Powder preparation superconducting

After fabrication, the samples are tested for the superconductive transition temperature and critical current density by conventional methods. X-ray diffraction is the technique most consistently used to roughly determine the quality of the powders prepared. In general, the XRD analysis must be free of second phases (in particular, unreacted BaCO ) to work well. A program to examine these materials by electron microscopy is being pursued. Initial results show considerable unreacted material... 

Pohtis, C., Amorphous superconducting NbaGe and NbsGei- Ak powders prepared by mechanical alloying, Physica, 135B, 286-289, 1985. 

Yttrium—barium—copper oxide, YBa2Cu202 is a newly developed high T material which has been found to be fully superconductive at temperatures above 90 K, a temperature that can be maintained during practical operation. The foremost challenge is to be able to fabricate these materials into a flexible form to prepare wines, fibers, and bulk shapes. Ultrapure powders of yttrium—barium—copper oxide that are sinterable into single-phase superconducting... 

Immediately after the isolation of macroscopic quantities of Cgo solid [298], highly conducting [299] and superconducting [141] behaviors were verified for the K-doped compounds prepared by a vapor-solid reaction (Haddon, Hebard, et al.). Crystallographic study based on the powder X-ray diffraction profile revealed that the composition of the superconducting phase is KsCeo and the diffraction pattern can be indexed to be a face-centered cubic (fee) structure with a three-dimensional electronic pathway [300]. The lattice parameter (a = 14.24 A) is apparently expanded relative to the undoped cubic Ceo = 14.17 A). The superconductivity has been observed for many A3C60 (A alkali metal), e.g., RbsCeo (Tc = 29 K... 

Superconducting films of C60 compounds with alkali metals (M3C6o) were first prepared at AT T Bell Laboratories by means of this vapor diffusion method (Haddon and others 1991). Meanwhile, pure K3C60 can be prepared simply by mixing powdered potassium metal and C6o in toluene (Wang et al. 1991). 

This means that the interaction occurs via the mechanism completely excluding the formation of non-superconductive green phase Y2BaCuO,. Using the powder thus synthesized, one succeeded in preparing the ceramics with the density approaching the theoretical one (6.15 g/cm ). 

K. They combined three experiments on two different morphologies (microwave absorption and magnetic susceptibility x of powders, and dc resistivity of films) to assert superconductivity imder unusually difficult, poorly controlled chemical conchtions. As the powder is prepared by a solid-state reaction, only the initial composition is known, so no claim could be made about the... 

Powder samples of were prepared as described previously 3, 5, 8). Briefly, the appropriate amounts of Cgo powder and K were sealed in pyrex tubes in vacuo. The tubes were heated at 225°C for 3 to 5 days to form the potassium fulleride compounds. The samples were then resealed in s20 torr HC(g) and annealed at temperatures between 250° and 300°C for periods ranging from 10 to 25 days. NMR measurements are reported for four samples, with nominal stoichiometries Kj sC o (sample I), Kj 0 60 (sample II), Kj.iQo (sample III), and K60C60 (sample IV). Superconducting fractions, as determined by magnetic flux expulsion measurements, were 16, 33, 32, and 0% for samples I to IV, respectively. In all cases, a single superconductivity transition at 19.0 0.5 K was observed. NMR spectra were obtained at a fiequency of 100.5 MHz (9.39-T field). Frequencies are reported in parts per million with respect to tetrameth-ylsilane. The total mass of carbon that contributes to the NMR spectrum was determined for each sample by comparing... 

In 1979, White [3.2] observed that, by milling elemental Nb and Sn powders, the distinct X-ray diffraction peaks of the elements disappeared and typical diffuse peaks of an amorphous pattern showed up. But these samples did not show the superconducting transition temperature of vapor-quenched amorphous Nb-Sn alloys. In 1983, Koch et al. reported on the Preparation of amorphous Ni60Nb40 by mechanical alloying [3.3]. After the detection of amorphization by solid-state reaction in evaporated multilayer films by Schwarz and Johnson [3.4] (see also Chap. 2), Schwarz et al. [3.5] proposed after investigating glass formation in Ni-Ti alloys, that amorphization by mechanical alloying is also based on the solid-state reaction process. Within the last couple... 

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