acoustic response of piezoelectric membranes j dreu.mme.wsu.edu/2010/files/34.pdfwsu reu 2010...
TRANSCRIPT
Acoustic Response of Piezoelectric
Thanks to Bruce Chang for getting me started on the bulge testingequipment and to everyone in the MEMS lab who were there to bounceideas off of.
This work was supported by the National Science Foundation’s REUprogram under grant number DMR-0755055.
INTRODUCTION
The acoustic response of piezoelectric membranes has beenshown to improve when bulged by a differential pressure.Research to this point has been focused on transducersusing lead zirconate titanate (PZT) as the piezoelectriclayer.
MembranesA.S. Wixom, D.F. Bahr, M.J. Anderson
WSU REU 2010 – Materials Science and Engineering
OBJECTIVES
•Develop transducers using the piezoelectric polymerpolyvinylidene flouride (PVDF)
•Investigate the effect of pre-stress in the membrane onacoustic response
•Compare PZT and PVDF transducers
TRANSDUCER CONSTRUCTION
•Sheets of 28 μm think PVDF film are cut into transducerblanks
•Silver ink painted onto the film is patterned to formelectrodes and leads
•Transducers are stretched to stress the membrane
•A two part puck is used to mount and clamp the active areaof the transducer
•Testing is completed using the bulge testing apparatus topressurize the transducers
PRE-STRESS EFFECT
•Pressure-deflection Curve is regressed to the equation
where
•Results for several PVDF membranes and a PZT membranefor comparison
•Response to a harmonic input was measured at differingpressurization levels
Trans. ID γo (kPa/μm) δ (mPa/μm^3) σ (Mpa) E (Gpa)
2A 0.0062 0.484 3.85 5.29
2B 0.0116 0.473 7.24 5.18
2C 0.0215 0.447 13.4 4.89
PZT 0.2968 64.6 90.0 102
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
-10 -5 0 5 10
Dis
pla
cem
en
t (μ
m/V
)
Pressure (kPa)
Pre-Stress Peak - 500 Hz
2A
2B
2C
• Total Harmonic Distortion (THD)
21a
tdo
3wwP o
42)1( a
tEd
P = Applied pressurew = Center displacement of membrane
γo = Initial edge tensionδ = Sheet stiffness
a = Membrane span (radius)
t = Membrane thicknessd1, d2 = Geometry constants
E = Young’s Modulusν = Poisson’s Ratio
σ = Initial stress in membrane
Reduced pre-stressresults in anincreasing peak inresponse due to asmaller levels ofpressure differential
PVDF vs. PZT
•Frequency Response
PZT
Pressure (kPa)
Dis
pla
cem
ent
(μm
/V)
Pre
ssu
re (
kPa)
Frequency (Hz)
Frequency (Hz)
PVDF – 2C
Pressure (kPa) Frequency (Hz)
Frequency (Hz)
Dis
pla
cem
ent
(μm
/V)
Pre
ssu
re (
kPa)
PVDF has a lower resonant frequency as well as a lower overall response whencompared to PZT. The symmetry of layers in PVDF results in zero bending actuationcausing the dead spot to occur at zero pressure differential where the PZT occurs ata slight negative pressure due to non-symmetrical layers.
Cut and patternedtransducer beingstretched to produceuniform biaxial stressin the active area.
Mounted transducerbeing tested on bulgetesting set-up.
THD has a strong dependence on resonance, so in order to comparedifferent transducers, a normalized frequency was defined.
B
R
f
fff
f_bar = Normalized frequencyf = Frequency of interestfR = Resonant frequencyfB = Lower half-bandwidth of resonance
An example of these values is shown below for both PVDF and PZTtransducers.
fB
fR
PVDF – 2C
Dis
pla
cem
ent
(μm
/V)
Frequency (Hz)
fB
fR
PZT
Dis
pla
cem
ent
(μm
/V)
Frequency (Hz)
Restricting measurements to negative normalized frequencies,those between -1 and 0 are within the resonance band with fullresonance occurring at 0. THD measurements for both PZT andPVDF transducers at different normalized frequencies and pressureswere gathered.
0.1
1
10
100
-2.5 -2 -1.5 -1 -0.5 0
THD
(%
)
Normalized Frequency
THD Comparison - 1 kPa
PVDF-2C
PZT
0.1
1
10
100
-2.5 -2 -1.5 -1 -0.5 0
THD
(%
)
Normalized Frequency
THD Comparison - 3 kPa
PVDF-2C
PZT
1
10
100
1000
-2.5 -2 -1.5 -1 -0.5 0
THD
(%
)Normalized Frequency
THD Comparison - 5 kPa
PVDF-2C
PZT
The PVDF transducer tends tohave slightly lower levels of THD.The peak in the PZT THD is due tothe smaller resonance regionbelow the main resonantfrequency.
CONCLUSION
Reduction of pre-stress in piezoelectric membranesproduces a peak in acoustic response for low levels ofdifferential pressure. When comparing PVDF and PZTtransducers, we see that PVDF has a smaller response but italso has less THD and therefore produces a cleaner signal.
Future work can be done examining the power required tooperate piezoelectric acoustic transducers. Also, thesensitivity of piezoelectric membranes acting as free fieldmicrophones should be investigated.