压电薄膜麦克风-eTouch

High-sensitivity piezoelectric microphones based on stacked cellular polymerfilms(L)

Joachim Hillenbrand a)and Gerhard M.Sessler b)

Department of Telecommunications,Darmstadt University of Technology,Merckstrasse25,

64283Darmstadt,Germany

͑Received30July2004;revised1September2004;accepted7September2004͒

Improvements of the sensitivity of piezoelectric microphones based on charged cellular polymer

films are reported.The improvements are achieved by͑1͒an increase of the piezoelectric

d33-coefficient of the cellular polypropylenefilms by pressure expansion and͑2͒stacking of the

films.Microphones consisting of a singlefilm of such material have sensitivities of about2mV/Pa

at1kHz,independent of size,while for a microphone withfive stackedfilms a sensitivity of10.5

mV/Pa was measured.The equivalent noise level is about37dB͑A͒for the single-film transducer

and26dB͑A͒for the stacked version.Advantages of these new piezoelectric transducers include

their simple design,low cost,and small weight,as well as a large range of shapes and sizes

possible.©2004Acoustical Society of America.͓DOI:10.1121/1.1810272͔

PACS numbers:43.38.Fx,43.38.Ar,43.38.Kb͓AJZ͔Pages:3267–3270

I.INTRODUCTION

Cellular polypropylene͑PP͒,after appropriate electrical

charging,is highly piezoelectric.1–3In particular,the piezo-

electric d33-coefficient of this material reaches values of

about150pC/N in the audio frequency range and is thus

aboutfive times as high as that of polyvinylidenefluoride ͑PVDF͒,the best conventional piezoelectric polymer.It was therefore suggested to use charged cellular PP in electro-

acoustic and electromechanical transducers.In particular,

implementations of loudspeakers,4microphones5,6and

hydrophones6have been described in the literature.Although

of considerably simpler design than conventional transduc-

ers,the previously implemented new devices did not yet

reach the electro-acoustic performance of older capacitive

and piezoelectric systems.

Recently,the piezoelectric activity of cellular PP has

been significantly increased by thickness-expansion of the

polymer.7–11Microphones with such improvedfilms,show-

ing sensitivities of about2.2mV/Pa at1kHz,have already

been described by the present authors.12The use offilm

stacks in such microphones,not yet implemented experimen-

tally,is a method to further enhance the sensitivity of these

transducers.This suggests to build advanced microphones

and to examine their electro-acoustic properties.

In the present letter,the implementation and character-

ization of such microphones are described.In particular,the

cellularfilms are briefly specified in Sec.II,the microphone

design and measuring methods are outlined in Sec.III,

electro-acoustic measurements on the new systems are re-

ported in Sec.IV,and the properties of these microphones

are discussed in Sec.V.

II.CELLULAR PP

The upper part of Fig.1shows a SEM photograph of the

cross sectional area of a cellularfilm͑HS01by VTT,Tam-pere,Finland͒.Thefilm is usually charged on its surface by

a corona discharge.Due to the ensuing electricfield in the

interior of thefilm,discharges occur in the voids and charg-

ing as shown in the lower part of thefigure is achieved.A

charge distribution of this kind in a nonhomogeneous mate-

rial causes the piezoelectric effect.2

All piezoelectricfilms used in the present microphones

were made of a commercial cellular PPfilm͑VHD40by

Treophan,Neunkirchen,Germany͒.Expansion of thesefilms

is achieved by a pressure treatment consisting in the appli-

cation of an increased gas pressure for some time,followed

by pressure reduction to atmosphere.This results in an in-

crease of the thickness of the lenslike voids which originally

extends up to about5␮m and approximately up to about10␮m after expansion.According to the Paschen law,the in-creased thickness after expansion lowers the electricfield

required for breakdown.Thus,for a given voltage,more

voids experience stronger breakdown,causing an increase of

the piezoelectric coefficient.Since metallization after the ex-

pansion causes shrinking of thefilm,a second expansion

after metallization softens the material and thus results in

another increase of d33.10

The frequency response of the d33-coefficient of such

samples was measured,utilizing the inverse piezoelectric ef-

fect,by sinusoidal electrical excitation and interferometric

measurement of the resulting surface deflection.A typical

response of an expanded sample is shown in Fig.2.In the

audio frequency range,d33-coefficients of about420pC/N

were found.These values are larger than those previously

reported for nonexpanded samples by a factor of3.The slow

decrease of d33up to about30kHz is due to an increase of

Young’s modulus,while the resonance at approximately140

kHz is determined by Young’s modulus and the mass of the

film.9

The pressure dependence of d33was determined quasis-

tatically by measuring the generated charge upon pressure

application.9Typical results show that up to pressures of a

few kPa d33increases slightly.This indicates that the stress–

a͒Electronic mail:j.hillenbrand@nt.tu-darmstadt.de b͒Electronic mail:g.sessler@nt.tu-darmstadt.de