Piezo Tube

Everything you need to know about Piezo Tubes 

This article looks into piezo tubes, one of the most used piezoelectric components in the industry today. Piezo tubes come in many different types and size and have multiple uses. We at PD (PiezoData Inc.) believes that our customers should know more about piezo tubes that they buy from us hence, here is everything that you need to know about piezo tubes.

1.0 What is a piezo tube

Piezoelectric tubes also commonly referred to as piezo tubes are cylinders of radially poled piezoelectric materials. The piezo tubes usually have four external electrodes, along with a continuous internal electrode. Due to the piezoelectric nature of the materials used, when a voltage is applied to one end of the external electrodes of the piezo tubes, the wall of the actuator can expand resulting in a vertical contraction. This results in large lateral deflection of the lip of the tube. The contraction and deflection allow engineers to deploy piezo tubes in places where very minute movement is a requirement or the precision required is really high.  

A standard piezo tube

Where; OD – Outer Diameter ID – Inner Diameter P – Polarization Direction L – Length

2.0 What are the common type of piezo tubes?

Though piezo tubes come in many different shapes, they exist in three different types of configuration. The different configuration of piezo tubes helps them to be used for different purposes. The typical piezo tube configurations are; Piezo tube with a plain electrode – Under this configuration, the electrode is on the outer diameter of the tube covering its inner material completely. The standard piezo tubes have three main resonance modes. These include wall, length, and circumferential modes. Piezo tube with split electrodes on the outer diameter – This configuration is like the configuration of the plain electrodes. However, under this configuration, there is a single split between the electrode on the outer diameter. The split electrode piezo tube has one main resonance, which is the circumferential mode. Piezo Tube with striped electrodes – This piezo tube configuration is done when the very minimal piezoelectric effect is required on the tube or in the application used. There are multiple strips on the outer diameter of the material which gives these tubes the slight piezoelectric effect that they need. The striped electrode piezo tubes can easily achieve frequencies of 15kHz with just only a two-inch tube.

3.0 What is the piezoelectric response of piezo tubes under an electric field?

Typically, all types of piezo tubes have a similar response under an electric field. When a piezoelectric tube is being exposed to an electric field along with radial directions, the piezo tube tends to experience some deflection along with a change in its dimension. It is important for both the users and manufacturers of the piezo tubes to know the strain of their piezo tubes before manufacturing or using their products. The strain component for any given piezo tube under an electric field can be determined using the following formula;   Under the above formula, a, b and c are the integration constants. The constants can be determined via the boundary conditions. Under the electric field, the piezo tubes change their size and dimension, and studies have shown that the thin-walled piezo tubes expand more than the piezo tube of normal thickness.

4.0 What are the industrial usages of piezo tubes?

Piezo tubes can address different industrial needs such as the high power and sensitivity, good stability and sensitivity under cryogenic temperatures as well as resistance to high temperature and extreme pressures. Owing to their properties, they are suited for the following applications;

• Accelerometers
• Torpedo Decoys
• Hydrophones
• Pressure sensors
• Oil and gas exploration
• Scientific equipment
• Augmented reality
• Print head transducers
• Scientific equipment
• Fiber optic stretcher

5.0 What are the construction and fabrication of piezo tubes?

5.1        Mounting

The most common mounting option for the piezo tubes is in the cantilever configuration. Under this mounting position, one end of the piezo tube is fixed whereas; the other end is free to move. The base of the piezo tube can be bonded via insulating surfaces such as two-part epoxy like Araldite or Loctite Superglue Gel.

5.2 Electrodes

The piezo tubes can be bonded to a conductive surface by carefully removing a small amount of electrode. The piezo tubes can be connected to electrodes. The Nickel thin-film electrodes are the most commonly used electrodes. Let’s say, in a given piezo tube, the internal electrode is continuous, whereas, the external electrodes are quartered. When connecting any electrode to a piezo tube, it must be kept in mind that the electrode clearance specification is the length of the external electrode that is removed from both the ends. This provides good electrical clearance for the mounting surface. Though Nickel electrodes are the electrodes of choice for most application, they can be swapped for either Gold or Copper. Copper is a more economical option whereas; Gold provides excellent resistance against corrosion and electrical conductivity.

5.3 Connecting Wires

The piezo tubes can be connected with wires by using conductive epoxy or Rosin flux and standard solder. It is strongly recommended that after the flux application, a one-second contact with a 300˚C iron should be done in order to have a stronger adhesion. Some piezo tubes can also be bought with attached wires.

5.4 Vacuum and Cryogenic compatibility

Since none of the piezo tubes contains any outgassing material, all piezo tubes are fully vacuumed compatible. As for the very low cryogenic temperatures, different piezo tubes have different behavior according to the piezoelectric material that they have. For most of the piezoelectric materials, the sensitivity under very low temperatures tends to be lower by at least a factor of four. However, at cryogenic temperatures, applying voltage can be increased by +/-2 kV/mm.

5.5 OEM Customization

Though most of the piezo tubes available in the market can cater to the demands of many users, if the users require, some manufacturers can get the piezo tubes custom build for their customers. The OEM customization options include custom dimensions and thickness of the piezo tubes along with custom electrode configuration of different piezoelectric materials. The customers can also demand custom wiring connectors and different wiring arrangements as per their requirements. The mounting platform for the piezo tubes can also be designed and fabricated easily.

6.0 What are the electric current requirements for piezo tubes?

To determine the current required for the piezo tubes, the following formula is used; Under this formula, I is the effective capacitance while dV/dt is the rate of change of voltage. For a sine wave, the peak current is equal to the; Where V_p-p is the peak to peak voltage. For the triangle wave, the maximum current can be determined by the following equation;

7.0 How to handle and store your piezo tubes?

7.1 Storing

Though each manufacturer has a different storage requirement for their piezo tubes, the typical storing relative humidity for storage must be lower than 60% whereas, the temperatures should be lower than 40˚C. There is no significant decrease in the solderability of the piezo tube electrodes when stored for several months. However, it must be noted, that storing your piezo tubes for long times may result in thick-film electrodes to easily become tarnished. This occurs due to the reaction of the silver surface of the electrode with the atmospheric sulfur. Tarnishing of the surface may also make it more difficult to wet the electrode with sulfur. When stored for a long time, thin-film electrodes can easily become discolored from the UV radiations or sunlight. This is only a visual effect that does not decrease the conductivity or the solderability of the piezo tube electrodes. Still, it is recommended that the piezo tubes must be stored away from direct sunlight.

7.2  Destruction of piezo tubes by impurities

If piezo tubes are not properly insulated, they can be contaminated with conductive fluids such as finger sweat or conductive materials like metal dust. These contaminations can lead to damage from electrical flashovers during their operations. Contaminated piezo tubes are harmful to both the device used in as well as for the person operating it. To avoid contamination of piezo tubes by impurities, the user must wear insulating gloves when handling them. The piezo tubes must also be prevented from encountering any conductive materials or conductive liquids. In the case where the piezo tube has been accidentally been contaminated, the tubes can be cleaned in accordance with the instructions stated in the next section.

7.3 How to clean piezo tubes

Piezo tubes though highly versatile and robust needs to be cleaned once in a while for their effective performance as dust and other residuals collected on them may hinder the piezo tube performance. Contaminated piezo tubes are also not safe for operation. Before cleaning the piezo tubes, it must be made sure that the tube has been fully discharged and the components are not connected to the electrodes directly. A charged or live piezo tube may pose a risk for the cleaning person. The traditional method of cleaning the piezo tube includes using organic solvents like isopropanol to remove contaminations such as oil residues. After all, cleaned, the drying must be done residue-free. Piezo tubes can also be cleaned using the ultrasonic bath. To do so, first, lower the energy input to the tubes to a minimum and make sure not to use any other liquid than isopropanol for the cleaning purposes. When using the ultrasonic baths on the piezo tubes, the bath time should not exceed more than two minutes. After the piezo tubes are completely cleaned, dry the piezo component completely. This can be done effectively in a drying cabinet. The recommended drying period is 30 minutes under 40˚C. Make sure not to dry the component for a long duration as it may damage the piezo tube internally.

7.4 Safe handling of piezo tubes

Let it be piezo tubes or any other piezo components, safe handling is a must, or they may pose a serious threat to the users. Piezo tubes tend to hold dangerous residual and voltage charge, which can be easily altered with compressive stresses or temperature changes. Depending on their size, capacity, and material, when a piezo tube is electrically charged, it can have a high amount of electrical voltages present in them. Touching live piezo tubes, especially the ones with high ratings, can result in serious injury or even death from electric shock. Therefore, it is highly recommended that before touching the used piezo tube, the tubes must be fully discharged, and the operator must use insulating latex gloves or powder-free nitrile gloves. The operator must also keep the solder connectors (outer electrodes) short-circuited.

8.0 What are the piezo tube constants?

There are several constants that are associated with the piezoelectric components, and for the better understanding of the piezo tubes, it is important that we have at least the surface level understanding of them. Since the materials for the piezo tubes are piezoelectric, they are considered as anisotropic. Therefore, the physical constants are connected to both the direction of the electric current or the applied force as well as the directions that are perpendicular to the applied force. Each constant for the piezo tubes has two subscripts that tell the readers about the direction of the two related quantities. The direction of the positive polarization is normally made to coincide with the z-axis, as shown in the image below. The numbers associated with the axes helps us better understand the subscripts with the constants.

The direction of forces affecting a piezoelectric element

8.1  Piezoelectric charge constant for piezo tubes

The piezoelectric charge constant for the piezo tubes is, d. It is the polarization generated per unit of mechanical stress (T) that is applied to the piezo tubes or in another case; it is the mechanical stress (S) that is experienced by the piezo tube per unit of the electric field applied. The piezoelectric charge constant, d, has two subscripts. The first subscript tells us the direction of polarization that is generated in the tube when the electric field is zero. The latter subscript is the direction of the induced strain of applied stress, respectively. The following are some of the values used for the piezo tube electric charge constant; d₃₃ – There is an induced polarization in direction 3, whereas, the per-unit stress applied in direction 3 as well. It can also be said that the induced strain is in direction 3 per unit electric field applied is also in direction 3. d₃₁ – There is an induced polarization in direction 3, whereas, the per-unit stress applied in direction 1. It can also be said that the induced strain is in direction 3 per unit electric field applied is also in direction 1.

8.2 Piezoelectric voltage constant for piezo tubes

The piezoelectric voltage constant, g, for the piezo tube is the electric field that is generated by a tube per unit of mechanical stress applied on it or the mechanical strain that is experienced by the piezo tubes per unit of electric displacement applied. Just like the piezoelectric current constant, the voltage constant also has two subscripts. The first subscript shows the direction of the electric field or the direction of the applied electric displacement. The second subscript indicates the direction of the induced strain of applied stress. The following are some of the common values for the voltage constant; g₃₃ – The induced electric field is in the direction 3 while the per-unit stress is in direction 4 as well. It can also be said that the induced strain is in direction 3, while the per unit displacement applied is also in direction 3. g₁₅ –  The induced electric field is in the direction 1, while the per-unit shear stress applied is in direction  5.

9.0 How is piezo tubes at PD (PiezoData Inc.) R&D Co., Ltd

PD (PiezoData Inc.) uses proprietary machining process to fabricate piezo tubes. By using our state-of-the-art machining capabilities, we are able to machine, length grind, electrode, and polarize each piezo tube with precise tolerances. Though machining is considered to be more costly as compared to pressing, extruding and other bulk processing methods, only machined piezo tubes like ours are able to meet the tight rules and requirements of straightness, wall thickness, and concentricity for all the precision application. Moreover, we can machine almost any type of design or geometry that you require for your industrial needs. Contact PD (PiezoData Inc.) today to get your custom-made piezo tubes delivered right to your doorstep.