Статьи на перевод PVDF_P(VDF-TrFE) / Ferroelectric P(VDF-TrFE) as a Large-Scale Piezoelectric Sensor

Abstract — Piezoelectric poly(vinylidene fluoride – trifluoroethylene) copolymer is embedded as a functional material in a table-tennis racquet. Together with the developed electronics, this piezoelectric sensor is able to monitor the position where the ball contacts the racquet. In order to build up the device, conventional circuit boards with adapted electrode structures are used as substrates for the piezoelectric table-tennis racquet. This leads to challenges because such substrates are usually not flat in the micrometer range as well as the heights of copper electrodes cause irregularities in the final surface topology. Here, the adaptation of the circuit boards, the processing and polarization of the piezoelectric polymer layer, the designed electronics for signal detection and the properties of the table tennis racquet are discussed.

INTRODUCTION

Large-scale and flexible piezoelectric sensors could be used for a broad range of application. Suitable piezoelectric transducer materials are e. g. (i) ferroelectric polymers [1, 2], (ii) ferroelectric ceramic-polymer composites [3-5] as well as (iii) the recently developed piezoelectric polymer foams (so-called ferroelectrets [6- 8]). Polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene (P(VDF-TrFE)) are the most important ferroelectric polymers (i) in regard to basic research and application integration. A broad variety of applications are demonstrated or implemented, among them touch sensors, ultrasonic transducer [9, 10] or sophisticated sensor applications involving the recording of organ-pipe breathing [11]. Both materials can be processed from solution, however, usually a stretching is necessary to transform PVDF into the ferroelectric phase. Finally, they show piezoelectric in-plane (31) and thickness coefficients in the order of about 20 pC/N. Ceramic-polymer composites (ii), e. g. the 0-3 composition [12], can be processed by means of similar techniques as thin layers on substrates or as free-standing films. Usually many attempts are necessary in order to receive optimal ceramic particle dispersion in the polymer matrix. Ferroelectrets (iii) are currently processed as thin free-standing films e. g. by extrusion and stretching. The advantages of ferroelectrets are their very high piezoelectric 33 and low piezoelectric 31 activities which could simplify signal processing in suitable applications.

Typically, a piezoelectric polymer sensor is prepared as free-standing electrically charged polymer film covered with two electrodes which were deposited by a

metal evaporation process. Since the electronics industry is growing, the application of a sensor directly onto different kind of circuit boards is often needed. Thus, for a certain range of sensors, a connection via e. g. gluing is necessary. Such processes can be avoided if the piezoelectric polymer materials are processed by solution techniques. Therefore, especially the P(VDF-TrFE) copolymer is a suitable transducer material because it can be processed as large-scale layer by means of well controllable technologies such as spin coating, solvent casting or by air brushing directly onto electronic circuit boards or other substrates. Challenges arise from the preparation of the polymer layer onto the circuit board due to its topography.

Fig. 1: Schematic sketch of the different layers in a piezoelectric table-tennis racquet.

Here, we demonstrate the preparation of large-scale piezoelectric transducers by using a P(VDF-TrFE) copolymer as a piezoelectric material. In detail, the layer preparation on circuit boards, the electrical connection and polarization as well as the signal processing of P(VDF-TrFE) as piezoelectric sensor in a table-tennis racquet are discussed.

CONCEPT FOR

A PIEZOELECTRIC TABLE-TENNIS RACQUET

The aim of this study was the preparation of a table tennis racquet with a full covering piezoelectric sensor area on one side of the racquet which can measure the contact with a table-tennis ball. All electrical connections should end in a connecting pad in the handle of the tabletennis racquet and transmitted wireless to a computer. The structured electrode is placed on the circuit board

which is processed by etching the not required copper areas. The structured electrode was covered by the piezoelectric polymer layer. The structured electrode with the deposited P(VDF-TrFE) layer is schematically shown in Figure 1 as well as in Figure 2 (upper picture) as a photograph. After the polarization process was applied to the P(VDF-TrFE) layer, a copper electrode covering the whole area is evaporated. This metal layer is not shown in Figure 1 in order to not overload the picture. Then, a standard polymer sheet for table tennis racquets is glued onto the area covered with the piezoelectric layer. Finally, the typical racquet form was cut out.

THE CIRCUIT BOARD

FOR THE TABLE TENNIS RACQUET

The aim of the classical concept, which contains the preparation of a circuit board made of layer-by-layer covering, was to prevent glue layers and glue connections. Therefore, challenges arise when using typical circuit board structuring techniques.

Furthermore, thickness measurements with a thickness gauge were performed over the whole circuit board at the different positions marked in the upper picture of Figure 2 with dots. The results show varying thicknesses between 1,631 and 1,670 mm which demonstrate that the substrate is not homogeneous. These thickness values are taken in order to estimate the polymer thickness after similar measurements were performed on the circuit boards covered with the P(VDFTrFE) layer (c. f. next paragraph). The structured electrode, which is already covered with the P(VDFTrFE) layer, is schematically shown in Figure 2 (upper picture). The lower picture of Figure 2 shows the rear of the circuit board with the connections of the rows and columns. In addition, the upper picture of Figure 2 contains the positions (plotted dots) where thickness measurements were performed and where the corona-tip was positioned during the charging process.

The structuring techniques lead to well defined and mechanically stiff structured copper electrodes. However, in terms of further processing thin polymer sheets, problems arise due to the heights of the copper lines in comparison to the circuit board substrate. As it can be seen in the results of the surface profile measurements (Veeco Dektak 150) presented in Figure 3, the height of the copper lines is about 35 µm.

The height of the copper lines is in the range of usually deposited thicknesses of piezoelectric P(VDFTrFE) layer. Thus, the preparation of polymer layers with thicknesses less than 50 µm failed, because the concentration of P(VDF-TrFE) was not high enough. An example of height variations measured on a thin polymer layer is also shown in Figure 3. The still visible structure of the underlying electrode yields the result that the copper lines are mostly covered by P(VDF-TrFE) at its tops, whereas the vertical separations are not coated which leads to short-circuit contacts.

Fig. 2: Structured electrodes of the table-tennis racquet. Upper picture: electrode structure covered with the P(VDF-TrFE) layer. The dots are symbolizing the

positions of the thickness measurements and of the corona-tip during the various charging processes. Lower picture: Rear of the racquet circuit board with copper lines for the connection of rows and columns.

Two changes are made in order to overcome this problem. First, the “ditches” between the copper lines where closed with a solder mask. Second, a larger amount of P(VDF-TrFE) solution was used in order to prepare the polymer layer (c. f. the next paragraph). Unfortunately, no surface profiles were recorded on the modified circuit boards as well as on the processed thicker polymer layers.

Fig. 3: Surface profiles of the a structured circuit board as well as a structured circuit board with a deposited thin P(VDF-TrFE) layer. The curve of the circuit board with the polymer layer is shifted by adding 35 µm to each measurement point in order to increase readability.

PROCESSING AND FUNCTIONALIZING OF THE

PIEZOELECTRIC POLYMER LAYER

The circuit board containing the copper electrode as

shown in Figure 2 (upper picture) was first cleaned with acetone. Then, a frame was constructed around the circuit board by means of glue tape at the outer 2 mm on three sides as well as covering the long (ground) contact at the bottom of the table tennis racquet. P(VDF-TrFE) (75/25) was dissolved in a 1:1 solution mixture of methyl ethyl ketone (MEK) and dimethyl formamide (DMF). The solution was deposited onto the circuit board by drop casting. After drop casting, the solvents were removed and the layer was dried at room temperature for 30 min. The frame was removed and thermal treatments at 80°C

for one hour and 165°C for 5 min were performed. The polymer layer is slightly visible on the table-tennis racquet shown in Figure 2 (upper picture) especially at the edges of the circuit board because of the frame used during polymer deposition.

The thickness measurements with a thickness gauge as explained above yields an estimation of the polymerlayer thickness. As shown in Figure 4, the polymer thickness lies between 55 and 150 µm showing a tilt which is probably caused by the inhomogeneous thickness of the circuit board. However, at most measurement points the polymer thickness is significantly. Consequently, no short circuits were observed after the poling and metal evaporation process.

Fig. 4: Thickness profile of the P(VDF-TrFE) layer estimated from different thickness measurements on the circuit board of the table-tennis racquet.

In order to polarize the P(VDF-TrFE) layer, several corona poling processes were performed, one at each shown poling / thickness measurement point (c. f. Figure 2 (upper picture)), using a corona tip-voltage of +16 kV, a distance between corona-tip and polymer layer of 4 cm and a poling time of 30 s. As found earlier, these parameters are sufficient in order to polarize even thick P(VDF-TrFE) films [13].

The piezoelectric activities were determined by an experimental technique for dynamical excitation of a polymer-film sample. Here, the experimental setup is used to characterize the completed tennis-racquet as shown in Figure 7. An excitation with a force of 1.3 N is applied by means of a shaker (Brüel&Kjaer, model 4810) controlled via a function generator (Agilent 33210a). In order to arrange close contact of cylinder transporting the mechanical excitation and the table-tennis racquet, a bias force of 1.5 N is given to each point during the measurement. The amplified electrical signals between corresponding row or column and the ground level are measured together with the signals of the applied mechanical excitation via an oscilloscope (Tektronix TDS 320). Both curves are of sinusoidal shape and similar phase which demonstrates the piezoelectric nature of the signal. The calculation of “real” piezoelectric d33 coefficients is difficult because the signal will be strongly influenced by the additional layer, especially the (red) upper rubber layer which is relatively soft and spreads the force in different directions. Furthermore, a strong influence of the in-plane piezoelectric (31) effect is observed due to the vibration of the covering sheet and of the piezoelectric layer during mechanical excitation of a localized position at the table-tennis racquet. However, in order to get a characterization of the piezoelectric layer, the measured and, via a standard calculation for individual not coated samples, calculated piezoelectric coefficients are plotted in Figure 5.

Fig. 5: Measured piezoelectric activity by mechanical excitation at different row / column positions of the table-tennis racquet area.

Thereby, the values are plotted in arbitrary units because no clear separation of the piezoelectric 33 and 31 effects is possible and thus each value presents a superposition of different effects.

ELECTRONIC FOR

SIGNAL READOUT AND PROCESSING

The electronics for signal readout were divided into three parts (Fig. 6): preprocessing, location detector and intensity measurement.

Fig. 6: Schematic of the signal readout.

The preprocessing consists of current-to-voltage converters which were chosen for a minimization of interfering signals and a rectification of the sensor signal. In the location detection the column and row signals are written separately into a latch memory which is then locked for a certain time to exclude bouncing of the signal. During the experiment, the signal response time was favored against the signal amplitude for an indicator of impact location, because of piezoelectric properties described above. For the measurement of the impact intensity it is not sufficient to measure only the charge generated at the detected impact location. Due to the distribution of the impact energy all over the racquet for the intensity measurement it is necessary to collect all charges generated over the sensor area. Therefore column and row signals were integrated for an analogue output signal.

Fig. 7: First prototype of a piezoelectric table-tennis racquet and the electronics with two LED columns for the visualization of the x and y position when the ball contacts the racquet.

CONCLUSION AND OUTLOOK

In conclusion, a table tennis racquet containing a piezoelectric sensor function was constructed. The largescale piezoelectric sensor was processed by drop casting a dissolved P(VDF-TrFE) onto the racquet-like structured circuit board and by heating the complete system. Piezoelectric activities were successfully demonstrated at different x-y positions of the table-tennis racquet. For the detection of the impact location and intensity a specially adapted electronic read out circuit was developed to match the sensor out signals. Based on the developed electronics the sensor signal is measured and analyzed regarding the response time during impact. The detected and processed data allow the determination of the x-y position where the table-tennis ball contacts the racquet.

Beside the successful processing and demonstration of the table tennis racquet with a piezoelectric P(VDFTrFE) it could be concluded that a transducer material which shows piezoelectric in-plane and thickness activity is not optimal for the here needed signal generation because of the signal superposition. Other piezoelectric polymers such as ferroelectrets [6-8] maybe the better transducer-material choice due to the missing piezoelectric 31 and the strong piezoelectric 33 effect. However, for the application of ferroelectrets the circuit boards have to be processed in another way in order to permit the sticking of ferroelectret sheets onto the surface of the circuit board. In summary, P(VDF-TrFE) was a suitable transducer material for the processing on the here available circuit boards.

REFERENCES

[1]H. S. Nalwa (Ed.), Ferroelectric Polymers, New York: Marcel Dekker Inc., 1995.

[2]R. Gerhard-Multhaupt (Ed.), Electrets, 3rd Edition, Volume 2, Morgan Hill: Laplacian Press, 1999.