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Piezoelectric materials, of which piezoceramic materials are a subset, are particularly useful because they can generate an electrical charge from a physical force and/or mechanical deformation. One area in which this mechanism can be utilized is in a select number of sensors. In this article, we look at how piezoceramic materials can be used in sensor applications and how these sensors work.
Piezoelectric Materials
Piezoelectric materials are a class of materials that exhibit the ‘piezoelectric effect’ and can exist in many forms; provided they have an underlying regular crystalline lattice. Therefore, inorganic materials more commonly exhibit piezoelectric properties (although there are some exceptions).
The piezoelectric effect is the generation of an electric charge when a material is subjected to a physical force. In many instances, this force is a deformation or compression, or an induced stress/strain. There is also an inverse piezoelectric effect, which is essentially the opposite of the piezoelectric effect; a material either expands or contracts under an applied voltage.
For a crystalline material to be able to exhibit the piezoelectric effect, its crystalline lattice must be noncentrosymmetric and possess ions with a large effective charge. Ions with a smaller effective charge can generate a piezoelectric current, but it is often much smaller.
The presence of a physical force causes the material’s internal crystal lattice structure to change, and it is this mechanism which leads to the electrical charge being generated. In a normal crystalline lattice, ions are well-ordered (apart from atomic level defects, which are always present in one form or another). It is the distortion of this well-ordered lattice that generates the piezoelectric effect while still retaining an overall neutral charge. Under a physical force, the ions in the lattice are forced close together, creating a charge imbalance that resonates throughout the whole material. This results in the two ends of the crystal having opposite charges, producing a voltage. Once the force has been removed, the material returns to its normal state.
Piezoceramic Sensors
As the name suggests, piezoceramic sensors are sensors in which the active sensing material is both ceramic and piezoelectric. Ceramic materials are defined as inorganic, non-metallic materials. There are many examples which fall into this category, including inorganic oxides, nitrides and carbides. One common example of a piezoceramic used in sensors is lead zirconate titanate—commonly known as PZT.
As the piezoelectric effect is generated in many physical environments, piezoceramic sensors can be used to measure changes in physical factors, including pressure, acceleration, temperature, induced shock, strain, or force. A change in the environment will invoke some kind of force on the piezoceramic, resulting in an electrical charge which can be detected and measured by the sensor. One example is stress/strain gauges used in civil engineering, where the displacement of a structure (no matter how minute) can be picked up as it exerts a force on the sensor.
Most piezoceramic sensors are designed to measure more than one property, with pressure and acceleration being the most common. In accelerometers, a slightly different mechanism is utilized. Mechanisms relying on a direct force on the sensor are known as active sensors, whereas an accelerometer is a passive sensor. This means that a force is exerted by another component inside the sensor when there is a change in the parameter being measured. In accelerometers, a seismic mass is attached to the piezoceramic material and presses down with a relative force when the object being sensed accelerates.
One key advantage of using piezoceramic materials in sensors is that they can adapt to many different geometries without losing their functional properties. Whilst they generate an electrical charge under compression, the physical bulk structure doesn’t change that much, and this prevents them from breaking under prolonged deformation. Piezoceramic materials are also resistant to electromagnetic radiation, which enables them to be used in a number of environments for a range of applications.
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