Tham khảo tài liệu 'mems mechanical sensors - stephen beeby part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 90 Mechanical Transduction Techniques If a ferroelectric material is exposed to a temperature exceeding the Curie point it will lose its piezoelectric properties. Hence there is a limit beyond which they cannot be used as sensors or actuators . The Curie point of PZT type 5H is around 195 C and its maximum operating temperature is generally lower than this value. In addition to this the piezoelectric coefficients of the material also vary with temperature and this is referred to as the pyroelectric effect. This can be exploited in its own right and pyroelectric sensors based on modified PZT are often used as the basis of infrared sensor arrays. Owing to the anisotropic nature of piezoelectric materials a system of identifying each axis is required in order to specify its parameters. By convention the direction of polarization is taken as the 3-axis with the 1- and 2-axes being perpendicular. For example the material shown in Figure has the electrodes across the thickness of the material and hence this is the 3-axis. An important piezoelectric parameter is the charge coefficient dij C N . This relates the amount of charge generated on the surfaces of the material on the i-axis to the force applied on the j-axis. In the example given the force applied and the charge generated are both across the thickness of the material and hence this charge coefficient is denoted as d33. If a force F3 is applied to the piezoelectric sample then the charge generated is given by Q3 d 33 F3 and so the voltage produced from a rectangular block of area A thickness t and relative permittivity r is V Q3 d 33 F3t 3 C 0 rA where 0 is the permittivity of free space. For a 10 X 10-mm slab of PZT 5H d33 600 pC N er 3 000 of thickness 1 mm an applied force of 100N will produce an open circuit voltage of . Strictly the value of the relative permittivity is also dependent upon the direction in which it is used and the boundary conditions imposed upon the material. The .
