Powder Metallurgy and Metal Ceramics, Vol. 49, Nos. 1-2, 2010

THEORY, MANUFACTURING TECHNOLOGY, AND PROPERTIES OF POWDERS AND FIBERS FORMATION OF COPPER DEPOSITS UNDER ELECTROLYSIS WITH SURFACE-ACTIVE AGENTS I. B. Murashova,1,3 N. E. Agarova,2 A. B. Darintseva,1 A. B. Lebed,2 and L. M. Yakovleva2 UDC 669.343 The growth of dendritic deposit under galvanostatic electrolysis is modeled and a method is developed to analyze its formation in laboratory. The time dependences for cathodic overpotential are plotted and the rod electrode with deposit is video-recorded to establish that copper dendrites form at smaller tip-radius density from the solution with surface-active agent F2 than from the solution without additions. This results in higher fluidity of the powder. Nevertheless, dendrites formed from the solution with F2 addition have greater tip radii than the deposit from the pure solution. In addition, the dendritic deposit intensively grows for a longer time with F2 addition, thus increasing the yield of powder and decreasing the yield of cathodic scrap. The results from the laboratory experiment have been confirmed by commercial production. Keywords: electrolytic copper powder, galvanostatic electrolysis, surface-active agent, growth dynamics, deposit structure.

INTRODUCTION Surface-active agents (SAA) are widely used in engineering electrochemistry to improve the surface quality of a metal layer deposited. To produce metal powders by electrolysis, these agents may serve as additions to the electrolyte [1–4] or as its main components that substantially change the structure of the deposit [5]. In the production of electrolytic copper powders, the surface-active agent, which is identified as SAA F2 in [6], has proved to be effective and to influence the content of different-size particles in the product and powder characteristics such as bulk density and fluidity. It is challenging to optimize the parameters of SAA use in the production of copper powder. Experiments with an electrolyte containing an organic additive are difficult in plant conditions because of large-scale production and because the growth of the dendritic deposit and variation in cathodic overpotential with time can hardly be monitored. The growth of the deposit during electrolysis in a laboratory can be monitored, but electrolysis on a

1Ural

State Technical University, B. N. Eltsin Ural Polytechnic Institute, Ekaterinburg, Russia. Joint Stock Company, Verkhnyaya Pyshma, Russia.

2Uralélektromed’ 3To

whom correspondence should be addressed; e-mail: [email protected]; [email protected].

Translated from Poroshkovaya Metallurgiya, Vol. 49, No. 1–2 (471), pp. 3–10, 2010. Original article submitted July 29, 2009. 1068-1302/10/0102-0001 ©2010 Springer Science+Business Media, Inc.

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laboratory scale and in full-scale plant conditions differs. A technique for determining the properties of the laboratory deposit cannot be compared with industrial post-electrolysis treatment. Hence, the properties of powders following laboratory and industrial electrolysis may differ. Nevertheless, variation in the structural characteristics of the deposit can be monitored directly during electrolysis by combining video observation and continuous measurement of cathodic overpotential based on a dendritic deposit growth model [7].

MODEL REPRESENTATION AND PROBLEM STATEMENT The growth dynamics of a dendritic layer y on a cylindrical electrode with diameter d0 is described in [7] for galvanostatic electrolysis. If the depletion coefficient Kd is constant, the height y of dendrites on a surface with initial roughness y0 varies with time t as follows: y 2 + d0 y = y02 + d0 y0 +

V im d 0 K d t, zF 2πNrt2

(1)

where Kd equals the ratio of the set current I to the maximum current Im; im is the maximum current density; N is the distribution density of dendritic tips on the outer cylinder surface (growth front); rt is tip radius; V is the molar volume of metal; and F is the Faraday constant. If d0 and y0 are small, Eq. (1) transforms into the following quadratic equation:

y 2 + d0 y −

V im d 0 K d zF 2 πNrt2

t = 0.

(2)

This equation is solved to determine the time dependence of the length of dendrites: ⎛ ⎞ ⎜ V 2im d0 K d ⎟ y = 0.5⎜ − d0 + d0 + t ⎟. zF πNrt2 ⎜ ⎟ ⎝ ⎠

(3)

The formula shows that the dendrites grow nonuniformly with time; the growth front rapidly moves into the solution and then gradually slows down. According to (3), the growth rate of the dendritic deposit depends on the set current proportional to Kd, the concentration of metal ions in the solution (through im), the diameter of the cylindrical cathode, and the surface quality of the cathode (y0). According to [2], the directional development of the dendritic deposit continues until the maximum conditions are observed at the growth front; i.e., until Kd > 1. The growth rate of dendrites depends, besides Kd, on the structural parameters of the deposit (N and rt). Experimental data have confirmed that the height of the dendritic layer reached by time τfin increases with Kd; the maximum conditions at the growth front no longer exist by that time. This period is marked by the drop of cathodic overpotential, and the dendrites cease to grow [8]. The model does not account for the co-precipitation of hydrogen and metal, so it cannot be used to describe the variation in the cathodic overpotential η during the growth of the deposit. Nevertheless, the model can be used with the experimental dependences y(t) and η(t) to assess N and rt, which are in turn related to the size of particles obtained from the dendritic deposit after electrolysis and to the branching of the dendritic crystals determining the bulk density and fluidity of the powder. The objective of this paper is to develop a procedure for analyzing the growth dynamics of dendritic copper deposit from a pure electrolyte and that with SAA F2 in the production of GG copper powder. This procedure combines electrical measurement and video-recording of the process and assessment of the structural characteristics of the dendritic deposit growing on the cathode. The characteristics are assessed by comparing the tip-radius distribution of the deposit and the density of dendrites at the growth front for the pure electrolyte and that with SAA.

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EXPERIMENTAL PROCEDURE A glass laboratory electrolytic cell with a copper rod 1.3 mm in diameter and 15 mm in height was used for electrolysis. The solution corresponded to the electrolyte used in the production of GG powder; i.e., it contained 25 g/L of copper and 155 g/L of sulfuric acid. The experiments were conducted at 25°C. Chronovoltammetry was used to preliminary determine the maximum steady current density, which was 326.64 A/m2. In industrial conditions, electrolysis is conducted at a current density of 3026.32 A/m2 (for a smooth electrode), corresponding to a depletion coefficient of 9.3. With this depletion coefficient, the current load for the laboratory electrolysis cell was 186.13 mA. The electrolysis cell contained a cylindrical cathode inside a concentric copper foil anode (Fig. 1). A copper rod, which was similar to the cathode and immersed into a geber, was a reference electrode. The cell was fed from a P-5848 potentiostat. The current and cathodic overpotential were continuously measured at 0.5 sec intervals during electrolysis. The measured data were stored in the memory of APPA (I) and APPA (η) multimeters until the experiment was completed and then were transferred to a computer.

a

b

Fig. 1. Metering circuit (a) and video image of the electrode cell (b): APPA (η) and APPA (I)⎯multimeters for measuring and recording the overpotential η and current I; P5848⎯potentiostat with marking of electrodes (working, auxiliary, and reference electrodes); other explanations are in the text The electrode with deposit was illuminated through slots in the concentric anode when it was recorded by a Sony DCR-SR 200E video camera. The video-recording started at certain periods during the electrolysis and lasted for 30 sec each time. A Light Alloy 2.6 program was used to process the video material. To determine the actual diameter of the cylindrical cathode with deposit, the diameter values were averaged over the height of the electrode.

RESULTS AND DISCUSSION According to the model description of the growth of dendrites during galvanostatic electrolysis (Eqs. (1)– (3)), the dendritic deposit becomes thicker with decreasing rate (Fig. 2). The experimental data points in Fig. 2 are fitted by second-order polynomials (Table 1). The regression relations in Table 1 permit assessing the initial surface irregularity y0 by setting t = 0. We see that the growth of dendrites slows down with time, no matter what composition

3

d0 + 2y, y, mm

6

6

1

1

4

4

0

20

40

2

2

2

2

60

0

20

40

a

t, min

b

Fig. 2. Increase in the diameter d0 + 2y of the electrode with deposit (1) and in the height y of dendrites with time (2) in pure solution (a) and with SAA F2 (b)

TABLE 1. Fitting Equations for Experimental Data (d0 and y, mm; t, min) Solution without additives

Solution with SAA F2

d0 + 2y = 1.48 + 0.189 ⋅ t – 0.00213 ⋅ t2

d0 + 2y = 1.4838 + 0.1118 ⋅ t – 0.0009 ⋅ t2

y = 0.1 + 0.097 ⋅ t – 0.0011 ⋅ t2

y = 0.0919 + 0.0559 ⋅ t – 0.0004 ⋅ t2

the solution has; the dendritic deposit grows at a slower rate in the solution with SAA F2. The slower the dendritic growth, the slower the change in the crystallization conditions at the growth front. It takes a longer time for the limiting conditions at the growth front to cease to exist, for the hydrogen generation rate to drop to zero, and for the oscillations of overpotential to be damped. The same pattern is shown by the chronopotentiograms in Fig. 3. The active growth of the deposit lasted for 42 min in the solution without additives and for 56 min in the solution with SAA F2. These parameters will increase on the industrial scale, but the ratios between them will remain: the diffusion conditions at the growth front will exist for a longer time in the presence of SAA F2. When current is fed to the solution with SAA F2, the overpotential surge is especially great after the transition time. Undoubtedly, this is evidence of the higher rate of nucleation and of the smaller radius of curvature. Nevertheless, the limiting conditions at the growth front exist from the instant the current is switched on to the instant the overpotential drops and the oscillations of the overpotential are damped out. If the current efficiency hardly changes within the period, then Eq. (1) can be used to assess Nrt2. The production of copper powder testifies

Overpotential, V

that the current efficiency of dendritic deposit is quite high though the set current substantially exceeds the maximum current. The point is that hydrogen is vigorously liberated at the cathode after a short transition period 0.9

0.9

0.6

0.6

0.3

0

20

40

a

60

0.3

0

20

40

t, min

b

Fig. 3. Chronopotentiograms of the growth of dendritic deposit GG from electrolyte without additives (a) and with SAA F2 (b)

4

6

1

6

2

4

2

2

(y +d0 y, m ) .10

8

2 0

Fig. 4. Dependences

(y2

10

20

30

40 t, min

+ d0y)–t for the growth of dendritic deposit GG in pure solution (1) and in the presence of SAA F2 (2)

following the switching-on of current; the hydrogen bubbles intensively mix up the electrode space, increase the actual maximum current of convection diffusion, and promote higher metal yield. The dendritic deposit grows with time and the area of its growth front expands. The hydrogen generation rate drops and the effectiveness of convection transfer decreases. However, the decreased effectiveness of the convection transfer to the cathode is balanced by the expanding area of the growth front. If there is small change in the current efficiency, Eq. (1) can be used to assess Nrt2 since the dependence of (y2 + d0y) on t remains linear within the period, d0 and y0 being constants. The dependences for the crystallization of the dendritic copper deposit have been calculated using the known regression equations y(t) (Table 1, Fig. 4). According to Fig. 4, the dependence of (y2 + d0y) on t still remains linear when the dendritic deposit actively grows. Beyond this period, the calculated points depart from a straight line and tend to constant y, which indicates that the deposit ceases to grow. The slopes of the lines are used to determine Nrt2 for these systems. This parameter was 5.815 ⋅ 10–3 for the pure solution and 0.01163 for the solution with SAA F2, provided that rt and N are measured in m and m–2, respectively. In this connection, the variation in the tip radius and density of dendrites with time can be calculated. The dependence rt(t) permits assessing the tip-radius distribution of the deposit grown (Fig. 5b). The tip-radius distribution cannot substitute a particle-size analysis but shows how large the particles of the resulting powder are. The role of the tip radius as a structural parameter becomes evident when dendrites of different shapes are compared (Fig. 6). It is clear that the deposit (Fig. 6a) will form coarser particles and the dendrite (Fig. 6b) will easily disintegrate into smaller fragments after electrolysis. The dendrite particles (Fig. 6a) will remain globular after grinding and sieving. This powder has good fluidity. The bulk density and fluidity of the electrolysis product can also be assessed from Fig. 5a. The dendrite tips densely distributed at the growth front are evidence that the deposit is highly dendritic, which prevents the fluidity of the powder. Compacts from this powder will have good strength but the powder obtained in 0.6 1

1

Fraction

4

−9

N 10 , m

−2

6

.

2

0.4 2

0.2

2

0

0

10

20

30 40 Time, min

a

50

60

0

0

5

10 rt , μm

15

20

b

Fig. 5. Density of dendrites at the growth front of deposit GG (a) and tip-radius distribution of dendrites (b) in pure solution (1) and in the presence of SAA F2 (2)

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a

b

Fig. 6. Dendrites with close sizes and different tip shapes (and radii) the presence of SAA F2 will have higher fluidity. Actually, when SAA F2 was used in industrial conditions, the yield of fractions smaller than 160 µm increased from 66 to 80% and the bulk density of the powder increased from 1.12 to 1.67 g/cm3. The fluidity of the fraction smaller than 100 µm was 38.2 sec, while the powders obtained without SAA were not fluid. The amount of cathode scrap decreased in the process [8]; this agrees well with the results of Fig. 5b, according to which the deposit obtained in the presence of SAA F2 hardly contains dendrites with a greater radius of curvature unlike powder GG obtained from pure solution.

CONCLUSIONS A model for the growth of dendritic deposit under galvanostatic electrolysis is used to develop a method for analyzing the dynamics of its formation in laboratory conditions. Time dependences of cathodic overpotential are plotted and the rod electrode with deposit is video-recorded during electrolysis. The experimental data are processed to show tip-radius distribution of the deposit, which is related to the grain-size composition of the powder, and to show the extent of dendrites, which determines the compressibility and fluidity of the powder. Addition of surface-active agent F2 to the electrolyte has changed the growth dynamics of the dendritic deposit; as a result, the powder is more uniform in size and more fluid. This has been confirmed by commercial electrolysis.

REFERENCES 1. 2. 3. 4. 5.

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O. S. Nichiporenko, Copper Powders and Alloys [in Russian], Metallurgiya, Moscow (1988), p. 206. I. B. Murashova and A. V. Pomosov, “Electric deposition of metals as dendrites,” in: Advances in Science. Electrochemistry [in Russian], VINITI, Moscow (1989), Vol. 30, pp. 55–145. I. B. Murashova and A. V. Pomosov, “Electric deposition of fine copper in the presence of organic additions,” Izv. Vuzov. Khim. Khim. Tekhnol., 15, No. 5, 743–745 (1972). E. E. Usol’tseva, A. V. Pomosov, T. Yu. Kuz’mina, etc., “Electrolyte for producing copper powder by electrolysis,” USSR Inventor’s Certificate No. 1418349, Bulletin No. 31, Publ. August 23 (1988). E. M. Natanson and Z. R. Ulberg, Colloidal Metals and Polymers [Russian translation], Naukova Dumka, Kiev (1971), p. 348.

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7. 8.

A. P. Shpoltakov, A. T. Krest’yaninov, V. I. Chuprakov, et al., “Electrolyte for producing electrolytic copper powder,” Russian Federation Patent 2254209, Application 2004113925/02, May 5, 2004, Bulletin No. 17, Publ. June 20 (2005). T. N. Ostanina, I. B. Murashova, and E. E. Kuz’mina, “Dynamics of growth of dendritic deposits of lead on a cylindrical electrode,” Élektrokhimiya, 32, No. 11, 1329–1333 (1996). I. B. Murashova, P. V. Taushkanov, and N. G. Burkhanova, “Change in structural characteristics of loose copper deposit under galvanostatic electrolysis,” Élektrokhimiya, 35, No. 7, 835–840 (1999).

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