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- since 1996 -
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Technical Discussions
Doug Niessen
Doug Niessen
02:52 Sep-29-1998
PVDF face mtl.

I have a requirement to build a PVDF transducer for pulse echo work at low kilohertz freq.s.
The transducer will be coming in contact with sand and other earth materials. What material is
sugested to be used as a protective face?

Tank you,

Doug


 
 Reply 
 
Yoseph Bar-Cohen
R & D,
Jet Propulsion Lab (JPL), USA, Joined Nov 1998, 26

Yoseph Bar-Cohen

R & D,
Jet Propulsion Lab (JPL),
USA,
Joined Nov 1998
26
03:22 Sep-30-1998
Re: PVDF face mtl.
: I have a requirement to build a PVDF transducer for pulse echo work at low kilohertz freq.s.
: The transducer will be coming in contact with sand and other earth materials. What material is
: sugested to be used as a protective face?

: Thank you,

: Doug

--------------
Doug,

While PVF2 can induce waves at a broad frequency range it is not
an effective transmitter material for the low KHz frequency range. Generally,
the protective material that is located between the transmitter piezoelectric
material and the test medium needs to have the following properties:
- To reduce reflection losses its impedance value should be between the two media
- It should have minimum internal and bonding flaws and have low attenuation
- In case of contact transducer: good ware characteristics, chemical
resistance, formability and easy to process.

Since the frequency is low you can use thin homogeneous material to minimize
the effect of attenuation. There are not going to be much internal reflections to
interfere with your signal analysis.

Yosi




 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1286

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1286
04:44 Sep-30-1998
Re: PVDF face mtl.
: I have a requirement to build a PVDF transducer for pulse echo work at low kilohertz freq.s.
: The transducer will be coming in contact with sand and other earth materials. What material is
: sugested to be used as a protective face?

: Tank you,

: Doug

Doug:
I suspect you will be struggling to find a suitable material for a PVDF facing to use under the conditions you described.
Yosi pointed out, PVDF is a poor transmitter (only about 6.9 efficiency compared to 1 for quartz and 70 for PZT-5A).
But even worse is its receiver characteristics, normalising whith quartz as 1.0 again we find PVDF only 1.35. But
this is good compared to PZT-5A at 0.21. Combining the efficiencies of the transmit-receive PVDF still comes up short
with the Loop gain of PVDF 9.3 and PZT-5A 14.6.

Since the output of the pulse is relativley broadband and you are working in a VERY lossy medium you are going to lose
much of the upper frequency components so the broadband helps you very little.

The other item Yosi mentioned is the impedance matching matching effect. Most manufacturers use hard materials for
face plates (like Boron Silicate). For work on steel using standard PZT elements this is effective as the high acoustic
impedance of the face plate matches the impedance of the steel AND the PZT. In your application you again run into problems with
a low acoustic impedance in PVDF and a relatively high impedance for sand. You would normally want to find something in the middle
range between the sand and the PVDF. If the material you chose is too "hard" your losses due to reflections will be high and a
hard face of too high a density will also load the PVDF causing the bandwidth to broaden and amplitue displacement will drop causing
even further losses.

As a COMPROMISE, if you must use PVDF I would suggest you look for a thin durable plastic or rubber to bond to the front of the PVDF
probe. Your centre frequency is low so if the rubber is thin enough the losses in the material should be minimised and the loading
effect on the PVDF will be reduced. Also, you should be able to find rubbers and plastics will probably be closer matched in
acoustic impedance to the PVDF.

I do not know the application you have in mind but if you need to penetrate soils you might also want to consider microwaves.

Good luck!


 
 Reply 
 

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