Resonator materials

Ultrasonic resonators require special materials, referred to as "acoustic materials".

Requirements

The following factors may be important in choosing the material, depending on the application.

  1. Fatigue resistance. Most ultrasonic resonators fail by fatigue cracking. Materials with high fatigue strength can be run at high stress levels (higher amplitudes).
  2. Low loss. Materials with low loss can be run at high amplitudes without generating excessive heat. Materials with high loss must be run at lower amplitudes, at lower duty cycles, or with increased cooling; otherwise, the frequency of the stack may drift downward until the power supply can no longer drive the stack. Additionally, materials with high loss consume energy that would otherwise be available for the intended application.
  3. Cavitation resistance. Resonators that are immersed in liquids will experience cavitation erosion if the amplitude is high enough. Harder materials generally have better cavitation resistance. Details ...
  4. Wear resistance. Resonators that rub against other materials may experience wear. Harder materials generally have better wear resistance.
  5. Impact resistance (toughness). Required for inserting applications. This is not the same as hardness since a material may be hard on the surface but soft underneath.
  6. Chemical resistance. For chemical processing, resonators that need to be cleaned with caustics (food processing installations), medical devices (saline irrigation, body fluids; dental tools with irrigation/cooling).
  7. Yield strength. High yield strength is desirable to reduce deformation for impact-type applications (e.g., inserting) and where static stresses may be high (e.g., on threads).
  8. Repeatable modulus of elasticity (E). The modulus of elasticity (which, in part, determines the wave speed) should be repeatable among different lots of material. Otherwise, the tuned dimension of the resonator will not be consistent. This is a particular problem for titanium.
  9. Thermal conductivity. Thermal conductivity should be high where heat transfer must be maximized (e.g., in a front driver that transfers heat from the piezoelectric ceramics and in certain plastic welding applications where heat must be transferred from the part). In some cases thermal conductivity should be low (e.g., to minimize heat transfer from the load to the piezoelectric ceramics).
  10. Biological compatibility. This is a requirement for surgical instruments.
  11. Machinability. This is the ease with which the material can be machined.
  12. Safety. Some materials (such as beryllium) pose health risks if improperly machined.
  13. Availability.
  14. Cost.

Material for horns and boosters

The following table shows some common resonator materials and their characteristics. Click the material name for additional information.

Material Characteristics Typical uses
Titanium
Fatigue resistance ----------------------------------------------------------- High
Loss ----------------------------------------------------------- Moderate
Cavitation resistance ----------------------------------------------------------- Moderate
Wear resistance ----------------------------------------------------------- Moderate
Impact resistance ----------------------------------------------------------- Moderate
Chemical resistance ----------------------------------------------------------- Depends
Yield strength ----------------------------------------------------------- Moderate
Repeatable modulus E ----------------------------------------------------------- Moderate
Thermal conductivity ----------------------------------------------------------- Low
Biological compatibility ----------------------------------------------------------- High
Machinability ----------------------------------------------------------- Moderate
Cost ----------------------------------------------------------- High
Note: Small holes can be difficult to tap because titanium tends to seize the tap.
  • High amplitude horns for plastic and metal welding
  • Medical probes
  • Liquid processing
  • Horns in the food industry
  • Boosters (high gain)
Aluminum
Fatigue resistance ----------------------------------------------------------- Moderate
Loss ----------------------------------------------------------- Low
Cavitation resistance ----------------------------------------------------------- Low
Wear resistance ----------------------------------------------------------- Low
Impact resistance ----------------------------------------------------------- Low
Chemical resistance ----------------------------------------------------------- Depends
Yield strength ----------------------------------------------------------- Moderate
Repeatable modulus E ----------------------------------------------------------- High
Thermal conductivity ----------------------------------------------------------- High
Biological compatibility ----------------------------------------------------------- Low
Machinability ----------------------------------------------------------- High
Cost ----------------------------------------------------------- Low
  • Horns (particularly larger horns)
  • Boosters (medium & low gain)
  • Transducer front drivers
Tool steel
Fatigue resistance ----------------------------------------------------------- High
Loss ----------------------------------------------------------- High at high stress
Cavitation resistance ----------------------------------------------------------- High
Wear resistance ----------------------------------------------------------- High
Impact resistance ----------------------------------------------------------- High
Chemical resistance ----------------------------------------------------------- Depends
Yield strength ----------------------------------------------------------- High
Repeatable modulus E ----------------------------------------------------------- High
Thermal conductivity ----------------------------------------------------------- Moderate
Biological compatibility ----------------------------------------------------------- Low
Machinability ----------------------------------------------------------- Moderate
Cost ----------------------------------------------------------- Moderate
  • Inserting
  • Soldering
  • Metal welding
Stainless steel
Fatigue resistance ----------------------------------------------------------- Moderate
Loss ----------------------------------------------------------- High at high stress
Cavitation resistance ----------------------------------------------------------- High
Wear resistance ----------------------------------------------------------- High
Impact resistance ----------------------------------------------------------- High
Chemical resistance ----------------------------------------------------------- Depends
Yield strength ----------------------------------------------------------- High
Repeatable modulus E ----------------------------------------------------------- Moderate
Thermal conductivity ----------------------------------------------------------- Moderate
Biological compatibility ----------------------------------------------------------- High
Machinability ----------------------------------------------------------- Moderate
Cost ----------------------------------------------------------- Moderate
  • Block horns, especially where tips are to be brazed or soldered to the horn
  • Some medical probes
  • Transducer back drivers
Brass Tunes significantly shorter than most acoustic materials.
  • Horns where tips are to be soldered to the horn
Ceramic Brittle
  • High temperature applications
Ferro-Tic Machinable carbide
  • Horns subjected to high wear
Niobium (Nb) Niobium's Young's modulus is constant in the range of 20-1200 °C. (Eskin[1], p. 299) Thus, unlike other acoustic materials, the resonator's frequency remains relatively constant within this temperature range.
Niobium's wave speed is approximately 3460 m/sec so it tunes about 30% shorter than most acoustic materials (~5000 m/sec).
  • Molten metal processing applications (particularly molten aluminum)
Tungsten Young's modulus and density are approximately 3x steel.
  • Molten metal processing applications
  • Transducer back drivers
AlBeMet®
  • Low Poisson's ratio (0.17) and long thin-wire half-wavelength (240 mm @ 20 kHz) give superior amplitude uniformity.
  • Expensive

The above table assumes that the materials have been heat treated (as appropriate) for the best properties.

Notes —

  1. For titanium and aluminum, the loss increases approximately with the square of the stress (amplitude). For most other materials the loss increases substantially faster. Thus, while these other materials may perform acceptably at lower amplitudes, their loss may be excessive at higher amplitudes.
  2. Most acoustic metals have a wave speed near 5000 m/sec (about 200,000 inches/sec). However,  copper-based materials (e.g., brass) have substantially lower wave speeds which results in shorter tuned lengths.

Material for transducers

In addition to the above materials, the following may be used as component parts for transducers.

  • Piezoelectric ceramics
  • Ferromagnetic materials - nickel, Premendur
  • Electrode materials - nickel, brass, copper, beryllium copper
  • Insulators - Macor machinable ceramic
  • Tungsten

Materials for wear or appearance

Except for D-gun and carbide, the following have thin layers which don't affect tuning.

  • D-Gun
  • Carbide
  • Chrome
  • Nickel
  • Anodize