New functional material--oxide cathode of welding electric arc: Information 2.
Well compressed in dead hole of water-cooled copper holder powder insert is characterized by high density of physical contacts between separate powder components of the mixture, that's why heat flow heats compressed mass quickly and efficiently. Caused by this heating sintering process is to significant degree similar to the process, which proceeds in self-sintering Soderberg electrodes used in powerful ferroalloy furnaces for more than a century. Hence the name of oxide cathodes, which they received early 1990s--self-sintering thermo-chemical cathodes .
Like in Soderberg electrodes, in steady-state oxide cathode sintering process several structural zones are distinctive on semi-microscale. In Figure 1 typical picture of active insert diametric section is given. Three zones of oxide cathode are well seen under binocular microscope at natural illumination and 5--16 fold magnification.
[FIGURE 1 OMITTED]
By moving from open surface of the active insert into the depth we may single out the first zone 1, which is identified by color, structure, and shape. In general case longitudinal section of the zone has crescent form, because it copies form of the active insert crater.
Usually this zone has grey or dark-grey color and its structure is presented by a homogeneous molten material. It is possible to distinguish in it at high magnification (1000 times and more) two areas with different grain structures. Near very interface of the first zone with gas phase area of fine equiaxial crystals (5--15 [micro]m), having thickness from 1/4 to 1/3 of the considered zone thickness, is located. Greater part of the zone is presented by area of columnar comparatively coarse crystals (Figure 2), each of which achieves in diameter 50--60 [micro]m. Boundaries between columnar crystals are thin, their thickness is up to 1 [micro]m, they have brightly saturated light-yellow color against grey background of crystallites. Micro-X-ray diffraction analysis showed that boundaries of crystallites are enriched with barium. Columnar crystals are oriented along the normal to open surface of the emission film.
In liquid state in the process of arc burning the film is homogeneous, it emits into the arc column flow of electrons, which ensures its stable burning.
Such structure of solidified emission film is, in all probability, explained by different rates of solidification of mentioned areas of the first zone after extinguishing of the arc. First thin layer of liquid film, directed at the gas phase, is solidified. Here the highest rate of solidification is registered, that's why grains are the finest in this area. Second area of liquid emission film is solidified at lower rate and columnar crystallites oriented along the normal to the crater profile grow comparatively slowly deep into the insert.
General thickness of the emission film (first zone) on the insert axis is 400--600 [micro]m and does not depend upon the arc current within 300--650 A. Near copper wall of the dead hole width of the first zone gets wedge-like thinner down to 25--50 [micro]m. In Figure 1 position 4 designates the place, in which the first zone gets in direct contact with the third zone. Here in one point converge solid, liquid, and gas phases of oxide cathode and arc discharge.
Material structure of the first zone is dense, it does not have at mentioned magnification pores, micro-hardness of the zone is rather high and usually achieves 14000--16000 MPa.
Second zone differs from the first one by light-grey color with characteristic light yellowness. Its thickness varies from 1000 to 1200 [micro]m. The zone is arranged in the form of a bent strip of practically the same thickness from one to another wall of the hole. Structure of this zone is also molten and has metal gloss. Rather big number of pores (Figure 3) was detected in this zone, the biggest of which achieves 200 [micro]m in diameter. Shape of the pores is mainly spherical, their edges are smooth, which proves their gas or vapor origin. Exactly in this zone copper component of the active cathode insert transits into gas phase. Structure of the zone etching is multiphase one. Microhardness of the zone is inhomogeneous: from 16000--18000 MPa near first zone to 200--500 MPa near interface with third zone.
Third zone is much darker than the second one, boundary between them is clear (see Figure 3). Third zone has dark red tincture because of copper, which enters into composition of the active insert. At low magnification of microscope the zone is homogeneous, but at magnification above 50 separate components of powder mixture get well distinguishable.
Thickness of the third zone is not stable and by means of the cathode operation it changes from 3000 [micro]m (at first striking of the arc) to zero in case of the cathode resource exhaustion.
Pressure applied for powder compression into the dead hole of copper holder is so high that it causes cold welding of copper particles of the powder mixture with copper walls of the hole. It is very important peculiarity of oxide cathodes, due to which good heat withdrawal from the active insert to copper watercooled holder is ensured. In oxide cathodes with a compact zirconium insert heat transfer from the insert to the holder is a difficult issue, which explains their comparatively low resource.
Third zone, as well as the second one, is porous. Its general porosity is 4--7 vol.%, size of the pores varies from 5 to 50 [micro]m; the pores have quite different shapes. Boundaries of pores represent edges of separate particles of powder mixture components. Near contact with the second zone partial sintering of the mixture powder components is detected in case of using alumobarium cathode in the charge.
Temperature distribution over active insert section. Temperature of cathode spot on emission oxide film, consisting of pure de-stoichiometrized zirconium oxide, achieves, approximately, 4000 K . Evidently, in our case because of positive action of barium oxide, which reduces electron work function from emission film, it will be 500 degrees lower. At the bottom of the active insert, in the place of its contact with copper holder temperature is 413--421 K. So, the lowest temperature gradient, at which heat flow at the beginning of the cathode work is withdrawn from the active insert into copper holder, makes up 3500 - 421/5 616 degrees per 1 [micro]m, where 5 is the dead hole depth, [micro]m. Corresponding to this gradient specific heat flow into the dead hole bottom is 213 W/cm. This is minimum value of the flow; into side walls of the hole heat flows are directed, which are several times higher because of lower length of the flow way over the insert body from the place of the cathode spot tie to the wall.
[FIGURE 2 OMITTED]
Heat conductivity of the compressed powder mixture in the third zone is rather high due to the copper addition and makes up 0.3 W/(cm-K), which is even higher than a respective index for zirconium in compact state (0.27 W/(cm-K) at 400--500 K).
In the first zone the highest temperature gradient is registered, which makes up 1-104 deg/mm . So, over the insert axis within liquid emission film temperature varies from 3500 to 2300--2500 K.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
In the second zone one order lower temperature gradient is detected (890--900 deg/mm). Temperature over the insert axis is distributed as follows: near the boundary 2300--2500 K, and at the boundary with the first zone--1400--1430 K.
As far as the third zone is concerned, temperature here reduces from 1400--1430 to 413--421 K. Temperature gradient increases by means of the cathode erosion in the process of burning.
In the method of measuring local temperatures in the oxide cathode third zone we used rigid connection between parameters of elementary lattice pSiC and temperature .
For local temperature measuring silicon carbide chips, which were preliminary radiated by a flow of neutrons for their lattice to <<swell>>, are placed into certain places of the active insert third zone. After termination of the cathode operation chips are withdrawn and judgment is made about temperature in the place of their location on the basis of the lattice parameters. This method is fit for measuring temperature within the range 300--1500 K with error [+ or -] 6 deg.
[FIGURE 5 OMITTED]
Surface of the emission film on the side of electric arc looks in the same way as in case of purely zirconium cathode--both light and dark areas of the film are present (Figure 4). Place of the arc tie in the crater bottom is usually light, while crater slopes are dark. When inspecting non-etched film at high magnification of the cathode spot, one can see on screen of a scanning microscope granular structure with streaks of eutectics (Figure 5, a). At the same time such structure was not detected on the crater slopes (Figure 5, b).
Oxygen is distributed irregularly in the emission film material and composition of the film significantly differs from stoichiometric one; in addition, in center of the crater in place of the arc column tie oxygen content is somewhat higher than that on the crater slopes (Table).
X-ray diffraction and electron-graphic studies of the emission film showed that in addition to ordered solid solution of barium oxide in compound of zirconium with oxygen in the emission film barium metazirconate was unambiguously detected . This phase, as could be expected, is located over grains of solid solution, where it is displaced as a result of impurity segregation in process of the oxide film solidification and due to manifestation of surface-active properties of barium oxide in the zirconium oxide melt. Coefficient of barium oxide distribution in titanium dioxide (close analogue of zirconium) may achieve 150 .
Form of existence of active insert components in three phases. In order to track, in which form are in each of three phases (in solid phase--third zone of active cathode insert; in liquid phase--second and first zones of the insert, and in gas phase--arc discharge atmosphere) oxide cathode components, analysis of behavior of these components in the process of transition from one phase into the other was carried out , whereby not just action of temperature, but also of gas-vapor phase was taken into account. Results of this analysis are presented in Figure 6.
Temperature in the third zone is so low, that majority of the third phase components are in solid phase. Exceptions are copper and barium, if the latter is introduced into the oxide cathode charge in the form of rather low-melting alumobarium. As far as during compression of the powder charge certain amount of air is entrained together with it, it is quite possible that during heating of the insert aluminium and barium are oxidized to [Al.sub.2][O.sub.3] and BaO.
In second zone charge components are in the form of quasi-homogeneous alloy in liquid state. Solid aluminium and barium oxides transit from third zone into the second one without changes. At increased temperature in the second zone formation of solid zirconium nitride is possible. Copper in this zone transits into gas phase and is evaporated or partially retained in formed by it bubbles.
In the first zone homogeneous oxide alloy of zirconium and barium oxides with a certain share of aluminium oxide are formed as a result of interaction of the charge components. In this alloy barium monozirconate is also formed in liquid phase. Just insignificant amount of solid zirconium nitride, which passed from the second zone, may preserve in oxide melt. Thermo-dynamical analysis shows that probability of ZrN formation from the air nitrogen is rather low in comparison with formation of [Al.sub.2][O.sub.3]. High temperature of the first zone and electrolysis of the oxide melt cause disturbance of stoichiometry of oxides both in liquid solution BaO in Zr[O.sub.2] and in liquid chemical compound BaZr[O.sub.3].
Zirconium dioxide transits into the gas phase, which is equilibrium with oxide melt of the first zone, with partial dissociation and formation of ZrO and O, and [Al.sub.2][O.sub.3] with formation of AlO, [Al.sub.2]O, Al, and O. Barium oxide, in its turn, disintegrates into Ba and O.
If we take into account that air in the arc column at comparatively low temperatures, which is peculiar for arc discharge with oxide cathode, will be present in the form of completely dissociated oxygen and partially dissociated nitrogen, N2, N and O have to be added to the gas atmosphere. Copper practically does not change its composition when transiting into the gas phase, just certain aggregation of separated particles is possible .
By means of missile technology development thermodynamics of high-temperature oxide systems was thoroughly studied and presented in known publications JANAF Tables as well as in . Results of calculation of partial pressures of the gas phase separate components are presented in .
So, gas phase above emission film of the cathode represents a multi-component system similar to the one presented in Figure 6.
Barium compounds in the emission film reduce electron work function from oxide cathode and thus increase thermal coefficient of efficiency of the arc, which acts as converter of electric energy into heat one. Voltaic equivalent in these cathodes does not exceed 2 V, and presence of barium in the arc atmosphere increases stability of the arc discharge burning and reduces gradient of electric voltage in the arc column.
[FIGURE 6 OMITTED]
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[2.] Zhukov, M.F., Kozlov, N.P., Pustogarov, A.V. et al. (1982) Near-electrode processes in arc discharges. Novosibirsk: Nauka.
[3.] Nikolaenko, V.A., Karpukhin, V.I. (1986) Measurements of temperature using the irradiated materials. Moscow: Energoatomizdat.
[4.] Lakomsky, V.I. (1997) Oxide cathodes of electric arc. Zaporozhie: Internal.
[5.] Leonov, A.I., Kostikov, Yu.P., Strykanov, V.S. (1988) Influence of surface microstructures on physical-chemical properties of oxides. Uspekhi Khimii, 57(8), 1233--1252.
[6.] Suvorov, A.V. (1970) Thermodynamical chemistry of vaporous state. Leningrad: Khimiya.
[7.] Kulikov, I.S. (1966) Thermal dissociation of compounds. Moscow: Metallurgiya.
[8.] Lakomsky, V.I., Taran, A.Ya. (1990) On composition of gas-vapor phase on the thermochemical cathode surface. Avtomatich. Svarka, 8, 20--23.
E.O. Paton Electric Welding Institute, NASU, Kiev, Ukraine
Weight share Stoichiometric coefficient Site of sampling of oxygen, % of oxygen 2--xZr[C.sub.2-x] Crater center 23.2 1.72 (light areas) 19.4 1.37 15.6 1.05 Crater slopes 14.9 1.00 (dark areas) 14.2 0.92 8.5 0.53
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|Title Annotation:||NEW MATERIALS|
|Publication:||Advances in Electrometallurgy|
|Date:||Apr 1, 2006|
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