The effect of the plasma composition on characteristics of the d.c. arc—IV

Spectrochimica Acta, vol. 26B,pp. 123to 126. PergamonPress19’71.Printedin NorthernIreland

The effect of the plasma composition on characteristics of the d.c. arc-IV Influence of lithium on an arc in nitrogen B. PAVLO~I~,

N. IKONOMOV,

V.

VUKANOVI~

and M. TODOROVI~

Faculty of Technology and Metallurgy, Faculty of Sciences, Institute of Chemistry, Technology and Metallurgy, Beograd, Yugoslavia (Received

28 October 1969)

Abstract-The radial temperature distribution of the arc burning in nitrogen was measured when a large quantity of LisCO, evaporates in the arc. In comparisonto the radial temperature distribution of the arc burning in nitrogen alone, a smaller temperature gradient was obtained. This can be interpretedby taking the variation of the electrondensity into account when solving the ELENBAAS-HELLER equation for the plasma energy balance.

known that the addition of elements with low ionization energy to the arc plasma increases its electron density and at the same time decreases its temperature. The addition of these elements makes the radial temperature distribution more uniform [e.g. l-31 as it must be in order to influence the mass transport processes in the plasma. In this paper, we ask if it is possible to interpret theoretically the more uniform radial temperature distribution in the arc if elements with low ionization energy are added. To answer this question we will consider the energy balance of the plasma and solve the ELENBAAS-HELLER equation [4, 51. We observed the freely IT IS WELL

burning arc in a nitrogen atmosphere, taking into account only the changed electrical conductivity

of the plasma. EXPERIMENTAL

To increase the electron density in the arc plasma we added L&$0, to the sample made of carbon powder mixed with traces of MgO and ZnO. The conditions of the experiment and the evaluation of the radial temperature distribution in the arc plasma were described in an earlier paper [a]. The experimental curves for the radial temperature distribution in a freeburning arc in nitrogen with and without lithium are given in Fig. 1. The upper curve 1 is for pure nitrogen, the lower one for nitrogen with lithium added. The addition of lithium decreases the temperature in the arc axis by about 900°K. For nitrogen alone, the radial temperature gradient ATIAr is 220”K/mm. This value drops to 166”K/mm when the same arc burns with increased electron density due to the addition of lithium. The measured radial distribution of the electron density in the arc burning in pure nitrogen atmosphere and nitrogen with added lithium is shown in Fig. 2. [l] V. VUKANOVI~, Emissionsspektroskopie.

Akademie-Verlag, Berlin (1964). [2] D. VUEANOVI~,Radovi Zavoda za$ziku, Beograd 5, 5 (1966). [3] I. A. KRINBERG, Zh. Prikl. Spektroskopii 4, 272 (1966). [4] V. VUEANOVI~, N. IKONOMOV and B.PAVLOVI~, Spectrochim.Acta 26B, 95 (1971). [5] F.BURHORN and R. WIENEC~E, 2. Phys.Chem.215, 269 (1960). 123

124

B. PAVLOVIC, N. IKONOMOV, V. VUKANOVI~

and M. TODOROVI~

AT/Ar

= 220°K /mm

5 6000 i

I 1 AT/Ar=

5000

I

I

.I.0

0.5

I

I

I. 5

2.0 r.

Fig.

I

I

2.5

167

I

I

1

3.5

3-o

“K/mm

4.5

4.0

mm

1. Experimental curves for the radial temperature distribution. 1. Arc burning in nitrogen; 2. Arc burning in nitrogen with lithium added.

N,+ x+x,x

Li

-x-x-x_

-x-x-XIX

Lx\*_x

I

0

I

I

I

O-I

0.2

0.3

L

r, Fig.

2.

Experimental

Nz

-x,x,

mm

curves for the radial distribution

of the electron density.

The effect of the plasma composition on cha~eteristics

of the d.c. arc-IV

‘, 250,

0

-0 x

2co

-

b

IOO-

I I

0

/ 2

3

4

Fig. 3. Electrical conductivity

I 0.02

I

I

006

0.10

I

0.14

I

0.18

I

0.22

I 6

5

as a function of temperature.

I

I

0.26

0 30

I

0.34

I

0.38

I

0.42

I

I

I

0.46

0.50

0.54

P

Fig. 4. Radial temperature distributions. Curves 1, 2 and 3 correspond with the curves in Fig. 3. Curve 4 is the theoretical radial temperature distribution in an arc in nitrogen alone.

125

126

B. PAVLOVI~,N. IKONOMOV, V. VUKANOVI~and M. TODOROVIC! THEORETICAL CONSIDERATIONAND DISCUSSION

To show the effect of a change in the electrical conductivity on the radial temperature distribution of the arc, we solved the plasma energy balance as described in paper [4]. We assumed that the thermal conductivity was not considerably changed by the addition of lithium. As 5200°K was the lowest experimental temperature, the electron density n, had to be approximated for temperatures below 5200°K. We proceeded as follows. First, at 5200°K the total degree of ionization can be determined when lithium is present. The ratio between the number of lithium particles and the total number of particles present in the arc plasma can be estimated. It can be assumed that this ratio remains constant in the arc zones down to 1000°K. Accordingly, we calculated the electron density as a function of temperature and from this the electrical conductivity (Fig. 3, curve 1). As the electron density thus computed is obviously too high, we made another assumption, i.e. that the electron density below 5200°K equals the electron density in pure nitrogen. Data for n, in nitrogen were taken from [4]. The resulting electrical conductivities are shown in Fig. 3. The real values of the electrical conductivities in nitrogen with lithium added lie in between curves 1 and 3 and can be assumed to be represented by curve 2. The radial temperature distributions represented by curves 1, 2 and 3 in Fig. 4, correspond to the electrical conduct.ivity curves 1, 2 and 3 in Fig. 3. Curve 4 represents the theoretical radial temperature distribution for pure nitrogen for the same temperature in the arc axis, i.e. 5725% (cf. Fig. 1). We can notice that the theoretical curves, obtained by solving the ELENBAASWELLER equation and represented in Fig. 4, show that the addition of elements with low ionization energy contributes to a more uniform temperature distribution in the central zone of the arc plasma. Such a conclusion follows from our experimental measurements given in Fig. 1, too.