Positive thermal coefficient

Figure 63. Structure of a lithium-ion battery. PTC, positive thermal coefficient device.

The construction of the spiral-wound cylindrical battery is shown in Fig. 14.78. These batteries typically have several safety devices incorporated in their design to provide protection against such abusive conditions as short circuit, charging, forced discharge, and over heating. Two safety devices are shown in the figure—a pressure relief vent and a resettable thermal switch, called a positive thermal coefficient (PTC) device. The safety relief vent is designed to release excessive internal pressure to prevent violent rupture if the battery is heated or abused electrically. 

Positive thermal coefficient of expansion (PTC) device, placed in the battery header, acts as both a current fuse and a thermal fuse. When excessive current is drawn or the temperature increases due to some other cause, the resistance of the PTC increases ( 10 increase) resulting in decreased current as well as increased heat generation. 

Negative and positive electrode materials, electrolytes, and separators are four key components for lithium-ion batteries. However, other auxiliary materials such as electronic conductive agents, binders, solvents for slurry preparation, positive thermal coefficient (PTC) materials, current collectors, and cases are also important in the production of complete lithium-ion batteries. 

A particularly interesting test of the above rule of thumb would be blends containing the HIQ polymer described in Section 5.4.2 in which the unusual positive thermal coefficient of viscosity was attributed to the coexistence of isotropic and anisotropic domains of the coploymer, as determined by the distribution of copolymer ratios and therefore local chain stiffness. As temperature increased, the fraction of the high viscosity isotropic phase increases at the expense of the low-viscosity anisotropic phase. This polymer by itself as the LCP component would open a degree of freedom in varying the viscosity ratio. Furthermore, blending this HIQ LCP with more conventional LCPs with which it is misdble would expand the window of control of the LCP component. 

Positive temperature coefficient (PCT) thermistors are solids, usually consisting of barium titanate, BaTiOi, in which the electrical resistivity increases dramatically with temperature over a narrow range of temperatures (Fig. 3.38). These devices are used for protection against power, current, and thermal overloads. When turned on, the thermistor has a low resitivity that allows a high current to flow. This in turn heats the thermistor, and if the temperature rise is sufficiently high, the device switches abruptly to the high resisitvity state, which effectively switches off the current flow. 

This reaction can occur either thermally or photochemically, with the equilibrium position (thermal reaction) depending on the thermodynamic stability of the reactant and product, and the photostationary-state composition depending on the relative values of the absorption coefficients at the wavelengths used. 

CNTs are also valuable as field emitters because they have a small virtual source size [30], a high brightness, and a small positive temperature coefficient of resistance [31]. The latter means that they can run hot under high emission currents, but not go into thermal runaway. Emission from nanotubes can be visualized by electron holography in a TEM [32],... 

Dilatometric studies have demonstrated the negative thermal expansivity for many oriented crystalline polymers 64,170 176). The results of these experimental studies may be summarized as follows. Cold-drawing of PE below Tm 172) and solid-state extrusion under elevated pressure 170 1711 lead to a monotonous decrease of the positive thermal expansion coefficient with increasing draw ratio. At a certain degree of orientation, dependent on temperature, PM becomes negative with Pi < Pell (Fig. 16). This is the second way of reaching negative expansivity applied, e.g. to POM (w = 63 % Tdr = 423 K) 173>. 

H-bonds. In the former case, OH frequencies increase with increasing temperature, i.e., dvon/dT > 0, because the H-bonds are weakened as a result of the thermal expansion of the lattice. In the latter case, the temperature shift of the OH bands is negative, dvon/ dT < 0, since such bonds are strengthened with increasing amplitude of the H2O libra-tional modes. However, for symmetrically bifurcated H-bonds, i.e., if all O, H, and the acceptor groups are located in one plane and the distances to the two acceptors are equal (see Fig. 2), a positive temperature coefficient (as in the case of linear bonds) must be produced This is, in fact, found, e.g., in BaCl2 H20 . ... 

These standards have been widely used by the Li-ion battery manufacturers and users to evaluate the battery safety characteristics to ensure battery safety in applications. Various safety devices including thermal control devices such as the positive temperature coefficient switches and electronic control devices such as various IC protection circuits have been successfully used for Li-ion batteries and battery packs. With an adequate combination of various safety devices, one can protect a Li-ion battery from overcharge, overdischarge, hard short circuit, impact, and other safety concerns throughout its applications. 

For, by our convention, the products of the reaction have positive stoichiometric coefficients, and the sum over the positive terms is the total enthalpy of the products. Similarly, the sum over the negative terms is the total enthalpy of the reactants and so the sign convention gives the difference. If AH is positive, the enthalpy of the products is greater than that of the reactants and heat will be absorbed as the reaction goes on such a reaction is called endothermic. On the other hand, an exothermic reaction, for which AH is negative, evolves heat. The thermally balanced form of the reaction = 0 is thus... 

In general, all materials have a positive thermal expansion coefficient that is they increase in volume when heated. Thermal expansion results from thermal excitation of the atoms that compose the material [16]. At absolute zero, atoms are at rest at their equilibrium positions (i.e., at r0 in Fig. 1). As they are heated, thermal energy causes the atoms to vibrate around their equilibrium positions. The amplitude of vibration increases as heating is continued. Asymmetry in the shape of the potential well causes the average interatomic distance to increase as temperature increases, leading to an overall increase in volume [15]. 

As seen from equation (3.7.32), this coefficient is dependent both on the melt temperature and on the melting point of the metal-oxide. In contrast with the derivative from equation (3.7.31), the value of the thermal coefficient of solubility is positive, and consequently the Le Chatelier-Shreder equation predicts an increase in the solubility of any substance together with elevation of the solvent temperature. The data on the oxide solubilities obtained for the molten CsCl-KCl-NaCl eutectic at different temperatures allow us to verify the formulated conclusions. For this purpose, we write equation (3.7.32) in the form of finite differences, taking into account the fact that pPMe0 = — log PMeQ ... 

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