Preparation temperature, effect superconductivity

Nanocrystalline materials comprising sub-100 / metal particles, when compressed to 50% of their bulk density, show properties (specific heat, thermal conductivity, saturation magnetization and critical temperature for superconductivity) provocatively different from those of their crystalline or glassy counterparts.(48) It is well known that the interfaces of mechanically reduced composites are effective in interacting with dislocations and with flux lines in superconducting composites. Precursor materials for the preparation of ultrafine filamentary composites can also be imagined. Here the combinations of interphasial boundaries and dislocations can... 

The effect of fast neutron fluence on thermal conductivity and thermopower has been determined by Uher and Huang (70). For fluences to 3 x 1018 n/cm2 Tc decreases in Y-Ba-Cu-O to a temperature of 86 K, the thermal conductivity decreases and is without a peak above Tc and the thermopower starts from a negative value and approaches zero and becomes positive. As will be seen below the more usual value of thermopower is positive in the superconducting material but these authors note the variability dependent on sample preparation conditions. 

Roberts ( 1 1) surveyed the superconductive properties of the elements and recommended a critical temperature of 1.175 0.002 K for Al(cr). Since this temperature is so low, the effects of superconductivity on the thermodynamic functions are not considered. The entropy contribution due to superconductivity will be less than 0.002 J X mol . The data of Giauque and Meads (j ) and Downie and Martin (3) agree at temperatures up to 150 K but drift apart by 0.2 J X mol at 200 X and 0.17 J X mol at 300 K, with the Downie and Martin study being lower. The Takahashi (4, 5) study is even lower at 298 X. The high temperature heat capacity values are derived from the enthalpy study of Ditmars et al. (9). Their curve is intermediate between those derived from previous studies (4, 5, 6, 7, 8) and implies a flatter Cp curve near the melting point (in comparison to previous interpretations). Numerous other heat capacity and enthalpy studies are available but were omitted in this analysis. A detailed discussion of the Group IIIA metals (B, Al, and Ga) is in preparation by the JANAF staff. 

To elucidate the isotope effect on superconductivity, we have studied potassium-doped K3 C(jo and samples were synthesized by heating stoichiometric amounts of solvent-free C o and Cso with K metal (3 1, K Cso) in sealed quartz tubes (10" Torr) typically, 1-2 mg of was used. The temperature was ramp from 200 to 400 C over a 1-week period during the reaction. " Kj Cjo and K3 C4o samples were prepared simultaneously in the same furnace to minimize differences due to the preparative conditions. 

More recently, (6-7) several groups have observed superconductivity in the unsubstituted phase La2CuOA y at temperatures just below the metal to insulator phase transition. However, in this case the Meissner effect measurements were consistent with only a small fraction of the sample being in the superconducting state. While the magnetic and electron transport properties of La2CuOA y had been measured some time ago, (8) superconductivity was not previously observed. Thus, the presence or absence of superconductivity in samples of La2CuOA y is a sensitive function of the method of sample preparation. 

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