Primary tungsten sources

Primary Tungsten Sources. Marketable ore concentrates of both types usually contain between 65 and 75% WO3. This guarantees a low percentage of foreign material in contrast to low-grade concentrates, which are in use in fully integrated companies (from mine to APT). Those contain typically between 6 and 40% WO3 and a correspondingly... 

The electron sources used in most sems are thermionic sources in which electrons are emitted from very hot filaments made of either tungsten (W) or lanthanum boride (LaB ). W sources are typically heated to ca 2500—3000 K in order to achieve an adequate electron brightness. LaB sources require lower temperatures to achieve the same brightness, although they need a better vacuum than W sources. Once created, these primary electrons are accelerated to some desired energy with an energy spread (which ultimately determines lateral resolution) on the order of ca 1.5 eV. 

As a specific illustration reference may be drawn to molybdenum reserve scenario in the United States. The reserves are mainly grouped under five categories (i) primary, (ii) byproduct of copper ores, (iii) co-product of copper-molybdenum ores, (iv) by-product of tungsten ores, and (v) by-product of uranium ores. These have been presented and briefly elaborated in Table 1.14. It may finally be recorded by way of summary that the present day molybdenum sources in the world today seem to be principally of two main kinds first, the large-tonnage, low-grade, disseminated type of deposit in which molybdenite is the principal economic mineral second, the deposits in which molybdenite occurs as a by-product in... 

As primary sources, continuous sources such as a tungsten halogenide or a deuterium lamp can be used. They have the advantage that multielement determinations are possible. However, because of the low radiant densities saturation is not obtained and the power of detection is not fully exploited. With line sources such as hollow cathode sources and electrodeless discharge lamps much higher radiances can be obtained. Even ICPs into which a concentrated solution is introduced can be used as a primary source, through which multielement determinations become possible. 

I. Raw Materials [5.1,5.2,5.6]. There exist three different types of tungstenbearing materials which are currently in use as raw materials for APT production scheelite ore concentrates and wolframite ore concentrates are the primary sources, and various tungsten-containing scrap materials act as secondary sources. 

For the primary energy range 5-10 keV used in conventional AES, the electron emitter is thermionic, usually a hot tungsten filament, and focusing of the electron beam is carried out electrostatically. Typically, such an electron source would be able to provide a spot size on the specimen of about 0.5 pm at 10 keV and a beam current of about 10 A. The beam can normally be rastered over the specimen surface, but such a source would not be regarded as adequate for SAM. Sources for SAM may be of either the thermionic or the field emission type, the latter being partic-... 

Other sources are incandescent electric lamps (tungsten), the Welsbach mantle, hot glass, and the quartz-jacketed high-pressure mercury arc. The tungsten ribbon filament lamp makes a good secondary standard (compared to a primary blackbody) in the near... 

In a dispersive instrument, the specimen is the target for bombardment by high energy X-rays from the source, generally an X-ray tube containing a target such as tungsten, onto which electrons are accelerated by a 50 kV potential difference. These primary X-rays excite the specimen to produce X-rays... 

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