8) Pyroelectricity and IR detection

Pyroelectricity

ability of a material to generate a charge when uniformly heated; all ferroelectrics are pyroelectrics (ferro can switch while pyro can’t)

(piezoelectrics can generate charge when non-uniformly heated due to thermal stresses)

Equation for the dielectric displacement

D = ε_0E + P_tot; P_tot = P_s + P_ind; P_ind = χ ε_0 Ε, ε_r = 1 + χ

General pyroelectric coefficient

p_g = p + E dε/dT

Define the change in polarisation

ΔP_i = p_gΔ

Why are second order transitions preferred for IR sensor application

Ps changes rapidly around phase transition which gives a large pyroelectric coeff; second order tranistions have no thermal hysterisis so produce the same change whether heating up or cooling down, the properties change predictable and continuously which enables a more reproducible response required in sensors

Voltage responsivity

v_r = pη/C_pAεω, η=fraction of incident energy absorbed, C_p=specific heat capacity, A=area, ω=frequency radiation is physically pulsed at (with chopper fan)

Why is the radiation physically pulsed with a chopper fan

continuous radiation won’t produce a response since only ΔT is measured

Figure of merit

F_V = p/C_p ε or F_D = p/ (C_P sqrt(εtanδ))

What is PZFNTU and why is it used

based on lead zirconate (naturally antiferroelectric), add Pb(FeNb)O3 converts it to a low permittivity (increases voltage in pyro) ferroelectric; had a phase transition at 40C making p unstable; adding PbTiO3 shifted it to 100C (outside device operating temp); Uranium used to control electrical resistance and further reduces permittivity; preferred as it is cheap and easy to manufacture

Bulk applications of pyroelectrics

charge easily transformed into a signal for sensing; imaging is harder: need thermally isolated array of pyro disks which can individually detect a ΔT and behave as pixels

How does F_D compare in thin materials vs bulk

much smaller in thin film than same material in bulk; films are mechanically clamped by substrate which limits small changes in dimensions thereby limiting changes in polarisation; films may not grow in direction that optimises pyroelectric coeff

Microstructural effects in pyroelectrics

porous can be superior as decreases permittivity with little affect on pyroelectric coeff therefore optimising F_D; but very hard to do all the processing required for flipchips with a porous structure

Thin film pyroelectrics

very fragile and air was not a good enough thermal insulator so never commercialised (bulk prevailed); also due to substrate clamping