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.

 

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

    
     Piezoelectric Charge Constant
 

    The piezoelectric charge constant, d, is the polarization generated per unit of mechanical stress (T) applied to a piezoelectric material or, alternatively, is the mechanical strain (S) experienced by a piezoelectric material per unit of electric field applied. The first subscript to d indicates the direction of polarization generated in the material when the electric field, E, is zero or, alternatively, is the direction of the applied field strength. The second subscript is the direction of the applied stress or the induced strain, respectively. Because the strain induced in a piezoelectric material by an applied electric field is the product of the value for the electric field and the value for d, d is an important indicator of a material's suitability for strain-dependent (actuator) applications.

 
  d33 induced polarization 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 field applied in direction 3		 
  d31 induced polarization 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 field applied in direction 3		 
  d15  induced polarization 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 field applied in direction 1		 
  
    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.

 
  g33 induced 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		 
  g15 induced 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		 
 
     Permittivity
     The permittivity, or dielectric constant,                                                                                , for a piezoelectric ceramic material is the dielectric displacement per unit electric field.                              T is the permittivity at constant stress,                              S is the permittivity at constant strain. The first subscript to                              indicates the direction of the dielectric displacement; the second is the direction of the electric field.
    The relative dielectric constant, K, is the ratio of                              , the amount of charge that an element constructed from the ceramic material can store, relative to the absolute dielectric constant,                              0 , the charge that can be stored by the same electrodes when separated by a vacuum, at equal voltage (                              0 = 8.85 x 10-12 farad / meter).
 
                                                                                                               T11

permittivity for dielectric displacement and electric field in direction 1 (perpendicular to direction in which ceramic element is polarized), under constant stress

 
                                                                                                               S33

permittivity for dielectric displacement and electric field in direction 3 (parallel to direction in which ceramic element is polarized), under constant strain

 
     
    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 ceramic elements 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, kp , 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, kt , 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 kt 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)		 
  kt 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)		 
  kp 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)		 
 
     Dielectric Dissipation Factor
 

    The dielectric dissipation factor (dielectric loss factor), tan                                                                                                                                                                , for a ceramic material is the tangent of the dielectric loss angle. tan                                                                                                                                                                is determined by the ratio of effective conductance to effective susceptance in a parallel circuit, measured by using an impedance bridge. Values for tan                                                                                                                                                                typically are determined at 1 kHz.

 
     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, fs. The relationship between the radial mode resonance frequency constant, NP , and the diameter of the ceramic element, D                                        , is expressed by:
     NP = fs D                                  
       At higher resonance, another impedance minimum, the axial resonance frequency, is encountered. The thickness mode frequency constant, NT , is related to the thickness of the ceramic element, h, by:
     NT = fs h
       A third frequency constant, the longitudinal mode frequency constant, is related to the length of the element:
     NL = fs l

 
    Curie temperature
 

   Temperature at which the permittivity of ferroelectric ceramics reaches its peak. Above this temperature the ceramic material will not exhibit piezoelectric properties.

 
     Mechanical quality factor
 

    Amplitude magnification of oscillating piezoelectric parts in a resonant state. This is a non-dimensional factor indicating the mechanical loss of the component under dynamic operating conditions.

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