Ultrasonic Transducer Technical Overview

An ultrasonic transducer is a device that converts energy from one form to another, specifically between electrical energy and acoustic (sound) energy at ultrasonic frequencies—frequencies above the range of human hearing (typically above 20 kHz). In simpler terms, it’s a device that can generate and receive sound waves at frequencies that humans can’t hear.

1. Classification

Ultrasonic transducers are categorized based on their specific applications, materials, frequency, power, and shape. A broad classification based on application includes:

• Ultrasonic Cleaning Transducers: These are designed for removing contaminants from surfaces using ultrasonic cavitation.
• Ultrasonic Welding Transducers: Used for joining materials by applying high-frequency vibrations, generating heat and causing fusion.
• Ultrasonic Beauty Transducers: Employed in cosmetic procedures, such as skin cleansing and enhancing product penetration.
• Ultrasonic Atomizer Transducers: Create fine mists or fogs for humidification or inhalation devices.
• Ultrasonic Vibration Transducers: Utilize ultrasonic vibrations for various applications, including material processing and sieving.
• Ultrasonic Cell Disruptor Transducers: Used for lysing cells in biological and chemical research, enabling the release of intracellular contents.

Another classification approach is based on the piezoelectric ceramic material used:

• PZT4 Series: Typically characterized by black chips.
• PZT8 Series: Usually identified by yellow chips.

Frequency-based classifications are also common:

• Low-Frequency Series: Ranging from 17 to 23 kHz.
• Medium-Frequency Series: From 25 to 28 kHz.
• High-Frequency Series: From 33 to 60 kHz.
• Ultra-High Frequency Series: Spanning 68 to 200 kHz.

Power-based categories include:

• 50W Series: Which may include 60W transducers.
• 100W Series: Encompassing 80W and other similar power ratings.
• Other Unconventional Power: Transducers with non-standard power outputs.

Finally, transducers can be classified by their physical shape:

• Straight: Transducers with a cylindrical or rectangular form.
• Horn Shape: Transducers with a tapered shape to amplify the ultrasonic vibrations.

2. Naming Methods

Ultrasonic transducers are often designated using a combination of letters and numbers that specify key features. A common naming convention is as follows, using a hypothetical example from Beijing Ultrasonic:

Example: BJ-18 50D-35HN PZT8

Component	Description	Example
BJ	Enterprise Code	BJ
C	Application, “Cleaning”	C
18	Frequency in KHz	18
50	Power in Watts	50
D	Number of Piezoelectric Ceramics	D
35	Diameter of Piezoelectric Ceramic	35
H	Shape of Transducer	H
N	Special Feature, “No Hole”	N
PZT8	Piezoelectric Ceramic Type	PZT8

This detailed code allows for precise identification of transducer characteristics. Note that specific naming conventions can vary from manufacturer to manufacturer.

3. How It Works?

The core function of an ultrasonic transducer is to convert energy between electrical and acoustic forms. This conversion process relies on the piezoelectric effect. Here’s a breakdown:

Piezoelectric Element: At the heart of the transducer is the piezoelectric element, made from materials like PZT (lead zirconate titanate). When an alternating electrical field is applied, the piezoelectric material expands and contracts.

Sound Wave Generation: The expansion and contraction of the piezoelectric element create mechanical pressure waves, which are the sound waves.

Directional Control: To focus the sound waves in a specific direction, transducer design is crucial.

• Backing Layer: This layer, usually made of dense, sound-absorbing materials like tungsten-loaded polymers, prevents sound from propagating in the opposite direction. The backing layer can be carefully engineered with specific shapes to ensure complete absorption of the waves.
• Matching Layer: This layer, often one or more layers with an intermediate acoustic impedance, is positioned between the piezoelectric element and the medium into which the sound will propagate. It helps to maximize the transfer of acoustic energy by reducing reflections, similar to an anti-reflection coating on glass.

By controlling these elements, a transducer efficiently converts electrical energy into focused ultrasonic sound waves and then back into electrical energy.

4. Bonding Process of Piezoelectric Ultrasonic Transducer

The bonding process is critical for the performance and longevity of a piezoelectric ultrasonic transducer. Below are the key steps and considerations:

Step	Description	Importance
Glue Selection	Choose a glue with low curing shrinkage and low thermal expansion to minimize stress on the piezoelectric element. Epoxy adhesives (A+B types) are commonly used.	Reduces stress and enhances transducer reliability.
Surface Preparation	Sandblast the bonding surface to increase adhesion strength. Then, clean the surface with acetone or alcohol.	Ensures a strong bond between the piezoelectric element and the transducer body.
Alignment	Ensure vertical correction when bonding with a nail process. This prevents uneven pressure distribution on the piezoelectric element during the bonding.	Ensures even performance from the piezoelectric element.
Pre-stress Loading	Apply controlled pre-stress during curing with precise consistency. This helps to maintain desired mechanical and electrical characteristics for best acoustic performance.	Improves long-term stability and conversion efficiency.
Impedance Control	Implement impedance control during the bonding process to minimize the transducer’s impedance and enhance electro-acoustic conversion.	Optimizes energy transfer and overall transducer efficiency.
Impedance Testing	After curing, perform impedance testing to verify the consistency and integrity of the bond, ensuring even load energy across all connected parts.	Guarantees consistent performance among transducers and minimizes potential performance flaws.
Wiring and Insulation	Use flexible electrode wires and secure, reliable welding methods. Strengthen insulation to prevent electrical shorts.	Prevents mechanical failures and electrical malfunctions.
Insulation Test	Perform an insulation test after wiring to prevent any risk of electrical shorts or damage.	Ensures safety and reliability of the transducer
Debugging	Debug machine in hot water (40-60°C) and maintain temperature below 80°C during operation.	Minimizes risks related to high operating temperatures.

5. Frequency Selection

The selection of an appropriate operating frequency depends on the specific application and the nature of the materials or objects to be processed.

Frequency Range	Characteristics	Typical Applications
Low (17-23 kHz)	Generates larger cavitation bubbles; more effective on heavy soils and durable objects.	Magnetic sector, vehicle maintenance, textile industries.
Medium (25-28 kHz)	Provides a balance between cleaning power and surface sensitivity; effective on moderate dirt.	Most mechanical processing, electroplating, food industries.
High (33-60 kHz)	Creates smaller cavitation bubbles; ideal for fine cleaning of delicate parts and surfaces.	Precision parts, glass, lenses, electronic components, circuit boards.
Ultra High (68-200 kHz)	Produces the smallest cavitation bubbles with low physical impact and extremely high level of cleaning.	Highly sensitive materials, microfluidics and biomedical applications.

In summary, low frequencies are better for removing large contaminants, while higher frequencies are better for precision cleaning.

Ultrasonic transducers are versatile devices with a wide range of applications due to their ability to convert electrical energy into mechanical vibrations and vice-versa. Understanding the different classifications, naming conventions, working principles, bonding processes, and frequency selection criteria is crucial for selecting and utilizing the correct transducer for a given application. The use of piezoelectric materials, coupled with precise design and bonding techniques, allows for the efficient generation and manipulation of ultrasonic waves, making these transducers invaluable in numerous industries.