Physics Functional Materials

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Physics Functional Materials

Composite Materials

Prof. Subhasis Ghosh, School of Physical Sciences, Jawaharlal Nehru University, New Delhi

Development Team

Principal Investigator

Paper Coordinator

Content Writer

Content Reviewer

Pr o f . Ashi sh Ga r g , Pro f . Ka l lo l Mo nda l , Pro f . Ra ju Ku ma r Gupta , Pr o f . Sha sha n k S he kha r , De p a rt me n t o f M a t e ria l s Sc i e nc e an d

En g i ne e ri ng , De p a rt men t o f C he mi c a l Eng i ne e ri ng , In d ia n In sti t u t e o f Te c h n o lo g y K an p u r

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Description of Module Subject Name Physics

Paper Name Functional Materials Module Name/Title Composite Materials Module Id 5_1

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Contents

A. Pyroelectricity……….2

Learning Objectives: ………..……2

Lecture 1………..3

1.1 Theory………3

1.2 Mathematical analysis………4

1.3 Variation of pyroelectric coefficient with temperature………..6

1.4 Figures of merit (FOM) for pyroelectric materials ……….………….10

1.5 Determining pyroelectric coefficient and related parameters---7

1.6 Materials with non-uniform spontaneous polarization---9

1.7 Generating pyroelectricity---10

1.8 Pyroelectric thermal energy harvesting- potential and challenges---11

1.9 Pyroelectric Materials – structure and properties ---13

1.10 Applications of pyroelectric materials ---18

1.11 Challenges to pyroelectric materials---22

1.12 Conclusion---23

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Learning objectives

The purpose of this chapter is to make students familiar with the following:

 Theoretical description of pyroelectric effect

 Mathematical description of pyroelectric effect

 Determining pyroelectric coefficient and related parameters

 Brief study of non-uniform spontaneous polarization

 Generating pyroelectricity

 Pyroelectric thermal energy harvesting

 Properties and applications of Pyroelectric Materials

 Potential and challenges to pyroelectric thermal energy harvesting

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Lecture 1 1.1 Theory

Pyroelectricity is electric response of a polar dielectric with a change in temperature. The change in temperature causes movement of atoms from their neutral position, such that the polarization of the material changes giving rise to a voltage across the material. This effect is temporary, if the temperature stays constant at its new value; the pyroelectric voltage gradually disappears due to leakage current. Within the same temperature limits, the charges developed by the effect of heating or cooling are equal and opposite.

Pyroelectric materials exihibit spontaneous polarization i.e. polarization in the absence of electric field which cannot be changed or reversed on applying electric field. It does so in ferroelectric materials. All pyroelectric materials are also piezoelectric. However, some piezoelectric materials have a crystal symmetry that does not allow pyroelectricity.

Figure 1. Various dielectric materials

Pyroelectric effect takes place below Curie temperature. When the material is heated above Curie temperature, the atoms come back to their equilibrium positions. The electrocaloric effect is considered to be the physical inverse of the pyroelectric effect. It is a phenomenon in which a material shows a reversible temperature change under an applied electric field.

Pyroelectricity should not be confused with thermoelectricity: In the case of pyroelectricity, the whole crystal is changed from one temperature to another, resulting in a temporary

DIELECTRICS

PIEZOELECTRICS

PYROELECTRICS

FERROLELECTRICS

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voltage across the crystal. Whereas in case of thermoelectricity, the two ends of the device are subjected to two different temperatures, resulting in a permanent voltage across the device as long as there is a temperature difference.

1.2 Mathematical analysis

A thin piece of pyroelectric material (Figure 2) is electroded and is connected to a high input impedance amplifier, a field effect transistor (FET). The pyroelectric current, ip generates a voltage V across the electrical admittance YE. A unity-gain voltage amplifier couples the high impedance source of current (the pyroelectric element) to a low input impedance following circuit. Suppose p’ is a component of pyroelectric coefficient p orthogonal to electrode surface of area A. The generated current is independent of thickness since the current is associated with the unbound surface charge.

Charge Q = p′A∆T (1)

Pyroelectric current i_p = Ap^′dT

dt (2)

Pyroelectric voltage V = i/Y_E (3)

where, Y_E = (G_A+ G_E) + jω(C_A+ C_E) (4)

is the electrical admittance. GA, GE are shunt and sample conductances; CA, CE are shunt and sample capacitances, respetively.

The equivalent capacitance of these dielectrics is given as C =∈^σ A/d (5) and stored energy (=1/2 CV²) as E =¹

2 p²

∈^σAh(∆T)² (6) where d is material thickness, ∈^σis permittivity at constant stress.

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Figure 2. Pyrolelctric material showing flow of pyroelectric current due to change in temperature (PIR-motion detector)

The pyroelectric coefficient measured under an applied field E is different from its true value as explained below.

In a dielectric material the presence of an electric field E causes the bound charges in the material (atomic nuclei and their electrons) to separate slightly resulting in local electric dipole moment. When an electric field E is applied to a material, the total dielectric displacement D i.e. charge per unit area of the plates on both side of pyroelectric material is

D = P_S+∈₀ E (7)

Where ε0 is permittivity of vacuum and Ps is the spontaneous polarization-volume density of electric dipole moments.

But P = χ ∈₀ E (8)

Therefore, D =∈₀ E(1 + χ) (9)

𝐷 = ∈_r∈₀ E (10)

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where ∈_r is relative permittivity, or dielectric constant (for isotropic material), or dielectric tensor (for anisotropic material), χ is electric susceptibility.

Generalized pyroelectric coefficient, ∆ pg , assuming E constant, is given as

∆p_g =^∂D

∂T= ^∂Ps

∂T + E^∂∈0

∂T

(11)

True pyroelectric coefficient p under constant electric field and stress is:

p = ^∂𝑃𝑆

∂T σ,E (12)

as it defines variation of spontaneous polarization with temperature.

The equation for polarization in x-direction is given as:

P_x= P_x0+ ϵ₀ ∑ ϵ_xkE_k

k=x,y,z

+ ∑ ( ∑ d_xklσ_kl

l=xyz

)

k=x,y,z

+ ∑ ( ∑ ( ∑ μ_xijk

i=x,y,z

∂u_jk

∂i )

j=x,y,z

)

l=x,y,z

(13)

where Px0 is the permanent polarization in the x direction; the second term is the electric field induced polarization (ε0 is the permittivity of vacuum, εxk are components of the dielectric tensor and Ek is the electric field in direction k); the third term is the piezoelectric effect induced polarization (dxkl are components of the piezoelectric tensor and σkl are components of the stress tensor); and the fourth term is the flexoelectric polarization (μxijk are the components of the flexoelectric tensor, ujk are the components of the strain tensor).

The derivative of Px from above equation with respect to time gives primary pyroelectric coefficient. If pyroelectricity is measured under stress free conditions, thermal expansion of the material leads to a secondary pyroelectric coefficient. The total pyroelectric coefficient is given as

p_i^X = p_i^X+ d_ijk^Xc_jklm^XEα_lm^E

(14)

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Where, first and second terms refer to primary and secondary pyroelectric coefficients, respectively. d_ijk^X is the piezoelectric strain tensor under a constant stress; c_jklm^XE is the elastic stiffness tensor under constant stress and electric field; α_lm^E is the thermal expansion tensor under a constant electric field.

1.3 Variation of pyroelectric coefficient with temperature

Pyroelectric coefficient increases with temperature. It also depends on the order of phase transition and is larger for second order transitions.

Figure 3.Variation of pyroelectric coefficient with temperature¹

1.4 Figures of merit (FOM) for pyroelectric materials

To evaluate pyroelectric materials and their interference with electronic readout circuit, three figures of merit are employed-

 current response FOM, Fi=p for the charge sensitive readout

 voltage response FOM, Fv=p/ϵ’ for the readout circuit in voltage mode

 For detectors, the signal to noise ratio (SNR) is a critical parameter and FOM, FD

=p/√ϵ’’ measures the SNR of the sensor.

Where, ϵ’ and ϵ’’ are the real and imaginary parts of permittivity, respectively.

We can calculate FOM for the PIR detector shown in Figure 2. We can get ip in terms of the average IR power (W(t) = W₀exp(iωt) signal sinusoidally modulate) incident on the pyroelectric element. Considering equation i_p = ApdT/dt, we can see that ip is maximised by

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firstly maximising the active area of the element and by using material with a larger p. In order to determine current responsivity Ri , we need to make a balance between power input ɳW(t), power retained (HdT/dt, where H is the element thermal capacity), and power lost to the environment (GT where G is thermal conductance ). Thus we get

R_i = i_p

W₀ = ɳpAω G_T(1 + ω²τ_T²)^1/2

(15)

At high modulation frequencies (ω > ¹

τT, where τ_T = H G⁄ _T , the thermal time constant of the element)

R_i = ɳAp/H

(16)

This will give FOM, Fi.

For device in voltage readout mode, to find the voltage responsivity Rv, we need to consider the electrical admittance presented to ip and we get

R_V= ^ip

YW0 = ^RGɳPAω

GT(1+ω²τT2))^1/2(1+ω²τE2) (17)

where, τ_E is electrical time constant (τ_E = R_G(C_E+ C_A)). This response when plotted on a log/log graph is flat between the frequencies 1/τ_E and 1/τ_T, rising response at low frequencies, and then a 1/ɷ roll-off. In the high frequency region, we get

R_V= ^ɳp

c′∈∈0Aω (18)

Where (CE>>CA) and F_V= _c′∈∈^p

0 (19)

For signal to noise ratio, we define specific detectivity D* as,

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D^∗ =^RVA1/2

∆V_N (20)

VN the total RMS electrical noise i.e. sum of all the noise contributions in the circuit.

Therefore, F_D = ^p

c′√∈∈₀tan δ (21)

1.5 Determining pyroelectric coefficients and related parameters

A) TECHNIQUES APPLIED TO BULK SAMPLES-

 BY CONTINUOUS RAMPING –

This method involves measuring the pyroelectric current flowing between two contacts on a sample, which is continuously heated or cooled. A uniform temperature distribution is obtained in the sample, when the sample heating rate is smaller than the time required for thermal diffusion. Equivalent circuit is shown in Figure 4.

Figure 4. Equivalent electrical circuit for continuous ramping with the pyroelectric material and a high-impedance voltmeter connected in parallel

 BY PERIODIC TEMPERATURE CHANGE –

The periodic heating induces temperature waves which are attenuated exponentially as a function of both depth in the sample and frequency of modulation. This results in heat regions

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having different polarizations and thus produces an AC pyroelectric current. This technique is preferred over others as it reduces effect of trapped charges that interfere in accurate measurement of pyroelectric response. It can be classified as under-

a) Periodic pulse technique:

The sample to be measured is subjected to periodic, step-like heating. Sample is heated by radiation only but cools by both radiation and conduction. This method is extremely attractive when heating and cooling rates can be very large with respect to the temperature ramping techniques.

b) Continuous oscillation method:

The sample is subjected to a continuous, sinusoidally modulated heat source. The basic idea is that if the source frequency is low enough to consider the sample homogeneously heated, then the component of the current originated from pyroelectricity is 90◦ out of phase with the thermal wave. This allows the accurate measurement of the pyroelectric effect.

B) TECHNIQUES AS APPLIED TO THIN FILMS-

 SUBSTRATE SUPPORTED THIN FILMS

a) Heat and cool the substrate considering the film and the substrate as one. Because the heat capacity of the substrate is much larger than that of the film, the film will be homogeneously heated. This is achieved by choosing the modulation frequency to be much lower than the inverse of the thermal diffusion time through the film.

b) Holeman method: With increase in modulation frequency, the pyroelectric response depends only on the properties of the film and not of the substrate This occurs if the modulation frequency is at least one order of magnitude higher than inverse of the thermal diffusion time through the film. However, it is necessary to know the thermal properties of the film. Thermal diffusion coefficient of the film is obtained by plotting the pyroelectric response versus frequency.

 SELF SUPPORTED, EDGE CLAMPED FILMS

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These structures maximize the temperature change with respect to a given amount of heat input which results in very high responsivity. They are applicable for high performance infrared detectors and/or detector arrays. The important thing to consider here is the mechanisms by which heat is transferred through them. Depending on the material of the self-supported film, any one of these may dominate-

(a) thermal conduction through the self-supported film;

(b) radiative transfer to the environment from the detector area or from the whole film;

(c) heat transfer to the gas surrounding the sample.

There are two major measurement geometries for pyroelectric measurements-

1) A small area in the center of the membrane is illuminated :

This method is used to produce the highest possible detectivity from a given material. The lumped parameter model is applied as under-

a) When radiation is dominant-radiation resistance can be estimated from the Stefan- Boltzmann law.

b) When the thermal conduction is dominant-thermal conductance is calculated as

G = 2πλd/ln(R/r) (22)

Where, λ is the thermal conductivity of the material of membrane, d is thickness of the film, R is the film radius, r is the radius of the region covered by the contacts.

2) The membrane is uniformly (total area) illuminated:

This technique is used in detector arrays. The total current must be calculated from the derivative of the temperature with time at each point in the film and then summed over entire film- area.

1.6 Materials with non-uniform spontaneous polarization

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D = Pi + Ps+∈∈₀ E (23)

Here, Pi is polarization of interior and Ps is polarization of surface. Pyroelectric current due to temperature change is given by

J =^∂Q

∂t (24)

J = − ^Cs

ϵϵ0∫₀^d∂P(x)_∂t dx = −^Cs

ϵϵ0∫₀^d∂P(x)_∂T ^∂T_∂tdx (25)

Techniques applied to get spatially averaged pyroelectric coefficient are- A) The periodic pulse technique applied to a very thick film :

 Considering pyroelectric coefficient α vary linearly with depth z -

α = α₀(1 + bz ) (26) α0 is pyroelectric coefficient at surface, b is a constant.

 Considering pyroelectric coefficient to be constant -

For a time shorter than the thermal diffusion time through the film, the pyroelectric current is constant, given as

I = Aα ^Fd

C_fD_f (27)

B) Continuous temperature oscillation techniques:

 Pyroelectric current generated in response to sinusoidally modulated heating: Lumped model

Body of thermal capaticance H, having surroundings of thermal conductance G, heated by sinusoidally modulated heat source F(1 + exp(jwt))

Pyroelectric current is given as

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G ω

√1+ω²τth2 (28)

Where, τ_th= ^H

G is a thermal time constant.

For low frequencies (τ_th²ω² ≪ 1), pyroelectric current increases linearly with frequency.

I_low= AαFω/G (29)

It reaches a constant value of

I_low= AαF/H (30)

Therefore, if either G or H is known, pyroelectric coefficient can be determined.

1.7 Generating pyroelectricity

Pyroelectricity can be generated from different types of materials-

1. Polar materials, in which the permanent dipole moment of the structures changes with temperature. This is called primary pyroelectricity.

2. Dielectric media in an external electric field. Since the dielectric constant is temperature dependent, a change in temperature results in a change in surface charge density.

3. Piezoelectric materials, where heating produces changes in mechanical stress, which in turn generates changes in surface charge density. This is often called secondary pyroelectricity.

4. Non-piezoelectric materials where thermally induced non-uniform deformation can also produce changes in the surface charge density.

1.8 Thermodynamics of pyroelectric thermal energy harvesting

The non-zero spontaneous polarization PS of pyroelectric materials causes the material to attract charged particles at room temperature. If the material is placed between two electrode plates and the dielectric material within a capacitor is connected to an external circuit, then

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the plates will charge until the surface charge on the pyroelectric material is neutralized (Figure 5a). The current remains zero. When the temperature across the capacitor is increased, the dipoles begin to oscillate about their equilibrium axis, thus reducing PS. The quantity of induced charges in the electrodes is thus reduced. A current will flow in the external circuit as long as the temperature across the capacitor changes (Figure 5b). On reducing temperature, spontaneous polarization will be enhanced since the electric dipoles oscillate with smaller angles due to the lower thermal activity. A current will flow in the opposite direction once the temperature is reduced due to realigning of the dipoles in the material (Figure 5c). Cycling the temperature across the capacitor induces an alternating current in the external circuit. The magnitude of the electrical current and energy conversion efficiency are dependent on the rate of change in the temperature. Pyroelectric nanogenerators work on this concept.

The efficiency of the thermal to electrical energy conversion is thermodynamically limited by Carnot efficiency as,

ɳ_carnot = 1 −T_L T_H

⁄ (31)

TH and TL are the temperatures of heat source and sink, respectively.

In the real world, the efficiency for the conversion of heat (Qin) into electrical power (Wout) is defined as:

ɳ =^Wout

Q_in = ^WE−WP

CV∆T+QInt+Q_Leak

(32)

where, WE is the generated electrical power, WP is the power lost in the temperature cycle, CV

is the heat capacity of the pyroelectric device, QInt are the intrinsic heat losses in the thermal cycle and QLeak are the heat leakages between the hot and cold sources.

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Figure 5. A pyroelectric material used as the dielectric in a capacitor, a) electric dipoles under room temperature, b) heated and c) cooled conditions. The angles marked in the diagrams represent the degrees to which the dipole would oscillate as driven by statistical thermal fluctuations²

If the pyroelectric capacitor is part of a thermal engine, much higher efficiencies and output powers can be achieved through considering the Ericsson thermal energy conversion cycle modified by Olsen process. This cycle works by allowing large temperature swings across the pyroelectric capacitor while applying alternating voltages on the capacitor electrodes. The Olsen cycle consists of two isothermal and two isoelectric field processes as shown in Figure 6. Its operating principle is to charge a capacitor via cooling under low electric field and to discharge it under heating at a higher electric field.

(a)

(b)

(c)

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Figure 6. The Ericsson temperature and voltage cycle developed by Olsen to generate electrical energy in the pyroelectric thermal energy converter^3-11

Pyroelectric current is given by equation 2. If Vappl is the external applied voltage across the pyroelectric capacitor, output work is given as

W_out= ∮ V_appldq = ∫ V_applpA^dT

dtdt (33)

Hence, the faster the temperature can be cycled back and forth across the device, the greater shall be the energy conversion efficiency. Energy harvesting of waste heat from pyroelectric materials can be achieved by utilizing the Olsen cycle.

The pyroelectric generator is modelled as a current source with a capacitor and a resistor in parallel. However, one of the main problems of pyroelectric energy harvesting is heating process followed by cooling process, which produces charge accumulation in different directions. This problem can be resolved by using a full bridge diode rectifier circuit.

 ENERGY HARVESTING DEVICES

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Pyroelectric materials are preferred over thermoelectric devices for the conversion of thermal energy into electrical energy, since they depend on temporal temperature gradient (dT/dt) instead of a spatial temperature gradient (dT/dx), which is more difficult to attain at micro/nanoscale. Also, they do not require bulky heat sinks to maintain temperature gradient and operate with high thermodynamic efficiency. Using a limited temperature gradient, a maximum efficiency of 1.7% of Carnot efficiency can be expected using a thermoelectric module whereas a pyroelectric device may reach efficiency up to 50% of Carnot efficiency.

Many pyroelectric materials are stable up to a very high temperature (~1200°C), and thus providing advantage over thermoelectrics for harvesting energy from high-temperature sources. Further, methods have been developed to convert stationary spatial gradients to transient temperature gradients, facilitating development of thermoelectric- pyroelectric hybrid energy harvesters. For instance a micro heat engine that acts as a thermal energy shuttle between a heat sink and a heat source. In this configuration, an oscillating thermal field is created across a pyroelectric generator. It was demonstrated that using this micro thermomechanic–pyroelectric energy generator (μTMPG), 3 μW power could be harvested for a temperature difference of 79.5 K from pyroelectric generators.

Devices based on pyroelectric energy harvesting have an advantage over piezoelectric energy harvesting devices as unlike the latter they require low or no maintenance since they do not include any moving parts.

1.9 Pyroelectric Materials – structure and properties

Pyroelectric materials are non –centro-symmetric polar dielectrics. Most inorganic pyroelectrics are perovskite structured.

Figure 7. A unit cell of perovskite (calcium titanate - CaTiO3)¹³

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General requirements of pyroelectric materials are high pyroelectric coefficient, physical and chemical stability, low relative permittivity, low piezoelectric response, low cost, high quality and ease of processing.

Pyroelectric materials can be classified in four main areas as-single crystals, ceramics, polymers and thin film materials. These have been tabulated as under-

Table 1. Classification of Pyroelectric materials

MATERIAL STRUCTURE FEATURES APPLICATIONS

SINGLE CRYSTALS Triglycine

sulphate(TGS) (NH2CH2COOH)3

H2SO4

Sulphate Can withstand

temperatures above Curie point without depoling

Thermal imaging

cameras, IR

detectors

LiTaO3 single crystal

Hexagonal Mechanical and chemical stability, high optical damage threshold and wide transparency range

Optical modulators, pyroelectric

detectors, SAW substrates,

transducers, pyroelectric anemometers LiNbO3 (Lithium

Niobate)

Trigonal Thermal imaging,

telecom market i.e.

mobile telephones

and optical

modulators Sr1-xBaxNb2O6

(Strontium Barium Niobate)

Thermal imaging

ZnO wurzite nanowires

CERAMICS

PbTiO3 perovskite High pyroelectric

coefficient

Pyrodetector, substitute for TGS

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Composite Materials Lead zirconium

titanate (PZT)

perovskite High sensitivity and permittivity

Electroceramics, ultrasound

transducers, ceramic capacitors

POLYMERS Polyvinylidene

fluoride (PVDF) based polymers

polymer Large areas of thin film, chemically inert

toward most acids, solvents, oxidants, halogens, and alcohol

Burglar alarms, commercial cooling devices

POLYMER CERAMIC NANOCOMPOSITES 0.75Pb(Mg1/3Nb2/3)

O30.25PbTiO3

Lead magnesium niobate-lead

titanate (PMN-PT)

High dielectric permittivity, high piezoelectric

properties, high electrostriction

Microatcuators and energy harvestors, sensors and multi- layer capacitors, optical devices

THIN FILMS

Langmuir Blodgett organic amphiphilic molecules which have a hydrophobic tail and a hydrophilic head group. Examples: Z-type Liquid crystal LB film, [Ru(PPh2biP)2]PhOC16/behenic acid, WTA/Docosylamine superlattice.

Sol-gel technique to get nanocrystalline and large-area homogeneous thin films.

Examples: PZT, P(VdF-TrFE), Ba0.64Sr0.36TiO3

 ENHANCING THE PERFORMANCE OF PYROELECTRIC MATERIAL

The performance of pyroelectric sensor /array is derived from FOM and other parameters.

High value of permittivity and maximized FOM are found in ferroelectric perovskite ceramics, the PbZrxTi1-xO3(PZT) system. They are stable and can be manufactured relatively easily. Doping is done to optimise the FOM and related parameters thereby enhancing the performance and reducing the cost of pyroelectric materials.

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Another important parameter is electrical resistivity. In Figure 2, resistor RG is connected across the pyroelectric element. It serves for three functions. Firstly, it fixes the electrical time constant τ_E of the device. Secondly, it serves as bias resistor that stabilizes the FET amplifier. Thirdly, it determines the voltage responsivity RV. For the perovskite oxide group ABO3, to which PZT belongs, doping the ‘B’ site with uranium controls resistivity, dielectric constant and loss of PZT.

Structuring the pyroelectric materials in an advantageous way also enhances their performance. For example, the temperature range over which the FR(LT) to FR(HT) phase transition occurs is increased by laminating and co-sintering layers of ceramic with different compositions forming a functionally-gradient material. A porous layer is included in the centre of a laminated stack. This way FV can be increased from 5×10^-2 m²C^-1 to about 8×10² m²C^-1. Pyroelectric response of detectors is also affected by substrate. According to a study⁴³, Pb(Zr0.3Ti0.7)O3 thin film was deposited on Pt/Ti/Si3N4/SiO2/Si substrate. Four substrate processing alternatives were studied.

1.10 Applications of pyroelectric materials

 PIR – SENSOR:

A pyroelectric infrared sensor (PIR sensor) absorbs infrared (IR) light radiating from objects.

The resultant temperature change gives rise to pyroelectric current. Some of its applications are-

1) PIR-based motion detectors (PID):

PIR detectors have gained importance due to their wide –wavelength response, lack of need for cooling and good sensitivity. Change in temperature of the detector against the surrounding temperature when an intruder comes in its vicinity leads to a voltage which can be used to trigger burglar alarms and in automatically activated lighting systems.

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Figure 8. Motion-detector attached to outdoor door, automatic light¹⁴

Figure 9. A schematic presentation of a pyroelectric infrared (PIR) sensor with dual sensing elements aligned in a motion plane and the output signal in case of walking (a) in different directions; (b) at different distances; (c) at different speed levels¹⁵

Detector arrays market for infrared imaging is expanding. These can interpret scenes from comparatively low-resolution, non-invasive images sensitive to motion and provide cost- effective solutions. They are being used in counting of people, and if provided with artificial intelligence they can serve for security and health care. They have the potential to be used in very-large-volume applications in environments where video systems are limited. The integration of ferroelectric thin films directly onto silicon substrates has reduced array fabrication costs and increased performance through reduced thermal mass and improved thermal isolation.

Infrared or thermal imaging: Infrared (IR) imaging is as similar as using visible light to make a photograph. A special lens focuses the infrared light emitted by all of the objects in view.

The focused light is scanned by detector array which create a detailed temperature pattern called a thermogram within nearly 1/30th of a second. The thermogram is converted to electrical signals which are sent to a signal-processing to translate them into data for the display.

Thermal imaging systems are being used in firefighting, medicine and surveillance. The pyroelectric vidicon tube and the detector arrays are the examples of thermal imaging devices. They are used in military, industrial and space fields.

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Figure 10. Flowchart showing working of thermal imaging device

2) PIR sensor based indoor –location aware system for smart home.

Figure 11. Technology applied in smart home¹⁶

3) Radiometry- A radiometer measures the power generated by a radiation source and is used to calibrate UV, visible and infrared radiation sources.

4) Detection and protection of wildlife. Pyroelectric IR devices can detect wild animals and their movements by using the difference of IR radiation in the environment.

5) Infrared imaging pollution monitoring- Pyroelectric devices can be used for testing the level of IR radiation that passes through a gas sample. The pollutant gas can be easily detected since the wavelength at which a gas absorbs usually uniquely identifies that gas.

6) Other applications- PIR remote based thermometer, fire detectors, pyroelectric generators, spectrometry, pyrometry, thermometry, direction sensing, remote temperature measurement, laser diagnostics.

OBJECT LENS

DETECTOR (PYROELECTRIC

MATERIAL)

SIGNAL

PROCESSING MONITOR

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7) Model of a MEMS-based Surface Acoustic Wave Hydrogen Sensor: Hydrogen is a very important element in industry. However, its careful handling and monitoring are crucial since it is explosive in air. Surface Acoustic Wave (SAW) sensors can be used for hydrogen detection due to their small size, low fabrication cost, ease of integration and high sensitivity. Pyroelectric sensors incorporated with palladium electrodes for hydrogen detection can be used. There are AC and DC pyroelectric hydrogen sensors.

The adsorption and dissociation of hydrogen molecules on the Pd surface causes transfer of thermal energy.

Figure 12. Layout of SAW hydrogen sensor⁴²

 CMOS Integrated Infrared Sensor

CMOS Integrated Microsensors (CIMS) technology combines silicon bulk micromachining with sol- gel pyroelectric thin films materials and a commercial CMOS process to provide an integrated infrared sensor.

 Pyroelectric nanogenerators

The working principle of pyroelectric nanogenerator is based on two different cases: the primary pyroelectric effect, and the secondary pyroelectric effect. The primary pyroelectric effect dominates in PZT, BTO (Barium titanate), and some other ferroelectric materials. The secondary pyroelectric effect dominates the pyroelectric response in ZnO, CdS, and some other wurzite-type materials. Pyroelectric nanogenerators are used as an active sensor, which can work without a battery. They have the potential to charge batteries using waste heat released from various sources in the environment. A polarized (PVDF) film-based nanogenerator can harvest both thermal and mechanical energies i.e. it can be used as pyroelectric - piezoelectric hybrid nanogenerator.

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Recent advances in nanotechnology have enabled energy harvesting from 1D materials such as nanowires and nanofibers. The advantages of nanowires over bulk materials are their enhanced mechanical properties due to lower defect density. As such, nanowires can withstand larger strains. This makes them attractive for nanogenerators, which are promising for wearable electronics. Nanofibers are often mechanically soft and can be readily incorporated into flexible devices.

Figure 13. Nanowires and nanowire composites; (a) vertical bundle of ZnO nanowires sandwiched between two electrodes⁴⁴ and (b) flexible pyroelectric nanogenerator based on KNbO3 nanowire composites⁴⁵

 MEMS based pyroelectric thermal energy harvester

A resonating pyroelectric capacitor thermal energy converter has been shown in Figure 14.

The cantilevered pyroelectric capacitor is fabricated with two metal films, which act as the electrodes of a capacitor, and a pyroelectric material which acts as the dielectric between the metal electrodes. The cantilever can be anchored to either the hot or cold surfaces. The cantilever structure initially gets heated through the anchor, causing the cantilever to bend towards the lower cold heat sink where it rapidly loses heat, and bends back towards the upper heated surface. This way the cycle gets repeated.

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Figure 14. A bimaterial cantilever structure, which alternately contacts the hot and cold surfaces, generating an electrical current in the pyroelectric capacitor¹²

 Liquid flow measurement using a pyroelectric anemometer

They are used for fluid flow in microchannels whose hydraulic diameters range from fractions of a micron to a hundred microns. They can also be used for micro-chromatographs, compact fluid flow-injection-analysis systems, bioreactors, and heat exchangers for heat sinking VLSI systems. Resistance-based thermal anemometers are limited to flow rates of at most three orders of magnitude. Whereas a pyroelectric anemometer can measure gas flows of at least five orders of magnitude, require minimal calibration, can be used for both liquid and gas flow rates.

1.11 Challenges to pyroelectric materials

One of the technical challenges in pyroelectric harvesting systems is to develop methods to generate temperature oscillations to harvest power. Cyclic pumping is used to transform a temperature gradient into a time variable temperature. If the pumping power consumption is a small fraction (<2%) of the harvested energy, the process becomes feasible. Another challenge is that the inherent large thermal mass of materials generally restricts pyroelectric harvesters to low frequency operation (<1 Hz) compared to vibration harvesters. A potential solution to this problem is using radiative heat transfer between the pyroelectric element and hot and cold surfaces at the nanoscale. It has a faster response compared to convection. If the

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surfaces are separated by a distance smaller than a characteristic wavelength (given by Wien's displacement law λmax=b/T) the net radiation flux in a vacuum between two surfaces can be increased by several orders of magnitude.

In thermal imaging, the challenge is to produce robust and useable imagers with adequate features at a less price. Designing of cost-effective imagers using new pyroelectrics is in progress. The use of image fusion (visible with thermal) is one of the keys to meet this challenge.

1.12 Conclusions

Characteristics like low cost, low power, wide operating temperature range etc. make pyroelectric devices ideal for applications where the cost, power and cooling requirements of photoconductive or photovoltaic detectors are impractical. Their applications range from consumer and commercial level such as intruder alarms, where low cost and high reliability are needed, through professional products such as gas analysis equipment and process control, to military requirements such as surveillance equipment where good performance, low power and a high degree of environmental stability and reliability are crucial.

Improvements in manufacturing techniques may give significant reductions in dielectric loss and will improve the availability and reduce the cost of the pyroelectric materials. With rapidly increasing demand for energy harvesting from environmental sources, incorporation of nano- and micro- structures into devices is gaining momentum. Incorporating pyroelectric nanowires and nanofibers into flexible textile and wearable electronics for hybrid pyroelectric- piezoelectric energy harvesting is a challenge. These energy harvesters could power personal electronics and replace batteries. It is anticipated that, over the next few years pyroelectric infrared detectors will have great importance in the society due to ongoing developments.