Direct current arc

Direct-current arcs into which no material is introduced have many appHcations as heat sources. Industrial processing of metals using plasma torches has been carried out in the former USSR (126). Thermal plasmas also are used in surface and heat treatment of materials (127,128). Metals can be... 

Direct Current Arc It is considered to be one of the most versatile excitation modes used extensively for quantitative spectrochemical emission analysis. Figure 24.2 represents the different essential components of the circuit for a direct current are... 

It is worthwhile noting that the arc-gap temperature in this case is considerably lower than the direct-current arc, due to the stop-and start nature of the source, which ultimately offers a much lower sensitivity. 

In addition to fullerenes, many other carbon structures formed of six-, five- and seven-membered rings (predominantly six) are likely to be discovered. Special mention must be made of carbon nanotubes formed in the direct-current arc evaporation of graphite (lijima, 1991). The nanotubes are essentially made up of graphite sheets and have an inner core of around 1 nm with a variable number of graphite sheaths (Fig. 

Apparatus. Plasma Jet. A section of the plasma arc gun is shown in Figure 1. A direct current arc was struck between the thoriated tungsten rod, which acted as the cathode, and the copper tube anode. Both electrodes were... 

In CG-direct-current plasma (DCP), a direct-current arc is struck between two electrodes as an inert gas sweeps between the electrodes carrying the sample. Carrier gases such as helium, argon, and nitrogen have been used. 

Carbon sulfide telluride was prepared by allowing a direct-current arc to burn between a graphite and a graphite/tellurium electrode. The electrodes were immersed into carbon disulfide kept at 0°3-5. 

Alternating or direct current arcs and spark discharge are common methods of excitation for emission spectroscopic analysis of rare earth elements. Emission spectra of rare earth elements contain a large number of lines. The three arbitrary groups are (i) spectra of La, Eu, Yb, Lu and Y, (ii) more complicated spectra of Sm, Gd and Tm, (iii) even more complicated spectra of Ce, Nd, Pr, Tb, Dy and Er. Rare earths have been analyzed with spectrographs of high resolution and dispersion up to 2 A/mm. Some salient information is presented in Table 1.36. 

Emission spectrography includes an excitation source (in this instance a direct-current arc), an optical unit using a dispersion system to provide monochromatic images of the input slit on its focal surface, and a detection system (in this instance, a photographic emulsion). 

Figure 3.2 Direct-current arc (after G.L. Moore, Introduction to Inductively Coupled Plasma. Atomic Emission Spectrometry, p. 35. C 1989, with the permission of Elsevier Science, Amsterdam).

Spitsberg A. T. and Otis J. S. (2000) Difficulties with direct-current/arc optical emission spectrometric analysis of molybdenum powder for chromium, iron and nickel, Appl Spectrosc 11 1707-1711. 

The elements, arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver, are listed as contaminants for characteristics of toxicity by the EPA [2J. Chromium, mercury, and selenium were not detected in the phosphogypsum. The detection limits for direct-current arc emission spectrograf ic analysis are 0.001% for chromium, 0.05% for mercury, and 0.10% for selenium. Barium, cadmium, lead, and silver were detected at concentrations far less than allowable by EPA requirements, even assuming that 100% of these elements would be... 

S. W. Reeve, W. A. Weimer, and D. S. Dandy, Diamond growth using remote methane injection in a direct current arc jet chemical vapor deposition reactor, Appl Phys. Lett., 63(18) 2487-2489 (1993)... 

The decomposition is accomplished using electrically heated filaments, microwave plasma discharge, or direct-current arc discharge. Polycrystalline diamond is deposited as a thin, hard film. 

Golightly DW, Dorrzape AF Jr, Mays RE, Fries TL and Conklin NM (1987) Analysis of geological materials by direct-current arc emission spectrogra-phy and spectrometry. In Baedecker PA ed. Methods for Geochemical Analysis, chapter A. US Geol Surv Bull 1770. 

A number of electrical excitation-sources are available for emission spectroscopy. In most commercial spectrochemical instruments, more than one excitation source is contained in a single power-supply cabinet a typical combination may include a spark, a direct-current arc, and an alternating-current arc. A list of the various electrical excitation-sources, some of their characteristics, their approximate cost and the types of samples generally required is given in Table 11.1. Because of the actual or potential widespread use in emission spectroscopy, only the arc, spark, and inductively coupled plasma discharges will be described here in detail. 

Figure 7.7 (a) Diagram of a direct current arc source. The solid sample is packed into the cupped... 

The first design of a plasma torch was given in 1957 by Gage who used a direct current arc struck between a cathode rod and a nozzle anode. Forced gas flow extended the arc in the anode nozzle which was strongly cooled. A thermal arc pinch effect was produced by the joint action of the cold wall arc channel and the cold gas sheet around a very high temperature conducting core (the arc coliunn). 

Direct current plasma (DCP). A spectroscopic source in which plasma is maintained by an electric field (a direct current arc between three electrodes). 

Direct current arc excitation is preferred for qualitative analysis since it is simple to use and is more sensitive than flame or spark excitation. It is common practice to use 1-5 mg of sample placed in a cup electrode that serves as the lower electrode and is the anode (plus electrode). The counter electrode is mounted immediately above the anode and a dc voltage of 200-250 V is applied. A current of 10-15 A is desirable. Electrode spacing should be maintained constant at about 3-5 mm. Current and voltage also should be maintained constant. 

FIGURE 8-3. Effect of potassium on the spectral line density of phosphorus. [From W. G. Schrenk and H. E. Clements, Influence of Certain Elements on Line Intensity in the Direct Current Arc, Anal. Chem., 23, 1467 (1951). Used by permission of the American Chemical Society.]... 

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