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Positive temperature co-efficient of resistivity materials (PTCR) are a family of semiconductors that exhibit special properties concerning electrical conductivity. In particular, their electrical resistivity increases as temperature rises. However, the unique behavior of PTCR materials is more complex than this, as their behaviour is characterized by a slow increase in resistivity up to a certain temperature, which is dependent on the actual material. This temperature is called the Curie Temperature.
When a PTCR material reaches its Curie Temperature, its resistivity increases by several orders of magnitude over a very small temperature range (see figure 1). Thus, the amount of current that can flow is very small compared to that which can flow at significantly lower temperatures. After this sharp rise, the resistivity approaches an almost constant value. It should be noted that the increase in temperature can be induced in some part by the flow of current through the material.
Materials exhibiting PTCR properties are generally semi-conducting titanate ceramics, including barium titanate, lead titanate and strontium titanate. In their pure forms, they are insulators. However, when small amounts of dopants are added, they become semi-conducting.
Other characteristics of PTCR materials include:
- A polycrystalline n-type semiconductor
- They have surface acceptor sites at their grain boundaries
- They are ferroelectric
- They have ohmic electrodes
These semi-conducting materials undergo a slow increase in resistance as the temperature increases. Within the region of the materials’ characteristic Curie Temperature, their resistivity increases dramatically over a very small temperature range. After this rapid increase, the resistivity approaches a maximum as the temperature rises further.
Temperature Controlled Heaters
When operated in the region of the Curie Temperature, the PTCR will maintain an almost static temperature despite large variations in ambient temperature and voltage. This allows them to be self-thermostatting.
Heating elements for hair dryers and domestic heaters can be made from PTCR materials, and have an advantage over metallic elements as they will not overheat and burn out if the air flow is interrupted.
PTCR units can be used to determine temperature by monitoring resistance. If small units can be suitably placed in windings of transformers or electric motors, they can be connected to identify overheating in the system and automatically trigger external switching devices. Alternatively, if the PTCR units are incorporated directly into the system, they will automatically reduce the current flow if overheating occurs.
Transient Current Generators
When PTCR elements have a voltage applied to them, they initially allow high currents to flow through them, which in turn heats them up. As the temperature of the PTCR increases, so does the resistivity, hence restricting current flow. Thus, a PTCR element connected in series with a coil will serve to negate an alternating magnetic field in a few seconds. An example of this application is a degaussing circuit in a color television.
Ambient Thermal State Indicators
This application utilizes the fact that a PTCR device will dissipate a different amount of heat if immersed in liquids rather than air, and the amount of heat dissipated will also vary with the surrounding fluids flow rate. At the same time, the amount of power drawn by the device is proportional to the heat dissipated. Due to this fact, PTCR materials can be used in liquid level meters and flow meters.
This article was updated on 26th June, 2019.