heat generation using Piezo elements - Page 1
Forum section: electronics / projects

author: Buk
author_rank: Contributor
timestamp: September 25, 2021, 09:53:39 pm
content: Warning: I barely know what I am asking; and nothing about what I am trying to do!I want to generate heat -- 200°C -- into a 1mmOD x 0.5mmID brass tube in <1 second.My starting point is this paperhttp://gianchandani.engin.umich.edu/wp-content/uploads/sites/360/2018/01/BiomedMicroDev_2011_Cauterization.pdfthat uses4 off 300Ø x 135 µm PZT-5 elements to cauterize injection tracts from a 20 gauge hyperdermic needle.My application is non-medical.I *think* that a stack of thin piezoceramic rings surrounding the microbore brass tube, excited using a (guessimate) 100V p-p sinewave whilst being constrained axially, will generate heat internally, and by conduction, into the brass tube they surround.Does anyone here have experience of using piezo ceramics in this way?The 'electronics' part of this is that I wish to drive this heating using a single (or serial dual) Li-ion battery and a boost converter.But first, I need to understand the piezo heat generation part of the equation.Any suggestions, links, "you're talking outta you're ass", seriously considered.Thanks, Buk.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 25, 2021, 10:13:38 pm
content: To what end? Does it have to be a tube -- what goes inside it? Can it be filled with things, or does it have to be open? Evidently at least some PZT micro-pucks would be permissible, but what about say a wire heating coil? And what's being heated, will volatiles boil off? Phase changes require far more energy than heat capacity alone; this is critical to the power requirement.It looks to me like they're depending on the mechanical lossiness of tissue to do the job -- it's not just squishy but smooshy as well, and therefore absorbs mechanical energy. Evidently, as it "cooks", the losses go up, evidenced by the lower impedance. If you're just heating metal, you may find the losses aren't there, and it will take much more vibration than expected to deliver the heat.Cool find by the way, that paper!Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 25, 2021, 10:51:04 pm
content: Welding plastics with ultrasonic vibrations is a common process and these also work with Piezo elements.I do not know how much overlap there is with your application, but searching in that direction may lead to useful results.These generate heat by introducing physical motion and friction in the plastic. Heat is also just motion of atoms.The design of these things is not trivial though. There is a simple looking metal part to convey the movement of the piezo element to the plastic but this is a sort of mechanical lens that focuses and thus amplifies the vibrations. Itss also a tuned mass system. At such high frequencies and small movements the metal is not a solid material, but it's more like a river though which the ultrasonic vibrations flow.

author: Buk
author_rank: Contributor
timestamp: September 26, 2021, 04:03:01 am
content: Quote from: T3sl4co1l on September 25, 2021, 10:13:38 pmTo see if it can be done and explore the pros and cons of doing it.

author: Buk
author_rank: Contributor
timestamp: September 26, 2021, 04:54:08 am
content: Quote from: Doctorandus_P on September 25, 2021, 10:51:04 pmIndeed. However, thetranducers for ultrasonic weldingare invariably very large.The concept is to generate heat very locally directly into/thru the walls of the microbore tube -- approx. same diameter & wall thickness as the 20 gauge needle in the paper -- to heat a liquid within it.The cited paper states: "The temperature of PZT tends to increase while converting the electrical energy to mechanical energy due to dielectric losses in PZT, and damping in both PZT and surrounding material. Off-resonance, the dielectric loss tends to be the major source of heat generation, while at resonance the damping tends to be the major contributor . The high electrical impedance and low thermal conductivity of PZT (Table 1) reduce the losses due to parasitic resistance and conduction through connecting wires, thereby making this method highly power efficient. The product of density and specific heat capacity of PZT, which determines the thermal time constant, is almost same as other micro-heater materials".Part of the concept -- using ring(s) around the tube -- is that the ultrasonic vibrations will be focused at a point in the middle of the tube, and heat the liquid there through direct excitation rather than via conduction through the walls of the tube.The part of the problem that is giving most trouble is understanding which polarisation orientation is most likely to create the focused energy required.

author: Weston
author_rank: Regular Contributor
timestamp: September 26, 2021, 05:47:45 am
content: What are you making? Sounds like a fancy vape or something.I believe the primary (or at least the intended) heating mechanism in the paper you linked is due to the acoustic dissipation in the tissue, not due to resistive losses in the piezo element. Otherwise its a whole lot of work for nothing better than a resistive heater.Heating due to acoustic loss requires an acoustically lossy material. Most liquids (lets say ethylene glycol) are going to be a lot less lossy than tissue, so its not going to heat very well.Another issue of concern is that the curie temperature of most PZT materials, where it stops being piezoelectric, is in the range of 200-300C.Consider looking at something like induction heating. You can tightly control the heat zone with the coil design. And it would heat the tube itself, so insulation between the coil and the tube would not increase the response time. If you made the tube of the right ferrous material with a well controlled curie temperature you could even have the temperature self regulate. Its how the Metcal soldering irons work.

author: Buk
author_rank: Contributor
timestamp: September 26, 2021, 06:05:59 am
content: Quote from: Weston on September 26, 2021, 05:47:45 amExactly. Its the possibility of transmitting energy through the walls of the tube and focusing it within the liquid in the microbore tube that is driving force of my investigation.There are a variety of possible applications.I found a little further information here:https://www.hindawi.com/journals/ijce/2011/670108/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2021, 10:05:07 am
content: Ah, liquid, alright. And the hope is that this is more effective than flowing it through a heated tube? Presumably which would have to be longer to transfer the heat, and be limited by Leidenfrost effect (critical heat flux).I wonder if liquids heat directly due to cavitation. Might work above a threshold level or something?Tim

author: Buk
author_rank: Contributor
timestamp: September 26, 2021, 10:39:15 am
content: The paper I found and linked in the post above your descibes 4 effects of ultrsound on liquids:Depending on the specific application, any or all of these might be useful.My goal for now is to work out what size (thickness) and polorization of PZT ring(s) (plus suitable circuit to drive the thing(s)) to buy in order to get a setup for exploration.Don't take this image too seriously, its pure guesswork, but it serves for description:Assume the 4 yellow disks are piezo elements, physically constrained -- sandwiched between the nut and the base -- and a current flows through top to bottom.Depending on the polarisation they would either- (mostly) oscilate vertically thus stretching the nozzle within its elastic limit.- (mostly) oscilate radially and transmit energy into/through the walls of the tube. (I think)But other arrangements are also possible. Alternate layers in shear mode.I am really guessing whilst reading everthing I can find at this point.I posted here because I saw a few threads here dealing with piezo elements and circuits (mostly for actuation/movement) and at least one member (Conrad Hoffman) with real world experience of using piezo elements.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 26, 2021, 12:46:14 pm
content: Might it make more sense to make the tube out of heat resistant glass or quartz and then use a laser to heat the liquid?

author: mikerj
author_rank: Super Contributor
timestamp: September 26, 2021, 12:57:34 pm
content: Quote from: T3sl4co1l on September 25, 2021, 10:13:38 pmWhoa, could you turn down the technical terms a bit? We aren't all experts you know.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 26, 2021, 02:53:33 pm
content: Heve you ver tried blocking a piezo buzzer's hole with your finger tip. Ouch that burns!I learned this the hard way. That was at 2.7KHz. At US frequencies you need to adjust your resonant cavity. The buzzers use a Helmholtz Resonatorhttps://www.puiaudio.com/pages/design-of-helmholtz-chamber.You will aslo need to acoustically couple your transducer to the tube. They trandcucer has a much lower accoustic output impedance than the tube, so a matching "transformer"is needed or power will be reflected back. Its exactly the same process as electrical transmisson line matching. See this article:https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiv9fam75zzAhXOQkEAHcn4DBkQFnoECAMQAQ&url=https%3A%2F%2Fassets.press.princeton.edu%2Fchapters%2Fs9912.pdf&usg=AOvVaw3SZtH1KcS7NMdVYo2R-Z2oIn the end the tube's impedance may be just too high to get any useful power at the tip.Googolise "Sonicator Microprobe", a tiny blender, and "In Vivo Ultrasonic Microprobe"

author: Buk
author_rank: Contributor
timestamp: September 26, 2021, 03:26:01 pm
content: Quote from: Terry Bites on September 26, 2021, 02:53:33 pmMy only hands on experience with ultrasound is using for flaw detection in steel. Here the piezo head is basically just pressed onto the surface.Here's a breif description of the process and transducer design.Quote from: Terry Bites on September 26, 2021, 02:53:33 pmThanks for the search terms.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 26, 2021, 04:41:49 pm
content: Wot, no cold jelly!

author: Buk
author_rank: Contributor
timestamp: September 26, 2021, 06:33:18 pm
content: Nah! And the pictures were boring too

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2021, 07:31:33 pm
content: Quote from: mikerj on September 26, 2021, 12:57:34 pmOkay, "squishy" is probably too generic, but comparing a fairly low-loss ~ideal liquid* to something like a viscoelastic rubber, a solid but it flows under pressure, but reluctantly (lossy) (which I at least would call "smooshy"), is precisely what I was going for.*Granted, I'm not sure what the losses are like at these frequencies. The wavelength is very small and time scale very short. Probably for something like water, that makes it look more like a stiff body than a flowing fluid? Which would be a diffusion mode too, right? If it were stiffness proportional to frequency it should have a simple pole (break / cutoff frequency), which seems odd for a bulk material.Tim

author: Buk
author_rank: Contributor
timestamp: September 27, 2021, 11:56:24 am
content: Quote from: T3sl4co1l on September 26, 2021, 07:31:33 pmIf you have any experience of Ultrasonic cleaners, in the base of the tank you have something like this:The transducer is bonded to the base of the (usually stainless steel) tank; as the piezo elements oscillate they cause the tuned masses they are sandwiched between to vibrate in unison and that energy is tranfered through the wall of the tank into the liquid.My goal is to transfer energy through the thin walls of a microbore tube containing liquid, which will focus the energy -- think ripples on a pond in reverse -- at a point in the middle of the tube to cause cavition -- vapour bubbles -- at that point. As the bubbles burst, they transfer their energy to the (tiny amount of) surrounding liquid and thus quickly raise its temperature locally.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2021, 09:43:16 pm
content: Yes -- difference being, at the ~30kHz a typical ultrasonicator operates at, the waves seem to propagate quite nicely. I suppose to be fair, even at some MHz they do, else nebulizers would have a much harder time working. (Hmm, want to say their beams dissipate quite quickly, say in the span of a few mm; though whether that's due to simple spreading and reflection, I don't know.) But yeah, cavitation should do the trick.Hm, I wonder if it would be worth adding lowpass filter sections, ahead/behind the active section. A ~1mm tube of fluid will act as a waveguide, carrying energy away from the source and thus reducing the amount of cavitation produced. And not necessarily dissipating that energy as heating in the fluid, it might go mostly into the sidewalls or something instead (which will still conduct back into the fluid so that's okay, as long as it's low enough power density that it behaves itself). Maybe it's not enough to matter? I wonder what such a section would look like, anyway; you don't have much opportunity to change the I.D. of the tubing, assuming some requirements for flow; maybe it's enough to use spirals of soft (silicone?) tubing, and just let the waves disperse. Oh, also a lot of energy can be carried on hard metal parts; soft tubing connections should serve as shock absorbers, again tending to keep the energy in the active region. As opposed to, say it's a benchtop unit with two hose barbs on the front, you might not want metal tubes going back from them to the heating cell.Heh, one interesting side effect of this, I would think: as the fluid is heated, some cavities will not collapse, but stay inflated by some amount of dissolved gas that's come out. Like when you raise tap water to near boiling, in a clean vessel, at first it's bubbling slowly, but add a nucleation source and a fog of fine bubbles are released. In this case, that nucleation has already happened, so it should be about as degassed as it's going to get, at whatever temperature it's been raised to. Put more succinctly: the state of dissolved gas should be closer to equilibrium.Which also means, with a ready source of nucleation, the threshold of critical heat flux should be lower, for example.And I wonder what effect a slightly frothy load will have. The density will be lower, so a higher mechanical impedance. Cavitation won't be entirely by spontaneous nucleation and collapse, but by (mostly-)adiabatic cycling of the bubbles, but that should still transfer plenty of heat.Y'know, probably it's a lot like induction heating, where the Q factor might not be all that low, it's not like wiring up a resistor, you pretty much always have to deal with the reactance -- but it's also not crazy, that reactance can be tuned out easily enough, and there you go. At least up to a modest variation in load characteristics, which may require adaptive tuning or frequency control to account for, depends. Biggest difference is you're doing it in space, all the propagating modes matter. It's ultimately a one-port to the circuit, probably the impedance doesn't change much, easy enough to drive; what would change more is the spacial distribution, or coupling efficiency to how much cavitation, that sort of thing I would think.Tim

author: Buk
author_rank: Contributor
timestamp: September 27, 2021, 10:20:34 pm
content: Quote from: T3sl4co1l on September 27, 2021, 09:43:16 pmThat's an interesting thought. I've already realised that as piezo chips shrink in one dimension as they expand in the other, and vice versa, I need to accomodate that through the use of something like a wave washer. The movement isn't a lot, but the forces are suprisingly high, and ceramic can be pretty brittle.It would certainly be possible to isolate the section of tube where the heat is being generated using a couple of short sections of silicon rubber tubing or similar. Something for me to consider if the pipe itself shows sign of heating too much.Quote from: T3sl4co1l on September 27, 2021, 09:43:16 pmAlso interesting. I just reada (very short) paperabout the spray from a "ultrasonic fountain" which I take to mean nebuliser, actually melting a (thin) piece of plastic mounted well above the surface of the liquid.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2021, 04:22:15 am
content: Neat. From a very small amount of experimentation I've seen that ultrasonic fields push on the surface, presumably a momentum thing if it's not other effects, and at higher field strength the surface itself breaks up (nebulization). (Which I suppose might be a surface cavitation thing, or a narrow-neck instability sort of thing. Hm, I never looked it up actually!) Sounds like they're doing something intermediate (perhaps including how the generator is focused?), forming a stable jet. Which is probably self-focusing from the cone shape (internal reflection). So that sounds reasonable, that there would be a lot of ultrasonic heat available, and plastic is well known to be ultrasonically weldable, yup.I don't think clamping the assembly should be that big of a deal. It's brittle, and it's not the strongest ceramic as things go, but it's solid material, it'll do. Needs at least enough clamping force, or gluing force if permanently assembled, to handle peak stress/strain, and preferably the expansion rates should be similar. Which at a glance, looks like it shouldn't need anything strange, like, steel or aluminum parts should do.Tim

author: Buk
author_rank: Contributor
timestamp: September 28, 2021, 01:37:13 pm
content: Quote from: T3sl4co1l on September 28, 2021, 04:22:15 amI don't antisipate any great problems with the mechanics of the thing. I'm trying to avoid the use of bonding agents as I'd like to be able to try different size (thickness) PZT elements. The hardest problem is trying to make a good physical connection between the PZT and the walls of the tube. Given that a radially poloraised ring will expand away from the centre when energised, and there will be tolorences involved, I'll need some kind of moveable joint between the between the ID of the ring and the OD of the tube. I've tentatively come up with this arrangement:The two silver components in the middle with the angled interface are split ring collets. The washer at the bottom is a cone (Belleville) spring washer that should allow the two collets to slide axially as they expand and contract.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2021, 07:28:44 pm
content: Seems reasonable. Hm, that'll put a lot of tension into the ring, which it's probably weaker in; a compression ring around the outside (shrink fitted, say) might be wise. Which, if it's poled that way, would that also serve as the other terminal?Tim

author: Buk
author_rank: Contributor
timestamp: September 29, 2021, 12:24:31 am
content: Good thought. It would have to have some give, as the OD also expands when energised.The washer I was looking at only produces 20N for a 0.04mm deflection and the piezo only produces a few microns of movement.