You could use a ferroelectric with a phase transition near room temperature. All ferroelectrics are piezoelectric. If you heat them to above the Curie temperature, the piezoelectricity is gone. That would work for example in BaTiO3. Above TC the system is cubic, has inversion symmetry and no piezoelectricity. 

Hi Christian

Thanks for the suggestion. I understand the principle.

Your suggestion would meet with other practical problems, e.g. I use the piezomaterial above room temperature and I would like only to turn off a portion of the sensor, not all of it.

It seems that the Curie temperature for some reason is hard to use on the piezomaterials I use. You can heat them up to near melting but the piezoeffect does not go away, at least not by orders of magnitude.

Sincerely

Claes

What about to try using the ferroelectric effect, using an external electrical field material can be leaded to saturation state, then there are less dipoles available for piezo effect

Hi

Thanks for the suggestion. I am not sure I am able to use it though.

To explain a bit better what I do with the piezoelectric cable - I wind it around pipes to measure the hopp strain (or radial expansion/contraction) as this is proportional to pressure fluctuation inside. I do this in an industry setting.

The piezocable has very good properties for this measurement. My reason for desensitizing portion of the sensor is simply that I am not allowed to cut the cable in certain environments. Therefore, if it somehow is possible to make one section desensitized it would allow me to do somethings I otherwise am not able to do. This is mostly a practical problem as the sensor does the job very well.

Perhaps differentlly put - does anyone know why thermal aging does not seem to work that well on piezofilm/piezocable, i.e. there seems not to be a clear cut Curie temperature for the material?

Sincerely

Claes

Hi

I don't have an answer regarding the Curie temperature of your material. But may I suggest that if you employ RF heating by coating certain parts of the wire with metallic paste and heating them to a high enough temperature ? maybe if you increase the holding time at elevated temperature it will be enough to desensitize the wire. AC frequency could also help with randomizing the domains.

Also, it is better if you can look up the composition of the material to see where exactly the Curie temperature lies and heat it atleast 20K beyond that temperature.

I hope it helps.

Hi Aditya

Thanks for the suggestion. OK, RF heating is a new thought and worth a try.

One practical problem with TC is that at least the piezocble jacket material melts at about 130 C.

I will ask the supplier what is the TC for the material. I would not be surprised if it is higher than 130 C.

One more question - I took a quick look at Ebay for RF heating units. Most of them seem to relate to beaty treatments of various kinds.

http://www.ebay.com/itm/Ddiathermy-Heat-Wavelength-Facial-Lift-Rejuvenation-Radio-Frequency-Skin-Care-/291435359497?hash=item43dae6ad09:g:5Q4AAMXQbflSM9c-

Do you have any suggestions on the type of application/product where I can find RF heaters?

SIncerely

Claes

What I meant with RF heating is the coil type induction furnace. You need not buy one as most are easily available in mechanical workshops. It heats up fairly quick and the whole process should not take more than an hour at best.

However, the wire cladding is an issue because if the wire is made of lead-based material Tc could be well beyond 500K. Please consult your manufacturer if they allow you to remove the cladding for the purpose.

In principle the RF heating mentioned by Aditya works for metals due to Eddy currents, which I think it is not the case, on the other hand if you heat piezos around TC, then  the material goes to an unpoled state, so you have to pole it again

Dear All

The piezocable has a metal core, a thin layer of piezofilm or copolymer, a cable braid and then an outer sheet of cover material.

The cover material melts at about 130 C. Ageing of the piezofilm starts at about 60 C.

The 'problem' with desensitizing a section of the cable (length) is that even extended periods of high temperature, say 120C, does not seem to desensitize the piezofilm much. 

Taking the cable apart implies destroying the cable which demotes the purpose of desensitizing it in the first place.

Adityas suggestion migh be the way as it should be able to, at least, heat the cable braid and, I hope (perhaps in vain?), do a better job at depolarization than temperature alone. 

Then again, this may simply be a problem that cannot be solved, at least not this way.

Comments, thoughts, anyone?

 Sincerely

Claes

I am familiar with the piezoelectric cable you are talking about; in fact, I even know the fellow who invented it (not me). I'm fairly certain that your cable is PVDF homopolymer. PVDF has a Curie temperature of 168C but a melting point of 150C; yes, that's right, the melting point is lower than the Curie temperature, that's not a mistake. Heating will make a mess and probably cause shorts in the cable.

Hi Henry

Thx for taking the time to reply.

Piezocables are nifty devices. I use them to mesure pulsation inside pipes and sometimes also to measure flow velocity, though they can be used for very much more than these applications.

I am mostly using the PVDF spiral wrap piezocable made by MSI. There exists also a Korean PZT based cable that can take 150 C that I use for higher temperatures. I have used coplymer cables from Ormal in the past.

I have been in contact with MSI and they pretty much told me what you and other are telling me.

The Curie info was news to me. As far as I understand, ageing of the PVDF starts at about 60 C. The cable’s outer casing material melts at 130 C. When used above 60 C I have therefore boiled the cable a while (a bit like like Uncle Benz rice).

The wrap is very thin and it does not take much to make a mess also by means other than heating.

I simply asked the question to eliminate the possibility of there being some other, unconventional, way of doing a local desensitizing.

Sincerely

Claes