Selection of PTC heating elements

- Apr 26, 2020-

The Curie point is a characteristic temperature of ferroelectric materials. For PTC materials, it is used to characterize the temperature at which the volume resistivity of PTC suddenly changes, and the Curie point series is used to classify the materials. Studies have shown that BaTiO-based materials, only polycrystalline ceramic materials, have an essential PTC effect and are determined to be grain boundary effects. In this type of ceramics, the micro grains are
Semiconductors are not dielectrics, so they do not have ferroelectricity. In the grain boundary layer, a layer forming a gradient composition is not a semiconductor material, but a non-uniform, gradient composition of ferroelectric Boundary layer. The ferroelectric has white-hair polarization, and a ferroelectric phase transition will occur under a certain characteristic temperature eagle (the phase transition of the basic ferroelectric hegemony involved here—the paraelectric cubic). The temperature (experimental value) is called the Curie point Tc. Below Tc, a large amount of ferroelectric polarization participates in conduction in the boundary layer, and the resistivity of the material is very small. When the Curie point is reached, the phase transition causes the boundary layer to establish a barrier to conduction, and the resistivity of the material increases abruptly. Experiments have found that the temperature at which PTC resistivity increases significantly corresponds exactly to the ferroelectric phase transition temperature Tc, so it is also called the Curie point accordingly.
The method of determining the Curie point is currently not uniform.
2. Choose the right Curie point material
As a heater, the temperature of the system is often considered as the main consideration. The Curie point of the selected material can be considered from the following aspects
1) The system itself is a PTC element, or a pair with a PTC element, which is considered to be a weak thermal load unit of the PTC element.
2) The system itself is centered on the PTC heater
The resulting temperature field, the temperature of the system just below the Curie point. The larger the system, the higher the system temperature requirements and the higher the Curie point of the selected material.
3) The Curie point of the PTC element for current limiting and insulation is generally higher than the insulation temperature of the system, but it is also special. The distribution of the temperature field of the system and the position of the current limiting element in the system A or the thermal design of the element itself must be considered. Determined by experiment.
The determination of the Curie point is also closely related to the number of PTC components used in the system, and the temperature of the operating point is often different due to the p-T characteristics of the PTC area. In fact, within a certain range. The thermal design of the heater or the dissipative design of the system is even more important.