Crystallization behavior of electroless Co-Ni-B alloy plated in magnetic field in presence of cerium

Crystallization behavior of electroless Co-Ni-B alloy plated in magnetic field in
presence of cerium

XUAN Tian-peng(???), ZHANG Lei(? ?), HUANG Qin-hua(???)

Institute of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China

Received 8 July 2005; accepted 7 November 2005


Abstract: The electrochemical property, chemical composition and crystal structure of electroless Co-Ni-B-Ce alloy plated in
general state as well as in magnetic field were studied using potentiometer, plasma emission spectrometer, X-ray diffractometer,
transmission electron microscope. The results show that the static potential and polarizability of electroless Co-Ni-B alloy are
remarkably improved as the plating is carried out in magnetic field in the presence of a little amount of cerium in plating bath.
Because of the action of magnetic field and rare earth element cerium, the boron content in alloy decreases, while cobalt and nickel
contents increase. As a result, the amorphous Co-Ni-B alloy transforms to the microcrystalline Co-Ni-B-Ce alloy when the plating is
in general state, and the Co-Ni-B alloy makes a crystalline transformation because of the action of magnetic field and rare earth
element cerium.

Key words: Co-Ni-B alloy; crystallization behavior; crystalline transformation; crystal structure; cerium; magnetic field



eletroless Co-Ni-B coating under different conditions are
1 Introduction
presented in this paper. The mechanism of magnetic field

and rare earth element cerium effect is also studied.
Electroless Co-Ni-B soft magnetic thin film has the

property of high microhardness, wearability and
2 Experimental
excellent soft magnetism[1, 2], and possesses many

advantages such as simple preparation and low cost. It
The sample material was red copper sheet with the
can be used as a new type of soft magnetic film in the
size of 20 mm×5 mm×1 mm. The starting formulation
electronic industry and computers. However, the bath
of plating bath was: CoCl2·6H2O 7 g/L; NiCl2·6H2O
stability, depositing speed and behaviour reproduction of
3 g/L; Na2C4H4O6·2H2O 60 g/L; Na2B4O4·10H2O 4 g/L;
electroless Co-Ni-B alloy coating are not very ideal. This
NaBH4 1.0 g/L; Ce 0.8 g/L, temperature 20-25?; pH
influences the research and application of the coating
value 10; loadage 0.4 dm2/L. Rare earth element cerium
directly. In order to solve these problems, rare earth
was added into plating bath in the form of chloride.
element cerium is added in the electroless Co-Ni-B alloy
Electroless Co-Ni-B-Ce plating bath was placed in
plating bath to improve the processing property and
open magnetic field with a copper sample appended in
service performance of the alloy coating[3?5]. On the
the middle of two magnetic poles. The line of
basis of this experiment, the effect of magnetic field on
magnetization was cut vertically by the copper sheet. The
preparing process and crystal structure of electroless
magnetic field induction was (400±10) ×10?4 T. The
Co-Ni-B-Ce alloy coating is studied. Although magnetic
cathode polarization curve of Co-Ni-B-Ce alloy was
field has obvious effect on the crystallization, the
measured by the electrochemical online system of the
figuration of the alloy as well as the mechanical, physical,
potentiometer (type TD3690). The working electrode
electrochemical properties of the material[6?12], no
was pure copper sheet with an area of 1 cm2 and
report has been found on the electroless Co-Ni-B alloy
insulated with single side. The material of auxiliary
plated in magnetic field by far. The electrochemical
electrode was platinum sheet. The reference electrode
property, chemical composition and structure of was HgO electrode. The scanning speed of the potentio-

Foundation item: Project(50371023) supported by the National Natural Science Foundation of China
Corresponding author: XUAN Tian-peng; Tel: +86-551-2903124; E-mail: xtpxm@mail.hf.ah.cn
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XUAN Tian-peng, et al/Trans. Nonferrous Met. Soc. China 16(2006)

meter was 3.6 mV/s. The chemical composition was
After being added in electroless Co-Ni-B plating bath,
determined by ICP/AES (Inductive Coupling Plasma/
rare earth element cerium with lively character will
Appearance Emission Spectrometre) with the type of
superiorly be adsorbed on the surface of matrix where
Plasma-Spec. The structure was examined by X-ray
crystal defects exist (such as end of dislocation, grain
diffractometry with Dmax/rb revolving anode and H-800
boundary, vacant position). In this way, rare earth
transmission electron microscope.
element can reduce the surface energy of matrix,

accelerate the adsorption of metallic complex ions and
?
3 Results and discussion
reductant ions BH to the surface of metal. At the
4

same time, rare earth element can also accelerate the
3.1 Cathode polarization curves of Co-Ni-B-Ce alloys
exchange of electrons on the interface, improve the static
The cathode polarization curves of Co-Ni-B-Ce
potential and decrease the polarizability. Furthermore,
alloys are shown in Fig.1. In general state, the static
the reaction speed of electrode is increased and the
potential of electroless Co-Ni-B alloy is ?0.512 V and
reduction ability of electroless Co-Ni-B alloy is also
the polarization curve is very steep with great
reinforced, so the depositing speed of alloy is promoted
polarizability (curve a). When the rare earth element
remarkably[4].
cerium is added in plating bath, the static potential of
With the magnetic field being applied to electroless
electroless Co-Ni-B-Ce alloy increases to ?0.496 V in
Co-Ni-B-Ce plating, the solution on the boundary
general state. Moreover, the polarization curve becomes
surface moves to the catalyzing surface of the sample.
smooth and the polarizability decreases(curve b). In the
Such phenomenon is called magnetohydrodynamic
magnetic field, the static potential of electroless
(MIID). Under the influence of magnetic field, the
Co-Ni-B-Ce alloy increases even more with the value
moving of charged ions to a certain direction accelerates
reaching ?0.457 V, then the polarization curve becomes
the convection of electroless plating bath, and slight stir
more smooth with the least polarizability(curve c). In the
action of plating bath occurs. And the thickness of
electrode reaction of electrochemical process, the lower
diffusion layer and the concentration polarization are
the static potential is, the more difficultly the alloy
decreased, so the ability of mass transmission in plating
deposits. With the polarizability increasing, the bath is improved, and the structure of electrical double
resistance to the reduction and deposition of alloy
layer is changed[6]. Consequently the static potential of
becomes greater, and the electrode reaction goes on more
alloy increases and the polarizability ameliorates, the
difficultly[13].
discharging, reducing and depositing of metallic ions are

facilitated on the surface of matrix.

3.2 Chemical composition of electroless Co-Ni-B-Ce
alloy coatings
The chemical composition of the electroless
Co-Ni-B-Ce alloy coatings is shown in Table 1. It shows
that the boron content decreases obviously, and the
cobalt and nickel contents of coatings increase with the
increasing of cerium in the coating.

Table 1 Chemical composition of electroless Co-Ni-B-Ce alloy
coatings (mass fraction, %)
Alloy coating
Co
Ni
B
Ce

Co-Ni-B (in general state)
93.21
0.69
6.1
0
Fig.1 Cathode polarization curves of electroless Co-Ni-B-Ce
Co-Ni-B-Ce(in general state) 96.19
1.02
2.65
0.14
alloys: (a) Co-Ni-B (in general state); (b) Co-Ni-B-Ce (in
Co-Ni-B-Ce (in magnetic field) 97.33 1.31 1.19
0.17
general state); (c) Co-Ni-B-Ce(in magnetic field)


It is generally believed that rare earth element can
The electronic structure of rare earth element
not be deposited easily from aqueous solution directly
cerium atom is [Xe]4f15d16s2. Its 4f electron encloses
because of the lower electronegativity. But under the
atomic nuclear imprecisely, so the shielding factor is less
inducement effects of the appropriate complexing agent
than that of other inner electrons with the same principal
and transition metal, the electrode potential of rare earth
quantum number. Accordingly cerium atom possesses
element increases in the positive direction while the
larger valid nuclear charge number and represents
electrode potential of transition metal increases in the
powerful adsorptive capacity to the electrons around it.

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XUAN Tian-peng, et al/Trans. Nonferrous Met. Soc. China 16(2006)
365
negative direction. Then the reduction and co-deposition
electroless Co-Ni-B-Ce alloy coating in general state
of rare earth metal and transition metal can be achieved.
(Fig.3(b)) shows that the inner halos are clear and
Moreover, the rare earth metal ion is one of the few
distinct, and the exterior slender rings do not take on the
cations which can be characteristically adsorbed on the
character of broadening. This can be considered micro-
surface of electrode[14]. After being adsorbed on the
crystalline structure. Fig.3(c) shows the diffraction
surface of matrix, the rare earth metal ion facilitates the
pattern of electroless Co-Ni-B-Ce alloy plated in
adsorption, discharge, reducing and deposition of the
magnetic field, which is made up of many clear scattered
metal ions on the surface of the sample. Thus electroless
halos. It indicates that the coating transforms to
Co-Ni-B alloy coating containing rare earth element
crystalline with the presentation of diffraction halos of
cerium is formed.
Co(100), Co(002), Co(101),Co(110) and so on. The TEM
Rare earth elements have low electronegativity and
morphologies of electroless Co-Ni-B-Ce alloy coatings
great activity. In alkaline plating bath, rare earth element
in different conditions are shown in Fig.4. From Fig.4(a),
cerium easily turns into cation to react as reductant,
it is known that the electroless Co-Ni-B coating shows
which can accelerate the reduction of metallic ions and
black and white regions with indistinct borders, taking on
reduce the consumption of reductant NaBH4[4], then the
the amorphous morphology character. The morphology
quantity of reduced boron atoms decreases accordingly.
of electroless Co-Ni-B-Ce alloy coating plated in general
When coating contains a little amount of cerium, the
state is changed (Fig.4(b)), the borders between strong
boron content decreases, while the cobalt and nickel
and weak regions are clear and fine crystal grains can be
contents of coating increase.
examined faintly. The clear massive crystalline grains of
Under the effect of Lorentz force which is generated
electroless Co-Ni-B-Ce alloy coating plated in magnetic
by the magnetic field, metal ions move in the direction of
field are observed in Fig.4(c).
line of magnetization[8]. This is useful for the reduction

and deposition of the ions of transition metal and rare
earth metal. Therefore, the quantity of cerium atoms in
Co-Ni-B-Ce coating plated in magnetic field is more
than that of coating plated in general state. Accordingly
the boron content decreases, while the cobalt and nickel
contents of coating plated in magnetic field increase
further.

3.3 Crystal structure of electroless Co-Ni-B-Ce alloy
coatings
The X-ray diffraction(XRD) patterns of electroless
Co-Ni-B-Ce alloy coatings in different conditions are
shown in Fig.2. The XRD pattern of electroless Co-Ni-B
alloy coating comes forth a diffraction peak with the

shape of steamed bread at 2?=45?, which is the character
Fig.2 XRD patterns of electroless Co-Ni-B-Ce alloy coatings:
of amorphous diffraction. In general state, the pattern of
(a) Co-Ni-B(in general state); (b) Co-Ni-B-Ce(in general state);
electroless Co-Ni-B-Ce coating shows a resemblance
(c) Co-Ni-B-Ce(in magnetic field)
with that of electroless Co-Ni-B coating, but the

half-width height of its diffraction peak decreases by
The difference between the negativity of cobalt or
about 1?. So it presents certain trend to crystallization,
nickel and that of boron is so great that the interaction
which possesses the character of microcrystalline
between them is intensive. Thus their alloy will form into
structure. When magnetic field is applied to the plating,
amorphous state easily. What’s more, the higher the
electroless Co-Ni-B-Ce coating has come into the
content of the metalloid is, the greater the probability and
structure of crystallization with the presentation of
the stability of forming amorphous state are. At the same
diffraction peaks of Co(100), Co(002), Co(101),Co(110)
time the co-deposition of Ni, Co and B increases the
and so on.
degree that the atoms accumulate randomly, which also
The TEM electron diffraction patterns of electroless
enhances the trend of forming amorphous state[15].
Co-Ni-B-Ce alloy coatings under different conditions are
After being added in electroless Co-Ni-B plating
shown in Fig.3. From Fig.3(a), it can be seen that the
bath, rare earth element cerium can adsorb the other
diffraction pattern of electroless Co-Ni-B alloy coating is
depositing atoms to the surface. And metallic atoms
a broadening diffraction halo, which indicates that such
deposit along the lattice orientation of matrix. Then the
film is amorphous structure. The diffraction pattern of
three-dimensional epitaxial growth occurs. The regular

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XUAN Tian-peng, et al/Trans. Nonferrous Met. Soc. China 16(2006)


Fig.3 TEM electron diffraction patterns of electroless Co-Ni-B-Ce coatings: (a) Co-Ni-B(in general state); (b) Co-Ni-B-Ce(in general
state); (c) Co-Ni-B-Ce(in magnetic field)


Fig.4 Bright field TEM images of electroless Co-Ni-B-Ce coatings: (a) Co-Ni-B(in general state); (b) Co-Ni-B-Ce(in general state);
(c)Co-Ni-B-Ce(in magnetic field)

distribution of atoms and the decrease of boron content
2) In electroless Co-Ni-B-Ce alloys which are
for the existence of cerium will make the trend of
plated in general state and in magnetic field, the boron
forming amorphous descend, and it shows some trend of
content decreases, and the cobalt and nickel contents of
crystallization[16]. When magnetic field is applied to
alloy coatings increase with the increasing of cerium
electroless Co-Ni-B-Ce plating process, the contents of
content of the alloys.
Ce, Co and Ni increase further and the B content of
3)The operating condition and chemical
electroless Co-Ni-B-Ce coating decreases. In this case
composition of electroless Co-Ni-B-Ce alloy affect the
the adsorption, reduction and deposition of atoms are
crystalline structure of alloy remarkably. The crystal
significantly affected. This restrains the trend of forming
structure of electroless Co-Ni-B alloy coating in general
amorphous in alloy coating, so the coating transforms to
state is amorphous. The alloy containing rare earth
crystalline from microcrystalline.
element cerium transforms to microcrystalline state.

When magnetic field is applied to the electroless plating
4 Conclusions
process, the electroless Co-Ni-B-Ce alloy transforms to

crystalline structure.
1) When rare earth element cerium is added to

plating bath and plating is carried out in magnetic field,
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