An analysis is made of the electrophysical processes occurring at the end surface of rod electrodes during constant and alternating arc discharge in hydrogen. Experiments are reported on the effect of surface temperature of tungsten electrodes on their erosion. The influence of activating additions of thorium oxide, the structure of the tungsten, and the gas surrounding the electrode on the specific thermal loading and the erosion of the electrodes is discussed

Bolotov, A. V.

Yukhimchuk, S. A.

English

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NASA TECHNICAL MEMORANDUM NASA TM-77893
INFLUENCE OF THE TEMPERATURE OF ELECTRODE MATERIAL
ON ITS DISINTEGRATION UNDER THE ACTION
OF AN ARC DISCHARGE IN HYDROGEN
A. V. Bolotov, S. A. Yukhimchuk
Translation of "Vliyaniye temperatury materiala elektroda na
yego razrusheniye pod deystviyem opornykh pyaten dugi v
vodorode," Fiz. i Khim, Obrab. Mater.. No. 2, March-April 1973,
pp. 19-23.
At»ASA-TI3-77893) INFLDItSCE OF THE N86-256S5
TlHPEfiATORE OF ELECTRODE MftlEKJfi.L ON ITS
J3ISINTE.GEATIOK UNDER THE ACTION OF AN AEC
DISCHARGE. IN HYCBGGEN (National Aeronautics Unclas
Space Administration) Up G3/33 43590
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, D.C. 20546 AUGUST 1985
https://ntrs.nasa.gov/search.jsp?R=19860016223 2020-03-20T15:27:08+00:00Z
S T A N D A R D T I T L E P A C E
1. Report No.
NASA TM-77893
2. Co He: 3. R.cipi»nl'i Cololop Nc
*. Till, ond Suo.iti. INFLUENCE OP THE TEMPERATURE' OF
ELECTROD MATERIAL ON ITS DISINTEGRATION UNDER
THE ACTION OF AN ARC DISCHARGE IN HYDROGEN
5. Report Del.
August 1985
6. Pvrlotminp Or$oniiol»on Cod*
7. Author(s)
A. F. Bolotov, S. A. Yukhimchuk
8. Performing Orponiiolion R«por1 No.
10. Work Unit No.
9. Performing Of £oni zotion Nome end Ado"'* 11
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National Aeronautics and Space Administration
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X. Sponsoring Ap*ncy Cod*
is. Suppi.m.niory NOLI Translation of "Vliyaniye temperatury materiala elektroda na
yego razrusheniye pod deystviyem opornykh pyaten dugi v vodorode," Fiz. i Khim.
Obrab. Mater., No. 2, March-April 1973, pp. 19-23.
U. Abiiroei
 An anaiySis is ma(je Of the electrophysical processes occurring at
the end surface of. rod electrodes during constant and alternating arc discharge
Ln hydrogen. Experiments are reported on the effect of surface temperature of
-ungsten electrodes on their erosion. --The influence of activating additions of
-horium oxide, the structure of the tungsten, and the gas surrounding the
lectrode on the specific thermal loading and the erosion of the electrodes is
liscussed.' . . . • • -
ORIGINAL PAGE IS
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INFLUENCE OF THE TEMPERATURE OF ELECTRODE MATERIAL
ON ITS DISINTEGRATION UNDER THE ACTION
OF AN ARC DISCHARGE IN HYDROGEN
A. V. Bolotov, S. A. Yukhimchuk
Study of materials in terms of tungsten erosion in an /12*
arc discharge reveals the lack of systematized information on
the causes of electrode disintegration. Goryachev and Zelenin
[1] feel that a tungsten cathode's stability is profoundly
influenced by its rake of tool and voltage on the arc.
Sergeyev et al. [2] consider that gas flow rate and current
magnitude are the influencing factors. Obviously, this
approach does not identify physical conditions which determine
the magnitude and nature of erosion. The authors of reference
[3] state that the primary factor affecting erosion is specific
heat flux across the arc spot. In their opinion, electrode
wear is caused by vaporization. However, they do not show how
the extent of erosion changes when an electrode's thermal
condition changes or what factors determine minimum
disintegration and maximum current load.
In considering the problem of electrode disintegration, one
must assume an energy balance on the boundary between the solid
body and the current-carrying plasma. Ways to decrease erosion
are best determined by identifying the primary parameters which
define an electrode's working conditions and the extent of its
disintegration. Let us consider the energy balance [4] on the
cathode
Qk + Vi^k + V -<P] " °n
where Q. is total heat flux to the cathode; Q is heatK n
*Numbers in the margin indicate pagination in the foreign text
flux from the cathode; I is the arc current; S. is the
current's ion component; U, is potential cathodic voltage
drop; U. is ionization potential; (p is cathode material work
function, and on the anode,
Qa+ VUr + Ua + <? > Se ' Qn
where Q is total heat flux to the anode; U is the
cl 1C
potential characterizing the amount of heat energy transmitted
to the anode by electrodes; U is am
a
is the current's electron component.
nodic potential drop; S
 e
We feel that the primary reason for disintegration is heat
flux to the electrode, especially its density [4]. Then arc
current, as shown in (1) and (2), has a considerable effect on
Qk and Qa.
The type of electrons emitted from a cathode is /20
related to its surface temperature. Emphasizing the important
role which the temperature of electrode T's working surface
plays in the nature of processes at the point where the arc
closes, we note that T is a function of total heat flux to the
electrode. Changing the tungsten's dimensions, configuration,
and cooling rate may have a significant effect on Q , T ,
and electrode erosion. Thus, it is apparent that the magnitude
of current, the electrode's dimensions and heat flux to it, and
its working surface temperature determine its rate of
disintegration.
The effect of these parameters on erosion was studied on a
setup consisting of two pure or thoriated (VT-15) tungsten rods
cooled on their ends and placed on the same axis with 20-25 mm
between the ends and the arc path. Time for one closure was
about 15 min, erosion magnitude was measured with accuracy to
-410 g. Electrode length varied from 6 to 60 mm. Erosion,
electrode temperature, and heat flux to the electrode were
derived as a function of current load imposed on the rod's
cross section.
The erosion curves of the cathode and AC arc electrode
(figure 1) have typical maxima and minima. Visual inspections
showed that, at current loads at which rod end temperature is
below 1650"C, the arc closure contracted, the area of the
bright spot (defined as the surface emitting electrons) is
small, and therefore, specific heat flux to the electrode is
maximum. Contraction of the arc discharge on the electrode
indicates the possible existence of autoelectron or thermo-
autoelectron emission.
G, g/sec
2.0 4.0 £.0
Figure 1. Tungsten rod erosion as a function of current
density: 1 - AC, L = 60 mm, WTh09; 2 - AC, L = 60
c ^ tr
mm, W; 3 - AC, LQ = 40 mm, W; 4 - Cathode, Lg = 60 mm,
WTh02; 5 - Cathode, L3 = 60 mm, W; 6 - Cathode, L3 =
40 mm, W; 7 - Cathode, L = 6 mm, W; 8 - Cathode, Lg =
10 mm, W.
The current's ion component may constitute more than 75% of
total current [5]. Heat flux concentration causes hot spots
and rapid tungsten disintegration. An increase in electrode
temperature leads to a sharp increase in the area of the bright
cathode spot and of the surface which possesses high
thermoelectronic emission efficiency. If the compensating
effect of the electron current on the positive three-dimensional
charge is to be complete and the cathode is to have a strong
field, the following condition must be satisfied [5]:
where j. is ion current density; j is electron current
density; m., ,
respectively.
 and m are the mass of the ion and electron,
From inequality (3) we find that, for a hydrogen arc, /21
the limit for ratios Je/jj will equal 0.297. This probably
occurs during the shift from autoelectron to thermoelectron
emission. It has been noted that, when an electrode operates
in these conditions, there is a chaotic shift in the arc spot
caused by sharply pronounced gas turbulence, nonuniform
emission properties of individual segments of the electrode
working surface, etc. Reference [6] has established that, when
an arc spot shifts, the rate of change in the temperature of
the tungsten cathode in the arc closure area, as recorded by a
BFEP-1 unit, reaches 10 deg/sec from 1,400 to 4,000°C.
Let us consider cathode working conditions when I = 100
9
a and T = 2,000'C. According to data [7], current density
4 2in the cathode spot on tungsten equals (3-7)xlO a/cm .
Projecting the spot through a lens on a matte-finish screen has
made it possible to establish that the bright area where the
discharge comes in contact with the cathode equals about 1
2
mm . Using thermoelectron emission current density as a
function of temperature [7], let us plot the graph for the
increase in thermoemission current in the spot after it moves
to the new location (figure 2).
Apparently, at this time there existed auto- and auto-
thermoelectron emission, which leads to high specific heat flux
and an increase in electrode erosion. For alternating current,
movement of the spot leads to even greater tungsten
disintegration, which is caused by abrupt cooling of the
electrode as current passes zero. The spot may not stabilize
in one place in 0.4 sec, since the compressed cathode spot
possesses high mobility, while thermal collisions lead to the
appearance of electrode material vapor jets and an arc shift on
the electrode. The time it takes the discharge to travel along
the electrode's surface can be considerable, and tungsten
erosion increases.
Figure 2. Increase in
thermoemission current
in an arc spot.
•sec
0
6
7.0
Js/Sei
Z.D
Figure 3. Voltaic equivalent of heat
flux as a function of current load: 1
- Anode, L = 40 mm, WTh02; 2 -
Anode, Lg = 40 mm, W; 3 - AC, Lg =
40 mm, W; 4 - AC, Lg = 40 mm; 5 -
Cathode, L = 40 mm, W; 6 - Cathode,
L = 40 mm, WThO-.
e ^
An increase in the overall temperature of the working end
of the rod reduces the time required for the arc spot to
concentrate and the magnitude of the current density in the
spot. After the spot stabilizes in one place, its tendency to
shift diminishes. Note that, in this example, thermoemission
becomes significant only at spot temperatures above 3,800'C.
The type of working gas has a considerable effect on the
extent of heat fluxes to the electrode. Thus, if the voltaic
equivalent of heat flux for a tungsten cathode in helium equals
4-6 V; it equals 3-4 V in argon. Figure 3 shows the voltaic
equivalent (Ug) for tungsten in hydrogen, obtained by
experiment. The high value for U_. is due to recombination ofh>
hydrogen atoms on the surface of the electrode. Adsorption of
hydrogen on tungsten affects the nature of near-electrode
process, increasing the magnitude of the potential barrier. It
is assumed that a change in work function must be a function of
the extent to which the electrode surface is shielded by the
adsorbing gas [8]:
l
° (4)
where 6 is the number of places on the surface accessible for
adsorption; 0 is the percentage of adsorbed places;u „ is the
dipole moment of the atom-surface adsorption system.
A(e<p) for hydrogen is about 0.5 eV and decreases as
temperature increases because of the sublimation of hydrogen
from the tungsten surface.
The action of tungsten in argon, whose dipole moment
equals zero, occurs without an arc spot at rod end temperatures
above 3,000°C [9]. In these experiments, the spots had sharply
pronounced boundaries and they could not be eliminated by
changing electrode end temperature conditions. An increase in
the overall temperature of the rod's working end leads to a
change in the nature of emission processes, an increase in the
current's electron component, redistribution of heat flux near
the arc closure over a larger area, reduction in cathodic
voltage drop because of the shift from autoelectron to
thermoelectron emission, a decrease in the potential barrier on
the boundary between the current-carrying plasma and the solid
body, and, in general, "a reduction in electrode erosion.
An increase in current load on the tungsten rod to a value
at which the electrode melts causes the electrode material to
spray, forming bands of solidified metal droplets on the quartz
tube. The cathode's erosion curve exhibits an upward bend.
8
The causes for and the nature of electrode disintegration in
alternating current are similar to those for a cathode.
The increase in erosion in alternating current as compared
to direct current results from accelerated dynamic processes in
the near-electrode area and the presence of anodic conditions
in one of the half-periods.
The change in the tungsten work function and anodic
voltage drop on the tungsten when current load and electrode
temperature increase is slight. Therefore, when current rises,
heat flux to the rod increases continuously, having a decisive
effect on electrode disintegration.
Experiments have shown that the anode spot, defined as the
bright surface, increases more rapidly than I ; on an anode,
this is about equal to the electron component [7], From this
one might conclude that specific heat flux to the anode
decreases. The increase in erosion under these circumstances
makes it possible to assume that disintegration is not local,
but is determined by the vaporization of tungsten from the
high-temperature area near the arc closure. The thorium-
activated end of the tungsten exhibits a three- to fourfold
reduction in work function as compared with that of pure
tungsten for a cathode and AC electrode at no more than
1,800'C. If the electrode reaches T greater than 1,800SC,
its erosion increases sharply. This is because of the
disintegration of the activating coating, which is /23
superior in terms of the rate at which the activator diffuses
from deep layers and because of thorium depletion in the
working end of the electrode to depths of several millimeters
[9]. Electrode material work function increases to a level
found under identical conditions in pure tungsten. An increase
in rod working end temperature results in a change in its
structure: crystals grow [10]. An increase in grains reduces
the length of their boundaries on the electrode surface,
creating conditions for redistribution of impurities located
along the crystal boundaries to a smaller area, and this
decreases tungsten disintegration. Thorium oxide films on the
crystal boundaries inhibit their growth [10], creating
conditions for increased electrode material erosion in
vaporizing conditions above 1,800°C. Heating relatively
easy-to-fuse (as compared with tungsten) ThO- causes the
thorium oxide films to melt around the crystals and to expel
them in thermal collisions associated with the presence of
mechanical forces occurring during local and rapid heating of
the material [11]. Note that the function a thoriated
electrode in hydrogen is complicated because the thorium oxide
on the electrode's working surface is reduced to pure thorium,
the work function of which is greater than that of its oxide.
Analysis of this material shows that the level of tungsten
rod electrode erosion depends on the thermal conditions for the
working surfaces of the rods, which is determined by current
load, heat flux to the electrode, temperature of its working
end, and its dimensions. Depending on thermal conditions, the
erosion curve for the cathode and AC electrode has two
extrema. Thus, at a rod end temperature of about 1,650°C,
tungsten disintegration is maximum; at about 3,000'C, minimum.
10
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11


Influence of the temperature of electrode material on its disintegration under the action of an arc discharge in hydrogen

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