance R, and amplifier
noise. At 0.1 Hz, noise voltage of the above system was Vn = lOOyV, giving
a noise equivalent power of
NEP = Vn/R = 10~9W,
and the detectivity normalized for amplifier bandwidth of f = 1 Hz was thus,
D*(500°K, 0.1 Hz, 1 Hz) = ^AT/NEP = 3 x 108 cm Hz1/2 W~i.
The responsivity is constant with radiant power so long as the detector
is not thermally saturated.
Being a thermal detector, the PVF radiometer has a wide and flat spectral
response limited only by the absorptivity of the blackening material used.
- 6 -
The effect of operating temperature is to increase the responsivity
with increasing temperature up to the temperature at which the PVF film
was initially charged, viz., 80-100°C. This temperature is analogous
to the Curie temperature of crystalline pyroelectrics, above which the
material loses its spontaneous polarization.
Application
The high detectivity of the PVF radiometer qualifies it as a
sensitive infrared detector. It has the pyroelectric detector advantages
of room temperature operation and wide, flat spectral response, and
additionally, the PVF radiometer can easily be made with a large detector
area. The sensitivity is comparable to existing TGS pyroelectric and
thermistor bolometer detectors.
The small PVF radiometers have been used for wide spectral measure-
ment of shock tube radiation. Spectral response from the IR to UV
was obtained by using gold-blacked PVF films without any windows, and
megahertz frequency response required for following the shock radiation
waveforms was obtained by using the current mode configuration.
Large (5 cm2) area PVF radiometers have been used to detect non-
uniform beams from a C02 gas dynamic laser.
-7-
REFERENCES
1. Russell, L. and Beam, B.: "Thermodielectric Radiometer", J. Space-
craft and Rockets, 5, 1501 (1968); U.S. Patent #3,657,644.
2. Phelan, R.; Mahler, R. and Cook, A.: Applied Physics Letters, 19,
337 (1971). .
3. Cohen, J.; Edelman, S. and Vezzetti, C.: Nature Physical Science,
233, 12 (1971).
4. Nakamura, K. and Wada, Y.: J'. Polymer Science, A-2, 9, 161 (1971).
5. Bergman, J. Jr.; McFee, F. and Crane, G.: Applied Physics Letters.
18, 203 (1971).
6. Reedyk, C. and Perlman, M.: J. Electrochemical Society. 115, 49 (1968)
7. Sessler, G. and West, J.: J. Electrochemical Society, 115, 836 (1968).
8. Simhony, M. and Sanlov, A.: J. Applied Physics, 42, 3741 (1971).
6 i
FIGURE 1 - Equivalent Circuit of the PVF Pyroelectric Radiometer
+8.4
blackened
front elec-
trode i
1
 .
(TrvL/«ViX '
' ^
<J <inu ">10 4
? ;
1
k
J 10K
^
>
CRO
-8.4
FIGURE 2 - PVF Radiometer Schematic
,
FIGURE 3 - PVF Radiometers


PVF pyroelectric radiometer

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