Electrical modelling
The Hybrid piezo-pyro effect is almost close to the algebraic sum of the pure piezo and pure pyro effects. Therefore, we can say the total charges induced by the hybrid effect
Q = Q t + Q M 1
Pyroelectric effect induced charge, voltage and energy
The pyroelectric effect induced charges can be calculated using Eq. 1.
Qt = ρΔTA 2
where ΔT is the temperature change and A is the surface area of the harvester. Assuming that during one cycle of cooling or heating process, pyroelectric effect induced charge is QT. In a random n th cycle, the charge will be distributed between the internal and external capacitors Eq 2
ΔQ + Qp (n+1) +QE(n-1) = QP (n) + QE(n) 3
where ΔQ is the charge induced in the nth cycle; QP and QE are the charges stored in external capacitor (CE) and internal capacitor (Cp), respectively, typically, CE Cp.
QP (n) = V(n) CP 4
QE (n) = V(n) CE
QP (n-1) = V (n-1) CP
QE (n-1) = V (n-1) CE
where V is the voltage across CE. It should be noted that the sign in third equation in Eq. 3 depends on the signs of temperature change ΔT. That is, if both the nth and n1th cycles are cooling or heating process, the current generated will be in the same direction; however, if the nth cycle is cooling (heating) but the n-1th cycle is heating (cooling), then the current generated in these two cycles will be opposite and a negative sign will be used. By substituting Eq. 4 in Eq.3, we can get (Batra et al., 2011, Cuadras et al., 2010)
Vn = +()Vn-1 = + () Vn-1 5
Assume V1=0 and with CE >>Cp, Eq. 4 reduces to
Vn = = 6
where Qn is the accumulated charges on CE after nth thermal cycle.
The energy stored in CE is thus,
En = Ce ( )2 7
Piezoelectric effect induced charge, voltage and energy
The dynamic motion of the cantilever can be described using the mass-spring-damper equation
mt z ̈+ bz + k1Z + k2 Z3 = A cos(ωt ) 8
where mt , b, k1, k2 are total effective mass ,damping coefficient, linear stiffness, and nonlinear stiffness respectively. z(t) is the tip displacement. The first derivative of the z with respect to the time t represents the velocity, and the second derivative denotes the acceleration. A is the amplitude of the external periodic driving force, and ω is the driving frequency. When ω= ω0 (ω0 being the natural frequency of the cantilever; ω = here we only analyse the fundamental resonance), the deformation of the cantilever reaches to the maximum. Here we set the driving frequency same as the nature frequency of the cantilever to ensure an optimal match between the excitation and the beam. Under the external driving, the cantilever vibrates, the deflected cantilever can be treated as an arc. The curvature 1/r (r is the radius) of this arc is expressed with respect to the tip deflection z
= 9
The axial strain on the surface of the piezoelectric layer along the x-axis can be obtained in terms of curvature and tip deflection
єx(t) = 10
The axial stress σ in the piezoelectric layer is derived through the stress-strain relation, that is
σx (t)= Yεx(t) 11
The piezoelectric constitutive equations relate four field variables stress components Σ, strain components S, electric field components E, and the electric displacement components D, which can be described as [] = | | [], We shall focus on the generated electric displacement D from the mechanical stress Σ. Assuming no external electric field is applied to the cantilever, D= dΣ, d being the piezoelectric constant. The electrical charges generated from a tip deflection z(t) can therefore be obtained.
QM = ѡLd33σx (t) = (t) 12
By substituting the Eq 12 and Eq 6 in Eq 1 we will get the total charge induced by the piezo pyro effect,
Q = Q t + Q M = ρA∑∣ΔT∣ + ѡLd33σx (t)
Similarly, by Eq 6 and Eq 7 we can calculate the voltage and the energy.