Overview: The article reviews PTC thermistors in detail, highlighting their structures, working principles, challenges, and applications. It focuses on the significance of PTC thermistors as current-limiting components in electrical circuits.

The term "thermistors" comes from a combination of the words "thermal" and "resistor," signifying that these devices are sensitive to changes in temperature. These semiconductor devices have temperature-dependent resistance, which makes them popular in temperature-sensing applications. There are two types of thermistors: PTC and NTC thermistors.

What is a PTC thermistor?

PTC thermistors are devices whose electrical resistance rises with temperature. Hence, they have a positive temperature coefficient. Specifically, by multiple orders of magnitude, PTC thermistors increase their electrical resistance within a restricted temperature range. Because of this characteristic, PTC thermistors are frequently employed as current-limiting components in various electrical circuits.

Types of PTC Thermistor

There are four categories of materials that make up the PTC thermistor:

  • Barium titanate
  • Ceramic composites
  • Vanadium (III) oxide
  • Polymer composites 

Barium Titanate PTC Thermistor

The simplest and least expensive to make is barium titanate. Specifically, they are well known for their varied mechanical and physical properties. As a result, the practical use of barium titanate PTC thermistors has increased. This PTC thermistor is a more desirable option because of its current-limiting characteristics in relation to the influence of current pulses.

Structure

A PTC thermistor's resistive layer is made up of many grains of semiconductor barium titanate that interact with each other. Barium titanate's crystal structure is made up of tetragonal cells at ambient temperature (below Curie temperature (TC), as depicted in Fig. 1a). The lattice represents titanium atoms Ti+4 as white nodes, oxygen atoms O2 as gray nodes, and barium atoms Ba+2 as black knots.

Fig. 1: Illustration of barium titanate structure Source: MDPI

  • The illustration shows barium titanate as an asymmetrical crystal structure, and the titanium atom is displaced from the center of the tetragonal cell.
  • This structure enables spontaneous polarization when an electric field is applied, inducing dipole moments.
  • The material has an extremely high permittivity at the moment.
  • At the grain boundary, the height of surface potential barriers is low.
  • Impurities are added to the barium titanate structure via a doping procedure to be used as thermistors.
  • Adding impurities increases the donor and acceptor levels in the grain boundaries, increasing the barium titanate's conductivity multiple times.

La, Sb, Zn, Sn, Ce, Zr, and Hf are employed as doping agents for barium titanate. Alternatively, the PTC thermistor's resistance can be greatly increased above the Curie point and below the Curie point by adding various impurities, such as Fe, Mn, V, Cr, Ni, and Co. Additionally, the phase transition temperature can be regulated, especially with the use of Fe or Mn.

Working

When the temperature of the PTC thermistor approaches the Curie point,

  • The tetragonal structure of the cell transforms into a fully symmetrical perovskite form (as seen in Figure 2b).
  • Spontaneous polarization disappears.
  • It loses the dipole moment.
  • Permittivity drops quickly.
  • At the same time, the height of the potential barriers on the grains increases.
  • The resistance of the PTC thermistor goes up.

Ceramic Composites PTC Thermistor

The ceramic composite PTC thermistor also shares similar properties and methods of action because they also have the majority of barium titanate or other composites with similar perovskite structures. These include SrTiO3 added with niobium, lanthanum, lead titanate, and several additional elements.

Vanadium (III) Oxide PTC Thermistor

The current-limiting characteristics of vanadium (III) oxide are distinct. It seems highly promising to use the characteristics of vanadium (III) oxide for current limitation applications. When compared to barium titanate PTC thermistors, chromium doping can significantly reduce the resistance of vanadium (III) oxide. As a result, PTC thermistors based on vanadium (III) oxide can be used in high-current circuits.

Structure

Fig. 2a illustrates the monoclinic structure of the V2O3 cell before the phase transition temperature. As seen in Fig. 2b, the transition from a monoclinic to a rhombohedral lattice happens at temperatures over the Curie point. White circles indicate oxygen atoms, whereas black circles indicate vanadium atoms. 

Working

The average distance between vanadium atoms and oxygen atoms stays constant, whereas the distance between neighboring vanadium atoms rises with temperature. Simultaneously, the Mott transition model for charge carriers explains the variation in electrical resistance.

Fig. 2: Illustration of vanadium oxide crystal structure Source: MDPI

Challenges

There are certain restrictions for this vanadium PTC thermistor. During the phase transition, the volume of V2O3 unit cells first changes by 0.8% to 3.5%. This makes microcracks in the PTC thermistor, which in turn breaks down materials during many phase transitions. Second, the cost of generating such a PTC thermistor goes up because making vanadium (III) oxide is more complicated than making barium titanate.

Polymer composites PTC Thermistor

The devices consist of a crystalline polymer with conductive particles dispersed throughout the element's volume. Because these particles are in contact with each other within the polymer, they form a conductive grid at standard device temperatures. 

If the temperature increases to the point where the polymer crystallites melt and turn amorphous, the device switches. When the crystalline polymer particles melt, they cause a break in the conductive chains, which causes the resistance of the whole device to change in a way that is not linear. It is important to remember that this kind of shift is reversible and can happen again.

Summarizing the Key Points

  • PTC thermistors exhibit a significant increase in electrical resistance with temperature, making them valuable as current-limiting components in various electrical circuits.
  • Different types of PTC thermistors, including barium titanate, ceramic composites, vanadium (III) oxide, and polymer composites, offer unique structures and working principles.
  • Challenges such as volume fluctuations during phase transitions and complex manufacturing processes impact the performance and cost of PTC thermistors.
  • Understanding the structural transformations and properties of PTC thermistors is essential for optimizing their performance and applications in high-current circuits.

Reference

Evgeniy Safonov et al., “The Specifics of PTC Thermistor Applications for Limiting Surge Currents,” Energies 17, no. 2 (January 9, 2024): 318, https://doi.org/10.3390/en17020318.

Rizos N. Krikkis, “Multiplicity Analysis of a Thermistor Problem—A Possible Explanation of Delamination Fracture,” J 6, no. 3 (September 4, 2023): 517–35, https://doi.org/10.3390/j6030034.