Pergamon NanoShuchred Materials, Vol. 5, Nos. l/8, pp. 793-799.1995

Ekvier Science Ltd Copyright Q 1995 Acta Metallurgica Inc.

Printed in the USA. AU rights reserved 09659773195 $9.50 + .oo

0%5-9773(95)00290-l

INVESTIGATION OF NANOMETER POWDERS PRODUCED BY AN ELECTROHYDRODYNAMIC TECHNIQUE

X.F. Yu*, Z.Q. Hu*, X. Liu** and Z. Jiang

*State Key Lab of RSA, Institute of Metal Research, Academia Sinica, Shenyang 110015, Peoples Republic of China

**Northeastern University, Shenyang, llOOOf5, Peoples Republic of China

(Accepted August 1995)

Abstract-Sn-Binanometerpowders wereproduced by an electrohydrodynamic technique. The effects of processing variables on the size of Sn-Bi nanometer powders were studied. Experimental results indicate that with an increase in high voltage and a decrease in diameter of the capillary nozzle, the size of Sn-Bi nanometer powders decreases. In addition, thefeedpressure for delivering the molten alloy to the tip of the capillary nozzle is also an important factor in controlling the size of the nanometer powders.

INTRODUCTION

The electrohydrodynamic (EHD) technique requires an intense electric field to generate charged liquid droplets from the liquid state, and these droplets are accelerated by the same electric field to a collector. The use of the EHD technique to produce nanometer-sized particles was introduced by Perel et al. in 1977 (1,2,3). In recent years, we have also studied EHD processing of nanoscale powders: sub-micron Sn-Bi-Pb powders were produced.

In this study, we report on the use of a fine quartz capillary tube (CO. 1 mm dia) to synthesize nauosized powders of Sn-Bi alloy.

EXPERIMENTAL

The experimental apparatus employed to produce nanometer powders is shown schemati- cally in Figure 1. Specific experimental parameters are shown in Table 1. The diameter of the quartz crucible tube is 9.2 - 9.5 mm. One end of this tube is drawn into a capillary. The cathode and anode are made of stainless steel.

793

794 XF Yu, ZQ Hu, X LIU AND Z JIANG

TABLE 1 Experimental Parameters

Diameter of the capillary nozzle Length of the capillary nozzle Height of the cathode tube Diameter of the cathode tube External diameter of the circular anode Internal diameter of the circular anode Vacuum in the chamber Distance between cathode and anode

:Z; co.1 10 15

:ZZ; 4.2 5

:Z; 3 (Ton.) 10-5 (m@ 4

A master ahoy of Sn-60 Wt.% Bi eutectic was prepared by vacuum melting. Small pieces were placed in a quartz jet tube, sealed up, and placed in the heater. When the vacuum in the chamber reached 10e5 torr, the Sn-Bi alloy was heated to 35OC, andthe molten alloy was delivered to the tip of the capillary nozzle to which a high voltage was applied. Charged liquid droplets were formed, which were scattered and accelerated by the electric field onto a collector. The droplets solidified in flight to produce the nanometer powders.

Powders were observed under a transmission electron microscope, Phillips EM-420. Size distribution of the nanometer powders was analyzed by an image analyzer.

1. seal plate 2. baseplate shield 3. collector 4. anode 5. stainless steel shell 6. droplet source 7. high voltage wire 8. procelain insulator 9. heat shield 10. emitter 11. high voltage source 12. shield 13. droplet beam 14. sealing ring 15. gas pressure source 16. vacuum set 17. heat power source

Figure 1. Schematic of experimental EHD apparatus.

NANOMETER Powoms PFKWCED BY ELECTROHYDROD~NAMIC TECHNICWE 795

High voltage, kV

Figure 2. Relationship between the emitting frequency of liquid droplets and the high voltage applied.

30

25

g . w 20

._ VI

.g I5 =o e 8 LZ 10

5

0

I-

O 5 10 15 20

High voltage, kV

Figure 3. Effect of high voltage on the mean size of the nanometer powders.

EXPERIMENTAL RESULTS AND DISCUSSION

Effects of High Voltage on the Emitting Frequency of Liquid Droplets and the Mean Size of the Nanometer Powders

Figures 2 and 3 show, under the experimental conditions of Table 1, the relationships between the emitting frequency of liquid droplets, the mean size of the nanometer powders ,and the high voltage, respectively. From these results it can be seen that with an increase of the high voltage, the emitting frequency of liquid droplets increases and the mean size of the nanometer powders decreases. The relationship between the mean size of the nanometer powders and the high voltage is given by (4)

6&cGV r = E(#J-JI -I- 2v@J77$7

where, r is the minimum mean size of the nanometer powders: v the surface tension of the liquid droplet; so the permittivity of free space; p the coefficient related to field strength: G the volumetric flow rate; V the high voltage; and I the EHD source current.

796 XF Yu, ZQ Hu, X LIU AND Z JIANG

m 2 -0.06

$- -0.0s

M 5 -0.10 8 t&

-0.12 0

\a 0 1

0.1 0.2 (

= 3 -0.06

- 1.3 -0.12 0 2 4 6 8 10

Diameter of the capillary nozzle, mm Length of the capillary nozzle, mm

Figure 4. Relationship between feed pressure Figure 5. Relationship between feed pressure and diameter of the capillary nozzle. and length of the capillary nozzle.

Effects of Diameter and Length of the Capillary Nozzle on the Feed Pressure for Delivering a Molten Alloy to the Tip of the Capillary Nozzle

Figures 4 and 5 show the relationships between the feed pressure and the diameter and length of the capillary nozzle, respectively. It canbe seen that the feed pressure decreases with an increase in the diameter of the capillary nozzle and a decrease in the length of the capillary nozzle. For a quartz jet tube with fixed capillary nozzle diameter and length, an appropriate feed pressure is necessary for emitting stable liquid droplets.

30

._ CA S ._ Ti kil = IO

g201 -J

0 3 z 0

0 0 0.1 0.2 0.3 Diameter of the capillary nozzle, mm

Figure 6. Relationship between mean size of the nanometer powders and diameter of the capillary nozzle.

NANOMETER POWDERS PRODUCED BY ELECTFIOHYDROCWNAMIC TECHNIQUE 797

Effect of Diameter of the Capillary Nozzle on the Mean Size of the Nanometer Powders

The relationship between the mean size of the nanometer powders and the diameter of the capillary nozzle is shown in Figure 6. The mean size of the nanometer powders increases as the diameter of the capillary nozzle increases. The size of the liquid cone produced in the capillary nozzle is determined by the diameter of the capillary nozzle, which affects field strength at the liquid cone tip and the size of the scattered liquid droplets. With decreasing diameter of the capillary nozzle, the size of the liquid cone in the capillary nozzle decreases, and the effect of the high voltage electric field on the scattered liquid droplets increases, so the mean size of the nanometer powders decreases.

Effect of Feed Pressure on the Mean Size of the Nanometer Powders

There are three kinds of resistance when a liquid-alloy flows through the capillary nozzle and begins to form a jet.

(i) The pressure drop for a flow in the capillary, Pi. Generally speaking, laminar flow occurs when the viscous liquid flows through the capillary. It is assumed that the flow of a molten alloy in a capillary is a stable laminar flow. According to the Darcy-Weisbach equation, the pressure drop Pl is given by (5)

where f is the friction factor: D the diameter of the capillary: L the length of the capillary: V,, the average-velocity across the section; and p the mass density of the molten alloy.

(ii) Additional pressure, P2, produced by surface tension of the liquid cone formed at the tip end of the quartz capillary.

p = 2ocose 2- r

where cr is the surface tension of the liquid cone, r the surface curvature radius of the liquid cone, and 0 the contact angle of the molten alloy and material of the jet nozzle.

(iii) Gas pressure, P3, out of the quartz capillary in the working chamber. In addition, the static pressure of a molten alloy, P,, affects the molten alloy itself

where p is the mass density of the molten alloy, g the acceleration of gravity,and H the height from the upper surface between the molten alloy to the lower end of the jet nozzle.

Only when the combined forces of the feed pressure, P, and the static pressure, Pg, are slightly larger than the total resistance, that is

798 XF Yu, ZQ Hu, X Ltu AND Z JIANG

Figure 7. TEM microgmph of Sn-Bi nanometer powders.

% 14

12

10

8

6

4

2

0 1.0 1.5 2.0 2.5 3.0 3.5

Powders size, nm

Figure 8. Particle size distribution of Sn-Bi nanometer powders shown in Figure 7.

will the molten alloy begin to form a jet. The feed pressure is the important condition for achieving a continuous and stable jet. The control for the feed pressure value and its stable state directly affects the mean size and distribution of the nanometer powders. If the feed pressure value is unstable, the mean size of the nanometer powders is not uniform. Its specific value must be determined by experiments.

NAN-R POWOERS PRODUCED BY ELE~TR~RODWMC TECHNIQUE 799

Morphology and Size Distribution of the Nanometer Powders

A TEM photomicrogmph of the Sn-Bi nanometer powders is shown in Figure 7, and the corresponding image analysis in Figure 8. Most of the nanoparticles are < 3 nm in size.

CONCLUSIONS

Sn-Bi nanometer powders can be produced by the elecbohydrodynamic technique. Critical variables in the powder synthesis are high voltage, shape and size of the cathode and anode, diameter and length of the quartz capillary nozzle, and the feed pressure.

1.

2.

3.

4.

5.

REFERENCES

J. Perel, J.F. Mahoney, B.E. Kalensher, K.E. Vickers, and R. Mehrabian, Rapid Solidification Processing: Principles and Technologies, eds. R. Mehrabian, B.H. Kear, and M. Cohen, Claitor Publishing Division, Baton Rouge, LA, p. 258 (1978). J. Perel, J.F. Mahoney, B.E. Kalensher, and R. Mehrabian, Twenty-fifth Sagamore Army Materials Conference on: Recent Advances in Metal Processing, Bolton Landiig, New York, p. 79 (1978). J. Perel, J.F. Mahoney, P. Duwez, and B.E. Kalensher,RapidSolidification Processing: Principles and Technologies 2, eds. R. Mehrabian, B.H. Kear, and M. Cohen, Claitor Publishing Division, Baton Rouge, LA, p. 287 (1980). H.S. Hu, X. Liu, L. Zou, L.Z. Cheng, G.L. Jia, G.Z. Zhaug , X.Q. Dong , and Z.Q. Hu, 94 Autumn Materials Investigation and Discussion Conference of the Peoples Republic of China, p. 1475 (1994) (in Chinese). J.J. Bertin, Engineering Fluid Mechanics, Prentice-Hall, Inc., Englewood Cliffs, New Jersey, p. 224 (1984).