What Are Piezoelectric Constants

Jump To: Charge Constant | Voltage Constant | Permittivity Constant | Elastic and Compliance | Young Modulus | Electromechanical Coupling Factor | Dielectric Dissipation | Frequency Constant | Common Constants & Equations | Symbols

Because a piezoelectric ceramic is anisotropic, physical constants relate to both the direction of the applied mechanical or electric force and the directions perpendicular to the applied force. Consequently, each constant generally has two subscripts that indicate the directions of the two related quantities, such as stress (force on the ceramic element / surface area of the element) and strain (change in length of element / original length of element) for elasticity. The direction of positive polarization usually is made to coincide with the Z-axis of a rectangular system of X, Y, and Z axes (Figure 1.6). Direction X, Y, or Z is represented by the subscript 1, 2, or 3, respectively, and shear about one of these axes is represented by the subscript 4, 5, or 6, respectively. Definitions of the most frequently used constants, and equations for determining and interrelating these constants, are summarized here. The piezoelectric charge constant, d, the piezoelectric voltage constant, g, and the permittivity, e, are temperature-dependent factors that can significantly influence the output signal and output voltage in a piezoelectric device.

Figure 1.6 – The direction of positive polarization usually is made to coincide with the Z-axis.

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Piezoelectric Voltage Constant

The piezoelectric voltage constant, g, is the electric field generated by a piezoelectric material per unit of mechanical stress applied or, alternatively, is the mechanical strain experienced by a piezoelectric material per unit of electric displacement applied. The first subscript to g indicates the direction of the electric field generated in the material, or the direction of the applied electric displacement. The second subscript is the direction of the applied stress or the induced strain, respectively. Because the strength of the induced electric field produced by a piezoelectric material in response to an applied physical stress is the product of the value for the applied stress and the value for g, g is important for assessing a material’s suitability for sensing (sensor) applications, such as in pressure sensors and piezoelectric sensors.

g₃₃induced electric field in direction 3 (parallel to direction in which ceramic element is polarized) per unit stress applied in direction 3

or

induced strain in direction 3 per unit electric displacement applied in direction 3

g₃₁ induced electric field in direction 3 (parallel to direction in which ceramic element is polarized) per unit stress applied in direction 1 (perpendicular to direction in which ceramic element is polarized)

or

induced strain in direction 1 per unit electric displacement applied in direction 3

g₁₅induced electric field in direction 1 (perpendicular to direction in which ceramic element is polarized) per unit shear stress applied about direction 2 (direction 2 perpendicular to direction in which ceramic element is polarized)

or

induced shear strain about direction 2 per unit electric displacement applied in direction 1

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Elastic And Compliance

Elastic compliance, s, is the strain produced in a piezoelectric material per unit of stress applied and, for the 11 and 33 directions, is the reciprocal of the modulus of elasticity (Young’s modulus, Y). s^D is the compliance under a constant electric displacement; s^E is the compliance under a constant electric field. The first subscript indicates the direction of strain, the second is the direction of stress.

s^E₁₁elastic compliance for stress in direction 1 (perpendicular to direction in which ceramic element is polarized) and accompanying strain in direction 1, under constant electric field (short circuit)

s^D₃₃elastic compliance for stress in direction 3 (parallel to direction in which ceramic element is polarized) and accompanying strain in direction 3, under constant electric displacement (open circuit)

Electromechanical Coupling Factor

The electromechanical coupling factor, k, is an indicator of the effectiveness with which a piezoelectric material converts electrical energy into mechanical energy, or converts mechanical energy into electrical energy. The first subscript to k denotes the direction along which the electrodes are applied; the second denotes the direction along which the mechanical energy is applied, or developed.

k values quoted in ceramic suppliers’ specifications typically are theoretical maximum values. At low input frequencies, a typical piezoelectric ceramic can convert 30 – 75% of the energy delivered to it in one form into the other form, depending on the formulation of the ceramic and the directions of the forces involved.

A high k usually is desirable for efficient energy conversion, but k does not account for dielectric losses or mechanical losses, nor for recovery of unconverted energy. The accurate measure of efficiency is the ratio of converted, useable energy delivered by the piezoelectric element to the total energy taken up by the element. By this measure, piezoelectric technology in well designed systems can exhibit efficiencies that exceed 90%.

The dimensions of a ceramic element can dictate unique expressions of k. For a thin disc of piezoelectric ceramic the planar coupling factor, k_p , expresses radial coupling – the coupling between an electric field parallel to the direction in which the ceramic element is polarized (direction 3) and mechanical effects that produce radial vibrations, relative to the direction of polarization (direction 1 and direction 2). For a disc or plate of material whose surface dimensions are large relative to its thickness, the thickness coupling factor, k_t , a unique expression of k₃₃ , expresses the coupling between an electric field in direction 3 and mechanical vibrations in the same direction. The resonance frequency for the thickness dimension of an element of this shape is much higher than the resonance frequency for the transverse dimensions. At the same time, strongly attenuated transverse vibrations at this higher resonance frequency, a result of the transverse contraction / expansion that accompanies the expansion / contraction in thickness, make k_t lower than k₃₃ , the corresponding factor for longitudinal vibrations of a thin rod of the same material, for which a much lower longitudinal resonance frequency more closely matches the transverse resonance frequency.

k₃₃ factor for electric field in direction 3 (parallel to direction in which ceramic element is polarized) and longitudinal vibrations in direction 3 (ceramic rod, length >10x diameter)

k_t factor for electric field in direction 3 and vibrations in direction 3 (thin disc, surface dimensions large relative to thickness; kt < k33)

k₃₁ factor for electric field in direction 3 (parallel to direction in which ceramic element is polarized) and longitudinal vibrations in direction 1 (perpendicular to direction in which ceramic element is polarized) (ceramic rod)

k_p factor for electric field in direction 3 (parallel to direction in which ceramic element is polarized) and radial vibrations in direction 1 and direction 2 (both perpendicular to direction in which ceramic element is polarized) (thin disc)

Piezoelectric Frequency Constant

When an unrestrained piezoelectric ceramic element is exposed to a high frequency alternating electric field, an impedance minimum, the planar or radial resonance frequency, coincides with the series resonance frequency, f_s. The relationship between the radial mode resonance frequency constant, N_P , and the diameter of the ceramic element, DΦ , is expressed by:

N_P = f_s DΦ

At higher resonance, another impedance minimum, the axial resonance frequency, is encountered. The thickness mode frequency constant, N_T , is related to the thickness of the ceramic element, h, by:

N_T = f_s h

A third frequency constant, the longitudinal mode frequency constant, is related to the length of the element:

N_L = f_s l

Symbols

A surface area of ceramic element (m2 )

B bandwidth (frequency)

d piezoelectric charge constant (C / N)

DΦ diameter of ceramic disc or rod (m)

ε₀ permittivity of free space (8.85 x 10-12 farad / m)

ε^T permittivity of ceramic material (farad / m) (at constant stress

F force

f_m minimum impedance frequency (resonance frequency) (Hz)

f_n maximum impedance frequency (anti-resonance frequency) (Hz)

f_p parallel resonance frequency (Hz)

f_s series resonance frequency (Hz)

g piezoelectric voltage constant (Vm / N)

h height (thickness) of ceramic element (m)

k electromechanical coupling factor

k_eff effective coupling factor

K^T relative dielectric constant (at constant stress)

l initial length of ceramic element (m)

N frequency constant (Hz*m)

Par₁ value for parameter Par at t1 (days)

Par₂ value for parameter Par at t2 (days)

Q_m mechanical quality factor

ρ density of ceramic (kg / m3 )

s elastic compliance (m2 / N)

S strain

t₁ time 1 after polarization (days)

t₂ time 2 after polarization (days)

tan δ dielectric dissipation factor

T stress

T^o temperature

T_C Curie point (°C)

ν> velocity of sound in the ceramic material (m / s)

w width of ceramic element (m)

Y Young’s modulus (N / m2 )

Z_m minimum impedance at fm (ohm)