Low Cost PZT/Polymer Composites as a Sensor Material Umut GUNAYDIN, Celaletdin ERGUN Istanbul Technical University, Mechanical Engineering Department, Taksim Istanbul, TURKEY. Abstract This research aims to develop PZT/Polymer composites for low cost sensor applications. The sensors were prepared with mixing the PZT powders and the acrylic polymer in the form of paintable state. After applying these composites on aluminum substrates, they were polarized under 250V, 500V and 750 V (0.8-2.5 kv/mm) DC electric field to give them piezoelectric properties. Then, the composites were characterized for their piezoelectric performances in terms of electrical signal measurements induced by mechanical stimulations. Consequently, the performances of coated composites were evaluated for simple sensor applications. Key words: 0-3 PZT-Polymer composites, PZT paint, piezoelectric paint, Piezo coating Introduction Piezoelectric materials can transform mechanical motions into electrical signals (direct effect) and vice versa; electric signals into mechanical motion (indirect effect). This property of piezoelectric materials was first discovered in 880 by Jacques and Pierre Curie on certain crystals. Since then, natural piezoelectric crystals, such as quartz, Rochelle salt, tourmaline, etc., were the exclusive source of piezoelectricity. Since mid 960s, man-made piezo materials have replaced with natural piezoelectric materials in many applications. The most known manmade piezoelectric materials are; PZT (leadzirconate-titanate), PT (lead titanate), PLZT (lanthanum modified PZT), BT (barium titanate), LM (lead metaniobate), LNN(lead nickel niobate) etc. PZT, is the more common of them, and can be used as sensor, generator, actuator and transducer in many applications. The source of piezoelectricity in PZT ceramics is because of its non-centrosymmetric crystals. This asymmetry of the atomic arrangement in the crystal lattice makes them electrically dipolar. When a force acts on a piezoelectric ceramic, the dipole moment of the crystal changes and produces an electrical signal. Conversely, if a voltage is applied on a piezoelectric ceramic, the ceramic becomes longer or shorter in length. In both cases, the magnitude of the generated electric signal and the strain depends on the direction and the amount of the acting energy. PZT as a sensor material can be used in the form of bulk or thin/thick films. However, due to its ceramic nature, processing, such as shaping into complex geometries, sintering operations or film depositions, is highly dependent on high-tech processing technologies. Therefore, the decrease in processing cost and flexibility in many different sensor applications will be appreciated. Piezoelectric composite coatings, which can also be named as Piezoelectric Paint or PZT Paint [-4], in which PZT powders are dispersed in the polymer matrix, may be used as an alternative and low cost sensor material. The aim of this study is to develop and characterize 0-3 PZT-Polymer composites
that can be used in low cost sensor applications. Experimental Preparation of Samples The PZT used in this study was obtained from commercial source (PZT type 856, APC International Ltd, USA). First of all the PZT powders were milled by using a ball milling machine (Retsch PM00) with 5mm balls for 30 minutes at 00 rpm speed. Then the PZT powders were sieved to 200 mesh (75µ) size. Subsequently, the powders were mixed with liquid acrylic polymer in two different weight ratios: 80 wt.% PZT + 20 wt.% acrylic polymer (80PZT) and 70 wt.% PZT +30 wt.% acrylic polymer (70PZT). These mixtures were then applied on to aluminum substrates with a dimension of 200 mm x 30 mm x mm by using a paint brush. The dimensions of the composite coating layer were 20 mm x 20 mm x 0.3 mm. Masking tapes were used to adjust the dimensions of the coated layers. All samples were cured at room temperature at least for 2 hours. Since the substrate material was aluminum, substrate itself was used as one of the electrodes either for polarization or signal measurements. The upper electrodes of samples were prepared by using conductive silver paint (Fame, Turkey). The dimensions of these electrodes were 6mm x 6 mm. The sizes of electrodes were also adjusted by using masking tapes. The conducting copper cables (Ø 0.mm) were mounted on the silver electrodes with also silver conductive paint. After drying this layer at least for 5 hours, at room temperature, another acrylic layer was applied over all the elements to secure the assemble. Fig. Cross section of composite coating Polarization The composite samples were polarized under three different DC electric fields for 6 different polarization times. DC electric fields were provided by using a high voltage generator (GW Insteck GPT-85). Polarization times were increased with consecutive polarization steps after each measurement. So, for the next polarization time, the sample was depolarized over the previous polarization time with the necessary increment over the previous polarization time. For example, a sample which was polarized for 60 seconds was tested for signal performance, and then polarized 240 seconds more to reach a total of 300 seconds polarization time. The voltage levels and polarization times is shown in Table. 80 PZT 70 PZT Voltage Polarization Time (seconds) 250V 60, 300, 900, 500, 200, 2700 500V 60, 300, 900, 500, 200, 2700 750V 60, 300, 900, 500, 200, 2700 250V 60, 300, 900, 500, 200, 2700 Table Polarization Steps Three different samples were used for each measurement to minimize the error. 2
70PZT samples were only polarized under 0,8 kv/mm electric field because current leakage was occurred at higher electric fields. Signal Performance Tests For signal performance test, a test rig was designed to standardize the measurements. The substrate was fixed from one of its ends to this rig, while the other end was free. Mechanical stimulation was applied to form the free end as a constant displacement by an arm mechanism. The induced strain on the coating was measured with a strain gage. This induced mechanical strain has an average of 950-2000µε. Since the thickness of the coating is relatively very small, this strain can be recognized as plane strain. the first 0. second was evaluated to compare the piezo performance of each case. Results and Discussion The output electric signals of 80 PZTs and 70 PZTs at 250V are shown in Fig. 3. For the polarization time of 60 sec., the difference between the outputs is nearly 20%. But as the polarization time increases, the differences increase to about 40 to 45%. The main reason for this difference should be the decrease in piezoelectric properties associated with the decrease of PZT content. After 900 sec. the output voltages for each composition became nearly constant. Consequently, the substrate was released and left to vibrate until stop. The electric signals generated by the coating as a response to mechanical stimulus resulted to vibration were measured using a DAQ (NI, USB 922 DAQ). Two different software (Measurement & Automation and Signal Express programs (NI)) were used for process the signals. The electric signals recorded from a vibrating substrate via piezo coating are given in Fig.2. Volt (AC),2 0,8 0,6 0,4 0,2 0 250V 250V 60 300 900 500 200 2700 Polarization time (seconds) Fig. 3 Electric signal outputs of 70 PZT and 80 PZT composite coatings polarized at 250V Fig. 2 Recorded electric signals Eight random measurements were taken from each sample for each case. Measurements were recorded as AC voltage output from coatings and represented by +peak and -peak values. The voltage values corresponding to 3 The polarizations of 70 PZTs could not be carried out at the voltage levels 500V and 750V, because current leakages occurred at these electric field levels. Leakages should be occurred due to the decrease in dielectricity of the coating related to the decrease in the amount of PZT and an increase in the amount of polymer. The polarizations of 80 PZTs were done successfully at 500V and 750V. Fig. 4 shows the signal performances of 80 PZTs. The
maximum out put voltage were observed at the polarization voltage of 750V for every polarization time. The output difference between the samples polarized at 500V and 750V was about 7-0%. Volt (AC),4,2 0,8 0,6 0,4 0,2 0 250V 500V 750V 60 300 900 500 200 2700 Polarization Time (seconds) Fig. 4 Electric signal outputs of 80 PZT composite coatings polarized at 250V, 500V and 750V. On the other hand, the difference in output voltages between samples polarized at 250V and 750V was the highest for the polarization time of 60sec with a value of 50%. But, as the polarization time increases, the difference showed a decrease to around 3-23%. All output electric signals were recorded under an average of 950-2000µε mechanical strain, which was measured with strain gage. Since mechanical strain values of 40 to 4000µε should be responded by piezoelectric sensors in terms of sensitivity [2], the sensor developed in this research seems to be capable to service in many structural sensing applications. Conclusions The results of this research can be summarized as follows: 70 wt.% PZT +30 wt.% acrylic polymer composite coatings could not be polarized above 250V due to the dielectric limitations, but polarization voltage of 250V is still enough the coating to have a piezoelectric response. 80 wt.% PZT + 20 wt.% acrylic polymer composite coatings were polarized successfully at 250V, 500V and 750V. The output voltage values were increased with both the polarization time and the polarization voltage. An optimum output performance was observed at 900sec. for both 80 wt.% PZT + 20 wt.% acrylic polymer and 70 wt.% PZT +30 wt.% acrylic polymer composite coatings at every polarization voltage. The best output voltages in 80 wt.% PZT + 20 wt.% acrylic polymer composite coatings were obtained after polarization at 750V for all polarization times. Acknowledgements This research was founded by Istanbul Technical University, BAP Project # 3342, and Project #3904. References. Klein K. A., Safari A., Newnham R.E. & Runt J., 986, Composite Piezoelectric Paints, Proc. IEEE 6 th Int. Symposium on Applied Ferroelectrics (ISAF-86), Pennsylvania, pp 285-287 2. Hale J. M., 2004, Piezoelectric Paint: Thick-Film Sensors For Structural Monitoring of Shock And Vibration, 7th Biennial Conference on Engineering Systems Design and Analysis, July 9-22, Manchester, United Kingdom 3. White J.R., de Poumeyrol, B., Hale J.M. &Stephenson R, 2004, Piezoelectric Paint: Ceramic- 4
Polymer Composites for Vibration Sensors, Journal of Materials Science, 39, pp 305-34 4. Egusa S. & Iwasawa N., 996, Piezoelectric Paints: Application of Piezoelectric Paints to Damage Detection in Structural, Journal of Reinforced Plastics and Composites, 5, pp 806-87 5. Tressler, J. F., Alkoy, S., Newham, R. E., 995, Piezoelectric Sensors and Sensor Materials, Journal of Electro Ceramics 2:4, pp 277-272. 5