7. Practical application of pyroelectrics

Question 1

What is the most common application of pyroelectrics

Answer 1

As thermal detectors (as they release charge when experiencing a temperature change)

Question 2

What are 3 features of a good pyroelectric element

Answer 2

• Absorb and retain as much heat as possible during charge release monitoring
• Low thermal capacity (for largest possible temp rise for a given heat input)
• Retain charge for long enough for it to be amplified (long enough electrical time constant)

Question 3

What is configured to amplify the signal in a thermal detector?

Answer 3

A FET is configured as a source follower (due to low noise, high input impedance and low output impedance) It effectively transforms the output of the element from a a high to low impedance)

Question 4

What two factors compromise to give the thermal time constant in a thermal detector?

Answer 4

Time constant = Thermal capacity / Conductance to surroundings

Compromise low thermal capacity to give large temperature change, and retaining enough heat at the element.

Question 5

Which is larger, the thermal or electrical time constant in a thermal detector?

Answer 5

The thermal time constant

Question 6

What is current responsivity?

Answer 6

The pyroelectric current generated per unit watt of input power (from the incident heat)

Question 7

What are the 3 sources of noise in the output of pyroelectric detectors?

Answer 7

• Thermal noise (statistical fluctuations in heat flow)
• Amplifier noise
• Electrical noise (Johnson noise) due to random movement of electrons (fluctuations in energy dissipation) THIS IS THE DOMINANT FORM

Question 8

What is the equation for the current produced by a pyroelectric material when subjected to a temperature change?

Answer 8

i = Ap dT/dt

A-Area
p-pyroelectric constant
dT/dt- rate of change of temperature

Question 9

What is the equation for the thermal time constant of a pyroelectric element?

Answer 9

Thermal time constant (tau t) = H / Gt
H- Thermal capacity
Gt- Thermal conductance of the surroundings

Question 10

What is the equation for the electrical time constant of a pyroelectric element?

Answer 10

Electrical time constant (tau e) = Rg (Ce + Ca)
Rg-Gate resistance
Ce-Element capacitance
Ca-Amplifier capacitance

Question 11

What does C contain when referring to pyroelectric elements?

Answer 11

The sum of the capacitance for the element and the amplifier.

Question 12

State the piezoelectric equations of state

Answer 12

1.   P= χE+dσ
P-change in polarisation 
χ-electric susceptibility 
E-electric field
d-piezoelectric constant 
σ-stress

2. S=cσ+dE
S-Strain
c-elastic constant 
d-piezoelectric constant 
E-electric field

(The piezoelectric equations of state describe the basic principles of operation of
piezoelectric devices. Most devices operate on the basis of varying stress, s, or electric field
and hence the most useful form of the equations of state are derived from Gibbs free energy
considerations)

Question 13

What is the conditions under which the piezoelectric equations reduce to hookes law?

Answer 13

With d = 0, the piezoelectric equations reduce to Hooke’s law, with S prop to σ and P=χ ε0 E

Question 14

Outline the principle of operation of passive piezoelectric devices

Answer 14

Passive piezoelectric devices use the direct piezoelectric effect in which an applied stress
causes a change in polarisation in the absence of an applied electric field. This is expressed in
reduced suffix notation by ;

Pi = dij . σ

Where i = 1 to 3 and j = 1 to 6. Hence each of 6 applied stresses contribute to the change in
polarisation along a given direction via the d coefficients in passive operation.

Question 15

Outline the principle of operation of active piezoelectric devices

Answer 15

Active piezoelectric devices use the converse piezoelectric effect in which an applied
electric field causes a strain, i.e. ;

Sj = dij . Ei

An applied field along one direction can result in a uniaxial or shear strain along others.