PTCR effect

The PTCR effect is complex and not fully understood in terms of the grain boundary states and stmcture. Both the PTCR effect and room temperature resistivities are also highly dependent on dopant type and ionic radius. Figure 11 (32) illustrates this dependence where comparison of the PTCR behavior and resistivity are made for near optimum concentrations of La ", Nd ", and ions separately substituted into BaTiO. As seen, lowest dopant concentration and room temperature resistivity are obtained for the larger radius cation (La " ), but thePTCR effect was sharpest for the smallest radius cation (Y " ), reflecting dual site occupancy of the Y " ion. 

Hayashi, K., Yamamoto, T., and Sakuma, T. (1996). Grain orientation dependnce of the PTCR effect in niobium-doped barium titanate. J. Am. Ceram. Soc. 79 1669-1672. 

The PTCR may serve as a spectacular example of this effect, which is an exclusive property of polycrystalline materials (Figure 4.46). In other words, the PTCR effect can only be displayed by a crystal involving at least one interface (biciystal). The extent of the PTCR effect can be modified by the chemical composition of the grain boundaries. 

The PTCR effect refers to anomalies observable in some semiconductors or ionic conductors, in that the resistivity increases significantly with the temperature (PTCR = positive temperature coefficient of the resistance). Trivial but powerful PTCR effects rely on composites of insulating... 

Fig. 4. Effect of dopant additions on the resistivity versus temperature behavior of BaTiO PTCR ceramics. A, undoped B, doped with 0.134 mol % Cr ...

The class of ferroelectric materials have a lot of useful properties. High dielectric coefficients over a wide temperature and frequency range are used as dielectrics in integrated or in smd (surface mounted device) capacitors. The large piezoelectric effect is applied in a variety of electromechanical sensors, actuators and transducers. Infrared sensors need a high pyroelectric coefficient which is available with this class of materials. Tunable thermistor properties in semiconducting ferroelectrics are used in ptcr (positive temperature coefficient... 

Glass JT, Cahen JGL, Stoner GE, Taylor EJ (1987) The effect of metallurgical variables on the electrocatalytic properties of PtCr alloys. J Electrochem Soc 134(l) 58-65... 

Let us now turn briefly to space charge effects in electronic conductors and concentrate on the important case of the depletion layers, insofar as this is of importance for our considerations. It was mentioned above that depletion layers in extrinsic conductors can be fairly large on account of lack of significant screening . The resultant grain boundary resistances can be enormously high, so that, as in a zinc oxide varistor, in the substrate material Si3N4 or the PTCR material BaTiOa,... 

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