Very High Temperature Ultrasonic Transducer

Is there a particular reason why 'crystal' type (piezoelectric) tansducers must be used?

From what little searching I've done, magnetostrictive transducers are much more able to cope with high temperatures. I don't know whether they go to such high frequencies, but then would a level probe need to run at over 100kHz?

It is ultrasonic application for generating ultrasound wave and to sense ultrasound waves and it is all to be done at high temperature in very hot fluid.

I repeat, magnetostrictive ultrasound transducers are more able to cope with high temperature operation than 'crystal' or ceramic piezoelectric devices.

Shyam,

I suppose that you are pretty well informed about crystallographic phase change in most (all?) typical piezo-electric materials below 650°C.

The only maybe suitable material I ever made (but not tested for its piezo coeffcients) was aluminum-nitride. As this is an inherently stable material I suppose that it may be suitable for your temperature.

The photo below shows a stack of aluminum-nitride with aluminum layers inbetween.

We made it for testing the potentials for cutting-tool coatings made from AlSiTiN multilayers. This was done by DC sputtering with argon and nitrogen.

Multilayer stack and high temperature metalizing and metallic shielding would be necessary but not impose principal questions.

RHABE

I suspect your temperature is beyond the range of any transducer material (including magnetostrictive). However, you might consider generating the ultrasonic wave outside your high temperature environment and use of a waveguide to conduct it into the hot fluid.

Good idea, good answer

RHABE

Application is for immersible use and depths are high to few meters and external mounted not possible. We do have transducers up to 500C and looking for up to 650C use.

A water column with a constant feed. I know of no makers of this application although i have used a 300 Khz pitch/catch transducer with a standoff by spraying a mist of water in the air column for hot CS slabs. The wave is carried by the mist.

Ron

It is a high pressure on liquid Sodium metal at 500C to 650C range. Hence, transducer must take that temperature for few hours. So far, it was possible up to 400C and for short period up to 500C with quality degradation of the transducer performance and life degradation. Next step is to find something worth for 650C real use.

There is a column of 6 meter to 7 meter liquid Sodium tank in which probe is to be used for sonic waveform generation and reflection measurement.

I greatly appreciate ideas projected by various people here, however I am still looking for something extraordinary and do know what exactly it takes for this type of technology.

I guess you're doing something complex (like maybe trying to judge the purity of the liquid sodium) rather than something simple (like level measurement). It would seem to me that there are alternate means (other than an immersion device) to measure liquid level.

Or, maybe there's a slag layer on top of the liquid sodium and you're trying to measure that (and maybe the height of the sodium). We had similar desires in steelmaking, and I know with our research department (pieces of which might still exist, but our company is gone), they either had or were working on things like that.

If that is the kind of application (that is slag thickness/level measurement), why don't you check around with other metal industries and see what they use. (And, if that's it, I might try and contact some remnants of our research department and see what they had or were working on.)

There are solid, liquid interfaces and then there are voids at different levels and sensor searches these levels by ultrasonic reflections.

Yes, there can be alternative method for each of these. However, ultrasonic technique is very well known and it is very effective. Only problem is the high temperature that damages the transducer.

So why not follow the suggestion from welderman:

there must be a flange to bring in the sensor.

Through this flange any waveguide is much easyer to feedthrough than a sensor that is composed of many different materials.

The waveguide may be a thin tube filled with liquid sodium and surrounded by a sheath of argon.

Any objections?

RHABE

No, I am not discarding any good suggestion. It is a great idea. I have not yet looked at ultrasonic waveguide that can effectively take the signal to several meters. Perhaps if surrounding interface is reflecting type and beam is highly focused then it may be possible.

Access is limited to about 3cm diameter hole through which this sensor enters into the molten Sodium tank. Assuming that we have a protective shield, some space for heat isolation or filled with heat non-conductor and then in the center we have waveguide for ultrasonic wave then it looks it may be feasible.

Ultrasonic waveguides are possible over lengths of km and more, think about earthquakes

-also small ones- that can be detected many km away from the source.

Waveguides are invers to optical waveguides as velocity of sound is high in dense materials and velocity of light is low in dense materials.

As boys we used railroad tracks to hear very distant trains.

RHABE

Those are subsonic vibrations we generally measure for earth quakes. Ultrasonic may be generated locally due to pressure wave of low speed. I do not have much idea about that kind of energy involved as nuclear explosions can hardly travel and go undetected.

I am still considering your point and will work on details.

Subsonic, sonic and ultrasonic are only a matter of wavelength, velocity and damping.

And in elastic rigid media a complicated matter of mode coupling between compression waves into shear waves and Rayleigh waves (at planes with discontinuity of density and or elasticity).

As your sodium will be liquid the internal compression waves will be leaking to the shielding tube and being reflected there show some additional damping.

This additional damping can be made small with high frequency waves that are easily constrained as beams by a limited size of a plane emitter surface.

But this will be limited by the may be too high damping of the liquid if frequency is high enough.

Any turbulence or convection will generate stray "light".

So I thought about a thin walled tube to contain the liquid inner core. The walls will act to produce total internal reflection if more dense/higher in velocity of sound. As in optical prisms where rays from inside can't pass to the outside if the outside has lower index of refraction.

The shielding of the tube by an inert gas seems to be not a necessity.

RHABE

All i can think of , without taking time from my current assingments, is a super cooled transducer. A remote coolant system with a high flow rate and good heat transfer.

I believe this is a better application than looking for new materials.

Ron

That thing works only on hot surfaces

Natural quartz crystal is the highest temperature material in common use. It has limited life over 500 degrees C. You could try to invent an electrostatic transducer for higher temperature operation, but it would be a formidable challenge. I still think an external transducer with a waveguide (possibly ceramic) would be the most direct approach.

I have passion for research as that is my life and it is an endless process.

I will consider all possibilities that may or may not work as final solution is somewhere there in exploring new thoughts. It does not matter how odd these thoughts may be as long they sound logical, it is the way to proceed.

Would it be possible to extend the heat tolerance of your best transducer by using an insulating material to protect it (or part of it)?

I came across several products which jewelers use to protect stones from open flame temperatures while soldering. Don't know how they would behave in liquid sodium, but I think they are not reactive materials.

"Heat Shield" is described as a clay-like material which is also water soluble, protecting gems to 2760 C.

"Extra Hands" is a fibrous putty-like insulator that protects up to 1674 C.

Heat shield works well on sideways to protect the sensor for limited time. Face which transmits and receives signal is often matched for impedance to let the ultrasound get coupled with transducer and media. I am not sure if there is some non-corrosive ceramic that will stop heat and allow transfer of ultrasound waves.

Stop heat: not possible. Best nonporous ceramic is MgO

Heat conductivity along Transducer connection to the outside will transport some heat to the outside.

Heat conductivity across sensor interface will transport some heat to the sensor.

Think about these as thermal resistances that act in analog to voltage dividing here as temperature dividing.

Thermal resistance: degree/Watt

Include distributed thermal resistance along sensor connection to fluid.

RHABE

The hot sodium is contained in a tank of some kind. Consider an external transducer attached to the outside of the tank with an impedance matching thermal isolator between the transducer and the tank wall. The transducer frequency to be chosen by the smallest void you need to detect. Operate the transducer as a sonar, looking for acoustic reflections from voids.