Transition temperature impurity dependence

The ductile-to-britde transition temperature (DBTT) is dependent on purity, history, grain size, etc. Furthermore, the potential utility of the metal is impaired by the fact that the ductility below this transition is essentially nil. To achieve measurable ductility, impurities should be below O, 2000 ppm N, 100 ppm C, 100 ppm H, 20 ppm Si, 1500 ppm S, 150 ppm. 

This expression leads to a temperature dependence for Q that is distinctly different from that implicit in Equation (5) above. For small values of h, Q decays steeply near the critical temperature and then levels off, creating a characteristic defect tail (Fig. 8). These defect tails are commonly observed in the temperature evolution of properties in impure solids that are proportional to the square of the order parameter, such as birefringence and spontaneous strain. To the extent that the sudden strengthening in the dependence of Q with temperature near Tc diverges from the dependence at lower temperatures, two transition temperatures must be differentiated the critical temperature extrapolated from lower temperatures (r ), and the observed f as measured experimentally ff"). 

Below it is shown that in quasi-one-dimensional conductors the impurities suppress both the dielectric and superconducting transitions. This is related to the fact that the BCS formula for the superconducting transition temperature cannot be applied to quasi-one-dimensional conductors. For the three-dimensional case the transition temperature is determined by the density of electron states, its dependence on the impurity concentration being weak. In contrast, in the quasi-one-dimensional case this temperature depends on the amplitude of the electron pair jump from one thread to another and from the type of the one-dimensional correlation function. 

Kainuma et al., 1994). As already noted with respect to TijAl, the phase equilibria, i.e. equilibrium compositions and transition temperatures, depend in a sensitive way on impurity contents, in particular on oxygen (Kahveci and Welsch, 1986 Huang and Siemers, 1989 Froes etal., 1991 Saunders and Chandrasekaran, 1992). 

The photoconductivity spectrum for a 1000 A film [146] is shown in Figure 20. The salient features are the peaks at 382.0 and 355.0 nm and the absence of photoresponse in the visible. The temperature dependence of the peaks (measured from room temperature to 103°K) indicated an activation energy of 0.02 eV. The minimum in photocurrent at 375 nm is at the same wavelength as the first strong absorption peak in thin films prepared in the same manner [96]. If the photocurrents arise from band-to-band transitions, the photocurrent minimum might then be due to a competing absorption process near 375 nm (3.3 eV), such as a transition involving impurities or the creation of excitons [96]. 

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