Moulson A.J., Herbert J.M. Electroceramics: Materials, Properties, Applications

Подождите немного. Документ загружается.

7.5 Measurement of the Pyroelectric

Coe⁄cient

The pyroelectric coeﬃcient can be measured in a variety of ways but a direct

method is illustrated in Fig. 7.6. From Eq. (7.12)

422 PYROELECTRIC MATERIALS

Fig. 7.5 Variation in the resistivity of PZFNTU with the uranium content.

Fig. 7.6 Circuit for the measurement of the pyroelectric coeﬃcient.

TEAMFLY

Team-Fly

p ¼

ð7:32Þ

where

¼ I



1 þ



in which I

is the measured current and R

and R

are the input resistance of the

ampliﬁer and the leakage resistance of the crystal respectively. Provided that

 R

, which is usually the case, then I

 I

and p can be determined by

changing the temperature of the crystal at a known rate.

7.6 Applications

Pyroelectric materials respond to changes in the intensity of incident radiation

and not to a temporally uniform intensity. Thus humans or animals moving

across the ﬁeld of view of a detector will produce a response as a result of the

movement of their warm bodies which emit infrared radiation (l  10 mmÞ.To

obtain a response from stationary objects requires the radiation from them to be

periodically interrupted. This is usually achieved by a sector disc rotating in front

of the detector and acting as a radiation chopper.

All pyroelectric materials are piezoelectric and therefore develop electric

charges in response to external stresses that may interfere with the response to

radiation. This can largely be compensated for by the provision of a duplicate of

the detecting element that is protected from the radiation by reﬂecting electrodes

or masking, but which is equally exposed to air and mounting vibrations. The

principle is illustrated in Fig. 7.7. The duplicate is connected in series with the

detector and with its polarity opposed so that the piezoelectric outputs cancel.

This results in a small reduction in sensitivity (53 dB) but compensation is an

APPLICATIONS 423

Fig. 7.7 ‘Dummy’ element to compensate for piezoelectrically induced currents.

essential feature in many cases. Using a compensating element of larger area than

that of the detecting element, e.g. larger by a factor of 3, does not aﬀect the

piezoelectric voltage cancellation but results in a response and detectivity closer

to that of the detecting element alone. It may also be necessary to ensure that the

detector and compensator are so packaged as to reduce the transmission of

vibrations to them.

7.6.1 Radi ometry

The radiant power dW emitted into a solid angle dO from a small area dA varies

with the angle y measured from the normal to dA and is given by

dW ¼

sZT

dA dO cos y ð7:33Þ

The situation is illustrated in Fig. 7.8 where dA is a small part of a uniformly

radiating surface AB of emissivity Z. Provided that y is small, which is also the

condition that r is approximately constant for all radiating elements, Eq. (7.33)

becomes

dW 

ZsT

dAdO

AdO



ZsT

ð7:34Þ

If the radiometer is designed so that the imaged uniformly emitting surface

covers the detector surface, then both A ¼

dA and the solid angle dO subtended

424 PYROELECTRIC MATERIALS

Fig. 7.8 Radiometer optics.

by the aperture at a point on the emitting surface are known. Therefore if W is

measured and Z is known, T is determined.

The radiation has to be chopped, and so the detector receives pulses of

radiation from the source and the chopper alternately. Therefore the temperature

is measured relative to the temperature of the chopper surface. In practice,

radiometers are calibrated so that the temperature can be read directly from a

scale. A particular commercial model which operates over the temperature range

0–600 8C uses a LiTaO

single crystal as the pyroelectric detector.

7.6.2 Pollutant control

The amount of speciﬁc impurities in a gas can be monitored by passing radiation

from an infrared source through a tube containing a gas sample and then

through a narrow passband optical ﬁlter corresponding to a frequency at which

the pollutant absorbs radiation to a greater extent than any other constituent of

the gas. For example, CO

absorbs strongly at about 4.3 mm. The experimental

arrangement is illustrated in Fig. 7.9. The response from the infrared detector

can be compared with that obtained when using a diﬀerent ﬁlter corresponding

to a wavelength to which the gas is transparent. A variety of pollutants can be

detected by changing ﬁlters.

7.6.3 Intruder alarm

In a common type of intruder alarm the detector is positioned at the focus of an

encircling set of parabolic mirrors as shown in Fig. 7.10. A moving object causes

a succession of maxima and minima in the radiation from it reaching the

detector.

The wavelength l

of radiation emitted with maximum power from a black

body at temperature T is given by Wien’s displacement law:

APPLICATIONS 425

Fig. 7.9 Apparatus for the detection of gaseous pollutants.

T ¼ 2944 mmK (7:35Þ

For a black body at 300 K, l

¼ 9:8 m m. A ﬁlter is included which cuts out

radiation of wavelength shorter than about 5 mm and so prevents the device from

responding to changes in background lighting levels. Such a detector is capable

of responding to a moving person up to distances of 100 m.

7.6.4 Thermal imaging

In thermal imaging the infrared radiation emitted by surfaces at diﬀerent

temperatures is focused onto a sensitive plate in an exactly analogous manner to

the formation of a photographic image using visible light. There are two

atmospheric ‘windows’ in the infrared extending from 3 to 5 mm and from 8 to

14 mm. As mentioned above, the power radiated from a black body at 300 K

peaks around 10 mm, and the power density in the 8–14 mm band is

approximately 25 times that radiated in the 3–5 mm band (about 150 W m

2

compared with about 6 W m

2

). For this reason infrared imaging using

pyroelectrics exploits energy in the 8–14 mm band.

The pyroelectric elements used in the devices described so far are commonly

square plates with sides about a millimetre long and thicknesses around 30 mm.

Because entire scenes are focused onto the plates in thermal imaging, they have

to be larger, typically squares of side about 1 cm; the thicknesses are the same as

for the simpler devices.

426 PYROELECTRIC MATERIALS

Fig. 7.10 Faceted mirror in an intruder alarm: m indicates the positions of the source giving

radiation maxima at the detector.

The eﬀect of a spatially variable radiation ﬂux falling on a pyroelectric plate is

to produce a corresponding charge distribution. In imaging devices this charge

distribution is detected by a process giving rise to a current which is ampliﬁed

and electronically processed to produce a television picture corresponding to the

infrared radiation from the original scene. A widely used device is the infrared

vidicon shown schematically in Fig. 7.11. It is used in many systems, both civil

and military. A UK-manufactured camera is used for locating survivors buried

in collapsed building rubble.

The radiation from the scene is chopped and focused, using germanium optics,

onto an infrared absorbing electrode carried on the front face of the pyroelectric

plate. The voltages developed across the plate are typically a few millivolts. The

metal grid, positioned close to the detector, collects most of the electrons as it is

scanned. Some electrons pass through the grid; the number depends on the local

potential at the surface of the plate. The charge deposited on the plate produces a

signal at the front electrode which is processed into the television picture.

In the particular mode of operation outlined, when the radiation is interrupted

by the chopper, the detector cools, giving negative signals and causing the local

surface potential to go negative with respect to the cathode. In order for the

detector to be properly addressed by the electron beam it is biased to a potential

of approximately þ300 mV. This is achieved by the deposition of charge from

gas molecules deliberately introduced into the tube and ionized by the electron

beam. Circuitry in synchronism with the chopper reverses the sense of the

negative output signal current when the chopper is closed and adds this to the

positive signal obtained when the chopper is open. This both removes any ﬂicker

in the picture due to detector or bias non-uniformity and increases the sensitivity

for imaging moving objects.

The charge on the surface of the plate must be removed before the electron-

beam scan is repeated. This will occur automatically if the bias is high enough to

ensure that suﬃcient electrons can reach the plate to neutralise the pyroelectric

APPLICATIONS 427

Fig. 7.11 Pyroelectric vidicon.

charges. Suﬃcient ions must be created in the residual gas to provide the required

bias.

Lateral ﬂow of heat in the pyroelectric detector plate tends to even out the

temperature diﬀerences, blurring the charge pattern and hence the ﬁnal image.

This process will depend on the thickness h of the plate and the thermal

diﬀusivity L ¼ l=c

of the plate material. Vidicon performance improves as both

h and L are reduced in value, and a suitable ﬁgure of merit is

vid

eLh

leh

ð7:36Þ

According to Eq. (7.36) PVDF is an attractive candidate for vidicon targets, and

it has been used, although it does not approach the performance obtained using

TGS and other members of that family.

The eﬀect of thermal diﬀusion can be countered by reticulating the target.

Reticulation involves the formation of grooves that cut through the thickness of

the target almost completely (or completely) and divide it into small square

regions which are nearly (or completely) separate, as illustrated in Fig. 7.12.

Suitable patterns for the grooves can be formed by depositing a metal, or some

other material resistant to ion-beam etching, on the target surface and then

partially removing it in the required pattern by photo-engraving. The target is

then exposed to an ion beam that etches away the bare pyroelectric material far

more rapidly than the metal or other protective layer. This method has been used

428 PYROELECTRIC MATERIALS

(a)

Fig. 7.12 Reticulation to improve thermal isolation: (a) before milling; (b) after milling.

(b)

to divide TGS crystals some 30 mm thick into islands 20 mm wide separated by

5 mm wide grooves.

As an attractive alternative to the vidicon the array of elements can be

connected directly to individual voltage detectors on a silicon integrated circuit

(usually referred to as a ROIC: ‘read-out integrated circuit’) via solder ‘bumps’.

The resulting signals can then be electronically processed to yield a picture in the

standard way. Fig. 7.13 shows a section through part of such an imaging array.

Arrays can be fabricated in the following way. A wafer,  15 mm thick by

 75 mm diameter is prepared from a hot-pressed ceramic block (e.g. ‘PST’ or

‘BST’) by diamond sawing, lapping and polishing. Both faces are coated with

metal ﬁlms by sputtering after which a polymer ﬁlm is ‘spun’ onto one face. The

coated wafer is then bonded to a glass support for the reticulation process carried

out by laser-assisted etching under potassium hydroxide. This gives an element

pitch of down to 40 mm with groove widths down to 10 mm. The structure is

then ﬂip-chip bonded to the integrated circuit after which the glass support layer

is removed to expose the l/4 polymer/metal ﬁlm radiation-absorbing structure.

To reduce expense, eﬀorts are made to exploit integrated thin ﬁlm

technologies. For example, arrays have been produced via thin ﬁlm deposition

of the pyroelectric onto a sacriﬁcial layer, e.g. a suitable metal or polysilicon,

which is then selectively etched away. Thermal isolation of the pyroelectric

element is achieved through engineering a gap between it and the ROIC silicon

wafer. Vias in the supporting layer permit electrical connections to be made

between the detector and the wafer via solder bonds. Imaging arrays have been

produced in this way incorporating sputtered PST and sol-gel formed PZT ﬁlms.

APPLICATIONS 429

Fig. 7.13 Ferroelectric pyroelectric imaging array structure (after R.W. Whatmore and R.

Watton [2]).

incidence focused infrared radiation

The ideal route would be one in which the pyroelectric detector material is laid

down in thin ﬁlm form by a route compatible with the production of the silicon

ROIC. There are obvious parallels with the development of FeRAMS (see

Section 5.7.5) and the substantial eﬀort now devoted to their development will

have a positive impact on the manufacture of pyroelectric arrays. Challenges lie

in the requirement to process the deposited ﬁlms at temperatures not too high for

the underlying integrated circuit, and the need to engineer the temperature

diﬀusion characteristics within the element and its surroundings so as to optimise

image deﬁnition.

The type of infrared imager described is now widely exploited for both civilian

and military use and typical image quality currently achieved is illustrated in

Fig. 7.14.

Pyroelectric infrared detectors are inferior in detectivity by one or two orders

of magnitude compared with photoconductors such as cadmium mercury

telluride, as shown in Fig. 7.15. However, such materials require temperatures of

200 K for eﬃcient operation and generally respond to rather narrow bands at the

infrared wavelengths. Pyroelectric devices can discriminate temperature diﬀer-

ences of 0.1 K but ﬁnd many useful applications in which the discrimination is

limited to about 0.5 K. They have the great practical advantage of operating at

normal ambient temperatures.

430 PYROELECTRIC MATERIALS

Fig. 7.14 A thermal image produced by an infrared camera exploiting uncooled pyroelectric

ceramic detection technology. (Courtesy of QinetiQ Limited.)

The uncooled thermal imaging technology exploiting ferroelectric ceramics

is being challenged by other technologies, signiﬁcantly by one based

upon ‘resistance bolometer’ materials such as vanadium oxide (VO

) and

amorphous silicon.

Problems

1. Explain why it is that the sun appears to the eye as a uniformly bright disc. Would it

appear equally bright were its distance from us doubled?

2. Check by integration that Eq. (7.33) is consistent with the expression W ¼ sT

for the

total power radiated from unit area of a black body.

3. Make an order of magnitude estimate of the time taken for a plate of pyroelectric

material 30 mm thick to achieve a uniform temperature if the face is suddenly

illuminated with infrared radiation (l  4.0 W m

; r ¼7.45 Mg m

;

c ¼0.33 kJ kg

). [Answer:  0:1 ms]

4. A TGS plate of uniform thickness (30 mm) receives a uniform radiation ﬂux for 1 ms at

normal incidence. Using data given in Table 7.1, calculate the lower radiation intensity

PROBLEMS 431

Fig. 7.15 Detectivities D of some photoconductive and pyroelectric detector materials:

curve 1, PbS; curve 2, InAs; curve 3, HgCdTe; curve 4, PbSe; curve 5, InSb; curve 6,

L-alanine-

doped TGS; curve 7, LiTaO

; curve 8, PZ; curve 9, PVDF. (Curves 1–5 measured at 200 K;

curves 6–9 measured at 300 K.)