Formation of monodisperse refractory metal particles by impulse discharge

23

Powder Technology, 74 (1993) 23-30

Formation discharge

of monodisperse

refractory

metal particles by impulse

A. V. Suslov, E. L. Dreizin and M. A. Trunov Odessa State University, 2 Petra Velikogo St., Odessa-270100 (Russian Federation) (Received

March 13, 1991; in revised form September

21, 1992)

Abstract

A new method of forming monodisperse metal particles has been developed, in which the tip of a consumable wire electrode is fused and then formed into a separating droplet by an impulse discharge. The properties of the particles make them suitable for application in calibrating devices for particle size analysis, specialized applications in powder metallurgy and creating new tools. An experimental study has been carried out to test optimum conditions for dispersion. The physical processes occurring during impulse discharge when a droplet of molten metal forms and detaches have been examined to obtain conditions for producing monodisperse particles. It has been shown that initiation of the impulse discharge under conditions required for production of monodisperse droplets is possible when a non-equilibrium system containing ‘hot’ electron and ‘cool’ molecular subsystems is formed in the interelectrode space. The interaction between electrons obtaining their energy from the electric field and the surface of the consumable electrode provides the heat and force required to form and separate the metal droplets. The high energy concentration on the melting electrode gives rise to monodisperse metal powders with unique properties.

Introduction

Monodisperse systems represent ideal models for investigation of processes occurring in disperse systems, so that their formation is of particular importance. Monodisperse metal particles, especially those metals with high melting points, have further interesting applications. These particles can be used to develop methods of powder metallurgy, to make microbearings and new equipment for plastic deformation of surfaces, to create necessary surface microrelief by expanding microballs, etc. Spherical metal particles of the same diameter are also indispensable for calibrating devices used for measuring size distribution in disperse systems. The most widely used methods of producing metal powders are based on spraying the melts using compressed gas or a pressurized liquid [l, 21. Other methods are those of centrifugal spraying; in this case the melt is extruded from a flat rotating crucible [3] or projected from the surface of a flat rotating disk, to form metal drops with an average size of 150-300 pm. The main drawback of this technique is an excessively wide spread of particle diameters, ranging from 5 to 500 pm. It is possible to reduce the size distribution of particles somewhat by using rotating or vibrating [4,5] electrode discharge spraying. However, even in this case the particle size range is 5 to 300 Wm.

0032-5910/93/$6.00

Much more effective is metal dispersion by means of pressure vibrations applied to the stream of melt flowing out of a die, to enhance the Rayleigh (capillary) instability of the stream; the particle size variation is then not more than l-2% of adjusted diameter [6]. This technique enables one to form particles of fusible metals, e.g. Sn and Pb. The analysis of the above methods of obtaining metal particles shows that the least explored area here is refractory metals. The authors of this paper suggested a method of monodisperse metal particle formation [7] based upon melting a wire electrode by a pulsed gas discharge. During the pulse the fused area of the electrode takes the shape of a drop which is then broken away from the wire before the end of the pulse. The drop is cooled in a regulated gaseous or liquid environment, which influences the resulting properties of the particles. This method can give spherical particles of refractory metals with a narrow size distribution. Fig 1 shows the particles.

Experimental

Setup and measurements Figure 2 shows a schematic diagram of the experimental setup to study the physical processes accom-

0 1993 - Elsevier Sequoia. All rights reserved

16- 9

01

30

Fig. 1. Micrograph of copper particles obtained by pulse discharge (average diameter 280 pm).

u2

+

8

I

I

in -

Fig. 2. Schematic diagram of the experimental setup for investigation of process to form monodisperse particles.

panying formation of the monodisperse metal particles. The electric discharge is excited between the consumable wire electrode, 1, and the non-melting electrode, 2, and is fed from the power supply, 3. The high-voltage unit to initiate discharge was built into the power supply. The power supply gave a pulsed output voltage, and the duration of the pulses was varied form 20 to 1000 ps. Two power supplies were used with different output curves of current versus voltage shown in Fig. 3. The consumable electrode, 1, is driven into the discharge zone by the electrode feed unit, 4. The switch 5 controls the direction of the discharge current flow. The initial position of the electrodes before each discharge pulse is regulated by the electronic optical device, 6, its output being connected to the input of the clock driver, 7. The clock driver includes a comparator and a calibrated constant voltage supply and generates an

55

80

Current

105

(A)

130

7

155

Fig. 3. Output curves of current versus voltage of power supply: curve 1: unit 1 (first mode of operation); curve 2: (second mode of operation).

output pulse of appropriate amplitude and duration to synchronize the pulse generator, 8, with the movement of the wire. The clock driver, 7, is connected to the master input of the pulse generator, 8, which determines the discharge pulse duration and controls the mode of operation of the power supply unit, 3. The power supply output voltage U, is fed to the input of a storage oscilloscope to measure the discharge voltage. The calibrated resistor, 9, is connected in series with the discharge gap, while U, (which is proportional to the discharge current) is also supplied to the input of the storage oscilloscope. Thus the current I= UJR and the discharge voltage U= U,- U, (R is a calibrated resistance, 9) were measured simultaneously. The drop 10, which is thus formed during the pulse duration, breaks away from the electrode, and starts moving in the gas. The initial temperature of the drop was measured by the brightness pyrometric sensor, 11. Selection of modes of operation facilitating dispersion

In order to develop the dispersion technology one must clearly understand the physical processes of forming and separating the drops of the molten metal. With this aim in view the authors investigated the dispersion process under different operating conditions. Heating and melting of the electrode, formation and separation of the drop were studied using a high-speed motion picture camera. To prevent the film spoiling by radiation from the arc, the contrast lighting of the interelectrode gap by a helium-neon laser were used, with a proper interference light-filter between the object and the camera. At the same time, the discharge current was recorded by the storage oscilloscope. Two different modes of operation were investigated. The first used a power supply with steep output curve of current versus voltage and 35-55 A range of discharge

25

current (see Fig. 3, curve l), with pulse duration of some tens of milliseconds. In this mode of operation particles of Cu and MO were obtained with initial temperature close to the melting point of the metal. Analyses of the oscillograms and of the simultaneous photographs showed that there were two principal phases of the discharge: the first included electrode melting and drop formation with constant arc spacing and arc current, while the second consisted of drop separation and extinction of the arc, accompanied by increase in the interelectrode gap and reduction of the current. In this mode of operation, the discharge potential difference varied from 4 to 10 V, with the interelectrode gap up to 5 mm. One of the peculiarities of the process (revealed by the high-speed film) is the fact that, after separation of the drop, a residual droplet of melt remained at the tip of the electrode. The droplets thus formed had initial velocity -0.1 m s-‘, parallel to the axis of the wire. In this case the shape and separation of the drops is independent of the arc’s polarity, but when the melting electrode was a cathode, brief (100 ps) fluctuations of the arc current were observed which did not occur with the reverse polarity. This can be attributed to the wellknown phenomenon named cathode spot wandering [8]. When a large electrode (which is not melted during the arc pulse and whose vapour does not take part in arc plasma formation) is the cathode, this wandering of the cathode spot does not affect the arc current significantly. If the melting electrode is the cathode, then the cathode spot wandering must make evaporation into arc zone non-uniform; this has a strong effect on the interelectrode plasma structure, and hence the current. Changing the arc current changed the mean particle size (as shown by the experimental points in Fig. 4), while the degree of monodispersity was unchanged.

The variation of particle diameter was not more than 10-E% (for different metals). Fig. 5(a) shows histograms for copper and molybdenum powders. A source with a smooth output curve of current versus voltage was implemented to obtain the second mode of operation (see Fig. 3, curve 2). The discharge current was controlled within the range 50-155 A. The pulse duration was 0.1-l ms, which was much less than that for the first mode of operation. The current pulse was nearly square. The interelectrode potential differences were l-4 V, the interelectrode gap was 0.01-0.1 mm; the particles obtained were of Cu, MO, Fe, W, Al and Ni. The particular feature of this mode of operation is that dispersion only occurs when the melting electrode is the anode, and the diameter of the drops formed is about twice that of the electrode; the initial velocity of the drops was N 1 m s-l, perpendicular to the axis of the wire. Fig. 5(b) shows typical size distributions for copper and molybdenum particles obtained in the second mode of operation. It is seen that size spread of particles has been decreased and does not exceed 5-6% of average diameter. The initial particle

.oo, 80-1

601 40 i 20

I Diameter

360

(pm)

460

800 -700

cu

Ip

g -600

3'0

6'0

7'0

i 250

Fig. 4. Effect of electric current during pulse discharge on the average diameter of particles: A, experimental points; -, calculated curve.

1\

Diameter

350

(pm)

I

450

Fig. 5. Histograms for copper and molybdenum powders: (a) first mode of operation; (b) second mode of operation.

26

temperature could be much higher than the metal melting point.

Analysis of drop-forming processes Heating the melting electrode is an essential process to ensure drop-forming in the melt. In order to get a drop of radius, r, and temperature, T, one needs a quantity of heat, Q:

Q

rp,r3[C,(T, - To) + A + C,(T- Tm)]

(1)

where p1 is the liquid metal density, C, and C, are the heat capacities of solid and liquid metals, respectively, A is the latent heat of melting, T,,, is the melting point of the metal, and T,, is the ambient temperature. To analyze the drop-forming process, we compared the quantity of heat, Q, to the total energy, W, supplied to the interelectrode gap during the pulse: W=IUr, where T is the pulse duration. The ratio Q/W was 0.01 for the first mode; i.e. most of the energy supplied to the interelectrode gap is consumed in the discharge column, which is typical for an electric arc [8]. We also made an estimate of the upper bound value of heat losses Q- for radiation and heat removal into the cold area of the wire: Q- = 2moHu(T4

6 - T:)T+ r y &C&T,

- T,,)

in the liquid zone of the molten electrode, the latter is an order of magnitude greater than all the rest. It counteracts the surface tension force retaining’the drop. Analysis of the photographs enables simulation of the molten electrode configuration shortly before the break-away, and it is shown in Fig. 6(a). In this case the surface tension force d;F has two components acting on the length element dl, normal (dF,“) and parallel (dFap) to the wire electrode axis. We note that the only retaining component is dFap; hence the total retaining force takes the form: F

s

=

s

&7P= 6

L

25.2 r

0

(3)

where 6 is the surface tension. The integration is performed around the circle L, which is the intersection of the cylindrical wire and the spherical drop. The separating element of the pinch-effect force, F,, is calculated by the formula [8]:

where h is the relative permeability, and p is the magnetic permeability of the metal. Equating Fd to F,, we obtain the equation which relates the drop radius and the current:

(2)

where H is the length of the wire melted during a single pulse, (T is the Stefan-Boltzman constant, K is the thermal conductivity of the metal, and r, is the wire radius The calculated value for the first mode is Q-/Q =0.4. Comparability of Q- and Q shows that one cannot consider heating of the wire as an adiabatic process. Similar estimates made for parameters describing the second mode give QlW=O.9 and Q-lQ=O.O5. These values (obtained neglecting heat losses in the wire) show that the energy consumption in the discharge column is less than 10% of the energy fed to the electrodes and the process of droplet heating may be considered as an adiabatic one. Therefore, the electric discharge in the second mode of operation cannot be regarded as an arc. Analysis of the forces leading to drop separation provides additional argument in favor of this conclusion. The process of drop separation taking place in the first mode can be presented in terms of well-known concepts concerning forces affecting a molten electrode in an arc discharge [9]. Among the forces acting, including the reaction pressure of the plasma vapour, ion and electron pressure, and the pinch-effect arising

w---r (4

4X

Fig. 6. Idealized configurations of the molten electiode shortly before droplet detachment: (a) first mode of operation, (b) second mode of operation.

27

Figure 4 shows the relationship with the experimental values of I and r. The discrepancy is due to the fact that separation is caused not only by the pinch-effect force, since the additional forces actually will lower the theoretical curve towards the experimental points. However, the agreement between the slopes of the estimated and experimental curves confirms the predominance of the pinch-effect. The increasing divkrgence for increasing drop size is assumed to be caused by the growing role of gravity during separation. It was impossible to study the processes of drop formation and separation by the high-speed motion picture camera in the second mode of operation due to the pulse’s short duration. Therefore we used an ad hoc method of recording phases in the process of consumable electrode melting, by comparing photographs of the consumable electrode initial configuration with that observed after application of a discharge pulse of a certain duration. The pulse duration was changed from 0.1 r to r in steps of 0.05 7. Figure 7 shows the successive phases of electrode melting. It should be noted that, unlike the first mode of operation, there

Fig. 7. Successive

phases

of the electrode

melting.

is no residual droplet on the wire after separation of the main drop. The drop that has been formed at a certain moment of time solidifies without changing its shape after the discharge voltage is removed. Analysis of the pictures enabled simulation of the molten electrode configuration immediately before drop detachment, as shown in Fig. 6(b). In this case, the surface tension force which retains the drop acts along the curve defined by the intersection of the wire cylinder with the spherical drop, which can be approximated as a circle of diameter &I (see Fig. 6(b). The total surface tension force is then: s cos(a)

Introducing IABI = 26; Fs = 21rr~6

(6)

the geometric expression: cos(a)=m,

then: (7)

It is difficult to determine the precise value of the pinch-effect force since the drop and the wire are not axis-symmetric. This leads to a complex configuration of the magnetic field, so that it is impossible to formulate the pinch-effect analytically. The absence of a residual droplet on the edge of the wire after drop formation and separation, noted above, implies that pinch-effect forces arising in the molten zone of the electrode are not impressed forces as far as the detaching drop is concerned, because it contains practically all the metal melted during the pulse, so that they do not affect the separation process. The fact that discharge polarity influences the drop formation significantly also shows that the separation is driven by some other force, since the pinch-effect force does not depend upon polarity. Thus estimated forces show that the surface tension force can be compared only to that of the pressure of electrons from the discharge column. On the assumption that the whole length of the interelectrode spacing can be regarded as a near-electrode zone, the electron pressure force is:

where m and e are electron mass and charge, respectively. The electron pressure force acts along the discharge axis, which is nearly perpendicular to the axis of the wire. During drop formation, the resultant force on the drop is firstly directed from the drop center to intersect the contact area of the forming drop with the wire, so that the drop position is stable. The resultant force then transfers out of this area and the drop starts to leave the equilibrium position, and subsequently breaks away. Thus, a condition beyond which the drop

28

0.50

600

E E 0.40 -

,,I_

,_..-.----.----...._...________ 1

;,’

,:’

;,’ ,,,’ ,,,1’ :;’ ,,,

2

_,_-

3 _,__ ___....... -----------.--: ,f _;; .f ;;\: ,,1’

: 0 go.30 L

,I

,,

;’

,’

” ,,:,;’

o.201y!yo

200

300

Diameter

~

I

400

(pm)

Fig. 8. Effect of drop diameter on force: -, variation of Fax at constant temperature; --1, 2, 3, variation of F,, at various magnitudes of parameter P=I@ 1, P=120 AVln; 2, P=lOO /IV’“*, 3 9 P=80AV’“.

position becomes unstable can be defined which will determine the beginning of the separation process. Assuming that the process is short, this will be interpreted simply as the drop separation condition. Let us draw the axis OX, running through the drop center and perpendicular to the line OA. Then the separation condition depends on the force components on this axis as:

while equality describes the critical condition. Substituting the dependence of forces on the parameters describing the drop and taking into account the fact that the force Fe is perpendicular to the wire axis, we obtain: z

J

~W-ro)ro er2

=27rroS

J3 r

Figure 8 shows the dependence of Fsx on drop diameter at constant temperature (the solid line), with three curves (dashed lines) relating Fex to the drop diameter at different values of current and voltage. Figure 9 shows the estimated dependence of separated drop diameter on the parameter P=Z(U)‘n with the experimental results and shows that, at various values of P, condition (8) is met for various diameters of droplets. The agreement obtained is satisfactory, if we take into account the calculation error.

Interpretation

of phenomena

observed

The above analysis shows that the first mode of drop formation can be described in terms of the well-known concepts of melting a metal electrode in the electric arc [8, 91. However, some features observed while

Y$500 .z & 400 a, 4 $00 (d .3 a 200 100

I

160

Parameter

180

140

160

P (A ,V “‘)

Fig. 9. Effect of parameter P=Ifi on particle diameter: A, experimental points: -, calculated curve.

operating in the second mode (which provided greater monodispersity and gave monodisperse powders for any metal and alloy with a wider range of initial temperature change) are beyond the scope of recognized features of arc discharge. They are: - strong dependence of the process on the polarity of the discharge current; - heating and separation of the drops lies within the range of interelectrode gap of 20-30 pm; - low voltage in the interelectrode gap; - high efficiency (all the energy supplied to the electrodes is consumed in the anode heating); - the electron pressure force predominates in drop separation (which is calculated on the assumption of absence of the arc column). The last of these is similar to the above supposition that the whole of the interelectrode gap can be regarded as a near-electrode zone. The above features are described within the framework of concepts stating that there exists a thermodynamic non-equilibrium system comprised of a subsystem of ‘hot’ electrons and ‘cold’ gas molecules in the interelectrode spacing. Such a system exists because the electrons that generate the discharge current acquire high energy when accelerated by the electric field. The heating of the molecular subsystem is a result of collisions between molecules and electrons and it can be reduced to a minimum under both of two conditions: (1) Electron-molecule collisions are elastic. In this case a molecule in a single collision acquires part of the total electron energy, that is not more than 2m/M, [lo], where M is the molecular mass. The elastic collision mode occurs if the electron energy does not exceed the level of plasma-forming gas ionization energy. (2) The number of collisions that an electron undergoes while crossing the interelectrode gap is considerably less than that needed to achieve thermodynamic equi-

29

librium (determined as N=OSMlm). The number of collisions equals the ratio of the total electron path to the electron mean free path. Introducing the notion of electron drift velocity in the electric field we get the expression:

,A!!2 It vdr

d

where 2)dr is the drift velocity of the electron, (v) is mean velocity of the electron, h is interelectrode gap, and d is the mean free path. Assuming that the electron density is small and that they do not collide; neglecting the inelastic electronmolecular collision; considering that the electron distribution function within the pulses spacing is close to spherically symmetric and can be described (on the assumption of a strong field) by the Druyvestayn distribution [ll], then the drift velocity can be expressed as:

vdr=[3ag3,4)Jy; +$4(&)1’z (lo)

agreement with experimental results and is not typical for gas discharges. The experimental energy density in the anode spot is of the order of 30 000 W mm-’ which is comparable to the energy densities in electron and laser beams. High concentration of energy flux makes it possible to reduce to a minimum the energy loss due to heat removal into the cold area of the wire and to ensure high initial temperatures of the metal drop. In this case the behaviour of the cooling drop can strongly differ from the usual one; e.g. intensive gas dissolution in the drop occurs resulting in changes in the properties of solidified particles. Figure 10 shows a photo-micrograph of a microtomed copper particle which was frozen from the initial temperature of 2000 “C. One can clearly see numerous spherical inclusions of Cu,O formed out of oxygen dissolved in the overheated drop volume. Hence, the conclusion is that the second mode of operation enables not only to obtain monodisperse metals but also to produce particles with specific preselected properties.

Conclusions

M

(11)

where E is energy transfer from electric field to the electron subsystem, and E’ is energy transfer from electrons to the molecular subsystem. Thus, in the second mode of operation for ~‘/e=O.l, we obtain 36 pm for h in air that is in agreement with experimental data. The part of total discharge energy inputted into anode material is 90% in this estimate and this is in

It is possible to produce monodisperse particles of refractory metals by means of pulsed electric low-voltage gas discharge having specific properties. The size spread of particles does not exceed 5-6% of average diameter. The particle initial temperature can be much higher than the metal melting point. Drop formation and separation result from interaction of the metal wire with the thermodynamically inequilibrium system of electrons and heavy particles. The electrons obtain their energy from the electric field and transfer it to the droplet forming on the tip of the consumable wire anode. The electric power consumed by heating gas molecules is less than 10-E% and the energy density in the anode spot is of the order of magnitude of 3 X lo4 W mm-* which is comparable with energy densities in electrons and laser beams. This method of producing monodisperse refractory metals makes it possible to change the properties of the particles obtained and to form metal powders with unique properties.

List of symbols

c d

dl

Fig. 10. Photo-micrograph

of a microtomed

copper

particle.

> F,

heat capacity, J/(kg K) free path distance, m length element, m charge of an electron, C strength of electric field, V/m force of the electron pressure, N

30

J-P F8

h H I L ii : P

Q Qr r0

R S T To T, & Odr

W

force of the pinch effect, N force of the surface tension, N interelectrode gap, m length of the wire melted during a single pulse, m current, A line limited the area of integration, m mass of an electron, kg mass of a gas molecule, kg concentration of gas molecules, mm3 number of collisions, parameter IUrn, AV’” quantity of heat, J heat losses, J radius of the drop, m radius of the wire electrode, m resistance, CI cross-section of the elastic collision of electron and molecule, mz temperature, K ambient temperature, K melting point of a metal, K voltage, V mean velocity of electron, m s- ’ drift velocity of electron, m s-’ total energy supplied to the interelectrode gap during a pulse, J

Greek letters ff angle, ’ s surface tension, N/m energy transferred from electric field to the E electron subsystem, J energy transferred from the electron subsystem E’ to the molecular subsystem, J

7

nK P PO

P

(T

pulse duration, s thermal conductivity, mz s-l specific melting heat, J/kg magnetic permeability, relative magnetic permeability, H/m density, kg rnv3 Stefan-Boltzmann constant, W/(m” K4)

Subscripts 1 liquid S solid

References 1 0. S. Nichiporenko,J. Powder Metall., 9 (1976) 5 (in Russian). 2 R. A. Rickc and T. M. Clyne, J. Mater. Sci. Len., 7 (1985) 814. 3 0. S. Nichiporenko, A. G. Cipunov and U. P. Temovoy, J. Powder Metall., 1 (1984) 1 (in Russian). 4 P. Klark, Some Pilot Papers on Powder Metallurgy and Joining, London, 1975. 5 C. Matei, E. Risak and W. Havmann, P/M MPZF, Chicago, 1975. 6 V. V. Blajenkov, A. S. Dimitriev and V. V. Shishov, Trans. Moscow Energetic Inst., 615 (1983) 3 (in Russian). 7 A. V. Suslov and E. L. Dreizin, Soviet Powder Metall. Met. Ceram., 29 (1990) 939. 8 W. Finkelnburg and H. Maecker, Elektrische Biiden und Thermisches P&ma, Berlin, 1956. 9 G. A. Merlin and P. V. Denisov, Welding Erg., 12 (1969) 5 (in Russian). 10 S. V. Dresvin, A. V. Donskoy, V. M. Goldfarb and V. S. Klubnichkin, Physics and Engineering of Low-temperature Plasma, Atomizdat, Moscow, 1972 (in Russian). 11 L. G. H. Huxley and R. W. Crompton, i%e Diffusion and Drift of Electronics in Gases, Wiley Interscience, New York, 1974.