Background

Positive temperature co-efficient of resistivity or PTCR materials area family of semiconductors which exhibit special properties in relation to electrical conductivity. In particular, their electrical resistivity increases with increasing temperature.

However, the unique behaviour of PTCR materials is more complicated than just having the resistivity increase as the operating temperature increases. PTCR materials behaviour is characterised 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.

Figure 1. Electrical resistance versus temperature behaviour for positive temperature co-efficient of resistivity (PTCR), negative temeparature co-efficient of resistivity (NTCR) materials and critical temperature thermistors



Materials exhibiting PTCR properties are generally semiconducting 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 semiconducting.

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
Key Properties

These semiconducting materials undergo a slow increase in resistance as the temperature increases. In the region of their 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 rise further.
Applications
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. They have the advantage over metallic elements that they will not overheat and burn out if the air flow is interrupted.
Thermostat Elements

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 in such a way they can trigger off external switching devices if the system begins to overheat. Alternatively, if they are incorporated directly into the system, they will automatically reduce the current flowing 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 wills serve to negate an alternating magnetic field in a few seconds. An example of this application is a degaussing circuit in a colour television.
Ambient Thermal State Indicators

This application utilises the fact that a PTCR device will dissipate a different amount of heat if immersed in liquids other 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.