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Plasma of Free Burning Electric Arc between Composition Electrodes in Air

A.N.Veklich1, I.L.Babich1, V.Ye.Osidach1, L.A.Kryachko2, R.V.Minakova2
1Taras Shevchenko Kyiv National University, 64, Volodymyrs`ka Str., Kyiv 01033, Ukraine,
E-mail: van@univ.kiev.ua
2Institute of Materials Technology Problems NAS of Ukraine, 3, Krizhanivs`ky Str., Kyiv 03142, Ukraine
E-mail: 29min@ipms.kiev.ua
Abstract. The radial profiles of the temperature and electron density in the plasma of the electric arc between electrodes
from composition materials on the base of silver (Ag-Ni) and copper (Cu-Mo) were studied by optical spectroscopy
techniques. The electron density and the temperature in plasma as initial parameters were used in the calculation of the
plasma composition in local thermodynamic equilibrium (LTE) assumption. So, it is possible to determine the amounts of
metal vapors in plasma. The structural changes in a working layer of electrodes are investigated by metallographic
techniques.
Keywords: Electric Arc, Plasma, Metal Vapors, Local Thermodynamical Equilibrium, Electrode Working Layer.
PACS: 52.80.Mg; 52.70.Kz
1. INTRODUCTION
A wide application of composition materials in the electrical device industry inspired a renewed interest in the
investigation of plasma of an electric arc discharge between such material electrodes. A study of arc plasma
parameters will be able to facilitate the optimization of various manufacturing equipments.
Electrical and thermophysical properties of composition materials on silver or copper base predetermine the wide
popularity of their application, especially, for contacts of switching devices. Processes occurred in free burning
electric arc discharges can be serve as model of processes between contacts of switching devices.
2. EXPERIMENTAL SETUP
The free burning arc was ignited in air between the end surfaces of the non-cooled electrodes. The diameter of
the rod electrodes was of 6 mm. The arc discharge gap lak was of 2, 4, 6 or 8 mm.
To avoid the metal a droplet appearing a pulsing mode was used: the current pulse up to 30 A was put on the
“duty” weak-current (3.5 A) discharge. The pulse duration ranged up to 30 ms.
The quasi-steady mode was investigated.
Because of the discharge spatial and temporal instability the method of the single tomographic recording of the
spectral line emission was used. A 3000-pixel CCD linear image sensor (B/W) Sony ILX526A accomplished fast
scanning of spatial distributions of radiation intensity.
In a combination with a monochromator it allows recording the radial distribution of nonstationary arc radiation
intensity in the average cross section of the discharge simultaneously.
In a combination with a Fabry-Perot interferometer the spectrometer provides simultaneous registration of spatial
and spectral distribution of radiation intensities. Thus, the spectrometer allows measuring contours of spectral lines
in different spatial points of plasma volume.
The control of the CCD linear image sensor was realized by the IBM personal computer [1]. The hardware and
software was especially designed for laboratory and industry plasma research.
3. RESULTS AND DISCUSSIONS
The temperature profiles T(r) in electric arc between composition electrodes are obtained from relative intensities
of spectral lines. The radial profiles of electron densities Ne(r) are obtained from width of spectral line in a case of
prevail Stark broadening. On the base of experimentally obtained temperature profiles and radial profiles of
intensities of various atom spectral lines the ratio of these atom concentrations in plasma were calculated in the
assumption of the equilibrium of the energy level population.
The obtained electron density and the temperature in plasma as initial parameters were used in the calculation of
the plasma composition in LTE assumption. We used the Saha’s equation for copper/silver, nitrogen, oxygen and
molybdenum/nickel atoms, dissociation equation for nitrogen and oxygen molecules, the equation of plasma
electrical neutrality and Dalton’s law as well. As an additional experimentally obtained parameter we used the ratio
of atom concentrations.
3.1 Investigation Of The Electric Arc Discharge Between Ag-Ni Electrodes In Air
We used composition materials Ag-Ni (90/10) as well as Ag-Ni (70/30). The temperature profiles T(r) in electric
arc between Ag-Ni electrodes are obtained from relative intensities of spectral lines AgI 520.9, 768.7, 827.3 nm. The
radial profiles of electron densities Ne(r) are obtained from width of spectral line AgI 466.8 nm [2]. The ratio of
atom concentrations Ag and Ni in plasma was calculated from radial profiles of intensities of spectral lines AgI
520.9 and NiI 551.0 or 500.0 nm.

plasma of the discharge gap. The content of silver is predominated in plasma of the electric arc by the order of
magnitude in all investigated modes of electric arc.
The structural changes in a working layer of electrode take place during the electric arc discharge. These changes
were studied metallographicly by analysis of microvolumes of a working layer.
The microstructure analysis reveals more significant silver evaporation in comparison with nickel due to the
strong ejection of such kind vapors. The initial size of nickel particles is about of 1 μm. The heating of such smallgrained
particles leads to their agglomeration because of thermal conductivity decreasing in the condition of silver
evaporation. Such lengthy conglomerations are convex upwards at the electrode surface and the arc discharge is
attached to these areas. But as soon as the amount of such formations at the electrode surface is negligible therefore
the silver evaporation must be predominated into the plasma.
Furthermore as the pressure of saturated vapors of silver above the electrode surface some orders exceeds the
pressure of nickel vapors the predominance of the silver atom concentrations at the discharge axis is natural.
3.2 Investigation Of The Electric Arc Discharge Between Cu-Mo Electrodes In Air
We used composition materials Cu-Mo (50/50). The temperature profiles T(r) in electric arc between Cu-Mo
electrodes were obtained from relative intensities of spectral lines CuI 521.8 and 510.5 nm. The radial profiles of
electron densities Ne(r) are obtained from width of spectral line CuI 515.3 nm. The ratio of atom concentrations Cu
and Mo in plasma was calculated from radial profiles of intensities of spectral lines CuI 521.8 and MoI 603.1. In
figures 5, 6 and 7 radial profiles T(r), Ne(r) and NCu/NMo in the average cross section of the discharge are shown.

In figure 8 equilibrium plasma composition is shown. It is possible to calculate the content of copper and
molybdenum in plasma of the discharge gap. In figure 9 contents of copper and molybdenum in plasma are shown. It
is visible the content of copper is predominated by two orders of magnitude in plasma of the electric arc. The
increasing of the content of metal vapors at the discharge periphery can be possible explained by the phenomenon of
the ambipolar diffusion (demixing). The additionally confirmation of such assumption is the spatial separation of
different plasma components according to their ionization energies [3, 4].
Furthermore as the pressure of saturated vapors of copper above the electrode surface some orders exceeds the
pressure of molybdenum vapors the predominance of the copper atom concentrations in plasma is natural.
It is interesting to note that the content of copper in plasma of the electric arc between copper electrodes is about
0.6 % at the discharge axis [5]. In this case of Cu-Mo electrodes the content of copper prevails by the order of
magnitude.
In paper [6] was found the working layer at Cu-Mo electrode surface is formed by the influence of the electric
arc discharge. The structural changes in a working layer of electrode were studied metallographicly by analysis of
microvolumes of a working layer too. Such secondary modified surface of electrode has a capillary structure.
The low thermal conductivity of such modified surface of Cu-Mo electrodes in comparison with singlecomponent
Cu electrodes results in copper overheating and its rejection into the discharge gap. Such erosion
mechanism can be makes for decreasing of plasma temperature of arc discharge caused by the intensive copper
rejection.

4. CONCLUSIONS
The radial profiles of temperature and electron densities in the plasma of electric arc discharge between
composition Ag-Ni and Cu-Mo electrodes were measured by optical spectroscopy techniques. On the base of this
initial data the calculation of equilibrium composition of a plasma mixture was carried out. The content of metal
vapors is defined.
The structural changes in a working layer of electrode were studied metallographicly by analysis of microvolumes
of a working layer. The processes occurred in the discharge gap are determined by erosion of the electrode
material and condition of its surface.
The obtained results allow to make some conclusions concerning of processes of the electrode material erosion in
arc. The spectroscopy techniques yield results, which are not at variance with metallographic technique results.
The carried out investigation found that secondary structures of working layers of different kind composition
materials are essentially differed. So, the surface of working layer on Ag-Ni electrodes has lengthy conglomerations
which cause the thermal conductivity decreasing of electrodes. It leads to more significant silver evaporation. In a
case of Cu-Mo electrodes the working layer has a capillary structure which results in essential copper evaporation
into discharge gap.
To clarify the mass transfer processes in the arc discharge gap between different kind of composition electrodes
the complex spectroscopy investigations as well as metallographic studies must be carried out.
The further complex studies of composition materials will allow to optimize the contact and electrode content.
REFERENCES
1. A. N. Veklich, V. Ye. Osidach, Problems of Atomic Science and Technology. Series: Plasma Physics (11) 2, 229-231 (2005).
2. M. S. Dimitrijevic, S. Sahal-Brechot, Atomic Data and Nuclear Data Tables, 2002, (submitted).
3. V. L. Granovsky, Electric Current in Gas. Steady State Current, Moscow: Nauka, 1971, pp. 450-459 (in Russian).
4. P. Andanson, B. Cheminat, Rev. Phys. Appl. 14, 775-782 (1979).
5. I. L. Babich, A. N. Veklich, V.A. Zhovtyansky, Ukrainian Journal of Physics 44, 954-959 (1999).
6. A. N. Veklich, I. L. Babich, L. A. Kryachko, R. V. Minakova, V. Ye. Osidach Electric Contacts and Electrodes, Kiev: IPM
NANU, 2004, pp.105-116 (in Russian).

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