Solid-phase relationships have been determined for the Bi203-Sr0-Cu0 system at 800°C in air with and without Li2C03 mineralizer. Two ternary compounds were detected with Bi : S r :Cu ratios of approximately 2 : 2 : I and a solid-solution phase I 1 -x:9+x:5 with 0 1~1 0 . 4 . The solid-solution phase is a 9 K superconductor at x =0.4. Two additional compounds with Bi:Sr:Cu ratios of approximately 4:9:1 and 2:7:2 are apparently stabilized by the presence of Li2C0,, but do not form without it. X-ray crystallographic data are provided for each compound. Ramifications for quaternary Bi :Sr :Ca : Cu-0 superconductors are discussed. [
The mechanism for the slow rate of reaction between portland cement and water during the early stage is not well understood, but it probably is controlled by either the rate that the reactants diffuse through a barrier that surrounds the unreacted cement grains or by the rate that nuclei of the stable product form and grow or by both rates. New evidence using environmental scanning electron microscopy is presented about the structure of a layer that forms around the particles of cement. Preliminary observations that relate mixing to the structure of the layer are also presented. [
IntroductionScanning électron microscopes offer several unique advantages and they hâve evolved into complex integrated instruments that often incorporate several important accessories. Their principle advantage stems from the method of constructing an image from a highly focused électron beam that scans across the surface of a spécimen. The beam générâtes backscattered électrons and excites secondary électrons and x-rays in a highly localized "spot." Thèse signais can be detected, and the results of the analysis are displayed as a spécifie intensity on a screen at a position that represents the position of the électron spot. As with a télévision image, after a given period, information about the entire field of view is displayed on the screen, resulting in a complète image. If the spécimen is thin, the same type of information can be gathered from the transmitted électrons, and a scanning transmission image is thus constructed.The scanning électron microscope is highly versatile and widely used. The quality of the image is related to its resolution and contrast, which, in turn, dépend on the diameter of the focused beam as well as its energy and current. Because électron lenses hâve inherently high aberrations, the usable aperture angles are much smaller than in a light microscope and, therefore, the électron beam remains focused over a relatively large distance, giving thèse instruments a very large depth of focus.Scanning électron microscopes are versatile in their ability to detect and analyze a lot of information. As a resuit, modem versions of thèse instruments are equipped with a number of detectors. Developments are sometimes related to placing the detectors in a geometrically attractive position close to the spécimen.
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