Pyroelectric themal materials

8/13/2019 Pyroelectric themal materials

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THERMAL IMAGING:

• Detecting radiation in the infrared range electromagnetic radiation (roughly 9,000 – 14,000

nanometers) and produce images of that radiation.

• The amount of radiation emitted by an object increases with temperature; therefore, therm

imaging allows one to see variations in temperature.

• In thermal imaging warm objects stand out well against cooler backgrounds; humans and

other warm blooded animals become easily visible against the environment, day or night.


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PYRO ELECTRIC EFFECT:

• The pyroelectric effect, whereby a change in temperature in a material engenders a re

electric charge, has been known as a physically observable phenomenon for many ce

• The effect occurs in any material (single crystal, ceramic or polymer) which possesse

symmetry. Thus, of the 32 possible point group symmetries, there are 10 for which th

possessing them are pyroelectric.


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PYRO ELECTRICITY:• Pyroelectricity is the property presented by certain materials that exhibit an electric pola

when a temperature variation ∆T is applied uniformly:

∆P = γ⋅∆T

where γ is the pyroelectric coefficient at constant stress.

• Pyroelectric crystals actually have ,a spontaneous polarization, but the pyroelectric effec

be observed during a temperature change.

• If a pyroelectric crystal with an intrinsic dipole moment (top) is fashioned into a circuit

electrodes attached on each surface (FIG. 1), an increase in temperature T prompts the s

polarization PS to decrease as the dipole moments, on average, diminish in magnitude.

horizontal tilting of the dipoles, (pictured at bottom of FIG. 1), signifies the effect. A cu

to compensate for the change in bound charge that accumulates on the crystal edges.


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Schematic drawing showing the origin of the pyroelectric cu

FIG. 1


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SIMPLE MODEL OF THE PYROELECTRIC EFFEC

•In the microscopic scale, the pyroelectric effect occurs because of the asymmetric int

potential caused by electrically charged atoms within the crystal structure of the mate

• This can be viewed schematically as presented in FIG. 2, which shows a two dimens

cations and anions. The cations are displaced relative to the unit cells “centre” to givelectrical dipole moment (or spontaneous polarization PS along the line (x1 – x2).


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FIG. 2 Schematic two-dimensional presentation of pyroelectricity

• The potential energy of any cat ion along this line will be an asymmetric form as ill

FIG. 3. Any excitation caused by an increase in lattice temperature will make it cha

quantised energy level (E1 to En) within the well, and lead to a change in its mean e

position in the lattice along the line A-B in FIG. 3.

• This gives a change in the overall electrical dipole moment, which appears as the m

pyroelectric effect.

[1]


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• Potential energy of cation in lattice of FIG. 3 along the line x1-x2, E1 to En represent t

energy levels for the cation and the locus A-B is the change in its equilibrium position

in energy.

FIG. 3

[1]


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PYRO ELECTRIC MATERIALS:

Poly vinylsidene fluoride:

• The PVDF molecules have a repeat unit of (-CH2-CF2-) in which the carbon-hydrogen and t

electrically polar carbon-fluorine bonds can take up a number of stable configurations determ

the polymer's treatment.

• The two most important are shown in figure

• Structure of PVDF molecular and unit cell forms:

(a) and (c) alpha-phase; (b) and (d) beta-phase.

• If the alpha-phase(a) is stretched and electrically poled,

the beta-phase forms in which the molecular groups are in an

all-trans configuration (c) and the molecules are assembled to give a polar unit cell(d).

• This form of PVDF is also pyroelectric.


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Barium Titanate:

(a) (b)

(a) Cubic crystal structure of BaTio3

(b) Cubic structure above 130 c

(c) Tetragonal structure below 130 c

• Above 130 c the Barium Titanate is not ferroelectric and hence the there is no polarizati

• Below 130 c the crystal is polarized due to separation of center of masses of positive an

[2]


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Pyro electricity in Barium Titanate:

• The Barium Titanate is said to be pyroelectric because when the temperature increases

expands and the relative distance of ions change.

• The Ti 4+ shifts with change in temperature which results in change in polarization.

[2]

PYROELECTRIC MATERIAL IN THERMAL SENSOR


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PYROELECTRIC MATERIAL IN THERMAL SENSOR:

APPLICATION C ti P l


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APPLICATION: Counting People:

• There are many situations where numbers of people entering, leaving, or passing through an

must be monitored.

• Most notably, this is the case in retail outlets and shopping malls, where retailers desire to mo

the conversion rates of people entering their premises leading to sales actually made.

Where the white spots are the humans

can be spotted out and they can be iden

TECHNOLOGY DEVELOPMENT FOR FUTURE APPLICATIO


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TECHNOLOGY DEVELOPMENT FOR FUTURE APPLICATIO

• The future of array design is moving in the direction of designing arrays with sufficient n

elements for imaging and detection, but with reduced pitch and reduced silicon area.

• In this way, it is possible to build pyroelectric arrays which can be assembled in small, in

standard leadless packages, enabling high-volume production throughput and cost reducti

well over 50%.• In this way, the mass market for intelligent detection and thermal imaging can be properly

addressed.

• In thermal imaging, the challenge is to produce robust and useable imagers with adequafeatures at a price that is acceptable for inclusion in the average tool box.

• Work is progressing to design cost-effective imagers using new pyroelectric array design

•The use of image fusion (visible with thermal) is one of the keys to meeting this challeng


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REFERENCES:

• [1]Pyroelectric devices and materials,R W Whatmore,Plessey Research

Limited, Allen Clark Research Centre, Caswell, Towcester, Northants,

• [2]Principles of electronic materials and devices third edition, S.O.kas

• [3] Pyroelectricity-group of dielectric physics-Professor Ryszard Popra

• [4] Applications of Pyroelectric Materials in Array-Based Detectors,AHolden, Member , IEEE.

Pyroelectric themal materials

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