Reports: ND1052695-ND10: Giant Enhancement in Effective Piezoelectric Sensitivity of Piezoelectric Polymers
printer friendlyTemperature and Substrate Dependence of Piezoelectric Sensitivity for PVDF Films
The piezoelectric sensitivity, via both the direct and converse effects, for commercial polyvinylidene fluoride (PVDF) films is measured. The PVDF film was mounted on two different substrates, nylon and aluminum. The average effective sensitivity for the PVDF mounted on nylon was 29 pm/V for both the direct and the converse effect. The effective sensitivity of the piezoelectric increased by a factor of five when mounted on aluminum and measured by the direct effect. This change in effective sensitivity is analyzed with respect to the change in the substrate’s thermal properties and mechanical compliance. The piezoelectric sensitivity for the PVDF mounted on Al was further tested for temperature dependence. The sensitivity increased at approximately 2% per degree Celsius over the frequency range 0.5 to 200 Hz for both the direct and converse effects.
Piezoelectric materials produce an electric field proportional to an applied mechanical stress. Conversely, these materials deform mechanically at the same rate when an electric field is applied. PVDF is a piezoelectric polymer that is used for sonic and ultrasonic transducers, sensors and actuators. The piezoelectric sensitivity of PVDF is dependent on temperature. Between the glass transition temperature (-40 deg C) and the breakdown of the piezoelectric film (80 deg C), the magnitude of the sensitivity increases with temperature.
The PVDF films, purchased from Goodfellow, were 28 microns thick and 12.5 mm wide. Charges for the direct effect were measured using a Kistler 5010 Charge Amplifier and a TDS 3014B digital oscilloscope. An SRS SR560 high-low pass filter was used to minimize noise in the experiments. For the converse effect, an SRS DS345 function generator, a SR850 DSP lock-in amplifier and a fiber optic displacement sensor (MTI 2000 Fotonic Sensor) were used. The temperature of the PVDF film was controlled using a Cambion (Cambridge, MA) thermoelectric module and measured using an Omega 871A Digital Thermometer with Type K thermocouples. The PVDF films were mounted on several substrates for the purpose of evaluating possible enhancement of effective properties.
Analysis of boundary conditions was conducted to evaluate possible reasons for the enhanced sensitivity of PVDF on aluminum substrate. The difference between adiabatic and isothermal conditions was studied as was the role of pyroelectric effect; also the effect of mechanical constraint by the substrate. Specifically an important consideration in the measurement of the piezoelectric sensitivity is that the film is fixed to a substrate of different stiffness. The PVDF film can only expand and contract perpendicular to the surface if the substrate is stiff. Therefore, the measured piezoelectric sensitivity is only an effective coefficient. None of these effects can account for the observed enhancement of sensitivity.
The PVDF sensitivity in response to the direct effect increased 500% when mounted on an aluminum substrate. Analysis of the change in mechanical and thermal properties of the substrate cannot account for all of this increase. Nevertheless the observed enhancement of properties may be useful in applications.
