Temperature coefficient negative

MMC = multicomponent components PTC = positive temperature coefficient NTC = negative temperature coefficient. 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

VPO reactions of typical alkanes may be considered conveniently in three temperature regions. Under some circumstances, particularly at pressures not greatly exceeding atmospheric, a curious and fundamentally important phenomenon known as the negative temperature coefficient (NTC) region is... 

Data-based (DDC) or programmable (PLC) controllers with universal inputs and outputs can be used. It is essential that they are configured before use. In some cases the input may be used only for temperature measurement from special types of thermistors. (Thermistors are constructed from semiconductor materials where the resistance changes reversibly proportional to the temperature, i.e., a negative temperature coefficient.)... 

Thermistors are temperature-dependent resistances, normally constructed from metal oxides. The resistance change with temperature is high compared with the metallic resistances, and is usually negative the resistance decreases with temperature increase. The temperature characteristics are highly nonlinear. Such thermistors, having a negative temperature coefficient, are called NTC thermistors. Some thermistors have a positive temperature coefficient (PTC), but they are not in common use for temperature measurement. 

As expected from their structures, the elements are poor conductors of electricity solid F2 and CI2 have negligible conductivity and Br2 has a value of 5 X 10 ohm cm just below the mp. Iodine single crystals at room temperature have a conductivity of 5 x 10 ohm cm perpendicular to the be layer plane but this increases to 1.7 x 10 ohm cm" within this plane indeed, the element is a two-dimensional semiconductor with a band gap g 1.3eV (125kJmol" ). Even more remarkably, when crystals of iodine are compressed they become metallic, and at 350kbar have a conductivity of 10" ohm" cm", The metallic nature of the conductivity is confirmed by its negative temperature coefficient. 

Sirokman, G., Wilson, T., and Hastings, J. W. (1995). A bacterial luciferase reaction with a negative temperature coefficient attributable to protein-protein interaction. Biochemistry 34 13074-13081. 

Blum and Kearns (1) showed in the American cockroach that topically applied pyrethrum shows a negative temperature coefficient of action between 15° and 35°C. The 24-hour LDso for pyrethrum is about 1 g. at 15°C. and about 6 Mg- at 35°C. 

A negative temperature coefficient of the inner-layer potential drop was observed, -0.8 mV KT1. Estimates of dipole potential drops due to solvent molecules22,23,29,30 gave much larger values for DMSO than for H20, which can be explained by a strong preferential orientation of DMSO at the Hg surface.26,8,304... 

In this chapter it is of interest to discuss the dependence of BEo=0/BT on AX. Data for a number of faces of Ag and Au are available and constitute the basis for some correlations. In particular, Trasatti and Doubova32 have shown that a common correlation exists (Fig. 25) between BEas0/BTmd AX for single-crystal faces of Ag and Au in the sense that BEg /BT becomes less positive as AX increases. As a limiting case, a negative temperature coefficient has been found393 for Ag(110), which exhibits the highest AX. 

Pyrethroids, such as p,p -DDT, are toxic because they interact with Na+ channels of the axonal membrane, thereby disturbing the transmission of nerve action potential (Eldefrawi and Eldefrawi 1990, and Chapter 5, Section 5.2.4 of this book). In both cases, marked hydrophobicity leads to bioconcentration of the insecticides in the axonal membrane and reversible association with the Na+ channel. Consequently, both DDT and pyrethroids show negative temperature coefficients in arthropods increasing temperature brings decreasing toxicity because it favors desorption of insecticide from the site of action. 

The kinetics of deuterium isotope exchange between diphenyl phosphine and t-butylthiol have been studied by H n.m.r. spectroscopy.274 A negative temperature coefficient was observed for the reaction of a perf1uoroalky1 phosphite with a fluorinated aldehyde.275 The kinetics for the reaction of alcohols with phosphoryl trichloride bore strong similarities to those of carboxylic acid derivatives.276 An interesting report desribed the solvolysis of ary 1 hydroxymethyl-phosphonates. It was shown that a phosphoryl group does not prevent carbocation formation on an immediately adjacent carbon atom.277... 

Adverse effects of fenvalerate on survival of terrestrial arthropods were observed at 0.002 to 0.015 pg whole-body topical application, O.llkg/ha aerial application, 5.4 mg/kg in the soil, 50 mg/kg in the diet, and 1.4 g/ant mound (Table 20.4). Synthetic pyrethroids are more effective in biological systems at low temperatures. The relative sensitivity of insects when compared with mammals is attributed in part to this negative temperature coefficient. Thus, warm-blooded animals are less affected than insects and other poikilotherms (Klaassen etal. 1986). Fenvalerate, for example, showed a negative correlation between temperature and toxicity to crickets (Acheta pennsylvanicus), being up to 1.9 times more toxic at 15°C than at 32°C (Harris etal. 1981). A similar case is made for honey bees (Apis mellifera) (Mayer et al. 1987) and for many species of aquatic invertebrates and fish (Mayer 1987). 

Barium titanate can be made into an w-type semiconductor by heating in a vacuum. Such materials do not exhibit the PTC effect. [Materials heated in vacuum show negative temperature coefficient (NTC) thermistor behavior see Chapter 8]. Samples for use in PCT thermistors are heated in oxygen to prevent any chance of reduction. 

How do negative temperature coefficient (NTC) thermistors respond to temperature changes ... 

Negative temperature coefficient thermistors are made from impurity-doped transition-metal oxides. Donor doping, for example, Fe203 doped with Ti02, produces n-type thermistors. The favored mechanism is the formation of electrons ... 

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