A question re: piezoelectric materials and resonators

In summary, both types of resonators have a similar resonance frequency, but the TFBAR mode generates a shear acoustic wave instead of a longitudinal acoustic wave.
  • #1
Dishsoap
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TL;DR Summary
Why do bulk acoustic resonators have higher resonant frequencies than PMUTs?
I'm new to the world of acoustics, and I've been reading up on various methods for wireless communication/sensing through ultrasound, especially with piezoelectric materials such as AlN. Fundamentally, I can't seem to find a difference between thin-film bulk acoustic resonators (1 um thick) and PMUTs, which are also about 1 um thick in AlN. They have similar structures (top and bottom electrode, sometimes suspended), and yet the resonance frequency of a PMUT is about 0.5-1 MHz and 10-100 for a TFBAR. I think I am missing something fundamental but I cannot figure out what.
 
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  • #2
PMUT = Piezoelectric micromachined Ultrasound Transducer. 0.5-1 MHz.
TFBAR = thin-film bulk acoustic resonators. 10-100 MHz. Both 1 um thick.

Is the device being specified as a resonator or a transducer?
Maybe the resonant frequency of TFBAR is being compared with the bandwidth of the loaded PMUT.
 
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  • #3
Baluncore said:
Is the device being specified as a resonator or a transducer?

I think that is where I am misunderstanding - I don't see why it matters. The geometry, thickness etc. can be the same, but why would the resonance frequency change depending on whether it is being used as a resonator or a transducer? In both cases, an acoustic wave is being converted into an AC voltage, right?
 
  • #4
Dishsoap said:
In both cases, an acoustic wave is being converted into an AC voltage, right?
Yes, or vice versa.
It is also possible that the mode of operation might be different.
See; http://mems.usc.edu/fbar.htm
"A bulk-micromachined FBAR with Thickness Field Excitation () uses a z-directed electric field to generate z-propagating longitudinal or compressive wave. In an LFE-FBAR, the applied electric field is in y-direction, and the shear acoustic wave (excited by the lateral electric field) propagates in z-direction, as illustrated ()"
 
  • #5
Baluncore said:
Yes, or vice versa.
It is also possible that the mode of operation might be different.
See; http://mems.usc.edu/fbar.htm
"A bulk-micromachined FBAR with Thickness Field Excitation () uses a z-directed electric field to generate z-propagating longitudinal or compressive wave. In an LFE-FBAR, the applied electric field is in y-direction, and the shear acoustic wave (excited by the lateral electric field) propagates in z-direction, as illustrated ()"

Oh, that could be - I'll have to examine these papers further. Thank you!
 
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1. What are piezoelectric materials and how do they work?

Piezoelectric materials are materials that can generate an electric charge in response to mechanical stress, or vice versa. This phenomenon is known as the piezoelectric effect, and it occurs because the material's crystal structure has a polarized arrangement of atoms that can shift when pressure is applied. This shift creates an electric charge, which can be harnessed for various applications.

2. What are some common uses for piezoelectric materials and resonators?

Piezoelectric materials and resonators have a wide range of uses, including in medical devices such as ultrasound machines, in sensors for measuring pressure, force, and acceleration, in actuators for precise movement, and in energy harvesting devices to convert mechanical energy into electrical energy.

3. How are piezoelectric materials and resonators made?

Piezoelectric materials are typically made from crystals such as quartz, tourmaline, or ceramics such as lead zirconate titanate. These materials are shaped into desired forms and then polarized by applying a strong electric field. Resonators, on the other hand, are made by sandwiching a piezoelectric material between two electrodes and shaping it into a specific geometry to achieve the desired frequency and amplitude of vibration.

4. Can piezoelectric materials and resonators be used in harsh environments?

Yes, piezoelectric materials and resonators are known for their durability and ability to withstand harsh environments. They are often used in aerospace and defense applications, where they are exposed to extreme temperatures, pressures, and vibrations.

5. What are the advantages of using piezoelectric materials and resonators compared to other materials?

Piezoelectric materials and resonators have several advantages over other materials, including their high sensitivity, fast response time, and low power consumption. They are also small in size and can be easily integrated into electronic systems, making them ideal for use in miniaturized devices. Additionally, they are highly reliable and have a long lifespan, making them a cost-effective choice for many applications.

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