Transition temperatures, metal

The molecule BEDT-TTF (bis-ethylenedithia tetrathiofulvalene Figure 18.11) with acceptor molecule Cu(NSC)2, was found to be conducting and even superconducting at a quite high critical temperature (Tc = 10.4 K) under ambient pressure. An insulator-metal transition temperature is observed at 135 K at ambient pressure. 

The transition temperature depends on the - especially interstitial -impurities in the metal. Transition temperatures measured under comparable test-conditions (creep rate, grain size), depending on kind and concentration of the impurity elements, are compiled in Table 1-7. 

The temperature of the metal-to-insulator transition in TTF—TCNQ is 53 K. For systems with increased interchain coupling, the transition temperature for the onset of metallic conduction increases roughly as the square of the interaction between the chains. This behavior is tme as long as the coupling between chains remains relatively weak. For compounds with strong interactions between stacks, the material loses its quasi-ID behavior. Thus, the Peieds distortion does not occur even at low temperatures, and the materials remain conductive. 

Phase transitions are involved in critical temperature thermistors. Vanadium, VO2, and vanadium trioxide [1314-34-7] V2O3, have semiconductors—metal transitions in which the conductivity decreases by several orders of magnitude on cooling. Electronic phase transitions are also observed in superconducting ceramics like YBa2Cu30y but here the conductivity increases sharply on cooling through the phase transition. 

Rhenium hexafluoride is a cosdy (ca 3000/kg) material and is often used as a small percentage composite with tungsten or molybdenum. The addition of rhenium to tungsten metal improves the ductility and high temperature properties of metal films or parts (11). Tungsten—rhenium alloys produced by CVD processes exhibit higher superconducting transition temperatures than those alloys produced by arc-melt processes (12). 

The modification shows that the transition temperature may vary significantly from those stated above and in general shift to higher temperatures as the gas pressure in the synthesis process increases. The transition from one zone to the next is not abmpt, but smooth. Hence, the transition temperatures should not be considered as absolute but as guidelines. Furthermore, not all zones are found in all types of deposit. For example. Zone T (see Fig. 7) is not prominent in pure metals, but becomes more pronounced in complex alloys, compounds, or in deposits produced at higher gas pressures. Zone 3 is not often seen in materials with high melting points. 

After long periods of time at operating temperature, the britde—ductile transition temperature in autoclave steels increases (13). At temperatures much above 200°C for the solutions and fiHs used in ordinary hydrothermal processes, pressures and hence stresses in autoclaves can cause faHure of metal in the brittie state. OrdinarHy, the brittie region is weH below these temperatures but careful monitoring of the brittie—ductile transition of the steel is necessary for safe autoclave use over many years. 

Special Alloys. AHoys of tin with the rater metals, such as niobium, titanium, and 2kconium, have been developed. The single-phase alloy Nb Sn [12035-04-0] has the highest transition temperature of any known superconductor (18 K) and appears to keep its superconductivity in magnetic... 

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. 

When pressure tests are conducted at metal temperatures near the ductile-to-brittle transition temperature of the material, the possibility of brittle fracture shall be considered. 

Adhesive strength is evaluated at room temperature as well as at the extreme temperatures of —65°F and 180°F. Aircraft structure can reach —65°F at cruise altitudes and 180°F on the ground in a hot, sunny location. The types of toughened epoxies commonly used for metal bond adhesives have glass transition temperatures not much greater than 200°F, so properties fall off drastically at higher temperatures. 

A couple of theoretical studies [5,19] have hitherto attempted to estimate the Peierls transition temperature (Tp) for metallic CNT. A detailed theoretical check with respect to the stability of metallic wavefunction in tube (5, 5) has also... 

It will be intriguing to theoretically examine the possibility of superconductivity in CNT prior to the actual experimental assessment. A preliminary estimation of superconducting transition temperature (T ) for metallic CNT has been performed considering the electron-phonon coupling within the framework of the BCS theory [31]. It is important to note that there can generally exist the competition between Peierls- and superconductivity (BCS-type) transitions in lowdimensional materials. However, as has been described in Sec. 2.3, the Peierls transition can probably be suppressed in the metallic tube (a, a) due to small Fermi integrals as a whole [20]. 

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