Electro-acoustic transducers with cellular polymer electrets

Transcription

1 Proeedings of 20 th International Congress on Aoustis, ICA August 2010, Sydney, Australia Eletro-aousti transduers with ellular polymer eletrets Yoshinobu Yasuno, Hidekazu Kodama, Munehiro Date and Eiihi Fukada Kobayasi Institute of Physial Researh, , Higashi-motomahi, Kokubunji, Tokyo , Japan PACS: BS ABSTRACT Eletroaousti transduers utilizing the piezoeletri d 33 oeffiient of ellular polypropylene eletrets generally employ two proesses, orona harging and expansion of the voids to inrease the piezoeletri onstant. However, earlier works noted instability of the transduer performane due to hanges in the material struture beause the seond proess applied to expand the voids and required a staked struture. Additionally, the transduers should be driven by a few hundreds of volts to generate a suffiient sound-pressure level. The effetive frequeny band beame narrower with the inreased number of staked sheets. Therefore, onsumer appliations were limited. This paper desribes ultrasoni transduers of porous polypropylene eletrets that exhibit 250 to 350 pc/n of d 33 without applying seond proess. First, the piezoeletri onstant of the sample was evaluated by the dieletriresonane method. The hange of piezoeletri oeffiient was studied for utilizing stable sample. Next, in order to realize a robust transduer, a low-voltage drive should be possible for transmitters, and flat frequeny harateristis with high sensitivity for reeivers should be realized in a pakage with simple struture. Transmitters and reeivers are designed experimentally. The material that stabilizes the piezoeletri d 33 -oeffiient of 250 to 350 pc/n is estimated to determine the optimal frequeny band and driving method. This paper will also report temperature stability as utilization researh and appliation in an airborne ultrasoni range. INTRODUCTION In reent years, eletrets of ellular strutured polymers have reeived attention as a new piezoeletri material with a high piezoeletri d 33 oeffiient over pc/n. In partiular, researh on porous polypropylene film has been arried out for both improvement of physial harateristis and appliation [1]-[5]. The aousti impedane is low, and some researh has verified the validity in the ultrasoni range in air, but high-voltage operation is required and is not pratial [6]. This study evaluated piezoeletri oeffiients of ellular polypropylene by means of dieletri spetrosopy. Next, a transmitter-and-reeiver devie was developed as a simple experimental struture in the stable domain of a piezoeletri oeffiient to estimate performane and temperature stability in a low-voltage drive. As an appliation, short distane domain measurement by a ombined transmitter and reeiver ombination is reported. SAMPLE A porous PP sheet (provided by YUPO Corporation) was used as a sample. This sample s thikness was 85 μm, and its density was 4 kg/m 3. Aluminium eletrodes were deposited on both surfaes of the sample. A orona disharge was applied to one side of the sample, with -10 k in a grid eletrode and -18 k in a needle eletrode. The other side eletrode was onneted to a ground terminal while harging. Both eletrodes were shorted after the disharge. Dieletri Resonane to Measure Piezoeletri Properties Figure 1 depits the frequeny spetra of the real part ε /ε 0 and imaginary part ε /ε 0 of the relative omplex permittivity measured before harging, just after harging, a day after harging, and 6 days after harging. These frequeny harateristis were aused by the thikness mode of the piezoeletri response [7]. The permittivity in fixed ondition ε S, the elasti onstant 33, the piezoeletri onstants e 33 and d 33 (= e 33 / 33 ), and the eletromehanial oupling fator k t (= (e 33 2 / 33 ε S ) 1/2 ) were obtained by fitting the experiment results with the following equation: ε = ε S / [1 k t 2 {tan(ωh/2v)}/(ωh/2v)], (1) where ε is omplex permittivity, ω is angular frequeny, and h is the thikness of the sample. Here, v is the aousti veloity defined as v 2 = 33 /ρ, where ρ is density. In Fig. 2, ε S /ε 0, 33, e 33, d 33, and k t were plotted against storage time in hours. For the sample just after harging, ε S /ε 0 = 1.45, k t = 0.094, 33 = 0.95 MPa, e 33 = 330 μc/m 2, and d 33 = 350 pc/n. One day after harging, ε S /ε 0 inreased to 1.50, and 33 inreased to 1.0 MPa. However, k t dereased to 0.064, e 33 dereased to 231 μc/m 2, and d 33 dereased to 233 pc/n. Thereafter, they were almost onstant. ICA

2 23-27 August 2010, Sydney, Australia Proeedings of 20th International Congress on Aoustis, ICA 2010 Material speifiation and struture of the Ultrasoni transduer The struture of the transmitter (Tx) depited in Fig. 3, and the main speifiations for the porous polypropylene experiment are presented in Table 1. The drive voltage is diretly impressed to both sides of the eletrode of the Tx; a reeiver (Rx) is operated through an impedane onverter (FET), and metal shielding is applied. Table 1. Speifiations of speimens. Thikness [μm] Basis weight [g/m 2 ] Piezoeletri oeffiient d 33 [pc/n] Eletromehanial Coupling oeffiient k t oid ontent [%] Piezo-eletret film Fig. 1. Frequeny spetra of the real part ε /ε 0 and imaginary part ε /ε 0 of relative permittivity of the samples before harging, just after harging, 1 day after harging, and 6 days after harging 20 Osillator Cellular PP PC-Board Eletri ondution tape Copper plate Rubber sheet <enlarged view > Fig. 3. Struture of the Experimental Transmitter. Performane of the Ultrasoni Transmitter Conventionally, an ultrasoni Tx for the airborne ultrasoni range is a resonated adapting PZT, and the resonane frequeny is 40 khz. With standard performane, the output sound pressure of db/10p-p at a distane of 300 mm is used for the appliation apparatus for whih the distane from the sound soure is assumed to be 0.2 to 5 m.and the following were evaluated: 1) Frequeny harateristis of output sound pressure level (Fig. 4) 2) Input-output response of Tx (Fig. 5) 3) Sound pressure level vs. distane (Fig. 6). SPL[dB] Fig. 2. Plots of permittivity ε S /ε 0, eletromehanial oupling oeffiient k t, elasti onstant 33, piezoeletri onstant e 33, and piezoeletri onstant d 33 against storage time EXPERIMENTAL MODEL OF THE ULTRASONIC TRANSMITTER The ultrasoni transduer was developed as an experiment, using the same material as for the piezoeletri d 33 oeffiient desribed above [8] Frequeny[kHz] Fig. 4. Sound pressure level of the Cellular PP Transmitter. (r=300 mm, in=20 p-p ) 2 ICA 2010

3 23-27 August 2010, Sydney, Australia Proeedings of 20th International Congress on Aoustis, ICA 2010 SPL [db] 1 10 Input voltage [rms] Fig. 5. Sound Pressure level vs. Input voltage. (r=300 mm, f=145 khz) MEASUREMENT Short Distane Domain Measurement An ultrasoni burst signal that input three waves into the TTL iruit was generated from the Tx through Analog SW, and the Rx reeived the signal. The time required to travel from the Tx to the Rx was measured, and the distane was determined from the aousti veloity. Figure 8 depits an example of the result. Here, the propagation time from the Tx to the Rx was 630 μse, and the distane was determined to be m from the sound veloity of m/se at 25. This value was in agreement with the atual measurement of m. Furthermore, the distane between the atual Tx and Rx was hanged, and the distane alulated from the propagation delay time is presented in Fig. 9. The obtained data was in good agreement with the ideal value (red dotted line). Tx signal SPL [db] Distane r [m] Rx signal Fig. 6. Sound pressure level vs. Distane from the Transmitter. (in=20 p-p, f=145 khz) Fig. 8. Example of short range measurement. Tx-Rx Mutual Charateristis Both a Tx and a Rx were used in an experiment with porous polypropylene film. An ultrasoni signal was sent from the Tx, and the Rx reeived it at a distane of 300 mm. The response level of the Rx is the open-iruit output voltage of the soure-grounded iruit in J-FET. The response level is defined as the ratio of Rx output voltage to Tx input voltage (Fig. 7). As for the peak of sensitivity, the response level of - 56 db is determined from a reeiving output of 11 mrms and a transmitting input of 7.1rms at 150 khz. Sine the response levels of a ommerial PZT devie are - to - db at 300 mm and 200 khz, a more sensitive transduer was realized. Response Level [db] Frequeny [khz] Fig. 7. Cellular PP Tx-Rx Mutual Charateristis(r=300 mm) Response level=20*log(rx Output oltage/ Tx Input oltage) Measured distane[m] Atual distane[m] Fig. 9. Atual distane vs. measured distane. Operation of the Tx-Rx Combined Type In the previous setion, Tx and Rx operation was heked using separate devies. The operation iruit of Tx and Rx that shared a devie was then investigated as an experiment. A brass board ( mm x 1 mm) was plaed as a target, and a range-finding experiment was onduted. Figure 10 depits this operation iruit, where R1 is kω, R2 is 39 kω, and R3 is 2.2 kω. Sine the apaity of a transduer is pf, R1 is set so that it may beome a time onstant shorter than the delay time for measuring distane. As R2 determines the high ut off frequeny of the Rx, it is set so that the required frequeny range an be seured. Figure 11 depits an example of the measurement results. The 20 p-p ultrasoni signal of 145 khz is sent to the target. A delay time of 5 μse se was measured at 200 mm in both diretions, and the alulated values from the aousti veloity for that time and survey were in good agreement. ICA

4 23-27 August 2010, Sydney, Australia Proeedings of 20th International Congress on Aoustis, ICA 2010 r Ultrasound transduer DISCUSSION Short Distane Domain Measurement Target Funtion generator R1 R2 R3 Osillosope Non-ontat and short-range detetion has many uses. However, in order to realize stable measurement with a simple struture, a ommon devie must be used. Experimentally, sine the amplitude of the burst signal differed greatly from the amplitude of the refleted signal when a Tx-Rx ombined devie was used, detetion was attempted in a basi iruit ombined with an impedane onverter of the reeiver (Fig. 10). Improvement is required for the optimal iruit of operation to demonstrate a devie s performane. Fig. 10. Tx-Rx ombined operation iruit. Attribution Analysis of Temperature Charateristis Change The ommon struture portion of the Tx and the reeiver is analyzed from the transmitter harateristi hange. The resonane frequeny of the transmitter dereases with inreasing heat. A strutural model of porous polymer is onsidered (Fig. 13). Assuming that hange of resonane frequeny by temperature has resulted from the dimensional hange of the volume of air layer, the volume of ideal gas is proportional to the absolute temperature at the ondition of pressure regularity, aording to Charles's Law. S 1 S 2 solid layer air layer Fig. 11. Distane measurement by Tx-Rx ombination. Temperature Charateristis of the Ultrasoni Transmitter The Tx output sound pressure level with an input voltage of 20 p-p and a frequeny of 40 khz to 200 khz was measured. Distane r from Tx to the mirophone was set to 50mm, measuring 40 to khz with the 1/4in mirophone, and to 200 khz with the 1/8in mirophone (Fig. 12). The sound pressure level went up with a rise in heat, and the resonating point peak hanged from 1 khz to 125 khz. This hange of resonane ould be presumed from the hange of thikness of the sample. The output beomes unstable at high temperatures; thus, observation was not possible at temperatures above C. Sound pressure level [db] Frequeny [khz] Fig. 12. Temperature harateristis of the sound pressure level of Tx. (r=50 mm, in =20 p-p ) Cellular polymer film ross-setion Fig. 13. Eletro-mehanial simplified model. For example, the hange in volume for temperatures of 10 C and 50 C is alulated as follows. T = T (1) Therefore, the volume ratio of the air layer is = (2) However, the resonane frequeny is 1 E f 0 =, (3), 2πS ρ where E is Young's modulus, ρ is density, and S is thikness. Resonane frequeny inreases 0.81 times at 50 C, ompared with 10 C, and the thikness of the air layer hanges by 1.24 times (Eq. (3)) in the domain in whih Young's modulus and others remain onstant. The thikness of the air layer inreases ompared with the volume ratio of Eq. (2), and a horizontal ross setion is assumed to beome 0.91 times smaller. As mentioned above, the hange of air layer is onsidered due to the temperature hange of a piezoeletri transduer. The temperature hange of the piezoeletri transduer is a key fator in ontrolling the spae. However, as overall sensitivity dereases at temperatures below 0 C and the peak of resonane frequeny annot be judged, it is neessary to onsider other fators. 4 ICA 2010

5 23-27 August 2010, Sydney, Australia Proeedings of 20th International Congress on Aoustis, ICA 2010 CONCLUSION 1) The stability of a porous polypropylene film was defined by measuring the time-dependent hange of piezoeletri oeffiients. 2) As an experiment, the ultrasoni transduer was evaluated for frequeny, input and output, and deay-bydistane harateristis. The sound pressure level reahed 96 db/20 p-p at a distane of 300 mm and a frequeny of 150 khz. 3) In Tx and Rx mutual harateristi, performane at 150 khz exeeded the overall harateristis of a 200 khz range PZT devie. 4) The possibility of range-finding from propagation delay time was onfirmed in the short-distane domain. 5) The dimensional hange of the air layer with temperature is a key fator in the struture of the Tx and Rx ommon portion, and ompensation for this hange should be onsidered neessary for a robust transduer. ACKNOWLEDGEMENT The authors would like to aknowledge the kind help of Prof. G. M. Sessler and Dr. M. Paajanen at the initial stage of the study. REFERENCES [1] J. Lekkala and M. Paayanen, EMFi - New Eletret Material for Sensor and Atuators, Pro. 10th International Symp. Eletrets, (1999). [2] G. S. Neugshwandtner, R. Shwodiauer, M. ieytes, S. Bauer-Gogonea, S. Bauer, J. Hillenbrand, R. Kressmann, G. M. Sessler, M. Paajanen, and J. Lekkala, Large and broadband pieezoeletriity in smart polymer-foam spae-harge eletrets, Appl. Phys. Lett., vol. 77, no. 23, pp ,(2000). [3] J. Hillenbrand and G. M. Sessler, Piezoeletriity in Cellular Eletret Films, IEEE Trans. Dieletr. Eletr. Insul., vol. 7, no. 4, pp ,(2000). [4] M. Paajanen, J. Lekkala and H. alimaki, Eletromehanial Modeling and Properties of the Eletret Film EMFI, IEEE Trans. Dieletr. Eletr. Insul., vol. 8, no. 4, pp , (2001). [5] X. Zhang, G. M. Sessler, and J. Hillenbrand, Improvement of Piezoeletri Coeffiient of Cellular Polypropylene Films by Repeated Expansions, J. Eletrostatis, vol. 62, pp. 94-,(2007). [6] L.Reinhard, et al., 19 th ICA, Madrid, (2007). [7] Axel Mellinger, Dieletri Resonane Spetrosopy: a ersatile Tool in the Quest for Better Piezoeletri Polymers, IEEE Trans. Dieletr. Eletr. Insul., vol. 10, no. 5, pp , (2003). [8] Hidekazu Kodama, Yoshinobu Yasuno, Munehiro Date, and Eiihi Fukada, A Study of Time Stability of Piezoeletriity in Porous Polypropylene Eletrets, 2009 IEEE International Ultrasonis Symposium, Roma, Sep. (2009). ICA