Negative Temperature Coefficient Of Electrical Resistivity Research Articles

Martensitic transformation and anomalies in resistivity of (Ti–50Ni)1−xCx (x=0.1, 0.5at.%) shape memory alloys

Abstract Phase transformation of solid solution (Ti–50Ni) 1− x C x ( x = 0.1, 0.5 at.%) alloys have been studied by using differential scanning calorimetry, physical property measurement system and optical microscope. The transformation temperature decreases due to the existence of titanium carbide (TiC) particles compared with that of near-equiatomic Ti–Ni shape memory alloy. The resistivity vs. temperature curves show hysteresis. Thermoelastic martensitic transformation occurred in two alloys despite the difference in TiC content. Nevertheless, the resistivity results show different martensitic transformation routes. A one-step B2 → B19′ transformation occurred in the low TiC content alloy and an R transformation appeared in another alloy, suggesting that the martensitic transformation routes depended on the TiC content. The cumulative effect of the TiC particles causes the local stress field and lattice distortion to restrain the transformation of the B19′. On the other hand, the TiC content has an effect on the temperature coefficient of electrical resistivity (TCR) of alloys. The Ti–Ni–0.5C alloy shows a negative TCR in the range 100–300 K during which transformation occurs. Another alloy shows the opposite result. The cause of the negative TCR is briefly discussed.

Microstructure and temperature coefficient of resistivity for ZnO ceramics doped with Al 2O 3

The ZnO doped with Al 2O 3 showed the minimum electrical resistivity at an Al 2O 3 content of 0.25%. It was found that the ZnO doped with Al 2O 3 exhibited the resistance-temperature characteristic reversion from negative temperature coefficient of electrical resistivity (NTCR) to positive temperature coefficient of electrical resistivity (PTCR) at temperatures of about 450 and 250 °C for ceramics sintered at 1400 and 1300 °C, respectively. Both NTCR and PTCR features were weakened for samples with lower Al 2O 3 content or sintered at a higher temperature. Microstructure studies exposed that the densification process was significantly depressed when the Al 2O 3 addition levels were over about 1%.

Negative Temperature Coefficient of Electrical Resistivity in B2-Type Ti–Ni Alloys

A negative temperature coefficient of electrical resistivity (TCR) has been observed in Ti50-XNi50+X (at.%; X=1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5) in the temperature range between 20 and 350 K. The resistivity vs temperature curve has no hysteresis and the relative resistivity at 20 K (ρ20 K/ρ350 K) decreases with increasing Ni content. X-ray diffraction and magnetic susceptibility measurements show that a negative TCR is not caused by structural phase transition nor by magnetic transition. The Debye temperature of Ti48Ni52, obtained from the analysis of specific heat measurements, is quite low (217 K) and its spin relaxation process has a time duration between 400 µs and 20 ms, suggesting that some lattice instability exists. Based on these results, the origin of a negative TCR is discussed.

Investigation of the PTCR effect in ZnO–NiO two-phase ceramics

A positive temperature coefficient of electrical resistivity (PTCR) in ZnO–NiO ceramics was observed and an attempt to explain its origin was made. According to electrical measurements, supported by dilatometric and microstructural investigations, the percolation principle was used for this purpose. A series of samples with different ZnO ss and NiO ss contents were prepared. The electrical resistivity of the samples was measured at different temperatures. ZnO ss represents a low ohmic and NiO ss represents a high ohmic phase. Monophase samples exhibited a strictly negative temperature coefficient of electrical resistivity (NTCR), while in some two-phase samples PTCR behaviour was observed. Results of electrical, dilatometric measurements and microstructural investigations indicated a possible explanation of PTCR behaviour on the basis of the percolation model.

Origin of negative temperature coefficient of resistivity in polycrystalline Ti-N films

A negative temperature coefficient of electrical resistivity (TCR) has been observed in Ti-N films prepared by reactive evaporation or sputtering. Some samples measured had a resistivity minimum and the minimum temperature changing with varying nitrogen contents. To study the origin of this anomalous temperature dependence of resistivity, the crystallographic structures of Ti-N films were measured by x-ray diffraction. The resistivity data could be analyzed in terms of the existing theory based on the structural Kondo effect. The results demonstrated that the negative TCR originated from higher concentration of interstitial nitrogen atoms in the Ti-Nx solid-solution phase, and nitrogen vacancies and/or intrinsic stacking faults in the fcc TiNx compounds.

Negative-temperature coefficients of electrical resistivity in amorphous La-based alloys

A negative-temperature coefficient ($\ensuremath{\alpha}=\frac{{\ensuremath{\rho}}^{\ensuremath{-}1}d\ensuremath{\rho}}{\mathrm{dT}}$) of electrical resistivity, $\ensuremath{\rho}(T)$, has been observed in many amorphous and disordered metallic conductors. The origin of this anomalous temperature dependence of resistivity is still unclear. To explain the negative-$\ensuremath{\alpha}$ anomaly, there are two theoretical approaches (i.e., the Ziman-type theory and the structural Kondo model) currently discussed in the literature. In an attempt to distinguish between these two approaches the resistivity of several liquid-quenched amorphous La-based alloy systems containing Al, Au, Ga, or Ge, has been analyzed as a function of composition and temperature. It is concluded that the resistivity data are inconsistent with the Ziman-type theory and are in favor of the structural Kondo-type model. This conclusion is based on the fact that (1) resistivity varies as $\ensuremath{-}\mathrm{ln}T$ ($T\ensuremath{\gtrsim}100$ K) in alloys with negative $\ensuremath{\alpha}$, and (2) the occurrence of negative $\ensuremath{\alpha}$ is independent of the valence of the La-based alloys.