Maximum transition temperature

Notch sensitivity Effect of low temperature Maximum transition temperature ( O... 

While in this system the lattice gas model is believed to be a good approximation of reality, the fact that the maximum transition temperature occurs for 0.48 instead of 0 = 1/2 shows that a model with strictly pairwise interaction is not adequate. Calculations with a reasonable value for the strength of the trio-interaction in Fig. 1 (/ , = (pt/experimental phase diagram at temperatures near the maximum transition temperature, but fail to reproduce the apparent widening of the ordered phase regime at lower temperatures. 

Carlo simulation (Fig. 18), since this case seemed to have qualitative similarity with the experiment (Fig. IS) the maximum transition temperature of the (2 X 1) phase and the (3 x 1) phase with 0 % 2/3 are comparable, while the transition temperature of the (3 x 1) phase with 0 1/3 is much smaller (due to the strong trio-interaction) the initial interpretation was that in the real system (Fig. IS) this phase was not seen at all because at low temperatures (T 140 K) the chemisorbed layer can no longer be equilibrated, since surface diffusion is frozen out. 

Gloom for Oxide Superconductors Dismayed at the progress through the years, even with the most promising room-temperature metallic, binary oxides, many scientists abandoned the search for new high temperature oxide superconductors. Also, it should be mentioned that a deep-rooted prejudice had developed which claimed that the BCS theory had imposed a maximum transition temperature limit of 25 K for all superconducting materials, and that this temperature had already been achieved in certain alloys of niobium. Some scientists, however, were steadfast in their determination to break this barrier, optimistic in their outlook, and they continued their search for this unusual phenomenon in other metallic oxide systems. 

The maximum transition temperature of -0.45 K was observed on a Nb-doped phase containing approximately 102Ocm s... 

A series of three solid solutions La2 x(Ba,Sr,Ca)xCu04 g was then prepared (152) the superconducting transition temperatures of the resulting products were determined from a.c. susceptibility measurements. The three-component phase diagram for this chemical system was plotted as a function of Tc. The maximum transition temperature of 37.0 K was observed for the composition, La18Sr0 2-Cu04 g, an end-member in the phase diagram. 

Tc,max is the maximum transition temperature in family of materials, c is known to be proportional to the condensate density. The universal trends found are summarised in Figure 8 and compared with our prediction for the doping sequence given in Figure 3. The density of condensed electrons continues to increase beyond Tc>max in the same way as found experimentally. In particular the experimental data and our prediction both have a highest value for [Pg.300]

Meissner-efTect and microwave-absorption measurements on bulk samples show that Rb ,C6o is superconducting with a maximum transition temperature of 28 K. This is a 10-K (60%) increase over the K-doped material. Only Bao.6Ko.4Bi03 and the cuprate superconductors have higher transition temperatures. 

Fig. 32. (a) Phase diagram of the square lattice gas with nearest neighbor repulsion /nn > 0 and next-nearest neighbor attraction 7I)m, < 0, in the plane of variables temperature and coverage, for three choices of R = 7nnn/7nn. Insert shows the variation of the maximum transition temperature (at 9 = 1 /2) and of the tricritical temperature Tt with R. From Binder and Landau (1981). 

Summing up what has been described above, the maximum transition temperatures observed for the additive elements Y and La occur at a valence electron per atom of 3.7, and for Ho, Er, Lu and Sc this is 3.9. Cooper et al. (1970) have demonstrated that a new peak of the superconducting transition temperatures exists at the general valence electron per atom concentration of 3.7 to 3.9 for a number of crystal structures of which the PU2C3 structure is one. 

At 200 kbar, TdP) saturates. The maximum transition temperature is 13 K, several degrees higher than for all other elemental supereonduetors. Zarubina (1973) reported a saturation of TdP) at approximately 11 K. The origin of the diserepaney is not elear at present. 

Subsequently, various cuprates with cations of 6s electron configuration (Tl, Bi, Pb, Hg) were discovered with steadily increasing transition temperatures, but still considerably distant from room temperature. A maximum transition temperature of Tc = 138 K under ambient conditions was identified for the compound Hgag Tlo.2Ba2Ca2Cu3O8.33 (Dai et al, 1995), and the pure Hg cuprate of this structure type shows under high pressure even Tc = 164 K (Gao et al, 1994). The important perovskite-related superconducting compounds, together with their transition temperatures and references to their discoverers, are summarized in Table 9.1. 

The dynamic mechanical properties of VDC—VC copolymers have been studied in detail. The incorporation of VC units in the polymer results in a drop in dynamic modulus because of the reduction in crystallinity. However, the glass-transition temperature is raised therefore, the softening effect observed at room temperature is accompanied by increased brittleness at lower temperatures. These copolymers are normally plasticized in order to avoid this. Small amounts of plasticizer (2—10 wt %) depress T significantly without loss of strength at room temperature. At higher levels of VC, the T of the copolymer is above room temperature and the modulus rises again. A minimum in modulus or maximum in softness is usually observed in copolymers in which T is above room temperature. A thermomechanical analysis of VDC—AN (acrylonitrile) and VDC—MMA (methyl methacrylate) copolymer systems shows a minimum in softening point at 79.4 and 68.1 mol % VDC, respectively (86). 

In the case of a crystalline polymer the maximum service temperature will be largely dependent on the crystalline melting point. When the polymer possesses a low degree of crystallinity the glass transition temperature will remain of paramount importance. This is the case with unplasticised PVC and the polycarbonate of bis-phenol A. 

Specific gravity Average thermal expansion > coefficient J Transition point Softening point Maximum service temperature Bending... 

The hiding of the pyroelectric coefficients seems to be correlated to the maximum c parameter, which in turn corresponds to the transition temperature. The shift along the Oz direction, Az, of the niobium atoms, which are located within the octahedrons, is responsible for the compound s polar properties. When c is at its maximum, this shift is enhanced and leads apparently to maximum spontaneous polarization P The value of Ps increases in the temperature range of 300 to 490K and then decreases at temperatures above 490°K. 

---