PTC thermistors

Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

Motor winding temperature detection (by PTC thermistors and RTDs) 12/306... 

A typical characteristic curve of a PTC thermistor having a Curie point of 120°C is shown in Figure 12.37. 

Industrial Power Engineering and Applications Handbook Table 12.1 Curie points lor a lew PTC thermistors... 

Kolfertz, G., Full thermal protection with PTC thermistors of three-phase squirrel cage motors, Siemens Review, 32, No. 12 (1965). 

As a first example, let us consider a metallic thermistor inserted in fig. 3, whose resistance is, in a first approximation, expressed as R(T)=Ro(l+aT). R(T) is the resistance of a PTC thermistor at a given temperature T, Ro is the resistance at To, and I represents a suitable DC (or AC current), while A is the constant gain of a low noise amplifier, operating in a suitable bandwidth. Let us suppose that the injected current I does not induce, through the heating process, a detectable change of the resistance value. 

Composite-based PTC thermistors are potentially more economical. These devices are based on a combination of a conductor in a semicrystalline polymer—for example, carbon black in polyethylene. Other fillers include copper, iron, and silver. Important filler parameters in addition to conductivity include particle size, distribution, morphology, surface energy, oxidation state, and thermal expansion coefficient. Important polymer matrix characteristics in addition to conductivity include the glass transition temperature, Tg, and thermal expansion coefficient. Interfacial effects are extremely important in these materials and can influence the ultimate electrical properties of the composite. 

Fig. 4.15 (a) Silicon carbide varistors, and (b) zinc oxide varistors. (Courtesy of P.D. Devices Ltd. UK.) (c) PTC thermistors for motor protection and (d) NTC thermistors inset, catheter fitted with biomedical NTC thermistor for measuring blood temperature. (Courtesy of Thermometries.)... 

The fabrication route for PTC thermistors is typical of that employed for modern electroceramics except in so far as special attention is given to maintaining high purity and to the firing schedule. 

Fig. 4.23 Current-voltage characteristic for a PTC thermistor in thermal equilibrium.

Just as in the case of NTC thermistors (see Section 4.4.1), the temperature distribution within a PTC thermistor can be far from uniform in certain nonequilibrium applications. For example, thin discs in motor-starter units have... 

With the chemisorption of, for example, oxygen a surface density of electron acceptor states leads to the establishment of a Schottky barrier. The process is essentially the same as that which occurs in the case of the PTC thermistor (see Figs 4.21 and 4.10). The electron potential barrier height (p) is given by... 

Under the same simplifying assumptions as for the PTC thermistor (where the surface states are shared between two grains) the depleted layer thickness d is given by... 

The perovskite structure is, of course, of special significance in the electroceramics context since the ferroelectric perovskites are dominant in the ceramic capacitor, PTC thermistor and electromechanical transducer industries. The structure favours the existence of soft modes (low frequency phonons) as evidenced by its tendency to instability, for example the ferroelectric-paraelectric transition. Instability is evident in the case of the T23 compound which exhibits a tetragonal-orthorhombic transition in the region of 700 °C (the exact temperature depends on the oxygen content). Extensive twinning, very reminiscent of ferroelectric domain structures, is observed. 

Suggest a basic composition for a PTC thermistor that will switch at around 30 °C. 

Small amounts of Y or La are used to dope BaTiC>3 which is the main component of all PTCs (positive temperature coefficient). Demand for PTC thermistors is high. Yttrium iron garnets are used in soft ferrites at very high frequencies (microwave region) for radar equipment. 

Crytal chemitry. The effect of solid solution on the transition behavior of perovskite (ABX3) structures has been intensively scrutinized for more than 50 years. These materials have merited continuous attention because of their enormous technological versatility. As multilayer capacitors, piezoelectric transducers, and positive temperature coefficient (PTC) thermistors they generate a market of over 3 billion every year (Newnham 1989, 1997). In addition to ease of fabrication, these compounds exhibit a number of attributes required of ideal actuators (1) They display very large field-induced strains (2) They offer quick response times and (3) Their strain-field hysteresis can be chemically controlled to be very large or negligibly small, depending on the application. Details of their technical applications can be found in Jaffe et al. (1971) and Cross (1993). 

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