PTC resistors

When microelectronics and solid state devices developed over the last five to four decades, the development of solid-state sensors followed suit, resulting in the introduction of NTC and PTC resistors to monitor temperature, and first Reed relais and inductive sensors to determine position and distance, or tachometers for rotational measurements in washing machines and dish washers over the past two decades. 

NTC and PTC resistors for temperature determination are built into nearly all large appliances... 

In recent several years, super-capacitors are attracting more and more attention because of their high capacitance and potential applications in electronic devices. The performance of super-capacitors with MWCNTs deposited with conducting polymers as active materials is greatly enhanced compared to electric double-layer super-capacitors with CNTs due to the Faraday effect of the conducting polymer as shown in Fig. 9.18 (Valter et al., 2002). Besides those mentioned above, polymer/ CNT nanocomposites own many potential applications (Breuer and Sundararaj, 2004) in electrochemical actuation, wave absorption, electronic packaging, selfregulating heater, and PTC resistors, etc. The conductivity results for polymer/CNT composites are summarized in Table 9.1 (Biercuk et al., 2002). 

Electroceramics. Titanates like barium, strontium, calcium and lead titanate prepared from finely divided, high-purity TiOz hydrolysates are used in capacitors, PTC-resistors and piezoelectric materials. The specifications of the Ti02 starting materials with respect to purity, reactivity, and sintering properties are expected to become more stringent. The market is estimated to be several thousand of tonnes a year as TiOz. A strong annual growth is expected. 

Donor-doped BaTi03 is the basis of positive temperature coefficient (PTC) resistors (see Section 4.4.2). The insulating dielectrics formed with high donor concentrations have a low oxygen vacancy content and are therefore less prone to ageing and degradation. 

Barriers of the Schottky type control the behaviour of voltage-dependent resistors (VDRs), PTC resistors and barrier-layer capacitors. Their behaviour is by no means as well understood as that occurring in semiconductors such as silicon but, where appropriate in the text, simplified models will be presented to indicate the principles involved. 

PTC resistors could be classified as critical temperature resistors because, in the case of the most widely used type, the positive coefficient is associated with the ferroelectric Curie point. 

Positive temperatnre coefficient (PTC) resistor operates by a phase change in the conductive polymer placed internally to terminal connection which increases its resistance to minimize cnrrent flow to the cell terminals when either the current flow exceeds the design point or the cell internal temperature exceeds the set point. 

Figure 12.4 Thermal shock cracks in PTC-ceramics. (a) In a quenched specimen (b) In a PTC resistor after soldering. The reason for the...

In order to establish the model of intergranular impedance for doped barium titanate, it is important to notice that miorostructure properties of BaTiOj based materials, expressed in their grain boundary contacts, are of basic importance for electric properties of these materials. The barrier character of the grain boundaries is especially pronounced for doped BaTiOs materials which are used as PTC resistors. Basically two types of dopants can be introduced into BaTiOs large ions of valence 3+ and higher, can be incorporated into Ba positions, while the small ions of valence 5+ and higher, can be incorporated into the Ti sublattice [9-11], Usually, the extent of the solid solution of a dopant ion in a host structure depends on the site where the dopant ion is incorporated into the host structure, the compensation mechanism and the solid solubility limit [12], For the rare-earth-ion incorporation into the BaTiOs lattice, the BaTiOs defect chemistry mainly depends on the lattice site where the ion is incorporated [13], It has been shown that the three-valent ions incorporated at the Ba -sites act as donors, which extra donor charge is compensated by ionized Ti vacancies (V -), the three-valent ions... 

FIGURE 2.21 Typical resistance/temperature curve for a PTC resistor. 

Negative temperature coefficient (NTC) resistors for example, spinel structure oxides, manganese(ll. 111) oxide - Mn304 Positive temperature coefficient (PTC) resistors for example, aluminum oxide AI2O3, magnesium oxide - MgO... 

Concerns about short-circuit currents, especially internal, are very similar to those associated with Ni-Cd systems. Temperature rise to due short circuit can lead to very high temperatures at the battery case, although most cells will withstand a great deal of short-circuit abuse. These temperatures can be sufficient to cause burns or ignite flammable materials [34]. To guard against this, many modem batteries, especially those contained in a multicell pack, include integrated safety devices such as PTC resistors, thermal fuses, and thermistors to sense pack temperature remotely. 

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