Materials Science and Engineering A 393 (2005) 191–195
Pore formation in iron micro-spheres by plasma procedure
Ioan Bica
DepartmentofPhysics,WestUniversityofTimisoara,Bd.V.Parvan,No.4,1900Timisoara,Romania
Received 19 April 2004; received in revised form 6 October 2004; accepted 7 October 2004
Abstract
The paper presents the mechanism of hollow Fe micro-sphere production in Ar plasma jet. The required condition for cavity (pore) formation
and the dependence of the pore radius on the plasma jet velocity are derived. Technological data concerning the micro-sphere production are
given and comparison between the model predictions and experiment is made.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Iron micro-spheres; Pores; Superficial energy; Inertial forces
1. Introduction
Micro-spheres are particles, hollow inside, with a diameter
ranging between 1 and 100 m. They are interesting from a
scientific and practical standpoint. The scientific interest lies
in the formation [1] and the formation mechanisms [2] of
micro-spheres. The practical interest for iron micro-spheres is
related to the transport of working substances, as exemplified
in [3]. Interesting applications of iron micro-particles are the
biological ones, as shown in [4–6].
In all cases, it is necessary for the micro-sphere wall to
have pores. They are needed for the introduction of work-
ing substances in the micro-spheres. The production of iron
micro-spheres with pores, in plasma jet, is a reality [1]. How-
ever, the production of pore micro-spheres in increased quan-
tities and with a pre-established pore diameter is a necessity.
Consequently, in what follows, we show the mechanisms
of iron micro-sphere formation as well as the dependence of
the pore diameter on the velocity of the plasma jet.
2. Model
2.1. Stages in micro-sphere formation
The carbon-steel electrode is introduced uniformly in the
plasma. In the electrode-plasma interaction, the electrode is
E-mailaddress: ibica2@yahoo.com.
melted. The electrode velocity is correlated with that of the
plasma jet. At the temperature T
0
of the plasma, the metal
drops change into vapors. The temperature of the vapors in-
stantaneously reaches the temperature of the plasma. The
molar concentration C
0
of the vapors is low by comparison
with that of the gas.
Consequently, the vapors can be assimilated to an ideal
gas. If m
0i
is the mass of vapors, then the radius r
0i
of the
sphere of vapors results, according to [7], from the relation:
r
0i
=
3
4π
m
0i
µ
1
C
0
1/3
(1)
in which µ is the molar mass of the iron vapors.
The vapor spheres (Fig. 1a) are driven by the plasma jet.
The vapor-gas interface reaches regions of temperatures T
1
equal to the temperature of the dew point temperature for
iron, and changes into a liquid membrane (Fig. 1b). The molar
concentration of the so-formed liquid phase drops is [7]:
C
c
= C
0
exp
E
ℜ· T
(2)
where E is the activating energy of condensation, ℜ the
universal constant of ideal gases, and T = T
0
- T
1
the un-
dercooling.
The process of liquid membrane formation takes place at
constant pressures. The liquid membrane vapor sphere radius
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2004.10.011