What is the Difference Between Piezoelectric and Piezoresistive

Piezoelectric and piezoresistive are two effects are triggered by mechanical stress. Mechanical stress is the internal force an object experiences when an external force acts on it. Higher force or smaller area means greater stress. When mechanical stress is applied on some substances, certain properties may vary.

What is the difference between piezoelectric and piezoresistive? Piezoelectric effect generates a voltage when mechanical stress is applied, whereas piezoresistive effect causes a change in electrical resistance of a material due to mechanical stress.

Key Areas Covered

1. What is Piezoelectric Effect
      – Definition, Features 
2. What is Piezoresistive Effect
      – Definition, Features 
3. Similarities Between Piezoelectric and Piezoresistive
      – Outline of Common Features
4. Difference Between Piezoelectric and Piezoresistive
      – Comparison of Key Differences
5. FAQ: Piezoelectric and Piezoresistive
      – Answers to Frequently Asked Questions

Key Terms

Piezoelectric, Piezoresistive, Piezoelectric Effect, Piezoresistive Effect

What is Piezoelectric Effect

The piezoelectric effect is the ability of certain materials to convert mechanical stress (pressure, squeezing, vibration) into electrical energy. This effect is due to the internal structure of these materials. Examples of piezoelectric materials usually include ceramics or crystals with a non-symmetrical arrangement of positive and negative charges. The internal balance of these structures is disrupted when a mechanical stress such as squeezing or vibration is applied. This causes a shift in positive and negative charges, which in turn creates a voltage across the material. This phenomenon is reversible. When an electric field is applied, the material can be deformed slightly. We call this the converse piezoelectric effect.

Figure 1: Piezo Bending Principle

These piezoelectric materials have many applications in day-to-day life. They are used in microphones and guitar pickups, where the sound energy (vibrations) is converted to electrical signals that can be heard as music or speech. Ultrasound imaging relies on piezoelectric crystals to send and receive sound waves within the body, creating detailed pictures of organs and tissues. Cigarette lighters also use this effect to generate a spark when the button is pressed.

What is Piezoresistive Effect

The piezoresistive effect describes a change in a material’s electrical resistance due to applied mechanical stress. When a certain substance is subjected to stretching or squeezing, its ability to conduct electricity changes. The major substances involved in this phenomenon are semiconductors. Semiconductors are materials that conduct electricity up to a certain degree. When squeezed or pressed, the internal structure of the semiconductors gets altered, affecting the movement of electrons (which are the charged particles). There are two main ways in which this phenomenon happens: by geometric change or by conductivity change.

The geometric change is the physical change in shape due to stress that alters the pathway of electrons in the material. In the same manner, squeezing a semiconductor can restrict the flow of electrons, increasing its resistance. In the conductivity change, the stress itself directly influences the material’s ability to conduct electricity. Moreover, in some semiconductors, pressure can promote the creation of more charge carriers (electrons or “holes” where electrons were), making it easier for electricity to flow and decreasing resistance.

Figure 2: Piezoresistive Sensor

This change in resistance is small but can be accurately measured. Hence, piezoresistive substances are used in sensitive sensors called strain gauges. These strain gauges are useful in microelectromechanical systems, medical devices, and various other industrial applications.

Similarities Between Piezoelectric and Piezoresistive

1. Mechanical stress trigger both effects.
2. Both phenomena rely on specific material properties.
3. Both effects involve a change in the material’s internal structure due to the applied stress.
4. While not always the case, both effects can exhibit some level of reversibility.

Difference Between Piezoelectric and Piezoresistive

Definition

• Piezoelectric effect is the ability of certain materials to convert mechanical stress (pressure, squeezing, vibration) into electrical energy, while piezoresistive effect is the change in a material’s electrical resistance due to applied mechanical stress.

Effect

• Piezoelectric generates voltage (electricity) when mechanically stressed while piezoresistive causes a change in electrical resistance of the material under stress but doesn’t directly produce electricity.

Materials

• Piezoelectric materials include crystals, ceramics, and some biological materials, while piezoresistive materials primarily include semiconductors, such as silicon.

Stress

• In piezoelectric materials, disruption of internal charge distribution due to stress leads to a voltage across the material. In piezoresistive effect, stress alters the material’s geometry or conductivity, affecting how easily electricity flows.

Nature

• Piezoelectric effect can generate a measurable voltage without external amplification. Meanwhile, in piezoresistive effect, resistance changes are typically smaller and require specialized circuits for detection.

Conclusion

The basic difference between piezoelectric and piezoresistive is their reaction to mechanical stress. Piezoelectric effect generates a voltage when mechanical stress is applied, whereas piezoresistive effect causes a change in electrical resistance of a material due to mechanical stress.

FAQ: Piezoelectric and Piezoresistive

1. What is a piezoresistive sensor used for?

Piezoresistive sensors most commonly help in measuring pressure. They work by detecting changes in electrical resistance of a material caused by applied force or pressure. This change in resistance is then converted into a voltage signal that can be measured and used to determine the amount of pressure being applied.

2. What is the difference between piezoelectric and reverse piezoelectric?

When a mechanical stress or force applied to a piezoelectric material, it generates electricity. Reverse piezoelectric effect is the opposite of this. In reverse piezoelectric effect, when a voltage is applied to the substance, it physically deforms or vibrate.

3. What do you mean by piezoresistive effect?

The piezoresistive effect is a phenomenon where the electrical resistance of a material changes upon application of mechanical strain.  That is, when a certain object which is piezoelectric is squeezed or stretched or bent  its ability to conduct electricity changes.

4. What are piezoresistive materials examples?

Piezoresistive materials are those that exhibit a change in electrical resistance when subjected to mechanical strain.  Some examples of such type of material are semiconductors(silicon), metals and certain polymers.

5. What are the applications of piezoresistivity?

Piezoresistivity is useful in various industrial applications such as pressure monitoring, tire pressure sensors, engine oil pressure monitoring, air bag development in automotive industry, touch screens, blood pressure monitors, catheters, scientific research, robotics and microphones.

Reference:

1. “Piezoelectricity.” Science Direct.
2. “Piezoresistance.” Science Direct.

Image Courtesy:

1. “Piezo bending principle” By User:Sonitron Support – (CC BY-SA 3.0) via Commons Wikimedia
2. “Piezo resistive mems preasure sensor” By MichaelFrey – Own work, based on Research Gate  (CC BY-SA 4.0) via Commons Wikimedia